The dielectric spectral response of Ge1-xSnx thin film alloys with relatively high Sn contents (0.15 ≤ x ≤ 0.27) and thickness from 42 to 132 nm was characterized by variable angle spectroscopic ellipsometry over the wavelength range from 0.190 to 6 μm. The Ge1-xSnx thin films were deposited on Ge substrates by molecular beam epitaxy using an electron-beam source for Ge to achieve a substrate temperature below 150 °C to prevent the surface segregation of Sn. From the measured dielectric function, the complex refractive index was calculated indicating an increase in the real index with the Sn content at mid-infrared wavelengths. The ellipsometry revealed that the band structure critical point energies red-shifted with the increasing Sn content. The optical absorption coefficient was calculated from the imaginary index and showed a strong absorption into, and beyond, the mid-infrared with the increasing Sn content.

Using Fourier-transform infrared ellipsometry, the authors provide spectroscopic evidence about the valence band (VB) structure of diamond-like α-tin. The mid-infrared dielectric function of α-tin grown pseudomorphically on InSb or CdTe by molecular beam epitaxy shows a strong E¯0 peak near 0.41 eV. This peak is assigned to allowed intravalence band transitions from the Γ7− (electron-like) VB to the Γ8+v heavy hole VB and/or interband transitions from Γ7− to the Γ8+c light “hole” conduction band. The strength of this peak requires a hole density in the mid-1018 cm−3 range at room temperature, which might be caused by unintentional doping, by thermal electron-hole pair generation, or by the possibility that the L6+ conduction band might have an energy slightly lower than the Γ8+ VB maximum. Alternatively, this E¯0 peak might be enhanced by the M-shape of the Γ7− VB caused by interactions with the Γ7+ split-off hole VB. A sum-rule analysis of the dielectric function between 0.16 and 6.5 eV is consistent with a high-frequency dielectric constant of 24, which has at most a weak temperature dependence between 100 and 300 K.

Semiconductor-based mid-wave infrared photon detectors that functionalize space-based imaging systems are susceptible to both cumulative ionization and displacement damage, especially due to proton irradiation. Here, the dark current density and quantum efficiency of a mid-wave infrared detector utilizing a strain-balanced InGaAs/InAsSb superlattice active region are examined as a function of a 63 MeV proton radiation dose. Proton-irradiation is performed in an incremental stepwise dose up to a total ionizing dose of 100 krad(Si) or an equivalent proton fluence of 6.1 × 1011 protons/cm2. All characterization work is conducted with the detectors held at an operating temperature of 130 K throughout the experiment to limit thermal annealing effects. Prior to irradiation, the quantum efficiency of the top-side illuminated device without anti-reflection coating is 59.5%. The quantum efficiency is largely independent of temperature below 150 K, indicative of an electron minority carrier. As irradiation progressed the typical linear increase in inverse quantum efficiency with increasing proton fluence was observed, which led to a quantum efficiency damage factor of 1.12 × 1013 e cm2/ph. This value is shown to be an order of magnitude lower than typically observed in III-V nBn devices and is likely due to the higher mobility of minority electrons in the active region of this device. A full analysis of the characterization results suggests that displacement damage creates a significant population of donor states that modify the doping profile, in addition to Shockley–Read–Hall recombination centers that generally form as a result of proton irradiation.

Tin-rich Sn1−xGex alloys with Ge contents up to 6% were grown pseudomorphically on InSb (001) substrates by molecular beam epitaxy at room temperature. The alloys show a germanium-like lattice and electronic structure and respond to the biaxial stress within continuum elasticity theory, which influences bands and interband optical transitions. The dielectric function of these alloys was determined from 0.16 to 4.7 eV using Fourier-transform infrared and spectroscopic ellipsometry. The E1 and E1 + Δ1 critical points decrease with the increasing Ge content with a bowing parameter similar to the one established for Ge-rich Sn1−xGex alloys. On the other hand, the inverted direct bandgap E¯0 is nearly independent of the Ge content, which requires a bowing parameter of about 0.8 eV, much lower than what has been established using photoluminescence experiments of Ge-rich relaxed Sn1−xGex alloys.