B. Pradeep

Structural, electrical and optical properties of silver selenide thin films

We study the structural, electrical and optical properties of silver selenide thin films prepared by reactive evaporation. From the Hall effect study, at room temperature we find a mobility of 2000 cm2 V−1 s−1 and a carrier concentration of 1018 cm−3. The thermoelectric power is measured from room temperature up to 200 °C. X-ray diffraction indicates that the as-prepared films are polycrystalline in nature. The composition and morphology are determined using energy dispersive x-ray analysis and scanning electron microscopy (SEM). The optical bandgap, which is direct-allowed, is 1.58 eV.

Structural analysis and optical and electrical characterization of the ordered defect compound CuIn5Se8

Thin polycrystalline films of the ordered defect compound CuIn5Se8 and CuInSe2 are prepared on soda lime glass substrates at temperature 623 K by coevaporation of the constituent elements. X-ray diffraction, scanning electron microscopy and energy dispersive analysis of x-rays are done on the films for structural, morphological and composition determination. The lattice constants and the anion displacements for the compounds are calculated. The deformation parameter values show that the compounds have slightly distorted tetragonal unit cells. Assuming the atomic positions, the x-ray structure factor calculations have been made and the emergence of (110) reflection in the x-ray diffraction pattern of the ordered defect compound CuIn5Se8 is explained. High resistivity measured for the compound film is attributed to the compensated defect levels in it. Hot probe measurements show the conductivity of the films to be n-type. A band gap Eg = 1.32 eV is observed for the compound and the increase in band gap compared to Eg = 0.98 eV for CuInSe2 is explained as a consequence of the lowering of the valence band due to weaker SeΓ15(p)–CuΓ15(d) repulsion in the defect compound and electronic passivation due to 2VCu–InCu pairing.

Investigations of the electrical properties in CuInSe2 and the related ordered vacancy compounds

The conductivity mechanisms in polycrystalline CuInSe2 and the ordered vacancy compounds CuIn3Se5, CuIn5Se8 and CuIn7Se12 were studied by low temperature conductivity measurements and room temperature Hall and photoconductivity measurements. The polycrystalline films were structurally, morphologically and compositionally characterized using x-ray diffraction, atomic force microscopy and energy dispersive analysis by x-rays, respectively. In general, the films showed Motts variable range hopping conduction in the temperature range below 200 K while above 200 K thermally activated conductivity due to grain boundary effects and defect levels was observed. To investigate the reason for the metallic behaviour shown by Cu-rich films, the films were etched and the change in behaviour was studied.

Transport properties of silver selenide thin films from 100 to 300 K

Abstract The Hall coefficient, Hall mobility and electrical conductivity of n-type low temperature phase of silver selenide (β-Ag2Se) thin films prepared by reactive evaporation have been studied from 100 to 300 K. Thermoelectric power was measured from 150 to 300 K. Hall effect study shows that it has a mobility of 2000 cm2 V−1 s−1 at room temperature and it showed a maximum at 100 K and it has a carrier concentration of 1018 cm−3 at room temperature. X-ray diffraction (XRD) study at room temperature indicates that the as-prepared films are polycrystalline in nature. The lattice parameters were found to be a=4.353 A, b=6.929 A and c=7.805 A. The electrical conductivity measurements showed that there exist two different activation energies below 300 K with energies 1.3 and 15.8 m eV.

Optoelectronic properties and Seebeck coefficient in SnSe thin films

SnSe thin films of thickness 180 nm have been deposited on glass substrates by reactive evaporation at an optimized substrate temperature of 523 ± 5 K and pressure of 10−5 mbar. The as-prepared SnSe thin films are characterized for their structural, optical and electrical properties by various experimental techniques. The p-type conductivity, near-optimum direct band gap, high absorption coefficient and good photosensitivity of the SnSe thin film indicate its suitability for photovoltaic applications. The optical constants, loss factor, quality factor and optical conductivity of the films are evaluated. The results of Hall and thermoelectric power measurements are correlated to determine the density of states, Fermi energy and effective mass of carriers and are obtained as 2.8 × 1017 cm−3, 0.03 eV and 0.05m 0 respectively. The high Seebeck coefficient ≈ 7863 μV/K, reasonably good power factor ≈ 7.2 × 10−4 W/(mK2) and thermoelectric figure of merit ≈ 1.2 observed at 42 K suggests that, on further work, the prepared SnSe thin films can also be considered as a possible candidate for cryogenic thermoelectric applications.

