Integrated Electroluminescence Intensity Research Articles

A Waveguide-Integrated Two-Dimensional Light-Emitting Diode Based on p-Type WSe2/n-Type CdS Nanoribbon Heterojunction.

Transition metal dichalcogenides (TMDs) have emerged as two-dimensional (2D) building blocks to construct nanoscale light sources. To date, a wide array of TMD-based light-emitting devices (LEDs) have been successfully demonstrated. Yet, their atomically thin and planar nature entails an additional waveguide/microcavity for effective optical routing/confinement. In this sense, integration of TMDs with electronically active photonic nanostructures to form a functional heterojunction is of crucial importance for 2D optoelectronic chips with reduced footprint and higher integration capacity. Here, we report a room-temperature waveguide-integrated light-emitting device based on a p-type monolayer (ML) tungsten diselenide (WSe2) and n-type cadmium sulfide (CdS) nanoribbon (NR) heterojunction diode. The hybrid LED exhibited clear rectification under forward biasing, giving pronounced electroluminescence (EL) at 1.65 eV from exciton resonances in ML WSe2. The integrated EL intensity against the driving current shows a superlinear profile at a high current level, implying a facilitated carrier injection via intervalley scattering. By leveraging CdS NR waveguides, the WSe2 EL can be efficiently coupled and further routed for potential optical interconnect functionalities. Our results manifest the waveguided LEDs as a dual-role module for TMD-based optoelectronic circuitries.

Wave-shaped temperature dependence characteristics of the electroluminescence peak energy in a green InGaN-based LED grown on silicon substrate

This study aimed to investigate temperature dependencies at different injection currents (ICs) of the electroluminescence (EL) spectra from a green InGaN/GaN light-emitting diode (LED) based on multiple quantum wells (MQWs) grown on a Si substrate in a wide range of ICs (0.001–350 mA) and temperatures (6–350 K). The results show that the temperature-changing characteristic of the EL peak energy gradually evolves from an approximately V-shaped temperature dependence into a wave-shaped (three-step blueshift) dependence with increasing IC. Finally, it emerges as an approximately inverted V-shaped temperature dependence. The behavior reflects the fact that the emission related to InGaN is significantly influenced by the changing recombination dynamics of carriers with rising temperature or IC. This is attributed to the presence in the MQW active region of a stronger carrier localization effect across three zones with different average In contents. Moreover, with the decline of the temperature at lower ICs, the temperature behavior of the external quantum efficiency (EQE) value is dominated by the deactivated non-radiative centers. This phenomenon occurs not only in the higher temperature range but also at lower temperatures due to more In-content-induced structural defects, which are confirmed by measurements of the integrated EL intensity as well as the EQE dependence on IC.

Fabrication and characterization of asymmetric metal/Ge/metal diodes with Ge-on-insulator substrate

We fabricated asymmetric metal/Ge/metal diodes with p-type Ge-on-insulator (GOI) and bulk Ge substrates, from which direct band gap electroluminescence (EL) was observed at room temperature. The integrated EL intensity for p-GOI diodes is approximately 16 times that for bulk p-Ge diodes, owing to the structure of GOI that confines carriers in the very thin Ge layer, and consequently enhances both the electron density in the conduction band and the efficiency of radiative recombination. Micro-photoluminescence spectra of p-GOI show defect-related peaks in the range of 0.70–0.75 eV, of which the intensity and energy depend on the measurement position, which implies fluctuation of defect type and density in p-GOI. Those defects contribute to the low-energy part of the broad EL spectra in p-GOI diodes.

Experimentally study electron overflow from quantum wells into p-side GaN and its effect on the efficiency droop in InGaN/GaN blue-light-emitting-diodes

We experimentally study the electron overflow in InGaN/GaN blue light emitting diodes (LEDs) by measuring the electroluminescence (EL) emission spectra from p-type InGaN/GaN superlattices (SLs) adjacent to the quantum well (QW) active region. The integrated EL intensity ratio of the SL peak to quantum well peak was calculated to reveal the dependence of electron overflow from active region on current density. Our experimental results indicate that the electron overflow from the active region at both low and high current densities, and no saturation is observed even at the maximum current density (280 A/cm2) applied. It is found that both direct electron leakage and Auger assisted leakage contribute to the electron overflow. Further analyses reveal that at low current densities (less than 150 A/cm2) direct electron leakage dominates the mechanism for electron overflow, and simultaneously for efficiency droop, while electron overflow is mainly caused by Auger assisted leakage at high current density. These results identify the main mechanisms for efficiency droop of LEDs at different current density regimes.

