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Abstract

High efficient green light emitting diodes (LED) on the basis of GaN/InGaN exhibit indium-rich nanoclusters inside the quantum wells (QW) due to InN–GaN phase decomposition. By direct measurements of the variations in the electronic structure, we show for the first time a correlation between indium-rich nanoclusters and local energy band gap minima. Our investigations reveal the presence of 1–3 nm wide indium rich clusters in these devices with indium concentrations x as large as x∼0.30–0.40 that narrow the band gap locally to energies as small as 2.65 eV. These clusters are able to act as local traps for migrating photon-emitting carriers and seem to boost the overall device performance.

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... Another problem about the AlGaN-based DUV LEDs is the commonly observed multiple or asymmetrical emission, even in the high-performance devices [11,[13][14][15]. Similar phenomenons are generally observed in the InGaN alloy system and have been widely ascribed to the local Indium cluster induced by compositional segregation [16][17][18][19]. However, few studies concern the mechanism contributing to the multiple or asymmetrical spectra of AlGaN DUV materials. ...
... Similar multiple or asymmetrical spectra and inhomogeneous emission distributions were commonly observed in the InGaN alloy system. This phenomenon has been mainly ascribed to the local Indium cluster induced by compositional segregation [16][17][18][19], or the compositional pulling effect caused by mismatch strain [22,32]. For AlGaN alloy system, the Al composition has been found to segregate around dislocation lines between the crystal grain boundaries due to the non-conforming orientation of crystal columns including tilt and twist [33][34][35]. ...
Article
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A systematic study was carried out for strain-induced microscale compositional pulling effect on the structural and optical properties of high Al content AlGaN multiple quantum wells (MQWs). Investigations reveal that a large tensile strain is introduced during the epitaxial growth of AlGaN MQWs, due to the grain boundary formation, coalescence and growth. The presence of this tensile strain results in the microscale inhomogeneous compositional pulling and Ga segregation, which is further confirmed by the lower formation enthalpy of Ga atom than Al atom on AlGaN slab using first principle simulations. The strain-induced microscale compositional pulling leads to an asymmetrical feature of emission spectra and local variation in emission energy of AlGaN MQWs. Because of a stronger three-dimensional carrier localization, the area of Ga segregation shows a higher emission efficiency compared with the intrinsic area of MQWs, which is benefit for fabricating efficient AlGaN-based deep-ultraviolet light-emitting diode.
... The maps for In and Ga show much larger scatter, with 2-3 times higher standard deviations. The lateral scale of the indium-rich regions visible in (e) and (f) extends to only 0.5-3nm, which would be in agreement with [30] . Given the projection through the ~40nm thick specimen such In-rich clusters, should they exist and not be an artefact from electron irradiation, must consist of almost pure InN. ...
... Any algorithm fitting just one bandgap by a square-root function then would have to fail at an interface between two materials where both bandgaps overlap to differing degrees. The additional intensity ~0.5eV below the nominal bandgap reported in VEELS from an In 0.14 Ga 0.86 N quantum well [30], on the other hand, cannot be attributed to a delocalisation effect because the bandgap from the surrounding GaN barrier would have been higher instead of lower than the bandgap of InGaN; it may be the result of a too large bandgap assumed for InN and a resulting offset in the relationship between composition and actual bandgap for InGaN alloys. ...
Article
Phase separation of InxGa1−xN into Ga-rich and In-rich regions has been studied by electron energy-loss spectroscopy (EELS) in a monochromated, aberration corrected scanning transmission electron microscope (STEM). We analyze the full spectral information contained in EELS of InGaN, combining for the first time studies of high-energy and low-energy ionization edges, plasmon, and valence losses. Elemental maps of the N K, In M4,5 and Ga L2,3 edges recorded by spectrum imaging at 100 kV reveal sub-nm fluctuations of the local indium content. The low energetic edges of Ga M4,5 and In N4,5 partially overlap with the plasmon peaks. Both have been fitted iteratively to a linear superimposition of reference spectra for GaN, InN, and InGaN, providing a direct measurement of phase separation at the nm-scale. Bandgap measurements are limited in real space by scattering delocalization rather than the electron beam size to ∼10 nm for small bandgaps, and their energetic accuracy by the method of fitting the onset of the joint density of states rather than energy resolution. For an In0.62Ga0.38N thin film we show that phase separation occurs on several length scales.
... The light emission at longer wavelengths from the LT-sample is likely caused by a non-uniform In content in the well. It was often reported that the luminescence spectra of LED in the green or even red color range is related to the nanostructure of the alloy [41][42][43][44][45][46][47][48][49]. The appearance of composition fluctuations with a locally smaller bandgap on the nanoscale leads to improved carrier confinement in band tail states, which dominate the luminescence spectra. ...
... The appearance of composition fluctuations with a locally smaller bandgap on the nanoscale leads to improved carrier confinement in band tail states, which dominate the luminescence spectra. It was shown that In-rich nanoclusters also arise in quantum wells and are able to act as local traps for photonemitting carriers of especially green LED [44][45][46][47][48]. In clusters with 1-3 nm lateral size with local In content up to 30-40% are observed in a layer with an average composition of 17-20% In. ...
Article
The growing interest in modern energy-saving illuminants for general lighting, multimedia applications and automotive industry demands for alternative low-cost substrates for MOVPE LED growth. Nitride MOVPE growth is possible on the Si (111) plane, which makes Si substrates attractive as an alternative to sapphire substrates. A novel technology is presented using patterned Si (100) substrates, in which MOVPE-grown LED structures are fabricated on Si {111} facets tilted by 54.7°. Structural and optical properties are discussed and correlated to epitaxial growth conditions. It is shown that crystal quality reaches already a reasonable level for preliminary LED operation.
... The lateral scale of the Inrich regions visible in fig. 3.15(B) extends to only 0.5-3 nm, which would be in agreement with findings by other groups (Jinschek et al., 2006). Given the projection through the "40 nm thick specimen such In-rich clusters, should they exist and not be an artefact from electron irradiation (Doppalapudi et al., 1998;Lin et al., 2000;Park et al., 2005;Singh et al., 1997), must consist of almost pure InN. ...
... These plasmon peaks can be determined by applying fourier-log deconvolution to low-loss spectra (Egerton et al., 1985). Wang et al., (2015) have compared the relationship between x and plasmon peak position and FWHM in ternary alloys of In x Ga 1-x N with the literature from Bosman et al., (2009), Egerton, (2009), Jinschek et al., (2006, Kong et al., (2012) and Specht et al., (2006) and have used a linear least-squares fit for the expression for E p px q as shown in eqn. 5.3 (ref: to fig. 3 from Wang et al., (2015)). ...
... i.e. completely random replacement of gallium by indium atoms, leads to significant localization of hole states in InGaN [71,72]. In addition, during the preparation of InGaN layers, the growth conditions can be such, that indium is preferentially incorporated close to other indium atoms, resulting in additional, macroscopic fluctuations of the indium composition [73], macroscopic clusters of higher indium content [74][75][76] or phase separation [77,78]. In these cases, even deeper localization potentials are created, which may also bind electrons [79,80]. ...
... In particular, of the suggested explanations (section 5.4.1 on page 71) for the green gap, a) and b) can be excluded. The growth of InGaN QWs with high In content, as required for green emission wavelengths, is certainly challenging [74,75,77,78], and many groups observe higher defect densities [214,[235][236][237][238]. Nevertheless it is in principle possible to produce material with a similar crystal quality as in the blue (lower indium composition) spectral region. This is attested by the almost identical magnitude and thermal evolution of the SRH recombination coefficient A. ...
... The mostly cited bandgap of 0.7 eV for InN [63] may therefore very well be its optical bandgap, but it is not at all clear if this is also its fundamental bandgap [81]. Local indium fluctuations in combination with band gap (Eg) variations were observed in high efficiency green light emitting In x Ga 1 À x N/GaN diodes by Jinschek et al. [82] . The authors analyzed VEELS data to find emission energies that peak at Eg¼3.13 eV, with a variance towards lower energies of ΔEg¼0.20 eV (asymmetric distribution). ...
... From the letter of Baloch et al. [34], the listed indium concentration of 22% and assigned errors ranging from 3% to 5.4% are used to add a single data point at x/y ¼ 0.22/ $ 0.04 to the Fig. 8(full black circle). A striking agreement of this data point with previously published strain results is immediately revealed that is also consistent with the previous indium fluctuation measurements by VEELS [82]. These outstanding agreements of independent data sets strongly supports the cluster model but are in stark contradiction to the opposite statement that was made in the letter. ...
Article
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Ternary InxGa1-xN alloys became technologically attractive when p-doping was achieved to produce blue and green light emitting diodes (LED)s. Starting in the mid 1990th, investigations of their chemical homogeneity were driven by the need to understand carrier recombination mechanisms in optical device structures to optimize their performance. Transmission electron microscopy (TEM) is the technique of choice to complement optical data evaluations, which suggests the coexistence of local carrier recombination mechanisms based on piezoelectric field effects and on indium clustering in the quantum wells of LEDs. We summarize the historic context of homogeneity investigations using electron microscopy techniques that can principally resolve the question of indium segregation and clustering in InxGa1-xN alloys if optimal sample preparation and electron dose-controlled imaging techniques are employed together with advanced data evaluation.
... High defect densities in InGaN layers, grown on GaN/sapphire templates, increase the impact of the non-radiative recombination and reduce the light emission. However, efficient luminosity, observed in highly defected InGaN QWs, is due to In-rich regions; the potential minima for localized carriers provide efficient radiative recombination channels, in spite of the surroundings with a high dislocation density [4][5][6][7][8][9][10][11][12]. Indium segregation in InGaN alloys [13][14][15] or some spinodal decomposition [16][17][18] are named as the origin of these potential fluctuations [6,[10][11][12]. ...
... However, efficient luminosity, observed in highly defected InGaN QWs, is due to In-rich regions; the potential minima for localized carriers provide efficient radiative recombination channels, in spite of the surroundings with a high dislocation density [4][5][6][7][8][9][10][11][12]. Indium segregation in InGaN alloys [13][14][15] or some spinodal decomposition [16][17][18] are named as the origin of these potential fluctuations [6,[10][11][12]. For the green or yellow light emitting QWs, the situation becomes even more complicated, as lower growth temperatures and higher In concentrations lead to more defected areas and a higher density of non-radiative recombination channels. ...
