Identification of defect species in ZnO thin films through process modification and monitoring of photoluminescent properties

Abstract

Photoluminescence (PL) from defect-rich ZnO thin films was investigated in relation to transparent conductive properties. By varying the sputter deposition and post-treatment conditions, the correlation between deep-level luminescence and changes in the optical and electrical properties was examined, and the defects that were the origin of the donors and acceptors were identified. Slightly oxygen-deficient and transparent conductive films deposited on glass substrates without a supply of oxygen source gas had a resistivity of 3 × 10−3 Ω cm; these films exhibited only band edge emissions peaking at 380 nm in the PL spectra. Abundant defects were introduced through postannealing in an H2 atmosphere at 300−400 °C. The permeating H2 reacted with the O and Zn atoms constituting the crystal network, and the resulting PL spectra exhibited a continuous defect-emission band ranging from violet to red. The spectra included transitions of the conduction band (CB) → zinc vacancies (VZn) (400 nm), zinc interstitials (Zni) → valence band (VB) (440 nm), CB → oxygen vacancies (VO) (560 nm), and CB → oxygen interstitials (Oi) (620 nm). Similar PL spectra from disordered crystals were obtained by sputter deposition at 300 and 400 °C under a reducing atmosphere. The films deposited on the sapphire substrate above 300 °C were nonemissive because they were strongly oxygen deficient compared to those on the glass substrate. When the films on sapphire were postannealed, only emissions from the CB → VO transition appeared. Thus, VO is the primary defect in films on sapphire, whereas every type of intrinsic defect (Zni, Oi, VO, and VZn) builds up in ZnO films on glass. Electronically excited modifications induced by argon plasma irradiation were investigated in order to discriminate the influence from that of thermal processes. After prolonged plasma exposure, emissions corresponding to CB → VO (540 nm) and Zni → VB (420−470 nm) transitions predominated as a result of preferential sputtering of oxygen atoms. The improvement in electric conduction by the plasma treatment is attributed to hydrogen atoms trapped at newly created VO sites.