Heterogeneous integration of high-k complex-oxide  gate dielectrics on wide band-gap high-electron- mobility transistors.pdf
ARTICLE Heterogeneous integration of high-k complex-oxide gate dielectrics on wide band-gap high-electron- mobility transistors Jongho Ji1,10, Jeong Yong Yang2,10, Sangho Lee 3,4,10, Seokgi Kim5, Min Jae Yeom2, Gyuhyung Lee2,6, Heechang Shin1, Sang-Hoon Bae 7,8, Jong-Hyun Ahn1, Sungkyu Kim5, Jeehwan Kim 3,4,9✉, Geonwook Yoo 2,6✉& Hyun S. Kum 1✉ Heterogeneous integration of dissimilar crystalline materials has recently attracted con- siderable attention due to its potential for high-performance multifunctional electronic and photonic devices. The conventional method for fabricating heterostructures is by hetero- epitaxy, in which epitaxy is performed on crystallographically different materials. However, epitaxial limitations in monolithic growth of dissimilar materials prevent implementation of high quality heterostructures, such as complex-oxides on conventional semiconductor plat- forms (Si, III-V and III-N). In this work, we demonstrate gallium nitride (GaN) high-electron- mobility transistors with crystalline complex-oxide material enabled by heterogeneous inte- gration through epitaxial lift-off and direct stacking. We successfully integrate high-κ com- plex-oxide SrTiO3 in freestanding membrane form with GaN heterostructure via a simple transfer process as the gate oxide. The fabricated device shows steep subthreshold swing close to the Boltzmann limit, along with negligible hysteresis and low dynamic on-resistance, indicating very low defect density between the SrTiO3 gate oxide and GaN heterostructure. Our results show that heterogeneous integration through direct material stacking is a pro- mising route towards fabricating functional heterostructures not possible by conventional epitaxy. https://doi.org/10.1038/s44172-024-00161-z OPEN 1 Department of Electrical and Electronic Engineering, Yonsei University, Seoul, South Korea. 2 Department of Electronic Engineering, Soongsil University, Seoul, South Korea. 3 Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA. 4 Research Laboratory of Electronics, Massachusetts Institute of Technology, Cambridge, MA, USA. 5 Department of Nanotechnology and Advanced Materials Engineering, Sejong University, Seoul, South Korea. 6 Department of Intelligent Semiconductors, Soongsil University, Seoul, South Korea. 7 Department of Mechanical Engineering and Materials Science, Washington University in St. Louis, St. Louis, MO, USA. 8 Institute of Materials Science and Engineering, Washington University in St Louis, St Louis, MO, USA. 9 Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA. 10These authors contributed equally: Jongho Ji, Jeong Yong Yang, Sangho Lee. ✉email: jeehwan@mit.edu; gwyoo@ssu.ac.kr; hkum@yonsei.ac.kr COMMUNICATIONS ENGINEERING |   (2024) 3:15 | https://doi.org/10.1038/s44172-024-00161-z | www.nature.com/commseng 1 1234567890():,; R ecent advances in producing ultrathin freestanding single- crystalline membranes have enabled heterogenous inte- gration of dissimilar crystalline materials in a single elec- trical or photonic device, opening a path towards creation of devices with enhanced performance and functionalities1,2. The focus of recent studies was mainly on the membrane generation method and a rough demonstration of a prototype device fabri- cated via heterogeneous integration3–5. However, to advance this ﬁeld to the next stage, it is pivotal to demonstrate the possibility of creating a device with state-of-the-art performance. Many aspects of heterogeneous integration of dissimilar materials are unknown, but perhaps the most important is the interface quality between the transferred single-crystalline membrane and the host heterostructure. To verify this, we have fabricated gallium nitride (GaN)-based high-electron-mobility transistors (HEMT) with a heterogeneously integrated SrTiO3 gate oxide layer and char- acterized its performance. We ﬁnd that with careful fabrication and transfer of the