Abstract: This work reports on InAs/GaAs quantum dots (QDs) intermixing, induced by phosphorous ion implantation and subsequent rapid thermal annealing. The implantation process was carried out at room temperature at various doses (5 x 10(10)-10(14) ions/cm(2)), where the ions were accelerated at 50 keV. To promote the atomic intermixing, implanted samples are subjected to rapid thermal annealing at 675 degrees C for 30 s. Low temperature photoluminescence (PL) measurements are carried out to investigate the influence of the interdiffusion process on the optical and electronic properties of the QDs. PL emission energy; linewidth and integrated intensity are found to exhibit a drastic dependence on the ion implantation doses. The band gap tuning limit has been achieved for an implantation dose of 5 x 10(13) ions/cm(2). However, our measurement reveals that the accumulated defects for implantation doses higher than 10(12) ions/cm(2) drive the system towards the degradation of the QDs structure's quality. (C) 2008 Elsevier B.V. All rights reserved.

Abstract: In this work, low-temperature photoluminescence (PL) and photoluminescence excitation (PLE) experiments have been carried out to investigate the optical and electronic properties of InAs/GaAs quantum dots (QDs) subjected to room-temperature proton implantation at various doses (5 x 10(10) - 10(14) ions cm(-2)) and subsequent thermal annealing. The energy shift of the main QD emission band is found to increase with increasing implantation dose. Our measurements show clear evidence of an inhomogeneous In/Ga intermixing at low proton implantation doses (<= 5 x 10(11) ions cm(-2)), giving rise to the coexistence of intermixed and non-intermixed QDs. For higher implantation doses, a decrease of both the PL linewidth and the intersublevel spacing energy have been found to occur, suggesting that the dot-size, dot-composition and dot-strain distributions evolve towards more uniform ones.

Abstract: The effect of post-growth rapid thermal annealing on the photoluminescence properties of long wavelength low density InAs/GaAs (001) quantum dots (QDs) with well defined electronic shells has been investigated. For an annealing temperature of 650 degrees C for 30 s, the emission wavelength and the intersublevel spacing energies remain unchanged while the integrated PL intensity increases. For higher annealing temperature, blue shift of the emission energy together with a decrease in the intersublevel spacing energies are shown to occur due to the thermal activated In-Ga interdiffusion. While, this behaviour is commonly explained as a consequence of the enrichment in Ga of the QDs, the appearance of an additional exited state for annealing temperatures higher than 650 degrees C suggests a variation of the intermixed QDs's volume/diameter ratio toward QDs's enlargement. (C) 2007 Elsevier B.V. All rights reserved.

Abstract: Strain-driven phase separation of InAs self-assembled quantum dot's InGaAs heterocapping alloy is investigated by temperature-dependent photoluminescence (PL) spectroscopy and tuned by rapid thermal annealing (RTA) as a means to control the optical properties of such a structure. The integrated PL intensity is found to exhibit an anomalous increase with increasing temperature up to 100 K. This behavior is attributed to the strain-driven phase separation-induced formation of small potential barriers surrounding the quantum dots (QDs) and supported by a rate equation model for the carrier dynamics. After RTA at 650 degrees C during 50 s, an enhancement of the integrated PL intensity, an improvement of the heterocapping alloy PL properties together with the suppression of the anomalous increase of the PL intensity with temperature has been observed. Acting as a reverse phenomenon for the strain-driven alloy decomposition, the thermal induced intermixing is expected to alter only the heterocapping alloy. However, for an annealing temperature (T.) of 750 degrees C, the QDs PL peak is shown to exhibit a weak blue shift and a line width narrowing indicating the beginning of In/Ga interdiffusion. At higher annealing temperature (850 degrees C), large blue shift and broadening of the PL peak occur. (C) 2007 Elsevier B.V. All rights reserved.

