Papers by Keyword: Point Defect

Abstract: Admittance spectroscopy (AS) and deep level transient spectroscopy (DLTS) have been applied to B-doped thin polycrystalline diamond films deposited on p+-silicon by hot filament chemical vapour deposition. Films with two boron concentrations (1.5×10^19 cm-3 and 4×10^19 cm-3) were selected to study the effect of B concentration on the electronic states in CVD-diamond. We have investigated whether these deep states arise from point or extended defects. DLTS and AS find two hole traps, E1 (0.29±0.03 eV) and E2 (0.53±0.07 eV), in both films. A third level, E3 (0.36±0.02 eV) was also detected in the more highly doped film. The defect levels E1 and E2 exhibited behaviour typical of extended defects, which we suggest may be due to B segregated to the grain boundaries. In contrast, the defect level E3 exhibited behaviour characteristic of an isolated point defect, which we attribute to B-related centres in bulk diamond.

Abstract: Thick and low-doped epilayers with a low Z_1/2 center concentration were grown on 8^o off-cut 4H-SiC(0001)Si-face substrate. Two post-growth processes, namely, the C⁺-implantation/annealing process or the thermal oxidation/Ar annealing process, were applied to the thick epilayers. The dependence of the Z_1/2 center concentration and the carrier lifetime on process conditions was investigated. Under proper conditions, both processes could eliminate the Z_1/2 center to a depth of 100 μm or more, and considerably improved the carrier lifetime while maintaining the surface roughness comparable to that of the as-grown sample. The effect of the post-growth processes applied on C-face is also presented.

Abstract: Relatively little is known about the transition metal defects in silicon carbide (SiC). In this study we applied highly convergent and sophisticated density functional theory (DFT) based methods to investigate important transition metal impurities including titanium (Ti), vanadium (V), niobium (Nb), chromium (Cr), molybdenum (Mo) and tungsten (W) in cubic 3C and hexagonal 4H and 6H polytypes of SiC. We found two classes among the considered transition metal impurities: Ti, V and Cr clearly prefer the Si-substituting configuration while W, Nb, and Mo may fractionally form a complex with carbon vacancy in hexagonal SiC even under thermal equilibrium. If the metal impurity is implanted into SiC or when many carbon impurities exist during the growth of SiC then complex formation between Si-substituting metal impurity and the carbon vacancy should be considered. This complex pair configuration exclusively prefers the hexagonal-hexagonal sites in hexagonal polytypes and may be absent in cubic polytype. We also studied transition metal doped nano 3C-SiC crystals in order to check the effect of the crystal field on the d-orbitals of the metal impurity.

Abstract: The C-C stretch vibration associated with the dicarbon antisite in 4H SiC has been observed out to the fifth harmonic in the low temperature photoluminescence spectrum. The anharmonicity is accounted for reasonably well by fits to the data based on the Morse potential. We combine the observations from experiment, the analytically tractable Morse potential, and results obtained from first principles calculations on this defect to obtain an estimate of the thermal expansion coefficient of the C-C bond. This local thermal expansion coefficient is considerably smaller than the linear thermal expansion coefficient of bulk 4H SiC, in striking contrast with the recent result for the nitrogen-vacancy center in diamond that the local thermal expansion coefficient is larger than the bulk value.

Abstract: Abstract. Ca3CO4O9/x wt.%SiO2 composites were prepared by using field-activated pressure-assisted synthesis(FAPAS) process. The effects of nano-SiO2 addition on the microstructure and thermoelectric properties of the Ca3CO4O9 were investigated. With an increasing of the of SiO2 content, the size of Ca3Co4O9 particles decreased. The Seebeck coefficient decreased after the doping of nano-SiO2. The electrical conductivity firstly increased and then decreased with increasing amount of nano-SiO2 and the highest value belong to the Ca3CO4O9/0.5 wt.% SiO2 - sample. With the highest electric conductivity and slightly decreased Seebeck coefficient, the Ca3CO4O9/0.5 wt.%SiO2-sample achieved the highest power factor of 0.349 mw/mk2 at 800 °C, which is 8% higher than the value of pure Ca3CO4O9 reported in the literature.

