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Interpretation of the two-components observed in high resolution X-ray diffraction ω scan peaks for mosaic ZnO thin fi lms grown on c-sapphire substrates using pulsed laser deposition
Abstract
X-ray scattering methods were applied to the study of thin mosaic ZnO layers deposited on c-Al2O3 substrates using Pulsed Laser Deposition. High Resolution (HR) studies revealed two components in the ω scans (transverse scans) which were not resolved in conventional “open-detector” ω rocking curves: a narrow, resolution-limited, peak, characteristic of long-range correlation, and a broad peak, attributed to defect-related diffuse-scattering inducing a limited transverse structural correlation length. Thus, for such mosaic films, the conventional ω rocking curve Full Width at Half Maximum linewidth was found to be ill-adapted as an overall figure-of-merit for the structural quality, in that the different contributions were not meaningfully represented. A “Williamson-Hall like” integral breadth (IB) metric for the HR (00.l) transverse-scans was thus developed as a reliable, fast, accurate and robust alternative to the rocking curve linewidth for routine non-destructive testing of such mosaic thin films. For a typical ZnO/c-Al2O3 film, the IB method gave a limited structural correlation length of 110nm±9nm. The results are coherent with a thin film containing misfit dislocations at the film-substrate interface.
... This so-called correlation of defects defines an associated correlation length, determining the area of zero average net displacement. Even with relatively high defect densities, coherent x-ray scattering can be preserved if the position and, eventually, the orientation of the defects are correlated, leading to very narrow FWHMs in ω rocking curves [204][205][206][207][208][209][210][211][212][213][214][215][216][217]. Besides defects, also the finite size of crystallites lead to peak broadening. ...
... The narrow peaks have approximately the same FWHM than the symmetric MgO (200) reflection of the substrate, as shown in the inset of Figure 4.12(a). At this stage, we assume that this narrow peak is linked to a coherent scattering phenomenon with a homogeneous spatial distribution of some crystalline defects displaying long correlation distances, as explained in the previous section, and as observed previously in other material systems [204][205][206][207][208][209][210][211][212][213][214][215][216][217]. Interestingly, the peak area of the broad contribution increases as a function of thickness with respect to the narrow one, indicating that this contribution has an origin independent of the correlated defects. ...
... Using two pseudo-voigt functions we fitted the transverse ω scans of all samples before normalization in order to evaluate the absolute intensities of both contributions. As shown in tions [205,206,[209][210][211], misfit dislocations at the film-substrate interface [204,[206][207][208][213][214][215][216], misorientations [209,215,216] or grain boundaries [212,217]. ...
Cubic group II-nitride materials are almost unknown in the scientific community, even though many of them consists of earth-abundant elements. Zn3_ N2_ and Mg3_ N2_, for instance, are non-toxic and have band gaps of around 1 eV and 3 eV, respectively, making them attractive candidates for the application in optoelectronic devices. However, in order to evaluate these materials for device applications it is necessary to grow them in a sufficiently high quality, and characterize their fundamental physical properties.In this work we develop the epitaxial growth of Zn3_ N2_ and Mg3_ N2_ thin films by MBE. To prevent the decomposition of Mg3N2 thin films in air, we employed polycrystalline MgO capping layers grown in-situ in the MBE chamber, thereby increasing the Mg3_ N2_ lifetime from a few minutes to several weeks in air and enabling further characterization. The film orientation of both Zn3_ N2_ and Mg3_ N2_ on MgO (100) substrates can be tuned from epitaxial (100) to (111) by varying the growth conditions. Apart from that, epitaxial Zn3_ N2_ (100) is obtained on YSZ (100), whereas epitaxial Mg3_ N2_ (111) is obtained on YSZ (100) and MgO (111) substrates. The epitaxial relationships and structural qualities are analyzed for all thin film-substrate combinations using reflection high-energy electron diffraction, x-ray diffraction and/or transmission electron microscopy, giving thereby a detailed insight into group-II nitride epitaxy and their film microstructure, as never reported before in the literature. Besides their structural properties, we explored some of the fundamental physical properties of Zn3_ N2_ and Mg3_ N2_. Due to the sensitivity of II-nitrides towards humidity, a reliable determination of the optical band gap of Zn3_ N2_ and Mg3_ N2_ is challenging. By employing photoluminescence, diffuse reflectance and transmittance measurements we measure the band gap of our MBE-grown Mg3_ N2_ thin films to be 2.9 eV at room temperature, as well as the band gap of MBE-grown Zn3_ N2_ as a function of carrier concentration (1.0 eV at room temperature, blue-shifted due to the Moss-Burstein effect). While Zn3_ N2_ thin films were found to be n-type and degenerate, with carrier concentrations between 10〖19〗^ cm〖-3〗^ and 10〖21〗^ cm〖-3〗^, consistent with previous literature work, Mg3_ N2_ thin films turn out to be electrically insulating. Finally, we monitored the out-of-plane lattice dilatation as a function of temperature for Zn3_ N2_ and Mg3_ N2_ thin films, enabling the extraction of their linear thermal expansion coefficients therefrom, an important parameter for the design of epitaxial heterostructures. We established average values of 1.5 × 10〖-5〗^ K〖-1〗^ and 1.1 × 10〖-5〗^ K〖-1〗^ in a temperature range from 300 K to 700 K - 800 K for Zn3_ N2_ and Mg3_ N2_, respectively, three to four times higher than III-nitrides.
... These two broadening mechanisms can be distinguished by choosing an asymmetric reflection where the arc deviates significantly from the k x direction or by analyzing a series of symmetric reflections, which will be affected differently by the two broadening mechanisms depending on their distance from the origin. The latter is called the Williamson-Hall like analysis and is described in detail in [DLRHT11]. ...
... This narrow peak is likely a result of partially coherent scattering between individual crystallites with limited displacement towards each other and is regularly observed in the material system studied in this thesis. [DLRHT11] Besides symmetric reflections, asymmetric reflections with non-zero h or k can be measured by tilting the sample in ω or ψ direction until the respective planes are in reflection geometry. Measuring the position of such an asymmetric reflection gives access to the in-plane lattice constant a using equation 4.6 since h, k or both are non-zero. ...
... These peaks indicate the existence of long-range structural correlations as they are observed in unconventional mosaic crystals whose individual mosaic blocks can scatter X-rays coherently with respect to each other. [DLRHT11] The slightly deviating angles relative to the broad peak and the dashed line indicate a small misalignment to the substrate planes, likely a result of the small unintentional miscut of the substrate. The FWHM is resolution limited by the instrument and will be analyzed in greater detail on a more suitable sample later in this chapter. ...
The subject of this thesis is the fabrication and characterization of magnetic topological insulator layers of (V,Bi,Sb)Te exhibiting the quantum anomalous Hall effect. A major task was the experimental realization of the quantum anomalous Hall effect, which is only observed in layers with very specific structural, electronic and magnetic properties. These properties and their influence on the quantum anomalous Hall effect are analyzed in detail. First, the optimal conditions for the growth of pure BiTe and SbTe crystal layers and the resulting structural quality are studied. The crystalline quality of BiTe improves significantly at higher growth temperatures resulting in a small mosaicity-tilt and reduced twinning defects. The optimal growth temperature is determined as 260C, low enough to avoid desorption while maintaining a high crystalline quality. The crystalline quality of SbTe is less dependent on the growth temperature. Temperatures below 230C are necessary to avoid significant material desorption, though. Especially for the nucleation on Si(111)-H, a low sticking coefficient is observed preventing the coalescence of islands into a homogeneous layer. The influence of the substrate type, miscut and annealing sequence on the growth of BiTe layers is investigated. The alignment of the layer changes depending on the miscut angle and annealing sequence: Typically, layer planes align parallel to the Si(111) planes. This can enhance the twin suppression due to transfer of the stacking order from the substrate to the layer at step edges, but results in a step bunched layer morphology. For specific substrate preparations, however, the layer planes are observed to align parallel to the surface plane. This alignment avoids displacement at the step edges, which would cause anti-phase domains. This results in narrow Bragg peaks in XRD rocking curve scans due to long-range order in the absence of anti-phase domains. Furthermore, the use of rough Fe:InP(111):B substrates leads to a strong reduction of twinning defects and a significantly reduced mosaicity-twist due to the smaller lattice mismatch. Next, the magnetically doped mixed compound V(BiSb)Te is studied in order to realize the quantum anomalous Hall effect. The addition of V and Bi to SbTe leads to efficient nucleation on the Si(111)-H surface and a closed, homogeneous layer. Magneto-transport measurements of layers reveal a finite anomalous Hall resistivity significantly below the von Klitzing constant. The observation of the quantum anomalous Hall effect requires the complete suppression of parasitic bulklike conduction due to defect induced carriers. This can be achieved by optimizing the thickness, composition and growth conditions of the layers. The growth temperature is observed to strongly influence the structural quality. Elevated temperatures result in bigger islands, improved crystallographic orientation and reduced twinning. On the other hand, desorption of primarily Sb is observed, affecting the thickness, composition and reproducibility of the layers. At 190C, desorption is avoided enabling precise control of layer thickness and composition of the quaternary compound while maintaining a high structural quality. It is especially important to optimize the Bi/Sb ratio in the (V,Bi,Sb)Te layers, since by alloying n-type BiTe and p-type SbTe charge neutrality is achieved at a specific mixing ratio. This is necessary to shift the Fermi level into the magnetic exchange gap and fully suppress the bulk conduction. The Sb content x furthermore influences the in-plane lattice constant a significantly. This is utilized to accurately determine x even for thin films below 10 nm thickness required for the quantum anomalous Hall effect. Furthermore, x strongly influences the surface morphology: with increasing x the island size decreases and the RMS roughness increases by up to a factor of 4 between x = 0 and x = 1. A series of samples with x varied between 0.56-0.95 is grown, while carefully maintaining a constant thickness of 9 nm and a doping concentration of 2 at.% V. Magneto-transport measurements reveal the charge neutral point around x = 0.86 at 4.2 K. The maximum of the anomalous Hall resistivity of 0.44 h/e is observed at x = 0.77 close to charge neutrality. Reducing the measurement temperature to 50 mK significantly increases the anomalous Hall resistivity. Several samples in a narrow range of x between 0.76-0.79 show the quantum anomalous Hall effect with the Hall resistivity reaching the von Klitzing constant and a vanishing longitudinal resistivity. Having realized the quantum anomalous Hall effect as the first group in Europe, this breakthrough enabled us to study the electronic and magnetic properties of the samples in close collaborations with other groups. In collaboration with the Physikalisch-Technische Bundesanstalt high-precision measurements were conducted with detailed error analysis yielding a relative de- viation from the von Klitzing constant of (0.17 0.25) * 10. This is published as the smallest, most precise value at that time, proving the high quality of the provided samples. This result paves the way for the application of magnetic topological insulators as zero-field resistance standards. Non-local magneto-transport measurements were conducted at 15 mK in close collaboration with the transport group in EP3. The results prove that transport happens through chiral edge channels. The detailed analysis of small anomalies in transport measurements reveals instabilities in the magnetic phase even at 15 mK. Their time dependent nature indicates the presence of superparamagnetic contributions in the nominally ferromagnetic phase. Next, the influence of the capping layer and the substrate type on structural properties and the impact on the quantum anomalous Hall effect is investigated. To this end, a layer was grown on a semi-insulating Fe:InP(111)B substrate using the previously optimized growth conditions. The crystalline quality is improved significantly with the mosaicity twist reduced from 5.4 to 1.0. Furthermore, a layer without protective capping layer was grown on Si and studied after providing sufficient time for degradation. The uncapped layer on Si shows perfect quantization, while the layer on InP deviates by about 5%. This may be caused by the higher crystalline quality, but variations in e.g. Sb content cannot be ruled out as the cause. Overall, the quantum anomalous Hall effect seems robust against changes in substrate and capping layer with only little deviations. Furthermore, the dependence of the quantum anomalous Hall effect on the thickness of the layers is investigated. Between 5-8 nm thickness the material typically transitions from a 2D topological insulator with hybridized top and bottom surface states to a 3D topological insulator. A set of samples with 6 nm, 8 nm, and 9 nm thickness exhibits the quantum anomalous Hall effect, while 5 nm and 15 nm thick layers show significant bulk contributions. The analysis of the longitudinal and Hall conductivity during the reversal of magnetization reveals distinct differences between different thicknesses. The 6 nm thick layer shows scaling consistent with the integer quantum Hall effect, while the 9 nm thick layer shows scaling expected for the topological surface states of a 3D topological insulator. The unique scaling of the 9 nm thick layer is of particular interest as it may be a result of axion electrodynamics in a 3D topological insulator. Subsequently, the influence of V doping on the structural and magnetic properties of the host material is studied systematically. Similarly to Bi alloying, increased V doping seems to flatten the layer surface significantly. With increasing V content, Te bonding partners are observed to increase simultaneously in a 2:3 ratio as expected for V incorporation on group-V sites. The linear contraction of the in-plane and out-of-plane lattice constants with increasing V doping is quantitatively consistent with the incorporation of V ions, possibly mixed with V ions, at the group-V sites. This is consistent with SQUID measurements showing a magnetization of 1.3 per V ion. Finally, magnetically doped topological insulator heterostructures are fabricated and studied in magneto-transport. Trilayer heterostructures with a non-magnetic (Bi,Sb)Te layer sandwiched between two magnetically doped layers are predicted to host the axion insulator state if the two magnetic layers are decoupled and in antiparallel configuration. Magneto-transport measurements of such a trilayer heterostructure with 7 nm undoped (Bi,Sb)Te between 2 nm thick layers doped with 1.5 at.% V exhibit a zero Hall plateau representing an insulating state. Similar results in the literature were interpreted as axion insulator state, but in the absence of a measurement showing the antiparallel magnetic orientation other explanations for the insulating state cannot be ruled out. Furthermore, heterostructures including a 2 nm thin, highly V doped layer region show an anomalous Hall effect of opposite sign compared to previous samples. A dependency on the thickness and position of the doped layer region is observed, which indicates that scattering at the interfaces causes contributions to the anomalous Hall effect of opposite sign compared to bulk scattering effects. Many interesting phenomena in quantum anomalous Hall insulators as well as axion insulators are still not unambiguously observed. This includes Majorana bound states in quantum anomalous Hall insulator/superconductor hybrid systems and the topological magneto-electric effect in axion insulators. The limited observation temperature of the quantum anomalous Hall effect of below 1 K could be increased in 3D topological insulator/magnetic insulator heterostructures which utilize the magnetic proximity effect. The main achievement of this thesis is the reproducible growth and characterization of (V,Bi,Sb)2Te3 layers exhibiting the quantum anomalous Hall effect. The detailed study of the structural requirements of the quantum anomalous Hall effect and the observation of the unique axionic scaling behavior in 3D magnetic topological insulator layers leads to a better understanding of the nature of this new quantum state. The high-precision measurements of the quantum anomalous Hall effect reporting the smallest deviation from the von Klitzing constant are an important step towards the realization of a zero-field quantum resistance standard.
