Excitation Power-dependent Photoluminescence Research Articles

Stabilization of the Cubic Crystalline Phase in Organometal Halide Perovskite Quantum Dots via Surface Energy Manipulation.

Surface functionalization of nanoscale materials has a significant impact on their properties. We have demonstrated the effect of different passivating ligands on the crystal phase of organometal halide perovskite quantum dots (PQDs). Using static and dynamic spectroscopy, we studied phase transitions in CH3NH3PbBr3 PQDs ligated with either octylaminebromide (P-OABr) or 3-aminopropyl triethoxysilane (P-APTES). Around 140 K, P-OABr underwent a structural phase transition from tetragonal to orthorhombic, established by the emergence of a higher energy band in the photoluminescence (PL) spectrum. This was not observed in P-APTES, despite cooling down to 20 K. Additionally, time-resolved and excitation power-dependent PL, as well as Raman spectroscopy over a range of 300-20 K, revealed that recombination rates and types of charge carriers involved are significantly different in P-APTES and P-OABr. Our findings highlight how aspects of PQD phase stabilization are linked to nanoscale morphology and the crystal phase diagram.

Dynamics of carrier tunneling and recombination in asymmetric coupled InGaN multiple quantum wells.

In this work, dynamics of carrier tunneling and recombination in InGaN-based asymmetric coupled multiple quantum wells (AC-MQWs) are systematically studied by excitation power-dependent and temperature-dependent photoluminescence (PL) measurements. With different pumping wavelengths of 405 and 325 nm, distinctly different PL spectral evolutions are observed, which could be well explained by the proposed anomalous carrier "reverse tunneling" based on the forbidden 1h→2e transitions in the AC-MQWs. The forbidden transitions are identified through the well agreement between the measured photo-modulated reflectance (PR) spectrum and the calculated interband transition energies. Our results indicate that, by ingeniously designing the MQW structure of the InGaN-based optoelectronic devices, it is possible to realize a specific interband optical transition which is even not allowed by the selection rule, and thereby effectively improve the carrier distribution across the QWs through the conventional and/or anomalous "reverse" carrier tunneling.

Detecting Spatially Localized Exciton in Self-Organized InAs/InGaAs Quantum Dot Superlattices: a Way to Improve the Photovoltaic Efficiency

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.

The effect of an InP cap layer on the photoluminescence of an InxGa1–xAs1–yPy/InzAl1– zAs quantum well heterostructure

The effect of an InP cap on the photoluminescence (PL) spectrum of an InGaAsP/InAlAs quantum well (QW) is investigated using excitation power and temperature dependent PL. An as-grown sample with the InP cap layer shows an inverted interface created between InP and InAlAs that has a transition energy very close to the transition energy of the QW; consequently, there is an overlap between them. On the other hand, the QW sample with the cap layer etched away does not have a feature due to the inverted interface; even at very low power, the only observed feature is due to the QW transition.

Localized states emission in type-I GaAsSb/AlGaAs multiple quantum wells grown by molecular beam epitaxy

As an important candidate for novel infrared semiconductor lasers, the optical properties of GaAsSb-based multiple quantum wells (MQWs) are crucial. The temperature- and excitation power-dependent photoluminescence (PL) spectra of the GaAs0.92Sb0.08/Al0.2Ga0.8As MQWs, which were grown by molecular beam epitaxy, were investigated and are detailed in this work. Two competitive peaks were observed from 40 K to 90 K. The peak located at the low-energy shoulder was confirmed to be localized states emission (LE) and the high-energy side peak was confirmed to be free-carrier emission by its temperature-dependent emission peak position. It is observed that the LE peak exhibited a blueshift with the increase of laser excitation power, which can be ascribed to the band filling effect of localized states. Our studies have great significance for application of GaAsSb-based MQWs in infrared semiconductor lasers.

Investigation of Cu2ZnSnS4 thin films with controllable Cu composition and its influence on photovoltaic properties for solar cells

The effects of Cu composition on the structural, morphological, optical properties of Cu2ZnSnS4 (CZTS) films prepared by RF magnetron sputtering and a post sulfurization, and CZTS solar cells performance have been investigated. The crystal structure characterized by XRD patterns, Raman and far-infrared spectra reveal that the secondary phases (e.g. SnS and Cu2SnS3) can be suppressed effectively by manipulating the Cu content in CZTS thin films. Furthermore, morphological studies indicate that the CZTS thin film with low Cu content shows a bilayer structure, while that with high Cu content exhibits the grain growth throughout the entire absorber film with some voids at the Mo/CZTS interface. Finally, a Cu-poor CZTS solar cell with 3.46% is fabricated. Temperature and excitation-power dependent Photoluminescence measurement reveals the radiative recombination between electrons in conduction band tails and holes bound to the neighboring acceptors levels in Cu-poor CZTS device. The diode parameters of Cu-poor CZTS solar cell are extracted and used to discuss the possible reason for limiting efficiency.

