GeSn Thin Films Crystallized on Silica for Infrared Phototransistors

Group IV GeSn alloy has recently attracted substantial attention in integrated photonics and quantum material applications due to its compatibility with Si complementary metal-oxide semiconductor (CMOS) processes [1,2]. Ge-rich GeSn alloy is known by its direct bandgap with Sn compositions above approximately 8%. In addition, GeSn alloy offers a tunable bandgap by adjusting the Sn composition, which extends the direct-gap absorption edge to short-wave infrared (SWIR) and even mid-infrared (MIR) ranges [3]. Lasing in a direct-bandgap GeSn alloy with a Sn composition of 13% has been demonstrated as an example of its optoelectronic applications [4]. However, the growth of high-crystallinity GeSn on Si is challenging due to mismatch lattice constants of GeSn and Si, as well as their low solid solubility under thermal equilibrium [5]. Thick Ge buffer layers (&gt;500 nm thick) can be used in chemical vapor deposition (CVD) and molecular-beam epitaxy (MBE) to manage lattice mismatch. Nevertheless, these layers are not suitable for back-end-of-line (BEOL) CMOS device integration due to the requirement for high-quality Ge buffers to be annealed at temperatures exceeding 600°C [6]. Low-temperature crystallization of GeSn alloy has recently emerged as a possible approach for facile BEOL CMOS integration, as it requires lower annealing temperature (~500°C) and maintains reasonable material quality with large grain sizes [7]. Here we present the synthesis and characterization of crystallized GeSn films with 11% and 13% Sn composition and thicknesses of 30 nm and 100 nm on amorphous fused silica substrate for IR phototransistors. The GeSn films are deposited on the substrates through Physical Vapor Deposition and later crystallized by rapid thermal annealing process. Clear grain boundaries are observed on 30 nm-thick GeSn annealed at 500°C, with the apparent grain size being approximately 10-20 μm. On the other hand, 100 nm-thick GeSn annealed at 470°C shows radiation patterns, with grain sizes up to 300 μm. X-ray Diffraction (XRD) and electron back-scatter diffraction (EBSD) both demonstrate a very strong (111) preferred orientation, with subgrains also identified in the EBSD analyses. Further coupled with Raman Spectroscopy, the exact Sn compositions of the GeSn films are determined to be 10.6% and 12.7% [8]. Based on these high-crystallinity GeSn thin films, we demonstrate a 30 nm GeSn-on-silica phototransistor with Sn composition with responsivities of ~1.0 mA/W at 1600 nm, which is on the same order as GeSn photodetectors grown by MBE when normalizing the GeSn infrared absorption layer thickness [9]. Furthermore, the photocurrent/dark current ratio can be maximized via gating voltage to optimize the device performance, a feature in accessible in regular photodiodes or photoconductors. Extending similar approach towards BEOL CMOS integration, solid phase epitaxy of GeSn on thin low-temperature Ge buffer/Si seed layer [10] can be applied to potentially improve the quality of GeSn and thereby improve the performance of GeSn phototransistor.<br/><br/>This work has been supported by μ-ATOMS, an Energy Frontier Research Center funded by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences (BES), under the award DE-SC0023412. SNL is managed and operated by NTESS under DOE NNSA contract DE-NA0003525.<br/><br/>Reference:<br/>[1] Atalla et al. ACS Photonics 2022, 9, 4, 1425–1433.<br/>[2] Tran et al. ACS Photonics 2019, 6, 11, 2807–2815.<br/>[3] Chang et al. Sensors 2023, 23(17), 7386.<br/>[4] Wirths et al. Nature Photon 9, 88–92 (2015).<br/>[5] Predel, B. Ge-Sn (Germanium-Tin) Landolt-Börnstein - Group IV Physical Chemistry 5F(Ga-Gd – Hf-Zr) (1996).<br/>[6] Miao et al. Nanomaterials 2021, 11, 2556.<br/>[7] Wang et al. Frontiers in Physics 7, 134 (2019).<br/>[8] Li et al. Appl. Phys. Lett. 108, 102101 (2016).<br/>[9] Petluru et al. In Preparation (2024).<br/>[10] Michel et al. Nature Photon 4, 527–534 (2010).