We evaluated the potential of polycrystalline (poly-) GeSn as channel material for the fabrication of thin film transistors (TFTs) at a low thermal budget (<600 C). Poly-GeSn films with a grain size of $50 nm showed a carrier mobility of $30 cm 2 V À1 s À1 after low-temperature annealing at 475-500 C. Not only carrier mobility but also thermal conductivity of the films is important in assessing the self-heating effect of the poly-GeSn channel TFT. The thermal conductivity of the poly-GeSn films is 5-9 W m À1 K À1 , which is significantly lower compared with 30-60 W m À1 K À1 of bulk Ge; this difference results from phonon scattering at grain boundaries and Sn interstitials. The poly-GeSn films have higher carrier mobility and thermal conductivity than poly-Ge films annealed at 600 C, because of the improved crystal quality and coarsened grain size resulting from Sn-induced crystallization. Therefore, the poly-GeSn film is well-suited as channel material for TFTs, fabricated with a low thermal budget. V

Thermally induced crystallization processes for amorphous GeSn thin films with Sn concentrations beyond the solubility limit of the bulk crystal Ge-Sn binary system have been examined by X-ray photoelectron spectroscopy, grazing incidence X-ray diffraction, and (scanning) transmission electron microscopy. We paid special attention to the behavior of Sn before and after recrystallization. In the as-deposited specimens, Sn atoms were homogeneously distributed in an amorphous matrix. Prior to crystallization, an amorphous-to-amorphous phase transformation associated with the rearrangement of Sn atoms was observed during heat treatment; this transformation is reversible with respect to temperature. Remarkable recrystallization occurred at temperatures above 400 °C, and Sn atoms were ejected from the crystallized GeSn matrix. The segregation of Sn became more pronounced with increasing annealing temperature, and the ejected Sn existed as a liquid phase. It was found that the molten Sn remains as a supercooled liquid below the eutectic temperature of the Ge-Sn binary system during the cooling process, and finally, β-Sn precipitates were formed at ambient temperature.

Thin-film transistors (TFTs) on insulator substrates are widely used in applications from liquid crystal displays to sensor devices. However, insulator substrates with low heat conductivity lead to unfavorable self-heating effects in the channel regions. Herein, the carrier and heat transport properties of polycrystalline GeSn films on SiO 2 /Si substrates were improved by suppressing Sn segregation in the films to fabricate GeSn channel TFTs. Alloying with 5.5% Sn enabled the formation of larger grains than those in poly-Ge films after low-temperature annealing (below 520 °C) without Sn segregation. In addition, the films had a hole mobility of 40 cm 2 V −1 s −1 at a hole density of 1.1 × 10 18 cm −3 and a thermal conductivity of 12.1 Wm −1 K −1 at room temperature. The temperature dependences of the carrier and heat transport properties of the poly-GeSn films were investigated to accurately simulate a device with a poly-GeSn channel TFT. This was achieved by using the carrier transport measurements and numerical simulations of the heat transport in the Debye model. The simulated device allowed an accurate assessment of the self-heating effects of the TFT and thus provides a design guide for TFTs.

Functional surface creation technologies have garnered increasing attention over the years. These technologies can provide various functions to a material by establishing a fine structure on the material surface and responding to the needs of industrial products with distinguished functions or high values. In addition, by creating a “composite fine structure,” which is composed of two kinds of structures with different scales, the enhancement of functions and emergence of new functionalities can be expected. Hence, our study combined a micrometer-scale V-shaped groove structure using an ultra-precision cutting and nanometer-scale ultra-fine periodic structure (LIPSS) using a short-pulsed laser. Then, we clarified the creation principle and studied the functionality of the structure, specifically, its wettability. As a result, it was found that optical behavior inside the V-shaped groove changed; therefore, the composite structure changed depending on the groove angle, laser polarization direction, and number of times of irradiation. In addition, it was found that the water wettability changed depending on the type of formed micro-nano composite structures. Moreover, the wettability could be controlled by depending on how the structure is used.

Fine structures provide materials with various functionalities such as friction reduction and water repellency. A short-pulsed laser is a suitable method to effectively fabricate fine structures through self-organization, called as LIPSS (laser induced periodic structures). However, this method has difficulty in control of LIPSS since the principles and the phenomena have not been clarified completely. It has been reported that LIPSS follow debris on a material surface, hence, the short-pulsed laser assisted by mechanical processing was proposed, and the straightness and the aspect ratio of LIPSS were improved by laser irradiation on the surface with the straight microgooves. The short-pulsed laser assisted by mechanical processing was developed to control LIPSS and fabricate zigzag LIPSS, and the effects of the surface geometry before laser irradiation on LIPSS were investigated. Firstly, laser irradiation was conducted on the microgrooved surfaces with changing the angle of laser polarization to investigate the effects of the angle between groove direction and laser polarization on LIPSS. When the angle were from 45° to 90°, LIPSS followed the microgrooves regardless of the laser polarization. Based on the results, the laser was irradiated on the surfaces with zigzag microgrooves created by ultraprecision cutting, and zigzag LIPSS were fabricated as following zigzag microgrooves under the proper conditions. This study demonstrated the importance of the initial surface shape and the effectiveness of the short-pulsed laser assisted by mechanical machining to control LIPSS freely since the surface plasma waves are propagated from unevenness on the material surface.

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