... The transverse scan is like a rocking curve measurement but uses the monochromator on the detector side, as opposed to a double-axis scan with a wide-open detector for a classical rocking curve. [26] In the case of large and moderate mismatch with GaAs and InAs, we focus primarily on the film morphology determined by the transverse scans, the atomic arrangement at the interface, and dislocation network as much of the starting mismatch strain is relaxed even for ultrathin films. On the other hand, we use x-ray reciprocal space maps (RSMs) to more accurately study strain relaxation in PbSe on the low-mismatch InAsSb/GaSb template. ...
... [29,30] Two-component transverse scan signatures have since been observed in a range of epitaxial films, spanning metals to ceramics. [26,31] The intensity of the coherent scatter component is predicted to reduce with increasing order (hkl) of the reflection, also seen in our experiments (and more clearly in section III.B), but this expectation is for very thin films whose thickness is on the order of the average misfit dislocation spacing. [30] Remembering that the favored Burgers vector of dislocations in PbSe is 2 〈110〉 (similar to III-V zincblende materials), a semi-coherent interface with a square network of edge misfit dislocations to relieve 8% strain would correspond to a dislocation spacing of approximately 5 nm, much smaller than the film thickness. ...
PbSe and related IV-VI rocksalt-structure semiconductors have important electronic properties that may be controlled by epitaxial strain and interfaces, thus harnessed in an emerging class of IV-VI/III-V heterostructures. The synthesis of such heterostructures and understanding mechanisms for strain-relief is central to achieving this goal. We show that a range of interfacial defects mediate lattice mismatch in (001)-oriented epitaxial thin films of PbSe with III-V templates of GaAs, InAs, and GaSb. While the primary slip system {100}<110> for dislocation glide in PbSe is well-studied for its facile glide properties, it is inactive in (001)-oriented films used in our work. Yet, we obtain nearly relaxed PbSe films in the three heteroepitaxial systems studied with interfaces ranging from incoherent without localized misfit dislocations on 8.3% mismatched GaAs, a mixture of semi-coherent and incoherent patches on 1.5% mismatched InAs, to nearly coherent on 0.8% mismatched GaSb. The semi-coherent portions of the interfaces to InAs form by 60{\deg} misfit dislocations gliding on higher order {111}<110> slip systems. On the more closely lattice-matched GaSb, arrays of 90{\deg} (edge) misfit dislocations form via a climb process. The diversity of strain-relaxation mechanisms accessible to PbSe makes it a rich system for heteroepitaxial integration with III-V substrates.
... However, if the position and, eventually, the orientation of the defects are correlated, coherent scattering can be preserved even in the presence of large defect densities, leading to extremely narrow linewidths in transverse ω scans. [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] While XRD gives a global view of the crystal's microstructure and lattice parameters, transmission electron microscopy (TEM) enables a local insight into the film structure that allows the identification of lattice defects and the determination of their spatial arrangement. Unfortunately, TEM is a destructive technique that requires time-consuming sample preparation. ...
... At this stage, we assume that the coherent x-ray scattering phenomenon leading to this peak is associated with a regular distribution of some crystalline defect displaying very long correlation distances, as observed previously in other material systems. [11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] Interestingly, while an increase of the overall diffuse scattering intensity with respect to the coherent one is observed as a function of sample thickness, the integrated intensity of the coherent peak itself remains roughly constant. This observation points toward its origin being located close to the film-substrate interface, with the initially correlated defects being overgrown by a disordered Mg 3 N 2 displaying correlations at much smaller spatial scales. ...
The epitaxial growth of Mg3N2 thin films by molecular beam epitaxy has been recently achieved. This work presents the structural properties of the films, including grain sizes and lattice rotations, as assessed by x-ray diffraction and transmission electron microscopy. The films’ microstructure consists of well-aligned columnar grains 10 nm in diameter that nucleate at the film/substrate interface and display a significant column twist, in the order of 2.5°. As growth proceeds, tilted and twisted mosaic blocks overgrow these columns, as observed in many other epitaxial semiconductors. Yet, the rocking curves on symmetric reflections display extremely narrow peaks (∼50 arc sec), revealing a long-range spatial correlation between structural defects that should not be mistakenly considered a proof of high crystalline quality.
... substrate reflection shows a FWHM of 0.08 • , which also points toward a limited angular resolution of the chosen optical setup. It should be noted, the broad peak FWHMs of this work cannot be directly compared to the rocking curve FWHMs of significantly thicker samples due to the difference in the line shape broadening mechanisms35,39 .To confirm epitaxial growth and to determine the epitaxial relationships between film and substrate, in-plane pole figures were measured on the reference sample(Figure 4(a) for the β-Ga 2 O 3 (201) reflection at 2θ = 18.95 • , and Figure 4(b) for the β-Ga 2 O 3 (111) and α-Al 2 O 3 (104) reflections at 2θ = 35.18 • , respectively. ...
Deposition of epitaxial oxide semiconductor films using physical vapor deposition methods requires a detailed understanding of the role of energetic particles to control and optimize the film properties. In the present study, Ga 2O 3 thin films are heteroepitaxially grown on Al 2O 3(0001) substrates using oxygen ion beam sputter deposition. The influence of the following relevant process parameters on the properties of the thin films is investigated: substrate temperature, oxygen background pressure, energy of primary ions, ion beam current, and sputtering geometry. The kinetic energy distributions of ions in the film-forming flux are measured using an energy-selective mass spectrometer, and the resulting films are characterized regarding crystalline structure, microstructure, surface roughness, mass density, and growth rate. The energetic impact of film-forming particles on the thin film structure is analyzed, and a noticeable decrease in crystalline quality is observed above the average energy of film-forming Ga + ions around 40 eV for the films grown at a substrate temperature of 725 °C.
... The presence of diffuse scattering close to Bragg peaks indicates a high degree of crystalline disorder and lattice disruptions. 21,22 Quantitative analysis of polish-induced defect can be obtained by measuring the diffuse scatter on the X-ray rocking curves. For C-plane sapphire (Fig. 3), the highest intensity of diffuse scatter was observed on the coupon which was mechanical polished using the largest size diamond abrasives of d mean = 3 μm (around 10 1 a.u.) and gradual decreasing intensities of diffuse scatter were observed with the lower abrasive sizes of d mean = 1 μm and 0.25 μm. ...
Single-crystal sapphire is known to be among the hardest insulators. Its mechanical properties and chemical inertness make it a challenging material to polish for the atomic-level surface smoothness required for its applications. Mechanical polish with diamond abrasives renders high removal rates but creates unacceptable levels of polish-induced gouges. Chemical mechanical polish on the other hand results in atomic smoothness but is a slow process. Hence, a combination of the two is used in the industry. In this work, we have attempted to characterize gouging and subsurface damage using atomic force microscopy, X-ray diffraction, and cross-section transmission electron microscopy on C-plane and A-plane sapphire induced by diamond abrasive mechanical polish and chemical mechanical polish with colloidal silica.
... The periodicity of the oscillations can be used to extract the crystalline ordered volume, with the agreement between t Laue = 500 ± 10 Å and the XRR derived thickness of t U = 514 ± 6 Å suggesting that crystalline order is maintained throughout the full thickness of the U layer. The rocking curve also adopts the distinctive two-component line shape common to many high quality thin films [30,31]. ...
Thin layers of orthorhombic uranium (α-U) have been grown onto buffered sapphire substrates by dc magnetron sputtering, resulting in the discovery of new epitaxial matches to Ti(00.1) and Zr(00.1) surfaces. These systems have been characterized by x-ray diffraction and reflectivity and the optimal deposition temperatures have been determined. More advanced structural characterization of the known Nb(110) and W(110) buffered α-U systems has also been carried out, showing that past reports of the domain structures of the U layers are incomplete. The ability of this low symmetry structure to form crystalline matches across a range of crystallographic templates highlights the complexity of U metal epitaxy and points naturally toward studies of the low temperature electronic properties of α-U as a function of epitaxial strain.
... Further structural investigations via RSM to explain the peak asymmetry observed in the 2θ −ω XRD scan are not possible due to the low intensity of the asymmetric (246) peak. The ω scan of a YAlIG film annealed at T A = 800 • C, given in the inset of Fig. 1(a) film materials [44][45][46][47][48]. The sharp peak is associated with areas of higher structural order, i.e., an epitaxially strained crystalline layer close to the substrate/film interface. ...
Magnetic garnets such as yttrium iron garnet (Y3Fe5O12, YIG) are widely used in spintronic and magnonic devices. Their magnetic and magneto-optical properties can be modified over a wide range by tailoring their chemical composition. Here, we report the successful growth of Al-substituted yttrium iron garnet (YAlIG) thin films via radio frequency sputtering in combination with an ex situ annealing step. Upon selecting appropriate process parameters, we obtain highly crystalline YAlIG films with different Al3+ substitution levels on both single crystalline Y3Al5O12 (YAG) and Gd3Ga5O12 (GGG) substrates. With increasing Al3+ substitution levels, we observe a reduction of the saturation magnetization as well as a systematic decrease of the magnetic ordering temperature to values well below room temperature. YAlIG thin films thus provide an interesting material platform for spintronic and magnonic experiments in different magnetic phases.
... A hybrid monochromator 2-bounce Ge (220) mirror is used as the incident beam optical module, and a 0.27 • parallel plate collimator is used as the diffracted beam optical module. The curve data of the XRD and XRC measurement was fitted using a Voigt peak shape [32]. The function was derived by integrating over the Lorentz profile and weighing with a Gauss function using the numerical software. ...
