Power-dependent Photoluminescence Research Articles

Surface morphology and optical properties of InGaN quantum dots with varying growth interruption time

The effect of different growth interruption time on the surface morphology and optical properties of InGaN quantum dots (QDs) grown on 2-inch silicon substrates is investigated. The surface becomes rougher and the photoluminescence intensity has been enhanced significantly when employing the growth interruption method. Temperature-dependent photoluminescence and excitation power-dependent photoluminescence both present unchanged peak energy and line-width of QDs. The sharp increase of PL intensity in medium temperature regime is attributed to the fingerprint of the existence of InGaN QDs. The shape of the QDs are further confirmed by the transmission electron microscopy with a size of 3 nm by 4 nm. Among the samples, a growth interruption time of 30 s gives the best optical performance.

Investigation of Chemical Origin of White-Light Emission in Two-Dimensional (C4H9NH3)2PbBr4 via Infrared Nanoscopy.

The broadband light emission in low-dimensional organic lead halide perovskites (OHPs) is a fascinating property for white light-emitting diodes (LEDs). However, unique emission has been observed in highly distorted low-dimensional OHPs such as (110) and (111) perovskites. Herein, we report the first observation of white-light emission under ambient (21 °C) conditions in a rectangular microsheet of (C4H9NH3)2PbBr4, a (100) perovskite. The origin of white-light emission in (C4H9NH3)2PbBr4 was revealed as defect-assisted radiative recombination via excitation power-dependent photoluminescence measurement. Additionally, the origin of the defect was confirmed to be organic cation vacancies formed by intercalated water molecules via infrared nanoscopy. This result can help to improve the performance of white LEDs using low-dimensional OHPs.

Three-Dimensional Resonant Exciton in Monolayer Tungsten Diselenide Actuated by Spin-Orbit Coupling.

The intricate features of many-body interactions and spin-orbit coupling play a significant role in numerous physical phenomena. Particularly in two-dimensional transition metal dichalcogenides (2D-TMDs), excitonic dynamics are a key phenomenon that promises opportunities for diverse range of device applications. Here, we report the direct observation of a visible-range three-dimensional resonant exciton and its associated charged exciton in monolayer tungsten diselenide, as compared to monolayer molybdenum disulfide. A comprehensive experimental study that includes high-resolution TEM, Raman, high-resolution spectroscopic ellipsometry over a wide temperature range down to 4 K, high-energy temperature, and excitation power-dependent photoluminescence spectroscopy has been conducted. It is supported by first-principles calculations to unravel the influence of spin-orbit coupling in the formation of the resonant exciton and to identify its in-plane and out-of-plane features. Furthermore, we study the impact of temperature and thickness on the spin-orbit coupling strength in 2D-TMDs. This work is crucial in creating a platform in the fundamental understanding of high-energy resonant exciton in layered two-dimensional systems and that such high-energy optoelectronic features make them an increasingly attractive candidate for novel electronic and optoelectronic applications particularly in the aspects of solar cells and light-emitting diodes via the manipulation of excitonic states.

Observation of bandgap renormalization in mesoscopic zinc oxide particles

Deep UV photoluminescence (PL) spectroscopy was employed to study the optical properties of mesoscopic zinc oxide (ZnO) particles using the third harmonic laser (λ ~ 260 nm) generated from a Ti: sapphire pulse laser for optical excitation. PL spectrum measured at 15 K exhibits excitonic emissions at 3.362 and 3.375 eV due to the donor bound exciton (DoX) and free exciton (FX) transitions, respectively. The dominant emission peak is observed at around 3.308 eV due to the recombination of free electrons from the conduction band to the holes bound to an acceptor state (FA) along with longitudinal optical (LO) phonon replicas. From the power dependent PL measurement, we observed a redshift of FX and DoX emission peaks with increasing excitation laser power, showing a bandgap renormalization effect. Temperature dependent PL measurements were also performed to explore the bandgap renormalization effect. Fitting of our power dependent PL data with Vashishta, and Kalia model shows the clear trend in a good agreement with the model.