Article
The paper reports on fully strained green light emitting InGaN/GaN multiple quantum wells, grown by metalorganic vapor phase epitaxy, using metal precursor multiple flow interruptions during InGaN quantum well growth. Optimization of the interruption timing (pulse t(1) = 20 s, pause t(2) = 12 s) lets us reach the integrated photoluminescence enhancement for the growth at temperature 780 degrees C. The enhancement, as a function of pause duration, appeared to be pulse duration dependent: a lower enhancement can be achieved using shorter pulses with optimized relatively shorter pauses. Indium evaporation during the interruption time was interpreted as the main issue to keep the layers intact. Quantum wells revealing the highest photoluminescence enhancement were inspected for interface quality, layer thickness, growth speed, strain, surface morphology and roughness by TEM, XRD and AFM techniques, and compared with the one grown in the conventional mode.
... In order to improve the efficiency of InGaN based LEDs, an In-rich GaN matrix could be used as an efficient radiative recombination centre rather than ordinary layers [7]. Recent investigations on InGaN showed that phase separation and composition fluctuation might be actually beneficial and significantly improve the luminescence efficiency due to the presence of localized states [8,9]. In this work, we have obtained a strong green-tored room temperature (RT) emission from the In-rich phase InAlGaN quaternary layer. ...
Article
Full-text available
A strong room temperature green-to-red photoluminescence (PL) emission has been obtained from an In-rich InAlGaN quaternary layer grown on a GaN template using metal organic chemical vapor deposition. The PL decay kinetics could be described by a stretched exponential with the stretching parameter β = 0.40 ± 0.02. The decay time increased with wavelength while β was constant, which indicates significant disorder in the material related to spatial fluctuation of local In concentration. By the addition of blue emission via the insertion of an InGaN layer, a white light emission has been demonstrated from In-rich InAlGaN/InGaN heterostructures.
... Indeed, only relative intensity variations in TEM and HAADF-STEM reflect variations in composition. The difficulty lies in the fact that these relative intensity variations cannot be quantitatively interpreted directly from the TEM or STEM image [12]. In order to do that, one needs a means of establishing an absolute scale of composition. ...
Article
Full-text available
Using elastic scattering theory we show that a small set of energy dispersive x-ray spectroscopy (EDX) measurements is sufficient to experimentally evaluate the scattering function of electrons in high-angle annular dark field scanning transmission microscopy (HAADF-STEM). We then demonstrate how to use this function to transform qualitative HAADF-STEM images of InGaN layers into precise, quantitative chemical maps of the indium composition. The maps obtained in this way combine the resolution of HAADF-STEM and the chemical precision of EDX. We illustrate the potential of such chemical maps by using them to investigate nanometer-scale fluctuations in the indium composition and their impact on the growth of epitaxial InGaN layers.
... In addition, experimentally determined values of E max reported in various publications are also included in figure 3 for comparison with our measurements. The literature data well agree with the linear trend, albeit they were measured in epitaxially strained thin films [20][21][22][23][24]. ...
Article
Full-text available
We demonstrate the potential of low-loss electron energy-loss spectroscopy in transmission electron microscopy as a quick and straightforward method to determine the local indium compositions in (In,Ga)N/GaN nanowires. The (In,Ga)N/GaN nanowire heterostructures are grown by plasma assisted molecular beam epitaxy on Si(111) substrates in a self-assembled way, and on patterned GaN templates in an ordered way. A wide range of indium contents is realized by varying the substrate temperatures. The plasmon peak in low-loss electron energy-loss spectroscopy exhibits a linear relation with respect to indium concentration in (In,Ga)N nanowires, allowing for a direct compositional analysis. The high spatial resolution of this method in combination with structural information from transmission electron microscopy will contribute to a basic understanding of the lattice pulling effect during (In,Ga)N/GaN nanowire growth.
... High current density flows through these areas. Our model in this part corresponds very closely with the Kovarsky paper conclusions [42] in which it was said that the device properties depend critically on the metal cation composition of the emitting layer, i.e. generally the In x Ga 1-x N mole fraction and with Jinschek paper conclusions [43] where it was said that in green LEDs there is the presence of 1-3 nm wide indium rich clusters with indium concentration x as large as x ~ 0.3 -0.4 that narrow the band gap locally to energies as small as 2.65 eV. These clusters were able to act as local traps for migration photon-emitting carries and seemed to boost the overall device performance. ...
Article
Full-text available
Light-emitting diodes (LEDs) degradation during 10 000 hours and the influence of ultrasonic action on the InGaN LEDs were investigated. The model of LED degradation is suggested and based on 1) common LED is the combination of parallel Small-LEDs (S-LED) which correspond to the areas with different concentration of In atoms; 2) redistribution of In atoms in quantum-dimensional active region of blue InGaN LED under strong piezoelectric effect and spontaneous polarization induced by ultrasonics; 3) the ultrasonics which is used in creating LEDs can make defects in heterostructures and they (during LEDs work) are heated by current or ultrasonic, can be increased and that's why nonradiation recombination decreases LEDs efficiency. It can be said that great current density flows through areas with low In concentration and therefore S-LEDs are "burned out" and irradiate less. At the areas with average In concentration the densities decrease. That is why electroluminescence spectrum, radiation power, luminous intensity characteristics are shifted to the long wave region. All that also exactly corresponds with our experimental results of LEDs degradation investigation during 10 000 hours
... It is known that carrier localization can originate from the compositional fluctuation of In in the InGaN well ~Ruterana et al., 2002; Cheng et al., 2004!, well-width fluctuation of InGaN ~Grandjean et al., 2001; Brandt et al., 2002!, and likely growth of quantum dots ~Graham et al., 2005!; however, these remain poorly understood ~Narayan et al., 2002!. Although various indirect experimental results have been reported as an evidence of compositional fluctuation or In clustering in nm scale ~Jinschek et al., 2006; Van der Laak et al., 2007!, the problems still remain to be solved. To observe the compositional localization phenomenon as direct evidence, high-resolution transmission electron microscopy ~HRTEM! ...
Article
In this paper, we have observed an atomic-scale structure and compositional variation at the interface of the InGaN/GaN multi-quantum wells (MQW) by both scanning transmission electron microscopy (STEM) using high-angle annular dark-field mode and atom probe tomography (APT). The iso-concentration analysis of APT results revealed that the roughness of InGaN/GaN interface increased as the MQW layers were filled up, and that the upper interface of MQW (GaN/InGaN to the p-GaN side) was much rougher than that of the lower interface (InGaN/GaN tot he n-GaN side). On the basis of experimental results, it is suggested that the formation of interface roughness can affect the quantum efficiency of InGaN-based light-emitting diodes.
... Actually, it have been reported that InGaN/GaN MQWs with high In content can lead to strong carrier localization due to the compositional fluctuation of In within the active layers. 21,22 At the In-rich regions, it may form In clusters and quantum-dots-like structures, which form deep traps acting as localized radiative recombination centers and trap the excitons. However, it seems difficult to understand the carrier localization outside the In-rich regions because the In content there is relatively low and the potential fluctuation can be very weak. ...
Article
Full-text available
The effect of carrier localization in InGaN/GaN multiple quantum wells (MQWs) light-emitting diodes is investigated by photoluminescence (PL) and time-resolved PL (TRPL) measurements. PL results show that two peaks obtained by Gaussian fitting both relate to the emission from localized states. By fitting the TRPL lifetimes at various emission energies, two localization depths corresponding to the In-rich regions and quasi-MQWs regions are obtained. Using a model we proposed, we suggest that compositional fluctuations of In content and variation of well width are responsible for carrier localization in In-rich regions and quasi-MQWs regions, respectively.
... Established reasons for this matter include a reduced electron-hole-overlap in the QWs due to the quantumconfined Stark effect (QCSE) [4][5][6][7][8] and general difficulties in growing homogeneous InGaN material. [9][10][11][12][13][14][15] Also high pumping conditions are known to cause a further reduction of the efficiency that is attributed to Auger recombination. [16][17][18][19] In this letter, we report on an additional high excitation loss mechanism arising from confined hole continuum (CHC) states promoting carrier leakage, which is of utmost importance for the implementation of efficient LDs emitting in the green spectral region. ...
Article
We investigate industrial-grade InGaN/GaN quantum wells (QWs) emitting in the green spectral region under high, resonant pumping conditions. Consequently, an ubiquitous high energy luminescence is observed that we assign to a polarization field Confined Hole Continuum (CHC). Our finding is supported by a unique combination of experimental techniques, including transmission electron microscopy, (time-resolved) photoluminescence under various excitation conditions, and electroluminescence, which confirm an extended out-of-plane localization of the CHC-states. The larger width of this localization volume surpasses the QW thickness, yielding enhanced non-radiative losses due to point defects and interfaces, whereas the energetic proximity to the bulk valence band states promotes carrier leakage.
... In the LED or LD structure, the optoelectronic properties are very sensitive to the indium content distribution in the InGaN well layer, and the physical mechanism is still in controversy. In the meantime, the indium segregation phenomenon in InGaN well layer has been studied for several years [29][30][31][32][33][34] . Due to the indium segregation, the nonuniform distribution of indium content will seriously change the potential profile of InGaN/GaN MQWs. ...
Article
Full-text available
The indium segregation in InGaN well layer is confirmed by a nondestructive combined method of experiment and numerical simulation, which is beyond the traditional method. The pre-deposited indium atoms before InGaN well layer growth are first carried out to prevent indium atoms exchange between the subsurface layer and the surface layer, which results from the indium segregation. The uniform spatial distribution of indium content is achieved in each InGaN well layer, as long as indium pre-deposition is sufficient. According to the consistency of the experiment and numerical simulation, the indium content increases from 16% along the growth direction and saturates at 19% in the upper interface, which cannot be determined precisely by the traditional method.
