Distribution Of Extended Defects Research Articles

Structural and luminescence imaging and characterisation of semiconductors in the scanning electron microscope

The scanning electron microscopy techniques of electron backscatter diffraction (EBSD), electron channelling contrast imaging (ECCI) and cathodoluminescence (CL) hyperspectral imaging provide complementary information on the structural and luminescence properties of materials rapidly and non-destructively, with a spatial resolution of tens of nanometres. EBSD provides crystal orientation, crystal phase and strain analysis, whilst ECCI is used to determine the planar distribution of extended defects over a large area of a given sample. CL reveals the influence of crystal structure, composition and strain on intrinsic luminescence and/or reveals defect-related luminescence. Dark features are also observed in CL images where carrier recombination at defects is non-radiative. The combination of these techniques is a powerful approach to clarifying the role of crystallography and extended defects on a material’s light emission properties. Here we describe the EBSD, ECCI and CL techniques and illustrate their use for investigating the structural and light emitting properties of UV-emitting nitride semiconductor structures. We discuss our investigations of the type, density and distribution of defects in GaN, AlN and AlGaN thin films and also discuss the determination of the polarity of GaN nanowires.

Nature and spatial distribution of extended defects in Czochralski-grown Ca3RE2(BO3)4 (RE = Y, Gd) orthoborate single crystals

Pure and Nd-doped Ca3Y2(BO3)4 and Ca3Gd2(BO3)4 orthoborate (space group Pnma) single crystals are considered for applications as materials for optoelectronics, in particular for ultrafast pulse lasers. In the present work, Czochralski-grown single crystals of such materials were studied. All of them were nearly free of defects such as impurity phases or gas bubble inclusions, previously observed and reported in similar orthoborates. Laboratory high-resolution x-ray diffraction together with synchrotron radiation-based white-beam x-ray topography have been employed as experimental techniques. In this paper we perform for the first time a systematic study of extended defects present in those crystals, including the spatial distribution of the defects. The crystal quality of the studied Ca3RE2(BO3)4 samples is relatively good—the mean values of double-axis rocking-curve full width at half maximum are low enough low (in the order of 32–131 arcsec) and comparable with literature results obtained for other borate crystals. The chemical composition is generally uniform along the samples. The nature of the extended defects is recognized—they are mainly crystal blocks with low misorientation angles ⩽0.06°), micromosaics, dislocations, and crystallographic plane bending. A detailed analysis of the bend for each crystal is presented. Simple criteria for classifying bulk crystals in terms of bending profile are formulated.

Non-Destructive In Situ Study of Plastic Deformations in Diamonds: X-ray Diffraction Topography and µFTIR Mapping of Two Super Deep Diamond Crystals from São Luiz (Juina, Brazil)

Diamonds from Juina, Brazil, are well-known examples of superdeep diamond crystals formed under sublithospheric conditions and evidence would indicate their origins lie as deep as the Earth’s mantle transition zone and the Lower Mantle. Detailed characterization of these minerals and of inclusions trapped within them may thus provide precious minero-petrogenetic information on their growth history in these inaccessible environments. With the aim of studying non-destructively the structural defects in the entire crystalline volume, two diamond samples from this locality, labelled JUc4 and BZ270, respectively, were studied in transmission mode by means of X-ray Diffraction Topography (XRDT) and micro Fourier Transform InfraRed Spectroscopy (µFTIR). The combined use of these methods shows a good fit between the mapping of spatial distribution of extended defects observed on the topographic images and the µFTIR maps corresponding to the concentration of N and H point defects. The results obtained show that both samples are affected by plastic deformation. In particular, BZ270 shows a lower content of nitrogen and higher deformation, and actually consists of different, slightly misoriented grains that contain sub-grains with a rounded-elongated shape. These features are commonly associated with deformation processes by solid-state diffusion creep under high pressure and high temperature.

Reduction of Dislocation Density in Bulk Silicon Carbide Crystals Grown by PVT on Profiled Seeds

The distribution of extended defects in silicon carbide (SiC) crystals grown on profiled seeds by the sublimation (physical vapor transport) method has been studied by optical microscopy in combination with chemical etching. It is established that free lateral growth on protruding relief elements (mesas) is accompanied by a sharp decrease in the density of threading dislocations and micropipes. The decreased density of dislocations is retained after growing a thick layer that involves the overgrowth of grooves that separated individual mesas.

