Bistable behavior of the nitrogen impurity in SiC nanoclusters.

Nitrogen doped SiC nanoclusters of various shapes and polytype structures were modeled in the framework of density functional theory using the PBE0 hybrid functional. While in the bulk crystal NC impurity may be introduced in a large concentration and shows features of the conventional shallow effective-mass donor, in the nanocluster this defect can exist in two forms with sharply different electronic structures. The first one resembles the shallow donor confined in the nanocluster with a small and nearly isotropic lattice relaxation, and spin density spread over distant shells. The second form resembles a small bound polaron structure with a large and anisotropic lattice relaxation, which leads to a strong localization of the spin density on one of the nearest silicon atoms. Relative energies of these two states depend on the location of nitrogen in the lattice as well as on the shape and polytype structure of the nanocluster. The calculations show that the position of the impurity in the vicinity of the hexagonal double layers and cluster surface facilitates the formation of small-bound-polaron-like states. As follows from the calculations, the two impurity states can be experimentally distinguished by the values of the hyperfine and quadrupole coupling parameters that can be measured in magnetic resonance experiments. They are also characterized by rather different electronic absorption spectra. We have demonstrated that for negatively charged SiC nanoclusters the polaron-like impurity states can be formed even if they do not exist in neutral clusters. This provides a reason to believe that the nitrogen impurity occupying particular sites in SiC nanoclusters is an analog of the DX centers in the bulk crystal. In addition, the formation of the polaron-like states depends on the method of the surface passivation. In particular, the substitution of the Si-H surface bond with the Si-NH3 one facilitates a bistable behavior of the NC impurity located in the central part of the nanocluster. We have also shown that the effect of the self-purification manifests itself in different ways depending on the nanocluster shape and polytype structure.

Comparison of nitrogen-vacancy complexes in diamond and cubic SiC: dose dependencies and spin-Hamiltonian parameters.

At present the nitrogen-vacancy (NV) complex in diamond is the most promising defect for application in the area of quantum computing. This provides a stimulus for an extensive search of other defects in semiconductors with similar properties. Recently it was shown that the NCVSi defect complex in SiC is perspectively appropriate for this goal as well. In the present work we perform comparative ab initio studies of NV complexes in diamond and 3C-SiC. We focus both on radiospectroscopic characterization of these defects and on the calculation of the equilibrium concentration of complexes in irradiated crystals. In particular a full set of spin-Hamiltonian parameters including g-tensors, hyperfine tensors and the spin-spin part of zero-field splitting constant Dss were calculated for both negative and neutral charge states as well as for excited quartet states of neutral complexes. Comparison of calculated values with the available experimental data and results of other calculations show good agreement, especially in the case when hybrid and meta-hybrid functionals were used. This makes the unambiguous identification of negative NV complexes in both materials possible. Our calculations reveal that the ground states of neutral complexes are a difficult case for both DFT calculations and experimental observations. This is caused by multi-determinantal behavior of wave function for such complexes, which leads to a large amount of spin contamination and to the broken symmetry solution which appeared for single Slater determinant DFT calculations. Based on the calculated minimum of free energy of neutral and negative complexes in SiC and diamond we obtained the equilibrium concentrations of these complexes depending on the vacancy concentration produced by irradiation. We show that in some dose regions both negative and neutral complexes coexist, while in other regions only one charge state prevails. Comparison of the calculated and experimental dose dependencies for diamond shows good qualitative agreement.

Mechanism of strain-influenced quantum well thickness reduction in GaN/AlN short-period superlattices.

We report on the mechanism of strain-influenced quantum well (QW) thickness reduction in GaN/AlN short-period superlattices grown by plasma-assisted molecular beam epitaxy. Density functional theory was used to support the idea of a thermally activated exchange mechanism between Al adatoms and Ga surface atoms that is influenced by the strain state of the GaN QWs. These ab initio calculations support our experimentally observed reduction in QW thickness for different intrinsic strains.

Barium dithionate as an EPR dosemeter.

Electron paramagnetic resonance (EPR) dosimetry is growing in popularity and this success has encouraged the search for other dosimetric materials. Previous studies of gamma-irradiated barium dithionate (BaS(2)O(6) x 2H(2)O) have shown promise for its use as a radiation dosemeter. This work studies in greater detail several essential attributes of the system. Special attention has been directed to the study of EPR response dependences on microwave power, irradiation temperature, minimum detectable dose and post-irradiation stability.