Experimental Control of Macroscopically Large, Schrodinger Cat like Quantum Coherent State of Bose-Einstein Condensate of Excitons as Qubits

In this work, we varied the number density of photo generated and bias driven excitons by changing the applied bias voltage and photo excitation intensity. Phase coherent periodic oscillations in photo generated capacitance as a function of the bias voltage and light intensity are measured respectively over a macroscopically large area. We then argue that presence of coherent resonant tunnelling in this well-dot heterostructure strongly restricts the available momentum space of the charge carriers within this quantum well. Consequently, the measured average electric polarization vector of the associated indirect excitons spontaneously increase below 100 K as excitonic dipoles chooses a particular phase and collectively orients along the direction of applied bias. As a result, these excitons continuously undergo Bose-Einstein condensation below a transition temperature as the density threshold is approached at regular bias intervals. Moreover, periodic presence and absence of splitting of excitonic peaks in the optical spectra based on photocapacitance confirm such tunneling induced variations in quantum coupling of electrons between quantum well and quantum dots. Observation of negative quantum capacitance due to screening of charge carriers by the quantum well indicate periodic variation of Coulomb correlations of interacting excitons with increasing bias as a precursor to condensation and vice versa. Generation of density dependent enhancement of quantum interference beats in photocapacitance oscillation with bias even under incoherent white light further confirm the presence of stable, long range spatial correlation among these indirect excitons as well as the existence excitonic matter waves. We also detected collective Rabi oscillations of these macroscopically large, multipartite, two-level, coupled and uncoupled quantum states of excitonic condensate which can be used as qubits.Comment: 43 pages, Manuscript + 11 Figure

Role of interface potential barrier, Auger recombination and temporal coherence in In0.5Ga0.5As/GaAs quantum dots based p-i-n light emitting diodes

© 2018 IOP Publishing Ltd. In this work, we investigate the mechanisms that control the electroluminescence from p-i-n heterostructures containing self-assembled In 0.5 Ga 0.5 As quantum dots embedded inside a GaAs/Al 0.3 Ga 0.7 As quantum well as a function of temperature and applied bias. Our results reveal that the carrier dynamics at the interface between the quantum dot and the quantum well play a crucial role in the electroluminescence emission. At low temperatures, two distinct emission bands are observed. Initially at low bias current, we observe broad emissions from the quantum wells and wetting layers. Another dominant and sharp emission at lower energy arises from the quantum dots, but only at higher bias currents. We discuss how a potential barrier between the quantum dots and quantum well can control the density of injected carriers undergoing optical recombination. We have also investigated the role of carrier capture and escape, quantum-confined stark effect and band-filling effects in the electroluminescence emission. In addition, we demonstrate how measurements of temporal coherence of individual spectral peaks, can detect the presence of Auger recombination in quantum dots under high injection currents. Interestingly, a significant increase in the temporal coherence of quantum dot emissions is observed, which could be due to a decrease in Auger recombination with increasing temperature

Adsorption Thermodynamics, Modeling, and Kinetics Studies for the Removal of Lead Ions Using ZnO Nanorods

In the present investigation, zinc oxide nanorods (ZnO-NR) were synthesized via the hydrothermal method using ZnCl$_2$ as a zinc ion precursor in the presence of cetyltrimethylammonium bromide. Synthesized ZnO-NR was featured using advanced techniques including XRD, PL, SEM, and UV-visible spectroscopy. The role of these assynthesized ZnO-NR was evaluated for the sequestration of lead ions in batch mode. The elimination of lead ions was achieved at pH 6-7 using a 0.06 g adsorbent dose, 25 min contact time, 25 mg/L initial lead ion concentration, 323 K temperature, and 200 rpm agitation speed. A thermodynamic study revealed the endothermic nature of lead ion sequestration onto ZnO-NR. The lead ion sequestration followed kinetic (pseudo-second-order) and isotherm (Langmuir) models. The lead ions were eliminated up to 142 mg/g at the optimum level of affecting variables. The ZnO-NR might be a potential adsorbent for lead ion removal from industrial effluents

Effect of indium doping on the electrical and structural properties of TiO2 thin films used in MOS devices