Origin of photoluminescence from antimony selenide

Antimony selenide (Sb2Se3) absorber material has great potential for low-cost photovoltaics due to its excellent optoelectronic properties and low processing temperatures. This study presents detailed temperature and excitation power dependent photoluminescence (PL) analysis of Sb2Se3 polycrystals revealing the dominating radiative recombination mechanisms and related defects in the studied material. The low-temperature (T = 10 K) PL spectrum consisted of three bands at 0.94 eV, 1.10 eV and 1.24 eV, the last one located close to the low-temperature band gap of Sb2Se3 1.32 eV. The PL bands at 1.24 eV and 0.94 eV were found to originate from the donor-acceptor pair recombination, the first one at 1.24 eV involving more distant pairs while the second one at 0.94 eV resulting from the deep acceptor – deep donor pair recombination. Third PL band at 1.10 eV is proposed to be related to the grain boundaries.

Optical transitions in lysozyme mediated zinc oxide nanoparticles probed by deep UV photoluminescence

Lysozyme mediated zinc oxide nanoparticles (L:ZnO) synthesized by sol-gel method were characterized at room temperature using x-ray diffraction, scanning electron microscopy and absorption spectroscopy for structural, morphological and optical properties, respectively. A deep UV photoluminescence (PL) spectroscopy was employed to probe the optical transitions in the L:ZnO nanoparticles at low temperature (15 K). In the excitation power dependent PL measurements, we observed band edge emissions at 3.318, 3.359, and 3.374 eV due to recombination of a free electron from the conduction band to a hole in the acceptor level (FA), donor bound exciton (DoX) and free exciton (FX), respectively. A dominant emission peak around 2.40 eV is confirmed to be a donor acceptor pair (DAP) from the power dependent PL experiment. Based on the observed PL emission peaks, energy level diagram is constructed for the optical transitions in the L:ZnO particles and the DAP is assigned to the electronic transition from a deep donor level to the acceptor level related to the FA transition.

Strain relaxation of InGaN/GaN multi-quantum well light emitters via nanopatterning.

Strain in InGaN/GaN multiple-quantum well (MQW) light emitters was relaxed via nanopatterning using colloidal lithography and top-down plasma etching. Colloidal lithography was performed using Langmuir-Blodgett dip-coating of samples with silica particles (d = 170, 310, 690, 960 nm) and a Cl2/N2 inductively coupled plasma etch to produce nanorod structures. The InGaN/GaN MQW nanorods were characterized using X-ray diffraction (XRD) reciprocal space mapping to quantify the degree of relaxation. A peak relaxation of 32% was achieved for the smallest diameter features tested (120 nm after etching). Power-dependent photoluminescence at 13 K showed blue-shifted quantum well emission upon relaxation, which is attributed to reduction of the inherent piezoelectric field in the III-nitrides. Poisson-Schrödinger simulations of single well structures also predicted increasing spectral blueshift with strain relaxation, in agreement with experiments.

Surface depletion field in 2D perovskite microplates: Structural phase transition, quantum confinement and Stark effect

Surface depletion field would introduce the depletion region near surface and thus could significantly alter the optical, electronic and optoelectronic properties of the materials, especially low-dimensional materials. Two-dimensional (2D) organic—inorganic hybrid perovskites with van der Waals bonds in the out-of-plane direction are expected to have less influence from the surface depletion field; nevertheless, studies on this remain elusive. Here we report on how the surface depletion field affects the structural phase transition, quantum confinement and Stark effect in 2D (BA)2PbI4 perovskite microplates by the thickness-, temperature- and power-dependent photoluminescence (PL) spectroscopy. Power dependent PL studies suggest that high-temperature phase (HTP) and low-temperature phase (LTP) can coexist in a wider temperature range depending on the thickness of the 2D perovskite microplates. With the decrease of the microplate thickness, the structural phase transition temperature first gradually decreases and then increases below 25 nm, in striking contrast to the conventional size dependent structural phase transition. Based on the thickness evolution of the emission peaks for both high-temperature phase and low-temperature phase, the anomalous size dependent phase transition could probably be ascribed to the surface depletion field and the surface energy difference between polymorphs. This explanation was further supported by the temperature dependent PL studies of the suspended microplates and encapsulated microplates with graphene and boron nitride flakes. Along with the thickness dependent phase transition, the emission energies of free excitons for both HTP and LTP with thickness can be ascribed to the surface depletion induced confinement and Stark effect.

