Dislocations In Epilayers Research Articles

Interfacial Dislocation Reduction by Optimizing Process Condition in SiC Epitaxy

It is known that generation of interfacial dislocation on SiC epitaxy depends mainly on misfit strain between substrate and the epilayer. In this paper, we investigate the impact of temperature profile, doping profile of the epilayer and resistivity of the substrates on the formation of interfacial dislocation in epilayers. Our preliminary results show that thermal profile during the epitaxy plays a key role in formation of interfacial dislocations in epilayers. We demonstrated reduction or elimination of interfacial dislocation in epilayers by optimizing the temperature profile of the wafers during the epitaxial growth.

Stacking Fault Manifolds and Structural Configurations of Partial Dislocations in InGaN Epilayers

The configurations of basal stacking fault (BSF) manifolds often observed in III‐nitride alloy epilayers, particularly InGaN, are considered herein. Using high‐resolution transmission electron microscopy (HRTEM), it is shown that the folds and steps of intrinsic BSFs can acquire a Shockley‐like partial dislocation character depending on the relative senses of the BSF stackings. This can lead to the introduction of extra geometrically necessary threading dislocations in the epilayer on account of variant coexistence. Moreover, it is demonstrated that the overlap of two I1 BSFs can transform into a single I2 BSF, which is terminated by a glissile Shockley dislocation. Shockley and Shockley‐like partial dislocations can acquire line directions comprising either <> a‐line or <100> m‐line segments. Using atomistic calculations, the core structures of the m‐line partials are provided and their visibility is examined by HRTEM as well as the possibility of their discrimination from the a‐line ones.

Electron-Beam-Induced Current Study of Dislocations and Leakage Sites in GaN Schottky Barrier Diodes

This work aims to clarify the electrical activities of threading dislocations and their relation with leakage sites in homoepitaxial GaN Schottky barrier diodes based on the electron-beam-induced current (EBIC) technique and transmission electron microscopy (TEM). First, the recombination activities of threading dislocations in epilayers grown on different substrates are compared by EBIC. The dislocation type is characterized based on etch pit measurements and TEM. The dislocation density and character are strongly affected by defects in the substrate. The recombination strength of dislocations is revealed to be correlated with their type. It is found that single dislocations including both edge and mixed type exhibit weak (< 5%) EBIC contrast, while dislocation clusters show strong contrast (up to 30%). Second, leakage sites in Schottky diodes are visualized by EBIC under reverse bias. There is no direct correlation between the initial leakage sites and threading dislocations; whereas, instead of dislocations, a variety of initial leakage/breakdown sites are found, including grown-in pit defects as initial breakdown sites and hillocks at the Schottky interface acting as strong leakage sites.

On the mechanism of cross-hatch pattern formation in heterostructures with a small lattice mismatch

The mechanism of cross-hatch (CH) pattern formation on the surface of GaMnAs/InGaAs/GaAs (001) heterostructures with a small lattice mismatch, related to the peculiarities of glide process of dislocations in epilayers, is considered. Electrically active defects in these heterostructures and the surface morphology of epilayers were investigated by electron beam induced current imaging, transmission electron microscopy (TEM), atomic force microscopy and differential interference contrast microscopy in combination with selective chemical etching. The studies revealed a high-density system of extended defects (EDs) in the bulk of InGaAs epilayers in addition to misfit dislocations at the epilayer/substrate interface. These defects possess a number of properties reflecting their unusual core structure: pronounced crystallography, recombination activity, existence of elastic strains in the core, and at the same time the absence of contrast in the TEM images. Model experiments performed on InGaAs epilayers give grounds to consider that the revealed EDs are similar in properties to a new type of EDs generated by moving dislocations in plastically deformed Si and SiGe crystals. The formation of CH surface structure is assumed to be directly related to the emergence of the EDs during the strain relaxation process in heterostructures based on III-V and IV-group materials.

