Metastable and equilibrium defect structure of II–VI/GaAs interfaces

Defect characterization by high-resolution electron microscopy is presented for (001) epitaxy of the CdTe/GaAs and ZnTe/GaAs heterojunctions, before and after a long vacuum anneal. The annealed interface structure consists of a periodic array of perfect edge Lomer misfit dislocations, with spacing corresponding to a strain-free thin film. Since this is the most efficient manner to obtain complete relaxation, it represents the equilibrium microstructure. The as-deposited films are very thick, three to four orders of magnitude greater than the critical thicknesses, which are both less than a monolayer for these large lattice mismatch systems. Their microstructure corresponds to a metastable distribution of defects in that the thin film residual strain is nearly zero and neighboring defects can react to form Lomer misfits. A variety of defects exist both in the form of perfect misfit dislocations at the interface and extended defects into the thin film. The extended defects result from formation of stacking faults bounded by either Shockley or Frank partials, and more complicated defect structures due to interacting perfect and partial dislocations on intersecting slip planes. The purpose of this paper is to investigate the means by which complete stress relaxation can occur through dislocation reactions during annealing of very thick as-deposited films.     

Relaxation Dynamics and Threading Dislocations in ZnSe and ZnS y Se1−y /GaAs (001) Heterostructures

The design of lattice-mismatched semiconductor devices requires a predictive model for strains and threading dislocation densities. Previous work enabled modeling of uniform layers but not the threading dislocations in device structures with arbitrary compositional grading. In this work we present a kinetic model for lattice relaxation which includes misfit–threading dislocation interactions, which have not been considered in previous annihilation–coalescence models. Inclusion of these dislocation interactions makes the kinetic model applicable to compositionally graded structures, and we have applied it to ZnSe/GaAs (001) and ZnS _y Se_1?y /GaAs (001) heterostructures. The results of the kinetic model are consistent with the observed threading dislocation behavior in ZnSe/GaAs (001) uniform layers, and for graded ZnS _y Se_1?y /GaAs (001) heterostructures the kinetic model predicts that the threading dislocation density may be reduced by the inclusion of grading buffer layers employing compositional overshoot. This “dislocation compensation” effect is consistent with our high-resolution x-ray diffraction experimental results for graded ZnS _y Se_1?y /GaAs (001) structures grown by photoassisted metalorganic vapor-phase epitaxy.     

Defects and defect reduction processing in semiconductor heterostructures

Defects such as dislocations and interfaces play a crucial role in the performance of heterostructure devices. The full potential of GaAs on Si heterostructures can only be realized by controlling the defect density. The nature of misfit dislocations at the heterointerface has been studied and a mechanism for the formation of 60° and 90° misfit dislocations has been proposed. Threading dislocations in the epilayer are the most prominent defects and their density has to be controlled to fully utilize the properties of semiconductor heterostructures. Various processes to reduce defect densities in the epilayers have been discussed and in particular, the use of strained layer superlattices to reduce the threading dislocation density has been presented in this paper. Several superlattice structures have been used to reduce the density of threading dislocations in the GaAs epilayer. In this study, we have optimized the use of strained layer superlattices with respect to the position, period and number to reduce and control the dislocation density. The use of strained layer superlattices in conjunction with rapid thermal annealing was found to be the most effective method in reducing threading dislocation density. Transmission electron microscopy has been used to study the dislocation density reduction and the interaction of threading dislocations with the strained layers.     

Low threading dislocation density GaAs on Si(100) with InGaAs/GaAs strained-layer superlattice grown by migration-enhanced epitaxy

This paper reports a promising approach for reducing the density of threading dislocations in GaAs on Si. In _x Ga_1-x As/GaAs strained-layer superlattices (SLSs) grown by migration-enhanced epitaxy at 300° C on GaAs/Si acted as barriers to threading dislocations. Unlike conventional high-temperature-grown SLSs, the low-temperature-grown SLSs were hardly relaxed by the formation of misfit dislocations at GaAs/SLS interfaces, and this allowed them to accumulate considerable strain. New threading dislocation generation due to the misfit dislocation was also suppressed. These factors caused effective bending of threading dislocations and significantly reduced the dislocation density. For the samples that had an SLS withx = 0.3, the average etch-pit density was 7 × 10⁴ cm^-2, which is comparable to that of GaAs substrates.     