... The periodicity of the oscillations can be used to extract the crystalline ordered volume, with the agreement between t Laue = 500 ± 10Å and the XRR derived thickness of t U = 514±6Å suggesting that crystalline order is maintained throughout the full thickness of the U layer. The rocking curve also adopts the distinctive two-component lineshape common to many high quality thin films [30,31]. ...
Thin layers of orthorhombic uranium ({\alpha}-U) have been grown onto buffered sapphire substrates by d.c. magnetron sputtering, resulting in the discovery of new epitaxial matches to Ti(00.1) and Zr(00.1) surfaces. These systems have been characterised by X-ray diffraction and reflectivity and the optimal deposition temperatures have been determined. More advanced structural characterisation of the known Nb(110) and W(110) buffered {\alpha}-U systems has also been carried out, showing that past reports of the domain structures of the U layers are incomplete. The ability of this low symmetry structure to form crystalline matches across a range of crystallographic templates highlights the complexity of U metal epitaxy and points naturally toward studies of the low temperature electronic properties of {\alpha}-U as a function of epitaxial strain.
... The FWHM of each peak was determined by numerically fitting of the data to the sum of two Pseudo-Voigt functions as 0.04 • and 0.46 • . A similar behaviour has been reported for different heteroepitaxially relaxed thin film materials [44][45][46][47][48]. The sharp peak is associated with areas of higher structural order, i.e. an epitaxially strained crystalline layer close to the substrate/film interface. ...
Magnetic garnets such as yttrium iron garnet (YFeO, YIG) are widely used in spintronic and magnonic devices. Their magnetic and magneto-optical properties can be modified over a wide range by tailoring their chemical composition. Here, we report the successful growth of Al-substituted yttrium iron garnet (YAlIG) thin films via radio frequency sputtering in combination with an ex situ annealing step. Upon selecting appropriate process parameters, we obtain highly crystalline YAlIG films with different Al substitution levels on both, single crystalline YAlO (YAG) and GdGaO (GGG) substrates. With increasing Al substitution levels, we observe a reduction of the saturation magnetisation as well as a systematic decrease of the magnetic ordering temperature to values well below room temperature. YAlIG thin films thus provide an interesting material platform for spintronic and magnonic experiments in different magnetic phases.
... where K is a proportionality constant, usually taken to be 0.92 [34]. An estimate of the coherent domain size in the ab direction is obtained from the width of the rocking curve of the (005) peak using the Scherrer formula [35] ...
We present a way to reach the maximum possible critical current density, Jc , for YBa2Cu3O 6+x (YBCO) thin films. This value is found to be around ten times the currently reached values. It is found that the Jc (0 T) is governed by the mean free path of the electrons, as is the critical temperature, Tc . The Jc in field, on the other hand, is governed by flux pinning sites and can be enhanced by optimizing the size and distribution of the non-superconducting nanoinclusions. By optimizing both the mean free path and the pinning structure, the maximum values can be reached.
... Films that had tF > 40 nm show a simple broad peak. The two components imply the existence of two different structural correlation lengths throughout the films [57][58][59][60][61][62]. The sharp peak reveals an atomically long range-ordered area. ...
Magnetic binary oxides with the rocksalt structure constitute an important class of materials for potential applications as electronic or electrochemical devices. Moreover, they often become a theoretical playground, due to the simple electronic and crystal structures, in the quest for novel phenomena. For these possibilities to be realized, a necessary prerequisite would be to grow atomically ordered and controllably-strained binary oxides on proper substrates. Here we systematically explore the use of pulsed laser deposition technique (PLD) to grow three basic oxides that have rocksalt structure but different chemical stability in the ambient atmosphere: NiO (stable), MnO (metastable) and EuO (unstable). By tuning laser fluence FL, an epitaxial single-phase NiO thin-film growth can be achieved in a wide range of growth temperatures 10 ≤ TG ≤ 750°C. At the lowest TG, the out-of-plane strain raises to 1.5%, which is five times higher than in NiO film grown at 750°C. MnO thin films that had long-range order were successfully deposited on the MgO substrates after appropriate tuning of deposition parameters. The growth of MnO phase was strongly influenced by FL and the TG. EuO films with satisfactory quality were deposited by PLD after oxygen availability had been minimized. Synthesis of EuO thin films at rather low TG = 350°C prevented thermally-driven lattice relaxation and allowed growth of strained films. Overall, PLD was a quick and reliable method to grow binary oxides with rocksalt structure in high quality that can satisfy requirements for applications and for basic research.
... Figures 4(c) and (d) showed that the rocking curve consisted of two different components. A broad component which was due to the mosaic-structured area throughout the film and a narrow component which showed a longer range atomically ordered area [36][37][38]. The mosaic-structured area is a typical feature of the vacuum grown films as there is no decelerating force to reduce the kinetic energy of the ablated particles resulting in defective structured films. ...
Recently, lattice dynamics of the highly strained Europium monoxide, as a promising candidate for strong ferroelectric-ferromagnet material, applied in the next-generation storage devices, attracted huge attention in the solid-state electronics. Here, the authors investigate the effect of tensile strain on dielectric properties of pulsed laser deposited EuO thin films from 10 to 200 K. A nearly 3% out-of-plane lattice compression is observed as the film thickness was reduced to ~ 8 nm, which could originate from the lattice mismatch between film and a LaAlO3 substrate. The temperature and frequency dependence of capacitance and loss factor of the films reveals the dominant role of electronic and ionic polarization below 100 K. The interdigitated capacitor fabricated on strained film shows almost 50% increase of capacitance compared to the relaxed one, which corresponds to a considerable increase of dielectric constant induced by in-plane tensile strain. Softening of the in-plane polarized transverse optical (TO) phonon modes of EuO lattice due to tensile strain might have the major contribution to this behavior according to the Lyddane–Sachs–Teller relation
... Figures 4(c) and (d) showed that the rocking curve consisted of two different components. A broad component which was due to the mosaic-structured area throughout the film and a narrow component which showed a longer range atomically ordered area [36][37][38]. The mosaic-structured area is a typical feature of the vacuum grown films as there is no decelerating force to reduce the kinetic energy of the ablated particles resulting in defective structured films. ...
The effect of strain on the dielectric properties of EuO thin films grown on LaAlO3 001 substrate is studied as a function of temperature from 5 to 200 K. The grown samples show high quality crystalline features based on the XRD, XRR and also AFM studies. Decreasing the film thickness to 10 nm is accompanied by an out-of plane compression of the lattice z-plane spacing up to 2.8%, which could be arisen from the lattice mismatch between film and substrate. In-plane dielectric properties are measured through interdigiteted electrode fabricated on the surface of the grown films and the measured capacitance of the strained film shows 1.5 times increase compared to the relaxed one. The frequency and temperature dependence of the capacitance reveals the dominant role of the lattice polarization in the measured capacitance of interdigital capacitors. Softening of the TO phonon modes due to the in-plane lattice tensile strain of EuO structure could be the reason for this behavior based on the Lyddane-Sachs-Teller relation.
... For the film grown at 700 ºC it consists of two different components (see fig.1b). A sharp peak, which corresponds to an atomically long-range ordered area inside the film and a broad peak, which reflects a mosaic structure [26,[31][32][33][34][35]. As the growth temperature decreases, the sharp peak disappears and the width of the broad peak increases (Fig. 1c). ...
The dielectric properties of NiO thin films grown by pulsed laser deposition have been studied as a function of strain at temperature from 10 to 300 K. Above 150 K, the contribution of space-charge polarization to the dielectric permittivity of NiO films becomes dominant, and the more defective films, which were grown at low temperatures shows a drastical increase in the dielectric constant up to room temperature. While the atomically-ordered film, which was grown at high temperature doesn’t show any considerable change in the dielectric constant in the range from 10 to 300 K. Below 100 K, the effect of strain on the dielectric constant becomes clear. An increase in dielectric permittivity is observed in the strained films while the relaxed film doesn’t show any remarkable deviation from its bulk value. The low-temperature dielectric behavior of NiO thin films can be interpreted based on the effect of strain on the lattice dynamics of rocksalt binary oxides.
... For the film grown at 750 ºC it consists of two different components (see fig.1b). A sharp peak, which corresponds to an atomically long-range ordered area inside the film and a broad peak, which reflects a mosaic structure [26,[31][32][33][34][35]. As the growth temperature decreases, the sharp peak disappears and the width of the broad peak increases (Fig. 1c). ...
The dielectric properties of NiO thin films grown by pulsed laser deposition have been studied as a function of strain at temperature from 10 to 300 K. Above 150 K, the contribution of space-charge polarization to the dielectric permittivity of NiO films becomes dominant and the more defective films, which were grown at low temperatures show a drastical increase in the dielectric constant up to room temperature. While the atomically-ordered film, which was grown at high temperature doesn't show any considerable change in the dielectric constant in the range from 10 to 300 K. Below 100 K, the effect of strain on the dielectric constant becomes clear. An increase in dielectric permittivity is observed in the strained films while the relaxed film doesn't show any remarkable deviation from its bulk value. The low-temperature dielectric behavior of NiO thin film can be interpreted based on the effect of strain on the lattice dynamics of rocksalt binary oxides.
... A mixture of these two peaks forms the rocking curves of other grown films. These two components imply two different structural correlation lengths in the films [54][55][56][57][58][59]. The narrow peak, width of which is limited by instrumental resolution, reveals a long range-ordered phase. ...
Here we explore systematically the growth of three basic oxides with rocksalt structure having different chemical stability in the ambient atmosphere: NiO (stable), MnO (metastable) and EuO (unstable). It is shown that by tuning laser fluence, an epitaxial single-phase nickel oxide thin-film growth can be achieved in a wide range of temperatures from 10 to 750 {\deg}C. At the lowest growth temperature, the out-of-plane strain raises to 1.5%, which is five times bigger than that for a thick NiO film grown at 750 {\deg}C. The growth of metastable MnO phase presents a bigger challenge and is strongly influenced by the laser fluence as well as the temperature of substrates. In particular, by properly tuning the deposition parameters, long-range ordered MnO thin films are successfully deposited on the MgO substrates. The unstable EuO phase is found to be strongly sensitive to oxygen contents during the growth. Controlling this parameter, EuO films with satisfactory quality are also deposited by PLD. Overall, it is proven that PLD is a quick and reliable method to grow binary oxides with rocksalt structure in high quality that would satisfy requirements for applications as well as needs for basic research. Specific details of the growth are highlighted.
... 34 Similarly, also the large L k $ 3 lm confirms the good crystallographic property of sample A. On the other hand, for sample B, both a tilt and L k are in the range of the values generally reported for heteroepitaxially grown ZnO layers. 34,38,39 The RSM results of the asymmetrical À10.5 reflection are shown in Figs. 5(a) and 5(b). ...
Homoepitaxial ZnO growth is demonstrated from conventional RF-sputtering at 400 °C on both Zn and O polar faces of hydrothermally grown ZnO substrates. A minimum yield for the Rutherford backscattering and channeling spectrum, χmin, equal to ∼3% and ∼12% and a full width at half maximum of the 00.2 diffraction peak rocking curve of (70 ± 10) arc sec and (1400 ± 100) arc sec have been found for samples grown on the Zn and O face, respectively. The structural characteristics of the film deposited on the Zn face are comparable with those of epilayers grown by more complex techniques like molecular beam epitaxy. In contrast, the film simultaneously deposited on the O-face exhibits an inferior crystalline structure ∼0.7% strained in the c-direction and a higher atomic number contrast compared with the substrate, as revealed by high angle annular dark field imaging measurements. These differences between the Zn- and O-face films are discussed in detail and associated with the different growth mechanisms prevailing on the two surfaces.
... The FWHM of the sharper component remained constant with growth temperature, whereas that of the broader component decreased, suggesting that the lms had a thin, near-perfect epitaxial layer adjacent to the substrate overlaid with a thicker, slightly misoriented layer. 24 Cross-sectional FIB-SEM images acquired aer impedance measurements indicated that the lms were dense. A representative image is shown in Fig. 4. It can be presumed that the lms were also dense as grown. ...
