InAs quantum dot superluminescent diodes with trench structure

Using a trench structure, we have realized improved reliability for processing InAs quantum-dot-based J-shaped superluminescent diodes (SLDs) with shallow-etched waveguides. The observed drastic decrease of output power in shallow-etched-waveguide SLDs is recovered with the deep-etched waveguide. The output power increases with decreasing separation between the waveguide and the trench. The maximum output power of the SLDs with the trench structure exceeds 25 mW. The trench structure should help to achieve low production costs while retaining high reliability.     

Electrically driven quantum dot/wire/well hybrid light-emitting diodes

Electrically driven quantum dot, wire, and well hybrid light-emitting diodes are demonstrated by using nanometer-sized pyramid structures of GaN. InGaN quantum dots, wires, and wells are formed at the tops, edges, and sidewalls of pyramids, respectively. The hybrid light-emitting diodes containing low-dimensional quantum structures are good candidates for broad-band highly efficient visible lighting sources.     

Spin-polarized light-emitting diodes based on heterostructures with a GaAs/InGaAs/GaAs quantum well and ferromagnetic GaMnSb injection layer

Light-emitting diode structures with an InGaAs/GaAs quantum well and a ferromagnetic GaMnSb layer as a p-type semiconductor have been manufactured and investigated. A magnetic-field-induced circular polarization of electroluminescence in these structures has been obtained, the degree of which (0.012 at 0.37 T) is almost constant in the temperature interval of 10–50 K. Circular polarization is determined by the injection of spin-polarized holes from the ferromagnetic GaMnSb layer.     

Efficient CdSe/CdS quantum dot light-emitting diodes using a thermally polymerized hole transport layer

We report multilayer nanocrystal quantum dot light-emitting diodes (QD-LEDs) fabricated by spin-coating a monolayer of colloidal CdSe/CdS nanocrystals on top of thermally polymerized solvent-resistant hole-transport layers (HTLs). We obtain high-quality QD layers of controlled thickness (down to submonolayer) simply by spin-coating QD solutions directly onto the HTL. The resulting QD-LEDs exhibit narrow ( approximately 30 nm, fwhm) electroluminescence from the QDs with virtually no emission from the organic matrix at any voltage. Using multiple spin-on HTLs improves the external quantum efficiency of the QD-LEDs to approximately 0.8% at a brightness of 100 cd/m(2) (with a maximum brightness over 1,000 cd/m(2)). We conclude that QD-LEDs could be made more efficient by further optimization of the organic semiconductors.     

1.3 μm InAs quantum dot resonant cavity light emitting diodes

Resonant cavity light emitting diodes (RCLEDs) containing nine sheets of self-organized InAs quantum dot (QD) active layers and operating at around 1.3 μm are demonstrated. The structure was grown directly on GaAs substrates, which includes selectively oxidized AlO_x current apertures and intracavity metal contacts. It was found that the average operating resistance is 60 Ω, while the average turn-on voltages is 1.6 V. It was also found that temperature coefficient of these RCLEDs was about 0.11 nm/°C.     

Bright and efficient full-color colloidal quantum dot light-emitting diodes using an inverted device structure

We report highly bright and efficient inverted structure quantum dot (QD) based light-emitting diodes (QLEDs) by using solution-processed ZnO nanoparticles as the electron injection/transport layer and by optimizing energy levels with the organic hole transport layer. We have successfully demonstrated highly bright red, green, and blue QLEDs showing maximum luminances up to 23,040, 218,800, and 2250 cd/m(2), and external quantum efficiencies of 7.3, 5.8, and 1.7%, respectively. It is also noticeable that they showed turn-on voltages as low as the bandgap energy of each QD and long operational lifetime, mainly attributed to the direct exciton recombination within QDs through the inverted device structure. These results signify a remarkable progress in QLEDs and offer a practicable platform for the realization of QD-based full-color displays and lightings.     

Decoherence of nuclear spin quantum memory in a quantum dot

Recently, an ensemble of nuclear spins in a quantum dot have been proposed as a long-lived quantum memory. A quantum state of an electron spin in the dot can be faithfully transfered into nuclear spins through controlled hyperfine coupling. Here we study the decoherence of this memory due to nuclear spin dipolar coupling and inhomogeneous hyperfine interaction during the storage period. We calculated the maximum fidelity of writing, storing, and reading operations. Our results show that nuclear spin dynamics can severely limit the performance of the proposed device for quantum information processing and storage based on nuclear spins.     

