High-Mobility Thin-Film Transistors Based on Polycrystalline GeSn Formed by Selectively Nucleated Solid-Phase Growth
Authors: Maeda Shintaro , Moto Kenta , Huang Linyu , Morimoto Atsuki , Yamamoto Keisuke , Suemasu Takashi , Toko Kaoru 
Published: December 2024

High-speed thin-film transistors (TFTs) are important for realizing higher performance and multifunctionality in advanced display devices. Ge has been a preferred TFT channel material owing to its high carrier mobility and relatively low crystallization temperature. Remarkable progress has been made in Ge-based TFTs in recent years. Today, polycrystalline (poly-) Ge thin films are formed on insulating substrates by various techniques and at low temperatures. Among these methods, solid-phase crystallization (SPC) methods have made progress in recent years. In the amorphous precursor of Ge, the grain size of the polycrystalline film after SPC has been enlarged by densification and elemental doping[1,2]. Reflecting the reduction of grain boundary scattering, high electron mobility (380 cm2 V−1 s −1) and hole mobility (690 cm2 V−1 s −1) have been achieved[3,4]. However, to achieve such high carrier mobility, a thick (>100 nm) Ge film was essential. This is because the thinner the Ge film, the poorer the crystallinity, such as smaller crystal grain size[5,6]. In addition, acceptor defects in Ge complicate the low carrier concentration of the poly-Ge layer. These features limit TFT performance because the reduction of off-current requires a thin film and a low carrier concentration[7]. Therefore, in this study, we investigated grain size enlargement by nucleation control in SPC of amorphous Ge thin films (≤50 nm). We selectively controlled nucleation by introducing an amorphous multilayer structure because the nucleation frequency depends on the deposition conditions (Fig. 1). Each layer was prepared by controlling deposition temperature as nucleation or nucleation suppression layers, respectively. The inverse pole figures (IPF) in Fig. 1 show that the grain size changes dramatically depending on the position of the nucleation layer. In the bottom structure, the incubation time until SPC begins and grain size are comparable to that of the conventional single-layer structure, suggesting a dominance of heterogeneous nucleation at the substrate interface in the single-layer structure. The middle structure exhibited a long incubation time owing to suppressed nucleation at the substrate and surface; however, it also provided an extremely slow lateral growth rate. Reflecting the balance between nucleation frequency and lateral growth, the top structure exhibited the largest grain size. A small amount (2%) of Sn doping further increased the grain size to 12 μm (Fig. 2(a)), which is the largest among poly-Ge(Sn) thin films (<100 nm). As a result of Hall measurements, all samples showed p-type conduction owing to acceptor defects. Especially in the case of this GeSn film, the large grain size effectively reduced acceptor defects and improved hole mobility. The resulting hole mobility (260 cm2 V−1 s −1) and hole concentration (3 × 1017 cm−3 ) values are realized. We then performed cross-sectional TEM observations of this sample. Within the field of view (∼1 μm wide), the {111} twin grain boundaries were observed at a few locations near the GeSn/SiO2 interface, as shown in Fig. 3(b). This may be due to the roughness of the SiO2 substrate or thermally induced strain during the crystallization process, which is consistent with the mobility degradation of thinner Ge layers. The selected-area electron diffraction (SAED) pattern (Fig. 3(c)) shows clear spots attributed to GeSn. Fig. 3(b), (d) show no interfacial trace of the GeSn bilayer structure. As a result of TFT fabrication using this film, it was confirmed that it operates as a typical p-channel accumulation-mode TFT. Its on-off current ratio is higher than that of conventional single Ge film[7] (Fig. 3(a)). The low hole concentration and high hole mobility caused by selective nucleation and Sn doping likely contributed to the increase in on-current and decrease in off-current. Compared to other methods, the TFT in this study has a relatively high on-off current ratio and very high field-effect mobility (250 cm2 V−1 s −1), demonstrating the highest performance among polycrystalline Ge-based TFTs (Fig. 3(b)). The introduction of a new crystal growth process in this study greatly contributes to the high performance of polycrystalline Ge-based TFTs, and is expected to spread to a variety of material systems. [1] K. Toko et al.,
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Figure 1