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For that purpose, the interdigital transducers (IDTs) of SAW device with ... Keywords: SAW; GaN; SiC; waveguide; dispersion; triple transit echo; sensor; oscillator ...
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Procedia Engineering 25 (2011) 1101 – 1104

Proc. Eurosensors XXV, September 4-7, 2011, Athens, Greece

AlGaN/GaN Based SAW-HEMT Devices for Chemical Gas Sensors Operating in GHz Range Ivan Rýgera, Gabriel Vankoa*, Tibor Lalinskýa, Martin Valloa, Martin Tomáškab, Adrian Ritomskýc a

b

Institute of Electrical Engineering of the Slovak Academy of Sciences, Bratislava, Slovakia, Slovak University of Technology, Faculty of Electrical Engineering and Information Technology, Bratislava, Slovakia, c Institute of Informatics, Slovak Academy of Sciences, Bratislava, Slovakia,

Abstract This work presents the process technology and high frequency analysis of AlGaN/GaN based SAW (Surface Acoustic Wave) – HEMT (High Electron Mobility Transistor) devices to be applied for chemical gas sensors operating in GHz range. For that purpose, the interdigital transducers (IDTs) of SAW device with submicrometer finger width and spacing were designed and to improve the sensor detection sensitivity. The sensor signal processing electronics containing microwave SAW oscillator was designed, simulated and successfully tested. The automated measurement system for SAW oscillator frequency measurement was built. An integrated SAW-HEMT device for multifunctional, high sensitive and remote gas sensing in the local ambient is also demonstrated.

© 2011 Published by Elsevier Ltd. Open access under CC BY-NC-ND license. Keywords: SAW; GaN; SiC; waveguide; dispersion; triple transit echo; sensor; oscillator;

1.Introduction This work is motivated by the need for a new material basis for chemical and gas sensors operating in extreme conditions. Recently, we have presented a new approach to design and fabrication of such devices using AlGaN/GaN based SAW-HEMT structures [1]. This approach permits to integrate two different principles of sensing and detection of gases or vapor toxic chemicals: - sensing principle based on SAW and AlGaN/GaN HEMT. Moreover, integrated HEMT sensing device can also serve as a

* Corresponding author. Tel.: +421-2-5922-2739; fax: +421-2-5477-5816. E-mail address: [email protected]

1877-7058 © 2011 Published by Elsevier Ltd. Open access under CC BY-NC-ND license. doi:10.1016/j.proeng.2011.12.271

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Ivan Rýger / Procedia Engineering 25 (2011) 1101 – 1104

selective thermal heater. It can increase the operating temperature of chemical absorbing layer as the gate electrode of HEMT. SAW chemical sensors operating in GHz range can be designed and integrated with wireless remote sensing applications. Excellent high sensitive gas detection (0.8 % of a monolayer coverage on the surface) should be achieved for AlGaN/GaN based SAW sensors operating in harsh conditions. To increase selectivity, it is typical to employ an array of SAW devices, with different coatings used to identify specific chemicals or bio-agents. In this work, we present a SAW-HEMT device operating in GHz range. Their ability for gas sensing is also demonstrated. For that purpose, a simple sensor chamber containing the measured gas was built. 2.Experiment An undoped AlGaN/GaN heterostructure grown by metal-organic chemical vapor-phase deposition (MOCVD) on SiC substrate was used to define SAW-HEMT devices. The GaN layer thickness was 1.5 μm. The HEMT device with the chemical absorbing gate layer is integrated in space between IDTs (Fig. 1a, b). SAW signals were excited directly on GaN layer after selective etching of AlGaN barrier layer in the area under IDTs. Two IDTs, each with 20 pairs of interdigital fingers of the width and spacing 1, 0.8 and 0.5 μm were applied. The center to center distance and transducer aperture were 100 Ȝ and 50 Ȝ, respectively. Direct-writing electron beam lithography (EBL) in combination with Ni/Au e-beam evaporation and lift-off were carried out to form the Schottky fingers of the IDTs. A thin laser ablated ZnO layer (~ 10 nm) was also applied immediately after “MESA”-isolation step for surface passivation of SAW-HEMT sensing devices (Fig. 1a).

(a)

(b)

Fig. 1. (a) A cross-section of AlGaN/GaN based SAW-HEMT sensing devices; (b) SEM micrograph of the fabricated input SAW IDT and the part of long gate HEMT

The experiment setup consists of a sensor chamber ensuring constant operating conditions, two input valves, nitrogen source and chamber with ethanol. In sensor chamber, the microwave SAW oscillator sensor electronics was placed. Gas sealing inlet and outlet were fed via rubber. The rubber sealed glass bottle was used as bubbler. Nitrogen inlet tube was immersed to alcohol and outlet tube was placed above level of liquid. The RF output of the oscillator was connected to a frequency counter controlled by a personal computer. A simple program for automated data mining was written. The oscillator consists of a SAW pass-band filter in feedback loop of an amplifier, reflective matching networks and buffer amplifier with pi-attenuators to suppress load influence on measured frequency. Main amplifier uses MMIC Agilent MSA-2086. A stabilizing inductance was added at output of MMIC in order to improve Rollet’s k-stability factor at frequencies above 2 GHz. The low frequency stability was ensured by using small coupling capacitors. To test the circuit designed, we used small signal AC analysis and time-domain TRAN analysis in simulation software HSPICE.

