K. Ryczko

Photoreflectance investigations of the energy level structure in GaInNAs-based quantum wells

In this paper, we present the application of photoreflectance (PR) spectroscopy to investigate the energy level structure of GaInNAs-based quantum wells (QWs). Series of single GaInNAs/GaAs QWs wit ...

Photoreflectance investigations of oscillator strength and broadening of optical transitions for GaAsSb–GaInAs/GaAs bilayer quantum wells

GaAsSb–GaInAs/GaAs bilayer quantum wells which consist of two adjacent layers of GaAsSb and GaInAs sandwiched between GaAs barriers have been investigated by photoreflectance (PR) spectroscopy. The oscillator strengths of optical transitions in such multiheterointerface structures have been determined from the experiment and compared with the results of envelope function calculations. Additionally, the broadening of the PR features has been analyzed and a correlation has been found with the character of the transitions: the broadening increases significantly when the type of the transition changes from direct to indirect.

Optically probed wetting layer in InAs/InGaAlAs/InP quantum-dash structures

Photoluminescence and photoreflectance measurements have been performed to investigate molecular-beam-epitaxy-grown InAs/InGaAlAs/InP structures with different-size InAs quantum dashes. Optical features related to all relevant parts of the structure have been detected and recognized, including a line which has been attributed to the ground-state wetting layer quantum well transition. The spectral position of the latter is independent of the nominal InAs layer thickness in contrast to quantum-dash emission peak, which shifts sequentially to the red due to an increase of the islands’ size. The interpretation has been supported by energy level calculations showing that the wetting layer has to be approximately 2 ML thick and that only one state is confined in such a thin well for each kind of carriers, i.e., electrons, heavy, and light holes.

Emission wavelength tuning of interband cascade lasers in the 3–4 μm spectral range

GaSb-based type-II quantum well (QW) structures and interband cascade lasers (ICLs) are investigated with regards to the dependence of emission wavelength on active QW thicknesses. Experimentally derived photoluminescence data and electrically driven ICL device data accompanied by theoretical calculations yield an average tuning rate of 0.55 μm per monolayer InAs in the range between 2.97 and 4.16 μm. Together with a temperature dependent ICL tuning behavior of 1.88 nm/K, the presented results provide the means for reliable and accurate emission wavelength control of ICLs in the 3–4 μm wavelength span which is of major importance for gas sensing applications.

Wetting layer states of InAs∕GaAs self-assembled quantum dot structures: Effect of intermixing and capping layer

The authors present a modulated reflectivity study of the wetting layer WL states in molecular beam epitaxy grown InAs/GaAs quantum dot QD structures designed to emit light in the 1.3‐1.5 m range. A high sensitivity of the technique has allowed the observation of all optical transitions in the QD system, including low oscillator strength transitions related to QD ground and excited states, and the ones connected with the WL quantum well QW. The support of WL content profiles, determined by transmission electron microscopy, has made it possible to analyze in detail the real WL QW confinement potential which was then used for calculating the optical transition energies. We could conclude that in spite of a very effective WL QW intermixing, mainly due to the Ga‐In exchange process causing the reduction of the maximum indium content in the WL layer to about 35% from nominally deposited InAs, the transition energies remain almost unaffected. The latter effect could be explained in effective mass envelope function calculations taking into account the intermixing of the QW interfaces described within the diffusion model. We have followed the WL-related transitions of two closely spaced QD layers grown at different temperatures, as a function of the In content in the capping layer. We have shown that changing the capping layer from pure GaAs to In0.236Ga0.764As has no significant influence on the composition profile of the WL itself and the WL QW transitions can be usually interpreted properly when based on the cap-induced modification of the confinement potential within a squarelike QW shape approximation. However, some of the observed features could be explained only after taking into consideration the effects of intermixing and InGaAs cap layer decomposition.

