Detecting Spatially Localized Exciton in Self-Organized InAs/InGaAs Quantum Dot Superlattices: a Way to Improve the Photovoltaic Efficiency
© The Author(s). 2017
Received: 17 March 2017
Accepted: 28 June 2017
Published: 11 July 2017
This paper reports on experimental and theoretical investigations of atypical temperature-dependent photoluminescence properties of multi-stacked InAs quantum dots in close proximity to InGaAs strain-relief underlying quantum well. The InAs/InGaAs/GaAs QD heterostructure was grown by solid-source molecular beam epitaxy (SS-MBE) and investigated via photoluminescence (PL), spectroscopic ellipsometry (SE), and picosecond time-resolved photoluminescence. Distinctive double-emission peaks are observed in the PL spectra of the sample. From the excitation power-dependent and temperature-dependent PL measurements, these emission peaks are associated with the ground-state transition from InAs QDs with two different size populations. Luminescence measurements were carried out as function of temperature in the range of 10–300 K by the PL technique. The low temperature PL has shown an abnormal emission which appeared at the low energy side and is attributed to the recombination through the deep levels. The PL peak energy presents an anomalous behavior as a result of the competition process between localized and delocalized carriers. We propose the localized-state ensemble model to explain the usual photoluminescence behaviors. The quantitative study shows that the quantum well continuum states act as a transit channel for the redistribution of thermally activated carriers. We have determined the localization depth and its effect on the application of the investigated heterostructure for photovoltaic cells. The model gives an overview to a possible amelioration of the InAs/InGaAs/GaAs QDs SCs properties based on the theoretical calculations.
KeywordsInAs quantum dots Molecular beam epitaxy Optical transitions Photoluminescence Picosecond time-resolved photoluminescence Spectroscopic ellipsometry Localized-state ensemble model
Self-assembled quantum dots (QDs) have been widely investigated for possible applications in optoelectronics due to the nature of three-dimensional carrier confinement and the δ-like density of states. Recently, QD structures were proposed to realize the intermediate band solar cells (IBSCs), which introduce extra photo-carriers through the valence-IB and IB-conduction band absorptions . The GaAs-based IBSCs with QDs that have smaller energy band gap than GaAs form tandem structures which are able to absorb photons at energies lower than the GaAs energy gap resulting in higher energy conversion efficiencies . The formation of QD intermediate band needs a close-packed multiple layer structure of high-density QDs [3, 4]. However, the crystal quality of InAs QDs degrades as the QD layer number increases and layer spacing decreases owing to the buildup of internal compressive strain. The excessive strain will induce dislocations and defects that thread up from the QDs toward the surface. Therefore, the performance of an InAs/GaAs QD SC also degrades as the number of QD layers increases . To overcome these problems, a strain compensation growth technique has been demonstrated with GaAsN, GaAsP, and GaP buffer layer for InAs/GaAs material systems [6–8]. Another technique to overcome these problems is to cover InAs/GaAs QDs layer with a thin InGaAs strain-reduced layer. Compared to InAs/GaAs QDs, this layer causes a redshift to the photo-response due to the presence of a small lattice mismatch between InAs and InGaAs. The temperature-dependent photoluminescence study provides useful information about the multi-stacked InAs/GaAs QDs SC which is of considerable practical and theoretical interest. Classically, the band gap of a semiconductor material reduces monotonically with increasing temperature. Special materials, such as InAs/GaAs QDs, have shown an anomaly in the PL at low temperatures due to thermally activated carrier transfer mechanisms within the ensemble of the quantum dots. However, these abnormalities disappear progressively after post-growth intermixing processes in the InAs/InGaAs/GaAs QD heterostructures as shown by Ilahi et al. . Heterostructures similar to those of the present study have been investigated for their efficiency in photovoltaic applications by Sayari et al. . Many models have been proposed during the last decades, such as the Passler, Vina, and Varshni one. In order to produce reliable devices, temperature behavior of such kind of InAs/InGaAs/GaAs QD heterostructures must be well understood and this is by the use of the best fitting model. We hereby use the Passler classical model corrected to the thermal redistribution coefficient, in order to better understand the observed S-shape temperature dependence of the excitonic band gap. Our study gives rise to a self-consistent precise picture for carrier localization and transfer in an InAs/InGaAs/GaAs QD heterostructure, which is an extremely technologically important energy material for fabricating high-efficiency photovoltaic devices.
