Transport and optical properties of resonant tunneling structures

Transport properties of a GaAs/AlAs superlattice-like double barrier diode are studied in this work as a function of the sample temperature. An activation energy of about 60meV obtained from the Arrhenius plot is in good agreement with the confined level in the central well. Numerical simulations also confirm the importance of bound levels in the gammaand X bands for the resonant tunneling process. The enhancement of photoluminescence as the temperature is increased is also studied. This behavior is associated to the transport properties of holes in the collector contact, which control the supply of minority carriers, which tunnel into the well. The description of the observed results requires the modi-cation of simple known models to take into account the two contributions to the pair generation rate in the well, responsible of the photoluminescence at zero and finite bias.331333Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES

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

Optical properties of large area WS2 grown by chemical vapor deposition

Transition metal dichalcogenides (TMDs) have attracted great attention for fundamental physics and possible application as optoelectronic devices [1-5]. Monolayers of TMDs are direct gap semiconductors with optical transition in inequivalent at K and K´ valleys and distinct optical selection rules due to the combination of spin-orbit interaction and broken inversion symmetry. As a consequence, optical excitation with circularly polarized light results in circularly polarized emission. Actually, recent studies for TMDs have evidenced important valley polarization degree and large excitonics effects as well particularly at lower temperatures [1-5]. In this work, we have investigated optical properties from large are a monolayers of WS2 on 295nm SiO2/Si grown by Van der Waals Epitaxy Chemical Vapor Deposition. Particularly , we have investigated polarization resolved photoluminescence (PL ) spectra for different light excitation intensities and temperatures using a 532 nm solid state laser. The s + and s - light excitation and detection were obtained using appropriate quarter wave plates and linear polarizers. At lower temperatures, we have observed different PL peaks for the WS2 monolayer. The temperature and laser power dependence of PL spectra evidences that the observed peaks are associated to neutral (X), charged excitons (X- ) and biexcitons (XX). Therefore, our results reveal important many -body interactions in atomically thin WS2 semiconductor

Experimental Detection and Control of Trions and Fermi-Edge Singularity in Single-Barrier GaAs/AlAs/GaAs Heterostructures Using Photocapacitance Spectroscopy

© 2018 American Physical Society. We show how photocapacitance spectra can probe and manipulate two dimensional excitonic complexes and Fermi-edge singularities as a function of applied bias even at a temperature of 100 K. For lower density regimes (1×1011cm-2), we observe a sharp spectral transition from trions to asymmetrically shaped Fermi-edge singularities in photocapacitance spectra above a particular reverse bias. However, these signatures of indirect excitonic states are absent from photoluminescence spectra. Such dissimilarities clearly point out that different many body physics govern these two spectral measurements. We also argue why such quantum-confined dipoles of spatially indirect trions can have thermodynamically finite probability to survive even around 100 K. Finally, our observations demonstrate that photocapacitance spectroscopy, which was rarely used to detect trions in the past, can also be useful to detect the traces of these spatially indirect excitonic complexes as well as Fermi-edge singularities. This is mainly due to the enhanced sensitivity of these capacitive measurements to "dipolar" changes of excitonic complexes in these heterojunctions. Thus, our studies clearly open up future possibilities for electro-optical modulation and detection of trions and Fermi-edge singularities in several other heterostructures for next-generation optoelectronic applications

Spin polarization of carriers in InGaAs self-assembled quantum rings inserted in GaAs-AlGaAs resonant tunneling devices

In this work, we have investigated transport and polarization resolved photoluminescence (PL) of n-type GaAs-AlGaAs resonant tunneling diodes (RTDs) containing a layer of InGaAs self-assembled quantum rings (QRs) in the quantum well (QW). All measurements were performed under applied voltage, magnetic fields up to 15 T and using linearly polarized laser excitation. It was observed that the QRs’ PL intensity and the circular polarization degree (CPD) oscillate periodically with applied voltage under high magnetic fields at 2 K. Our results demonstrate an effective voltage control of the optical and spin properties of InGaAs QRs inserted into RTDs

Electric field inversion asymmetry: Rashba and Stark effects for holes in resonant tunneling devices

