Ultra-Low Temperature (~180°C) Solid-Phase Crystallization of GeSn on Insulator Triggered by Laser-Anneal Seeding

Solid-phase crystallization (SPC) of amorphous-GeSn (a-GeSn) films on insulating substrates has been developed combining with laser-annealed seeding, to realize next generation thin-film devices. By this technique, we have realized crystallization of GeSn (>10%) at low temperatures (~180^oC), which is applicable for flexible thin-film devices on low cost plastic substrates with low softening temperatures (~200^oC). In addition, the starting point of crystallization can be controlled by seeding, which is big advantage in device designing.

Introduction

In order to realize next generation flexible devices, novel functional materials have to be crystallized at controlled positions on insulating substrates below the softening temperature of low cost plastic substrates (~200^oC). GeSn is attracting material for this purpose, since GeSn has higher carrier mobility than Si or Ge, and moreover, GeSn shows direct band structure by introducing substitutional Sn concentration more than 8%.  Under such background, we have investigated solid-phase crystallization (SPC) of GeSn [1] and realized growth of GeSn at 180^oC. However, to initiate the SPC at controlled positions, the samples have to be annealed above the melting point of Sn (~232^oC) to form seeding regions, which is not applicable on flexible plastic substrates.

In line with this, we propose an idea to use laser-annealing (LA) to form crystal seeds. By forming seeds by LA, lateral SPC is expected at process temperature below 200°C. Moreover, by decreasing the process temperature, we can expect that GeSn crystals with high substitutional Sn concentration should be obtained.

Experimental Procedure

In the experiment, amorphous Ge_0.8Sn_0.2 layers (100 nm thickness) were deposited on quartz substrates by a molecular beam technique. The samples were irradiated with a KrF excimer laser (wavelength: 248 nm, energy: 160 mJ/cm²) with 100 pulses in 1 sec to form seed regions, followed by thermal annealing (180 - 220 ^oC) for lateral SPC as shown in Fig. 1.

Results and Discussion

Nomarski images of samples as LA and after annealing at 180^oC for 48h are shown in Figs. 2(a) and 2(b), respectively. The laser annealed seed region is formed as shown in Fig. 2(a). From the seed, a bright region spreads after 180^oC annealing.

The Raman spectra obtained at points A, B, and C in Figs. 2(a) and 2(b) are shown in Fig. 2(c). From these spectra, we can clearly say that lateral SPC is initiated from only the seed and no spontaneous nucleation occurs.

From the length of lateral SPC region, growth rates of GeSn at various annealing temperatures (180, 200, 220 ^oC) are evaluated. It is found that the growth rate is about 1 μm/h at 180 ^oC, which is nearly 10⁶ time as large as that of Ge [2]. The activation energy of the growth rate of Ge_0.8Sn_0.2 was estimated to be ~1.5 eV, which was smaller than that of Ge (~2.0 eV). This is attributed to the weakening atomic bonds by Sn atoms. This is a very useful technique to realize next generation flexible thin-film devices.

Substitutional Sn concentration in grown layer obtained after annealing at 180^oC for 48h is evaluated by Raman measurement, which is shown in Fig. 3. Substitutional Sn concentration in SPC region shows higher value (>10%) than that of LA region (~7.5%), which is attributed to difference of non-equilibrium conditions between SPC and LA. Interestingly, substitutional Sn concentration shows very high values exceeding 15% near the growth edge. The Sn concentration decreases with increasing distance from the edge. Decrease in the Sn concentration from >15% to ~10% is attributed to the post annealing effect of grown regions. Detailed physics will be discussed at the presentation.

References

[1] H. Chikita, et al., Appl. Phys. Lett. 105 (2014) 202112.

[2] K. Toko, et al., Appl. Phys. Lett. 94 (2009) 192106.