D. S. Abramkin

Atomic structure and energy spectrum of Ga(As,P)/GaP heterostructures

The atomic structure and energy spectrum of Ga(As,P)/GaP heterostructures were studied. It was shown that the deposition of GaAs of the same nominal thickness leads to the formation of pseudomorphic GaAs/GaP quantum wells (QW), fully relaxed GaAs/GaP self-assembled quantum dots (SAQDs), or pseudomorphic GaAsP/GaP SAQDs depending on the growth temperature. We demonstrate that the atomic structure of Ga(As,P)/GaP heterostructures is ruled by the temperature dependence of adatom diffusion rate and GaAs-GaP intermixing. The band alignment of pseudomorphic GaAs/GaP QW and GaAsP/GaP SAQDs is shown to be of type II, in contrast to that of fully relaxed GaAs/GaP SAQDs, which have the band alignment of type I with the lowest electronic states at the indirect L valley of the GaAs conduction band.

Quantum dots formed in InSb/AlAs and AlSb/AlAs heterostructures

The crystal structure of new self-assembled InSb/AlAs and AlSb/AlAs quantum dots grown by molecularbeam epitaxy has been investigated by transmission electron microscopy. The theoretical calculations of the energy spectrum of the quantum dots have been supplemented by the experimental data on the steady-state and time-resolved photoluminescence spectroscopy. Deposition of 1.5 ML of InSb or AlSb on the AlAs surface carried out in the regime of atomic-layer epitaxy leads to the formation of pseudomorphically strained quantum dots composed of InAlSbAs and AlSbAs alloys, respectively. The quantum dots can have the type-I and type-II energy spectra depending on the composition of the alloy. The ground hole state in the quantum dot belongs to the heavy-hole band and the localization energy of holes is much higher than that of electrons. The ground electron state in the type-I quantum dots belongs to the indirect XXY valley of the conduction band of the alloy. The ground electron state in the type-II quantum dots belongs to the indirect X valley of the conduction band of the AlAs matrix.

Formation and crystal structure of GaSb/GaP quantum dots

The atomic structure of GaSb/GaP quantum dots grown via molecular beam epitaxy on a (100) GaP surface at epitaxy temperatures of 420–470°C is investigated. It is established that, depending on morphology of the GaP growth surface, the deposition of 1 ML of GaSb leads to the formation of strained Ga(Sb, P)/GaP or fully relaxed GaSb/GaP quantum dots. The obtained heterostructures exhibit high photoluminescence efficiency.

New system of self-assembled GaSb/GaP quantum dots

The atomic structure and energy spectrum of self-assembled GaSb/GaP quantum dots are discussed. It is shown that the quantum dots consist mainly of fully relaxed GaSb and have type-I band alignment with the ground electron state at the indirect valley of the GaSb conduction band.

Heterostructures with diffused interfaces: Luminescent technique for ascertainment of band alignment type

The experimental ascertainment of band alignment type for semiconductor heterostructures with diffused interfaces is discussed. A method based on the analysis of the spectral shift of photoluminescence (PL) band with excitation density (Pex) that takes into account state filling and band bending effects on the PL band shift is developed. It is shown that the shift of PL band maximum position is proportional to ℏωmax ∼ (Ue + Uh)·ln(Pex) + b·Pex1/3, where Ue (Uh) are electron (hole) Urbach energy tail, and parameter b characterizes the effect of band bending or is equal to zero for heterostructures with type-II or type-I band alignment, respectively. The method was approved with InAs/AlAs, GaAs/AlAs, GaSb/AlAs, and AlSb/AlAs heterostructures containing quantum wells.

Formation of low-dimensional structures in the InSb/AlAs heterosystem

Low-dimensional quantum-well and nanoisland heterostructures formed in the InSb/AlAs system by molecular-beam epitaxy are studied by transmission electron microscopy and steady-state photoluminescence spectroscopy. The structures are grown under conditions of alternate In and Sb deposition (the socalled atomic-layer epitaxy mode) and the simultaneous deposition of materials (the traditional molecularbeam epitaxy mode). In both modes of growth, at a nominal amount of the deposited material in a single layer, large-sized (200 nm–1 μm) imperfect islands arranged on the InxAl1 – xSbyAs1–y quantum-well layer are formed. In the heterostructures grown under conditions of atomic layer epitaxy, the islands are surrounded by ring-shaped arrays of much smaller (~10 nm), coherently strained islands consisting of the InxAl1 – xSbyAs1 – y alloy as well. The composition of the alloy is defined by the intermixing of Group-V materials in the stage of InSb deposition and by the intermixing of materials because of the segregation of In and Sb atoms during overgrowth of the InSb layer by an AlAs layer.

Determining the structure of energy in heterostructures with diffuse interfaces

A standard way of determining the type of energy structure in heterostructures, based on analyzing the spectral shift of the band of steady-state photoluminescence (PL) as a function of the excitation power density, is developed with allowance for the effect of the fluctuation tails of the density of energy states. The technique is validated for InAs/AlAs, GaAs/AlAs, and AlSbyAs1–y/AlAs heterostructures with quantum wells.

Coexistence of type-I and type-II band alignment in Ga(Sb, P)/GaP heterostructures with pseudomorphic self-assembled quantum dots

Band alignment of heterostructures with pseudomorphic GaSb1 − xPx/GaP self-assembled quantum dots (SAQDs) lying on a wetting layer was studied. Coexistence of type-I and type-II band alignment was found within the same heterostructure. Wetting layer has band alignment of type-I with the lowest electronic state belonging to the XXY valley of GaSb1 − xPx conduction band, in contrast to SAQDs, which have band alignment of type-II, independently of the ternary alloy composition x. It is shown that type-I-type-II transition is a result of GaP matrix deformation around the SAQD.

Trapping of charge carriers into InAs/AlAs quantum dots at liquid-helium temperature

The problem of how the probability of trapping of charge carriers into quantum dots via the wetting layer influences the steady-state and time-dependent luminescence of the wetting layer and quantum dots excited via the matrix is analyzed in the context of some simple models. It is shown that the increase in the integrated steady-state luminescence intensity of quantum dots with increasing area fraction occupied by the quantum dots in the structure is indicative of the suppression of trapping of charge carriers from the wetting layer into the quantum dots. The same conclusion follows from the independent decays of the time-dependent luminescence signals from the wetting layer and quantum dots. The processes of trapping of charge carriers into the InAs quantum dots in the AlAs matrix at 5 K are studied experimentally by exploring the steady-state and time-dependent photoluminescence. A series of structures with different densities of quantum dots has been grown by molecular-beam epitaxy on a semi-insulating GaAs (001) substrate. It is found that the integrated photoluminescence intensity of quantum dots almost linearly increases with increasing area occupied with the quantum dots in the structure. It is also found that, after pulsed excitation, the photoluminescence intensity of the wetting layer decays more slowly than the photoluminescence intensity of the quantum dots. According to the analysis, these experimental observations suggest that trapping of excitons from the wetting layer into the InAs/AlAs quantum dots at 5 K is suppressed.

Luminescence of a near-surface GaAs/AlAs heterojunction in AlAs-based heterostructures

It is necessary to protect the surface of AlAs-based heterostructures from oxidation using a GaAs cap layer because of the high reactivity of aluminum. Thus, the surface region of these heterostructures always contains a GaAs/AlAs heterojunction. Here, it is demonstrated that, under nonresonant photoexcitation, the photoluminescence spectrum of AlAs-based heterostructures features a band associated with this heterojunction. The intensity of this band is determined by the thickness and doping type of the GaAs cap layer.