| AVS 63rd International Symposium & Exhibition | |
| Advanced Surface Engineering | Wednesday Sessions |
| Session SE+2D+EM-WeA |
| Session: | Multifunctional Thin Films and Coatings |
| Presenter: | Nalin Fernando, New Mexico State University |
| Authors: | N.S. Fernando, New Mexico State University R. Hickey, University of Delaware J. Hart, University of Delaware R. Hazbun, University of Delaware D. Zhang, University of Delaware J. Kolodzey, University of Delaware S. Zollner, New Mexico State University |
| Correspondent: | Click to Email |
Ge-Si-Sn alloys are interesting for CMOS applications for a variety of reasons. For example, adding Sn to Si-Ge lowers the band gap, which reduces parasitic contact resistance. Also, the lattice constant of Ge-Sn alloys increases when adding Sn. Therefore, such alloys could be used to impart strain on PMOS devices with Ge channels. We used deformation potential theory to determine the compositional dependence of the direct, indirect, E1, and E1+ Δ1 band gaps of pseudomorphic Ge1-x-ySixSny on Ge and theoretical predictions are validated through spectroscopic ellipsometry measurements of the band gaps of pseudomorphic Ge1-ySny on Ge grown by MBE.
The band structure of Ge is a strong function of strain and alloy composition, and a transition from an indirect to a direct band gap has been observed for y~6-10% for relaxed Ge1-ySny indicating the possibility of widespread applications of Ge-based photonic devices. The pseudomorphic nature of the Ge-based alloy layer on a substrate is important to keep dislocation densities low at the interface to improve the performance of the device. Band gap engineering of Ge by controlling strain and alloying with Si and Sn has attracted great interest since a Ge1-x-ySixSny ternary alloy with two compositional degrees of freedom allows decoupling of the lattice constant and electronic structures.
The pseudomorphically grown Ge1-x-ySixSny layer on Ge experiences a biaxial stress due to the lattice mismatch between the alloy layer and the Ge. The strain resulting from the stress affects the band structure of the alloy. Deformation potential theory is used to determine the compositional dependence of the band gaps of pseudomorphic Ge1-x-ySixSny on Ge as a function of Si (x) and Sn (y) compositions. The predictions of the deformation potential theory are validated for pseudomorphic Ge1-ySny (for Si=0) on Ge through measurements of the optical properties. The complex pseudodielectric functions of pseudomorphic Ge1-ySny alloys grown on Ge by MBE were measured using ellipsometry in the 0.1-6.6 eV energy range for Sn contents up to 11%, to investigate the compositional dependence of the band gaps. Critical point energies (CP) and related parameters were obtained by analyzing the second derivative spectrum of the dielectric function. Our experimental results for Egdir, E1 and E1+ Δ1 gaps are in good agreement with the theoretically predicted CP energies. We will discuss the strain and compositional dependence of the band gaps and the effects of the growth temperature of the Ge buffer layer on Si to the band gaps.
This work was supported by AFOSR (FA9550-13-1-00222). FTIR measurements were performed at CINT.