Takehide Miyazaki

Topology and energetics of metal-encapsulating Si fullerenelike cage clusters

On the basis of a topological discussion as well as an ab initio calculation, we show that it is possible to construct a fullerenelike Si cage by doping a transition metal atom in the cage center. The cage is a simple 3-polytope which maximizes the number of its inner diagonals close to the metal atom. Our topological argument also reveals how closely the structure of the fullerenelike Si cages studied is related to that of fullerenes themselves.

Germanium-Vacancy Single Color Centers in Diamond

Atomic-sized fluorescent defects in diamond are widely recognized as a promising solid state platform for quantum cryptography and quantum information processing. For these applications, single photon sources with a high intensity and reproducible fabrication methods are required. In this study, we report a novel color center in diamond, composed of a germanium (Ge) and a vacancy (V) and named the GeV center, which has a sharp and strong photoluminescence band with a zero-phonon line at 602 nm at room temperature. We demonstrate this new color center works as a single photon source. Both ion implantation and chemical vapor deposition techniques enabled fabrication of GeV centers in diamond. A first-principles calculation revealed the atomic crystal structure and energy levels of the GeV center.

A theoretical study of a sulfur impurity in diamond

Abstract An isolated substitutional sulfur in diamond is found to be a deep donor. Some complex sulfur defects with nitrogen, boron and a vacancy may have donor levels in the range of 0.4–0.5 eV. However, formation energies of the defects of our interest are all high. Thus, we expect that, although sulfur can be a relatively shallow donor in diamond, its doping efficiency is quite low.

Theoretical study of the structural evolution of small hydrogenated silicon clusters: Si6Hx

Abstract Density functional calculations were performed for the structural properties and energetics of small hydrogenated silicon clusters: Si6Hx (0 ⩽ x ⩽ 14). We find that the structures of Si6Hx can be classified into several distinct families in terms of the arrangement of silicon atoms. In particular, we find a series of structures which are intermediate between compact and tetrahedral atomic arrangements. Based on calculated formation energies we address the relative stability of the Si6Hx clusters.

Perfect selective alignment of nitrogen-vacancy centers in diamond

Nitrogen-vacancy (NV) centers in diamond have attracted significant interest because of their excellent spin and optical characteristics for quantum information and metrology. To take advantage of the characteristics, the precise control of the orientation of the N-V axis in the lattice is essential. Here we show that the orientation of more than 99 % of the NV centers can be aligned along the [111]-axis by CVD homoepitaxial growth on (111)-substrates. We also discuss about mechanisms of the alignment. Our result enables a fourfold improvement in magnetic-field sensitivity and opens new avenues to the optimum design of NV center devices.

Atomistic mechanism of perfect alignment of nitrogen-vacancy centers in diamond

Nitrogen-vacancy (NV) centers in diamond have attracted a great deal of attention because of their possible use in information processing and electromagnetic sensing technologies. We examined theatomistic generation mechanism for the NV defect aligned in the [111] direction of C(111) substrates. We found that N is incorporated in the C bilayers during the lateral growth arising from a sequence of kink propagation along the step edge down to [-1,-1,2]. As a result, the atomic configuration with the N-atom lone-pair pointing in the [111] direction is formed, which causes preferential alignment of NVs. Our model is consistent with recent experimental data for perfect NV alignment in C(111) substrates.

Theoretical study of sulfur–hydrogen–vacancy complex in diamond

We present an ab initio study of sulfur (S)–hydrogen (H)–vacancy (V) complexes in diamond. An S–H–V defect may become a much shallower donor than an isolated substitutional S defect when S in the complex is either three or five connected. Upon annealing the S-doped crystal, preferential formation of other deep-level defects would deactivate the shallow S-complex donors.

Ab initio dynamics of field emission from diamond surfaces

We propose a new interpretation of the efficiency of field emission, which is understood based on the concept of electron affinity. We use time-dependent density functional theory to simulate field emission from clean and chemically modified diamond (001) surfaces under applied electric fields. We find that the emission efficiency is governed by the self-consistent electrostatic potential (VSCF) at the surface rather than by the sign of the electron affinity, which is determined by VSCF in the vacuum region far from the surface. We resolve the paradox that the emission efficiency of a clean (001) surface with positive electron affinity is even higher than that of a H/OH-co-terminated (001) surface with negative electron affinity.

Efficiency of multiple atom doping in wide band gap semiconductors

We present a thermodynamic model calculation for the efficiency of an impurity-atom-complex formation suggested for doping of wide band gap semiconductors, where the complexes with various sizes coexist at local equilibrium as suggested by Van de Walle and Neugebauer [J. Appl. Phys. 95, 3851 (2004)]. When the size of a target complex becomes large, the effect of configurational entropy of smaller complexes shows up in the energetics and may even hamper stabilization of the target. Thus one should design a target complex with the size being as small as possible and the binding energy as large as possible, in order to make the concept of multiple atom doping a real technology.

Structure determination of W-capsulated Si cage clusters by x-ray absorption fine structure spectra

Local structure of transition metal (TM)-encapsulated Si clusters TMSin is investigated by x-ray absorption fine structure. For a series of W-encapsulated Si cluster (WSin) films, the extended x-ray absorption fine structure (EXAFS) results indicate that the W atoms are coordinated by 9–10 Si atoms with a large structural disorder. The x-ray absorption near-edge structure (XANES) spectra exhibit intense white lines for the cluster films, which distinguishes the different local structures from that of amorphous W–Si alloy. Multiple-scattering XANES calculations indicate that the intense white lines are reproduced by the WSin cage clusters with a W atom in the Sin cage centre. Combining the XANES and EXAFS results, it is revealed that the WSin cage structure (n = 8 or 12) is formed, characterized by strengthened covalency consistent with considerable bond contraction.