# Co-Integration of High-Frequency Devices on Epitaxial ScAlN for Tunable RF SAW Filters

Nagesh Bhat, Seif El Whibi, Edouard Lebouvier, N. Defrance, Jean-Claude de Jaeger, Zahia Bougrioua, Florian Bartoli, Valentina Gallardo-Mödinger, Maxime Hugues, Yvon Cordier, et al.

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Nagesh Bhat, Seif El Whibi, Edouard Lebouvier, N. Defrance, Jean-Claude de Jaeger, et al. Co-Integration of High-Frequency Devices on Epitaxial ScAlN for Tunable RF SAW Filters. Wocsdice Exmatec 2025, Jun 2025, Cadiz, Spain. [hal-05467164](https://hal.science/hal-05467164v1)

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# Co-Integration of High-Frequency Devices on Epitaxial ScAlN for Tunable RF SAW Filters

Nagesh Bhat<fnref n="1" />,*, Seif El Whibi<fnref n="1" />, Edouard Lebouvier<fnref n="1" />, Nicolas Defrance<fnref n="1" />, Jean-Claude De Jaeger<fnref n="1" />, Zahia Bougrioua<fnref n="1" />, Florian Bartoli<fnref n="2" />, Valentina Gallardo-Mödinger<fnref n="2" />, Maxime Hugues<fnref n="2" />, Yvon Cordier<fnref n="2" />, Sami Hage-Ali<fnref n="3" />, Ulrich Youbi<fnref n="3" />, Thierry Aubert<fnref n="3" />, Omar Elmazria<fnref n="3" />, Marie Lesecq<fnref n="1" />

## Abstract

With the rise of 6G and growing data demands, high-performance materials for RF components are essential. Scandium-doped aluminum nitride (ScAlN) offers a promising barrier layer for next-generation high electron mobility transistors (HEMTs), providing enhanced carrier density. This piezoelectric material, with high electromechanical coupling coefficients (K<fnref n="2" />), is also a strong candidate for surface acoustic wave (SAW) filtering applications, particularly in the 1-10 GHz range, due to high SAW velocities. We demonstrate the monolithic integration of ScAlN/GaN HEMTs with interdigital transducers (IDTs), highlighting the potential of this material system for high-performance RF applications.

## Introduction

ScAlN (Scandium-doped AlN) has emerged as a promising barrier material for GaN-based high electron mobility transistors (HEMTs), offering significant advantages due to its high polarization. The ternary compound ScₓAl₁-xN provides a bandgap range from 6.1 eV to 3.9 eV, depending on the scandium content, with a theoretically higher two-dimensional electron gas (2DEG) density of 5×10¹<fnref n="3" /> cm⁻<fnref n="2" /> when lattice-matched with GaN. This results in enhanced device performance in comparison to traditional AlGaN, InAlN and AlN barriers [1,2,3] are very promising, although challenges such as phase separation and compositional fluctuations persist. ScAlN-based HEMTs were primarily demonstrated on SiC substrates, showing potential for RF and power applications [4]. In addition to HEMTs, ScAlN is highly suited for piezoelectric applications, particularly in surface acoustic wave (SAW) and bulk acoustic wave (BAW) devices. ScAlN exhibits a piezoelectric modulus (d₃₃) five times greater than AlN [5,6], making it ideal for sensors, transducers, and RF filters. The electromechanical coupling (K<fnref n="2" />) of ScAlN is significantly higher than AlN, with values of up to 7.8% for SAW [7] and 19% for BAW devices [8], enhancing performance in wireless sensing applications. Finally, high SAW velocities can be reached with ScAlN, making it suitable for high-frequency applications, in the GHz range [9].

In this work, we demonstrate monolithic integration of ScAlN/GaN HEMTs with SAW IDTs addressing various technology challenges. Our future objective is 1.5% frequency tuning in the 3-10 GHz range unlike various platforms previously reported, as listed in the table 1.

<table><caption>Table 1: Various state-of-the-art SAW filter platforms</caption><thead><tr><td>Approach</td><td>Frequency</td><td>Tuning</td><td>Bias</td></tr></thead><tbody><tr><td>Ni-ZnO / GaN [10]</td><td>1,25 GHz</td><td>0,9%</td><td>25 V</td></tr><tr><td>CN trans. / LiNbO<sub>3</sub> [11]</td><td>20MHz</td><td>2,5%</td><td>40V</td></tr><tr><td>Graphène/LiNbO<sub>3</sub> [12]</td><td>11MHz</td><td>0,1%</td><td>1V</td></tr><tr><td>AlGaN-GaN/ZnO [13]</td><td>586 MHz</td><td>0,78%</td><td>40V</td></tr></tbody></table>

## Material growth and device technology

### Material growth

The whole device heterostructure was grown by ammonia-assisted molecular beam epitaxy (NH3-MBE) on a 2-inch AlN-on-Sapphire template. An AlGaN layer is first regrown on the template followed by a highly resistive 2 µm GaN buffer is deposited at 800°C. A 7 nm Sc0.15Al0.85N barrier is grown at 670°C. To minimize intermixing and alloy scattering, a 1.6 nm AlN exclusion layer is introduced between ScAlN and GaN. Finally, a 2 nm GaN cap layer is added to protect ScAlN from oxidation during device processing.

