Papers by Author: E.R. Glaser

Abstract: In this work we report the measurement of minority carrier lifetimes using the time resolved photoluminescence technique. It was found that 4H-SiC homo-epilayers grown using chlorine-based precursors have longer carrier lifetimes if used in conjunction with a tantalum carbide coated (TaC-coated) graphite susceptor rather than a SiC-coated graphite susceptor. Longer carrier lifetimes were obtained by optimal combinations of precursor gases and susceptor type. The controllable variation in lifetime from 250 ns to 9.9 s was demonstrated.

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 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.

Abstract: We have recently explored the nature and stability of native defects in high-purity semi-insulating 4H-SiC bulk substrates grown by PVT and HTCVD methods after post-growth anneal treatments up to 2400oC using electron paramagnetic resonance (EPR) and low-temperature photoluminescence (PL) experiments. In the present study we have extended these investigations to SI 4H-SiC subjected to the same post-growth high-temperature anneal treatments, where significantly enhanced carrier lifetimes have been reported for such conditions, but cooled at different rates ranging from ~2-25oC/min. Previously, the intensities of the native defects decreased monotonically with anneals from 1200–1800oC; however, it was recently observed that several of these defects reappear after annealing at 2100oC and above. Our results illustrate the effects of the post-growth anneal treatments and cool-down rates on the concentrations of native defects.

Abstract: The understanding of the structure and associated defect level of point defects in SiC is important because the material is to be used both as a semiconductor and semi-insulator. Production of the latter is achieved by compensation of unavoidable impurities using defects that require more energy for ionization than the unintentional donors or acceptors. The purpose of the present work is to measure the defect energy level of one center in high resistivity 4H SiC using photo-induced electron paramagnetic resonance (photo-EPR). The center is identified as SI-5, an EPR signal that others have attributed to the negative charge state of the carbon vacancy-carbon antisite pair, − C Si V C . Samples containing this defect exhibit two different photo thresholds, which depend on the resistivity activation energy, Ea. For samples with Ea less than 0.8 eV, a photothreshold at 0.75+/- 0.05 eV is observed, but for those with Ea greater than 0.8 eV, the threshold is between 2 and 2.5 eV. Previous work focused on the former case. Here, the SiC substrates with the larger Ea are emphasized, showing that the photo-threshold likely measures the neutral to negative defect level, − / 0 C Si V C .

Abstract: Epitaxial growth of 3-in, 4° off-axis 4H SiC with addition of HCl has been presented. Good surface morphology with a low defect density has been obtained, even for epi thickness of 38 µm. Comprehensive characterization techniques conducted on the epi material obtained in this process have independently confirmed the high purity and low density of crystalline imperfections. Low temperature PL displays clear free exciton I77 recombination while no L1 line is discernable. DLTS measurements have confirmed a low concentration of Z1/2 and EH6/7 below or in the range of 1011 cm-3. Time resolved PL at room temperature performed on a 38 µm thick epi wafer gives long carrier lifetime in the range of 1.5 to above 5 µsec. PiN diodes with diode area up to 25 mm2 have demonstrated blocking voltages above 900V, with a max electric field of above 2.5 MV/cm.

Abstract: 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.

Abstract: We have employed low-temperature photoluminescence to estimate the total residual N concentration in semi-insulating (SI) SiC substrates where all N shallow donors are compensated in the dark. The ratio of the nitrogen-bound exciton line (Qo) to the free excitonic emission (I77) as a function of excitation power density (Pexc) was tracked for several SI 4H-SiC samples with varying residual N concentration (~ 7x1014 – 5.2x1016 cm-3). Most notably, a linear relationship was found between Qo/I77 and [N] for [N] < 1x1016 cm-3 while a sub-linear behavior was observed for samples with higher N levels. This technique should be particularly valuable to map [N] where the levels are close to or below the present SIMS detection limit of ~ 5-7 x 1014 cm-3. Results obtained for a limited number of low n-type and SI 6H-SiC substrates are also presented.

Abstract: High temperature anneals were used to study the evolution of native defects in semiinsulating (SI), ultrahigh purity SiC using electron paramagnetic resonance (EPR), infrared and visible photoluminescence (PL) and COREMA (Contactless Resistivity Mapping) measurements. In EPR we observe a defect that we tentatively identify as VC-CSi-VC. The EPR intensities of this defect and the UD1 IRPL increase significantly with annealing in all samples.