Quantum Confinement in InAs/GaAs Systems with Self-Assembled Quantum Dots Grown Using In-Flush Technique (original) (raw)
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The vertical coupling effect on the electronic states in self-assembled InAs/GaAs quantum dots
Solar Energy Materials and Solar Cells, 2006
A series of self-organized InAs/GaAs quantum dots (QDs) were grown by molecular beam epitaxy to investigate the dependence of transition energy on GaAs spacer layer thickness. The latter was varied of 60, 45, 30, 15, and 10 monolayers (MLs) for the five different samples. The photoluminescence (PL) measurements were carried out. The electronic states in coupled selfassembled InAs/GaAs QDs are investigated through PL properties with the aid of the theoretical calculation. First the energy levels of electrons and holes are calculated by solving the threedimensional Schro¨dinger equation by considering the vertical coupling effect between a finite numbers of QDs. Based on the results the energies transitions between electrons and holes levels are calculated. Modification of PL spectra by increasing number of layers was found and attributed to an increasing vertical coupling. 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 MLs. Moreover, the observed behavior PL lines is analyzed theoretically. r
Optical properties of self-assembled InAs quantum dots on high-index GaAs substrates
Superlattices and Microstructures, 1997
In this work we have studied the optical properties of InAs quantum dots (QDs) grown by molecular-beam epitaxy on GaAs (211)A, on (n11)A/B (where n is 1, 5 and 7), and on reference (100) substrates. Investigation of orientation and polarity effects by means of photoluminescence (PL) are also presented. The PL spectra reveal interesting differences in amplitude, integral luminescence, peak position and peak shape. The PL temperature dependence indicates an additional lateral confinement on (100), (n11)B, (211)A and (111)A surfaces. Our results also show an enhancement of the QD onset thermal quenching energy by a factor of ∼ 3 for these orientations. In contrast, the structures grown on (711)A and (511)A surfaces do not exhibit QD formation.
Control of structural and excitonic properties of self-organized InAs/GaAs quantum dots
Physica E: Low-dimensional Systems and Nanostructures, 2006
A systematic dependence of excitonic properties on the size of self-organized InAs/GaAs quantum dots is presented. The bright exciton fine-structure splitting changes from negative values to more than 0.5 meV, and the biexciton binding energy varies from antibinding to binding, as the height of truncated pyramidal dots increases from 2 to above 9 InAs monolayers. A novel mode of metalorganic vapor phase epitaxy was developed for growing such quantum dots with precise shape control. The dots consist of pure InAs and feature heights varying in steps of complete InAs monolayers. Such dot ensembles evolve from a strained, rough twodimensional layer with a thickness close to the critical value for the onset of the 2D-3D transition. Dots with a common height represent subensembles with small inhomogeneous broadening. Tuning of subensemble emission energy is achieved by varying the mean lateral extension of the respective QDs. Detailed knowledge of the structural properties of individual dots enable realistic k. p calculations to analyze the origin of the observed excitonic properties. The binding energies of charged and neutral excitons increase due to correlation by the gradually increasing number of bound states for increasing dot size. The monotonously increasing magnitude of the fine-structure splitting with dot size is shown to be caused by piezoelectricity. The identification of key parameters allows to tailor exciton properties, providing a major step towards the development of novel applications.
Inverted electron-hole alignment in InAs-GaAs self-assembled quantum dots
Physical review letters, 2000
New information on the electron-hole wave functions in InAs-GaAs self-assembled quantum dots is deduced from Stark effect spectroscopy. Most unexpectedly it is shown that the hole is localized towards the top of the dot, above the electron, an alignment that is inverted relative to the predictions of all recent calculations. We are able to obtain new information on the structure and composition of buried quantum dots from modeling of the data. We also demonstrate that the excited state transitions arise from lateral quantization and that tuning through the inhomogeneous distribution of dot energies can be achieved by variation of electric field. PACS numbers: 73.61. -r, 68.90. + g, 73.50.Pz, 78.66. -w 0031-9007͞00͞84(4)͞733(4)$15.00
Hole emission processes in InAs/GaAs self-assembled quantum dots
Physical Review B, 2002
We present a study of the hole emission processes in InAs/GaAs quantum dots using capacitance and admittance spectroscopies. From the conductance mapping, the hole levels show a quasicontinuous distribution, instead of the clear shell structures that have been observed in electron systems. According to a comparative analysis of the capacitance and admittance spectroscopies, the hole emission process is identified to be via thermally activated tunneling through the wetting layer as an intermediate state. An energy level diagram of the quantum dot is also constructed, which shows the hole in our quantum dots to be more weakly confined. We propose a general thermally activated tunneling model to explain our results and those in other works. The conclusion is that both the localization energy and the electric field are important for the carrier emission processes. This model is further extended to predict which carrier type ͑i.e., electron or hole͒ will be more relevant during the exciton dissociation processes in quantum dots.
Quantum size effect in self-organized InAs/GaAs quantum dots
Physical Review B, 2000
The quantum size effect of exciton transitions is investigated experimentally and theoretically for selforganized InAs/GaAs quantum dots ͑QD's͒. Photoluminescence excitation ͑PLE͒ experiments are reported for a series of samples with QD's varying in average size, revealing size-dependent excitation resonances. Temperature-dependent measurements show that the PLE spectra mirror the absorption spectra of QD's with a certain ground state transition energy. The observed PLE resonances are identified based on their energy, relative intensity, and sensitivity to size variations in comparison to results of eight-band k•p calculations for pyramidal InAs/GaAs QD's with ͕101͖ side facets. Band mixing, strain, and the particular geometry of the three-dimensional confinement lead to a rich fine structure with a variety of ''forbidden'' excitonic transitions. A good agreement between experiment and theory is found for large QD's (E det Շ1.1 eV), whereas the agreement becomes worse for smaller QD's. The discrepancies arise, most likely, from the uncertainties in the size-and growth-dependent variations of the QD shape and composition as well as Coulomb-induced localized wetting layer states.
1997
The structural and the optical propertics of lnAs layers grown on high index GaAs surfaces by molecular beam epitaxy are investigated in order to understand the formation and the self-organization of quantum dots (Ql)s) on novel index surfaces. Four different GaAs substrate orientations have been examined, namely, (111)B, (311)A, (311)B and (100). The (100) surface was used as a reference sample. STM pictures exhibit a uniform QI) coverage for all the samples with the exception of (111)B, which displays a surface characterized by very large islands and where STM pictures give no evidence of QD formation. The photoluminescence (PL) *Corresponding author. spectra of GaAs (100) and {311 } samples show typical QD features with PL peaks in the energy range 1.15-1.35eV with comparable efficiency. No significant quenching of PL up to temperatures as high as 70 K was observed. These results suggest that the high index substrates are promising candidates for production of high quality selfassembled QD materials for application to photonics. ,~i 1997 Elsevier Science Ltd.