Comparison of delta and uniform doped p-type and n-type ZnO films

Abstract

ZnO is a wide band gap semiconductor with unique optical, electrical, magnetic and thermal properties. The major road block for the fabrication of ZnO based optoelectronic devices is the difficulty of achieving high and stable p-type conduction in ZnO. Highly conductive n-type ZnO materials are obtained easily with extrinsic dopants. Most of the p-type and n-type ZnO materials reported in literature were produced by the standard procedure of uniform doping. In the literature, delta doping technique has enhanced p-type conduction in wide band semiconductors like GaN, AlGaN and ZnSe as compared to uniform doping method. There are few reports about delta doped n-type ZnO materials. The goal of this study is to obtain reliable and highly p-type conductive ZnO through delta doping and to investigate the difference of p-type and n-type conductivities in delta and uniform doped p-type and n-type ZnO films. The properties of p-type and n-type ZnO thin films doped with acceptors (lithium and phosphorous) and donor (aluminum) have been characterized. All the films were prepared on sapphire substrates by rf (radio frequency) magnetron sputtering. Uniform doping of the films was accomplished by simultaneous sputtering from ZnO and Li3PO4 targets for p-type doping and from ZnO and Al targets for n-type doping. For delta doping, the deposition from Li3PO4or Al targets was interrupted periodically. Hall effect measurements on O2 annealed p-type films revealed p-type conductivities with an average Hall concentration of 3.76 x 1013 cm-3 in uniform doped film and 1.3 x 1016 cm-3 in delta doped films. The as-grown delta doped n-type film exhibited an average Hall concentration of 2.79 x 1019 cm-3 whereas uniform doped n-type film showed an average concentration of 1.82 x 1018 cm-3. The electrical conductivity in n-type films was enhanced upon annealing in (N2 + O2) gas ambient. X-ray diffraction scans indicated that the crystal quality of delta and uniform doped p-type and n-type films improved significantly after annealing. The l