Anisotropic Zeeman splitting and Stark shift of In1-yMnyAs1-xNx oblate quantum dots

The electronic structure, Zeeman splitting, and Stark shift of In1-yMnyAs1-xNx oblate quantum dots are studied using the ten-band k center dot p model including the sp-d exchange interaction between the carriers and the magnetic ion. The Zeeman splitting of the electron ground states is almost isotropic. The Zeeman splitting of the hole ground states is highly anisotropic, with an anisotropy factor of 918 at B=0.1 T. The Zeeman splittings of some of the electron and hole excited states are also highly anisotropic. It is because of the spin-orbit coupling which couples the spin states with the anisotropic space-wave functions due to the anisotropic shape. It is found that when the magnetic quantum number of total orbital angular momentum is nearly zero, the spin states couple with the space-wave functions very little, and the Zeeman splitting is isotropic. Conversely, if the magnetic quantum number of total orbital angular momentum is not zero, the space-wave functions in the degenerate states are different, and the Zeeman splitting is highly anisotropic. The electron and hole Stark shifts of oblate quantum dots are also highly anisotropic. The decrease of band gap with increasing nitrogen composition is much more obvious in the smaller radius case because the lowest conduction level is increased by the quantum confinement effect and is closer to the nitrogen level. (C) 2007 American Institute of Physics.

Highly anisotropic Zeeman splittings of wurtzite Cd1-xMnxSe quantum dots

The electronic structure and Zeeman splittings of wurtzite Cd1-xMnxSe quantum spheres are studied using the k center dot p method and mean-field model. It is interesting to find that the Zeeman splittings of some hole states in quantum spheres are highly anisotropic due to the spin-orbit coupling and wurtzite crystal structure. The anisotropy of the Zeeman splittings of hole ground states in large dots is large, while that in small dot is small because the hole ground states vary with radius. An external electrical field can change the Zeeman splitting significantly, and tune the g factor from nearly 0 to about 100.

Plasmons in vertically coupled InAs/GaAs quantum dots

We investigate plasmon excitations in a quantum wire that consists of an infinite one-dimensional array of vertically coupled InAs/GaAs strained quantum dots (QDs). The research is carried out in the framework of random-phase approximation using effective-mass theory. Our formalism is capable of studying plasmons with strong tunneling among QDs, which frustrate the conventionally adopted tight-binding approximation. Based on this formalism, a systematic study on the intraminiband or intrasubband plasmon in vertically coupled InAs/GaAs strained QDs is presented. It is found that an increase of the dot spacing will inevitably reduce the plasmon energy. In contrast, the role of dot height is relatively complex and depends on the dot spacing. The results demonstrate the possibility to engineer collective excitations in low dimensional systems by simply changing their geometric configuration.

Defects around self-organized InAs quantum dots measured by slow positron beam

Self-organized InAs quantum dots (QDs) have been fabricated by molecular beam epitaxy. The authors try to use a slow positron beam to detect defects in and around self-organized QDs, and point defects are observed in GaAs cap layer above QDs. For the self-organized InAs QDs without strain-reducing layer, it is free of defects. However, by introducing a strain-reducing layer, the density of point defects around larger sized InAs QDs increased. The above results suggest that low energy positron beam measurements may be a good approach to detect depth profiles of defects in QD materials. (c) 2007 American Institute of Physics.

Ordered InAs quantum dots with controllable periods grown on stripe-patterned GaAs substrates

GaAs (001) substrates are patterned by electron beam lithography and wet chemical etching to control the nucleation of InAs quantum dots (QDs). InAs dots are grown on the stripe-patterned substrates by solid source molecular beam epitaxy, A thick buffer layer is deposited on the strip pattern before the deposition of InAs. To enhance the surface diffusion length of the In atoms, InAs is deposited with low growth rate and low As pressure. The AFM images show that distinct one-dimensionally ordered InAs QDs with homogeneous size distribution are created, and the QDs preferentially nucleate along the trench. With the increasing amount of deposited InAs and the spacing of the trenches, a number of QDs are formed beside the trenches. The distribution of additional QDs is long-range ordered, always along the trenchs rather than across the spacing regions.

Electrical and optical properties of InAs/GaAs quantum dots doped by high energy Mn implantation

Mn ions were doped into InAs/GaAs quantum dots samples by high energy. implantation and subsequent annealing. The optical and electric properties of the samples have been studied. The photoluminescence intensity of the samples annealed rapidly is stronger than that of the samples annealed for long time. By studying the relationship between the photoluminescence peaks and the implantation dose, it can be found that the photoluminescence peaks of the quantum dots show a blueshift firstly and then move to low energy with the implantation. dose increasing. The latter change in the photoluminescence peaks is probably attributed to that Mn ions entering the InAs quantum dots, which release the strain of the quantum dots. For the samples implanted by heavy dose (annealed rapidly) and the samples annealed for long time, the resistances versus temperature curves reveal anomalous peaks around 40 K.

Chemical trends of defect formation in Si quantum dots: The case of group-III and group-V dopants

Using first-principles methods, we have systematically calculated the defect formation energies and transition energy levels of group-III and group-V impurities doped in H passivated Si quantum dots (QDs) as functions of the QD size. The general chemical trends found in the QDs are similar to that found in bulk Si. We show that defect formation energy and transition energy level increase when the size of the QD decreases; thus, doping in small Si QDs becomes more difficult. B-Si has the lowest acceptor transition energy level, and it is more stable near the surface than at the center of the H passivated Si QD. On the other hand, P-Si has the smallest donor ionization energy, and it prefers to stay at the interior of the H passivated Si QD. We explained the general chemical trends and the dependence on the QD size in terms of the atomic chemical potentials and quantum confinement effects.