Visualizing carrier transitions between localization states in a InGaN yellow–green light-emitting-diode structure

InGaN-based light-emitting diodes (LEDs) have higher luminescence efficiency than other materials used for the blue and green LEDs in spite of their relatively high dislocation density. Localization theory has been used to explain this phenomenon, but the direct observation of localization states in the InGaN active region has been rarely reported. In this paper, we propose an LED structure to obtain higher luminescence efficiency in the yellow-green LEDs and directly observe the transition of carriers between different localization states. The localization states were investigated and confirmed by temperature-dependent photoluminescence and excitation power-dependent photoluminescence. The value of the external quantum efficiency also exhibited a higher radiative efficiency of the quantum well with a higher degree of localization states. These results offer a promising means of realizing high-luminescence LEDs.

Observation of photoluminescence edge emission in CuSbSe2 absorber material for photovoltaic applications

CuSbSe2 has great potential for being an earth-abundant absorber material in efficient thin film solar cells. In this study, the radiative recombination in CuSbSe2 polycrystalline material was studied by temperature and laser power dependent photoluminescence spectroscopy which revealed the band-to-band and band-to-deep acceptor transitions, with the last one involving a deep acceptor defect with an activation energy of 165 ± 6 meV. The observation of the edge emission in CuSbSe2 refers to the high quality of the synthesized material.

Tailoring of Bound Exciton Photoluminescence Emission in WS2 Monolayers

Temperature‐ and laser power‐dependent photoluminescence (PL) properties of the asymmetric defect‐bound exciton band in defective WS2 monolayers, grown by chemical vapor deposition, are studied. Based on PL mapping, a monolayer region with an intensive band emission at about 1.9 eV is chosen for further studies. The band is thermally quenched above 180 K, and the thermal activation energy is found to be = 33 ± 4 meV. At = 15 K, the band intensity reveals a sublinear dependence with increasing excitation power and the peak position shows a blueshift of about 15 meV per decade of laser power. It is shown that the band is related to the deep defect states within the band gap of WS2.

Optical properties demonstrating strong coupling of compactly arranged Ge quantum dots.

Coupled quantum dots (QDs) have been extensively investigated for their unique collective properties and potential applications in optoelectronic devices. Herein, dense Ge QDs with a compact arrangement in-plane are readily obtained. Systematic studies on power-dependent photoluminescence (PL) from the QD ensemble demonstrate a PL peak with a superior intensity, a constant peak energy and width as a function of the excitation power. Moreover, the temperature-dependent PL spectra exhibit a pronounced red-shift and a rapid PL quenching with increasing temperature. Such PL properties are attributed to the formation of miniband and the delocalization of holes in the QD ensemble due to the strong coupling between closely adjacent QDs.

Epitaxial Ge0.81Sn0.19 Nanowires for Nanoscale Mid-Infrared Emitters.

