I.E. Itskevich

Photoluminescence spectroscopy of self-assembled InAs quantum dots in strong magnetic field and under high pressure

We have investigated the photoluminescence spectrum of self-assembled InAs quantum dots embedded in a GaAs matrix in magnetic field B up to 23 T and under hydrostatic pressure up to 8 kbar. A strong anisotropy in the diamagnetic shift is found depending on whether B is applied parallel or perpendicular to the growth direction. In the former case, the spatial extent of the carrier wave function in the dot is estimated to be 60 A. The pressure coefficient for the dot emission line is (9.1±0.2) meV/kbar, about 20% smaller than for the Γ-point band gap in bulk GaAs.

Stark shift in electroluminescence of individual InAs quantum dots

We have fabricated light-emitting-diode heterostructure devices, in which a layer of InAs self-assembled quantum dots is embedded, with an active area of submicron size. In the electroluminescence spectra of these devices, we observed isolated narrow peaks due to emission from individual dots. From the shift of the peaks in an electric field (the quantum confined Stark effect), we show that the ground and excited states in the dots have different spatial alignments of the electron and hole.

Quantum Confined Stark Effect and Permanent Dipole Moment of InAs–GaAs Self‐Assembled Quantum Dots

New information on the electron and hole wavefunctions in InAs–GaAs self-assembled quantum dots (SAQD) is obtained from photocurrent spectroscopy of the quantum confined Stark effect. We show that the hole is localised towards the top of the dot, above the electron, an alignment opposite to that predicted by all previous theories. By means of envelope function calculations we show that this alignment can only arise if the nominally InAs dots have a graded GaxIn1–xAs composition with x decreasing from base to apex, and a relatively flat, non-pyramidal shape. In the light of these results most if not all previous modelling of the electronic structure of InAs SAQD needs to be re-examined. The mechanisms involved in the photocurrent process are also discussed.

Magnetotunnelling and Photoluminescence Spectroscopy of Self-Assembled InAs Quantum Dots

Self-assembled InAs quantum dots (QDs) in AlAs and GaAs matrices are investigated by tunnelling and optical spectroscopy. Tunnelling through an individual QD in the AlAs barrier of a n-i-n single-barrier device is used to probe the properties of both the emitter two-dimensional electron gas (2DEG) and the QD. The Landau fan of the 2DEG is mapped at magnetic field parallel to the tunnelling current, BVertI, as well as the spin splitting in the QD at B⊥I. An electron g-factor in the QD is determined as g=1.2±0.5. In the photoluminescence spectrum of InAs QDs in B up to 23 T, a strong anisotropy in the diamagnetic shift is found. The spatial extent of the carrier wave function in the dot is estimated as 60 A. Under hydrostatic pressure up to 8 kbar, the pressure coefficient for the dot emission line is (9.1±0.2) meV/kbar, about 20% smaller than for the Γ-point bandgap in bulk GaAs.

Electroluminescence from Individual InAs Self-Assembled Quantum Dots

Emission lines from individual InAs self-assembled quantum dots (SAQD) have been observed in electroluminescence spectra from small-area p–i–n light-emitting-diode heterostructure devices containing an embedded layer of SAQD. In the energy range which corresponds to the blue edge of the luminescence from the dot ensemble, the lines emerge from a zero background with increasing positive bias, a signature that each line is due to exciton recombination from an individual dot. In magnetic field parallel to the growth direction, each line splits into two Zeeman components which exhibit pronounced circular polarizations. The measured value of the g-factor and the observed diamagnetic shift are in good agreement with earlier data obtained from photoluminescence spectroscopy.

Electron Capture on DX Centers near \varGamma-X Crossover in AlGaAs:Te

Relaxation of the persistent photoconductivity in AlGaAs:Te has been studied directly, in the real time, under quasihydrostatic pressure up to 11 kbar. Effect of the capture barrier broadening on the relaxation rate is reduced significantly at low electron concentration. The relaxation equation used for capture kinetics description in the wide electron concentration range appears to be not valid near the Γ–X crossover.

High magnetic field studies of tunnelling through X-valley-related silicon donor states in GaAs/AlAs heterostructures

Four resonances have been observed in the current-voltage characteristics of a GaAs/AlAs single-barrier diode incorporating a δ-layer of silicon donors in the barrier. High magnetic fields applied perpendicular to the plane of the barrier are used to examine the nature of the resonant features in the current-voltage characteristics of the diode. The diode has been modelled, showing that the voltage positions and magnetic field dependence of the lower bias resonances are consistent with tunnelling through strain-field split ground states of silicon impurities below the X-conduction band minima of AlAs.

High Pressure as a Tool to Study Electron Localization

We have used high pressure to investigate resonant tunnelling in a single-barrier, n-i-n GaAs/ AlAs/GaAs diode with an embedded layer of InAs self-assembled quantum dots (SAQD). We have obtained convincing evidence for resonant tunnelling through individual r-valley-related electron states that we associate with the SAQD. The tunnel current through a SAQD was used as a local probe of a localized phase of a two-dimensional electron system in the accumulation layer of the diode. We have found evidence that at low densities, the localized electrons form relatively large, high-density clusters.

MBE growth and magnetotunnelling transport properties of a single GaAs/AlAs/GaAs barrier incorporating InAs quantum dots

We describe the growth and investigation of a novel type of structure in which self-organised InAs quantum dots are incorporated in the AlAs tunnelling barrier of an n-i-n single-barrier GaAs/AlAs/GaAs heterostructure. The lowtemperature current-voltage curves exhibit a series of pronounced peaks which are absent in a control sample grown without InAs in the barrier. By studying their behaviour in a magnetic field B, we attribute these peaks to single-electron tunnelling through single discrete zero-dimensional states of individual InAs dots in the barrier.