High Ge Content Research Articles

Selective Etching of Si versus Si1-xGex in Tetramethyl Ammonium Hydroxide Solutions with Surfactant.

We investigated the selective etching of Si versus Si1−xGex with various Ge concentrations (x = 0.13, 0.21, 0.30, 0.44) in tetramethyl ammonium hydroxide (TMAH) solution. Our results show that the Si1−xGex with a higher Ge concentration was etched slower due to the reduction in the Si(Ge)–OH bond. Owing to the difference in the etching rate, Si was selectively etched in the Si0.7Ge0.3/Si/Si0.7Ge0.3 multi-layer. The etching rate of Si depends on the Si surface orientation, as TMAH is an anisotropic etchant. The (111) and (010) facets were formed in TMAH, when Si was laterally etched in the <110> and <100> directions in the multi-layer, respectively. We also investigated the effect of the addition of Triton X-100 in TMAH on the wet etching process. Our results confirmed that the presence of 0.1 vol% Triton reduced the roughness of the etched Si and Si1−xGex surfaces. Moreover, the addition of Triton to TMAH could change the facet formation from (010) to (011) during Si etching in the <100>-direction. The facet change could reduce the lateral etching rate of Si and consequently reduce selectivity. The decrease in the layer thickness also reduced the lateral Si etching rate in the multi-layer.

(Digital Presentation) Strain Engineering of Heteroepitaxial SiGe/Ge on Si with Various Crystal Orientations

Strained SiGe layers with high Ge concentrations are grown on the Ge and Ge-on-Si substrates with various crystal orientations, and structural properties are studied. Particularly, initial stages of defect formation and related surface morphology are investigated. It is found that cracks are generated in the tensile-strained layer and surface ridge roughness is formed around the cracks although the total relaxation ratio of the SiGe layer is almost 0 %. Due to annealing the tensile strain is partially relaxed by creating additional trench-like roughness with the initially formed ridge roughness left unchanged. This comparative investigation of strain relieving defect formation provides quite useful information toward applications to strained SiGe-based optoelectronic and spintronic devices with enhanced performances.

Increasing the critical thickness of SiGe layers on Si substrates using sputter epitaxy method

SiGe layers were formed on Si substrates by using the sputter epitaxy method. The growth temperatures were 400 and 560 °C, and the growth rates of the SiGe layers were about 2.7–5.4 nm/min, depending on the Ge composition ratio. The samples formed by gas-source molecular beam epitaxy (GSMBE) were also fabricated for comparison. The lattice-mismatch ratio was investigated using X-ray diffraction, the surface roughness was observed by an atomic force microscope, and the crystallinity of the SiGe layers was observed by a transmission electron microscope. The critical thickness of the SiGe layers was estimated from the lattice-mismatch ratio, and the critical thickness at a growth temperature of 400 °C was higher than that in the GSMBE and previous reports. The surface roughness of the Si1-xGex layers (x = 0.24–0.51) with no strain relaxation was flat and below 1 nm. Without any buffer techniques, high Ge content SiGe layers with no strain relaxation were directly formed on Si substrates using the sputter epitaxy method. It is considered that the sputter epitaxy method can grow SiGe layers at relatively low temperatures and can increase the critical thickness.

(Invited) Thin Epitaxial Single-Crystal Si on Sige Followed By in-Situ Deposition of High-k Dielectrics – Novel Gate Stacks for Achieving Extremely Low Dit and Highly Reliable SiGe MOS

