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Misfit dislocation formation in the AIGaN/GaN heterointerface
Journal of Applied Physics
Abstract
Heteroepitaxial growth of AlxGa1−xN alloy films on GaN results in large tensile strain due to the lattice mismatch. During growth, this strain is partially relieved both by crack formation and by the coupled introduction of dense misfit dislocation arrays. Extensive transmission electron microscopy measurements show that the misfit dislocations enter the film by pyramidal glide of half loops on the 1∕3〈123〉∕{112} slip system, which is a well-known secondary slip system in hcp metals. Unlike the hcp case, however, where shuffle-type dislocations must be invoked for this slip plane, we show that glide-type dislocations are also possible. Comparisons of measured and theoretical critical thicknesses show that fully strained films can be grown into the metastable regime, which we attribute to limitations on defect nucleation. At advanced stages of relaxation, interfacial multiplication of dislocations dominates the strain relaxation process. This work demonstrates that misfit dislocations are important mechanisms for relaxation of strained III-nitride heterostructures that can contribute significantly to the overall defect density.
... Critical thickness measures signify the onset of relaxation during pseudomorphic growth of thin films [43]. In the Griffith's model, cracks are assumed to be generated when the consequent reduction in elastic energy is larger than creating a new surface due to defects [44,45]. While the model was shown to overestimate critical thicknesses by about 10%-20%, we continue to refer to the model in our evaluation. ...
... The value of Z is 3.951 for surface cracks and 1.976 for channeling cracks [43]. The values of ν (0.31) and M (460 GPa) were taken from [45]. The barrier strain ε a, AlGaN was calculated using equation (10) to be 0.766%. ...
... Moreover, these defects are likely to manifest themselves as channeling cracks. These channeling cracks would promote misfit dislocation (MD) generation, which in turn would serve to relax local stress [45]. This defect generation picture is consistent with the findings of Whiting et al, who reported the observation of channeling cracks near and under the ohmic contacts, with defects exhibiting depths ranging from 20 to 30 nm and an average length of approximately 50 nm [38]. ...
This study explores the impact of alloyed ohmic contact separation on ungated GaN high electron mobility transistors (HEMTs) lattice stress by employing Raman spectroscopy and solid mechanics simulations for comprehensive analysis. Focusing on the substantial stresses exerted by ohmic contacts, our research introduces a novel mechanical calibration procedure. The proposed procedure demonstrates that the stress in the GaN buffer can be precisely modelled using Raman measurements taken from patterns of varying length, which in return reveals the impact of ohmic contacts on stress. We show that this technique shows a good alignment to the Raman measurement results. Moreover, we identify ohmic contact edges as potential sites for defect generation due to the accumulation of substantial elastic energy, a finding supported by experimental observations of crack formations in related studies. Our calibrated mechanical model not only enhances the understanding of stress distributions within GaN HEMTs but also lays the groundwork for future improvements in electro-thermo-mechanical simulations.
... The growth of AlGaN layers beyond the critical layer thickness results in the introduction of misfit dislocations and/ or cracking in the film, and, consequently, the device yield is reduced. [6][7][8][9][10][11][12][13][14] To enable the growth of thick and high Al-molefraction AlGaN layers while suppressing cracking, various approaches have been reported to tackle the epitaxial growth of AlGaN-on-GaN-based heterostructures. For example, approaches based on epitaxial lateral overgrowth (ELO) were demonstrated for the growth of thick AlGaN layers on GaN substrates beyond the critical layer thickness. ...
... The critical thickness for crack formation for Al x Ga 1Àx N layers with x ¼ 0.16 on GaN was calculated to be around 200 nm. 5,9 Thus, the cracks were mainly formed during the growth of the lower n-cladding layer. Additionally, some cracks are bent as they approach the other primary cracks, and the directions are converted into 1100 h i around the intersections. ...
... The thicknesses of n-and p-side CLs were 690 and 540 nm, respectively. Since the calculated critical thicknesses for Al x Ga 1Àx N layers with x ¼ 0.13 and 0.17 were around 310 and 180 nm, 5,9 respectively, the CLs far beyond the limitation of critical layer thickness were realized. ...
The non-planar growth of UV-A laser diode heterostructures composed of AlGaN layers with high Al-mole-fractions and thicknesses exceeding the critical layer thickness was performed on patterned c-plane GaN (0001) substrates with stripe-shaped mesa structures. This approach suppressed the surface cracking at the top of the mesas via an anisotropic relaxation of the in-plane strain along the direction normal to the mesa stripes. Stimulated emission and laser operation at room temperature with emission in the UV-A range under pulsed current injection were demonstrated for laser diodes fabricated on the mesas.
... 41 These misfit strains are exceptionally large relative to other epitaxial layers such as epitaxial Si 1−x Ge x on Si ( m = 1.1% for x = 0.3) 42,43 and Al x Ga 1−x N on AlN ( m = 0.9% for x = 0.37). 44 Thus, the computed stored elastic strain energy per unit volume w and in-plane biaxial tensile stresses in the Co layers deposited on the two seed layers are extremely large. Values of misfit strain, strain energy density, and biaxial in-plane stress are provided in Table I. ...
... To calculate h , c the pyramidal II slip system á ñ 1122 1 3 1123 {¯}¯¯was chosen as it has the highest resolved shear stress making it the most effective slip system for strain relaxation for a basal plane, i.e., (0001), oriented epitaxial film. 27,44 To calculate h c where the energy of both threading and misfit dislocations are considered, the following expression is used 27 ...
... This growth into a metastable regime is likely due to an additional kinetic barrier for the nucleation or propagation of the threading dislocations. 44 Having grown well beyond this critical thickness, the film then relaxed through surface roughening. If deposited using a high temperature technique such as vacuum deposition, however, it is unlikely the epitaxial Co film would be able to grow past a nanometer of thickness before the film would break up or dewet. ...
Co electrodeposition was performed onto single crystal Ru(0001) and polycrystalline Ru films to study the influence of such seed layers on the growth of epitaxial Co(0001). The effect of misfit strain on the electrodeposited Co(0001) films was studied using 60 and 10 nm-thick Ru(0001) seed layers, where the misfit strains of the Co layer on the two Ru(0001) seed layers are 7.9% and 9.6%, respectively. Despite a large misfit strain of 7.9%, the planar growth of Co(0001) was achieved up to a thickness of 42 nm before a transition to island growth was observed. Epitaxial Co films electrodeposited onto 10 nm Ru(0001) showed increased roughness when compared with Co electrodeposited onto the 60 nm seed layer. Co electrodeposition onto polycrystalline Ru resulted in a rough, polycrystalline film with faceted growth. Electrochemical experiments and simulations were used to study the influence of [Co ²⁺ ] and solution pH on the throughput of the electrodeposition process. By increasing [Co ²⁺ ] from 1 to 20 mM, the deposition rate of Co(0001) increased from 0.23 nm min ⁻¹ to 0.88 nm min ⁻¹ at an applied current density of −80 μ A cm ⁻² .
... Three candidate families of slip systems with inclined slip planes to the basal plane with c + a type threading segments, with b ¼ 1/3 11 23 h i, and a-type misfit segments in the basal plane are listed in Table I. [49][50][51]53,54,57 The resolved shear stresses on the threading dislocations for the representative slip systems are also listed in the table. These stresses are seen to range from as low as 2.6 GPa for the 1/3 11 2 3 h i{2 1 11}system to as high as 4.5 GPa for the 1/3 11 2 3 h i{11 22} system. ...
... The latter is the so-called pyramidal II system. [49][50][51]57 Using the candidate slip systems listed in Table I, the critical thickness for strain relaxation by the combined threading plus misfit dislocation configurations can be computed using different approaches. 42,[56][57][58] As was the case earlier in this section, these approaches also ignore the differences in elastic constants of the film and the substrate but they do account for the elastic anisotropy of the Ru layer. ...
... [49][50][51]57 Using the candidate slip systems listed in Table I, the critical thickness for strain relaxation by the combined threading plus misfit dislocation configurations can be computed using different approaches. 42,[56][57][58] As was the case earlier in this section, these approaches also ignore the differences in elastic constants of the film and the substrate but they do account for the elastic anisotropy of the Ru layer. However, they assume the deviation from full isotropy to be small. ...
Defects in epitaxial Ru(0001) films on c-plane sapphire, with nominal thicknesses of 10–80 nm, deposited at 350 °C and step-annealed to 950 °C, were characterized using transmission electron microscopy. The variation of Ru and sapphire lattice parameters with temperature is such that the misfit strain for the observed 30° rotated-honeycomb epitaxial relationship is essentially constant with temperature at 1.5%, resulting in a biaxial stress of 10.0 GPa and an energy density of 150 MJ m⁻³ in unrelaxed films. Stress relaxation occurs by the formation of defects. For the 20–80 nm thick films, the defects are a- and c-type dislocations and stacking faults, argued to be of I 2 type. In addition, the films show the surprising presence of { 11 2 ¯ 1 } 1 / 3 ⟨ 11 2 ¯ 6 ¯ ⟩ deformation twins. The 10 nm-thick films were found to be defect free. The critical thickness for misfit strain relaxation via the formation of threading and misfit dislocations is computed as 7 ± 2 nm, depending on the choice of the dislocation core radius. Energetic analysis of twin formation, using both the infinite-matrix and the finite-matrix (Mori–Tanaka) approaches, provides values of the twin aspect ratios, assumed to be ellipsoidal, and shows that the latter but not the former approach can qualitatively explain the formation of the observed twins. In addition to providing the maximum strain relief compared to other potential twin types, { 11 2 ¯ 1 } 1 / 3 ⟨ 11 2 ¯ 6 ¯ ⟩ twins do not require lattice shuffles and have a boundary that is a special boundary, namely, a 35° tilt boundary with a-type dislocations every other {0002} plane, that may also favor their formation.
