Excitation Power-dependent Photoluminescence Measurements Research Articles

An efficiently excited Eu3+ luminescent site formed in Eu,O-codoped GaN

For the development of III-nitride-semiconductor-based monolithic micro-light-emitting diode (LED) displays, Eu,O-codoped GaN (GaN:Eu,O) is a promising material candidate for the red LEDs. The luminescence efficiency of Eu-related emission strongly depends on the local atomic structure of Eu ions. Our previous research has revealed that post-growth thermal annealing is an effective method for reconfiguring luminescent sites, leading to a significant increase in light output. We observed the preferential formation of a site with a peak at ∼2.004 eV by the annealing process. In this study, we demonstrate that it is a previously unidentified independent site (OMVPE-X) using combined excitation–emission spectroscopy and time-resolved photoluminescence measurements. In addition, we perform excitation power-dependent photoluminescence measurements and show that this OMVPE-X site dominates the emission at a low excitation power region despite its small relative abundance, suggesting a high excitation efficiency. Most importantly, applying our annealing technique to an LED exhibits a reasonably increased electroluminescence intensity associated with OMVPE-X, confirming that this site has a high excitation efficiency also under current injection. These results demonstrate the importance of OMVPE-X as a notable luminescent site for brighter and more efficient GaN:Eu,O-based LEDs.

Twist-angle-dependent momentum-space direct and indirect interlayer excitons in WSe2/WS2 heterostructure.

Interlayer excitons (ILEs) in the van der Waals (vdW) heterostructures of type-II band alignment transition metal dichalcogenides (TMDCs) have attracted significant interest owing to their unique exciton properties and potential in quantum information applications. However, the new dimension that emerges with the stacking of structures with a twist angle leads to a more complex fine structure of ILEs, presenting both an opportunity and a challenge for the regulation of the interlayer excitons. In this study, we report the evolution of interlayer excitons with the twist angle in the WSe2/WS2 heterostructure and identify the direct (indirect) interlayer excitons by combining photoluminescence (PL) and density functional theory (DFT) calculations. Two interlayer excitons with opposite circular polarization assigned to the different transition paths of K-K and Q-K were observed. The nature of the direct (indirect) interlayer exciton was confirmed by circular polarization PL measurement, excitation power-dependent PL measurement and DFT calculations. Furthermore, by applying an external electric field to regulate the band structure of the WSe2/WS2 heterostructure and control the transition path of the interlayer excitons, we could successfully realize the regulation of interlayer exciton emission. This study provides more evidence for the twist-angle-based control of heterostructure properties.

Research on the photoluminescence of spectral broadening by rapid thermal annealing on InAs/GaAs quantum dots

Photoluminescence (PL) test was conducted to investigate the effect of rapid thermal annealing (RTA) on the optical performance of self-assembled InAs/GaAs quantum dots (QDs) at the temperatures of 16 and 300 K. It was found that after RTA treatment, the PL spectrum of the QDs sample had a large blue-shift and significantly broadened at 300 K. Compared with the as-grown InAs QDs sample, the PL spectral width has increased by 44.68 meV in the InAs QDs sample RTA-treated at 800 °C. The excitation power-dependent PL measurements showed that the broadening of the PL peaks of the RTA-treated InAs QDs should be related to the emission of the ground state (GS) of different-sized InAs QDs, the InAs wetting layer (WL) and the In0.15Ga0.85As strain reduction layer (SRL) in the epitaxial InAs/GaAs layers.

Photoluminescence investigation of the properties of GaAsSb in the dilute Sb regime

GaAsSb epitaxial layers are grown using liquid-phase epitaxy (LPE) technique over oriented GaAs substrates. The layers contain up to 1.84% Sb, as estimated from high-resolution X-ray diffraction (HRXRD) measurements. Detailed photoluminescence (PL) studies are made on the as-grown and high-temperature-annealed layers to reveal various electronic transitions in the materials. Excitation power-dependent PL measurements are done to investigate the origin of each of the transitions. The bandgap energy, obtained from PL, shows a reduction of approximately 17 meV per at% increase in Sb content, which agrees well with the bandgap reduction, calculated with the band anticrossing (BAC) model. Analysis of the temperature-dependent PL data, using Bose–Einstein model, indicates that both the electron–phonon interaction and the average phonon temperatures strongly depend on the Sb content in GaAsSb.

