Carrier Recombination in InAs/GaAs Self-Assembled Quantum Dots under Resonant Excitation Conditions (original) (raw)
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Influence of the InAs coverage on the phonon-assisted recombination in InAs/GaAs quantum dots
Surface Science, 2002
In this work, a study of the relaxation mechanisms in InAs self-assembled quantum dots have been performed by means of photoluminescence (PL), resonant PL (RPL) and PL excitation. The observed phenomenology is different for samples with different InAs coverage. At low InAs coverages, two resonant Raman scattering lines are observed when the excitation energy is chosen inside the emission band. These lines are related to the TO and LO GaAs phonons. Multi-phonon relaxation of carriers (GaAs phonons) is clearly observed in RPL and excitation PL in the lowest InAs coverage sample. Neither such mechanism nor resonant Raman scattering is present in samples with larger InAs coverages, for which the average quantum dot size and areal density remain essentially constant. Ó
Fröhlich interaction in InAs/GaAs self-assembled quantum dots
Physical Review B, 2001
The phonon bottleneck in self-assembled InAs/GaAs quantum dots ͑SAD's͒ is observed directly in continuous-wave photoluminescence experiments when exciting one GaAs longitudinal optical ͑LO͒-phonon energy above the ground level of the smallest dot. To overcome the phonon bottleneck, selective photoluminescence ͑PL͒ experiments are performed and multiple phonon-assisted radiative bands are observed. We found that no real crystal states are involved in the experimentally observed phonon emission. Under nonresonant excitation at 5 K, the SAD's photoluminescence band is centered at 1.315 eV. As proven by our photoluminescence experiments at high excitation densities, there are no excited states in such small dots. We interpret the phonon-assisted PL as being due to enhanced Fröhlich interaction between strain-induced polarized excitons in the SAD's and LO phonons. Further experimental support for this model is found from the cleaved-side PL measurements. A light-hole ground state is observed, instead of the theoretically predicted heavy-hole one.
Phonon-enhanced intraband transitions in InAs self-assembled quantum dots
Journal of Luminescence, 2000
Phonon-mediated intraband carrier relaxation was studied in InAs/GaAs self-assembled quantum dots (QDs) by combining resonant photoluminescence and photoluminescence excitation experiments with high spectral resolution. The phonon-related resonances were found to dominate both the luminescence and excitation spectra. Spectral widths of 1LO-phonon peaks are surprisingly small and comparable with the inverse lifetime of the LO phonons in QDs, whereas, the multiphonon resonances are much broader most likely re#ecting the width of n-phonon density of states.
Resonant excitation of intraband absorption in InAs/GaAs self-assembled quantum dots
We have investigated the infrared absorption between confined levels in the conduction and valence bands of undoped InAs/GaAs self-assembled quantum dots. The intraband absorption, which is measured by photoinduced spectroscopy, is analyzed under resonant and nonresonant optical excitation of the quantum dots. The assignment of electron and hole intraband transitions is achieved on the basis of experimental results obtained with n-and p-doped quantum dots. A careful analysis of the absorption spectra shows that several hole transitions and one electron transition with a large broadening are evidenced in the mid-infrared spectral range. We show that the amplitude of the intraband absorption depends on the pump excitation wavelength and exhibits a maximum when the dots are populated via the wetting layer. The spectral shape of the hole intraband absorption is very weakly dependent on the excitation wavelength. The amplitude of the photoinduced hole intraband absorption exhibits a sublinear behavior with the pump intensity. This feature is explained by the state filling of the quantum dots.
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.
Photocurrent Spectroscopy of InAs/GaAs Self-Assembled Quantum Dots
physica status solidi (b), 2001
Photocurrent ͑PC͒ spectroscopy is employed to study several important aspects of the interband optoelectronic properties of InAs/GaAs self-assembled quantum dots ͑SAQDs͒. PC spectroscopy is first shown to be a highly sensitive, quantitative technique to measure the interband absorption spectra of SAQDs. Up to four well-defined features are observed in the spectra arising from transitions between confined hole and electron levels. The transition energies are shown to agree well with those observed in electroluminescence, with negligible Stokes shifts found, contrary to previous reports. Large quantum-confined Stark shifts of the transitions are observed in PC spectroscopy as discussed in detail elsewhere ͓Fry et al. Phys. Rev. Lett. 84, 733, ͑2000͔͒. Discrete interband transitions are observed superimposed on a broad background signal, shown to arise in part from field-dependent transitions into tail states of the two-dimensional wetting layer and the GaAs cladding region. A field-independent contribution to the background is also found, possibly from dots with larger size and shape fluctuations than those which give rise to the resolved interband transitions. By comparison of photocurrent signals from quantum dots and the wetting layer within the same sample, it is demonstrated that the quantum-dot oscillator strength is not significantly modified relative to that of a quantum well of the same surface area, consistent with performance found from quantum-dot laser devices. Polarization studies for in-plane light propagation are reported. The measurements show that the observed interband transitions involve predominantly heavy-hole-like levels, consistent with an assumption of theoretical modelling of Stark-effect results. Finally carrier escape mechanisms from the dots are deduced, with tunneling found to dominate at low temperature, and thermally activated escape becoming increasingly important at temperatures above ϳ100 K. Carrier escape is shown to occur from a common level, the ground state, demonstrating that excited-state to ground-state relaxation is faster than direct excited-state escape.
