Low Intensity Ultraviolet light detection using a ZnO/LiNbO 3 SAW Oscillator Sanjeev Kumar*, Parmanand Sharma, Vikas Gulia and K. Sreenivas Department of Physics and Astrophysics, university of Delhi, Delhi-110007, India *Email: Sanjeev@scientific.net ABSTRACT Fabrication and characterization of a UV light detector based on surface acoustic wave (SAW) oscillator principal utilizing a ZnO/LiNbO 3 bilayer structure is reported. The response characteristics of the SAW oscillator is found to depend on V cc, the power supply to the oscillator circuit. The SAW oscillator output showed a decrease in amplitude and a downshift shift in frequency of oscillation under a UV light illumination due to acousto-electric interactions between the photo generated charge carriers and the electric potential associated with SAW s. The change in amplitude and the shift in frequency showed a linear variation with UV light intensity. A low level UV intensity ~ 450 nw/cm was easily detectable. Results show its immense application in wireless UV light detection. Keywords: Photoconductivity, acousto-electric interactions, ZnO/LiNbO 3 SAW device, UV detectors 1. INTRODUCTION Surface acoustic wave devices (SAW) are well recognized and widely accepted for the efficient processing of electrical signals. This has stimulated the research on many new devices for signal processing applications. A SAW device can be utilized as an optical detector if the optical radiations bring a change in the electrical characteristics at its surface due to the interaction of optically generated charges with the electric potential associated with the surface acoustic waves, or more specifically the aousto-electric interactions. SAW device structures built on either photoconducting piezoelectric bulk substrates (GaN), or alternate layered structure fabricated with photoconducting overlayer (e.g. ZnO) on a bulk piezoelectric substrate such as LiNbO 3, LiTaO 3, or PZT are therefore useful for making SAW optical detectors. A good combination of the optoelectronic properties of a photoconducting overlayer and the piezoelectric substrate is therefore necessary for making sensitive light detectors. Recently, Palacious et al. 1 demonstrated the remote collection of photogenerated carriers swept by acoustic waves in a GaN based SAW device and, Ciplys et al. reported on the ultraviolet photoresponse of a GaN SAW device and observed a downshift in frequency ~ 60 KHz with UV illumination. Sharma et al. 3 reported a downshift in frequency (170 KHz) for the ZnO/LiNbO 3 hybrid SAW device, and an upshift in frequency (~ 10KHz) for a ZnO thin film SAW device operating at higher harmonics (310 MHz) under UV illumination. The large shift in frequency in the case of ZnO/LiNbO 3 structure was attributed to the large K of the piezoelectric LiNbO 3 substrate, and was utilized successfully for UV light detection by configuring the hybrid SAW structure in the form of an oscillator 4. A change in the amplitude of the oscillator output due to acoustoelectric interaction under UV illumination was used as the detecting signal 4. It is known that with an increase in the conductivity of the sensing layer the SAW velocity decreases monotonically and the attenuation goes through a peak. 5 The changes in the SAW velocity ( v) and the attenuation ( ) due to acousto-electric interactions are given by Γ 5 v v o = K 1 (1 + σ / σ ) m (1) K π 1 Γ = () λ (1 + σ / σ ) m
where, K, λ, v, σ, and σm are the coupling coefficient, wavelength, SAW velocity on free surface, and sheet o conductivity and critical sheet conductivity, respectively. Since the changes in v and Γ depend on the photoconductivity of the ZnO overlayer in the ZnO/LiNbO 3 bilayer structure, a much stronger acoustoelectric effect can be expected if a highly photoconducting ZnO overlayer is used along with a high K material like LiNbO 3, and the resulting changes in the SAW frequency can be detected easily. In the present work, the fabrication and performance of a SAW UV light detector configured in the form of an oscillator using ZnO/LiNbO 3 bilayer structure have been investigated. The changes in the amplitude and the frequency shift of the output signal of the SAW oscillator have been examined. The influence of V cc, the power supply to the oscillator circuit is shown to be significant for achieving enhanced responsivity and a higher sensitivity.. EXPERIMENTAL Photoconductivity in sputtered ZnO films depends on the microstructural defects incorporated during the growth process, and can be enhanced by depositing films using a highly unbalanced magnetron sputtering technique 6. In the present study ZnO thin films were deposited on a commercially available LiNbO 3 SAW filter (central frequency 37 MHz), using RF magnetron sputtering technique by lowering the strength of the central magnets in a 6 magnetron electrode. A metallic Zn target was sputtered in 100% O gas ambient at a pressure of 50 mtorr, and RF power of 600 Watts. The fabricated ZnO/LiNbO 3 hybrid SAW device was connected in the positive feedback loop of high frequency amplifier to design a SAW oscillator. Details on the construction of a SAW oscillator circuit have been reported elsewhere 4. The gain of the high frequency amplifier was controlled by the power supply V cc to the electronic circuitry. It is observed that at V cc =11 V, the gain of the amplifier was appropriate to overcome the losses in the ZnO/LiNbO 3 hybrid structure, and the circuit oscillates at a frequency of 36.3 MHz. 3. RESULTS AND DISCUSSIONS 3.1 Photoconductivity of ZnO overlayer in ZnO/LiNbO 3 bilayer SAW structure The 71nm thick ZnO overlayer used in the ZnO/LiNbO 3 hybrid structure was tested for its photoconductivity in the presence of UV light (λ=365nm; Intensity =1.0mW/cm ). The photoresponse curve shown in Fig. 1 reveals a Figure 1: Photoresponse curve of ZnO overlayer under UV light illumination (1.0 mw/cm ); inset: AFM image (area 00 nm x 00nm) of a 71 nm thick photoconducting ZnO overlayer, variation of photocurrent with UV light intensity for ZnO overlayer. continuous rise in the photocurrent due to the desorption of the chemisorbed oxygen at the surface of the ZnO film under UV illumination, and the observed slow recovery is entirely due to trapping of the charge carriers between the shallow level and the conduction band 6. The photocurrent was found to vary linearly with UV intensity as shown in