Structural Perfection of Silicon Carbide Crystals Grown on Profiled Seeds by Sublimation Method

— The distribution of extended defects in silicon carbide (SiC) crystals grown on profiled seeds by the sublimation (physical vapor transport) method has been studied by optical microscopy in combination with chemical etching and AFM. It is established that free lateral growth on protruding relief elements (mesas) is accompanied by a sharp decrease in the density of threading dislocations and micropipes. The decreased density of dislocations is retained after growing a thick layer that involves the overgrowth of grooves that separated individual mesas.

Defect creation in InGaAs/GaAs multiple quantum wells–I. Structural properties

We present a systematic study of extended defect creation in InGaAs/GaAs multiple quantum well (MQW) structures. Three sets of samples, grown by molecular beam epitaxy, were characterized by high-resolution x-ray diffraction and transmission electron microscopy. First, in a temperature series, optimal deposition temperature of 505°C was found for the In composition of 20% as determined from dislocation loop (DL) density and inspection of diffuse scattering patterns. InGaAs decomposition and lateral layer thickness undulations were observed above this optimal temperature. Second, increase of MQW periodicity from ×5 to ×10 revealed a thickness-related cumulative deterioration, characterized by increased likelihood of defect intersection with continued MQW growth, as suggested by an increase of the secondary DL density from ~1.6×107cm−2 to ~6×108cm−2. Additional strained layers experienced an ever-degrading quality of the growth surface. Third, a set consisting of samples with three different MQW periods was investigated. Different stages, suggested by a model of defect creation, were obtained for these different MQW periods, allowing specification of particular type, density, and spatial distribution of extended defects for each stage of defect creation.

Photoluminescence Imaging of Semiconductors

The development of energy-related devices requires new materials and fabrication processes that enable both an enhancement in performance and a decrease in manufacturing cost. Polycrystalline semiconductors are promising for photovoltaic applications because they can be grown at low temperatures on inexpensive substrates. Metamorphic epitaxy of semiconductor alloys on latticemismatched substrates is also increasingly used to access new semiconductor alloys for visible light emitting diodes used in high efficiency solid-state lighting. With these advancements, defects play a progressively more prominent role in device performance, and it is important to be able to probe their behavior in order to understand and ultimately control their effects. Information about the spatial distribution of extended defects and compositional fluctuations as well as their influence on carrier transport is particularly crucial for materials engineering efforts. Characterization techniques must therefore be able to provide real time feedback with micron-scale spatial resolution. We present a novel photoluminescence (PL) imaging technique with both of these capabilities and discuss the information that can be obtained in a number of semiconductor systems.

Comparison of Carrier Lifetime Measurements and Mapping in 4H SiC Using Time Resolved Photoluminscence and μ-PCD

Carrier lifetime measurements and wafer mappings have been done on several different 4H SiC wafers to compare two different measurement techniques, time-resolved photoluminescence and microwave induced photoconductivity decay. The absolute values of the decay time differ with a factor of two, as expected from recombination and measurement theory. Variations within each wafer are comparable with the two techniques. Both techniques are shown to be sensitive for substrate quality and distribution of extended defects.

Microscopic distribution of extended defects and blockage of threading dislocations by stacking faults in semipolar (11¯01) GaN revealed from spatially resolved luminescence

Spatial distribution of extended defects in semipolar (11¯01)-oriented GaN layers grown on patterned (001) Si substrates with striped grooves of varying width was investigated by optical means only using near-field scanning optical microscopy (NSOM) and cathodoluminescence (CL). A high density of basal and prismatic stacking faults was observed in the c− wings, and the threading dislocations in c+ wings, which appear as dark patterns in the NSOM and CL images, were found to bend toward the surface during the initial stages of growth. In the case when growing c+ front of GaN made contact with the SiO2 masking layer during growth, stacking faults were found to form also in the c+ wings. These additional stacking faults effectively blocked propagation of dislocations along the c+ direction, resulting in high quality stripes virtually free of defects. As revealed by optical means only without the need for any structural investigation, such control over the threading dislocation density using select growth geometries is potentially advantageous for improving semipolar (11¯01)GaN.

Defect Generation Mechanisms in PVT-Grown AlN Single Crystal Boules

A systematic study on the density and distribution of extended defects in a typical single crystal AlN boule grown by the physical vapor transport (PVT) method has been carried out in order to gain a detailed understanding of the formation of defects such as dislocations and low angle grain boundaries (LAGBs). Boule surface studies reveal that LAGBs are nucleated during initial stages of growth and propagate to the end of growth. Basal plane dislocations (BPDs) are generated during growth due to thermal gradient stresses. Higher BPD densities are found near the LAGBs at the boule edges due to additional stresses from constrained growth. Threading edge dislocations (TEDs) are typically replicated from the seed, and LAGBs composed of arrays of threading dislocation walls are formed to accommodate the c-axis rotation between different groups of threading screw dislocation (TSD) mediated growth centers.

