Comparative study of InAs quantum dots grown on different GaAs substrates by MOCVD

We present a comparative study of InAs quantum dots grown on Si-doped GaAs (10 0) substrates, Si-doped GaAs (10 0) vicinal substrates, and semi-insulating GaAs (10 0) substrates. The density and size distribution of quantum dots varied greatly with the different substrates used. While dots on exact substrates showed only one dominant size, a clear bimodal size distribution of the InAs quantum dots was observed on GaAs vicinal substrates, which is attributed to the reduced surface diffusion due to the presence of multiatomic steps. The emission wavelength is blueshifted during the growth of GaAs cap layer with a significant narrowing of FWHM. We found that the blueshift is smaller for QDs grown on GaAs (10 0) vicinal substrates than that for dots on exact GaAs (100) substrates. This is attributed to the energy barrier formed at the multiatomic step kinks which prohibits the migration of In adatoms during the early stage of cap layer growth. (c) 2005 Elsevier B.V. All rights reserved.

Realization of highly uniform self-assembled InAs quantum wires by the strain compensating technique

Self-assembled InAs quantum wires (QWRs) on InP(001) substrate have been grown by molecular-beam epitaxy, using a strain compensating technique. Atom force microscope, Transmission electron microscopy, and high-resolution x-ray diffraction are used to characterize their structural properties. We proposed that, by carefully adjusting composition of InAlGaAs buffer layer and strain compensating spacer layers, stacked QWRs with high uniformity could be achieved. In addition, the formation mechanism and vertical anti-correlation of QWRs are also discussed. (c) 2005 American Institute of Physics.

Research progress of electronic properties of self-assembled semiconductor quantum dots

Self-assembled semiconductor quantum dot is a new type of artificially designed and grown function material which exhibits quantum size effect, quantum interference effect, surface effect, quantum tunneling-Coulumb-blockade effect and nonlinear optical effect. Due to its advantages of less crystal defects and relatively simpler fabrication technology, this material may be of important value in the research of future nanoelectronic device. In the order of vertical transport, lateral transport and charge storage, recent advances in the electronic properties of this material are brefly introduced, and the problems and perspectives are analyzed.

The structural and photoluminescence character of InAs quantum dots grown on a combined InAlAs and GaAs strained buffer

The structural and photoluminescence (PL) properties of the InAs quantum dots (QDs) grown on a combined InAlAs and GaAs strained buffer layer have been investigated by AFM and PL measurements. The dependence of the critical thickness for the transition from 2D to 3D on the thickness of GaAs layer is demonstrated directly by RHEED. The effects of the introduced-InAlAs layer on the density and the aspect ratio of QDs have been discussed.

The effect of a combined InAlAs and GaAs strained buffer layer on the structure and optical property of InAs quantum dots

The character of InAs quantum dots (QD) directly deposited on a combined InAlAs-GaAs (XML) strained buffer layer (SBL) has been investigated. This growth technique realizes high-density QD (5.88 x 10(10) cm(-2)) by changing the thickness of GaAs in InAlAs-GaAs SBL. The dependence of the density and the aspect ratio of QD on the GaAs thickness has been discussed in detail. The photoluminescence (PL) measurements demonstrate an obvious redshift with the increase of GaAs thickness. In addition, the deposition of InAs QDs grown on the combined InAlAs-GaAs SBL has an important effect of the QD properties. The ordered QD array can be observed from the sample deposited by atomic layer epitaxy, of which the PL peak shows an obvious redshift in comparison to the molecular beam epitaxy (MBE) QDs when the GaAs thicknesses are equal. (c) 2004 Elsevier B.V. All rights reserved.

Electronic structure of self-assembled InAs quantum discs in a magnetic field with varying orientation and two-electron quantum-disc qubits

We have studied the electronic structure of vertically assembled quantum discs in a magnetic field with varying orientation using the effective mass approximation. We calculate the four energy levels of single-electron quantum discs and the two lowest energy levels of two-electron quantum discs in a magnetic field with varying orientation. The change of the magnetic field as an effective potential strongly modifies the electronic structure, leading to splittings of the levels and anticrossings between the levels. The calculated results also demonstrate the switching between the ground states with the total spin S = 0 and 1. The switching induces a qubit controlled by varying the orientation of the magnetic field.

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.

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.

