Impurity effects in ZnO and nitrogen-doped ZnO thin films fabricated by MOCVD

We studied the role of impurities in nitrogen-doped ZnO thin film to understand the difficulty of producing p-type ZnO via nitrogen doping. The ZnO:N films were fabricated by low-pressure metal-organic chemical vapor deposition (MOCVD) using diethylzinc (DEZ) and nitric oxide (NO) precursors. Although very high levels of nitrogen incorporation were observed (∼10 21 cm −3), acceptor concentrations were typically low (10 14–10 17 cm −3). The investigation suggests that the low carrier concentrations are possibly due to compensation and passivation effects by hydrogen and carbon impurities unintentionally incorporated into the films from the metal-organic precursor. X-ray photoelectron spectroscopy (XPS) demonstrated that carbon was a bulk impurity in MOCVD-grown films. Secondary-ion mass spectrometry (SIMS) analysis confirmed the presence of carbon and indicated that hydrogen was also a bulk impurity. The concentration of carbon contaminant was found to increase with nitrogen doping. Both XPS and Fourier transform infrared spectroscopy (FTIR) data indicated that defect complexes (CH x , NH x , and NC x ) are likely present in MOCVD-grown ZnO films. First-principles calculations predict that the N O–H and (NC) O defect complexes are neutral and 1 + charge state; therefore, the existing carbon and hydrogen passivate the nitrogen acceptor species. Thus, we believe a low hole concentration in MOCVD-fabricated ZnO:N films are partially due to inadvertent passivation by hydrogen and carbon.