Experimental and theoretical study of the quantum-confined Stark effect in a single InGaN/GaN quantum dot under applied vertical electric field

We present a study of the effect of externally applied vertical electric field on the optical properties of single InGaN/GaN quantum dots via microphotoluminescence spectroscopy. This is achieved by incorporating the quantum dot layer in the intrinsic region of a p–i–n diode structure. We observe a large blue energy shift of 60 meV, which is explained by the partial compensation of the internal piezoelectric field. The energy shift dependence on the applied field allows the determination of the vertical component of the permanent dipole and the polarizability. We also present theoretical modelling of our results based on atomistic semi-empirical tight-binding simulations. A good quantitative agreement between the experiment and the theory is found.     

Charged magneto-exciton states in semiconductor quantum dots

By embedding a layer of self-assembled quantum dots into a field-effect structure, we are able to control the exciton charge in a single dot. We present the results of photoluminescence experiments as a function of both charge and magnetic field. The results demonstrate a hierarchy of energy scales determined by quantization, the direct Coulomb interaction, the electron–electron exchange interaction, and the electron–hole exchange interaction. For excitons up to the triply charged exciton, the behavior can be understood from a model assuming discrete levels within the quantum dot. For the triply charged exciton, this is no longer the case. In a magnetic field, we discover a coherent interaction with the continuum states, the Landau levels associated with the wetting layer.     

Splitting and storing excitons in strained coupled self-assembled quantum dots

We present a novel self-assembled quantum dot structure designed to spatially separate and store photo-generated electrons and holes in pairs of strain coupled quantum dots. The spatial separation of electron–hole pairs into quantum dots and strain-induced quantum dots has been investigated and verified by photoluminescence experiments. Results from time-resolved PL demonstrates that at low temperatures (3 K) the electron–hole pair can be stored for several seconds.     

Insight into optical properties of strain-free quantum dot pairs

Self-assembled GaAs/AlGaAs quantum dot pairs (QDPs) are grown by molecular beam epitaxy using high temperature droplet epitaxy technique. A typical QDP consists of dual-size quantum dots as observed based on atomic force microscopy image. The average height of quantum dot is 5.7 nm for the large quantum dots and 4.6 nm for the small ones. The average peak-to-peak distance of the two dots is about 75 nm. The optical properties of GaAs QDPs are studied by measuring excitation power-dependent and temperature-dependent photoluminescence. Unique photoluminescence properties have been observed from both excitation power-dependent and temperature-dependent measurements. Excitation power-dependent as well as temperature-dependent PL measurements have suggested lateral exciton transfer in the QDPs.     

SiGe (Ge-dot) heterojunction phototransistors for efficient light detection at 1.3–1.55 μm

The aim of this work is to develop a Si/SiGe HBT-type phototransistor with several Ge dot layers incorporated in the collector, in order to obtain improved light detectivity at 1.3–1.55 μm. The MBE grown HBT detectors are of n–p–n type and based on a multilayer structure containing 10 Ge-dot layers (8 ML in each layer, separated by 60 nm Si spacer) in the base-collector junction. The transistors were processed for normal incidence or with waveguide geometry where the light is coupled through the edge of the sample. The measured breakdown voltage, BV_ceo, was about 6 V. Compared to a p–i–n reference photodiode with the same dot layer structure, photoconductivity measurements show that the responsivity is improved by a factor of 60 for normal incidence at 1.3 μm. When the light is coupled through the edge of the device, the detectivity is even further enhanced. The measured photo-responsivity is more than 100 and 5 mA/W at 1.3 and 1.55 μm, respectively.     

Effect of dimensional and magnetic quantization hybridization in the optical absorption spectra of nanoheterosystems with <Emphasis Type="Italic">D</Emphasis><Superscript>(−)</Superscript>-states

The magnetoabsorption of light by quantum dot — D^(–)-center complexes synthesized in a transparent dielectric matrix — is theoretically studied with allowance for dispersed quantum dot (QD) sizes. In the effective mass approximation, an analytical expression for the impurity magnetoabsorption coefficient of light polarized in the quantizing magnetic polarization field direction is derived.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 67–72, October, 2004.     

