Yuki Tojo

Single-crystalline laterally graded GeSn on insulator structures by segregation controlled rapid-melting growth

Single-crystalline laterally graded GeSn-on-insulator (GeSnOI) structures are essential to achieve novel device-arrays with various direct-energy-band gaps, which can be merged with high-density Si large-scale-integrated-circuits. We investigate the seeding rapid-melting-growth of narrow stripes with a-Ge/Sn/a-Ge stacked-structures. This achieves laterally graded GeSn crystalline layers on Si substrates covered with SiO2 films. Stripe-length dependent GeSn lateral-profiles are quantitatively explained by Scheil equation, which enables precise designing of GeSn lateral-profiles. High-crystallinity GeSn stripes without dislocations or stacking faults are also demonstrated.

Growth-rate-dependent laterally graded SiGe profiles on insulator by cooling-rate controlled rapid-melting-growth

Laterally graded SiGe-on-insulator is the key-structure for next-generation Si-technology, which enables advanced device-arrays with various energy-band-gaps as well as 2-dimensional integration of functional-materials with various lattice-constants. Segregation kinetics in rapid-melting growth of SiGe stripes are investigated in wide ranges of stripe-lengths (10–500 μm) and cooling-rates (10–19 °C/s). Universal laterally graded SiGe-profiles obeying Scheil-equation are obtained for all samples with low cooling-rate (10 °C/s), which enables robust designing of lateral-SiGe-profiles. For samples with high cooling-rates and long stripe-lengths, anomalous two-step-falling profiles are obtained. Dynamical analysis considering the growth-rate-effects enables comprehensive understanding of such phenomena. This provides the unique tool to achieve modulated lateral-SiGe-profiles beyond Scheil equation.

High-quality formation of multiply stacked SiGe-on-insulator structures by temperature-modulated successive rapid-melting-growth

Laterally and vertically modulated SiGe-on-insulator (SGOI) structures are essential to integrate functional device-arrays with various energy-band-gaps and/or lattice-constants. We develop the temperature-modulated successive rapid-melting-growth (RMG) method, where Si-concentration dependent RMG processing is combined with non-destructive crystallinity-analysis. First, SGOI is formed by segregation-controlled RMG of SiGe by using Si-substrate as crystalline-seed. Polarized-Raman-scattering measurements non-destructively reveal the lateral-epitaxial-growth of SGOI with graded SiGe-concentration profiles. Second, Ge-on-insulator (GOI) is stacked on SGOI by using SGOI as crystalline-seed, where RMG temperature is selected between the melting-points of Ge and underlying SGOI. This achieves defect-free, multiply-stacked GOI on graded-SGOI structure, which demonstrates 3-dimensionally modulated SiGe-concentration profiles on Si-platform.

Low-Temperature (~300°C) Epitaxial-Growth of SiGe(Sn) on Si-Platform by Liquid-Solid Coexisting Annealing

To develop a new low-temperature crystallization technique, annealing characteristics of a-GeSn/c-Si structures are examined as a function annealing temperature (200-1000 o C) and Sn concentration (10-30%). By Sn-doping (~26%) into a-Ge, SiGe-mixing and epitaxial-growth temperatures are significantly decreased, which enables SiGe(Sn) epitaxial-growth at 300 o C. These phenomena are attributed to liquid-phase-growth through partial-melting-channel running across liquid-solid coexisting region. This technique facilitates integration of multi-functional devices on Si-platform.

Segregation-Free Giant Single-Crystalline SiGe-on-Insulator by Super-Cooling-Controlled Rapid-Melting Growth

Seedless rapid-melting growth of SiGe-on-insulator (SGOI) is investigated. By controlling the cooling rate, segregation-free giant SGOI (~400 μm) is achieved. TEM observations reveal that the crystallinity of SGOI is very high. This technique is useful for integration of advanced multi-functional devices on Si platform.

Formation of Large Grain SiGe on Insulator by Si Segregation in Seedless-Rapid-Melting Process

INTRODUCTION SiGe-on-insulator and Ge-on-insulator structures are essential to realization of advanced LSIs. Recently, we developed SiGe mixing triggered liquid-phase epitaxy (LPE) of a-Ge [1,2], which achieved high quality giant Ge single crystals (length: ~1cm) on insulating substrates [3]. In course of the study, we encountered a phenomenon that large grain Ge single crystals (~5 m) grew on insulating substrates without any seed [2]. On the other hand, SiGe has solidification-completing temperatures significantly different from solidification-initiating temperatures in melting growth. Thus, longer growth times are expected for SiGe compared with pure Ge. This triggered an idea that much larger crystal grains should be obtained, if a-SiGe films were melt-grown instead of a-Ge. To examine this idea, we investigate rapid-melting growth of a-SiGe on insulating substrates without any seed.