Şenay Katircioğlu

THE ELECTRONIC BAND STRUCTURE OF AlN, AlSb, AlAs AND THEIR TERNARY ALLOYS WITH In

The electronic band structure of AlN, AlSb, AlAs and their ternary alloys with In has been investigated by ETB. The ETB method has been formulated for sp3d2 basis and nearest neighbor interactions of the compounds and its energy parameters have been derived from the results of the present first principles calculations carried on AlN, AlSb and AlAs. It has been found that the present ETB energy parameters can produce the band structure of the compounds and their ternary alloys with In successfully.

DECOMPOSITION OF SiH4 ON THE SA TYPE STEPPED Si(100) SURFACE

The density functional theory method is used to explore the mechanism of dissociative adsorption of silane (SiH4) on the SA type stepped Si(100) surface. Two reaction paths are described that produce silyl (SiH3) and hydrogen atom fragments adsorbed on the dimer bonds present on each terrace. It has been found that the initial stage of the dissociation of SiH4 on the SA type stepped Si(100) surface shows similarity to the dissociation of SiH4 on the flat Si(100) surface; SiH3 and hydrogen fragments bond to the Si dimer atoms by following the first reaction path.

FIRST-PRINCIPLES CALCULATIONS FOR THE STRUCTURAL AND ELECTRONIC PROPERTIES OF ScxAl1-xN ALLOYS

The first-principles calculations based on Density Functional Theory (DFT) within generalized gradient approximation (GGA) of Engel–Vosko–Perdew–Wang and modified exact exchange potential of Becke–Johnson have been introduced for the structural and electronic properties of the ScxAl1-xN alloys, respectively. The present lattice constants calculated for the ScAlN alloys and the end compounds (AlN and ScN) are found to be in very good agreement with the available experimental and theoretical ones. The stable ground state structures of the ScxAl1-xN alloys are determined to be wurtzite for the Sc concentration less than ~0.403 and rock-salt for the higher Sc concentrations. The present electronic band structure calculations within Becke–Johnson scheme are found to be capable of providing energy band gaps of the AlN and ScN compounds very close to the ones of the available experiments and expensive calculations. According to the calculations of Becke–Johnson potential, the ScxAl1-xN alloys in the wurtzite and zinc-blende structures are direct band gap materials for the Sc concentrations in the ranges of (0.056 ≤ x ≤ 0.833) and (0.03125 ≤ x ≤ 0.0625, 0.375 ≤ x ≤ 0.96875), respectively. However, the ScAlN alloys in the rock-salt phase are determined to be direct band gap materials for total range of the Sc concentration considered in this work. While the energy gaps of the RS-AlScN alloys are found to be extending from near ultraviolet to near infrared with a large (negative) bowing, the ones of the WZ-AlScN and ZB-AlScN alloys are determined to be varying in a small energy range around near ultraviolet with a small (negative) bowing.

THE ELECTRONIC BAND STRUCTURE OF GaN, GaAs AND InxGa1-xAs1-yNy ALLOYS

The electronic band structure of GaN and GaAs has been investigated by ETB to obtain the band gap bowing of InxGa1-xAs1-yNy alloys lattice matched to GaAs. The ETB method has been formulated for sp3d2 basis and nearest neighbor interactions of the compounds, and its energy parameters have been derived from the results of the present first principles calculations carried out on GaN and GaAs. It has been found that the present ETB energy parameters are capable of producing the electronic band structure of corresponding compounds and the large bowing parameter of InGaAsN alloy.

ADSORPTION AND DISSOCIATION OF PH3 ON SiGe(100) (2 × 1) SURFACE

The most stable structures for the adsorption and dissociation of phosphine (PH3) on SiGe(100) (2 × 1) surface have been investigated by relative total energy calculations based on density functional theory. According to the optimization calculations, PH3 is adsorbed on the Si (down) and Ge (down) site of the Ge–Si and Ge–Ge dimers on SiGe surface, respectively. The PH2 and H products have been found to be thermodynamically favored in the dissociation path of PH3 on SiGe surface when the system is thermally activated. Although PH3 is adsorbed on the Ge–Ge and Ge–Si dimers directly, it dissociates on the SiGe surface by passing through a transition state. The asymmetric Ge–Si and Ge–Ge dimers on SiGe surface are found to be approximately symmetric after the dissociation of PH3 on the surface. The present work has showed that PH2 prefers to be adsorbed on Ge site of the Ge–Si dimer. Therefore, the adsorption of PH2 on Ge site of the Ge–Si dimer, while PH3 being dissociated on the Si site, has indicated the migration of PH2 on SiGe surface.

ADSORPTION SITES OF THE PRODUCTS OF GeH4 ON ASYMMETRIC Si(100)(2×1) SURFACE

The decomposition of GeH4 on Si(100)(2×1) was investigated on different adsorption models of fragments using density functional theory method. The most probable adsorption model of fragments corresponding to the growth steps of SiGe film has been obtained by geometry optimization and single value total energy calculations. The relative adsorption energies of GeH3, GeH2 and GeH have been found to be -5.6, -5.1, and -4.5 eV for their most probable adsorption models respectively. It has been found that, the asymmetric dimer bond rows of Ge on Si(100) surface can be constructed by following the adsorption models corresponding to the relative adsorption energies of GeH3, GeH2 and GeH.

DECOMPOSITION OF CH4 ON SA TYPE STEPPED Si(100) SURFACE

Density functional theory method is used to explore the mechanism of dissociative adsorption of methane (CH4) on SA type stepped Si(100) surface. Two reaction paths are described that produce CH3 and hydrogen atom fragments adsorbed on the dimer bonds present on each terraces. It has been found that, in the initial stage of the carbonization of stepped Si(100) surface, the CH3 and H fragments bound to the Si dimer atoms by following the first reaction path.

ADSORPTION OF HYDROGEN AND OXYGEN ON SINGLE AND DOUBLE LAYER STEPPED SI(100) SURFACES

We have investigated the electronic band structure of hydrogen and oxygen adsorbed single and double layer stepped Si(100) surfaces by Empirical Tight Binding (ETB) method. The total electronic energies of the H,O-SA, DA, DB type stepped Si(100) systems are calculated with limited number of hydrogen and oxygen atoms separately to find out the most probable adsorption sites of the adatoms in the initial stage of hydrogenation and oxidation.