Anomalous photoluminescence of InAs quantum dots implanted by Mn ions

The photoluminescence (PL) of Mn-implanted quantum dot (QD) samples after rapid annealing is studied. It is found that the blue shift of the PL peak of the QDs, introduced by the rapid annealing, decreases abnormally as the implantation dose increases. This anomaly is probably related to the migration of Mn atoms to the InAs QDs during annealing, which leads to strain relaxation when Mn atoms enter InAs QDs or to the suppression of the inter-diffusion of In and Ga atoms when Mn atoms surround QDs. Both effects will suppress the blue shift of the QD PL peaks. The temperature dependence of the PL intensity of the heavily implanted QDs confirms the existence of defect traps around the QDs. (c) 2006 Elsevier B.V. All rights reserved.

Optical transitions of positively charged excitons and biexcitons in single InAs quantum dots

National Basic Research Program of China 2007CB924904;Chinese Academy of Sciences KICX2.YW.W09-1

Temperature-dependent bimodal size evolution of InAs quantum dots on vicinal GaAs(100) substrates

The effects of growth temperature on the bimodal size distribution of InAs quantum dots on vicinal GaAs(100) substrates grown by metal organic chemical vapor deposition are studied. An abnormal trend of the bimodal size evolution on temperature is observed. With the increase of the growth temperature, while the density of the large dots decreases continually, that of the small dots first grows larger when temperature was below 520 degrees C, and then exhibits a sudden decrease at 535 degrees C. The trend is explained by taking into account the presence of multiatomic steps on the substrates. Photoluminescence (PL) studies show that quantum dots on vicinal substrates have a narrower PL linewidth, a longer emission wavelength, and a larger PL intensity than those of the dots with exact substrates. (c) 2006 American Institute of Physics.

MBE InAs quantum dots grown on metamorphic InGaAs for long wavelength emitting

GaAs-based InAs quantum dots using InGaAs composition-graded metamorphic layers have been investigated by molecular beam epitaxy. Emission with the wavelength similar to 1.5 mu m from the dots was obtained at room temperature with the relatively large full width at half maximum. The emission wavelength is relatively stable when subjected to fast annealing. The number density of dots reached similar to 6 x 10(10) cm(-2). Undulated morphology was observed on the surface of the sample, which has some influence on the dot size and distribution. In epilayers, misfit dislocations were confined within the step-graded InGaAs metamorphic buffer layer. (c) 2006 Elsevier B.V. All rights reserved.

Effect of GaAS(100) 2 degrees surface misorientation on the formation and optical properties of MOCVD grown InAs quantum dots

The influence of GaAS(1 0 0)2 degrees substrate misorientation on the formation and optical properties of InAs quantum dots (QDs) has been studied in compare with dots on exact GaAs(1 0 0) substrates. It is shown that, while QDs on exact substrates have only one dominant size, dots on misoriented substrates are formed in lines with a clear bimodal size distribution. Room temperature photoluminescence measurements show that QDs on misoriented substrates have narrower FWHM, longer emission wavelength and much larger PL intensity relative to those of dots on exact substrates. However, our rapid thermal annealing (RTA) experiments indicate that annealing shows a stronger effect on dots with misoriented substrates by greatly accelerating the degradation of material quality. (c) 2005 Elsevier B.V All rights reserved.

Exciton states of vertically stacked self-assembled InAs quantum disks in an axial magnetic field

We have studied the exciton states of vertically stacked self-assembled quantum disks within the effective mass approximation. The ground energies of a heavy-hole and a light-hole excitons as functions of the vertical disk separation are presented and discussed. The transition energy of a heavy-hole ground-state exciton is calculated and compared with the experimental data. The binding energies are discussed in terms of the probability of ground wave function. The ground energies of a heavy-hole and a light-hole excitons as functions of the applied axial magnetic field are calculated and the effect of disk size (radius of disks) on exciton energies is discussed. (c) 2006 Elsevier B.V. All rights reserved.

Optical anisotropy and strain evolution of GaAs surfaces at the onset of the formation of InAs quantum dots

By using reflectance difference spectroscopy we have studied the in-plane optical anisotropy of GaAs surfaces covered by ultrathin InAs layers. The strain evolution of the GaAs surface with the InAs deposition thickness can be obtained. It is found that the optical anisotropy and the surface tensile strain attain maximum values at the onset of the formation of InAs quantum dots (QDs) and then decrease rapidly as more InAs QDs are formed with the increase of InAs deposition. The origin of the optical anisotropy has been discussed.