Crystal growth of undoped ZnO films on Si substrates under different sputtering conditions
Introduction
Zinc oxide, a wide direct band gap semiconductor, attracts as much attention as GaN in the optoelectronics research field. ZnO not only has the same crystal structure as GaN, but also has strong exciton binding energy of 60 meV [1], which is 2.4 times that of GaN. Therefore, it has many potential applications in short-wavelength light emitting devices such as light emitting diodes (LEDs) [2], photodetectors [3], electroluminescence devices and the next generation UV semiconductor laser. ZnO films have been grown by many methods, such as chemical vapour deposition (CVD) [4], molecular beam epitaxy (MBE) [5], [6], [7] and metal organic CVD (MOCVD) [8], [9], [10]. We adopt sputtering for the growth of ZnO films. Sputtering methods have several advantages. They can deposit large area films of well-controlled compositions economically and the growth rate is high enough for thick films and low enough for ultrathin films by changing the sputtering rate [11].
Now, sapphire substrates have been widely used to grow ZnO films. We use silicon as substrate because silicon is not only of interest for the integration of optoelectronic devices but also cheaper and easier to cleave in comparison to sapphire. In this paper, ZnO films were grown on (0 0 1) Si substrate by sputtering. The effects of the radio frequency (RF) power and O2/Ar flux gas ratio in the sputtering were studied.
Section snippets
Experiments
ZnO films were deposited on (0 0 1) silicon substrates by RF magnetron sputtering. A 99.999% purity Zn target with 100 mm diameter and 4 mm thickness was used. Silicon substrates were cleaned by ultrasonic with a sequence of methylbenzene, acetone and alcohol and etched in the mixture solution of a H2SO4:H3PO4=3:1 at 160°C for 10 min. The distance between the substrate and the target was 140 mm. After the chamber of the coating system were pumped to a pressure of about 2×10−3 Pa, a 99.995% pure
Structure of films
Fig. 1 showed the X-ray diffraction (XRD) spectra of ZnO films at 300, 400, 500, 600 and 650 W, respectively, when k=1:1. All films had a high ZnO (0 0 2) diffraction peak, which indicated that the samples were strongly c-axis oriented. As the RF power increased, ZnO (0 0 2) diffraction peak intensity of the samples was enhanced. However, an increase of the RF power to 650 W caused the degradation of (0 0 2) to the peak intensity at 600 W. The values of full-width at half-maximum (FWHM) for the (0 0 2)
Conclusion
ZnO films were deposited on Si (0 0 1) substrates by RF magnetron sputtering. It was found that all ZnO films had strongly c-orientation and the samples had better crystal quality when the RF power was increased and the value of k was changed. When k=3:1 and the RF power was 600 W, the FWHM of ZnO (0 0 2) diffraction peak was as small as 0.3°. The intense luminescence, including two stronger UV emission peaks (at 3.30 and 3.24 eV) and the deep-level emission, had been observed at room temperature.
Acknowledgements
This work was supported by NSFC-RGC (No. 59910161983) and NSFC (No. 60177007, 60176026) and 863 project with contract number of 2002AA311130.
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