gate oxide membrane, the device performance in regard to the oxide/HEMT interface show excellent quality, matching or exceeding that of conventionally deposited amor- phous gate oxides as well as in-situ grown SiN oxides6–8. GaN-based HEMT is one of the most promising structure for high-power and RF applications owing to its superior properties, such as high electron mobility and high breakdown ﬁeld9–11. To further improve the performance and reliability beyond conven- tional GaN HEMTs with Schottky metal gate, metal-oxide- semiconductor (MOS)-HEMTs structures have been proposed to suppress gate leakage and passivated the GaN surface from ambi- ent. In this regard, MOS-GaN HEMTs with various dielectric materials such as Al2O3 and HfO2 have been demonstrated12–14. In MOS-HEMT, the crystallization and interfacial quality of the gate dielectric material play a crucial role in determining the performance15. Among various dielectric materials, complex-oxide materials have attracted considerable interest due to their diverse functional properties such as high dielectric constant (high-κ), ferroelec- tricity, magnetism, and superconducting properties making complex-oxides an attractive material system for developing next- generation devices16,17. However, epitaxial limitations in mono- lithic growth of dissimilar materials make it difﬁcult to integrate single-crystalline complex-oxide materials with other material platforms such as GaN. The epitaxy of complex-oxides on sub- strates with different lattice constants and thermal expansion coefﬁcients results in growth of polycrystalline ﬁlms with inferior material properties. In other words, epitaxial limitations make it difﬁcult to integrate complex-oxide materials onto conventional semiconductors while maintaining its excellent functional properties18. To address this challenge, recent advances in fabricating single- crystalline freestanding complex-oxide membrane techniques have paved the way to seamlessly integrate complex-oxides with any arbitrary semiconductor platforms19–21. In this work, we demonstrate state-of-the-art GaN HEMTs utilizing hetero- geneously integrated single-crystalline strontium titanium oxide (SrTiO3, abbreviated as STO) gate dielectric ﬁlms. Epitaxial lift- off and direct stacking of freestanding membrane enables inte- gration of crystalline complex-oxide materials on GaN HEMT platforms. STO, a representative perovskite material, exhibits ultrahigh dielectric constant (κ ~ 300 at room temperature) and comparable breakdown ﬁeld with conventional dielectric materials22,23. This material was recently utilized to demonstrate 2D-FETs successfully24. The fabricated devices show excellent electrical characteristics such as negligible hysteresis (ΔV), low subthreshold swing (SS) close to the Boltzmann limit, and low dynamic on-resistance. We conclude that the pristine interface between the transferred complex-oxide membrane and GaN is attributed to the superior performance of the devices. Results Structure of the STO/GaN HEMTs. Figure 1a–c shows a 3D schematic illustration, photograph, and optical microscope image of the fabricated STO/GaN HEMT device. Centimeter-scale freestanding STO membrane with a thickness of ~25 nm was transferred onto the AlGaN/GaN HEMT heterostructure as the gate insulator. The channel width, channel length, and gate length are 60 µm, 30 µm, and 4 µm, respectively. Figure 1d shows the energy band diagram of the fabricated device (the characteriza- tion and fabrication procedure are shown in Supplementary Figs. 1, 2, respectively). With an estimated valence band offset of ~0.5 eV and conduction band offset of ~0.1 eV, the STO gate insulator forms a type-I straddling band alignment with AlGaN25–31. Typically, the type-I negative band alignment of gate Fig. 1 Structure of the STO/GaN HEMTs. a Schematic illustration, b photograph, c optical microscopy image and d energy band diagram of fabricated AlGaN/GaN high-electron-mobility transistors (HEMT) with SrTiO3 (STO) gate dielectric. ARTICLE COMMUNICATIONS