The effect of current crowding on the internal quantum efficiency of InAsSb/InAs light-emitting diodes

The effect of current crowding on the internal quantum efficiency (IQE) of InAsSb/InAs light-emitting diodes (LEDs) operating in the middle-infrared (mid-IR) range (?? = 3?5 ??m) has been studied. Calculations based on a modified model of recombination coefficients show that current crowding leads to a significant decrease in the IQE of LEDs, which is especially pronounced in longer-wavelength devices (23% at ?? = 3.4 ??m versus 39% at ?? = 4.2 ??m). The obtained results indicate that the effect of current crowding should be taken into consideration as an additional nonthermal mechanism of IQE decrease in mid-IR LEDs.     

Fabrication of white light-emitting diodes based on UV light-emitting diodes with conjugated polymers-(CdSe/ZnS) quantum dots as hybrid phosphors

White light-emitting diodes (LEDs) were fabricated using GaN-based 380-nm UV LEDs precoated with the composite of blue-emitting polymer (poly[(9,9-dihexylfluorenyl-2,7-diyl)-alt-co-(2-methoxy-5-{2-ethylhexyloxy)-1 ,4-phenylene)]), yellow green-emitting polymer (poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(1,4-benzo-{2,1',3}-thiadiazole)]), and 605-nm red-emitting quantum dots (QDs). CdSe cores were obtained by solvothermal route using CdO, Se precursors and ZnS shells were synthesized by using diethylzinc, and hexamethyldisilathiane precursors. The optical properties of CdSe/ZnS QDs were characterized by UV-visible and photoluminescence (PL) spectra. The structural data and composition of the QDs were transmission electron microscopy (TEM), and EDX technique. The quantum yield and size of the QDs were 58.7% and about 6.7 nm, respectively. Three-band white light was generated by hybridizing blue (430 nm), green (535 nm), and red (605 nm) emission. The color-rendering index (CRI) of the device was extremely improved by introducing the QDs. The CIE-1931 chromaticity coordinate, color temperature, and CRI of a white LED at 20 mA were (0.379, 0.368), 3969 K, and 90, respectively.     

Electrical characterization of InAs/GaAs quantum dot structures

The electrical properties of InAs quantum dots (QD) in InAs/GaAs structures have been investigated by space charge spectroscopy techniques, current–voltage and capacitance–voltage measurements. Au/GaAs/InAs(QD)/GaAs Schottky barriers as well as ohmic/GaAs/InAs(QD)/GaAs/ohmic structures have been prepared in order to analyze the apparent free carrier concentration profiles across the QD plane, the electronic levels around the QD and the electrical properties of the GaAs/InAs(QD)/GaAs heterojunction. Accumulation and/or depletion of free carriers at the QD plane have been observed by Capacitance–Voltage (C–V) measurements depending on the structure parameters and growth procedures. Similarly, quantum dot levels which exhibit distributions in energy have been detected by Deep Level Transient Spectroscopy (DLTS) and Admittance Spectroscopy (AS) measurements only on particular structures. Finally, the rectification properties of the InAs/GaAs heterojunction have been investigated and the influence of the related capacitance on the measured capacitance has been evidenced.     

Argon-plasma-induced InAs/InGaAs/InP quantum dot intermixing

We report the first study of argon (Ar)-plasma-enhanced intermixing of InAs/InGaAs/InP self-assembled quantum dots (QDs) in an inductively coupled plasma reactive ion etch system. The Ar-plasma exposure creates point defects, which propagate into the QD structure and enhance the intermixing between the QDs and their barrier layers, hence tuning the energy bandgap of the QDs. By optimizing the plasma exposure time and the annealing temperature, we observe (i) a blueshift of 160?nm and an increase in the photoluminescence (PL) intensity of the QD samples immediately after Ar-plasma exposure for 90?s, and (ii) a further increase in the blueshift of 330?nm, accompanied by 2.5-times increase in the PL intensity and 37?nm narrowing in the PL linewidth after subsequent rapid thermal annealing at 720?°C. The ability to generate a large blueshift without degrading the material quality shows that Ar-plasma exposure is an efficient post-growth technique for tuning the energy bandgap of QD structures.     

InAs/GaAs multiple quantum dot structures grown by LP-MOVPE

Structures with self-organised InAs quantum dots in a GaAs matrix were grown by the low pressure metal–organic vapour phase epitaxy (LP-MOVPE) technique. Photoluminescence and atomic force microscopy were used as the main characterisation methods for the growth optimisation. The properties of multiple-stacked quantum dot structures are influenced by the thickness of the GaAs separation layers (spacers) between quantum dot-containing InAs layers, by the InAs layer thickness, by arsine partial pressure during growth, and by group III precursor flow interruption time.     

