Silicon germanium (SiGe) is expected to be applied for the optical devices such as infrared sensors, because of its narrower band gap than Si. Therefore, it is important to understand the optical properties of SiGe. In general, SiGe films epitaxially grown on Si or Ge substrates are the candidate materials for the optical devices. Inhomogeneous dislocation and strain in the SiGe films may reduce process yields. Although there are some literatures regarding to the distribution of dislocations and strain in the SiGe films [1, 2], the optical properties have not been evaluated sufficiently yet. In this study, we investigate the distribution of optical properties in the SiGe films by spectroscopic ellipsometry.
Experiments
Graded SiGe buffer layers (10% Ge / μm) and strain-relaxed SiGe layers (6 μm) with Ge fraction of 20% and 50% were grown on 200 mm diameter Si (001) substrates by molecular beam epitaxy. It is widely recognized that the dislocation density and strain are reduced in the SiGe layer epitaxially grown on a graded buffer layer on a Si (001) substrate. Since the optical properties are affected by strain [3], this structure, which can reduce strain, is useful for applying SiGe films in optical devices. In order to investigate the distribution of the optical properties, strain and Ge fraction in the SiGe films, we measured several points from the center to the edge of the wafer by spectroscopic ellipsometry and X-ray diffraction (XRD). The spectroscopic ellipsometry measurements were performed with an incident angle of 70°, a 70 W xenon lamp as the light source, and a wavelength range of 200 to 1600 nm. For XRD, we measured the (004) plane using a Cu Kα X-ray source with a wavelength of 1.5406 Å. The lattice parameters were calculated based on the 004 diffraction peak from the SiGe films.
Results and Discussion
Figure 1 (a) and (b) show the distribution of the extinction coefficient in the SiGe films with Ge fraction of 20% and 50%, respectively. The distributions of the extinction coefficient in the infrared region are enlarged in these figures. We found that the extinction coefficient of SiGe (50%) is higher than that of SiGe (20%) in the near-infrared region, indicating that the higher Ge fraction is able to absorb longer wavelength light. From Fig. 1, it is clear that the SiGe wafer (50%) has a larger variation than the SiGe wafer (20%), especially in the long wavelength region.
Figures 2 and 3 show XRD peak of SiGe films and the distribution of the lattice parameter in the SiGe films with Ge fraction of 20% and 50%, respectively. The lattice parameters of Si1-xGex assuming strain-free were calculated using Vegard's law (dx [Å] = 0.027x2 + 0.2x + 5.431).
From Fig. 3, it is clear that the SiGe wafer (50%) shows a larger variation than the SiGe wafer (20%) with respect to the lattice parameter. Since the lattice parameter of SiGe varies with the Ge fraction and strain state, first the cause of this variation can be cited that of the Ge fraction. The lattice parameters at the edge of the wafers are much larger than those at the center of the wafers, suggesting that variations in Ge fraction at the edge of the wafers. The same behavior has been observed in previous studies regarding SiGe wafers [1, 2]. Also, it is considered that the variations of the lattice parameter vary not only due to Ge fraction, but also residual strain. Although sample structures are designed to induce strain relaxation in SiGe films, it is considered that the strain in the epitaxially grown SiGe film on the graded buffer layer cannot be fully relaxed based on the results of XRD. These variations of Ge fraction and strain may cause the variation of the extinction coefficient of SiGe films.
The result that the SiGe wafer (50%) has a larger variation than the SiGe wafer (20%) indicates that the inhomogeneity of the optical properties and the lattice parameter increase and it may reduce process yields with increasing Ge fraction, for SiGe films on Si substrates.
References
[1]. A. Ogura et al., Jpn. J. Appl. Phys. 45, 3007 (2006).
[2]. Y. F. Tzeng et al., Appl. Phys. Express 3, 106601 (2010).
[3]. Y. Ishikawa et al., Appl. Phys. Lett. 82, 2044 (2003).