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Group-IV alloy semiconductor materials are much attractive for photovoltaic application, as those realizes multijunction photovoltaic with multi-heterostructure like group-III-V compound semiconductors [1]. In addition, group-IV alloy materials are expected to be familiar to Si integrated electronics. Ge1–x–ySixSny is one of candidate materials suitable for multijunction photovoltaic, since ternary alloy semiconductor provides energy band design independent on the lattice constant. The energy band gap of Ge1–x–ySixSny can be controlled from 0.66 eV to 1.0 eV with the epitaxial layer whose lattice is matching to Ge substrate. We previously reported the epitaxial growth of Ge1–x–ySixSny layers and also investigated the crystalline and electronic properties of them [2, 3]. However, the junction properties of Ge1–x–ySixSny/Ge heterostructure have not been well understood and it is necessary to clarify the crystalline and electronic properties of group-IV ternary alloy hetero junctions. In this study, we examined the crystal growth of pseudomorphic epitaxial layers of Ge1–x–ySixSny ternary alloy on Ge(001) substrates to form heterojunction structure for photovoltaic application. We also investigated the electronic and optoelectronic properties of these Ge1–x–ySixSny/n-Ge heterojunction diodes. Substrate used was n-type Ge(001) wafer. After chemical cleaning of substrate, a 200 nm-thick undoped Ge1–x–ySixSny layer was grown at 150–350 °C on substrate using molecular beam epitaxy system in ultrahigh vacuum chamber. Ge and Sn were deposited using Knudsen cells and Si was done using electron gun evaporation. Then, we prepared a mesa-shaped structure with wet etching. A 300 nm-thick SiO2 layer was deposited with coating of spin-on-glass and annealing at 450 °C. Finally, an Al electrode was prepared on contact holes to form heterojunction diode structure. We confirmed the crystalline structure of epitaxial layers with X-ray diffraction reciprocal space mapping. A Ge1–x–ySixSny layer with a high Sn content of 12% can be prepared with lattice matching epitaxy on Ge at a low temperature below 250 °C. It is also found that the small misfit between Ge1–x–ySixSny and Ge effectively impacts on the high crystalline quality and high thermal stability of substitutional Sn atoms in Ge matrix. Absorption spectroscopy and current-voltage characteristics of undoped Ge1–x–ySixSny/n-Ge heterojunction diodes were also performed. We observed good rectifying property of the diodes meaning that the majority carrier of these undoped Ge1–x–ySixSny ternary alloy layers is hole unintentionally generated due to defects. We also found the extension of the energy band gap of Ge1–x–ySixSny layer with Si and Sn contents. Photovoltaic property of Ge1–x–ySixSny/Ge heterojunction diodes were also measured. We can see the increasing of the open circuit voltage, Voc from 0.08 to 0.12 V for undoped Ge/n-Ge to undoped Ge0.54Si0.36Sn0.10/Ge heterojunction photovoltaic cell structure, which indicates the enhancement of Voc with increasing the energy bandgap of epitaxial layer. We also achieved the improvement of Voc by 0.14 V with 500 °C-annealing in hydrogen (H2) ambient, which suggests the annihilation or termination of point defect in the heterojunction structure. In conclusions, heterojunction structure with Ge1–x–ySixSny ternary alloy semiconductor layer provides the energy band engineering for photovoltaic applications. Understanding and controlling the defect properties in epitaxial growth and heterojunction formation should be key factors for the enhancement of performance of group-IV multijunction photovoltaic. References [1] S. Asaba et al., in Ext. Abstra. of Int. Conf. on Solid State Devices and Materials (SSDM2013), pp. 1172-1173, Fukuoka, Japan, Sept. 2013. [2] T. Asano et al., Solid State Electronics 110, 49 (2015). [3] T. Yamaha et al., Appl. Phys. Lett. 108, 061909 (2016).

