Microtwin Density Research Articles

Reduction of Structural Defects in the GaSb Buffer Layer on (001) GaP/Si for High Performance InGaSb/GaSb Quantum Well Light-Emitting Diodes.

Monolithic integration of GaSb-based optoelectronic devices on Si is a promising approach for achieving a low-cost, compact, and scalable infrared photonics platform. While tremendous efforts have been put into reducing dislocation densities by using various defect filter layers, exploring other types of extended crystal defects that can exist on GaSb/Si buffers has largely been neglected. Here, we show that GaSb growth on Si generates a high density of micro-twin (MT) defects as well as threading dislocations (TDs) to accommodate the extremely large misfit between GaSb and Si. We found that a 250 nm AlSb single insertion layer is more effective than AlSb/GaSb strained superlattices in reducing both types of defects, resulting in a 4× and 13× reduction in TD density and MT density, respectively, compared with a reference sample with no defect filter layer. InGaSb quantum well light-emitting diodes were grown on the GaSb/Si templates, and the effect of TD density and MT density on their performance was studied. This work shows the importance of using appropriate defect filter layers for high performance GaSb-based optoelectronic devices on standard on-axis (001) Si via direct epitaxial growth.

The Effect of Microstructure and Temperature Gradient on Radiation-Induced Swelling of Austenitic Steel

Radiation porosity through-thickness of the fuel pin cladding, made of 16Cr-19Ni-2Mo-2Mn-Nb-Ti-V-P-B steel, has been studied with scanning electron microscopy using backscattered electron (BSE) detector. The examined sample was irradiated at a temperature around 480 °С up to an integral damage dose of 87 dpa. It was shown that, due to the temperature gradient through the cladding thickness, the average size of radiation voids reduces, and their concentration increases from internal to external surface. Local nonuniformity of radiation porosity is observed in regions close to internal and external surfaces. It was shown that, non-uniformity of radiation porosity is determined by the material structure, microtwin density and high concentration of low-angle inter-granular boundaries, in particular.

Эволюция микроструктуры и ее параметров с деформацией в поликристаллических сплавах медь - алюминий с разной энергией дефекта упаковки

Evolution of the dislocation structure during active plastic deformation was carried out in copper-aluminum alloys with Al content of 0.5-14 at. % using transmission electron microscopy. Analysis of the types of the dislocation substructure as a function of the alloying element content and deformation degree was conducted. The following parameters of the defect substructure were measured: average scalar dislocation density, curvature-torsion of the crystal lattice and microtwin density. The effect of stacking fault energy on accumulation of defects in the alloys was observed and evaluated.

Characterization of antimonide based material grown by molecular epitaxy on vicinal silicon substrates via a low temperature AlSb nucleation layer

We report on the characterization of GaSb layers grown on silicon substrates using an AlSb nucleation layer. In particular, we investigate the influence of the AlSb layer thickness when this nucleation layer is grown at low temperature (400°C). X-ray diffraction techniques, atomic force microscopy and transmission electron microscopy were used to characterize the material properties. We demonstrate that there exists a correlation between the micro-twin density, the surface roughness and the broadening of the ω-scan GaSb peaks. Moreover, the AlSb thickness has a strong influence on the micro-twin density, and must be carefully optimized to improve the GaSb quality.

(Invited) High Quality 3C-SiC for MOS Applications

In this work, the growth of high quality 3C-SiC films on Si substrates grown by a hot-wall chemical vapor deposition (CVD) reactor is presented. An increased crystal quality means a reduced crystallographic defect density affecting 3C-SiC films which can be achieved by reducing the growth rate during 3C-SiC heteroepitaxy. In particular, the micro-twin density was observed to decrease with decreasing growth rate allowing for the reduction of theboth the rocking curve and transverse optical Raman mode peak width. Si substrates, both of on- and off-axis orientation, are considered with improvement in crystal quality observed when off-axis substrates are used. Finally, stacking faults and microtwins at the SiC/SiO2 interface are seen to cause non-uniformity in the oxide layer due to the different oxidation rate observed.

