Crystallization of Ge Thin Films on Sapphire(0001) by Thermal Annealing
Authors: Sugawa Hibiki , Ohta Akio , Kobayashi Masato , Taoka Noriyuki , Ikeda Mitsuhisa , Makihara Katsunori , Miyazaki Seiichi 
Published: December 2020

Introduction>
A formation of single crystalline semiconductor such as Silicon on Sapphire substrate has been intensively studied for applying the high frequency devices with low power consumption and high-speed operation. Recently, two-dimensional honeycomb crystals consisted with Si and Ge atoms so called sillicene and germanene have been attracted much attention as a post-graphene material [1,2]. One of the key issues to exhibit their unique electronic properties for device application is the formation of silicone and germanene on the insulating substrate [3]. Therefore, the purpose of this work is to get an insight into the formation of ultra-thin Ge crystal on Sapphire substrate. We have investigated an influence of thermal annealing on the surface morphology and the solid-phase-crystallization of amorphous Ge thin films formed on Sapphire(0001) substrate.

Experimental procedure
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Surface treatment of Sapphire(0001) substrate was performed by the ultrasonic cleaning with acetone and subsequent annealing at 1000 °C for 5 hours in N 2 ambience to obtain the cleaned flat surface. Then, a Ge thin film with a thickness of 8 nm was deposited by RF magnetron sputtering with a power density of 254 W/cm2 at Ar pressure of ~0.9 Pa. In some samples, a SiO2 capping layer with a thickness of ~ 15 nm was formed by ALD using BEDAS gas and O2 plasma at a substrate temperature of 300ºC. After that, annealing in N2 ambience at temperatures of 600 °C and 650 °C for 30 minutes were carried out to crystalize the Ge thin films. During the annealing, ramp rate was set at 20 °C/min. The surface morphology and crystallinity of the Ge thin layer for the samples were evaluated by atomic force microscope (AFM) and Raman scattering spectroscopy.

Results and discussion
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Figure 1 shows the AFM topographic images of the Ge thin film formed on the Sapphire(0001) substrate after the annealing at 650 °C. It was also confirmed that atomically flat surface of Sapphire(0001) with step and terrace structure was obtained by the surface treatment, and the flatness was almost unchanged even after the deposition of Ge thin film and the SiO2 capping layer (data not shown). For the samples after the annealing at 650 °C, the SiO2 capping layer was effectively suppressed an increase in the surface roughness due to the Ge (or GeO) desorption and migration, and line shapes originated from the crystalline structure near the surface were partially detected in the AFM image (Fig. 1(a)).
An impact of the annealing on the crystallinity of the Ge thin film for the samples with SiO2 capping layer was evaluated from Raman scattering spectra excited by green laser with a wavelength of 532 nm as shown in Fig. 2. A broad Raman feature around 275 cm-1 due to the amorphous Ge (a-Ge) was dominant for the samples before the annealing and after the annealing at 600 °C. After the annealing at 650 ºC, a sharp signal of the TO phonon mode in the crystalline phase (c-Ge : ~300 cm-1) was clearly observed, which peak width was slightly larger than that of a reference spectrum of Ge substrate. Also, the peak tailing toward the lower Raman shift side from c-Ge signals originated from poly-crystalline were detected. From these results, it was found that the crystal nucleation in the 8 nm-thick amorphous Ge film on Sapphire(0001) substrate occurs at around 650 °C. In addition, the peak shift of c-Ge signals as compared to the reference Ge wafer was detected, which was likely to be responsible for the tensile stress caused by the lattice mismatch between Ge and Sapphire.
In summary, the Ge thin films with a thickness of ~8nm on the cleaned Sapphire(0001) substrate was found to be crystallized with keeping the surface flatness using SiO2 capping layer by the annealing at 650 ºC.

References >

[1] K. Ito et al., Jpn. J. Appl. Phys. 57, 06HD08 (2018). [2] J. Yuhara et al., ACS Nano 12, 11632 (2018). [3] Z. Guo, et al., J. Phys. Soc. of Jpn., 82, 063714 (2013).

Acknowledgement >

This work was supported in part by JSPS Grant-in Aids for Scientific Research Nos. 19H02169.

Figure 1