(Invited) Low-Temperature Growth of Orientation-Controlled Large-Grain Ge-Rich SiGe on Insulator at Controlled-Position for Flexible Electronics

Abstract

I NTRODUCTION Germanium-rich silicon-germanium (Ge-rich SiGe) (Ge fraction: 50−100%) provides higher carrier mobility and superior optical properties compared with Si. In the field of large-scale integrated circuits, Ge-rich SiGe devices are being employed to break through the scaling limit. Such an approach is also very useful in the fields of advanced thin-film electronics, such as system-in-displays. In addition, to improve the usability of the system-in-displays, Ge-rich SiGe-based high-performance thin-film devices should be integrated on flexible plastic substrates (softening temperature: ~350°C). For this purpose, a technique for low-temperature (≤300oC) growth of orientation-controlled large-grain (≥10 μm) Ge-rich SiGe on insulator should be developed. In addition, position control of the orientation-controlled large-grains is useful to obtain high-performance devices without degradation by grain boundaries. Various growth techniques were investigated to achieve SiGe on insulator. For example, solid-phase crystallization enables growth of poly-SiGe films on insulator. However, high-temperature annealing at a temperature above 500oC is necessary to induce crystallization of SiGe.For low-temperature growth of SiGe on insulator, we have been investigating metal-induced crystallization using gold as catalyst [1-4]. These efforts enable low-temperature growth of orientation-controlled large-grain Ge-rich SiGe on insulator at controlled-positions. LOW-TEMPERATURE GROWTH OF ORIENTATION-CONTROLLED LARGE-GRAIN BY INTERFACE-NUCLEATION In order to obtain poly-SiGe films on insulator at low temperatures, we have developed gold-induced crystallization (GIC), where a-SiGe/Au stacked structures are employed, as shown Fig. 1 [1]. Owing to bond-modulation by catalysis, GIC proceeds at ~250oC for SiGe with the whole Ge fractions (0−100%). However, for these samples, layer-exchange of the stacked structures results in formation of randomly-oriented poly-SiGe films on insulator.To achieve orientation-controlled large-grains by GIC, randomly-oriented bulk nucleation in Au layers should be suppressed and instead, preferentially-oriented nucleation at Au/insulator interfaces should be dominated. This domination of interface nucleation can be achieved through retarding Si and Ge supply into Au layers by introducing diffusion barrier between a-SiGe and Au layers, because interface nucleation is energetically favorable compared to bulk nucleation. Use of anisotropy of free energy of SiGe nuclei is a key for orientation control. Based on these ideas, we have examined control of nucleation site by modulating atomic diffusion in the layer-exchange process, as shown in Fig. 2(a). As a result, orientation-controlled large-grain Ge-rich SiGe crystals are realized on amorphous insulator at a low temperature (≤300oC) [2]. The growth of orientation controlled large-grains (≥50 μm) on flexible substrates becomes possible, as shown in Figs. 2(b)-2(d) [3]. The large grains provide high carrier mobility (160 cm2/Vs) [3]. POSITION CONTROL BY SPATIALLY-MODULATED NUCLEATION In order to control the positions of large grains, micro-opening in diffusion barrier is examined, as shown in Fig. 3(a) [4]. By decreasing the diameter of the micro-opening below 3 μm, a single interface-nucleus is selectively generated. This enables growth of orientation-controlled large-grains (≥10 μm) at controlled positions, as shown in Figs. 3(b) and 3(c). This technique will facilitate fabrication of high-performance Ge-rich SiGe thin-film devices for flexible electronics.