(Invited) Gold-Induced Low-Temperature (<300°C) Growth of Quasi-Single Crystal SiGe on Insulator for Advanced Flexible Electronics
Low-temperature (≤300oC) formation of quasi-single crystal SiGe, i.e., orientation-controlled large-grain crystal (≥10 μm) SiGe, on insulator is desired for realization of advanced flexible electronics, where high-speed transistors and high-efficiency optical devices are integrated on flexible plastics substrates (softening temperature: ~350°C). This is because SiGe provides higher carrier mobility and superior optical properties compared with Si.
Recently, we developed gold-induced layer-exchange crystallization (GIC) using a-Ge/Au stacked structures.[1-3] This enabled selective formation of (100)- or (111)-oriented large-grain Ge (≥20 μm) through controlling nucleation during GIC by using anisotropic surface energy on substrates. We have developed this technique onto plastic and realized orientation-controlled large-grain crystals on flexible plastic substrates with low concentration holes (~2×1017 cm-3), which enables high carrier mobility (~160 cm2/Vs).[2,3] However, reports of orientation-controlled growth by GIC are limited to pure Ge crystals so far.
In the present study the GIC technique is developed to SiGe. Consequently, quasi-single crystal SiGe is achieved on insulating substrates at low temperatures (~300oC).
The sample structure and experimental procedure are schematically shown in Fig. 1. In the experiment, fused-quartz chips were employed as substrates. On the substrates, a-SiGe (Ge concentration: 0-100%, thickness: 100 nm) /Au (thickness: 100 nm) stacked structures were formed. Here, amorphous Al2O3 interface-layers (thickness: 4-7 nm) were introduced between a-SiGe and Au layers as diffusion barrier to make interface-nucleation dominate over random bulk-nucleation in Au layers. These samples were annealed at 250-300oC to induce crystallization.
The grown layers were analyzed by Nomarski optical microscopy, electron backscattering diffraction (EBSD), and micro-probe Raman scattering spectroscopy (excitation-laser spot-diameter: ~1 μm). The Nomarski optical microscopy observations were carried out from the back sides of the samples through the transparent quartz substrates to characterize the bottom layers.
RESULTS AND DISCUSSION
The growth features of samples with various Ge fractions and Al2O3 interface layer thicknesses were investigated by Nomarski optical microscopy observations after annealing at 250°C for 50 h. Typical Nomarski micrographs of samples with Ge concentrations of 50% and 100% are shown in Figs. 2(a)-2(b) and 2(c)-2(d), respectively. In these micrographs, dark and bright contrast regions are observed. Micro-probe Raman measurements revealed that the dark regions dominantly observed in Figs. 2(a), 2(c), and 2(d) are crystal, while bright regions dominant in Fig. 2(b) are not crystal. These results indicate that layer exchange crystallization occurs for the samples shown in Figs. 2(a), 2(c), and 2(d), while it does not proceed for the sample shown in Fig. 2(b). EBSD measurements revealed that the grown layers for Fig. 2(d) consisted of (111)-oriented large-grains (≥10 μm). On the other hand, grown layers for Figs. 2(a) and 2(c) consisted of randomly-oriented small-grains (1-3 μm).
Here, we examined the Nomarski micrographs of a no-grown Si0.5Ge0.5 sample (diffusion barrier thickness: 5 nm), shown in Fig. 2(b). It is found that nuclei are formed for the Si0.5Ge0.5 sample, though lateral growth does not proceed from the nuclei. From this finding, it is speculated that for the Si0.5Ge0.5 sample (diffusion barrier thickness: 5 nm), (111)-oriented nucleation occurs at Au/SiO2 interface. However, lateral growth does not proceed even after long time annealing. Thus, layer-exchange is not observed. It is suggested that lateral crystallization in GIC is retarded by Si introduction.
This consideration triggers an idea that (111)-oriented large-grains can be obtained even for the Si0.5Ge0.5 sample by generating lateral growth from nuclei. Thus, we examine annealing at a higher temperature of 300°C. A Nomarski micrograph of Si0.5Ge0.5 sample (diffusion barrier thickness: 5 nm) after annealing at 300°C for 50 h is shown in Fig. 3(a). Micro-probe Raman measurements reveled layer-exchange crystallization in the entire area. The crystal structures of the grown layer are examined by EBSD measurements. The result is shown in Fig. 3(b), which reveals (111)-oriented large-grains (≥10 μm). As a result, quasi-single crystal SiGe is realized on insulator. This technique will facilitate realization of advanced flexible electronics.
Part of this work was supported by JSPS Core-to-Core Program, A. Advanced Research Networks.
 J.-H. Park et al., Appl. Phys. Lett. 103, 082102 (2013).  J.-H. Park et al., Jpn. J. Appl. Phys. 53, 020302 (2014).  J.-H. Park et al., Appl. Phys. Lett. 104, 252110 (2014).