Materials Science and Engineering: B, 2021
Photoluminescence (PL) measurements are presented for self-assembled InAs/GaAs quantum dots (QDs) grown by molecular beam epitaxy (MBE) at a growth temperature of 510°C. Two well-defined sub-bands were observed from the 8K-PL spectrum obtained under very low excitation density and unambiguously clarified as optical emissions from the ground state (GS) and first excited state (FES) of the dots. Temperature-dependent PL measurements were investigated in the 8-270K temperature range. Differently from the FES transition, a sigmoidal temperature-dependent variation was observed from the integrated PL intensity of the GS transition. This anomalous behavior was assigned to the carrier exchange between excited states and GS of the dots. A simple rate equation model which takes into account the effects of the thermal escape and re-trapping of photo-injected carriers was proposed to describe the temperature-dependent variation of the integrated PL intensity. A good agreement between the model simulation and the experimental results was obtained for temperatures ranging from 8 to 270K and which supports the argument for the carrier exchange between the excited states and the ground state. ⁕Corresponding author.
Absorption and Emission Spectroscopy of Self-Assembled Inas/Gaas Quantum Dots
Absorption and emission processes are studied in self-organised InAs/GaAs quantum dots using the spectroscopic techniques of photocurrent (PC) and electroluminescence (EL). Two types of dots are studied, grown using different deposition rates. Clear evidence for state filling and saturation is observed in EL spectra recorded as a function of injection current. The dependence of the transition intensities on current is found to be well described by a recently proposed model. Magneto-optical measurements in fields up to 14T provide information on the nature of the optical transitions, the spatial extent of wavefunctions and allow the determination of effective masses.
Journal of Luminescence, 2019
In this work we study, using photoluminescence (PL) and photomodulated transmission experiments (PT) we investigate the eletronic states of InAsP/GaAs QDs as a function of excitation power. PL measurements were performed from 15 K to 290 K, with the excitation power varying from 2 to 30 W /cm 2. We were able to identify the recombination from the QDs ground state from PL experiments and the contribution of the InAsP wetting layer (WL) from PT experiments which has never been observed for these QDs. Specifically, for the InP material, WL recombination is so close in energy to the QD transition that it is possible that both are seen simultaneously as a single PL band. Also, for the InAsP samples with higher As contents excited QD states were identified, since the confining potential is deeper in this situation. The WL presents an isotropic behavior, as expected. Modulated transmission shows spectral structures with different ranges of line widths, which are consistent with the QD and WL assignments given after the PL analysis.
Dynamics of photo-excited carriers in self-assembled quantum dots
Carrier dynamics in InAs/GaAs self-assembled quantum dots have been studied by using time-resolved photoluminescence experiment. We have studied a series of doped quantum dot structures by looking at the role of the experimental conditions, such as the laser excitation intensity, the crystal temperature and the intersublevel energy, on the carrier relaxation time. For all samples, we have found two distinct relaxation regimes. At a crystal temperature of 77K, a rise time of the quantum dot emission signal of a few tens of ps has been measured under low photocarrier densities (≤ 2 photocarriers per dot). This rise time decreases significantly, down to few ps, as the laser intensity increases. These results show that carrier-carrier scattering processes play a significant role at high photo-excited carrier densities. Under the low-excitation regime, the dot emission rise time depends on the dopant type, on the doping level on the degree of intermixing and on the temperature. Our results obtained on structures having a relatively low density of dots indicate that transport processes (diffusion and localization at the InAs/GaAs interfaces) limit the dot capture efficiency at low temperatures (T < 80 K). The experimental conditions and the dot structural parameters that give rise to ultrafast capture and intra-dot relaxation times are discussed.