K. Kuldová

Optical characterisation of MOVPE grown vertically correlated InAs/GaAs quantum dots

Structures with vertically correlated self-organised InAs quantum dots (QDs) in a GaAs matrix were grown by the low-pressure metal-organic vapour phase epitaxy (MOVPE) and characterised by different microscopic techniques. Photoluminescence in combination with photomodulated reflectance spectroscopy were applied for characterisation of QDs structures. We show that combination of both methods allows detecting optical transitions originating both from QDs and wetting (separation) layers, which can be than compared with those obtained from numerical simulations. On the basis of obtained results, we demonstrate that photoreflectance spectroscopy is an excellent tool for characterisation of QDs structures wetting layers and for identification of spacer thicknesses in vertically stacked QDs structures.

Type I–type II band alignment of a GaAsSb/InAs/GaAs quantum dot heterostructure influenced by dot size and strain-reducing layer composition

The aim of this work is to red shift quantum dot (QD) photoluminescence (PL) towards telecommunication wavelengths by engineering the metalorganic vapour phase epitaxy (MOVPE) prepared structure of InAs/GaAs QDs covered by a GaAsSb strain-reducing layer. Our results proved that type I or type II band alignment can be controlled by both GaAsSb composition and QD size. Maintaining type I heterostructure is important for high luminescence efficiency and emission wavelength stability of the QD structure. The simulation of electron structure in InAs QDs covered with a GaAsSb strain-reducing layer as well as experimental results suggest the importance of increasing QD size for obtaining a longer wavelength PL from the type I heterostructure. The PL maximum wavelength 1371 nm was achieved for the MOVPE prepared type I QD structure with 14% of Sb in GaAsSb. This type of structure exhibits seven times higher PL intensity, twice narrower PL peak and 85 meV redshift in comparison with similarly prepared QDs covered by GaAs.

Elongation of InAs∕GaAs quantum dots from magnetophotoluminescence measurements

The authors have used magnetophotoluminescence for the determination of the lateral anisotropy of buried quantum dots. While the calculated shifts of the energies of higher radiative transitions in magnetic field are found to be sensitive to the lateral elongation, the shift of the lowest transition is determined mainly by the exciton effective mass. This behavior can be used for a fairly reliable determination both the effective mass and the elongation from spectra containing at least two resolved bands.

Magneto-photoluminescence study of electronic transitions in InAs/GaAs quantum dot layers

Abstract Magneto-photoluminescence of single- and multi-layered self-organised MOCVD grown InAs quantum dots in GaAs has been investigated at 77 K in magnetic fields up to B=27 T in Faraday configuration. From one up to six peaks are resolved at B=0 T in the photoluminescence spectra when the excitation intensity increases from 10 mW up to 1 W (514.5 nm line of Ar+ laser). Simple one particle Fock–Darwin model of two-dimensional electrons confined in a parabolic well describes satisfactorily the evolution of magneto-photoluminescence only for some peaks. The other peaks exhibit slight decrease in energy with increasing magnetic field (∼5 meV at 27 T).

Lasers with δ InAs layers in GaAs

Abstract Electroluminescence of lasers with different numbers (1, 3, 5, 7) of δ InAs layers in GaAs prepared by Low Pressure Metal–Organic Vapor Phase Epitaxy was investigated in a broad temperature range from 10 to 400 K. The dependence of the electroluminescence spectra on the number of δ InAs layers and on the separation of these δ InAs layers was studied under pulse excitation in a wide range of current densities. Results show that by increasing the number of δ InAs layers and decreasing the distance between these layers it is possible to decrease the lasing emission energy below 1.15 eV. Our δ InAs lasers operate even at temperatures above 100 °C, they exhibit weak temperature dependence of threshold current density with values lower than 0.2 kA cm−2 and their differential quantum efficiency lies between 12 and 18%.

On the correlations between the excitonic luminescence efficiency and the QW numbers in multiple InGaN/GaN QW structure

In this work, we compare the luminescence results obtained on InGaN/GaN multiple quantum well (QW) structures with different numbers of QWs. Structures are designed for scintillating applications, where large QW number covering particle penetration depth is necessary, and fast luminescence response is required. Special attention is devoted to increase the intensity of fast excitonic QW emission and to decrease the luminescence of the QW defect band, which has slower luminescence response and is undesired for fast scintillator applications. We found that increasing the In content in QWs suppresses the defect band luminescence and decreasing the QW growth rate increases the photoluminescence (PL) intensity of excitonic luminescence. We also show that increasing the number of InGaN further improves the PL properties of InGaN QWs. The photoluminescence and cathodoluminescence characteristics are compared and discussed.

Devices based on InGaN/GaN multiple quantum well for scintillator and detector applications

Fast scintillators are necessary for electron microscopes, as well as in many other application fields like medical diagnostics and therapy and fundamental science. InGaN/GaN multiple quantum well structures (QW) are perspective candidates due to strong exciton binding energy, high quantum efficiency, short decay time in order of ns and good radiation resistance. The aim of our work is to prepare scintillator structure with fast luminescence response and high intensity of light. InGaN/GaN multiple QW structures described here were prepared by metal-organic vapour phase epitaxy and characterized by high resolution X-ray diffraction measurements. We demonstrate structure suitability for scintillator application including a unique measurement of wavelength-resolved scintillation response under nanosecond pulse soft X-ray source in extended dynamical and time scales. The photo-, radio- and cathodo-luminescence (PL, RL, CL) were measured. We observed double peak luminescence governed by different recombination mechanisms: i) exciton in QW and ii) related to defects. We have shown that for obtaining fast and intensive luminescence response proper structure design is required. The radioluminescence decay time of QW exciton maximum decreased 4 times from 16 ns to 4 ns when the QW thickness was decreased from 2.4 nm to 2 nm. We have proved suitability of InGaN/GaN structures for fast scintillator application for electron or other particle radiation detection. For x-ray detection the fast scintillation response would be hard to achieve due to the dominant slow defect luminescence maximum.

Electron states and magnetophotoluminescence of elongated InAs/GaAs quantum dots

We have calculated the electronic structure of a single InAs/GaAs quantum dot in a perpendicular magnetic field. Considerable sensitivity of the shift of energy levels in magnetic field to a lateral elongation of the dots is demonstrated and a possibility to retrieve the elongation from magnetophotoluminescence spectra is discussed. Sensitivity analysis shows that spectra with at least two well resolved bands are needed for a reliable determination of the elongation.