Design of Dislocation-Compensated ZnS y Se1−y /GaAs (001) Heterostructures

The understanding of lattice relaxation and dislocation dynamics in lattice-mismatched semiconductors makes it possible to design metamorphic device structures utilizing the dislocation compensation mechanism for reduced defects, improved performance, and enhanced reliability. We have developed a dislocation dynamics model accounting for misfit–threading interactions and have applied it to ZnS _y Se_1?y /GaAs (001) heterostructures.1 Dislocation compensation involves the removal of threading dislocations associated with one sense of misfit dislocations by bending them over to create misfit dislocations of the opposite sense at an intentionally mismatched interface. Here we investigated the design of dislocation-compensated ZnS _y Se_1?y /GaAs (001) heterostructures and considered the sulfur mole fraction tolerances applicable to such structures. We considered two types of structures: type A involved a uniform-composition (ungraded) layer on top of a uniform-composition buffer, while type B involved a uniform-composition layer on a linearly graded buffer. For each structure type we studied the requirements on the thickness and compositional profile of the buffer layer to optimize the removal of mobile threading dislocations from the top uniform (device) layer as well as the allowed tolerance in compositional overshoot to achieve structures with low threading dislocation density. We show for both types of structure that (i) for given compositional overshoot at the buffer–device layer interface, there is an optimum buffer thickness which minimizes the dislocation density; and (ii) for given buffer thickness there is an optimum overshoot which minimizes the dislocation density.     

Influence of the misfit-dislocation screw component on the formation of threading dislocations in semiconductor heterostructures

In heterostructures with the (001) interface and diamond and sphalerite crystal lattices, the total relief of mismatch stresses by introducing two mutually perpendicular arrays of 60° misfit dislocations (MDs) was shown to be possible only if their screw components were of the same type. In the opposite case, it was necessary to introduce additional MD arrays that increased the probability of formation of threading dislocations in an epitaxial film. When the process is nonoptimal and two mutually perpendicular arrays are introduced with opposite types of screw components, excess energy of long-range shear stresses is accumulated. Examples of nonoptimal introduction of misfit dislocations are the operation of the Frank-Read and Hagen-Strunk modified dislocation sources. The relaxation process was simulated and investigated experimentally.     

AlGaSb/GaSb quantum wells grown on an optimized AlSb nucleation layer

Five-period AlGaSb/GaSb multiple quantum wells (MQW) are grown on a GaSb buffer.Through optimizing the AlSb nucleation layer,the low threading dislocation density of the MQW is found to be (2.50±0.91)×108cm-2 in 1-μm GaSb buffer,as determined by plan-view transmission election microscopy (TEM) images.High resolution TEM clearly shows the presence of 90°misfit dislocations with an average spacing of 5.4 ran at the AlSb/GaAs interface,which effectively relieve most of the strain energy.In the temperature range from T = 26 K to 300 K,photoluminescence of the MQW is dominated by the ground state electron to ground state heavy hole (el-hhl) transition,while a high energy shoulder clearly seen at T > 76 K.can be attributed to the ground state electron to ground state light hole (e1-lh1) transition.     

The thermal stability of lattice mismatched InGaAs grown on patterned GaAs

Patterning and etching substrates into mesas separated by trenches before the growth of mismatched (by about 1% or less) epitaxial layers considerably reduces the interface misfit dislocation density when the layer thickness exceeds the critical thickness. Such films are in a metastable state, since misfit dislocations allow the epitaxial layers to relax to an in-plane lattice parameter closer to its strain-free value. Thermal annealing (from 600 to 850° C) has been used to study the stability of these structures to explore the properties of the misfit dislocations and their formation. The misfit dislocation density was determined by counting the dark line defects at the InGaAs/GaAs interface, imaged by scanning cathodoluminescence. InGaAs epitaxial layers grown on patterned GaAs substrates by organometallic chemical vapor deposition possess a very small as-grown misfit dislocation density, and even after severe annealing for up to 300 sec at 800° C the defect density is less than 1500 cm⁻¹ for a In_0.04Ga_0.96As, 300 nm thick layer (about 25% of the dislocation density found in unpatterned material that has not been annealed). The misfit dislocation nucleation properties of the material are found to depend on the trench depth; samples made with deeper (greater than 0.5 μm) trenches are more stable. Molecular beam epitaxially grown layers are much less stable than the above material; misfit dislocations nucleate in much greater numbers than in comparable organo-metallic chemical vapor deposited material at all of the temperatures studied.     