ENGINEERING | https://doi.org/10.1038/s44172-024-00161-z 2 COMMUNICATIONS ENGINEERING |   (2024) 3:15 | https://doi.org/10.1038/s44172-024-00161-z | www.nature.com/commseng which showed no defects, airgaps, unexpected interfacial layers or residues at the STO/GaN interface. We suspect that the dangling bonds of the transferred STO membrane may form atomic bonding with the underlying substrate (in our case, GaN HEMT heterostructure) by the thermal annealing process, which con- sistent with the previous works55–59 (the mechanism of the interface bonds formation is schematically illustrated in Supple- mentary Fig. 5). Moreover, the selected area electron diffraction (SAED) pattern (inset) of the STO region, veriﬁed the crystalline nature of the transferred STO membrane. We believe these results, along with the electrical analysis of the gate oxide inter- face, strongly support that the reliable operation and excellent performance of the fabricated STO gate oxide HEMT is a result of a pristine interface between highly crystalline STO gate oxide and GaN. Conclusions In summary, we heterogeneously integrated crystalline complex- oxide material on AlGaN/GaN HEMT as a gate dielectric by epitaxial layer transfer approach. Using STO with ultrahigh-κ properties as the gate oxide, we fabricated a MOS-HEMT device. The fabricated devices exhibited a negligible hysteresis (ΔV) at a drain current of 1 µA/mm, and a minimum SS value of 62 mV dec−1. We attribute these results to an extremely clean interface between the STO and GaN, free from interface and border traps. The dynamic on resistance measurements were performed for further interface quality analysis, in which our device only showed a maximum resistance increase of 7%. Finally, we conﬁrmed through TEM that no unwanted inter- facial layer, residues, or airgaps between STO/GaN exists, and that the transferred STO maintains its crystalline nature. Our results demonstrate the potential of heterogeneous integration of complex-oxide materials with mature semiconductor technolo- gies, substantially expanding the possibility of creating high performance electrical and photonic devices with novel functionalities. Methods Material preparation and device fabrication. A GaN HEMT, consisting of GaN capping layer (3 nm), Al0.26Ga0.74N barrier (25 nm), AlN interlayer (1 nm), GaN channel layer (2 μm) and GaN buffer layer (1 μm), was grown via metal-organic chemical vapor deposition (MOCVD) on a Si (111) substrate. The electron mobility and sheet carrier concentrations of the two-dimensional electron gas (2DEG) formed between the AlGaN/GaN interface were >1300 cm2 V−1 s−1 and ~1013 cm−2 at T = 300 K. The integration of STO with AlGaN/GaN HEMT structure begins with the successive epitaxial growth of a water-soluble strontium aluminum oxide (Sr3Al2O6, abbreviated as SAO) sacriﬁcial layer, followed by the growth of STO gate oxide on a single-crystalline STO (001) substrate by pulsed-laser deposition (PLD). It has been reported that the epitaxial growth of oxide ﬁlm on the SAO template allows the transfer of single-crystalline STO membranes without fundamental thickness limitation60. The single-crystallinity of the epitaxially grown STO ﬁlm on SAO/ STO substrate is conﬁrmed using X-ray diffraction (XRD) and electron backscatter diffraction (EBSD) map, as shown in Supplementary Fig. 1a–c. After deposition of a poly(methyl methacrylate) (PMMA) supporting layer on the as-grown STO/ SAO/STO substrate via spin-coating, the stack was immersed in deionized (DI) water for ~24 hours to completely dissolve the SAO sacriﬁcial layer61. The freestanding STO membrane with a thickness of ~25 nm was then transferred onto the AlGaN/GaN HEMT structure followed by removal of the supporting layer by acetone and rinsed by isopropyl alcohol (the complex-oxide membrane transfer procedure schematically illustrated in Sup- plementary Fig. 2). The transferred membrane was patterned through standard photolithography and etched using a combination of ion milling and etching in diluted