G. Sęk

Semiconductor quantum dot microcavity pillars with high-quality factors and enlarged dot dimensions

Vertical-emitting AlAs∕GaAs microcavity pillars with a type of GaInAs quantum dots within a one λ cavity have been realized based on high reflectivity distributed Bragg reflectors. High-quality factors were achieved due to an improved fabrication technology with a maximum quality factor of 27 700 for a micropillar with a diameter of 4μm. The dot dimensions could be enlarged by one order of magnitude using a low strain Ga0.7In0.3As nucleation layer.

Photoreflectance evidence of multiple band gaps in dilute GaInNAs layers lattice-matched to GaAs

Dilute Ga1−xInxNyAs1−y∕GaAs quantum wells with high In-content, which are under compressive strain, have been shown previously to exhibit multiple band gaps, likely due to the presence of different nitrogen nearest-neighbor environments, i.e., N‐Ga4−mInm(0⩽m⩽4) short-range-order clusters. Here, photoreflectance (PR) measurements on lattice-matched dilute GaInNAs-on-GaAs layers with low indium and nitrogen content are reported, which give evidence that these layers also exhibit several distinct band gaps. These distinct band gaps, which were found to coexist, are associated with different nitrogen bonding configurations, as revealed by Raman spectroscopy. Thus, the metastable nature of GaInNAs seems to be a persistent intrinsic property, irrespective of strain and indium content. The annealing-induced blueshift of GaInNAs band gap energy, which is usually observed in this system, has been associated with the change in the intensity of PR resonances related to different N‐Ga4−mInm configurations.

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-probed excited states in InAs∕InGaAlAs quantum dashes grown on InP substrate

Photoreflectance (PR) measurements have been performed on InAs∕In0.53Ga0.23Al0.24As quantum dashes (QDashes) molecular-beam epitaxy grown on InP substrate. The PR features related to all relevant parts of the structure have been detected, including the ground and excited state optical transitions in QDashes. QDash ground state transition shifts from 1.5 to almost 2μm with the increase in the thickness of InAs layer, corresponding to the increase in the average size of the dashes. Excited state transitions have been clearly observed at the energy of about 150meV above the ground state transition energy.

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.

Photoreflectance study of δ-doped semiconductor layers by a fast Fourier transformation

Abstract Photoreflectance (PR) spectroscopy has been applied to the investigation of Si δ-doped GaAs, Al 0.35 Ga 0.65 As and AlAs layers grown by metal–organic vapor phase epitaxy (MOVPE) on GaAs substrates. The observation of Franz–Keldysh oscillations (FKO) and the application of fast Fourier transform (FFT) has allowed us to determine the internal electric field and, hence, the potential barrier between surface and δ-doped region of the layer. The FFT of the photoreflectance spectra has exhibited two separate heavy and light hole frequencies showing that the FKO in the PR signal are always the superposition of these two components.

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.

Experimental evidence on quantum well–quantum dash energy transfer in tunnel injection structures for 1.55μm emission

Here comes a report on the investigation of the energy transfer in InP-based tunnel injection structures, consisting of InAs∕InAlGaAs quantum dashes (QDashes) and an InGaAs∕InAlGaAs quantum well (QW), designed for 1.55μm emission at room temperature. Temperature dependent photoluminescence excitation (PLE) spectroscopy was used to experimentally confirm that the carriers created in the well reach the quantum dash layer by the tunneling through a thin InAlAs∕InAlGaAs barrier and recombine there radiatively. A measurable QW-QDash energy transfer has been detected up to 130K. The electronic structure of the whole complex system obtained by modulation spectroscopy exhibits full conformity with the PLE measurement results.

Eight-band k⋅p calculations of the composition contrast effect on the linear polarization properties of columnar quantum dots

We present eight-band k⋅p calculations of the electronic and polarization properties of columnar InzGa1−zAs quantum dots (CQD) with high aspect ratio embedded in an InxGa1−xAs/GaAs quantum well. Our model accounts for the linear strain effects, linear piezoelectricity, and spin-orbit interaction. We calculate the relative intensities of transverse-magnetic (TM) and transverse-electric (TE) linear polarized light emitted from the edge of the semiconductor wafer as a function of the two main factors affecting the heavy hole—light hole valence band mixing and hence, the polarization dependent selection rules for the optical transitions, namely, (i) the composition contrast z/x between the dot material and the surrounding well and (ii) the dot aspect ratio. The numerical results show that the former is the main driving parameter for tuning the polarization properties. This is explained by analyzing the biaxial strain in the CQD, based on which it is possible to predict the TM to TE intensity ratio. The conclu...