Materials Science Forum Vols. 778-780

Abstract: Diffusion of lithium, sodium and potassium in SiC has been studied by secondary ion mass spectrometry. The alkali metal diffusion sources have been introduced by ion implantation. Subsequent anneals have been carried out in vacuum or in Ar atmosphere in the temperature range 700 °C - 1500 °C for 5 min to 16 h. The bombardment-induced defects in the vicinity of the ion implanted profile are readily decorated by the implanted . In the bulk, the diffusing alkali metals are most likely trapped and detrapped at boron and/or other defects during diffusion. The diffusivity of the studied alkali metals decreases as the mass increases, Li⁺<Na⁺<K⁺, but the sodium mobility in SiC is substantial already at 1100 °C.

Abstract: Carrier lifetime measurements and wafer mappings have been done on several different 4H SiC wafers to compare two different measurement techniques, time-resolved photoluminescence and microwave induced photoconductivity decay. The absolute values of the decay time differ with a factor of two, as expected from recombination and measurement theory. Variations within each wafer are comparable with the two techniques. Both techniques are shown to be sensitive for substrate quality and distribution of extended defects.

Abstract: Free carrier lifetimes and diffusion coefficients were determined in 6H-SiC bulk crystals grown by PVT on 6H-and 4H-SiC seeds varying the temperature from 300 K to 650 K and at excess carrier densities ΔN₀ from 10¹⁷ cm^-3 to 10¹⁹ cm^-3. Carrier generation was achieved by using a single or two-photon absorption of picosecond pulses at 351 and 532 nm, respectively. Fast and slow recombination transients revealed the decay time of free carriers and the presence of deep acceptor traps. The thermal trap activation energy E_a = 0.33 eV was determined in the 6H/4H sample and ascribed to the boron, while the presence of deeper traps is suggested in the 6H/6H sample. At room temperature and reaching conditions of trap saturation regime (ΔN₀ 10¹⁹ cm³), both crystals revealed the bipolar diffusion coefficient D_a 4 cm²/s. For comparison, we also determined the photoelectrical parameters in commercial 6H-SiC crystals grown by the Lely and PVT techniques.

Abstract: Suppression of basal plane dislocations (BPDs) from critical epitaxial drift layer has occurred mainly by converting BPDs in the substrate into threading edge dislocations before the BPDs enter the drift layer. As optimized epitaxial growth has produced drift layers free of BPDs originating from the substrate over a large fraction of the wafer, other sources of BPDs have become important. One source of BPDs introduced during epitaxial growth is from inclusions, which mainly consist of misoriented 4H-SiC. Inclusions are surrounded by a local cluster of BPDs and in thick, low-BPD epitaxy the outermost BPDs glide centimeters from the inclusion forming a much larger damaged area. The details of BPD migration from inclusions are discussed.

Abstract: This paper demonstrates high-resolution photoluminescence (PL) imaging and discrimination of threading dislocations in 4H-SiC epilayers. Threading screw dislocations (TSDs) and TEDs are distinguished by differences in PL spot size and spectrum. We found that TEDs are further discriminated into six types according to their Burgers vector directions by the appearance of PL imaging. Cross-sectional PL imaging reveals inclination angles of threading dislocations across a thick epilayer.

Abstract: Polarization characteristics of luminescence from partial dislocations (PDs) in 4H-SiC have been investigated by room-temperature photoluminescence (PL) imaging. After expansion of Shockley stacking faults by high-power laser irradiation, PL from PDs tilted by 6° from their Burgers vector (6°-PDs) was observed with almost the same PL peak energy as that of 30°-Si (g) PDs. The PL from the 30°-Si (g) and 6°-PDs which were mobile under illumination were both found to be polarized perpendicular to their dislocation lines. In contrast, the PL from immobile 30°-C(g) PDs was not polarized. The present results suggest that the carriers bound to the 30°-Si (g) and 6°-PDs have anisotropic wave functions and those bound to 30°-C(g)PDs have isotropic wave functions.

Abstract: Basal Plane Dislocations (BPD) were reduced in 4H-SiC epilayers by high temperature annealing in the range of 1600 °C to 1950 °C using two annealing techniques. Samples annealed at > 1750 °C showed almost complete elimination of BPDs propagating from the substrate. However, surface morphology was degraded for MW annealed samples above 1800 °C, with new BPDs being generated from the surface. A new capping technique was developed along with application of high N₂ overpressure to preserve the surface morphology and avoid formation of new BPDs.

Abstract: Synchrotron X-ray Beam Topography (SWBXT) and KOH etching observations are presented of interfacial dislocations (IDs) and half-loop arrays (HLAs) which can form under certain growth conditions during homoepitaxy of 4H-SiC on off-cut substrates. The HLAs and IDs are observed to form from pairs of opposite sign basal plane dislocations in the substrate which intersect the substrate surface in screw orientation. These dislocations glide in opposite direction in the epilayer once critical thickness has been exceeded. Half-loop arrays are formed at the same time as the screw-type basal plane dislocations (BPDs) side-glide inside the epilayer. From knowledge of the formation mechanism of the HLAs [, if the line of the HLA is extended to intersect the original threading dislocation line direction, then the distance between this intersection point and the ID along the line direction of the original BPD provides a measure of the critical thickness. It is also calculated that the critical thickness in this case is largely determined by the mutual attractive force between the pairs of opposite sign threading BPDs in the substrate. In addition we observed both interfacial dislocations and HLAs generated from: (a) surface sources of BPDs; (b) micropipes; (c) 3C inclusions; and (d) substrate/epilayer interface scratches.

Abstract: Nomarski optical microscopic, KOH etching and synchrotron topographic studies are presented of faint needle-like surface morphological features in 4H-SiC homoepitaxial layers. Grazing incidence synchrotron white beam x-ray topographs show V shaped features which transmission topographs reveal to enclose 1/4[0001] Frank-type stacking faults. Some of these V-shaped features have a tail associated with them and are referred to as Y-shaped defects. Geometric analysis of the size and shape of the V-shaped faults indicates that they are fully contained within the epilayer and appear to be nucleated at the substrate/epilayer interface. Detailed analysis shows that the positions of the V-shaped stacking faults match with the positions of c-axis threading dislocations with Burgers vectors of c or c+a in the substrate and thus appear to result from the deflection of these dislocations onto the basal plane during epilayer growth. Similarly, the Y-shaped defects match well with the substrate surface intersections of c-axis threading dislocations with Burgers vectors of c or c+a in the substrate which were deflected onto the basal plane during substrate growth. Based on the observed morphology of these defect configurations we propose a model for their formation mechanism.

Abstract: This paper demonstrates optical second-harmonic generation (SHG) and two-photon excited photoluminescence (2P-PL) imaging of 3C-SiC inclusions forming triangular and carrot-type defects in 4H-SiC epilayers. Triangular defects exhibit clear SHG images because 3C-SiC is SHG active, but not 4H-SiC host crystal in c-axis incidence. A carrot defect provides SHG and 2P-PL images in different regions in a basal-plane fault area. The spectrums of the SHG and 2P-PL are also investigated, and their emission mechanisms discussed.