We report experimental evidence of excitonic spin-splitting, in addition to the conventional Zeeman effect, produced by a combination of the Rashba spin-orbit interaction, Stark shift and charge screening. The electric-field-induced modulation of the spin-splitting are studied during the charging and discharging processes of p-type GaAs/AlAs double barrier resonant tunneling diodes (RTD) under applied bias and magnetic field. The abrupt changes in the photoluminescence, with the applied bias, provide information of the charge accumulation effects on the device.Comment: 4 pages, 2 figure

Aharonov-Bohm interference in quantum ring exciton: effects of built-in electric fields

We report a comprehensive discussion of quantum interference effects due to the finite structure of excitons in quantum rings and their first experimental corroboration observed in the optical recombinations. Anomalous features that appear in the experiments are analyzed according to theoretical models that describe the modulation of the interference pattern by temperature and built-in electric fields.Comment: 6 pages, 7 figure

Investigation of optical and electrical properties of erbium-doped TiO2 thin films for photodetector applications

We have investigated the electrical and optical properties of erbium (Er3+) doped TiO2 thin films (Er:TiO2 TFs) grown by sol–gel technique on glass and silicon substrates. The samples were characterized by field emission gun–scanning electron microscopes (FEG–SEM), energy dispersive X-ray spectroscopy (EDX), atomic force microscopy (AFM), X-ray diffraction (XRD), photoluminescence (PL) and current–voltage measurement techniques. FEG–SEM and AFM images showed the morphological change in the structure of Er:TiO2 TFs and EDX analysis confirmed the Er3+ doped into TiO2 lattice. Broad PL emissions in visible and infrared regions were observed in undoped TiO2 samples and associated to different mechanisms due to the anatase and rutile phases. PL spectra revealed sharp peaks at 525 nm, 565 nm, 667 nm and 1.54 µm which are related to Er3+ emissions in Er:TiO2 samples. The undoped TiO2 and Er:TiO2 TFs based UV-photodetectors were fabricated, and various device parameters were investigated. The doped devices exhibit high photoresponse upon illuminating 350 nm UV light at 2 V bias with faster response time compared to undoped device

Experimental evidences of quantum confined 2D indirect excitons in single barrier GaAs/AlAs/GaAs heterostructure using photocapacitance at room temperature

We investigated excitonic absorptions in a GaAs/AlAs/GaAs single barrier heterostructure using both photocapacitance and photocurrent spectroscopies at room temperature. Photocapacitance spectra show well defined resonance peaks of indirect excitons formed around the C-AlAs barrier. Unlike DC-photocurrent spectra, frequency dependent photocapacitance spectra interestingly red shift, sharpen up, and then decrease with increasing tunneling at higher biases. Such dissimilarities clearly point out that different exciton dynamics govern these two spectral measurements. We also argue why such quantum confined dipoles of indirect excitons can have thermodynamically finite probabilities to survive even at room temperature. Finally, our observations demonstrate that the photocapacitance technique, which was seldom used to detect excitons in the past, is useful for selective detection and experimental tuning of relatively small numbers ( 1011/cm2) of photo-generated indirect excitons having large effective dipole moments in this type of quasi-two dimensional heterostructures

Distinctive g-factor of moire-confined excitons in van der Waals heterostructures

We investigated experimentally the valley Zeeman splitting of excitonic peaks in the photoluminescence (PL) spectra of high-quality hBN/WS2/MoSe2/hBN heterostructures at near-zero twist angles under perpendicular magnetic fields up to 20 T. We identify two neutral exciton peaks in the PL spectra: the lower energy one exhibits a reduced g-factor relative to that of the higher energy peak, and much lower than the recently reported values for interlayer excitons in other van der Waals (vdW) heterostructures. We provide evidence that such a discernible g-factor stems from the spatial confinement of the exciton in the potential landscape created by the moire pattern, due tolattice mismatch and/or inter-layer twist in heterobilayers. This renders magneto-PL an important tool to reach deeper understanding of the effect of moire patterns on excitonic confinement in vdW heterostructures.Comment: 6 pages, 3 figures. Submitte