Carrier tunneling in asymmetric coupled quantum dots

Exciton spin relaxation at low temperatures in InAlAs–InGaAs asymmetric double quantum dots embedded in AlGaAs layers has been investigated as a function of the barrier thickness by the time-resolved photoluminescence measurements. With decreasing the thickness of the AlGaAs layer between the dots, the spin relaxation time change from 3 ns to less than 500 ps. The reduction in the spin relaxation time was considered to originate from the spin-flip tunneling between the ground state in InAlAs dot and the excited states in InGaAs dot, and the resultant tunneling leads to the spin depolarization of the ground state in InGaAs dot.     

Fabrication and optical properties of quantum dots and wires

We describe photoluminescence measurements made on mesa geometry quantum dots and wires with exposed side walls fabricated by laterally patterning undoped GaAs/AlGaAs quantum wells using electron beam lithography and dry etching. At low temperature the photoluminescence efficiency of many but not all of the GaAs quantum dot arrays scales with the volume of quantum well material down to lateral dimensions of 50nm. This behaviour contrasts with that found in wires produced at the same time where the intensity falls off rapidly with decreasing wire width for dimensions below 500nm but is recovered by overgrowth with indium tin oxide, possibly as a result of strain. Narrow overgrown wires exhibit anisotropy in polarized excitation spectra which is discussed in relation to strain and lateral confinement effects.     

Optical pumping of the electronic and nuclear spin of single charge-tunable quantum dots

We present a comprehensive examination of optical pumping of spins in individual GaAs quantum dots as we change the net charge from positive to neutral to negative with a charge-tunable heterostructure. Negative photoluminescence polarization memory is enhanced by optical pumping of ground state electron spins, which we prove with the first measurements of the Hanle effect on an individual quantum dot. We use the Overhauser effect in a high longitudinal magnetic field to demonstrate efficient optical pumping of nuclear spins for all three charge states of the quantum dot.     

Millisecond fluorescence in InAs quantum dots embedded in AlAs

The temperature dependence of steady-state and time-resolved photoluminescence from self-assembled InAs quantum dots embedded in AlAs has been studied. Millisecond-long nonexponential photoluminescence decay is observed in the temperature range of 4.2–50 K. At higher temperatures, the decay time decreases to a few nanoseconds. The experimental results are interpreted using a model of singlet–triplet splitting of exciton levels in small dots in a dense quantum dot system with local carrier transfer between dots.     

Optical properties of 1.3 μm room temperature emitting InAs quantum dots covered by In0.4Ga0.6As/GaAs hetero-capping layer

Room temperature 1.3 μm emitting InAs quantum dots (QDs) covered by an In_0.4Ga_0.6As/GaAs strain reducing layer (SRL) have been fabricated by solid source molecular beam epitaxy (SSMBE) using the Stranski–Krastanov growth mode. The sample used has been investigated by temperature and excitation power dependent photoluminescence (PL), photoluminescence excitation (PLE), and time resolved photoluminescence (TRPL) experiments. Three emission peaks are apparent in the low temperature PL spectrum. We have found, through PLE measurement, a single quantum dot ground state and the corresponding first excited state with relatively large energy spacing. This attribute has been confirmed by TRPL measurements which allow comparison of the dynamics of the ground state with that of the excited states. Optical transitions related to the InGaAs quantum well have been also identified. Over the whole temperature range, the PL intensity is found to exhibit an anomalous increase with increasing temperatures up to 100 K and then followed by a drop by three orders of magnitude. Carrier’s activation energy out of the quantum dots is found to be close to the energy difference between each two subsequent transition energies. PACS 68.65.Ac; 68.65.Hb; 78.67.Hc     

Photoluminescence of charged magneto-excitons in InAs single quantum dots

We calculated the photoluminescence spectra of charged magneto-excitons in single two-dimensional parabolic quantum dots, using an unrestricted Hartree–Fock method. The calculated luminescence spectra explain well the observed red shifts of transition energies of InAs/GaAs single quantum dot by additional electron capture in a dot. The magnetic-field-induced transition of the ground state configuration of trapped electrons causes drastic change in the photoluminescence spectra. The dependence of photoluminescence intensities of charged excitons on the excess energies of photogenerated carriers above the bulk GaAs energy gap is studied phenomenologically, by calculating the steady state electron population probability in a dot.     

Electroluminescence of excitons in an InGaAs quantum well

We report on electrical injection of excitons in a quantum well placed in the intrinsic region of a p–i–n photodiode. Both the narrow linewidth of the electroluminescence at 70 K and the evolution of the emission spectra with increasing current are signatures of the excitonic character of the emission. This structure is ready to be integrated in semiconductor microcavities in order to evidence the strong coupling regime under electrical injection.     