Misfit accommodation at epitaxial interfaces

The papers presented in this special volume address the critical issues of the mechanisms by which epitaxial films accommodate the interfacial misfit through strain relaxation. In this concluding paper, we wish to highlight some of the important concepts associated with the misfit accommodation at epitaxial interfaces. Towards the goal of achieving defect free epitaxial layers, efforts are continuously made to understand the mechanisms of generation of misfit dislocations. Thermodynamic equilibrium considerations are important in this respect. However, nucleation and growth is considered to be the limiting step for the misfit dislocation generation in the semiconductor heterostructures. Strained superlattice structures have been used to reduce the propagation of threading dislocations. Similarly, patterned substrates are also employed to reduce the propagating dislocation density in the epilayers. These different ideas are used to produce defect free epitaxial structures for device construction. The present state of our understanding of dislocation nucleation and generation are critically examined in this paper and suggestions for future work are given in this paper.     

Dislocation density reduction in GaAs epilayers on Si using strained layer superlattices

Defects such as dislocations and interfaces play a crucial role in the performance of heterostructure devices. The full potential of GaAs on Si heterostructures can only be realized by controlling the defect density. The reduction of threading dislocations by the use of strained layer superlattices has been studied in these heterostructures. Several superlattice structures have been used to reduce the density of threading dislocations in the GaAs epilayer. In this study, we have optimized the use of strained layer superlattices with respect to the position, period and number to reduce and control the dislocation density. The use of strained layer superlattices in conjunction with rapid thermal annealing was found to be a most effective method for reducing the threading dislocation density. Transmission electron microscopy has been used to study the dislocation density reduction and the interaction of threading dislocations with the strained layers. A model has been developed based on energy considerations to determine the critical thickness required for the bending of threading dislocations.     

Microstructure of Heteroepitaxial ZnTe Grown by Molecular Beam Epitaxy on Si(211) Substrates

The interface of ZnTe/Si(211) grown by molecular beam epitaxy was investigated by high-resolution transmission electron microscopy. Several types of defects such as misfit dislocations, stacking faults, agglomerations of vacancies, and precipitates were observed and studied by electron microscopy at the ZnTe/Si interface. The distribution of misfit dislocations at the interface was revealed with the assistance of the fast Fourier transformation filtering technique. A stick-and-ball interface model including misfit dislocation geometry is proposed. The possible origins of the stacking faults, vacancies, and precipitates are discussed.     

Structural properties of nitrogen-doped ZnSe epitaxial layers grown by MBE

We have used transmission electron microscopy (TEM) and high-resolution x-ray diffraction (HRXRD) techniques to investigate the structural properties of ZnSe doped with nitrogen, in the concentration range of 1 × 10¹⁸ to 2 × 10¹⁹cm⁻³. The nitrogen-doped layers contain substantial residual compressive strain at layer thicknesses where undoped ZnSe would be completely relaxed. The residual strain is clearly observed both in the inequality of the lattice constants (measured by HRXRD) parallel and perpendicular to the growth direction, and in the reduction of the misfit dislocation density (measured by TEM) relative to undoped ZnSe. In addition to the reduction in dislocation density, the misfit dislocations form a regular rectangular grid, rather than the irregular array seen in undoped ZnSe. The effective relaxed ZnSe lattice constant, as measured by x-ray diffraction, decreases as the nitrogen concentration increases. For the highest nitrogen concentration, this reduction in lattice constant, however, is greater than can be explained by the shorter Zn-N bond distance of theoretical predictions.     

Morphology and defect structures of GaSb islands on GaAs grown by metalorganic vapor phase epitaxy

The initial nucleation of GaSb on (001) GaAs substrates by metalorganic vapor phase epitaxy has been investigated using transmission electron microscopy (TEM) and high resolution electron microscopy (HREM). TEM results showed that the GaSb islands experience a morphological transition as the growth temperature increases. For growth at 520°C, the islands are longer along the [110] direction; at 540°C, they are nearly square, and at 560°C, they are longer along the direction. Possible mechanisms are proposed to describe such a transition. TEM and HREM examination showed that lattice misfit relaxation mechanisms depend on the growth temperature. For the sample grown at 520°C, the lattice mismatch strain was accommodated mainly by 90° dislocations; for the sample grown at 540°C, the misfit strain was relieved mostly by 90° dislocations with some of 60° dislocations, and for the sample grown at 560°C, the strain was accommodated mainly by 60° dislocations which caused a local tilt of the GaSb islands with respect to the GaAs substrate. The density of threading dislocations was also found to be dependent on the growth temperature. Mechanisms are proposed to explain these phenomena.     

