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.
A semiconductor thin film with high
carrier mobility was fabricated
on a flexible film. During the solid-phase crystallization process
of the densified amorphous Ge layer, the interfacial reaction with
the GeO
x
underlayer is controlled by the
GeO
x
thickness (0–300 nm) and the
growth temperature (375–450 °C). The appropriate amount
of oxygen diffusion from GeO
x
to Ge produces
large Ge grains (up to 13 μm in diameter) with high crystal
quality. The use of a high heat-resistant polyimide film allows postannealing
at 500 °C and improves the hole mobility of Ge to 690 cm2 V–1 s–1 while maintaining
flexibility. This hole mobility is higher than that of any other semiconductor
thin films directly formed on insulators, including single-crystal
Ge grown at high temperatures, and even higher than that of a Si wafer.
The findings open up the possibility of incorporating high-speed transistors
on flexible devices that surpass Si metal–oxide–semiconductor
field-effect transistors.
Polycrystalline Ge thin films have attracted increasing attention because their hole mobilities exceed those of single-crystal Si wafers, while the process temperature is low. In this study, we investigate the strain effects on the crystal and electrical properties of polycrystalline Ge layers formed by solid-phase crystallization at 375 °C by modulating the substrate material. The strain of the Ge layers is in the range of approximately 0.5% (tensile) to -0.5% (compressive), which reflects both thermal expansion difference between Ge and substrate and phase transition of Ge from amorphous to crystalline. For both tensile and compressive strains, a large strain provides large crystal grains with sizes of approximately 10 μm owing to growth promotion. The potential barrier height of the grain boundary strongly depends on the strain and its direction. It is increased by tensile strain and decreased by compressive strain. These findings will be useful for the design of Ge-based thin-film devices on various materials for Internet-of-things technologies.
Low-temperature synthesis of polycrystalline (poly-) Ge on insulators is a key technology to integrate Ge-CMOS into existing devices. However, Fermi level control in poly-Ge has been difficult because poly-Ge has remained naturally highly p-type due to its defect-induced acceptors. We investigated the formation of n-type poly-Ge (thickness: 100-500 nm) using the advanced solid-phase crystallization technique with Sb-doped densified precursors. Sb doping on the order of 10 20 cm À3 facilitated lateral growth rather than nucleation in Ge, resulting in large grains exceeding 15 lm at a low growth temperature (375 C). The subsequent heat treatment (500 C) provided the highest electron mobility (200 cm 2 /V s) and the lowest electron density (5 Â 10 17 cm À3) among n-type poly-Ge directly grown on insulators. These findings will provide a means for the monolithic integration of high-performance Ge-CMOS into Si-LSIs and flat-panel displays.
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