We investigated the effect of Indium (In) doping on the structural and electrical properties of Ti/Au/ TiO2:In/n-Si metal-oxide-semiconductor (MOS) devices. Sputtering grown TiO2 thin films on Si substrate were doped using two In-films with 15 nm and 50 nm thicknesses leading to two structures named Low Indium Doped (LID) sample and High Indium Doped (HID) sample, respectively. XRD analysis shows no diffraction pattern related to Indium indicating that In has been incorporated into the TiO2 lattice. Current-Voltage (I-V) characteristics show that rectification ratio at 2V is higher for HID sample than for LID sample. Evaluated barrier height, ϕB0 , decreased while the ideality factor, n, increased with decreasing temperature. Such behavior is ascribed to barrier inhomogeneity that was assumed to have a Gaussian Distribution (GD) of barrier heights at interface. Evidence of such GD was confirmed by plotting ϕB0versus n. High value of mean barrier ϕ̅B0 and lower value of standard deviation (σ) of HID structure are due to indium doping which increases the barrier homogeneities. Finally, estimated Richardson constants A* are in good agreement with theoretic values (112 A/cm2K2), particularly, for the HID structure

Effect of erbium-doping concentration on the electrical, structural and morphological properties of heterostructures based on TiO2 thin films

Effect of erbium (Er) doping on the electrical, structural and morphological properties of TiO2 thin films deposited by the combination of a simple sol–gel process and spin-coating technique on p-type silicon substrates, has been investigated. A systematic study of the effect of concentration of Er on the properties of heterostructures was carried out. Raman spectroscopy and atomic force microscopy have been used to study the structural and morphology properties of devices based on Er-doped TiO2/Si heterostructures. Deep level transient spectroscopy (DLTS) has been also employed to study the electrically active defects within the band gap of Er-doped TiO2 thin films. DLTS that has proved to be a powerful tool in analysing traps in semiconductors devices showed that undoped TiO2-based devices exhibit five defects. However, three defects have been detected in the low erbium-doped TiO2 devices and only one defect was observed in the higher erbium-doped devices. These results provide strong evidence that Er doping annihilates oxygen-related defects and demonstrate the effective proof of doping process in TiO2 thin film. This finding contributes to the improved activities (e.g., photocatalytic) of TiO2 since the increase in charge traps can reduce bulk recombination and consequently, separates photogenerated electrons and holes more efficiently. Furthermore, it is found that the overall electrical properties of the devices are improved by increasing Er doping concentration. This study provides an important understanding of the deep and shallow level defects in Er-doped TiO2 thin films, which is essential for the manufacturing of future devices including UV detectors

The role of defects on the performance of quantum dot intermediate band solar cells

Electrically active defects present in three InAs/GaAs quantum dots (QDs) intermediate band solar cells grown by metalorganic vapor phase epitaxy have been investigated. The devices’ structures are almost identical, differing only in the growth temperature and thickness of the GaAs layers that cover each InAs QD layer. These differences induce significant changes in the solar energy conversion efficiency of the photovoltaic cells, as previously reported. In this work, a systematic investigation was carried out using deep level transient spectroscopy (DLTS) and Laplace DLTS measurements on control samples and solar cell devices, which have clearly shown that electrically active traps play an important role in the device figures of merit, such as open circuit voltage, short circuit current, and shunt resistance. In particular, it was found that the well-known EL2 defect negatively affects both the open circuit voltage and shunt resistance, more in structures containing QDs, as a consequence of the temperature cycle required to deposit them. Other unidentified defects, that are absent in samples in which the QDs were annealed at 700 °C, contribute to a reduction of the short circuit current, as they increase the Shockley-Read-Hall recombination. Photoluminescence results further support the DLTS-based assignments

Effect of growth techniques on the structural, optical and electrical properties of indium doped TiO2thin films

We have investigated the effect of the growth techniques on the structural, the electrically and optically active defects in Indium doped TiO2 thin films grown by pulsed laser deposition (PLD) and sputtering techniques. X-ray diffraction (XRD) and Raman spectroscopy patterns revealed both rutile and anatase phases for the sputtering samples. On the other hand, only the anatase phase was observed for the PLD samples. The photoluminescence (PL) spectra have unveiled several peaks which were explained by defect related optical transitions. Particularly, the PL bands are fully consistent with anatase/rutile TiO2 phases and the formation of In2O3 during the preparation of our samples. It was also observed that at −4 V reverse bias, the PLD samples have lower leakage currents (∼1.4 × 10−7 A) as compared to the sputtering samples (∼5.9 × 10−7 A). In addition, the PLD samples exhibited lower ideality factors and higher barrier heights as compared to those grown by sputtering. Finally, the Deep Level Transient Spectroscopy (DLTS) measurements have shown only one defect in the PLD samples whereas five defects have been detected in the sputtering samples. Therefore, our results provide strong evidence that the PLD technique is better suited for the growth of In-doped TiO2 thin films

