Broad Electroluminescence Spectra Research Articles

Preparation of Si nanoparticles from bamboo leaves and measurement of their photoluminescence and electroluminescence spectra

In this study, Si nanoparticles were prepared by the Mg reduction of SiO2 extracted from bamboo leaves, and their optical properties were investigated. Si nanoparticles were filtered under reduced pressure to separate the residue and filtrate specimens. Although RT photoluminescence (PL) was observed for both Si nanoparticle specimens, the PL intensity of the filtrate specimens was approximately seven times higher than that of the residue specimens. An electroluminescence (EL) device was fabricated using the Si nanoparticles of the filtrate as the active layer, and its EL properties were evaluated. With an applied voltage of 15 V, red luminescence was observed in the active layer. The Si nanoparticles from the bamboo leaves had relatively broad EL spectra with a peak wavelength approximately 700 nm, similar to that of the PL spectra. These results indicated that Si nanoparticles from bamboo leaves can be recycled for light-emitting devices.

A Method to Realize Efficient Deep-Red Phosphorescent OLEDs with a Broad Spectral Profile and Low Operating Voltages.

Organic light-emitting diodes (OLEDs) used as phototherapy light sources require sufficient spectral distribution in the effective wavelength ranges and low operating voltages. Herein, a double emitting layer structure consisting of a red-emitting Ir(piq)2acac and a deep-red Ir(fliq)2acac was designed to generate a broad electroluminescence spectrum. An efficient TCTA:CN-T2T exciplex system was used as the host of the emitting layer, facilitating effective energy transfer from the exciplex host to the red and deep-red phosphors. The materials used in the exciplex host were also used as the carrier transport layers to eliminate the energy barriers and thus increase the current density. The hole injection layer structures were varied to examine the hole injection capabilities and the carrier balance. The resulting optimized phosphorescent OLEDs with a broad spectral profile exhibit a 90% coverage ratio in the target ranges from 630 to 690 nm, together with a high peak efficiency of 19.1% (10.2 cd/A and 13.8 lm/W). The proposed device only needs 5.2 V to achieve a power density of 5 mW/cm2, implying that the device could be driven via two series-connected button cell batteries. These results illustrate the feasibility of our design concepts and demonstrate the realization of a portable and lightweight OLED phototherapy light source.

Towards low cost fabrication of inorganic white light emitting diode based on electrodeposited Cu2O thin film/TiO2 nanorods heterojunction

Employing low cost hydrothermal and electrodeposition methods is one of the keys to minimize the high cost fabrication of inorganic LEDs. Therefore, we are presenting here for the first time the synthesis of TiO2 nanorods/Cu2O thin film heterojunction employing these methods to fabricate LED. The crystal structure, the optical and the morphology properties of grown samples and the diode device was investigated employing Uv–vis spectroscopy, X-ray diffraction, photoluminescence and scanning electron microscopy. Results confirm the formation of pure tetragonal rutile phase of TiO2 nanorods and polycrystalline structure of Cu2O thin film. Moreover, I-V and electroluminescence measurements were studied to investigate the LED heterojunction performance. The fabricated heterojunction showed diode characteristic behavior and broad electroluminescence spectrum that takes place at the visible region. Consequently, the TiO2 /Cu2O heterojunction provided the prospects to fabricate low cost white inorganic light emitting diode with a large scale area.

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.

Semipolar InGaN/GaN nanostructure light-emitting diodes on c-plane sapphire

The fabrication of electrically injected triangular-nanostripe core–shell semipolar III–nitride LEDs (TLEDs) is demonstrated using interferometric lithography and catalyst-free bottom-up selective-area metal–organic chemical vapor deposition (MOCVD). This alternative approach enables semipolar orientations on inexpensive, c-plane sapphire substrates, in comparison with planar growth on free-standing GaN substrates. Transmission electron microscopy and energy dispersive X-ray spectroscopy reveal nonuniform quantum well thickness and composition, respectively, as a function of location on the triangular stripes. The broad electroluminescence spectra, wavelength shift with increasing current density, and nonlinear light vs current characteristics are well correlated with the observed quantum-well nonuniformities.

Homoleptic Copper(I), Silver(I), and Gold(I) Bisphosphine Complexes

AbstractA series of homoleptic copper(I), silver(I), and gold(I) complexes of two bisphosphine ligands {1,2‐bis(diphenylphosphino)benzene, dppb; bis[2‐(diphenylphosphino)phenyl]ether, POP} have been prepared. Whilst all three [M(dppb)2]BF4 complexes are tetracoordinate, this geometry is found only for the silver(I) complex with POP. Instead, [Cu(POP)2]+ and [Au(POP)2]+ adopt a trigonal coordination geometry with an uncoordinated phosphorus atom. A close inspection of the P–M bond lengths reveals an interesting trend. From the copper to silver and gold complexes, a substantial elongation is found. On the other hand, from the silver to gold compounds, a decrease in the M–P bond length is found. Indeed, gold(I) has a smaller van der Waals radius than silver(I) as a result of its peculiar relativistic effects. Electrochemical investigations revealed two oxidation processes for all of the [M(dppb)2]BF4 and [M(POP)2]BF4 complexes. The first oxidation is likely metal‐centered, whereas the second one corresponds to ligand‐centered processes in all cases. The emission properties of these compounds in solution, in frozen rigid matrices at 77 K, and in the solid state at room temperature have been systematically investigated. Although all of them are weak emitters in solution, remarkably high emission quantum yields were found in the solid state, in particular for [Cu(dppb)2]BF4 and [Ag(dppb)2]BF4. Finally, these two compounds were used for the fabrication of light‐emitting devices. Interestingly, both the copper(I) and the silver(I) complex afford quite broad electroluminescence spectra with white light emission.

