Low-Temperature Formation of Sn-Doped Ge on Insulating Substrates by Metal-Induced Crystallization
Authors: Sakai Takatsugu , Matsumura Ryo , Sadoh Taizoh , Miyao Masanobu 
Published: February 2020

Low-temperature formation of Sn-doped Ge on insulator is desired to realize next generation flexible electronics. To achieve this, metal-induced crystallization (MIC) of a-GeSn is investigated. By MIC of a-GeSn with initial Sn concentration of 5%, Sn-doped Ge is obtained at low temperatures (<250℃). The Sn concentration (0.5-2.0%) in the grown layers can be controlled by modulating the annealing temperatures in the range of 150-250℃.

Introduction

A technique for low-temperature (≤250°C) formation of high-quality crystalline Ge films on insulator is expected to realize advanced high performance thin film transistors (TFT) on flexible plastic substrates (softening temperature: ~300oC). To improve quality of Ge crystals, Sn-doping (<4%) is very effective, because it enables passivation of point defects in Ge [1]. This triggered the recent research of solid-phase crystallization of Sn-doped Ge (Sn concentration: 2%) on SiO2layers [2]. However, the growth temperature (425°C) reported to date is higher than the softening temperatures of plastic substrates. To decrease the growth temperature of Sn-doped Ge, in the present study, we investigate metal-induced crystallization (MIC) of a-GeSn. This successfully decreases the growth temperature below 250°C.

Experimental Procedure

Fig.1 schematically shows the procedure of sample preparation. Amorphous Ge1-xSnx layers (x: 0-0.2, thickness: 100 nm) were deposited on quartz substrates at room temperature. Then, Au layer patterns were formed on the amorphous GeSn layers. The samples were annealed at 150-250oC for 10-60 min to induce lateral growth in a N2ambient. The growth characteristics were analyzed by Nomarski optical microscopy and microprobe Raman spectroscopy.

Results and Discussion

Figs. 2(a) and 2(b) show Nomarski optical micrographs of Ge1-xSnx (x: 0.05 and 0.2, respectively) samples after annealing at 250oC for 10 min. Au pattern regions are located in the left side areas of the micrographs. In the case of the low Sn concentration (5%), different contrast regions are observed around the Au patterns, as indicated by the broken lines. On the other hand, the sample with the high Sn concentration (20%) shows no change around the Au pattern after annealing. The Raman spectra obtained from these samples are summarized in Fig. 2(c). In Raman spectra #1 for the low Sn concentration (5%), a sharp peak due to Ge-Ge bonding in c-Ge is clearly observed. On the other hand, no peaks are detected in spectra #3 for the sample with the high Sn concentration (20%). These results indicate that lateral growth is generated from the Au patterns for samples with the low Sn concentration (5%), while it is not generated for the high Sn concentration (20%).The lateral growth lengths at 150 and 250oC for samples with various Sn concentrations were estimated using theNomarski optical microscopy combined with line-scanning of microprobe Raman measurements. The results are summarized as a function of the annealing time in Fig. 3. In the case of low Sn concentrations (<5%), lateral growth lengths increase in a short annealing time (10 min). In addition, the growth lengths increase with increasing annealing temperature and exceed 20 μm at 250°C, which is sufficient for device fabrication.The substitutional Sn concentrations in grown layers for samples with the initial Sn concentration of 5% are evaluated by peak shift of Raman spectra. Substitutional Sn concentrations of 0.5−2.0% are obtained in grown layers, which is useful to passivate point defects in Ge [1]. The substitutional Sn concentration depended on the annealing temperature. This indicates that the substitutional Sn concentration can be controlled by modulating annealing temperature. This technique will be useful to realize next generation flexible electronics.

References

[1] O. Nakatsuka, et al., Jpn. J. Appl. Phys. Lett. 49, 04DA10 (2010).

[2] W. Takeuchi, et al., Appl. Phys. Lett. 107, 022103 (2015).

Figure 1