The most often studied acceptors in ZnO are group-V N, As, and Sb atoms. However, stable and reproducible p-type material with high concentration and high mobility is yet to be achieved. Motivated by the recent reports [1,2] we have studied the efficiency of Ag as an acceptor.

Ag-doped ZnO thin films were fabricated in a two step process in which ZnO/Ag multilayer was first sputter-deposited on sapphire substrate and then heat treatment was performed to obtain a homogeneous distribution of dopant. The Ag content was 1-5 at. %. High-resolution X-ray diffraction, secondary mass spectrometry, and atomic force microscopy analyses assessed a good crystalline quality with a uniform composition. Transport measurement revealed that after moderate temperature (400-500^oC) anneals in N₂ flow ZnO:Ag is p-type with carrier concentration above 5x10¹⁸cm^-3, which compares favorably with group V dopants. Mobility is few cm²/Vs The transmittance of 500 nm thick p-ZnO films in the visible wavelength spectrum was >60%.

In parallel, acceptor level of Ag was calculated based on the density functional theory using the ESPRESSO code [3]. The calculated level of Ag is 0.2-0.25 eV above the top of the valence band. This confirms our experimental results, and shows that ionization energy of Ag is comparable to those of N, As, and Sb. Formation of Ag-related bands with the increasing Ag content was analysed using the Coherent Potential Approximation.

Part of the research was supported by the grant from the Polish Ministry of Science and Higher Education “Bandgap engineering of ZnO-based alloys” N50703131/0743 and the national scientific network (“New materials and sensors for optoelectronics, informatics, energetics and medicine) 72/E-67/SN-033/2006.

[1] Y. Yan, M.M. Al-Jassim, S-H Wei, Appl. Phys. Lett. 89, 181912 (2006).

[2] B. D. Ahn, H. S. Kang, J. H. Kim, Gun Hee Kim, H. W. Chang, S. Y. Lee, J. Appl. Phys. 100, 093701 (2006)

[3] www.pwscf.org