... This technique is already successfully employed to study the band structure information, especially to find the bandgap of the systems [28,[55][56][57]. Compared to other optical techniques, such as vacuum ultraviolet spectroscopy, spectroscopic ellipsometry (spatial resolution ∼ 0.2 μm), the superior spatial resolution down to ∼1 nm feasible by EELS measurements [58]. However accurate determination of the small bandgap values by this technique is hindered by various retardation losses, which alters scattering distribution. ...
Article
The two-dimensional (2D) transitional metal dichalcogenides (TMDS) have become an intensive research topic recently. The alloys of these TMDs have offered continuous tunability of the bandstructure and carrier concentration, providing a new opportunity for various device applications. Here the rich variations in optical excitations in RexMo1-xS2 alloy at nanoscale region are shown. The alloy bandgap and charge response are probed by low-loss high-resolution transmission electron energy loss spectroscopy (HR-EELS). Concurrent density functional theory calculations revealed many electronic structures from n-type semiconductors to metallic and p-type semiconducting nature with band bowing effect. The alloying-induced Peierls distortion leads to a change in crystal symmetry and decreased interlayer coupling. These alloys undergo indirect to direct bandgap transition with the function of Re concentration. These unique correlated structural and electronic properties of these 2D alloys can be potentially applicable for various electronic and optoelectronic devices.
... Random alloy fluctuations are of a bigger scale (> 1nm), and they stand for average local composition variations, which stem from the large difference in the bandgaps of the InGaN alloy constituents (InN and GaN)even the slight variations in the average composition can lead to potential fluctuations of tens of meV in the valence band [75]. Another cause of localization might be the cation segregation, which can basically be seen as the formation of In clusters or even quantum dots [76,77]. It was believed to be one of the main causes of strong localization in the early days of researches. ...
Thesis
LEDs based on InGaN already present high internal quantum efficiencies, exceeding 90%. Nevertheless, there is still room for improvement, especially in the green emission region or at high non-equilibrium carrier densities. However, finding the optimal conditions for the growth of InGaN LEDs is burdened by a large number of variables in device fabrication and characterization; a single parameter to guide the technological efforts toward perfecting the InGaN LED design would ease the process. The diffusion coefficient of carriers might become such a parameter due to the negative correlation between carrier diffusivity and the peak quantum efficiency, which is revealed in this work. This tendency is seen among a large number of diverse samples, therefore diffusivity is suggested as a universal quantum efficiency limiting parameter. Reduction of the carrier diffusion coefficient by adjusting the quantum well thickness and modifying the growth procedures looks promising for further improvements in InGaN-based LEDs. The drop of quantum efficiency at higher carrier densities, known as the efficiency droop, is addressed in this work as well, with results pointing to a simultaneous occurrence of different mechanisms – diffusivity-driven non-radiative recombination at the defects and Auger recombination. It is suggested that whichever prevails is determined by localization conditions and the quality of the material, with Auger recombination having the main role in samples with high efficiency.
... In TEM, the composition of semiconductor alloys can be assessed quantitatively using strain-state analysis [1,2], cathodoluminescence [3,4], energy-filtered TEM [5,6], and energy-dispersive X-ray spectroscopy [7,8]. These techniques either provide high chemical precision or high resolution. ...
Article
Atomically-resolved mappings of the indium composition in InGaN/GaN multi-quantum well structures have been obtained by quantifying the contrast in HAADF-STEM. The quantification procedure presented here does not rely on computation-intensive simulations, but rather uses EDX measurements to calibrate the HAADF-STEM contrast. The histogram of indium compositions obtained from the mapping provides unique insights into the growth of InGaN: the transition from GaN to InGaN and vice versa occurs in discreet increments of composition; each increment corresponds to one monolayer of the interface, indicating that nucleation takes longer than the lateral growth of the step. Strain-state analysis is also performed by applying Peak-Pair Analysis to the positions of the atomic columns identified the quantification of the contrast. The strain mappings yield an estimate of the composition in good agreement with the one obtained from quantified HAADF-STEM, albeit with a lower precision. Possible improvements to increase the precision of the strain mappings are discussed, opening potential pathways for the quantification of arbitrary quaternary alloys at atomic scales.
... Localization of free carriers in the local potential minima is believed to be responsible for low nonradiative recombination rate at dislocations [2]. Quantum dot-like indium clusters were believed to be the main source of local potential minima [3], but later it has been shown that both random indium density fluctuations and monoatomic variations in quantum well thickness can result in carrier localization even at room temperature [4]. Recently, it was demonstrated that localization is much stronger for holes than electrons due to higher effective mass of the former [5][6][7]. ...
... In TEM, the composition of semiconductor alloys can be assessed quantitatively using strain-state analysis [1, 2], cathodoluminescence [3,4], energy-filtered TEM [5,6], and energy-dispersive X-ray spectroscopy [7,8]. These techniques either provide high chemical precision or high resolution. ...
Preprint
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Atomically-resolved mappings of the indium composition in InGaN/GaN multi-quantum wellstructures have been obtained by quantifying the contrast in HAADF-STEM. The quantificationprocedure presented here does not rely on computation-intensive simulations, but rather uses EDXmeasurements to calibrate the HAADF-STEM contrast. The histogram of indium compositionsobtained from the mapping provides unique insights into the growth of InGaN: the transitionfrom GaN to InGaN and vice versa occurs in discreet increments of composition; each incrementcorresponds to one monolayer of the interface, indicating that nucleation takes longer than thelateral growth of the step. Strain-state analysis is also performed by applying Peak-Pair Analysisto the positions of the atomic columns identified the quantification of the contrast. The strainmappings yield an estimate of the composition in good agreement with the one obtained fromquantified HAADF-STEM, albeit with a lower precision. Possible improvements to increase theprecision of the strain mappings are discussed, opening potential pathways for the quantification ofarbitrary quaternary alloys at atomic scales.
Article
We report on the spatial variation of optical properties in thick, In-rich InGaN layers, grown by a novel droplet elimination by radical beam irradiation (DERI) technique. The increase of layer thickness causes layer relaxation and results in double-peaked photoluminescence spectra. Spatially resolved measurements show that the defects in the strained sub-layer are distributed inhomogeneously. An increase in the layer thickness results in faster nonradiative recombination due to increasing density of nonradiative recombination centers, as evidenced by time-resolved free carrier absorption, and facilitates larger indium incorporation in the upper part of the layer.
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Quaternary AlInGaN layers were grown on conventional GaN buffer layers on sapphire by metal organic vapour phase epitaxy at different surface temperatures and different reactor pressures with constant precursor flow conditions. A wide range in compositions within 30-62% Al, 5-29% In, and 23-53% Ga was covered, which leads to different strain states from high tensile to high compressive. From high-resolution x-ray diffraction and Rutherford backscattering spectrometry, we determined the compositions, strain states, and crystal quality of the AlInGaN layers. Atomic force microscopy measurements were performed to characterize the surface morphology. A critical strain value for maximum In incorporation near the AlInGaN/GaN interface is presented. For compressively strained layers, In incorporation is limited at the interface as residual strain cannot exceed an empirical critical value of about 1.1%. Relaxation occurs at about 15 nm thickness accompanied by strong In pulling. Tensile strained layers can be grown pseudomorphically up to 70 nm at a strain state of 0.96%. A model for relaxation in compressively strained AlInGaN with virtual discrete sub-layers, which illustrates the gradually changing lattice constant during stress reduction is presented.
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Electron energy loss spectroscopy (EELS), in both the low loss and the core loss regions of the spectra, have been used to study local alloys composition, electronic transitions and bonding environment of different species in nanowires consisted of a GaN base-part and a superlattice upper part of InxGa1–xN nanodisks (NDs)/GaN barriers. Experimental valence (low loss) electron energy loss spectroscopy (VEELS) spectra were taken from the GaN base-part, the GaN barriers and the InxGa1–xN disks. Electron energy loss near edge structure (ELNES) spectra presenting the C K-edge, the N K-edge, the In M4,5-edge were obtained from the GaN base-part and spacer and the InxGa1–xN NDs. Variation of In concentration as well as different strain states change the intensity and result in the broadening and shifting of the edges in the spectra. The TELNES.2 program of the WIEN2k code (Blaha et al., Comp. Phys. Commun. 59, 399 (1990) [1]) was implemented in order to study the electronic properties of InxGa1–xN. The N K-edge that represents the energy loss of the electron transition from the 2p to the 1s state is unique for each structure. Details on the bonding environment of the structure were extracted from the simulations of the ELNES spectra. Moreover, the influence of the In content in the InxGa1–xN NDs on the N-K edge was interpreted. (© 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
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The high intensity of light emitted in InxGa1−xN/GaN heterostructures has been generally attributed to the formation of indium-rich clusters in InxGa1−xN quantum wells (QWs). However, there is significant disagreement about the existence of such clusters in as-grown InxGa1−xN QWs. We employ atomically resolved CS-corrected scanning transmission electron microscopy and electron energy loss spectroscopy at 120 kV—which we demonstrate to be below the knock-on displacement threshold—and show that indium clustering is not present in as-grown In0.22Ga0.78N QWs. This artifact-free, atomically resolved method can be employed for investigating compositional variations in other InxGa1−xN/GaN heterostructures.
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Indium kinetics and evidence for indium segregation on the GaN (0001) surface are investigated via in situ spectroscopic ellipsometry. Indium deposition exhibits two stable states at coverages of 1.0 and 1.7 ML within the temperature range of 630–688 °C. Formation of each layer is governed by two kinetic processes: nuclei formation and nuclei-mediated layer adsorption. The measured desorption activation energies of nuclei of the first (2.04 eV) and second (2.33 eV) monolayers are lower than the desorption activation energies of the aggregated first (2.64 eV) and second (2.53 eV) monolayers, respectively. This suggests that adatoms preferentially interact with the nuclei and laterally aggregate.
