Si-Ge Mixing Research Articles

Morphology and optical properties of Ge nanocrystalline films grown by nonequilibrium epitaxy on Si (001) surface

Optical properties and morphology of thin films with Ge and SiGe nanocrystallites are studied by using the methods of Raman scattering, multi-angle ellipsometry, and X-ray measurements. Our observations showed that low-temperature Ge epitaxy on Si(001) surface at 350 °C and at high growth rate leads to the formation of thin films consisting of Ge nanocrystallites with the porosity of about 50%. Deposition of Si adatoms on the film surface stimulates the reconstruction of the surface with nanocrystallites and results in the increase of the degree of Ge crystallinity, coalescence of adjacent nanocrystallites, and in a slight Si-Ge mixing. The change in the values of the optical constants due to silicon deposition is described by means of Bruggeman approximation. The main reasons for these changes are the film crystallization and the appearance of Si crystalline phase in the structures with Si capping layer. The prepared Ge nanocrystalline solids, as well as the films covered with Si, may be used in quantum dot solar cells due to the high absorption capacity of the films and devices working in the hopping regime.

Lateral growth of single-crystal Ge on insulating substrate using amorphous Si seed by rapid melting growth

A rapid melting growth of Ge on insulating substrate was developed using amorphous Si seed. Spontaneous nucleation process and lateral growth process occurred at different annealing temperature. Single-crystal Ge stripes on the insulator substrate were obtained as the Ge stripe was dominated by the lateral growth process along the stripes. Different single-crystal Ge stripe has different orientation. No preferential orientation was observed. Spatial gradient of solidification temperature along the Ge stripes induced by Si–Ge mixing in seeding region is the key to cause lateral growth of Ge stripes. Saturation of Si concentration along the Ge stripes at higher annealing temperature was observed, which was explained by limited Si volume in the seeding region.

Ultra-low temperature (≤300 °C) growth of Ge-rich SiGe by solid-liquid-coexisting annealing of a-GeSn/c-Si structures

Ultra-low temperature (≤300 °C) growth of Ge-rich SiGe on Si substrates is strongly desired to realize advanced electronic and optical devices, which can be merged onto Si large-scale integrated circuits (LSI). To achieve this, annealing characteristics of a-GeSn/c-Si structures are investigated under wide ranges of the initial Sn concentrations (0%–26%) and annealing conditions (300–1000 °C, 1 s–48 h). Epitaxial growth triggered by SiGe mixing is observed after annealing, where the annealing temperatures necessary for epitaxial growth significantly decrease with increasing initial Sn concentration and/or annealing time. As a result, Ge-rich (∼80%) SiGe layers with Sn concentrations of ∼2% are realized by ultra-low temperature annealing (300 °C, 48 h) for a sample with the initial Sn concentration of 26%. The annealing temperature (300 °C) is in the solid-liquid coexisting temperature region of the phase diagram for Ge-Sn system. From detailed analysis of crystallization characteristics and composition profiles in grown layers, it is suggested that SiGe mixing is generated by a liquid-phase reaction even at ultra-low temperatures far below the melting temperature of a-GeSn. This ultra-low-temperature growth technique of Ge-rich SiGe on Si substrates is expected to be useful to realize next-generation LSI, where various multi-functional devices are integrated on Si substrates.

Low Temperature (~300°C) Epitaxial Growth of SiGe by Liquid-Solid Coexisting Annealing of A-GeSn/Si(100) Structure

To develop a new low-temperature crystallization technique, annealing characteristics of a-GeSn/Si (100) structures are investigated. It is revealed that epitaxial growth accompanying Si-Ge mixing is generated at temperatures in the liquid-solid coexisting region of the Ge-Sn system. The annealing temperature necessary for epitaxial growth is significantly decreased by increasing annealing time and/or Sn concentration. Consequently, epitaxial growth at 300°C becomes possible. These findings are expected to be useful to realize next-generation large-scale integrated circuits, where various multi-functional devices are integrated.

Effects of high temperature rapid thermal annealing on Ge films grown on Si(001) substrate

Tensile strain, crystal quality, and surface morphology of 500 nm thick Ge films were improved after rapid thermal annealing at 900 °C for a short period (< 20 s). The films were grown on Si(001) substrates by ultra-high vacuum chemical vapor deposition. These improvements are attributed to relaxation and defect annihilation in the Ge films. However, after prolonged (> 20 s) rapid thermal annealing, tensile strain and crystal quality degenerated. This phenomenon results from intensive Si—Ge mixing at high temperature.