Investigation of the electrical and optical properties of advanced semiconductors materials and devices

This thesis reports on an investigation of deep level defects in InAs quantum dots intermediate band solar cells (QD-IBSCs) and InAs/InGaAs quantum dot lasers structures that have applications in photovoltaics and optoelectronic technologies. In fact, it is of paramount importance to understand the characteristics of the defects and their influence on the quality of materials and the performance of devices. In this work, the effect of electrically active defects on the structural, optical and electrical properties of a set of intermediate band solar cells grown by Metal Organic Vapour Phase Epitaxy (MOVPE) and self-assembled InAs/InGaAs QDs based laser structures grown by Molecular Beam Epitaxy was studied using Atomic Force Microscopy, Transmission Electron Microscopy, Photoluminescence, current-voltage (I-V), capacitance-voltage (C-V), deep level transient spectroscopy (DLTS), and Laplace DLTS (LDLTS) techniques. In addition, the effect of Gamma irradiation on the properties of laser structures was also investigated. Electrically active defects present in a set of p-i-n InAs/GaAs QD-IBSCs grown by MOVPE on GaAs substrates have been studied systematically. The device’s structures are almost identical, differing only in the growth temperature and thickness of the GaAs layers that cover each InAs QD layer. In order to distinguish between the roles played by the growth temperature and the insertion of the QDs in the active region of the devices, reference solar cells with the equivalent temperature growth sequence as the ones used for the fabrication of the QD-IBSCs were grown and their DLTS results were compared. These differences induce significant changes in the solar energy conversion efficiency of the photovoltaic cells. DLTS and Laplace DLTS measurements on control samples and the studied solar cell devices have clearly shown that electrically active traps play an important role in the device figures of merit, such as open circuit voltage, short circuit current, and shunt resistance. In particular, it was found that the well-known EL2 defect negatively affects both the open circuit voltage and shunt resistance, more in structures containing QDs, as a consequence of the temperature cycle required to deposit them. Other unidentified defects, that are absent in samples in which the QDs were annealed at 700 °C, contribute to a reduction of the short circuit current, as they increase the Shockley-Read-Hall recombination. Extensive work on InAs QDs grown on GaAs substrates has been reported in the literature, however, research in the use of different substrate materials such as silicon to achieve an ideal and full integration of photonic and electronic systems is still under development. In this work we have investigated the effect of the substrate material (Si and GaAs) and strain reducing layer (SRL) on the electrical and optical properties of InAs QDs based laser structures grown by MBE. Two InAs QD laser structures with similar active regions grown on GaAs and Si substrates using SRL consisting of InAs QDs/6nm In0.15Ga0.85As SRL have been investigated. Atomic Force Microscopy (AFM), Transmission Electron Microscopy (TEM), photoluminescence (PL), I-V, C-V, DLTS and LDLTS have been used for the characterization of the grown samples. The formation of fairly similar QDs in both structures was evidenced, with higher strain for QDs in the sample grown on a Si substrate. A red shift of the InAs QD PL peak energy was observed for the sample grown on a Si substrate as compared to the sample grown on a GaAs substrate, which was associated with residual biaxial strain from the Si/GaAs heterointerface. This red-shift of the PL peak energy is accompanied by a broadening of the PL spectrum from ~31 meV to a value of ~46 meV. This broadening is attributed to the QD size inhomogeneity increase for samples grown on a Si substrate. DLTS and LDLTS showed that InAs QD laser devices grown on Si substrate showed more defects than the structures grown on GaAs substrates, suggesting that QDs/GaAs lasers should have better properties than QDs/Si lasers. However, the use of Si substrates to grow InAs QDs’ based lasers opens up new possibilities for controlling the density and size of QDs, and therefore the emission of InAs QDs, for photonic devices integration using Si substrates when using specific type of SRL/substrate. A significant number of lattice defects are produced in semiconductors as a result of radiation in space, which reduce the devices’ performance. The effect of gamma irradiation on the electrical properties of InAs QD lasers grown on GaAs and Si substrates was studied. The number of traps either decreased, remained the same or new defects were created after irradiation. Moreover, the DLTS and Laplace DLTS have been able to reveal a close connection between the grown-in defects and those induced by radiation