... Quantification of the critical thickness in the case of III-Nitrides on (0001)-oriented substrates has been attempted by several authors, but the discrepancies between experiment and theory are considerable. [7][8][9][10][11] In this system, the complex glide geometry of the hexagonal crystal lattice (three types of Burgers vectors instead of one in the cubic lattice and at least five different types of glide planes instead of one in the cubic lattice) adds additional degrees of freedom and makes quantification even more challenging. Moreover, one has to keep in mind that for (0001)-oriented thin wurtzite films, the primary slip-systems of the hexagonal lattice are usually not active because there are no resolved shear stresses which are necessary to enable dislocation glide. ...
... On the one side TEM studies of Al x Ga 1-x N/GaN and In x Ga 1-x N/ GaN heterostructures have revealed the presence of a þ ctype misfit dislocation networks. [11][12][13][14][15] Their formation has been explained by glide mechanisms of a þ c-type dislocations on either 1122 f g or 1101 f g pyramidal planes and the process has been quantitatively modeled using a classical Matthews-Blakeslee approach. However, the high kinetic barriers for such glide configurations caused by the large Burgers-vector of a þ c-type dislocations and the small interplanar spacing of their slip-planes ( 1 3 1123 h i j 1122 f g and 1 3 1123 h i j 1101 f g are secondary slip-systems in the wurtzite lattice) limit the efficiency of the plastic relaxation process: accordingly the amount of strain relaxation by this mechanism is typically low corresponding to a large spacing of a þ c-type misfit dislocations in the interfacial plane. ...
... On the other side, several authors found a high degree of strain relaxation due to dislocation networks formed of atype misfit dislocations. 8,11,14,[16][17][18][19] Their presence has been explained by either a punch-out mechanism 15 or a cooperative strain relaxation mechanism. 8,18,19 These models, however, are only of qualitative nature. ...
In this work, we present an experimental and theoretical study of the process of plastic strain relaxation of (0001)-oriented wurtzite heterostructures. By means of transmission electron microscopy and atomic force microscopy, we show that plastic relaxation of tensile strained AlxGa1-xN/GaN heterostructures proceeds predominantly by nucleation of a-type misfit dislocations in the 13⟨112¯0⟩|0001 slip-system driven by a three-dimensional surface morphology, either due to island growth or due to cracking of the layer. Based on our experimental results, we derive a quantitative model for the dislocation nucleation process. With the shear stress gradients at the nucleation sites of a-type misfit dislocations obtained by the finite element method, we calculate the critical thickness for plastic relaxation of strained wurtzite films and heterostructures as dependent on the surface morphology. The crucial role of the growth mode of the film on the strain relaxation process and the resulting consequences is discussed in the paper.
... The misfit dislocations will initiate from the surface and slip into the epitaxial layer to relax the misfit strain when the epitaxial layers exceed a critical thickness [7][8][9]. The strain relaxation and mechanical deformations in hexagonal lattice systems are related to the dislocation slip systems, which have been widely analyzed [10][11][12]. Generally, previous studies have analyzed the stress relaxation process by directly observing the dislocation state at the multilayer interface after the growth process, which is easily affected by different growth conditions and high TDs in the substrate. ...
... The electron barrier layers (EBL) in GaN-based devices, typically composed of AlGaN materials, confine electrons within the active region to prevent leakage and mitigate the performance degradation of devices [21]. Additionally, AlGaN materials are known to be used in LED and LDs for stress relaxation and prevention of dislocation elongation [10,22]. Due to the larger lattice constants of GaN crystal (a = 0.3189 nm, c = 0.5185 nm) compared to AlN (a = 0.3112 nm, c = 0.4982 nm), high Al-component AlGaN can induce dislocations bending by providing tensile stress. ...
The slip systems and motion behavior of dislocations induced by nano-indentation technique in GaN-based LDs were investigated. Dislocations with burgers vector of b = 1/3 <11 2 ¯ 3> were introduced on either {11 2 ¯ 2} <11 2 ¯ 3>, or {1 1 ¯ 01} <11 2 ¯ 3> pyramidal slip systems in the upper p-GaN layer. Besides, {0001} <11 2 ¯ 0> basal slip system was also activated. The AlGaN/InGaN multi-layers in device can provide mismatch stresses to prevent dislocations from slipping through. It was observed that the density of dislocations induced by the indenter significantly decreased from the upper to the lower regions of the multi-layers. The a + c dislocations on pyramidal slip planes were mostly blocked by the strained layers.
... 16 Nevertheless, dislocation gliding has come into focus recently as a possible cause of dislocation motion under temperature-induced stress during HTA thermal cycle annealing of AlN. 17 Furthermore, strain relaxation through dislocation half-loop formation has been previously observed for InGaN layers 18 as well as AlGaN layers grown on GaN. 19 This process involved glide of mixed-type dislocations with the Burgers vector 1/3⟨11−23⟩ on pyramidal {11−22} planes and resulted in the formation of misfit dislocation arrays. Similar to the classical model sketched in Fig. 2(b), the authors observed half-loops with straight TD lines propagating through the whole layer and connected by the horizontal misfit segments in the heterointerfacial plane. ...
... Diagrams showing distribution of TD inclination angles measured in the[11][12][13][14][15][16][17][18][19][20] zone axis in (a) AlN:uid and (b) AlN:Si layers grown on HTA-AlN. In AlN: uid, a rather homogeneous distribution of inclination angles ranging from −10°u p to +10°is visible. ...
In this work, we compare the defect structure in unintentionally doped and Si-doped AlN layers grown by metalorganic vapor phase epitaxy (MOVPE) on high-temperature annealed (HTA) sputtered AlN templates on sapphire substrates. Since the HTA process leads to a reduction of the in-plane lattice constant of the AlN layers, further homoepitaxial overgrowth results in compressively strained AlN layers. With increasing MOVPE-AlN layer thickness, strain relaxation takes place mostly by formation of dislocation half-loops of an irregular shape, which accumulate at the homoepitaxial MOVPE-AlN/HTA-AlN interface. We suggest that these dislocations nucleate at the layer surface and move down to the homoepitaxial interface at high temperatures. The formation of these irregular and hardly controllable defects can be avoided by introduction of Si-doping into the MOVPE-AlN layers. Si-doping enlarges the inclination of threading dislocation lines stemming from the HTA-AlN template, producing an alternative mechanism for strain relaxation.
... The MDs in AlGaN/GaN are known to be dissociated into two partial dislocations, 1/2a + 1/2c. 25 The atomic structure of a 1/2a dislocation is also proposed to be very similar to a-plane stacking faults, 26,27 which have been reported as being related to radiative emission. 28 Moreover, emission from MDs in some other materials, such as ZnSe/GaAs, has been previously reported. ...
... In general, the steps are found to have a height of ∼0.5 nm, exactly corresponding to the c-axis component of the reported Burgers vector, a + c, of a MD. 25 However, we also found that a subset of the steps is larger and exhibits a height of 2c (∼1.0 nm). Such a 2c-high step will reveal a semipolar (1122) facet, which could lead to preferential nucleation of Ga adatoms rather than Al counterparts on the step edge during overgrowth (whereas a c-high step formed from a standard MD does not generate any welldefined facet on the growth surface as the uppermost atoms at the step edge may deviate from their natural sites due to reconstruction). ...
We report the discovery and characterization of single-photon-emitting carrier localization centers that are spontaneously formed along misfit dislocations in AlGaN. The emitters exhibit extremely narrow linewidths, which are in some cases narrower than our resolution limit of 35 µeV. Spectral analysis reveals a record-low inhomogeneous broadening (smaller than 20 µeV), which can be characterized as almost spectral-diffusion free. Such narrow linewidths allow for an unprecedented discussion of the homogeneous linewidths of quantum emitters in the III-nitrides and, in the current case, provide a lower bound on the pure-dephasing time T2 of ∼200 ps. These experimental results will pave the way to further improve the performance of III-nitride low-dimensional nanostructure-based quantum emitters.
... The result is shown in Figure 3.2. This method has frequently been used to determine stress gradients within GaN films, see for example references [60], [102] . Note that curvature measurements determine the global stress throughout the thickness of the GaN film. ...
... This divergence is probably due to a change in the structure of TDs with respect to the film surface, in terms of distribution and correlation. The presence of misfit dislocations, as reported for other AlGaN/GaN interfaces, [102], [142] might further broaden the diffraction profiles of the highly etched sample (2). However, their impact is probably small compared to that of the high number of TDs. ...
CEA-Leti develops power electronics components with high energy efficiency, based on semiconductors of the III-N group (GaN, AlGaN, InGaN…), in particular in partnership with ST microelectronics. In order to minimize the costs and improve the compatibility with the standards of microelectronics industry, CEA-Leti chose to produce epitaxial thin films of GaN grown on silicon substrates. However, these two materials have large mismatches of coefficient of thermal expansion and lattice parameter. The resulting GaN layers are affected by strong gradients of mechanical stress and dislocation density throughout their thickness. As component performances and wafer fragility are linked to crystalline quality and stress state of these thin films, it is important to have access to effective, rapid and non-destructive metrology tools. To this end, this PhD focuses on the study of GaN layers by X-ray diffraction (XRD), which is an acknowledged and widely used technique for the analysis of epitaxial films. The effect of stress gradient on XRD measurements is an asymmetrical broadening of diffraction peaks. We suggest extracting this gradient by reproducing the experimental XRD signal, by means of a simulation of the diffracted intensity stemming from a distorted crystal. A good agreement between simulations and measurements is obtained when modelling local variations of the strain profile throughout the surface plane. For the quantification of dislocations extending through the thickness of GaN layers, we suggest a simple methodology, based on the measurement of the micro-strain field surrounding the dislocations. The study shows how to use this type of measurement on GaN layers with strong stress gradient. In addition, the results are compared to the dislocation densities obtained with alternative characterization techniques such as transmission electron microscopy, cathodoluminescence or XRD, via an analysis of crystal lattice misorientations. The studies of stress gradient and dislocation density, initially carried out on complete stacks of III-N layers, are enlighten by means of in-depth measurements on progressively etched films of GaN.