Features of molecular-beam epitaxy and structural properties of AlInSb-based heterostructures

AlInSb layers are grown on highly lattice-mismatched GaAs (100) substrates by molecular-beam epitaxy (MBE) and studied in situ by reflection high-energy electron diffraction and ex situ by scanning and transmission electron microscopy (SEM and TEM). It is shown that one feature of AlInSb/GaAs heterostructure features is a high probability of forming microtwins; methods for decreasing their concentration are proposed. To initiate AlInSb growth on GaAs substrates under high lattice-mismatch (∼14.5%) conditions and to stimulate the transition to 2D growth, the GaAs layer surface was preliminarily exposed to an antimony flux followed by deposition of an intermediate AlSb buffer layer. The optimization of initial MBE growth stages of Sb-containing layers on the GaAs surface allows a decrease in the defect density in the GaAs/AlInSb heterostructures more than by two orders of magnitude, including a drastic decrease in the microtwin density. Optimal MBE growth conditions for AlxAl1 − xSb are determined in a wide composition range (0 < x < 0.3). The TEM and SEM studies confirm the high structural quality of grown GaAs/AlInSb heterostructures. Hall-effect measurements showed the dependence of the carrier mobility and concentration on the aluminum content in AlInSb layers and allowed preliminary conclusions on scattering mechanisms.

Microtwin reduction in 3C–SiC heteroepitaxy

In this letter, we have studied the density of microtwins in the vicinity of the film surface, by using in-plane grazing incidence diffraction technique, to suggest a reduction trend of such defects at different growth rates. In (100) 3C–SiC heteroepitaxy, microtwin density decreases when a slower growth rate process is performed. A reduction ratio of microtwins of 0.16, 0.2, and 0.28 for 2 μm/h, 5 μm/h, and 10 μm/h, respectively, has been obtained. The relationship between microtwin density and growth rate, or Si/H2 ratio, found with this technique is in perfect agreement with the pre-existent literature.

Micro-twin Defects in InSb/AlInSb layers grown on (001) GaAs ∼ Application of the [formula omitted]-directional TEM analysis ∼

Methods for the reduction of micro-twin defects in InSb/AlInSb epilayers were investigated by using 〈1̄16〉-directional TEM analysis. The use of a 2°off-axis GaAs (001) substrate which has been known as an effective way to reduce micro-twin densities was combined with another approach, the use of an As2 beam for oxide desorption from a GaAs substrate surface. The combined method yields a micro-twin density of 3.3×102/cm, which is 3.0 times smaller than for a similar previous method with an Sb2 flux. This study also demonstrates the usefulness of 〈1̄16〉-directional TEM analysis for investigating micro-twin defects in (001) thin films.

Structural defects in (100) 3C-SiC heteroepitaxy: Influence of the buffer layer morphology on generation and propagation of stacking faults and microtwins

Growth of 3C-SiC on (100) Si has been performed via chemical vapor deposition under two pressure regimes (low and atmospheric pressure) in the early stage of growth. Atomic force microscopy (AFM) and transmission electron microscopy (TEM) have been conducted to study the initial stage of growth while X-ray diffractometry (XRD) and TEM have been used to analyze thicker films and to detect and quantify defects, resulting in a comprehensive and detailed investigation of 3C-SiC structural defects. We have found out that the secondary nucleation of 3C-SiC island growth leads to a higher defect generation rate and, at the same time, to a more effective defect elimination rate. Hillocks found on the surface of thin samples grown under reduced pressure conditions are more pronounced as they seem to be a consequence of twins created in the early stage of growth. Finally, a different initial nucleation density (in the two pressure regimes considered) does not strongly influence stacking fault and microtwin density when growth of thick 3C-SiC films is performed. A very strong influence is indeed observed when 3C-SiC thickness is limited to hundreds of nanometers.

Substructure of titanium alloys after cyclic heat treatment

The fundamental structure parameter determining the properties of titanium alloys is the grain size. The grain size can be controlled by means of the lattice strain and the internal work-hardening which accompany the phase transitions occurring during repeated heating↔cooling cycles. During cyclic heat treatments, there are conditions facilitating the cumulation of internal work-hardening. The structural research carried out has shown that in the pre-treatment cycles the structural defects refer mainly to the α phase and manifest itself by an increase in the microtwin density. Larger numbers of cycles increase the dislocation density mainly in the α phase and then in the β phase.

