A.K. Sharma

Bio: A.K. Sharma is an academic researcher from Sikkim Manipal University. The author has contributed to research in topics: Superlattice & Seebeck coefficient. The author has an hindex of 2, co-authored 2 publications receiving 29 citations.

Simple theoretical analysis of the thermoelectric power in quantum dot superlattices of non-parabolic heavily doped semiconductors with graded interfaces under strong magnetic field

Abstract: We study theoretically the thermoelectric power in the presence of a large magnetic field (TPM) in heavily doped III–V, II–VI, PbTe/PbSnTe, strained layer and HgTe/CdTe quantum dot superlattices (QDSLs) with graded structures on the basis of newly formulated electron energy spectra and compare the same with that of the constituent materials. It has been found, taking heavily doped GaAs/Ga1−xAlxAs, CdS/CdTe, PbTe/PbSnTe, InAs/GaSb and HgTe/CdTe QDSLs as examples, that the TPM increases with increasing inverse electron concentration and film thickness, respectively, in different oscillatory manners and the nature of oscillations is totally band structure dependent. We have also suggested the experimental methods of determining the Einstein relation for the diffusivity–mobility ratio, the Debye screening length and the electronic contribution to the elastic constants for materials having arbitrary dispersion laws.

The Einstein relation for the diffusivity–mobility ratio in nonlinear optical, optoelectronic and the related materials

Abstract: In this paper an attempt is made to study the Einstein relation for the diffusivity-mobility ratio (DMR) in nonlinear optical compounds on the basis of a newly formulated electron energy spectrum taking into account the combined influences of the anisotropies in the effective electron mass and the spin orbit splitting constant together with the inclusion of crystal field splitting in the Hamiltonian within the framework of k.p formalism. The corresponding results for III–V, ternary and quaternary types of optoelectronic materials form a special case of our generalized analysis. We have also studied the DMR in II–VI, Bi, IV–VI and stressed materials on the basis of various band models as applicable for such specialized materials. It has been found taking n-Cd3As2, n-CdGeAs2, n-InAs, n-InSb, n-Hg1−xCdxTe and n-In1−xGaxAsyP1−y lattice matched to InP, CdS, Bi, PbS, PbTe, PbSe and stressed InSb as examples of the aforementioned compounds that the DMR increases with increasing electron concentration in various manners and the rate of increase is greatly influenced by the presence of the different energy band constants of the said materials together with the fact that the rates of variation are totally band structure dependent. An experimental method of determining the DMR in degenerate samples having arbitrary dispersion laws has been suggested and the present simplified analysis is in agreement with the suggested relationship. In addition, the well-known results for nondegenerate wide gap materials have been obtained as special cases of our generalized theory under certain limiting conditions.

Interpretation of electron diffusion coefficient in organic and inorganic semiconductors with broad distributions of states

Abstract: The carrier transport properties in nanocrystalline semiconductors and organic materials play a key role for modern organic/inorganic devices such as dye-sensitized (DSC) and organic solar cells, organic and hybrid light-emitting diodes (OLEDs), organic field-effect transistors, and electrochemical sensors and displays. Carrier transport in these materials usually occurs by transitions in a broad distribution of localized states. As a result the transport is dominated by thermal activation to a band of extended states (multiple trapping), or if these do not exist, by hopping via localized states. We provide a general view of the physical interpretation of the variations of carrier transport coefficients (diffusion coefficient and mobility) with respect to the carrier concentration, or Fermi level, examining in detail models for carrier transport in nanocrystalline semiconductors and organic materials with the following distributions: single and two-level systems, exponential and Gaussian density of states. We treat both the multiple trapping models and the hopping model in the transport energy approximation. The analysis is simplified by thermodynamic properties: the chemical capacitance, Cμ, and the thermodynamic factor, χn, that allow us to derive many properties of the chemical diffusion coefficient, Dn, used in Fick’s law. The formulation of the generalized Einstein relation for the mobility to diffusion ratio shows that the carrier mobility is proportional to the jump diffusion coefficient, DJ, that is derived from single particle random walk. Characteristic experimental data for nanocrystalline TiO2 in DSC and electrochemically doped conducting polymers are discussed in the light of these models.

A simple theoretical analysis of the effective electron mass in III-V, ternary and quaternary materials in the presence of light waves

Abstract: We present a simple theoretical analysis of the effective electron mass (EEM) at the Fermi level for III–V, ternary and quaternary materials, on the basis of a newly formulated electron energy spectra in the presence of light waves whose unperturbed energy band structures are defined by the three-band model of Kane The solution of the Boltzmann transport equation on the basis of this newly formulated electron dispersion law will introduce new physical ideas and experimental findings under different external conditions It has been observed that the unperturbed isotropic energy spectrum in the presence of light changes into an anisotropic dispersion relation with the energy-dependent mass anisotropy In the presence of light, the conduction band moves vertically upward and the band gap increases with the intensity and colours of light It has been found, taking n-InAs, n-InSb, n-Hg1−xCdxTe and n-In1−xGaxAsyP1−y lattice matched to InP as examples, that the EEM increases with increasing electron concentration, intensity and wavelength in various manners The strong dependence of the effective momentum mass (EMM) at the Fermi level on both the light intensity and wavelength reflects the direct signature of the light waves which is in contrast with the corresponding bulk specimens of the said materials in the absence of photo-excitation The rate of change is totally band-structure-dependent and is influenced by the presence of the different energy band constants The well known result for the EEM at the Fermi level for degenerate wide gap materials in the absence of light waves has been obtained as a special case of the present analysis under certain limiting conditions, and this compatibility is the indirect test of our generalized formalism

Electronic structure of (BP)n/(BAs)n (0 0 1) superlattices

Abstract: An accurate ab initio full potential linear muffin-tin orbital method has been used to investigate the structural, electronic and optical properties of BP, BAs and their (BP) n /(BAs) n superlattices (SLs). The exchange-correlation potential is treated with the local density approximation of Perdew and Wang (LDA-PW). The calculated structural properties of BP and BAs compounds are in good agreement with available experimental and theoretical data. It is found that BP, BAs and their alloys exhibit an indirect fundamental band gap. The fundamental band gap decreases with increasing the number of monolayer n . The optical properties show that the static dielectric constant significantly decreases in superlattices compared to their binary compounds.

A thermodynamic model for heat transport and thermal wave propagation in graded systems

Abstract: We study the effects of a composition gradient and of a non-vanishing heat flux on the phase velocity of thermal waves along a graded system. We take into account non-local and non-linear effects by applying a generalized heat transport equation. We compare the results for high-frequency and low-frequency waves. For low frequency, we discuss the conditions in which thermal waves may propagate in Si x Ge 1 − x and ( Bi 1 − x Sb x ) 2 Te 3 systems. For high frequency, we discuss the influence of the relaxation of the flux of the heat flux on the heat wave propagation.