Growth of High Sn Concentration Germanium-Tin Films on Insulators by Microsecond Laser Annealing
Authors: Matsumura Ryo , Fukata Naoki 
Published: July 2021

1. Introduction
Thanks to their high carrier mobility, germanium based group IV semiconductors are promising materials to realize next generation electronics [1]. Especially, germanium tin alloys (GeSn) having high Sn concentration (> 6 at. %) are attracting much attention due to their direct band structure [2]. To apply such significant advantage to thin-film devices such as thin-film transistors (TFTs) or three dimensional large-scale integrated circuits (3D-LSIs), various groups are working on to crystallize these materials on insulating substrates by various approaches such as germanium condensation [3], solid phase crystallization [4], and so on.
Recently, we have established a technique to grow highly strained germanium based materials on insulating substrates by microsecond annealing using novel high speed continuous wave laser annealing (CWLA) system [5]. Here, in this presentation, we will introduce our recent achievements in GeSn crystal growth by the microsecond annealing technique.
2. Experimental Procedure
In the experiment, amorphous GeSn films with Sn concentration of 15% (thickness: 100 nm) were firstly deposited on quartz substrates by molecular beam deposition. Then the samples were annealed by the high speed CWLA system, which can scan laser light (focused to 20 μm diameter) on samples with very high speed up to 15 m/s. The laser irradiation time (tirr) can be varied from 1.5 to 6.0 μs by controlling the scanning speed. Here, Nd:YVO4 solid state laser with wavelength of 532 nm was used as a light source, and laser power was controlled from 350 to 1000 mW. After the laser annealing, the samples were characterized by microscopic Raman spectroscopy and electron back scattered diffraction (EBSD) method.
3. Results and Discussions
Fig. 1(a) shows Nomarski microscopic image after laser annealing (500 mW, tirr: 1.5 μs), which shows bright contrast at annealed region. Raman signals measured at surface of GeSn thin film before and after LA (tirr: 1.5 μs) are shown in Fig. 1(b). We can see clear peak around 300 cm-1 due to Ge-Ge atomic bond at the annealed region, which supports that GeSn film has been successfully crystallized by the annealing.
To discuss about the growth feature of GeSn films, EBSD measurements were carried out. Mapping images of Ge and Sn phase signals at surfaces of samples are summarized in Fig. 2 as a function of tirr. Here, we can clearly see that the area of Sn phase signal is expanding by increasing tirr. These results (Raman and EBSD) suggest that by increasing tirr, Sn precipitation toward surface of GeSn occurs after crystallization of GeSn films, due to excess annealing. As a result, by increasing tirr, substitutional Sn concentration in the films decreases after LA [Fig. 3], showing that short time annealing is essential to prevent the precipitations. Fig. 4 summarizes substitutional Sn concentration in GeSn films grown with shortest irradiation time (1.5 μs) as a function of laser power. By minimizing the irradiation time and tuning laser power to minimum value for crystallization, we could realize GeSn thin films with a high substitutional Sn concentration (13 %) on insulating substrates. These are very attracting results to realize next generation electronics.
Detailed physics and further tin concentration improvement methods will be discussed in the presentation.
[References]
[1] J.R. Haynes, W. Shockley, Phys. Rev. 81 835–843 (1951).
[2] S. Gupta, K. C. Saraswat, et.al., J. Appl. Phys. 113 073707 (2013).
[3] S. Nakaharai, S. Takagi, et. al., J. Appl. Phys. 105 024515 (2009).
[4] K. Toko, et. al., Sci. Rep., 7, 16981 (2017)
[5] R. Matsumura, N. Fukata, ECS J. Solid State Sci. Technol, 9 063002 (2020).

Figure 1