Partial dislocations and stacking faults in 4H-SiC PiN diodes

Using plan-view transmission electron microscopy (TEM), we have identified stacking faults (SFs) in 4H-SiC PiN diodes subjected to both light and heavy electrical bias. Our observations suggest that the widely expanded SFs seen after heavy bias are faulted dislocation loops that have expanded in response to strain of the 4H-SiC film, while faulted screw or 60° threading dislocations do not give rise to widely expanded SFs. Theoretical calculations show that the expansion of SFs depends on the Peach-Koehler (PK) forces on the partial dislocations bounding the SFs, indicating that strain plays a critical role in SF expansion.     

Comparison of Continuously- and Step-Graded ZnS y Se1−y /GaAs (001) Metamorphic Buffer Layers

The design of metamorphic buffer layers for semiconductor devices with reduced defect densities requires control of lattice relaxation and dislocation dynamics. Graded layers are beneficial for the design of these buffers because they reduce the threading dislocation density by (1) allowing the distribution of the misfit dislocations throughout the buffer layer therefore reducing pinning interactions, and (2) enhancing mobility from the high built-in surface strain which helps to sweep out threading arms. In this work, we considered heterostructures involving a linearly-graded (type A) or step-graded (type B) buffer grown on a GaAs (001) substrate. For each structure type, we studied the equilibrium configuration and the kinetically-limited lattice relaxation and non-equilibrium threading dislocations by utilizing a dislocation dynamics model. In this work, we have also considered heterostructures involving a constant composition ZnS _y Se_1?y device layer grown on top of a GaAs (001) substrate with an intermediate buffer layer of linearly-graded (type C) or step-graded (type D) ZnS _y Se_1?y . For each structure type, we studied the requirements on the thickness and compositional profile in the buffer layer for the elimination of all mobile threading dislocations from the device layer by the dislocation compensation mechanism.     

Role of interface chemistry and growing surface stoichiometry on the generation of stacking faults in ZnSe/GaAs

The existence of Zn-As and vacancy-contained Ga-Se interfacial layers are suggested by transmission electron microscopy of Zn-and Se-exposed (or - reacted) ZnSe/GaAs interfaces, respectively. A very low density of faulted defects in the range of ∼10⁴cm² was obtained in samples with Zn passivation on an Asstabilized GaAs-(2 × 4). However, the density of As precipitates increases as the surface coverage of c(4 × 4) reconstruction increased on the Zn-exposed Asstabilized GaAs-(2 × 4) surface and this is associated with an increase of the density of extrinsic-type stacking faults bound by partial edge dislocations with a core structure terminated on additional cations. On the other hand, densities of extrinsic Shockley-and intrinsic Frank-type stacking faults are of ∼5 × 10⁷/cm² in samples grown on Se-exposed Ga-rich GaAs-(4 × 6) surfaces. Annealing on this Se-exposed Ga-rich GaAs-(4 × 6) generated a high density of vacancy loops (1 × 10⁹/cm²) and an increase of the densities of both Shockley-and Frank-type stacking faults (>5 × 10⁸/cm²) after the growth of the films. Furthermore, we have studied the dependence of the generation and structure of Shockley-type stacking faults on the beam flux ratios in samples grown on Zn-exposed As-stabilized GaAs-(2 × 4) surfaces. Cation-and anion-terminated extrinsic-type partial edge dislocations were generated in samples grown under Zn-and Se-rich conditions, respectively. However, an asymmetric distribution on defect density under varied beam flux ratios (0.3 ≤ P_Se/P_Zn ≤ 10) is obtained.     

