Microstructure and photoluminescence properties of ZnO thin films grown by PLD on Si(1 1 1) substrates
Introduction
One important advantage of ZnO is that it is a II–VI semiconductor of wurtzite structure with a wide direct-band-gap of 3.2 eV [1] at room temperature. Wide and direct band gap semiconductors are of interest for blue and ultraviolet optical devices, such as light-emitting diodes and laser diodes [2], [3], [4]. The other notable advantage of ZnO is its high exciton binding energy (60 meV) at room temperature, which is much higher than that of ZnS (20 meV) and GaN (21 meV). This makes ZnO an ideal material to realize room temperature excitonic devices. Recently, many methods are being used to obtain ZnO thin films, including metal-organic chemical vapor deposition (MOCVD) [5], molecular beam epitaxy (MBE) [6], rf magnetron sputtering [7] and pulse laser deposition (PLD) [8]. Pulsed laser deposition (PLD) technique is more useful in obtaining high quality thin films of metal oxide materials. The plasma fabricated by the pulsed laser ablation is very energetic, and its mobility can be easily controlled by changing processing parameters. For these practical reasons, PLD technique has been wildly applied for the formation of the high quality thin films.
However, in general the excimer lasers with shorter wavelength and high purity ZnO target (99.99% purity) have been widely used to grow ZnO thin films [9], [10], [11]. Few reports were published on laser (pulse duration of 100 ns, wavelength of 1064 nm) and Zn target (99.99% purity) applied by our knowledge. It was reported [12] that the films (with many droplets) were obtained by lasers with wavelength of 1064 nm. However, in our experiment it is possible to obtain relatively high quality ZnO thin films through control the process parameters including laser energy density, oxygen pressure and substrate temperature. In this paper we study the structural and optical properties of ZnO thin films obtained at different substrates temperatures by laser (wavelength of 1064 nm) ablation of Zn target in oxygen active atmosphere.
Section snippets
Experimental
Si(1 1 1) substrate was used as the underlay. Si substrates were rinsed three times in acetone with ultrasonic vibration, each rinse for 15 min; and then rinsed in ethanol for 15 min before they were put into the deposition chamber. The experiments were performed in a deposition system. Before deposition, the vacuum chamber of the deposition system was evacuated by turbo-molecular pump down to 5×10−4 Pa and then filled with oxygen (99.99% purity) at a working pressures of 11 Pa. The laser energy
The structure of ZnO film
Fig. 1 shows the XRD of the ZnO thin films on Si(1 1 1) substrate obtained by laser ablation of Zn target in oxygen reactive atmosphere at different substrate temperature (300∼550 °C) with a fixed oxygen pressure of 11 Pa. Three main diffraction peaks of ZnO thin films appear at 2θ=31.81°, 34.46° and 36.27°, which are corresponding to the (1 0 0) plane, the (0 0 2) plane and the (1 0 1) plane of ZnO thin films, respectively. It is seen that the films obtained with the substrate temperature at 300∼350 °C
Conclusion
ZnO film on Si substrate was obtained by ablation of Zn target in oxygen atmosphere, with a Nd-YAG laser of 1064 nm wavelength as the pulse laser source. The influences of the substrate temperature on the structure and optical properties of ZnO film were studied.
- 1.
ZnO film with mainly (0 0 2) texture formed at the substrate temperature 500 °C, the average grain diameter parallel to the surface of the films was about 240 nm.
- 2.
All samples show a typical luminescence behavior. The intensity and FWHM of UV
Acknowledgements
The authors would like to thank Prof. Y.C. Liu for many discussions and The Advanced Center for Optoelectronics Functional Materials Research, Northeast Normal University for PL measurement.
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