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aPhys. Status Solidi C 7, No. 7–8, 1829–1831 (2010) / DOI 10.1002/pssc.200983616

Piezoelectric actuated epitaxially grown AlGaN/GaN-resonators F. Niebelschuetz1*, K. Brueckner1, K. Tonisch1, R. Stephan1, V. Cimalla2**, O. Ambacher2, and M. A. Hein1 1 Institute of Micro- and Nanotechnologies, Ilmenau University of Technology, Ilmenau, Germany 2 Fraunhofer Institute for Applied Solid-State Physics IAF, Freiburg, Germany

Received 7 October 2009, accepted 2 December 2009 Published online 2 June 2010

Keywords AlGaN/GaN, MEMS resonators, piezoelectricity, acoustic vibrations * Corresponding author: e-mail florentina.niebelschuetz@tu-ilmenau.de, Phone: +49 3677 69 3352, Fax: +49 03677 69 3355 ** e-mail volker.cimalla@iaf.fraunhofer.de, Phone: +49 761 5159 304, Fax: +49 761 5159 71304

In this work we present a novel concept of piezoelectri-cally actuated MEMS resonators based on AlGaN/GaN-heterostructures. The 2DEG, which is confined at the AlGaN/GaN interface, serves as back electrode for pie-zoelectric actuation and read-out. Using the inverse pie-zoelectric effect longitudinal in-plane mechanical oscilla-tions could be exited by applying a RF signal to the drive contacts. Through the charges, which were generated by the direct piezoelectric effect a sensor signal could be de-tected at the sense contacts of the integrated resonator

structure. This enables the measurement of longitudinal resonant frequencies up to the 4th mode with values from 3 to 63 MHz. The investigations were carried out in vari-ous ambient pressure conditions, which exemplarily demonstrate the sensitivity of the quality factor to envi-ronmental parameters. Thereby the AlGaN/GaN-resonators showed the highest sensitivity in the region of viscous damping, which leads to the possibility of sensor applications concerning the property determination of gaseous or even liquid surrounding conditions.

© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction As a result of the combination of pyro- and piezoelectric properties in group III-nitrides, AlGaN/GaN-heterostructures are a promising material sys-tem for sophisticated and highly integrated sensor concepts [1]. Current leading applications for group III-nitride de-vices include high power/high temperature devices [2], light emitting diodes and photo detectors in the UV-range [3] and high electron mobility transistors (HEMT) [4]. However, the AlGaN/GaN-heterostructure, known from HEMTs, offers further promising possibilities for micro-electromechanical systems (MEMS) [1].

Until now piezoelectric actuated MEMS resonators have been realized by multilayer beams containing the pie-zoelectric layer, for example ZnO, AlN or PZT and metal-lic layers as top and back electrode. This actuation type is favourable due to the possibility of downscaling devices while maintaining the electromechanical coupling strength [5]. For the presented novel concept the 2DEG confined in the AlGaN/GaN-heterostructures serves as back electrode for piezoelectric actuation and read-out. In doing so the 2DEG provides both the conductivity to form the back

electrode of a piezoelectric transducer configuration and the sensitivity to enable the sensing of changes in the sur-rounding environment.

In this work we demonstrate the electrical excitation and detection of longitudinal acoustic vibration modes within the processed doubly-clamped AlGaN/GaN-resonators. The acoustic resonances have been excited and detected electrically in the fundamental and higher order modes at frequencies up to 63 MHz. Additionally investi-gations were carried out in various ambient pressure condi-tions, which exemplarily demonstrate the sensitivity of the MEMS resonators to environmental parameters.

2 Design and theory Figure 1 shows the schematic

design of a doubly clamped resonator based on AlGaN/GaN-heterostructures using the 2DEG as back electrode for piezoelectric actuation.

If RF-signals are applied to the drive contacts, a verti-cally orientated electric field is created between the driving top electrode and the conductive 2DEG charge.

1830 F. Niebelschuetz et al.: Piezoelectric actuated epitaxially grown AlGaN/GaN-resonators

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Figure 1 Schematic design of a piezoelectric actuated AlGaN/GaN-resonator using the 2DEG as back electrode.

Due to the inverse piezoelectric effect, a longitudinal

in-plane mechanical oscillation is excited, which can be detected at the sensing electrode using the direct piezoelec-tric effect. These longitudinal vibration modes can be de-scribed by lateral standing bulk acoustic waves, which oc-cur at frequencies where integer multiples of the half wavelength coincide with the geometric dimensions of the resonator. Thus, the resonant frequencies fn of the nth har-monic resonant longitudinal mode are given by equation:

( )lcncfn 2// 11 ⋅== λ , (1)

where n is the mode number, c1 the phase velocity, λ the wavelength and l the beam length [6]. The phase velocity c1 depends on the excited wave mode and can be estimated as:

GaNGaNEc ρ/1 = , (2)

with EGaN as Young’s modulus and ρGaN as mass density of the dominating beam material GaN. In case of very thin beams, i.e. a thickness t much smaller than the lateral acoustic wavelength λ, this phase velocity c1 is representa-tive for Lamb wave resonators [7] to which the doubly clamped AlGaN/GaN-resonators can be referred to.

