Defect studies in 4H- Silicon Carbide PVT grown bulk crystals, CVD grown epilayers and devices

Silicon Carbide [SiC]which exists as more than 200 different polytypes is known for superior high temperature and high power applications in comparison to conventional semiconductor materials like Silicon and Germanium. The material finds plethora of applications in a diverse fields due to its unique properties like large energy bandgap, high thermal conductivity and high electric breakdown field. Though inundated with superior properties the potential of this material has not been utilized fully due to impeding factors such as defects especially the crystalline ones which limit their performance greatly. Lots of research has been going on for decades to reduce these defects and there has been subsequent improvement in the quality as the diameter of SiC commercial wafers has reached 150mm from 25mm since its inception. The main focus of this thesis has been to study yield limiting defect structures in conjunction with several leading companies and national labs using advanced characterization tools especially the Synchrotron source. The in depth analysis of SiC has led to development of strategies to reduce or eliminate the density of defects by studying how the defects nucleate, replicate and interact in the material. The strategies discussed to reduce defects were proposed after careful deliberation and analysis of PVT grown bulk crystals and CVD grown epilayers. Following are some of the results of the study: [1] Macrostep overgrowth mechanism in SiC was used to study the deflection of threading defects onto the basal plane resulting in stacking faults. Four types of stacking faults associated with deflection of c/c+a threading defectshave been observed to be present in 76mm, 100mm and 150mm diameter wafers. The PVT grown bulk crystals and CVD grown epilayers in study were subjected to contrast studies using synchrotron white beam X- ray topography [SWBXT]. The SWBXT image contrast studies of these stacking faults with comparison of calculated phase shifts for postulated fault vectors by macrostep overgrowth of surface outcrops, has revealed faults to be of four types of which one of the following are discussed in detail which is the Shockley faults. The fault vector were determined by taking into account the contrast from stacking faults in SWBXT undergoing phase shift as the X-ray wave fields cross the fault plane. The deflected dislocations onto the basal plane were responsible for the stacking faults and were observed to be detrimental to the devices grown on them as they replicate to the epilayer. In the wafers studied at different stages of the SiC crystal boule resulted in reduction of threading defects as they at certain stage get deflected out of the crystal causing drop of defects density. [2] A novel technique known as the Ray Tracing Simulation was used to determine the sense of c/c+a dislocations obtained via Grazing- Incidence X-ray Topography. Determination of the complete sense and burgers vector of these dislocations was very important to augment our proposed models on stacking faults associated with these defects. Orientation contrast mechanism in X- ray diffraction topography was previously determined to be the dominant factor in SiC by our group and the same principles were used for the simulation. The results were surmised after extensive comparison between experimental and simulation images for the c+2a defects. [3] With the BPD density down to a record level of few hundred per square centimeter in several wafers in multiple regions made it possible to observe the conversion of sessile Threading Edge Dislocations [TED] to glissile BPDs with this repeating multiple times. Previously the high density of Basal Plane Dislocations [BPD] prevented from discerning the details accurately in the SiC images taken by SWBXT. The contribution of SWBXT in accurately categorizing the nature of dislocations in SiC has enabled the crystal growth community to incorporate strategies to mitigate their influence. One of them has been recognizing BPDs as deformation induced defects which have led to the development of strategies to reduce stress imperative for the motion of BPDs to levels below critical resolved shear stress. This in turn has provided an opportunity for last five years to resolve important defect interactions in the crystals with one of them being the operation of single- ended Frank Read source for the first time in SiC. [4] Failure analysis of SiC bipolar devices using SWBXT and correlation with defect density has been studied to determine how the defect density affect breakdown voltage of high power junction diodes. It was observed that the screw dislocation density unlike in failure analysis studies performed previously did not affect the breakdown voltage for these Junction Barrier Schottky (JBS) rectifiers. The defects that were detrimental were the triangular defects, stacking faults and micropipes in bipolar devices observed on 4H- SiC patterned wafers. | 129 page

Hydrothermal coating of ZnO onto calcium alumino silicate beads and their application in photodegradation of amaranth dye

Hydrothermal coating of ZnO nanoparticles onto the surface of calcium alumino silicate beads was carried out under hydrothermal conditions (T, 220 degrees C; P, 300 psi; duration, 12 h). The reagent grade ZnO and calcium alumino silicate beads (0.5-1.0 mm in diameter, specially prepared as supporting material for ZnO) were used as starting materials along with 1M NaOH as a mineraliser leading to the formation of a new class of photocatalytic material. The effect of the hydrothermal experimental parameters on the coating of ZnO nanoparticles, and the grain morphology, etc. was investigated and thus obtained ZnO coated calcium alumino silicate beads were characterised using X-ray diffraction, SEM, Fourier transform infrared and positron annihilation spectroscopy. Sunlight and ultraviolet light mediated photocatalytic degradation of amaranth dye was studied using hydrothermally prepared ZnO coated calcium alumino silicate beads. The effect of various parameters such as initial dye concentration, catalytic loading, pH of the medium, time duration and light source on the photodegradation of amaranth dye was investigated. Silk industrial effluents containing amaranth dye as a major constituent along with other dyes and dyeing auxiliaries were treated using ZnO coated calcium alumino silicate beads