The formation of Ge 1− x − y Si x Sn y /Ge 1− x Sn x /Ge 1− x − y Si x Sn y double heterostructures with strain-controlled Ge 1− x − y Si x Sn y layers and their crystalline properties were investigated. We achieved the epitaxial growth of double heterostructures consisting of a Ge 1− x Sn x layer with a Sn content of 9% sandwiched between compressive- or tensile-strained Ge 1− x − y Si x Sn y layers. The strain sign of the Ge 1− x − y Si x Sn y epitaxial layer influenced the crystallinity of the double heterostructures. Compressive-strained Ge 1− x − y Si x Sn y layers provided double heterostructures with higher crystallinity than the tensile-strained ones. The magnitude of strain in the Ge 1− x − y Si x Sn y layers also affected the surface roughness of the double heterostructures. Low surface roughness was achieved by decreasing the magnitude of strain in the Ge 1− x − y Si x Sn y layers. Moreover, the strain sign and/or Si content in Ge 1− x − y Si x Sn y influenced the thermal stability of the double heterostructures. Compressive-strained Ge 1− x − y Si x Sn y and/or a low Si content in Ge 1− x − y Si x Sn y improved the thermal stability of the double heterostructures to withstand annealing temperatures as high as 400 °C.

Germanane (GeH) is an attractive layered material with potential applications in channel of field-effect transistors and can be topochemically synthesized from CaGe 2 . So far, the film growth method of CaGe 2 has been limited to molecular beam epitaxy. In this study, we demonstrate CaGe 2 epitaxial film growth on Ge(111) substrates at 675–800 ° C by a scalable close-spaced evaporation method using a conventional lamp annealing furnace. Detailed investigation of the film structure by X-ray diffraction reveals that the CaGe 2 films consist mainly of the metastable 2H polytype and of a minor amount of the stable 6R polytype. The CaGe 2 film deposited at 800 ° C is converted to GeH by low temperature HCl treatment while the other CaGe 2 films transform to crystalline and nanocrystalline Ge. The reasons why only the CaGe 2 film synthesized at 800 ° C is converted to GeH is discussed in terms of the film microstructure observed by scanning electron microscopy. The absence of GeH in low-temperature deposited films after HCl treatment is attributed to the exfoliation of island crystals during HCl treatment. We thus show that close-spaced evaporation is a rapid and scalable method to prepare CaGe 2 epitaxial films as a precursor of GeH films and that the roughness of CaGe 2 should be improved for efficient conversion to GeH.

Toward creation of a direct gap group-IV semiconductor, we have developed a new method of growing Si1-xSnx layers with various Sn content including a very high Sn content around 40% by using solid phase epitaxy. Crystallographic analyses indicate that grown Si1−xSnx layers are nearly lattice-matched to the substrates, specifically, the substitutional Sn contents of ~20 and ~40% are achieved for case of growth on (001)-oriented Ge and InP substrates, respectively. Although band structure of the Si0.6Sn0.4 layers grown on InP is still indirect band gap, the difference between the indirect and direct band gap energies decreases from 3.01 (Si) to 0.14 eV (Si0.6Sn0.4) following Sn substitution.

We have investigated the epitaxial growth and crystalline properties of Ge1 − xSnx layers on a Si(110) substrate. We found that the twin growth in the Ge epitaxial layer deposited on the Si(110) using molecular beam epitaxy at a low temperature of 200 °C can be effectively suppressed by the incorporation of 2.0% Sn. We also examined the strain relaxation of annealed Ge1 − xSnx/Si(110) samples. The degree of strain relaxation is enhanced by the annealing process, and the threading dislocation in the Ge1 − xSnx layers decreases from 1011 cm− 2 to 1010 cm− 2 because of the propagation of misfit dislocations. We also observed misfit dislocations formed at the Ge1 − xSnx/Si interface, which would effectively promote isotropic strain relaxation in the Ge1 − xSnx layers.