Systematic study of GeSn heterostructure-based light-emitting diodes towards mid-infrared applications

Temperature-dependent characteristics of GeSn light-emitting diodes with Sn composition up to 9.2% have been systematically studied. Such diodes were based on Ge/GeSn/Ge double heterostructures (DHS) that were grown directly on a Si substrate via a chemical vapor deposition system. Both photoluminescence and electroluminescence spectra have been characterized at temperatures from 300 to 77 K. Based on our theoretical calculation, all GeSn alloys in this study are indirect bandgap materials. However, due to the small energy separation between direct and indirect bandgap, and the fact that radiative recombination rate greater than non-radiative, the emissions are mainly from the direct Γ-valley to valence band transitions. The electroluminescence emissions under current injection levels from 102 to 357 A/cm2 were investigated at 300 K. The monotonic increase of the integrated electroluminescence intensity was observed for each sample. Moreover, the electronic band structures of the DHS were discussed. Despite the indirect GeSn bandgap owing to the compressive strain, type-I band alignment was achieved with the barrier heights ranging from 11 to 47 meV.

Electroluminescence properties of LEDs based on electron-irradiated p-Si

The electroluminescence (EL) in n+–p–p+ light-emitting-diode (LED) structures based on Si irradiated with electrons and annealed at high temperature is studied. The LEDs are fabricated by the chemical- vapor deposition of polycrystalline silicon layers doped with high concentrations of boron and phosphorus. Transformation of the EL spectra with current in the LEDs is well described by six Gaussian curves. The peak positions of these curves are current-independent and equal to 1233, 1308, 1363, 1425, 1479, and 1520 nm. The dependences of the integrated EL intensity and of the full-width at half-maximum (FWHM) of the lines on current are examined.

Correlation of Electroluminescence With Open-Circuit Voltage From Thin-Film CdTe Solar Cells

Electroluminescence (EL) measurements of several CdTe solar cells, a GaAs solar cell, and an AlGaAs LED were performed. The mean integrated EL intensity, normalized to exposure time and injection-current density, correlates well with the difference between ideal open-circuit voltage and measured open-circuit voltage, nearly independently of cell details or uniformity. This trend holds for devices of different absorber materials but does not hold for devices which exhibit poor superposition between light and dark current–voltage ( J–V ) curves.

Polarization‐resolved electroluminescence study of InGaN/GaN dot‐in‐a‐wire light‐emitting diodes grown by molecular beam epitaxy

The polarization properties of light emission from InGaN nanowire (NW) light‐emitting diodes (LEDs) have been studied with the use of goniometric setup. A maximum polarization ratio of ∼0.7 has been obtained from the edge emission of NW array‐based LEDs and the light is mainly polarized parallel to the c‐axis of NWs. The nearly isotropic polarization response from a core–shell NW LED structure is also observed, and it is found that the degree of polarization is strongly depended on the NW diameter. With the growth of the AlGaN shell, the resulting diameter of core–shell NWs becomes larger and is comparable to the emission wavelength, thus weakening the optical confinement effect and the polarization behavior. The size‐dependent polarization properties of NW structures are further verified by the finite‐difference time‐domain simulation. NWs with diameters much less than the emission wavelength render a strong contrast between the p‐ and s‐polarized light emissions.(Left) FE‐SEM image of MBE‐grown NWs covered with polyimide. (Right) Plot of integrated electroluminescence intensity as a function of polarizer angle for a NW LED.

Near-infrared superluminescent diode using stacked self-assembled InAs quantum dots with controlled emission wavelengths

A near-infrared superluminescent diode (SLD) using stacked InAs/GaAs quantum dots (QDs) was developed. The emission wavelength of each QD layer was controlled by varying the thickness of a strain-reducing layer deposited on the QD. The controlled ground state emission peaks enabled formation of a dipless broadband spectrum with the contributions of the first excited state emissions. The bandwidth of the resulting emission was approximately 170 nm with a peak wavelength of 1280 nm. The integrated electroluminescence intensity exhibited a superlinear relation with respect to the injected current density, indicating an SLD emission behavior owing to contributions of stimulated emissions from QDs. The developed broadband SLD was found to be suitable as a potential light source for optical coherence tomography (OCT) leading to improved resolution of OCT images. The axial resolution estimated from the Fourier-transformed spectrum is 4.1 µm.

Optimization of the electroluminescence from SiNx-based light-emitting devices by modulating the size and morphology of silver nanostructures

A maximal enhancement of ~6.5 times of the external quantum efficiency (EQE) for SiNx-based light-emitting devices (LEDs) is achieved by magnetron sputtering a silver nanostructures layer onto the active matrix. The enhancement of EQE is affected by the dimension and morphology of silver nanostructures, which can be controlled by the sputtering time and the post treatment of rapid thermal annealing. The optimal size of silver nanostructures is about 100 nm in diameter by comparing the integrated electroluminescence intensity under the same input power. The optimization of EQE for SiNx-based LEDs is discussed by considering the contributions of the enhancement of light-extraction efficiency induced by the surface roughening of the front electrode, internal quantum efficiency due to the coupling between excitons and localized surface plasmons, and carrier injection efficiency. Our work may provide an alternative approach for the fabrication of Si-based light sources with promising luminescence efficiency.