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The reason the InGaN/GaN quantum well system emits intense light even though the dislocation density is high is assessed. First, the evidence from electron microscopy for nanometre-scale In-rich clusters in InGaN quantum wells is presented. Such clusters would localize the excitons away from the dislocations and hence the dislocations would not quench the light emission, consistent with observations. However, it is then shown that InGaN damages extremely rapidly under an electron beam, and that the damage causes In-rich clusters to form. No evidence is found of gross indium clustering at low beam currents and short exposure times in the electron microscope. However, at such low electron doses the image is noisy, and low-level indium clustering in InGaN quantum wells could possibly exist, but be masked by the noise. A different technique, 3-dimensional atom probe field ion microscopy, has therefore been used to image the InGaN quantum wells. This reveals that InGaN is a random alloy, with the local statistical fluctuations in indium content expected in a random alloy, but with no indium clustering. Since In-rich clusters are clearly not necessary for bright light emission from InGaN quantum wells, another mechanism must be responsible for the exciton localization observed. It is shown that thickness fluctuations in the InGaN quantum wells of only one monolayer, observed in electron microscopy, result in an exciton confinement energy of 58 meV, sufficient to localize the carriers at room temperature. An alternative localization mechanism due to randomly formed In-N-In chains proposed by others is discussed. It is concluded that In-rich clusters in InGaN quantum wells do not exist in the specimens we have studied, and in any case they are not necessary to localize the excitons and for bright light emission.
Article
Vertically aligned GaN nanorod arrays with nonpolar InGaN/GaN multi quantum wells (MQW) were grown by MOVPE on c-plane GaN-on-sapphire templates. The chemical and structural properties of single nanorods are optically investigated with a spatial resolution beyond the diffraction limit using tip-enhanced Raman spectroscopy (TERS). This enables the local mapping of variations in the chemical composition, charge distribution, and strain in the MQW region of the nanorods. Nanoscale fluctuations of the In content in the InGaN layer of few percents can be identified and visualized with a lateral resolution below 35 nm. We obtain evidence for the presence of indium clustering and the formation of cubic inclusions in the wurtzite matrix near the QW layers. These results are directly confirmed by high-resolution TEM images, revealing the presence of stacking faults and different polymorphs close to the surface near the MQW region. The combination of TERS and HRTEM demonstrates the potential of this nanoscale near-field imaging technique, establishing TERS as very potent, comprehensive, and non-destructive tool for the characterization and optimization of technologically relevant semiconductor nanostructures.
Article
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High-resolution monochromated electron energy loss spectroscopy (EELS) at subnanometric spatial resolution and <200 meV energy resolution has been used to assess the valence band properties of a distributed Bragg reflector multilayer heterostructure composed of InAlN lattice matched to GaN. This work thoroughly presents the collection of methods and computational tools put together for this task. Among these are zero-loss-peak subtraction and nonlinear fitting tools, and theoretical modeling of the electron scattering distribution. EELS analysis allows retrieval of a great amount of information: indium concentration in the InAlN layers is monitored through the local plasmon energy position and calculated using a bowing parameter version of Vegard Law. Also a dielectric characterization of the InAlN and GaN layers has been performed through Kramers-Kronig analysis of the Valence-EELS data, allowing band gap energy to be measured and an insight on the polytypism of the GaN layers.
Indium nitride (InN) is a promising yet technologically challenging material with a high defect density and unusual material properties. Its high electron mobility may be utilized in high power electronic devices, and its high absorbance and low energy optical response make it a promising candidate for multi-junction, high-efficient solar cell technology. Studies of absorption and photoluminescence optical response of epitaxial InN resulted in a large correction of the fundamental bandgap from the originally proposed 1.9 eV to now below 0.7 eV. Yet, it is still debated if the commonly measured optical transitions below the original high bandgap values are actually caused by a large concentration of defects, on the order of 1020/cm3, rather than reflecting a low fundamental bandgap. Many applications of this material, e.g. in high-efficient solar cell technology, are primarily dependent on the successful production of a contacted p-n junction, which has not yet been achieved. This contribution addresses the controversy in the bandgap discussion of InN. Valence electron energy loss spectroscopy (VEELS) of InN allows spatially resolved analysis of the density of states in the transmission electron microscope (TEM). Standard optical characterization is compared with results from TEM characterization.
Article
InN, high indium content InGaN, and Mg-doped InGaN were grown by metal modulated epitaxy (MME). Transient reflection high-energy electron diffraction intensities were analyzed during the growth of InN and found to be similar to that previously reported for GaN and AlN. The x-ray diffraction rocking curve and background electron concentration of InN grown by MME were found to be respectable in comparison to recent reports in literature. InGaN alloys grown by MME were also investigated, and a method for detecting indium surface segregation was demonstrated. It was found that the shutter modulation scheme could be modified to prevent phase separation by indium surface segregation, and a range of single-phase InGaN samples with indium contents throughout the miscibility gap were grown. Using the discovered method of suppressing phase separation, several p-InxGa1 - xN samples were grown with indium contents from x = 0 to 0.22. A maximum hole concentration of 2.4 × 1019 cm-3 was detected by Hall effect characterization, demonstrating feasibility of these p-InGaN layers for use in several device applications.
Article
Solid-state lighting (SSL) is now the most efficient source of high color quality white light ever created. Nevertheless, the blue InGaN light-emitting diodes (LEDs) that are the light engine of SSL still have significant performance limitations. Foremost among these is the decrease in efficiency at high input current densities widely known as “efficiency droop.” Efficiency droop limits input power densities, contrary to the desire to produce more photons per unit LED chip area and to make SSL more affordable. Pending a solution to efficiency droop, an alternative device could be a blue laser diode (LD). LDs, operated in stimulated emission, can have high efficiencies at much higher input power densities than LEDs can. In this article, LEDs and LDs for future SSL are explored by comparing: their current state-of-the-art input-power-density-dependent power-conversion efficiencies; potential improvements both in their peak power-conversion efficiencies and in the input power densities at which those efficiencies peak; and their economics for practical SSL.
Chapter
Typical devices based on compound semiconductors including GaN and related materials are made of layered multiheterostructures, which require epitaxial growth on appropriate substrates. Owing to the fundamental difficulty of growing GaN bulk crystals using procedures established for other semiconductors, this chapter starts by discussing some recently developed alternatives to produce GaN wafers. Then, the most prominent epitaxial method for this purpose is described: metalorganic vapor‐phase epitaxy (MOVPE). After presenting some basics about MOVPE, we discuss various specifics about GaN MOVPE including high‐temperature growth, deposition on foreign substrates, and methods for reducing the very high threading dislocation density developing on such foreign substrates. Moreover, doping difficulties for GaN and related materials are briefly discussed, particularly Mg doping for achieving p‐type conductivity. The growth of ternary layers such as AlGaN, InGaN, and AlInN provides further challenges. Particularly, InGaN quantum wells find, on the one hand, important applications in all kinds of light‐emitting devices but show strong problems concerning composition uniformity, thermal instability, and decomposition tendencies. Therefore, the last part of this chapter is dedicated to this topic.
Article
This article demonstrates for the first time the merits of an immediate InAlGaN capping layer over self-assembled InxGa1-xN/GaN quantum dots (QDs) coaxially grown on the m-plane and r-plane of n-GaN nanowires on Si (111) substrate using metal organic chemical vapor deposition. For comparative analysis, we prepared InGaN/GaN QD samples both with and without quaternary capping. InAlGaN capping layer acted as a strain-driven phase separation alloy. Inhomogeneous surface strain over the dots helped this quaternary alloy in forming an indium concentration gradient over InxGa1-xN QDs and thus, indium out-diffusion from the dots was reduced. Quaternary alloy capped samples exhibited vertically stacked, highly dense, pyramidal InxGa1-xN/GaN QDs of improved carrier confinement grown as the active region on n-GaN NWs. In contrast, the nonexistence of InAlGaN capping over InGaN/GaN QDs caused deformation of the dots due to In-Ga inter-diffusion between the dots and the GaN barrier layer. Three kinds of InxGa1-xN/GaN QDs of different x with an InAlGaN capping layer were fabricated coaxially on n-GaN nanowire, whose emission wavelength were 380 nm, 450 nm and 510 nm respectively. These coaxially fabricated InxGa1-xN/GaN QDs on defect free n-GaN nanowires have various excellent characteristics and can be widely applicable to new optoelectronics semiconductor devices.
Article
The localization effect is studied in blue-violet light emitting InGaN/GaN multiple quantum wells (MQWs) with varying InGaN growth rate. The temperature-dependent photoluminescence (PL) measurement shows that for higher-growth-rate samples two emission peaks appear in their PL spectra. Further analysis reveals that two different localization luminescence states (i.e., deep and shallow localization states) exist in the InGaN QWs with higher QW growth rate, and the competition of radiative recombination between the two localization states determines the relative intensity of the two emission peaks. It is also found that, as InGaN growth rate reduces, the deep localization state depth is almost unchanged while the shallow localization state weakens. When the QW growth rate reduces to a certain value, the shallow localization state disappears and only a single main peak induced by deep localization state appears in the PL spectra. Finally, it is noted that an intermediate InGaN growth rate results in a better light emission efficiency of the MQW.
Thesis
This thesis is mainly focused on the development of III-N materials for HEMTs power transistors, as well as quantum wells and optronics applications that result to a lesser extent. Following a reminder of the properties of nitrides, the different possible applications, the principle of the MOCVD and the different characterizations used for this work, we first treated the growth of GaN at low temperature, that is to say below 1050degres C. The manufacture of multiple quantum wells involving the alternation of GaN and InAlN or InGaAlN layers forces us to work at these temperatures, which generates the appearance of a defect in surface of the GaN which is called V-defect. An advanced experimental study allowed us to understand how these defects appear and evolve according to the growth parameters. A model based on surface energies could be developed and explains the evolution of these defects. Then we defined the influence of many MOCVD growth parameters by MOCVD and derived, from the multiple trends highlighted, the models and explanations justifying this or that physical and chemical property of the material. Downstream, these are electrical characterizations and mainly resistivity measurements that have been processed to compare the performance of our indium-based samples to those of AlGaN/GaN type. The problem of gallium pollution in vertical MOCVD reactors has been highlighted and we have proposed different solutions to limit or even annihilate it. Finally, we have tried to develop protective layers based on SiN and GaN in order to protect our indium-based alloys for the next technological steps required to manufacture a component, for example.