Rashba effect in an asymmetric quantum dot in a magnetic field

We derive an expression for the total spin-splitting energy in an asymmetric quantum dot with ferromagnetic contacts, subjected to a transverse electric field. Such a structure has been shown by one of us to act as a spintronic quantum gate with in-built qubit readers and writers (Phys. Rev. B61, 13813 (2000)). The ferromagnetic contacts result in a magnetic field that causes a Zeeman splitting of the electronic states in the quantum dot. We show that this Zeeman splitting can be finely tuned with a transverse electric field as a result of nonvanishing Rashba spin–orbit coupling in an asymmetric quantum dot. This feature is critical for implementing a quantum gate.     

Optical spectroscopy of a single InAs/GaAs quantum dot in high magnetic fields

We report on the measurements of the photoluminescence from the s-shell of a single InAs/GaAs quantum dot in magnetic fields up to 23 T. The observed multiline emission is attributed to different charge states of a single dot. Characteristic anticrossing of emission lines is explained in terms of hybridization of final states of a triply charged exciton (X⁻³).     

Lifetime dispersion in a single quantum dot

We present a decay formula for photoluminescence of a single quantum dot. We apply the formula to time-resolved photoluminescence (PL) measurements for a single InAs/GaAs quantum dot. The formula works very well for the PL decays of excitons and biexcitons in the system. The physical basis of the formula originates from the temporal dispersion of lifetimes. PACS 78.67.Hc; 78.47.+p; 78.55.Cr; 71.35.-y     

Influence of GaAsSb structural properties on the optical properties of InAs/GaAsSb quantum dots

The optical properties of InAs quantum dots with GaAsSb buffer, capping and cladding layers of different alloy compositions are studied by photoluminescence techniques. Fully strained GaAsSb layers show that the inclusion of a buffer layer gives a blue-shift to quantum dot emission, while for quantum dots capped with GaAsSb a clear red-shift is seen. Power-dependent photoluminescence suggests a transition from type-I to type-II can be achieved by GaAsSb at Sb composition between 11–13%, while the transition for the GaAsSb cladding layer occurs at around 11%. At low Sb composition, good crystal quality and energy barrier are detected by temperature-dependent photoluminescence, while high-level dislocation and defects exist under high antimony content, as evidenced by X-Ray Diffraction and Transmission Electron Microscopy.     

Picosecond timescale carrier dynamics of InAs quantum dots: The role of a continuum background

We have investigated the ultrafast carrier dynamics in Molecular Beam Epitaxy (MBE)-grown InAs/InGaAs/GaAs quantum dots emitting at 1.3 μm by means of time resolved photoluminescence upconversion measurements with a time resolution of about 200 fs. The detection energies scan the spectral region from the energy of the quantum dot excitonic transition up to the barrier layer absorption edge. We found, under high excitation intensity, that the intrinsic electronic states are populated mainly by carriers directly captured from the barrier.     

Variation of co-tunneling and Kondo effects by control of the strength of coupling between a vertical dot and a two-dimensional electron gas

We have fabricated a vertical quantum dot with lateral coupling, modulated by a split gate voltage, to a two-dimensional electron. We thereby control not only electron configurations but also the strength of coupling between the dot and the lateral lead, by applying gate voltages. We have measured the conductance enhancement when the applied bias exceeds the single-electron excitation energy, in the Coulomb blockade regime. This conductance enhancement disappears as the split gate voltage decreases (reducing the coupling). This indicates that this enhancement is caused by inelastic co-tunneling. Furthermore, we observed a conductance enhancement at zero source–drain bias with stronger coupling. An anomaly is observed that we attribute to Kondo resonance between the dot and the leads.     

Excitons in cylindrical GaAs–Ga1−xAlxAs quantum dots under applied electric field

The exciton binding energy and photoluminescence energy transition in a GaAs-Ga_1−xAl_xAs cylindrical quantum dot are studied with the use of the effective mass approximation and a variational calculation procedure. The influence of these properties on the application of an electric field along the growth direction of the cylinder is particularly considered. It is shown that for zero applied field the binding energy and the photoluminescence energy transition are decreasing functions of the quantum dot radius and height. Given a fixed geometric configuration, both quantities then become decreasing functions of the electric field strength as well.