hydroﬂuoric acid (HF) solution. We believe that the HF etch removes most of the defects caused by the ion milling in the gate oxide membrane, leading to excellent characteristics. Mesa isolation was conducted by inductively coupled plasma reactive ion etching (ICP-RIE) using a mixture of BCl3/Cl2 gas. A Ti/Al/Ti/W (20/120/20/30 nm) stack was deposited for the source/drain electrode via e-beam evaporation, followed by rapid thermal annealing at 500 °C for 2 min in N2 environment. Finally, a Ni ﬁlm ( ~ 500 nm) was deposited as the gate electrode via plasma sputtering (the low magniﬁcation cross- sectional image of the fabricated device is shown in Supplemen- tary Fig. 6). Electrical and material characterizations. The electrical prop- erties of the fabricated devices were measured using a semi- conductor parameter analyzer (Keithley−4200A-SCS) with a 4255-RPM module to apply pulsed signals. The structural and interfacial analysis was performed using a scanning electron microscope (SEM), focused ion beam (FIB), and transmission electron microscopy (TEM). SEM measurements were performed using JEOL high-resolution SEM (IT-500HR), ZEISS SEM with an EBSD detector. The cross-sectional TEM specimens of the fabricated devices were prepared using a Ga-focused ion beam milling (ZEISS crossbeam 540) technique. TEM measurements were performed using a JEOL ARM 200 F (NEOARM). Data availability The data that support the ﬁndings of this study are available from the corresponding author upon reasonable request. Fig. 4 TEM characterization of STO/GaN interface. a Plan-view scanning electron microscope (SEM) image of HEMT device, black box represents region of focused ion beam (FIB) milling for transmission electron microscope (TEM) analysis. b Cross-sectional TEM image of SrTiO3 (STO)/GaN interface. The inset shows selected area electron diffraction (SAED) pattern of the transferred STO. COMMUNICATIONS ENGINEERING | https://doi.org/10.1038/s44172-024-00161-z ARTICLE COMMUNICATIONS ENGINEERING |   (2024) 3:15 | https://doi.org/10.1038/s44172-024-00161-z | www.nature.com/commseng 5 Received: 29 August 2023; Accepted: 5 January 2024; References 1. Ji, J., Park, S., Do, H. & Kum, H. S. A review on recent advances in fabricating freestanding single-crystalline complex-oxide membranes and its applications. Phys. Scr. 98, 052002 (2023). 2. Ji, J. et al. Understanding the 2D-material and substrate interaction during epitaxial growth towards successful remote epitaxy: a review. Nano Converg. 10, 19 (2023). 3. Chiabrera, F. M. et al. Freestanding perovskite oxide ﬁlms: Synthesis, challenges, and properties. Ann. Phys. 534, 2200084 (2022). 4. Bouaziz, J., Cancellieri, C., Rheingans, B., Jeurgens, L. P. H. & La Mattina, F. Advanced epitaxial lift-off and transfer procedure for the fabrication of high- quality functional oxide membranes. Adv. Mater. Interfaces 10, 2200084 (2023). 5. Puebla, S. et al. Combining freestanding ferroelectric perovskite oxides with two-dimensional semiconductors for high performance transistors. Nano Lett. 22, 7457–7466 (2022). 6. Jiang, H. et al. Investigation of in situ SiN as gate dielectric and surface passivation for GaN MISHEMTs. IEEE Trans. Electron Devices 64, 832–839 (2017). 7. He, J. et al. Normally-OFF AlGaN/GaN MIS-HEMTs with Low RON and Vth hysteresis by functioning in-situ SiNx in regrowth process. IEEE Electron Device Lett. 43, 529–532 (2022). 8. Zhang, H. et al. Effect of in-situ SiNx grown with different carrier gas on structural and electrical properties of GaN-based MISHEMTs. Appl. Phys. Lett. 122, 173501 (2023). 9. Hoo Teo, K. et al. Emerging GaN technologies for power, RF, digital, and quantum computing applications: Recent advances and prospects. J. Appl. Phys. 130, 160902 (2021). 10. Amano, H. et al. The 2018 GaN power electronics roadmap. J. Phys. D: Appl. Phys. 51, 163001 (2018). 11. Millan, J., Godignon, P., Perpina, X., Perez-Tomas, A. & Rebollo, J. A survey of wide bandgap power semiconductor devices. IEEE Trans. Power Electron. 29, 2155–2163 (2014). 