Features of polarized Raman spectra for homogeneous and non‐homogeneous compressively strained Ge_1−ySn_y alloys

Abstract

Raman spectra of homogeneous and non‐homogeneous, compressively strained, Ge‐rich Ge1−ySny alloys (with y < 13%), deposited by molecular beam epitaxy (MBE) techniques using various low‐temperature process conditions, were investigated using polarized micro‐Raman spectroscopy. It is demonstrated that collecting Raman spectra using both polarizations ( bold-italicz(),xxtruez¯ and bold-italicz(),xytruez¯) provides more insight into the structural features of GeSn alloys than Raman spectroscopy using an unpolarized probe beam. Acquiring Raman spectra using polarization offers the advantages of enhancing the behaviour of some modes as well as eliminating the overlap of longitudinal optical (LO) modes with unwanted acoustical phonons from the Ge. Raman spectra of samples, grown at extremely low temperatures and identified by X‐ray diffraction as non‐homogeneous, exhibit a lowering of the Ge–Ge peak position, an increase in linewidth and a decrease of the dichroic ratio. Another type of non‐homogeneity appears when the metastable GeSn separates into two phases after obtaining a Sn content‐dependent thickness. Raman microscopy confirmed that the segregates, obtained as a result of phase separation and Sn diffusion to the surface, are of metallic tin origin. Copyright © 2017 John Wiley & Sons, Ltd.

Evolution of optical phonons in epitaxial Ge1−ySny structures

“…The main peak at 296.7 cm −1 corresponds to the Ge–Ge LO phonon along the z direction. [ 17,25,26 ] A weak Raman response at 285 cm −1 on the low‐wavenumber shoulder of the Ge–Ge LO(z) phonon mode corresponds to a disorder‐related phonon. [ 17 ] The Ge–Sn and Sn–Sn LO(z) phonons are weakly observed approximately at 259 and 183 cm −1 , respectively, owing to the incorporation of a small amount of Sn (5.2%) into Ge.…”

“…[ 17,25,26 ] A weak Raman response at 285 cm −1 on the low‐wavenumber shoulder of the Ge–Ge LO(z) phonon mode corresponds to a disorder‐related phonon. [ 17 ] The Ge–Sn and Sn–Sn LO(z) phonons are weakly observed approximately at 259 and 183 cm −1 , respectively, owing to the incorporation of a small amount of Sn (5.2%) into Ge. [ 17 ] The weak 160 and 228 cm −1 modes correspond to two‐phonon overtone states of the transverse acoustic phonons of Ge.…”

“…[ 12–14 ] In addition, a large lattice mismatch between the GeSn layer and Si substrate results in the formation of defects in GeSn during epitaxial growth. [ 8,10 ] In response, low‐temperature epitaxial growth techniques have been introduced to improve the crystalline quality of GeSn, [ 14–18 ] and further, by inserting a Ge buffer layer between the GeSn layer and Si substrate, defects can be trapped at the GeSn/Ge interface, resulting in a high crystalline quality of GeSn. [ 8,19 ] Post‐growth treatments, such as thermal annealing and plasma treatment, have also been shown to be helpful in modifying the structural, electrical, and optical properties of GeSn.…”

Recent advances in linear and nonlinear Raman spectroscopy. Part XII

“…Perova et al described features of polarized Raman spectra for homogeneous and nonhomogeneous compressively strained Ge 1‐y Sn y alloys. They demonstrate that collecting Raman spectra using both polarizations (zxxz and zxyz) provides more insight into the structural features of GeSn alloys than Raman spectroscopy using an unpolarized probe beam . Vasquez et al provided an assessment of Cr doping and size effects on the Raman‐active modes of rutile TiO 2 by UV/Visible polarized Raman spectroscopy.…”

“…They demonstrate that collecting Raman spectra using both polarizations (zxxz and zxyz) provides more insight into the structural features of GeSn alloys than Raman spectroscopy using an unpolarized probe beam. [182] Vasquez et al provided an assessment of Cr doping and size effects on the Raman-active modes of rutile TiO 2 by UV/Visible polarized Raman spectroscopy. Raman spectra are highly sensitive to lattice distortions, doping, size effects, or excitation source, among other factors.…”