Highly oriented Ge0.81Sn0.19 nanowires have been synthesized by a low-temperature chemical vapor deposition growth technique. The nanostructures form by a self-seeded vapor-liquid-solid mechanism. In this process, liquid metallic Sn seeds enable the anisotropic crystal growth and act as a sole source of Sn for the formation of the metastable Ge1-xSnx semiconductor material. The strain relaxation for a lattice mismatch of ε = 2.94% between the Ge (111) substrate and the constant Ge0.81Sn0.19 composition of nanowires is confined to a transition zone of <100 nm. In contrast, Ge1-xSnx structures with diameters in the micrometer range show a 5-fold longer compositional gradient very similar to epitaxial thin-film growth. Effects of the Sn growth promoters' dimensions on the morphological and compositional evolution of Ge1-xSnx are described. The temperature- and laser power-dependent photoluminescence analyses verify the formation of a direct band gap material with emission in the mid-infrared region and values expected for unstrained Ge0.81Sn0.19 (e.g., band gap of 0.3 eV at room temperature). These materials hold promise in applications such as thermal imaging and photodetection as well as building blocks for group IV-based mid- to near-IR photonics.

GaAs1-xSbx/GaAs single quantum well for long wavelength photonic devices

Carrier dynamics in GaAs1-xSbx/GaAs single quantum well (SQW) is investigated in this report. With Sb incorporation (x = 0.352, 0.405), the photoluminescence (PL) emission peaks exhibit characteristics of GaAs1-xSbx in low temperature and GaAs above 200 K. In power dependent PL, the intensities reveal sublinear power relationship as localized and free excitons are involved at 10 K. The power exponent is in agreement with the degree of localization energy present in SQW and carrier kinetics for various recombination mechanisms are also discussed by deriving respective rate equations.

Photoluminescence of CuInSe2/GaN and CuInSe2/InN

The power and temperature dependent photoluminescence (PL) of epitaxially grown In-rich CuInSe2 (CIS) and Cu-rich CIS deposited on N-polar GaN and InN were investigated in this paper. The In-rich CIS/GaN has two PL emissions characterized by a donor-acceptor pair (DAP) peak at 0.92 eV and an excitonic peak at 1.08/1.1 eV. On the other hand, the Cu-rich CIS/GaN has four PL emissions characterized by two DAP peaks at 0.86 eV and 0.94 eV, free-to-bound recombination peak at 0.97 eV and an excitonic peak at 1.03 eV. Identification of these defects in the CIS absorber layer is crucial for the improvement of the device efficiency. The PL emission of the CIS/InN closely resembles that of the Cu-rich CIS/GaN. For all the samples, the PL intensity increased with excitation power while the PL intensity decreased with temperature. The obtained power coefficients and activation energies support the proposed mechanism causing the luminescence. The defects present in CIS/GaN and CIS/InN were also identified. The results from this study are consistent with those of CIS single crystals and show that the CIS absorber layer can be incorporated with III-nitride materials, and its absorption can be extended beyond the spectrum covered by plain CIS.

Understanding the Impact of Bismuth Heterovalent Doping on the Structural and Photophysical Properties of CH3 NH3 PbBr3 Halide Perovskite Crystals with Near-IR Photoluminescence.

A comprehensive study unveiling the impact of heterovalent doping with Bi3+ on the structural, semiconductive, and photoluminescent properties of a single crystal of lead halide perovskites (CH3 NH3 PbBr3 ) is presented. As indicated by single-crystal XRD, a perfect cubic structure in Bi3+ -doped CH3 NH3 PbBr3 crystals is maintained in association with a slight lattice contraction. Time-resolved and power-dependent photoluminescence (PL) spectroscopy illustrates a progressively quenched PL of visible emission, alongside the appearance of a new PL signal in the near-infrared (NIR) regime, which is likely to be due to energy transfer to the Bi sites. These optical characteristics indicate the role of Bi3+ dopants as nonradiative recombination centers, which explains the observed transition from bimolecular recombination in pristine CH3 NH3 PbBr3 to a dominant trap-assisted monomolecular recombination with Bi3+ doping. Electrically, it is found that the mobility in pristine perovskite crystals can be boosted with a low Bi3+ concentration, which may be related to a trap-filling mechanism. Aided by temperature (T)-dependent measurements, two temperature regimes are observed in association with different activation energies (Ea ) for electrical conductivity. The reduction of Ea at lower T may be ascribed to suppression of ionic conduction induced by doping. The modified electrical properties and NIR emission with the control of Bi3+ concentration shed light on the opportunity to apply heterovalent doping of perovskite single crystals for NIR optoelectronic applications.

