Synthesis, stability range, and fundamental properties of Si-Ge-Sn semiconductors grown directly on Si(100) and Ge(100) platforms

This paper reports a comprehensive experimental and theoretical account of synthesis, optical response, transport properties, and thermodynamic stability for a new family of Ge_1-x-ySi_xSn_y semiconductor alloys based entirely on group IV elements. Device quality layers are grown directly on both Ge(100) and Si(100) wafers using low-temperature chemical vapor deposition (CVD) of commercially available sources such as trisilane, digermane, and stannane, thereby making the process suitable for direct industrial scale up and applications. This soft chemistry process is extended to demonstrate fabrication of p- and n-type layers on Si and determine their transport properties by both contactless optical methods and conventional Hall experiments. Spectroscopic analyses by UV-IR ellipsometry and Raman scattering show that the alloys possess fundamental optical and bonding properties identical to those of the materials previously grown on Ge-Sn buffers. Transmission electron microscopy (XTEM), Rutherford backscattering (RBS), and high resolution X-ray diffraction (HRXRD) characterizations demonstrated that precise tuning of the composition to achieve a Si/Sn ratio of ∼3.7 yields strain-free films with Ge-like unit cell dimensions. In the case of growth on Ge(100) the films exhibit the expected flawless registry afforded by the perfect chemical and structural matching with the underlying platform. When grown on Si(100) the lattice misfit with the substrate is compensated by periodic edge-type dislocations at the interface. Independent variation of the Si/Sn ratio from ∼1.5-4 produces a range of tetragonally distorted films on Si(100) with significant compressive strains (<0.60%) and in-plane lattice constants that are found to be "pinned" near the Ge value of 5.658 ⋯ The composition/temperature phase diagrams for SiGeSn and GeSn systems are obtained from first principles by calculating the Gibbs free energy via density functional theory. The stability fields of the alloys are used to predict accessible compositions, and these are compared in detail with the results from experimental studies. The principle outcome is that the mixing entropy stabilizes the ternary alloys with respect to the binaries at the same Sn content, in agreement with experimental observations.