APPLIED PHYSICS LETTERS 90, 213506 共2007兲
Surface acoustic wave device on AlGaN / GaN heterostructure using two-dimensional electron gas interdigital transducers King-Yuen Wong,a兲 Wilson Tang, Kei May Lau, and Kevin J. Chen Department of Electronic and Computer Engineering, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong
共Received 1 March 2007; accepted 30 April 2007; published online 23 May 2007兲 Surface acoustic wave 共SAW兲 devices using two-dimensional electron gas 共2DEG兲 as interdigital transducers 共IDTs兲 on AlGaN / GaN heterostructure has been demonstrated using a planar isolation technique based on the fluoride-based 共CF4兲 plasma treatment technique. The rf characteristics of the SAW filters with planar 2DEG IDTs are compared with SAW devices that are made of metal IDTs or hybrid metal/2DEG IDTs. © 2007 American Institute of Physics. 关DOI: 10.1063/1.2742589兴 The wide-band-gap III-nitride semiconductors, with high surface acoustic wave 共SAW兲 propagation velocity and strong piezoelectric effect, are attractive for SAW devices such as voltage-controlled SAW filters and ultraviolet sensors.1–3 Their inherent chemical inertness and biocompatibility also make them attractive for SAW-based biosensors.4 In addition, high-quality single-crystal heterostructures have been obtained and used to fabricate high-performance high electron mobility transistors 共HEMTs兲. Thus, monolithic integration of SAW devices with active HEMTs on AlGaN / GaN heterostructures can potentially lead to compact system-on-chip solutions for wireless transceivers and sensors. This integration requires the removal5 or depletion6,7 of two-dimensional electron gas 共2DEG兲 in the heterojunction channel in the SAW device region. The presence of the charge conductivity in 2DEG channel can screen the electric field and prohibit the acoustoelectric transductions in the interdigital transducers 共IDTs兲. In all the GaN-based SAW devices reported up to date, metal films are deposited and patterned to form metal IDTs. However, the metal IDTs introduce several inherent effects associated with the mass loading and geometric discontinuities.8,9 First, the average phase velocity under the IDTs would be reduced from the free-surface value and a reduction in the resonance frequency would be observed. Moreover, the amplitude and phase ripples across the passband could be increased due to signal reflection from the metal IDTs. In this work, we demonstrate planar SAW filters using 2DEG-based IDTs on AlGaN / GaN heterostructure 共shown in Fig. 1兲 by fluoride-based 共CF4兲 plasma treatment technique,10–12 which is used to pattern the 2DEG IDTs on a planar surface without removing the top AlGaN layer. The SAW devices were characterized by on-wafer S-parameter measurement. The rf characteristics of the SAW filters with planar 2DEG IDTs were compared with metal IDTs. The AlGaN / GaN epitaxial heterostructures used in this work were grown on sapphire substrates in an Aixtron 2000HT metal-organic chemical vapor deposition system. The structure consisted of a low temperature GaN nucleation layer, a 2.5 m thick unintentionally doped GaN buffer layer, and an AlGaN barrier layer with nominal 35% Al coma兲
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position. The barrier layer was composed of a 2 nm undoped spacer, a 17.5 nm carrier supplier layer with Si doping at 3 ⫻ 1018 cm−3, and a 3 nm undoped cap layer. Device fabrication commences with Ohmic contacts that connect the 2DEG IDTs and probing pads. These Ohmic contacts were formed by e-beam deposition of Ti/ Al/ Ni/ Au and subsequent rapid thermal annealing at 850 ° C for 30 s. The 2DEG IDTs were then defined by photolithography and the subsequent CF4 plasma treatment in a reactive ion etching system with rf plasma power of 300 W for 100 s. In the plasma-treated regions, a large amount of immobile negatively charged fluorine ions were incorporated into the AlGaN layers near the surface, depleting the electrons in the channel.11 This approach was able not only to achieve the same level of active device isolation as the mesa formation but also to permit the patterning of 2DEG and the acoustoelectric transductions in the 2DEG-IDTs.12 At last, the SAW filter with Schottky metal IDTs were patterned by photolithography followed by e-beam evaporation of Ni/ Au and lift-off. The width w and the spacing p of all the IDTs are both 2.5 m. The wavelength is 10 m 共4 ⫻ p兲. Both IDTs for signal excitement and sensing have 20 pairs of single unapodized electrodes. The center-to-center separation of the two IDTs L is 2.4 mm and the IDT aperture lengths are 200 m. The propagation direction of the SAW was chosen ¯ 0兴 of the sapphire substrate. For comparison to be along 关112 purposes, we had fabricated SAW filter pairs 共that include exciting IDT and receiving IDT兲 with three configurations: metal-metal, metal-2DEG, and 2DEG-2DEG. The micrographs of the SAW filters with metal-metal and 2DEG-2DEG configurations are shown in Figs. 2共a兲 and 2共b兲, respectively. The 2DEG-2DEG IDTs were fabricated on a planar surface without any groove mark and the IDTs appeared invisible. We characterized the metal-2DEG hybrid IDT SAW filter. The on-wafer rf characteristics of the fabricated SAW filters between 400 and 1200 MHz at an input power of 10 dBm were measured by an Agilent 8753E network analyzer. The frequency and time-domain transfer characteristics were plotted in Fig. 3. The center frequency was determined as the maximum peak of the frequency response. The center frequency and 3 dB bandwidth were 754.28 and 20.85 MHz. Without impedance matching at input and output, the insertion loss and sidelobe attenuation were 74.05 and 9.46 dB, respectively. The phase velocity vph evaluated from the
0003-6951/2007/90共21兲/213506/3/$23.00 90, 213506-1 © 2007 American Institute of Physics Downloaded 23 May 2007 to 143.89.46.81. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
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Appl. Phys. Lett. 90, 213506 共2007兲
FIG. 1. 共Color online兲 Schematic of the proposed 2DEG IDT SAW filter on AlGaN / GaN/sapphire structure.
