Microstructure and Optical Properties of Aluminum Nitride Thin Films
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unique properties. In addition, AIN thin films are transparent in the visible and near infrared regions. Owing to these properties, AIN thin films are also expected for the promising materials in optoelectronics devices. Optical properties of AIN films have been studied using various deposition techniques, such as chemical vapor deposition (CVD) [1,21, reactive sputtering 13-61, excimer laser ablation [71 and ion beam sputtering (IBSD) [8,9]. Most of these researches focused on the influence of deposition conditions on optical properties, such as the optical band gap energy and the refractive index. The present authors synthesized AIN thin films by ion beam assisted deposition (IBAD) method and studied the microstructure and nanometerscale surface morphology [10-13]. We reported that the crystallographic orientation state and surface morphology of AIN films depend on the nitrogen ion beam energy. In other words, the orientation state and morphology of AIN films can be controlled by regulating deposition conditions. Thus, it is attractive to study the dependence of optical properties of AIN films on the ion beam energy. In the present paper, using the IBAD method, AIN thin films are synthesized with changing the ion beam energy and the influence of the 539 Mat. Res. Soc. Symp. Proc. Vol. 403 01996 Materials Research Society
nitrogen ion beam energy on the transmission spectrum and the refractive index of films is studied. EXPERIMENTAL Nitrogen gas (99.999% pure) and aluminum (99.99% pure) were used as ion source and target, respectively. Film synthesis was carried out on the substrates of silicon single crystal for the microstructure study and fused silica glass for optical property measurements. After evacuating the vacuum chamber to about 2.7 x 10- 4 Pa, pure nitrogen gas was introduced to the ionization chamber and nitrogen ions were generated by an arc discharge. Then a nitrogen ion beam was obtained with electric field lenses for focusing and accelerating. The kinetic energy of the nitrogen ions is varied from 0.05
to 1.5 keV and the deposition rate was kept at 0.07 nm/s. The current density was approximately 70 [tA/cm 2 . Aluminum was evaporated by electron bombardment from a 10 kW electron gun and the evaporation rate was monitored by a quartz sensor. The substrate temperature was kept at room temperature. The thickness of the synthesized films was measured with a stylus device (Rank Taylor Hobson Ltd.). The crystallography of the films was determined by X-ray diffraction (XRD) using a Cu target (RINT 2500, RIGAKU Co.). The sample was rotated at the speed of 50 rpm in order to get strong X ray intensities. Optical transmission spectrum was measured in the wavelength region from 220 to 2200 nm using UV-visible spectrometer (JASCO U-best 570). RESULTS AND DISCUSSION Figure 1 shows typical transmission spectra of AIN films, of 300 nm in thickness, synthesized with the ion beam energy of (a) 0.05 keV, (b) 0.5 keV and (c) 1.5 keV. All of the films are transparent in the visible and near infrared region, and the
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