Papers by Keyword: Intrinsic Defect

Abstract: High-purity, semi-insulating 6H-SiC substrates grown by high-temperature chemical vapor deposition were studied by electron paramagnetic resonance (EPR). The carbon vacancy (VC), the carbon vacancy-antisite pair (VCCSi) and the divacancy (VCVSi) were found to be prominent defects. The (+|0) level of VC in 6H-SiC is estimated by photoexcitation EPR (photo-EPR) to be at ~ 1.47 eV above the valence band. The thermal activation energies as determined from the temperature dependence of the resistivity, Ea~0.6-0.7 eV and ~1.0-1.2 eV, were observed for two sets of samples and were suggested to be related to acceptor levels of VC, VCCSi and VCVSi. The annealing behavior of the intrinsic defects and the stability of the SI properties were studied up to 1600°C.

Abstract: Density functional theory (DFT) with local density approximation has been used to calculate the formation energy (EF) of the neutral vacancy in germanium single crystal. It was shown that careful checking of convergence with respect to the number of k-points is necessary when calculating the formation energy of the intrinsic point defects in Ge. The formation energy of the single neutral vacancy was estimated at 2.35 eV which is in excellent agreement with published experimental data.

Abstract: Reduction in deep level defects and increase of carrier lifetime in 4H-SiC epilayer was observed after carbon ion implantation into the shallow surface layer of 250 nm and subsequent annealing above 1400 °C. The concentration of Z1/2 and EH6/7 traps was determined by deep level transient spectroscopy 4 μm below the implanted layer. After annealing, concentration of both traps decreased from 1013 cm-3 range to below the detection limit. Minority carrier lifetime almost doubled in the implanted samples compared to the unimplanted samples. We suggest that carbon interstitials from the implanted layer in-diffuse into the layer underneath during annealing and annihilate with carbon vacancies. Our results indicate that Z1/2 and EH6/7 traps are most likely carbon vacancy related.

Abstract: The authors have investigated deep levels in electron-irradiated n- and p-type 4H-SiC epilayers by deep level transient spectroscopy (DLTS). By low-energy electron irradiation at 116 keV, the Z1/2 and EH6/7 concentrations are increased in n-type samples, and the concentrations are almost unchanged after annealing at 950°C for 30 min. In p-type samples, the unknown centers, namely EP1 and EP2, are introduced by irradiation. By annealing at 950°C, the unknown centers are annealed out. The HK4 center (EV + 1.44 eV) is increased by the electron irradiation and subsequent annealing at 950°C. The dependence of increase in the trap concentrations by irradiation (NT) on the electron fluence reveals that NT for the Z1/2 and EH6/7 centers is in proportional to the 0.7 power of electron fluence, while the slope of the plot is 0.5 for the HK4 center. The Z1/2 and EH6/7 centers show similar annealing stage and are thermally stable up to 1500-1600°C, while the HK4 center is annealed out at about 1350°C. The Z1/2 and EH6/7 centers may be derived from a same origin (single carbon vacancy: VC) but different charge state. The HK4 center may be a complex including VC.

Abstract: Nitrogen (N) donors in SiC are partially deactivated either by Si+-/N+-co-implantation or by irradiation with electrons of 200 keV energy and subsequent annealing at temperatures above 1450°C; simultaneously the compensation is decreased. The free electron concentration and the formation of energetically deep defects in the processed samples are determined by Hall effect and deep level transient spectroscopy. A detailed theoretical treatment based on the density functional theory is conducted; it takes into account the kinetic mechanisms for the formation of N interstitial clusters and (N-vacancy)-complexes. This analysis clearly indicates that the (NC)4-VSi complex, which is thermally stable up to high temperatures and which has no level in the band gap of 4HSiC, is responsible for the N donor deactivation.

Abstract: A variety of 4H-SiC samples from undoped crystals grown by the physical vapor transport technique have been studied by temperature dependent Hall effect, optical and thermal admittance spectroscopy and thermally stimulated current. In most samples studied the activation energies were in the range 0.9 - 1.6 eV expected for commercial grade HPSI 4H-SiC. However, in several samples from developmental crystals a previously unreported deep level at EC-0.55 ± 0.01 eV was observed. Thermal admittance spectroscopy detected one level with an energy of about 0.53 eV while optical admittance spectroscopy measurements resolved two levels at 0.56 and 0.64 eV. Thermally stimulated current measurements made to study compensated levels in the material detected several peaks at energies in the range 0.2 to 0.6 eV.

Abstract: Deep levels in as-grown p-type 4H-SiC epilayers have been investigated by DLTS. Three deep hole traps (HK2, HK3 and HK4) can be detected by DLTS in the temperature range from 350K to 700K. They are energetically located at 0.84 eV (HK2), 1.27 eV (HK3) and 1.44 eV (HK4) above the valence band edge. The activation energy of the traps does not show any meaningful change regardless of applied electric field, indicating that the charge state of the deep hole traps may be neutral after hole emission (donor-like). By the low-energy electron irradiation, the HK3 and HK4 concentrations are significantly increased, suggesting that the origins of the HK3 and HK4 may be related to carbon displacement. Study on the thermal stability of these hole traps has revealed that the trap concentrations of HK3 and HK4 are reduced to below the detection limit (1-2 × 1011 cm-3) by annealing at 1350°C. The HK2 is thermally more stable than HK3 and HK4, and becomes lower than the detection limit by annealing at 1550°C.

Abstract: We have investigated the electrically active deep level defects in p- and n-type 4H-SiC after low energy electron irradiation. Intrinsic defects were created by irradiation with 200 keV electrons, with energy sufficient to move only the carbon atoms in SiC lattice. Defect spectra were compared between the p- and n-doped samples prepared under identical irradiation conditions. We probed both conduction and valence band sides of the band-gap by using capacitance transient techniques with electrical and optical trap filling. We have found that the defect spectrum in the p-type epilayers differs significantly from the n-type. The Z1/Z2, EH1 and EH3 electron traps which are usually present in irradiated n-type material could not be detected in p-type samples. An electron trap at 1.6 eV below the conduction band edge is present in both n- and p-type samples at the same energy position and with similar concentration, therefore it is probably related to the same type of defect. We have also found a new hole trap in p-type epilayers at energy EV + 0.66 eV.

Abstract: The migration of carbon interstitials in n-type 4H-SiC has been revealed with optical and electrical measurements. Furthermore, clear evidence is found that carbon interstitials are involved in the formation of the Z- and S-centers detected by DLTS within the electronic band gap of n-type 4H-SiC.