Analysis of Microscopic Strain and Crystalline Structure in Ge/Ge1−X
Sn
x
Fine Structures by Using Synchrotron X-ray Microdiffraction
Authors: Ike Shinichi , Nakatsuka Osamu , Inuzuka Yuki , Washizu Tomoya , Takeuchi Wakana , Imai Yasuhiko , Kimura Shigeru , Zaima Shigeaki 
Published: February 2020

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 <110> 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).