Papers by Keyword: Crystal Defect

Abstract: In this study, we present positron lifetime calculations for diamond, including the defect-free bulk, vacancies and vacancy clusters, as well as grain boundaries, using Two-Component Density Functional Theory. Our results show a positron bulk lifetime of 103 ps and a mono-vacancy lifetime of145 ps, which is in agreement with experimental data. We calculated positron lifetimes for vacancy clusters from 2 up to 40 missing atoms, resulting in lifetimes between 168 to 365 ps. From these results, we derived a function that correlates the cluster size with the respective positron lifetime.Furthermore, we computed the positron lifetime of 124 ps for a grain boundary, which is between the bulk and mono-vacancy lifetime. Our results will be used to interpret measured positron lifetime spectra in diamond.

Abstract: A systematic study has been made on a Cu-40%Zn alloy treated by an electric current pulse (ECP) and by the examination of the microstructure and the crystallographic features of both the parent and the product phases. The β precipitates under ECP show a Kurdjumov Sachs Orientation Relation (K-S OR) in the vicinity of the grain boundaries (GBs), but a Nishiyama Wasserman (N-W) OR within the grains. Along the GBs the {111}_α /<110>_α dislocation arrays were spotted, whereas the {111}_α /<112>_α stacking faults were observed in the grain interiors. A closer examination of the lattice strain required for the phase transformation revealed that the maximum lattice deformation under the K-S OR is a shear on the {111}_α plane in the <110>_α direction. The dislocations arrays existing along the GBs offer the pre-strain that favors the precipitation of β particles obeying the K-S OR. Oppositely, the stacking faults within the grains provide pre-stains for the formation of the β precipitates respecting the N-W OR. This study sheds some light on the mechanisms by which crystal defects initiate phase transformation in a Cu-40%Zn alloy.

Abstract: KDP (KH₂PO₄) crystal is a kind of excellent nonlinear optical crystals, which has been widely used in nonlinear optical and Inertial Confinement Fusion (ICF) engineering. KDP crystal with the characteristics of low hardness, high brittleness, easy deliquescence and temperature-sensitive is easy to crack during the crystal growth, taken out from crystallizer, and the process of slicing. Stress concentration caused by the initial internal stress redistribution and the growth defect in KDP crystal is an important reason of KDP crystal cracking during sawing process. The numerical simulation model of the KDP crystal containing spherical cavity defect and sawing with fixed abrasive wire saw is established by finite element method in this paper. The effects of initial internal stress, spherical cavity defect on sawing stress are investigated. The maximum tensile stress near the defect during the sawing process is simulated and analyzed. The results show that sawing stress changes smoothly during sawing process, and the fixed abrasive wire saw slicing belongs to low stress cutting way. The sawing stress at sawing kerf is increased obviously. The crystal defect leads to local stress concentration during sawing process. The coupling effect of sawing stress with initial internal stress and the effect of stress concentration are enhanced when the sawing kerf approaches to the defect.

Abstract: A new type of defects, vacant broken line defects, was found to occur in a-face grown crystals of 4H-Silicon Carbide. We characterized the vacant broken line defects by high voltage transmission electron microscope (HV-TEM). The HV-TEM image revealed that the edges of broken line defects were connected by a bundle of dislocations, which elongated to the growth direction on the basal plane. The analysis by gb method for determining Burgers vector indicated that the dislocations were not pure screw dislocations, but complex of screw and edge dislocations. The vacant broken line defect was considered to be a quasi-stable state of a bundle of basal plane dislocations in a-face growth, similar to a micropipe defect in c-face growth.

Abstract: An approach was proposed to calculate the directions of crystal defects in the research of synchrotron radiation topography (SRT). The calculating method was deduced based on the theory of homogeneous coordinates. When spherical coordinates of a crystal defect had been calculated in a radiograph, the directions of the defect on any other radiograph, taken after the specimen had been turned around any axis in the SRT experiments, could be calculated. The directions of crystal defects can be determined by the methods previously been used only if the images of the same defects are similar in the topographs, but it is often not the case in the SRT experiments. With the method proposed in this paper, it is convenient to analyze the defects in different radiographs on which the images of the same defects are dissimilar. An example was taken to show the use of the method.

Abstract: The impacts of threading dislocations, surface defects, donor concentration, and schottky Schottky barrier height on the reverse IV characteristic of silicon carbide (SiC) junction barrier schottky Schottky (JBS) diodes were investigated. The 100 A JBS diodes were fabricated on 4H-SiC 3-inch N-type wafers with two types of threading dislocation density. The typical densities are were 0.2×104 and 3.8×104 cm-2, respectively. The improvement of vIt was found that variations in the leakage current and the high yield of large area JBS diodes werecould be were obtained improved by using a wafer with a low threading dislocation density. In the range of low leakage current, the investigation shows showed a correlation between leakage current and threading dislocation density.

Abstract: Anodic oxidation was performed to 4H-SiC in order to suppress the negative impacts on the Schottky diode characteristics. The electrochemical deposition of ZnO before and after the oxidation revealed a reduction in the number of areas with low Schottky barrier height. Before and after the oxidation, current-voltage characteristics of Ni Schottky contacts was compared, and it was found out that the characteristics were improved after the oxidation. These results suggest that the anodic oxidation is a promising technique to suppress the negative influence of the crystal defects.

Abstract: The present work reports a method to prevent the condensation defects on contact hole patterns by improving the rinsing process after a dry etching. In general, residual gases on the surface after the dry etching can be easily removed by using a DI water rinse. However, the residual gas can not be completely removed in high aspect ratio contact holes, resulting in the condensation defect. In this work, in order to completely remove the residual gas inside the contact holes, several rinse processes were employed such as a megasonic rinse, a sequential rinse and a hot temperature rinse. These proposed rinse methods were effective in eliminating the residual dry etching gases in the high aspect ratio contact holes and thus were able to remove condensation defects on contact holes.

Abstract: It was experimentally shown that an ONO gate dielectric carefully formed on 4H-SiC has extremely high reliability even under a negative electric field at least up to a junction temperature of 300°C, making it promising for power MOS and CMOS applications. Medium charge to failure of –30 C/cm2 was achieved for fully processed polycrystalline Si gate MONOS capacitors with an equivalent SiO2 thickness of teq = 44 nm and a 200-μm diameter. The medium time to failure of these capacitors projected for –3 MV/cm exceeds 86 and 6.3 thousand years at room temperature and 300°C, respectively. A parasitic memory action did not appear even when Eox of -6.6 MV/cm was applied for 5000 seconds.

Abstract: The main electronic characteristics of silicon carbide (SiC) are its wide energy gap, high thermal conductivity, and high break down electric field which make of it of one of the most appropriate materials for power electronic devices. Previously we reported on a new electrical conductivity evaluation method for nano-scale complex systems based on our original tight-binding quantum chemical molecular dynamics method. In this work, we report on the application of our methodology to various SiC polytypes. The electrical conductivity obtained for perfect crystal models of 3C-, 6H- and 4H-SiC, were equal to 10-20-10-25 S/cm. For the defect including model an extremely large electrical conductivity (of the order of 102 S/cm) was obtained. Consequently these results lead to the conclusion that the 3C-, 6H-, and 4H-SiC polytypes with perfect crystals have insulator properties while the electrical conductivity of the crystal with defect, increases significantly. This result infers that crystals containing defects easily undergo electric breakdown.