Yoji Imai

Electronic structures of semiconducting alkaline-earth metal silicides

Electronic structures and densities of states of the following alkaline-earth metal silicides have been calculated using the first-principle pseudopotential method, Mg/sub 2/Si, BaSi/sub 2/, Ca/sub 2/Si and Sr/sub 2/Si. Energetics of these in their equilibrium structures in comparison with other hypothetical structures (i.e. Ca/sub 2/Si-type Mg/sub 2/Si and Mg/sub 2/Si-type Ca/sub 2/Si) are also considered to clarify the structural change of alkaline-earth metal silicides with the promotion to the heavier elements. The semiconducting behaviors of these could be predicted though the energy band gaps calculated were about 40% of the actual measured values. Energetics of the non-stoichiometry and the atomic site of dopants for Mg/sub 2/Si are also discussed.

Energetics of alkaline-earth metal silicides calculated using a first-principle pseudopotential method

Abstract The variations of total energies of the following alkaline-earth metal silicides with the cell volumes have been calculated; Mg2Si, Ca2Si, CaSi, CaSi2, Sr2Si, SrSi, SrSi2, BaSi, and BaSi2. The structural change of the alkaline-earth metal disilicides with the promotion to the heavier elements could be relatively well explained though the predicted lattice constants of the equilibrium phases were a bit smaller and the estimated energies of formation of silicides were about 40–60% of the actual measured values. The semiconducting behavior of Mg2Si, Ca2Si, Sr2Si, and BaSi2 could be predicted.

Comparison of electronic structures of doped ZnO by various impurity elements calculated by a first-principle pseudopotential method

Electronic band calculations of ZnO doped with Li, Al, Ga, In, Si, Ge, Y, Ti, V, Cr, Mn, Fe, Co, Ni, and Cu have been done within a framework of local density approximation, and partial densities of states (PDOSs) of dopants have been evaluated. PDOS of Al(Ga, In) of ZnO : Al(Ga, In) has delocalized nature, which is considered to be responsible for good conductivity of Al-doped ZnO. PDOS values of Cu and Li of ZnO : Cu and ZnO : Li are quite small at their Fermi levels (EFs), and this may be one of the possible reasons for their insulating nature as well as the compensation effects. PDOS values of Si and Ge of ZnO : Si and ZnO : Ge at their EFs are relatively large but are less delocalized compared to that of ZnO : Al(Ga, In). As for ZnO : Y, ZnO : Ti, and ZnO : V, PDOS values of Y(Ti, V) are small at their EFs. ZnO : Cr and ZnO : Mn have isolated impurity levels between the energy gap of ZnO and their EFs are located on these levels, seemingly resulting in poor conductivities. EFs of ZnO : Fe(Co, Ni) are located near the top of the valence band, and PDOS values of Fe(Co, Ni) at their EFs are relatively large. Their semiconducting nature might be caused by strong correlation between electrons which is beyond the description of a band calculation.

Comparison of electronic structures of doped ZnS and ZnO calculated by a first-principle pseudopotential method

Electronic band calculations of doped and undoped ZnO and ZnS have been done using density functional theory under the local density approximation so as to clarify the reason of the difference in behaviors of doped ZnO and ZnS. The reason why the electrical conductivity of ZnS is difficult to be increased by doping was discussed. In the case of doped ZnS, an impurity level was generated at deep position below the bottom of the conduction band of the host ZnS lattice and Fermi level was located at this impurity level. On the contrary, the shape of the density of states curve and the band structures of doped and undoped ZnO are alike with each other and the donor band is hybridized with the conduction band of the ZnO host material. This seems to result in contribution of doped electrons to electrical current in the case of doped ZnO.

Comparison of density of states of transition metal disilicides and their related compounds systematically calculated by a first-principle pseudopotential method using plane-wave basis

Density of states of transition metal disilicides and their related compounds of the following types of structures have been calculated by a first-principle pseudopotential method using plane-wave basis, CASTEP; CaF2, α- and β-FeSi2, TiSi2, ZrSi2, CrSi2, MoSi2, Mn4Si7, Ru2Si3, and AlB2. The principle of “DOS at Fermi level would be hopefully smaller in energetically-favored structure” is valid in most of silicides except for IIIa elements. The broadening of the energy difference between the bonding and anti-bonding states by promotion to the heavier element in the same group in periodic table is dependent on the crystal structure and this may cause the transition of the structure in the same group. Semiconducting property of ReSi2 is probably due to the vacancies located at silicon sites because DOS at Fermi level has relatively large value for stoichiometric ReSi2. Also, it is pointed out that the formula of Mn11Si19 is not consistent with the intrinsic semiconducting nature.