The Reduction of The Defect Density in CdTe Buffer Layers for The Growth of HgCdTe Infrared Photodiodes on Si (211) Substrates

CdTe epilayers were grown by molecular beam epitaxy on As-passivated nominal (211) Si substrates using thin interfacial ZnTe layers. By using thin recrystallized (initially amorphous) ZnTe buffei layers, we utilized migration enhanced epitaxy (MEE) in the ZnTe layer and overcome the tendency toward three dimensional nucleation. The threading dislocation densities in 8–9 tm thick CdTe films deposited on the recrystallized amorphous ZnTe films were in the range of 2 to 5 × 105 cm−2. In addition to the reduction of threading dislocation density, the interface between the ZnTe layers and the Si substrate is much smoother and the microtwin density is an order of magnitude lower than in regular MEE growth. In order to understand the initial nucleation mechanism of the ZnTe on the As precursor Si surface, we also grew ZnTe epilayers on Te precursor treated Si substrates. The growth mode, microtwin density, and threading dislocation density are compared for films grown on Si substrates with different surface precursors and grown by different growth methods.

The Reduction of The Defect Density in CdTe Buffer Layers for The Growth of HgCdTe Infrared Photodiodes on Si (211) Substrates

CdTe epilayers were grown by molecular beam epitaxy on As-passivated nominal (211) Si substrates using thin interfacial ZnTe layers. By using thin recrystallized (initially amorphous) ZnTe buffer layers, we utilized migration enhanced epitaxy (MEE) in the ZnTe layer and overcome the tendency toward three dimensional nucleation. The threading dislocation densities in 8–9 μm thick CdTe films deposited on the recrystallized amorphous ZnTe films were in the range of 2 to 5 × 105cm−2. In addition to the reduction of threading dislocation density, the interface between the ZnTe layers and the Si substrate is much smoother and the microtwin density is an order of magnitude lower than in regular MEE growth. In order to understand the initial nucleation mechanism of the ZnTe on the As precursor Si surface, we also grew ZnTe epilayers on Te precursor treated Si substrates. The growth mode, microtwin density, and threading dislocation density are compared for films grown on Si substrates with different surface precursors and grown by different growth methods.

Structural Characterization of GaP/GaAs/GaP Heterostructure by TEM

ABSTRACTThe defect structure in a GaP/GaAs/GaP heterostructure deposited on a (001) GaAs substrate with a GaAs buffer layer has been characterized by cross sectional TEM. The buffer layer presents dislocations and (α-δ)-fringe contrast parallel to (001) interface plane. HREM study reveals uniformly distributed amorphous capsules in the first GaP/GaAs buffer layer interface. The dominant defects are microtwins which are propagated into the overall heterostructure. Microtwin density is different in the GaP and GaAs layers.The different stress signs may explain the density difference.

Low temperature microstructure and phase transitions in La 2−xSr xCuO 4 and La 2−xBa xCuO 4

We have studied microstructural phenomena and structural phase transitions of La 2− x Sr x CuO 4 (with 0≤ x≤0.15) and La 2− x Ba x CuO 4 (with x=0.125) at low temperatures by convergent beam electron diffraction and electron microscopy. Microtwin density in the orthorhombic phase is found to increase with increasing dopant concentration. At the lowest temperature (∼10 K) in the present experiment, La 2− x Sr x CuO 4 remains orthorhombic and La 2− x Ba x CuO 4 exhibits a fine mixture of orthorhombic and tetragonal phases with a typical domain size ≤300 Å. The new phase transition does not alter the twin domain configurations that already exists in the orthorhombic phase. A new strain-relieving mechanism for the new phase transition with small variations of lattice parameters among the orthorhombic and tetragonal domains is observed. Convergent beam electron diffraction studies indicate that the samples are relatively homogeneous and free of distinct second phases, and variation of short range (∼ lattice parameter) lattice ordering, however, is commonly observed on a scale of ∼ 100 Å.

The Effect of Rapid Thermal Annealing on Heteroepitaxial Structures

ABSTRACTWe present evidence on the types of structural changes caused by the rapid thermal annealing of two types of heteroepitaxial layers: CaF2/CoSi2/Si(111) and Si(100) on A12O3 (1102). We find that grains in a film can be merged into a single crystal and that the microtwin density can be dramatically lowered. We also find a number of changes in the structure of the heteroepitaxial interfaces.

