Zhichuan Niu

InAs/GaAs single-electron quantum dot qubit

The time evolution of the quantum mechanical state of an electron is calculated in the framework of the effective-mass envelope function theory for an InAs/GaAs quantum dot. The results indicate that the superposition state electron density oscillates in the quantum dot, with a period on the order of femtoseconds. The interaction energy E-ij between two electrons located in different quantum dots is calculated for one electron in the ith pure quantum state and another in the jth pure quantum state. We find that E-11]E-12]E-22, and E-ij decreases as the distance between the two quantum dots increases. We present a parameter-phase diagram which defines the parameter region for the use of an InAs/GaAs quantum dot as a two-level quantum system in quantum computation. A static electric field is found to efficiently prolong the decoherence time. Our results should be useful for designing the solid-state implementation of quantum computing

Complex quantum ring structures formed by droplet epitaxy

Well-defined complex quantum ring structures formed by droplet epitaxy are demonstrated. By varying the temperature of the crystallizing Ga droplets and changing the As flux, GaAs/AlGaAs quantum single rings and concentric quantum double rings are fabricated, and double-ring complexes are observed. The growth mechanism of these quantum ring complexes is addressed. (c) 2006 American Institute of Physics.

Formation of GaAs∕AlGaAs and InGaAs∕GaAs nanorings by droplet molecular-beam epitaxy

GaAs/AlGaAs lattice-matched nanorings are formed on GaAs (100) substrates by droplet epitaxy. The crucial step in the formation of nanorings is annealing Ga droplets under As flux for proper time. The observed morphologic evolution of Ga droplets during annealing does not support the hypothesis that As atoms preferentially react with Ga around the periphery of the droplets, but somehow relates to a dewetting process similar to that of unstable films. Photoluminescene (PL) test results confirm the quantum-confinement effect of these GaAs nanorings. Using similar methods, we have fabricated InGaAs/GaAs lattice-mismatched rings. (c) 2005 American Institute of Physics.

Growth of thicker zinc-blende CrSb layers by using (In,Ga)As buffer layers

Zinc-blende CrSb (zb-CrSb) layers with room-temperature ferromagnetism have been grown on (In,Ga)As buffer layers epitaxially prepared on (001) GaAs substrates by molecular-beam epitaxy. Compared with the typical thickness [2-3 ML (ML denotes monolayers)] of zb-CrSb grown directly on GaAs, the thickness of zb-CrSb grown on (In,Ga)As has been increased largely; the maximum can be up to similar to 9 ML. High-resolution cross sectional transmission electron microscopy images show that the zb-CrSb layer is combined with (In,Ga)As buffer layer without any dislocations at the interface

Photoluminescence from self-assembled long-wavelength InAs/GaAs quantum dots under pressure

The photoluminescence from self-assembled long-wavelength InAs/GaAs quantum dots was investigated at 15 K under hydrostatic pressure up to 9 GPa. Photoemission from both the ground and the first excited states in large InAs dots was observed. The pressure coefficients of the two emissions were 69 and 72 meV/GPa, respectively. A nonlinear elasticity theory was used to interpret the significantly small pressure coefficients of the large dots. The sequential quenching of the ground and the excited state emissions with increasing pressure suggests that the excited state emissions originate from the optical transitions between the first excited electron states and the first excited hole states.

GaAs-based room-temperature continuous-wave 1.59 {mu}m GaInNAsSb single-quantum-well laser diode grown by molecular-beam epitaxy

Starting from the growth of high-quality 1.3μmGaInNAs∕GaAs quantum well (QW), the QW emission wavelength has been extended up to 1.55μm by a combination of lowering growth rate, using GaNAs barriers and incorporating some amount of Sb. The photoluminescence properties of 1.5μm range GaInNAsSb∕GaNAs QWs are quite comparable to the 1.3μm QWs, revealing positive effect of Sb on improving the optical quality of the QWs. A 1.59μm lasing of a GaInNAsSb∕GaNAs single-QW laser diode is obtained under continuous current injection at room temperature. The threshold current density is 2.6kA∕cm2 with as-cleaved facet mirrors.

Electron and hole transport through quantum dots

The transmission through quantum dots (QDs) is calculated using the recursion method. In our calculation, the effect of finite offset is taken into account. The results show that the shapes of the QDs determine the number of resonant tunneling peaks and the distances between the peaks decrease as the radii of the QDs increase. The intensities of the conductance are strongly dependent on the barrier widths. The conductance peaks are split when transmitting through two QDs. The theoretical results qualitatively agree with the available experimental data. Our calculated results should be useful for the application of QDs to photoelectric devices

Electron transport through coupled quantum dots

The transmission through coupled quantum dots (CQDs) is calculated using the coupled-channel recursion method. Our results reveal that the conductance peaks move to high energy as the CQDs radius decreases or the period increases. If we increase the transverse momentum the conductance peaks move to high energy. Applying this characteristic, we can design a switch device using CQDs by applying a static electric field perpendicular to transmission direction. The theoretical results qualitatively agree with the available experimental data. Our calculated results may be useful for the application of CQDs to photoelectric devices.

High-indium-content InxGa1−xAs/GaAs quantum wells with emission wavelengths above 1.25 μm at room temperature

High-indium-content InxGa1-xAs/GaAs single/multi-quantum well (SQW/MQW) structures have been systematically investigated. By optimizing the molecular-beam epitaxy growth conditions, the critical thickness of the strained In0.475Ga0.525As/GaAs QWs is raised to 7 nm, which is much higher than the value given by the Matthews and Blakeslee model. The good crystalline quality of the strained InGaAs/GaAs MQWs is proved by x-ray rocking curves. Photoluminescence measurements show that an emission wavelength of 1.25 mum at room temperatures with narrower full width at half maximum less than 30 meV can be obtained. The strain relaxation mechanism is discussed using the Matthews-Blakeslee model

1.55 {mu}m GaInNAs resonant-cavity-enhanced photodetector grown on GaAs

We report the design, growth, fabrication, and characterization of a GaAs-based resonant-cavity-enhanced (RCE) GaInNAs photodetector operating at 1.55μm. The structure of the device was designed using a transfer-matrix method (TMM). By optimizing the molecular-beam epitaxy growth conditions, six GaInNAs quantum wells were used as the absorption layers. Twenty-five (25)- and 9-pair GaAs∕AlAs-distributed Bragg reflectors were grown as the bottom and top mirrors. At 1.55μm, a quantum efficiency of 33% with a full width at half maximum of 10nm was obtained. The dark current density was 3×10−7A∕cm2 at a bias of 0V and 4.3×10−5A∕cm2 at a reverse bias of 5V. The primary time response measurement shows that the device has a rise time of less than 800ps.