Jiri Pangrac

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.

Microwave radiation absorption and Shubnikov-de Haas oscillations in semimetal InAs/GaSb/AlSb composite quantum wells

Abstract. Strong Shubnikov-de Haas (SdH) oscillations were observed in the derivative of microwave absorption (f=10  GHz) in InAs/GaSb/AlSb composite quantum wells (CQWs) using electron-paramagnetic resonance spectroscopy at low temperatures (2.7 to 20 K) and in the magnetic field up to 14 kOe. CQWs were grown on n-GaSb:Te(100) and n-InAs:Mn(100) substrates with various widths of QWs by MOVPE. A predominant contribution to the bulk n-GaSb substrate in SdH oscillations was manifested. Two frequencies of the SdH oscillations connected with warping of the Fermi surface of GaSb were found from Fourier analysis. An unusual angular indicatrix was observed in dependence on the orientation of the samples grown on n-GaSb in the magnetic field. The obtained results can be explained by bulk inversion asymmetry, which is a feature of substances with a lack of inversion centers. For CQWs grown on n-InAs:Mn substrate, only several SdH oscillations with higher period were observed. We succeeded in extracting a contribution of the two-dimensional carriers of InAs QW∼H⊥, where H⊥=H·cos Θ, from bulk substrate oscillations using a special spline interpolation from the angular dependence of SdH oscillatory amplitudes in the angle range of 0 to 90 deg.

Superlinearity and temperature dependence of electroluminescence in heterostructures with deep AlSb/InAs1-x Sbx /AlSb quantum well

We report on superlinear electroluminescent structures based on AlSb/InAs1-xSbx/AlSb deep quantum well grown by MOVPE on n-GaSb:Te substrate. Dependence of the electroluminescence (EL) spectra and optical power on the drive current in nanoheterostructures with AlSb/InAs1-xSbx/AlSb quantum well at 77 – 300 K temperature range was studied. Intensive two-band superlinear EL in the 0.5 - 0.8 eV photon energy range was observed. Optical power enhancement with the increasing drive current at room temperature is caused by the contribution of the additional electron-hole pairs due to the impact ionization by the electrons heated at the high band offset between AlSb and the first electron level Ee1 in the InAsSb QW. Study of the EL temperature dependence at 90 – 300 K range enabled us to define the role of the first and second heavy hole levels in the radiative recombination process. It was shown that with the temperature decrease, the relation between the energies of the valence band offset and the second heavy hole energy level changes due to the temperature transformation of the energy band diagram. That is why the EL spectrum revealed radiative transitions from the first electron level Ee1 to the first hole level Eh1 in the whole temperature range (90 – 300 K) while the emission band related with the transitions to the second hole level occurred only at T < 200 K.

Lattice-Mismatched Epitaxial Growth On Porous III-V Substrates

There is a limited number of III-V semiconductor substrates which are available at acceptable quality and cost. Restriction to lattice-matched systems would greatly limit the number of applications. Development of vapor phase growth techniques allowed to precisely control the layer thickness and uniformity on the atomic level. Still, when the critical layer thickness is exceeded, misfit dislocations are created having negative impact on the performance, reliability and lifetime of semiconductor devices (1). The ability to grow pseudomorphic layers thicker than the critical layer thickness is be beneficial in many applications. One of the unexplored approaches to meet this need consists in the growth on a porous substrate, which was theoretically predicted to be capable of accommodating elastic strains at the heteroepitaxial interface (2). Focus of this paper is on the InGaAs/GaAs heterostructures, which allow to vary the lattice mismatch in a wide range up to 7.4 %. The InGaAs system is flexible in terms of the range of optical wavelengths that can be emitted and absorbed; by varying the indium concentration, emission or detection wavelengths ranging from 1.1 to 3 μm may be achieved. InGaAs is a material widely used in electronic and optoelectronic devices such as high electron mobility transistors (3), laser diodes (4), infrared detectors (5), and photovoltaic cells (6).

Strained InxGa1−xAs/GaAs multiple quantum wells grown by MOVPE

Luminescence properties of strained InxGa1−xAs/GaAs multiple quantum wells of different thickness and In content, prepared by metal organic vapour phase epitaxy were studied. The influence of the quantum well material composition on the shape of luminescence spectra was investigated. The experimental results were fitted by the Model Solid Theory. This fit was improved by the use of adjustedQ parameter.