1. Background and purpose

Ge_1-xSn_x has a higher electron and hole mobility than Si, which attract much attention as a next-generation channel material [1]. Moreover, it is expected further mobility enhancement by applying appropriate strain. However, the strain field in the nano-fabricated channel region changes complicatedly depending on the channel shape, therefore accurate strain evaluation is strongly desired. In this study, we evaluate the anisotropic three-dimensional strain relaxation in the Ge_1-xSn_x stripe-shaped mesa structures by oil-immersion Raman spectroscopy and X-ray diffraction (XRD) measurement using synchrotron radiation.

2. Experiment

The Ge_1-xSn_x layers were epitaxially grown on Ge (001) substrate for 34, 55 and 45 nm with Sn concentrations of 3.2, 2.1, and 1.3%, respectively by home-made metal organic chemical vapor deposition (MOCVD) using t-C₄H₉GeH₃ (tertiary-butyl-germane) and (C₂H₅)₄Sn (tetra-ethyl-tin) as precursors[2]. After the epitaxial film formation, stripe-shaped mesa structures were fabricated by electron beam lithography and dry etching. Figure1 shows the schematic diagram of sample. The length (L) in the major axis directions along the stripe was fixed at 10 μm and the width (W) in the minor axis directions perpendicular to the stripe were varied as 0.1, 0.2, 0.5 and 1.0 μm. In-plane biaxial strain was measured by oil-immersion Raman spectroscopy, and the strain of the in-plane direction and the direction perpendicular to the substrate surface were measured by XRD using synchrotron radiation. In the oil-immersion Raman spectroscopy, the wavelength of the excitation light source was 532 nm, the focal length of the spectrometer was 2000 mm, the objective lens was immersion lens with the numerical aperture NA = 1.4, and the oil in the medium had the refractive index of n = 1.5. In the XRD measurement, the wavelength of the incident X-ray was 1.23968 Å. Figure2 shows an SEM image of a stripe-shaped Ge_1-xSn_x mesa structure with L = 10μm. As shown in Fig. 2, the X-ray diffraction intensity was sufficiently obtained by covering many stripes with the same shape in the X-ray irradiation area.

3. Results and Discussion

Figure3 and 4 show the stress along of the in-plane major axis and the minor axis directions determined by oil-immersion Raman spectroscopy, where L was fixed and W were varied as 0.1, 0.2, 0.5, 1.0 μm. As shown in Fig. 3, the stress increases with increasing Sn concentration. The stress of the major axis direction is not relaxed by W reduction regardless of Sn concentration. On the other hand, as shown in Fig. 4, the stress of the minor axis direction perpendicular to the stripe is relaxed at W = 0.2 μm or less regardless of Sn concentration. Since oil-immersion Raman spectroscopy cannot evaluate the strain perpendicular to the substrate surface, XRD measurement was performed. Observed XRD profiles for W = 0.1 and 0.2 μm indicated lattice shrinkage in the direction perpendicular to the surface. From the above, we have comfilmed the relaxation due to the nano-fabrication of the stripe Ge_1-xSn_x mesa structure occurs in the in-plane minor axis direction and the direction perpendicular to the substrate surface while preserving the strain of the in-plane major axis direction, for the sample W = 0.2 μm or less.

Acknowledgements

The synchrotron radiation experiments were performed at the BL19B2 of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2016A1529, 2017A1702, 2017B1843 and 2017B1930).

References

[1] K. J.Kuhn. et al., IEDM Tech.Dig. 171 (2012).

[2] K. Suda. et al., Jpn.J.Appl.Phys.53,110301 (2014).