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3.Results and discussion An AGILENT Technologies, E8363B network analyzer with 50 Ohm terminal microprobes was used to measure scattering parameters of the SAW structure. In Fig. 2 we can see the measured transmission coefficient of SAW sensor in band pass filter mode. The frequency and attenuation of peak for structures with IDT width and spacing of 1 Pm are 2,05 GHz and -20 dB, respectively. The filter geometric center frequency is 2,09 GHz. Summary of measured excitation frequencies and phase velocities of main modes for different IDT periods are listed in Table 1. Our measured velocities are in good agreement with theoretically calculated data by Takagaki et. al. [2].

Table 1.– measured dispersive phase velocity Finger width and spacing [μm]

1

0.8

0.5

Ȝ [μm]

4

3.2

2

k.h [rad]

3/4ʌ

15/16ʌ

3/2ʌ

fc [GHz]

1.7485

2.0935

2.9045

vph [m.s-1]

6994.0

6699.2

5809.0

Fig. 2.- Transmission coefficient for different IDT geometries

When measuring samples with three different IDT finger periods, we noticed the dispersion of acoustic velocity due to the waveguide effect of thin GaN layer (with lower acoustic phase velocity) placed on almost semi-infinite SiC substrate (with higher acoustic phase velocity). At short wavelength the phase velocity approaches to Rayleigh velocity of top layer. On the other hand, when scaling the wavelength up, the phase velocity approaches to shear velocity in the semi-infinite substrate [3]. Moreover, if the acoustic waveguide thickness is comparable to wavelength, the multiple modes are excited. An additional peak in transmission coefficient appeared at approximately 3.5355GHz at structures with a width and spacing of 0.5um. The main excitation peak belongs probably to guided Sezawa mode (also known as generalized Rayleigh-type mode) [2]. As seen in Fig. 2, the characteristic is deformed from ideal curve described by function sinc(x). By performing Fast Fourier Transform we obtained transmission coefficient in time domain. Several equidistantly located impulses appeared. These pulses are the result of triple transit echo[4][5][6][7]. If we apply time-gating method to the measured data and select the first excited pulse, the deformation disappears. Low-loss filter employing bi-directional the input and output transducer creates natural FabryPerot resonator with triple transit mode spacing proportional to effective transit length. This effect may cause oscillator mode-hoping due to sharp steps in group delay characteristic. Triple transit echo amplitude falls three times faster with increasing the insertion loss. For this reason, the effect could not be seen in former plasma treated samples with high pass-band attenuation [1]. Measurement procedure (taken from the article [8] and adapted to our needs): x 1. Approximately 1 hour stabilization time for sensor electronics temperature after connecting to the power source is needed. x 2. In next step, valve on nitrogen bottle was opened to completely flush the sensor chamber. Nitrogen pressure was set slightly above atmospheric pressure (1.2bar).

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Ivan Rýger / Procedia Engineering 25 (2011) 1101 – 1104

3. Valve was reverted to ethanol gas mixture inlet and the sensor response was measured. 4. Repeating the steps 2-3.

Fig. 3. The response of sensor to ethanol vapour

From measured sensor response (Figure 3.) we can see that sensor with no chemical absorbing layer coating is also sensitive to condensed ethanol vapors, even if the response is small, but not negligible. SAW sensor exhibits large sensitivity to local atmospheric pressure changes. This is supported by sharp peaks in sensor response affected by pressure change during the valve reverting. 4.Conclusion The process technology and high frequency analysis of AlGaN/GaN based SAW – HEMT devices to be applied for chemical gas sensors operating in GHz range is presented. The frequency and attenuation of peak for structures with IDT width and spacing of 1 μm are 2,05 GHz and -20 dB, respectively. The sensor signal processing electronics containing microwave SAW oscillator was designed, simulated and successfully tested. An integrated SAW-HEMT device for multifunctional remote gas sensing in the local ambient was also demonstrated. Acknowledgement This work was supported in part by the following projects: - Slovak Research and Development Agency under the contracts APVV-0655-07, APVV-0450-10, APVV-0199-10, VEGA project 2/0163/09 and SKFR-0041-09 bilateral project. References [1] T. Lalinský, I. Rýger, G. Vanko, M. Tomáška, I. Kostiþ, Š. Hašþík, M. Vallo, Procedia Engn. 5 (2010), 152-155 [2] Y. Takagaki, P. V. Santos, E. Wiebicke, O. Brandt, H.-P. Scho ¨nherr, and K. H. Ploog, PHYSICAL REVIEW B 66, 155439 (2002) [3] Hess, P., Physics Today, March 2002, pp. 42-47 [4] Komatsu, Y. , Yaganisawa, Y.,IEEE TRANSACTIONS ON ELECTRON DEVICES, MARCH 1977 [5] Wang, W., et. al., Sensors and Actuators B 125 (2007) 422–427 [6] Huang, M. Y, et. al., United States Patent 4237432 [7] De Vries, United States Patent 4392116 [8] Hao, H. C., Sensors and Actuators B 146 (2010) 545–553