Pulsed Laser Deposition of High Quality ZnO Thin Films
- PDF / 1,378,366 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 31 Downloads / 246 Views
PULSED LASER DEPOSITION OF HIGH QUALITY ZnO THIN FILMS S. AMIRHAGHI*, V. CRACIUN*, F. BEECH*, M. VICKERS**, S. TARLING**, P. BARNES** AND I. W. BOYD* *Electronic & Electrical Engineering, University College London, London WC1E 7WE, UK "**Departmentof Crystallography, Birkbeck College, London WC1E 7HX, UK ABSTRACT Thin films of ZnO have been grown on silicon and glass substrates by the pulsed laser deposition method. The effects of the oxygen partial pressure, substrate temperature and laser wavelength on the structural and optical properties of the films have been studied. The KrF excimer laser (at 248 nm) was found to produce better quality thin films than the frequency doubled Nd:YAG laser (532 nm). Layers produced at substrate temperatures as low as 3001C were c-axis oriented with a FWHM value for the 002 XRD reflection less than 0.20 and exhibited optical transmission higher than 80% in the visible region. INTRODUCTION ZnO is a wide band-gap (Eg=3.2 eV) n-type semiconductor with a hexagonal wurtzite structure that is optically transparent in the visible region and electrically conductive. It also exhibits large piezoelectric and piezooptic coefficients and is relatively cheap and non-toxic. These qualities have attracted considerable interest in recent years - thin ZnO films being used in acoustoelectric and acoustooptic devices [1, 2], solar cells [3], liquid crystal displays [4], gas sensors [5] and window coatings [6]. Many deposition techniques for obtaining ZnO thin films have been reported, including sputtering [1, 2], chemical vapour deposition (CVD) [7], laser CVD [4], metalorganic CVD [3], and chemical spraying [8]. The pulsed laser deposition (PLD) technique, first used in 1965 [9], was applied to ZnO film growth in 1983 [10], where a pulsed CO 2 laser was shown to produce highly oriented films. In this paper we present a systematic investigation of the structural properties, morphology and optical properties of ZnO thin films deposited by the PLD technique at different oxygen partial pressures and substrate temperatures. EXPERIMENT The experimental set-up for laser ablation and the preparation of the targets have already been described elsewhere [11]. ZnO pellets of 99.99% purity were ablated either by a frequency-doubled Nd:YAG laser operating at 532 nm with a pulse duration of 4 ns or by a KrF excimer laser operated at 248 nm with a pulse duration of 20 ns. In both cases, the laser fluence was set at 2.5-3 J/cm 2 and the repetition rate was 5 Hz. The deposition chamber was evacuated to pressures in the 10-6 - 10-7 torr range. During deposition, oxygen partial pressures from between 2*10-6 to 2*10-2 torr were employed. The thin layers were deposited either on Coming glass 7059 or (100)Si substrates which were chemically cleaned prior to film growth.
Mat. Res. Soc. Symp. Proc. Vol. 285. @1993 Materials Research Society
490
(-4
0 0
31 35
*40 45
50 55
60
65
70- 75
20 (degrees) Fig. 1. XRD patterns of ZnO films deposited at 400°C for different deposition times. 0 Each film was deposited at a
Data Loading...