Libraries of (La0.8Sr0.2)0.95MnO3+δ (LSM) thin film microelectrodes with systematically varied thickness or growth temperature were prepared by pulsed laser deposition, and a novel robotic instrument was used to characterize these libraries in automated fashion by impedance spectroscopy. The measured impedance spectra are found to be described well by an electrochemical model based on a generalized transmission model for a mixed conducting oxide, and all trends are consistent with a reaction pathway involving oxygen reduction over the LSM surface followed by diffusion through the film and into the electrolyte substrate. The surface activity is found to be correlated with the number of exposed grain boundary sites, which decreases with either increasing film thickness (at constant growth temperature) or increasing film growth temperature (at constant thickness). These findings suggest that exposed grain boundaries in LSM films are more active than exposed grains towards the rate-limiting surface process, and that oxygen ion diffusion through polycrystalline LSM films is faster than many prior studies have concluded. This journal is
... There are many examples in the literature of hetero-epitaxial systems presenting two-component rocking curves [37][38][39][40][41][42]. Most of them present sharp components which are extremely thin, limited by the instrumental resolution, being observed generally in triple-axis setup (an additional channel-cut crystal inserted in the diffracted beam path) [43]. In those cases, it has been shown that the crystalline quality of the investigated systems is given only by the broad component of the rocking curves, while the observed extremely narrow features cannot be linked with it [37,38]. ...
... The slight lowering of fringes contrast in this scan may be an indication of a weak plastic relaxation process. The evolution of the GaPN(004) Bragg peak integral breadth has also been plotted since itcan been relied to the diffraction planes correlation length, also called crystallite size (through the Scherrer law, for instance).261 If the diffraction planes are correlated from one interface to the other one, then this correlation length should give the sample thickness. ...
This thesis focuses on optimizing the heterogeneous growth of IIIN- V solar cells on GaP (001) and GaP nanolayers on Si (001). The goal is to build high efficiency solar cells on low-cost substrate for the realization of concentrated photovoltaic powerplant. The main results shows: - AlGaP as prenucleation layer increase the annihilations of anti-phase boundaries at the GaP/Si interface (harmful for the electronic properties of the devices). - Similarities between the growth of GaAsN and GaPN giving strategies to improve the GaAsPN electrical properties - Clear correlations between the optical and electrical properties of dilute nitride solar cells, giving interesting tools to optimize the growth of those materials using optical measurements. - The realization of a GaAsPN solar cell on GaP with a yield of 2.25%. This results is encouraging given the thin GaAsPN absorber used in this cell
... A more precise evaluation has been carried out based on the Williamson-Hall evaluation method (Williamson & Hall, 1953;Herres et al., 1996;Kirste et al., 2005), as described by Durand et al. (2011b). Herres and co-workers attributed the line profile broadening to three mechanisms: the tilt, the average crystallite size and the inhomogeneous strain. ...
This study is carried out in the context of III–V semiconductor monolithic integration on silicon for optoelectronic device applications. X-ray diffraction is combined with atomic force microscopy and scanning transmission electron microscopy for structural characterization of GaP nanolayers grown on Si. GaP has been chosen as the interfacial layer, owing to its low lattice mismatch with Si. But, microtwins and antiphase boundaries are still difficult to avoid in this system. Absolute quantification of the microtwin volume fraction is used for optimization of the growth procedure in order to eliminate these defects. Lateral correlation lengths associated with mean antiphase boundary distances are then evaluated. Finally, optimized growth conditions lead to the annihilation of antiphase domains within the first 10 nm.
... So far several attempts have been made to evaluate the influence of microstructure on the physical properties of MCS materials. Most of the techniques used in these studies are limited to the data obtained from the scanning and transmission electron microscopy (TEM and SEM) [10,11], atomic force microscopy (AFM) [12,13] and the old X-ray diffraction analysis (Scherrer formula [14,15] and Williamson-Hall plot [16,17]). However, the defect density in crystalline nanomaterials is very high [18] and hence, in many cases, the resolution of electron microscopy images is not clear enough to detect the linear or planar defect. ...
... 27 The APB doesn't break the lateral long range order given by epitaxy but the Antiphase Domains (APD) gives a broad peak around antiphase reflections with a weaker peak maximum intensity, so that weak "antiphase" reflections (APR) act as APB sensors. Then, mean correlation length of defects has been determined by means of Williamson-Hall-Like (WHL) plots [38]. Weak (002) and (006) Bragg reflections give correlation length of defects including APB while strong (004) Bragg reflection gives correlation length of defects without the APB contribution [39;40]. ...
Lattice-matched GaP-based nanostructures grown on silicon substrates is
a highly rewarded route for coherent integration of photonics and
high-efficiency photovoltaic devices onto silicon substrates. We report
on the structural and optical properties of selected MBE-grown
nanostructures on both GaP substrates and GaP/Si pseudo-substrates. As a
first stumbling block, the GaP/Si interface growth has been optimised
thanks to a complementary set of thorough structural analyses.
Photoluminescence and time-resolved photoluminescence studies of
self-assembled (In,Ga)As quantum dots grown on GaP substrate demonstrate
a proximity of two different types of optical transitions interpreted as
a competition between conduction band states in X and Γ valleys.
Structural properties and optical studies of GaAsP(N)/GaP(N) quantum
wells coherently grown on GaP substrates and GaP/Si pseudo substrates
are reported. Our results are found to be suitable for light emission
applications in the datacom segment. Then, possible routes are drawn for
larger wavelengths applications, in order to address the chip-to-chip
and within-a-chip optical interconnects and the optical telecom
segments. Finally, results on GaAsPN/GaP heterostructures and diodes,
suitable for PV applications are reported.
... These profiles display two components in the transverse scans: a narrow, resolution-limited, peak, characteristic of long-range correlation, and a broad peak, attributed to defect-related diffuse-scattering inducing a limited transverse structural correlation length. 45 Where Sz is the longitudinal diffraction vector modulus and β(S) the peak integral breadth. ...
Selected results obtained in the framework of MBE grown nanostructure
for photonics on silicon are repsented in this paper. We present first a
comprehensive study of GaAsPN/GaPN quantum wells (QWs) grown onto GaP
substrates, in the light of a comparison with their N-free GaAsP/GaP QWs
counterpart system. High density of small InGaAs/GaP Quantum Dots are
presented next with their PL properties. Finally, RT photoluminescence
properties of GaAsPN/GaPN QWs onto Si substrate are presented and
discussed in term of carrier injection efficiency. However, for future
development, optical properties of the active area must be improved and
are tightly bound to the structural perfection of the GaP/Si template
layer. To address this point, structural analyses including X-Ray
Diffraction (lab setup and synchrotron) and Transmission Electron
Microscopy have been performed, with a particular care for typical
III-V/Si defect characterisation. First results of Si buffer layer
growth are also presented as a perspective for future low defect MBE
grown GaP/Si template layers.
... There is still a difference in the perfection of the crystals as a function of growth direction, but it is not correct to assign the sharp component to a part of the film near the substrate. Durand et al. 20 have also analyzed in more detail data taken from ZnO films on c-Al 2 O 3 . However, the data available from such studies far exceed what we have accumulated on our samples, and our analysis is therefore less complete. ...
Thin films (250–4500 A° ) of epitaxial UO2 were produced by reactive sputtering on two different substrate
materials: LaAlO3 and CaF2. Using the large enhancement present with resonant x-ray scattering using photons
at the uraniumM4 absorption edge, antiferromagnetic (AF) order was found in all films. The ordering temperature
T N is the same as the bulk, but the films show second-order (continuous) transitions in contrast to the first-order
bulk transition. For LaAlO3-based films, an additional strong diffuse magnetic disorder is observed, which is
reminiscent of the second-length scale, associated with structural disorder and/or strain. By using a formulation
accounting for the strong absorption and coherent nature of the photons, the energy widths at the UM4 resonances
can be related to the thickness of the AF region. The LaAlO3-based films do not order magnetically over more
than ∼600 A° , whereas the CaF2-based film orders throughout. Further, for thicker films (>1000 A° ) the fitting
procedure shows that the AF order is located at the top of the LaAlO3-based film. This points to the formation in
thicker films of a nonmagnetic layer of UO2 adjacent to the substrate, which may have tetragonal symmetry.
... Two main parameters are extracted from this and are reported in the Table I: (1) the quality factor (QF) which is the ratio of integrated intensity of the thin versus the broad contribution; (2) the integral 28 which is a variant of the Williamson-Hall plot method has been applied to these transverse scans. [29][30][31] This allows the separation of short range correlation length from micromosaicity in the broadening. Moreover, when QFs are much smaller on APR than on SR reflections, we can treat them separately and different correlation lengths can be given as shown in Table I. ...
We report a structural study of molecular beam epitaxy-grown lattice-matched GaP/Si(0 0 1) thin layers with an emphasis on the interfacial structural properties, and optical studies of GaAsP(N)/GaP(N) quantum wells coherently grown onto the GaP/Si pseudo substrates, through a complementary set of characterization tools. Room temperature photoluminescence at 780 nm from the (GaAsPN/GaPN) quantum wells grown onto a silicon substrate is reported. Despite this good property, the time-resolved photoluminescence measurements demonstrate a clear influence of non-radiative defects initiated at the GaP/Si interface. It is shown from simulations, how x-ray diffraction can be used efficiently for analysis of antiphase domains. Then, qualitative and quantitative analyses of antiphase domains, micro-twins, and stacking faults are reported using complementarity of the local transmission electron microscopy and the statistical x-ray diffraction approaches.
This study presents a comprehensive analysis of the microstructural, optical, and electrical transport properties of heteroepitaxial cubic spinel (111)‐oriented Zn 2 GeO 4 thin films grown on cubic spinel (111) MgAl 2 O 4 substrates by pulsed laser deposition. The in‐plane epitaxial relationships are [1–10] Zn 2 GeO 4 //[1‐10] MgAl 2 O 4 and [11–2] Zn 2 GeO 4 //[11–2] MgAl 2 O 4 , indicating a cube‐on‐cube epitaxy. A 316 nm thick (111) Zn 2 GeO 4 epitaxial film has a surface root‐mean‐square (RMS) roughness of about 0.9 nm and a narrow rocking curve of the (444) reflex with a full width at half maximum of about 0.36°. Temperature‐dependent Hall effect measurements indicate that the nominally undoped films exhibit n‐type semiconductor behavior. The high‐quality 316 nm thick epitaxial film, with a direct optical bandgap of about 4.9 eV at room temperature, shows a notable decline in resistivity from about 60 to about 4 Ω cm, as temperature increases from 100 to 300 K. Concurrently, the Hall electron carrier mobility rises gradually from ≈0.5 to 5.5 cm ² V ⁻¹ s ⁻¹ as temperature increases from 100 to 300 K. In contrast, the Hall electron carrier concentration demonstrates minimal temperature dependence, with a value of ≈10 ¹⁷ cm ⁻³ . The native n‐type conductivity is likely the result of unintentional dopants introduced during thin‐film fabrication.
Abstract Reducing the leakage current through the gate oxide is becoming increasingly important for power consumption reduction as well as reliability in integrated circuits as the semiconducting devices continue to scale down. Here, this work reports on the high‐k dielectric SrHfO3 (SHO) based devices with ultralow leakage current density via pulsed laser deposition (PLD). The ultralow current density is achieved by optimizing the growth conditions and the associated structural properties. In the optimized conditions, the dielectric properties of the 50‐nm‐thick SHO capacitors are measured: high dielectric constant (κ = 32), low leakage current density ( 4 MV cm−1). The surprisingly low leakage current density of SHO is ascribed to the large bandgap (≈6 eV), the large conduction band offset (CB offset > 3 eV) with respect to the semiconductor, and the low density of defect states inside the bandgap. The optimized SHO dielectric with high dielectric constant and ultralow leakage current density is proposed for future low‐power consumption devices based on Si as well as perovskite oxide semiconductors.