Spectroscopic ellipsometry (SE) was performed at room temperature between 1 and 6 eV, using a J.A. Woollam variable angle spectroscopic ellipsometer (VASE) M-2000. The SE measurements were performed at angles of incidence ranging from 45° to 60°. In PL measurements, an argon ion (Ar+) laser with a wavelength of 514.5 nm was used as an excitation source to generate electron-hole pairs. The luminescence light from the samples was dispersed by a high-resolution spectrometer and detected by a thermoelectrically cooled Ge photo-detector with a built-in amplifier. For the excitation power-dependent and temperature-dependent PL measurements, the samples were mounted in a closed-cycle, temperature-controlled helium cryostat. The PL spectra were taken in the nominal output power range of 1.5–350 mW and the temperature range of 11–300 K. The time-resolved PL measurements were performed in a variable-temperature (10–240 K), closed-cycle helium cryostat. The 514 nm line was used as an excitation wavelength, from a mode-locked Ti: sapphire picosecond pulse laser at a repetition rate of 80 MHz with a 1.2 ps pulse width.
Results and Discussions
For further understanding the recombination process in InAs/InGaAs/GaAs multi-stacked QDs, we have studied the time-resolved PL using the photocounting time-correlated technique. It was predicted theoretically that the exciton decay lifetime of QDs is sensitive to temperature . Experimental measurements have shown that the lifetimes are indeed a constant of temperature below a critical temperature . Markus et al.  reported a constant lifetime of about 950 ps over a wide range of temperature within the experimental error.
Parameters used to fit the energy evolution using empirical Passler (a) and modified Passler (b) model (LSE)
E 0 (eV)
E ch-E 0 (eV)
α (10−4 eV/K)
τ r/τ tr
19 × 10−3
4 × 10−3
In conclusion, we have successfully fabricated GaAs-based SC with a multi-stack of InAs QDs by capping an InGaAs layer on the QDs and inserting GaAs spacer layers. The two major spectral features observed in the dielectric function spectra of the InAs/InGaAs/GaAs QD heterostructure at 3 and 4.5 eV are attributed to the E 1 and E 2 CP structures of GaAs and InAs, respectively. The PL spectrum of the InAs QDs in the GaAs matrix is intense and presents an asymmetric shape, which indicates the growth of a high-quality, multi-stacked InAs QD structure. The contribution of larger and relatively smaller QDs to the PL spectrum is also demonstrated. The luminescence measurements were successfully modeled and re-interpreted using the developed LSE model. The theoretical study has quantitatively interpreted the observed temperature-dependent spectra, and has shed light on the complicated spontaneous emission mechanisms in multi-stacked InAs/InGaAs/GaAs QDs, based on the fitting parameters. This study suggests a way to improve the efficiency of InAs/GaAs QD structures for their use in photovoltaic applications. These results help to improve the understanding of the temperature-dependent carrier dynamics in strain-engineering QDs in order to improve the efficiency of the investigated structure. Further to this work, we will study the effect of orientation as well as the increase in the number of InAs/GaAs QDs of the multi-stack structure on the localization depth.
This work was supported by the Université de Monastir, Laboratoire de Micro-Optoélectronique et Nanostructures (LMON), Faculté des Sciences, 5019, Monastir, Tunisia.
All sources of funding for the research reported should be declared. We admit that no other funding should be declared except grants from Université de Monastir, Laboratoire de Micro-Optoélectronique et Nanostructures, which were added in the Acknowledgements section, for the funding, interpretation of the data, and writing of the manuscript.
ME, TH, MHHA, NC, and CB-C carried out the PL measurements and data analysis and drafted most of the manuscript. ME and LS participated in the MBE growth of InAs/InGaAs QDs pair sample. AS, ES, and AA Al-G participated in the acquisition of the SE measurements and the discussions of the results. ME, TH, MHHA, AS, ES, NC, LS, FS, AA Al-G, CB-C, and HM participated in the discussions and supervised the writing of the manuscript. All authors read and approved the final manuscript.
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