Jaime A. Freitas

Efficient Incorporation of Mg in Solution Grown GaN Crystals

Detailed spectrometry and optical spectroscopy studies carried out on GaN crystals grown in solution detect and identify Mg as the dominant shallow acceptor. Selective etching of crystals with higher Mg levels than that of the donor concentration background indicates that Mg acceptors incorporate preferentially in the N-polar face. Electrical transport measurements verified an efficient incorporation and activation of the Mg acceptors. These results suggest that this growth method has the potential to produce p-type doped epitaxial layers or p-type substrates characterized by high hole concentration and low defect density.

CL/EBIC-SEM Techniques for Evaluation of Impact of Crystallographic Defects on Carrier Lifetime in 4H-SiC Epitaxial Layers

The effect of various types of in-grown stacking faults and threading screw/edge type dislocations on carrier lifetime and diffusion lengths in 4H-SiC epitaxial films was investigated through cathodoluminescence decays and charge collection efficiencies of electron beam induced current signals at specific defects sites. Most stacking faults yielded ~40% reduction in the carrier lifetime. Moreover, drastic lifetime reductions were observed in regions containing surface triangular defects and bulk 3C polytype inclusions. Dislocations of both types serve as efficient recombination centers, though stronger reduction in diffusion lengths was observed in the vicinity of screw type dislocations.

Mapping of Defects in Large-Area Silicon Carbide Wafers via Photoluminescence and its Correlation with Synchrotron White Beam X-Ray Topography

Comparative studies of defect microstructure in 4H-SiC wafers have been carried out using photoluminescence (PL) imaging and grazing-incidence Synchrotron White Beam X-ray Topography. Images of low angle grain boundaries on the PL images correlate well with SWBXT observations, and similar correlation can be established for some micropipe images although the latter is complicated by the overall level of distortion and misorientation associated with the low angle grain boundaries and the fact that many of the micropipes are located in or close to the boundaries. This validation indicates that PL imaging may provide a rapid way of imaging such defect structures in large-scale SiC wafers.

Activation of Implanted Al and Co-Implanted Al/C or Al/Si in 4H-SiC

Rsh, CL, EPR, RBS, and TEM measurements were made on samples implanted with Al, Al and C, or Al and Si to a depth of 0.3 μm with a concentration of 10 20 cm -3 and annealed with an AlN cap. Rsh measurements suggest that at the lower annealing temperatures the implanted C facilitates the chemical reactions for the incorporation of Al into a Si site and Si impedes it, but for annealing temperatures >1600°C the co-implants have little effect because the activation energy for these reactions is no longer the rate determining step. The CL measurements show that the peak near 3.0 eV associated with a free electron recombining with a hole bound to an Al acceptor decreases as the annealing temperature increases suggesting defects are trapping out the holes. This can explain the lower mobilities, lower electrical activation, and our inability to detect the EPR peak associated with AlSi in these heavily implanted samples. Our RBS and TEM measurements suggest that these defects are extended residual defects that nucleate and grow, as opposed to being annealed out, and that they could be stacking faults.

Evolution of D1-Defect Center in 4H-SiC during High Temperature Annealing

The behavior of the D1 center in semi-insulating 4H-SiC substrates revealed by low-temperature photoluminescence was investigated after post-growth high temperature anneals between 1400 and 2400oC. The influence of different post-anneal cooling rates was also studied. The optical signature of D1 was observed up to 2400oC with intensity maxima at 1700 and 2200oC. We propose that the peak at 1700°C can be related to the formation and subsequent dissociation of SiC native defects. It was found that changes in the post-annealing cooling rate drastically influence the behavior of the D1 center and the concentrations of the VC, VSi, VC-VSi and VC-CSi lattice defects.

Electrical Properties of Undoped 6H- and 4H-SiC Bulk Crystals Grown by Halide Chemical Vapor Deposition

Undoped 6H- and 4H-SiC crystals were grown by Halide Chemical Vapor Deposition (HCVD). Concentrations of impurities were measured by various methods including secondary-ion-mass spectrometry (SIMS). With increasing C/Si ratio, nitrogen concentration decreased and boron concentration increased as expected for the site-competition effect. Hall-effect measurements on 6H-SiC crystals showed that with the increase of C/Si ratio from 0.06 to 0.7, the Fermi level was shifted from Ec-0.14 eV (nitrogen donors) to Ev+0.6 eV (B-related deep centers). Crystals grown with C/Si > 0.36 showed high resistivities between 1053 and 1010 4cm at room temperature. The high resistivities are attributed to close values of the nitrogen and boron concentrations and compensation by deep defects present in low densities.

Variations in the Effects of Implanting Al at Different Concentrations into SiC

SiC samples implanted at 600°C with 1018, 1019, or 1020 cm-3 of Al to a depth of ~ 0.3 μm and annealed with a (BN)AlN cap at temperatures ranging from 1300 – 1700°C were studied. Some of the samples have been co-implanted with C or Si. They are examined using Hall, sheet resistivity, CL, EPR, RBS, and TEM measurements. In all instances the sheet resistance is larger than a comparably doped epitaxial layer, with the difference being larger for samples doped to higher levels. The results suggest that not all of the damage can be annealed out, as stable defects appear to form, and a greater number or more complex defects form at the higher concentrations. Further, the defects affect the properties of the Al as no EPR peak is detected for implanted Al, and the implanted Al reduces the AlSi peak intensity in bulk SiC. CL measurements show that there is a peak near 2.9941 eV that disappears only at the highest annealing temperature suggesting it is associated with a complex defect. The DI peaks persist at all annealing temperatures, and are possibly associated with a Si terminated partial dislocation. TEM analyses indicate that the defects are stacking faults and/or dislocations, and that these faulted regions can grow during annealing. This is confirmed by RBS measurements.

Observation of Thermal-Annealing Evolution of Defects in Ion-Implanted 4H-SiC by Luminsescence

4H-SiC samples implanted at 600°C with 1020 cm-3 of B or B and C to a depth of ~0.5 μm, capped with (BN/AlN), and annealed at temperatures ranging from 1400°C – 1700°C were studied using variable temperature cathodoluminescence. New emission lines, which may be associated with stacking faults, were observed in the samples co-implanted with B and C, but not in the samples implanted only with B. For both the B and B and C co-implanted samples, the intensity of the line near 3.0 eV decreases with increasing annealing temperature, TA, and this line is not observed after annealing at 1700°C. The D1 defect related emission lines are observed in the luminescence spectra of all samples and their relative intensities seem to vary with the implantation-annealing schedule and excitation conditions.