D Bhattacharyya

Bandgap and optical transitions in thin films from reflectance measurements

Abstract A new formulation and method are presented for evaluating bandgap, optical transitions and optical constants from the reflectance data for films deposited onto a non-absorbing substrate.

Some aspects of surface roughness in polycrystalline thin films : optical constants and grain distribution

A modified formulation and new method have been suggested to derive meaningful information on the optical constants, grain size and grain distribution of polycrystalline thin films where surface roughness plays an important part through the contributions arising out of scattered transmission and diffuse reflection. It was observed that this method successfully described the experimental observations in polycrystalline CuInSe2 and CdSe films.

Effect of substrate on the structural and optical properties of chemical-bath-deposited CdS films

Abstract CdS films were grown on glass and ITO/glass substrates by the electroless chemical-bath deposition technique. Structural and optical studies were carried out by X-ray diffraction (XRD), ellipsometry and absorption spectroscopy of the samples. Critical investigation of the characteristic X-ray peak for hexagonal CdS reveals that, under similar deposition and annealing conditions, the peak always occurs at a slightly higher angle for films grown on glass substrates than for those deposited on ITO/glass substrates. The shift in the peak is attributed to the additional stresses present in thin CdS films deposited onto ITO/glass substrates due to the large lattice mismatch between CdS and ITO. The stress decreases on annealing due to the possible appearance of compressive stresses in the films arising from the difference between the thermal expansion coefficients of CdS and the substrate materials. Ellipsometric studies also show that the refractive indices of the films deposited onto ITO/glass substrates differ from those of the films deposited onto glass substrates. This difference decreases with increasing annealing temperature. The stresses generated in the films were also studied using the optical spectra of the samples; it was assumed that the below bandgap optical absorption in the films is due to the structural imperfections present. The strains obtained from optical analysis are consistent with the findings of the structural studies by XRD.

Alternative route for studying the grain boundary scattering in semiconductor films of high resistivity

Abstract An alternative technique for studying the grain boundary scattering phenomena for highly resistive semiconducting films is presented. It is shown that the grain boundary potential, the density of trap states and the carrier concentration of the films can be obtained by measuring reflectances of the films deposited on non-absorbing substrates.

Nanostructured ZnTe films prepared by D.C. magnetron sputtering

Abstract ZnTe films in nanostructured form have been deposited by high pressure d.c. magnetron sputtering of a ZnTe target onto different substrates kept at various temperatures ranging from 223–373 K. Shift of the band gap to higher energies depended on the relative magnitudes of substrate temperature and gas pressure during deposition.

Surface photovoltage measurement in CdS/CdTe solar cell: Grain boundary effect

The surface photovoltage (SPV) measurements of CdS/CdT solar cells, prepared by electrodeposition technique, is presented. Minority carrier diffusion length (L) and lifetime (τ) along with the surface space-charge width (W) in the polycrystalline CdTe absorber layers were estimated. A simple modification of the existing theory of surface photovoltage (SPV) measurements is considered to include the effect of grain boundaries in the polycrystalline material. It was observed that the value of the diffusion length, derived from the Goodmans expression, was much higher (almost double) than the value obtained by considering the effect of barrier height at the grain boundaries of the polycrystalline absorber layer. This indicated that for SPV measurements in polycrystalline material the consideration of the effect of grain boundaries becomes essential. The use of Goodmans simplified expression may not yield unambiguous results.

Determination of effective mass of electrons in ternary compound semiconductors

Abstract A method for evaluating the effective electron mass in compound semiconductors of the form AB x C 1−x or A x B 1−x C from the variation of band gaps in them is presented here. The effective masses of a number of ternary semiconductor films have been computed by using this method.

Absorption tail of polycrystalline semiconductor films

Abstract A comprehensive description of the broadening of the absorption tail in polycrystalline semiconductor films has been presented by considering intrinsic inhomogeneity due to random distribution of grains and grain boundary regions and the fluctuation in potential from local thermal vibrations present at the grains and grain-grain interfaces. The model has been applied to describe the absorption data of a number of polycrystalline films. The results of the analysis on CdSe and ZnTe, prepared by hot wall evaporation, and on diamond films, prepared by plasma deposition of acetylene and hydrogen, are presented here.

Electrical conductivity of nanostructured ZnTe films

Abstract Nanostructured ZnTe films have been deposited by high pressure d.c. magnetron sputtering onto glass substrates. The variation of electrical conductivity with temperature was measured in the temperature range of 100–300 K. The conductivity is found to vary as T − 1 2 within a temperature range 100–150 K, indicating variable range hopping within a Coulomb gap.

Electrodeposited CdTe films: Space charge limited conduction

Abstract Of use in the optoelectronic field, CdTe has been prepared by vapour transport and electrodeposition processes and after a brief review of reported work an account is given of a study of the structural and electrical properties of CdTe films prepared by electrodeposition. The effect of the defect states on the carrier transport in CdTe films has been discussed critically by studying the space charge limited conduction in CdTe films electrodeposited onto Ni coated glass substrates at deposition potentials ( V d ) in the range − 675 to − 725 mV, with respect to a saturated calomel electrode (SCE). The density of the trap states within the band gap of the material was obtained. It was observed that with the variation of the deposition potential from − 675 to − 725 mV (wrt SCE), the density of trap states in the films varied from 3 × 10 16 to 5 × 10 15 cm −3 while the corresponding variation of resistivity (ρ) with deposition potential ( V d ) was within 1.0–20.1 × 10 4 Ω- cm . The minority carrier diffusion length, determined by a surface photovoltage technique, was ~ 0.35 μ m.