Non-Thermal Equilibrium Formation of Ge1-xSnx (0�≤x�≤0.2) Crystals on Insulator by Pulsed Laser Annealing

Introduction

Formation of crystalline GeSn film on insulator with high Sn concentration exceeding thermal equilibrium solid-solubility is strongly expected to realize high-speed thin film transistors. To explore non-thermal equilibrium processing [1], we examined crystallization of amorphous-GeSn (a-GeSn) with various modulated Sn concentration by pulsed laser annealing (PLA). In the present study, we focus on the influence of the Sn concentration on crystallization of a-GeSn by PLA.

Experimental Procedure

Schematic sample structure and PLA conditions are shown in Fig.1. A-Ge_1-xSn_x (0≦x≦0.2) films (thickness: 100 nm) were deposited on quartz substrates by a molecular beam technique. These samples were irradiated with a pulsed laser (energy : 10 - 230 mJ/cm², pulse period : 1 sec, pulse number : 100 shot). The laser annealed regions were analyzed by Nomarski microscopy and Raman scattering spectroscopy.

Results and Discussion

　Nomarski micrograph and Raman spectra of Ge_0.8Sn_0.2 samples before and after PLA with various energy are summarized in Fig. 2. In the samples after PLA, the contrast-changed regions were observed, compared with the as-deposited sample. Here, the contrast becomes stronger with increasing energy from 50 mJ/cm² to 170 mJ/cm², and finally the GeSn layer was damaged for energy of 230 mJ/cm². Raman spectra reveales that Ge_0.8Sn_0.2 is crystallized on insulating substrates by 50 - 170 mJ/cm² PLA.

The process-window to achieve crystalline GeSn is summarized in Fig. 3(a), where the energy for starting crystallization and the energy just before damaging are indicated as E_cryst. and E_damage , respectively. Substitutional Sn concentration estimated from shift of Ge-Ge peak obtained by Raman measurements is shown in Fig. 3(b). The process window necessary for crystallization is significantly expanded by introducing Sn, i.e. 16 mJ/cm² for Ge, ~ 130 mJ/cm² for GeSn.

High Sn concentration exceeding thermal equilibrium solid-solubility is obtained by PLA, for all GeSn samples. Interestingly, for samples with initial Sn concentration of 5 % and 10 %, almost all Sn atoms are incorporated into substitutional site, while for 20 % sample, only half of Sn atoms are incorporated into substitutional site. The detailed physics will be discussed in the presentation.

References

[1] M. Kurosawa, et al., Appl. Phys. Lett. 104, 061901 (2014).