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We examined the formation of locally strained Ge nanostructures sandwiched between Ge1−x Sn x stressors using metal-organic chemical vapor deposition method. We have investigated the microscopic local strain and stress in the Ge/Ge1−x Sn x heterostructures using synchrotron microdiffraction and finite element method calculation. The microdiffraction measurement for an asymmetric lattice plane enables directly quantitative evaluation of the strain value of an individual Ge fine line structure with a few tens of nanometers width. An in-plane compressive strain value of 0.9% is achieved for a 30 nm-width Ge line with Ge1−x Sn x stressors, which corresponds to a compressive stress of 1.2 GPa.

Research and development of GeSn and related group-IV semiconductor materials have been widely extended in recent years for not only electronic transistors but also various optoelectronic applications [1,2]. Design and engineering of the energy band structure of group-IV semiconductor materials are swiftly blossoming with establishing the crystal growth technology of GeSn and related materials. We have successively developed the crystalline growth technology of GeSn and GeSiSn thin films on various substrates mainly by using molecular beam epitaxy [1-7]. Recently, we also achieved the epitaxial growth of GeSn layer by using metal organic chemical vapor deposition method [8]. In this presentation, we will report our recent achievements of our study for the crystalline growth and electronic properties including energy band structure of GeSn and related group-IV materials. Controlling the composition of elements and the strain structure of the group-IV semiconductor alloys promises prospective energy band engineering technology. Increasing in the Sn content over about 10% or tensile strain over about 1% achieve the indirect-to-direct crossover, which is a strong driving force of the optical applications of GeSn material. In addition, we recently achieved the formation of polycrystalline SiSn thin layers with a very high Sn content of 20%, and demonstrated that the direct bandgap decreases to 1.05 eV with Si-Sn alloying, which is just 0.22 eV higher than the indirect bandgap of the poly-SiSn even by using Si matrix material [9]. Also, the ternary alloy GeSiSn promises the control of the energy band structure independently on the lattice constant by changing each content of three elements. We found that unstrained GeSiSn/Ge heterostructure realizes a type-I energy band alignment without using strain structure, that promises various electronic and optoelectronic applications [10]. We will demonstrate the experimental results of the energy band engineering with GeSn and related materials. The energy band engineering also provides controlling technology of the electronic property at the interface such as metal/Ge contact. Recently, we found that the Sn/Ge or GeSn/Ge contacts effectively reduces the Schottky barrier height at the metal/n-Ge interface [11,12]. In our presentation, we will also discuss the influence of the energy band structure on the interface properties for Ge and GeSn. This work was partially supported by Grant-in-Aid for Scientific Researches of the JSPS and the JSPS Core-to-Core Program, A. Advanced Research Networks. References [1] S. Zaima, Jpn. J. Appl. Phys. 52, 030001 (2013). [2] S. Zaima et al., Sci. Technol. Adv. Mater., submitted. [3] S. Zaima et al., ECS Trans. 41, 231 (2011). [4] S. Zaima et al., ECS Trans. 50, 897 (2012). [5] O. Nakatsuka et al., ECS Trans. 58, 149 (2013). [6] S. Zaima et al. ECS Trans. 64, 147 (2014). [7] O. Nakatsuka et al. ECS Trans. 64, 793 (2014). [8] Y. Inuzuka et al., ECS Solid State Lett., submitted. [9] M. Kurosawa et al., Appl. Phys. Lett. 106, 171908 (2015). [10] T. Yamaha et al., in Abstr. of ICSI-9, Montreal, Canada, May, 2015. [11] Suzuki et al., Jpn. J. Appl. Phys. 53, 04EA06 (2014). [12] A. Suzuki et al., SSDM2015, Hokkaido, Japan, Sept. 2015, to be submitted.

In this study, we have examined the local growth of Ge1 − x Sn x heteroepitaxial layers on micrometer-scale-patterned Ge substrates with molecular beam epitaxy method. We have investigated the strain relaxation behavior and microscopic local strain structure in both Ge and Ge1 − x Sn x by using x-ray microdiffraction and finite element method calculation. We found that the anisotropic strain relaxation of embedded Ge1 − x Sn x layers preferentially occurs along the direction which is perpendicular to the stripe line. Microdiffraction method revealed that the elastic strain relaxation of the embedded Ge1 − x Sn x layer occurs near the edge region. We demonstrated that the uniaxial compressive strain of 0.2% is locally induced in Ge with Ge1 − x Sn x stressors.