Fabrication and Characterization of High-Quality Factor GaN-Based Resonant-Cavity Blue Light-Emitting Diodes

High-quality factor (Q >; 1700) GaN-based blue resonant-cavity light-emitting diodes (RCLEDs) incorporating an InGaN/GaN multiquantum well active region, two high-reflectivity dielectric-distributed Bragg reflectors, and a thin indium tin oxide (ITO) layer are fabricated by a two-step substrate transfer technique. Electroluminescence measurements showed a narrow linewidth of 0.26 nm at the wavelength of 450.6 nm by precisely placing the ITO layer at the node position of the electric field, corresponding to a high Q-value of 1720. Further, adopting a chemical-mechanical polishing (CMP) technique to polish the GaN surface after the removal of sapphire substrate, an even higher Q-value of 2170 was obtained. This improvement was attributed to the exclusion of the defect-rich buffer layer and the achievement of a smooth surface with a root mean square roughness below 1 nm. The integrated electroluminescence intensity was enhanced by 40% as compared with the RCLEDs without CMP at a current density of 8 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> .

Luminescence at 1.5 µm from Si/GeSn nanodot/Si structures

Broad photoluminescence (PL) around 1.5 µm (∼0.8 eV) from Si/GeSn nanodot (ND)/Si structures was enhanced by high temperature post-annealing. This PL was related to radiative electronic states created in the Si-capping layer owing to the underlying GeSn NDs. A light-emitting diode was fabricated containing the Si-capped GeSn NDs as an active layer. Broad electroluminescence (EL) was observed even at room temperature (RT) near 0.8 eV which is consistent with the optical fibre communication wavelength (∼1.5 µm). The temperature dependence of the integrated EL intensity revealed two activation energies of 3 and 33 meV. The light emission with a higher activation energy of 33 meV leading to the RT luminescence was considered to have the same origin as that of broad PL. This demonstrates that the Si/GeSn ND/Si structures are promising as a group IV-based light-emitting material compatible with Si technology.

Size-dependent electroluminescence from Si quantum dots embedded in amorphous SiC matrix

Si quantum dots (QDs) were formed by thermal annealing the hydrogenated amorphous silicon carbide films (a-SiCx:H) with different C/Si ratio x, which were controlled by using a different gas ratio R of methane to silane during the deposition process. By adjusting x and post annealing temperature, the QD size can be changed from 1.4 to 4.2 nm accordingly, which was verified by the Raman spectra and transmission electron microscopy images. Size-dependent electroluminescence (EL) was observed, and the EL intensity was higher for the sample containing small-sized Si QDs due to the quantum confinement effect (QCE). The EL peak energy as a function of the Si QDs size was in good agreement with a modified effective mass approximation (EMA) model. The calculated finite barrier potential of the Si QDs embedded in SiC matrix is 0.4 and 0.8 eV for conduction and valence band, respectively. Moreover, the current-voltage properties and the linear relationship between the integrated EL intensity and injection current indicate that the carrier transport is dominated by Fowler–Nordheim tunneling and the EL mechanism is originated from the bipolar recombination of electron-hole pairs at Si QDs. Our results demonstrate Si QDs embedded in amorphous SiC matrix has the potential application in Si-based light emitting devices and the third-generation solar cells.

Green Electroluminescence From ZnCdO Multiple Quantum-Well Light-Emitting Diodes Grown by Remote-Plasma-Enhanced Metal–Organic Chemical Vapor Deposition

Zn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.80</sub> Cd <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.20</sub> O/Zn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.92</sub> Cd <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.08</sub> O multiple quantum-well (MQW) light-emitting diodes (LEDs) on <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">p</i> -type 4H-SiC substrates, using remote-plasma-enhanced metal-organic chemical vapor deposition, have been fabricated and characterized. Band-edge green electroluminescence (EL) at 500 nm has been obtained. The ZnCdO MQW LEDs show a rectifying current-voltage behavior with a turn-on voltage of 6.0 V. The integrated EL intensity linearly increases with increasing injection current density up to 1.9 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . Furthermore, the presence of an EL blue-shift of 20 nm with increasing current and a large EL spectral width in comparison to PL are discussed in terms of Cd compositional fluctuations.