... Because the Burgers vector without c-type components is on (0001), b ez is always equal to zero. In contrast to the (0001) slip, the edge components of the Burgers vectors may exist along any in-plane direction, as shown in equations (24) and (25). The magnitude of the Burgers vector for the 110m ...
... Thus, F q is roughly predicted by combining our calculation results with the reported results. For a 540 nm thick Al 0.17 Ga 0.83 N/GaN (0001) heterostructure, MDs due to the conventional slip system 1122 ⟨1123⟩ are experimentally confirmed by transmission electron microscopy (TEM) [25]. In the 1122 ⟨1123⟩ system for θ = 0 • , the calculated F p at the epilayer thickness that satisfies F a = F l is 1.28 µN. ...
We establish a calculation method to determine the critical layer thickness of the lattice relaxation in wurtzite heterostructures with arbitrary pairs of growth planes and slip systems. The calculation, which is based on the force balance model, takes the friction force and thermal assistance for the dislocation motion into account. Experimentally, Al x Ga1 − x N/AlN heterostructures are fabricated. The established method well reproduces the crystallographic orientations of experimentally observed dislocation lines.
... Our investigation focuses on the InGaN/GaN system, but the results can be directly applied to other compressively strained wurtzite layers like GaN or AlGaN layers grown on AlN substrates. Although, the strain relaxation mechanism by the introduction of misfit dislocations is not dominant in the case of tensile strained layers, it has also been reported for tensile strained AlGaN layers grown on GaN [30] and some conclusions drawn here can also be transferred to this material system. ...
... The mis t dislocations will initiate from the surface and slip into the epitaxial layer to relax the mis t strain when the epitaxial layers exceed a critical thickness [7][8][9] . The strain relaxation and mechanical deformations in hexagonal lattice systems are related to the dislocation slip systems, which have been widely analyzed [10][11][12] . Generally, previous studies have analyzed the stress relaxation process by directly observing the dislocation state at the multilayer interface after the growth process, which is easily affected by different growth conditions and high TDs in the substrate. ...
The slip systems and motion behavior of dislocations induced by nano-indentation technique in GaN-based LDs were investigated. Dislocations with burgers vector of b = 1/3 < 113> were introduced on either {112} <113>, or {101}<113> pyramidal slip systems in the upper p-GaN layer. Besides, {0001}<110> basal and {110}<0001 > cylindrical slip systems were also activated. The AlGaN/InGaN multi-layers in device can provide mismatch stresses to prevent dislocations from slipping through. It was observed that the density of dislocations induced by the indenter significantly decreased from the upper to the lower regions of the multi-layers. The a + c dislocations on pyramidal slip planes and the c dislocations on {110} planes were mostly blocked by the strained layers.
... 10,11 Such dislocations have also been observed at the interface between relaxed Al x Ga 1-x N deposited on GaN. 12 Most of the available data in the literature on (a+c) dislocations in nitride semiconductor epitaxial layers are related to threading dislocations with mixed character since their line direction is roughly oriented along the [0001] growth directions of these layers. 7,[13][14][15] These reports show that typically 50% of the observed (a+c) dislocations are dissociated. ...
In hexagonal materials, ( a+c ) dislocations are typically observed to dissociate into partial dislocations. Edge ( a+c ) dislocations are introduced into (0001) nitride semiconductor layers by the process of plastic relaxation. As there is an increasing interest in obtaining relaxed InGaN buffer layers for the deposition of high In content structures, the study of the dissociation mechanism of misfit ( a+c ) dislocations laying at the InGaN/GaN interface is then crucial for understanding their nucleation and glide mechanisms. In the case of the presented plastically relaxed InGaN layers deposited on GaN substrates, we observe a trigonal network of ( a+c ) dislocations extending at the interface with a rotation of 3° from <100> directions. High‐resolution microscopy studies show that these dislocations are dissociated into two Frank–Shockley 1/6<203> partial dislocations with the I 1 BSF spreading between them. Atomistic simulations of a dissociated edge ( a+c ) dislocation revealed a 3/5‐atom ring structure for the cores of both partial dislocations. The observed separation between two partial dislocations must result from the climb of at least one of the dislocations during the dissociation process, possibly induced by the mismatch stress in the InGaN layer.
... 7 AlGaN materials are known to be used in LED and LDs to relax stresses and prevent dislocations elongation. 8,9 Due to the larger lattice constants of GaN crystal (a=1.>189 nm, c=1.5185 nm) compared to AlN (a=1.>11} ...
The multiplication and motion behavior of dislocations induced by surface damage or the failure of devices in GaN-based LDs were simulated by nano-indentation technique. The results shown that dislocations with burgers vector of b = 1/3 < 113> were introduced on either {112} <113>, or {101} <113> slip systems, and dislocations with burgers vector of b = 1/3 < 110> were introduced inevitably on {0001} <110> slip system. It is worth noting that dislocations on {101} slip planes underwent decomposition when passing through the AlGaN/InGaN interface, the screw component slip to the plane {0001} and the edge component were nailed at the interface. Inversely, dislocations on {112} slip planes exhibited smooth traverse through AlGaN and quantum well layers.
... In the process of growing InGaN films on GaN substrates, large lattice mismatches lead to huge strains, resulting in plastic relaxation of the misfit strain. This has been evidenced as an occurrence of parallel networks of straight misfit dislocations aligned along the <10-10> orientation in compressively strained InGaN/GaN [5][6][7][8][9], as well as in tensile-strained AlGaN/GaN [10] heterostructures. III-nitride films grown on GaN templates are short of the main slip planes, due to the absence of resolved shear Crystals 2023, 13, 1027 2 of 8 stress, and the plastic strain relaxation requires activation of the secondary slip plane. ...
We have investigated the interface dislocations in InxGa1−xN/GaN heterostructures (0 ≤ x ≤ 0.20) using diffraction contrast analysis in a transmission electron microscope. The results indicate that the structural properties of interface dislocations depend on the indium composition. For lower indium composition (up to x = 0.09), we observed that the screw-type dislocations and dislocation half-loops occurred at the interface, even though the former do not contribute toward elastic relaxation of the misfit strain in the InGaN layer. With the increase in indium composition (0.13 ≤ x ≤ 0.17), in addition to the network of screw-type dislocations, edge-type misfit dislocations were generated, with their density gradually increasing. For higher indium composition (0.18 ≤ x ≤ 0.20), all of the interface dislocations are transformed into a network of straight misfit dislocations along the <10–10> direction, leading to partial relaxation of the InGaN epilayer. The presence of dislocation half-loops may be explained by a slip on basal plane; formation of edge-type misfit dislocations are attributed to punch-out mechanism.
... 2 For AlGaN on (0001) GaN, the tensile stress leads to cracking when a certain surface energy has accumulated, 3,4 although some glide can occur via secondary slip systems. 5 The stress relaxation could also be affected by dislocations (as shown for InGaN 6 ), or by anisotropic stress field, e.g., from patterned growth. 7 For non-polar AlGaN on (10 10) GaN, the anisotropic lattice and thermal mismatches lead to different stress on the surface in [0001] and [1 210] directions, and glide is possible. ...
The stress relaxation with increasing thickness of metal-organic vapor phase epitaxy grown Al0.19Ga0.81N on quasi-bulk (101¯0) m-plane GaN substrates was investigated by x-ray diffraction. The anisotropic in-plane stress leads to an orthorhombic distortion of the lattice, which requires special mathematical treatment. Extending earlier works, we developed a method to calculate the distortion along [12¯10], [0001], and [101¯0] and obtained the lattice parameters, Al content, and strain values. The stress relaxation along the two in-plane directions involves two different mechanisms. First, the stress along [12¯10] relaxes by the onset of misfit dislocations through the {101¯0}⟨12¯10⟩ slip system while for thicker layers the stress along [0001] relaxes by crack formation. Comparing the cathodoluminescence emission at room temperature with the expected bandgap showed that both tensile in-plane strains along [12¯10] and [0001] decrease the bandgap.
... During the heteroepitaxial growth of InGaN thin films on GaN buffers, large lattice mismatches lead to huge strains, resulting in plastic relaxation of misfit strain. This has been evidenced as occurrence of parallel networks of straight misfit dislocations aligned along with <10-10> orientations in compressively strained InGaN/GaN [5][6][7][8][9] as well as in tensile strained AlGaN/GaN [10] heterostructures. III-nitride epilayers grown on (0001) substrates suffer from the absence of primary gliding planes due to the lack of resolved shear stress, and possible plastic strain relaxation needs the activation of secondary glide planes. ...
We have investigated the interfacial dislocations in InxGa1–xN/GaN (0 ≤ x ≤ 0.20) heterostructures using diffraction contrast analysis in a transmission electron microscopy. The analysis indicate that the structural properties of the interface dislocations depend on the indium composition. For lower indium composition up to x = 0.09, we observed that the screw-type dislocations and dislocation half loops occurred at the interface even though the former do not contributes toward elastic relaxation of the misfit strain in the InGaN layer. With the increase of indium composition (0.13 ≤ x ≤ 0.17), in addition to the network of screw-type dislocations, edge-type misfit dislocations were found generated with their density gradually increasing. For higher indium composition (0.18 ≤ x ≤ 0.20), all the interfacial dislocations are transformed into a network of straight misfit dislocations along the <10-10> directions leading to partially relaxation of the InGaN epilayer. The presence of dislocation half loops may be explained by slip on the basal plane, the formation of misfit dislocations are attributed to punch-out mechanism.
