Papers by Author: Nguyen Tien Son

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: EPR spectra originating from phosphorus shallow donors occupying silicon sites in 3C-, 4H-, and 6H-SiC are identified by using CVD grown films in which the interference from the signals from the nitrogen shallow donors is practically absent. Phosphorus donors occupying both silicon and carbon sites are observed in high-energy phosphorus ion implanted semi-insulating 6H-SiC which was also free from the interference from the signals from the nitrogen shallow donors.

Abstract: We present new electron-paramagnetic-resonance (EPR) data on the HEI4/SI5 center in 4H-SiC. So far, the SI5 (SI-5) center has been observed only in as-grown SiC substrates; however, we found that it can be created by electron irradiation to commercial n-type 4H-SiC. The artificially created SI5 center, which we had preliminary called HEI4, was found to be identical with the SI5 center in as-grown SiC. A high-intensity HEI4/SI5 spectrum of irradiated SiC revealed clear hyperfine structures of 29Si and 13C, which enabled us to identify the origin of this center as a carbon antisite-vacancy pair in the negative charge state (CSi-VC –). We assessed its electronic levels using photo-EPR.

Abstract: The negative carbon vacancy antisite complex is analysed by ab initio theory in view of the SI5 EPR-center. The complex occurs in a Jahn-Teller distorted ground state and a meta stable state. This and the calculated hyperfine structure agree nicely with the temperature dependent EPR spectra of SI5. An interpretation of the photo-EPR experiments is proposed.

Abstract: Electron paramagnetic resonance (EPR) studies of the P6/P7 centers in 4H- and 6H-SiC are reported. The obtained principal values of the hyperfine tensors of C and Si neighbors are in good agreement with the values of the neutral divacancy (VCVSi 0) calculated by ab initio supercell calculations. The results suggest that the P6/P7 centers, which were previously assigned to the photo-excited triplet states of the carbon vacancy-carbon antisite pairs in the double positive charge state (VCCSi 2+), are related to the triplet ground states of the C3v/C1h configurations of VCVSi 0.

Abstract: Only recently the well-resolved hyperfine structure of the P6/P7 EPR center has been experimentally observed. Based on the calculated hyperfine tensors we assign the P6/P7 center to the high spin state neutral divacancy, which is the ground state in agreement with the experiment. We propose a mechanism to explain the loss of divacancy signal at high tem- perature annealing in semi-insulating SiC samples. We discuss the possible correlation between the divacancy and some photoluminescence centers.

Abstract: Semi-insulating SiC grown by the HTCVD technique are studied by luminescence and absorption measurements and the results are compared to PAS and SIMS results. We have found that metal impurities are present but only in very small concentrations. The semi-insulating properties are instead determined by the intrinsic defects, mostly the silicon vacancy in hydrocarbon rich grown material and the carbon vacancy in the hydrocarbon poor grown material. The hydrocarbon poor material is stable upon annealing both from a vacancy concentration point of view and from a resistivity point of view. The hydrocarbon rich grown material does not stand the annealing at 1600 °C and the resistivity is decreased; from the absorption and PAS measurements we have observed that the decrease in silicon vacancy concentration fits the growth of the vacancy clusters.

Abstract: Interaction of boron and aluminum with interstitial carbon is studied using first principles calculations. It is shown that carbon can form very stable complexes with Al and B, forming a family of negative-U bistable defects with deep levels. The influence of this effect on the activation rate of p-type implants is discussed.

Abstract: Electron paramagnetic resonance (EPR) was used to study 4H- and 6H-SiC doped with P during chemical vapour deposition (CVD) growth. In 6H-SiC, three spectra with C3v symmetry and spin S=1/2, labelled Ph, Pc1 and Pc2, were detected. The g-values and the 31P hyperfine (hf) constants were determined for Ph: g||=2.0046, g^=2.0028, and A||=0.103 mT, A^<0.05 mT; for Pc1: g||=2.0039, g^=2.0025, and A||=0.615 mT, A^=0.43 mT; for Pc2: g||=2.0038(5), g^=2.0025, and A||=0.40 mT, A^=0.22 mT. The hf interaction with nearest 13C neighbours were also observed for the Pc1 and Pc2 centers, confirming that in CVD grown material the shallow P donor occupies the Si site. The Ph, Pc1 and Pc2 centers are assigned to the ground states of the shallow P at the hexagonal (Ph) and quasi-cubic sites (Pc1 and Pc2) in 6H-SiC. In 4H-SiC, an EPR spectrum of C3v symmetry with a larger anisotropy in the g-values (g||=2.0065 and g^=2.0006) was observed. The temperature dependence of the spectrum is similar to that of Ph in the 6H polytype. Its 31P hyperfine constants are determined as A||=0.294 mT and A^=0.21 mT.

Abstract: Shallow N donors in n-type 4H-SiC were studied by electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR). For the N donor at the cubic site (Nk) in 4H-SiC, the hyperfine (hf) constants of the interaction with the nearest neighbour (NN) 29Si atom along the c axis were determined as A = 41.07 MHz and A^ = 41.31 MHz. For other three NN Si atoms in the basal plane, the hf tensor has C1h symmetry and the principal values Axx = 5.94 MHz, Ayy = 5.06 MHz and Azz = 14.25 MHz. Our EPR and ENDOR observations unambiguously confirm that the N donor occupies the C site in 4H-SiC lattice and also reveal a considerable amount of the spin density of Nk (~23.9%) which was not obtained in previous studies.