Density Of Misfit Dislocations Research Articles

Design engineering of non-linear graded InAlAs metamorphic buffer layers for efficient reduction of misfit dislocation density

Metamorphic InxAl1-xAs (xmax > 0.75) buffer layer (MBL) grown on GaAs with an optimized non-linear graded composition profile along the growth direction is proposed for enhanced reduction of misfit dislocation (MD) density in highly mismatched III-V/GaAs metamorphic heterostructures. The equilibrium distributions of MD density throughout such InxAl1-xAs/GaAs MBLs are calculated. The influence of the initial composition (xmin) of MBL and the elastically strained thin GaAs layer embedded into the InxAl1-xAs MBL on the MD density distribution was studied. An optimum value of the inverse step (Δ), representing the difference between the top In content of the InxAl1-xAs MBL and that of a In0.75Al0.25As virtual substrate (VS) grown atop is determined. It was theoretically shown that Δ above 0.04 results in relaxation of the elastic stresses in VS via formation of the MDs, while the lower Δ values allow growing the VS completely free of MDs. Finally, the metamorphic In0.75Al0.25As/graded-InAlAs/GaAs heterostructures differing from each other by only the composition profile of the graded-InAlAs MBL were grown by MBE on GaAs(001) and studied by atomic force microscopy. The structure with proposed optimized non-linear graded (x = 0.05–0.77) MBL demonstrated the ∼ 1.5 times less MD density as compared to that with the convex-graded (x = 0.05–0.81) MBL, which was estimated to be at the level of ∼ 2·107 cm−2.

Strain accumulation and relaxation on crack formation in epitaxial AlN film on Si (111) substrate

The formation of cracks is often observed in the epitaxial growth of ultrawide-bandgap aluminum nitride (AlN) semiconductor films on economical and versatile silicon (Si) substrates due to the significant differences in in-plane lattice parameters and thermal expansion coefficients between the film and the substrate, which hampers the development of template, buffer layer, and device structure with a relatively thick AlN layer for devices. The present study aims to elucidate the conditions of crack formation through a simple but comprehensive estimation of strain energy accumulation and relaxation by lattice strain, misfit dislocation density, and crack formation. Strain energy in the epitaxial film from lattice and thermal mismatches is evaluated by an elastic strain equation tailored to the epitaxy of the hexagonal crystal structure. The effects of temperature, thickness, and dislocation density on the lattice and dislocation strain energies of the film are also considered. Finally, the comparison in the changes in the total strain energy and cleavage energy with decreasing temperature shows that cleavage energy is higher than strain energy if the film is thinner than 400 nm but becomes lower than the strain energy if the film is thicker than 400 nm during cooldown, suggesting the crack formation, which matches well with experimental observations.

Designing L21-Co2TiAl precipitate-strengthened ferritic alloys with excellent high-temperature mechanical properties

Recently, Fe–Cr–Ni–Al–Ti ferritic alloys strengthened by L21-type Ni2TiAl precipitates have demonstrated superior high-temperature strength compared to conventional ferritic steels. However, it still shows a lower high-temperature strength than that of Ni-based superalloys. Therefore, in the current study, we designed new ferritic alloys to improve the high-temperature strength by introducing L21-Co2TiAl precipitates, known as having better high-temperature properties than the Ni2TiAl structure. Specifically, the Ni element has been replaced with the Co element. In addition, the Ti element has been adjusted because the structural evolutions of precipitates, such as composition, lattice misfit and size rely on the Ti content. For instance, the addition of 2 wt% Ti forms a semi-coherent Co2TiAl precipitate with a relatively low density of misfit dislocations. More Ti addition up to 6 wt % leads to an increase in the lattice parameter of the precipitate, while that of the Fe matrix remains unchanged, which results in an increased lattice misfit. The increased lattice misfit causes a higher amount of misfit dislocations, which leads to a reduction of the elastic strain field around the precipitates. It was found that the semi-coherent interface with a high level of the elastic strain field (2 wt % Ti), as compared to the precipitates with a high amount of misfit dislocations (4 and 6 wt % Ti), plays a crucial role in enhancing the mechanical properties at 973 K. In addition, the designed ferritic alloys show superior strength, compared to the previously-reported ferritic alloys.

