Deep-UV Sensors Based on SAW Oscillators Using Low-Temperature

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IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control ,

vol. 58, no. 8,

August

2011

Correspondence Deep-UV Sensors Based on SAW Oscillators Using Low-TemperatureGrown AlN Films on Sapphires Chipta P. Laksana, Meei-Ru Chen, Yen Liang, An-Jyeg Tzou, Hui-Ling Kao, Erik S. Jeng, Jyh Shin Chen, Hou-Guang Chen, and Sheng-Rui Jian Abstract—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.

I. Introduction

D

etection of UV radiation is widely used in military, environmental, and industrial applications such as flame monitors, water purification systems, money counting and counterfeit-detection machines, sunlight exposure meters, photochemical phenomena detection, etc. The detectors generally use photodiodes, which currently are sensitive to other light sources in addition to the UV rays and, therefore, the UV rays can be detected only by the cooperation of the photodiodes and UV-ray pass filters. To fabricate lightweight, low-power consumption, lownoise devices, and to avoid the use of filters that are needed for traditional photodetectors to accomplish visibleblind operation, UV detectors based on wide-band-gap semiconductors have been widely studied during the last decade. AlN is very robust to harsh environments, because it is a wide-band-gap, radiation-resistant, temperature- and chemical-resistant material. The largest direct band gap (6.2 eV) in III-nitride materials has made it a good candidate for use in a visible-blind (wavelength < 400 nm) UV detector. The unique UV optoelectronic properties of AlN have not yet been superseded by other materials, especially in the deep UV range (wavelength < Manuscript received July 15, 2010; accepted June 6, 2011. This work was supported by the National Science Council of R.O.C. under grant No. NSC 97-2221-E-033-052. C. P. Laksana, M.-R. Chen, Y. Liang, A.-J. Tzou, E. S. Jeng, and J. S. Chen are with the Department of Electronic Engineering, Chung Yuan Christian University, Chung-Li, Taiwan. H.-L. Kao is with the Department of Electronic Engineering, Chung Yuan Christian University, Chung-Li, Taiwan (e-mail: hlkao@ cycu.edu.tw). H.-G. Chen and S.-R. Jian are with the Department of Materials Science and Engineering, I-Shou University, Kaohsiung, Taiwan. Digital Object Identifier 10.1109/TUFFC.2011.1997 0885–3010/$25.00

200 nm) which is important for satellite communications and lithography applications. In addition, the excellent piezoelectric and acoustic properties of AlN, such as high acoustic wave velocity of 6000 m/s, high piezoelectricity, and electro-mechanical coupling (k2), have recently motivated an intense study of SAW devices. The greatest advantage of incorporating SAW devices in the sensors, in addition to their characteristics of small size, high reliability, low cost, high quality, and good reproducibility, is that the RF output signal allows remote wireless operation. Several UV detectors using GaN- and ZnO-based SAW devices have been demonstrated [1]–[5]. A perturbation by the SAW of a photo-generated charge carrier system in the materials leads to a variety of interesting effects, which have potential applications in new types of UV sensors. In this paper, we report, for the first time to our knowledge, deep-UV (wavelength < 200 nm) detection using AlN-based SAW devices. The growth of AlN films has been reported using various techniques such as molecular beam epitaxy [6], chemical vapor deposition [7], [8], laser evaporation [9], and reactive sputtering [10]–[12]. The sputtering method often offers the advantages of lower growth temperature; however, to obtain c-axis-oriented AlN thin films, the process of conventional magnetron sputtering usually results in a rough surface morphology caused by plasma damage [13]. In our earlier works, high-quality epitaxial AlN films have been obtained on GaN/sapphire substrates using a helicon sputtering system [14]. In this report, AlN films were deposited on c-plane sapphire substrates directly at temperatures of 300°C to 450°C. SAW devices were fabricated on AlN/sapphire. The SAW characteristics such as frequency response, insertion loss, and side lobe rejection have been investigated and discussed. UV detectors using AlN-based SAW oscillators are demonstrated. The results reveal the feasibility of using the SAW devices on AlN/ sapphire for remote deep-UV detection. II. Experimental AlN thin films with thickness of 1 µm were deposited on c-plane sapphire substrates using a helicon sputtering system. The helicon sputtering method is a magnetron sputtering system enhanced with an inductively coupled radio frequency plasma, in which the sputtered atoms can be ionized efficiently with the coil power, and the plasma can be sustained in a gas pressure lower than conventional magnetron sputtering. The mean free path of the atoms during sputtering can be increased to be larger than the target to substrate distance, therefore results in a smooth surface and good crystalline structure. The cathode of aluminum target was 5 cm in diameter with purity of 99.999% and kept 15 cm away from the substrate holder.

© 2011 IEEE

laksana et al.: deep-UV sensors using low-temperature-grown AlN films on sapphires

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TABLE I. Deposition Conditions of AlN Films Using Helicon Sputtering System. Base pressure Ar:N2 Sputtering pressure RF Power Coil power Growth temperature