Detection of Effect of Strain on the Valence Band Structure of SiGe by HXPES with High Spatial Resolution

Since the aggressive scaling of the Si-based metal–oxide–semiconductor field effect transistors (MOSFET) is reaching its limits, the strained-Ge and III–V semiconductors are being studied extensively as one of the alternative channel materials due to their higher carrier mobility [1,2]. Recently, the channel strain engineering has become an essential technology for the future complementary metal oxide semiconductor. The process-induced local strain is currently the mainstream technology [3]. We reported that a selective-ion-implantation technique was developed as a novel method of creating uniaxially strained Si/Ge heterostructures [4]. We investigated the effect of the strain on the binding energy of valence band top, Si 1s and Ge 2p core level by Hard X-ray photoemission spectroscopy (HXPES) with the high spatial resolution.

The sample was prepared as follows. A SiO₂ film was firstly deposited as a mask on a Si(100) substrate by plasma-enhanced chemical vapor deposition (PECVD), followed by the patterning of stripes with a standard photolithography process. Ar⁺ ion were selectively implanted into the Si substrate through the SiO₂ mask windows at an energy of 25 keV with a dose of 1×10¹⁵ cm^-2. The line widths of the implanted and unimplanted regions were fixed to be 20 and 2μm, respectively. A SiGe layer with a Ge content of 26% and thickness around 70 nm was pseudomorphically grown on the selectively ion-implanted Si substrate. Subsequently, postgrowth annealing was carried out to promote strain relaxation at 900�C. The hard X-ray (7.94 keV photons) excited Si 1s, Ge 2p, and valence band spectra arising from the sample were measured at photoelectron take-off angle (TOA) of 85 degrees with the energy resolution of 100 meV at undulator beam line (BL47XU at SPring-8 (JASRI, Proposal No. 2012B1269)) using high resolution electron energy analyzer R-4000 with the spatial resolution [11(�|)0] direction is about 0.8µm [5].

 The Si 1s photoelectron spectra are changing in the position of a sample as shown in Fig. 1 (a). From Fig. 1 (b) and (c), the binding energies of Si 1s and Ge 2p core level are reduced at the position of 8, 30, and 52µm. Here, spectra are shown in figures as a parameter at the position of the [11(�|)0] direction. On the other hand, the change of the binding energy of valence band top is less than that of peak of Si 1s and Ge 2p as shown in Fig. 1(d). The comparison of the position of the valence band top in the case in relation to the binding energy of Ge 2p is shown in Fig. 2. Focus on the position of the valence band top, the position of the valence band of the uniaxial strained SiGe is lower than that in the case of the biaxial strained SiGe (50% relaxation rate). The change in the valence band position due to the difference in stress is consistent with the calculated results [6].

Acknowledgments

 We acknowledge Prof. N. Usami and Y. Hoshi for their supports for the MBE growth. This work was partly supported by the MEXT-Supported Program for the Strategic Research Foundation at Private Universities 2009-2013, by Grant-in-Aid for Scientific Research (Grant No. 25600079) from MEXT, Japan, and by the Strategic Information and Communications R&D Promotion Programme (SCOPE) from MIC, Japan.

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