Remote plasma-enhanced chemical vapor deposition of GeSn on Si: Material and defect characterization

Germanium–tin (GeSn) alloys at sufficiently high Sn concentration, above several atomic percent, are the only group IV semiconductor exhibiting a direct bandgap and have generated much recent interest for optoelectronic applications into the mid-infrared region. Because the large lattice mismatch between GeSn and Si results in considerable strain for thin layers and a high defect density for thicker strain-relaxed layers, most reported GeSn growths incorporate a Ge buffer layer rather than depositing directly on Si substrates. Published reports of GeSn growth directly on Si utilize specialized precursors such as higher order germanes (Ge2H6, Ge3H8, or Ge4H10) or SnD4. In this paper, we report GeSn films with up to 10.6% Sn grown directly on Si substrates by remote plasma-enhanced chemical vapor deposition using GeH4 and SnCl4 precursors. These alloys have been characterized in detail using x-ray diffraction (XRD), transmission electron microscopy (TEM), and Rutherford backscattering spectrometry with channeling (RBS-C), as well as Raman spectroscopy (RS) and optical microscopy. The films studied are almost fully relaxed, with small residual strain observed, particularly in thinner films, and contain a high interface density of misfit dislocations that increases with Sn concentration. The defect density decreases toward the surface. Good agreement is found between the various characterization methods for the Sn content (XRD and RBS-C), lattice parameter measurement (XRD and TEM), and defect characterization (RBS-C, TEM, and RS). Such characterization of GeSn grown directly on Si substrates is essential to allow growth parameters to be optimized for the realization of the attractive optoelectronic properties of these alloys.

Improved wavelength stability and heat dissipation of InGaN-based light-emitting diodes using a graphene interlayer on patterned sapphire substrate

This study presents a straightforward strategy that embeds a graphene interlayer between InGaN-based light-emitting diodes (InGaN LEDs) and patterned sapphire substrate (PSS substrate) for substantial improving device performances of wavelength stability and heat dissipation. The InGaN LEDs on the graphene interlayer/PSS substrate (Gr-LED) have lower piezoelectric fields of 1.16 MV/cm than 1.60 MV/cm for InGaN LEDs on the PSS substrate (Ref-LED). The low piezoelectric field diminishes the screen of the polarization field resulting in a stable electroluminescence peak wavelength. At 100 mA driving current, the chip temperature of Gr-LED shows a decrease in around 24.4 °C relative to Ref-LED. The thermal resistances measured in a vacuum environment are 351 and 128 °C/W for Ref-LED and Gr-LED, respectively. The low thermal resistance of Gr-LED is believed to be due to a low misfit dislocation density of the aluminum nitride nucleation layer that increases the vertical direction of heat transport to PSS. This work demonstrates that the graphene/PSS substrate is a promising substrate for high-power InGaN LEDs.

Reduction of Misfit Dislocation Density in Metamorphic Heterostructures by Design Optimization of the Buffer Layer with Non-Linear Graded Composition Profile

Reduction of Misfit Dislocation Density in Metamorphic Heterostructures by Design Optimization of the Buffer Layer with Non-Linear Graded Composition Profile

Reduction of misfit dislocation density in metamorphic heterostructures by design optimization of the buffer layer with non-linear graded composition profile

