High-carrier mobility semiconductors on insulators are essential for fabricating advanced thin-film transistors, allowing for three-dimensional integrated circuits or high-performance mobile terminals. We investigate the low-temperature (375–450 °C) solid-phase crystallization (SPC) of Ge on a glass substrate, focusing on the precursor conditions. The substrate temperature during the precursor deposition, T d, ranged from 50 to 200 °C. According to the atomic density of the precursor and the T d dependent SPC properties, the precursor conditions were determined by three regimes: the low-density regime (T d < 100 °C), high-density regime (100 ≤ T d ≤ 125 °C), and nucleation regime (T d > 125 °C). The use of the precursor in the narrow high-density regime enabled us to form SPC-Ge with a hole mobility of 340 cm2/Vs, the highest value among semiconductor thin films grown on insulators at low temperature (<900 °C). The origins of the high hole mobility were determined to be both a large grain size (5 µm) and a low energy barrier height (6.4 meV) for the grain boundary. The findings from and knowledge gained in this study, that is, the influence of the precursor conditions on subsequent crystal growth, will be universal and applicable to various materials.

To improve the performance of electronic devices, extensive research efforts have recently focused on the effect of incorporating Sn into Ge. In the present work, we investigate how Sn composition x (0 ≤ x ≤ 0.12) and deposition temperature Td (50 ≤ Td ≤ 200 °C) of the Ge1−xSnx precursor affect subsequent solid-phase crystallization. Upon incorporating 3.2% Sn, which is slightly above the solubility limit of Sn in Ge, the crystal grain size increases and the grain-boundary barrier decreases, which increases the hole mobility from 80 to 250 cm2/V s. Furthermore, at Td = 125 °C, the hole mobility reaches 380 cm2/V s, which is tentatively attributed to the formation of a dense amorphous GeSn precursor. This is the highest hole mobility for semiconductor thin films on insulators formed below 500 °C. These results thus demonstrate the usefulness of Sn doping of polycrystalline Ge and the importance of temperature while incorporating Sn. These findings make it possible to fabricate advanced Ge-based devices including high-speed thin-film transistors.

The hole mobility of the solid-phase-crystallized Ge layer is significantly improved by controlling the deposition temperature of Ge (50-200 °C) and the Ge thickness (50-500 nm) and by applying post annealing at 500 °C. The resulting hole mobility, 450 cm 2 /Vs, is the highest value to date among that of semiconductor layers directly formed on glass. The mechanism of the mobility enhancement is discussed from the perspective of three carrier scattering factors: grain boundary scattering, interface scattering, and impurity scattering. The high-hole mobility Ge layer formed by the simple fabrication process will be useful for high-speed thin-film transistors.

The highest recorded hole mobility in semiconductor films on insulators has been updated significantly. We investigate the solid-phase crystallization of a densified amorphous Ge layer formed on GeO2-coated insulating substrates. The resulting polycrystalline Ge layer with a glass substrate consists of large grains (~10 μm) and exhibits a hole mobility as high as 620 cm2 V−1 s−1, despite a low process temperature (500 °C). Even for the Ge layer formed on a flexible polyimide substrate at 375 °C, the hole mobility reaches 500  cm2 V−1 s−1. These achievements will aid in realizing advanced electronics, simultaneously allowing for high performance, inexpensiveness, and flexibility.

Crystalline Ge was directly achieved on a flexible plastic by layer exchange between Ag and amorphous Ge layers. The key factor for the layer exchange was limiting the diffusion of Ag to Ge by lowering the growth temperature (250 °C) and controlling the condition of an interlayer (1-nm-thick SiO2) between Ag and Ge. The layer exchange using Ag provided much faster nucleation and lateral growth rates of Ge compared with the conventional solid-phase crystallization and Al-induced layer exchange. A principle to determine the materials for layer exchange is proposed from the perspective of the diffusion and solubility of metals and semiconductors.

Metal-induced layer exchange (MILE) is an advanced crystallization technique for fabricating high-quality semiconductor thin films on insulating substrates at low temperatures. Here, we focused on Ag as a catalytic metal for crystallizing amorphous germanium thin films on glass through MILE. The layer exchange between Ag and aGe was not simple because Ag diffused into aGe so fast that it crystallized the top aGe layer before the completion of layer exchange. To suppress Ag diffusion into aGe , we explored the kind of interlayer material between them as well as the growth temperature. Preparing SiO2 and GeO2 interlayers allowed for a complete layer exchange, resulting in the formation of polycrystalline Ge thin films on glass at temperatures as low as 250 °C. These findings are essential for further understanding and controlling the layer exchange phenomenon.

The grain size and hole mobility of polycrystalline Si1-xGex thin films formed on glass by solid-phase crystallization were significantly improved after preparing the amorphous precursors by heating the substrate. By just controlling the deposition temperature of the precursors (50-350 °C) for each SiGe composition, the grain size reached over 2 μm across the whole composition range. Reflecting the enlargement of the grain size, the hole mobility values were improved by approximately one order of magnitude. These values are comparable to those of single-crystal SiGe formed by Ge condensation and are the highest among SiGe on insulators synthesized at low temperature (<900 °C). The SiGe on insulator technology obtained in this study will greatly contribute to the development of SiGe-based electronic and optical devices.

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