Properties of phosphorus-doped ZnO and (Zn,Mg)O thin films via pulsed laser deposition

The room temperature transport and optical properties of phosphorus-doped ZnO and (Zn,Mg)O thin films are studied. Pulsed laser deposition (PLD) has been employed to grow epitaxial and polycrystalline layers on c-plane (0001) sapphire substrate. The ZnO:P film properties show a strong dependence on the deposition ambient at different growth temperatures. The resistivity of the samples deposited in O₃/O₂ mixture is two orders of magnitude higher than the films grown in oxygen and O₂/Ar/H₂ mixture. The photoluminescence (PL) spectra of the as-deposited films are composed of both the near band-edge and broadband visible emission, which peak at 3.29 and 1.87 eV, respectively. It has been shown that growing in the O ₂/Ar/H₂ mixture ambient significantly increases the band edge emission while inhibiting the visible emission. The opposite effect on the PL emissions is shown for the films grown in pure oxygen and O ₃/O₂ mixture. There is an inverse correlation between the intensity of the visible broadband emission and the carrier density. The enhanced UV emission in the films grown in O₂/Ar/H₂ mixture may result from hydrogen passivation of the deep level emission centers. For the P-doped (Zn,Mg)O grown at 500°C, increasing the oxygen partial pressure from 20 to 200 mTorr yields a carrier type conversion from n-type to p-type without post-annealing. The films grown at 150 mTorr oxygen partial pressure are p-type and exhibit a hole concentration of 2.7 × 10 ¹⁶ cm^-3, a mobility of 8.2 cm²/Vs and a resistivity of 35 Ω-cm. All the films exhibit good crystallinity with c-axis orientation. These results indicate the importance of oxidation conditions in realizing p-type (Zn,Mg)O:P films.