Equilibrium distributions of misfit dislocation density along the growth direction of metamorphic buffer layers InxAl1-xAs/GaAs with maximum In content xmax≥0.77 and different non-linear graded composition profiles x propto z1/n are calculated. The effect of the initial In composition (n=2) of InxAl1-xAs buffer layer with convex-graded (xmin) composition profile on misfit dislocation density as well as amount of residual stresses at its top part is considered. Using computational approach, it was shown that a dislocation-free region is formed under thin tensile-strained GaAs layer (1-10 nm) inserted into InAlAs metamorphic buffer layer, which agrees with experimental data obtained early by transmission electron microscopy. Novel non-linear graded composition profile of metamorphic buffer layer has been proposed, which results in twice reduction of misfit dislocation density as compared to the convex-graded one. In addition, equilibrium distributions of misfit dislocation density in the HEMT heterostructures with two-dimensional electron channel InxAl1-xAs, which are based on In0.75Ga0.25As/In0.75Al0.25As metamorphic buffer layer of various designs, are calculated. The values of inverse steps (Delta), representing the difference between the maximum In content of InxAl1-xAs (xmax) and In content of In0.75Al0.25As virtual substrate, at which relaxation of the elastic strains in 2D channel In0.75Ga0.25As/In0.75Al0.25As doesn't occur, are calculated for metamorphic buffer layers InxAl1-xAs with convex-graded and optimized non-linear graded composition profiles. Keywords: metamorphic heterostructures, metamorphic buffer layer, misfit dislocations, elastic stresses, In(Ga,Al)As/GaAs.

Снижение плотности дислокаций в метаморфных гетероструктурах путем оптимизации конструкции буферного слоя с нелинейным профилем изменением состава

Equilibrium distributions of misfit dislocation density along the growth direction of metamorphic buffer layers InxAl1-xAs/GaAs with maximum In content xmax ≥ 0.77 and different non-linear graded composition profiles (x ∝ z1/n) are calculated. The effect of the initial In composition (xmin) of InxAl1-xAs buffer layer with convex-graded (n = 2) composition profile on misfit dislocation density as well as amount of residual stresses at its top part is considered. Using computational approach, it was shown that a dislocation-free region is formed under thin tensile-strained GaAs layer (1–10 nm) inserted into InAlAs metamorphic buffer layer, which agrees with experimental data obtained early by transmission electron microscopy. Novel non-linear graded composition profile of metamorphic buffer layer has been proposed, which results in twice reduction of misfit dislocation density as compared to the convex-graded one. In addition, equilibrium distributions of misfit dislocation density in the HEMT heterostructures with two-dimensional electron channel In0.75Ga0.25As/In0.75Al0.25As, which are based on InxAl1-xAs/GaAs metamorphic buffer layer of various designs, are calculated. The values of inverse steps (∆), representing the difference between the maximum In content of InxAl1-xAs (xmax) and In content of In0.75Al0.25As virtual substrate, at which relaxation of the elastic strains in 2D channel In0.75Ga0.25As/In0.75Al0.25As doesn’t occur, are calculated for metamorphic buffer layers InxAl1-xAs with convex-graded and optimized non-linear graded composition profiles.

Direct Observation of Carrier Transportation between Localized States in InGaN Quantum Wells

Despite the large misfit dislocation densities, indium gallium nitride (InGaN) demonstrates high luminous efficiency both for electroluminescence and photoluminescence. The mechanism behind it has been interpreted as the existence of potential minima (i.e., localized states), which will screen the non-radiative recombination centers to avoid carriers being trapped by the defects. The existence of localized states has been testified by many experiments. However, almost all of the observations are indirect observations, and some experiments, such as those focused on whether the indium clusters observed by transmission electron microscopy are localized states, still remain controversial. Here, we report the direct observation of carrier transportation between localized states driven by temperature-dependent photoluminescence (TDPL) and excitation power-dependent PL in InGaN quantum wells. This enriches the experimental evidence on the existence of localized states.

Influence of growth temperature and miscut angle of m-plane sapphire substrate on the semi-polar (11–22) AlN film grown by HVPE

The high-quality semi-polar (11-22) AlN thin films were grown on m-plane sapphire substrates by hydride vapor phase epitaxy (HVPE). The surface morphology and crystalline quality of the AlN film were greatly influenced by the growth temperature and the substrate miscut angle. As the temperature increased, the grain size on the surface increased and the grain density decreased. In addition, the higher growth temperature also resulted in smaller values of the full width at half maximum (FWHM) of X-ray rocking curves (XRC) when temperature was more than 1,460 °C. At high temperature of 1,530 °C, the introduction of 1° -off miscut angle to the substrate resulted in smooth surface, low density of stacking faults and low FWHM of XRC. The misfit dislocation density was calculated from the tilt angle of epilayer measured by X-ray reciprocal space mappings along [−1−123] AlN. The misfit dislocation density of the sample grown on 1° -off substrates was 6.7 × 105 cm−2. The improvement of crystal quality is believed to be due to the enhancement of adatom mobility at higher temperatures and also the appropriate miscut variation.