... Based on experimental work, several strain relaxation mechanisms of InGaN layers have been proposed so far. It has been shown that (0001)-oriented InGaN epilayers grown on GaN substrates can relax plastically by glide of 1/3 <1123> dislocations on {1122} pyramidal planes resulting in a trigonal network of straight misfit dislocations aligned along 〈1100〉 directions [6][7][8][9] . Several authors have also reported the creation of 1/3 <1120> misfit dislocations in nitride structures [10][11][12][13] . ...
Cross-sectional transmission electron microscopy studies often reveal a-type dislocations located either below or above the interfaces in InGaN/GaN structures deposited along the [0001] direction. We show that these dislocations do not emerge during growth but rather are a consequence of the stress state on lateral surfaces and mechanical processing, making them a post-growth effect. In cathodoluminescence mapping, these defects are visible in the vicinity of the edges of InGaN/GaN structures exposed by cleaving or polishing. Finite element calculations show the residual stress distribution in the vicinity of the InGaN/GaN interface at the free edge. The stress distribution is discussed in terms of dislocation formation and propagation. The presence of such defects at free edges of processed devices based on InGaN layers may have a significant negative impact on the device performance.
... In other words, it is possible to completely reduce the dislocations in the GaN channel layer when a buffer layer is inserted in between the GaN and Si. In the top region of buffer layer, vertical SDs can be bent to form horizontal dislocations (HDs) which are classified into the misfit dislocation (MD) [25] and edge dislocation (ED) [26]. X-ray diffraction (XRD) patterns have been used to evaluate the densities of vertical SDs and horizontal EDs in the GaN layer [27][28][29]. ...
Second harmonic generation (SHG) intensity, Raman scattering stress, photoluminescence and reflected interference pattern are used to determine the distributions of threading dislocations (TDs) and horizontal dislocations (HDs) in the c-plane GaN epitaxial layers on 6 inch Si wafer which is a structure of high electron mobility transistor (HEMT). The Raman scattering spectra show that the TD and HD result in the tensile stress and compressive stress in the GaN epitaxial layers, respectively. Besides, the SHG intensity is confirmed that to be proportional to the stress value of GaN epitaxial layers, which explains the spatial distribution of SHG intensity for the first time. It is noted that the dislocation-mediated SHG intensity mapping image of the GaN epitaxial layers on 6 inch Si wafer can be obtained within 2 hours, which can be used in the optimization of high-performance GaN based HEMTs.
... directions has been evidenced in compressively strained InGaN/GaN [32][33][34]153] and tensile strained AlGaN/GaN [28,154] heterostructures. The formation of dislocations has been attributed to the surface nucleation and glide of (a+c)-type dislocations on {1122} pyramidal planes, i.e., in 1/3〈1123〉{1122} slip system ( Fig. 3.3(e)) [32]. ...
Relaxed InGaN layers deposited on GaN substrates are of potential interest as pseudo-substrates for growth of InGaN heterostructures. Such layers have attracted attention for achieving higher indium contents in InGaN quantum wells and tailoring piezoelectric fields. This would allow to shift emission wavelengths of nitride LEDs further towards the red spectral range while reducing the charge carrier separation in polar devices due to the piezoelectric polarization field. To achieve relaxed InGaN pseudo-substrates with good structural quality, it requires understanding and control of the dislocation formation process.
In this work, we study the strain relaxation mechanism of InGaN epitaxial films grown on (0001)-oriented GaN substrates. The series of InxGa1-xN films, with x ranging from 0.12 to 0.2, were grown by nitrogen plasma assisted molecular beam epitaxy (PAMBE) or metalloorganic vapor phase epitaxy (MOVPE) on various types of substrates. The analysis of structural defects and strain relaxation mechanism was performed mainly in terms of transmission electron microscopy and X-ray diffractometry.
We demonstrate that the InGaN layers relax by the surface nucleation of (a+c)-type misfit dislocations that glide on pyramidal planes in the 〈112¯3〉{112¯2} slip system down to the interface. These form a trigonal network consisting three sets of dislocations aligned roughly along 〈11¯00〉 directions. This relaxation mechanism leads to partially relaxed InGaN layers with relatively smooth surfaces and threading dislocation densities below 10^9 cm^(-2). We reveal that the substrate misorientation promotes the formation of a particular set of Burgers vectors. As a result, there are differences in dislocation distribution leading to anisotropic state of relaxation – degree of relaxation depends on the crystallographic direction. We propose the adjusted procedure of the XRD strain analysis of such anisotropic relaxed InGaN layers. The preferential formation of dislocations also induces macroscopic tilt of the layer with respect to the substrate which reduces misorientation of the final structure. We show that the degree of relaxation of partially relaxed InGaN can be enhanced by post-growth thermal treatment. We also evidenced the upper limits for plastic relaxation in InGaN grown on GaN/sapphire templates. We attribute this upper limit to the blocking of newly created dislocation half-loops gliding on pyramidal planes by preexisting threading dislocations via repulsive interactions. The achievable degree of relaxation is higher for layers grown on substrates with lower threading dislocation density.
Additionally, we found that near the edges of the sample, a-type dislocations form under the InGaN/GaN interface. We show that these dislocations do not contribute to the strain relaxation of InGaN layers and are due to the elastic relaxation of the free edges of InGaN layer leading to the plastic deformation of underlaying GaN layer. We also discuss how other defects like stacking faults, threading dislocations and V-shaped pinholes, which may be present in InGaN epitaxial layers, influence the relaxation process.
... 6(b) and 6(c), we observe three misfit dislocation half-loops, the one in the middle of the cyan rectangle along the [ 1 120] direction, the one in the right of the cyan rectangle along the [0 110] direction, and the one in the middle of the orange rectangle along the [1 100] direction, indicative of the 1/3h11 23i/{1 101} and the 1/3h11 23i/{11 22} slip systems shown in the schematics, respectively. 8,31 Specifically, 18% of the mixedtype dislocations exhibit bright "tails," indicating misfit segments formed at the GaN/AlGaN interface due to the bending of the threading dislocations. Correlation with the ECP in Fig. 6(a) reveals that half of the misfit segments are along h11 20i directions and the other half along h1 100i directions. ...
We implemented invisibility criterion and black–white contrast orientation analysis into low-tilt electron channeling contrast imaging (ECCI) for dislocation-type discrimination in GaN and AlGaN layers grown on a Si(111) substrate. Our ECCI and x-ray diffraction (XRD) analysis attained consistent threading dislocation densities for GaN and AlGaN grown on Si, but demonstrated drastic discrepancy in the percentage of edge-type dislocations, potentially due to the lack of appropriate consideration of mixed-type [Formula: see text] dislocations in XRD. Further ECCI analysis of GaN/AlGaN heterointerface revealed mixed-type [Formula: see text] dislocation half-loops and dislocation bending due to compressive strain relaxation, validating that not all the dislocations originated from the mosaic or columnar structure. As a result, XRD analysis based on the mosaic block model does not give reliable edge-to-screw dislocation ratio. The observation of classic van der Merwe–Matthews-type dislocation half-loop nucleation and dislocation gliding could be associated with potential GaN/AlGaN optoelectronic device degradation issues.
... State-of-the-art AlGaN/GaN HEMTs have a defect density of the order of 10 8 -10 10 /cm 2 . At the same time, when the thickness of the Al x Ga 1-x N alloy layer exceeds the critical limit, the interface between the AlGaN barrier and the GaN channel suffers from crystal defects, resulting in misfit dislocation [20]. e effect of dislocation on electron transport can be analyzed by introduction of dislocation scattering. ...
In this work, the electron mobility in the MgZnO/ZnO heterostructure at room temperature is theoretically studied by considering interface roughness (IFR), dislocation (DIS), and polar optical phonon (POP) scattering. Analytical formulae are introduced to calculate the critical thickness and dislocation density in the barrier layer of MgZnO/ZnO heterostructures. The calculated critical thickness for the MgZnO/ZnO heterostructure is much smaller than that for the AlGaN/GaN heterostructure system. At room temperatures, POP scattering is found to be the most important scattering mechanism. On the other hand, the change of electron mobility limited by IFR as a function of the barrier thickness in the MgZnO layer is found to be quite different to that limited by DIS. High-density (> 10 13 c m − 2 ) 2DEG can be obtained in the MgZnO/ZnO interface by increasing the thickness and Mg composition in the MgZnO layer.
... A crack was generated on the In y Al 1−y N thin films due to stress and lattice mismatch between GaN and InAlN. The crack phenomenon was previously investigated using the tensile strain in AlGaN on GaN [10][11][12] or on the AlN [13] template. Different types of cracks were intensively studied, including surface crack, substrate damage, and de-cohesion [14]. ...
InyAl1 − yN epifilms with alloy compositions (y) ranging from 0.13 to 0.47 were grown on a GaN template using a low-temperature molecular-beam-epitaxy technique. Because of the larger lattice constant of high-indium-content InyAl1 − yN epifilm, the compressive stress accumulated during epitaxy. When the compressive stress exceeded the elastic limit of the InyAl1 − yN epifilms, a “concave-valley” crack with an equivalent width (~25 nm) was generated to form a prismatic domain. Our calculation and fitting results showed that no cracking occurred when the lattice mismatch was <1% because the stress was within the elastic limit. By contrast, the cracks appeared when the lattice mismatch was >2.4% because the stress exceeded the elastic limit.
... This relaxation commonly occurs via slip, i.e., via the formation of threading screw dislocations that glide to the interface and deposit misfit dislocations at the interface that relax the strain. [44][45][46] Deformation twinning is an alternative plastic deformation mode to slip. During twinning, the original (parent) lattice is reoriented by atom displacements that are equivalent to a simple shear of the lattice points, or some integral fraction of these points. ...
