Anisotropic in-Plane and out-of-Plane Strain Relaxation in Carbon-Doped Silicon Nanowires Evaluated by X-Ray Reciprocal Space Mapping

Abstract

1. Background and purpose Since carbon (C) has a smaller lattice constant than silicon (Si), Si doped with C approximately 1% or less (Si:C: carbon-doped silicon) also has a smaller lattice constant than Si. By using Si:C as the source / drain material of the n-type metal-oxide-semiconductor field-effect transistor, the tensile strain can be applied to the channel region to improve electron mobility [1]. However, the nano-fabrication of Si:C causes strain relaxation, which hinders the improvement of electron mobility. We have reported the in-plane biaxial strain relaxation for Si:C nanowires evaluated with Raman spectroscopy, assuming that there is no out-of-plane strain relaxation [2]. In this study, we evaluated anisotropic strain relaxation, including not only in-plane but also out-of-plane direction, for Si:C nanowire by reciprocal lattice space mapping (RSM) measurement using synchrotron radiation X-rays. 2. Experimental method Si:C thin films were grown on (001) Si substrate by molecular beam epitaxy, and then nano-fabricated into nanowire by electron beam lithography and dry etching. The film thicknesses were 43, 50, and 37 nm, with C concentration of 0.60%, 0.83%, and 1.1%, respectively, confirmed by cross-sectional transmission electron microscope and secondary ion mass spectrometry measurements. In the Si:C nanowire, the length direction was parallel to the [110] direction and the width direction was parallel to the [-110] direction, respectively. The nanowire width (W) was varied as 1000, 500, 200, and 100 nm, while the nanowire length (L) was fixed at 10 μm. In order to obtain sufficient signals for RSM, 30,000 identical nanowires in the case of W = 1,000 and 500 nm, and 50,000 identical nanowires in the case of W = 200 and 100 nm were fabricated in approximately 1.5 mm × 1.5 mm areas, respectively. The X-ray energy was set to 10 keV. The RSMs of Si:C nanowires were obtained around 337 diffractions for C concentrations of 0.60% and 0.83% samples (Si:C0.83%), and around 115 diffraction for C concentration of 1.1% sample, respectively. 3. Results and Discussion Figures 1 (a) and (b) show the RSMs of the unprocessed and the nanowires with W of 500 nm of Si:C0.83% films, respectively. In Fig. 1 (a) and (b), the profiles near qx = 4.91 Å- 1, qz = 8.11 Å- 1 correspond to the hem of the 337 diffraction profiles for the Si substrate, and the peaks around qz = 8.165 Å- 1 are the diffraction profiles for the Si:C0.83% film or the Si:C0.83% nanowires, respectively. Here, qx and qz are components of scattering vector in the in-plane and out-of-plane directions, respectively. Figure 1 (a) shows that the profiles for Si substrate and Si:C0.83% film were obtained on the same qx, which indicates that the in-plane lattice constant of Si:C0.83% film before nano-fabrication was equal to that of Si substrate, and the tensile strain has been applied to Si:C0.83% film. Figure 1 (b) shows that qx increases and qz decreases for the Si:C0.83% nanowires with W of 500 nm as compared with the Si:C0.83% film, which indicates that...