Fig. 1. Thickness of ZnO overlayer (71nm) was found sufficient, because at a higher thickness a mass loading effect was evident in the frequency response of the SAW device. Grain size and film thickness are reported to influence the photoconductivity in ZnO films 6,7. Atomic force microscopy study [Fig.1, inset ] revealed that the average grain size of the films in the present work (71nm) was ~ 40nm, and is much smaller than the earlier reported 6 grain size of 00nm for the photoconducting ZnO films that were quite thick (1µm). The thinner ZnO films in the present study showed an improved responsivity to UV light due to a large surface atom to bulk volume ratio. The improvement in photoresponse is in agreement with the report of Takahashi et al. 7 where thinner films of ZnO exhibited a large photoresponse in comparison to the thick films. 3. Performance of ZnO/LiNbO 3 SAW oscillator based UV detector ZnO/LiNbO 3 SAW oscillator was resonating at a frequency of 36.3MHz. The SAW oscillator was characterized by studying the effect of UV light illumination on its output signal in frequency and amplitude mode. 3..1 Amplitude mode The amplitude of the SAW oscillator output was found to be very sensitive to UV light and showed a significant decrease in amplitude (from 54 to 43 mv) under a UV illumination of ~ 450 nw/cm at V cc =11 V. When the UV intensity was increased to 3.54 µw/cm and the V cc was left unchanged, the oscillator output seized to zero in a time less than 5 sec due to an increase in the attenuation of the SAW device, and the condition required for the oscillation was not met. 4 Since the amplitude change of the SAW UV oscillator was so fast, the changes in the amplitude within a 5 sec. exposure were recorded. The SAW oscillator was tested at various V cc and at higher UV intensity. It was noted that the change in oscillator amplitude varies almost linearly with UV intensity [Fig. ]. Inset of Fig. shows a linear variation of the change in oscillator amplitude under very low UV intensity (0.4-.4 µw/cm ) at V cc = 1 V. The present UV detector in amplitude mode is highly sensitive and low level of UV intensity can be easily detected. Figure : Variation of change in SAW oscillator output amplitude with UV light intensity at different V cc. Inset shows the variation of amplitude change at V cc = 1 V with low UV intensity (<.4 µw/cm ) 3.. Frequency mode The frequency spectrum of the SAW oscillator output showed two major modes and some other modes [Fig. 3]. When UV light was illuminated on the ZnO/LiNbO 3 SAW oscillator, a shift in frequency as well as a decay in amplitude was clearly visible as shown in Fig. 3. A frequency downshift of ~ 8 KHz was observed when UV illumination (~34 µw/cm ) was chopped after 5 sec. The observed frequency response of the SAW oscillator dies out under UV illumination, whenever the V cc is kept low enough to obtain a higher sensitivity. A minimum shift in frequency ~ 69 Hz was observed under a UV illumination of 450 nw/cm at V cc = 18.5 V[Fig. 3]. It was observed that at a lower V cc, the decrease in amplitude was very rapid as compared to the frequency shift under UV illumination, and the amplitude seized to zero (within 3 sec) much before than the actual downshift in frequency could be recorded. The oscillator output shows a linear variation of the shift in frequency with UV
intensity at V cc = 4 V [Fig. 4 ]. It can be seen that the SAW oscillator exhibits higher frequency responsivity at lower V cc as shown in Fig. 4. The device exhibits interesting characteristics through the linear variation of shift in frequency and finds potential application in wireless UV light detection. Figure 3: Frequency spectrum of SAW oscillator based on ZnO/LiNbO 3 bilayer showing a downshift in frequency ~8 KHz under UV light intensity (34 µw/cm ); ~69 Hz at UV light intensity (450 nw/cm ). Figure 4: Variation of shift in frequency of SAW oscillator output with UV light intensity at V cc = 4 V. variation of frequency responsivity with V cc 4. CONCLUSIONS In summary, the fabrication and characterization of a UV detector based on ZnO/LiNbO 3 SAW oscillator have been demonstrated. The shift in frequency of oscillations and the decrease in amplitude of the oscillator output are found be very fast. A low level UV intensity of 450 nw/cm could be easily detected and shows promise for the fabrication of a wireless UV light detector. The results show that the present SAW UV detector can be operated in either amplitude mode or frequency mode, and can be effectively utilized for the development of a wireless detector for low level UV intensity measurements. ACKNOWLEDGEMENTS The authors wish to thank the Ministry of Information Technology, Govt. of India for financial support, and the author (SK) thanks DST, India for a research fellowship (JRF).
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