Under oblique incidence of circularly polarized infrared radiation the spin-galvanic effect has been unambiguously observed in (001)-grown n-type GaAs quantum well (QW) structures in the absence of any external magnetic field. Resonant inter-subband transitions have been obtained making use of the tunability of the free-electron laser FELIX. It is shown that a helicity dependent photocurrent along one of the 110 axes is predominantly contributed by the spin-galvanic effect while that along the perpendicular in-plane axis is mainly due to the circular photogalvanic effect. This strong non-equivalence of the [110] and [110] directions is determined by the interplay between bulk and structural inversion asymmetries. A microscopic theory of the spin-galvanic effect for direct inter-subband optical transitions has been developed being in good agreement with experimental findings.
Photoluminescence ͑PL͒ has been observed from dilute InN x As 1−x epilayers grown by molecular-beam epitaxy. The PL spectra unambiguously show band gap reduction with increasing N content. The variation of the PL spectra with temperature is indicative of carrier detrapping from localized to extended states as the temperature is increased. The redshift of the free exciton PL peak with increasing N content and temperature is reproduced by the band anticrossing model, implemented via a ͑5 ϫ 5͒ k·p Hamiltonian.
There is a large number of technologically important semiconducting optoelectronic materials with narrow band-gaps in the "finger-print" region of the infra-red (IR) spectrum. However, in many instances their band-structures have not been very well characterised, making it difficult to engineer their properties. Part of the reason is that the key non-destructive optical characterisation tool, modulation spectroscopy, becomes increasingly difficult as one attempts to look further out into the IR. To date, conventional diffraction-grating-based modulation spectroscopy has been applied predominantly below ~4 µm. We have developed a new photo-modulation system, based on a Fourier transform spectrometer, that permits such measurements out to much longer wavelengths. We discuss the advantages and technical difficulties of implementing such a system, and give the results obtained so far for narrower-gap materials, including bulk-like GaSb, InAs and InSb, comparing these with what can be obtained with conventional modulation spectroscopy arrangements. We apply our new technique to measure the bandgap in dilute-N InSbN, achieving what we believe are the first modulation spectroscopy measurements in the mid-IR beyond ~6 µm. optical property (e.g. transmittance, or more usually, reflectance R) is probed by a spectrally-resolved source while its dielectric function is being simultaneously externally modulated at frequency ν mod . In the case of reflectance, the modulated (AC) component, ∆R, is usually detected with sensitive lock-in techniques, yielding sharp, differential-like oscillatory spectra. In electro-modulated reflectance an applied AC electric field modulates any high resistance region in the structure directly. The most widely used version, photo-modulated reflectance (PR) [1] uses a chopped laser pump beam with photon energy above the band-gap. When the laser is on, generated carriers are captured by traps, thus reducing the inbuilt/surface electric fields. When the laser is off, the trap population, and hence field, are restored [2]. This mechanism modulates the Stark shift of, say, the quantum well (QW) levels which modulates the dielectric function, and thus the relative reflectivity, ∆R/R. Uninteresting broad background or systemresponse features in R are removed. At room temperature PR can give equivalent energy resolution to that obtained by PL and PLE at liquid helium temperatures, and probes a far wider range of critical point transition energies, yielding all these in a single spectrum, highlighting not only ground-state but also many higher-order critical point interband optical transitions, from which transition energies, and thus band-structure, can be accurately obtained. Unlike PLE, PR can be carried out using a white light blackbody radiator source (e.g. tungsten filament lamp), a simple diffraction grating spectrometer (e.g. 0.5 metre), and a small mechanically-chopped laser (such as a 3 mW, 633 nm HeNe) and so is applicable over a fairly wide wavelength range using the same equipment.Thus, modula...
Photoluminescence ͑PL͒ has been used as a means of unambiguously observing band gap reduction in InNAs epilayers grown by molecular beam epitaxy. The observed redshift in room temperature emission as a function of nitrogen concentration is in agreement with the predictions of the band anticrossing ͑BAC͒ model, as implemented with model parameters derived from tight-binding calculations. The temperature dependence of the emission from certain samples exhibits a signature non-Varshni-like behavior indicative of electron trapping in nitrogen-related localized states below the conduction-band edge, as predicted by the linear combination of isolated nitrogen states ͑LCINS͒ model. This non-Varshni-like behavior tends to grow more pronounced with increasing nitrogen content, but for the highest nitrogen concentration studied, the more familiar Varshni-like behavior is recovered. Although unexpected, this observation is found to be consistent with the BAC and LCINS models. With consideration given to the effects of conduction-band nonparabolicity on the emission line shapes, the BAC model parameters extracted from the measured PL transition energies are found to be in excellent agreement with the predictions of the aforementioned tight-binding calculations.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.