maximum peak 共at 754.28 MHz兲 in the frequency response yields a value of 7542.8 m / s 共vph = f ⫻ = 754.28 MHz ⫻ 10 m兲. The first Sezawa and Love modes were identified in Fig. 3, according to the calculated dispersion relations in a GaN/Sapphire layered structure.13 For the time-domain transfer response, the traveling time of the main travel path T was 427.29 ns. The experimental group velocity was 5614.16 m / s 关Vg = L / T = 共2.4 mm/ 427.49 ns兲兴. The phase velocity was higher than the group velocity because the acoustic wave propagated in the AlGaN / GaN/sapphire dispersive layered structure.14 The transfer 共S21兲 and reflection 共S11兲 characteristics of the SAW filters fabricated using the metal-metal and 2DEG2DEG IDTs are plotted in Fig. 4. The Rayleigh mode was observed in addition to the Sezawa mode and the Love mode in the metal-metal IDTs because of the smaller insertion loss. The center frequency and the insertion loss of the Sezawa mode evaluated from the S21 responses were 747.04 MHz and 45.08 dB for the metal-metal IDTs and 761.25 MHz and 86.05 dB for the 2DEG-2DEG IDTs, respectively. The center
frequency of the metal-metal IDT SAW device was lower than that of the metal-2DEG IDT by 0.96% and that of the 2DEG-2DEG IDT by 1.9%, respectively. This center frequency shift is caused by the difference in density and elastic properties between the metal IDTs and the substrate.8 It was reported that the phase velocity of the propagating waves in the layered structure is decreased by about 1.5% due to the mass loading in metallized regions.9 Since no mass loading is presented in the 2DEG IDT design, its effect on center frequency is negligible. It should be noted, however, that the insertion loss of the 2DEG IDTs was higher than that of the metal IDTs. The higher insertion loss is mainly due to the resistive loss of the 2DEG IDTs. The leakage current between the IDT fingers is ruled out because a good isolation has been achieved in the planar process using the CF4 plasma treatment11,12 and the S11 of 2DEG IDT SAW device also shows similar open circuit characteristics as the metal IDT SAW device. Further increase in the 2DEG density can lead to reduced insertion loss in the 2DEG IDTs. Time-domain measurement using the Agilent 8753E network analyzer was used to characterize the reflection effects of the metal and 2DEG IDT SAW devices. The reflection time was calculated from the time-domain transmission measurement. The measured reflection coefficients in time domain from 800 ns to 1 s 关共the traveling time of the main travel path兲 ⫻ 2兴 were then transformed to the frequency domain by the Fourier transform. The reflection coefficient magnitudes from SAW devices with metal-metal and metal2DEG configurations in frequency domain are compared in Fig. 5. The average reflection coefficients of the frequency responses were, 6.18⫻ 10−4 for metal-to-metal IDTs, 3.84 ⫻ 10−4 for metal-to-2DEG IDTs, respectively. There are four distinct types of reflection mechanisms: piezoelectric short-
FIG. 3. 共Color online兲 Frequency and time transfer characteristics of the FIG. 2. SEM images of SAW filters with 共a兲 metal IDTs and 共b兲 2DEG SAW filter with metal-2DEG IDTs. The first Sezawa and Love modes are IDTs. labeled S and L, respectively. Downloaded 23 May 2007 to 143.89.46.81. Redistribution subject to AIP license or copyright, see http://apl.aip.org/apl/copyright.jsp
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FIG. 5. 共Color online兲 Reflection coefficients of the SAW filters with metalto-metal IDTs and metal-to-2DEG IDTs.
the plasma treatment can be used to pattern 2DEG IDTs on a planar surface. Such a process also allows the monolithic integration of AlGaN / GaN HEMT and SAW devices for integrated SAW filters and sensors. Radio-frequency characteristics were investigated. The 2DEG IDT SAW devices are free of the features that are associated with the mass loading and geometric discontinuities in the conventional metal IDTs. 1
FIG. 4. 共Color online兲 Transfer 共S21兲 and reflection 共S11兲 characteristics of SAW filters composed of 共a兲 metal-metal IDTs and 共b兲 2DEG-2DEG IDTs. The first Sezawa, Love, and Rayleigh modes are labeled S, L, and R, respectively.
ing, electrical regeneration, geometric discontinuity, and mass loading.15 The piezoelectric shorting and electrical regeneration are negligible for GaN, as the electromechanical coupling factor of GaN 共0.066%兲 共Ref. 16兲 is small. The higher reflection coefficient of the metal IDTs was caused by the mass loading and geometric discontinuity. Furthermore, the reflection coefficient ripple of metal-to-metal IDTs increased in the passband of the IDTs as the reflection of the acoustic waves from the reflection IDTs was at maximum at the center frequency. However, the fringe reflection can be reduced by the 2DEG IDTs, as they present no mass loading and geometric discontinuities. In conclusion, GaN-based SAW devices using 2DEG IDTs have been demonstrated on AlGaN / GaN heterostructure with a planar process that takes advantage of the CF4 plasma treatment. Without removing the top AlGaN layer,
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