R. Ferrini

Optical functions from 0.02 to 6 eV of AlxGa1−xSb/GaSb epitaxial layers

The complex refractive index n=n+ik and the dielectric function e=e1+ie2 at room temperature of AlxGa1−xSb films with 0⩽x⩽0.5, grown by molecular beam epitaxy on a GaSb substrate, were determined from 0.02 to 6 eV by using the complementary data from fast Fourier transform far-infrared, dispersive, and ellipsometric spectrometry. The effect of the native oxide was accounted for and the self-consistency of the optical functions was checked in the framework of the Kramers–Kronig causality relations. In the restrahlen region the dielectric function was well fitted by classical Lorentz oscillators; in the transparent region below the fundamental gap E0, the refractive index was modeled by a Sellmeier dispersion relation, and in the interband region the dielectric function near the critical points was analyzed through standard line shapes. Interpolating the fitting parameters or the interband dielectric spectra, it was possible to obtain the optical functions for any concentration x between 0.0 and 0.5.

Optical functions of bulk and epitaxial GaSb from 0.0025 to 6 eV

Abstract The complex refraction index and dielectric functions of GaSb bulk (both p and n -type) and MBE film were accurately determined from 0.0025 to 6 eV by using reflectance and ellipsometric spectroscopies. These functions, which satisfy the Kramers-Kronig causality relations, appear independent on the doping (≤2×10 17 cm −3 ) in the interband transition region. In particular, in comparison with previous data from literature, the E 0 fundamental gap is well evidenced for all the samples. Moreover, the optical functions around the gap are very different from those previously reported and the refractive index was modeled by a Sellmeier dispersion relation. The free-carriers instead strongly influence the far-infrared restrahlen region due to the phonon-plasmon coupling. In this range the optical functions were well fitted by Drude-Lorentz oscillators.

Optical evaluation of the ionized EL2 fraction in proton (24 GeV) irradiated semi-insulating GaAs

Semi-insulating SI GaAs samples from a zone refined crystal were irradiated with high energy protons (24 GeV/c, fluences up to 1.64×1014 p/cm2). Optical spectra in transmittance and reflectance were accurately measured in the energy range of 0.6–1.4 eV to determine, through the absorption coefficient, the concentrations of both neutral and ionized EL2 defects as a function of the proton fluence. Both these concentrations have been shown to increase linearly with the proton fluence; this behavior well explains the remarkable decrease of the charge collection efficiency observed in proton irradiated GaAs detectors at doses associated with high luminosity beams at a new particle collider accelerator (e.g., the LHC at the CERN laboratory).

Interband optical properties of molecular-beam epitaxially grown GaAs1−xSbx on GaAs substrates

Spectroscopic ellipsometry (SE) from 0.8 to 5 eV and photoreflectance (PR) from 0.7 to 1.5 eV were used to study the interband optical response at room temperature of epitaxial GaAs1−xSbx layers with 0<x<0.5. The samples were grown by molecular-beam epitaxy at 520 °C on (001)-GaAs substrates and characterized by low-temperature photoluminescence and x-ray diffraction. The complex dielectric function e(ω) of GaAs1−xSbx vs x was derived from the ellipsometric spectra after mathematically removing the oxide overlayer effects. The SE and PR spectra were analyzed with their energy derivatives in terms of standard analytical line shapes: in particular the E0, E1, E1+Δ1. E0′, and E2 critical point energies were derived as a function of x. On this basis the energy-shift model is appropriate to interpolate ex(ω) for any x<0.5, thus allowing a nondestructive optical diagnostic of epitaxial heterostructures based on GaAs1−xSbx.

Photoreflectance of GaSb/Al0.4Ga0.6Sb single quantum wells

We report a photoreflectance study conducted in the 0.7–1.2 eV photon energy range and at temperatures from 80 to 300 K of GaSb/Al0.4Ga0.6Sb single quantum wells grown by molecular beam epitaxy. We observed clear and well-resolved structures, which could be attributed to the interband optical transitions originating in both the GaSb buffer and the quantum wells, and which could be fitted by standard critical-point line shapes. Our results demonstrate that even unintentionally doped GaSb-based quantum systems can be studied and characterized by photoreflectance, especially at low temperatures.

Infrared reflectance study of n-type GaSb epitaxial layers

Abstract Reflectance measurements from 50 to 5000 cm −1 were made at room temperature on several GaSb layers grown by molecular beam epitaxy on GaAs substrates, and n -doped from ∼10 16 to more than 10 18 cm −3 . A two-layer model, joined with Drude-Lorentz dielectric functions accounting for phonons and plasmons, was applied to calculate the reflectance spectra, which fitted the experimental data very well. With the optical parameters extracted it was possible to obtain the carrier concentration and mobility to be compared with the Hall measurements. Both the optical and transport analysis require a two-valley conduction model, because of the presence in GaSb of Γ and L conduction valleys, separated by only 80 meV. Populations and free-carrier scattering times in each valley were derived as a function of the phenomenological plasma frequency. Some discrepancies between optical and transport results were found and are discussed.

Optical study of intersubband transitions in GaSb/AlGaSb systems for QWIPs

Abstract The realization of long-wavelength normal-incidence photodetectors relying on interconduction subband absorption in GaSb-based multi-quantum well (MQW) systems has been proven to be possible. High efficiencies can be achieved based on the size-induced Γ -to-L subband crossover in GaSb wells with an electron spill-over from the Γ to the ellipsoidal L conduction subbands. A systematic study of the intersubband absorption in n-GaSb/AlGaSb MQWs as a function of the n-doping and temperature is presented. In particular, medium-infrared transmittance measurements at normal-incidence evidenced sharp bands, corresponding to a peak fractional absorption per well which is higher than that reported in literature on similar systems.