Determination of the types and densities of dislocations in GaN epitaxial layers of different thicknesses by optical and atomic force microscopy

The change in the dislocation density on the surface of GaN epitaxial layers, which were grown by hydride vapor-phase epitaxy on sapphire substrates with c and r orientations, has been investigated by optical and atomic force microscopy (AFM). It is shown that the observed decrease in the density of threading dislocations with an increase in the layer thickness is related to the annihilation of mixed dislocations. The experimental and theoretical data on the change in the density of mixed dislocations with an increase in the epitaxial-layer thickness are in good correspondence.     

Influence of Microdefects on the Generation of Dislocations in Homoepitaxial Gallium Phosphide Layers Grown on LEC Substrates

GaP LEC substrates doped with sulphur (N_D – N_A roughly (3–7) × 10¹⁷ cm⁻³) were characterized by transmission electron microscopy. This material was found to contain microdefects such as perfect perismatic dislocation loops, and spherical precipitates. Cross-sectional TEM investigations perfomed have shown that above all perfect dislocation loops lying directly at the substrate/layer interface are sources for the formation of extended dislocations propagating through the epitaxial layer. Using the methods of selective photoetching and AB-etching on (110) cleavage faces this phenomenon was observed, too.     

InP/GaP晶格失配界面的电特性

本文研究了InP/GaP晶格失配界面的电特性。HRTEM图象表明在界面存在90°位错缺陷的应变缓释。ECV表明界面存在高密度载流子层。AFM图象表明本研究中获得了粗糙度为2.48nm的良好InP异质外延层。并对于InP界面给出了一个基于费米能级钉扎的模型来解释观察到的电性质。     

Propagation of dislocations during epitaxy

The reduction of dislocation density in epitaxial layers relative to the original density in the substrate is a well-known phenomenon which seems particularly pronounced for liquid-phase processes and in compound semiconductors. Several mechanisms have previously been suggested to account for this reduction of dislocation density. We are here proposing a simple alternative in order to explain the formation of loops and the propagation parallel to the substrate surface. The initial stages of growth are assumed to consist of advancing terraces or ridges parallel to the surface. Dislocations may be dragged along with this parallel growth, and annihilation of dislocations results. We discuss the details of this model in terms of dislocation configurations and present consequences and methods for possible experimental verifications.     

Calculation of displacement fields and simulation of HRTEM images of dislocations in sphalerite type A(III)B(V) compound semiconductors

The displacement fields of different kinds of both perfect and dissociated dislocations have been calculated for an isotropic continuum, and by means of linear elasticity. Additionally, the corresponding HRTEM images have been simulated by the well-established EMS program package in order to predetermine the structural aspects of dislocations, and then to compare it with experimental HRTEM micrographs. The latter ones resulted from plastically deformed GaP single crystals and InAs/(001)GaAs single epitaxial layers. It could be established that using the simple approach of linear elasticity and isotropy results can be obtained which correspond well to the experimental images. So, the structure of various Shockley partial dislocations bounding a stacking fault can be detected unambiguously. The splitting behaviour of perfect 30° dislocations (separation into a 0° and 60° partial) and 90° dislocations (separation into two 60° partials) both with line direction along 〈112〉, 60° dislocations (separation into 30°/90° and 90°/30° configuration) and screw dislocations (separation into two 30° partials) along 〈110〉 are discussed in the more detail. Moreover, the undissociated sessile Lomer dislocation, glissile 60° dislocation and edge dislocation have been considered too.     

Epitaxial growth of silicon carbide layers by sublimation “Sandwich method” (II) structural defects and growth mechanism

Structural defects of α-SiC epitaxial layers grown by sublimation “sandwich-method” in vacuum at the temperatures ranging from 1600 to 2100 °C have been investigated by X-ray topography and optical microscopy methods. It was shown, that perfect SiC layers with the homogeneous polytype structure and small dislocation density (≦ 10² cm⁻²) may be obtained on the substrates with any crystallographic orientation at the conditions close to quasi-equilibrium one. The presence of impurities and silicon deficiency in the vapour phase, lead usually to the deterioration of morphological and structural perfection of SiC layers. There are the following structural defects: uncoherent polytype inclusions (mainly β-SiC), pores, dislocations, specific stacking faults. Morphological peculiarities of the SiC epitaxial layers and possible growth mechanisms are discussed.     