PbSe and related IV-VI rocksalt-structure semiconductors have important electronic properties that are controlled by epitaxial strain and interfaces and can be harnessed in the emerging class of IV-VI/III-V hybrid heterostructures. The synthesis of such heterostructures and understanding mechanisms for strain relief in these materials is central to achieving this goal. We show that a range of interfacial defects mediate lattice mismatch in (001)−oriented epitaxial thin films of PbSe and III-V templates of GaAs, InAs, and GaSb. While the primary slip system {100}<110> in PbSe is well studied for its facile dislocation glide even at low temperatures, it is inactive in (001)−oriented films in our work. Yet, we obtain nearly relaxed PbSe films in the three heteroepitaxial systems studied with interfaces ranging from incoherent without localized misfit dislocations on 8.3% mismatched GaAs, a mixture of semicoherent and incoherent patches on 1.5% mismatched InAs, to nearly coherent on 0.8% mismatched GaSb. The semicoherent portions of the interfaces to InAs form by 60° misfit dislocations gliding on higher-order {111}<110> slip systems. On the more closely lattice-matched GaSb, arrays of 90° (edge) misfit dislocations form via a climb process. This diversity of strain-relaxation mechanisms accessible to PbSe makes it a convenient material for epitaxial integration in hybrid heterostructures.
By applying the restricted random dislocation distribution model, solved using a discrete Hankel transform approach, it is shown that the shapes and characteristics of the [Formula: see text]-scans of the ZnO 00.2 reflection are mainly determined by the strain field introduced by dislocations with a screw component. On the other hand, no clear evidence of mosaicity or interfacial region contributions is found despite the layers exhibiting a [Formula: see text] axis oriented residual columnar structure and a highly defective interface being present. The applied model not only permits a more precise estimation of the densities of dislocations, which present a screw component with respect to methods based on the analysis of the 00.2 peak [Formula: see text]-scan FWHM, but also gives an indication of their distribution characteristics with the extracted values, as well as their homogeneous/quasi-regular spacing, being confirmed by cross-sectional transmission electron microscopy observations.
Cette thèse porte sur l'analyse de de défauts structuraux d'un pseudo-substrat GaP/Si. L'objectif principal concerne la qualité structurale de la couche épitaxiale de GaP sur un substrat de Si, en tant que pierre angulaire du développement d'une cellule solaire tandem à haut rendement avec des dispositifs bas-couts (jonctions silicium et CIGS) et une couche intermédiaire Ill-V. Tout d'abord, l'étude porte sur la caractérisation de la distribution des dislocations dans le substrat vicinal GaP sur Si(001), avec une méthode de diffusion des rayons X ayant une résolution sous-micrométrique, appelée K-Map. Les informations locales d'inclinaison et de déformation sont obtenues par une analyse de l'ensemble de données complexes 5D. Cette étude révèle une distribution anisotrope des dislocations dans différentes directions cristallographiques, liée aux marches à la surface du substrat de Si, et une distribution non homogène des dislocations, liée à la tendance à la formation de regroupements de marches. La deuxième par1ie de l'étude porte sur la croissance et la caractérisation de CIGS sur GaP/Si, en vue de développer des cellules solaires tandems associant une cellule du dessous en silicium monocristallin et une cellule du dessus en CIGS. On observe (par XRD, EDX et HRTEM) que le CIGS est déposé par croissance épitaxiale sur le pseudo substrat GaP/Si et présente une qualité structurale proche d'un monocristal. Le premier essai, non optimisé, d'une cellule solaire simple jonction de CIGS sur pseudo-substrat GaP/Si, permet d'obtenir un EQE très encourageant, similaire à celui obtenu avec la même cellule déposée sur substrat de verre avec des paramètres de dépôts optimisés.
Magnetic binary oxides with cubic rocksalt structure are the best candidates in which to
experimentally investigate the accuracy of the new mechanisms proposed for the
electronic behavior under complex condition. Recently, the effect of strain on the lattice
dynamics of magnetic binary oxides attracted a huge attention due to the theoretically
predicted emergent phenomena, e.g. colossal dielectric permittivity, ferroelectricity and
multiferroicity under epitaxial strain. Moreover, the idea of a purely magnetic orderinduced anisotropy in the phonon properties demonstrated the substantial contribution
of spin-phonon interaction in the lattice dynamics of this group of materials. Since then,
there has been increasing interest in spin-phonon coupling within the context of
magnetoelectric multiferroics and spintronics, but the mechanisms underlying this
phenomenon remained unclear. It was proposed for the binary oxides that the actual size
of the exchange-driven phonon splitting Δ𝜔 is solely determined in sign and magnitude
by the non-dominant exchange interaction J1, while the contributions of the dominant
superexchange coupling J2 are canceled. Recently, this idea was experimentally
confirmed. On the other hand, ab initio study of magnetic-exchange interactions in
antiferromagnetic transition metal monoxides revealed a substantial effect of strain on
the J1. Consequently, strain can directly affect the lattice dynamics of binary oxide
through influence on phonon modes frequency 𝜔 and indirectly on magnetic exchange
interaction J1, which in turn affects the exchange-driven phonon splitting Δ𝜔.
Here, we systematically explore the use of pulsed laser deposition technique (PLD) to
grow three basic oxides that have rocksalt structure but different chemical stability in the
ambient atmosphere: NiO (stable), MnO (metastable) and EuO (unstable). By tuning
laser fluence FL, an epitaxial single-phase NiO thin-film growth can be achieved in a wide range of growth temperatures 10 ≤ TG ≤ 750 °C on SrTiO3 and MgO substrates. At the
lowest TG, the out-of-plane strain raises to 1.5%, which is five times higher than in NiO
film grown at 750 °C. MnO thin films that had atomically long-range order were
successfully deposited on the MgO and TiO2 substrates after appropriate tuning of
deposition parameters. The growth of MnO phase was strongly influenced by FL and the
TG. Finally, EuO films with satisfactory quality were deposited by PLD after oxygen
availability had been minimized. Synthesis of EuO thin films at rather low TG = 350 °C
prevented thermally-driven lattice relaxation and allowed growth of strained films.
Overall, PLD was a quick and reliable method to grow strained binary oxides with rocksalt
structure in high quality that can satisfy requirements for applications and for basic
research. Thereafter, (obtaining high-quality rocksalt binary oxides with a range of strain) the effect
of strain and magnetic interaction on the lattice dynamics and dielectric properties was
systematically investigated in the range of applied voltage frequencies from 20 Hz to 2
MHz at different temperatures from 2 to 300 K.
The temperature and frequency dependence of capacitance and loss factor of the films
reveals the dominant role of electronic and ionic polarization below 100 K. Compressive
(tensile) strain increases (decreases) the frequency of transverse optical (TO) phonon
mode in rocksalt binary oxides as was predicted recently resulting in the decrease
(increase) of dielectric constant. This result implies the substantial effect of strain on the
TO phonon mode, which is a basic component in the dielectric properties of rocksalt
binary oxides according to the Lyddane–Sachs–Teller relation. ( 𝜖(0)
𝜖(∞)= ∏𝜔𝐿𝑗2𝜔𝑇𝑗𝑗 2
, where 𝜔𝐿
and 𝜔𝑇 are the frequencies of the LO and TO phonon modes, respectively, 𝜖(0) is the lowfrequency dielectric permittivity and 𝜖(∞) is the high-frequency limit for electronic
dielectric permittivity). While compressive strain increase the TO phonon frequency
smoothly the tensile strain decrease it sharply which is in good agreement with first
principle calculation.
The interdigitated capacitor fabricated on strained EuO film shows almost 50% increase
of capacitance compared to the relaxed one, which corresponds to a considerable increase of dielectric constant induced by in-plane tensile strain. Tensile strain of 1.5% increases
the dielectric constant of NiO by almost two times.
The effect of strain on spin-phonon interaction in rocksalt binary oxides reveals
interesting phenomena. MnO films grown on (001)-oriented MgO substrates exhibit
homogenous biaxial compressive strain, which increases as the film thickness is reduced.
For that reason, in paramagnetic phase, the frequency 𝜔 of doubly-degenerate phonon
increases with the strain, and splits below Néel temperature TN due to the magneticexchange interaction. The phonon splitting Δ𝜔 in the MnO (001) films is 20% larger than
that of the bulk MnO. On the other hand, the films grown on (110)-oriented MgO
substrates exhibit a huge phonon splitting Δ𝜔 already at room temperature due to the
anisotropic in-plane compressive strain. Below TN, the lower-frequency phonon splits in
the IR spectra and the higher-frequency phonon strongly hardens in AFM phase; these
features are evidences for a spin-order-induced structural phase transition from
tetragonal to a lower symmetry phase. Total phonon splitting is 55 cm-1
in (110)-oriented
MnO film, which is more than twice (~ 120%) the value in bulk MnO, but since the
splitting is present already in paramagnetic phase, we cannot clearly correlate it with the
value of exchange coupling constant J1. Nevertheless, at least observation of enhanced
phonon splitting in strained MnO (001) films show that the exchange coupling J1 could
be enhanced by the compressive strain which supports recent theoretical predictions. On
contrary, the compressive strain causes the reduction of exchange-driven phonon
splitting Δ𝜔 in NiO films which indicates that the relation between exchange-driven
splitting Δ𝜔 and atomic distance a, i.e. 𝜕|Δ𝜔|
𝜕𝑎 is positive for antiferromagnets with J1 < 0
(NiO) and negative for those with J1 > 0 (MnO). That means for antiferromagnets with J1
< 0 the decrease of atomic distance reduces the exchange-driven splitting Δ𝜔 while for
those with J1 > 0 it increases Δ𝜔. This partly unravels the ambiguous nature of spinphonon coupling in rocksalt magnetic binary oxides.
Losses in metals and mass production compatibility represent the most serious challenges limiting the widespread practical application of plasmonic devices. Many recent studies have focused on developing alternative low-loss materials, but silver is by far potentially the most preferred plasmonic material because it has the lowest optical losses among all the metals and the longest propagation length of surface plasmon polaritons at optical and near-IR frequencies. In this paper, a a two-step process (SCULL process) for atomically flat sub-50-nm-thick single-crystalline silver film deposition is demonstrated, which is reproducible in a few hours under standard cleanroom enviroment. Thin film growth is self-controlled by quantum size effects, which enables the deposition of perfect silver films on non-ideally lattice-matched substrates, even with imperfect standard deposition tools and process deviations. Because of single-crystalline silver films unique properties, including a 35 nm minimum thickness, sub-100 pm surface roughness, and ultra low-loss dielectric permittivity, they opened fundamentally new possibilities in nanophotonics and quantum technology. Using SCULL films, the all-silver ultrabright room-temperature single-photon quantum emitters were demonstrated. The SCULL process has been approved for silver, gold and aluminum film evaporation, and it could be used for a variety of atomically flat sub-100 nm single-crystalline metal films deposition.
p-Type transparent conductors and semiconductors still suffer from remarkably low performance compared to their more widespread n-type counterparts, despite extensive investigation into their development. In this contribution, we present a comparative study on the defect chemistry of potential p-type transparent conducting oxides Mg-doped and Ni-doped Cr2O3. Conductivities as high as 28 S cm⁻¹ were achieved by Ni-doping. By benchmarking crystallography and spectroscopy characterization against density functional theory calculations, we show that the incorporation of Ni into Cr2O3 contributes to the composition of the valence band, making the formed holes more delocalized, while Mg states do not interact with the valence band in Mg-doped Cr2O3. Furthermore, it is experimentally proven that Ni has a higher solubility in Cr2O3 than Mg, at least in the highly non-thermodynamic deposition conditions used for these experiments, which directly translates into a higher acceptor concentration. The combination of these two effects means that Ni is a more effective acceptor in Cr2O3 than Mg and explains the improved conductivity observed for the former.
Uranium dioxide thin films were deposited on single crystal TiO2, Al2O3, YSZ, ZnO and NdGaO3 substrates to optimize conditions for the growth of high quality single crystal films. X-ray diffraction results show that all the films have one growth direction and well defined peaks in the specular scans with the expected symmetry for each growth orientation. The UO2/Al2O3, TiO2, and ZnO films have high concentration of misfit dislocations that increase with the lattice mismatch. The UO2 film on YSZ is found to be in registry with the substrate. The film has narrow mosaic component that is imposed upon a broader component arises from the diffuse scattering due to defects in the film. Meanwhile, UO2/NdGaO3 film shows a splitting of the X-ray diffraction peaks which is attributed to the in-plane asymmetry of the orthorhombic substrate.