Room temperature direct band gap emission characteristics of surfactant mediated grown compressively strained Ge films

Compressively strained Ge films have been grown on relaxed Si0.45Ge0.55 virtual substrates using molecular beam epitaxy in the presence of Sb as a surfactant. Structural characterization has shown that films grown in the presence of surfactant exhibit very smooth surfaces with a relatively higher strain value in comparison to those grown without any surfactant. The variation of strain with increasing Ge layer thickness was analyzed using Raman spectroscopy. The strain is found to be reduced with increasing film thickness due to the onset of island nucleation following Stranski–Krastanov growth mechanism. No phonon assisted direct band gap photoluminescence from compressively strained Ge films grown on relaxed Si0.45Ge0.55 has been achieved up to room temperature. Excitation power and temperature dependent photoluminescence have been studied in details to investigate the origin of different emission sub-bands.

Enhanced optical output and reduction of the quantum-confined Stark effect in surface plasmon-enhanced green light-emitting diodes with gold nanoparticles.

We report the optical properties of localized surface plasmon (LSP)-enhanced green light-emitting diodes (LEDs) containing gold (Au) nanoparticles embedded in a p-GaN layer. The photoluminescence (PL) and electroluminescence (EL) intensities of a green LED with Au nanoparticles were enhanced by the coupling between excitons and LSPs. Excitation power-dependent PL and injection current-dependent EL measurements revealed that the blue-shift of PL and EL peaks with increasing carrier density was smaller for the LSP-enhanced LED compared with that for a conventional LED. The increased optical output power and decrease in blue-shift of the LED with Au nanoparticles were attributed to the increased radiative recombination efficiency of carriers induced by the LSP-coupling process and the compensation of the polarization-induced electric fields with LSP-enhanced local fields, both of which suppressed the quantum-confined Stark effect.

Photoluminescence of localized excitons in ZnCdO thin films grown by molecular beam epitaxy

Abstract We have investigated the luminescence characteristics of Zn 1− x Cd x O thin films with different Cd contents grown by molecular beam epitaxy system. The temperature-dependent photoluminescence (PL) and excitation power-dependent PL spectra were measured to clarify the luminescence mechanisms of the Zn 1− x Cd x O thin films. The peak energy of the Zn 1− x Cd x O thin films with increasing the Cd concentration is observed as redshift and can be fitted by the quadratic function of alloy content. The broadened full-width at half-maximum (FWHM) estimated from the 15 K PL spectra as a function of Cd content shows a larger deviation between the experimental values and theoretical curve, which indicates that experimental FWHM values are affected not only by alloy compositional disorder but also by localized excitons occupying states in the tail of the density of states. The Urbach energy determined from an analysis of the lineshape of the low-energy side of the PL spectrum and the degree of localization effect estimated from the temperature-induced S-shaped PL peak position described an increasing mean exciton-localization effects in ZnCdO films with increasing the Cd content. In addition, the PL intensity and peak position as a function of excitation power are carried out to clarify the types of radiative recombination and the effects of localized exciton in the ZnCdO films with different Cd contents.

Photoluminescence Characterization of Vertically Coupled Low Density InGaAs Quantum Dots for the application to Quantum Information Processing Devices

Vertically coupled low density InGaAs quantum dots (QDs) buried in GaAs matrix were grown with migration enhanced molecular beam epitaxy method as a candidate for quantum information processing devices. We performed excitation power-dependent photoluminescence measurements at cryogenic temperature to analyze the effects of vertical coupling according to the variation in thickness of spacer layer. The more intense coupling effects were observed with the thinner spacer layer, which modified emission properties of QDs significantly. The low surface density of QDs was observed by atomic force microscopy, and scanning transmission electron microscopy verified the successful vertical coupling between low density QDs.

