Anomalous coarsening of self-assembled InAs quantum dots on vicinal GaAs (100) substrates

The formation process of InAs quantum dots (QDs) on vicinal GaAs (1 0 0) substrates is studied by atomic force microscopy (AFM). It is found that after 1.2 MLs of InAs deposition, while the QDs with diameters less than the width of the multi-atomic steps are shrinking, the larger QDs are growing. Photoluminescence measurements of the uncapped QDs correspond well to the AFM structure observations of the QDs. We propose that the QDs undergo an anomalous coarsening process with modified growth kinetics resulting from the restrictions of the finite terrace sizes. A comparison between the QDs on the vicinal GaAs (1 0 0) substrates and the QDs on the exact GaAs (1 0 0) further verifies the effect of the multi-atomic steps on the formation of QDs.

Shape stability of InAs self-assembled islands on vicinal GaAs(001) substrates

We have grown InAs self-assembled islands on vicinal GaAs( 001) substrates. Atomic force microscopy and photoluminescence studies show that the islands have a clear bimodal size distribution. While most of the small islands whose growth is limited by the width of one multi-atomic step have compact symmetric shapes, a large fraction of the large islands limited by the width of one step plus one terrace have asymmetric shapes which are elongated along the multi-atomic step lines. These results can be attributed to the shape-related energy of the islands at different states of their growth. (C) 2008 Elsevier B. V. All rights reserved.

Quantum-confined Stark effect and built-in dipole moment in self-assembled InAs/GaAs quantum dots

Quantum-confined Stark effect and built-in dipole moment in self-assembled InAs/GaAs quantum dots (QDs), which are grown at relative low temperature (460degreesC) and embedded in GaAs p-i-n structure, have been studied by dc-biased electroreflectance. Franz-Keldysh oscillations from the undoped GaAs layer are used to determine the electric field under various bias voltages. Stark shift of -34 meV for the ground-state interband transition of the QDs is observed when the electric field increases from 105 to 308 kV/cm. The separation of the electron and hole states in the growth direction of 0.4 nm, corresponding to the built-in dipole moment of 6.4x10(-29) C m, is determined. It is found that the electron state lies above that of the hole, which is the same as that predicted by theoretical calculations for ideal pyramidal InAs QDs. (C) 2004 American Institute of Physics.

Electronic structure of self-assembled InAs quantum disks in an axial magnetic field and two-electron quantum-disk qubit

We have studied the single-electron and two-electron vertically assembled quantum disks in an axial magnetic field using the effective mass approximation. The electron interaction is treated accurately by the direct diagonalization of the Hamiltonian matrix. We calculate the six energy levels of the single-electron quantum disks and the two lowest energy levels of the two-electron quantum disks in an axial magnetic field. The change of the magnetic field strongly modifies the electronic structures as an effective potential, leading to the splitting of the levels and the crossings between the levels. The effect of the vertical alignment on the electronic structures is discussed. It is demonstrated that the switching of the ground-state spin exists between S=0 and S=1. The energy difference DeltaE between the lowest S=0 and S=1 states is shown as a function of the axial magnetic field. It is also found that the variation of the energy difference between the lowest S=0 and S=1 states in the strong-B S=0 state is fairly linear. Our results provide a possible realization for a qubit to be fabricated by current growth techniques. (C) 2004 American Institute of Physics.

Electronic transport through self-assembled quantum disks

In this paper we study a single electron tunneling through a vertically stacked self-assembled quantum disks structure using a transfer matrix technique in the framework of effective mass approximation. In the disks, the electron is confined both laterally and vertically; we separate the motion in the vertical and lateral directions within the adiabatic approximation and treat the energy levels of the latter as an effective confining potential. The influence of a constant applied electric field is taken into account using an exact Airy-function formalism and the current density is calculated at zero temperature. By increasing the widths of the barriers, we find the peaks of the current density shift toward lower voltage region; meanwhile, they can become even sharper. (C) 2004 Elsevier Ltd. All rights reserved.

Luminescence properties of self-assembled InAs/GaAs quantum dots covered by InAlAs and InGaAs combination strain-reducing layer

We report the photoluminescence (PL) and structural properties of self-assembled InAs/GaAs quantum dots (QDs) covered by In0.2Al0.8As and In0.2Ga0.8As combination strain-reducing layer (SRL). By introducing a thin InAlAs layer, the ground state emission wavelength redshifts, and the energy splitting between the ground and first-excited states increases to 85 meV at 10 K. The energy splitting further increases to 92 meV and the temperature dependence of full width at half maximum (FWHM) changes for QDs with different SRL after the multi-stacking. These results are attributed to the fact that the combination layer has different effects on QDs compared to the InGaAs SRL.

Effect of the InAlAs and InGaAs combination strain-reducing layer on 1.3 mu m emission self-assembled InAs/GaAs quantum dots

Self-assembled InAs quantum dots (QDs) with differing deposition thicknesses covered by InxAl1-xAs (x = 0.2, 0.3) and In0.2Ga0.8As combination strain-reducing layers (CSRLs) were grown by molecular beam epitaxy. Their structural and optical properties were investigated by atomic force microscopy and photoluminescence spectroscopy, respectively. The emission peak position of InAs QDs capped by CSRL can reach 1.34 mum at room temperature with a relatively larger energy splitting of 93 meV between the ground and first excited states.