Transparent conductive oxides (TCOs) combine high optical transparency with metal like conductivity and represent in striking evidence the pairing of two mutually exclusive physical properties. Epitaxial films produced by state-of-the art crystal growth technologies in scientific laboratories around the globe satisfy the needs for supreme structural materials due to lattice-matched growth settings. This thesis presents molecular beam epitaxial growth and doping studies of indium oxide films on lattice-matched ZrO2:Y(111) and single-crystalline In2O3(111) substrates. This thesis has shown that high power oxygen plasma molecular beam epitaxy allows the growth of nanocrystals and perfectly aligned nanowires on YSZ(111) with supreme crystal quality and uniformity. The self-assembled growth of In2O3:Sn nanowires is caused by the interplay of a continuous thin wetting layer and the benefits of homoparticle assisted growth triggered by tin. Flat films of highly doped In2O3:Sn on YSZ(111) has been obtained by layer-by-layer growth under low oxygen-pressures (ρ ≈ 6.6 x 10^-7 Torr). High n-type doping and oxygen-supply strengthen the film surface stability and suppress dewetting and film reconstructions on YSZ(111). First time surface and doping studies of homoepitaxial In2O3:Mg/In2O3:Sn films on In2O3(111) grown under varying oxygen pressures testify the absence of (111)-surface reconstructions throughout a wide growth window. HRXRD measurements reveal excellent structural quality of single and multilayer films on In2O3(111) even in case of high doping. It could be proven that peak-splitting in 2θ/θ-scans is based on high Mg and Sn concentrations.
This thesis focuses on the heterogeneous growth optimization of III-V nanostructures on Si (001) substrate displaying a miscut toward [110]. The main purpose concerns the integration of efficient light sources on Si substrate for high-speed optical interconnects inter-and intra-chip, as a cornerstone for the development of optoelectronic integrated circuits (OEIC).First, this study focuses on the optimisation of nitrogen incorporation in GaPN on GaP(001) substrate, while reachingthe lattice-matching condition with Si. This study is also interesting for the growth of any GaPN-based dilute nitridecompounds, such as GaAsPN, which are very attractive for long wavelength laser applications and high-efficiency photovoltaic applications on Si substrates. In a second step, we studied the growth of an active layer based on (In,Ga)As quantum dots (QD) on GaP (001) substrate. These QD display a high density and good uniformity in size. Room temperature photoluminescence is also obtained on these QD, which is very promising for the fabrication of integrated optoelectronic sources on a silicon substrate. In the third part, this study focuses on the homoepitaxial growth of Si by UHV/CVD necessary to bury residual contaminants initially present on the Si surface, and to obtain a Si surface suitable for the subsequent heteroepitaxial growth of optimal structural quality GaP layer. This includes the formation of double atomic steps, by step bunching and favors by the substrate miscut, in order to limit the structural defects. Finally, the GaP/Si interface is optimized, while obtaining a flat GaP surface and a minimum defects density. A methodology to quantify the structural defects (anti-phase domains, micro-twins) by X-ray diffraction using Synchrotron and laboratory sources is presented. This study reveals an anisotropic behavior of the micro-twins, linked to the miscut direction of the Si substrate, and a dramatic reduction of the micro-twins density at high growth temperature. The growth of thin GaP layers on Si substrates, with thickness less than the critical one and obtained with a purposely dedicated growth cluster composed of a Si UHV/CVD chamber connected under UHV with a III-V MBE chamber, shows a significant reduction of the structural defects and provides a GaP/Si pseudo-substrate with a flat surface suitable for subsequent growth of efficient light sources.
We report on the quantitative study of microtwins (MT) defects in the GaP/Si(0 0 1) thin film grown by Molecular Beam Epitaxy and the optical properties of GaAsP(N)/GaP(N) quantum wells grown on top of the GaP/Si pseudo-substrates. A 780 nm photoluminescence at room temperature from the GaAsPN quantum wells is measured on silicon. Time-resolved photoluminescence has been performed and evidences the influence of non-radiative defects originated from the GaP/Si interface. The structural defects such as MT are quantitatively analyzed by synchrotron X-ray diffraction (XRD) combined with transmission electron microscopy (TEM) analyses. We show that the XRD measurements are in good agreement with TEM observation and reveal a strong contribution of MT in the [1 1 1] direction. The MT density appears to be directly correlated with the growth temperature.
In the context of III-V monolithic integration on silicon, synchrotron X-ray diffraction has been employed in this study using a bi-dimensional large area hybrid pixel detector (XPAD third generation) to characterize defects in the GaP layers. Despite a very coherent interface (low plastic relaxation) of GaP/Si, 2 types of defect are detected. Micro-twins contributions are evidenced and quantitatively evaluated from additional reflections analysis. Antiphase domains are evidenced using the Williamson-Hall-like plot method applied to transverse scans extracted directly from single XPAD images taken on specular GaP reflections.
We have investigated quantitatively anti-phase domains (APD) structural properties in 20nm GaP/Si epilayers grown by molecular beam epitaxy, using fast, robust and non destructive analysis methods. These analyses, including atomic force microscopy and X-ray diffraction, are applied to samples grown by various molecular beam epitaxy growth modes. Roughness, lateral crystallite size of the epilayer, ratio of Anti-Phase Domains and their relationship are discussed. It is shown that both these analysis methods are useful to clarify the physical mechanisms occurring during the heterogeneous growth. Low temperature migration enhanced epitaxy is found to guarantee smoother surface than conventional molecular beam epitaxy. Effect of annealing temperature on Anti-Phase Boundaries (APB) thermodynamics is discussed. The modification of the thermodynamic equilibrium through a thermal activation of APBs motion is expected to play an important role in the dynamic evolution of surfaces during thermal annealing and growth.
Effect of hydrogen on the chemical bonding and band structure at the Al2O3/In0.53Ga0.47As interface Appl. Phys. Lett. 99, 232103 (2011) Vapor-liquid-solid growth of Ge nanowhiskers enhanced by high-temperature glancing angle deposition Appl. Phys. Lett. 99, 223107 (2011) The significance of in situ conditions in the characterization of GaSb nanopatterned surfaces via ion beam sputtering J. Appl. Phys. 110, 074301 (2011) The microstructure of dislocation clusters in industrial directionally solidified multicrystalline silicon This paper examines the structural properties of gallium phosphide layers by high resolution x-ray diffraction and atomic force microscopy measurements. GaP layers are grown on misoriented and nominally exactly oriented silicon (001) substrates by metalorganic vapor phase epitaxy. Structural characterization is performed by reciprocal lattice map and transverse scan measurements of (00l)-reflections (l ¼ 2, 4, 6). Transverse scan line profiles of GaP layers on exactly oriented and misoriented substrates are compared thoroughly and antiphase disorder related satellite peaks are observed on exactly oriented substrates. In addition, results imply that antiphase disorder is self-annihilated on misoriented substrates. The dependence of crystallographic tilt on growth temperature indicates structural coherence. Williamson-Hall-like plot of transverse scans reveals the lateral correlation length of crystalline defects of 79 nm which gives the average size of the mosaic crystallites. In addition, the mosaicity of the GaP layer is 0.042 . V C 2012 American Institute of Physics. [doi:
The mosaic structure of an (Al,Ga)N layer grown on (0001) sapphire showing natural ordering was studied by high-resolution X-ray diffraction (HRXRD) reciprocal-space mapping. The direction-dependent mosaicity of the layer has been elaborated using maps of symmetrical and asymmetrical reflections. The reciprocal-lattice points show significant broadening depending on the direction in reciprocal space, the diffraction order and the reflection type (fundamental or superstructural). The evaluation followed two paths: (i) a procedure based on the Williamson–Hall plot and (ii) a new approach based on the statistical diffraction theory (SDT). Here, the transformed Takagi equations were implemented for the simulation of the reciprocal-space maps (RSM) for symmetrical and asymmetrical reflections. The reconstruction comprised the mosaic block size, their average rotation angle and the spatial distribution of some components of the microdistortion tensor. The results based on the SDT modelling agree well with those obtained by the Williamson–Hall method, while providing a higher degree of precision and detail.
The main ZnO physical properties are reviewed and some of them compared
to those of GaN. As a result of these attractive properties, the various
applications it could be thought of for ZnO are summarized. A critical
review is then proposed of the different techniques used for the growth
of bulk ZnO crystals and of ZnO epitaxial films. The result are
discussed from the assessment of their structural and electrical
properties. The key issue of p-type doping is finally discussed in the
light of the most recent results.
We report the realization of p-type behavior in ZnO thin films, which are prepared by codoping method using Ga (donor) and N (acceptor) as the dopants. Especially using active N formed by passing N2O gas through an electron cyclotron resonance (ECR) plasma source is quite effective for the acceptor doping. We have observed a room temperature resistivity of 2 Omega cm and a hole concentration of 4 x 10(19) cm(-3). These values are enough high for practical applications in various oxide electronic devices.
The semiconductor ZnO has gained substantial interest in the research community in part because of its large exciton binding energy (60 meV) which could lead to lasing action based on exciton recombination even above room temperature. Even though research focusing on ZnO goes back many decades, the renewed interest is fueled by availability of high-quality substrates and reports of p-type conduction and ferromagnetic behavior when doped with transitions metals, both of which remain controversial. It is this renewed interest in ZnO which forms the basis of this review. As mentioned already, ZnO is not new to the semiconductor field, with studies of its lattice parameter dating back to 1935 by
Bunn [Proc. Phys. Soc. London 47, 836 (1935)
], studies of its vibrational properties with Raman scattering in 1966 by
Damen et al. [Phys. Rev. 142, 570 (1966)
], detailed optical studies in 1954 by
Mollwo [Z. Angew. Phys. 6, 257 (1954)
], and its growth by chemical-vapor transport in 1970 by
Galli and Coker [Appl. Phys. Lett. 16, 439 (1970)
]. In terms of devices, Au Schottky barriers in 1965 by
Mead [Phys. Lett. 18, 218 (1965)
], demonstration of light-emitting diodes (1967) by
Drapak [Semiconductors 2, 624 (1968)
], in which Cu2O was used as the p-type material, metal-insulator-semiconductor structures (1974) by
Minami et al. [Jpn. J. Appl. Phys. 13, 1475 (1974)
], ZnO/ZnSe n-p junctions (1975) by
Tsurkan et al. [Semiconductors 6, 1183 (1975)
], and Al/Au Ohmic contacts by
Brillson [J. Vac. Sci. Technol. 15, 1378 (1978)
] were attained. The main obstacle to the development of ZnO has been the lack of reproducible and low-resistivity p-type ZnO, as recently discussed by
Look and Claflin [Phys. Status Solidi B 241, 624 (2004)
]. While ZnO already has many industrial applications owing to its piezoelectric properties and band gap in the near ultraviolet, its applications to optoelectronic devices has not yet materialized due chiefly to the lack of p-type epitaxial layers. Very high quality what used to be called whiskers and platelets, the nomenclature for which gave way to nanostructures of late, have been prepared early on and used to deduce much of the principal properties of this material, particularly in terms of optical processes. The suggestion of attainment of p-type conductivity in the last few years has rekindled the long-time, albeit dormant, fervor of exploiting this material for optoelectronic applications. The attraction can simply be attributed to the large exciton binding energy of 60 meV of ZnO potentially paving the way for efficient room-temperature exciton-based emitters, and sharp transitions facilitating very low threshold semiconductor lasers. The field is also fueled by theoretical predictions and perhaps experimental confirmation of ferromagnetism at room temperature for potential spintronics applications. This review gives an in-depth discussion of the mechanical, chemical, electrical, and optical properties of ZnO in addition to the technological issues such as growth, defects, p-type doping, band-gap engineering, devices, and nanostructures.
The x-ray scattering from relaxed heteroepitaxial layers with the misfit dislocations randomly distributed at the interface between the layer and the substrate is analyzed theoretically and experimentally. The profiles of the x-ray-diffraction peaks and the reciprocal space maps of the intensity are measured and simulated for several heteroepitaxial structures in a wide range of dislocation densities. At large dislocation densities, the peak position is governed by the mean distortions and the peak width is due to the mean-square variations of the distortions. The peak widths calculated for uncorrelated distribution of dislocations exceed the widths of the peaks measured on the heteroepitaxial structures with large mismatch. It is shown that the spatial correla-tions of the dislocations reduce the peak width and explain the discrepancy. For small dislocation densities, the coherent and the diffuse components of the intensity are measured and simulated. It is shown that the position of the coherent peak does not follow the mean distortions. Satellites of the diffuse peak are observed and explained.