Silicon-germanium (SiGe) is replacing silicon (Si) as the channel layer for the p-type metal-oxide-semiconductor (pMOS) in the aggressively scaled complementary MOS (CMOS) technology. Many research efforts involving SiGe gate stacks have resulted in the attainment of low interfacial trap densities (Dits). Si-passivated SiGe gate stacks are the most promising method to achieve both low Dit and high device reliability. However, regardless of the growth methods, most publications reported the pileup of Ge atoms on top of the grown Si even after extensive treatments. The subsequent process in forming gate stacks, including oxidation, inevitably caused the segregated Ge to form GeOx, leading to degradation of the interfacial quality of the gate stacks.We have used molecular beam epitaxy (MBE) to grow thin single-crystal Si of 0.79nm thickness (six MLs) epitaxially on epi-Si1-xGex(001) under ultra-high vacuum (UHV). Different from its counterpart of chemical vapor deposition (CVD), MBE enables epi-Si growth on Ge at growth temperatures below 300 °C. The measurements using reflection high-energy electron diffraction (RHEED), high-resolution synchrotron radiation X-ray diffraction (HR-XRD), and the scanning transmission electron microscopy with high-angle annular-dark-field (STEM-HAADF) have revealed high crystallinity of the epi-Si, the Si1-xGex layers and abrupt interfaces of the high-k/epi-Si/Si1-xGex layers. Moreover, our unique capability of epi growth/analysis enabled the study of the electronic surface structures of epi-Si/epi-Ge using in-situ high-resolution synchrotron radiation photoemission spectroscopy (SRPES) via a portable sample transfer station under UHV. We have used SRPES to demonstrate that the as-grown Si1−xGex(001)-2×1 surfaces with Ge content ranging from 0.1 to 0.9 are all terminated with buckled Ge–Ge dimers, and the surface electronic structure is similar to that of epi-Ge(001)-2×1. The direct deposition of epi-Si on as-grown SiGe removed the top buckled Ge-Ge dimers, which diffused into the epi-Si and some segregated to the top of the Si. As a result, the topmost layer beneath the epi-Si was 1 ML of Ge, and the interfaces of high-k/epi-Si/Si1−xGex are similar to those of the high-k/epi-Si/Ge. Again, the hetero-interfaces of our gate dielectrics are different from those grown using CVD.The well-controlled interfaces have enabled the achievement of low interfacial trap densities (Dit) of (3 - 6) × 1011 eV-1cm-2 in these high-k/epi-Si/Si1-xGex samples. The minimum Dit values remained at 3 × 1011 eV-1cm-2 regardless of the Ge content, demonstrating the effective passivation of the low-temperature deposited epi-Si. The limited Ge segregation during the low-temperature growth of epi-Si may attribute to the attainment of the low Dit values. We have benchmarked the minimum Dit (Dit, min) of our work with other state-of-the-art SiGe gate stacks. The Dit,min values from the other efforts increased with increasing Ge content, which may be caused by the GeOx formation that was difficult to control in the high-Ge-content (HGC) Si1-xGex.The effective charge sheet densities for the Si1-xGex gate stacks were extracted via examination of capacitance-voltage (C-V) hysteresis with decreasing stress voltage in the accumulation region of the MOS capacitors. We have attained very high acceleration factors of 8-12, indicating high reliability of the HfO2/epi-Si/Si1-xGex pMOS gate stacks.Furthermore, we have used a scavenging method, consisting of a cyclic process of an O2 exposure followed by a UHV annealing, to eliminate the segregated and diffused Ge in epi-Si before high-k deposition. Upon O2 exposure at room temperature, both the Si and Ge surface atoms are simultaneously oxidized to give rise to four Si charge states and Ge suboxides, respectively. The subsequent in-situ annealing at 500 °C under UHV moved the oxygen atom in the Ge suboxides to bond with the nearby Si atom. The annealing also caused the diffused Ge inside the epi-Si to segregate to the surface. The processes of O2 exposure followed by annealing were repeated three times resulting in an oxidized Si/Ge surface having only the four Si oxidized states without GeOx, but with a minimal amount of segregated Ge.

