The present status and prospects for further development of polycrystalline or amorphous transparent conducting oxide (TCO) semiconductors used for practical thin-film transparent electrode applications are presented in this paper. The important TCO semiconductors are impurity-doped ZnO, In2O3 and SnO2 as well as multicomponent oxides consisting of combinations of ZnO, In2O3 and SnO2, including some ternary compounds existing in their systems. Development of these and other TCO semiconductors is important because the expanding need for transparent electrodes for optoelectronic device applications is jeopardizing the availability of indium-tin-oxide (ITO), whose main constituent, indium, is a very expensive and scarce material. Al- and Ga-doped ZnO (AZO and GZO) semiconductors are promising as alternatives to ITO for thin-film transparent electrode applications. In particular, AZO thin films, with a low resistivity of the order of 10−5 Ω cm and source materials that are inexpensive and non-toxic, are the best candidates. However, further development of the deposition techniques, such as magnetron sputtering or vacuum arc plasma evaporation, as well as of the targets is required to enable the preparation of AZO and GZO films on large area substrates with a high deposition rate.

https://iopscience.iop.org/article/10.1088/0268-1242/20/4/004/pdf

Transparent conducting oxide semiconductors for transparent electrodes

Most research to develop highly transparent and conductive thin films has focused on n-type semiconductors consisting of metal oxides. Historically, transparent conducting oxide (TCO) thin films composed of binary compounds such as SnO2 and In2O3 were developed by means of chemical- and physical-deposition methods. Impurity-doped SnO2 (Sb- or F-doped SnO2, e.g., SnO2:Sb or SnO2: F) and In2O3: Sn (indium tin oxide, ITO) films are in practical use. In addition to binary compounds, ternary compounds such as Cd2SnO4, CdSnO3, and CdIn2O4 were developed prior to 1980, but their TCO films have not yet been used widely.

https://www.cambridge.org/core/services/aop-cambridge-core/content/view/S0883769400027159

New n-Type Transparent Conducting Oxides

Tin doped indium oxide (ITO) films are highly transparent in the visible region, exhibiting high reflectance in the infrared region, and having nearly metallic conductivity. Owing to this unusual combination of electrical and optical properties, this material is widely applied in optoelectronic devices. The association of these properties in a single material explains the vast domain of its applicability and the diverse production methods which have emerged. Although the different properties of tin doped indium oxide in the film form are interdependent, this article mainly focuses on the electrical aspects. Detailed description of the conduction mechanism and the main parameters that control the conductivity is presented. On account of the large varieties and differences in the fabrication techniques, the electrical properties of ITO films are discussed and compared within each technique.

/pdf/tin-doped-indium-oxide-thin-films-electrical-properties-1emmfeswwg.pdf

Tin doped indium oxide thin films: Electrical properties

Transparent conducting p-type NiO thin films prepared by magnetron sputtering

Heavily doped zinc oxide films are used as transparent and conductive electrodes, especially in thin film solar cells. Despite decades of research on zinc oxide it is not yet clear what the lower limit of the resistivity of such films is. Therefore, the electrical parameters of zinc oxide films deposited by magnetron sputtering, metal organic chemical vapour deposition and pulsed laser ablation are reviewed and related to the deposition parameters. It is found that the lowest resistivities are in the range of 1.4 to 2×10-4 Ω cm, independently of the deposition method. The highest reported Hall mobilities are about 60 cm2 V-1 s-1. The thin film electrical data are compared with the corresponding values of single crystalline zinc oxide and with that of boron and phosphorous doped crystalline silicon. From this comparison it can be seen that the dependence of the Hall mobilities on the carrier concentration n are quite similar for silicon and zinc oxide. In the region n>5×1020 cm-3, which is most important for the application of zinc oxide as a transparent and conductive electrode, phosphorous doped silicon has a mobility only slightly higher than zinc oxide. The experimental data on the electron and hole mobilities in silicon as a function of the impurity concentration have been described by a fit function (Masetti et al 1983), which can also be applied with different fitting parameters to the available zinc oxide mobility data. A comparison of the experimental data with the well known ionized impurity scattering theories of Conwell-Weisskopf (1946) and Brooks-Herring-Dingle (1955) shows that these theories are not able to describe the data very well, even if the non-parabolic band structure is taken into account. As in the case of silicon, an additional reduction of the mobility also occurs for zinc oxide for concentrations n>5×1020 cm-3, which can be ascribed qualitatively to the clustering of charge carriers connected with increased scattering due to the Z-2 dependence of the scattering cross section on the charge Z of the scattering centre. The presented review of the charge carrier transport in zinc oxide indicates that a physical limit due to ionized impurity scattering is reached for homogeneously doped layers. Due to the universal nature of this limitation it is suggested that it also applies to the other important materials indium-tin (ITO) and tin oxide. Experiments are proposed to overcome this limit.

https://iopscience.iop.org/article/10.1088/0022-3727/34/21/301/pdf

Resistivity of polycrystalline zinc oxide films: current status and physical limit

The present state of the art of transparent, electrically conducting films, with special reference to In2O3, SnO2 and Cd2SnO4, has been reviewed. Various production techniques currently in use, and typical parameters used in the processes have been discussed in detail. Electrical and optical properties of these films have been reported as a function of various parameters, e.g. substrate temperature, doping, oxygen pressure, etc. Finally, the applications of these films in research and industry have been discussed in detail.

J. C. Joshi

Papers

Semiconducting transparent thin films: their properties and applications