Papers by Author: Björn Magnusson

Abstract: In this work, quenching effect in the photoluminescence (PL) spectrum of divacancy defects in 4H SiC is investigated. Quenching in PL occurs when photoexcitation with an energy below a certain threshold is applied. In order to understand this phenomenon, we carried out Kohn-Sham density functional theory (DFT) calculations. In accordance with recent experimental results, we propose a physical model which explains the quenching phenomenon.

Abstract: X-ray topography shows that selective KOH etching after CVD growth of n-type epilayers on highly N doped 4H SiC substrates can be used to reliably map pure and mixed Threading Screw Dislocations (TSD). The influence of the mapping grid density and the wafer position in the crystal on the average TSD density are investigated. A reliable mapping of TSD contributed to the development of 100mm SiC wafers with average TSD density down to 200 cm^-2.

Abstract: A commonly observed unidentified photoluminescence center in SiC is UD-1. In this report, the UD-1 center is identified to be tungsten related. The identification is based on (i) a W-doping study, the confirmation of W in the samples was made using deep level transient spectroscopy (DLTS), (ii) the optical activation energy of the absorption of UD-1 in weakly n-type samples corresponds to the activation energy of the deep tungsten center observed using DLTS. The tungsten-related optical centers are reported in 4H-, 6H-, and 15R-SiC. Further, a crystal field model for a tungsten atom occupying a Si-site is suggested. This crystal field model is in agreement with the experimental data available: polarization, temperature dependence and magnetic field splitting.

Abstract: SiC MESFETs were scaled both laterally and vertically to optimize high frequency and high power performance. Two types of epi-stacks of SiC MESFETs were fabricated and measured. The first type has a doping of 3×1017 cm-3 in the channel and the second type has higher doping (5×1017 cm-3) in the channel. The higher doping allows the channel to be thinner for the same current density and therefore a reduction of the aspect ratio is possible. This could impede short channel effects. For the material with higher channel doping the maximum transconductance is 58 mS/mm. The maximum current gain frequency, fT, and maximum frequency of oscillation, fmax, is 9.8 GHz and 23.9 GHz, and 12.4 GHz and 28.2 GHz for the MESFET with lower doped channel and higher doping, respectively.

Abstract: Electron paramagnetic resonance (EPR) was used to study high-purity semi-insulating 4H-SiC irradiated with 2 MeV electrons at room temperature. The EPR signal of the EI4 defect was found to be dominating in samples irradiated and annealed at ~750°C. Additional large-splitting 29Si hyperfine (hf) lines and also other 13C and 29Si hf structures were observed. Based on the observed hf structures and annealing behaviour, the complex between a negative carbon vacancy-carbon antisite pair (VCCSi–) and a distance positive carbon vacancy ( ) is tentatively proposed as a possible model for the EI4 defect.

Abstract: The photoluminescence (PL) from the I 1 centre is observed in p-, n-type as well as in compensated samples, using above band gap excitation. The PL from I 1 in the two polytypes 4H and 6H is very similar, the difference being the position of the main peak, in 4H 1.1521 eV and 1.1057 eV in 6H. We here suggest I-1 to be Mo related based on intentional doping, SIMS results and comparison with earlier reports of Mo in SiC using magnetic resonance techniques. From PL measurements, we analyze the electron structure of the defect, and suggest it be the neutral Mo (4d2) residing on a Si site, the luminescence coming from the transition between the 3A2 multiplet of the first excited electronic configuration and the ground state 3A2.

Abstract: A model is presented for the silicon vacancy in SiC. The previously reported photoluminescence spectra in 4H and 6H SiC attributed to the silicon vacancy are in this model due to internal transitions in the negative charge state of the silicon vacancy. The magnetic resonance signals observed are due to the initial and final states of these transitions.

Abstract: Low temperature infrared photoluminescence (PL) performed on a large set of bulk SiC substrates has revealed distinct series of lines between 0.8 and 1.5 eV for samples with nitrogen levels between ~ 1016 and 1017 cm-3. Semi-insulating and intentionally N-doped wafers grown by PVT and HTCVD were investigated. Two groups of PL lines clustered near 1.0 and 1.35 eV, respectively, were observed in n-type 4H-SiC. Not surprisingly, a multiplicity of features at slightly different energy positions was found for this emission from the 6H- and 15R-SiC polytypes. Both sets of lines were not observed for substrates with N doping concentrations greater than 3x1017cm-3. Thus, it appears this IR emission can serve as optical “fingerprints” of bulk n-type substrate with moderate levels of N impurities. Models for the possible origins of these lines will also be discussed.

Abstract: The novel technique microwave detected photo induced current transient spectroscopy (MD-PICTS) was applied to semi-insulating 6H-SiC in order to investigate the properties of inherent defect levels. Defect spectra can be obtained in the similar way to conventional PICTS and DLTS. However, there is no need for contacting the samples, which allows for non-destructive and spatially resolved electrical characterization. This work is focused on the investigation of semi-insulating 6H-SiC grown under different C/Si-ratios. In the corresponding MD-PICTS spectra several shallow defect levels appear in the low temperature range. However the peak assignment needs further investigation. Additionally different trap reemission dynamics are obtained for higher temperatures, which are supposed to be due to different compensation effects.

Abstract: Semi-insulating (SI) 4H-SiC substrates doped with vanadium (V) in the range 5.5×1015 –1.1×1017 cm–3 were studied by electron paramagnetic resonance. We show that only in heavily V-doped 4H-SiC vanadium is responsible for the SI behavior, whereas in moderate V-doped substrates with the V concentration comparable or slightly higher than that of the shallow N donor or B acceptor, the SI properties are thermally unstable and determined by intrinsic defects. The results show that the commonly observed thermal activation energy Ea~1.1 eV in V-doped 4H-SiC, which was previously assigned to the single acceptor V4+/3+ level, may be related to deep levels of the carbon vacancy. Carrier compensation processes involving deep levels of V and intrinsic defects are discussed and possible thermal activation energies are suggested.