Materials Science Forum Vols. 556-557

Abstract: Aluminum ions (Al+) were implanted at room temperature or at 500°C into n-type 4HSiC. The implantation damage (displaced Si atoms) and the electrical activation of Al+ ions (concentration of Al acceptors) were determined by Rutherford backscattering in channeling mode and Hall effect, respectively, as a function of the annealing temperature.

Abstract: This paper presents results of investigations about the influence of Hydrogen (introduced by annealing or plasma implantation), and Helium (ion implantation followed by a proper annealing for creating nanocavities) on the electrical properties of 4H-SiC n-type epitaxial samples. First, 4HSiC epitaxial layers were hydrogenated either by annealing under H2 ambient or by a RF plasma treatment. This last process took place before or after the deposition of Schottky contacts. Two different annealing temperatures were imposed (300°C and 400°C), as well as two plasma hydrogen doses for the same low energy. An improvement of electrical characteristics (25 % increasing of the minority carrier diffusion length, lowering of ideality factor, better switching characteristic) is detected for samples annealed at 400°C. The treatment of 4H-SiC surface in hydrogen plasma through Ni metal also increases the diffusion length, but not sufficiently to have an effect on I-V characteristics. A second set of 4H-SiC epitaxial layers were secondly implanted with He+ ions at two distinct temperatures. An annealing at 1700°C during 30 minutes under argon atmosphere was then carried out. C-V measurements revealed the presence of a high charge density zone around the nanocavities, containing fixed negative charges, opposite in sign to the donor atoms.

Abstract: We report a comparison of continuous-wave photoluminescence spectra with spatiallyresolved micro-photoluminescence data collected at low temperature on as-grown stacking faults in a 4H-SiC epitaxial layer. We find that the defects have a large triangular shape (50 μm x 50 μm x 50 μm) and that the maximum signal wavelength shifts when scanning across one triangular defect. These results show that the built-in electric field in the stacking fault well can be screened, more or less depending on the incoming light intensity.

Abstract: D-band electron paramagnetic resonance (EPR) measurements as well as X and Q-band field-swept Electron Spin Echo (ESE) and pulsed Electron Nuclear Double Resonance (ENDOR) studies were performed on a series of n-type 4H-SiC wafers grown by different techniques including sublimation sandwich method (SSM), physical vapor transport (PVT) and modified Lely method. Depending on the C/Si ratio and the growth temperature the n-type 4H-SiC wafers revealed, besides a triplet due to nitrogen residing on the cubic site (Nc), two nitrogen (N) related EPR spectra with g||=2.0055, g⊥=2.0010 and g||=2.0063, g⊥=2.0005 with different intensities. In the samples with low C/Si ratio the EPR spectrum with g|| =2.0055, g⊥=2.0010 consists of a triplet with low intensity which is tentatively explained as a N-related complex, while in the samples with high C/Si ratio the triplet is transformed into one structureless line of high intensity, which is explained as being due to an exchange interaction between N donors. In the samples grown at low temperature with enhanced carbon concentration the EPR line with g||=2.0063, g⊥=2.0005 and a small hyperfine (hf) interaction dominates the EPR spectrum. It is attributed to N on the hexagonal lattice site. The interpretation of the EPR data is supported by activation energies and donor concentrations obtained from Hall effect measurements for three donor levels in this series of 4H-SiC samples.

Abstract: We carried out mapping of the excess carrier lifetime for a bulk p-type 4H-SiC wafer by the microwave photoconductivity decay (μ-PCD) method, and we compared the lifetime map with structural defect distribution. Several small regions with short lifetimes compared with surrounding parts are found, and they correspond to regions with high-density structural defects. Excess carrier decay curves for this wafer show a slow component, which originates from minority carrier traps. From temperature dependence of the excess carrier decay curve, we found decrease of the time constant of the slow component with increasing temperature. We compared the activation energy of the time constant with that obtained from the numerical simulation, and concluded that the energy level for the minority carrier trap would be 125 meV from the conduction band.

Abstract: The resonant frequencies and quality factors of MEMS and NEMS depend critically on the layer quality and the residual stress in the SiC/Si heterostructure. It is demonstrated, that FTIRellipsometry is a suitable technique for monitoring the inhomogeneous residual stress inside SiC/Si heterostructures containing thin layers and their variation with during processing.

Abstract: We report on admittance spectroscopy (AS) investigations taken on aluminum (Al)- doped 6H-SiC crystals at low temperatures. Admittance spectra taken on Schottky contacts of highly doped samples (NA ≥ 7.2×1017 cm-3) reveal two series of conductance peaks, which cause two different slopes of the Arrhenius plot. The steep slope is attributed to the Al acceptor, while the flatter one - obtained from the low temperature peaks - is attributed to the activation energy ε3 of nearest neighbor hopping. We propose a model, which explains the unexpected sharpness of the low temperature conductance peaks and the disappearance of these peaks for low acceptor concentrations. The model is verified by simulation, and the AS results are compared with corresponding results obtained from resistivity measurements taken on 4H- and the identical 6HSiC samples.

Abstract: The influence of different cooling rates on deep levels in 4H-SiC after high temperature annealing has been investigated. The samples were heated from room temperature to 2300°C, followed by a 20 minutes anneal at this temperature. Different subsequent cooling sequences down to 1100°C were used. The samples have been investigated using photoluminescence (PL) and IV characteristics. The PL intensities of the silicon vacancy (VSi) and UD-2, were found to increase with a faster cooling rate.

Abstract: Recently published experimental results for 4H–SiC diodes up to 700 °C are used to deduce the hole lifetime temperature-dependence in n-base for high temperature range. The reverse recovery measurements are interpreted by the nonisothermal drift-diffusion simulator DYNAMIT. The uncertainties from lifetimes unknown behavior in emitter layers and consequences from possible nonuniform lifetime distribution in n-base are analyzed. Results show that up to temperature 400 °C nearly quadratic dependence of lifetime versus temperature τ ~ T 2 holds. At higher temperatures lifetime growth is accelerated approximately to quartic dependence τ ~ T 4.

Abstract: From the temperature dependence of the hole concentration p(T) in a lightly Al-doped 4H-SiC epilayer irradiated with several fluences of 200 keV electrons, the density of Al acceptors with 0.2 V E + eV decreases significantly with increasing fluence, whereas the density of unknown defects with 0.37 V E + eV increases with fluence and then decreases slightly. Although only C vacancies increase with fluence because 200 keV electrons can displace only C atoms, only the increase in the density of C monovacancies cannot explain the changes of p(T) by 200 keV electron irradiation. It may be necessary to consider the relationship between C vacancies and Al acceptors.