Crystal-field and spin–orbit interactions in the valence band of the ordered vacancy compound CuIn4Se7

The optical absorption spectra of the ordered vacancy compound CuIn4Se7 thin film have been analysed with special consideration to the threefold optical structure at the fundamental gap. The thin films for this study have been prepared by vacuum co-evaporation and the structural and compositional characterization done using x-ray diffraction in conjunction with energy dispersive x-ray analysis. The quasi-cubic model adapted for crystals with tetragonal distortion has been used to determine the crystal field splitting and spin–orbit splitting parameters. The effects of p–d hybridization and structural changes on the parameters are explained. The percentage of hybridization of the orbitals is determined using the linear hybridization model.

Rapid room temperature crystallization of TiO2 nanotubes

This paper reports a very efficient and reproducible technique for room temperature crystallization of titanium dioxide in a record time of 5 minutes, by application of alternating voltage square pulses. The well aligned tubular formation of crystalline nanotubes is confirmed through various characterization techniques, and the mechanism of crystallization is explained based on the pulse induced electrophilic–nucleophilic reaction. This ultrafast pulsed crystallization method provides a viable alternative for the widely used post fabrication thermal annealing and is highly applicable in device fabrication.

Photoluminescence Study of Copper Selenide Thin Films

Thin films of Copper Selenide of composition of composition Cu7Se4 with thickness 350 nm are deposited on glass substrate at a temperature of 498 K±5 K and pressure of 10−5 mbar using reactive evaporation, a variant of Gunther’s three temperature method with high purity Copper (99.999%) and Selenium (99.99%) as the elemental starting material. The deposited film is characterized structurally using X‐ray Diffraction. The structural parameters such as lattice constant, particle size, dislocation density; number of crystallites per unit area and strain in the film are evaluated. Photoluminescence of the film is analyzed at room temperature using Fluoro Max‐3 Spectrofluorometer.

Optical and electrical properties and phonon drag effect in low temperature TEP measurements of AgSbSe2 thin films

Polycrystalline thin films of silver antimony selenide have been deposited using a reactive evaporation technique onto an ultrasonically cleaned glass substrate at a vacuum of 10?5 torr. The preparative parameters, like substrate temperature and incident fluxes, have been properly controlled in order to get stoichiometric, good quality and reproducible thin film samples. The samples are characterized by XRD, SEM, AFM and a UV?vis?NIR spectrophotometer. The prepared sample is found to be polycrystalline in nature. From the XRD pattern, the average particle size and lattice constant are calculated. The dislocation density, strain and number of crystallites per unit area are evaluated using the average particle size. The dependence of the electrical conductivity on the temperature has also been studied and the prepared AgSbSe2 samples are semiconducting in nature. The AgSbSe2 thin films exhibited an indirect allowed optical transition with a band gap of 0.64 eV. The compound exhibits promising thermoelectric properties, a large Seebeck coefficient of 30 mV/K at 48 K due to strong phonon electron interaction. It shows a strong temperature dependence on thermoelectric properties, including the inversion of a dominant carrier type from p to n over a low temperature range 9?300 K, which is explained on the basis of a phonon drag effect.

Structural and optical characterization of reactive evaporated tin diselenide thin films

Tin diselenide thin films with thickness of the order of 300 nm are deposited on glass substrate at a substrate temperature of 523±5 K and pressure of 10−5 mbar using reactive evaporation, a variant of Gunthers three temperature method. High purity tin (99.999%) and selenium (99.99%) were used as the elemental starting materials. The deposited film is characterized structurally using X-Ray Diffraction (XRD). The structural parameters such as lattice constant, particle size, dislocation density, number of crystallites per unit area and strain in the film are evaluated. Optical absorption spectrum of the film is analysed using UV-Vis- NIR Spectrophotometer.