Raman scattering of folded acoustic phonons in self-assembled Si/Ge dot superlattices

Self-assembled Si/Ge dot multilayers with small, uncorrelated dots fabricated by molecular beam epitaxy in the Stranski-Krastanov growth mode are studied by Raman scattering of folded longitudinal acoustic (FLA) modes. The FLA Raman spectra are analyzed and modeled with respect to mode frequencies and the spectral envelope of mode intensities. The deduced average superlattice properties are consistent with results from atomic force microscopy. The simple Rytov model used for Si/Ge layer structures reproduces very well the frequencies of the FLA modes up to 150 cm(-1). The nonlinearity of phonon dispersion curves in bulk Si for large momenta, however, becomes important for modeling the higher frequencies of observed FLA modes up to 22nd order. The effective dot layer width and an activation energy for thermal intermixing of 2.1+/-0.2 eV are determined from the spectral envelopes of FLA mode intensities of as-grown and annealed Si/Ge dot multilayers. (C) 2004 American Institute of Physics.

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. (C) 2004 American Institute of Physics.

Structural and optical properties of InAs/In0.52Al0.48As self-assembled quantum wires on InP(001)

Six-stacked InAs/In0.52Al0.48As self-assembled quantum wires (QWRs) on InP(001) by molecular-beam epitaxy (MBE) have been studied by high-resolution transmission electron microscopy (HRTEM) and polarized PL measurements. We obtained the chemical lattice fringe (CLF) image of InAs self-assembled QWRs embedded in the In0.52Al0.48As matrix by the interference between the (002)-diffracted beam and the transmitted beam in the image plane of the objective lens. The results show that the InAs QWRs were bounded by (113), (001) and (114) facets. Both the size and strain distribution in QWRs were determined. It was found that with the growth of successive periods, the height and height fluctuation of InAs QWRs decreased from the bottom period to the upper one. Some suggestions are put forward for further improving the uniformity of the stacked InAs QWRs. (c) 2005 Elsevier B.V. All rights reserved.

Artificial nanograting woven by self-assembled nanowires

We report on a new simple route to realize a high resolution nanograting. By adopting an InAlGaAs matrix and strain-compensated technique, we have proved that a uniform self-assembled InAs nanowire array can be fabricated by molecular beam epitaxy (MBE). A nanograting woven by self-assembled semiconductor nanowires shows a conspicuous diffraction feature. The good agreement between the theoretical and experimental values of diffraction peak positions indicates that a uniform nanowire array is a promising nanograting. This simple one-step MBE growth method will open exciting opportunities for the field of clever optics design.

1.3 mu m InAs/GaAs self-assembled quantum dots grown on In0.2Ga0.8As-GaAs combined strain-buffer layer

Optical properties and surface structures of InAs/CaAs self-assembled quantum dots (QDs) grown on 2 nm In-0.2 Ga0.8As and x ML GaAs combined strain-buffer layer were investigated systematically by photoluminescence ( PL) and atomic force microscopy (AFM). The QD density increased from similar to 1.7 x 10(9) cm(-2) to similar to 3.8 x 10(9) cm(-1) due to the decreasing of the lattice mismatch. The combined layer was of benefit to increasing In incorporated into dots and the average height-to-width ratios, which resulted in the red-shift of the emission peaks. For the sample of x = 10 ML, the ground state transition is shifted to 1350 nm at room temperature.

Thermal quenching of luminescence from buried and surface InGaAs self-assembled quantum dots with high sheet density

Variable-temperature photoluminescence (PL) spectra of Si-doped self-assembled InGaAs quantum dots (QDs) with and without GaAs cap layers were measured. Narrow and strong emission peak at 1075 nm and broad and weak peak at 1310 nm were observed for the buried and surface QDs at low temperature, respectively. As large as 210 meV redshift of the PL peak of the surface QDs with respect to that of the buried QDs is mainly due to the change of the strain around QDs before and after growth of the GaAs cap layer. Using the developed localized-state luminescence model, we quantitatively calculate the temperature dependence of PL peaks and integrated intensities of the two samples. The results reveal that there exists a large difference in microscopic mechanisms of PL thermal quenching between two samples. (c) 2005 American Institute of Physics.

Raman study on self-assembled InAs/InAlAs/InP(001) quantum wires

The phonons of self-assembled InAs/InAlAs/InP quantum wires (QWRs) have been studied by Raman scattering. The QWR LO phonons show an unusual frequency shift with the increase of the InAs deposited thickness due to dislocations. The QWR LO phonons are found to follow the selection rule of the LO phonons in bulk zinc-blende semiconductors. Because of the intermixing of In/Al atoms and the multiplication of dislocations, the post-growth thermal annealing treatment leads to a shift of the QWR LO phonons to lower frequency.

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.