Materials Science Forum Vols. 527-529

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: The accuracy of Secondary Ion Mass Spectrometry (SIMS) depth profiles of aluminum (Al) dopant in silicon carbide (SiC) has been investigated. The Al SIMS profile differs in shape depending on whether it was obtained using a cesium (Cs+) or oxygen (O2 +) primary ion beam, and depends in the former case on which secondary ion is followed. The matrix signals indicate that the CsAl+ secondary ion yield changes during the Cs+ depth profile, probably because of the work function lowering due to the previously-implanted Al. These same matrix ion signals are used for a depth-dependent empirical correction to increase the accuracy of the Al concentration profile. The physics of these phenomena and the accuracy of the correction are discussed.

Abstract: We report on investigation of p-type doped, SiC wafers grown by the Modified- Physical Vapor Transport (M-PVT) method. SIMS measurements give Al concentrations in the range 1018 to 1020 cm-3, with weak Ti concentration but large N compensation. To measure the wafers’ resistivity, carrier concentration and mobility, temperature-dependant Hall effect measurements have been made in the range 100-850 K using the Van der Pauw method. The temperature dependence of the mobility suggests higher Al concentration, and higher compensation, than estimated from SIMS. Additional LTPL measurements show no evidence of additional impurities in the range of investigation, but suggest that the additional compensation may come from an increased concentration of non-radiative centers.

Abstract: We report density functional calculations using the full-potential linearized muffin-tin orbital method on early first row transition metal doped Silicon Carbide in both cubic (3C) and hexagonal (4H) polytypes. The energy levels in the gap for Ti, V and Cr are in good agreement with the available photoluminescence experiments. Our calculation shows that the Ti impurity is active for 4H but not for 3C, while V and Cr impurities are active for both polytypes. The magnetic interactions are very different for Cr and Mn. Cr shows a very local exchange interaction that decays rapidly, which is similar for different polytypes and different sites. The exchange interaction for Mn is quite long range and is very sensitive to the location of the Mn pairs.

Abstract: The purpose of this study is to determine the vanadium defect levels in semi-insulating 4H-SiC and 6H-SiC using optical admittance spectroscopy (OAS). OAS data show several distinct peaks for the vanadium-doped SI 4H-SiC and 6H-SiC. Comparison of the data for the two polytypes suggests that peaks at 0.67 ± 0.02 eV and 0.70 ± 0.02 eV in 6H substrates and 0.75 ± 0.02 eV in 4H substrates are related to V3+/4+ levels at the cubic sites. A peak at 0.87 ± 0.02 eV in the 6H sample is assigned to the same defect level at the hexagonal site and the associated transition in 4H was observed at 0.94 ± 0.02 eV in our spectra. The donor levels are thought to be related to peaks at 1.94 ± 0.05 eV and 1.87 ± 0.05 eV in 4H and 6H samples, respectively. The differences between the values obtained from the optical admittance measurements and those reported in the literature are attributed to thermal relaxation and/or contributions from defect complexes.

Abstract: In this paper, we report on a photoluminescence (PL) and EPR study of several semiinsulating (SI) 4H SiC samples showing the different compensation regimes due to the presence of V4+ and V3+of different concentration. The samples which contain only V3+ indicates the compensation regime NV≅ND-NA>0 with the Fermi level located in the upper half of the band gap. The presence of V4+ along with V3+ in the other two set of samples indicates the SI behavior of the samples with compensation regime NV>NA-ND>0. Considering that the samples revealed EPR spectrum of vanadium V3+, position of the Fermi level should be also in the upper half of the band gap and mixed conductivity could be expected. UD-3 PL spectrum was observed in vanadium doped SI 4H SiC presented in the samples in V3+/V4+ and V4+/V5+ charge states with compensation regime NV>NA-ND>0. The data obtained prove that the PL and EPR are suitable techniques in determination SI yield in SiC crystal.

Abstract: Silicon carbide (SiC) was investigated for deep band gap states of europium by means of deep level transient spectroscopy (DLTS). The knowledge of the properties of optoelectrically active impurities or defects is essential for a detailed understanding of the energy-transfer process resulting in the observable excitations [1]. SiC-samples of the polytypes 4H as well as 6H are ion-implanted by different europium- isotopes in order to obtain a chemical identification of the characterized energy levels. Here the concentration sensitivity of the DLTS is applied to observe the elemental transmutation of the incorporated radioactive tracer atoms 146Eu (t1/2=4.51 d) and 147Eu (t1/2=24.6 d). DLTS on samples implanted with stable Eu-ions (153Eu) was carried out for comparison and manifestation of the results. From these studies 5 Eu-related deep band gap levels are established: in 4H-SiC two levels at EV+0.86(2) eV and EC−0.47(2) eV, and in 6H-SiC three levels at EV+0.88(2) eV, EC−0.29(2) eV and EC−0.67(2) eV.

Abstract: We present comprehensive cathodoluminescence measurements from thin amorphous a- SiC films doped with rare earths. The a-SiC films were prepared by rf magnetron sputtering using a high purity SiC wafer in high purity argon atmosphere (5N, pressure approx. 0.2 mbar). The rare earth doping (Tb, Dy and Eu concentrations were below 2%) was performed by placing respective rare earth metal pieces of appropriate size onto the Silicon Carbide wafer. The rare earth ion emissions cover the colors green (Tb), yellow (Dy) and red (Eu). The optical and related structural properties of the films are correlated by means of high resolution transmission electron microscopy in combination with cathodoluminescence measurements in a scanning electron microscope. In addition, the corresponding compositions are determined by energy-dispersive x-ray analysis. The cathodoluminescence spectra of the rare earth 3+ ions are recorded in the visible at 20°C in the asgrown condition and after annealing treatments in the temperature range from 300°C to 1050°C by steps of 150°C. The anneal-related changes in the cathodoluminescence emission spectra and in the microstructure of the films are addressed. The SiC films show amorphous structure almost independent of the annealing treatment. Optimal annealing temperature for emissions of Tb3+ doped a-SiC were derived to be 600°C whereas Dy3+ and Eu3+ emissions increase at least up to 1050°C.