Materials Science Forum Vols. 556-557

Abstract: The commercial availability of SiC-based devices has been limited by a number of factors including wafer supply, cost, wafer size and crystal quality. Recently a number of vendors of 4H and 6H SiC wafers have emerged and 100 mm diameter wafers are commercially available. All vendors now claim to meet or exceed the existing SEMI specifications M55-0304. Both SEMI and manufacturers’ specifications focus on wafer dimensions and gross physical defects such as micropipes that are visible by optical inspection. In this paper we describe high-density XRD and PL mapping measurements on a series of nominally identical, semi-insulating 4H wafers from a range of manufacturers. We show very large variations in crystal quality, polytypism and doping/contamination within-wafer, from wafer-to-wafer and vendor-to-vendor. It is logical to assume that these variations may be responsible for observed variations in device properties and yield.

Abstract: We carried out the characterization of the crystallinity of the solution growth self-standing silicon carbide (SiC) crystals, which were grown from Si-C-Ti ternary solution with Accelerated Crucible Rotation Technique (ACRT). The self-standing crystal exhibited homogeneous green color without cracks and inclusions. The crystallinity of the self-standing crystal was characterized by various precise XRD diffraction measurements, such as の-scan rocking curve measurement, X-ray topography and reciprocal lattice mapping. The Full Width at Half Maximum (FWHM) of the の-scan rocking curves was about 20 arcsec. The X-ray topography showed a large area with a homogeneous orientation. The reciprocal lattice mapping exhibited a sharp single peak indicating the excellent crystallinity. Finally we confirmed rather high crystallinity of the self-standing crystals by etch pits measurement using molten KOH etching.

Abstract: Nitrogen (N) donors in SiC are partially deactivated either by Si+-/N+-co-implantation or by irradiation with electrons of 200 keV energy and subsequent annealing at temperatures above 1450°C; simultaneously the compensation is decreased. The free electron concentration and the formation of energetically deep defects in the processed samples are determined by Hall effect and deep level transient spectroscopy. A detailed theoretical treatment based on the density functional theory is conducted; it takes into account the kinetic mechanisms for the formation of N interstitial clusters and (N-vacancy)-complexes. This analysis clearly indicates that the (NC)4-VSi complex, which is thermally stable up to high temperatures and which has no level in the band gap of 4HSiC, is responsible for the N donor deactivation.

Abstract: Use of a transmission electron microscope to irradiate silicon carbide samples has been demonstrated as a useful additional characterisation technique. The photoluminescence spectra of crystal defects introduced in this way have been found to be extremely rich in detail, involving more than 50 zero phonon lines. It is perhaps disappointing that relatively few of these optical centres have been identified conclusively. Indeed, controversy exists over most of the interpretations that have been advanced. As a step towards clarifying this situation we have been studying many of the more important photoluminescent systems by investigating the dependence of the results on the sample n- and p-doping levels, their stoichiometry, the source of supply, the electron dose, the subsequent annealing history, and by exploiting two new aspects of the technique that will be introduced here. A brief review will be given of new results obtained for some of the major optical centres. Most of the irradiations have been performed at room temperature using 300 kV electrons but some were carried out at 750°C.

Abstract: The high-temperature persistent PL defect known as DII is commented on within this study, seen for the first time in low-energy electron irradiated 4H SiC. The local vibrational modes associated with the defect have been identified and the temperature dependence, spatial variation and electron-energy/electron-dose variation of this defect have all been investigated.

Abstract: Excess carrier lifetimes in 4H SiC epitaxial wafers were characterized by microwave photoconductive decay (o/PCD). The measured decay compromised of surface and bulk recombination curves have fast and slow components. Measured lifetimes are not changed with various surface passivation techniques. High resolution lifetime maps show good correlation with stress birefringence images and lower lifetime around extended material defects like grainboundaries, defect clusters, edge defects and polytype switching bands. Chlorosilane based CVD epiwafers show higher bulk lifetime values than standard silane based CVD materials due to less bulk lifetime defect density.

Abstract: The authors have investigated deep levels in electron-irradiated n- and p-type 4H-SiC epilayers by deep level transient spectroscopy (DLTS). By low-energy electron irradiation at 116 keV, the Z1/2 and EH6/7 concentrations are increased in n-type samples, and the concentrations are almost unchanged after annealing at 950°C for 30 min. In p-type samples, the unknown centers, namely EP1 and EP2, are introduced by irradiation. By annealing at 950°C, the unknown centers are annealed out. The HK4 center (EV + 1.44 eV) is increased by the electron irradiation and subsequent annealing at 950°C. The dependence of increase in the trap concentrations by irradiation (NT) on the electron fluence reveals that NT for the Z1/2 and EH6/7 centers is in proportional to the 0.7 power of electron fluence, while the slope of the plot is 0.5 for the HK4 center. The Z1/2 and EH6/7 centers show similar annealing stage and are thermally stable up to 1500-1600°C, while the HK4 center is annealed out at about 1350°C. The Z1/2 and EH6/7 centers may be derived from a same origin (single carbon vacancy: VC) but different charge state. The HK4 center may be a complex including VC.

Abstract: The dependence of donor-acceptor pair (DAP) emission properties on impurity concentrations of N and B in 6H-SiC epilayers was investigated. Doped samples were grown by closed sublimation technique, and impurity concentrations were confirmed by secondary ion mass spectrometry (SIMS). Photoluminescence (PL) measurement results indicate that p-type 6H-SiC with NA>ND had extremely low DAP emission efficiency, whereas n-type 6H-SiC with NA

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