On the origin of twist in 3D nucleation islands of tetrahedrally coordinated semiconductors heteroepitaxially grown along hexagonal orientations

In the first part of this paper, we present a model that explains and determines quantitatively the twists between nucleation islands in the case of a Volmer–Weber heteroepitaxial growth of tetrahedrally coordinated semiconductors along hexagonal orientations. These twists are caused by the network of the screw components of the 60° misfit dislocations. The orientations of the screw components are distributed randomly, and the maximum twist is obtained when all the screw components have the same orientation. The maximum twists are related to the density of misfit dislocations and, therefore, increase with the mismatch between the deposited materials and their substrate. In the second part of the paper, we study five systems having a large distribution of mismatches from 4% to 19%. For the four systems fulfilling the conditions necessary for the application of the model (plastic relaxation of grown islands), the measured maximum twists fit with the calculated values, thereby validating the model. The twists of nucleation islands are related to the mismatch and are, therefore, intrinsic to the material systems. The defects created at the coalescence of twisted islands determine the initial microstructure/defect distribution of the nucleation layer.

Effects of interdiffusion on shear response of semi-coherent {111} interfaces in Ni/Cu

Intermixing of chemical species as a result of interdiffusion during the manufacturing of metallic nanolaminates leads to diffuse interface structures which have distinct properties compared to corresponding atomically sharp interfaces. The effect of interdiffusion-induced changes to the interface structure on interface shear strength is complex due to the presence of solute atoms and a reduced misfit dislocation density. The shear responses of {111} Cu/Ni nanolaminate interfaces with varying levels of interdiffusion are studied using atomistic methods to elucidate the effect of interface structure changes on shear deformation mechanisms. Models with diffuse interfaces exhibit improved interface shear strength relative to the atomically sharp case; however, shear strength does not increase monotonically with solute concentration. The distribution of maximum changes in energy per misfit node and non-uniform misfit node displacements, filtered using microrotation vector analysis, suggest heterogeneous interface resistance to sliding. No strong correlation is found between solute concentration near misfit node centroids and misfit node displacements, indicating the importance of the longer-range misfit dislocation structure. Increased activation of misfit dislocation glide is associated with larger solute concentrations as a result of increased misfit node displacements. Analysis of change in energy during the shear deformation process, however, reveals that interface sliding is dominated by the misfit node behavior. These findings highlight the importance of modeling realistic diffuse interface structures and emphasize the competing effects of solute concentration and interface misfit dislocation density.

Impact of Long-Term Annealing on Photoluminescence from Ge1−xSnx Alloys

We report on the connection between strain, composition, defect density and the photoluminescence observed before and after annealing at 300 °C for GeSn samples with Sn content of 8% to 10%. Results show how the composition and level of strain influenced the separation between the indirect and direct optical transitions, while changes in the level of strain also influenced the density of misfit dislocations and surface roughness. The effect of annealing is observed to lower the level of strain, decreasing the energy separation between the indirect and direct optical transitions, while also simultaneously increasing the density of misfit/threading dislocations and surface roughness. The reduction in energy separation leads to an increase of photoluminescence (PL) emission, while the increase of misfit/threading dislocations density and surface roughness results in a decrease of PL. Consequently, the competition between these factors is observed to determine the impact of annealing on the PL. As a result, annealing increases the collected PL for small (≤40 meV) separation between the indirect to direct optical transitions in the as-grown sample while decreases the PL for larger (≥60 meV) separations. More generally, these numbers have a small dependence on the level of strain in the as-grown samples.

Effect of Half-Heusler Interfacial Structure on Thermal Transport Properties of (Ti, Zr)NiSn Alloys.

