Papers by Author: Sakwe Aloysius Sakwe

Abstract: The determination of dislocation density and in particular the dislocation distribution in SiC wafers is of particular interest for SiC crystal growth development and production. We present an image recognition tool allowing the wafer analysis with specific needs for SiC. In the first stage of expansion, micropipes are selected and counted from SiC wafers that have been etched by KOH.

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: In this paper we report, based on analysis of dislocation statistics, on the influence of growth temperature on the nucleation, propagation and annihilation mechanisms of dislocations. Using KOH defect etching and optical microscopy we have conducted dislocation tracking along lengths of crystals grown under various process temperature regimes to study their evolution and propagation mechanisms statistically. We further present the influence of growth temperature on the step structure of the growth front using AFM measurements. From the analysis of dislocation statistics and step structure in relation to temperature we derive the role of surface kinetics of the SiC gas species on the growth surface in dislocation evolution during PVT growth of bulk SiC.

Abstract: The origin of dislocation evolution during SiC crystal growth is usually related to lattice relaxation mechanisms caused by thermal stress. In this paper we discuss dislocation generation and dislocation propagation related to doping and suppression of basal plane dislocations, the latter being of particular interest for bipolar electronic devices. We have prepared alternating p-/n-/pdoped SiC crystals using the donor nitrogen and the acceptors aluminum or boron. In addition we determined the mechanical properties of n-type and p-type SiC; in particular we measured the critical shear stress by nano-indentation on c-plane and a-plane 6H-SiC surfaces. A considerably lower basal plane dislocation density is found in aluminum as well as in boron doped p-type SiC compared to nitrogen doped n-type SiC. It is concluded that the explanation of the reduced basal plane dislocation density in p-type SiC needs the consideration of electronic as well as mechanical effects.

Abstract: Wet chemical etching using molten KOH is the most frequently applied method to reveal structural defects in SiC. Until now etching kinetics of SiC in planes different from the polar cplane has not been reported. In this paper we report on defect etching of SiC in non-polar faces. Using a calibrated KOH defect-etching furnace with possibilities to set accurate etching temperatures we have etched SiC samples of various orientations to (i) study defect occurrence and their morphologies (ii) set KOH defect etching parameters for SiC for these orientations and (iii) investigate etching kinetics in relation to anisotropy/surface polarity. For non-polar planes of the same orientations a comparison in etching kinetics and defect morphologies in crystals grown in different directions is presented.

Abstract: We have carried out the growth and basic characterization of isotopically enriched 4HSi 13C crystals. In recent years the growth of 13C enriched 6H-SiC has been performed in order to carry out fundamental materials studies (e.g. determination of phonon energies, fundamental bandgap shift, carbon interstitial defect study, analysis of the physical vapor transport (PVT) growth process). For electronic device applications, however, the 4H-SiC polytype is the favored material, because it offers greater electron mobility. In this paper we present the growth of 4H-Si13C single crystals with up to 60% of 13C concentration. From a physical point of view we present first results on phonons as well as the fundamental bandgap energy shift due to 13C incorporation into the SiC lattice.

Abstract: The long term performance of today’s SiC based bipolar power devices suffer strongly from stacking fault formation caused by slip of basal plane dislocations, the latter often originating from the n-type doped SiC substrate wafer. In this paper, using sequentially p-type / n-type / p-type doped SiC crystals, we address the question, whether basal plane dislocation generation and annihilation behaves differently in n-type and p-type SiC. We have found that basal plane dislocations are absent or at least appear significantly less pronounced in p-type doped SiC, which may become of great importance for the stacking fault problem in SiC.

Abstract: We review the development of a modified physical vapor transport (M-PVT) growth technique for the preparation of SiC single crystals which makes use of an additional gas pipe into the growth cell. While the gas phase composition is basically fixed in conventional physical vapor transport (PVT) growth by crucible design and temperature field, the gas inlet of the MPVT configuration allows the direct tuning of the gas phase composition for improved growth conditions. The phrase "additional" means that only small amounts of extra gases are supplied in order to fine-tune the gas phase composition. We discuss the experimental implementation of the extra gas pipe and present numerical simulations of temperature field and mass transport in the new growth configuration. The potential of the growth technique will be outlined by showing the improvements achieved for p-type doping of 4H-SiC with aluminum, i.e. [Al]=9⋅1019cm-3 and ρ<0.2Ωcm, and n-type doping of SiC with phosphorous, i.e. [P]=7.8⋅1017cm-3.

Abstract: Etching temperature and time are important parameters in the etching of SiC single crystals in molten KOH for defect studies. However, comparison of results of different research groups is difficult because of the way temperature measurements are being carried out. Until now the temperature of the melt has been measured indirectly with a temperature sensor placed outside the melt on the outer walls of the crucible of the etching furnace, resulting in varying etching conditions for varying setup designs. In this paper we developed an etching furnace with the capability of measuring the absolute temperature in-situ directly in the KOH melt. A new thermoelement, resistant to hot molten KOH was developed. Temperature profile measurements of the molten KOH were carried out and a calibration curve of the furnace was obtained. Based on our temperature measurements, we found that etching at 530 °C for 5 minutes was optimal for defect characterisation, both for defect statistics and for distinguishing between the etch pit morphologies. At 550 °C the etch pits become too large, overlap each other and the etching is no longer defect selective.