Single-Crystal-Like Ge(110) Layers for High-Performance Flexible Thin-Film Transistors

<b>1. Introduction </b>Technology for building high-speed thin-film transistors (TFTs) at low temperatures is the key to fabricating high-performance flexible devices. Ge has been expected to be a channel material for TFTs owing to its high carrier mobility and relatively low crystallization temperature. To date, low-temperature syntheses of Ge layers have been achieved using various techniques; however, most polycrystalline (poly-) Ge has been poorly crystalline.<br/>We have developed an advanced solid-phase crystallization (SPC) technique using a densified amorphous (a-) Ge precursor [1] and updated the Hall carrier mobility of poly-Ge to 620 cm² V⁻¹ s⁻¹ for holes [2] and to 370 cm² V⁻¹ s⁻¹ for electrons [3]. The high-mobility Ge layers have been also developed on a plastic substrate [4]. The performance of the accumulation-mode p-channel TFTs exceeded the most of TFTs based on poly-Ge layers [5]; however, the TFTs characteristics varied for each device within the same chip.<br/>In this study, we clarified that the characteristic variation of the Ge-TFTs was caused by the variation of grain boundaries and crystal orientation. Additionally, to suppress the variation, we investigated metal-induced lateral crystallization (MILC) using the densified a-Ge precursors that enabled large-grained SPC-Ge. We demonstrate three-dimensionally orientation-controlled Ge rods formed at 325 °C.<br/><b>2. Experimental method </b>We deposited 100-nm-thick a-Ge layers on SiO₂ glass substrates at 125 °C using a molecular beam deposition system. The samples were annealed at <i>T</i>_anneal = 450 °C for 5 h in N₂ to induce SPC. The detailed flow for the TFT fabrication is described elsewhere [5]. For MILC, 5-nm-thick metal (Ag, Au, Bi, Co, Fe, Ni, Pd, and Pt) strips with a width of 5 µm were formed on the a-Ge layer. The samples were then annealed in an N₂ atmosphere at <i>T</i>_anneal = 325–350 °C for 0–100 h. The samples were characterized using optical microscopy, Raman scattering spectroscopy, electron backscatter diffraction (EBSD) analysis, and transmission electron microscopy (TEM) analyses with an energy dispersive X-ray (EDX).<br/><b>3. Results and Discussion </b>The EBSD analyses indicated that the channel region of the TFT was composed of randomly oriented grains with a few μm. The TFT characteristics showed a large variation with field-effect mobility of 10-190 cm² V⁻¹ s⁻¹ and on/off ratio of 10¹-10³. This variation became more prominent when the channel length was narrower, suggesting that the variation of the TFT characteristics was affected by the randomness inherent in polycrystalline channels.<br/>To control the grain boundaries and the orientation, we investigated the MILC using 8 different metals. According to the optical microscopy and Raman scattering spectroscopy, the lateral growth of crystalline Ge was observed over several μm in the vicinity of each metal pattern at <i>T</i>_anneal = 350 °C. EBSD measurements revealed that the grain size in the MILC region was a few μm. This is the first time that μm-order grains were detected in MILC-Ge, indicating that densification of a-Ge precursors was effective in MILC as well as SPC [1].<br/>Among the metals, here we focused on Ni. Lowering <i>T</i>_anneal to 325 °C enlarged the MILC region, which consisted of (110)-oriented rod-shaped crystals (15 μm length, 2 μm width). Furthermore, the longitudinal orientation of the growth direction was &lt;110&gt;. The amount of Ni in Ge was below the detection limit of EDX (&lt; 1%). The TEM measurement revealed that the high-crystal quality of the three-dimensionally orientation-controlled Ge rods, which will show single-crystal-like properties. We are now fabricating the flexible TFTs based on the Ge layers.<br/>[1] K. Toko <i>et al.</i>, Sci. Rep. <b>7,</b> 16981 (2016).<br/>[2] T. Imajo<i> et al.</i>, Appl. Phys. Express <b>12</b>, 015508 (2019).<br/>[3] M. Saito<i> et al.</i>, Sci. Rep. <b>9</b>, 16558 (2019).<br/>[4] T. Imajo<i> et al.</i>, Sci. Rep. <b>11</b>, 8333 (2021).<br/>[5] K. Moto <i>et al.</i>, Appl. Phys. Lett. <b>114,</b> 212107 (2019).