Hydrothermal preparation of ZnO:CNT and TiO2:CNT composites and their photocatalytic applications

ZnO:CNT and TiO2:CNT composites were fabricated under mild hydrothermal conditions (T = 150–240 °C) with an autogenous pressure. The as prepared composites were characterized using X-ray diffraction, Scanning electron microscopy and FTIR spectroscopy. Photocatalytic applications of the composites were investigated using indigo caramine dye. The effect of the catalyst content, pH of the medium, source and intensity of illumination on the photodegradation of the indigo caramine dye was studied and the efficiency of the composites were investigated based on different parameters like percent transmittance (%T), percent decomposition, and chemical oxygen demand of the dye solution to obtain optimum treatment conditions. The results obtained exhibit higher photocatalytic activity when compared to the reagent grade ZnO, TiO2 and hydrothermally prepared ZnO:AC and TiO2:AC composites

Effect of zeolite particulate filler on the properties of polyurethane composites

Polymer-Zeolites composites have been prepared, using castor oil based polyurethane (PU) as a host and AlPO4-5 as particulate filler. The prepared PU/zeolite composites have been characterized for mechanical properties such as tensile strength and tensile modulus. These PU composites exhibited an improved mechanical performance compared to the unfilled PU. Thermo gravimetric analyzer (TGA) curve shows that all the chain-extended PUs are stable up to 250 °C and maximum weight loss occurs at 490 °C. The thermal stability of composites increases with increase in zeolite content. Microcrystalline parameters and micro voids of composites have been measured by using wide-angle X-ray scattering (WAXS) and Positron Annihilation Lifetime (PALS) methods respectively. The microcrystalline parameters and micro-voids from PALS indicate the interaction of the filler with the matrix is stronger beyond 5% of the filler which reflect the mechanical performance as well. Surface morphology of composites has been studied using Scanning Electron Microscopy (SEM). The photomicrograph of SEM indicates a uniform distribution of zeolite filler in the PU matrix

Synchrotron X-ray Topography Studies of the Evolution of the Defect Microstructure in Physical Vapor Transport Grown 4H-SiC Single Crystals

Synchrotron White Beam X-ray Topography studies are presented of dislocation behavior and interactions in a new generation of 100mm diameter, 4H-SiC wafers grown using Physical Vapor Transport under specially designed low stress conditions. Such low stress growth conditions have enabled reductions of dislocation density by two or three orders of magnitude from lowest previously reported levels of 104-105cm-2 down to current levels of 102-103 cm-2. This provides a unique opportunity to discern the details of dislocation configurations and interactions which were previously precluded due to complications of image overlap at higher dislocation densities. Detailed topography analysis have revealed dislocation multiplication by the hopping Frank-Read source mechanism, interactions between threading c, a and c+a dislocations and deflections of threading dislocations resulting in stacking fault formation. These insights greatly aid in eliminating such undesirable defects or engineering their structures to minimize their impact leading to improved performance in device applications.</jats:p

Synchrotron X-Ray Topography Studies of the Evolution of the Defect Microstructure in Physical Vapor Transport Grown 4H-SiC Single Crystals

Abstract not Available.</jats:p

Simulation of Grazing-Incidence Synchrotron X-ray Topographic Images of Threading c+a Dislocations in 4H-SiC

ABSTRACTSynchrotron X-ray topography (SXRT) of various geometries has been successfully utilized to image c+a dislocations in 4H-SiC crystals. Although molten potassium hydroxide(KOH) can be used to reveal the location of such dislocations, it is not possible to determine their senses or their Burgers vector magnitude. A simple, non-destructive method has been proposed to determine the Burgers vector of these c+a dislocations called the ray tracing simulation, which has been successfully implemented previously in revealing the dislocation sense and magnitude of micropipes, closed-core threading screw dislocations (TSDs) and threading edge dislocations (TEDs) in 4H-SiC. In this paper, grazing incidence topography is performed using the monochromatic beam for the horizontally cut wafers to record pyramidal reflections of 11-28 type. Ray tracing simulation has been successfully implemented to correlate the simulated images with experimental images which are discussed in the paper.</jats:p