Determination of the optical energy gap of Ge[sub 1−x]Sn[sub x] alloys with 0<x<0.14 (original) (raw)
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Nonlinear behavior of the energy gap in Ge[sub 1−x]Sn[sub x] alloys at 4 K
Applied Physics Letters, 2007
The optical energy gap of Ge 1−x Sn x alloys ͑x ഛ 0.14͒ grown on Ge substrates has been determined by performing transmittance measurements at 4 K using a fast fourier transform infrared interferometer. The direct energy gap transitions in Ge 1−x Sn x alloys behave following a nonlinear dependence on the Sn concentration, expressed by a quadratic equation, with a so called bowing parameter b 0 that describes the deviation from a simple linear dependence. Our observations resulted in b 0 RT = 2.30± 0.10 eV and b 0 4 K = 2.84± 0.15 eV, at room temperature and 4 K, respectively. The validity of our fit is limited for Sn concentrations lower than 15%.
Ge[sub 1−x]Sn[sub x] alloys pseudomorphically grown on Ge(001)
Applied Physics Letters, 2003
The optical energy gap of Ge 1Ϫx Sn x alloys has been determined from transmittance measurements, using a fast-Fourier-transform infrared interferometer. Our results show that the change from indirect to direct band gap occurs at a lower critical Sn concentration (x c ) than the value predicted from the virtual crystal approximation, tight binding, and pseudopotential models. However, a close agreement between the experimental results and the predictions with deformation potential theory is observed. The concentration x c , which is theoretically expected to be 0.09, actually it is observed to lie between 0.10Ͻx c Ͻ0.13.
Optical Transitions in Direct-Bandgap Ge1–xSnx Alloys
ACS Photonics, 2015
A comprehensive study of optical transitions in direct bandgap Ge 0.875 Sn 0.125 group IV alloys via photoluminescence measurements as a function of temperature, compressive strain and excitation power is performed. The analysis of the integrated emission intensities reveals a strain-dependent indirect-to-direct bandgap transition, in good agreement with band structure calculations based on 8 band k•p and deformation potential method. We have observed and quantified valleyheavy hole and valleylight hole transitions at low pumping power and low temperatures in order to verify the splitting of the valence band due to strain. We will demonstrate that the intensity evolution of these transitions supports the conclusion about the fundamental direct bandgap in compressively strained GeSn alloys. The presented investigation, thus, demonstrates that direct bandgap group IV alloys can be directly grown on Ge-buffered Si(001) substrates despite their residual compressive strain.
Applied Physics Letters, 2000
The direct energy gap has been measured for coherently strained Sn x Ge 1Ϫx alloys on Ge͑001͒ substrates with 0.035ϽxϽ0.115 and film thickness 50-200 nm. The energy gap determined from infrared transmittance data for coherently strained Sn x Ge 1Ϫx alloys indicates a large alloy contribution and a small strain contribution to the decrease in direct energy gap with increasing Sn composition. These results are consistent with a deformation potential model for changes in the valence and conduction band density of states with coherency strain for this alloy system.
Band structure calculations of Si–Ge–Sn alloys: achieving direct band gap materials
Semiconductor Science and Technology, 2007
Alloys of silicon (Si), germanium (Ge) and tin (Sn) are continuously attracting research attention as possible direct band gap semiconductors with prospective applications in optoelectronics. The direct gap property may be brought about by the alloy composition alone or combined with the influence of strain, when an alloy layer is grown on a virtual substrate of different composition. In search for direct gap materials, the electronic structure of relaxed or strained Ge 1−x Sn x and Si 1−x Sn x alloys, and of strained Ge grown on relaxed Ge 1−x−y Si x Sn y , was calculated by the self-consistent pseudo-potential plane wave method, within the mixed-atom supercell model of alloys, which was found to offer a much better accuracy than the virtual crystal approximation. Expressions are given for the direct and indirect band gaps in relaxed Ge 1−x Sn x , strained Ge grown on relaxed Si x Ge 1−x−y Sn y , and for strained Ge 1−x Sn x grown on a relaxed Ge 1−y Sn y substrate, and these constitute the criteria for achieving a (finite) direct band gap semiconductor. Roughly speaking, good-size (up to ∼0.5 eV) direct gap materials are achievable by subjecting Ge or Ge 1−x Sn x alloy layers to an intermediately large tensile strain, but not excessive because this would result in a small or zero direct gap (detailed criteria are given in the text). Unstrained Ge 1−x Sn x bulk becomes a direct gap material for Sn content of > 17%, but offers only smaller values of the direct gap, typically ≤0.2 eV. On the other hand, relaxed Sn x Si 1−x alloys do not show a finite direct band gap.
Physical Review B, 2006
The E 0 , E 0 + ⌬ 0 , E 1 , E 1 + ⌬ 1 , E 0 Ј, and E 2 optical transitions have been measured in Ge 1−y Sn y alloys ͑y Ͻ 0.2͒ using spectroscopic ellipsometry and photoreflectance. The results indicate a strong nonlinearity ͑bow-ing͒ in the compositional dependence of these quantities. Such behavior is not predicted by electronic structure calculations within the virtual crystal approximation. The bowing parameters for Ge 1−y Sn y alloys show an intriguing correlation with the corresponding bowing parameters in the Ge 1−x Si x system, suggesting a scaling behavior for the electronic properties that is the analog of the scaling behavior found earlier for the vibrational properties. A direct consequence of this scaling behavior is a significant reduction ͑relative to prior theoretical estimates within the virtual crystal approximation͒ of the concentration y c for a crossover from an indirect-to a direct-gap system.
Optical properties of pseudomorphic Ge1−xSnx(x = 0 to 0.11) alloys on Ge(001)
Journal of Vacuum Science & Technology B, Nanotechnology and Microelectronics: Materials, Processing, Measurement, and Phenomena, 2014
The characterization of the optical properties of pseudomorphic Ge 1Àx Sn x /Ge/Si (x ¼ 0 to 0.11) alloys from the IR to UV is presented. The Ge 1Àx Sn x alloys were epitaxially grown on relaxed Ge grown on Si. Rutherford backscattering (RBS) and RBS ion channeling methods were used to confirm the Sn composition and substitutional nature of the Sn into the Ge lattice. The pseudomorphic nature of the Ge 1Àx Sn x on Ge is confirmed using high resolution x-ray diffraction (HRXRD) and transmission electron microscopy. Although HRXRD reciprocal space maps indicated that the Ge 1Àx Sn x was pseudomorphic to Ge, the shape of the Bragg peaks indicated that the sample surface was rough. The rough surface morphology is confirmed using atomic force microscopy. The complex dielectric function is reported in the IR, visible, and UV spectrum in the wavelength range of 0.2-5.06 eV. The E 1 , E 1 þ D 1 , E 2 , and E 0 critical points are extracted using second and third derivative line shape fitting and are compared with the elastic theory calculations of strained Ge 1Àx Sn x (x ¼ 0 to 0.11) alloys and fully relaxed Ge 1Àx Sn x (x ¼ 0 to 0.11) alloys. The E 0 critical point energies are observed to have slightly larger values than those calculated for completely relaxed Ge 1Àx Sn x alloys due to the presence of compressive strain. V