| Summary: | 博士 === 中原大學 === 電子工程研究所 === 102 === In this study, Aluminium Nitride thin films have been deposited directly on sapphire substrates at low temperatures (300-500℃) by a helicon sputtering system. The structural quality of AlN films was characterized by X-ray diffractometry and transmission electron microscopy (TEM). The full-width at half maximum (FWHM) of the (0002)-AlN rocking curve decreases with the growth temperature and is ranged within 0.58°-0.94°. The result suggests that the quality of the AlN films can be improved by increasing temperature. The films exhibit smooth surface with root-mean-square roughness as small as 0.6-0.7 nm evaluated by atomic force microscope. Even though the lattice-mismatch between AlN and sapphire is as high as 13.3%, the AlN epitaxial films have been obtained. The single crystalline characteristics of AlN film were confirmed by X-ray φ-scan reflection and SAD patterns of TEM. The growth mechanism of this large lattice-mismatched system can be explained by using the domain matching epitaxy (DME) model and the fourier-filtered image of the interface of AlN/sapphire. The dislocation density of about 10^10 cm^-2, which is low enough for optoelectronic applications, was calculated from the bright-field and dark-field TEM images of the sample. The band-edge transition (6.30 eV) of AlN film close to its theoretical value of energy bandgap was observed in the cathodoluminescence spectrum.
The metal-semiconductor-metal (MSM) photodetectors were fabricated on AlN/sapphire with various electrode spacing and were analyzed by the I-V characteristics. The low dark current of the AlN MSM devices was about 795 fA biased at 20V and was close to that of the best AlN MSM device fabricated by MOCVD in the reported literature, indicating the good quality of our AlN films. The ideality factor of 1.03 and effective Schottky barrier height of 1.01 eV derived from the dark current, reveal a good quality of Schottky contacts. The ratio of the photocurrent to dark current is more than two orders of magnitude with the radiation of the D2 lamp, showing fairly good sensitivity to the deep UV light. It is noted that the photocurrent increases almost linearly with the incident optical power. The linearity of the photoresponse is essential for photodetectors. The spectral response of the device displays the peak responsivity at 200 nm and a UV/visible rejection ratio of about two orders of magnitude.
The results of this low temperature deposition suggest the feasibility of epitaxial growth of AlN on sapphire substrates and the incorporation of the AlN films in optical devices at deep ultraviolet region.
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