12. Ando, Y., Kaneki, S. & Hashizume, T. Improved operation stability of Al2O3/ AlGaN/GaN MOS high-electron-mobility transistors grown on GaN substrates. Appl. Phys. Express 12, 024002 (2019). 13. Yeom, M. J. et al. Low subthreshold slope AlGaN/GaN MOS-HEMT with spike-annealed HfO2 gate dielectric. Micromachines 12, 1441 (2021). 14. Zhao, Y. P. et al. Comparative study on characteristics of Si-based AlGaN/ GaN recessed MIS-HEMTs with HfO2 and Al2O3 gate insulators. Chinese Phys. B 29, 087304 (2020). 15. Hashizume, T., Nishiguchi, K., Kaneki, S., Kuzmik, J. & Yatabe, Z. State of the art on gate insulation and surface passivation for GaN-based power HEMTs. Mater. Sci. Semicond. 78, 85–95 (2018). 16. Kum, H. S. et al. Heterogeneous integration of single-crystalline complex- oxide membranes. Nature 578, 75–81 (2020). 17. Disa, A. S. et al. Photo-induced high-temperature ferromagnetism in YTiO3. Nature 617, 73–78 (2023). 18. Bae, S. H. et al. Integration of bulk materials with two-dimensional materials for physical coupling and applications. Nat. Mater. 18, 550–560 (2019). 19. Han, S. et al. Freestanding membranes for unique functionality in electronics. ACS Appl. Electron. Mater. 5, 690–704 (2023). 20. Kum, H. et al. Epitaxial growth and layer-transfer techniques for heterogeneous integration of materials for electronic and photonic devices. Nat. Electron. 2, 439–450 (2019). 21. Bakaul, S. R. et al. Single crystal functional oxides on silicon. Nat. Commun. 7, 10547 (2016). 22. Neville, R. C., Hoeneisen, B. & Mead, C. A. Permittivity of strontium titanate. J. Appl. Phys. 43, 2124–2131 (1972). 23. Huang, J. K. et al. High-κ perovskite membranes as insulators for two- dimensional transistors. Nature 605, 262–267 (2022). 24. Yang, A. J. et al. Van der Waals integration of high-κ perovskite oxides and two-dimensional semiconductors. Nat. Electron. 5, 233–240 (2022). 25. Maruska, H. P. & Tietjen, J. J. The preparation and properties of vapor- deposited single-crystal-line GaN. Appl. Phys. Lett. 15, 327–329 (1969). 26. Yim, W. M. et al. Epitaxially grown AlN and its optical band gap. J. Appl. Phys. 44, 292–296 (1973). 27. Koide, Y. et al. Energy band-gap bowing parameter in an AlxGa1-xN alloy. J. Appl. Phys. 61, 4540–4543 (1987). 28. Jung, B. K. et al. GaN schottky barrier MOSFET using transparent source/ drain electrodes for UV-optoelectronic integration. Solid-State Electron. 73, 78–80 (2012). 29. Four, I. & Kameche, M. Optimization of DC and AC performances for Al0.26Ga0.74N/GaN/4H-SiC HEMT with 30nm T-gate. Int. J. Nanoelectron. Mater. 13, 361–372 (2020). 30. Robertson, J. & Falabretti, B. Band offsets of high K gate oxides on III-V semiconductors. J. Appl. Phys. 100, 014111 (2006). 31. Michaelson, H. B. The work function of the elements and its periodicity. J. Appl. Phys. 48, 4729–4733 (1977). 32. Jha, J., Meer, M., Ganguly, S. & Saha, D. Off-state degradation and recovery in Oxide/AlGaN/GaN heterointerfaces: importance of band offset, electron, and hole trapping. ACS Appl. Electron. Mater. 2, 2071–2077 (2020). 33. Meneghini, M. et al. Trapping in GaN-based metal-insulator-semiconductor transistors: Role of high drain bias and hot electrons. Appl. Phys. Lett. 104, 143505 (2014). 34. Yang, J. Y., Ma, J., Lee, C. H. & Yoo, G. Polycrystalline/Amorphous HfO2 bilayer structure as a gate dielectric for β-Ga2O3 MOS capacitors. IEEE Trans. Electron Devices 68, 1011–1015 (2021). 35. Kanamura, M. et al. Suppression of threshold voltage shift for normally-Off GaN MIS-HEMT without post deposition annealing. Proc. Int. Symp. Power Semicond. Devices ICs 411–414 (2013). 36. Kang, H. Y. et al. Epitaxial κ-Ga2O3/GaN heterostructure for high electron- mobility transistors. Mater. Today Phys. 31, 101002 (2023). 37. Wang, H. C. et al. High-Performance LPCVD-SiNx/InAlGaN/GaN MIS- HEMTs with 850-V 0.98-mΩ·cm2 for power device applications. IEEE J. Electron Devices Soc. 6, 1136–1141 (2018). 38. Wang, H. C. et al. AlGaN/GaN MIS-HEMTs with high quality ALD-Al2O3 gate dielectric using water and remote oxygen plasma as oxidants. IEEE J. Electron Devices Soc. 6, 110–115 (2017). 