F. Janiak

Effect of arsenic on the optical properties of GaSb-based type II quantum wells with quaternary GaInAsSb layers

Optical properties of molecular beam epitaxially grown type II “W” shaped GaSb/AlSb/InAs/GaIn(As)Sb/InAs/AlSb/GaSb quantum wells (QWs) designed for the active region of interband cascade lasers have been investigated. Temperature dependence of Fourier-transformed photoluminescence and photoreflectance was employed to probe the effects of addition of arsenic into the original ternary valence band well of GaInSb. It is revealed that adding arsenic provides an additional degree of freedom in terms of band alignment and strain tailoring and allows enhancing the oscillator strength of the active type II transition. On the other hand, however, arsenic incorporation apparently also affects the structural and optical material quality via generating carrier trapping states at the interfaces, which can deteriorate the radiative efficiency. These have been evidenced in several spectroscopic features and are also confirmed by cross-sectional transmission electron microscopy images. While arsenic incorporation into type II QWs is a powerful heterostructure engineering tool for optoelectronic devices, a compromise has to be found between ideal band structure properties and high quality morphological properties.

Fourier-transformed photoreflectance and fast differential reflectance of HgCdTe layers. The issues of spectral resolution and Fabry–Perot oscillations

Fourier-transformed photoreflectance and fast differential reflectance (FDR) spectroscopies have been used to investigate the optical properties of HgCdTe layers of various compositions that spectrally target a broad range of mid and long infrared wavelengths. For this spectral range, the extraordinary sensitivity of these modulation techniques has allowed for direct measurement of the fundamental band gap as a function of temperature and Cd-atom concentration. Additionally, employing the FDR technique has allowed us to detect the absorption-like spectra in this long-wavelength range within a collection time of less than 1 min. Finally, it is seen that, in order to obtain an unequivocal interpretation of the experimental data from Fourier-spectrometer-based measurements of differential reflectivity, a proper selection of the spectral resolution and number of scans is crucially important.

Increasing the optical transition oscillator strength in GaSb-based type II quantum wells

The oscillator strength of the fundamental optical transition in GaSb-based type II quantum well structures as one of the crucial parameters for the performance of interband cascade lasers was investigated. Modulation spectroscopy, supported by eight-band k·p calculations, has been employed as a sensitive probing technique allowing to determine the transition intensities of samples with various layer structures. The results show that altering the composition of the valence band well in a type II system can efficiently enhance the transition oscillator strength. Especially, the utilization of a quaternary GaInAsSb material for hole confinement turned out to be highly beneficial.

Band Offsets and Photoluminescence Thermal Quenching in Mid-Infrared Emitting GaInAsSb Quantum Wells with Quinary AlGaInAsSb Barriers

Optical transitions in Ga0.35In0.65As0.32Sb0.68/Al0.25Ga0.50In0.25As0.24Sb0.76 quantum wells grown by molecular beam epitaxy on GaSb substrates have been detected by photoreflectance. Based on comparison with energy level calculations, the chemical conduction band offset ratio has been determined to be 78%. This translates into 65% in the real structure (i.e., after strain inclusion) which is an evidence of the expected band offset ratio modification in a quinary barrier system in favor of enhanced confinement in the valence band, when compared to similar quantum wells but with quaternary barriers. This has allowed us to explain the main photoluminescence thermal quenching mechanisms and connect the carrier activation energies with delocalization of excitons at low temperatures and the escape of holes via the confined states ladder at room temperature.

Determination of energy difference and width of minibands in GaAs/AlGaAs superlattices by using Fourier transform photoreflectance and photoluminescence

In this work, Fourier transform photoreflectance (in a form of fast differential reflectance spectroscopy) has been used to study the interband optical transitions in molecular beam epitaxially grown GaAs/AlGaAs superlattices. The dependence of the measured features on the growth parameters (QW and barrier widths) has been studied. The minibands widths and energy differences between them have been obtained and matched to these coming from effective mass calculations. In addition, it has been shown that Fourier transform photoluminescence measurement might be used in the far infrared region (up to ∼15 μm) to the direct detection of the energies of intraband transitions between the electron minibands (subbands) in the superlattice and QW system.