The microstructure of the half-Heusler phase separation in half-Heusler (HH) MNiSn(M = Ti, Zr) intermetallic compounds has been investigated systematically in this study. Scanning electron microscopy observations from a range of (Tix, Zr1-x)NiSn have revealed the HH single phase at high temperature formed into many HH domains of various HH compositions with different Ti/Zr concentration ratios when x > 0.1. The formation of Ti-rich and Zr-rich HH domains with rather large size (up to several hundred μm in diameter) is thought to originate from a combination of the liquid solidification process and followed by an HH phase decomposition process within a miscibility gap between the TiNiSn and ZrNiSn HH phases. We have noticed that in addition to the mass and size difference based phonon scattering, sharp interfaces between the Ti-rich and Zr-rich HH domains containing high density of misfit dislocations could provide additional phonon scattering centers and reduced thermal conductivity of the alloys. Moreover, the cyclic heat treatment process at temperatures near the HH phase-decomposition's critical temperature could modify the HH domains' microstructure to become more diffuse, coherent with a more comprehensive length scale, and globular shape. These diffuse and coherent Ti-rich HH1/Zr-rich HH2 interfaces can provide an additional enhancement of phonon scattering and thereby result in a more considerable reduction of thermal conductivity than those of relatively less diffuse ones. We anticipate a similar approach of using cyclic heat treatment to modify the microstructure and consequently lead to further enhancement of phonon scattering can also apply to many other thermoelectric alloy systems possessing a miscibility gap.

Strain relaxation in semiconductor wafer bonding

The strain relaxation process in wafer-bonded semiconductor heterostructures is numerically investigated, in contrast to those formed by epitaxial growth. A kinetic model of strain relaxation in semiconductor layers is re-established for highly lattice-mismatched heterostructures. Numerical simulations are then performed by using the model to analyze the time evolution of the strain, the strain rate, and the misfit dislocation density. The calculation results present a slow strain relaxation behavior in the lattice-mismatched heterostructures wafer-bonded at lower temperatures than those for epitaxial growth, to suppress the thermodynamically preferred dislocation generation by sustaining the material system at a metastable state. The time constant of strain relaxation in a typical range of wafer bonding temperatures, normalized by the melting temperature, of 0.2–0.4 is found to be 3 × 105–2 × 1021 s for a lattice mismatch of 0.04. This relaxation time contrasts with 14 s for the case of heteroepitaxy at a typical normalized temperature of 0.6, thus evidencing the nonequilibrium crystalline stability in wafer bonding.

Enhanced fracture toughness in ceramic superlattice thin films: On the role of coherency stresses and misfit dislocations

Superlattice (SL) thin films composed of refractory ceramics unite extremely high hardness and enhanced fracture toughness; a material combination which is often mutually exclusive. While the hardness enhancement is well described by existing models based on dislocation mobility, the underlying mechanisms behind the increase in fracture toughness are yet to be unraveled. Here, we provide a model based on linear elasticity theory to predict the fracture toughness in (semi-)epitaxial nanolayers. As representative of cubic transition metal nitrides, a TiN/CrN superlattice structure on MgO (100) is studied. The density of misfit dislocations is estimated by minimizing the overall strain energy, each time a new layer is added on the nanolayered stack. The partly relaxed coherency stresses are then used to calculate the apparent fracture toughness (Kapp) by applying the weight function method. The results show that Kapp increases steeply with increasing bilayer period for very thin SLs, before the values decline more gently along with the formation of misfit dislocations. The characteristic Kapp vs. bilayer-period-dependence nicely matches experimental trends. Importantly, all critical stress intensity values of the SLs clearly exceed the intrinsic fracture toughness of the layer materials, evincing the importance of coherency stresses for increasing the crack growth resistance.

Epitaxial Growth of Active Si on Top of SiGe Etch Stop Layer in View of 3D Device Integration

We describe challenges of the epitaxial Si-cap/Si0.75Ge0.25//Si-substrate growth process, in view of its application in 3D device integration schemes using Si0.75Ge0.25 as backside etch stop layer with a focus on high throughput epi processing without compromising material quality. While fully strained Si0.75Ge0.25 with a thickness >10 times larger than the theoretical thickness for layer relaxation can be grown, it is challenging to completely avoid misfit dislocations at the wafer edge during Si-growth on top of strained Si0.75Ge0.25, even for thinner Si0.75Ge0.25 layers and when growing the Si-cap layer at a lower temperature. Extremely sensitive characterization methods are mandatory to detect the extremely low density of misfit dislocations at the wafer edge. Light scattering measurements are most reliable. The epitaxial Si-cap/Si0.75Ge0.25//Si-substrate layer stacks are stable against post-epi thermal processing steps, typically applied before wafer-to-wafer bonding and Si-substrate and Si0.75Ge0.25 backside removal.

