Naoyuki Kawabata

Orientation-controlled Si thin films on insulating substrates by Al-induced crystallization combined with interfacial-oxide layer modulation

Orientation-controlled Si films on transparent insulating substrates are strongly desired to achieve high-efficiency thin-film solar cells. We have developed the interfacial-oxide layer modulated Al-induced low temperature (<450 °C) crystallization technique, which enables dominantly (001) or (111)-oriented Si films with large grains (20–100 μm). These results are qualitatively explained on the basis of a model considering the phase transition of the interfacial Al oxide layers. This process provides the orientation-controlled Si templates on insulating substrates, which enables successive high quality epitaxial growth necessary for advanced Si thin-film solar cells.

Hybrid-orientation Ge-on-insulator structures on (100) Si platform by Si micro-seed formation combined with rapid-melting growth

Hybrid-integration of (111), (110), and (100) Ge-on-insulator (GOI) on an Si chip is essential to merge III-V semiconductor optical-devices as well as high-speed Ge transistors onto Si-large-scale integrated-circuits. We clarify important-parameters to control Ni-metal-induced lateral crystallization and Al-induced layer-exchange crystallization. This achieves artificial (110) and (111) Si micro-seed on insulating-film. Together with Si substrate as (100) Si seed, multi-crystal-seeds with different orientations are aligned on a Si chip. Then, SiGe-mixing triggered rapid-melting-growth of amorphous-Ge is examined from these multi-crystal-seeds. This enables simultaneous Ge lateral-crystallization with (111), (110), and (100) orientations. High-quality, hybrid-orientation GOIs without defects are demonstrated on Si platform.

(100) orientation-controlled Ge giant-stripes on insulating substrates by rapid-melting growth combined with Si micro-seed technique

Orientation-controlled single-crystal Ge stripes on insulating substrates are desired to achieve high-performance thin-film transistors. The rapid-melting growth process of amorphous Ge has been examined by using polycrystalline Si islands as the growth seed. Rotational growth is found for Ge stripes initiated from (110) and (111) orientations, however, the lateral-growth initiated from the (100) orientation propagates continuously keeping its orientation. Based on these findings, an advanced rapid-melting growth method is developed by combining with the Si(100) micro-seed technique. This enables single-crystal Ge(100) giant-stripes with 400 µm length on insulating substrates. High hole mobility exceeding 1000 cm2 V-1 s-1 is also demonstrated.

SiGe-Mixing-Triggered Rapid-Melting-Growth of High-Mobility Ge-On-Insulator

We have investigated the Si-seeding rapid-melting process and demonstrated the formation of giant Ge stripes with (100), (110), and (111) orientations on Si (100), (110), and (111) substrates, respectively, covered with SiO2 films. We revealed that crystallization is triggered by Si-Ge mixing in the seeding regions in this process. Based on this mechanism, we have proposed a novel technique to realize orientation-controlled Ge layers on transparent insulating substrates by using Si artificial micro-seeds with (100) and (111)-orientations. This achieved epitaxial growth of single crystalline (100) and (111)-oriented Ge stripes on quartz substrates. The Ge layers showed a high hole mobility exceeding 1100 cm2/Vs owing to the high crystallinity.

Low-temperature (≤250°C) crystallization of Si on insulating substrate by gold-induced layer-exchange technique

The gold-induced crystallization technique has been investigated to achieve poly-Si films on insulators at low temperatures (≤ 250°C). By annealing (∼250°C) the amorphous Si (a-Si)/Au stacked structures formed on insulating substrates, the positions of Si/Au layers are inverted, and Au/poly-Si stacked structures are obtained. On the other hand, by annealing (>400°C) the structures, mixed layers of c-Si and Au are obtained. These growth phenomena are explained on the basis of the eutectic reaction. This gold-induced layer-exchange growth technique at low-temperatures (∼250°C) is very useful to obtain poly-Si on flexible substrates, which are essential to realize flexible high-speed thin-films transistors and high-efficiency solar cells.

High-mobility defect-free ge single-crystals by rapid melting growth on insulating substrates

Single-crystal Ge films on insulating substrates are desired to achieve advanced 3-dimensional large-scale integrated circuits (3D-LSIs) and thin-film transistors (TFTs). We have developed the rapid-melting Ge growth seeded from Si substrates, which achieves giant Ge on insulator (GOI) structures with (100), (110), and (111) orientations. Driving force to initiate the lateral growth is clarified as the solidification temperature gradient originating from melting induced Si-Ge mixing. Combination with the artificial Si micro-seed technique and the rapid melting growth enables the single-crystal defect-free Ge on transparent insulating substrates. High hole mobility exceeding 1000 cm2/Vs is also demonstrated.