A new Fourier transform photo‐modulation spectroscopic technique for narrow band‐gap materials in the mid‐ to far‐infra‐red

Abstract

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...

Midinfrared photoreflectance study of InAs-rich InAsSb and GaInAsPSb indicating negligible bowing for the spin orbit splitting energy

“…We find that the sample with the highest Sb content ͑22.5%͒ has E 0 = 261± 5 meV ͑4.75 m͒, which represents the longest wavelength PR signal measured so far using grating spectroscopy. 20,21 The PR signal generally gradually decreases with increasing Sb content ͑see multiplication factors in Fig. 1͒.…”

Photoluminescence quenching mechanisms in type II InAs/GaInSb QWs on InAs substrates

“…The phase sensitive detection of the optical response was performed using a lock-in amplifier. A similar FT-based approach has been demonstrated to be an efficient tool for optical characterization in the mid-infrared spectral range of narrow-band gap materials (Hosea et al 2005;Shao et al 2007) as well as quantum cascade lasers (Dyksik et al 2016;Pierscinski et al 2014).…”

Interface Intermixing in Type II InAs/GaInAsSb Quantum Wells Designed for Active Regions of Mid-Infrared-Emitting Interband Cascade Lasers

“…In order to measure photoluminescence (PL) and photoreflectance (PR) spectra [20][21][22] in that spectral range, we utilized a setup based on a Fourier transform and vacuum spectrometer Bruker Vertex 80v operated in step-scan mode and equipped with an external chamber for experiments employing an additional mechanically modulated laser beam. In this case, a liquid-nitrogencooled InSb photodiode was used as a detector.…”