Article
We report the detection of an isolated energy level in the band gap of crystalline Yb2Si2O7 in the low-energy-loss region of its electron energy-loss (EEL) spectrum, obtained using a monochromated scanning transmission electron microscope. The experimental results are corroborated by first-principles calculations of the theoretical EEL spectrum. The calculations reveal that unoccupied Yb 4f orbitals constitute an isolated energy level about 1 eV below the conduction band minimum (CBM), resulting in a terrace about 1 eV wide at the band edge of the EEL spectrum. In the case of Yb2O3, no band edge terrace is present because the unoccupied f level lies just below the CBM. We also examined optical absorption properties of Yb2Si2O7 using UV-vis diffuse reflectance spectroscopy, which shows that the isolated energy level could not be detected in the band edge of the obtained absorbance spectrum. These findings demonstrate the utility of low-loss EEL spectroscopy with high energy resolution for probing semilocalized electronic features.
Article
The challenge for improving the internal quantum efficiency (IQE) of InGaN-based light emitting diodes (LED) in the green light range is referred to as the 'green gap'. However the IQE of InGaN-based LEDs often drops when the emission peak wavelength is adjusted through reducing the growth temperature. Although hydrogen (H2) can improve surface morphology, it reduces the indium incorporation significantly. Here, a novel usage of H2 treatment on the GaN barrier before the InGaN quantum well is demonstrated to enhance indium incorporation efficiency and improve the IQE simultaneously for the first time. The mechanism behind it is systematically investigated and explained in detail. The possible reason for this phenomenon is the strain relieving function by the undulant GaN barrier surface after H2 treatment. Test measurements show that applying 0.2 min H2 treatment on the barrier would reduce defects and enhance indium incorporation, which would improve the localization effect and finally lead to a higher IQE. Although further increasing the treatment time to 0.4 min incorporates more indium atoms, the IQE decreases at the expense of more defects and a larger polarization field than the 0.2 min sample.
Thesis
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This thesis explores the analytical capabilities of low-loss electron energy loss spectroscopy (EELS), applied to disentangle the intimate configuration of advanced semiconductor heterostructures. Modern aberration corrected scanning transmission electron microscopy (STEM) allows extracting spectroscopic information from extremely constrained areas, down to atomic resolution. Because of this, EELS is becoming increasingly popular for the examination of novel semiconductor devices, as the characteristic size of their constituent structures shrinks. Energy-loss spectra contain a high amount of information, and since the electron beam undergoes well-known inelastic scattering processes, we can trace the features in energy-loss spectra down to elementary excitations in the atomic electronic configuration.
Article
We demonstrate a method to determine the indium concentration, x, of Inx Ga1-x N thin films by combining plasmon excitation studies in electron energy-loss spectroscopy (EELS) with a novel way of quantification of the intensity of x-ray lines in energy-dispersive x-ray spectroscopy (EDXS). The plasmon peak in EELS of InGaN is relatively broad. We fitted a Lorentz function to the main plasmon peak to suppress noise and the influence from the neighboring Ga 3d transition in the spectrum, which improves the precision in the evaluation of the plasmon peak position. As the indium concentration of InGaN is difficult to control during high temperature growth due to partial In desorption, the nominal indium concentrations provided by the growers were not considered reliable. The indium concentration obtained from EDXS quantification using Oxford Instrument ISIS 300 x-ray standard quantification software often did not agree with the nominal indium concentration, and quantification using K and L lines was inconsistent. We therefore developed a self-consistent iterative procedure to determine the In content from thickness-dependent k-factors, as described in recent work submitted to Journal of Microscopy. When the plasmon peak position is plotted versus the indium concentration from EDXS we obtain a linear relationship over the whole compositional range, and the standard error from linear least-squares fitting shows that the indium concentration can be determined from the plasmon peak position to within Δx = ± 0.037 standard deviation.
Article
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Carrier localization phenomena in indium-rich InGaN/GaN multiple quantum wells (MQWs) grown on sapphire and GaN substrates were investigated. Temperature-dependent photoluminescence (PL) spectroscopy, ultraviolet near-field scanning optical microscopy (NSOM), and confocal time-resolved PL (TRPL) spectroscopy were employed to verify the correlation between carrier localization and crystal quality. From the spatially resolved PL measurements, we observed that the distribution and shape of luminescent clusters, which were known as an outcome of the carrier localization, are strongly affected by the crystalline quality. Spectroscopic analysis of the NSOM signal shows that carrier localization of MQWs with low crystalline quality is different from that of MQWs with high crystalline quality. This interrelation between carrier localization and crystal quality is well supported by confocal TRPL results.
Thesis
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InxGa1-xN are attractive semiconductor systems due to their emission wavelengths covering the range from ultraviolet to infrared. This makes them desirable for next generation visible light communication. The understanding of the relationship between emission wavelength and InxGa1-xN chemistry can benefit InGaN growth and device fabrication. Photoluminescence, electroluminescence and cathodoluminescence are often sufficient to demonstrate the emission wavelength from InxGa1-xN, therefore the relationship between In concentration and emission wavelength can be revealed if the In content in the sample is well understood. This research is started from the chemical analysis of InxGa1-xN (InxGa1-xN/GaN) thin film heterostructures, which were grown on sapphire substrates. The nominal concentration is not always very reliable and therefore needs to be measured by analytical transmission electron microscopy. Electron energy loss spectroscopy (EELS) and e n e r g y-dispersive X-ray spectroscopy (EDXS) in a JEOL JEM 2010 F field emission gun TEM h a v e been combined in the first part of this thesis, to evaluate the local indium concentration in those InxGa1-xN thin films. The quantification of In concentration from EDXS is based on our X-ray absorption correction method, which provided a consistent In content, quantified from Ga K and Ga L X-ray lines. The results can serve as a calibration point for evaluating the bulk plasmon energy in low-loss EELS, as a function of In concentration. An important aspect for growing high indium concentration InGaN heterostructure is phase separation. Phase separation means Ga-rich and In-rich regions form, rather than in growth where a perfect InGaN alloy is produced under the high temperatures of metal organic chemical vapour phase deposition (MOCVD). In the second part of this thesis, the EDXS absorption correction method is applied to analyse the In distribution from element maps, collected from a scanning transmission electron microscopy (STEM) mode, where the In-rich region (x>0.8) underneath the big island in In0.68Ga0.32N can be directly observed. To further analyse the iii components which form the In-rich area, and quantify the degree of phase separation, experimental low loss EELS spectrum were fitted with GaN, InN and InGaN reference spectra. The components in the In-rich area can be targeted by using reference spectra in the fitting routine, and their corresponding weights can represent the degree of phase separation. In this study, we have used the NION Ultra-STEM 100 TEM equipped with a monochromator, operating at 60kV to enhance the spectrum energy resolution and minimize electron beam induced damage. The result indicates the In-rich area is mainly formed of InN, rather than high In content InGaN ternary alloy. The averaged In concentration maps, which were calculated from EELS, correlated well with the EDXS mapping. Finally, the EDXS absorption correction method is applied to quantify the In and Al concentration in AlyInxGa1-x-yN nanowires. The analysis is mainly focused on the reliable quantification of In and Al content from low X-ray counts and noisy element maps. The result indicates a proper background subtraction for Ga L, Al K and Ga K, and geometry simulations for nanowires are necessary to obtain a consistent result with PL measurements. This approach will certainly benefit the beam sensitive material, and nanoparticle chemical analysis.
Article
In <04̄41>-oriented lithium niobate single crystals, striae of planar defect lying on nearly (2̄110) were formed during the Czochralski growth. A stria was a small angle tilt boundary including a set of undissociated edge dislocations with the Burgers vector of a/3[2̄110], arranged at similar intervals of about 1μm. The dislocations extended perpendicular to the growth interface, leaving from the basal plane, presumably for the reduction of their strain energy during the growth. The formation process was discussed in terms of thermal stress at the growth interface during crystallization.
Conference Paper
Geographic information systems (GIS) technology can greatly facilitate the development and use of statistical models for the assessment of regional integration. As one of the main megalopolis areas to lead China's participation in economic globalization, the National Capital Region of China (NCRC) is in the slow process of regional integration. From the NCRC located in the northeastern coast of China, relevant factors were collected and processed by applying GIS technology, such as traffic, topography and administrative division. An evaluation model of regional integration was established to discuss the obstacles of regional integration in the NCRC. The results show that the main obstacles of the NCRC include the lack of an overall regional planning, serious administrative institutional barriers, slowing development of hinterland, low transport accessibility and lack of industry cooperation, etc. At last, the paper proposes the development path of regional integration in the NCRC.
Article
With recent rapid advancement in electron microscopy instrumentation, in particular, bright electron sources and monochromators, valence electron energy-loss spectroscopy (VEELS) has become attractive for retrieving band structures, optical properties, dielectric functions and phonon information of materials. However, Cherenkov radiation and surface-loss contribution significantly alter fine structures of VEELS, even in simple semiconductors and insulators. This leads to the problem that dielectric function or bandgap structure of these materials cannot be determined correctly if these effects are not removed. In this work we present a solution to this dilemma. We demonstrate that energy-loss function and real part of inverse complex dielectric function can be retrieved from raw data of VEELS. Based on the calculated example of Si, the limitation of our approach is discussed. We believe that our approach represents an improvement over previous procedures and has a broad prospect for applications.
Article
The layer strain and its relaxation effects on the photoluminescence (PL) of InGaN layers are studied using confocal microscopy. The relaxation imposed structural changes are studied by X-ray diffraction (XRD) reciprocal space mapping and atomic force microscopy. Initial layer relaxation generated misfit dislocations were observed by confocal microscopy as intersecting parallel lines of lower PL intensity. The splitting of the PL spectrum into several PL bands indicated an onset of changes in the layer structure, which were confirmed by XRD measurements. The PL bands were attributed to two sub-layers of the sample: A relaxed upper sub-layer and a strained sub-layer underneath. Bright spots, approximately 250 nm in diameter, were observed on the background of the inhomogeneous PL intensity distribution due to fluctuations of In content. The bright spots correspond to column-like structures with relaxed lattice, In content as in the initial strained layer, and lower density of nonradiative recombination centers than that in the surrounding background.
