Epitaxial Si on Epitaxial-Gd₂O₃/Si(111) Virtual Substrate Using Low-Cost RF Sputtering for Silicon on Insulator (SOI) Application

There is an ever-growing need for 5G communication technology and the Internet of Things to achieve better-performing systems in speed and power consumption. Research has shown SOI technology to be very promising among different technologies (it offers low voltage and high-frequency operation capability) [1]. However, the costly smart-cut method for manufacturing SOI wafers is a challenge. Alternatively, the notion of using epitaxial rare earth (RE) oxides onto Si followed by epi-Si growth is a potential alternative to enable cheaper and high-volume manufacturing (HVM) of SOI wafers. Gd₂O₃ oxide is the most promising among all rare oxides due to its large bandgap, sufficient band offsets, stable oxidation state, and low lattice mismatch with Si(111). In this context, our group patented a methodology to produce semiconductors on an insulator substrate using an HVM-friendly RF magnetron sputter system [2,3].<br/>In the work, we demonstrate epitaxial-Si on epitaxial-Gd₂O₃/Si(111) substrate by RF magnetron sputtering for SOI application. The fabricated heterostructure is characterized through XPS, HRXRD, cross-sectional HRTEM, and EDS analyses, providing compelling evidence for the formation of the epi-Si(111) layer on the epi-Gd₂O₃surface.<br/>The cap-Gd₂O₃/Top-Si/BOX-Gd₂O₃ (capping layer (cap)/channel (Top-Si)/Buried-oxide (BOX)) stack is deposited on Si(111) substrate using RF sputtering. The box layer is deposited at 750 ^oC with 20 W RF power (~0.12nm/min) for 100 min, followed by intrinsic-Si deposition (~0.3 nm/min) at 100 ^oC temperature. The Gd₂O₃ capping layer is deposited at a higher temperature (750 ^oC) to crystallize the channel layer. The heterostructure is subsequently annealed at 850 ^oC to study the impact of rapid thermal annealing. The heterostructure thickness is 12 nm/15 nm/16 nm, confirmed using HRTEM. The capping layer is etched using an H₂SO₄: DI water wet etch, O₂ plasma ashing, followed by BHF etch. The conformal etching is verified by analyzing the XPS spectra. The absence of a Gadolinium (Gd3d and Gd4d) signature and a strong Si2p peak of bulk-Si confirms that the capping layer is properly etched. Next, HRXRD is employed to determine the structural nature of the BOX-Gd₂O₃.<br/>The full range <i>Φ</i> measurements at fixed <i>Χ </i>= 70.5^o and <i>2θ </i>= 28.44^oshow two sets of peaks 120^o apart correspond to the Si(-111) and c-Gd₂O₃(-222) planes. This observation indicates a 3-fold symmetrical nature of the peaks, establishing that the (222) planes of Gd₂O₃are grown epitaxially onto the Si(111) substrate. The Si peaks are found to be rotated by 60^o relative to Gd₂O₃, suggesting the presence of an A/B twinning relationship between Si and Gd₂O₃. The epitaxial quality of the BOX is further validated using a skew-symmetric ω-2θ scan.<br/>The HRTEM analysis demonstrates three key findings: (a) the formation of an atomically sharp interface at the BOX-Gd₂O₃/Si(111) interface, (b) the epitaxial nature of the BOX-Gd₂O₃and Top-Si layers, and (c) confirmation of the A/B/A stacking arrangement of Top-Si/BOX-Gd₂O₃/Si(111). The EDS analysis further confirms the formation of a flat and homogenous interface without significant Si(Gd) out-diffusion.<br/>In conclusion, we have developed a low-cost, high-volume, manufacturable state-of-the-art epitaxial Si-Gd₂O₃ on Si (111) substrate (SOXI) by RF sputtering. The epitaxial nature of the heterostructure is confirmed through HRXRD and HRTEM analyses. In the near future, a SOXI MOSFET demonstration should be done.<br/><br/>References:<br/>[1] J. Hartmann <i>et al.,</i> IEEE CSICS, 2014. [2] Amita <i>et al.,</i> DRC, 2018. [3] Amita <i>et al.,</i> TSF, 2021