Cooling Rate Dependent High Substitutional Sn Concentration (>10%) in GeSn Crystals on Insulator by Pulsed Laser-Annealing

Abstract

Introduction Crystalline GeSn thin-films on insulator with high substitutional Sn concentration (>10%) are deseired to realize high-speed thin film transistors. However, thermal equilibrium solid-solubility of Sn in Ge is only 1%. To solve this problem, we examined non-thermal equilibrium crystallization of amorphous-GeSn (a-GeSn) by pulsed laser-annealing (PLA). In the present study, we focus on the influence of pulse number on crystallization of a-GeSn by PLA. Experimental Procedure In the experiment, a-Ge1-xSnx (0≦x≦0.2) films (thickness: 100 nm) were prepared on quartz substrates by a molecular beam deposition technique. These samples were irradiated with a pulsed laser (wavelength: 248 nm, pulse duration: 80 ns, repetition rate: 100 Hz, fluence: ~140 mJ/cm2, pulse number: 1-200 shot). The laser-annealed regions were analyzed by Nomarski microscopy, Raman scattering spectroscopy, and transmission electron microscopy (TEM). Cooling rate after PLA was estimated by in-situ optical reflectivity measurements. Results and Discussion Figure 1(a) shows Raman spectra of Ge0.95Sn0.05 samples after PLA (laser flunence: 140 mJ/cm2) with various pulse numbers. In all spectra, sharp peaks due to Ge-Ge bonding in crystalline GeSn are observed. Thus, GeSn has been crystallized for all pulse numbers, which was confirmed by TEM observation. Substitutional Sn concentration in the grown layers is investigated from the difference of Ge-Ge Raman peak positions between GeSn samples and a crystalline Ge substrate [1]. Pulse number dependence of substitutional Sn concentration is shown in Fig. 1(b). Substitutional Sn concentration increases with decreasing pulse number, and GeSn crystals with very high substitutional Sn concentration (~13%) has been realized for the sample (initial Sn concentration: 20%) with a single shot. These results suggest that non-thermal equilibrium conditions are enhanced by lower pulse number. To explore non-thermal equilibrium growth by modulating pulse number, cooling rate after PLA is estimated by in-situ optical reflectivity measurements, as shown in Fig. 2(a). Reflectivity at 10th and 100th shots as a function of time is shown in the inset of Fig. 2(b). From the decreasing curve of reflectivity, time constants of cooling processes are estimated, which are summarized in Fig. 2(b). Interestingly, substitutional Sn concentration increases with decreasing time constant. The detailed physics will be discussed in the presentation.