ZnO has been identified as a viable candidate to replace expensive Sn doped In2O3 due to its high transparency and good conductivity. ZnO has been doped with many elements including Al and In to achieve conductivities near 104 Ω−1cm−1. One of the limiting factors in improving the conductivity beyond this is the limited solubility of these dopants in ZnO. The effect of codoping ZnO with both Al and In has been studied using a combinatorial technique. Cosputtering from three elemental targets in the presence of oxygen allowed for a range of ZnO(Al, In) compositions to be deposited on a single substrate. A deposition temperature of 250°XC was used. The films were analyzed using x-ray diffraction, electron microprobe, four-point resistivity measurements, Hall measurements, and reflection and transmission spectroscopy at optical wavelengths. Films that were codoped with both Al and In were found to have a significantly improved conductivity compared to singly doped samples, resulting mainly from an increase in mobility. This has been attributed to improved atomic structure of the codoped film. Al3+ has an ionic radius smaller than that of Zn2+ while In3+ has a larger radius; it is speculated that codoping compensates for these differences, reducing stress fields. The lattice parameter of the codoped film reflects this compensation effect.

Results of periodic ab initio density functional theory calculations on thin films of (i) wurtzite ZnO (hexagonal) which terminate with the non-polar (1010) surface, and with the polar (0001) and (0001) surfaces (ii) zinc blende (cubic) ZnO which terminate with the non-polar (110) and with the polar (111) surfaces. Thin (less than18 layer) films of wurtzite ZnO which terminate with the polar (0001) and (0001) surfaces are found to be higher in energy than corresponding films in which these polar surfaces flatten out forming a new planar ‘graphitic’-like structure in which the Zn and O atoms are coplanar and the dipole is removed. This is the lowest energy surface for ultra-thin films. For zinc-blende ZnO a graphitic-type solution, but with a different stacking of ZnO layers, is also comparable to energy to the non-polar (110) and polar (111) solutions. Consequences for crystal growth and the stabilization of thin films and nanostructures are discussed.