Experimentally measured resistivity of Co(0001) and Ru(0001) single crystal thin films, grown on c-plane sapphire substrates, as a function of thickness is modeled using the semiclassical model of Fuchs–Sondheimer. The model fits show that the resistivity of Ru would cross below that for Co at a thickness of approximately 20 nm. For Ru films with thicknesses above 20 nm, transmission electron microscopy evidences threading and misfit dislocations, stacking faults, and deformation twins. Exposure of Co films to ambient air and the deposition of oxide layers of SiO2, MgO, Al2O3, and Cr2O3 on Ru degrade the surface specularity of the metallic layer. However, for the Ru films, annealing in a reducing ambient restores the surface specularity. Epitaxial electrochemical deposition of Co on epitaxially deposited Ru layers is used as an example to demonstrate the feasibility of generating epitaxial interconnects for back-end-of-line structures. An electron transport model based on a tight-binding approach is described, with Ru interconnects used as an example. The model allows conductivity to be computed for structures comprising large ensembles of atoms (10⁵–10⁶), scales linearly with system size, and can also incorporate defects.
... The basic strain relaxation mechanism is the formation of misfit dislocation network in the layer/substrate interface. In the case of (0001)oriented nitride layers, the formation of misfit dislocation network by surface nucleation and glide of (a+c)-type dislocations on {1122} pyramidal planes have been evidenced [6][7][8]. ...
Relaxed InGaN layers have recently gained a lot of interest as potential pseudo-substrates for InGaN-based devices. In this work we study PAMBE grown thick In0.2Ga0.8N layers relaxed through the formation of (a+c)-type misfit dislocations. We show that either Ga/N flux ratio (ΦGa/ΦN) below 0.6 or high nitrogen flux (ΦN) equal to 1.4 μm h⁻¹ results in basal stacking fault (BSF) formation. Structural analysis has shown that stacking faults are I1-type in I3 and I4 configurations. We discuss the interactions between (a+c)-type dislocations and BSFs which depend on stacking fault configuration. Probably due to the lack of defect character of BSFs in I3 configuration, dislocations can cross them. Whereas BSFs in I4 configuration stop the dislocations a few nanometers above them, possibly due to the repulsive interactions between the descending dislocation and the Shockley partial dislocations surrounding the BSFs.
... Furthermore the most common substrate for growing GaN is 6H-SiC, and the lattice mismatch in this case is 3.5% [46]. The reason why high hole mobili-ties have not yet been observed for GaN grown on these substrates may be that GaN films are so thick that the crystal lattice accommodates the strain via misfit dislocations [48], which further decrease the mobility. Therefore to realize high-hole-mobility GaN one should devise strategies for preventing dislocation nucleation. ...
A fundamental obstacle toward the realization of GaN p-channel transistors is its low hole mobility. Here we investigate the intrinsic phonon-limited mobility of electrons and holes in wurtzite GaN using the ab initio Boltzmann transport formalism, including all electron-phonon scattering processes and many-body quasiparticle band structures. We predict that the hole mobility can be increased by reversing the sign of the crystal-field splitting, in such a way as to lift the split-off hole states above the light and heavy holes. We find that a 2% biaxial tensile strain can increase the hole mobility by 230%, up to a theoretical Hall mobility of 120 cm/Vs at room temperature and 620 cm/Vs at 100 K.
... [0001] [82]. Cependant, dans le cas ddune croissance du plan c, le glissement des dislocations sur les plans de base primaires et prismatiques secondaires de la structure hexagonale ne produit pas de relaxation significative [83] et nnest pas compatible avec la densité de dislocations « a » observée qui dépasse souvent 10 10 cm -2 . ...
Les puits quantiques InGaN/GaN montrent la plus grande efficacité connue dans le bleu-UV et le défi actuel dans ce type de matériau est de pousser leur émission vers les grandes longueurs d’ondes. Ceci serait possible en augmentant la composition en indium, mais il faut alors gérer les contraintes résultantes. Ce travail a mis en œuvre la microscopie électronique en transmission et la diffraction des rayons X pour déterminer la composition chimique à l’intérieur des couches InGaN, le taux de relaxation et le type de défauts présents. Les résultats montrent qu’il n’y a pas de fluctuations de composition en indium dans les couches d’InGaN étudiées avec des taux d’indium de l’ordre de 20%. Ainsi, la différence d’émission des échantillons pourrait s’expliquer par la variation d’épaisseur des puits quantiques InGaN et laprésence de défauts. En effet, plusieurs types de défauts ont été observés et caractérisés tels que les pinholes ou des domaines de défauts plans selon leur origine. Dans les multicouches InGaN/GaN avec couches AlGaN compensatrices de contrainte,la diffraction des rayons X a montré que lorsque l’épaisseur des couches d’AlGaN augmente en gardant constante l’épaisseur entre les couches actives d’InGaN (avec une valeur d’environ 16-17 nm), les puits quantiques sont totalement contraints dans le plan de croissance et en dehors. Par microscopie électronique, nous montrons queleur relaxation se fait par formation aussi bien de défauts en domaines plans que de dislocation de type a. Ces dislocations se propagent des pits quantiques vers la surface, et la densité des défauts augmente avec l’épaisseur des couches d’AlGaN.
... In the case of nitride heterostructures grown along the [0001] axis, which is the predominant growth orientation in commercial devices, the most crystallographically favorable slip system, namely the (0001) basal plane with <11-20> {0002} slip directions, lies parallel to the heterointerfaces, which results in the resolved misfit stress on the main slip plane to be zero, and the formation of regular networks of MDs is thus hindered [75]. However, MDs can be detected at heterointerfaces when shear stress is intentionally or unintentionally induced by 3D growth [76,77], by crack formation [78,79], or in close proximity to V-defects (illustrated by Figure 25) [80]. Therefore, the relaxation mechanism depends not only on the structure itself, but also on the growth conditions. ...
Intersubband (ISB) transitions are energy transitions between electronic states in a quantum well. GaN/AlGaN nanostructures have emerged as promising materials for new ISB optoelectronics devices, with the potential to cover the whole infrared spectrum. Their large conduction band offset (~1.8 eV for GaN/AlN) and sub-picosecond ISB recovery times make them appealing for ultrafast photonics devices in the short-wavelength infrared (SWIR, 1-3 µm), and mid-wavelength infrared (MWIR, 3-8 µm) regions. Moreover, the large energy of GaN longitudinal-optical phonon (92 meV, 13 µm) opens prospects for room-temperature ISB devices covering the 5-10 THz band, inaccessible to GaAs.The work described in this thesis has aimed at improving the performance and understanding of the material issues involved in the extension of the GaN/AlGaN ISB technology to the THz range. On the one hand, ISB photodetection requires n-type doping of the active nanostructures. In this work, we explore Si and Ge as potential n-type dopants for GaN. On the other hand, the presence of internal electric fields in the confinement direction of polar c-plane heterostructures constitutes one of the main challenges of the GaN-based ISB technology. In this thesis, we address the use of nonpolar a or m crystallographic orientations as an alternative to operate without the influence of these electric fields.Regarding the use of Si and Ge as n-type dopants for GaN, we show that the use of Ge as a dopant does not affect the morphology, mosaicity and photoluminescence properties of the doped GaN thin films. In the c-plane GaN/AlGaN heterostructures, no effect on the band-to-band properties was observed, but the structures with high lattice mismatch showed better mosaicity when doped with Ge. Regarding the alternative of nonpolar GaN, we compared GaN/AlN multi-quantum wells grown on a and m nonpolar free-standing GaN substrates. The best results in terms of structural and optical (both band-to-band and ISB) performance were obtained for m-plane structures. They showed room-temperature ISB absorption covering the whole SWIR spectrum, with optical performance comparable to polar c-plane structures, in spite of a too low structural quality to consider device processing. By introducing Ga in the AlN barriers, the lattice mismatch of the structure is reduced, leading to lower densities of cracks. Such m-plane structures showed room-temperature ISB absorption tunable in the 4.0-5.8 µm MWIR range, but still with structural defects. Finally, we extended the study to the far-infrared range, using AlGaN barriers with much lower Al content. As a result, the studied m-plane structures displayed an excellent crystalline quality, without extended defects, and showed low-temperature ISB absorption in the 6.3 to 37.4 meV (1.5 to 9 THz) range. This result constitutes an experimental demonstration of the feasibility of GaN devices for the 5-10 THz band, forbidden to GaAs-based technologies.
... State-of-the-art AlGaN/GaN HEMTs have a defect density of the order of 10 8 -10 10 /cm 2 . At the same time, when the thickness of the Al x Ga 1−x N alloy layer exceeds the critical limit, the interface between the AlGaN barrier and GaN channel suffers from crystal defects, resulting in misfit dislocation [18]. The effect of dislocation on electron transport can be analyzed by the introduction of dislocation scattering. ...
Based on the ensemble Monte Carlo method, we present a comparative study of the electron mobility of two-dimensional electron gases (2DEGs) formed in AlGaN/GaN abrupt-interface heterostructures (ABHs) and step-graded heterostructures (SGHs) at room temperature. We find that the electron mobility in SGHs is obviously higher than that in ABHs. The dependence of electron mobilities on the AlGaN barrier thickness is found to have a close relationship with the dislocation scatterings of electrons. On the other hand, our calculated results show that the mobility difference between SGHs and ABHs generally increases with AlGaN barrier thickness, which means that SGHs with a thicker barrier layer play a more prominent role in obtaining high mobility, compared with ABH counterparts.
V-defects are morphological defects that typically form on threading dislocations during epitaxial growth of ( 0001 )-oriented GaN layers. A V-defect is a hexagonal pyramid-shaped depression with six { 10 1 ¯ 1 }-oriented sidewalls. These semipolar sidewalls have a lower polarization barrier than the polarization barriers present between the polar c-plane quantum wells and quantum barriers and can laterally inject carriers directly into quantum wells in GaN-based light emitting diodes (LEDs). This is especially important, as the high polarization field in c-plane GaN is a significant factor in the high forward voltage of GaN LEDs. The optimal V-defect density for efficient lateral carrier injection in a GaN LED (∼10⁹ cm⁻²) is typically an order of magnitude higher than the threading dislocation density of GaN grown on patterned sapphire substrates (∼10⁸ cm⁻²). Pure-edge dislocation loops have been known to exist in GaN, and their formation into large V-defects via low-temperature growth with high Si-doping has recently been studied. Here, we develop a method for pure-edge threading dislocation half-loop formation and density control via disilane flow, growth temperature, and thickness of the half-loop generation layer. We also develop a method of forming the threading dislocation half-loops into V-defects of comparable size to those originating from substrate threading dislocations.
