Materials Science Forum Vols. 527-529

Abstract: A detailed investigation of the optical and electronic properties of the deep-level defect UD-4 is reported. This defect has recently been observed in 4H semi-insulating silicon carbide, but has hardly been studied yet. Both low temperature and temperature-dependent photoluminescence were collected from the defect. Zeeman spectroscopy measurements were performed as well as time-resolved photoluminescence.

Abstract: We observe new photoluminescence centers in electron-irradiated 6H-SiC with phonon replicas up to 250 meV and clear threefold isotope splitting of the highest energy mode. Based on ab initio calculations, we discuss the tri-carbon anti-site (C3)Si and the di-interstitial (C2)Hex as models for these centers.

Abstract: We applied a non-degenerate four wave mixing (FWM) technique to investigate carrier generation, diffusion and recombination processes in PVT-grown semi-insulating wafers of 6H-SiC at 300 K. The resistivity of samples, cut from different places of a boule as well as from different boules, varied in range from a few
⋅cm up to 1010
⋅cm. Interband excitation (at 355 nm) and below bandgap excitation (at 532 nm) allowed to study dynamics of the bipolar plasma and the contribution of deep levels to carrier generation and recombination. The nonequilibrium carrier lifetime was shorter in the samples of higher resistivity, in accordance with the increasing density of deep levels. The bipolar plasma diffusion in high-resistivity samples (~109
⋅cm) provided the value of the diffusion coefficient D = 4.4 cm2/s and hole mobility μh = (88 ± 6) cm2/Vs.

Abstract: The unusual behaviour of two optical centres with zero phonon lines close to 463nm has been investigated by means of low-temperature photoluminescence microscopy using 488nm and 325nm laser excitation. The experiments were performed on as-irradiated samples and also after annealing isochronally to various temperatures up to 1300°C.

Abstract: Several 4H SiC samples have been electron-irradiated at near threshold energies at low fluence, either along the [0001] or [000-1] direction. PL and EPR techniques have been used to investigate the dependence of the beam direction on defect generation and, together with a sample irradiated at a higher fluence, to correlate differences brought about by irradiating with a change in electron fluence. Attempts are made to correlate the information derived from the two techniques.

Abstract: Samples of 4H SiC, both n- and p-doped, have been irradiated with low-energy electrons in a transmission electron microscope. The dependence of the silicon vacancy-related V1 ZPL doublet (~860nm) on electron energy and electron dose has been investigated by low temperature photoluminescence spectroscopy. Furthermore, this luminescence centre has been studied across a broad range of samples of various doping levels. Some annealing characteristics of this centre are reported.

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.

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: Carrier lifetimes and the dominant electron and hole traps were investigated in a set of thick (9-104mm) undoped 4H-SiC epitaxial layers grown by CVD homoepitaxy. Deep trap spectra were measured by deep level transient spectroscopy (DLTS) with electrical or optical injection, while lifetimes were measured by room temperature time-resolved photoluminescence (PL). The main electron traps detected in all samples were due to Ti, Z1/Z2 centers, and EH6/EH7 centers. Two boron-related hole traps were observed with activation energies of 0.3 eV (boron acceptors) and 0.6 eV (boron-related D centers). The concentration of electron traps decreased with increasing layer thickness and increased toward the edge of the wafers. PL lifetimes were in the 400 ns-1800 ns range with varying injection and generally correlated with changes in the density of Z1/Z2 and to a lesser extent the EH6/EH7 electron traps. However, the results of DLTS measurements on p-i-n diode structures suggest that the capture of injected holes is much more efficient for the Z1/Z2 traps compared to the EH6/EH7 centers making the Z1/Z2 more probable candidates for the role of lifetime killers. A good fit of the thickness dependence of the measured lifetimes to the usual analytical form was obtained assuming that Z1/Z2 is the dominant hole recombination center and that the surface recombination velocity was 2500 cm/sec.

Abstract: Deep electron and hole traps were studied in a series of high purity 6H-SiC single crystals grown by Halide Chemical Vapor Deposition (HCVD) method at various C/Si flow ratios and at temperatures between 2000 oC and 2100 oC. Characterization included Low Temperature Photoluminescence (LTPL), Deep Level Transient Spectroscopy (DLTS), Minority Carrier Transient Spectroscopy (MCTS), and Thermal Admittance Spectroscopy (TAS) measurements. Concentrations of all deep traps were shown to strongly decrease with increased C/Si flow ratio and with increased growth temperature. The results indicate that the majority of deep centers in 6H-SiC crystals grown by HCVD are due to native defects or complexes of native defects promoted by Si-rich growth conditions. The observed growth temperature dependence of residual donor concentration and traps density is explained by increasing the effective C/Si ratio at higher temperatures for the same nominal ratio of C and Si flows.