Investigation on the origin of green luminescence from laser-ablated ZnO thin film
Introduction
Wide band gap semiconductor, ZnO, has been studied widely for applications, such as transparent electrodes, gas sensors, varistors and surface acoustic wave devices, owing to interesting optical, acoustical and electrical properties [1], [2], [3], [4]. Recently, there has been a surge of interest in an application of ZnO thin film for short wavelength light-emitting devices due to attractive properties such as wide band gap energy at room temperature (3.3 eV), large exciton binding energy (60 meV) and optical transparency [5], [6]. Many techniques, such as sputtering, ion beam-assisted reactive deposition, metal-organic chemical vapor deposition, molecular beam epitaxy and pulsed laser deposition (PLD) have been employed to deposit (0 0 2) highly textured ZnO films. Among the various growth techniques of ZnO thin film, PLD has advantages of being able to not only employ a relatively high oxygen pressure but also to achieve high-quality crystalline films with relatively high deposition rate at low temperatures due to the high energy (∼100 eV) of the ablated particles in the laser-produced plume [7].
ZnO thin film shows photoluminescence (PL) property in UV and green regions, and it depends on growth technique and growth condition. Therefore, luminescent properties of ZnO thin films including UV and green emissions have been studied extensively. The emission by excitons was concluded as an origin of UV emission in ZnO thin films [8], [9]. However, the exact origin of green emission is still in dispute, although defects such as oxygen vacancies, oxygen interstitials, zinc vacancies, zinc interstitials and oxide antisites are suggested as an origin of green PL by many other groups [10], [11], [12], [13].
In this study, the green luminescent property of ZnO thin film was systematically investigated by changing ambient pressures and by using different annealing gases. The forming gas in annealing process was used to form oxygen-deficient ZnO thin film, since hydrogen gas could desorb oxygen from the surface and the grain boundary of the film [14]. The characterization of annealed ZnO thin films was performed by the analysis of PL spectra and Hall measurement.
Section snippets
Experiment
ZnO thin films were deposited on (0 0 1) sapphire substrates at various ambient gas pressures by PLD. The deposition chamber was initially evacuated to the pressure in the range from 10−6 to 10−7 Torr by turbomolecular pump, and finally filled with 99.99% pure oxygen in the pressure range from 200 to 500 mTorr. The focused laser radiation illuminated on a rotating ZnO target at 33.7° incident angle. The energy density of a laser was 2.5 J/cm2. The target was a 99.999% pure ceramic ZnO target, and
Results and discussion
Fig. 1 shows XRD spectra of ZnO film deposited at 400 °C and 350 mTorr. All films were found to be c-axis-oriented, exhibiting only the (0 0 2) XRD peak of ZnO.
Fig. 2 shows green PL spectra of ZnO films deposited at various oxygen pressures. The intensity of green luminescence of the films deposited under 350 mTorr decreased as the oxygen pressure increased. Among the various techniques for the growth of ZnO thin films, PLD is found to be a very unique technique for the growth of oxide materials
Conclusions
In conclusion, the origin of green luminescence of ZnO thin film was investigated by the analysis of the structural, electrical and luminescent properties of ZnO thin films deposited at various ambient pressures and annealed in different gases of forming gas (N2:H2=95:5) and O2 gas. From the result of PL spectroscopy and Hall measurement, it is well agreed that the origin of the green luminescence of ZnO thin film comes mainly from the defect centers generated by the oxygen vacancies.
Acknowledgements
This work was supported by Korea Research Foundation Grant (KRF-2001-041-E00152).
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