3 Fabrication The Al0.31GaN0.69/GaN-heterostructures (30/650 nm)

have been epitaxially grown on Si-substrates by metal or-ganic chemical vapour deposition using SiN interlayers for stress reduction within the heterostructure [8]. Optical li-thography was used to define bridges with beam lengths l of 10 to 1000 µm and widths of 2 to 10 µm, respectively. An annealed and lift off patterned Ti/Al/Ti/Au layer sys-tem (20/80/30/100 nm) serves as back electrode for con-tacting the 2DEG and a Ti/Au layer system (10/50 nm) as top electrode [9]. A combination of a chlorine based ani-sotropic dry etching process in an inductive plasma system and a fluorine based dry etching process in an electron cy-clotron resonance system was utilized to pattern and re-lease the free standing AlGaN/GaN-resonators. The whole AlGaN/GaN MEMS process was developed to not affect the electrical properties of the integrated 2DEG [10]. Fig-ure 2 shows an array of in this way processed Al-GaN/GaN-resonators.

Figure 2 SEM-images showing an array of AlGaN/GaN-resonators (left) and magnification of one of the shortest realized AlGaN/GaN-resonators of 20 µm in length (right).

4 Experimental results First these structures were used to determine the reso-

nant frequencies and quality factors of the longitudinal vi-bration modes. For the piezoelectric actuation and read-out the AlGaN/GaN-resonators were contacted electrically by coplanar probes. The measurements inside of a vacuum wafer prober allow the variation of the ambient measuring conditions from 10-5 mbar to 103 mbar [6].

Figure 3 shows the resonant frequencies up to the fourth longitudinal mode with dependence on the beam length, from 175 ≤ l ≤ 1000 µm. The double-logarithmic scaling was chosen to emphasize the dependence of the resonant frequency on the beam length: fn~l-b with a char-acteristic exponent b. According to Fig. 3, the linear fit of the slopes resulting in values of b close to 1 following Equ. (1).

So the observed resonant frequencies can be described as 0th-order symmetric Lamb wave [11]. This assumption is supported by the determined value of c1 = 7500 m/s for the AlGaN/GaN-resonators, which is in very good agree-ment with results from literature obtained from measure-ments of AlGaN layers of a slightly lower tensile stress level [12].

Figure 3 Resonant frequency fn of the n-th longitudinal vibration mode in vacuum, for 1 ≤ n ≤ 4, versus beam length l on double-logarithmic scales. Solid lines represent the linear fit of the slopes resulting in values of b close to 1.

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The observed small deviation from the inverse propor-tionality, expected by Equ. (1), is caused by the dispersion relation of the wave mode, implying decreasing phase ve-locities for increasing frequencies.

Additionally the influence of the ambient pressures on the quality factor of the AlGaN/GaN-resonators were ana-lyzed. Therefore, electrical response was measured around the fundamental resonant frequencies f1 of 3.794 MHz and 7.536 MHz for the 1000 μm and the 500 μm long beam, respectively. The quality factors were determined by curve fitting assuming a Lorentzian frequency response superim-posed by a frequency-independent background signal. For the resonators analyzed in Fig. 4, the Q factors were de-termined to decrease from 2800 in vacuum to 1250 at nor-mal ambient conditions.

It is evident that the resonant vibration and with that the quality factor Q is unaffected at pressures below 5 mbar, where intrinsic losses and the molecular flow are the dominating damping effects, respectively. In the region above 5 mbar the damping mechanism of viscous flow de-creases the quality factor with a comparable sensitivity was already being observed for contour-mode MEMS resonators where longitudinal vibration modes are also ex-cited [13]. The sensitivity for longitudinal modes appears lower compared to flexural modes in the molecular domain (Q ~ p-1) [14] due to the smaller mechanical deflections.

The high Q factors maintained in gaseous and pre-sumably in liquid environments enables a much easier read-out of sensor signals, which makes the longitudinal modes advantageous for sensing applications in such envi-ronments despite the observed lower sensitivity.

The coupling of mechanical energy into the substrate at the clamping points seems to be a possible source for losses, which could explain the reported higher Q-factors for the flexible supported contour-mode devices and should be taken into account for the next beam designs.

Figure 4 Pressure dependence of the quality factor Q of the fun-damental longitudinal vibration mode. Measurements were car-ried out for 5 µm wide and 500/1000 µm long resonator beams.

5 Conclusion For the first time piezoelectrically actuated

AlGaN/GaN-resonators were presented, using the 2DEG as back electrode of the piezoelectric transducer. The epi-taxial growth and the piezoelectric actuation and read-out allow scaling and integration with high flexibility, which is a condition precedent to sensor applications. Acoustic resonances have been observed in the fundamental and higher order modes at frequencies up to 63 MHz Addition-ally investigations were carried out in various ambient pressure conditions, which exemplarily demonstrate the sensitivity of the quality factor to environmental parame-ters, whereby the AlGaN/GaN-resonators showed the highest sensitivity in the region of viscous damping.

Acknowledgements This work has been funded by the German Research Foundation (DFG), Priority Program 1157 ‘In-tegrated electroceramic functional structures’ (Grants HE3642/2 and AM105/2) and Priority Program 1165 ‘Nanowires and Nano-tubes’ (Grant CI148/2).

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