Defect structure of undoped and doped zinc oxide thin films

The defect structure of undoped and doped ZnO crystalline thin films grown by organometallic chemical vapor deposition at low temperature was studied. Structural and chemical properties of layers were examined by scanning electron microscopy, Auger electron spectroscopy, and x-ray diffraction. The transport properties of these layers were characterized by temperature-dependent Hall effect measurements.; The as-grown films had carrier concentrations of 10{dollar}sp{lcub}17{rcub}{dollar}-10{dollar}sp{lcub}19{rcub}{dollar} cm{dollar}sp{lcub}-3{rcub}{dollar}, mobilities of 20-80 cm{dollar}sp2{dollar}/Vs, and resistivities of 10{dollar}sp{lcub}-3{rcub}{dollar}-10{dollar}sp{lcub}-1{rcub} Omega{dollar}cm, respectively, at 300 K. Electrical properties of the as-grown films were dominated by oxygen vacancies which act as the shallow donors. After annealing in oxygen the resistivity of the films increased up to 10{dollar}sp8 Omega{dollar}cm.; Low temperature photoluminescence was measured over the spectral range of 1.5-3.5 eV. For the undoped layers, the luminescence spectra were dominated by strong exciton emission at 3.35 eV associated with impurity centers. In contrast, emission associated with excitons bound to native defects was observed in films that were annealed at high temperature in oxygen. Weaker deep level emission in the green (2.38 eV), and red (1.83 eV) spectral regions was also observed for undoped films. Based on electron irradiation experiments, the green emission band was associated with the singly charged oxygen vacancy centers. Upon annealing in oxygen, the green band was quenched.; The deep level defects in the undoped and annealed ZnO films were studied using spectral-dependent a.c. photoconductivity. For the annealed layers, Schottky defect pairs are generated. It is concluded that this defect associate is responsible for the self-compensation upon annealing.; To study the influence of doping on the defect structure, ZnO was doped with nitrogen resulting n-type layers with resistivities up to 390 {dollar}Omega{dollar}cm. A strong near band eage emission was observed and attributed to excitons bound to nitrogen centers. The energy level of nitrogen impurity was calculated using Haynes rule to be 0.85 eV. From spectral-dependent a.c. photoconductivity, a level at E{dollar}sb{lcub}rm v{rcub}{dollar} + (2.40 {dollar}pm{dollar} 0.05) eV was observed. This level was attributed to a defect associate involving nitrogen and an oxygen vacancy.