Abstract: The effect of rapid thermal annealing on InAs quantum dots (QDs) capped with In0.4Ga0.6As/GaAs layer has been investigated by photoluminescence (PL). An unusual red shift of the PL emission peak has been observed for an annealing temperature (T-a) of 650 degrees C together with a pronounced improvement of the PL from the quantum well like heterocapping layer (QW). This behavior is attributed to the strain induced phase separation of the hetero-capping alloy. However, for T-a = 750 degrees C, a blue shift of the QDs PL peak has been observed with respect to that of the as-grown sample. For this annealing temperature the PL intensity of the QW exceeds that of the QDs indicating a relatively prominent In/Ga interdiffasion. When annealed at 850 degrees C, only the PL arising from the QW can be detected in addition to a broadened low energy side band indicating the dissolution of the QDs at that temperature. (c) 2005 Elsevier B.V. All rights reserved.

Abstract: Vertically stacked multilayers of self-organized InAs/GaAs quantum dots (QDs) structures with different GaAs intermediate layer thicknesses varying between 2.8 and 17 nm are grown by solid source molecular beam epitaxy (SSMBE) and investigated by photoluminescence spectroscopy (PL). For 17 nm thick GaAs spacer, the PL spectra show two well separated features attributed to the formation of two QDs family with a bimodal size distribution indicating no correlation between the dots in different layers. In the meanwhile, the structures having thinner spacer thickness demonstrate single PL peaks showing an enhancement of high energy side asymmetrical broadening when increasing the excitation power. The corresponding emission energies exhibit a red shift when the spacer layer thickness decreases and correlated with the enhancement of the vertical electronic coupling as well as the rise of the QD's size in the upper layers induced by the build up of the strain field along the columns. The spacer thickness of 8.5 nm is found to yield the best optical properties. (c) 2005 Elsevier B.V. All rights reserved.

Abstract: Proton implantation followed by rapid thermal annealing (RTA) has been employed for the post-growth tuning of the band gap of molecular beam epitaxy grown InAs/GaAs quantum dots (QDs). To enhance QD intermixing, point defects are created by proton implantation at different doses (5 x 10(10)-10(14) cm(-2)) followed by rapid thermal annealing at 675 degrees C for 30 s. Low-temperature photoluminescence (PL) measurements have shown that the proton-implantation-induced intermixing alters both the optical transition energies and the PL full width at half maximum (FWHM). A purely proton-implantation induced band gap tuning limit of 131 meV has been achieved for an implantation dose of 5 x 10(13) cm(-2), keeping both the QDs' character and around 46% of the initial integrated PL intensity.

Abstract: Optical and morphological properties of solid source molecular beam epitaxy (MBE) grown InAs/GaAs quantum dots are investigated by photoluminescence spectroscopy (PL) and atomic force microscopy (AFM) as a function of the growth rate. The 10 K relatively high excitation density PL spectra of the investigated samples reveal the existence of multipeaks characterizing fundamental states and corresponding excited states. The emission energies are red-shifted by approximately 70 meV, when the InAs deposition rate is reduced from 0.069 ML/s to 0.013 ML/s. By further reducing the growth rate down to 0.003 ML/s an unexpected blue shift of the emission energy occurs. AFM observations, carried out on similar uncapped samples, show a monotonic decrease of the QDs density with decreasing QDs material's deposition rate. For a growth rate of 0.003 ML/s, islands density of 4.9 x 10(9) cm(-2) has been achieved with a room temperature PL full width at half maximum (FWHM) of 22 meV. For the later growth rate, the blue shift of the emission energy has been correlated with desorption of In atoms during InAs deposition. (c) 2006 Elsevier B.V. All rights reserved.

