High-quality crystalline Ge thin-films on a plastic substrate are essential to realize flexible system-in-displays, where high-speed transistors and multi-functional devices are integrated on a display panel. Ge is a promising channel material for high-speed operation, due to the higher carrier mobility compare to conventional Si devices. Furthermore, (100)- and (111)-oriented Ge films provide epitaxial templates for optical and spintronic functional materials, such as GaAs and Fe3Si, respectively, owing to good lattice matching. Thus, selective growth technique of (100)- and (111)-oriented Ge should be developed.
Ultra-low-temperature processing (≤250oC) is another challenge for these applications to avoid softening of plastic substrates. To achieve this, we have been investigated metal-induced-crystallization techniques (MIC). In MIC methods, the catalytic effects of metals enhance the crystal nucleation and subsequent nucleus growth. In the recent study, we reported the gold-induced crystallization technique (GIC) employing a-Ge/Au stacked structures, and demonstrated crystallization of a-Ge at ultra-low-temperatures (~250oC) through the exchange of layer positions [1]. However, the grown Ge layers consist of randomly-oriented small-grains, due to the significant bulk nucleation of Ge in Au layers.
To achieve orientation-control, randomly-oriented bulk nucleation should be suppressed, and instead, preferentially-oriented nucleation at Au/substrate interfaces should be dominated. This domination of interfacial nucleation can be achieved by retarding Ge atomic supply, because interfacial nucleation is energetically favorable compared to bulk nucleation. Here, (100)- or (111)-oriented interface nucleation can be obtained by employing Al2O3 or SiO2 substrates, respectively, due to the lowest interfacial energy [2,3]. Based on these ideas, the present study demonstrates selective growth of (100)- and (111)- Ge by controlling interfacial energy in the nucleation process of GIC.
The sample structure is schematically shown in Fig. 1. Quartz (SiO2) substrates and Al2O3 covered (30 nm) quartz substrates were employed. On these substrates, a-Ge/Au stacked structures (thickness: 100 nm, respectively) having the diffusion barriers [thin Al2O3 layers (0-10 nm)] at the a-Ge/Au interfaces were formed. These samples were annealed (250oC) in dry N2 ambient.
For samples with diffusion-barrier thickness of 0-7 nm, randomly-oriented small-grain Ge (1-2 μm) was obtained. This indicates the bulk nucleation cannot be suppressed by such thin diffusion barriers. Interestingly, for diffusion-barrier thickness of 8 nm, (100)- or (111)-oriented large-grains (20-50 μm) were achieved on quartz (SiO2) and Al2O3 covered substrates, respectively [Fig. 2. (a, b)]. Further increase of diffusion-barrier thicknesses resulted in significant retardation of growth. These results show that random bulk nucleation is effectively suppressed and thus interface nucleation is dominated by using diffusion barrier of ~8 nm thickness. Consequently, selective-growth of (100)- and (111)-oriented Ge becomes possible through the interfacial energy modulation by employing SiO2 and Al2O3 substrates, respectively.
In summary, interfacial-energy-controlled GIC process has been developed, which enables the selective formation of (100)- or (111)-oriented Ge crystals on insulators at an ultra-low-temperature (250oC). This technique is very useful to realize advanced new-functional flexible devices.
[1] J. H. Park, et al.: Thin Solid Films 520 (2012) 3293.
[2] J. Schneider, et al.: J. Cryst. Growth 287 (2006) 423.
[3] R. Jaccodine: J. Electrochem. Soc. 110 (1963) 524.
Fig. 1 Schematic sample structure.
Fig. 2 EBSD images of grown layer of the samples on quartz (SiO2) (a) and Al2O3-covered substrate (b).
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