Structural Perfection of Silicon Carbide Crystals Grown on Profiled Seeds by Sublimation Method

The distribution of extended defects in silicon carbide (SiC) crystals grown on profiled seeds by the sublimation (physical vapor transport) method has been studied by optical microscopy in combination with chemical etching. It is established that free lateral growth on protruding relief elements (mesas) is accompanied by a sharp decrease in the density of threading dislocations and micropipes. The decreased density of dislocations is retained after growing a thick layer that involves the overgrowth of grooves that separated individual mesas.

Structural perfection of silicon carbide crystals grown on profiled seeds by sublimation method

The distribution of extended defects in silicon carbide (SiC) crystals grown on profiled seeds by the sublimation sandwich method (physical vapor transport) has been studied by optical microscopy in combination with chemical etching. It is established that free lateral overgrowth of protruding relief elements (mesas) is accompanied by a sharp decrease in the density of threading dislocations and micropores. The decreased density of dislocations is retained after growing a thick layer that involves the overgrowth of grooves that separated individual mesas.

Characterization of dislocation arrays in AlN single crystals grown by PVT

AbstractThe density and distribution of extended defects in AlN single crystals grown by physical vapor transport (PVT) was studied by bright field and polarized light microscopy, defect‐selective etching, and high resolution X‐ray diffraction (HRXRD). Etch pits associated with dislocation arrays forming low angle grain boundaries (LAGB) were observed and two types of LAGB were identified. Reduction of LAGB in AlN grown under reduced radial temperature gradients, as estimated by simulation of the growth cell, was demonstrated.

Epitaxial lateral overgrowth of non-polar GaN(1 1̄ 0 0) on Si(1 1 2) patterned substrates by MOCVD

m-Plane GaN was grown selectively by metal–organic chemical vapor deposition (MOCVD) on patterned Si(1 1 2) substrates, where grooves aligned parallel to the Si〈1 1 0〉 direction were formed by anisotropic wet etching to expose the vertical Si{1 1 1} facets for growth initiation. The effect of growth conditions (substrate temperature, chamber pressure, and ammonia and trimethylgallium flow rates) on the growth habits of GaN was studied with the aim of achieving coalesced m-plane GaN films. The epitaxial relationship was found to be GaN(1 1̄ 0 0) || Si(1 1 2), GaN[0 0 0 1] || Si[1 1 –1], GaN[1̄ 1̄ 2 0] || Si[1 1̄ 0]. Among all growth parameters, the ammonia flow rate was revealed to be the critical factor determining the growth habits of GaN. The distribution of extended defects, such as stacking faults and dislocations, in the selectively grown GaN were studied by transmission electron microscopy in combination with spatially resolved cathodoluminescence and scanning electron microscopy. Basal-plane stacking faults were found in the nitrogen-wing regions of the laterally overgrown GaN, while gallium-wings were almost free of extended defects, except for the regions near the GaN/Si{1 1 1} vertical sidewall interface, where high dislocation density was observed.

Cathodoluminescence mapping and spectroscopy of Te-doped [formula omitted] grown by the vertical Bridgman method under an alternating magnetic field

Cathodoluminescence (CL) in the scanning electron microscope and wavelength dispersive X-ray microanalysis (WDX) have been used to assess the homogeneity of a whole Te-doped In x Ga 1− x Sb ingot grown by the vertical Bridgman method under an alternating magnetic field. In particular, WDX has been used to determine the chemical composition of the ingot along the growth axis and several radial directions, while CL has been used to investigate the effective incorporation of In into the alloy, the nature and distribution of extended defects influencing the luminescence of the material and the shape evolution of the growth interfaces along the growth axis. CL spectroscopy reveals that doping with Te influences the band gap energy of this ternary compound through the Moss–Burstein effect.

Mapping of extended defects in B-doped (001) homoepitaxial diamond films by electron-beam-induced current (EBIC) and cathodoluminescence (CL) combination technique

We have studied on a spatial distribution of extended defects with shell-shape pits in B-doped (001) homoepitaxial diamond films using electron-beam-induced current (EBIC) and cathodoluminescence (CL) combination technique. The EBIC image corresponded to the comprehensive one of band-A emission (430 nm) in CL measurements. In addition, both EBIC and CL images also corresponded to optical microscope image (Nomarski) with shell-shape pits. These experimental results indicate that the shell-shape pits are accompanied with localized electrically-active-defects originating from extended defects such as dislocation with band-A CL emission. It is suggested that extended defects such as dislocations result in the formation of shell-shape pits.