Rb4Ag2BiBr9: A Lead-Free Visible Light Absorbing Halide Semiconductor with Improved Stability.

Replacement of the toxic heavy element lead in metal halide perovskites has been attracting a great interest because the high toxicity and poor air stability are two of the major barriers for their widespread utilization. Recently, mixed-cation double perovskite halides, also known as elpasolites, were proposed as an alternative lead-free candidate for the design of nontoxic perovskite solar cells. Herein, we report a new nontoxic and air stable lead-free all-inorganic semiconductor Rb4Ag2BiBr9 prepared using the mixed-cation approach; however, Rb4Ag2BiBr9 adopts a new structure type (Pearson's code oP32) featuring BiBr6 octahedra and AgBr5 square pyramids that share common edges and corners to form a unique 2D layered non-perovskite structure. Rb4Ag2BiBr9 is also demonstrated to be thermally stable with the measured onset decomposition temperature of To = 520 °C. Optical absorption measurements and density functional theory calculations suggest a nearly direct band gap for Rb4Ag2BiBr9. Room temperature photoluminescence (PL) measurements show a broadband weak emission. Further, temperature-dependent and power-dependent PL measurements show a strong competition between multiple emission centers and suggest the coexistence of defect-bound excitons and self-trapped excitons in Rb4Ag2BiBr9.

Effect of incident laser power and incorporating polariton effects in GaN microrods

Optical property of GaN microrods was investigated by power dependent photoluminescence (PL). Two PL peak envelops were observed in near band edge region and this feature has been attributed to the exciton-photon coupling. Clear mode formation in PL spectrum was observed and modes were redshifted slightly with the increase in laser power for two microrods. Redshifting of PL peak envelops in near band edge region was huge in one microrod as compare to other microrods with the increase in laser power. This is attributed to both the electron-hole plasma effect and exciton-photon coupling which depends on the morphology. Modes in the defect related region were prominent than the modes in near band edge region at low laser power and with the increase in laser power, modes in the defect region disappears and modes in the near band edge region becomes more prominent. Cavity modes get guided in the cavity at higher power when the polariton formation takes place. Cavity modes are not guided at low power when modes are photonic.

Study on Optical Properties and Internal Quantum Efficiency Measurement of GaN-based Green LEDs

In this paper, the optical properties of GaN-based green light emitting diode (LED) are investigated and the internal quantum efficiency (IQE) values are measured by temperature dependent photoluminescence (TDPL) and power dependent photoluminescence (PDPL) methods. The "S-shaped” shift of peak wavelength measured at different temperature disappears gradually and the spectra broadening can be observed with increasing excitation power. The IQE calculation results of TDPL, which use the integrated intensity measured at low temperature as unity, can be modified by PDPL in order to acquire more accurate IQE values.

Optical properties of quasi-type-II structure in GaAs/GaAsSb/GaAs coaxial single quantum-well nanowires

The GaAsSb-based quantum well plays a very important role in optoelectronic devices due to its excellent wavelength tunability. When the dimension reduces, the quantum confinement effect will take place and the quantum well in nanowires will show many interesting characteristics. GaAsSb-based quantum-well nanowires are of contemporary interest. However, the properties of the quasi-type-II structure in a single quantum well nanowire have been rarely investigated. Here, we grow GaAs/GaAs0.92Sb0.08/GaAs coaxial single quantum-well nanowires and discussed their power-dependent and temperature-dependent photoluminescence. We find that due to the small band offset of conduction bands, both type-I like and type-II like emission exist in our nanowires. When electrons obtain enough thermal energy through collisions or surrounding environment, they will overcome the barrier and diffuse to the GaAs conduction band, which contributes to the type-II like recombination. These results show the optical property of the quasi-type-II quantum well in nanowires, which can pave the way toward future nanoscale quantum well devices.