Characterization of InAs quantum dots on lattice-matched InAlGaAs/InP superlattice structures

Self-assembled InAs quantum dots (QDs) in an InAlGaAs matrix, lattice-matched to InP substrate, have been grown by molecular beam epitaxy (MBE). Transmission electron microscopy (TEM), double-crystal X-ray diffraction (DCXRD) and photoluminescence (PL) are used to study their structural and optical properties. In InAs/InAlGaAs/ InP system, we propose that when the thickness of InAs layer deposited is small, the random strain distribution of the matrix layer results in the formation of tadpole-shaped QDs with tails towards random directions, while the QDs begin to turn into dome-shaped and then coalesce to form islands with larger size and lower density to release the increasing misfit strain with the continuous deposition of InAs. XRD rocking curves showing the reduced strain with increasing thickness of InAs layer may also support our notion. The results of PL measurements are in well agreement with that of TEM images. (C) 2004 Elsevier B.V. All rights reserved.

Thermal annealing effect on InAs/InGaAs quantum dots grown by atomic layer molecular beam epitaxy

The effect of rapid thermal annealing on the InAs quantum dots (QDs) grown by atomic layer molecular beam epitaxy and capped with InGaAs layer has been investigated using transmission electron microscopy and photoluminescence (PL). Different from the previously reported results, no obvious blueshift of the PL emission of QDs is observed until the annealing temperature increases up to 800 degreesC. The size and shape of the QDs annealed at 750 degreesC have hardly changed indicating the relatively weak Ga/In interdiffusion, which is characterized by little blueshift of the PL peak of QDs. The QD size increases largely and a few large clusters can be observed after 800 degreesC RTA, implying the fast interdiffusion and the formation of InGaAs QDs. These results indicate that the delay of the blueshift of the PL peak of QDs is correlated with the abnormal interdiffusion process, which can be explained by two possible reasons: the reduction of excess-As-induced defects and the redistribution of In, Ga atoms around the InAs QDs resulted from the sub-monolayer deposition of InGaAs capping layer. (C) 2004 Elsevier B.V. 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.

Structural and optical properties of 3D growth multilayer InGaN/GaN quantum dots by metalorganic chemical vapor deposition

Multilayer InGaN/GaN quantum dots (QDs) were grown on sapphire substrates through a three-dimensional growth mode, which was initiated by a special passivation processing introduced into the normal growth procedure. Surface morphology and photoluminescence properties of QDs with different stacking periods (from one to four) were investigated. The temperature dependences of the PL peak energies were found to show a great difference between two-layer and three-layer QDs. The fast redshift and the reversed sigmoidal temperature dependences of the PL energies for the former were attributed to the thermally activated carrier transfer from small to large dots. However, the increase of both the dot size and the spatial space among dots with the growing stacking periods reduced the carrier escape and retrapping. (C) 2004 Elsevier B.V. All rights reserved.

Research progress of self-organized Ge quantum dots on Si substrate

A review is presented on recent research development of self-organized Ge/Si quantum dots (QDs). Emphasis is put on the morphological evolution of the Ge quantum dots grown on Si (001) substrate, the structure analysis of multilayer Ge QDs, the optical and electronic properties of these nanostructures, and the approaches to fabricating ordered Ge quantum dots.

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.

Ordering growth of InAs quantum dots on ultra-thin InGaAs strained layer

InAs quantum dots (QDs) were grown On Ultra-thin In0.15Ga0.85As strained layers by molecular beam epitaxy on GaAs (00 1) substrates. Combining reflection high-energy electron diffraction, atomic force microscopy and transmission electron microscopy, we analyzed the stress field of dislocations in the strained layer/substrate interface. Specially, we revealed the relative position of QDs and dislocations. We found that the difference of the stress field around dislocations is prominent when the strained layer is ultra-thin and the stress field will directly affect the following growth. On the strained layer surface, In0.15Ga0.85As ridges will form at the inclined upside of dislocations. Then, InAs QDs will prefer nucleating on the ridges, there is relatively small stress between InAs and In0.15Ga0.85As. By selecting ultra-thin In0.15Ga0.85As layer (50 nm) and controlling the QD layer at just form QDs, we obtained ordered InAs QDs. (C) 2004 Elsevier B.V. All rights reserved.

The fabrication and properties of InAs/GaAs columnal islands

A columnal islands system, which was composed of three layers of self-assembled InAs/GaAs quantum dots (QDs), has been fabricated by solid-source molecular beam epitaxy (MBE) through S-K mode on a (100) semi-insulating GaAs substrate. The effects of the thickness of GaAs space layer, the growth interruption time and the amount of InAs deposition on the emission wavelength of columnal islands were presented. The image of atomic force microscopy (AFM) indicated the columnal islands with high uniformity in size and shape. At room temperature, the emission wavelength of columnal islands with different effective heights was achieved 1.32 and 1.4 mum; however, the emission wavelength of single-layer QDs with normal height was just 1. l mum. It provides a useful and intuitive approach to artificially control the emission wavelength of a QD material system.

