Materials Science Forum Vols. 600-603

Abstract: Low temperature infrared photoluminescence (PL) performed on a large set of bulk SiC substrates has revealed distinct series of lines between 0.8 and 1.5 eV for samples with nitrogen levels between ~ 1016 and 1017 cm-3. Semi-insulating and intentionally N-doped wafers grown by PVT and HTCVD were investigated. Two groups of PL lines clustered near 1.0 and 1.35 eV, respectively, were observed in n-type 4H-SiC. Not surprisingly, a multiplicity of features at slightly different energy positions was found for this emission from the 6H- and 15R-SiC polytypes. Both sets of lines were not observed for substrates with N doping concentrations greater than 3x1017cm-3. Thus, it appears this IR emission can serve as optical “fingerprints” of bulk n-type substrate with moderate levels of N impurities. Models for the possible origins of these lines will also be discussed.

Abstract: The boron diffusion in three kinds of group IV semiconductors: silicon, silicon carbide and synthetic diamond has been studied by secondary ion mass spectrometry. Ion implantation of 300 keV, 11B-ions to a dose of 21014 cm-2 has been performed. The samples are subsequently annealed at temperatures ranging from 800 to 1650 °C for 5 minutes up to 8 hours. In silicon and silicon carbide, the boron diffusion is attributed to a transient process and the level of out-diffusion is correlated to intrinsic carrier concentration. No transient, out-diffused, boron tail is revealed in diamond at these temperatures.

Abstract: The characteristics of boron diffusion in 3C-SiC at low temperature have been measured using spreading resistance technique and electroluminescence spectroscopy. The coefficient of boron diffusion in the temperature range of 1150 –1250°С has been found to be about 5.5 x 10-11–5.0 x 10-10 cm2/sec and the activation energy of boron diffusion was determined to be about 0.9 –1.15 eV. Electroluminescence spectra of 3C-SiC p-n junction structures showed peaks at 750 and 630 nm due to growth defects and carbon-silicon divacancies respectively.

Abstract: We report on the luminescence spectra related to Ti impurity in both 4H- and 6H-SiC polytypes. The spectrum depends strongly on the polarization. They are two families of lines in 4H and three in 6H. The main no-phonon line of each family is shown as a triplet and its phonon structure contains both sharp and broad replicas. The higher energy family has also extra lines at high energy appearing when the temperature increases. The spectra can be detected with excitation energy below the excitonic bandgap and even with excitation energy below the spectrum itself. Time-resolved photoluminescence reveals 0.1 ms long lifetime at low temperature.

Abstract: We report on the application of introducing gettering sites as an approach to control some phonons and charge carrier related properties in 4H-SiC epilayer. Helium implantation (at room temperature or 750°C) was first carried out, followed by a proper annealing and gold diffusion, in order to check the gettering efficiency. Raman measurements showed the presence of the desired defect, introduced by ion implantation at RT. The shift of the Fano interference allowed us to calculate the free carriers’ density in each sample. The lowest value was found for the sample implanted at RT. The gettering sites can act as majority carrier traps and reducers of recombination processes, which can be interesting for devices designed for the detection of radiations.

Abstract: We report a density functional theory study of N and NO passivation of surface dangling bonds at the Si-face of (0001) 4H-SiC. Results agree with many key experimental findings in nitrogen processed devices including: observed interface N coverage and characteristic changes in Dit. Dangling bonds at the SiC surface are fully passivated by 1/3 ML N or NO coverage. Upon passivation the surface is found to incur negligible strain and no reconstruction. Allowing atomic O to interact with the 1/3 ML N or NO passivated surface, we find oxygen prefers to incorporate into SiO2 rather than adsorb to the surface. This indicates the possibility of oxide deposition onto nitrogen passivated (0001) 4H-SiC surfaces.

Abstract: To elucidate the origin of giant step bunching on 4˚ off-axis 4H-SiC (0001) faces, we carried out hydrogen etching and epitaxial growth under various conditions. We found that giant step bunching occurs during hydrogen etching and epitaxial growth at extremely low or high C/Si ratios, i.e., with an excessive supply of SiH4 or C3H8. From these results, we have proposed that the origins of giant step bunching are asymmetry in the step kinetics in etching and Si or C cluster generation on terraces during growth.

Abstract: We have studied annealing of the main lifetime limiting defect Z1/2 in thick 4H-SiC epilayers by the application of carbon-implantation/annealing method. Examination of different implantation doses and annealing temperatures showed that finding the optimum conditions is crucial for obtaining carrier trap concentration below 1011 cm-3 in the whole 100 μm epilayer. The carrier lifetime increased from under 200 ns to over 1 μs at room temperature in the samples prepared by optimized carbon-implantation/annealing technique. Fabrication of pin diodes from the improved thick 4H-SiC epilayers confirmed the enhanced conductivity modulation and suitability of this technique for high-voltage bipolar SiC devices.

Abstract: X-ray diffraction (XRD) rocking curves were mapped across 4H-SiC, 3-inch, 8° off-cut substrates prior to and after epitaxial growth, where a pattern of slightly higher defectivity region was clearly seen. This same pattern was apparent in both cross-polarization images of the epiwafers and microwave photoconductivity decay (μ-PCD) lifetime maps of the epilayers, where the latter shows the lifetime in the high defectivity regions had drastically decreased. Within the short lifetime regions, electron trap concentrations were similar to that as in the long lifetime regions as determined by deep level transient spectroscopy; however, the extended defect density was significantly higher. Consequently, high spatial resolution XRD can be a valuable tool in preselecting substrates for epitaxial growth to produce low defect density material with long injected carrier lifetimes.

Abstract: Compared to silicon, there have been relatively few comparative studies of recombination and carrier lifetimes in SiC. For the first time, both generation and recombination carrier lifetimes are reported from the same areas in 20 m thick 4H SiC n-/n+ epi-wafer structures. The ratio of the generation to recombination lifetime is much different in SiC compared to Si. Activation energy calculated from SiC generation lifetimes shows that traps with energy levels near mid-gap dominate the generation lifetime. Comparison of both generation and recombination lifetimes and dislocation counts measured in the device area show no correlation in either case.