Characterization of defects in GaP and GaAsP graded heterojunctions by transmission electron microscopy

Misfit dislocations are observed in graded heterojunctions GaAs_1?xP_x by electron microscopy. Results are in agreement with previous work concerning the nature of the dislocations (Lomer and 60° dislocations) and their density dependence on the phosphorus gradient. The discussion concerns the formation of Lomer dislocations and the possibility of reducing the density of inclined dislocations which reach the surface of the epitaxial layer. GaP substrates, S-doped, are examined by transmission electron microscopy. Numerous defects such as Frank loops, perfect loops, helical dislocations and precipitates are observed. A GaP homoepitaxial layer realized on this substrate is free from these defects but exhibits stacking faults. A zinc diffusion does not produce additional defect but a 1000 Å thick amorphous layer is observed a at the surface.     

How to Avoid Plastic Deformation in GaAs Wafers during Molecular Beam Epitaxial Growth

Plastic deformation in two‐inch diameter GaAs wafers resulted from standard thermal treatments which accompanied epitaxial growth in molecular beam epitaxy (MBE) machines of three different makes. Synchrotron based X‐ray transmission topography was used to distinguish between thermal treatment induced dislocation bundles and misfit dislocations. Eradication of the wafer slip related dislocation bundles has been achieved by modifications to the sample holder of a user built MBE machine. These modifications are discussed, the extent of the problem is briefly outlined, and an extrapolation of the susceptibility of GaAs wafers of higher diameters to this type of plastic deformation is given.     

X-ray measurement of deformation and dislocation density in semiconductor strained layers

Double-crystal X-ray diffraction is commonly used to measure the misfit strain and relaxation of epitaxial semiconductor layers. In this paper, a framework is developed which links the measured parameters Δd / d and Δø to the deformation tensor of a semicoherent layer. Isotropic elasticity theory and the Frank-Bilby equation are used to derive an analytical expression for this deformation. By combining X-ray measurements of different planes, it is possible to obtain the misfit strain and details of the misfit dislocation array in a strained layer grown on a substrate of arbitrary orientation. In (001) layers, it is shown that the misfit strain and relaxation can be found from just six rocking curves, although the most accurate measurements require twelve rocking curves.     

Influence of a buried misfit dislocation network on the pyramid-to-dome transition size of Ge self-assembled quantum dots on Si(0 0 1)

The critical sizes of the pyramid-to-dome transition of Ge self-assembled quantum dots (SAQDs) grown on relaxed SiGe buffer layers were investigated for the relationship between the misfit strain built in dots and nucleation sites. The strain field of arrays of buried dislocations in a relaxed SiGe buffer layer provided preferential nucleation sites for quantum dots. Burgers vector analysis using plan-view transmission electron microscopy verified that the preferential nucleation sites of Ge SAQDs depended on the Burgers vector direction of corresponding dislocations. The measurement of the lateral distance between SAQDs and dislocations clarified that the location of SAQDs was at the intersection of the dislocation slip plane and the top surface. The samples are fabricated to contain low dislocation densities. The average dislocation spacing is larger than the surface migration length of Ge adatoms, resulting in two groups of SAQDs, those that are located along the dislocations, and those that are not. Atomic force microscopy observations showed a distinctively larger critical size for Ge SAQDs grown over the intersection of the dislocation slip plane and the top surface than those grown in regions between dislocations. These experimental observations indicate that the critical size of the pyramid-to-dome transition is strongly dependent on misfit strain in SAQDs with lower strain being associated with a larger critical size.     

Deformation Modes and Structure Evolution in Laser‐Shock‐Loaded Molybdenum Single Crystals of High Purity

This paper reports on the microstructural changes occurring within molybdenum single crystals after shock treatment with an excimer laser‐system in a confined ablation mode with different number of impacts. Using different complementary investigation methods (optical microscopy, scanning electron microscopy and transmission electron microscopy) it is found that slip and twinning are active modes of deformation during the shock‐induced plastic deformation. After laser treatment with a single laser pulse slip bands on {112} planes containing several microscopic twins are the dominating microstructural feature, whereas further laser‐shock‐processing leads to the formation of a homogeneous arrangement of screw dislocations, tangles and loops. Deformation modes and microstructure in laser shocked samples prove to be quite similar to those of explosive shocked specimens although the laser‐induced peak pressure is about an order of magnitude lower than that during explosive loading.It is shown that the laser shock‐induced hardening increases with increasing number of impacts in the range from 1 to 6 impacts. This shock‐induced strengthening is correlated with the overall dislocation density.     

