Elsevier

Applied Surface Science

Volume 252, Issue 2, 15 October 2005, Pages 420-424
Applied Surface Science

Single violet luminescence emitted from ZnO films obtained by oxidation of Zn film on quartz glass

https://doi.org/10.1016/j.apsusc.2005.01.018Get rights and content

Abstract

The photoluminescence (PL) emission properties of ZnO films obtained on quartz glass substrate by the oxidation of Zn films were studied. The strong single violet emission centering about 413–424 nm was observed in the room temperature PL spectra of the ZnO films. The intensity of violet emission increased and the peak position shift from 424 to 413 nm with increasing oxygen pressures. The violet emission was attributed to the electron transition from the valence band to interstitial zinc (Zni) level under low oxygen pressure conditions (50–500 Pa). Under high oxygen pressure conditions (5000–23,000 Pa), both interstitial zinc (Zni) and zinc Vacancy (VZn) were thought to be responsible for the violet emission.

Introduction

One important advantage of ZnO is that it is a II–VI semiconductor of wurtzite structure with a wide direct-bandgap of 3.3 eV [1] at room temperature. The most unique property of ZnO is its large exciton binding energy, 60 meV, which is about three times larger than that of ZnSe and of GaN [2]. Because of this large binding energy, the exciton is stable at room temperature even in bulk crystals. Owing to these properties, ZnO can be used as UV or blue emitting materials. Generally, the corresponding photoluminescence (PL) spectra obtained from ZnO thin films show defect-related deep-level emissions [3], [4], [5], [6] (green emission around 510 nm and red emission around 650 nm) as well as ultraviolet (UV) near-band-edge emission around about 380 nm, which strongly depends upon the preparation methods and growth conditions. Recently, many methods were used to obtain ZnO thin films, including metal-organic chemical vapour deposition (MOCVD) [7], molecular beam epitaxy (MBE) [8], rf magnetron sputtering [9], pulse laser deposition (PLD) [10] and the oxidation of the metallic Zn [11]. In these reports main investigation concentrated on the ultraviolet (UV) emission and yellow-green emission, only few reports were published on violet light emission [12], [13], [14] and there was no the report on single violet emission until now.

In this paper, we report a simple method for achieving polycrystalline ZnO thin films with single violet luminescent properties by the oxidation of metallic Zn films above melting (419 °C) temperature. In the room temperature PL spectra, we have observed a strong about 413–424 nm violet emissions without any accompanying the ultraviolet (UV) emission and deep-level emission. We also discussed the main defects responsible for single violet emission of ZnO film by investigating the relationships between the photoluminescence properties and the XRD chart of the ZnO film formed under different oxygen pressures.

Section snippets

Experimental

Metallic Zn films were deposited on quartz glass substrates by pulse laser deposition. The substrates were rinsed three times in acetone with ultrasonic vibration, each rinsed for 15 min, and then rinsed in ethanol for 15 min before they were put into the deposition chamber. The base pressure of the deposition chamber was kept at 5 × 10−4 Pa, and the deposition time of 15 min was maintained. The applied laser energy density was measured to be about 7.6 J/cm2. Zn (99.99% purity) targets were ablated by

The structure of ZnO film

Fig. 1 shows the XRD traces (θ–2θ) of the ZnO thin films on quartz glass substrate obtained by the oxidation of metal Zn film at 550 °C with the oxygen pressure from 50 to 23,000 Pa. It can be seen that the sample annealed at 550 °C with the oxygen ambient of 50 Pa shows a pattern of ZnO peaks, and very small Zn (1 0 1) peak was detected. When oxygen pressure increases above 500 Pa, Zn (1 0 1) peak disappears. The XRD patterns for the ZnO films indicated that they possess a polycrystalline hexagonal

Conclusion

Single violet emission centering about 413–424 nm was observed in the room temperature PL spectra of ZnO films prepared by oxidation of Zn films under 550 °C. From the above discussion, it may be concluded that the interstitial zinc defects in the ZnO films were responsible for the violet emission of ZnO film obtained by oxidation under low oxygen pressure and the zinc vacancy defects in the ZnO films were responsible for the violet emission of ZnO film obtained by oxidation under high oxygen

Acknowledgments

The authors would like to thank Prof. Y.C. Liu for many supports at optical experiment in Northeast Normal University and gratefully acknowledge the Foundation of National Key Basic Research and Development Program (973) no. 2004CB619301 for providing support of this work.

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