P-type nitrogen- and phosphorus-doped ZnO thin films grown by pulsed laser deposition on sapphire substrates

Abstract

Nitrogen- and phosphorus-doped ZnO thin films were grown by pulsed laser deposition using an electron cyclotron resonance (ECR) nitrogen plasma ion source or a ZnO:P₂O₅ doped target, as the dopant source, respectively. Both types of films were grown on sapphire substrates first coated at low temperature with a ZnO buffer layer. For the N-doped ZnO thin films, temperature-dependent Van der Pauw measurements showed consistent p-type behavior over the measured temperature range of 200-450 K, with typical room temperature acceptor concentrations and mobilities of 5 x 10¹⁵ cm^-3 and 5.61 cm²/Vs, respectively. The room-temperature photoluminescence spectrum of a N-doped ZnO thin film featured a broad near band-edge emission at about 3.1 eV photon energy with a width of 0.5 eV. XPS studies confirmed the incorporation of nitrogen in the samples. The ZnO:P layers (with phosphorus concentrations of between 0.01 and 1 wt %) typically showed weak n-type conduction in the dark, with a resistivity of 70 &OHgr;.cm, a Hall mobility of &mgr;_n ~ 0.5 cm²V^-1s^-1 and a carrier concentration of n ~ 3 x 10¹⁷ cm^-3 at room temperature. After exposure to an incandescent light source, the samples underwent a change from n- to p-type conduction, with an increase in mobility and a decrease in concentration for temperatures below 300K. Electrical measurements showed noticeable differences for both types of doped films when carried out in air or in vacuum. The results are discussed in terms of both the presence of surface conducting channels and the influence of photoconductive effects.