Distribution of Misfit Dislocations and Elastic Mechanical Stresses in Metamorphic Buffer InAlAs Layers of Various Constructions

The equilibrium distributions of the misfit dislocation density ρ(z) and elastic stresses e(z) are calculated along the direction of the epitaxial growth of the metamorphic InAlAs/GaAs(001) layer with higher In content (to 87 mol %) and various profiles of varying the composition: step, linear, and root. The calculations are performed using the method based on iteration searching for the minimum total energy of the system. It is shown that the largest differences between various constructions of the buffer layer are observed in the character of distributions ρ(z), rather than e(z). Unlike the traditional constructions with a step and linear gradients of the composition, which are characterized by a quite homogeneous distribution of misfit dislocations, in a buffer layer with a root composition gradient, the main part of such dislocations is concentrated in the lower part of the layer near the heteroboundary with a GaAs substrate, and their density sharply decreases by more than one order of value along the layer thickness, achieving the value minimum for all abovementioned constructions. In spite of the fact that the important effect of interacting the dislocations to each other is not taken into account in this work, the calculations enable us to establish the main peculiarities of the distributions ρ(z) and e(z) in various metamorphic buffer InAlAs layers, which were observed experimentally before. Thus, this approach can be effectively used when designing optimal constructions of the device metamorphic heterostructures.

Распределение дислокаций несоответствия и упругих механических напряжений в метаморфных буферных слоях InAlAs разной конструкции

Here we report on computing of the distribution of the equilibrium misfit dislocation density ρ(z) as well as the elastic strain ε(z) along the grow direction for metamorphic buffer layer InAlAs/GaAs(001) with high In content (x ≤ 0.87) and different design of composition profile: step-, linear- and convex-graded. For the computation, an approach based on the iterative finding the system total energy minimum have been used. It was shown, that the significant difference between different types of the buffer layer is observed for the ρ(z) distribution rather than for ε(z). In contrast to traditionally used step- and linear-graded metamorphic buffer layers, which are characterized by homogenous spreading of misfit dislocations, the main part of such dislocations in the convex-graded composition profile is concentrated at the bottom part of the buffer layer near to heterointerface InAlAs/GaAs, and the dislocation density drop by more than one order of magnitude along the layer thickness reaching near the surface the minimal value among the buffer types. Despite the fact, that the significant effect of interaction between misfit dislocations is not taken into account in the computation, the results obtained allowed one to determine the main features of the ρ(z) and ε(z) distributions in the different InAlAs metamorphic buffer layers, which were previously obtained experimentally. Thus, such an approach can be effectively utilized for the development of the metamorphic heterostructure based devices.

Epitaxial Growth of Active Si on Top of SiGe Etch Stop Layer in View of 3D Device Integration

We describe challenges of the epitaxial Si-cap / Si0.75Ge0.25 // Si-substrate growth process, in view of its application in 3D device integration schemes using Si0.75Ge0.25 as backside etch stop layer with a focus on high throughput epi processing without compromising material quality. While fully strained Si0.75Ge0.25 with a thickness >10 times larger than the theoretical thickness for layer relaxation can be grown, it is challenging to completely avoid misfit dislocations at the wafer edge during Si-capping, even for thinner Si0.75Ge0.25 layers. Extremely sensitive characterization methods are mandatory to detect the extremely low density of misfit dislocations at the wafer edge. Light scattering measurements are most reliable. The epitaxial Si-cap / Si0.75Ge0.25 // Si-substrate layer stacks are stable against post-epi thermal processing steps, typically applied before wafer to wafer bonding and Si-substrate and Si0.75Ge0.25 backside removal.