Hopping charge transport in hydrogenated amorphous silicon–germanium alloy thin films

Measurements of the dark conductivity and thermoelectric power in hydrogenated amorphous silicon–germanium alloys (a-Si1-xGex:H) reveal that charge transport is not well described by an Arrhenius expression. For alloys with concentrations of Ge below 20%, anomalous hopping conductivity is observed with a power-law exponent of 3/4, while the temperature dependence of the conductivity of alloys with higher Ge concentrations is best fit by a combination of anomalous hopping and a power-law temperature dependence. The latter has been attributed to charge transport via multi-phonon hopping. Corresponding measurements of the Seebeck coefficient reveal that the thermopower is n-type for the purely a-Si:H and a-Ge:H samples but that it exhibits a transition from negative to positive values as a function of the Ge content and temperature. These findings are interpreted in terms of conduction via hopping through either exponential band tail states or dangling bond defects, suggesting that the concept of a mobility edge, accepted for over five decades, may not be necessary to account for charge transport in amorphous semiconductors.

Channel Mobility Boosting in a Poly-Si Channel Using Ge Diffusion Engineering and Hydrogen Plasma Treatment

We demonstrated that channel mobility and cell current could be increased through Ge diffusion engineering and H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> plasma treatment in a poly-Si channel. Even though the Ge channel has a higher intrinsic mobility than the Si channel, a typical poly-SiGe channel has inferior channel characteristics due to high interface trap density (D <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">it</sub> ) and bulk trap density caused by Ge. We propose a novel technique to control the Ge profile along the depth direction of the channel to realize lower Ge concentration at the gate dielectric interface and higher Ge concentration at the channel bulk. This Ge profile could achieve reduced D <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">it</sub> and enhanced channel mobility. Furthermore, the bulk traps caused by Ge are effectively reduced by H <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> plasma treatment. These techniques increased the channel mobility to 32%, which can help increase the channel mobility of devices where poly-Si channels are used.

Elucidating the Germanium Distribution in ITQ-13 Zeolites by Density Functional Theory.

ITQ‐13 is a medium‐pore zeolite that can be prepared in all‐silica form and as silicogermanate with Si/Ge ratios as low as 3. Usually synthesised in the presence of fluoride, ITQ‐13 is among the very few systems containing fluoride anions in two distinct cage types, cube‐like d4r units and [4 ⋅ 56] cages. Here, dispersion‐corrected density functional theory (DFT) calculations are used to investigate the energetically most favourable Ge distributions for Si/Ge ratios between 55 and 6. The calculations show Ge atoms are incorporated at both the corners of d4r cages and at the basal plane of the [4 ⋅ 56] cages, in accordance with 19F NMR spectroscopy. Two Ge atoms at adjacent corners of [4 ⋅ 56] cages are stable at the highest Ge content considered (Si/Ge=6). Such a local environment has not yet been considered in the experimental literature. A calculation of the corresponding 19F NMR resonance points to overlap with other resonances, which might preclude its clear identification. Additional calculations investigate the variation of the dynamic behaviour of the fluoride anions as a function of the local environment as well as the selective defluorination of the [4 ⋅ 56] cages.

Opening Diffusion Pathways through Site Disorder: The Interplay of Local Structure and Ion Dynamics in the Solid Electrolyte Li6+xP1-xGexS5I as Probed by Neutron Diffraction and NMR.

