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.