Materials Science Forum Vols. 600-603

Abstract: The effects of measurement technique and measurement conditions (injection level, temperature) on the measured carrier lifetimes in n- 4H-SiC epilayers are investigated. For three optical measurement techniques, it is shown that the high and low injection lifetimes can vary dramatically. Differences in the lifetime for varying injection level and temperature are approached both experimentally and via carrier dynamics simulations, assuming Z1/Z2 as the dominant defect. Reasonable agreement between measured and calculated behavior is obtained, as is insight into the recombination kinetics associated with the lifetime limiting defect.

Abstract: We present an experimental equipment for studying the charge carrier distribution in the interior of bipolar 4H-SiC high power devices by means of laser absorption measurements. Since the light absorption coefficient in a semiconductor depends on the electron and hole concentration, the attenuation of a laser beam transmitted through a sample is an integral function of the local charge carrier density. In order to detect the tiny changes in the light intensity caused by the plasma-optical effect, a highly sensitive measurement set-up has been developed. Its crucial components are a low-noise blue laser and a high-speed and broad-band photo-diode amplifier circuit. Sample preparation is sophisticated and requires special care. We investigated charge carrier profiles in 4H-SiC pin-diodes in the high-injection regime at current densities between 175 A/cm² and 350 A/cm². The measured charge carrier profiles are in good agreement with computer simulations.

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.

Abstract: We characterized the 4H- and 6H-SiC bulk crystals with graded doping and epitaxial films with various carrier densities by Raman scattering spectroscopy. Electrical properties such as free carrier density were examined for the SiC crystals through Raman measurements of the A1 LO-phonon plasmon coupled (LOPC) mode and its line shape analysis. The peak frequency and band width of LOPC mode varied with carrier density in the range from 1016 to 1019 cm-3. The line shape analysis revealed that the carrier density in the SiC crystals can be simply estimated from measured frequency shift of LOPC mode for 4H- and 6H-SiC crystals.

Abstract: This paper presents the Raman scattering characteristics of poly 3C-SiC thin films deposited on AlN buffer layer by atmospheric pressure chemical vapor deposition (APCVD) using hexamethyldisilane (MHDS) and carrier gases (Ar + H2). The Raman spectra of SiC films deposited on AlN layer of before and after annealings were investigated according to the growth temperature of 3C-SiC. Two strong Raman peaks, which means that poly 3C-SiC admixed with nanoparticle graphite, were measured in them. The biaxial stress of poly 3C-SiC/AlN was calculated as 896 MPa from the Raman shifts of 3C-SiC deposited at 1180 °C on AlN of after annealing.

Abstract: We applied a picosecond transient grating technique for studies of nonequilibrium carrier dynamics in differently grown or doped SiC polytypes. Optical carrier injection in 4H-SiC at two different wavelengths (266 and 355 nm) allowed us to vary the depth of the photoexcited region and determine photoelectric parameters of high density (from ~1016 to ~1019 cm-3) carrier plasma in the temperature range 10 – 300 K. A strong decrease of carrier lifetime with increasing nonequlibrium carrier density was found in 4H-SiC samples at 300 K and fitted by bimolecular recombination with coefficient B = 3 × 10-11 cm3 s-1. In 3C-SiC epilayers, the opposite tendency was observed over a wide temperature range and attributed to recharging of defect states.

Abstract: A natural superlattice (NSL) in silicon carbide polytypes is shown to introduce a miniband structure into the conduction band, which leads to a number of effects in phenomena of quantum-mechanical transport and impact ionization when the electric field directed along an axis of NSL (axis C in crystal). These processes are absolutely traditional when the electric field is perpendicularly to this axis. The parallel field phenomena are explained by the effects of the Wannier–Stark localization (WSL) among them the Bloch oscillations effect is most prominent today.

Abstract: The thermal expansion of 6H Silicon Carbide with different dopant concentrations of aluminum and nitrogen was determined by lattice parameter measurements at temperatures from 300 K to 1575 K. All samples have a volume of at least 6 x 6 x 6 mm3 to ensure that bulk properties are measured. The measurements were performed with a triple axis diffractometer with high energy x-rays with a photon energy of 60 keV. The values for the thermal expansion coefficients along the a- and c-direction, α11 and α33, are in the range of 3·10-6 K-1 for 300 K and 6·10-6 K-1 for 1550 K. At high temperatures the coefficients for aluminum doped samples are approximately 0.5·10-6 K-1 lower than for the nitrogen doped crystal. α11 and α33 appear to be isotropic.

Abstract: Thermal anisotropy in 4H-, and 6H-SiC bulk single crystal wafers was studied by the PPE method. The thermal diffusivities of the [1-100] and [11-20] orientations (^c-axis) samples were higher than those of the [0001] orientation (//c-axis) samples. Moreover, the thermal anisotropies of the lattice component and the carrier component were analyzed by Raman measurement.

Abstract: Metal impurities are known to degrade dramatically the performances of silicon-based devices, even at concentrations as low as 1012 cm-3. A specific process, named proximity gettering, has been optimised by some authors in order to reduce the influence of these impurities [1]. This process consists in the building of a favourable impurity trapping zone in a non-active area of the device, by introducing implantation defects. This paper reports on the application of introducing such gettering sites as an approach to control phonon properties in 4H-SiC epilayer, and increase the thermal conductivity.