Cr-doped InAs self-organized diluted magnetic quantum dots with room-temperature ferromagnetism

Cr-doped InAs self-organized diluted magnetic quantum dots (QDs) are grown by low-temperature molecular-beam epitaxy, Magnetic measurements reveal that the Curie temperature of all the InAs:Cr QDs layers with Cr/In flux ratio changing from 0.026 to 0.18 is beyond 400 K. High-resolution cross sectional transmission electron microscopy images indicate that InAs:Cr QDs are of the zincblende structure. Possible origins responsible for the high Curie temperature are discussed.

Hydrogenic impurity in double quantum dots

The ground state binding energy and the average interparticle distances for a hydrogenic impurity in double quantum dots with Gaussian confinement potential are studied by the variational method. The probability density of the electron is calculated, too. The dependence of the binding energy on the impurity position is investigated for GaAs quantum dots. The result shows that the binding energy has a minimum as a function of the distance between the two quantum dots when the impurity is located at the center of one quantum dot or at the center of the edge of one quantum dot. When the impurity is located at the center of the two dots, the binding energy decreases monotonically. (c) 2006 Elsevier B.V. All rights reserved.

Fabrication of ultra-low density and long-wavelength emission InAs quantum dots

By optimizing the molecular beam epitaxy growth conditions of self-organized InAs/GaAs quantum dots (QDs), we obtained an ultra-low density system of InAs QDs (4 x 10(6)cm(-2)). Photoluminescence (PL) spectroscopy reveals the emission wavelength at room temperature to be longer than 1300 nm with a GaAs capping layer. (c) 2007 Elsevier B.V. All rights reserved.

Metamorphic growth of 1.25-1.29 mu m InGaAs quantum well lasers on GaAs by molecular beam epitaxy

We demonstrate 1.25-1.29 mu m metamorphic laser diodes grown on GaAs by molecular beam epitaxy (MBE) using an alloy-graded buffer layer (GBL). Use of Be in the GBL is effective to reduce surface/interface roughness and improves optical quality. The RMS surface roughness of the optimized metamorphic laser is only two atomic monolayers for 1 x 1 mu m(2). Cross-sectional transmission electron microscopy (TEM) images confirm that most dislocations are blocked in the GBL. Ridge waveguide lasers with 4 mu m wide ridge were fabricated and characterized. The average threshold current under the pulsed excitation is in 170-200 mA for a cavity length of 0.9-1.5 mm. This value can be further reduced to about 100 mA by high-reflectivity coating. Lasers can work in an ambient temperature up to at least 50 degrees C. (c) 2006 Elsevier B.V. All rights reserved.

Unveiling the morphology of buried In(Ga)As nanostructures by selective wet chemical etching: From quantum dots to quantum rings

The three-dimensional morphology of In(Ga)As nanostructures embedded in a GaAs matrix is investigated by combining atomic force microscopy and removal of the GaAs cap layer by selective wet etching. This method is used to investigate how the morphology of In(Ga)As quantum dots changes upon GaAs capping and subsequent in situ etching with AsBr3. A wave function calculation based on the experimentally determined morphologies suggests that quantum dots transform into quantum rings during in situ etching. (c) 2007 American Institute of Physics.

Extremely low density InAs quantum dots with no wetting layer

Extremely low density InAs quantum dots (QDs) are grown by molecular beam droplet epitaxy, The gallium deposition amount is optimized to saturate exactly the excess arsenic atoms present on the GaAs substrate surface during growth, and low density InAs/GaAs QDs (4x10(6) cm(-2)) are formed by depositing 0.65 monolayers (ML) of indium. This is much less than the critical deposition thickness (1.7 ML), which is necessary to form InAs/GaAs QDs with the conventional Stranski-Krastanov growth mode. The narrow photoluminescence line-width of about 24 meV is insensitive to cryostat temperatures from 10 K to 250 K. All measurements indicate that there is no wetting layer connecting the QDs.

Electric field tunable electron g factor and high asymmetrical Stark effect in InAs1-xNx quantum dots

The electronic structure, electron g factor, and Stark effect of InAs1-xNx quantum dots are studied by using the ten-band k center dot p model. It is found that the g factor can be tuned to be zero by the shape and size of quantum dots, nitrogen (N) doping, and the electric field. The N doping has two effects on the g factor: the direct effect increases the g factor and the indirect effect decreases it. The Stark effect in quantum ellipsoids is high asymmetrical and the asymmetry factor may be 319. (c) 2007 American Institute of Physics.

Geometrical-confinement effects on two electrons in elliptical quantum dots

The effects of the geometrical shape on two electrons confined in a two-dimensional parabolic quantum dot and subjected to an external uniform magnetic field have been calculated using a variational-perturbation method based on a direct construction of trial wave functions. The calculations show that both the energy levels and the spin transition of two electrons in elliptical quantum dots are dramatically influenced by the shape of the dots. The ground states with total spin S=0 and S=1 are affected greatly by changing the magnetic field and the geometrical confinement. The quantum behavior of elliptical quantum dots show some relation to that of laterally coupled quantum dots. For a special geometric configuration of the confinement omega(y)/omega(x)=2.0, we encounter a characteristic magnetic field at which spin singlet-triplet crossover occurs. (c) 2007 American Institute of Physics.