Nozomi Orita

Codoping method for the fabrication of low-resistivity wide band-gap semiconductors in p-type GaN, p-type AlN and n-type diamond: prediction versus experiment

We review our new valence control method of a co-doping for the fabrication of low-resistivity p-type GaN, p-type AlN and n-type diamond. The co-doping method is proposed based upon ab initio electronic structure calculation in order to solve the uni-polarity and the compensation problems in the wide band-gap semiconductors. In the co-doping method, we dope both the acceptors and donors at the same time by forming the meta-stable acceptor-donor-acceptor complexes for the p-type or donor-acceptor-donor complexes for the n-type under thermal non-equilibrium crystal growth conditions. We propose the following co-doping method to fabricate the low-resistivity wide band-gap semiconductors; p-type GaN: [Si + 2 Mg (or Be)], [H + 2 Mg (or Be)], [O + 2 Mg (or Be)], p-type AlN: [O + 2 C] and n-type diamond: [B + 2 N], [H + S], [H + 2 P]. We compare our prediction of the co-doping method with the recent successful experiments to fabricate the low-resistivity p-type GaN, p-type AlN and n-type diamond. We show that the co-doping method is the efficient and universal doping method by which to avoid carrier compensation with an increase of the solubility of the dopant, to increase the activation rate by decreasing the ionization energy of acceptors and donors, and to increase the mobility of the carrier.

Ab initio study of donor-hydrogen complexes for low-resistivity n-type diamond semiconductor

We performed local density approximation (LDA)-based ab initio calculations for isolated substitutional donor impurities (phosphorus single donor and sulfur double donor), hydrogen at various sites, and hydrogen-related complexes (phosphorus-hydrogen and sulfur-hydrogen) in diamond. Their stable atomic configurations and electronic structures were determined. The two donor impurities exhibit Jahn–Teller distortions, reducing their symmetries from Td to C3v. We found that the bond center site is the most stable site for hydrogen (and muonium) in diamond, and the tetrahedral interstitial site, the hexagonal interstitial site, and the antibonding site are metastable sites. This result is consistent with those of muon-spin-rotation (µSR) experiments. We also found that hydrogen passivates phosphorus donor. We propose a bonding model for the sulfur–hydrogen complex which produces a shallower single-donor level (1.07 eV below the bottom of the conduction band) than the isolated sulfur double-donor (1.63 eV). Frequencies of infrared active hydrogen vibration which will be observed by infrared absorption measurements and the cohesive energy of each complex was predicted. Finally, we propose a new doping method for the fabrication of low-resistivity n-type diamond.

The band structure of solid iodine under pressure and the mechanism of the pressure-induced insulator-to-metal transition

The band structures of solid iodine under ambient pressure and 15.3 GPa were calculated with the ab initio pseudopotential method. The apparently complicated band structures have been resolved by a detailed analysis, revealing the mechanism of the band overlap which causes the pressure-induced insulator-to-metal transition: The interlayer interaction as well as the interaction between the third-nearest neighbor atoms is responsible for the band overlap. The effect of the spin-orbit interaction on the band overlap is also discussed

Generalized Gradient Approximation

I investigated the metallization mechanism of niobium(Nb)-doped anatase titanium dioxide (TiO2). There are recent calculations for Nb-doped TiO2 using the local density approximation (LDA) or generalized gradient approximation (GGA) method. I performed GGA+U calculations to solve the problem. The obtained band gap and the defect state are expected to be improved by the GGA+U method. I calculated the electronic structures of undoped, one-Nb-doped, and two-Nb-doped TiO2 per supercell. It was found that metallization is caused not by Mott transition with an overlap between the impurity and conduction bands but by the direct doping of an excess electron of Nb to the hybridized bands consisting mainly of Ti and Nb d-orbitals.

+U

Abstract We performed local density approximation based ab initio calculations for isolated substitutional impurity (nitrogen, phosphorus, and sulfur), hydrogen at various sites, and hydrogen-related complexes (phosphorus–hydrogen and sulfur–hydrogen) in diamond. Atomic and electronic structures of these point defects are determined. Vibrational frequencies and cohesive energy of the hydrogen-related complexes are estimated. From the knowledge that we obtained with our ab initio calculations, we propose the co-doping method that enable the fabrication of low-resistive n-type diamond.

