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

Abstract

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