Origin of PL emission at 0.797 eV in near stoichiometric spray deposited CZTS thin films

Photoluminescence (PL) spectroscopy has been employed for analyzing defect levels in near stoichiometric Cu2ZnSnS4 (CZTS) thin films deposited using chemical spray pyrolysis (CSP) technique. An emission at 0.797 eV could be recorded at 15 K from the near stoichiometric CZTS thin films. This emission was persistent up to room temperature and could be identified as “donor to acceptor pair (DAP)” transition and also bears close connection with concentration of copper. Using temperature as well as excitation power dependent PL measurements and X-ray photoelectron spectroscopy (XPS) depth profile analysis, the emission at 0.797 eV could be identified as arising from transition between donor level (due to OSn defect) and acceptor level (created by Cuzn defect).

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.

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.

Monolithic multiple colour emission from InGaN grown on patterned non-polar GaN

A novel overgrowth approach has been developed in order to create a multiple-facet structure consisting of only non-polar and semi-polar GaN facets without involving any c-plane facets, allowing the major drawbacks of utilising c-plane GaN for the growth of III-nitride optoelectronics to be eliminated. Such a multiple-facet structure can be achieved by means of overgrowth on non-polar GaN micro-rod arrays on r-plane sapphire. InGaN multiple quantum wells (MQWs) are then grown on the multiple-facet templates. Due to the different efficiencies of indium incorporation on non-polar and semi-polar GaN facets, multiple-colour InGaN/GaN MQWs have been obtained. Photoluminescence (PL) measurements have demonstrated that the multiple-colour emissions with a tunable intensity ratio of different wavelength emissions can be achieved simply through controlling the overgrowth conditions. Detailed cathodoluminescence measurements and excitation-power dependent PL measurements have been performed, further validating the approach of employing the multiple facet templates for the growth of multiple colour InGaN/GaN MQWs. It is worth highlighting that the approach potentially paves the way for the growth of monolithic phosphor-free white emitters in the future.

Hole-Injection Layer-Free Perovskite Light-Emitting Diodes.

In this study, a dual-source vapor evaporation method was employed to fabricate the high-quality CsPbBr3 thin films with a good crystalline and high surface coverage. Temperature-dependent and excitation power-dependent photoluminescence measurements were performed to study the optical properties of the CsPbBr3 material. Further, based on the experimental data, the temperature sensitivity coefficient of band gap and exciton binding energy were estimated. More importantly, for the first time, we designed and prepared a hole-injection layer-free perovskite light-emitting diode (LED) based on the Au/MgO/CsPbBr3/n-MgZnO/n+-GaN structure, producing an intense green emission (∼538 nm) with a high purity. Besides, the device demonstrated a high luminance of 5025 cd/m2, an external quantum efficiency of 1.46%, a current efficiency of 1.92 cd/A, and a power efficiency of 1.76 lm/W. We studied in detail the current-voltage and electroluminescence properties of the prepared device and proposed the hole generation models and the carrier transport/recombination mechanisms to make these interesting characteristics certain. The results obtained would provide a new and effective strategy for the design and preparation of perovskite LEDs.

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.

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/GaN quantum well structures with greatly enhanced performance on a-plane GaN grown using self-organized nano-masks

Great improvement in crystal quality of a-plane (non-polar) GaN has been achieved using a simple but effective overgrowth technique based on self-organized nano-masks. This has been confirmed by a massive reduction in full width at half maximum of x-ray diffraction rocking curves measured along both symmetrical and asymmetrical directions. Taking the advantage of utilising the nano-masks, a quick coalescence with a thickness of less than 1 μm has been obtained, which is much less than that using any conventional overgrowth techniques. The dislocation density has been significantly reduced by more than one order magnitude compared with a standard a-plane GaN layer on sapphire. An InGaN/GaN multiple quantum well (MQW) structure grown on the high quality a-plane GaN has demonstrated an enhancement with a factor of 7 in optical efficiency, compared with a similar MQW structure grown on a standard c-plane GaN layer. The excitation-power dependent photoluminescence measurements have confirmed that the a-plane InGaN/GaN MQW structure does not suffer from quantum-confined Stark effect any more.