We have investigated the influence of tensile and compressive strain on the crystalline structures of Ge 1− x − y Si x Sn y epitaxial layers. The tensile strain in Ge 1− x − y Si x Sn y induces a non-uniform crystallinity of (2 2 0) lattice planes and surface roughening despite the strain magnitude is as small as 0.20%. In contrast, the unstrained or compressive strained Ge 1− x − y Si x Sn y layer exhibits a flat and uniform surface and high crystallinity. We found that the control of the sign of the strain is an important factor to obtain a high quality Ge 1− x − y Si x Sn y layer. Furthermore, substitutional Sn atoms in Ge 1− x − y Si x Sn y epitaxial layer with an Sn content of 10% are thermally stable for annealing at 500 °C.

Ge1-xSnx alloy thin films are much attractive for not only improving in Si nanoelectronics but also introducing new functional applications. High Sn content Ge1-xSnx promises to be direct semiconductor which promises the optoelectronic applications. We need to control the Sn precipitation, strain, dislocations, and point defects in Ge1-xSnx epitaxial layers for practical electronic and optoelectronic applications. We have been developing the growth technique of Ge1-xSnx epitaxial layers on various Si, Ge, Si on insulator (SOI), and Ge on insulator (GOI) substrates with molecular beam epitaxy (MBE) method [1-3]. Recently, we achieved high crystallinity Ge1-xSnx layers with a high Sn content up to 12% by lowering the growth temperature and controlling the lattice mismatch and strain [4]. In addition, the orientation of substrate has to be considered to improve on the crystalline quality and performance of Ge1-xSnx devices, because the substrate orientation significantly influence on the strain structure, defect structure, propagation of dislocations, energy band structures, and effective mass. The development of global- and local-growth technology of Ge1-xSnxepitaxial layers on not only Si(001) but also various substrates with (110) and/or (111) orientations is essentially required for various applications. Ge1-xSnx epitaxial growth on (110) and (111) substrates We examined the epitaxial growth of Ge1-xSnx layers on (110) and (111) Ge substrates [5-8]. We found that introducing Sn with a content as low as 2% into Ge is effective to reduce staking faults and twin defects in Ge epitaxial layers on (110) substrates even with the low-temperature growth of 150°C. We also found that the strain relaxation of Ge1-xSnxon (110) and (111) substrates takes place preferentially with a smaller Sn content than that grown on (001) substrate, that is attributed to the surface orientation dependence of the elastic modulus of epitaxial layers. The Sn incorporation into Ge also improves on the crystalline quality of epitaxial layers prepared with low-temperature MBE on Si(110) substrate [9]. Formation of twin defects in Ge epitaxial layers on Si(110) can be drastically suppressed by the incorporation of Sn with a content of 2%. The propagation of misfit dislocations can be effectively enhanced during the post deposition annealing by suppressing the formation of twin defects. Local growth of Ge1-xSnx epitaxial layers We examined the formation of locally strained Ge1-xSnx/Ge fine structure and characterized the microscopic local strain structures by using X-ray microdiffraction method [10]. The local growth of Ge1-xSnx with a Sn content of 6.5% on fine-structure-patterned-Ge is achieved with low-temperature MBE. The concentration of local stress in embedded Ge1-xSnx layer causes the degradation of crystallinity with tilting of the lattice plane and Sn precipitation. We can directly measure the individual local strain in Ge sandwiched with Ge1-xSnxlocal stressor by microdiffraction. We demonstrate increasing in the in-plane strain with shrinking the Ge width and increasing the Sn content. The maximum in-plane compressive strain is estimated to be 1.4% for a width of 25 nm. In the presentation, we will also talk about some electronic and optical properties of high Sn content Ge1-xSnx epitaxial layers and introduce some recent achievement in the epitaxial growth of Sn-related ternary alloys such as Ge1-x-ySixSny. Acknowledgements This work was partially supported by a Grant-in-Aid for Specially Promoted Research of MEXT, the JSPS through the FIRST Program, and the ALCA program from JST in Japan. The microdiffraction measurement was performed at SPring-8 with the approval of general proposals (No. 2013A1682 and No. 2013B1779/BL13XU). References [1] S. Zaima et al., ECS Trans. 41, 231 (2011). [2] S. Zaima et al., ECS Trans. 50, 897 (2012). [3] O. Nakatsuka et al., ECS Trans. 58, 149 (2013). [4] Y. Shimura et al., ECS Trans. 33, 529 (2010). [5] Y. Shimura et al., Appl. Phys. Express 5, 015501 (2012). [6] T. Asano et al., Thin Solid Films 531, 504 (2013). [7] T. Asano et al., Solid-State Electron. 83, 71 (2013). [8] T. Asano et al., Thin Solid Films, 557, 159 (2014). [9] S. Kidowaki et al., in Ext. Abstra of SSDM2012, p. 1043, Fukuoka, Japan, Sept., 2013. [10] S. Ike et al., Thin Solid Films, 557, 164 (2014).

