K. L. Chopra

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.

Effect of hydrogen plasma treatment on transparent conducting oxides

The effect of hydrogen plasma treatment on indium tin oxide (ITO), fluorine‐doped tin oxide (FTO), and indium‐doped zinc oxide (IZO) films has been studied. X‐ray photoelectron spectroscopy analysis shows that ITO and FTO surfaces get reduced to yield elemental indium and tin, respectively. Annealing of the plasma treated films in air leads to re‐oxidation of the reduced surface and the electro‐optical properties are recovered. In contrast, IZO films are not reduced by plasma treatment and show no changes in the electrical and optical properties. The surface of plasma treated IZO films shows a higher binding energy O(1s) peak probably due to OH or OH...O species which appear to form a protective layer against plasma degradation.

XPS and X-ray diffraction studies of aluminum-doped zinc oxide transparent conducting films

Abstract Aluminum-doped zinc oxide transparent conducting films are prepared by spray pyrolysis at different dopant concentrations. These films are subsequently characterized by X-ray diffractometric and X-ray photoelectron spectroscopic (XPS) techniques. The results are compared with those obtained from pure zinc oxide films prepared under identical conditions. X-ray diffraction measurements show an increase in lattice parameters (c and a) for aluminum-doped films while their ratio remains the same. This study also indicates that within the XPS detection limit the films are chemically identical to pure zinc oxide. However, a difference in the core-electron line shape of the Zn 2 p 3 2 photoelectron peaks is predicted. An asymmetry in Zn 2 p 3 2 photoelectron peaks has been observed for aluminium-doped films. The asymmetry parameters evaluated from core-electron line-shape analysis yield a value of the order of 0.04±0.01. The value is found to lie between those obtained for pure zinc oxide and has been attributed to the presence of excess zinc in the films.

Electrical and optical properties of undoped and antimony‐doped tin oxide films

Tin oxide films have been prepared on glass substrates by spray pyrolysis technique. The electrical and optical properties of undoped and antimony‐doped tin oxide films have been studied. The temperature dependence of electron mobility has been analyzed to establish the electron conduction mechanism. Optical properties near the plasma edge have been analyzed using Drude’s theory. The dependence of effective mass on carrier concentration has been explained on the basis of nonparabolicity of the conduction band. The shift in the Fermi energy, calculated on the basis of energy dependent effective mass, is consistent with the measured shift in the absorption edge.

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.

Highly transparent and conducting indium-doped zinc oxide films by spray pyrolysis☆

Abstract Spray pyrolysis has been used to deposit highly transparent and conducting films of indium-doped zinc oxide. The roles of various deposition parameters have been investigated and the optimum deposition conditions have been outlined. Without any post-deposition heat treatment, as-deposited films with a resistivity of about (8 − 9) × 10 -4 Ω cm and an average visible transmittance of about 85% have been obtained. The structural, electrical and optical properties have been studied. The electron transport properties suggest that the films are degenerate and the mobility data can be understood in terms of a grain boundary trapping model.

Electrical and optical properties of tin oxide films doped with F and (Sb+F)

This paper presents the structural, electrical, and optical properties of F‐ and (Sb+F)‐doped tin oxide films prepared by spray pyrolysis technique. Resistivity as low as 5.5×10−4 Ω cm with high optical transmission (≳80%) and high infrared reflection (∼90%) have been obtained in F‐doped tin oxide films. The figure of merit ΦTC = T10/Rsh (52.6×10−3Ω−1 at 0.65μm) of these films is the highest amongst the results reported on doped tin oxide films. The variation of mobility with doping concentration has been analyzed to understand the electron–conduction mechanism. The Drude theory has been used to explain the optical properties near the plasma edge.

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.

Indium-doped zinc oxide films as transparent electrodes for solar cells

Abstract Indium-doped zinc oxide films possess high conductivity and transparency with negligible absorption in the wavelength range of 0.4–0.8 μm which is the useful region for hydrogenated amorphous silicon solar cells. These films are thermally stable in both oxidizing and reducing ambients up to ∼800 K. These films do not degrade when exposed to hydrogen plasma. Pure ZnO films are rough, while In-doped ZnO films are very smooth. By making a double layer structure of In-doped ZnO/ZnO, the film surface has been texturized, which results in a large haze factor ( ∼16% ) at a wavelength of 0.7 μm.

Amorphous versus Crystalline GeTe Films. III. Electrical Properties and Band Structure

Various transport studies have been carried out on amorphous and crystalline GeTe films of 80 A to 10 μ thickness. Crystalline GeTe has a low resistivity (∼10−4 Ω·cm at 300°K) which increases with temperature slowly and nearly linearly at low temperatures (below ∼300°K) and rapidly at higher temperatures. The hole concentration (N∼1010−1021 cm−3) increases only slightly with temperature. Mobility varies as N−4/3. These results in conjunction with the tunnel‐spectroscopy and optical data show that crystalline GeTe is a degenerate (and thus metallic conduction), p‐type narrow band gap (∼0.1–0.2 eV) semiconductor with Fermi level ∼0.3–0.5 eV inside the valence band. The linear increase of the susceptibility mass with hole concentration, the constancy of the Hall coefficient up toωLτ=0.35 (ωL=Larmor frequency, τ=collision relaxation time), and the monotonic increase of thermopower with temperature indicate that conduction takes place only in a single valence band. Amorphous GeTe films exhibit activated conduc...