Properties of homoepitaxial ZnO and ZnO:P thin films grown by pulsed-laser deposition

We have investigated the morphology, crystalline quality, the transport and electronic properties of homoepitaxial ZnO and ZnO:P thin films grown by pulsed-laser deposition. Atomic surface steps are visible for growth temperatures of 650 degrees C and higher. The unit cell volume of undoped thin films is smaller than that of the hydrothermal substrates. Phosphorous doping increases the unit cell volume such that a perfect lattice match is achieved for a nominal phosphorous content of 0.01 wt.%. Undoped thin films have a net doping concentration below 10(15) cm(-3), whereas the phosphorous doping increases the free electron concentration at room temperature to 10(17) cm(-3) and above. Temperature dependent Hall effect measurements show that interstitial zinc with a thermal activation energy of 34 meV is a dominant donor in homoepitaxial ZnO:P thin films. The Hall mobility of such samples is similar to ZnO single crystals grown by seeded chemical vapor transport. Low temperature photoluminescence measurements reveal recombination of free excitons and excitons bound to interstitial zinc and excitons bound to neutral and ionized aluminum donors. Defect related deep luminescence is not observed for undoped homoepitaxial thin films. In contrast phosphorous doping introduces two broad recombination bands centered at 2.9 eV and 1.9 eV.