Chapter
The main goal of this chapter is to introduce the concept of “low-loss” or “valence loss” electron energy loss spectroscopy (VEELS) in the STEM. Much of the discussion will assume that the microscope is aligned to form the optimum probe size (as described in other chapters in this book) with only special attention being drawn to the monochromator and how its use modifies the electron optics of the microscope (i.e., how the probe is formed). VEELS is traditionally described by energy loss processes that are seen in the 0–50 eV region of the spectrum (Figure 16–1) and processes that are typically characterized as collective excitations. These collective oscillations can provide key insights into optical and electronic properties that are fundamentally different from the composition and structure information that is typically extracted from core-loss spectra. Here we will provide a basic physical model for these collective excitations that allows materials properties to be interpreted from experimental spectra acquired in the STEM.
Chapter
The InGaN/GaN quantum well system emits intense light even though the dislocation density is high. This is a puzzle since dislocations should quench the light emission. Photoluminescence (PL) experiments show that the excitons in the InGaN quantum well are localised on a nanometre scale, thus separating the carriers from most of the dislocations. Many papers report transmission electron microscopy (TEM) results showing that this localisation is caused by gross indium clustering in the InGaN quantum wells, but our TEM reveals no gross indium clustering. Three-dimensional atom probe field ion microscopy confirms that InGaN is a random alloy. Mechanisms are given for localisation on a nm scale. Confinement on a broader length scale (50 – 100 nm) can also occur in some InGaN quantum wells.
Article
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The electronic structure of InxGa1-xN alloys with (0less than or equal toxless than or equal to0.3) has been studied using synchrotron radiation excited soft x-ray emission and absorption spectroscopies. These spectroscopies allow the elementally resolved partial density of states of the valence and conduction bands to be measured. The x-ray absorption spectra indicate that the conduction band broadens considerably with increasing indium incorporation. The evolution of the band gap as a function of indium content derives primarily from this broadening of the conduction-band states. The emission spectra indicate that motion of the valence band makes a smaller contribution to the evolution of the band gap. This gap evolution differs from previous studies on the AlxGa1-xN alloy system, which observed a linear valence-band shift through the series (0less than or equal toxless than or equal to1). For InxGa1-xN the valence band exhibits a large shift between x=0 and x=0.1 with minimal movement thereafter. We also report evidence of In 4d-N 2p and Ga 3d-N 2p hybridization. Finally, the thermal stability of an In0.11Ga0.89N film was investigated. Both emission and absorption spectra were found to have a temperature-dependent shift in energy, but the overall definition of the spectra was unaltered even at annealing temperatures well beyond the growth temperature of the film.
Article
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The optical properties of wurtzite-structured InN grown on sapphire substrates by molecular-beam epitaxy have been characterized by optical absorption, photoluminescence, and photomodulated reflectance techniques. These three characterization techniques show an energy gap for InN between 0.7 and 0.8 eV, much lower than the commonly accepted value of 1.9 eV. The photoluminescence peak energy is found to be sensitive to the free-electron concentration of the sample. The peak energy exhibits very weak hydrostatic pressure dependence, and a small, anomalous blueshift with increasing temperature.
Article
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We report a study of the morphology and composition of InxGa1−xN/GaN multiple-quantum-well structures and their sensitivity to electron-beam damage. We have employed high-resolution transmission electron microscopy, energy dispersive x-ray analysis, and scanning transmission electron microscopy. Microstructural analysis was performed to investigate the dynamical effects of electron-beam irradiation on the relative indium distribution within the quantum wells. Exposure to relatively low incident beam illumination, corresponding to current densities at the specimen of ∼100 pA/cm2, was found to induce significant nanoclustering of indium within the multiple-quantum wells. These findings highlight the need for caution when reporting the presence of indium-rich clusters within InGaN/GaN multiple-quantum wells studied in the transmission electron microscope. © 2003 American Institute of Physics.
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of a paper presented at Microscopy and Microanalysis 2004 in Savannah, Georgia, USA, August 1–5, 2004.
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We report a direct observation of quantum dots formed spontaneously in a thick InGaN epilayer by high resolution transmission electron microscopy. Investigation of a (280 nm thick) In0.22Ga0.78N single layer, emitting in the blue/green spectral region, reveals quantum dots with estimated sizes in the range of 1.5-3 nm. Such sizes are in very good agreement with calculations based on the luminescence spectra of this specimen. (C) 2000 American Institute of Physics. [S0003-6951(00)00930-X].
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We report the first direct observation of phase decomposition in a luminescent alloy and show that this decomposition, allied to quantum confinement enhancements, accounts for the surprisingly high efficiency of InGaN-based diodes manufactured by Nichia Chemical Industries. Hence nanostructure, rather than composition, is responsible for the success of these devices. A common nanostructure, in the form of nearly pure InN quantum dots, occurs across a large range of average indium content in InGaN and leads to a universal scalability of the optical spectra. [S0031-9007(98)08055-7].
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A truncated-inverted-pyramid (TIP) chip geometry provides substantial improvement in light extraction efficiency over conventional AlGaInP/GaP chips of the same active junction area (∼0.25 mm<sup>2</sup>). The TIP geometry decreases the mean photon path-length within the crystal, and thus reduces the effects of internal loss mechanisms. By combining this improved device geometry with high-efficiency multiwell active layers, record-level performance for visible-spectrum light-emitting diodes is achieved. Peak efficiencies exceeding 100 lm/W are demonstrated (100 mA dc, 300 K) for orange-emitting (λ<sub>p</sub>∼610  nm ) devices, with a peak luminous flux of 60 lumens (350 mA dc, 300 K). In the red wavelength regime (λ<sub>p</sub>∼650  nm ), peak external quantum efficiencies of 55% and 60.9% are measured under direct current and pulsed operation, respectively (100 mA, 300 K). © 1999 American Institute of Physics.
Article
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The large difference in interatomic spacing between GaN and InN is found to give rise to a solid phase miscibility gap. The temperature dependence of the binodal and spinodal lines in the Ga{sub 1{minus}{ital x}}In{sub {ital x}}N system was calculated using a modified valence-force-field model where the lattice is allowed to relax beyond the first nearest neighbor. The strain energy is found to decrease until approximately the sixth nearest neighbor, but this approximation is suitable only in the dilute limit. Assuming a symmetric, regular-solutionlike composition dependence of the enthalpy of mixing yields an interaction parameter of 5.98 kcal/mole. At a typical growth temperature of 800{degree}C, the solubility of In in GaN is calculated to be less than 6{percent}. The miscibility gap is expected to represent a significant problem for the epitaxial growth of these alloys. {copyright} {ital 1996 American Institute of Physics.}
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Recent advances in fabrication technologies for the semiconducting nitrides of the group III elements have led to commercially available, high-efficiency solid-state devices that emit green and blue light. Light-emitting diodes based on these materials should find applications in flat-panel displays, and blue and ultraviolet laser diodes promise high-density optical data storage and high-resolution printing.
Article
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THE high angle elastic scattering of electrons in scanning transmission electron microscopy depends strongly on the atomic number Z, of the sample atoms, through the Z2 dependence of the Rutherford scattering cross-section1. The detection of scattered electrons at high angles and over a large angular range (75& ndash;150 milliradians) removes the coherent effects of diffraction, and the resulting incoherent image provides a compositional map of the sample with high atomic-number contrast1. If a fine electron probe is used, and the sample is a crystalline material oriented along one of its principal axes, individual columns of atoms can be imaged in this way2. Electrons scattered at low angles are not used in this detection scheme, and are thus available for simultaneous electron energy-loss spectroscopy3; in principle, this combination of techniques should allow the direct chemical analysis of single atomic columns in crystalline materials. Here we present electron energy-loss spectra from expitaxial interfaces between cobalt silicide and silicon, which confirm that atomic resolution can be achieved by this approach. The ability to correlate structure and chemistry with atomic resolution holds great promise for the detailed study of defects and interfaces.
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The optical properties of cubic and hexagonal GaN thin films, grown by electron-cyclotron resonance microwave plasma-assisted molecular-beam epitaxy on silicon and sapphire substrates, respectively, have been studied at photon energies up to 25 eV with conventional and synchrotron-radiation spectroscopic ellipsometry. The fundamental gaps of the two polytypes are located at different energies, namely at 3.25 and 3.43 eV for cubic and hexagonal GaN. Analysis of the dielectric function of the two phases in the region 4.5–9.5 eV with appropriate models yields the energy location and broadening of the observed critical points. These critical points are assigned to specific points in the zinc-blende and wurtzite Brillouin zones, respectively, making use of the latest published band-structure studies and a comparison is made between the corresponding results for GaN, GaAs, and GaP. Measurements in the temperature range from 80 to 650 K provide the temperature dependence of these parameters. The features observed in the reflectivity spectra of hexagonal GaN are discussed in relation to other works. Kramers-Kronig analysis of the reflectivity between 0 and 33 eV of the hexagonal polytype verifies the existence of a broad feature centered at 14 eV. Finally, average properties, such as the effective ir dielectric constant and the effective number of valence electrons per atom are calculated for the two polytypes and compared to GaAs and GaP.
Article
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Since 1993, InGaN light-emitting diodes (LEDs) have been improved and commercialized, but these devices have not fulfilled their original promise as solid-state replacements for light bulbs as their light-emission efficiencies have been limited. Here we describe a method to enhance this efficiency through the energy transfer between quantum wells (QWs) and surface plasmons (SPs). SPs can increase the density of states and the spontaneous emission rate in the semiconductor, and lead to the enhancement of light emission by SP-QW coupling. Large enhancements of the internal quantum efficiencies (eta(int)) were measured when silver or aluminium layers were deposited 10 nm above an InGaN light-emitting layer, whereas no such enhancements were obtained from gold-coated samples. Our results indicate that the use of SPs would lead to a new class of very bright LEDs, and highly efficient solid-state light sources.
