|
Zinc Oxide (ZnO) has emerged as a perspective material for thin film electronics (TCO), spintronics (DMS of high Tc), and optoelectronics (UV LED). The specific fundamental properties of ZnO (Eg = 3.37 eV, Eex → 60 meV) and the diversity of thin film deposition techniques offer strong prerequisites of a successful development of UV LEDs based on this material. However the problem of intense UV emission from ZnO films still exists: usually, the UV near band edge (NBE) excitonic emission in ZnO is assisted by broad deep level emission (DLE), so called "green-yellow luminescence". The origin of DLE is often explained by the presence of Zn atoms as interstitials (Zni) and/or oxygen vacancy (Vo). Reasonable UV PL could be obtained in the cases of using highly cost substrates, e.g. ScAlMgO4, SiC and high substrate temperature (Ts = 500 — 900°C) or by complicated deposition methods. Usually for UV emission as-grown ZnO films require additional treatment: thermal isochronal (IA), rapid thermal (RTA) or face-to-face (FTFA) annealing. We focused on obtaining UV emission from ZnO films deposited by convenient PEMOCVD technique onto low cost substrates. Films were grown simultaneously on Si(100), Si(111) and SiNx/Si(110) substrates at Ts = 150 / 500 °C. The films were investigated in term of structural (XRD, AFM, SEM) and optical (PL, CL at 5 — 300 K) properties. All the films, deposited at lowest Ts (150 °C) were of low crystal quality, which results in absence of PL. In contrast, films obtained at high Ts (500 °C) were found to be of high crystal quality but their PL spectra displays weak UV and strong broad DLE. The optimal Ts for high quality films was 350 °C while the best UV PL was revealed in ZnO films deposited on SiNx/Si(110): intense narrow peak of only UV NBE was observed at Eg = 3.27 eV with FWHM of 0.13 eV. The effect of IA, FTFA on PL properties is presented. We explain the data obtained in terms of oxygen diffusion and silicon oxidation during film growth and evidenced that by element’s depth analyses. The importance of ZnO stoichiometry for high PL properties is discussed. |