Liquid-Solid Coexisting Annealing of a-GeSn/Si(100) Structure for Low Temperature Epitaxial Growth of SiGe

To develop a new low-temperature crystallization technique, annealing characteristics of a-GeSn/Si(100) structures are investigated. It is revealed that epitaxial growth accompanying Si-Ge mixing is generated at temperatures in the liquid-solid coexisting region of the Ge-Sn system. The annealing temperature necessary for epitaxial growth is significantly decreased by increasing annealing time and/or Sn concentration. Consequently, epitaxial growth at 300°C becomes possible. These findings are expected to be useful to realize next-generation large-scale integrated circuits, where various multi-functional devices are integrated.

(Invited) Hybrid-Formation of Ge-on-Insulator Structures on Si Platform by SiGe-Mixing-Triggered Rapid-Melting Growth --- A Road to Artificial Crystal ---

Single-crystal Ge layers on insulating films (GOI) are desired to achieve advanced 3-dimensional large-scale integrated circuits and high-performance thin-film transistors. We have developed the rapid-melting Ge growth seeded from Si substrates, which achieves chip-size (~cm length) GOI structures with (100), (110), and (111) orientations. Driving force to initiate the lateral growth over insulating films is clarified as the solidification temperature gradient originating from melting-induced Si-Ge mixing. Combination with the artificial Si micro-seed techniques and the rapid melting growth enables the formation of hybrid-orientation GOI arrays, i.e., (100), (110), and (111) stripes, on (100) Si platform. High hole mobility (~1000 cm2/Vs) in hybrid-orientation GOI is also demonstrated.

Growth-Direction Dependent Rapid-Melting-Growth of Ge-On-Insulator (GOI) and its Application to Ge Mesh-Growth

Single crystal Ge-on-insulator (GOI) structures with various crystal orientations are necessary for realization of advanced high-speed and multi-functional devices. SiGe mixing triggered rapid-melting-growth of GOI is investigated as a function of seed-orientations and growth-directions. Single crystal growth of (100)-Ge strips is possible for all growth directions using (100)-oriented Si-seeds. However, rotational-growth is observed for some directions when Si-seeds with (110) and (111) orientations are employed. Such rotational-growth is completely suppressed by selecting the growth-directions deviating from the <111> direction by more than 35o. Based on this finding, growth of large mesh-patterned Ge layers with (100), (110), and (111) orientations are demonstrated.

Growth-direction-dependent characteristics of Ge-on-insulator by Si–Ge mixing triggered melting growth

The lateral liquid-phase epitaxy of Ge-on-insulator (GOI) using Si seeds has been investigated as a function of the Si-seed orientation and the growth direction. Giant single-crystalline GOI structures with ∼200 μm length are obtained using Si(1 0 0), (1 1 0), and (1 1 1) seeds. The very long growth is explained on the basis of the solidification temperature gradient due to Si–Ge mixing around the seeding area and the thermal gradient due to the latent heat around the solid/liquid interface at the growth front. In addition, growth with rotating crystal orientations is observed for samples with several growth directions. The rotating growth is explained on the basis of the bonding strength between lattice planes at the growth front. This rotating growth does not occur in any direction for (1 0 0) orientated seeds. Based on this finding the mesh-patterned GOI growth with a large area (250 μm × 500 μm) is 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 SiO2films. 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.

(Invited) High-Mobility Ge on Insulator (GOI) by SiGe Mixing-Triggered Rapid-Melting-Growth

High-quality orientation-controlled Ge on insulator (GOI) structures are essential to realize high-performance thin-film transistors (TFTs) and epitaxial templates for multifunctional 3-dimensional large-scale integrated circuits (3D-LSIs). We have investigated the Si-seeding rapid-melting process and demonstrated 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 transmission electron microscopy observations revealed no-defects in the laterally grown Ge regions. The Ge layers showed a high hole mobility exceeding 1100 cm2/Vs owing to the high crystallinity. This novel SiGe mixing-triggered growth technique opens up the possibility of the next-generation TFTs and multifunctional 3D-LSIs.

Giant growth of single crystalline Ge on insulator by seeding lateral liquid-phase epitaxy

Giant growth (~ 400 μm length) of single crystalline Ge on insulator (GOI) with (100), (110), and (111) orientations is demonstrated by lateral liquid-phase epitaxy (L-LPE) using Si(100), (110), and (111) substrates, respectively, as the seeds. The micro-probe Raman measurements and transmission electron microscopy observations showed that the growth regions were of very high crystal quality and were defect free. In addition, lateral diffusion of Si atoms was observed only in the regions near the seeding edges (~ 100 μm). Based on these findings, the trigger for the giant growth of the high-quality GOI was discussed considering the solidification temperature gradient due to Si–Ge mixing and the thermal gradient due to the latent heat at the growth front.