Papers by Author: Peter J. Wellmann

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: We report on investigation of p-type doped, SiC wafers grown by the Modified- Physical Vapor Transport (M-PVT) method. SIMS measurements give Al concentrations in the range 1018 to 1020 cm-3, with weak Ti concentration but large N compensation. To measure the wafers’ resistivity, carrier concentration and mobility, temperature-dependant Hall effect measurements have been made in the range 100-850 K using the Van der Pauw method. The temperature dependence of the mobility suggests higher Al concentration, and higher compensation, than estimated from SIMS. Additional LTPL measurements show no evidence of additional impurities in the range of investigation, but suggest that the additional compensation may come from an increased concentration of non-radiative centers.

Abstract: We study electronic Raman scattering of phosphorus and nitrogen doped silicon carbide (SiC) as a function of temperature in the range 7K < T < 300K. We observe a series of peaks in the Raman spectra which we assign to electronic transitions at nitrogen and phosphorus donors on different lattice sites. These transitions are identified as valley orbit transitions of the 1s donor ground state. From the polarization dependence of the observed peaks, we find that all electronic Raman signals have E2-symmetry of C6v for the hexagonal polytypes (6H-SiC and 4H-SiC) and E-symmetry of C3v for 15R-SiC. We find a reduction of the intensities of all valley-orbit Raman signals with increasing temperature and ascribe this reduction to the decreasing occupation of donor states.

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 have studied the impact of the chemical nature of additional gases fed into the modified physical vapor transport (M-PVT) growth cell. In particular experiments were carried out using helium, argon, nitrogen and propane in the growth setup. Numerical modeling was used to address the underlying physical and chemical effects that impact the global temperature field. It is found that chemical decomposition of complex gases plays a secondary role as heat source or sink. However, temperature variations related to varying gas compositions fed to the systems are primarily induced by changes of the graphite foam isolation properties.

Abstract: The development of the Continuous Feed Physical Vapour Transport (CF-PVT) process requires a perfect control of each phenomenon in the growth cell. Along this line, the present paper gives some inputs on the CF-PVT mass transfer regimes with respect to the process parameters, both from qualitative and quantitative viewpoints. For example, two boundary cases have been evidenced depending on the temperature. At low temperature, the growth is limited by the sublimation step between the source and the seed. In this case, the CF-PVT process can be roughly assimilated to the classical seeded sublimation technique. At high temperature, the process is limited by the feeding step, i.e. the CVD deposition and infiltration on the lower part of the source. Measurements are correlated to in-situ X-ray imaging. The ability of the X-ray imaging to in-situ qualify and quantify the mass transfer is discussed.

Abstract: Several SiC bulk crystals were grown with erbium and ytterbium as doping materials. Erbium contents determined by secondary ion mass spectroscopy (SIMS) ranged from 1.2 · 1014 cm-3 to 1.04 · 1015 cm-3, while ytterbium contents were below SIMS detection limit. Photoluminescence (PL) investigations of the characteristic 4f-4f-transition lines revealed a reduced luminescence yield in highly nitrogen and aluminum co-doped samples. Also, samples without intentional co-doping grown on the C-face showed less luminescence intensity than those grown on the Si-face. A stabilizing effect of erbium doping on the 4H polytype was observed.