Two-photon-excited, three-dimensional photoluminescence imaging and dislocation-line analysis of threading dislocations in 4H-SiC

This paper describes the three-dimensional imaging of threading edge dislocations (&amp;lt;a&amp;gt;TEDs), pure c threading screw dislocations (&amp;lt;c&amp;gt;TSDs), and c + a threading mixed dislocations (&amp;lt;c + a&amp;gt;TMDs) in 4H silicon carbide (4H-SiC) epilayers by two-photon-excited photoluminescence (2PPL), where inclinations of dislocations in epilayers with different offcut angles are investigated. A numerical model based on carrier diffusion explains the mechanism of the dark-contrast 2PPL imaging provided by band edge emission. More than 450 threading dislocations in 4H-SiC epilayers are visualized three-dimensionally and are classified into six &amp;lt;a&amp;gt;-type TEDs, two &amp;lt;c&amp;gt;-type TSDs, and twelve &amp;lt;c + a&amp;gt;-type TMDs according to the directions of extra half-planes and the chiral geometries (right- or left-handed screw). The 2PPL images reveal that all threading dislocation types incline in the step-flow direction, the extent of which is more prominent for a larger offcut angle. The &amp;lt;a&amp;gt;TEDs also incline in the directions of their extra half-planes. The inclinations of &amp;lt;c&amp;gt;TSDs perpendicular to the step-flow direction are influenced by chirality, while those of &amp;lt;c + a&amp;gt;TMDs are influenced by both the directions of the extra half-planes and the chirality. The origins of the dislocation inclinations are discussed taking into account the kinetic interactions between the dislocations and the advancing steps on the growing surface.

Effect of thicknesses of InP epilayers on InP/GaAs heterostructure

InP epilayers with different thicknesses were grown on GaAs substrates at a low temperature by using metalorganic chemical vapor deposition. Atom force microscope and double‐crystal X‐ray were carried out to investigate the morphology and the crystal quality. Transmission electron microscopy was performed to characterize the microstructure and morphology. It was found that with the epilayer thicknesses increased, the crystal quality was improved and the dislocations in epilayers were decreased. Moreover, the sample that has 150‐nm InP epilayer could observe large numbers of antidislocations in the gallium arsenide substrate. The formation mechanism of antidislocations was also discussed with the analysis of crystal structure and surface morphology. Furthermore, we proposed a mechanism to explain the motion and the reaction of antidislocations.

High resolution x-ray diffraction study of the substrate temperature and thickness dependent microstructure of reactively sputtered epitaxial ZnO films

Epitaxial ZnO films were grown on c-sapphire by reactive sputtering of zinc target in Ar–O2 mixture. High resolution x-ray diffraction measurements were carried out to obtain lateral and vertical coherence lengths, crystallite tilt and twist, micro-strain and densities of screw and edge dislocations in epilayers of different thickness (25–200 nm) and those grown at different temperatures (100–500 °C). ϕ-scans indicate epitaxial growth in all the cases, although epilayers grown at lower substrate temperatures (100 °C and 200 °C) and those of smaller thickness (25 nm and 50 nm) display inferior microstructural parameters. This is attributed to the dominant presence of initially grown strained 2D layer and subsequent transition to an energetically favorable mode. With increase in substrate temperature, the transition shifts to lower thickness and growth takes place through the formation of 2D platelets with intermediate strain, over which 3D islands grow. Consequently, 100 nm thick epilayers grown at 300 °C display the best microstructural parameters (micro-strain ~1.2 × 10−3, screw and edge dislocation densities ~1.5 × 1010 cm−2 and ~2.3 × 1011 cm−2, respectively). A marginal degradation of microstructural parameters is seen in epilayers grown at higher substrate temperatures, due to the dominance of 3D hillock type growth.

TEM study of dislocations structure in In0.82Ga0.18As/InP heterostructure with InGaAs as buffer layer

In order to improve the quality of detector, InxGa1−xAs (x=0.82) buffer layer has been introduced in In0.82Ga0.18As/InP heterostructure. Dislocation behavior of the multilayer is analyzed through plane and cross section [110] by transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM). The dislocations are effectively suppressed in InxGa1−xAs (x=0.82) buffer layer, and the density of dislocations in epilayer is reduced obviously. No lattice mismatch between buffer layer and epilayer results in no misfit dislocation (MD). The threading dislocations (TDs) are directly related to the multiplication of the MDs in buffer layer.

Identification of dislocations in 4H-SiC epitaxial layers and substrates using photoluminescence imaging

Dislocations in n- and p-type substrates as well as in epitaxial layers (epilayers) were clearly identified using a photoluminescence (PL) imaging technique. Dislocations in epilayers show large/small bright spots or lines in infrared PL images, which correspond to threading screw/edge dislocations (TSDs/TEDs) or basal plane dislocations (BPDs), respectively. In contrast, dislocations in substrates exhibit large/small dark spots or dark lines in infrared PL images, corresponding to TSDs/TEDs or BPDs, respectively. These different features (bright/dark contrast) of dislocations may originate from the different densities of point defects or impurities.