Abstract: Single InAs/GaAs quantum dots are studied using micro-photoluminescence spectroscopy. Single dot spectroscopy shows an antibinding biexciton. Some of these quantum dots are laterally coupled due to an anisotropic dot repartition. At low excitation power density, four excitons emissions are observed and are interpreted as the four possible exciton recombinations of two coupled quantum dots. (c) 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Abstract: Room temperature 1.3 mu m emitting InAs quantum dots (QDs) covered by an In0.4Ga0.6As/GaAs strain reducing layer (SRL) have been fabricated by solid source molecular beam epitaxy (SSMBE) using the Stranski-Krastanov growth mode. The sample used has been investigated by temperature and excitation power dependent photoluminescence (PL), photoluminescence excitation (PLE), and time resolved photoluminescence (TRPL) experiments. Three emission peaks are apparent in the low temperature PL spectrum. We have found, through PLE measurement, a single quantum dot ground state and the corresponding first excited state with relatively large energy spacing. This attribute has been confirmed by TRPL measurements which allow comparison of the dynamics of the ground state with that of the excited states. Optical transitions related to the InGaAs quantum well have been also identified. Over the whole temperature range, the PL intensity is found to exhibit an anomalous increase with increasing temperatures up to 100 K and then followed by a drop by three orders of magnitude. Carrier's activation energy out of the quantum dots is found to be close to the energy difference between each two subsequent transition energies.

Abstract: Vertically stacked self-assembled InAs/GaAs quantum dots (QDs) structures with different GaAs spacer layer thicknesses are investigated by photoluminescence spectroscopy (PL). For correlated structures, the PL full widths at half maximum (FWHM) is found to go throw a minimum and the PL intensity throw a maximum for a spacer layer thickness around 8.5 nm. The effect of post growth rapid thermal annealing (RTA) on the PL properties of the optimized structure is discussed. (c) 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Abstract: 1.3 mu m room temperature emitting multiple-stacked InAs/ GaAs( 001) quantum dots (QDs) are grown by molecular beam epitaxy ( MBE) and investigated by photoluminescence (PL), polarized photoluminescence (PPL), photoluminescence excitation (PLE), time resolved photoluminescence ( TRPL) and atomic force microscopy AFM. The PL measurement shows that two distinct sets of QDs coexist in the sample. The AFM image of the tenth QDs layer not only confirms the bimodal size distribution of the QDs but also shows that the large QDs are elongated along the [ 1- 10] direction. The former structural information has been verified by PPL. Through the excitation density dependent PL and the detection energy dependent PLE we have evidenced two kinds of QDs within the small size dots population: isolated QDs and laterally coupled QDs with vertically coupled large size QDs. The large size dot population is found to possess a long PL decay time confirming that they are electronically coupled. In the meanwhile the PL decay time of the small QDs is found to be similar to that of single layer QDs. These results would help improve understanding some fundamental properties of an interesting structure for optoelectronic applications.

Abstract: A molecular beam epitaxy (MBE) grown vertically stacked InAs/GaAs(001) quantum dots (QDs) structure emitting at 1.3 mum at room temperature has been investigated by photoluminescence (PL) experiment. The PL measurement has shown the coexistence of bimodal size distribution in the QD ensemble. Furthermore, the large size QDs are found to exhibit an interlayer vertical coupling in addition to a lateral coupling with neighboring small size QDs. A sigmoidal temperature dependence of the large size QD family's PL peak as well as an anomalous decrease of the corresponding PL line width at intermediate temperature range are shown to occur. This behavior has been interpreted in terms of in-plane carrier transfer and repopulation process facilitated by the intermediate small size QDs states. These results can help to improve understanding some fundamental properties of long wavelength vertically stacked InAs/GaAs QDs. (C) 2004 Elsevier Ltd. All rights reserved.

