Michael Rasp

Growth of 3″ and 4″ gallium arsenide crystals by the vertical gradient freeze (VGF) method

Abstract Results of the vertical gradient freeze (VGF) growth of Si-doped (3″) and semi-insulating (4″) GaAs crystals are shown. The VGF process conditions were optimised with the aid of numerical simulations using the so-called inverse modelling. Experimental results and predictions from the computer simulation (software CrysVUN++) are quantitatively compared with respect to the power versus time profiles of the heaters, and qualitatively with respect to the shapes of the solid–liquid interface and the growth rate.

Absorption mapping of doping level distribution in n-type and p-type 4H-SiC and 6H-SiC

An optical characterization method for determination of spatial doping level concentration in n-type 4H-SiC and p-type 6H-SiC is discussed. The absorption bands of free charge carriers at 460 nm in n-type 4H-SiC are used to determine its doping concentration. In p-type 6H-SiC, the band edge related absorption at 410 nm is a measure for the doping concentration. In both cases, Hall measurements are performed for calibration. Various examples of SiC-wafer mappings are shown and the relationships to crystal growth conditions, i.e. control of doping level and distribution, are investigated.

Analysis of electronic levels in SiC: V, N, Al powders and crystals using thermally stimulated luminescence

Abstract The production of semi-insulating SiC depends on two issues: (i) the incorporation of a transition metal or further defects in the SiC lattice providing near midgap electronic levels; and (ii) the control of shallow and middle deep acceptors and donors. In this work a synthesis process for SiC powders doped with vanadium (V) as an amphoteric dopant is proposed. The incorporation of vanadium as electrically active VSi is monitored via photoluminescence (PL) measurements. Thermally stimulated luminescence (TSL) is used as a characterization technique for shallow levels in the crystals and powders. Recorded TSL glow curves are compared with numerical solutions of trap emptying processes revealing quantitative information on the activation energy of the ionization of shallow donors and acceptors. TSL emission and photoionization studies give information on existing deep levels.

Anomalous interface shapes in the seed well during vertical gradient freeze growth of Si-doped GaAs

The growth mode in the seed well during vertical gradient freeze growth of Si-doped GaAs crystals has been studied via DSL etching and spatially resolved Fourier transform infrared spectroscopy. Pronounced faceted growth occurs and the shape of the phase boundary deviates remarkably from the isotherm. With ongoing solidification even melt inclusions are observed, which result in precipitates probably formed by Si. By taking into account segregation phenomena in the seed well, the observations are correlated to different enrichment of Si in front of the rough and the faceted parts of the phase boundary. The segregation is treated by a one-dimensional diffusion boundary layer model for both the rough and the atomically smooth interface. Interdiffusion between these two regions is treated as a perturbation by Ficks second law.

Quality Aspects for the Production of SiC Bulk Crystals

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.

Investigations on Polytype Stability and Dislocation Formation in 4H-SiC Grown by PVT

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.