Electrically tuned spin-orbit interaction in an InAs self-assembled quantum dot

Electrical control over electron spin is a prerequisite for spintronics spin-based quantum information processing. In particular, control over the interaction between the orbital motion and the spin state of electrons would be valuable, because this interaction influences spin relaxation and dephasing. Electric fields have been used to tune the strength of the spin-orbit interaction in two-dimensional electron gases, but not, so far, in quantum dots. Here, we demonstrate that electrical gating can be used to vary the energy of the spin-orbit interaction in the range 50-150 μeV while maintaining the electron occupation of a single self-assembled InAs quantum dot. We determine the spin-orbit interaction energy by observing the splitting of Kondo effect features at high magnetic fields.     

Photoluminescence and spin relaxation of MnZnO/GaN-based light-emitting diodes

This study investigates the spin relaxation of GaN-based light-emitting diodes with an MnZnO film by examining its photoluminescence (PL) and time-resolved magnetization modulation photoluminescence. PL measurements reveal that the application of a magnetic field produced a clear difference between the intensities of the right (σ⁺) and left (σ⁻) circular polarization components. The circular polarization was identified as P_circ = [I(σ⁺) − I(σ⁻)] / [I(σ⁺) + I(σ⁻)], where I(σ⁺) and I(σ⁻) are the intensities of the σ⁺ and σ⁻ components, respectively. The PL polarization was 3.6% in a 0.5 T magnetic field. In a magnetic field, the photo-ionized lifetime and spin-polarized lifetime values were approximately 13.64 and 54.54 ns, respectively. The right-circular-spin-polarization lifetime and the left-circular-spin-polarization lifetime values were about 39.09 and 40.01 ns, respectively.     

Multi-section quantum dot superluminescent diodes for spectral shape engineering

The design and operating parameters of a novel multi-contact quantum dot superluminescent diode incorporating a number of features which inhibit lasing are described and compared with that of a single-contact device. Such devices allow the independent tuning of emission power and spectral shape; hence the penetration depth and resolution in optical coherence tomography are decoupled. The emission spectrum of a device utilising chirped quantum dots is shown to be tuned to produce a broadband single Gaussian emission, centred at the required wavelength of 1050 nm, at higher output powers than a singlecontact device.     

Incorporation of gene therapy vector in chitosan stabilized Mn-doped ZnS quantum dot

Chitosan stabilized and water soluble (pH ≤ 6.5) ZnS:Mn²⁺ quantum dots (QDs) of ca. 3.6 nm having a strong orange fluorescence have been synthesized in an environment friendly method. Binding of plasmid DNA containing the bifunctional cytosine deaminase-uracilphosphoribosyl transferase (pCD-UPRT) gene with therapeutic importance in suicide gene therapy has been investigated and shown to follow the Langmuir reversible adsorption model. The biocompatibility of the composite on HT29 cells was confirmed by the viability assay. The chitosan stabilized ZnS:Mn²⁺ QDs synthesized in the present study could be a promising alternative to conventional organic fluorophore-tagged gene delivery systems for real-time monitoring in gene therapy applications.     

Nanostructured current-confined single quantum dot light-emitting diode at 1300 nm

A novel light-emitting-diode structure is demonstrated, which relies on nanoscale current injection through an oxide aperture to achieve selective excitation of single InAs/GaAs quantum dots. Low-temperature electroluminescence spectra evidence discrete narrow lines around 1300 nm (line width approximately 75 microeV) at ultralow currents, which are assigned to the emission from single excitons and multiexcitons. This approach, which enables the fabrication of efficient nanoscale active devices at 1300 nm, can provide single-photon-emitting diodes for fiber-based quantum cryptography.     

Influences of silicon doping in quantum dot layers on optical characteristics of InAs/GaAs quantum dot infrared photodetector

We have investigated the effects of silicon doping concentration within thirty-period self-assembled quantum dot (QD) layers on quantum dot infrared photodetectors (QDIPs). The lens-shaped quantum dots with the dot density of 1 × 10¹¹ cm^− 2 were observed by atomic force microscope (AFM). From the high ratio of photoluminescence (PL) peak intensities from dot layer to that from wetting layer, we have concluded that high dot density caused the short diffusion length for carriers to be easily captured by QDs. Moreover, the Si-doped samples exhibited the multi-state transitions within the quantum dots, which were different to the single level transition of undoped sample. Besides, the dominant PL peaks of Si-doped samples were red-shifted by about 25 meV compared to that of the undoped sample. It should result from the dopant-induced lowest transition state and therefore, the energy difference should be equal to the binding energy of Si in InAs QDs.