Article
THE high angle elastic scattering of electrons in scanning transmission electron microscopy depends strongly on the atomic number Z, of the sample atoms, through the Z2 dependence of the Rutherford scattering cross-section1. The detection of scattered electrons at high angles and over a large angular range (75-150 milliradians) removes the coherent effects of diffraction, and the resulting incoherent image provides a compositional map of the sample with high atomic-number contrast1. If a fine electron probe is used, and the sample is a crystalline material oriented along one of its principal axes, individual columns of atoms can be imaged in this way2. Electrons scattered at low angles are not used in this detection scheme, and are thus available for simultaneous electron energyloss spectroscopy3; in principle, this combination of techniques should allow the direct chemical analysis of single atomic columns in crystalline materials. Here we present electron energy-loss spectra from expitaxial interfaces between cobalt silicide and silicon, which confirm that atomic resolution can be achieved by this approach. The ability to correlate structure and chemistry with atomic resolution holds great promise for the detailed study of defects and interfaces.
Article
A high-energy resolution post-column spectrometer for the purpose of electron energy loss spectroscopy (EELS) and energy-filtered TEM in combination with a monochromated (S)TEM is presented. The prism aberrations were corrected up to fourth order using multipole elements improving the electron optical energy resolution and increasing the acceptance of the spectrometer for a combination of object area and collection angles. Electronics supplying the prism, drift tube, high-tension reference and critical lenses have been newly designed such that, in combination with the new electron optics, a sub-50meV energy resolution has been realized, a 10-fold improvement over past post-column spectrometer designs. The first system has been installed on a 200kV monochromated TEM at the Delft University of Technology. Total system energy resolution of sub-100meV has been demonstrated. For a 1s exposure the resolution degraded to 110meV as a result of noise. No further degradation in energy resolution was measured for exposures up to 1min at 120kV. Spectral resolution measurements, performed on the π* peak of the BN K-edge, demonstrated a 350meV (FWHM) peak width at 200kV. This measure is predominantly determined by the natural line width of the BN K-edge.
Article
Haloarculajaponica has a glycoprotein on its cell surface. The cell surface glycoprotein of H. japonica was purified and characterized. It had an apparent molecular mass of 180 kDa in sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The carbohydrate content was about 12% (wt/wt). The polypeptide portion contained a large proportion of acidic amino acids, and the sequence of 18 N-terminal amino acids was determined.
Article
Quantitative high resolution electron microscopy (HRTEM) is used to map the indium distribution in a GaN/In0.43Ga0.57N/Al0.1Ga0.9N quantum well at the atomic scale. Samples with atomically flat surfaces were prepared for microscopy by anisotropic chemical etching. The developed preparation procedure minimizes a possible confusion of thickness variations with local compositional fluctuations in the lattice images. An irregular distribution of indium is observed that is attributed to the formation of clusters with estimated diameters of 1-2 nm. The indium concentration gradient across GaN/In0.43Ga0.57N interfaces is measured to extend typically over a distance of 1nm. It is more than twice as large across the In0.43Ga0.57N/Al0.1Ga0.9N interface. Indium segregation into the Al0.1Ga0.9N layer during crystal growth is likely to cause this unusual large width of the In0.43Ga0.57N/Al0.1Ga0.9N interfaces. This introduces an asymmetric In distribution across the quantum well with respect to the growth direction.
Article
Current and temperature dependences of the electroluminescence of InGaN UV/blue/green single-quantum-well (SQW)-structure light-emitting diodes (LEDs) were studied. The emission mechanism of InGaN SQW-structure LEDs with emission peak wavelengths longer than 375 nm is dominated by carrier recombination at large localized energy states caused by In composition fluctuation in the InGaN well layer. When the emission peak wavelength becomes shorter than 375 nm, the conventional band-to-band emission mechanism becomes dominant due to poor carrier localization resulting from small In composition fluctuations. In addition, the quantum-confined Stark effect due to the piezoelectric field becomes dominant, which causes a low output power of the UV LEDs.
Article
The momentum and energy dependence of the matrix elements for direct and indirect transitions across the band gap is studied both theoretically and experimentally. The accepted theory for the inelastic scattering cross section of fast electrons by condensed matter is extended to show how the nature of a transition can change the shape of the measured energy-loss spectrum in the region of the onset. For the case of direct transitions the matrix element acts only as a multiplicative factor to the joint density of states (JDOS), and so an (E-Eg)1/2 term is observed in the spectrum. The matrix elements for indirect transitions are shown to be dependent on the momentum transferred by the incident fast electron to the crystal electrons. The product of the indirect matrix element and the JDOS contributes an (E-Eg)3/2 term to the spectrum. To test this theory it is shown that the matrix-element-weighted joint density of states should be extracted from the electron-energy-loss spectra via a Kramers-Kronig transformation. An objective method is proposed for plotting the data to determine both the principal band gaps and their direct or indirect nature. The method is tested and succeeds in well-known cases. It is now possible with the electron microscope to measure these fundamental electronic properties of semiconductors and insulators accurately by electron spectroscopy.
Article
Electron energy-loss spectroscopy has been used to study the electronic structure of GaN and InxGa1-xN. The fine structure on the ionization edges was seen to change with In concentration and these changes agreed well with the results of calculations using the full-potential linearized augmented plane-wave method. Best agreement was observed when the effects of the core hole were included. The anisotropy in the electronic structure of GaN was observed in both the experimental and calculated ionization edges. The low-loss region of the spectrum also varied with In concentration, with the most obvious change being a large shift in the plasmon energy. Interband transitions were found to have a large effect on the plasmon energy in this system. Band gaps and other optical properties were determined from the low-loss spectra.
Article
Quantitative comparisons have been made of the In concentration, strain, and internal electric field present in a pseudomorphic InGaN/GaN quantum well. Z-contrast scanning transmission electron microscopy was used for mapping In concentration with atomic resolution, variations of the c-lattice parameter of the InGaN layer were measured from (0001) lattice fringes in high-resolution transmission electron micrographs, and the internal electric fields were determined by differentiating phase images obtained by electron holography. Based on these measurements, it was concluded that local fluctuations of In concentration caused inhomogeneities in the internal electric field across the quantum well. The band structure of the quantum well would thus be altered not only by quantum dot effects but also by the additional modulation of the internal electric field, leading to further broadening of the light emission. © 2004 American Institute of Physics.
Article
We present an atomic-scale structural and compositional analysis of ultrathin layers in multiple quantum well InGaN/GaN, by high-angle annular dark field (HAADF) scanning transmission electron microscopy (STEM). A high-quality HAADF STEM image processed by two-dimensional smoothing and deconvolution provides precise atomic-column positions and clear contrast, thereby allowing us to map the strain field and In atom distribution in successive GaN and InGaN layers. We conclude from these maps that there is a local fluctuation of In atoms in the InGaN layers and the In-rich regions, considered as quantum dots, cause large expansion only along the [0001] direction. © 2003 American Institute of Physics.
Article
The high efficiency of luminescence from InGaN has underpinned widespread recent developments in blue-green optoelectronics. A loose consensus on the nature of the luminescence has emerged in the last three years. Localisation of excitation, whether by composition fluctuations or self-formed quantum dots, appears to tilt the balance in favour of radiative recombination, despite the presence of huge densities of extended defects found in ‘device-grade’ material by electron microscopy. The luminescence is lowered in energy with respect to the excitation by internal electric fields. What is not clear at present is the relationship between the composition and the structure: what exactly is responsible for optical effects in this material? We have argued previously that, contrary to accepted wisdom, current theoretical treatments fail to give a satisfactory account of the dependence of the optical energies on the composition of InGaN. We now advance a strong form of this argument: present theoretical treatments of light–matter coupling may be inadequate to take account of the complexities of structure inherent in this, or any other, luminescent material.
Article
The energy levels determining optical transitions within indium-rich InGaN quantum dots are estimated by calculating ground state energy levels in the regime of strong 3D confinement. Effects due to finite barrier heights and variation in band offset ratios are included and the consequences of the strong piezoelectric fields within the dots considered. It is shown that the lowest optical transitions in spherical quantum dots with radii up to 10 nm correspond to wavelengths covering the visible spectrum. These results are discussed in relation to the significant recent advances in light emitters based on InGaN layers.
Article
It is shown that the 0.7 eV band gap recently announced for InN is actually due to a sub band gap deep level trap with |sS like symmetry. This level had been known in the literature, but was previously misinterpreted as a deep level trap with |pS like symmetry. It is also shown that proper interpretation of the absorption data for InN requires that the energy dependence of the refractive index be taken into account.
Article
A survey of most recent studies of optical absorption, photoluminescence, photoluminescence excitation, and photomodulated reflectance spectra of single-crystalline hexagonal InN layers is presented. The samples studied were undoped n-type InN with electron concentrations between 6 × 1018 and 4 × 1019 cm—3. It has been found that hexagonal InN is a narrow-gap semiconductor with a band gap of about 0.7 eV, which is much lower than the band gap cited in the literature. We also describe optical investigations of In-rich InxGa1—xN alloy layers (0.36 < x < 1) which have shown that the bowing parameter of b ∼ 2.5 eV allows one to reconcile our results and the literature data for the band gap of InxGa1—xN alloys over the entire composition region. Special attention is paid to the effects of post-growth treatment of InN crystals. It is shown that annealing in vacuum leads to a decrease in electron concentration and considerable homogenization of the optical characteristics of InN samples. At the same time, annealing in an oxygen atmosphere leads to formation of optically transparent alloys of InN–In2O3 type, the band gap of which reaches approximately 2 eV at an oxygen concentration of about 20%. It is evident from photoluminescence spectra that the samples saturated partially by oxygen still contain fragments of InN of mesoscopic size.
Article
By the use of the electron beam plasma technique, it has been found that the solid solutions of Ga 1-x In x N can be synthesized over the entire composition region. From the optical measurements, the direct energy gap at 78 °K was determined to be 3.46 eV for GaN and 2.11 eV for InN. Also its composition dependence was found to deviate downward from linearity. From the infrared reflectivity measurement and the resultant K‐K dispersion analysis, the transverse optical frequencies for long‐wavelength phonons of GaN and InN were 563 and 478 cm<sup>-1</sup>, respectively. The optical phonons in this quasibinary system were concluded to exhibit a one‐mode–type behavior. The Brout sum rule was discussed for a large number of the diatomic crystals of A<sup>N</sup>B<sup>8-N</sup> type and its relation with respect to the reduced mass was derived as Σ i ω<sup>2</sup> i (k=0) =A μ<sup>-1.5</sup>. According to this relation, the longitudinal optical frequency of InN was deduced to be 694 cm<sup>-1</sup>. On the other hand, from the result of the annealing treatment for the solid‐solution alloys and the theoretical consideration, it was pointed out that this quasibinary system has a solid‐phase misibility gap in chemical equilibrium.