The strain engineering is a promising technology not only for low-power CMOS but also for optoelectronic applications of germanium (Ge) devices compatible with the conventional silicon (Si) process technology [1]. In terms of CMOS applications, a uniaxial compressive strained Ge has drawn attention as high-mobility channel material in p-MOSFET which is expected to transcend that of conventional strained Si channel [2]. We are focusing on germanium-tin (Ge1−x Sn x ) as a source/drain (S/D) stressors to apply a uniaxial compressive strain into Ge in analogy with Si1−x Ge x S/D for strained Si channel. However, there are few reports of the local epitaxial growth of Ge1−x Sn x on patterned Ge substrate and the local strain technology of the Ge/Ge1−x Sn x heterostructures. Since the recent device dimensions of Si CMOS devices have reached the sub-100 nm scale in accordance with Moore’s law, the investigation of the microscopic crystalline and the strain structures for various next generation semiconductor materials has grown increasingly important. In this study, we experimentally characterized the local strain structure in Ge/Ge1−x Sn x fine heterostructures by using synchrotron x-ray microdiffraction method [3,4]. We prepared Ge stripe structures with various line widths (25–100 nm) sandwiched with Ge1−x Sn x stressors. Patterned Ge(001) substrate with a SiO­ 2 cap layer was formed by anisotropic wet/dry etching as lines parallel along to the direction. The pitch of a Ge stripe is 500 nm. The Ge1−x Sn x layers were grown with solid-source molecular beam epitaxy (MBE) system and metal-organic chemical vapor deposition (MOCVD) system [5]. The Sn content in the Ge1−x Sn x was 2.9–6.5%. The x-ray microdiffraction measurement was performed at the beamline BL13XU in SPring-8 to analyze the local strain distribution and crystalline structure in Ge/Ge1−x Sn x samples. A synchrotron radiation light with an energy of 8 keV (λ=0.155 nm) was used. The cross-section size of an incident x-ray microbeam was estimated to be 0.16×0.20–0.82×0.26 μm2 in preliminary experiments. Transmission electron microscope observations revealed that Ge1−x Sn x were epitaxially grown on both sides of a Ge line, while the crystalline structure of Ge1−x Sn x on SiO2 on the top of Ge line was polycrystalline. We consider that the stressors of locally grown epitaxial Ge1−x Sn x can induce a local strain into the Ge line. In order to directly characterize the in-plane strain status in strained Ge region, we examined two-dimensional reciprocal space mapping (2DRSM) around the Ge1—1—3 asymmetric Bragg reflection with step scans of the microbeam position across the Ge lines with a step of 50 nm. Three diffraction peaks related to the bulk Ge, the epitaxial Ge1−x Sn x stressors, and the strained Ge are clearly observed in 2DRSM results. The intensity of diffraction peak of the strained Ge periodically varied corresponds to the stripe pitches. The diffraction peak position of the strained Ge in 2DRSM indicates an in-plane compressive strain is induced into a Ge line sandwiched with Ge1−x Sn x stressors. From the 2DRSM results for various Ge stripe widths, an in-plane strain value can be estimated from the lattice spacing with respect to that of bulk Ge. The in-plane compressive strain value in the Ge increases with narrowing the Ge line width and increasing the Sn content in the Ge1−x Sn x stressors, which is consistent with the simulated results of finite element method calculations. For the Ge0.943Sn0.057 stressors, the in-plane compressive strains were estimated to be 0.84%, 0.46% and 0.38% for 30, 60, and 100 nm-wide Ge lines, respectively. In summary, the formation of Ge/Ge1−x Sn x fine heterostructures were achieved by MBE and MOCVD, and experimentally characterized the local strain distribution in the Ge/Ge1−x Sn x by synchrotron radiation microdiffraction method. The microdiffraction enables a quantitative analysis of strain and an investigation of local crystalline structure in sub-100 nm-scale Ge lines sandwiched with Ge1−x Sn x stressors. This work was partly supported by the JSPS through the FIRST Program initiated by the CSTP, a Grant-in-Aid for Scientific Research (S) (Grant No. 26220605) and Core-to-Core Program ICRC-ACP4ULSI from the JSPS in Japan. The synchrotron radiation experiments were performed at SPring-8 with the approval of JASRI (Nos. 2012B1783, 2013A1682, 2013B1779, 2015A1874, and 2015B1813/BL13XU). [1] S. Zaima et al., Sci. Technol. Adv. Mater. 16, 043502 (2015). [2] T. Krishnamohan et al., IEDM Proc., 899 (2008). [3] S. Takeda et al., Jpn. J. Appl. Phys. 45, L1054 (2006). [4] Y. Imai et al., AIP Conf. Proc. 1221, 30 (2010). [5] Y. Inuzuka et al., Thin Solid Films 602, 7 (2016).