Solid electrolytes are at the heart of future energy storage systems. Li-bearing argyrodites are frontrunners in terms of Li+ ion conductivity. Although many studies have investigated the effect of elemental substitution on ionic conductivity, we still do not fully understand the various origins leading to improved ion dynamics. Here, Li6+xP1–xGexS5I served as an application-oriented model system to study the effect of cation substitution (P5+ vs Ge4+) on Li+ ion dynamics. While Li6PS5I is a rather poor ionic conductor (10–6 S cm–1, 298 K), the Ge-containing samples show specific conductivities on the order of 10–2 S cm–1 (330 K). Replacing P5+ with Ge4+ not only causes S2–/I– anion site disorder but also reveals via neutron diffraction that the Li+ ions do occupy several originally empty sites between the Li rich cages in the argyrodite framework. Here, we used 7Li and 31P NMR to show that this Li+ site disorder has a tremendous effect on both local ion dynamics and long-range Li+ transport. For the Ge-rich samples, NMR revealed several new Li+ exchange processes, which are to be characterized by rather low activation barriers (0.1–0.3 eV). Consequently, in samples with high Ge-contents, the Li+ ions have access to an interconnected network of pathways allowing for rapid exchange processes between the Li cages. By (i) relating the changes of the crystal structure and (ii) measuring the dynamic features as a function of length scale, we were able to rationalize the microscopic origins of fast, long-range ion transport in this class of electrolytes.

A pathway to highly conducting Ge-doped AlGaN

Ge doping in AlGaN was studied over a wide dopant concentration range. For high Ge concentrations, the formation of VIII–nGeIII was determined to be the main point defect limiting the conductivity. It was shown that the complex formation could be suppressed by controlling chemical potentials during growth, leading to a higher maximum achievable carrier concentration and selective stabilization of a certain complex type. Chemical potential of the growth species was varied by changing the V/III ratio and growth temperature. Free carrier concentrations as high as 4 × 1019 cm−3 were achieved in Al0.4Ga0.6N:Ge grown on sapphire substrates under “metal-rich” conditions. The ability to control the onset of self-compensation and to stabilize a certain charge state of the compensating defect is of great technological importance for application of AlGaN in various devices.

Bandgap engineered Cu2ZnGexSn1−xS4 solar cells using an adhesive TiN back contact layer

Kesterite-based solar cells are mainly restricted by their lower than expected open-circuit voltage (Voc) due to non-radiative recombination. Therefore, an approach to reduce bulk and interface recombination through band gap grading to induce a back surface field is attempted. This contribution presents the challenges in the formation of compositional grading of the wide bandgap material Cu2ZnGexSn1−xS4 (CZGTS) and successful fabrication of solar cells with an additional adhesive TiN interlayer. It is observed that the TiN interlayer improves adhesion between CZGTS and the back contact. The microstructure of the Cu2ZnSnS4 (CZTS) film is significantly affected by the concentration of Ge, and the existence of a Ge concentration gradient is strongly correlated to the formation of smaller Ge-rich and larger Sn-rich grains. The bandgap grading is exploited with a moderate Ge concentration of up to (Ge/(Ge+Sn) = 0.25) in CZTS. As the Ge profile stretched all the way to the front interface, the cliff-like band alignment at the front interface of the absorber could negate the beneficial effect of Ge inclusion in the bulk and back interface of the absorber. Ordering the absorber can introduce an additional downward shift in the valence band. In one of the samples, the increased ordering and high concentration of Ge in CZTS are suggested to enhance the hole barrier at the back interface. It is concluded that the effect of the bandgap grading with Ge can only be realized with optimization of interface band alignment and back contact formation.

Vertically-Stacked Si0.2Ge0.8 Nanosheet Tunnel FET With 70 mV/Dec Average Subthreshold Swing

A novel CMOS-compatible SiGe Tunnel Field-Effect Transistor (TFET) with a high current drivability is demonstrated, which features a vertically-stacked SiGe nanosheet (NS) with high Ge content and gate-all-around(GAA) structure to improve carrier injection efficiency and gate controllability. A multi-NS Si 0.2 Ge0.8 channel is formed by the Si selective etching and Ge condensation technique. The measured data shows both the 50 mV/dec minimum subthreshold swing (SS) and 70 mV/dec average SS over 3 decades of drain current change. Compared to previously reported devices, it is clear that the proposed SiGe nanosheet TFET can achieve steeper switching and low-level leakage current with a low drive voltage as an alternative to conventional MOSFET.