Article
Highly efficient InGaN/AlGaN double‐heterostructure blue‐light‐emitting diodes (LEDs) with an external quantum efficiency of 5.4% were fabricated by codoping Zn and Si into an InGaN active layer. The output power was as high as 3 mW at a forward current of 20 mA. The peak wavelength and the full width at half maximum of the electroluminescence of blue LEDs were 450 and 70 nm, respectively. Blue‐green LEDs with a brightness of 2 cd and a peak wavelength of 500 nm were fabricated for application to traffic lights by increasing the indium mole fraction of the InGaN active layer. © 1995 American Vacuum Society
Article
First‐principles calculations have been used to determine bowing parameters for disordered zinc‐blende Al 1-x Ga x N and Ga 1-x In x N. The direct transition at Γ is found to bow downward for both materials with parameters +0.53 and +1.02 eV, respectively, while the Γ‐to‐X transition bows upward for Al 1-x Ga x N (parameter -0.10 eV) and downward for Ga 1-x In x N (parameter +0.38 eV). The similarity of the calculated bulk zinc‐blende and wurtzite Γ‐point transitions also allows estimates to be made of the energy gap versus composition for wurtzite alloys. © 1995 American Institute of Physics.
Article
The electrical, optical, and structural properties of light emitting diodes (LEDs) fabricated from the III–V nitride material system have been studied. LEDs with external quantum efficiencies as high as 4% were characterized by transmission electron microscopy and found to contain dislocation densities in excess of 2×10<sup>10</sup> cm<sup>-2</sup>. A comparison to other III–V arsenide and phosphide LEDs shows that minority carries in GaN‐based LEDs are remarkably insensitive to the presence of structural defects. Dislocations do not act as efficient nonradiative recombination sites in nitride materials. It is hypothesized that the benign character of dislocations arises from the ionic nature of bonding in the III–V nitrides. © 1995 American Institute of Physics.
Article
Wurtzite (Al, Ga, In)N heterostructures grown by metal organic vaper phase epitaxy (MOVPE) were studied using quantitative analytical scanning transmission electron microscopy (STEM) and cathodoluminescence (CL). The STEM results can be summarized as follows. The interfaces in e.g. In0.12Ga0.88N/GaN single quantum wells (SQWs) appear to be asymmetric. The lower interface in the growth direction is more abrupt than the upper one, where a grading in the In concentration is significant. We found composition fluctuations in the nanometer range within the InGaN QWs, which are supposed to cause the localized exitonic behavior of the observed photoluminescence (PL) emission. In the sample with 17 nm thick InGaN QWs we observed relaxation effects, which are not present in the thin QWs of 2 nm thickness. CL results on both InGaN/GaN and AlGaN/GaN SQW structures show generally inhomogeneous emission intensity in panchromatic CL micrographs on a 1 μm scale, which is related to local variations of the interface quality. CL spectra recorded from defect sites in AlGaN/GaN SQWs are dominated by the so-called ‘yellow-emission’. In the samples containing InGaN layers, the grown-in hexagonal pyramids and mesa-like structures as well as micropipes were commonly observed.
Article
Electron energy loss spectroscopy in the TEM with sub-0.1-eV energy resolution requires the incorporation of an electron beam monochromator in the condenser column. At high beam currents, the statistical Coulomb interactions in the monochromator broaden the energy distribution (Boersch effect) and blur the apparent source, thus limiting the energy resolution and spatial resolution of the microscope. We have built a monochromator (Wien filter type) with a length of 50 mm and measured the blurring of the energy dispersed beam for various beam currents. At low beam currents, the observed blur agrees reasonably well with the theoretical size of the blur. We calculate the theoretical magnitude of the Boersch effect in the filter, and we conclude that the combined action of the Boersch effect and the blur does not impede an energy resolution better than 0.1 eV as long as the total beam current is below ≈30 nA.
Article
Valence electron energy loss spectroscopy (VEELS) was applied to determine band transitions in wurtzite InN, deposited by molecular beam epitaxy on (0001) sapphire substrates or GaN buffer layers. The GaN buffer layer was used as VEELS reference. At room temperature a band transition for wurtzite InN was found at (1.7±0.2 eV) and for wurtzite GaN at (3.3±0.2 eV) that are ascribed to the fundamental bandgap. Additional band transitions could be identified at higher and lower energy losses. The latter may be related to transitions involving defect bands. In InN, neither oxygen related crystal phases nor indium metal clusters were observed in the areas of the epilayers investigated by VEELS. Consequently, the obtained results mainly describe the properties of the InN host crystal.
Article
Annular objective apertures are fabricated for a CM300 transmission electron microscope using a focused ion beam system. A central beam stop in the back focal plane of the objective lens of the microscope blocks all electrons scattered up to a semi-angle of approximately 20 mrad. In this manner, contributions to the image from Bragg scattering are largely reduced and the image contrast is sensitive to the atomic number Z. Experimentally, we find that single atom scattering cross sections measured with this technique are close to Rutherford scattering values. A comparison between this new method and STEM-HAADF shows that both techniques result in qualitatively similar images although the resolution of ADF-TEM is limited by contrast delocalization caused by the spherical aberration of the objective lens. This problem can be overcome by using an aberration corrected microscope.
Article
Structural analysis was performed on a purple laser diode composed of In0.20Ga0.80N (3 nm)/ In0.05Ga0.95N (6 nm) multiple quantum wells, by employing transmission electron microscopy and energy-dispersive x-ray microanalysis, both of which are assessed from the cross-sectional direction. It was found that the contrast of light and shade in the well layers corresponds to the difference in In composition. The main radiative recombination was attributed to excitons localized at deep traps which probably originate from the In-rich region in the wells acting as quantum dots. Photopumped lasing was observed at the high energy side of the main spontaneous emission bands.
Article
REVIEW High-efficiency light-emitting diodes emitting amber, green, blue, and ultraviolet light have been obtained through the use of an InGaN active layer instead of a GaN active layer. The localized energy states caused by In composition fluctuation in the InGaN active layer are related to the high efficiency of the InGaN-based emitting devices. The blue and green InGaN quantum-well structure light-emitting diodes with luminous efficiencies of 5 and 30 lumens per watt, respectively, can be made despite the large number of threading dislocations (1 x 10(8) to 1 x 10(12) cm-2). Epitaxially laterally overgrown GaN on sapphire reduces the number of threading dislocations originating from the interface of the GaN epilayer with the sapphire substrate. InGaN multi-quantum-well structure laser diodes formed on the GaN layer above the SiO2 mask area can have a lifetime of more than 10,000 hours. Dislocations increase the threshold current density of the laser diodes.
Article
Spatially resolved electron-energy-loss scattering has been used to study changes in the inelastic scattering near the bulk band-gap energy for locations near the GaAs-Ga0.85In0.15As interface. We observe the expected bulk band gap on either side of the interface. At a single interface-misfit dislocation we observe scattering which is consistent with an excitation of transitions between a localized state near the dislocation and the crystal conduction band. Within this interpretation, the energy of the state is estimated to be 0.7+/-0.5 eV above the GaAs valence-band maximum.
Article
The low loss region of an EEL spectrum (<50 eV) contains information about excitations of outer shell electrons and thus the electronic structure of a specimen which determines its optical properties. In this work, dedicated electron energy loss spectroscopy (EELS) methods for the experimental acquisition and analysis of spectra are described, which give improved information about the electronic structure near the bandgap region at a spatial resolution in the range of nanometers. For this purpose, we made use of a cold field emission scanning transmission electron microscope (STEM) equipped with a dedicated EELS system. This device provides a subnanometer electron probe and offers an energy resolution of 0.35 eV. Application of suitable deconvolution routines for removal of the zero loss peak extracts information on the bandgap region while the Kramers-Kronig transformation deduces the dielectric properties from the measured energy loss function. These methods have been applied to characterize the optical properties of wide-bandgap materials for the case of III-nitride compounds, which are currently the most promising material for applications on optoelectronic devices working in the blue and ultraviolet spectral range. The obtained results are in excellent agreement with experimental measurements by synchrotron ellipsometry and theoretical studies. The potential of the superior spatial resolution of EELS in a STEM is demonstrated by the analysis of dielectric properties of individual layers of heterostructures and individual defects within wurtzite GaN.
Article
This article is a survey of hardware and software advances that promise to increase the power and sensitivity of electron energy-loss spectroscopy (EELS) and energy-filtered imaging (EFTEM) in a transmission electron microscope. Recent developments include electron-gun monochromators, lens-aberration correctors, and software for spectral sharpening, spectral processing and interpretation of fine structure. Future improvements could include the deployment of new electron sources. The expected enhancements in energy and spatial resolution are compared with fundamental limitations that arise from the natural widths of spectral peaks, the delocalization of inelastic scattering and the problem of electron-irradiation damage.
Article
With the development of monochromators for (scanning) transmission electron microscopes, valence electron energy-loss spectroscopy (VEELS) is developing into a unique technique to study the band structure and optical properties of nanoscale materials. This article discusses practical aspects of spatially resolved VEELS performed in scanning transmission mode and the alignments necessary to achieve the current optimum performance of approximately 0.15 eV energy resolution with an electron probe size of approximately 1 nm. In particular, a collection of basic concepts concerning the acquisition process, the optimization of the energy resolution, the spatial resolution and the data processing are provided. A brief study of planar defects in a Y(1)Ba(2)Cu(3)O(7-)(delta) high-temperature superconductor illustrates these concepts and shows what kind of information can be accessed by VEELS.
  • S Logothetidis
  • J Petalas
  • M Cardona