Optical properties of GaSb-based type II quantum wells as the active region of midinfrared interband cascade lasers for gas sensing applications

Photoreflectance and photoluminescence, supported by the energy level calculations in the eight-band k⋅p model including strain, have been used to study the optical properties of GaSb/AlSb/InAs/InGaSb/AlSb/GaSb type II quantum wells (QWs). The broad emission wavelength tunability in the midinfrared range has been demonstrated by the control of InAs layer thickness. The temperature dependent measurements have shown that the emission can still be efficient at room temperature in such structures, and that the temperature shift of the fundamental type II optical transition between 10 and 300 K can be significantly smaller than for type I QW systems.

The nature of optical transitions in Ga0.64In0.36As1−xNx/GaAs single quantum wells with low nitrogen content (x≤0.008)

Abstract Ga 0.64 In 0.36 As 1− x N x /GaAs single quantum wells (SQWs) with low nitrogen content have been investigated by both photoluminescence (PL) and photoreflectance (PR) at low and room temperatures. A huge broadening of the PR features has been observed at low temperature and a decrease in this broadening with the temperature increase was detected. This effect and the nature of the optical transitions observed in absorption and emission can be explained using a model which assumes band gap variation due to different nitrogen nearest-neighbour environments (different configurations). In the framework of this model, the large Stokes shift observed for quantum wells (QWs) with smooth interfaces is explained as originating from the potential fluctuations of conduction band edge in the QW layer.

Effect of nitrogen on the exciton binding energy in GaxIn1-xNyAs1-y/GaAs quantum well

Abstract The binding energy of ground state exciton in Ga x In 1− x N y As 1− y /GaAs single quantum well is studied theoretically. We have calculated the exciton binding energy by a variational envelope-function procedure using simple two-band model, including strains and the difference in dielectric constants between well and barrier materials. The influence of the well width and nitrogen and indium mole fractions on the value of binding energy has been analyzed. It has been observed that incorporation of small amounts of nitrogen (up to 5%) induces significant changes of the exciton binding energy.

Optical investigations of InGaAsN/GaAs single quantum well structures

Abstract The influence of nitrogen content on the optical properties of InGaAsN/GaAs single quantum wells (SQWs) with nitrogen concentration up to 5.2% and relatively high indium content have been investigated by photoreflectance (PR) and photoluminescence (PL) spectroscopy. The PR measurements have allowed us to observe, besides of the fundamental 11H optical transition, transitions between excited states in QWs. The identification of optical transitions present in PR spectra has been possible due to the theoretical calculations based on the envelope function model. We have found the relatively high red shift of all optical transitions with increasing nitrogen content. The temperature dependence of PL peak energy have been investigated, too. The thermal stability of PL peak energy is stronger for structures with higher N content. Fitting with Varshni empirical relation to PL data we have found that at low temperatures carriers are localised. For structures with high nitrogen content we observed S-shaped energy peak behaviour.

Eight-band k·p modeling of InAs/InGaAsSb type-II W-design quantum well structures for interband cascade lasers emitting in a broad range of mid infrared

Band structure properties of the type-II W-design AlSb/InAs/GaIn(As)Sb/InAs/AlSb quantum wells have been investigated theoretically in a systematic manner and with respect to their use in the active region of interband cascade laser for a broad range of emission in mid infrared between below 3 to beyond 10 μm. Eight-band k·p approach has been utilized to calculate the electronic subbands. The fundamental optical transition energy and the corresponding oscillator strength have been determined in function of the thickness of InAs and GaIn(As)Sb layers and the composition of the latter. There have been considered active structures on two types of relevant substrates, GaSb and InAs, introducing slightly modified strain conditions. Additionally, the effect of external electric field has been taken into account to simulate the conditions occurring in the operational devices. The results show that introducing arsenic as fourth element into the valence band well of the type-II W-design system, and then altering its composition, can efficiently enhance the transition oscillator strength and allow additionally increasing the emission wavelength, which makes this solution prospective for improved performance and long wavelength interband cascade lasers.