Self-assembled InAs QDs Research Articles

Ultrafast electronic spectroscopy on the coupling of Stranski-Krastanov and submonolayer quantum dots for potential application in near infrared light harvesting

Heterogeneously coupled self-assembled InAs QDs are always attractive to study their unique physical properties. Here, we have introduced Stranski-Krastanov (SK) grown InAs/GaAs QDs coupled to InAs/InGaAs submonolayer (SML) QDs to improve the light harvesting efficacy in the near infrared region (NIR) of the solar spectrum. Carrier migration mechanism from the SML stacks to the SK QDs and its dependence on the barrier thickness have been studied by temperature dependent steady state and picosecond resolved photoluminescence measurements. Both high and low energy excitations have been used to study the carrier migration and capture dynamics in the coupled heterostructures. It has been observed that the coupling is essentially determined by the spatial separation of both the QDs and the carrier lifetime reaches a maximum value at a certain barrier thickness. After successful optimization of the barrier thickness, we have used the SK-SML coupled structure as an active layer in a device to study its light harvesting efficacy. The SK-SML coupled device shows higher photocurrent, less saturation current and higher photovoltage compared to the uncoupled SK-QD one. In this study, we also find that the coupling in between the two classes of QDs increases the charge extraction efficiency and reduces the nonradiative recombination in the heterostructures. In brief, our experiments provide key information of the coupling mechanisms in the SK-SML coupled structure across various barrier thicknesses and its implication for improved device performance.

Microscopic Investigation of Mono-layer/Multi-layer self-assembled InAs QDs on GaAs1-xSbx/GaAs Composite Substrates for Photovoltaic Solar Cells

Microscopic Investigation of Mono-layer/Multi-layer self-assembled InAs QDs on GaAs_1-xSb_x/GaAs Composite Substrates for Photovoltaic Solar Cells

Carrier Dynamics in Self-Assembled InAs QD Laser Structures and Broad-Area InAs QD Lasers Grown by Molecular Beam Epitaxy

ABSTRACTWe investigated carrier dynamics in both proton-irradiated InAs-GaAs quantum dot laser structures and in high power broad-area InAs-GaAs quantum dot lasers with windowed n-contacts using time-resolved PL (TR-PL) techniques.

Wide-band emissions from highly stacked quantum dot structure grown using the strain-compensation technique

We developed a new scheme using modulated stacking of self-assembled InAs QDs on InP(311)B substrates with strain compensation to fabricate QDs in order to expand the potential bandwidths of QD active regions. A highly stacked QD structure was fabricated by the strain-compensation technique with no degradation of structural and optical qualities. The full-width at half-maximum of the photoluminescence of QDs is expanded to 240nm in the modulated stacking structure. Carrier transfer from the small QD layer to the large QD layer was observed in the stacking structure with a thin spacer layer.

Site-controlled InAs quantum dot formation grown on the templates fabricated by the Nano-Jet Probe method

We have developed a nanoprobe-assisted bottom-up technique that allows to fabricate site-controlled quantum dots (SC-QDs), using a specially designed atomic force microscope probe, referred to as the Nano-Jet Probe (NJP). Beginning with the fabrication of regular QD arrays by using the NJP, we vertically aligned self-assembled InAs QDs using the strain-induced stacking method and produced assembled QD stacks. In order to improve the optical property of the SC-QDs, an AlGaAs barrier layer was introduced in the spacer layer of the stacked structures. The photoluminescence measurements of the fabricated structures revealed that they had good crystallographic qualities.

Lateral electron tunneling through single self-assembled InAs quantum dots coupled to superconducting nanogap electrodes

The authors have fabricated superconductor-quantum dot-superconductor (SC-QD-SC) junctions by using SC aluminum electrodes with narrow gaps laterally contacting a single self-assembled InAs QD. The fabricated junctions exhibited clear Coulomb staircases and Coulomb oscillations at 40mK. Furthermore, clear suppression in conductance was observed for the source-drain voltage, ∣VSD∣<2Δ∕e, where Δ is the SC energy gap of Al. The absence of Josephson current that flows through QDs is due to the strong Coulomb interaction and non-negligible thermal fluctuation in our measurement system.

OPTICAL AND ELECTRICAL INVESTIGATION OF LOW DIMENSIONAL SELF-ASSEMBLEDInAsQUANTUM DOT FIELD EFFECT TRANSISTORS

Self-assembled InAs QD dot-in-a-well (DWELL) structures were grown on GaAs substrate by MBE system, and heterojunction modulation-doped field effect transistor (MODFET) was fabricated. The optical properties of the samples show that the photoluminescence of InAs/GaAs self-assembled quantum dot (SAQD) is at 1.265 μm at 300 K. The temperature-dependence of the abnormal redshift of InAs SAQD wavelength with the increasing temperature was observed, which is closely related with the inhomogeneous size distribution of the InAs quantum dot. According to the electrical measurement, high electric field current–voltage characteristic of the MODFET device were obtained. The embedded InAs QD of the samples can be regard as scattering centers to the vicinity of the channel electrons. The transport property of the electrons in GaAs channel will be modulated by the QD due to the Coulomb interaction. It has been proposed that a MODFET embedded with InAs QDs presents a novel type of field effect photon detector.

Photoluminescence Characteristics of InAs Quantum Dots Grown by STM/MBE Site-Control Technique

This paper describes micro-photoluminescence (PL) analysis of site-controlled QDs (SCQDs) grown using a novel in-situ MBE growth technique in which sites of self-assembled InAs QDs are controlled by forming nanometer deposits using a scanning tunneling microscope (STM) probe. We found from the temperature dependence of PL that the carrier collection at QDs at low temperature is limited by carrier diffusion in the wetting layer. The analysis of PL data considering this effect has indicated that individual QDs grown have high crystalline quality in spite of the addition of an artificial STM process during growth.

Excited states and energy relaxation in stacked InAs/GaAs quantum dots

Excited states and energy relaxation processes are studied for stacked InAs/GaAs QD's with GaAs cap layers grown by migration enhanced epitaxy. Photoluminescence excitation (PLE) spectra reveal the excited state spectrum as a function of size for self-assembled InAs QD's in multilayered samples with 36-ML spacers. The observed energy shifts and splittings are consistent with those of hole states numerically calculated for pyramidal QD's supporting assignment to the transition between the electron ground |000〉 and the |001〉 excited hole state. The optical results suggest the island shape uniformity to improve in multilayered samples, which is attributed to the contribution of the buried islands to the surface strain altering the island formation kinetics and energetics that also underlie vertical self-organization. Time-resolved photoluminescence (TRPL) results yield a lifetime of 40 ps for the first excited |001〉 hole state, attributed to multiphonon relaxation processes bridging the approximately 100 meV level separation, and ground-state lifetimes around 700 ps independent of the detection energy. At high excitation densities saturation of QD states leads to long-living excited-state PL and up to 1 ns delay in the ground-state PL decay, showing radiative decay to be the dominant recombination process in the QD's. The results presented contribute to the understanding of PLE spectra of an inhomogeneous QD ensemble, which is argued to be sensitive to the shape uniformity, the excited-state spectrum, and competing recombination processes.