Interaction of surface acoustic waves and ultraviolet light in ZnO films
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Interaction of surface acoustic waves and ultraviolet light in ZnO films Parmanand Sharma,a) Sanjeev Kumar, and K. Sreenivas Department of Physics and Astrophysics, University of Delhi, Delhi–110007, India (Received 10 October 2002; accepted 9 December 2002)
The frequency response of a 37 MHz bulk LiNbO3 surface acoustic wave (SAW) filter with a 200-nm-thick ZnO overlayer exhibited a downshift in the frequency with ultraviolet (UV) light due to acoustoelectric interactions between the photo-generated carriers in the semiconducting ZnO and the surface acoustic waves. In contrast, a 36 MHz ZnO thin film SAW delay-line with insulating ZnO films exhibited an upshift in the frequency. The response was more pronounced at higher harmonics (130–315 MHz) and was attributed to changes in the elastic/dielectric properties in the upper surface layer of ZnO. A linear change in the frequency with UV intensity shows immense applicability for wireless ultraviolet sensor applications.
Surface acoustic wave (SAW) sensors offer many attractive features. They are highly sensitive, and their frequency output allows the fabrication of wireless sensors for remote sensing applications. In a SAW device the measured response arises from perturbations in the wave propagation characteristics. Changes in the wave velocity and attenuation caused by acoustoelectric interactions and/or mass loading effects have been exploited for fabricating a variety of SAW sensors.1–5 Acoustoelectric interactions on a piezoelectric material can be utilized effectively for light-sensing applications when the near-surface region of a piezoelectric is made optically active by a suitable overlayer, so that the photo generated carriers can strongly interact with the piezoelectric fields and potentials accompanying the surface wave. SAW interaction with light has stimulated a new application area for developing integrated devices.6,7 Recently, Ciplys et al.8 reported the effect of ultraviolet (UV) radiation on the frequency response of a GaNbased SAW oscillator operating at 221.34 MHz. A downshift in the oscillator frequency under UV illumination was observed, and this was attributed to the interaction of optically generated charge carriers in GaN with SAWs. Several authors have reported that the relative change in
a)
Address all correspondence to this author. Present address: Department of Materials Science-Tmfy-MSE, Royal Institute of Technology, S-100 44 Stockholm, Sweden. e-mail: [email protected] J. Mater. Res., Vol. 18, No. 3, Mar 2003
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SAW velocity (⌬V/V) due to acousto-electric interactions in a piezoelectric semiconductor depends on the coupling coefficient (K), and the change can be less then or equal to K2/2.1,9,10 Materials like LiNbO3 with a high electromechanical coupling coefficient are most useful, and when combined with other materials possessing optoelectronic properties, can lead to an interesting class of light-sensing SAW devices. In recent years optoelectronic properties of zinc o
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