Journal of the Korean Physical Society, Vol. 68, No. 3, February 2016, pp. 448∼451 Similar Effects of the Electric Field and Annealing on the Near-band-edge Photoluminescence in ZnO Films Vadim Sh. Yalishev, ∗ Shavkat U. Yuldashev, Ziyodbek A. Yunusov and Tae Won Kang † Quantum-Functional Semiconductor Research Center, Dongguk University, Seoul 04620, Korea (Received 21 December 2015, in final form 30 December 2015) The response of the near-band-edge photoluminescence (PL) emission of ZnO thin films to an- nealing and the electric field’s action was investigated. These processes separately caused similar changes in the PL spectrum. The donor bound exciton emission at 3.36 eV, which is attributed to bulk defects, demonstrated invariance to any exposure, while the intensity of the 3.33-eV emis- sion line decreased after annealing in nitrogen gas and was restored after annealing in an oxygen atmosphere. On the other hand, application of an electrical field during laser illumination resulted in the same change in the PL spectrum. The transition of defects related to the 3.33-eV emission from a radiative to a non-radiative state (and inversely) through the capture (release) of electrons was proposed as the mechanism responsible for the observed changes in the optical properties. The desorption of oxygen from the surface of the ZnO film during annealing in N2 or the motion of pho- togenerated electrons toward the surface during laser illumination were suggested to be the cause of this capture process. PACS numbers: 78.20.Jq, 78.55.Et, 78.68.+m Keywords: ZnO films, Photoluminescence, Electric field and annealing effects DOI: 10.3938/jkps.68.448 I. INTRODUCTION The electrical and the optical characteristics of semi- conductors are often defined by their surface properties. Surface treatment is well known to be able to change significantly the surface conductivity. ZnO semiconduc- tor, which has been suggested as a potential material for use in various electrical and optical devices [1,2], has electrically active surface whose conductivity varies with ambient conditions [3,4]. Annealing in different environ- ments alters the conductivity in ZnO samples due to the modifications of the surface states [5,6]. Oxygen atoms absorbed on the ZnO surface are known to capture free electrons, which results in the creation of a negative sur- face charge and the formation of a depletion layer with reduced conductivity [7–9]. Due to heating, oxygen des- orbs, and the captured electrons return to the crystal, restoring the prime conductivity of the near-surface re- gion. On the other hand, the existence of a surface elec- tron accumulation layer has been revealed in ZnO [3, 4, 10]. The presence of this conducting surface channel has been suggested to be related to the type-conversion effects observed in epitaxially grown p-type ZnO films. Very little is known about the surface states in ZnO; therefore, thorough investigations on controlled ZnO sur- ∗ E-mail: yalvad@gmail.com † E-mail: twkang@dongguk.edu faces have to be performed in order to assess the possible formation of a surface electron accumulation layer. Be- cause photoluminescence (PL) often originates near the surface of a material, a PL analysis is an important tool in the characterization of surfaces. On the other hand, the investigation of PL signals under an electric field is a useful method of understanding the optical and the electronic properties of the materials. In the present work, we studied change in the PL sig- nals from ZnO films after they had been annealed in N 2 gas or had been exposed to an electric field. Both these actions were found to result in identical modifications of the near-band-edge (NBE) PL spectrum. In contradis- tinction to the emission related to bulk defects, whose intensity was almost unchanged, the 3.33-eV transition line was equally responsive to these actions, indicating the same change in the state of defects located in the near-surface region. II. EXPERIMENTS AND DISCUSSION The ZnO films studied in the present work were grown on SiO 2 /Si substrates (1 × 1 cm 2 ) with thickness of 400 nm by using a pulsed laser deposition technique with a KrF excimer laser (λ = 248 nm) at a repetition rate of 5 Hz. The deposition was carried out at an oxygen pres- sure of 100 mTorr while the substrate temperature was -448-