Coupling Behaviors of Surface Plasmon Polariton and Localized Surface Plasmon with an InGaN/GaN Quantum Well

Surface plasmon (SP) coupling behaviors of an InGaN/GaN quantum well (QW) with surface plasmon polariton (SPP) induced on a smooth Ag-film/GaN interface and localized surface plasmon (LSP) induced on randomly distributed surface Ag nanoparticles (NPs) are compared based on temperature-dependent, excitation power-dependent, and time-resolved photoluminescence (PL) measurements. By comparing the variations of PL excitation power-dependent emission at 10 and 300 K, the contribution of SP coupling to the increase of PL intensity can be differentiated from that of PL excitation-intensity increase at the QW due to the enhanced reflection at the Ag/GaN interface. The observed stronger SP coupling effect through SPP, when compared with that through LSP, is attributed to its larger available SP state number per unit area for coupling with the QW. Although the SP coupling strength through LSP on the surface Ag NPs is weakly dependent on PL excitation power, that through SPP at the smooth Ag-film/GaN interface increases significantly with PL excitation power. Such different behavior is due to the abundant SPP states available for coupling with the QW of a higher carrier density. It can also be attributed to the spectral blueshift of QW emission through the screening of the quantum-confined Stark effect and the band filling effect for QW coupling with SPP of a higher density of state.

Photoluminescence and surface photovoltage properties of ZnSe nanoribbons

ZnSe nanoribbons were synthesized with chemical vapor deposition route. The excitation power-dependent photoluminescence and surface photovoltage (SPV) techniques were used to study the optoelectronic properties of the as-grown ZnSe nanoribbons. Three deep defect (DD)-related emission bands, respectively, centered at 623nm (DD1), 563nm (DD2) and 525nm (DD3), emerge orderly with increasing the excitation power, which is attributed to the saturation of the DD states from deeper to shallower level. The SPV spectrum and the corresponding phase spectrum show that DD1 mainly acts as recombination center, while DD2 and DD3 can act as both the recombination center and electron traps. The influence of the trapping electrons on the SPV response dynamic was studied with transient SPV.

Auger recombination at low temperatures in InGaAs/InAlAs quantum well probed by photoluminescence

This paper reports abnormal effects of Auger recombination in InGaAs/InAlAs quantum well probed by steady state temperature and excitation-power dependent photoluminescence measurements. The transition from Auger recombination to Shockley–Read–Hall recombination is clearly manifested with rising temperature, and abnormal enhancement of Auger recombination at low temperatures is evidenced. Detailed analysis indicates that such enhancement is positively correlated with strong localization, and can be either relative and due to reduction of Shockley–Read–Hall recombination, or absolute and resulted from the confinement-induced uncertainty of momentum. It suggests that excitation-power dependent photoluminescence measurement is effective to assess the side-effect of the localization, and therefore a convenient evaluation routine of alloy quantum wells.

InGaN蓝光LED内量子效率的评测

The internal quantum efficiency (IQE) is one of the most important indexes to evaluate the performance of light emitting-diodes. This paper introduced several evaluation methods of IQE widely used in current research, including temperature-dependent photoluminescence (TDPL), excitation power dependent photoluminescence (EDPL), temperature dependent electroluminescence (TDEL), and fitting of efficiency-current curve. In addition, the application range and limit of these methods were also discussed based on the measurement results of practical samples. TDPL and EDPL apply to measurement of radiative recombination efficiency, and their results are consistent. The principle of TDEL is not reliable. Fitting of efficiency-current curve is reliable in principle, but it depends on the assumed parameters of samples. The believable results can be obtained based on the sufficient understanding on samples.

Emission mechanisms in Al-rich AlGaN/AlN quantum wells assessed by excitation power dependent photoluminescence spectroscopy

The optical properties of Al-rich AlGaN/AlN quantum wells are assessed by excitation-power-dependent time-integrated (TI) and time-resolved (TR) photoluminescence (PL) measurements. Two excitation sources, an optical parametric oscillator and the 4th harmonics of a Ti:sapphire laser, realize a wide range of excited carrier densities between 1012 and 1021 cm−3. The emission mechanisms change from an exciton to an electron-hole plasma as the excitation power increases. Accordingly, the PL decay time is drastically reduced, and the integrated PL intensities increase in the following order: linearly, super-linearly, linearly again, and sub-linearly. The observed results are well accounted for by rate equations that consider the saturation effect of non-radiative recombination processes. Using both TIPL and TRPL measurements allows the density of non-radiative recombination centers, the internal quantum efficiency, and the radiative recombination coefficient to be reliably extracted.