In the manufacture of semiconductor devices, cracking of heterostructures has been recognized as a major obstacle for their post-growth processing. In this work, we explore cracked GaN/AlN multi-quantum wells (MQWs) to study the influence of pressure on the recombination energy of the photoluminescence (PL) from the polar GaN QWs. We grow GaN/AlN MQWs on a GaN(0001)/sapphire template, which provides 2.4% tensile strain for epitaxial AlN. This strain relaxes through the generation and propagation of cracks, resulting in a final inhomogeneous distribution of stress throughout the film. The crack-induced strain variation investigated by micro-Raman spectroscopy and X-ray diffraction mapping revealed a correlation between the spacing of the cracks and the amount of strain between them. We have developed a 2D model that allows us to calculate the spatial variation of the in-plane strain in the GaN and AlN layers. The measured values of compressive in-plane strain in the GaN QWs vary from -0.4 % away from cracks, to -0.7 % near cracks. PL from the GaN QWs exhibits a clear correlation to the varying strain resulting in an energy shift of ∼ 140 meV. As a result, we can experimentally calculate a pressure coefficient of PL energy of ∼ -60.4 meV/GPa for the ∼ 7 nm thick polar GaN QWs. This agrees well with the previously predicted theoretical results by Kaminska et al. in 2016 [DOI: 10.1063/1.4962282], which were demonstrated to break down for such wide QWs. We will discuss this difference with respect to the reduction in both the expected point defects and extended defects resulting from not doping and growth on a GaN template, respectively. As a result, our work indicates that cracks can be utilized for investigating some fundamental material properties related to strain effects.
Aluminum scandium nitride barrier layers increase the available sheet charge carrier density in gallium nitride‐based high‐electron‐mobility transistors and boost the output power of high‐frequency amplifiers and high voltage switches. Growth of AlScN by metal‐organic chemical vapor deposition is challenging due to the low vapor pressure of the conventional Sc precursor Cp3Sc, which induces low growth rates of AlScN and leads to thermally‐induced AlScN/GaN‐interface degradation. In this work, novel Sc precursors are employed to reduce the thermal budget by increasing the growth rate of the AlScN layer. The AlScN/GaN interfaces are investigated by high‐resolution X‐ray diffraction, high‐resolution transmission electron microscopy, time‐of‐flight secondary ion mass spectrometry, capacitance–voltage, current–voltage and temperature‐dependent Hall measurements. Linearly graded interlayers with strain‐induced stacking faults, edge, and screw dislocations form at the AlScN/GaN interface at growth rates of 0.015 nms⁻¹. Growth rates of 0.034 nms⁻¹ and higher allow for abrupt interfaces, but a compositional grading in the barrier remains. Homogeneous barrier layers can be achieved at growth rates of 0.067 nms⁻¹ or by growing an AlN interlayer. The electrical properties of the heterostructures are sensitive to Sc accumulations at the cap/barrier interface, residual impurities from precursor synthesis, and surface roughness. This study paves the way for high‐performing devices.
GaN/AlGaN core-shell nanowires with various Al compositions have been grown on GaN nanowire array using selective area metal organic chemical vapor deposition technique. Growth of the AlGaN shell using pure N2 carrier gas resulted in a smooth surface for the nonpolar m-plane sidewalls with superior optical properties, whereas, growth using a mixed N2/H2 carrier gas resulted in a striated surface similar to the commonly observed morphology in the growth of nonpolar III-nitrides. The Al compositions in the AlGaN shells are found to be less than the gas phase input ratio. The systematic reduction in efficiency of Al incorporation in the AlGaN shells with increasing the Al molar flow in the gas phase is attributed to geometric loss, strain-limited Al incorporation, and increased gas phase parasitic reactions. Defect-related luminescence has been observed for AlGaN shells with Al content ≥ 30% and the origin of the defect luminescence has been determined as the (VIII-2ON)1- complex. Microstructural analysis of the AlGaN shells revealed that the dominant defects are partial dislocations. Growth of the nonpolar m-plane AlxGa1-xN/AlyGa1-yN quantum wells on the sidewalls of the GaN nanowires produced arrays with excellent morphology and optical emission, which demonstrated the viability of such a growth scheme for large area efficient ultraviolet LEDs as well as for next generation ultraviolet micro-LEDs.
We demonstrated high-electron-mobility transistors (HEMTs) with enhanced 2-dimentional electron gas (2DEG) mobility using a low-strain AlGaN barrier grown by metalorganic vapor phase epitaxy under nitrogen atmosphere. We investigated the effects of the growth temperature under nitrogen atmosphere on the electrical properties of AlGaN-HEMT structures, focusing on 2DEG mobility. At growth temperatures below 855°C, the 2DEG mobility decreased with decreasing growth temperature owing to an increase in the threading dislocation density. However, at growth temperatures above 855°C, the 2DEG mobility decreased with increasing growth temperature. This finding was attributed to the compressive strain in the GaN channel, which increased with increasing growth temperature owing to the increased tensile strain in the AlGaN barriers. We concluded that temperatures around 855°C are suitable for AlGaN barrier growth under nitrogen atmosphere. Finally, we achieved the highest 2DEG mobility of 2182 cm2V−1s−1 with a low sheet resistance of 406 Ω/sq. using an Al0.41Ga0.59N barrier.
Dans un micro-écran, chaque pixel est composé de trois diodes électroluminescentes (LEDs) émettant respectivement dans le bleu, le vert et le rouge. Pour les applications de réalité augmentée et de réalité virtuelle auxquelles sont destinées ces technologies d’affichage, les tailles de ces LEDs nécessitent d’être réduites à moins d’une dizaine de μm, ce qui restreint l’utilisation commune de plusieurs familles de matériau. Une approche monolithique est ainsi nécessaire. Les LEDs à base de nitrures d’éléments III pourraient théoriquement couvrir tout le spectre visible mais leur efficacité chute au-delà de 460 nm. Afin d’obtenir des LEDs efficaces à base de (Ga,In)N émettant à grande longueur d’onde, l’un des points clefs est l’augmentation de la concentration en In des puits quantiques à base d’In- GaN, tout en gardant une bonne qualité cristalline. Une des solutions envisagées, et probablement la plus efficace, est de disposer d’un substrat InGaN relaxé, c’est-à-dire un substrat plus en accord de maille avec l’InGaN de forte composition constituant les puits quantiques de la zone active. Ce désaccord de maille est en effet à l’origine d’une forte contrainte compressive dans ceux-ci, dont les répercussions sur le rendement radiatif et le taux d’incorporation d’indium sont préjudiciables. Ces travaux de thèse proposent d’explorer, en détail, les caractéristiques des structures émettrices à grande longueur d’onde à base d’InGaN crues sur ces pseudo-substrats InGaN relaxés et la possibilité de fabrication et d’amélioration de tels substrats. Sur les pseudo-substrats InGaN appelés InGaNOS fabriqués par Soitec, une structure adaptée à ce type de substrat permet d’émettre à une longueur d’onde de 624 nm, avec une efficacité quantique interne (IQE) optique de 6.5%, à température ambiante. L’estimation par des cartographies de déformation d’une teneur en indium de 39% dans ces puits quantiques à base d’InGaN, soit au-delà de la limite théorique de 25% sur GaN, est une première et atteste la pertinence de notre approche. Néanmoins, l’émission des puits quantiques demeure inhomogène et une couche d’InGaN donneur à forte teneur en indium a été développée en vue de la fabrication de pseudo-substrats InGaN de meilleure qualité cristalline et avec un plus grand paramètre de maille a. Finalement, un pseudo-substrat InGaN relaxé a été conçu. La couche d’InGaN relaxée en surface dispose d’un paramètre de maille de 3.209°A, obtenue au moyen d’un procédé de relaxation en trois étapes : la structuration des échantillons en mésas, la porosification du n-GaN sous-jacent et un recuit à haute température.
We demonstrated low-sheet-resistance AlGaN high electron mobility transistors (HEMTs) using a strain-controlled high-Al-composition AlGaN barrier grown by MOVPE. We systematically investigated the effects of the AlGaN-barrier’s growth conditions on the electrical characteristics. We found that growing an AlGaN barrier with nitrogen as a carrier gas increases dislocation density in the AlGaN barrier, resulting in reduction of strain in the AlGaN barrier and the top of the GaN channel. Reduction strain in the GaN channel can improve electron mobility. Moreover, dislocation and impurity scattering are prevented using an AlN spacer. Finally, we achieved low-sheet-resistance HEMT structures with a high Al composition of over 0.60. The lowest sheet resistance is 211 Ω/sq. with electron mobility of 1820 cm²V⁻¹s⁻¹ using an Al0.68Ga0.32N barrier.
c-oriented GaN micropillars created from single crystals containing ∼10³ or ∼10⁶ dislocations/cm² and a thick heteroepitaxially grown film containing ∼10⁹ were compressed to study methods to accommodate strain during heteroepitaxial growth. The yield stress in the 10³ samples was found to be the highest, and it was the lowest in the 10⁹ samples. The 10³ and 10⁶ pillars often failed catastrophically but the 10⁹ pillars almost never did. This was linked to the high stresses required to generate sufficient pyramidal dislocations to accommodate plastic strain and dislocation interactions, which precipitated axial fracture. Transmission electron microscopy analysis shows categorically that the first formed dislocations are ⅓ ⟨ 11 23 _ ⟩{1122} dislocations, and that a few ⅓ ⟨ 11 23 _ ⟩{0111} dislocations found were formed by a cross slip in the vicinity of where the former dislocations interacted. When compared with the similar stress patterns created in the heteroepitaxial growth of AlGaN films on GaN substrates, the analysis suggests that there is no pathway for creating basal plane dislocations during growth from the pyramidal dislocations, which require high applied stresses; the basal plane dislocations would provide relief for the mismatch strain while not penetrating the region where active devices are fabricated in the film. Rather, it will be necessary to find a method for creating shear stress in the basal plane during growth to form them directly.