In this study, we have examined the formation of uniaxially strained Ge microstructures with embedded Ge 1 − x Sn x epitaxial layers and the microscopic local strain structure in Ge and Ge 1 − x Sn x using synchrotron X-ray microdiffraction and the finite element method. We achieved local heteroepitaxial growth of Ge 0.947 Sn 0.053 layers on the Ge recess regions. Microdiffraction measurements reveal that an average uniaxial compressive strain of 0.19% is induced in Ge locally with Ge 1 − x Sn x stressors. In addition, we found that the Sn precipitation near the Ge 1 − x Sn x /Ge(001) interface occurs after post-deposition annealing at 500 °C without the introduction of dislocation. It is considered that the local Sn precipitation occurs preferentially due to the larger residual stresses near the Ge 1 − x Sn x /Ge interface.

We have investigated the substrate orientation dependence of the crystallinity and strain relaxation behavior of Ge and Ge 1 − x Sn x layers epitaxially grown on Ge(001), (110), and (111) substrates. The strain relaxation in Ge 1 − x Sn x layers on Ge(110) and (111) occurs from a strain value smaller than that on Ge(001). We obtained the critical strain energy of a Ge 1 − x Sn x layer regardless of Ge substrate orientation. As a result, we achieved the epitaxial growth of pseudomorphic Ge 1 − x Sn x layers with high substitutional Sn contents of 8.5% and 6.7% on Ge(110) and (111) substrates, respectively.

We have investigated the formation and optoelectronic properties of strain relaxed Ge1−x−y Si x Sn y /Ge1−x Sn x /Ge1−x−y Si x Sn y double heterostructures on ion-implanted Ge substrates. The strain relaxation of Ge1−x−y Si x Sn y and Ge1−x Sn x epitaxial layers was achieved using an ion-implanted Ge substrate. The maximal degree of strain relaxation (DSR) of the Ge1−x Sn x layers was evaluated to be 46%. In addition, we obtained a sharp and strong peak in the photoluminescence (PL) spectra from the sample with a DSR of 46%, while no strong peak was detected from a sample with a smaller DSR (22%). From the theoretical calculation of the energy band structure and the measurement temperature dependence of the PL intensity, the sharp and strong peak can be explained by the transition from an indirect to direct bandgap semiconductor due to the increase of the DSR and a concomitant increase of the Γ-valley electron population. Moreover, the PL intensity increases by the improvement of the crystallinity by a post deposition annealing process.

We have examined the heteroepitaxial growth of Ge1-xSnx and Ge1-x-ySixSny layers and investigated the crystalline and electrical properties of these Sn-related group-IV material thin films. We achieved the epitaxial growth of Ge1-xSnx on Ge(110) with suppressing the formation of twin defects. Ge1-x-ySixSny layers on Ge substrates show high thermal robustness without Sn precipitation even after annealing at 600°C. We also found that the incorporation of 0.1%-Sn and H2-annealing are effective to reduce the unintentional generation of holes due to the vacancy defects.

