III-NITRIDE BASED ULTRAVIOLET SURFACE ACOUSTIC WAVE SENSORS Introduction Due to a wide energy band gap, AlN, GaN, and their alloys are well suited for the fabrication of ultraviolet (UV) sensors, particularly, of visible-blind and solar-blind photodetectors. These materials possess strong piezoelectric properties making them attractive for surface acoustic wave (SAW) device applications. For sensing purposes, it is very convenient to use the SAW delay- line oscillator, which has been first demonstrated in 1969 [*] as temperature-sensitive device. Since then, various SAW sensors has been developed but not those for UV. *J. D. Maines, E. G. S. Paige, A. F. Saunders, A. S. Young, Electron. Lett. 5, 678 (1969). Making use of the unique combination of wide energy gap and piezoelectric properties, we were the first to implement the GaN- based surface acoustic wave III-nitride-based SAW oscillator and to apply it for UV sensing. D. Ciplys, R. Rimeika, M. S. Shur, S. Rumyantsev, R. Gaska, A. Sereika, J. Yang, M. Asif Khan, Appl. Phys. Lett. 80, 2020 (2002) Sapphire substrate ULTRAVIOLET RADIATION GaN layer AMPLIFIER SAW DELAY LINE SAW Input IDT Output IDT RF SPECTRUM ANALYZER Schematics of the SAW-based UV sensor Remote signal pickup is possible Transmission characteristic of the SAW delay line f 0 = 200 MHz K 2 = 0.1 % Frequency (MHz ) Transmission (dB) K 2 = 0.1 % GaN on sapphire K 2 = 0.5 % Bulk a-AlN Basic principles Amplitude condition for oscillations: amplifier gain must exceed the total insertion loss of the SAW delay line. Phase condition: the phase shift around the loop must be Calculation parameters: transducer aperture W = 1 mm, number of transducer electrode pairs N = 100, dielectric constant  = 10, source and load resistances R L = R s = 50 Ohm. K 2 is the electromechanical coupling constant. where L is the distance between IDTs, V is the SAW velocity,  is the phase shift introduced by the amplifier, cables and transducer circuitry. Any change in V or  leads to the change in oscillator frequency f. SAW oscillator frequency up-shift due to UV illumination of SAW transducers UV light: from mercury lamp through 330 nm filter Illuminated: entire surface of the sample, including transducer area Frequency shift is due to the change of SAW transducer parameters by UV D. Ciplys, R. Rimeika, A. Sereika, R. Gaska, M. S. Shur, J. W. Yang, and M. A. Khan, Electron. Lett. 37, 545 (2001). Possibilities of solar-blind operation SAW oscillator line widths measured under different illumination conditions We attribute the differences in the line widths to the different noise spectra of the artificial and natural UV sources. These differences might serve for the development of solar- blind UV sensors. Predicted optical wavelength cut-off as function of AlGaN composition Calculated using bowing parameter b: –0.39 eV for MOCVD-grown layers M. J. Bergmann et al, Appl. Phys. Lett. 75, 67 (1999) 1.08 eV for MBE-grown layers U. Ozgur et al, Appl. Phys. Lett. 79, 4103 (2001) (eV) Separation by wavelength Separation by line width D. Ciplys, R. Rimeika, M.S. Shur, R. Gaska, A. Sereika, J.Yang, M. Asif Khan, Electron. Lett. 38, 134 (2002) SAW line 1 Amplifier 1 UV light Spectrum analyzer Mixer SAW line 2 Amplifier 2 Differential SAW oscillator with improved thermal stability UV-induced frequency down-shift vs. optical power Output signal Schematics 365 nm The SAW oscillator frequency is temperature dependent. The temperature drift can be minimized by using the differential scheme The temperature coefficients of frequency (TCF): GaN on sapphire: -50 to -60 ppm/K H. H. Jeong et al, Physica Stat. Sol. (a) 188, 247 (2001) Bulk AlN: -19 ppm/K G. Bu, D. Ciplys, M. Shur, L. J. Schowalter, S. Schujman, R. Gaska, Electron. Lett. (accepted for publication in 2003) 1 mm 10 mm 1.3 mm Band gap width of GaN 3.4 eV Visible blind operation No frequency shift (with accuracy of 1 %) was observed at optical wavelengths above 400 nm SAW oscillator frequency down-shift due to UV illumination of SAW propagation path UV light: from Xenon lamp Illuminated: area between SAW transducers UV through 330 nm filter Wavelength tuning: 1 nm bandwidth monochromator Optical wavelength dependence of the frequency down-shift Optical power dependence of the frequency down-shift UV light spot spot between transducers Frequency shift is due to the change of SAW velocity by UV via screening the piezoelectric fields by photoconductivity where L 1 is the length of illuminated region, R s is the sheet resistivity. NATO Advanced Research Workshop UV Solid-State Light Emitters and Detectors June 17-21, 2003 Vilnius, Lithuania Acknowledgments The work at RPI was supported by the National Science Foundation (program monitors Dr. U. Varshney and Dr. James Mink); under a subcontract from DARPA (Project Manager Dr. Edgar Martinez and monitored by John Blevins at AFRL, contract F C-5417). The work at SET, Inc. was partially supported by the Office of Naval Research and monitored by Dr. Y.-S. Park. The work at SET, Inc. and USC was partially supported by NASA under contract NAG Authors also acknowledge the support by NATO Expert Visit grant.PST.EV D. Ciplys 1,3, A. Sereika 1, R.Rimeika 1, R. Gaska 2, M. Shur 3, J. Yang 4, M. Asif Khan 4 1 Vilnius University, Physics Faculty, Dept. of Radiophysics, Vilnius, Lithuania 2 Sensor Electronic Technology, Inc., Columbia, SC, USA 3 Rensselaer Polytechnic Institute, Dept. of Electrical, Computer, and Systems Engineering, Troy, NY, USA 4 University of South Carolina, Dept. of Electrical Engineering, Columbia, SC, USA