Thin films of (00l) oriented SrBi2Nb2O9 epitaxially grown on SrTiO3 by sol-gel spin coating have been studied by means of high-resolution x-ray diffraction reciprocal space mapping. It is shown that these materials contain highly localized heterogeneous strain fields due to imperfect stacking faults (i.e., faults that do not propagate throughout the crystallites building up the film). In the film plane, the strain fields are confined to 11 nm wide regions and characterized by a vertical displacement of 0.18c (where c is the cell parameter) showing that the stacking faults are mainly composed of one additional (or missing) perovskite layer. Prolonged thermal annealing at 700 °C strongly reduces the density of stacking faults and yields a more uniform strain distribution within the film volume without inducing significant grain growth.
A p-type ZnO was prepared on a sapphire substrate using P2O5 as a phosphorus dopant. As-grown n-type ZnO films doped with phosphorus showed electron concentrations of 1016–1017/cm3 and these films were converted to p-type ZnO by a thermal annealing process at a temperature above 800 °C under a N2 ambient. The electrical properties of the p-type ZnO showed a hole concentration of 1.0×1017–1.7×1019/cm3, a mobility of 0.53–3.51 cm2/V s, and a low resistivity of 0.59–4.4 Ω cm. The phosphorus-doped ZnO thin films showed a strong photoluminescence peak at 3.35 eV at 10 K, which is closely related to neutral acceptor bound excitons of the p-type ZnO. This thermal activation process was very reproducible and effective in producing phosphorus-doped p-type ZnO thin films, and the produced p-type ZnO was very stable. © 2003 American Institute of Physics.
A model that allows the quantitative analysis of heterogeneous strain fields in epitaxial thin films using x-ray diffraction (XRD) is presented. Particular emphasis is laid on the modelling of the two-component XRD profiles (i.e. profiles made of the superposition of a narrow coherent Bragg peak and a broad diffuse scattering profile) encountered in the XRD investigation of epitaxial thin films containing localized strain fields. The spatial properties of the strain field are included in a correlation function based on phenomenological parameters such as the defect correlation length ξ and the level of disorder σ∞. No assumption regarding the nature of the defect is hence required. The statistical properties of the strain field are described by means of Lévy-stable distributions which allow us to account for profile shapes ranging between the Gaussian and profiles exhibiting pronounced power law-type tails, as well as for asymmetrical profiles. The effects of finite size of the domains (crystallites) over which diffraction is coherent are rigorously taken into account by calculating the auto-correlation function of the crystallite shape including the size distribution effects. The effects of each parameter are presented and discussed in detail and the applicability of the model is illustrated with two examples.
Continued development of GaN-based light emitting diodes is being hampered by constraints
imposed by current non-native substrates. ZnO is a promising alternative substrate but it
decomposes under the conditions used in conventional GaN metal organic vapor phase epitaxy
(MOVPE). In this work, GaN was grown on ZnO/c-Al2O3 using low temperature/pressure MOVPE
with N2 as a carrier and dimethylhydrazine as a N source. Characterization confirmed the epitaxial
growth of GaN. The GaN was lifted-off the c-Al2O3 by chemically etching away the ZnO
underlayer. This approach opens up the way for bonding of the GaN onto a support of choice.
GaN-based optoelectronic devices are plagued by a tendency to non-radiative transitions linked to defects in the active layers. This problem has it's origin in a) intrinsic factors such as GaN's relatively low exciton binding energy (~24meV) and b) extrinsic factors including the poor availability of native substrates good enough to significantly suppress the defect density. Indeed, the quality and availability of large-area bulk GaN substrates is currently considered a key problem for the continuing development of improved GaN-based devices. Since development of bulk GaN substrates of suitable quality has proven very difficult, a considerable amount of effort is also being directed towards the development of alternative substrates which offer advantages compared to those in widespread use (c-sapphire and 6H SiC). ZnO is promising as a substrate material for GaN because it has the same wurtzite structure and a relatively small lattice mismatch (~1.8%). In this paper, we discuss use of ZnO thin films as templates for GaN based LED .
In this work, ZnO has been investigated as a substrate technology for GaN-based devices due to its close lattice match, stacking order match, and similar thermal expansion coefficient. Since MOCVD is the dominant growth technology for GaN-based materials and devices, there is a need to more fully explore this technique for ZnO substrates. Our aim is to grow low defect density GaN for efficient phosphor free white emitters. However, there are a number of issues that need to be addressed for the MOCVD growth of GaN on ZnO. The thermal stability of the ZnO substrate, out-diffusion of Zn from the ZnO into the GaN, and H 2 back etching into the substrate can cause growth of poor quality GaN. Cracks and pinholes were seen in the epilayers, leading to the epi-layer peeling off in some instances. These issues were addressed by the use of H 2 free growth and multiple buffer layers to remove the cracking and reduce the pinholes allowing for a high quality GaN growth on ZnO substrate.
This article deals with strain relaxation in SmNiO 3 epitaxial films deposited by chemical vapor deposition on SrTiO 3 substrates. Thanks to x-ray reciprocal space mapping, we demonstrate that the strain relaxation is driven both “chemically” and “mechanically” by the formation of oxygen vacancies and misfit dislocations, respectively. Besides, a careful interpretation of the resistivity measurements allows us to highlight a correlation between the formation of oxygen vacancies, the stabilization of Ni 3+ , and the metal-insulator transition in the SmNiO 3 films. Furthermore, using coplanar and grazing incidence diffraction, the shape of the strain gradient within the films is retrieved. This latter is calculated using a versatile scattering model involving B -spline functions. Finally, particular planar faults (Ruddlesden–Popper faults) that give rise to extended diffuse scattering on transverse scans are analyzed using a recent phenomenological model.
We have studied InAs/GaSb superlattices (SL´s) grown with either InSb-like or GaAs-like interfaces (IF´s) on top of a GaSb buffer layer on GaAs substrates. The InAs layer thickness was varied from 4 to 14 monolayers (ML) while the GaSb layer thickness was kept fixed at 10 ML. The type of IF bonds realized was verified by Raman scattering from mechanical IF modes. High-resolution x-ray diffraction using one- and two-dimensional mapping of symmetric and asymmetric reflections allowed us to determine independently the lattice parameters parallel and perpendicular to the growth direction. The GaSb buffer layer was found to be fully relaxed whereas the SL’s with InSb-like IF’s were coherently strained to the in-plane lattice parameter of the GaSb buffer for InAs layer thicknesses exceeding 6 ML. The strain distribution within the SL’s with GaAs-like IF’s was obtained from simulations of the x-ray reflection profiles. The SL’s were found to be coherently strained close to the GaSb buffer and showed increasing strain relaxation with increasing distance from the buffer layer. In addition, these simulations provide an accurate determination of the SL periods. Well-resolved Raman spectra of backfolded longitudinal acoustic ~LA! phonons were observed showing for SL’s with InSb-like IF’s folded LA phonon lines up to the seventh order. The spectrum of quasiconfined optical SL phonons was examined by Raman spectroscopy and by IR reflection. A detailed analysis of the IR reflection spectra allowed an independent determination of the individual layer widths within the SL stack, including the spatial extent of the GaAs-like IF mode.
We apply the Krivoglaz theory of x-ray scattering to thin epitaxial films containing misfit dislocations and reanalyze the seemingly puzzling x-ray scattering phenomena observed in several heteroepitaxial films. We show that the two-line shape scattering distribution and its dependence upon film thickness and momentum transfer can be understood in natural way and on a quantitative level. Extended diffuse x-ray scattering maps have been obtained from Nb(110)/Al2O3(1120) which are discussed within the framework of this theory disclose a particular dislocation network at the Nb-Al2O3 interface.
We have studied InAs/GaSb superlattices (SL's) grown with either InSb-like or GaAs-like interfaces (IF's) on top of a GaSb buffer layer on (100) GaAs substrates. The InAs layer thickness was varied from 4 to 14 monolayers (ML) while the GaSb layer thickness was kept fixed at 10 ML. The type of IF bonds realized was verified by Raman scattering from mechanical IF modes. High-resolution x-ray diffraction using one- and two-dimensional mapping of symmetric and asymmetric reflections allowed us to determine independently the lattice parameters parallel and perpendicular to the growth direction. The GaSb buffer layer was found to be fully relaxed whereas the SL's with InSb-like IF's were coherently strained to the in-plane lattice parameter of the GaSb buffer for InAs layer thicknesses exceeding 6 ML. The strain distribution within the SL's with GaAs-like IF's was obtained from simulations of the x-ray reflection profiles. The SL's were found to be coherently strained close to the GaSb buffer and showed increasing strain relaxation with increasing distance from the buffer layer. In addition, these simulations provide an accurate determination of the SL periods. Well-resolved Raman spectra of backfolded longitudinal acoustic (LA) phonons were observed showing for SL's with InSb-like IF's folded LA phonon lines up to the seventh order. The spectrum of quasiconfined optical SL phonons was examined by Raman spectroscopy and by IR reflection. A detailed analysis of the IR reflection spectra allowed an independent determination of the individual layer widths within the SL stack, including the spatial extent of the GaAs-like IF mode.
We present the results of high-resolution x-ray scattering studies of the structural coherence of niobium (110) films grown by molecular-beam epitaxy on sapphire (11{bar 2}0) substrates. In transverse scans of the out-of-plane (110) Bragg peak we find two components, the sharper of which implies mosaicities an order of magnitudes better than bulk single-crystal Nb, and transverse structural coherence lengths exceeding 10â´ A. In addition, we observe that the planes associated with the sharp component are exactly aligned with the sapphire (11{bar 2}0) planes. Upon hydrogen loading of the Nb film, we find evidence for a dramatic increase of the lateral coherence length.
The film growth of CeO 2 by metal-organic chemical vapor
deposition (MOCVD) was studied. Samples of different thicknesses ( d =
10-500 nm) on SrTiO 3 as well as on yttria stabilized
zirconia were prepared and analysed by X-ray diffraction (
{θ}/{2θ- scan}, ω-scan), scanning electron
microscopy (SEM), and Rutherford backscattering spectroscopy (RBS). The
films grew with a preferential orientation and a CeO
2[001]//substrate[001] was found. The rocking curves in the
ω-scans exhibited a two-component shape in very thin films. A
crossover depending on the substrate material was observed at higher
film thicknesses leading to a one-component curve. The complex behavior
in the film evolution of the CeO 2 system will be discussed,
and a model of the film growth mechanism will be proposed.
We report a regime of temperature film thickness and substrate miscut in which thin films of niobium (110) grow faceted on sapphire (112¯0), and with a surface morphology that contains long fingers oriented along the niobium in-plane [001] direction. The origins of these features are discussed.
Compact, solid-state UV emitters have many potential applications, and ZnO-based materials are ideal for the wavelength range 390 nm and lower. However, the most efficient solid-state emitters are p-n junctions, and p-type ZnO is difficult to make. Thus, the future of ZnO light emitters depends on either producing low-resistivity p-type ZnO, or in mating n-type ZnO with a p-type hole injector. Perhaps the best device so far involves an n-ZnO/p-AlGaN/n-SiC structure, which produces intense 390 +/- 1 nm emission at both 300 K and 500 K. However, development of p-ZnO is proceeding at a rapid pace, and a p-n homojunction should be available soon.
We report on plasma-assisted molecular beam epitaxy of ZnO on Al2O3 (0001) substrates with an MgO buffer layer. We found a thin MgO buffer greatly influence the initial growth of ZnO by facilitating the adhesion and promoting the lateral epitaxy of ZnO. Consequently, a flat surface with a (3×3) reconstruction of ZnO is observed and reflection high-energy electron diffraction intensity oscillations are recorded. Both in situ investigations and structural analysis revealed that the MgO buffer has a rocksalt structure and consists of a 2D wetting layer and 3D islands above. The change in surface energy by covering the substrate with an MgO wetting layer results in a drastic suppression of columnar growth and layer-by-layer epitaxy of ZnO is achieved. The resulting improvement in crystal quality of the ZnO epilayers is confirmed by a large decrease in the X-ray rocking curve widths of both symmetrical and asymmetrical diffraction peaks, as well as the presence of free exciton recombinations in low temperature photoluminescence and the low n-type background carrier concentration.
The growth of p-type ZnO film was realized for the first time by the simultaneous addition of NH 3 in carrier hydrogen and excess Zn in source ZnO powder. The resistivity was typically 100 Ω·cm. A model showing nitrogen incorporation suggests the possibility of realizing p-type ZnO film of low resistivity by optimizing thermal annealing.