Optical and structural investigation of a-plane GaN layers on r-plane sapphire with nucleation layer optimization

Nonpolar a-plane GaN epilayers are grown on several r-plane sapphire substrates by metal organic chemical vapour deposition using different nucleation layers: (A) a GaN nucleation layer deposited at low temperature (LT); (B) an AlN nucleation layer deposited at high temperature; or (C) an LT thin AlN nucleation layer with an AlN layer and an AlN/AlGaN superlattice both subsequently deposited at high temperature. The samples have been characterized by X-ray diffraction (XRD), atomic force microscopy and photoluminescence. The GaN layers grown using nucleation layers B and C show narrower XRD rocking curves than that using nucleation layer A, indicating a reduction in crystal defect density. Furthermore, the GaN layer grown using nucleation layer C exhibits a surface morphology with triangular defect pits eliminated completely. The improved optical property, corresponding to the enhanced crystal quality, is also confirmed by temperature-dependent and excitation power-dependent photoluminescence measurements.

Synthesis and characterization of Ag-doped p-type ZnO nanowires

P-type ZnO nanowires with silver (Ag) doping were synthesized via a chemical vapor deposition process. The incorporation of Ag was confirmed by selected-area energy-dispersive x-ray spectroscopy. The formation of acceptor states was demonstrated by temperature and excitation power-dependent photoluminescence measurements. Characterization of field-effect transistors using Ag-doped ZnO nanowires as channels showed p-type conductivity of the nanowires with a hole concentration of 4.9×1017 cm−3 and a carrier mobility of approximately 0.18 cm2 V−1 s−1.

Fabrication and characterization of wide band-gap CuGaSe 2 thin films for tandem structure

The effects of CuGaSe 2 (CGS) absorber layers with various film thicknesses have been investigated by using scanning electron microscopy (SEM) and photoluminescence (PL). SEM measurements show that the thickness of the CGS absorber layers grown on Mo-coated soda-lime glass substrate increases with an increase in Ga flux. Moreover, a trend in the grain size and surface roughness can be observed. Single broad peaks centered around 1.64 eV are observed in the PL spectra of the CGS samples. From the excitation power-dependent PL measurement, the single broad peaks are associated with the donor–acceptor pair transition from CGS absorber layers. In addition, we demonstrate that different film thicknesses of CGS absorber layer result in different defect concentration by temperature-dependent PL measurement.

The influence of a capping layer on optical properties of self-assembled InGaN quantum dots

Optical properties of InGaN quantum dots (QDs) with and without a GaN capping layer have been investigated, showing a major difference between each other. Compared with the InGaN QDs with a GaN capping layer, those grown under identical conditions but without the GaN capping layer showed much stronger photoluminescence (PL) emission and a ∼350 meV blueshift in emission energy. The excitation power-dependent PL measurements indicated that the emission energy of the QDs with the capping layer showed a large blueshift with increasing excitation power, while there was a negligible shift in the QDs without the capping layer. The major difference between them is attributed to existence of a strong quantum-confined Stark effect (QCSE) in the QDs with the capping layer, while there is no clear QCSE observed in the uncapped QDs. The transition energy has been calculated for both QDs within the framework of effective-mass approximation and variational approach, showing a good agreement with the experimental data. The results obtained should be highly taken into account in investigating the optical properties of InGaN QDs on a GaN surface.

Bi-induced acceptor states in ZnO by molecular-beam epitaxy

Bi-doped ZnO films were grown on Si (111) substrates by molecular-beam epitaxy. X-ray photoelectron spectroscopy and diffraction measurements reveal that Bi was incorporated into ZnO films without any phase separation. Room-temperature Hall effect measurements show a significant reduction of electron concentration and an increase of resistivity for Bi-doped ZnO films. In addition, a 3.222eV photoluminescence emission was observed particularly in the Bi-doped ZnO films and was identified as a donor-acceptor pair transition by temperature-dependent and excitation power-dependent photoluminescence measurements. These results indicate the possibility of generating acceptor states by Bi doping.