Abstract: The internal friction of the hyperstabilized martensite demonstrates very low values, both above and below the nominal martensitic transformation temperature, due to a pronounced pinning effect. Over a wide temperature range it is comparable with the level of damping in the parent phase. A study of the temperature dependence of the non-linear ultrasonic internal friction and its strain amplitude hysteresis indicates that the diffusion, assisted by dislocations/interfaces, is quite pronounced and in Ni-Fe-Ga and Cu-Al-Be alloys it operates at temperatures around 20 K. The renucleation of the lamellar parent phase during the reverse martensitic transformation close to 600 K is accompanied by an internal friction peak which demonstrates a substantial transitory contribution. After renucleation of the parent phase the samples recover a conventional martensitic transformation with the internal friction level in the martensite comparable to the one in non-stabilized samples. Observations of a relaxation peak in the parent phase of different alloys for temperatures just below the renucleation stage of the reverse transformation point to the essential role of diffusion in the nucleation of the parent phase in hyperstabilized martensites.

Abstract: Microstructural changes induced in a carbon fiber – magnesium matrix composite during thermal cycling in the range of 100 - 360 K are detected by an ultrasonic technique. The composite was comprised of Mg-2wt.%Si alloy matrix reinforced with long unidirectional carbon fibers (volume fraction of about 30%). Temperature variations of the elastic modulus of the composite are largely determined by elasticity of the carbon fibers stressed by the thermally expanded/contracted matrix. Anelastic properties of the composite (internal friction and modulus defect) are caused by dislocation motion in the matrix. Temperature spectra of anelasticity of the composite are controlled by a competition between creation of fresh mobile dislocations under the action of thermal stresses and immobilization of the fresh dislocations by atmospheres of mobile point defects.

Abstract: Coating and gradient structure can improve the quality of products, but growth-induced stress generates in the forming process of the coating and the gradient structure due to the existence of the defects and metastable phase. The existence of growth-induced stress can conversely affect the quality of the product, even lead to cracking and product failure. This paper shows the model of stress calculation according to the crystal structure’s defect theory.

Abstract: The results of an investigation of the point defects (PD) generation, redistribution and interaction with impurities in the Si-SiO₂ system during the process of its formation in use of of electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) spectra are presented. The type and density of the point defects that are generated in the Si surface layer during thermal oxidation depend on the oxidation conditions: temperature, cooling rate, oxidation time and impurity content. The difference between interface properties of n- and p-type wafers could be related to different Fermi level position at the interface and to different PD densities in volume.

Abstract: This paper reports experimental results on the production and annealing of oxygen-vacancy related (VO_n, 1<n<5) and carbon-related (C_iO_i, C_iO_iI, and C_iC_s) defects in Ge-doped Czochralski-grown silicon (Cz-Si) materials containing carbon. The samples were irradiated by 2 MeV fast electrons and the behavior of radiation-produced defects is studied by means of infrared (IR) spectroscopy, monitoring the relevant bands in spectra. Regarding the VO_n family, it was found that the presence of Ge affects the annealing temperature of VO defects as well as their fraction that is converted to VO₂ defects. Both effects are discussed in relation with an impact of Ge on the concentration of self-interstitials that take part in the annealing of VO defects via two reaction paths VO + I → O_i and VO + O_i → VO₂. Furthermore, two bands at 1037 and 1051 cm^-1 are attributed to the VO₅ defect, although three other bands at 762, 967 and 1005 cm^-1 are believed to be associated with V_nO_m clusters containing carbon, most likely having a VO_nC_s structure. Regarding carbon-related complexes, it has been established that the annealing of the 862 cm^-1 band belonging to the C_iO_i defect is accompanied by the emergence of the 1048 cm^-1 band previously assigned to the C_sO_2i center. The evolution of the C_iC_s and the C_iO_iI bands is monitored and the identification of bands at 947, 967 and 1020 cm^-1 making their appearance in IR spectra over the temperature range where C_iC_s and C_iO_iI defects are annealed out is discussed.