Papers by Author: Thomas L. Straubinger

Abstract: We report here an anisotropic increase in SiC bulk resistivity by annealing at 1150 °C, and discuss the implications for SiC devices. The increase in resistivity is resistivity dependent and can be (at least) partially reversed by a subsequent anneal at higher temperature. Ideal device performance is achievable with appropriate annealing steps during device processing.

Abstract: One of the most crucial defects for the device fabrication on silicon carbide (SiC) substrates are areas with low crystalline quality and micro-pipe clusters which can still occupy several percent of the area in commercial available 4H-substrates. These defects originate from the seed or are generated by modification changes during growth and can be easily detected under crossed polarizers. In this presentation the historic development at SiCrystal from Acheson material to wafers with 100mm diameter, state of the art micro-pipe density and excellent crystalline quality (FWHM < 20 arcsec) on whole area will be shown. Additionally the influence of carbon inclusions on surface quality and the present dislocation densities in 4H substrates will be discussed. While carbon inclusions were reduced to uncritical levels dislocation densities are still in the range of 104 cm-2. Therefore strategies for further reduction will be pointed out. Finally a resistivity limit (16 mΩcm) for stacking fault formation during annealing at 1150°C will be defined.

Abstract: We carried out investigations to elucidate the reasons for polytype changes in 4H. The aim was to sustain polytype stability throughout the entire process. The investigations were accompanied by studies on the formation of basal plane dislocations and their role as source for stacking faults. Several methods for the evaluation of material properties were applied to determine quality most precisely, e.g. KOH-defect-etching, optical microscopy, electron microscopy and X-ray-diffraction. We found out that several influences in growth conditions have to be controlled in a proper manner to achieve defect reduction. Based on these investigations we were able to improve our process and the crystal quality significantly. Best values for 3” 4H wafers show that EPD = 5x103 cm-2 , MPD < 0.1 cm-2 and FWHM-values < 15 arcsec can be achieved.

Abstract: For several years the major focus of material issues in SiC substrates was laid on the reduction of macroscopic defects like polytype inclusions, low angle grain boundaries and micropipes. Since then significant improvements have been achieved and micropipe densities could be reduced to values below 1 cm-2. Nevertheless the fabrication of high quality substrates at high volume and low cost is still challenging. Therefore preconditions for reproducible process and quality control will be discussed. Since it is obvious that dislocations are the main reason for degradation in power devices the prevailing attention has also been shifted to that field of material research. Intense studies were utilized on dislocation and stacking fault formation during sublimation growth. For this reason we systematically varied crucial parameters of the crystal growth process and applied several specific characterization methods, e.g. KOH-defect-etching, electron microscopy and optical microscopy, to evaluate resulting material properties. The investigations were accompanied by failure analysis on devices of the Schottky-type. We found out that for the improvement of substrate quality emphasis has to be laid on the reduction of thermoelastic stress in the growing crystal. The results of numerical calculations enabled us to derive moderate growth conditions with reduced temperature gradients and correspondingly low defect concentration.

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.