Sanghamitra Mukhopadhyay

Interface roughness scattering-limited electron mobility in AlAs/GaAs and Ga0.5In0.5P/GaAs wells

Interface roughness scattering-limited electron mobility is calculated for AlAs/GaAs and Ga0.5In0.5P/GaAs wells taking into account the effects of finite barrier potential, extension of the wave function into the barrier layer, energy band nonparabolicity, and screening of the scattering potential. The mobility in AlAs/GaAs wells varies with the well width L as Ln(n=4.7–4.8), but the variation cannot be fitted to a similar relation for Ga0.5In0.5P/GaAs wells. Experimental results for both the systems may be explained with an asperity height between 2.83 and 5.67 A, and a correlation length smaller than 170 A. It is concluded that the larger experimental mobility in Ga0.5In0.5P/GaAs wells is due to the smaller barrier potential in this system.

Electronic structure and magnetic properties of Cr/Sn multilayers

Electronic structure and magnetic properties of Cr/Sn metallic multilayers have been investigated using spin-polarized first principles tight-binding linear muffin-tin orbital (TB-LMTO) method within atomic sphere approximation (ASA). Calculations for an Fe/Cr multilayer have also been done for comparison. The experimentally observed reduced magnetic moment of thin Cr layer sandwiched between Sn or Fe layers has been explained from our band structure calculations. For Cr/Sn multilayer, self-consistent calculations have been done to determine the lattice relaxation for growth of multiple layers of Cr over one monolayer of Sn.

In‐plane effective mass in narrow quantum wells of nonparabolic semiconductors

A formula is derived for the in‐plane effective mass in narrow quantum wells, taking into account the effects of energy band nonparabolicity. The variation of the mass with the width of the well is studied by using the formula for four systems of wells. The mass is nearly the same as the velocity effective mass of the bulk material of the well in GaAs/Ga0.7Al0.3As wells. It is about 8% larger in InAs/InP wells, but is significantly larger in very narrow wells of Ga0.47In0.53As/InP systems. In the case of InAs/Ga0.58Al0.42Sb wells, the in‐plane mass differs from the well mass by large amounts for all well widths of interest.

BAND OFFSET IN INP/GA0.47IN0.53AS HETEROSTRUCTURES

Energy levels in InP/Ga0.47In0.53As quantum wells are calculated after reformulating the energy‐dependent effective mass to be used for taking into account the energy‐band nonparabolicity of both constituents. The required value of the ratio of the conduction‐band and valence‐band discontinuities is found to be close to 2/3, in agreement with the value found by other methods. The value of the nonparabolicity factor is also found to be the same as that used in earlier transport studies.

Spectroscopic features of dimer and dangling bond E ′ centres in amorphous silica

We performed first-principle embedded cluster calculations of the hyperfine parameters, g-tensors and optical excitation energies for the dimer and back-projected configurations of the centre in amorphous silica. The optical transition energies of these defects are calculated for the first time. We predict a strong optical transition at about 6.3 eV for the dimer configuration and a relatively weak transition at 5.6 eV for the back-projected configuration of the centre. These predictions could be used for further experimental identification of these centres. Our results support the dimer model of the centre, and for the first time provide a full range of spectroscopic parameters for the back-projected configuration of the centre in amorphous silica.

Grazing incidence x-ray scattering study of the structure of epitaxial Cr/Sn multilayers

The structure of epitaxial Cr/Sn multilayers has been studied experimentally using x-ray reflectivity and x-ray diffuse scattering measurements, as well as theoretically using linear muffin-tin orbital (LMTO) calculations. Measurements show a distinct variation in the structure of the multilayers as the Sn layer thickness increases from 0.4 to 0.6 nm. A decrease in the electron density of the Sn layer and an increase in the jaggedness of the interfaces accompany a partial transformation of the Sn layer from an epitaxial bcc structure to a β-Sn structure, as observed using in situ reflection high energy electron diffraction measurements [K. Mibu, S. Tanaka, and T. Shinjo, J. Phys. Soc. Jpn. 67, 2633 (1998)]. Present measurements along with the LMTO calculations support a structure for the multilayer in which Sn layers grow epitaxially with Cr in a bcc structure with a finite density of steps at the interfaces which causes the average electron density of a Sn layer to decrease. Diffuse scattering measuremen...

Cyclotron-resonance effective mass in narrow quantum wells

Abstract Values of the cyclotron-resonance mass are computed for Ga 0.47 In 0.53 As InP quantum wells as a function of the well width. The values are found to increase with decreasing well width partly due to the energy band nonparabolicity of the well material and partly due to the extension of the wave function into the barrier layer. On the basis of the computed results, it is suggested that cyclotron-resonance experiments with narrow quantum wells may be done to determine the electron mass in the forbidden gap and also to identify the energy-band minimum associated with the barrier potential.

Alloy scattering‐limited mobility in narrow quantum wells

Alloy scattering‐limited mobility is calculated for narrow quantum wells of GaAs/Ga0.7Al0.3As, Ga0.47In0.5As/InP, and Al0.48In0.5As/Ga0.47In0.53As systems with widths down to 0.5 nm, taking into account the energy‐band nonparabolicity and the effects of wave‐function penetration into the barrier layer. Values of mobility are found to be significantly different from those given by the formulas derived earlier for wide wells.

Deformation potential acoustic phonon scattering limited mobility in narrow quantum wells

The form factor and the effective mass are shown to be significantly altered in narrow quantum wells owing to the extension of the wavefunction into the barrier layers. The mobility is found to change as a result by a large factor (a factor of 12 for a well width of 0.5 nm). Modifications caused by the band non-parabolicity are also discussed.

Induced magnetic moments of X (X=Sn, Au, Cu, Mo, Co) in thin Fe/Cr/X/Cr multilayers

The magnetic moment induced on a monolayer of X (X=Sn, Au, Cu, Mo, and Co) embedded in thin layers of Cr (designated as Fe/Cr/X/Cr) is investigated by first principle spin-polarized electronic structure calculations. We find that in the multilayers the X (X=Sn, Au, Mo) atoms get a small magnetic moment which is predominantly 5d (Sn,Au), or 4d (Mo) in character. This is induced by exchange interaction with 3d Cr orbitals at the Cr/X interfaces. Co and Cu atoms are weakly influenced by interfacial Cr. Fe layers reduce the magnetic moments of Cr atoms at the Cr/Sn interface, and so the induced magnetic moment on a Sn atom decreases from 0.067μB/atom in Cr/Sn to 0.044μB/atom in Fe/Cr/Sn/Cr. These results are in correlation with Mossbauer observations. The magnetic moment of a Cr atom depends strongly on the nature of the exchange interaction with its neighbours. Also, magnetic moments on Cr sites decrease smoothly away from the Fe layer for Sn, Au, Cu, and Mo, but not for Co. Then the magnetic moment increases abruptly at the Cr/X interface for X=Sn, Au, Cu, and Co, but not for Mo. The decrease in Cr magnetic moment is a result of dissimilar exchange interactions at Fe/Cr and Cr/X interfaces while its large increase at the Cr/X interface is a result of ferromagnetic coupling with X atomic layer and interface states arise due to dissimilar electronic environment around the Cr atomic layer at the interfaces.