Effect of dislocation loops in macroscopically dislocation-free GaP substrates on the perfection of homo-epitactic deposits

Substrates and epitactic layers of GaP are etched under optimized conditions with the defect-revealing, preferential R-C etchant and subsequently examined using high-magnification (about 2500X) optical microscopy techniques. It is possible then to distinguish two new phenomena, viz. etch pits characteristic of dislocation loops in the substrate, and inclined dislocation dipoles in the layer. For layers grown on dislocation-free substrates we find that (i) the surface densities of both defects are equal (～5 × 10⁵cm^-2), and (ii) the average diameter of the dislocation loops in the substrate is roughly the same as the average distance between the two dislocations of the dislocation dipoles (0.5?1μm). Hence the perfection of these layers is determined by interfacial dislocation loops. Because the density of dislocation loops is only about a factor of two lower in highly (～1 × 10⁵ cm^-2) dislocated substrates, growth on these substrates results in layers which have even slightly lower dislocation density than layers grown on dislocation-free substrates. In the former case also single dislocations in the substrate propagate into the layer. Minority-carrier lifetime data indicate that minority-carrier recombination at dislocations is a restrictive factor for the luminescence quality of layers grown both on dislocation-free and highly-dislocated substrates.     

Misfit Strain Relaxation by Dislocations in InAs Islands and Layers Epitaxially Grown on (001)GaAs Substrates by MOVPE

The misfit dislocation configurations in InAs islands as well as in more or less continuous layers grown on (001) oriented GaAs substrates were studied by weak-beam and high-resolution electron microscopy. The islands are confined by {101} and {111} facets where the aspect ratio (height/lateral extension) can be affected by the growth conditions. It is possible to grow well-defined islands as well as relatively continuous layers by MOVPE under As-stabilized conditions. At constant deposition parameters the growth is characterized by islands of different sizes (but with constant aspect ratio) in various strain states depending on their dislocation content. Coherently strained islands without any dislocation can be observed for heights up to 23 ML InAs, or otherwise, up to a maximal island extension of about 12 nm (for the particular aspect ratio ≈︂0.585). With further increase of island height and lateral extension, the introduction of dislocations becomes favourable. Independent of the island size, the layer thickness and the dislocation density, a residual elastic strain of about ε^r = —0.8% remains after relaxation. This means, about 88% of the total misfit strain of ε = —6.686 × 10^—2 were compensated by Lomer dislocations. These sessile Lomer dislocations lie in the island interior only, where single 60° dislocations were observed exclusively in their near-edge regions. With increasing island size and/or layer thickness some close-spaced 60° dislocations occur additionally within the interfacial region. The Lomer dislocations that are always located 4 monolayers (ML) above the InAs/GaAs interfacial plane result from the well-known fusion of two 60° slip dislocations. These 60° dislocations have been nucleated 7 … 8 ML above the interface at surface steps on the {111} facets confining the islands. Based on our experimental observations a new mechanism is proposed that explains the origin of these 60° dislocations. Their further fusion to sessile Lomer dislocations that compensate the misfit strain most efficiently occurs in the way as commonly accepted.     

Optical defects in barium—strontium niobate single crystals

The distributions of edge dislocations and residual mechanical stresses in Ba_xSr_1-xNb₂O₆ (BSN) crystals are investigated and the explanation of the nature of the “growth column” is proposed. The “growth column” is a defect zone going through all of the crystal and usually repeating in its cross-section the contour of the seed crystal. The “growth column” boundary is the closed contour with extremely high edge dislocation density. These dislocations are connected with thermal stresses due to seed-melt contact or abrupt crystal widening. Under proper crystal seeding and widening conditions one can obtain the BSN crystals with dislocation densities less than 10 cm⁻² and without the “growth column”. The method of chemico-mechanical polishing of BSN crystals not forming a defect layer on the surface of the crystals have been developed. The high temperature diffusion annealing is shown to eliminate the growth striae in BSN crystals.     

Dynamic Aspects of Dislocation Behaviour Near Low-Angle Boundary in Molybdenum Single Crystals

The dislocation behaviour was inventigated in specimens of monocrystals of molybdenium during in — situ stretching in the direction [001] near low — angle twist boundary which was close to be parallel to the direction of the external force. Low-angle boundary is easily penetrable for the fast moving nonscrew dislocations and acts as an effective stopper for screw dislocations which are quite parallel to the forming boundary dislocations. The mixed tipe dislocations emission by the low-angle boundary was noticed. On the stage when plastic deformation is performed mainly by the motion of screw dislocations the “relay-race”-like transmission of the dislocations motion through the boundary was observed.     