Photoluminescence band of Hf origin in hafnium-implanted silicon in the energy range700meVto950meV

A new photoluminescence (PL) band in the energy range of $700\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}950\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ associated with hafnium implanted in silicon is reported. A shift in the position of photoluminescence peaks observed on the samples implanted with two different isotopes of Hf confirms the Hf-related origin of the observed photoluminescence band. Activation of the Hf-optical centers requires a $1000\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ anneal step. The intensity of the PL lines depends on the cooling conditions. The spectrum consists of 5 peaks in the rapidly quenched sample as opposed to 21 in the slowly cooled sample. Temperature- and excitation power-dependent PL measurements were performed to identify their nature. The $943.8\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ line was associated with an exciton complex, while the $896.6\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ line originates from impurity bound exciton. The $896.6\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ emission line appears to be related to a Hf-related deep level defect at ${E}_{\mathrm{C}}\text{\ensuremath{-}}0.22\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$, the $845\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ line to a deep level defect at ${E}_{\mathrm{C}}\text{\ensuremath{-}}0.27\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. The pressure measurements indicate that the $845\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$ peak could be an internal transition. It is also found that oxygen coimplantation enhances the PL intensity in rapidly quenched samples.

Study of the strain relaxation in InGaN/GaN multiple quantum well structures

The relationship between strain relaxation and quantum well number in InGaN/GaN multiple quantum well (MQW) structures has been investigated by x-ray diffraction (XRD) and low-temperature photoluminescence (PL) measurements. At low temperature, the PL emission energy of a MQW monotonically decreases with increasing quantum well number, and the large pumping induced blueshift of the emission energy decreases rapidly with an increasing quantum well number. Based on an excitation power dependent PL measurement and a previous calculation that was made assuming a fully strained InGaN/GaN MQW, it is found that only a two-period MQW shows no strain relaxation. Furthermore, the residual strain in a MQW with different quantum well numbers is estimated, which is used for an x-ray diffraction kinetic simulation. The simulation agrees well with our measured XRD data, which in turn supports the conclusion based on the PL measurement. Our results indicate that the strain relaxation starts from a three-period InGaN/GaN MQW. Since the strain relaxation generally has a strong influence on the performance of the optical devices, the results presented should definitely be taken into account in designing InGaN/GaN MQW based optical devices.

Modulation-doping influence on the photoluminescence from the two-dimensional electron gas ofAlxGa1−xN/GaNheterostructures

A photoluminescence (PL) investigation is carried out on an undoped ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}/\mathrm{G}\mathrm{a}\mathrm{N}$ heterostructure and several modulation-doped ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}/\mathrm{G}\mathrm{a}\mathrm{N}$ heterostructures with different doping levels at low temperature. The emission peaks of the first and second levels related to a two-dimensional electron gas (2DEG) are first observed in heavily modulation-doped ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}/\mathrm{G}\mathrm{a}\mathrm{N}$ heterostructures, which is supported by a calculation based on a self-consistent solution of one-dimensional Poisson and Schr\"odinger equations and an excitation-power dependent PL measurement. However, such emission peaks do not appear in undoped ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}/\mathrm{G}\mathrm{a}\mathrm{N}$ heterostructure, in which the 2DEG is confirmed by the observation of a clear Shubnikov--de Hass oscillation and quantum Hall plateaus. The formation of a 2DEG in an undoped ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}/\mathrm{G}\mathrm{a}\mathrm{N}$ heterostructure generally results from a strong piezoelectric field, which results in a rapid diffusion of photoexcited holes into the flatband region of GaN. Consequently, the recombination probability between the 2DEG and photoexcited holes becomes very low, which is the reason that the emission peaks related to the 2DEG become difficult to observe. In the case of heavily modulation-doped ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}/\mathrm{G}\mathrm{a}\mathrm{N}$ heterostructures, the piezoelectric field is screened out by silicon doping in the ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ layer, and the 2DEG formation is due to the electron transfer from ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}$ to GaN, which is the same as that happening in the ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}$ system. Therefore, the emission peaks related to the 2DEG can be observed. The difference of the 2DEG formation mechanism between undoped and modulation-doped ${\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{N}/\mathrm{G}\mathrm{a}\mathrm{N}$ heterostructures is observed.