Photoluminescence study of InGaN/GaN quantum dots grown on passivated GaN surface

We have measured photoluminescence (PL) and time-resolve photoluminescence (TRPL) from InGaN/GaN quantum dots (QDs) grown on passivated GaN surfaces by metalorganic chemical vapor deposition (MOCVD). Strong PL emission was observed from the QDs structure even at room temperature. By comparing the PL and TRPL dependence on temperature, a significant difference between the QD and wetting layer emissions was revealed. The QD emission is characterized by a strong exciton localization effect, which leads to a larger thermal activation energy, a nearly constant radiative lifetime independent of temperature and an unusual temperature behavior of the PL peak energy. (C) 2003 Elsevier B.V. All rights reserved.

Time-resolved photoluminescence of sub-monolayer InGaAs/GaAs quantum-dot-quantum-well heterostructures

Time-resolved photoluminescence (PL) of sub-monolayer (SML) InGaAs/GaAs quantum-dot-quantum-well heterostructures was measured at 5 K for the first time. The radiative lifetime of SML quantum dots (QDs) increases from 500 ps to 800 ps with the increase of the size of QDs, which is related to the small confinement energy of the excitons inside SML QDs and the exciton transfer from smaller QDs to larger ones through tunneling. The rise time of quantum-dot state PL signal strongly depends on the excitation power density. At low excitation power density, the rise time is about 35 ps, the mechanism of carrier capture is dominated by the emission of longitudinal-optical phonons. At high excitation power density, the rise time decreases as the excitation density increases, and Auger process plays an important role in the carrier capture. These results are very useful for understanding the working properties of sub-monolayer quantum-dot devices.

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.

Electron transport through hierarchical self-assembly of GaAs/AlxGa1-xAs quantum dots

The transmission of electrons through a hierarchical self-assembly of GaAs/AlxGa(1-)xAs quantum dots (QDs) is calculated using the coupled-channel recursion method. Our results reveal that the number of conductance peaks does not change when the barrier widths change, but the intensities decrease as the barrier widths increase. The conductance peaks will shift towards low Fermi energies as the transverse width of GaAs QD increases, as the thickness of GaAs quantum well increases, or as the height of GaAs QDs decreases. Our calculated results may be useful in the application of QDs to photoelectric devices. (c) 2005 American Institute of Physics.

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.

Intersubband optical absorption in quantum dots-in-a-well heterostructures

The theoretical analysis of intersubband optical transitions for InAs/ InGaAs quantum dots-in-a-well ( DWELL ) detectors are performed in the framework of effective-mass envelope- function theory. In contrast to InAs/ GaAs quantum dot (QD) structures, the calculated band structure of DWELL quantitatively confirms that an additional InGaAs quantum well effectively lowers the ground state of InAs QDs relative to the conduction-band edge of GaAs and enhances the confinement of electrons. By changing the doping level, the dominant optical transition can occur either between the bound states in the dots or from the ground state in the dots to bound states in the well, which corresponds to the far-infrared and long-wave infrared (LWIR ) peaks in the absorption spectra, respectively. Our calculated results also show that it is convenient to tailor the operating wavelength in the LWIR atmospheric window ( 8 - 12 mu m ) by adjusting the thickness of the InGaAs layer while keeping the size of the quantum dots fixed. Theoretical predictions agree well with the available experimental data. (c) 2005 American Institute of Physics.

Singlet-triplet transition in quantum dots confined by triangular and bowl-like potentials: the effect of electric fields

We theoretically investigate the energy spectra of two-electron two-dimensional (2e 2D) quantum dots (QDs) confined by triangular potentials and bowl-like potentials in a magnetic field by exact diagonalization in the framework of effective mass theory. An in-plane electric field is,found to contribute to the singlet-triplet transition of the ground state of the 2e 2D QDs confined by triangular or bowl-like potentials in a perpendicular magnetic field. The stronger the in-plane electric field, the smaller the magnetic field for the total spin of the ground states in the dot systems to change from S = 0 to S = 1. However, the influence of an in-plane electric field on the singlet-triplet transition of the ground state of two electrons in a triangular QD modulated by a perpendicular magnetic field is quite small because the triangular potential just deviates from the harmonic potential well slightly. We End that the strength of the perpendicular magnetic field needed for the spin singlet-triplet transition of the ground state of the QD confined by a bowl-like potential is reduced drastically by applying an in-plane electric field.