Polycrystalline Ge thin films are promising candidates for next‐generation thin‐film transistors. However, the difficulty in Fermi‐level control due to the high density of defect‐induced acceptors has been one of the main problems for device applications. Herein, GeO2 preparation and Sn addition are combined in the advanced low‐temperature (375 °C) solid‐phase crystallization of Ge layers that have been recently developed. The GeO2 underlayer works effectively at a low Sn composition (< 4%), enlarging the grain size by a magnification of ≈5. An appropriate amount of unsubstituted Sn reduces acceptor defects and achieves a minimum hole concentration of 3 × 1016 cm−3 by combining post‐annealing (500 °C). This hole concentration is the lowest level for undoped polycrystalline Ge‐based thin films, which allows control over their Fermi level widely by impurity doping. Although the lower hole concentration provides a higher energy barrier height of the grain boundary, the hole mobility is maintained at 190 cm2 V−1 s−1 owing to the large grains (15 μm diameter). This achievement paves the way for the implementation of poly‐Ge‐based thin films in various semiconductor devices, including advanced thin‐film transistors.
Solid-phase crystallization is rapidly developing as a method for obtaining high-quality polycrystalline Ge layers. We propose a four-step heating process, focusing on postdeposition annealing (PDA), which is performed between the film deposition and crystallization. PDA at an appropriate temperature (500 °C) reduces the grain size while improving the crystallinity of Ge (150 nm thickness), which leads to the reduction of grain boundary barrier height (5 meV). The resulting hole mobility (530 cm 2 V −1 s −1 ) is higher than that of any other semiconductor thin films (<300 nm) and even single-crystal Si wafers.
In article number http://doi.wiley.com/10.1002/pssr.202100509, Kaoru Toko and co‐workers illustrate how oxygen diffusion from the GeO2 layer during crystallization enables the synthesis of GeSn layers with large grains, which considerably reduces acceptor defects while maintaining a high hole mobility. These achievements will pave the way for advanced Ge‐based thin‐film transistors.
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