DIRECT CONVERSION OF TITANIUM ALKOXIDE INTO CRYSTALLIZED TIO2 (RUTILE) USING COATING PHOTOLYSIS PROCESS WITH ARF EXCIMER LASER

TiO2 (rutile) films have been successfully prepared by the coating photolysis process using ArF excimer laser irradiation at room temperature. The choice of the Ti-alkoxide type as the starting material and the effects of atmosphere on the product films have been studied by Fourier transformation infrared (FT-IR) and X-ray diffraction (XRD). When using Ti(O-i-C3H7)4 as the starting material, rutile films were obtained by the irradiation in a dry N2 atmosphere, whereas amorphous films were obtained by irradiation in air. When using Ti(OCH2CH(C2H5)-n-C4H9)4 as a starting material, rutile films were obtained by the irradiation even in the air atmosphere. Crystallization mechanism is also discussed.

Stoichiometry of tantalum oxide films prepared by KrF excimer laser-induced chemical vapor deposition

Abstract Tantalum oxide films have been prepared by photolysis of Ta(OCH 3 ) 5 vapor under KrF excimer laser irradiation at various laser fluences (50–450 J m −2 ). The composition and Stoichiometry of the films were determined by X-ray photoelectron spectroscopy (XPS). The variation of the shape of the Ta 4f XPS peak for surface erosion of samples by argon-ion etching indicated that films were deficient in oxygen atoms compared with the composition of the stoichiometric Ta 2 O 5 , though the X-ray diffraction pattern of films obtained at higher laser fluences (more than 300 J m −2 ) corresponded to that of the β-Ta 2 O 5 phase. The deficiency in oxygen increased with increasing laser fluence to reach the maximum value of about 20% in the laser fluence range 150 J m −2 to 250 J m −2 and then decreased. The carbon content in the films was less than the background contamination level (a few %) under the experimental conditions investigated. Various possible reasons for the laser fluence effect on the stoichiometry of films are discussed.

Screening of the possible boron-based n-type thermoelectric conversion materials on the basis of the calculated densities of states of metal borides and doped β-boron

Abstract In order to screen candidates for n-type boride-based thermoelectric materials, the energetics of solid solutions of metallic atoms (Zr, Cr, and V) in β-rhombohedral boron (β-boron) and the densities of states of metal-borides of the CrB-, FeB-, MoB-, AlB2-, ReB2-, CaB6-, and UB12- types of structures and some tetraborides (YB4, CrB4, WB4, and MgB4) have been calculated using a first-principle pseudopotential method within the local (spin) density approximation. It was properly concluded that Zr occupies the E site of β-boron while V and Cr occupy the A1 site. As for metal-monoborides and diborides, the ‘rigid band approach’ seems to be valid. The large negative Seebeck coefficient of FexCo1−xB is hopeful if Motts explanation for the trend of the Seebeck coefficient for transition metal elements is valid for these borides. Y- or La- doped Ca(Sr, and Ba)B6 can also be expected to be useful.

Direct Conversion of Metal Acetylacetonates and Metal Organic Acid Salts into Metal Oxides Thin Films Using Coating Photolysis Process with An ArF Excimer Laser

Metal oxide films of TiO2, In2O3, ZrO2, ZnO, SnO2 and Fe2O3 have been successfully prepared by the coating photolysis process using ArF excimer laser irradiation at room temperature. When using metal acetylacetonates (acac) as the starting material, TiO2, In2O3 and ZrO2 were obtained with ArF laser irradiation at 50 mJ/cm2 at a repetition rate of 5 Hz for 5 min. When using Zn-acac, Fe, Sn, or In 2-ethylhexanoate as the starting material, a two-step process consisting of preliminary weak and sufficiently strong irradiation was found to be effective for obtaining ZnO, Fe2O3, SnO2 and In2O3 films.

Thermoelectric properties and defect structure of La0.45Nd0.45Sr0.1FeO3−δ

Abstract A concept for the design of thermoelectric oxides, which is the method to decrease the thermal conductivity without decreasing the electrical conductivity using the complex oxide system, is proposed. The electrical transport properties and the defect structure of La 0.45 Nd 0.45 Sr 0.1 FeO 3− δ , which corresponds to Sr-doped LaFeO 3 –NdFeO 3 solid solution, were investigated to confirm the concept. La 0.45 Nd 0.45 Sr 0.1 FeO 3− δ was found to show a p-type behavior between 1073 and 1273 K at an oxygen partial pressure of 10 −4 to 1 atm. The values of hole mobility in La 0.45 Nd 0.45 Sr 0.1 FeO 3− δ were found to be in good agreement with those of La 0.9 Sr 0.1 FeO 3− δ . The heat of transport of the holes was also discussed using the small-polaron model.