39. Cai, Y. et al. Low ON-state resistance normally-OFF AlGaN/GaN MIS- HEMTs with partially recessed gate and ZrO charge trapping layer. IEEE Trans. Electron Devices 68, 4310–4316 (2021). 40. He, J., Hua, M., Zhang, Z. & Chen, K. J. Performance and VTH stability in E-Mode GaN fully recessed MIS-FETs and partially recessed MIS-HEMTs with LPCVD-SiNx/PECVD-SiNx gate dielectric stack. IEEE Trans. Electron Devices 65, 3185–3191 (2018). 41. Jiang, H., Tang, C. W. & Lau, K. M. Enhancement-Mode GaN MOS-HEMTs with recess-free barrier engineering and High-k ZrO2 gate dielectric. IEEE Electron Device Lett. 39, 405–408 (2018). 42. Chandrasekar, H. et al. Demonstration of wide bandgap AlGaN/GaN negative-capacitance high-electron-mobility transistors (NC-HEMTs) using barium titanate ferroelectric gates. Adv. Electron. Mater. 6, 2000074 (2020). 43. Yang, S. et al. High-quality interface in Al2O3/GaN/GaN/AlGaN/GaN MIS structures with in situ pre-gate plasma nitridation. IEEE Electron Device Lett. 34, 1497–1499 (2013). 44. Dutta Gupta, S. et al. Positive threshold voltage shift in AlGaN/GaN HEMTs and E-Mode operation by AlxTi1-xO based gate stack engineering. IEEE Trans. Electron Devices 66, 2544–2550 (2019). 45. Zhang, H. et al. Room-temperature organic passivation for GaN-on-Si HEMTs with improved device stability. IEEE Trans. Electron Devices 1–5 (2023) 46. Wang, T., Zong, Y., Nela, L. & Matioli, E. Enhancement-mode multi-channel AlGaN/GaN transistors with LiNiO junction Tri-gate. IEEE Electron Device Lett. 43, 1523–1526 (2022). 47. Wang, T., Nikoo, M. S., Nela, L. & Matioli, E. LiNiO gate dielectric with tri- gate structure for high performance E-mode GaN transistors. Proc. Int. Symp. Power Semicond. Devices ICs 135–138 (2021). 48. Wu, N. et al. Enhanced performance of low-leakage-current normally off p-GaN Gate HEMTs using NH3 plasma pretreatment. IEEE Trans. Electron Devices 70, 4560–4564 (2023). 49. Cheng, W.-C. et al. Low trap density of oxygen-rich HfO2/GaN interface for GaN MIS-HEMT applications. J. Vac. Sci. Technol. B 40, 022212 (2022). 50. He, J. et al. Distinguishing various inﬂuences on the electrical properties of thin-barrier AlGaN/GaN heterojunctions with in-situ SiNx caps. Mater. Sci. Semicond. Process. 132, 105907 (2021). 51. Lee, H. P. & Bayram, C. Improving current ON/OFF ratio and subthreshold swing of Schottky- Gate AlGaN/GaN HEMTs by postmetallization annealing. IEEE Trans. Electron Devices 67, 2760–2764 (2020). 52. Jin, D., & Del Alamo, J.A. Methodology for the study of dynamic ON- resistance in high-voltage GaN ﬁeld-effect transistors. IEEE Trans. Electron Devices 60, 3190–3196 (2013). 53. Meneghini, M. et al. Trapping phenomena and degradation mechanisms in GaN-based power HEMTs. Mater. Sci. Semicond. Process. 78, 118–126 (2018). 54. Zhu, J. J. et al. Improved interface and transport properties of AlGaN/GaN MIS-HEMTs with PEALD-grown AlN gate dielectric. IEEE Trans. Electron Devices 62, 512–518 (2015). 55. Yong, G. J. et al. Thermal stability of SrTiO3/SiO2/Si interfaces at intermediate oxygen pressures. J. Appl. Phys. 108, 033502 (2010). 56. Lee, M. et al. Self-sealing complex oxide resonators. Nano Lett. 22, 1475–1482 (2022). 57. Yoon, H. et al. Freestanding epitaxial SrTiO3 nanomembranes via remote epitaxy using hybrid molecular beam epitaxy. Sci. Adv. 8, eadd5328 (2022). ARTICLE COMMUNICATIONS ENGINEERING | https://doi.org/10.1038/s44172-024-00161-z 6 COMMUNICATIONS ENGINEERING |   (2024) 3:15 | https://doi.org/10.1038/s44172-024-00161-z | www.nature.com/commseng 58. Li, Y. et al. Stacking and twisting of freestanding complex oxide thin ﬁlms. Adv. Mater. 34, 2203187 (2022). 59. Hashizume, T. et al. Effects of postmetallization annealing on interface properties of Al2O3/GaN structures. Appl. Phys. Expr. 11, 124102 (2018). 60. Ji, D. et al. Freestanding crystalline oxide perovskites down to the monolayer limit. Nature 570, 87–90 (2019). 