Conversion of basal plane dislocations to threading edge dislocations in 4H-SiC epilayers by high temperature annealing

Conversion of basal plane dislocations (BPDs) to threading edge dislocations (TEDs) is found in 4H-SiC epilayers after being annealed simply at high temperatures. Grazing incidence reflection synchrotron x-ray topography for the dislocations in the epilayers before and after annealing confirmed that some of the BPDs in the epilayers had converted to TEDs from the epilayer surface by the annealing. Observations on the dislocation behavior during annealing are explained in detail, and the mechanism of BPD conversion is discussed. It is argued that the conversion proceeds through the cross slip of constricted BPD segments towards the surface on the prismatic plane driven by the image force as well as TED glide driven by the line tension. Certain kinetic processes during annealing may facilitate the formation of constriction.

Characterization of Basal Plane Dislocations in 4H-SiC Substrates by Topography Analysis of Threading Edge Dislocations in Epilayers

Burgers vector directions of threading edge dislocations (TEDs) in 4H-SiC epitaxial layer are distinguished by grazing incidence high resolution topography. Based on comparison between appearance of KOH etch pits and direction of TED Burgers vector, the size difference of the TED etch pits is found to be dependent on their Burgers vector directions. Examining TEDs in the epilayer by topography, the Burgers vector direction of basal plane dislocations (BPDs) in the substrate is identified. Correspondence between the topography contrast and the sense of a BPD is also investigated.

Growth and Electrical Characterization of 4H-SiC Epilayers

Homoepitaxial growth of 4H-SiC and characterization of deep levels obtained mainly in the authors’ group have been reviewed. The growth rate has been increased to 24 om/h with keeping very good surface morphology and low trap concentration on 8o off-axis 4H-SiC(0001) by hot-wall chemical vapor deposition at 1650oC. The increased growth rate has resulted in the enhanced conversion of basal-plane dislocations into threading edge dislocations in epilayers. The Z1/2 and EH6/7 concentrations can be decreased to about 1·1012 cm-3 by increasing the C/Si ratio during CVD. Extensive investigation on as-grown and electron-irradiated epilayers indicates that both the Z1/2 and EH6/7 centers may be attributed to the same origin related to carbon displacement, probably a carbon vacancy. Deep levels observed in as-grown and irradiated p-type 4H-SiC are also presented.

Do You Really Expect To Grow Epilayers On That? A Rationale For Growing Epilayers On Roughened Surfaces

AbstractWe describe surface preparation and epilayer growth techniques that readily reduce the density of Vf drift inducing basal plane dislocations in epilayers to less than 10 cm-2 and permit the fabrication of bipolar SiC devices with very good Vf stability. The optimal process route requires etching the substrate surface prior to epilayer growth to enhance the natural conversion of basal plane dislocations into threading edge dislocations during epilayer growth. The surface of this relatively rough “conversion” epilayer is subsequently repolished prior to growing the device structure. We provide details on processing parameters and potential problems as well as describe devices produced using this low basal plane dislocation growth processes.

Basic dislocation contrasts in SEM-CL/EBIC on III–V semiconductors

Abstract Scanning electron microscope cathodoluminescence (CL) and electron beam induced current (EBIC) contrasts of crystallographically defined single misfit dislocations in epilayers of the III–V semiconductor materials GaAs, GaAsP, GaP were analysed. The use of a realistic generation distribution allows an exact calculation and fit to experimental beam voltage dependent contrast profiles of selected surface-parallel dislocation lines. Differences in the recombination activity (defect strengths) derived from EBIC and CL contrasts are interpreted as a contribution of additional radiative recombination of the defect region to the CL signal.

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.

Growth of Lattice-Matched ZnSe-ZnS Strained-Layer Superlattices Onto GaAs as An Alternative to ZnSSe Alloys

ABSTRACTThis paper describes the results of the first attempt to reduce misfit dislocations in epilayers of a wide bandgap II-VI semiconductor on GaAs substrates by utilizing the ZnSe-ZnS strained-layer superlattice (SLS) structure. From a theoretical calculation, SLSs consisting of a 200A-ZnSe and a IOA-ZnS layer in one period can be grown as lattice-matched films to GaAs substrates. It has been found from the photoluminescence measurements and electron-beam-induced-current (EBIC) image observations that the generation of misfit dislocations can be markedly reduced, as expected.