Comparison of high-order silanes and island formation phenomena during SiGe epitaxy at 500 °C

High-order silane precursors, including trisilane, are candidates for the low-temperature epitaxy process owing to the low energy of Si–Si bond. Higher order silanes are regarded as being more reactive than lower order ones. We compared the SiGe epitaxial growth behaviors of high-order silane precursors on Si substrates in an ultra-high vacuum chemical vapor deposition chamber without a carrier gas. SiGe epitaxial layers with a thickness of 25 nm or more were grown using disilane, trisilane, or tetrasilane precursor at 500 °C. Interestingly, trisilane exhibited more severe island formation than tetrasilane, which has higher reactivity than trisilane, even at lower partial pressure. These islands were not eliminated by lowering pressure but could be suppressed by higher Ge content owing to enhanced surface diffusion.

In situtrace elements and S isotope systematics for growth zoning in sphalerite from MVT deposits: A case study of Nayongzhi, South China

AbstractZoning texture in sphalerite has been described in many studies, although its genesis and ore formation process are poorly constrained. In this investigation, we compare thein situtrace element and isotopic composition of colour-zoned sphalerites from Nayongzhi, South China, to explain the zoning growth process. Petrographic observations identified two broad types of zoned sphalerite, core–rim (CR) and core–mantle–rim (CMR) textures. Each zoned sphalerite displays two or three colour zones, including brown core, light colour bands and/or pale-yellow zones.In situlaser ablation inductively coupled plasma mass spectrometry trace-element analyses show that the three colour zones display variable trace-element compositions. Brown cores exhibit distinctly high Mn, Fe, Co, Ge, Tl and Pb concentrations, whereas pale-yellow and light colour zones have elevated Ga, Cd, Sn, In and Sb concentrations. Copper, Sb, In and Sn show slight variations between pale-yellow and light zones, the latter having higher In and Sn, but lower Cu and Sb abundances. Given the low concentration range of Pb, Ge, Tl, Mn Sb, Cd, etc., the colour of sphalerite is attributed mainly to Fe compositional variation. The δ34S values of sphalerite from Nayongzhi range from +22.3 to +27.9‰, suggesting reduced sulfur was generated by thermochemical sulfate reduction of marine sulfate in ore-hosted strata. Single-crystal colour-zoned sphalerite exhibits intracrystalline δ34S variation (up to 4.3‰), which is attributed to the δ34S composition of H2S in the original fluid. The lack of correlation between trace elements and δ34S values indicates episodic ore solution influxes and mixes with the reduced sulfur-rich fluid derived from the aquifers of the ore-hosted strata, which play a key role in the formation of the zoned Nayongzhi sphalerite. In conclusion,in situtrace element and S isotope studies of zoned sphalerite crystals might provide insight into the ore-forming process of MVT deposits.

Ab initio molecular dynamics and materials design for embedded phase-change memory

The Ge2Sb2Te5 alloy has served as the core material in phase-change memories with high switching speed and persistent storage capability at room temperature. However widely used, this composition is not suitable for embedded memories—for example, for automotive applications, which require very high working temperatures above 300 °C. Ge–Sb–Te alloys with higher Ge content, most prominently Ge2Sb1Te2 (‘212’), have been studied as suitable alternatives, but their atomic structures and structure–property relationships have remained widely unexplored. Here, we report comprehensive first-principles simulations that give insight into those emerging materials, located on the compositional tie-line between Ge2Sb1Te2 and elemental Ge, allowing for a direct comparison with the established Ge2Sb2Te5 material. Electronic-structure computations and smooth overlap of atomic positions (SOAP) similarity analyses explain the role of excess Ge content in the amorphous phases. Together with energetic analyses, a compositional threshold is identified for the viability of a homogeneous amorphous phase (‘zero bit’), which is required for memory applications. Based on the acquired knowledge at the atomic scale, we provide a materials design strategy for high-performance embedded phase-change memories with balanced speed and stability, as well as potentially good cycling capability.