High-quality crystalline Ge thin films on low-cost glass substrates are desired to reduce the fabrication cost of high-efficiency tandem solar cells. We applied an Al-induced crystallization technique to amorphous-Ge films (50-nm thickness) on SiO 2 glass substrates. The annealing temperature of the sample strongly influenced the grain size and the crystal orientation in the grown polycrystalline Ge layers: low annealing temperatures resulted in large grains and high (111)-orientation fractions. As a result, annealing at 325 °C provided 98% (111)-oriented grains with average diameters of 30 μm. Moreover, the grown Ge layers could be used as an epitaxial template for chemical vapor deposition. This large-grained Ge film on a SiO 2 substrate appears promising for use as a Ge light-absorbing layer, as well as an epitaxial buffer layer for group III–V compound semiconductors.

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.

We have developed the epitaxial growth technology of Ge 1−x Sn x and related group-IV materials. The crystalline properties and energy band structure have been investigated for integrating group-IV semiconductors into Si ULSI platform.

We comprehensively investigate Al-induced crystallization (AIC) of Si to achieve (0 0 1) and (1 1 1)-oriented Si films on quartz substrates. These phenomena are systematically explained by ‘preferential interfacial-nucleation model’ considering the nucleation sites and the phase transition of the interfacial Al oxide layers. Moreover, we demonstrate the lateral liquid phase epitaxy of Ge by using the orientation-controlled AIC-Si films as crystal seed. This technique enables high quality hetero-epitaxy of SiGe and Ge on transparent insulating substrates.

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.

We review the technology of Ge1−xSnx-related group-IV semiconductor materials for developing Si-based nanoelectronics. Ge1−xSnx-related materials provide novel engineering of the crystal growth, strain structure, and energy band alignment for realising various applications not only in electronics, but also in optoelectronics. We introduce our recent achievements in the crystal growth of Ge1−xSnx-related material thin films and the studies of the electronic properties of thin films, metals/Ge1−xSnx, and insulators/Ge1−xSnx interfaces. We also review recent studies related to the crystal growth, energy band engineering, and device applications of Ge1−xSnx-related materials, as well as the reported performances of electronic devices using Ge1−xSnx related materials.

The energy band alignment of Ge1−xSnx/Ge1−x−ySixSny heterostructures was investigated, and control of the valence band offset at the Ge1−xSnx/Ge1−x−ySixSny heterointerface was achieved by controlling the Si and Sn contents in the Ge1−x−ySixSny layer. The valence band offset in the Ge0.902Sn0.098/Ge0.41Si0.50Sn0.09 heterostructure was evaluated to be as high as 330 meV, and its conduction band offset was estimated to be 150 meV by considering the energy bandgap calculated from the theoretical prediction. In addition, the formation of the strain-relaxed Ge1−x−ySixSny layer was examined and the crystalline structure was characterized. The epitaxial growth of a strain-relaxed Ge0.64Si0.21Sn0.15 layer with the degree of strain relaxation of 55% was examined using a virtual Ge substrate. Moreover, enhancement of the strain relaxation was demonstrated by post-deposition annealing, where a degree of strain relaxation of 70% was achieved after annealing at 400 °C. These results indicate the possibility for enhancing the indirect-direct crossover with a strained and high-Sn-content Ge1−xSnx layer on a strain-relaxed Ge1−x−ySixSny layer, realizing preferable carrier confinement by type-I energy band alignment with high conduction and valence band offsets.

We investigated the crystalline structure of Ge 1-x-y Si x Sn y layers epitaxially grown on Ge(001) substrates. The unstrained and compressive strained Ge 1-x-y Si x Sn y layers with very flat surface and high crystallinity can be grown. We found that the control of the strain direction is important to form a high quality Ge 1-x-y Si x Sn y layer even with small misfit strain.

