M. Motyka

Contactless electromodulation spectroscopy of AlGaN∕GaN heterostructures with a two-dimensional electron gas: A comparison of photoreflectance and contactless electroreflectance

Photoreflectance (PR) and contactless electroreflectance (CER) spectroscopies have been applied to study optical transitions in undoped and Si-doped AlGaN∕GaN heterostructures at room temperature. Spectral features related to excitonic and band-to-band absorptions in GaN layer and band-to-band absorption in AlGaN layer have been resolved and analyzed. In addition, a broad spectral feature related to two-dimensional electron gas has been observed for the Si-doped heterostructure. It has been found that some of the mentioned optical transitions are not observed in CER spectra whereas they are very strong in PR spectra. This phenomenon is associated with different mechanisms of the modulation of built-in electric field in the investigated structure. A combination of PR and CER gives the possibility of a richer interpretation of both PR and CER spectra.

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.

Room temperature contactless electroreflectance characterization of InGaAs/InAs/GaAs quantum dot wafers

Contactless electroreflectance (CER) mapping has been performed on InGaAs capped InAs/GaAs quantum dot (QD) wafers of 2 inch diameter grown by molecular beam epitaxy. The CER spectra have revealed several features related to InAs self-assembled QDs and a quantum well (QW) formed of the InAs wetting layer and the InGaAs cap layer. The particular optical transitions have been identified based on theoretical calculations of the energy levels in the InAs/InGaAs/GaAs wetting layer related step-like QW, performed within the effective mass approximation. The influence of possible uncertainties in cap content or band offsets has also been analysed. The advantages of modulation spectroscopy, namely its absorption-like character and high sensitivity to optical transitions with even very low oscillator strength including those between the excited states, have allowed the energies of all the transitions along the wafer to be followed. The latter has shown that within experimental error the transition energies are independent of the position of the probing spot on the sample. It demonstrates not only a very high uniformity of the dot ensemble but also the wetting layer related QW and hence also the content and thickness of the InGaAs cap.

Photoreflectance and photoluminescence study of Ga0.76In0.24Sb/Gasb single quantum wells : Band structure and thermal quenching of photoluminescence

Photoreflectance (PR) and photoluminescence (PL) have been applied to study the band structure and PL thermal quenching for Ga0.24In0.76Sb/GaSb quantum wells (QWs) of the widths varying from 10 to 21 nm. In the case of PR spectra, a strong GaSb-related resonance followed by Franz–Keldysh oscillations and PR resonances associated with optical transitions between the QW ground and the excited states have been clearly observed. The QW transitions have been identified on the basis of theoretical calculations which were performed in the framework of the effective mass model. Satisfactory agreement between theoretical calculations and experimental data has been found for the conduction band offset of ∼80%–85% that is consistent with the theoretical predictions, which were obtained within the “model-solid” theory. In the case of PL measurements, a very efficient QW emission without any localization features has been observed in the temperature range of 10–280 K. It has been concluded that the thermal quenching o...

Contactless electroreflectance of GaInNAsSb/GaAs single quantum wells with indium content of 8%-32%

Interband transitions in GaInNAsSb∕GaAs single quantum wells (SQWs) with nominally identical nitrogen and antimony concentrations (2.5% N and 7% Sb) and varying indium concentrations (from 8% to 32%) have been investigated by contactless electroreflectance (CER). CER features related to optical transitions between the ground and excited states have been clearly observed. Energies of the QW transitions extracted from CER measurements have been matched with those obtained from theoretical calculations performed within the effective mass approximation for various conduction-band offsets (QC) and various electron effective masses. It has been found that the QC increases from 40% to 80% with the rise of the indium content from 8% to 32% and the electron effective mass is close to 0.09m0. The results show that the band gap discontinuity in GaInNAsSb∕GaAs SQWs can be broadly tuned with a change in the indium concentration.

Band gap discontinuity in Ga0.9In0.1N0.027As0.973−xSbx∕GaAs single quantum wells with 0⩽x<0.06 studied by contactless electroreflectance spectroscopy

Contactless electroreflectance (CER) spectroscopy has been applied to study optical transitions in Ga0.9In0.1N0.027As0.973−xSbx∕GaAs single quantum well (QW) with antimony content varying from 0% to 5.4%. CER features related to optical transitions between the ground and excited states have been clearly observed. Energies of the QW transitions have been matched with those obtained from theoretical calculations. It has been determined that the conduction band offset decreases from ∼55% to ∼45% with the increase in Sb content from 0% to 5.4%. This result demonstrates that the band gap discontinuity for Ga0.9In0.1N0.027As0.973−xSbx∕GaAs system can be simply tuned by a change in antimony content.

Contactless electroreflectance approach to study the Fermi level position in GaInNAs/GaAs quantum wells

A fruitful approach to study the Fermi level position in GaInNAs/GaAs quantum wells (QWs) has been proposed in this paper. This approach utilizes contactless electroreflectance (CER) spectroscopy and a very simple design of semiconductor structures. The idea of this design is to insert a GaInNAs quantum well (QW) into a region of undoped GaAs layer grown on n-type GaAs substrate. The possible pinning of the Fermi level in the GaInNAs QW region modifies band bending in this system. In CER spectra both QW transitions and GaAs-related Franz-Keldysh oscillations (FKOs) are clearly observed. The analysis of QW transitions allows one to determine the band gap discontinuity at GaInNAs/GaAs interface whereas the analysis of FKOs allows one to determine the built-in electric field in the GaAs cap layer, and, finally, one is able to find the Fermi level pinning in GaInNAs QW region.

Investigations of GaN surface quantum well in AlGaN∕GaN transistor heterostructures by contactless electroreflectance spectroscopy

AlGaN∕GaN transistor heterostructures, caped by ∼2nm GaN layer, were investigated by contactless electroreflectance (CER) spectroscopy at room temperature. Below the AlGaN-related transition CER spectra have shown a clear resonance at the energy of ∼3.7eV, i.e., at much higher energy than the GaN band gap energy. The observed feature has been connected with the optical transition within the GaN cap layer. It was concluded that a surface GaN quantum well has been created by the deposition of nominally undoped (or Si-doped) GaN cap layer on AlGaN∕GaN transistor heterostructures.