Sn Based Group Iv Materials For Photonic Devices

Author : Nalin S. Fernando
Publisher :
ISBN 13 :
Total Pages : 358 pages
Book Rating : 4.:/5 (1 download)

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Download or read book Optical Characterization of Group-IV Semiconductor Alloys Using Spectroscopic Ellipsometry and High Resolution X-ray Diffraction written by Nalin S. Fernando and published by . This book was released on 2017 with total page 358 pages. Available in PDF, EPUB and Kindle. Book excerpt: Germanium is a group IV semiconductor widely used in the semiconductor optoelectronic industry. It is an indirect band material with the conduction band minimum at the L point, which is 0.140 eV below the conduction band at the [gamma] point. However, the band structure of Ge is a strong function of temperature, strain, alloy compostition and dopant concentration. It has been reported that, at about ~2% tensile strain, Ge becomes a direct band gap material, indicating the possibility of wide spread applications of Ge-based photonic devices. Alloying Ge with Sn also makes it a direct band gap material, relaxed Ge1−[subscript y]Sn[subscript y] alloys become direct at 6-10% Sn. In addition, Ge1−[subscript x]−[subscript y]Si[subscript x]Sn[subscript y] ternary alloy with two compositional degrees of freedom allows decoupling of the lattice constant and electronic structures simultaneously. Band gap engineering of Ge by controlling strain, alloying composition and dopant concentration has attracted the interest of researchers in materials science. Hence, the knowledge of the compositional, strain, and temperature dependence of the Ge1−[subscript x]−[subscript y]Si[subscript x]Sn[subscript y] band structure is critical for the design of photonic devices with desired interband transition energy. This dissertation focuses on the optical characterization of the compostional, strain, and temperature dependence of the optical properties of Ge-Si-Sn alloys on Ge/Si substrates using spectroscopic ellipsometry. We use high resolution X-ray diffraction (HRXRD), X-ray reflectivity (XRR) and atomic force microscopy (AFM) to characterize the strain, compostion, thickness, surface roughness of the Ge-Si-Sn epilayers on Ge/Si substrates. The temperature dependent thermal expansion coefficent of Ge is larger than Si. Therefore a Ge film, which is relaxed at the growth temperature (~800 K) on Si substrate, likes to contract more rapidly compared to Si upon cooling down to lower temperatures, and will experience a temperature dependent biaxial tensile stress. We predict the strain dependence the E1 and E1+[delta]1 critical points of Ge on Si from 80 - 780 K using deformation potential theory and employing a model for the thermal expansion coefficients of Si and Ge. The predictions are validated experimentally, using spectroscopic ellipsometry. A psuedomorphically grown Ge1−[subscript x]−[subscript y]Si[subscript x]Sn[subscript y] layer on Ge also experiences a biaxial stress due to the lattice mismatch between the alloy layer and the Ge substrate. We use elasticity theory and deformation potential theory to predict the variation of the energy band structure of psuedomorphic Ge1−[subscript x]−[subscript y]Si[subscript x]Sn[subscript y] on Ge at the [gamma], L, and X symmetry points in the Brillouin zone as a function of Si (x) and Sn (y) compositions. The critical Si (x) and Sn (y) compositions needed for an indirect to direct band gap transition are identified. The effects of the substrate on the band gaps and indirect-direct transition are investigated when the active Ge1−[subscript x]−[subscript y]Si[subscript x]Sn[subscript y] is pseudomorphically grown on Ge buffered Si substrates and on GaAs. The theoretical predictions from deformation potential theory are experimentally validated using spectroscopic ellipsometry for psuedomorphic Ge1−[subscript y]Sn[subscript y] on Ge. The complex pseudodielectric functions of the samples were measured using spectroscopic ellipsometry in the 0.5-6.6 eV energy range. Temperature dependent measurements were taken in a UHV cryostat at temperatures from 80-780 K. A multilayer model consisting of a surface layer/epilayer/substrate was used for the treatment of the experimental data. The surface layer was modeled as a surface oxide layer-- GeO2, where the accurate optical constants were determined by multisample spectroscopic ellipsometry analysis of thermally grown GeO2. Critical point energies and related parameters were obtained by the analyzing second-derivative spectrum d2[epsilon]/d2[omega] of the dielectric function. Our experimental results are in excellent agreement with the theoretical predictions from deformation theory.