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Title: Lattice matched GeSn/InAlAs heterostructure: role of Sn in energy band alignment, atomic layer diffusion and photoluminescence
Germanium alloyed with α-tin (GeSn) transitions to a direct bandgap semiconductor of significance for optoelectronics. It is essential to localize the carriers within the active region for improving the quantum efficiency in a GeSn based laser. In this work, epitaxial GeSn heterostructure material systems were analyzed to determine the band offsets for carrier confinement: (i) a 0.53% compressively strained Ge 0.97 Sn 0.03 /AlAs; (ii) a 0.81% compressively strained Ge 0.94 Sn 0.06 /Ge; and (iii) a lattice matched Ge 0.94 Sn 0.06 /In 0.12 Al 0.88 As. The phonon modes in GeSn alloys were studied using Raman spectroscopy as a function of Sn composition, that showed Sn induced red shifts in wavenumbers of the Ge–Ge longitudinal optical phonon mode peaks. The material parameter b representing strain contribution to Raman shifts of a Ge 0.94 Sn 0.06 alloy was determined as b = 314.81 ± 14 cm −1 . Low temperature photoluminescence measurements were performed at 79 K to determine direct and indirect energy bandgaps of E g,Γ = 0.72 eV and E g,L = 0.66 eV for 0.81% compressively strained Ge 0.94 Sn 0.06 , and E g,Γ = 0.73 eV and E g,L = 0.68 eV for lattice matched Ge 0.94 Sn 0.06 epilayers. Chemical effects of Sn atomic species were analyzed using X-ray photoelectron spectroscopy (XPS), revealing a shift in Ge 3d core level (CL) spectra towards the lower binding energy affecting the bonding environment. Large valence band offset of Δ E V = 0.91 ± 0.1 eV and conduction band offset of Δ E C,Γ–X = 0.64 ± 0.1 eV were determined from the Ge 0.94 Sn 0.06 /In 0.12 Al 0.88 As heterostructure using CL spectra by XPS measurements. The evaluated band offset was found to be of type-I configuration, needed for carrier confinement in a laser. In addition, these band offset values were compared with the first-principles-based calculated Ge/InAlAs band alignment, and it was found to have arsenic up-diffusion limited to 1 monolayer of epitaxial GeSn overlayer, ruling out the possibility of defects induced modification of band alignment. Furthermore, this lattice matched GeSn/InAlAs heterostructure band offset values were significantly higher than GeSn grown on group IV buffer/substrates. Therefore, a lattice matched GeSn/InAlAs material system has large band offsets offering superior carrier confinement to realize a highly efficient GeSn based photonic device.  more » « less
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Journal of Materials Chemistry C
Medium: X
Sponsoring Org:
National Science Foundation
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