Characteristic analysis of temperature-dependent photocurrent and photoluminescence spectra in as-implanted p -ZnO crystals

The temperature-dependent photoresponse properties of As-doped p-ZnO crystals prepared by using the As-implantation method have been investigated by using photocurrent (PC) and photoluminescence (PL) spectroscopy. Two peaks, A and C, associated with band-to-band transitions were observed in the PC spectra. From the PL spectra, the neutral acceptor bound exciton emission, (A0, X), due to As-related acceptors was extracted. The temperature dependences of these peaks were well described by E g (T) = E g (0) — (3.10 × 10−4)T2/(143 + T). Also, the values of E g (T) were estimated to be 3.412, 3.418, and 3.468 eV at the valence-band states of Γ9(A), Γ7(B), and Γ7(C), respectively. Thus, the parameter of the crystal-field splitting was directly extracted by means of PC spectroscopy, and its value was 0.0503 eV at 13 K. However, the behavior of the PC was different from that generally observed in other semiconductors. The PC intensities progressively decreased with falling temperatures in regions below 150 K. From plots of log J PC vs. 1/T, where J PC is the PC current-density, a dominant level was observed at mid-range temperatures. By comparing the PC and the PL results, we confirmed an activation energy of 21.5 meV, corresponding to a partial dissociation energy of the (A0, X) emission. Consequently, trapping centers, which are imperfections related to As ions in As-implanted p-ZnO crystals, are thought to limit the PC intensity with decreasing temperatures below 150 K.