Invited talk abstract

The physical properties of silicon carbide (SiC) make the material well suited for high-temperature, high-power, and high-frequency applications. A striking property of SiC is its polytypism. The compound exists in several different polytypes of which the 3C, 4H, 6H, and 15R structures are the most common ones. The accompanying variation of the fundamental gap between 2.39 eV (3C) and 3.26 eV (4H) makes SiC interesting for heterostructure devices. Native defects play an important role in the modification of the material properties. They occur in as-grown crystals and layers but, in particular, in irradiated samples. In the paper the monovacancies are considered as prototypical native defects in the prototypical polytypes 3C and 4H. However, antisites and complexes of vacancies with impurities, e.g. boron, are also discussed.

The energetics, structure, and electronic states are investigated via ab initio calculations based on density functional theory and local spin density approximation. The generation of the C-site vacancies is generally accompanied by a remarkable Jahn-Teller distortion towards D_2d symmetry. Therefore a negative U behaviour is predicted. Due to the strong localization of the C dangling bonds, the Si-site vacancies exhibit an outward breathing relaxation and high-spin configurations except for the neutral vacancy. According to the formation energies the carbon vacancy, which is a double donor, dominates in p-type material independent of the polytype. In C-rich 3C-SiC, however, the electrically inactive C_Si antisite is the dominant native effect in the thermal equilibrium, at least for n-type conditions.

The Si vacancies give rise to ionization levels close to the valence band maximum of both 3C and 4H. Taking into account the inaccuracy of the calculations of about 0.1 eV and the binding energy of about 0.05 eV, the (+/0) ionization level may be related to the donor D_1 bound exciton. The L_1 photoluminescence (PL) line can be interpreted as a transition V_Si(+) + electron --> V_Si(0). It explains the polytype dependence and the occurrence of only one line in 4H, despite of the inequivalent lattice sites. The charge states of the vacancies can be also used to explain the characteristic PL lines at 1.12 eV (3C) and 1.35/1.44 eV (4H) in the infrared spectral region. The weak polytype dependence, the spin states, and the transition energies are related to intravacancy transitions between an excited state and the ground state of a charged Si vacancy.

The deep boron acceptor in hexagonal SiC is interpreted by a boron on a silicon position with an adjacent carbon vacancy. In addition, a shallow boron acceptor is discussed. In this paper we suggest to study the atomic geometry and the electronic structure of B adsorption on SiC(111)/(0001) surfaces, in order to understand bulk doping. We predict a B_Si-V_C (B_C) adsorption configuration under carbon (Si)-rich preparation conditions, in agreement with coimplantation experiments. It induces a surface band in a midgap position (close to the valence band maximum).