Hui-Ling Kao

Epitaxial AlN Thin Film Surface Acoustic Wave Devices Prepared on GaN/Sapphire Using Low-Temperature Helicon Sputtering System

High-quality epitaxial AlN films have been deposited on GaN/sapphire using helicon sputtering at a temperature of 300 °C. The surface acoustic wave (SAW) characteristics, in terms of insertion loss, stopband rejection, and electromechanical coupling coefficient, of SAW devices fabricated on AlN/GaN/sapphire are much superior than those fabricated on GaN/sapphire. The investigation of environmental effects, including temperature and relative humidity, shows that the ambient stability can be improved with the deposition of an AlN film on GaN/sapphire. An oscillator fabricated using an AlN/GaN/sapphire-based SAW device was developed for application in sensors. The composite structure of AlN on GaN may bring about the development of high-frequency components, which integrate and utilize their semiconducting, optoelectronic, and piezoelectric properties.

Investigation of layered structure SAW devices fabricated using low temperature grown AlN thin film on GaN/sapphire

Epitaxial AlN films have been grown on GaN/sapphire using helicon sputtering at 300 degrees C. The surface acoustic wave (SAW) filters fabricated on AlN/GaN/sapphire exhibit more superior characteristics than those made on GaN/sapphire. This composite structure of AlN on GaN may bring about the development of high-frequency components, which integrate and use their semiconducting, optoelectronic, and piezoelectric properties.

Deep-UVsensors based on SAW oscillators using low-temperature-grown AlN films on sapphires

High-quality epitaxial AlN films were deposited on sapphire substrates at low growth temperature using a helicon sputtering system. SAW filters fabricated on the AlN films exhibited excellent characteristics, with center frequency of 354.2 MHz, which corresponds to a phase velocity of 5667 m/s. An oscillator fabricated using AlN-based SAW devices is presented and applied to deep-UV light detection. A frequency downshift of about 43 KHz was observed when the surface of SAW device was illuminated by a UV source with dominant wavelength of around 200 nm. The results indicate the feasibility of developing remote sensors for deep-UV measurement using AlN-based SAW oscillators.

Room temperature fabrication of ZnO/ST-cut quartz SAW UV photodetector with small temperature coefficient.

Room-temperature fabricated ZnO/ST-cut quartz is adopted for SAW ultraviolet (UV) photodetector. The ST-cut quartz substrate and ZnO layer are used for SAW excitation and photodetection, respectively. High resolution x-ray diffraction (XRD) and photoluminescence (PL) measurement indicate that high quality ZnO films can be deposited on ST-cut quartz using radio frequency (RF) sputtering. As the SAW devices under UV illumination (6 mW/cm(2)), a downshift in frequency of about 35 KHz can be observed. The observed small temperature coefficient of frequency (TCF) indicates that SAW devices exhibit good temperature stability. The results present feasibility of using ZnO/ST-cut quartz SAW photodetectors in ultraviolet region.

Structural and optical properties of low temperature grown AlN films on sapphire using helicon sputtering system

AlN thin films have been deposited directly on c-plane sapphire substrates at low temperatures by a helicon sputtering system. The structural quality of AlN epitaxial films was characterized by x-ray diffractometry and transmission electron microscopy. The films exhibit smooth surface with root-mean-square roughness as small as 0.7 nm evaluated by atomic force microscope. The optical transmittance spectra show a steep absorption edge at the wavelength of 200 nm and a high transmittance of over 80% in the visible range. The band-edge transition (6.30 eV) of AlN film was observed in the cathodoluminescence spectrum recorded at 11 K. The spectral response of metal–semiconductor–metal photodetectors constructed with AlN/sapphire reveals 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 the epitaxial growth of AlN on sapphire substrates and the incorporation of the AlN films in the surface acoustic wave devices and the optical devices at deep ultraviolet region.