We present a Monte Carlo technique to calculate the x-ray diffraction profiles from films with arbitrarily correlated dislocation distributions. Both spatial integration and the integration over several dislocation ensembles are performed simultaneously. We explicitly consider the coexistence of misfit and threading dislocation ensembles with arbitrary correlations. Using these techniques, we are able to quantitatively reproduce the experimental lineshapes for both thin (less than 100 nm) and thick (more than 1000 nm) GaN epitaxial films on SiC. Our calculations explain the ubiquitous lineshape observed for thin, highly mismatched films, with a central coherent peak accompanied by exponentially decaying diffuse scattering.
An N-doped, p-type ZnO layer has been grown by molecular beam epitaxy on an Li-diffused, bulk, semi-insulating ZnO substrate. Hall-effect and conductivity measurements on the layer give: resistivity = 4×101 Ω cm; hole mobility = 2 cm2/V s; and hole concentration = 9×1016 cm−3. Photoluminescence measurements in this N-doped layer show a much stronger peak near 3.32 eV (probably due to neutral acceptor bound excitons), than at 3.36 eV (neutral donor bound excitons), whereas the opposite is true in undoped ZnO. Calibrated, secondary-ion mass spectroscopy measurements show an N surface concentration of about 1019 cm−3 in the N-doped sample, but only about 1017 cm−3 in the undoped sample. © 2002 American Institute of Physics.
The influence of lattice misfit on the growth of Ti (0001) is investigated in the limit of small negative (−1%) and large positive (+6.8%) misfit by choosing MgO (111) and Al2O3 (0001) as substrate materials. Reflection high energy electron diffraction imaging and intensity measurements during growth reveal two-dimensional nucleation of islands on MgO, in contrast to three-dimensional nucleation on Al2O3. X-ray analysis of 30-nm-thick films on MgO shows a two-component line shape in transverse scans of the (0002) and (0004) reflections, pointing to a high degree of structural coherence in the weak disorder limit. The surface morphology of films grown on MgO depends strongly on the substrate temperature during growth. © 1997 American Institute of Physics.
The evolution of surface roughness and strain relaxation as a function of film thickness of ZnO films grown on sapphire(0001) were studied by in situ synchrotron x-ray scattering and atomic force microscopy measurements. The well-aligned two-dimensional (2D) planar layer dominated in layer-by-layer growth at the highly strained initial growth stage. As the film thickness increased, the discrete nucleations on the 2D planar layer continuously grew until the ZnO film reached the strain relaxed steady-state regime. When the 3D islands were quickly developed by the strain relaxation, the dynamic scaling exponent β was roughly 1.579. The strain relaxed steady-state regime was described as β∼0.234.
THE measures of line-breadth most widely used in X-ray and electron diffraction are perhaps the half width and the integral breadth. The measure of dispersion most widely used in mathematical treatments of distributions is, however, the variance or mean-square breadth. Its comparative neglect in crystallographic problems has been due to the large variance of the wave-length spread and of the diffraction profile, the ‘tails’ of the lines approaching zero approximately as the square of the distance from the line centroids. In spite of this, however, the variance has been used in discussions of spectrometer1 and diffractometer2–4 aberrations. When the line profile is ‘truncated’, as it has to be in practical centroid determination5–8, the large contribution to the variance from the tails is eliminated, and it becomes a measure of line-broadening worth serious consideration. It has the great advantage over all other measures of breadth that it can be corrected for geometrical aberrations1 and wave-length spread by simple subtraction, instead of by ‘unfolding’9. Tournarie10 has criticized the use of half width and integral breadth on the ground that they give undue weight to a few isolated points of the diffraction profile, instead of treating all observed points on the same basis, and derived a relation between the crystallite size and the variance of the line profile over a deliberately restricted angular range. Tournarie's result is immediately obtained by considering the asymptotic expressions for the line profile given by Wilson11; in the present communication a unified treatment is outlined which not only recovers Tournarie's particle-size result but also gives analogous expressions applicable to certain types of ‘mistake’ and strain broadening.
We discuss the effects of misfit dislocations on the specular and diffuse scattering observed in X-ray reflectivity at the Bragg reflections of thin films. Using experimental results from ErAs/GaAs(001), it is shown that an interfacial displacement-difference correlation function can be used to model the scattering from misfit dislocations. We find that the diffuse scattering is correlation-length-limited for weak disorder whereas it has a rotational character for strong disorder. It is suggested that the correlation length arises from the elastic image field of a misfit dislocation.
A method is derived, using Fourier analysis, for finding the corrected distribution of intensity across an x-ray diffraction line, and the procedure is illustrated by a numerical example. The method may also be of use in spectrum analysis and statistical problems.
The powder diffraction method, by using conventional X-ray sources, was devised independently in 1916 by Debye and Scherrer in Germany and in 1917 by Hull in the United States. The technique developed steadily and, half a century later, the `traditional' applications, such as phase identification, the determination of accurate unit-cell dimensions and the analysis of structural imperfections, were well established. There was then a dramatic increase of interest in powder methods during the 1970s, following the introduction by Rietveld in 1967 of his powerful method for refining crystal structures from powder data. This has since been used extensively, initially by using neutron data and later with X-rays, and it was an important step towards extracting 3-dimensional structural information from 1-dimensional powder diffraction patterns, in order to study the structure of crystalline materials. Similarly, techniques which do not involve structural data have been introduced for modelling powder diffraction patterns, to extract various parameters (position, breadth, shape, etc.) which define the individual reflections. These are used in most applications of powder diffraction and are the basis of new procedures for characterizing the microstructural properties of materials. Many subsequent advances have been based on this concept and powder diffraction is now one of the most widely used techniques available to materials scientists for studying the structure and microstructure of crystalline solids. It is thus timely to review progress during the past twenty years or so.
Powder data have been used for the identification of unknown materials or mixtures of phases since the late 1930s. This is achieved by comparison of experimental data with standard data in crystallographic databases. The technique has benefited substantially from the revolution in the development of storage media during the last decade and from the introduction of fast search/match algorithms. Phase identification sometimes precedes a quantitative analysis of compounds present in a sample and powder diffraction is frequently the only approach available to the analyst for this purpose. A new development in quantitative analysis is the use of the Rietveld method with multi-phase refinement.
A major advance in recent years has occurred in the determination of crystal structures ab initio from powder diffraction data, in cases where suitable single crystals are not available. This is a consequence of progress made in the successive stages involved in structure solution, e.g. the development of computer-based methods for determining the crystal system, cell dimensions and symmetry (indexing) and for extracting the intensities of Bragg reflections, the introduction of high resolution instruments and the treatment of line-profile overlap by means of the Rietveld method. However, the intensities obtained, and hence the moduli of the observed structure factors, are affected by the overlap problem, which can seriously frustrate the determination of an unknown crystal structure. Although numerous structures have been solved from powder data by using direct or Patterson methods, the systematic or accidental total overlap of reflections continues to focus the attention of a number of crystallographers. New approaches for the treatment of powder data have been devised, based on maximum entropy methods and `simulated annealing', for example, to generate structural models. Additionally, resonant diffraction (anomalous scattering) is used as an aid to structure solution.
There has been spectacular progress in characterizing the microstructural properties which arise from various types of structural imperfection. The principal advance has been the 3-dimensional reconstruction of `anisotropic' (direction- or hkl-dependent) features or properties of polycrystalline materials. These include the shape of diffracting domains and the distribution of the size, structural `mistakes' induced during the formation or subsequent treatment of a sample and dislocations or other forms of lattice distortion. The main innovation here has been a comparison of experimental data with those derived from a physical model based on data from other techniques or from prior knowledge of the behaviour of the material.
Most aspects of powder diffraction are brought together in analysing data from experiments carried out under non-ambient conditions, a field that continues to expand as more intense sources of radiation become available. Such experiments can be carried out over a wide range of temperature and at ever increasing pressures. Chemical or solid-state reactions and other processes, such as phase transformations, can be followed in situ by means of time-resolved diffraction.
For the benefit of the reader who is unfamiliar with powder diffraction, a r?sum? of the basic principles underlying the various techniques and applications is included. Sources of radiation, modern instrumentation and detectors are also considered, since these have played a major role in the progress of powder diffraction during the past two decades. Numerous examples are discussed throughout the review, in order to illustrate the main applications and procedures. Powder diffraction is interdisciplinary and these are inevitably drawn from various branches of science. However, it should be remembered that, in the main, the use of powder diffraction is frequently a `means to an end', albeit an important stage in a study of polycrystalline materials.
This review was received August 1995
Some X-ray scattering methods (X-ray reflectometry and Diffractometry) dedicated to the study of thin-layered heterostructures are presented with a particular focus, for practical purposes, on the description of fast, accurate and robust techniques. The use of X-ray scattering metrology as a routinely working non-destructive testing method, particularly by using procedures simplifying the data-evaluation, is emphasized. The model-independent Fourier-inversion method applied to a reflectivity curve allows a fast determination of the individual layer thicknesses. We demonstrate the capability of this method by reporting X-ray reflectometry study on multilayered oxide structures, even when the number of the layers constitutive of the stack is not known a-priori. Fast Fourier transform-based procedure has also been employed successfully on high resolution X-ray diffraction profiles. A study of the reliability of the integral-breadth methods in diffraction line-broadening analysis applied to thin layers, in order to determine coherent domain sizes, is also reported.Examples from studies of oxides-based thin-layers heterostructures will illustrate these methods. In particular, X-ray scattering studies performed on high-k HfO2 and SrZrO3 thin-layers, a (GaAs/AlOx) waveguide, and a ZnO thin-layer are reported.
Both X-ray reflectometry and X-ray diffractometry techniques are used for the assessment of individual layer thicknesses inside complicated semi-conductor heterostructures, in particular for opto-electronic applications. The use of Fast Fourier transform-based numerical treatments applied to the reflectivity curve allows a fast determination of the individual layer thicknesses. We demonstrate the capability of this method by reporting X-ray reflectometry study on superlattices, multiple quantum wells, and other complicated structures. Typical layer thicknesses from 0.5 nm to 500 nm were successfully investigated. Fast Fourier transform-based procedure has also been employed successfully on high resolution X-ray diffraction spectra. We finally show the complementary of both techniques.
p-Type ZnO obtained by arsenic (As) doping is reported for the first time. Arsenic-doped ZnO (ZnO : As) films have been deposited on (0 0 1)-GaAs substrates by pulsed laser ablation. The process of synthesizing p-type ZnO : As films was performed in an ambient gas of ultra-pure (99.999%) oxygen. The ambient gas pressure was 35 mTorr with the substrate temperature in the range 300–450°C. ZnO films grown at 400°C and 450°C are p-type and As is a good acceptor. The acceptor peak is located at 3.32 eV and its binding energy is about 100 meV. Acceptor concentrations of As atoms in ZnO films were in the range from high 1017 to high 1021 atoms/cm3 as determined by secondary ion mass spectroscopy (SIMS) and Hall effect measurements.
A comparison between different simplified integral-breadth methods, often used in the Rietveld-refinement programs to calculate coherent domain size and lattice strain, is carried out. It is shown that systematic differences exist for both domain size and strain, when they simultaneously broaden diffraction lines. Among different approximations, the values of domain size exceptionally scatter and sometimes are completely false (negative or not real). A comparison to the alternative Fourier method shows that all the simplified integral-breadth methods overestimate domain size, but especially the Cauchy-Cauchy approximation. The root-mean-square strain and the upper limit of strain are related in the general case of the Voigt strain-broadened line profile. It is shown that they should not differ much as the profile changes between the Gauss and Cauchy extremes. The pure-Gauss size-broadened profile is incompatible with the definitions of surface-weighted domain size and column-length distribution function.
Motivated by x-ray-scattering measurements performed on
ErAs(001)/GaAs(001) and
In0.7Ga0.3P(001)/GaAs(001), we present a model
that explains the origin of a narrow peak and diffuse scattering, which
are frequently observed at Bragg reflections in epitaxial systems.
Central to the model is a correlation length for mosaiclike rotational
disorder that arises in lattice-mismatched epitaxial films. The adhesive
force between the film and the substrate is found to play a crucial role
and leads to a striking anisotropy in the line shapes.
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