Study for Metallization Mechanism of Niobium-Doped Anatase Titanium Dioxide

Abstract The band structures of high pressure phases of iodine are calculated on the basis of the local-density-functional formalism and a norm-conserving pseudopotential. The density of states (DOS) of the body-centered-tetragonal phase (BCT phase) at 49GPa is compared with that of a hypothetical face- centered-cubic phase (FCC phase) with the same atomic volume. It is found that the Fermi-level of the BCT phase is located off a peak of the DOS but that of the FCC phase is on a plateau, which originates from a saddle point singularity due to degenerate p-bands. This is in agreement with the experiment that the BCT structure is a stable phase at this atomic volume over the FCC structure. We argue from the band structures the reason why the FCC structure becomes a stable phase over the BCT structure at high pressures. We have calculated also the total energy of the FCC phase as a function of the atomic volume. The lattice constant at 64GPa is reproduced within 1% by the calculation.

Theoretical study of hydrogen-related complexes in diamond for low-resistive n-type diamond semiconductor

I investigated the mechanism by which the resistivity of niobium (Nb)-doped anatase titanium dioxide (TiO2) grown in an oxygen-reduced atmosphere decreases. For this purpose, I performed the generalized gradient approximation (GGA)+U calculation for the several oxygen-related defects in a Nb-doped TiO2 (TNO) supercell: an interstitial oxygen (Oint) atom in TNO, an Oint atom and an oxygen vacancy (VO) in TNO, and a VO in TNO. The obtained results showed that the Oint atoms trap the doped electrons and that the electrons are restored to the conduction bands by removing the Oint atoms. Therefore, removing the Oint atoms in oxygen-reduced atmosphere causes the lower resistivity. The defect formation enthalpies also indicated that the structure without the Oint atoms is stable in oxygen-reduced atmosphere.

Band structures and cohesive properties of high pressure phases of solid iodine

Previously, we performed local density approximation (LDA) based ab initio calculations for phosphorus-doped diamond and identified that the site symmetry of phosphorus is C3v [Jpn. J. Appl. Phys. 41 (2002) 1952]. Recently, however, Isoya et al. reported that the site symmetry is D2d, as found in their electron paramagnetic resonance (EPR) experiment [Physica B 376–377 (2006) 358]. We carried out recalculations to explain their result. In our new calculations, we took account of the shape optimization of the supercell and obtained the D2d symmetry at the phosphorus site in diamond.

Mechanism for Lower Resistivity of Niobium-Doped Anatase Titanium Dioxide Obtained in Oxygen-Reduced Atmosphere: Investigation by Generalized Gradient Approximation +U Method

Electronic structure and dynamics of defects in hydrogenated amorphous silicon (a-Si:H) are investigated based upon ab–initio molecular–dynamics simulations. It is shown that (i) the hydrogen–passivated dangling bond (Si-H), (ii) the positively-ionized three–centered bond (Si– H + –Si), (iii) the negatively–ionized three–coordinated dangling bond (D − ) and (iv) the five- coordinated floating bond (F 5 ) are the intrinsic defects in a–Si:H. Based upon the calculated result, we discuss the role of hydrogen and the origin of the photo–induced defect in a-Si:H.

Ab initio study for site symmetry of phosphorus-doped diamond

For two group-5B elements, arsenic and antimony, the total energies of the simple cubic (sc) and body centered cubic (bcc) structures are calculated within the local density functional formalism using ab initio pseudopotentials. Calculated equations of state for the high pressure phase, the sc arsenic and the bcc antimony, are in excellent agreement with the experimental ones. In order to see the difference between the phase transitions for both elements, the total energies are also calculated for hypothetical pseudopotentials, which are so constructed as to have an s (or p ) level between arsenics and antimonys ones. It is shown that the difference is mainly due to the p level.