The role of the V/III ratio in the growth and structural properties of metalorganic vapor phase epitaxy GaAs/Ge heterostructures

GaAs epitaxial layers have been grown on (001) 6† off-oriented toward (110) Ge substrates by metalorganic vapor phase epitaxy. In order to study the influence of V/III ratio on the growth mechanisms and the structural properties of the layers, the input flow of arsine was changed over a wide range of values, while keeping constant all other experimental settings. Optical microscopy in the Nomarski contrast mode, x-ray topography and high resolution diffractometry, transmission electron microscopy and Rutherford backscattering have been used to investigate the epilayers. It has been found that the growth rate increases and the surface morphology worsens with increasing V/III ratio. The abruptness of the layer-substrate interface has also been found to strongly depend on the V/III ratio, the best results being obtained under Ga-rich conditions. The main structural defects within the layers are stacking faults and misfit dislocations. Layers grown under As-rich conditions only contain stacking faults, probably originated by a growth island coalescence mechanism, whereas layers grown under Ga-rich conditions contain both misfit dislocations and stacking faults generated by dissociation of threading segments of interfacial dislocations. In spite of the different defects, the strain relaxation has been found to follow the same trend irrespective of the V/III ratio. Finally, the relaxation has been found to start at a thickness exceeding the theoretical critical value.     

Deposition,post-deposition annealing,and characterization of epitaxial Ge films grown on Si(100) by pyrolysis of GeH4

As a first step towards developing heterostructures such as GaAs/Ge/Si entirely by chemical vapor deposition, Ge films have been deposited on (100) Si by the pyrolysis of GeH₄. The best films are grown at 700° C and are planar and specular, with RBS minimum channeling yields of ≈4.0% (near the theoretical value) and defect densities of 1.3 x 10⁸ cm⁻². Variations of in-situ cleaning conditions, which affect the nature of the Si substrate surface, greatly affect the ability to get good epitaxial growth at 700° C. The majority of the defects found in the Ge films are extrinsic stacking faults, formed by dissociation of misfit and thermal expansion accommodation dislocations. The stacking fault density is not significantly reduced by post-deposition annealing, as is the case for the dislocations observed in MBE Ge films. It is suggested that lowering the CVD growth temperature through the use of high vacuum deposition equipment would result in dislocation defects like those of MBE films which could then be annealed more effectively than stacking faults. Films with defect densities equivalent to MBE Ge films (~2 x 10⁷ cm⁻²) could then probably be produced.     

Threading and misfit-dislocation motion in molecular-beam epitaxy-grown HgCdTe epilayers

Lattice mismatch between the substrate and the absorber layer in single-color HgCdTe infrared (IR) detectors and between band 1 and band 2 in two-color detectors results in the formation of crosshatch lines on the surface and an array of misfit dislocations at the epi-interfaces. Threading dislocations originating in the substrate can also bend into the interface plane and result in misfit dislocations because of the lattice mismatch. The existence of dislocations threading through the junction region of HgCdTe IR-photovoltaic detectors can greatly affect device performance. High-quality CdZnTe substrates and controlled molecular-beam epitaxy (MBE) growth of HgCdTe can result in very low threading-dislocation densities as measured by the etch-pit density (EPD ∼ 10⁴cm⁻²). However, dislocation gettering to regions of high stress (such as etched holes, voids, and implanted-junction regions) at elevated-processing temperatures can result in a high density of dislocations in the junction region that can greatly reduce detector performance. We have performed experiments to determine if the dislocations that getter to these regions of high stress are misfit dislocations at the substrate/absorber interface that have a threading component extending to the upper surface of the epilayer, or if the dislocations originate at the cap/absorber interface as misfit dislocations. The preceding mechanisms for dislocation motion are discussed in detail, and the possible diode-performance consequences are explored.     

钙钛矿型外延薄膜中两种分解失配位错的HRTEM研究

在（001）LaAlO3上生长钙钛矿型Ba0.3Sr0.7TiO3外延薄膜近界面层,用HRTEM观察到多种新型的分解失配位错,其中两种较复杂的分解失配位错已见报道。本文介绍在该薄膜中观察到的另外两种有趣的分解失配位错。这两种失配位错都分解成两个柏格斯矢量b=（1／2）（110）的不全位错,都与（1／2）（110）层错相伴。这些不全位错对薄膜应变松弛都有贡献,其发生与Ba0.3Sr0.7TiO3薄膜的岛状成核和莫扎克生长有关。