Luminescent emission of multi-junction InGaP/InGaAs/Ge PV cells under high intensity irradiation

Combined electroluminescence (EL) and photoluminescence (PL) measurements were conducted in order to investigate the presence of luminescent emission of InGaP/InGaAs/Ge at different operating conditions of the tandem. Luminescent emission from cell samples was observed at different sun concentrations, voltage biases and temperatures. A high intensity pulsed solar simulator was used to photoexcite the device which exhibited strong radiative recombination from both the top InGaP and middle InGaAs junctions. Luminescent emission from the device was investigated under a range of voltage biases and was clearly observed at the maximum power point voltage of the sample under test indicating its presence during typical operating conditions of the solar cells. Investigation of the emission was also performed at relatively high temperatures (up to 60°C) in order to mimic the outdoor operating conditions of a solar cell device. Luminescence was detected at high temperatures indicating that significant radiative recombination is present at even higher temperatures. Outdoor measurements under actual solar spectrum demonstrated the presence of luminescent emission in agreement with indoor testing. The significant amount of radiative recombination at the band-gap edges of the top junctions observed in our measurements gives evidence that optical coupling to the lower ones may occur. Finally, excitation power dependent PL was performed using monochromatic laser sources in order to investigate the impact of externally induced photocurrents of different intensity upon the radiative signal of each junction.

Photoluminescence properties and exciton dynamics in monolayer WSe2

In this work, comprehensive temperature and excitation power dependent photoluminescence and time-resolved photoluminescence studies are carried out on monolayer WSe2 to reveal its properties of exciton emissions and related excitonic dynamics. Competitions between the localized and delocalized exciton emissions, as well as the exciton and trion emissions are observed, respectively. These competitions are suggested to be responsible for the abnormal temperature and excitation intensity dependent photoluminescence properties. The radiative lifetimes of both excitons and trions exhibit linear dependence on temperature within the temperature regime below 260 K, providing further evidence for two-dimensional nature of monolayer material.

Characterization and Analysis of 1.3-μm InAs/InGaAs Self-Assembled Quantum Dot Lasers

High performance 1.3-μm GaAs-based InAs/InGaAs quantum dot (QD) lasers have been fabricated. The QD lasers have demonstrated low threshold current, high output power as well as high temperature operation. Temperature-dependent (20–100 °C) and excitation power-dependent (12–700 mW) photoluminescence (PL) measurements have been carried out on the QD laser structures. Both ground state (GS) and excited state (ES) luminescence has been observed in the PL spectra as well as in the lasing spectra. Rate equations were used to interpret the PL behavior of the QD structures. High radiative recombination efficiency in the QD structure has been verified even at 100 °C. The excitonic modal gain has been calculated and compared with the experimentally obtained modal gain value by the Hakki–Paoli method from the QD laser with good agreement.

A quantitative method for determination of carrier escape efficiency in GaN-based light-emitting diodes: A comparison of open- and short-circuit photoluminescence

We propose a method to quantitatively analyze the internal quantum efficiency (IQE) as well as the efficiencies of non-radiative recombination in the active region (NRA) and carrier escape out of the active region (ESC) by comparing open-circuit (OC) to short-circuit (SC) conditions of InGaN-based light-emitting diodes (LEDs). First, the IQE was extracted from excitation-power dependent photoluminescence at low temperature, and the electron-hole wavefunction overlaps were calculated under OC and SC conditions. Then, the NRA and ESC efficiencies were quantitatively deduced and also compared with photocurrent data. The proposed method would be useful for assessing and designing quantum barriers and analyzing leakage current in LEDs.

Time-integrated photoluminescence and pump-probe reflection spectroscopy of Si doped InN thin films

Temperature and excitation power dependent time-integrated photoluminescence of Si doped InN thin films are investigated. Photoluminescence (PL) spectra at low temperatures are described by single emission peak ensued due to “free-to-bound” recombination; whereas PL spectra at higher temperatures above 150 K are characterized by both “band-to-band” and “free-to-bound” transition. Carrier dynamics of Si doped InN thin films is studied using pump-probe reflection spectroscopy at room temperature. The hot electron cooling process is well described by electron-electron scattering. The dependence of the hot electron cooling rate on total electron density shows sublinear to linear behavior with increase of background electron density. The variation of the carrier recombination lifetime with total electron density implicates the dominance of the defect-related nonradiative recombination channel over other recombination processes.