### Device technology

Due to thermal budget, ohmic contacts fabrication is the critical part of the co-integration. So, the process starts by Ti/Al/Ni/Au (12/200/40/100nm) metal evaporation followed by rapid thermal annealing at 850°C for 30 seconds in N₂ ambience. An in-situ argon ion beam etching is performed prior to metal deposition to eliminate impurities and reach the 2DEG. The device isolation is performed by N⁺ ion implantations. Then the Ni/Au (40/300nm) gate electrode is evaporated and annealed at 400°C for 20 mins. This annealing improves the reliability and robustness. After the processing of HEMTs, interdigital transducers (IDTs) are fabricated on each side of the transistor, as shown in the fig. 1. The IDTs are defined using MMA/PMMA bilayer resist and e-beam lithography. After the evaporation of Al metal electrodes, the lift-off is performed using SVC 14 remover solution for 1 hour at 70°C to ensure high-resolution IDT formation. Finally, a Ti/Au pad is deposited to access transistor and IDT contacts.

<footnote n="1">¹Univ. Lille, CNRS, Centrale Lille, Univ. Polytechnique Hauts-de-France, UMR 8520 - IEMN-Institut d'Electronique de Microélectronique et de Nanotechnologie, F-59000, Lille, France</footnote>

<footnote n="2">² Université Côte d'Azur, CNRS, CRHEA, rue B. Gregory, 06560 Valbonne, France</footnote>

<footnote n="3">³ Université de Lorraine - CNRS, Institut Jean Lamour UMR 7198, Nancy, 54000, France</footnote>

<footnote>*nagesh.bhat@univ-lille.fr</footnote>
<figure><img src="image_2.png" /><figcaption>**Figure 1.** Schematic showing co-integration of HEMT and IDTs on MBE grown heterostructure.</figcaption></figure>

## Results and Discussion

A contact resistance of 0.6 ± 0.1 Ω.mm was extracted from transmission line measurements (TLM). The propagation of SAW through the transistor requires a contact stack as thin as possible to minimize waves reflection losses on it. This requires further contact thickness and annealing optimizations to achieve thin and low-resistance ohmic contacts. The I-V measurements performed on the Gated-TLM validate the transistor operation with a drain current of ~ 0.7 A/mm at V<sub>G</sub>= 1V and a ON-resistance R<sub>ON</sub> of ~ 5 Ω.mm, as shown in the Fig. 2. From Van Der Pauw measurements, a sheet carrier concentration of 2.76 ± 0.06 x 10<sup>13</sup> cm<sup>-2</sup>, and sheet resistance of 327 ± 15 Ω/□ were measured, while carrier mobility of 693 ± 21 cm<sup>2</sup>/V·s is estimated.

<figure><img src="image_3.png" /><figcaption>**Figure 2.** I-V measurement performed on a GTLM pattern for gate voltage I<sub>G</sub> from 1 V (black) to -10 V.</figcaption></figure>

The SEM analysis confirms that the actual IDT dimensions are in good agreement with the design requirement, i.e., a 400 nm finger width, and a SAW wavelength around 1.3 µm, taking into account a SAW velocity close to 4000 m/s (Fig. 3). Varied transistor lengths are considered from 2 to 40 µm corresponding to a phase shift between 10 to 90°. The first measurements of SAW on this device will be performed shortly.

<figure><img src="image_4.png" /><figcaption>**Figure 3.** SEM micrograph of ScAlN/GaN HEMT-SAW IDT integration, inset showing close up of IDT</figcaption></figure>

## Conclusions

The combination of high piezoelectric and electronic properties makes ScAlN a very promising material for SAW filter applications in the perspective of 6G technology. Successful integration of HEMTs with SAW IDTs was achieved on MBE-grown heterostructure. Our first measurements already demonstrate the transistor operation while SAW characterization will be carried out soon. Finally, the capability to tune the acoustic wave frequency with the transistor gate will be evaluated.

## Acknowledgments

The authors would like to thank the support of the French "Agence Nationale for the research" (ANR), through the 'Investissements d'Avenir' program GaNeX (ANR-11-LABX-0014), the project TWINS (ANR-23-CE51-0011) as well as the RENATECH. This work was also supported in part by the European Project GaN4AP (Gallium Nitride for Advanced Power Applications). The project has received funding from the Electronic Component Systems for European Leadership Joint Undertaking (ECSEL JU), under grant agreement No.101007310. This Joint Undertaking receives support from the European Union's Horizon 2020 research and innovation program, and Italy, Germany, France, Poland, Czech Republic, Netherlands.

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