Light emission and floating gate memory characteristics of germanium nanocrystals

AbstractWe report Ge nanocrystals (NCs) based dual functional light emitting and metal insulator semiconductor (MIS) flash memory devices, fabricated by rf sputtering. Transmission electron micrographs revealed the formation of spherically shaped Ge NCs. We have observed broad electroluminescence (EL) around 760 nm, which is attributed to electron–hole recombination in quantum confined Ge NCs. The dependence of integrated EL intensity on drive currents has also been studied. An anti‐clockwise hysteresis behaviour is observed in capacitance–voltage measurements of MIS devices for different sweep voltages, indicating net electron trapping in NC based floating gates.

Electroluminescence Studies of Modulation p-Doped Quantum Dot Laser Structures

Electroluminescence (EL) measurements have been performed on InAs/InGaAs/GaAs quantum dot (QD) structures with varying amounts of modulation p-doping. Temperature-dependent EL measurements show a reduction of the integrated EL intensity (IEL) with increasing temperature but with the size of this reduction decreasing with increasing doping level. An increase in the activation energy controlling the EL quenching is found with increasing p-doping. This is attributed to an increased coulombic attraction between the extrinsic holes and injected electrons. At room temperature and low injection current, a superlinear dependence of the IEL on the injection current is observed. This superlinearity decreases as the p-doping increases and this behavior indicates a reduction in the amount of nonradiative recombination. This reduction is believed to be caused by the saturation of nonradiative centers and/or reduced escape of electrons to the GaAs barrier due to the increased confinement potential.

Anomalous temperature-dependent behaviors of electroluminescence phenomena in disordered ITO/PEDOT/PF/Ca/Al polymer light-emitting diodes

In this study we systematically investigated the anomalous Berthelot-type optical property dependence of the electroluminescence (EL) that occurs in polymer light-emitting diode (PLED) structures. Devices were operated at 0.1 and 0.5 mA over a temperature range from 300 to 20 K. Initially, the emission intensity increased as the temperature decreased, until a maximum was reached at the turning-point temperature T t . After this, the intensity decreased with further decrease of temperature down to 20 K. A smaller reduction in intensity was exhibited by the device with 0.5 mA, a larger one exhibited by the device with 0.1 mA. The strong electron–electron scattering arising from high excitation, high microcrystalline randomization and high hopping frequency led to the corresponding dominance of the Berthelot-type process by a large T t . It can be seen in the Arrhenius plot of the integrated EL intensity that a smaller activation energy can be expected to facilitate the redistribution of the thermal carriers, overcoming the energy barrier and the large degradation of luminescence. This results in not only better than carrier confinement, but also enhances the quantum efficiency of PLED structures requiring lower excitation.

Investigation of Leakage Current of AlGaN/GaN HEMTs Under Pinch-Off Condition by Electroluminescence Microscopy

Results from on-wafer electroluminescence (EL) microscopy on AlGaN/GaN high-electron-mobility transistors with leakage currents varying over four orders of magnitude are presented. In the off-state region the integrated EL intensity is proportional to the leakage current and independent of gate width for the devices under study. The slope of the integrated EL–leakage current dependence is determined by the electrical field in the source–drain direction. The influence of the GaN cap thickness is small or even negligible for higher drain bias. Stress during accelerated aging results in enhanced degradation for areas of enhanced leakage current and/or electric field values.

Charging effect on electroluminescence performance of nc-Si/a-SiO2 films

Silicon nanocrystals are synthesized by reactive magnetron sputtering to distribute throughout the gate oxide layer. Electroluminescence (EL) from amorphous silicon dioxide (a-SiO2) film embedded with Si nanocrystals (nc-Si) has been studied under various gate voltages. Both the integrated EL intensity and the gate current first increase and then decrease with increasing gate voltage. The decrease in EL intensity is ascribed to the charging up of the nc-Si associated trapping centers. The EL intensity can be partially recovered by application of positive electric stress to release part of the trapped charges.

Dependence of the Electroluminescence on the Spacer Layer Growth Temperature of Multilayer Quantum-Dot Laser Structures

Electroluminescence (EL) measurements have been performed on a set of In(Ga)As-GaAs quantum-dot (QD) structures with varying spacer layer growth temperature. At room temperature and low injection current, a superlinear dependence of the integrated EL intensity (IEL) on the injection current is observed. This superlinearity decreases as the spacer layer growth temperature increases and is attributed to a reduction in the amount of nonradiative recombination. Temperature-dependent IEL measurements show a reduction of the IEL with increasing temperature. Two thermally activated quenching processes, with activation energies of ~ 157 meV and ~ 320 meV, are deduced and these are attributed to the loss of electrons and holes from the QD ground state to the GaAs barriers. Our results demonstrate that growing the GaAs barriers at higher temperatures improves their quality, thereby increasing the radiative efficiency of the QD emission.