61. Lu, D. et al. Synthesis of freestanding single-crystal perovskite ﬁlms and heterostructures by etching of sacriﬁcial water-soluble layers. Nat. Mater. 15, 1255–1260 (2016). Acknowledgements H.S.K. acknowledge support from the National Research Foundation of Korea (NRF) (grant no. RS-2023-00252720 and grant no. RS-2023-00222070) and the Department of Electrical and Electronic Engineering at Yonsei University (2022-22-0311). G.Y. acknowledge support from the Ministry of Science and ICT (NRF-2021R1A4A1033155) and Ministry of Trade, Industry, and Energy of Korea (RS-2022-00154729). J.K. acknowledge support from the Ofﬁce of the Director of National Intelligence (ODNI), Intelligence Advanced Research Projects Activity (IARPA), via [2021-210900005]. Author contributions H.S.K., G.Y., J.K., and J.J. conceived this work and designed the experiments. H.S.K. directed the team. J.J., J.Y.Y., S.K., S.-H. Bae, J-H.A., S.K., G.Y. and H.S.K. prepared the manuscript. J.J., J.Y.Y., S.L., S.K., M.J.Y., G.L., and H.S. performed the device fabrication and characterization. J.J. H.S., and S.L. carried out the complex-oxide ﬁlm growth and transfer fabrication. J.Y.Y., S.K., M.J.Y., and G.L. carried out GaN HEMT device fabri- cation and measurements. S.K. carried out the TEM measurements. All the authors contributed to the discussion and analysis of the results. Competing interests The authors declare no competing interests. Additional information Supplementary information The online version contains supplementary material available at https://doi.org/10.1038/s44172-024-00161-z. Correspondence and requests for materials should be addressed to Jeehwan Kim, Geonwook Yoo or Hyun S. Kum. Peer review information Communications Engineering thanks the anonymous reviewers for their contribution to the peer review of this work. Primary Handling Editors: Liwen Sang, Anastasiia Vasylchenkova and Ros Daw. Reprints and permission information is available at http://www.nature.com/reprints Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional afﬁliations. Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/ licenses/by/4.0/. © The Author(s) 2024 COMMUNICATIONS ENGINEERING | https://doi.org/10.1038/s44172-024-00161-z ARTICLE COMMUNICATIONS ENGINEERING |   (2024) 3:15 | https://doi.org/10.1038/s44172-024-00161-z | www.nature.com/commseng 7
Title: 
Heterogeneous integration of high-k complex-oxide gate dielectrics on wide band-gap high-electron-mobility transistors ​

Authors: Jongho Ji, Jeong Yong Yang, Sangho Lee, Seokgi Kim, Min Jae Yeom, Gyuhyung Lee, Heechang Shin, Sang-Hoon Bae, Jong-Hyun Ahn, Sungkyu Kim, Jeehwan Kim, Geonwook Yoo, Hyun S. Kum ​
Publisher: Nature Communications Engineering
Date: 
5 January 2024 ​

Abstract: Heterogeneous integration of dissimilar crystalline materials has recently attracted considerable attention due to its potential for high-performance multifunctional electronic and photonic devices. ​ The conventional method for fabricating heterostructures is by hetero-epitaxy, in which epitaxy is performed on crystallographically different materials. ​ However, epitaxial limitations in monolithic growth of dissimilar materials prevent implementation of high-quality heterostructures, such as complex-oxides on conventional semiconductor platforms (Si, III-V, and III-N). ​ In this work, we demonstrate gallium nitride (GaN) high-electron-mobility transistors with crystalline complex-oxide material enabled by heterogeneous integration through epitaxial lift-off and direct stacking. ​ We successfully integrate high-κ complex-oxide SrTiO3 in freestanding membrane form with GaN heterostructure via a simple transfer process as the gate oxide. The fabricated device shows steep subthreshold swing close to the Boltzmann limit, along with negligible hysteresis and low dynamic on-resistance, indicating very low defect density between the SrTiO3 gate oxide and GaN heterostructure. ​ Our results show that heterogeneous integration through direct material stacking is a promising route towards fabricating functional heterostructures not possible by conventional epitaxy.