Dinesh K. Pandya

Transparent conductors—A status review

Non-stoichiometric and doped films of oxides of tin, indium, cadmium, zinc and their various alloys, deposited by numerous techniques, exhibit high transmittance in the visible spectral region, high reflectance in the IR region and nearly metallic conductivity. The electrical as well as the optical properties of these unusual materials can be tailored by controlling the deposition parameters. These transparent conductors have found major applications in a vast number of active and passive electronic and opto-electronic devices ranging from aircraft window heaters to charge-coupled imaging devices. In this status review we present a comprehensive and up-to-date description of the deposition techniques, electro-optical properties, solid state physics of the electron transport and optical effects and some applications of these transparent conductors.

Growth Kinetics and Polymorphism of Chemically Deposited CdS Films

The kinetics of growth for chemical deposition of films from alkaline solutions of cadmium salts has been studied with respect to temperature of deposition and the relative concentrations of the various reactants in the solution. It has been established that the growth of the film takes place either by ion‐by‐ion condensation of Cd+2 and S−2 ions or by adsorption of colloidal particles of formed in the solution, depending on the various deposition parameters and the method of preparation. The former process of growth results in thin, hard, adherent and specularly reflecting films, whereas the latter results in thick, powdery and diffusely reflecting films. Occurrence of different polymorphic phases of (hexagonal and cubic) has been observed under different growth conditions. A model for growth mechanism has also been proposed.

Solution Growth of CdSe and PbSe Films

Thin films of CdSe and PbSe were prepared using an immersion technique. The effects of different parameters on the speed of deposition and the thickness of the final layer were studied. A growth model for these films was developed and the structure of the films is presented. 11 references.

Obliquely deposited amorphous Ge films. I. Growth and structure

The structure and growth of amorphous Ge films deposited at an angle of incidence ranging from 0° to 80° have been studied by electron microscopy and electron diffraction as a function of the film thickness, deposition temperature, deposition rate, and annealing temperature. Like crystalline films, the growth of amorphous films proceeds via random nucleation centers and is controlled by the small but finite adatom mobility. The small adatom mobility results in the formation of a large concentration of vacancies, vacancy clusters, voids, and associated dangling bonds in a‐Ge films. The adatom mobility is enhanced by the elevated deposition temperatures and oblique incidence and, consequently, the concentration of vacancy clusters and voids decreases. The formation of columns and decoration of structural defects associated with the cleaved NaCl single‐crystal substrates during the growth of a‐Ge films are a direct consequence of the enhanced adatom mobility. The observed thickness dependence of the resistiv...

Electrical and optical properties of single-phase CuInS2 films prepared using spray pyrolysis

Abstract The spray pyrolysis conditions required to prepare single-phase CuInS2 films of good optical quality were optimized. The as-deposited films had a sphalerite structure which transformed to the chalcopyrite structure on annealing at 670 K. Single-phase and two-phase (mixture of CuInS2 and CuxS) regimes were established and were correlated with the cation-to-anion and copper-to-indium ratios in the spray solution. The resistivity of both as-deposited and annealed single-phase films changed from about 103 Ωm to about 10−1 Ωm with increasing copper-to-indium ratio in the spray solution. An optical gap of about 1.38 eV was measured for both the sphalerite and the chalcopyrite structures. Photoconductivity was observed in the as-deposited films and was enhanced on annealing. The photosensitivity decreased as the copper excess in the spray solution was increased (up to 7%).

High temperature ferromagnetism in Mn-doped SnO2 nanocrystalline thin films

It has been possible to induce room temperature ferromagnetism, exhibiting high transition temperature, in tin oxide thin films by introducing manganese in a SnO2 lattice. The observed temperature dependence of the magnetization predicts a Curie temperature exceeding 550 K. A maximum saturation magnetic moment of 0.18±0.04 μB per Mn ion has been estimated for spray pyrolized Sn1−xMnxO2−δ thin films, with x=0.10. For Mn concentration (x) higher than 0.10, the films show linear behavior. The magnetization-versus-field studies indicate that the origin of ferromagnetism lies neither in ferromagnetic metal clusters nor in the presence of metastable phases. The structure factor calculations reveal that Mn has been incorporated in the SnO2 lattice. Also, the electron transport investigation indicates that there is a change of Mn occupancy from substitutional to interstitial sites of the SnO2 lattice when the Mn concentration exceeds 7.5 at. %. These films do not exhibit anomalous Hall effects at room temperature...

Photocontraction effect in amorphous Se1−xGex films

Anomalously large ( ~12%) photocontraction (photoinduced change in thickness) has been observed in amorphous Se 1-x Ge x films. The contraction depends strongly on the angle of deposition and post-deposition treatment of the films.

Electroless deposition of epitaxial Pb1−xHgxS films

Abstract Epitaxial films of Pb1−xHgxS (x = 0−0.33) were grown on single-crystal (111) Ge and (111) Si substrates by an electroless deposition process under conditions of controlled dilutions and reaction bath temperatures. Electron microscopy and electron diffraction studies show the predominance of [ 1 11] and [ 1 12] orientations in films grown on (111) Ge and (111) Si substrates respectively. The films grew initially as β′-Pb1−xHgxS up to a thickness of about 800 A; beyond this thickness the films were α′-Pb1−xHgxS.

Structural and Optical Properties of Solution Grown CdSe1 − x S x Films

CdSe 1-x S x films have been prepared in the entire composition range from CdSe to CdS by using a chemical solution growth technique. Optimum conditionsto deposit good quality films have been determined. Structural and opticalproperties of the films have been studied. Electron diffraction analysis showsthat the films have predominantly cubic (sphalerite) structure. The lattice parametervaries continuously from 5.81A for CdS to 6.06A for CdSe films. Thefilms have a direct bandgap, which varies continuously from 2.44 eV for CdS to 1.74 eV for CdSe films.

Electroless deposition of semiconductor films

Abstract An electroless (solution growth) technique for the deposition of thin films of a variety of multicomponent alloys ( e.g. Cd 1− x Zn x S, Pb 1− x Hg x S and Cd 1− x Pb x Se) of II–VI and IV–VI semiconductors has been developed. The technique involves the recombination on a suitably prepared substrate of two or more ions produced during a well-controlled chemical reaction. Films are obtained by an energetic ion-by-ion condensation process and exhibit nucleation and growth stages. The technique allows the preparation of thin films with polymorphic structures and variable compositions.