Temperature dependence of the energy gap and spin-orbit splitting in a narrow-gap InGaAsSb solid solution

Temperature dependence of the energy gap and the spin-orbit split off transition in a thick layer of narrow-gap InGaAsSb material with high In content has been determined by a combination of photoluminescence and photoreflectance. The respective temperature coefficients have been found to be equal for both the transitions and determined to be α = −0.41 meV/K. For the investigated In0.86Ga0.14As0.83Sb0.17 alloy, the separation energy of the split-off band has been obtained to be Δso = 0.460 eV and experimentally evidenced to be independent on temperature, which opens broad application prospects for these multinary (multicomponent) narrow gap compounds and their heterostructures.

Effect of Annealing-Induced Interdiffusion on the Electronic Structure of Mid Infrared Emitting GaInAsSb/AlGaInAsSb Quantum Wells

There has been investigated the effect of post-growth-annealing-induced interdiffusion process, and hence interface intermixing, on the electronic structure of Ga0.35In0.65As0.32Sb0.68/Al0.25Ga0.50In0.25As0.24Sb0.76 single quantum well designed to emit light in the range of about 3 µm. The band structure and optical transitions have been calculated based on the single band effective mass model and Ficks interdiffusion law. The calculation results are consistent with the experimentally observed transitions obtained by employing modulation spectroscopy. Our studies indicate that the intermixing processes in this kind of quantum wells are predominantly induced by the interdiffusion of group III atoms. The derived effective diffusion coefficient has been estimated to be of the order of 10-21 m2 s-1 for 480 °C annealing temperature.

Above GaSb barrier in type II quantum well structures for mid-infrared emission detected by Fourier-transformed modulated reflectivity

Modulation spectroscopy in its Fourier-transformed mode has been employed to investigate the optical properties of broken gap ‘W’-shaped GaSb/AlSb/InAs/InGaSb/InAs/AlSb/GaSb quantum well structures designed to emit in the mid infrared range of 3–4 μm for applications in laser-based gas sensing. Besides the optical transitions originating from the confined states in the type II quantum wells, a number of spectral features at the energy above the GaSb band gap have been detected. They have been analyzed in a function of InAs and GaSb layer widths and ultimately connected with resonant states in the range of AlSb tunneling barriers.

Recent advances in GaSb-based structures for mid-infrared emitting lasers: spectroscopic study

There are reviewed the optical properties of two kind of active regions of mid infrared laser devices both grown on GaSb substrates: GaInAsSb/AlGaInAsSb type I QWs for laser diodes and InAs/GaInAsSb type II QWs for interband cascade lasers. There are presented their crucial optical properties and the related current challenges with respect to the device performances. This covers such issues as spectral tenability of the emission via the structure parameters, the band gap discontinuities, carrier loss mechanisms and oscillator strengths. For that, spectroscopic techniques have been used (photoluminescence and its temperature dependence, and photoreflectance) and combined with the energy level calculations based on effective mass approximation and kp theory. Eventually, the potential for further material optimization and prospects for the improved device performances are also discussed.

Optical characterization of type II quantum wells for long-wavelength mid-infrared interband cascade lasers

We present result of optical studies on InAs/GaIn(As)Sb/InAs type II quantum wells predicted for the active region in interband cascade lasers, and further for laser-based gas sensors operating at room temperature in a broad wavelength range of mid infrared. Using photoreflectance spectroscopy supported by electronic structure calculations we determine the oscillator strength of the fundamental optical transition in structures with GaIn(As)Sb material of various compositions hole confinement layer. We show that incorporation of arsenic into this layer can affect several crucial properties significantly like transition wavelength and its probability, but also the structural material quality affecting the radiative efficiency. Also, by using photoluminescence we investigate one of the crucial parameters for the performance of interband cascade lasers, the spectral emission width of type II quantum wells constituting the laser active region.