Dopant Redistribution and Activation in Ga Ion-Implanted High Ge Content SiGe by Explosive Crystallization during UV Nanosecond Pulsed Laser Annealing

Explosive crystallization (EC) is often observed when using nanosecond-pulsed melt laser annealing (MLA) in amorphous silicon (Si) and germanium (Ge). The solidification velocity in EC is so fast that a diffusion-less crystallization can be expected. In the contacts of advanced transistors, the active level at the metal/semiconductor Schottky interface must be very high to achieve a sub-10−9 ohm·cm2 contact resistivity, which has been already demonstrated by using the dopant surface segregation induced by MLA. However, the beneficial layer of a few nanometers at the surface may be easily consumed during subsequent contact cleaning and metallization. EC helps to address such kind of process integration issues, enabling the optimal positioning of the peak of the dopant chemical profile. However, there is a lack of experimental studies of EC in heavily-doped semiconductor materials. Furthermore, to the best of our knowledge, dopant activation by EC has never been experimentally reported. In this paper, we present dopant redistribution and activation by an EC process induced by UV nanosecond-pulsed MLA in heavily gallium (Ga) ion-implanted high Ge content SiGe. Based on the obtained results, we also highlight potential issues of integrating EC into real device fabrication processes and discuss how to manage them.

Improving phase transition temperature of VO2 via Ge doping: a combined experimental and theoretical study

Enhancing the semiconductor–metal phase transition temperature (TSMT) of VO2 is of great consequence for further exploring the potential applications of VO2 at elevated temperatures. In this study, Ge4+-doped VO2 (GexV1−xO2) samples were prepared by the hydrothermal and annealing approach. X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), differential scanning calorimetry (DSC) and resistivity–temperature (R-T) analyses were used to investigate the influence of Ge doping on the lattice structures and phase transition properties of GexV1–xO2 samples. We found that the lattice parameter of GexV1−xO2 decreased with the Ge concentration increasing from 2 at% to 18 at%, which was further supported by density functional theory (DFT)-based first-principle simulations. TSMT firstly increased from 64.5 to 73.0 °C at 8 at% Ge and then decreased to 71.5 °C at higher Ge concentration. Furthermore, DFT analysis revealed that the impact of lattice distortion induced by Ge doping rather than the changes in electronic structure is more pronounced on modulating TSMT of GexV1−xO2. The present work has pointed out the direction that the TSMT could be enhanced and illustrated the physical reason behind the regulation of TSMT in ions-doped VO2 systems. The d (logρ)/dT vs T curves are plotted for GexV1−xO2 (0≤x≤0.18) samples (a) un-doped VO2 ; (b) 2%; (c) 8%; (d) 18%, the transition temperatures upon heating, Th, and cooling, Tc. The difference between Th and Tc gives the hysteresis width, ΔTt, while the FWHM determines the sharpness of the semiconductor-to-metal transition

Modes of occurrence of germanium and tungsten in the Spetsugli germanium ore field, Pavlovka brown coal deposit, Russian Far East

The main modes of Ge and W occurrence were established using alkaline extraction and analysis of organic matter, scanning electron microscopy and infrared spectroscopy. High-germanium-bearing coals of the Spetsugli deposit are characterized by different modes of Ge occurrence. The organic mode of Ge is dominant with diverse sources not limited to complex humates. However, minerals make an important contribution to the Ge concentration in the coal, a contribution which is probably much higher than was previously thought. Various micron and nanometer mineral phases of Ge are widespread in the coal seams, especially near the contact with the weathering crust developed on the greisenized granites. Identified as Ge bearing minerals in the coals are aluminosilicates, various ferrous minerals (hydrogoethite, goethite, jarosite) and polymineral phases. The aluminosilicate (silicate) phase is represented by film-like aggregates containing 0.15–0.4% of germanium. Ferrous mineral aggregates contain from 0.1 to 6.2% of germanium.Anomalously high Ge concentrations, together with W and As, were found in ferromanganese crusts formed in the paleo-bog conditions from material derived from the weathering of the greisenized granite in the basement. The presence of these secondary, Ge-rich, mineral phases in the coals demonstrates that the Ge mineralization in the deposit took place over a longer period of time than was previously thought and was not limited to the peat stage.Tungsten is mainly characterized by an organic mode of occurrence. After extraction of humus acids, 99% of tungsten on average passes into the alkaline extract. In the alkaline extract, about half is associated with the humic acids, and the rest is associated with the low-molecular organic acids. The contribution of W mineral forms in the coals of the Spetsugli deposit is not significant, with only a single grain of scheelite recorded. Tungsten in the coals was also found in the iron-manganese “crusts” associated with the weathered granite.