We achieved the world's first epitaxial growth of a Ge 1-x-y Sn x C y layer, and investigated the effect of Sn incorporation on the growth of Ge 1-x C x . Sn incorporation can decrease the epitaxial temperature of Ge 1-x C x layer. Also, Sn incorporation can make C atoms stable at the substitutional site. This Ge 1-x-y Sn x C y layer is expected to realize the energy band engineering independently on the lattice parameter and promises to extend the potential of group-IV semiconductor materials for nanoelectronics and optoelectronic applications.

Development of a low-temperature (≤250°C) formation technique of orientation-controlled large-grain (

The effect of hydrogen-surfactant-mediated molecular beam epitaxy (MBE) growth of Ge1− x Sn x layer on Ge(001) substrate on crystalline quality and photoluminescence (PL) property has been investigated. The effect of irradiation of atomic hydrogen (H) generated by dissociating molecular hydrogen (H2) were examined during the MBE growth. H irradiation significantly improves the surface morphology with the enhancement of the two-dimensional growth of the Ge1− x Sn x epitaxial layer. Enhanced diffuse scattering is observed in the X-ray diffraction profile, indicating a high density of point defects. In the PL spectrum of the H2-irradiated Ge1− x Sn x layer, two components are observed, suggesting the radiative recombination with both indirect and direct transitions, while one component related to the direct transition is observable in the H-irradiated sample. The postdeposition annealing in nitrogen ambient at as low as 220 °C decreases the PL intensity of the H-irradiated Ge1− x Sn x layer, although the intensity is recovered after annealing at 300 °C, suggesting the annihilation of point defects in the Ge1− x Sn x layer.

We have developed a new method of growing Si or Ge ultrathin films on a Ag(111) surface by using a Ag-induced layer exchange (ALEX) process toward the creation of 2D honeycomb sheets of Si and Ge, known as silicene and germanene, respectively. In the present paper, we clarify ALEX features, specifically the surface segregation of Si (or Ge) atoms from the underlying substrate, focusing on the annealing temperature and time. Hard X-ray photoelectron spectroscopy analyses demonstrate that surface-segregated Si (or Ge) exists on the Ag surfaces after the epitaxial growth of the Ag layer on Si(111) [or Ge(111)] substrates; the amount of segregated Si (or Ge) can be controlled by a subsequent annealing. Also, we find that the segregation of an ultrathin Si or Ge layer proceeds at an interface between Ag and the AlO x capping layer., Article Published 5 July 2016

We examined the molecular beam epitaxy of Ge1− x Sn x with in situ Sb doping on Ge substrates. The effects of Sb doping on the crystalline and electrical characteristics of Ge1− x Sn x epitaxial layer were investigated in detail. We found that Sb doping with a concentration of 1020 cm−3 remarkably improves the crystallinity, and surface uniformity of the Ge1− x Sn x epitaxial layer by changing the growth mode by the surfactant effect of Sb atoms. Low-temperature Ge1− x Sn x growth with in situ Sb doping realizes a very high electron concentration of 1020 cm−3, which is above the thermal equilibrium solid solubility, as a result of suppressing Sb segregation and precipitation.

We examined the epitaxial growth of an ultrahigh-Sn-content Ge1− x Sn x layer on a Ge substrate and investigated the impact of a Ge1− x Sn x interlayer on the Schottky barrier height (SBH) of the metal/Ge contact. In this study, we considered guidelines of the strain energy and growth temperature to realize a high-Sn-content Ge1− x Sn x layer while keeping the epitaxial growth and suppressing the Sn precipitation. By reducing the film thickness and keeping a low growth temperature, we formed an atomically flat and uniform Ge1− x Sn x epitaxial layer with a Sn content up to 46% on a Ge(001) substrate. We also performed the current density–voltage measurement for Al/Ge1− x Sn x /n-Ge Schottky diodes to estimate the SBH. We found that the SBH of Al/Ge1− x Sn x /n-Ge contact decreases with increasing Sn content in the Ge1− x Sn x interlayer. The shift of the pinning position towards the conduction band edge of Ge is one of the reasons for the SBH reduction of Al/Ge1− x Sn x /n-Ge contact because the valence band edge of Ge1− x Sn x would rise as the Sn content increases.

