Andreas Gällström

The silicon vacancy in SiC

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.

Theory of Neutral Divacancy in SiC: A Defect for Spintronics

We investigate the neutral divacancy in SiC by means of first principles calculations and group theory analysis. We identify the nature of the PL transitions associated with this defect. We show that how the spin state may be manipulated optically in this defect.

Deep levels and carrier compensation in V-doped semi-insulating 4H-SiC

Electron paramagnetic resonance was used to study semi-insulating (SI) 4H-SiC substrates doped with vanadium (V) in the range of 5.5× 1015 -1.1× 1017 cm-3. Our results show that the electrical acti ...

Deep levels in tungsten doped n-type 3C–SiC

Tungsten was incorporated in SiC and W related defects were investigated using deep level transient spectroscopy. In agreement with literature, two levels related to W were detected in 4H–SiC, whereas only the deeper level was observed in 6H–SiC. The predicted energy level for W in 3C–SiC was observed (EC−0.47 eV). Tungsten serves as a common reference level in SiC. The detected intrinsic levels align as well: E1 (EC−0.57 eV) in 3C–SiC is proposed to have the same origin, likely VC, as EH6/7 in 4H–SiC and E7 in 6H–SiC, respectively.

Electron paramagnetic resonance and theoretical studies of Nb in 4H- and 6H-SiC

High purity silicon carbide (SiC) materials are of interest from high-power high temperature applications across recent photo-voltaic cells to hosting solid state quantum bits, where the tight control of electrically, optically, and magnetically active point defects is pivotal in these areas. 4H- and 6H-SiC substrates are grown at high temperatures and the incorporation of transition metal impurities is common. In unintentionally Nb-doped 4H- and 6H-SiC substrates grown by high-temperature chemical vapor deposition, an electron paramagnetic resonance (EPR) spectrum with C1h symmetry and a clear hyperfine (hf) structure consisting of ten equal intensity hf lines was observed. The hf structure can be identified as due to the interaction between the electron spin S = 1/2 and the nuclear spin of 93Nb. Additional hf structures due to the interaction with three Si neighbors were also detected. In 4H-SiC, a considerable spin density of ∼37.4% was found on three Si neighbors, suggesting the defect to be a complex...

Transition Metal Defects in Cubic and Hexagonal Polytypes of SiC: Site Selection, Magnetic and Optical Properties from Ab Initio Calculations

Relatively little is known about the transition metal defects in silicon carbide (SiC). In this study we applied highly convergent and sophisticated density functional theory (DFT) based methods to investigate important transition metal impurities including titanium (Ti), vanadium (V), niobium (Nb), chromium (Cr), molybdenum (Mo) and tungsten (W) in cubic 3C and hexagonal 4H and 6H polytypes of SiC. We found two classes among the considered transition metal impurities: Ti, V and Cr clearly prefer the Si-substituting configuration while W, Nb, and Mo may fractionally form a complex with carbon vacancy in hexagonal SiC even under thermal equilibrium. If the metal impurity is implanted into SiC or when many carbon impurities exist during the growth of SiC then complex formation between Si-substituting metal impurity and the carbon vacancy should be considered. This complex pair configuration exclusively prefers the hexagonal-hexagonal sites in hexagonal polytypes and may be absent in cubic polytype. We also studied transition metal doped nano 3C-SiC crystals in order to check the effect of the crystal field on the d-orbitals of the metal impurity.

Contact-Less Electrical Defect Characterization of Semi-Insulating 6H-SiC Bulk Material

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.

Influence of Cooling Rate after High Temperature Annealing on Deep Levels in High-Purity Semi-Insulating 4H-SiC

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.

Investigation of Mo defects in 4H-SiC by means of density functional theory

We investigated Molybdenum (Mo) defects in 4H silicon carbide (SiC). This system can be suitable candidate for in vivo biomarker applications since it shows photoluminescence (PL) in the near-infrared (NIR) region. In order to reveal the origin of this Mo-related PL center we carried out ab initio density functional theory (DFT) calculations on two microscopic models.

Investigation of Intrinsic Carbon-Related Defects in 4H-SiC by Selective-Excitation Photoluminescence Spectroscopy

Emission of carbon-related defects is investigated by means of selectively-excited photoluminescence in high purity 4H-SiC electron-irradiated with very low dose. Two new centers with clearly associated phonon replicas are observed, one of which is tentatively assigned to the carbon split interstitial at hexagonal site. The temperature dependence of the spectrum is also studied and indicates that at least some of the observed luminescence lines arise from recombination of excitons bound to isoelectronic centers.