A fundamental obstacle toward the realization of GaN p-channel transistors is its low hole mobility. Here we investigate the intrinsic phonon-limited mobility of electrons and holes in wurtzite GaN using the ab initio Boltzmann transport formalism, including all electron-phonon scattering processes and many-body quasiparticle band structures. We predict that the hole mobility can be increased by reversing the sign of the crystal-field splitting in such a way as to lift the split-off hole states above the light and heavy holes. We find that a 2% biaxial tensile strain can increase the hole mobility by 230%, up to a theoretical Hall mobility of 120 cm2/V s at room temperature and 620 cm2/V s at 100 K.
Nitride semiconductors are ubiquitous in optoelectronic devices such as LEDs and Blu-Ray optical disks. A major limitation for further adoption of GaN in power electronics is its low hole mobility. In order to address this challenge, here we investigate the phonon-limited mobility of wurtzite GaN using the ab initio Boltzmann transport formalism, including all electron-phonon scattering processes, spin-orbit coupling, and many-body quasiparticle band structures. We demonstrate that the mobility is dominated by acoustic deformation-potential scattering, and we predict that the hole mobility can significantly be increased by lifting the split-off hole states above the light and heavy holes. This can be achieved by reversing the sign of the crystal-field splitting via strain or via coherent excitation of the A1 optical phonon through ultrafast infrared optical pulses.
Nitride semiconductors are ubiquitous in optoelectronic devices such as LEDs and Blu-Ray optical disks. A major limitation for further adoption of GaN in power electronics is its low hole mobility. In order to address this challenge, here we investigate the phonon-limited mobility of wurtzite GaN using the ab initio Boltzmann transport formalism, including all electron-phonon scattering processes, spin-orbit coupling, and many-body quasiparticle band structures. We demonstrate that the mobility is dominated by acoustic deformation-potential scattering, and we predict that the hole mobility can significantly be increased by lifting the split-off hole states above the light and heavy holes. This can be achieved by reversing the sign of the crystal-field splitting via strain or via coherent excitation the A optical phonon through ultrafast infrared optical pulses.
Large mismatch of lattice constants and thermal expansion coefficients between materials in AlGaN/GaN/Si heterostructures lead to high density of structural defects and material inhomogeneities in the epitaxial layers. The influence of growth conditions on the local electronic properties of the fabricated AlGaN/GaN/Si heterostructures was investigated using scanning capacitance microscopy (SCM). The significant differences of spatial electronic properties in the nanometer scale for the heterostructures grown with and without application of SiN nanomasking layer were revealed. Analysis of two dimensional images of the SCM signal and dC/dVAmp = f(VDC) spectrum allowed to conclude that negative charge accumulated at dislocations in the epitaxial layers could affect the surface electronic states of the AlGaN barrier layer but could not have major impact on the two dimensional electron gas formation at the AlGaN/GaN interface.
Les dispositifs à base de GaN et ses alliages sont de plus en plus présents dans notre quotidien avec le développement exponentiel des diodes électroluminescentes (LED). Bien que la majorité des productions commerciales soient pour le moment effectuées sur substrat saphir, le silicium, disponible en de plus grands diamètres et pour un coût moindre, est de plus en plus pressenti comme le substrat d’avenir pour le développement des technologies GaN. L’utilisation de ce substrat devrait aussi permettre le développement du marché de l’électronique de puissance du GaN basée sur les transistors à haute mobilité électronique (HEMT) dont les performances dépassent les limites des technologies silicium. Néanmoins, afin de permettre ou faciliter le développement de dispositifs avancés, certaines briques technologiques sont nécessaires comme le dopage par implantation ionique. L’utilisation du GaN soulève des problématiques nouvelles pour ces briques technologiques.Au cours de cette thèse nous avons donc cherché à implémenter le procédé de dopage par implantation ionique du GaN et son étude au sein du CEA-LETI en nous focalisant principalement sur le dopage p par implantation de Mg. Nous avons identifié les principales problématiques liées aux propriétés intrinsèques du matériau (difficulté du dopage p, instabilité à haute température…) et les solutions les plus prometteuses de la littérature. Nous avons ensuite cherché à mettre en place notre propre procédé en développant des couches de protection déposées in-situ pour permettre les traitements thermiques à haute température des couches implantées. Cela a rendu possible l’étude des cinétiques d’évolution des couches implantées pendant des recuits « conventionnels » (rampes < 10 °C/min, durée de plusieurs dizaines de minutes, T < 1100 °C) en utilisant notamment des caractérisations de photoluminescence (µ-PL) et de diffraction des rayons X (XRD). Nous avons aussi mis en évidence un effet de diffusion et d’agrégation à haute température du Mg implanté. Nous avons ensuite cherché à modifier le procédé d’implantation (implantation canalisée, co-implantation) pour favoriser l’intégration du dopant et limiter la formation de défauts. En parallèle nous avons évalué l’intérêt de recuits secondaires (recuits rapides (RTA), recuit laser, micro-ondes) afin de finaliser l’activation du dopant. Finalement nous avons aussi mis en place un procédé de caractérisation électrique de couche de GaN dopées au sein du laboratoire.
In this paper, we study the plastic relaxation of InGaN layers deposited on (0001) GaN bulk substrates and (0001) GaN/sapphire templates by molecular beam epitaxy. We demonstrate that the InGaN layers relax by the formation of (a+c)-type misfit dislocations gliding on pyramidal planes in the slip system ⟨112¯3⟩{112¯2} down to the interface where they form a trigonal dislocation network. Combining diffraction contrast and large angle convergent beam electron diffraction analyses performed using a transmission electron microscope, we reveal that all (a+c)-type dislocations belonging to one subset of the network exhibit Burgers vectors with the same c-component. This relaxation mechanism leads to partially relaxed InGaN layers with smooth surfaces and threading dislocation densities below 10⁹ cm⁻². Such layers are of potential interest as pseudo-substrates for the growth of InGaN heterostructures.
In this work, we have investigated the strain relaxation of InGaN layers grown on GaN templates by MOVPE and PAMBE using TEM. To this end we varied the indium composition from 4.1% to pure indium nitride and the corresponding mismatch was changing from less than 1% to 11.3%, the thickness of the InGaN layers was from 7 nm to 500 nm. When the indium composition is around 10%, one would expect mostly elastically strained layers with no misfit dislocations. However, we found that screw dislocations form systematically at the InGaN/GaN interface. Moreover, below 18% indium composition, screw and edge dislocations coexist, whereas starting at 18%, only edge dislocations were observed in these interfaces. Apart from the edge dislocations (misfit dislocations), other mechanisms have been pointed out for the strain relaxation. It is found that above an indium composition beyond 25%, many phenomena take place simultaneously. (1) Formation of the misfit dislocations at the heterointerface; (2) composition pulling with the surface layer being richer in indium in comparison to the interfacial layer; (3) disruption of the growth sequence through the formation of a random stacking sequence; (4) three dimentional (3D) growth which can even lead to porous layers when the indium composition is between 40% and 85%. However, pure InN is grown, the crystalline quality improves through a systematic formation of a 3D layer.
We demonstrated low-sheet-resistance metalorganic-vapor-phase-epitaxy-grown InAlN high-electron-mobility transistors using AlGaN spacers with excellent surface morphology. We systematically investigated the effects of AlGaN spacer growth conditions on surface morphology and electron mobility. We found that the surface morphology of InAlN barriers depends on that of AlGaN spacers. Ga desorption from AlGaN spacers was suppressed by increasing the trimethylaluminum (TMA) supply rate, resulting in the small surface roughnesses of InAlN barriers and AlGaN spacers. Moreover, we found that an increase in the NH3 supply rate also improved the surface morphologies of InAlN barriers and AlGaN spacers as long as the TMA supply rate was high enough to suppress the degradation of GaN channels. Finally, we realized a low sheet resistance of 185.5 Ω/sq with a high electron mobility of 1210 cm² V⁻¹ s⁻¹ by improving the surface morphologies of AlGaN spacers and InAlN barriers.
GaN micro-pyramids with AlGaN capping layer are grown by selective metal–organic–vapor phase epitaxy (MOVPE). Compared with bare GaN micro-pyramids, AlGaN/GaN micro-pyramids show wrinkling morphologies at the bottom of the structure. The formation of those special morphologies is associated with the spontaneously formed AlGaN polycrystalline particles on the dielectric mask, owing to the much higher bond energy of Al–N than that of Ga–N. When the sizes of the polycrystalline particles are larger than 50 nm, the uniform source supply behavior is disturbed, thereby leading to unsymmetrical surface morphology. Analysis reveals that the scale of surface wrinkling is related to the migration length of Ga adatoms along the AlGaN facet. The migration properties of Al and Ga further affect the distribution of Al composition along the sidewalls, characterized by the μ-PL measurement.
The surface energies and atomic structures for two nonpolar surfaces of GaN have been calculated within the local-density approximation. For the (101̅ 0) surface, which has Ga-N dimers in the surface layer, the calculated surface energy is 118 meV/Å2, and for the (112̅ 0) surface, which has Ga-N chains in the topmost layer, the energy is 123 meV/Å2. The relaxation mechanisms on both surfaces are a Ga-N bond contraction and a ∼7° buckling rehybridization in the surface layer. For the (101̅ 0) surface we find that under Ga-rich conditions a nonstoichiometric surface having Ga-Ga dimers is stable with respect to the ideal Ga-N dimer-terminated surface.