Microtwin morphology and volume fraction for silicon on sapphire

In order to understand the influence of various growth and processing procedures on the quality of silicon grown on sapphire (SOS), one needs a way to measure microtwin density as a function of distance from the interface. In previous studies, however, the technique best suited for determining such a density profile, transmission electron microscopy (TEM), has been applied to this problem in a relatively qualitative manner. We have chosen to apply TEM in a more quantitative fashion to this problem by keying our observations to microtwin morphology. These observations indicate that the microtwins occurring in SOS grown by chemical vapor deposition are faceted and grow contiguously from the interface. The volume fraction of silicon occurring as microtwins does not decrease away from the interface at as large a rate as indicated by previous TEM studies. Instead, the decrease in the number of twins away from the interface appears to be balanced by an increase in twin size. It also appears that the greatest volume fraction of twinned material occurs at least several hundred angstroms away from the interface rather that at the interface.

Microtwin morphology and density for silicon on sapphire

In order to understand the influence of various growth and processing procedures on the quality of silicon grown on sapphire (SOS), one needs a way to measure microtwin density as a function of distance from the interface. In previous studies, however, the technique best suited for determining such a density profile, transmission electron microscopy (TEM), has been applied to this problem in a relatively qualitative manner. We have chosen to apply TEM in a more quantitative fashion to this problem by keying our observations to microtwin morphology as determined by trace analysis and stereomicroscopy (Fig.1). These observations indicate that the microtwins occuring in SOS grown by chemical vapor depostition (CVD) are faceted and grow contiguously from the interface. The volume fraction of silicon occuring as microtwins does not decrease away from the interface at as large a rate as indicated by previous TEM studies (Fig.2). Instead, the decrease in the number of twins away from the interface appears to be balanced by an increase in twin size.

Enhanced performance of 2 μm N-MOSFETs in doubly recrystallised SOS films

The field-effect channel electron mobility in 0.5 μm-thick silicon-on-sapphire films is increased by up to 22%, while the back-channel leakage current is decreased 100-fold, following a double recrystallisation by means of Si ion implantation/amorphisation and solid-phase epitaxy. The improved electrical performance correlates with a more than 100-fold reduction in the microtwin density, as measured by X-ray diffraction.

Specular reflectance and surface roughness of silicon on sapphire

As part of an evaluation of the quality of silicon on sapphire (SOS) wafers, we have measured the specular reflectance over the wavelength region from 400 to 260 nm. The spectral data allow the separation of the effects on the reflectance produced either by the surface roughness or by any departure of the wafer surface from perfect crystallinity. A recently established optical evaluation technique employing reflectance at only two discrete ultraviolet (UV) wavelengths is unable to make this distinction. However, our results for virgin SOS wafers show that the observed deviations of the UV reflectance relative to that of well polished single crystal silicon can be entirely accounted for by the surface roughness. The relationships between UV reflectance and microtwin density observed for SOS films are not yet explained but are found to be specific to the process by which the SOS film was grown and to vary over much wider ranges than has been previously reported.

Silicon on Sapphire of Single Crystal Quality Obtained by Double Solid Phase Epitaxial Regrowth

AbstractA dual-implantation and annealing procedure has been refined which results in silicon films of single crystal perfection on sapphire substrates. This double solid phase epitaxy (DPSE) procedure consists of two self-implantation steps that serve to amorphize the defect structure of the parent silicon film with subsequent annealing steps employed to recrystallize the amorphized structure. High resolution cross-section transmission electron microscopy (HRXTEM) and ion channeling have been employed to study the microstructure of the silicon films, the sapphire substrates, and the interface. The resulting perfection of the silicon layer is shown to be a sensitive function of the first self-implant energy, dose, and silicon film thickness. Also, a correlation has been established between microtwin density and dechanneling profiles. HRXTEM of microtwins in silicon on sapphire (SOS) starting material demonstrates contrast features identified as rotational Moire' fringes between the microtwins and the matrix by means of optical diffractometry. Best results are achieved for the first implant energy of 170 keV 28Si+. Thus, the DSPE process is shown to give optimal results in defect reduction.