Abstract: We report on the electronic coupling effect on carrier dynamics in InAs/GaAs vertically stacked quantum dot (QD) layers. For this purpose, both pico-second and continuous-wave excitation techniques have been used. We show that for large numbers of QD deposition cycles (greater than or equal to10 planes), lateral coupling effects between the neighbouring QDs of the same InAs layer influence remarkably the electronic structure. Lateral coupling of vertically-aligned QDs results in the appearance of a new photoluminescence (PL) band. This new PL line is associated to the radiative recombination of excitons via electronic states induced by laterally coupled QDs (LCQDs). In the 20-layer sample, this effect is shining by dominating the PL spectrum. A strong increase of the radiative lifetime has been obtained for the laterally coupled QDs. Moreover, strong temperature and power excitation dependencies of the PL decay time have been found. (C) 2004 Elsevier Ltd. All rights reserved.

Abstract: Coherent InAs islands separated by GaAs spacer (d) layers are shown to exhibit self-organized growth along the vertical direction. A vertically stacked layer structure is useful for controlling the size distribution of quantum dots. The thickness of the GaAs spacer has been varied to study its influence on the structural and optical properties. The structural and optical properties of multilayer InAs/GaAs quantum dots (QDs) have been investigated by atomic force microscopy (AFM), transmission electron microscopy (TEM), and photoluminescence (PL) measurements. The PL full width at half maximum (FWHM), reflecting the size distribution of the QDs, was found to reach a minimum for an inter-dots GaAs spacer layer thickness of 30 monolayers (ML). For the optimized structure, the TEM image shows that multilayer QDs align vertically in stacks with no observation of apparent structural defects. Furthermore, AFM images showed an improvement of the size uniformity of the QDs in the last layer of QDs with respect to the first one. The effect of growth interruption on the optical properties of the optimized sample (E30) was investigated by PL. The observed red shift is attributed to the evolution of the InAs islands during the growth interruption. We show the possibility of increasing the size of the QDs approaching the strategically important 1.3 mum wavelength range (at room temperature) with growth interruption after InAs QD deposition.

Abstract: InAs/GaAs vertically stacked self-assembled quantum dot (QD) structures with different GaAs spacer layer thicknesses are grown by solid source molecular beam epitaxy (SSMBE) and investigated by transmission electron microscopy (TEM) and photoluminescence (PL) spectroscopy. An increase in the polarization anisotropy is observed when the spacer layer thickness decreases. For a 10 monolayer (ML) thick inter-dots GaAs spacer, the TEM image shows an increase in the QD size when moving to the upper layer accompanied by the generation of dislocations. Consequently, the corresponding temperature-dependant PL properties are found to exhibit an unusual behaviour. The main PL peak is quenched at a temperature around 190 K giving rise to a broad background correlated with the formation of a miniband in the growth direction due to the strong interlayer coupling. For a thicker GaAs spacer layer (30 ML), multilayer QDs align vertically in stacks with no apparent structural defects. Over the whole temperature range, the excitonic band energies are governed by the Varshni empirical relation using InAs bulk parameters and the PL line width shows a slight monotonic increase. For a thinner GaAs interlayer, the thermal activation energies of the carrier emission out of the quantum dots are found to be considerably small (about 25 meV) due to the existence of defects. By combining these structural and optical results, we can conclude that a thinner GaAs spacer has a poorer quality. (C) 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Abstract: Temperature dependence of the effective band gap (BG) energy of strained InxGa1-xAs/GaAs single-quantum well and multi-quantum well structures grown by solid source MBE at varied substrate temperature is investigated by photoluminescence spectroscopy between 10 K and room temperature. For low-temperature-grown heterostructure, the temperature-induced BG shrinkage exhibits a good correlation with that of unstrained material. However, no consensus is shown to occur for a relatively high-temperature-grown quantum wells (QWs). This discrepancy is interpreted in terms of indium segregation and reevaporation during epitaxy. The low-temperature range, where the well-known Varshni law fails to fit PL peak positions, is found to decrease with increasing QW width and is attributed to the interface-roughness-induced exciton localization. This study was propped by numerical solving of Schrodinger equation taking into account strain, indium segregation and desorption effects. (C) 2002 Elsevier Science B.V. All rights reserved.