We review recent advances in our understanding of the epitaxial growth and properties of SiGe/Si heterostructures for applications in high-speed field-effect transistors. Improvements in computing power and experimental methods have led to new calculations and experiments that reveal the complexity of 60 degrees misfit dislocations and their interactions, which ultimately determine the characteristics of strain-relaxed Sice films serving as a buffer layer for strained-layer devices. Novel measurements of the microstructure of relaxed SiGe films are discussed. We also present recent work on the epitaxial growth of SiGe/Si heterostructures by ultra-high-vacuum chemical vapor deposition. This growth method not only provides device quality buffer layers, but abrupt, high-concentration phosphorous-doping profiles, and high-mobility S0.20Ge0.80/Ge composite hole channels have also been grown. These achievements enabled the fabrication of outstanding n- and p-channel modulation-doped field-effect transistors that show enormous promise for a variety of applications.
Strain relaxation in III–V semiconductor (001) epitaxial strained layers is considered with particular reference to the InGaAs/GaAS system. Possible mechanisms of relaxation are briefly reviewed. It is pointed out that relaxation is, and never can be, complete and that results relating to the extent of relaxation of thick layers, and the asymmetry of such relaxation in the two 〈110〉 should be interpreted with caution. Any asymmetry introduced by the use of vicinal substrates is less than the scatter among experimental measurements of relaxation.
The process of epitaxial growth of a very thin layer onto a substrate crystal is considered for the particular situation in which the layer and substrate materials have the same crystal structure and orientation but different lattice parameters. Under these conditions, the layer grows with an intrinsic elastic strain determined by the mismatch in lattice parameters. The associated stress in the crystalline layer provides a driving force for the nucleation and motion of defects, primarily dislocations. The focus here is on the glide of a dislocation extending from the free surface of the layer to the layer-substrate interface, the so-called threading dislocation. A general definition of driving force for glide of a threading dislocation is introduced on the basis of work arguments. The definition is then applied to calculate the driving force for steady motion of an isolated threading dislocation in a strained layer, and the result includes Matthews' critical thickness concept as one of its features. Next, a kinetic equation for glide of a dislocation in semiconductor materials is proposed to estimate the glide rate of a threading dislocation in these low mobility materials. Finally, for the case of cubic materials, the general definition of driving force is applied to estimate the additional driving force on a threading dislocation due to an encounter with a dislocation on an intersecting glide plane. The results indicate that this effect is significant in blocking the glide of a threading dislocation for large mismatch strains and for layer thicknesses near the critical thickness.
This article reviews current experimental and theoretical knowledge of the relaxation of lattice-mismatch strain via misfit dislocations in heteroepitaxial semiconductor films. The energetics and kinetics of misfit dislocation nucleation, propagation, and interaction processes are described in detail. In addition, there is a brief review of the principal properties of dislocations in bulk semiconductors and an outline of existing models for strained layer stability.
Conventional mechanical property measurement techniques usually cannot be applied to ceramic thin films because of the small amount of material involved. A method is described to determine the ultimate tensile strength, fracture toughness, Weibull modulus, and surface energy of micron-sized ceramic films on substrates. This technique is based on measuring the radius of curvature of a coated substrate at room temperature, and equating the resulting calculated stress with a theoretical shear-lag stress distribution model using a force balance.
We describe a technique for measuring thin film stress using wafer curvature that is robust, compact, easy to setup, and sufficiently sensitive to serve as a routine diagnostic of semiconductor epilayer strain in real time during MBE or CVD growth. We demonstrate, using growth of SiGe alloys on Si, that the critical thickness for misfit dislocation can clearly be resolved, and that the subsequent strain relaxation kinetics during growth or post-growth annealing are readily obtained.
Heterostructures in the form of thin layers of one material grown on a substrate have been the subject of intense study for several years. In the case of semiconductor systems the aim is to grow epitaxial layers of, for example, Si1-xGex on Si, and devices based on such structures are already in use. Much is known, as is summarized in this review, about the stability of such systems against the insertion of dislocations, and about the critical thicknesses up to which strained layer structures are stable. The effects of dislocation nucleation and the dynamics of dislocation motion which lead to strain relaxation in metastable systems are also reviewed. The present state of theoretical understanding is compared with what is known experimentally. For metallic systems, which often exhibit magnetic properties, the underlying problems of lattice mismatch and strain relief are similar, but much of the interest has been concentrated on the commensurate-incommensurate transition in both structurally and magnetically modulated materials. The theory of this transition is reviewed, both for metallic systems and for epitaxial layers on graphite. In bringing together these different classes of systems within one review, it has been possible to demonstrate the parallels between them. It is hoped that, as a result, transfers of ideas between the fields will be promoted.
In situ optical reflectance transients reveal that the morphology evolution of the initial low-temperature buffer layer strongly influences the structural and electrical quality of the high-temperature GaN films. Moreover, the morphology evolution of that buffer layer, specifically evolution of the spatial and orientational distributions of the nuclei, is strongly affected by H2. The growth conditions for which surface smoothness is maintained throughout the two-step growth do not necessarily produce the best quality final GaN films; instead, there may be an optimal roughness and incubation period en route to the best quality final films.
Elastic constants for zinc-blende and wurtzite AlN, GaN, and InN are obtained from density-functional-theory calculations utilizing ab initio pseudopotentials and plane-wave expansions. Detailed comparisons are made with the available measured values and with results obtained in previous theoretical studies. These comparisons reveal clear discrepancies between the different sets of elastic constants which are further highlighted by examining derived quantities such as the perpendicular strain in a lattice-mismatched epitaxial film and the change in the wurtzite c/a ratio under hydrostatic pressure. Trends among results for the three compounds are also examined as well as differences between results for the zinc-blende and wurtzite phases. © 1997 American Institute of Physics.
The evolution of stress in gallium nitride films on sapphire has been measured in real time during metalorganic chemical vapor deposition. In spite of the 16% compressive lattice mismatch of GaN to sapphire, we find that GaN consistently grows in tension at 1050 °C. Furthermore, in situ stress monitoring indicates that there is no measurable relaxation of the tensile growth stress during annealing or thermal cycling. © 1999 American Institute of Physics.
We have studied the microstructure of InGaN layers grown on two different GaN substrates: a standard GaN film on sapphire and an epitaxial lateral overgrown GaN (ELOG) structure. These two materials exhibit two distinct mechanisms of strain relaxation. InGaN epilayers on GaN are typically pseudomorphic and undergo elastic relaxation by the opening of threading dislocations into pyramidal pits. A different behavior occurs in the case of epitaxy on ELOG where, in the absence of threading dislocations, slip occurs with the formation of periodic arrays of misfit dislocations. Potential slip systems responsible for this behavior have been analyzed using the Matthews-Blakeslee model and taking into account the Peierls forces. This letter presents a comprehensive analysis of slip systems in the wurtzite structure and considers the role of threading dislocations in strain relaxation in InGaN alloys. © 2003 American Institute of Physics.
We have directly measured the stress evolution during metal-organic chemical vapor deposition of AlGaN/GaN heterostructures on sapphire. In situ stress measurements were correlated with ex situ microstructural analysis to determine directly a critical thickness for cracking and the subsequent relaxation kinetics of tensile-strained AlxGa1−xN grown on GaN. Cracks appear to initiate the formation of misfit dislocations at the AlGaN/GaN interface, which account for the majority of the strain relaxation. © 2000 American Institute of Physics.
Multilayers composed of many thin films of GaAs and GaAs0·5P0·5 were grown epitaxially on GaAs surfaces inclined at a few degrees to (001). Examination of the multilayers by transmission and scanning electron microscopy has revealed that the interfaces between layers were made up of large coherent areas separated by long straight misfit dislocations. The Burgers vectors of the dislocations were inclined at 45° to (001) and were of type 1/2a <110>. Dislocations in adjacent interfaces were usually not independent of one another. They often lay on the same slip plane and when this was so they were clearly products of the same source. The layer thickness at which misfit dislocations were formed was in satisfactory agreement with the predicted thickness. However, the fraction of the total misfit accommodated by dislocations (once the critical thickness for dislocation generation was passed) was much smaller than predicted. This large discrepancy seems to arise from difficulties associated with the creation of misfit dislocations. Although there are many processes which can impede dislocation generation, the most important one in GaAs/GaAs0·5P0·5 multilayers appears to be the impaction of dislocations on one glide plane against dislocations in another.
Publisher Summary This chapter describes the mixed mode cracking in layered materials. There is ample experimental evidence that cracks in brittle, isotropic, homogeneous materials propagate such that pure mode I conditions are maintained at the crack tip. An unloaded crack subsequently subject to a combination of modes I and II will initiate growth by kinking in such a direction that the advancing tip is in mode I. The chapter also elaborates some of the basic results on the characterization of crack tip fields and on the specification of interface toughness. The competition between crack advance within the interface and kinking out of the interface depends on the relative toughness of the interface to that of the adjoining material. The interface stress intensity factors play precisely the same role as their counterparts in elastic fracture mechanics for homogeneous, isotropic solids. When an interface between a bimaterial system is actually a very thin layer of a third phase, the details of the cracking morphology in the thin interface layer can also play a role in determining the mixed mode toughness. The elasticity solutions for cracks in multilayers are also elaborated.
Two main assumptions which underlie the Stoney formula relating substrate curvature to mismatch strain in a bonded thin film are that the film is very thin compared to the substrate, and the deformations are infinitesimally small. Expressions for the curvature-strain relationship are derived for cases in which these assumptions are relaxed, thereby providing a basis for interpretation of experimental observations for a broader class of film-substrate configurations. (C) 1999 American Institute of Physics. [S0003-6951(99)04510-6].
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