Progress in Modeling of III-Nitride MOVPE

This review provides an introduction to III-Nitrides MOVPE process modeling and its application to the design and optimization of MOVPE processes. Fundamentals of the MOVPE process with emphasis on transport phenomena are covered. Numerical techniques to obtain solutions for the underlying governing equations are discussed, as well as approaches to describe multi-component diffusion for typical regimes during MOVPE. Properties of common industrial MOVPE reactor types like close spaced showerhead reactors, rotating disk reactors and Planetary Reactors are compared in terms of underlying working principles and generic process parameter dependencies.The main part of the paper is devoted to reviewing gas phase and surface reaction mechanisms during MOVPE. The process design in particular for MOVPE of III-Nitrides is determined by complex gas phase reaction kinetics. Advances in the modeling and predicting of these processes have contributed to understanding and controlling these phenomena in industrial scale MOVPE reactors. Detailed kinetics and simplified surface kinetic approaches describing the incorporation of constituents into multinary solid alloys are compared and a few application cases are presented. Differences in thermodynamic and kinetic properties of multi-layered structures of different compositions such as InGaN, AlGaN can cause enrichment of the adsorbed layer by certain group III atoms (indium in case of InGaN and gallium in case of AlGaN) that translate into specific features of composition profiles along the growth direction.An intrinsic feature of III-nitride materials is epitaxial strain that shows up in different forms during growth and affects both deposition kinetics and material quality. In case of InGaN MOVPE there is a strong interplay between indium content and strain that has direct influence on distribution of material composition in the epitaxial layers and multi-layered structures. Epitaxial strain can relax via different routes such as nucleation and evolution of the extended defects (dislocations), layer cracking and roughening of the surface morphology. Simulation approaches that address coupling of growth kinetics with strain and defect dynamics are discussed and exemplified.     

Correlations between photoluminescence and Hall mobility in GaN/sapphire grown by metalorganic chemical vapor deposition

We have investigated photoluminescence (PL) and electron Hall mobility for unintentionally doped GaN epitaxial layers grown by low-pressure metalorganic chemical vapor deposition on c-plane Al₂O₃ substrates. Four GaN films having identical dislocation density but remarkably different electron Hall mobility were exploited. At low temperature (12 K), a PL line associated with a bound exciton was observed and strong correlations were found between the Hall mobility and the PL intensity of the exciton transition. That is, relative PL intensity of the bound exciton to a donor-bound exciton monotonously increased with decreasing the electron mobility of the GaN films. This correlation was interpreted in terms of electrical compensation. Efforts to find the chemical origin of the PL line led to the conclusion that the BE line originated neither from threading dislocations nor from extrinsic point defects. Intrinsic acceptors such as Ga vacancy and GaN anti-site were suspected as plausible origin.     

Formation of dislocation structure in silicon layers on non-orienting substrates

A dislocation structure of Si layers crystallized from a floating grain on quartz glass and mullite ceramics substrates has been investigated by transmission electron microscopy (TEM) including the high-voltage one. The effect of the layer orientation on the crystallographic features of dislocation distribution and brittle fracture in Si-SiO₂ system has been considered. The dislocation structure is proved to form mainly at temperatures lower than 0.8 of absolute melting temperature (T_m) of Si. Dislocation sources are located inside the crystallizable layer, and they are dislocations appearing from grain as well as the dislocation bundles near the interface. The cross slip of screws plays an essential role in dislocation multiplication. The difference of thermal expansion coefficients of the layer and substrate determines the finite dislocation density near the interface and in the bulk of the layer.     

Lattice strain and misfit dislocations in GaAs-GaAlAsP heterojunctions

The lattice strain and misfit dislocations in a GaAs-GaAlAsP heterojunction were examined. The change in lattice strain with the composition ratio or the position of the crystal was measured, and it was found that some of the misfit dislocations introduced in the heterojunction were edge-type dislocations. The critical thickness of the epitaxial layer for the generation of misfit dislocations was also measured. The critical differential strain between the atomic layers was on the order of several angstroms. The distribution of lattice strain was analyzed by a two-dimensional simple cubic lattice model, and the distribution of differential strain was examined.     

Nucleation,Glide Velocity and Blocking of Misfit Dislocations in SiGe/Si

Relaxation of strained layer systems is still not well understood. It is time dependent and changes considerably for samples with different growth history. This has to be discussed in terms of nucleation, glide velocity and blocking of misfit dislocations. We have investigated these phenomena at samples with SiGe layer thicknesses ranging from 60 nm up to 120 nm grown by molecular beam epitaxy (MBE) or chemical vapor deposition (CVD) by means of X-ray topography. The samples were annealed at temperatures between 500° and 600° C. Nucleation of misfit dislocations is heterogeneous and the rate is obviously dependent on layer strain and thickness. A quantified nucleation rate was not yet accessible, mainly due to the preferential formation of dislocation bundles. The propagation velocities of misfit dislocation segments were measured during annealing by means of synchrotron radiation plane wave topography (reflection geometry). The values agree well with theory and there is no evidence that they depend on growth regime. It is shown that the interaction of propagating misfit dislocations with crossing ones may lead to blocking or cross slip in different glide systems. These results are corroborated by investigations with atomic force and transmission electron microscopy.