No abstract

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.

Magnetron sputtering of transparent conductive oxides (zinc oxide, indium tin oxide, tin oxide) is a promising technique which allows the deposition of films at low temperatures with good optical and electronic properties. A special advantage is the scalability to large areas. The principles underlying magnetron sputtering are reviewed in this paper. The growth process during magnetron sputtering is characterized by the bombardment of the growing film with species from the sputtering target and from the plasma. In addition to sputtered atoms with energies in the eV range, ions from the plasma (mostly argon) and neutral atoms (also argon) reflected at the target hit the growing film. Depending on the energy of these species and on the ion-to-neutral ratio the properties of the films vary. High energies ( 100 eV), which occur mainly at low sputtering pressures lead to damage of the growing film, connected with mechanical stress, small crystallites and bad electrical parameters. Ion assisted growth with low ion energies (below about 50 eV) is advantageous as is a high ion-to-neutral ratio. A compilation of resistivities of magnetron sputtered zinc oxide films yields a limiting resistivity of 2 × 10 −4 cm for polycrystalline films. Based on the correlation between plasma parameters and film properties new research fields are anticipated.

Highly-doped indium-tin oxide films exhibit resistivities ρ as low as 1. . Thus the grain barrier trap densities of ZnO and ITO are significantly different, which seems to be connected with the defect chemistry of the two oxides and especially with the piezoelectricity of zinc oxide.Keywords Transparent conductive oxides, carrier transport, degenerate semiconductors, grain barriers, electron mobility 2 Therefore, in the present study the carrier transport processes in ITO and ZnO are compared in order to get a deeper understanding of the differences between these TCO materials. For this purpose conductivity and Hall mobility measurements on ZnO:Al and ITO films were undertaken for films deposited on amorphous as well as single crystalline substrates (sapphire) in order to determine the dominant scattering processes (ionized impurities, grain barriers, crystallographic defects). Our own data are compared with literature data reported for ZnO and ITO to show the general trends. Theoretical and semiempirical models are used to fit the experimental data and to derive characteristic material parameters for these three oxides. Introduction Theoretical modelsThe theoretical models on ionized impurity scattering were already reviewed in 2001 by one of the authors when estimating the mobility limit of highly-doped zinc oxide [3]. In the following a short summary is given to lay the basis for the further discussion.Ionized impurity scattering. This scattering process is caused by ionized dopant atoms and dominates for carrier concentrations above about 10 19 cm -3. An analytical expression for the mobility µ ii of degenerately doped semiconductors, taking into account the non-parabolicity of the conduction band, was given by Zawadzki [7] and refined by Pisarkiewicz et al. [6]:where the screening functionwith the parameter ξ np =1-m 0 */m*, which describes the non-parabolicity of the conduction band (m*, m 0 * -effective masses in the conduction band and at the conduction band edge, 4 respectively). The prefactor in equ. (1) The fit parameters µ max , µ min and µ min -µ 1 describe the lattice mobility at low carrier concentrations, the mobility limited by ionized impurity scattering and the clustering mobility, discussed above (see Table 2). . The ZnO mobility values were fitted using the empirical formula (3) and the fit parameters are summarized in Table 2 together with the corresponding values for silicon. In the transition region from lattice to ionized scattering for 5 . 10 16 < N < 5 . 10 18 cm -3 a large scattering of the experimental ZnO data can be observed. Therefore, the data have been fitted in analogy to the silicon data, which exhibit a much higher accuracy [14].However, the exact transition does not influence the conclusions much since we are interested predominantly in ionized impurity scattering in the region N > 10 19 cm -3 .5 Neutral impurity scattering. Neutral shallow-impurity scattering is often discussed in papers about transport in TCO films at room temperature [17,18]. The mobility due to ...

A new technique of the simultaneous excitation of a magnetron sputtering discharge by rf and dc was used for the deposition of undoped ZnO- and Al-doped ZnO (ZnO:Al) films. By this technique, it was possible to change the ion-to-neutral ratio ji/jn on the substrates during the film growth by more than a factor of ten, which was revealed by plasma monitor and Langmuir probe measurements. While for a pure dc discharge the ions impinging onto a floating substrate have energies of about Ei≈17 eV, the rf discharge is characterized by Ar-ion energies of about 35 eV. Furthermore, the ion current density for the rf excitation is higher by a factor of about five, which is caused by the higher plasma density in front of the substrate. This leads to a much higher ion-to-neutral ratio ji/jn on the growing film in the case of the rf discharge, which strongly influences the structural and electrical properties of the ZnO(:Al) films. The rf-grown films exhibit about the three times lower specific resistances (ρ≈6×10−4 Ω cm), due to lower mechanical stress, leading to higher charge carrier concentrations and mobilities. Undoped ZnO films exhibited the largest compressive stress values up to 2.8 GPa. The aluminium-doped films have a better (001) texture and larger grains (dg≈38 nm), which can be attributed to the beneficial role of Al as a surfactant. The better crystalline film quality of the ZnO:Al films is the reason for the much lower compressive stress of &lt;0.5 GPa in these layers.