Two-Dimensional Growth Mode and Reduction of Dislocation Density in Nitride Layers

AbstractHeteroepitaxial two-dimensional (2D) and three-dimensional (3D) MOVPE growth of nitride layers on sapphire substrates using low temperature (LT) AlN nucleation layer are discussed. It is shown by Atomic Force Microscopy and Transmission Electron Microscopy that the 2D or 3D growth mode of successive AlGaN layers depends predominantly on the growth conditions of the underneath LT AlN nucleation layer.Modifications of growth conditions of LT AlN nucleation layer in respect of the growth in the conventional way, leads to a drastic change in properties of the successively grown at high temperature AlGaN layer. Main modification is connected with a drastic reduction of the growth rate of LT AlN nucleation layer at higher rector pressure. In this case the growth at high temperature starts evenly on the whole surface, retaining atomic flatness, and occurs in the 2D mode. Therefore, it is possible to grow even a very thin, continuous AlGaN layers directly on top of LT nucleation layer.The 2D growth mode leads to significant change in extended defect distribution. Edge dislocation density being close to 1010 cm−2, results in the effective relaxation of the internal stress. On the other hand mix dislocations density are close to 108 cm−2. All observed mix dislocations occur in pairs with dislocations with opposite sign. This suggests that all mix dislocations originate from dislocation half-loops. Both types of dislocations, edge and half loops can be effectively reduce in-situ, during the growth using annealing in the presence of silane. This process and precise control of the growth conditions leads to decreasing of mix dislocations density to 2·106 cm−2 and edge dislocations density to 5·108 cm−2.The newly developed two-dimensional growth mode of nitride layer opens up a new way to reduce strain, decrease density of dislocations and improvement of crystallographic properties.

Modeling of extrinsic extended defect evolution in ion-implanted silicon upon thermal annealing

A physically motivated model that accounts for the spatial and temporal evolution of extended defect distribution in ion-implanted Si is presented. Free physical parameters are extracted from experimental data and by means of a genetic algorithm (GA). Transmission electron microscopy (TEM) data and self-interstitial oversaturation measurements are combined in the same fitting procedure to eliminate unphysical solutions and find the optimum set of parameters. The calibration of parameters shows that binding energies of small self-interstitial clusters exhibit strong minima, as reported in other investigations. It is demonstrated that the calibrated model we propose is able to predict a wide variety of physical phenomena, from the oversaturation of self-interstitials via the mean-size distribution of {1 1 3} defects to the depth distribution of the density of the latter.

EBIC Characterization of III–Nitride Structures Using Multifractal Parameterization

Use of electron beam induced current (EBIC) method in combination of the multifractal analysis for investigation of GaN epitaxial layers with different degree of order of mosaic structure has revealed basic difference in distribution of extended defects with intense nonradiative recombination and small diffusion length of about 0.1 μm. The diffusion length of the layers with well ordered mosaic structure was determined to be 1.5–2 times longer than that of the layers with less-ordered mosaic structure. The difference between the extended defect distributions well correlates with peculiarities of carrier transfer in these layers.

Reduction of Planar Defect Density in Laterally Overgrown Cubic-GaN on Patterned GaAs(001) Substrates by MOVPE

The metalorganic vapor phase epitaxy (MOVPE) growth of cubic-GaN (c-GaN) layers has been performed on the [110]-stripe patterned GaAs (001) substrates. The surface morphology of the laterally overgrown layer was much improved with the formation of the flat (311)A surfaces as the growth proceeded. It is shown that the c-GaN layer thicker than 20 μm was grown on GaAs(001) substrates with the stripe direction in [110] for 90 min. Microstructure and extended defect distribution in the laterally overgrown c-GaN films have been analysed by transmission electron microscopy (TEM). It is found that the planar defect (stacking faults and twins) density drastically decreases at the region away from the substrate toward the top of the c-GaN stripe. On the other hand, the dislocations become dominant. The density of the dislocations was found to be lower than 10 8 cm -2 . These results suggest that the lateral overgrowth on [110]-stripe-patterned GaAs (001) substrates via MOVPE is an efficient method for obtaining a low planar defect density c-GaN layer.