We have demonstrated the crystal growth of Si1−x−ySnxCy ternary alloy layers on Si(001) substrates by radio-frequency magnetron sputtering. We have investigated the crystalline properties of the ternary alloy layers and clarified the influence of Sn and C on the crystalline structure of Si1−x−ySnxCy layers. We found that the epitaxial growth temperature of Si1−x−ySnxCy decreased and the island growth was suppressed by the introduction of Sn. The Sn precipitation was also effectively suppressed by C introduction. The substitutional C content of Si1−x−ySnxCy was estimated to be 2.7% and exceeded the thermal equilibrium solid solubility of C into the Si matrix.

We have investigated the formation of the locally strained Ge nanostructure with epitaxial Ge1−xSnx stressors and characterized the microscopic strain field in the Ge1−xSnx/Ge fine-heterostructures by synchrotron X-ray microdiffraction and finite element method (FEM) calculations. We achieved local epitaxial growth of Ge1−xSnx with Sn contents of 2.9% and 6.5%, sandwiching the 25 nm-wide Ge fine line structure. Microdiffraction measurements revealed that out-of-plane tensile strain induced in the Ge line effectively increased with decreasing Ge width and increasing Sn content of Ge1−xSnx stressors, which is in good agreement with FEM calculations. An out-of-plane tensile strain of 0.8% along the Ge[001] direction is induced in a 25 nm-wide Ge line, which corresponds to an in-plane uniaxial compressive strain of 1.4% in the Ge line sandwiched between Ge0.935Sn0.065 stressors.

We have investigated the crystalline and optical properties of epitaxial layers of the ternary alloy Ge1−x−ySnxCy grown on a Si substrate. We achieved the formation of epitaxial Ge1−x−ySnxCy layers with a C content as high as 2% even with a high C incorporation efficiency. X-ray photoemission spectra and Raman scattering spectroscopy measurements revealed that C atoms preferentially bond with Sn atoms in the Ge matrix, which is considered to enhance C introduction into substitutional sites in Ge with local strain compensation. We also demonstrated the control of the energy bandgaps of epitaxial Ge1−x−ySnxCy layers by controlling Sn and C contents.

(111)-oriented Ge thin films on insulators are essential for advanced electronics and photovoltaic applications. We investigate Al-induced crystallization of amorphous-Ge films (50-nm thickness) on insulators focusing on the annealing temperature and the diffusion controlling process between Ge and Al. The (111)-orientation fraction of the grown Ge layer reaches as high as 99% by combining the low-temperature annealing (325 °C) and the native-oxidized Al (AlOx) diffusion-control layer. Moreover, the transmission electron microscopy reveals the absence of defects on the Ge surface. This (111)-oriented Ge on insulators promises to be the high-quality epitaxial template for various functional materials to achieve next-generation devices.

Formation of laterally-graded single-crystal SiGe-on-insulator structures by rapid-melting-growth of a-SiGe is investigated. From micro-probe Raman scattering spectroscopy measurements, it is revealed that the Si concentration profiles in grown SiGe layers are significantly affected by the growth conditions and sample sizes. Based on the systematical analysis of the phenomena, effects of the growth rate on Si segregation kinetics in rapid-melting growth are clarified. These findings are expected to be useful to obtain epitaxial templates with laterally-variable lattice constants for 2-dimensional integration of various functional devices on an Si-platform.