(Invited) Strain Engineering of Si/Ge Heterostructures on Ge-on-Si Platform

Si/Ge heterostructures with high Ge contents have attracted increasing interests for last few decades because of their various high potential applications in the microelectronics industry. Strain engineering is an additional key technology not only for electronic devices with enhanced carrier mobilities but also for photonic devices since efficient light emission can be made possible via strain-induced direct band transition. In addition to the conventional (100) orientation, Si/Ge with a (111) orientation is very attractive owing to the higher electron mobility and applicability to spintronic devices via lattice matched epitaxial growth of high-quality ferromagnetic materials.As well known, however, it is a challenging issue to grow high-quality Ge-rich SiGe layers on the Si platform due to the large lattice mismatch between Si and Ge. Hence, strain engineered Si/Ge heterostructures with high Ge contents have to be created on Ge-on-Si virtual substrates, whereas strain states and crystallinities of the SiGe are expected to be highly influenced by those of the Ge-on-Si. Here, we show recent studies on heteroepitaxy of various Si/Ge heterostructures on Ge-on-Si with various surface orientations and discuss their strain states, stabilities, critical thickness and crystal qualities. Furthermore, their applications to optoelectronic and spin devices are overviewed.

(Invited) Strain Engineering of Si/Ge Heterostructures on Ge-on-Si Platform

Strain engineering of Si/Ge heterostructures with high Ge contents is allowed on the Ge-on-Si platform since a high quality Ge-on-Si has been available owing to the two-step growth method. Detailed studies on initial stages of the partial strain relaxation of strained SiGe layers on Ge-on-Si and bulk-Ge substrates clearly indicate that the strained SiGe grown on the Ge-on-Si reveals lower critical thickness than that on the Ge substrate. On the contrary, it is shown that this reduced critical thickness on the Ge-on-Si can be significantly enhanced beyond that on the Ge substrate by means of the substrate patterning. Additionally, our recent studies including strained Ge-on-Insulator fabrication and its surface passivation toward high-efficient light emitting devices are overviewed.

Topotactic conversion of Ge-rich IWW zeolite into IPC-18 under mild condition

Abstract A facile structural conversion process from Ge-rich IWW-type germanosilicate to its daughter structure IPC-18 zeolite has been developed under mild conditions of water medium and room temperature. The parent IWW zeolite with an extremely low Si/Ge ratio of 1.02, synthesized with 1,4-bis-(dimethyl-1-adamantylammonium)-butane as a novel structural directing agent, was critical for the success of this specific structural transformation. The high Ge content favored the breaking of double-four-ring (d4r) subunits under mild conditions and simultaneously the mild H2O medium suppressed the Si-assisted structural repairing. The HRTEM images collected along the (001) and (010) orientations of IWW and IPC-18 zeolite revealed the occurrence of structural transformation. The formation of IPC-18 zeolite structure was confirmed by the PXRD patterns and selected area electron diffraction patterns. The obtained IPC-18 zeolite demonstrated promising catalytic performance in the hydration of ethylene oxide to ethylene glycol with a high selective of 96%.