Dual-wavelength GaN-based LEDs grown on truncated hexagonal pyramids formed by selective-area regrowth on Si-implanted GaN templates

GaN-based blue light-emitting diodes (LEDs) with micro truncated hexagonal pyramid (THP) array were grown on selective-area Si-implanted GaN (SIG) templates. The GaN epitaxial layer regrown on the SIG templates exhibited selective growth and subsequent lateral growth to form the THP array. The observed selective-area growth was attributed to the different crystal structures between the Si-implanted and implantation-free regions. Consequently, LEDs grown on the GaN THP array emitted broad electroluminescence spectra with multiple peaks. Spatially resolved cathodoluminescence revealed that the broad spectra originated from different areas within each THP. Transmission electron microscopy showed the GaN-based epitaxial layers, including InGaN/GaN multi-quantum wells regrown at different growth rates (or with different In content in the InGaN wells) between the semi-polar and c-face planes of each THP.

Monolithically Integrated Mid-Infrared Quantum Cascade Laser and Detector

We demonstrate the monolithic integration of a mid-infrared laser and detector utilizing a bi-functional quantum cascade active region. When biased, this active region provides optical gain, while it can be used as a detector at zero bias. With our novel approach we can measure the light intensity of the laser on the same chip without the need of external lenses or detectors. Based on a bound-to-continuum design, the bi-functional active region has an inherent broad electro-luminescence spectrum of 200 cm⁻¹, which indicates its use for single mode laser arrays. We have measured a peak signal of 191.5 mV at the on-chip detector, without any amplification. The room-temperature pulsed emission with an averaged power consumption of 4 mW and the high-speed detection makes these devices ideal for low-power sensors. The combination of the on-chip detection functionality, the broad emission spectrum and the low average power consumption indicates the potential of our bi-functional quantum cascade structures to build a mid-infrared lab-on-a-chip based on quantum cascade laser technology.

Monolithically Integrated Mid-Infrared Quantum Cascade Laser and Detector

We demonstrate the monolithic integration of a mid-infrared laser and detector utilizing a bi-functional quantum cascade active region. When biased, this active region provides optical gain, while it can be used as a detector at zero bias. With our novel approach we can measure the light intensity of the laser on the same chip without the need of external lenses or detectors. Based on a bound-to-continuum design, the bi-functional active region has an inherent broad electro-luminescence spectrum of 200 cm−1, which indicate sits use for single mode laser arrays. We have measured a peak signal of 191.5 mV at theon-chip detector, without any amplification. The room-temperature pulsed emission with an averaged power consumption of 4 mW and the high-speed detection makes these devices ideal for low-power sensors. The combination of the on-chip detection functionality, the broad emission spectrum and the low average power consumption indicates the potential of our bi-functional quantum cascade structures to build a mid-infrared lab-on-a-chip based on quantum cascade laser technology.

High‐resolution optical coherence tomography using broadband light source with strain‐controlled InAs/GaAs quantum dots

AbstractRecently, there has been an increasing interest in broadband light sources to develop a biomolecular imaging technique called optical coherence tomography (OCT). We fabricated superluminescent diodes (SLDs) using three kinds of quantum dot (QD) layers with different emission wavelength in the active region. The emission wavelength was controlled by reducing the strain in QDs by using In0.1Ga0.9As strain‐reducing layer. The SLD device showed a broad electroluminescence spectrum with the center wavelength of 1104 nm and the spectral linewidth of 122 nm at the injection of 40 mA, which corresponds to the theoretical axial resolution of 4.4 µm. To estimate the actual resolution of the OCT system using fabricated SLD, we measured the interference signal in the Michelson interferometer. An axial resolution of 5.4 µm, which is close to the theoretical limit, was obtained (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Fabrication and Characterization of n-ZnO Nanonails Array/p+-GaN Heterojunction Diode

A novel heterojunctional structure of n-ZnO nanonails array/p(+)-GaN light-emitting diode was fabricated by Chemical Vapor Deposition method. A broad electroluminescence spectrum shows two peaks centered at 435 nm and 478 nm at room temperature, respectively. By comparing the photoluminescence and electroluminescence spectra, together with analyzing the energy band structure of heterojunction light emitting diode, it suggested that the electroluminescence peak located at 435 nm originates from Mg acceptor level of p(+)-GaN layer, whereas the electroluminescence peak located at 478 nm originates from the defects of n-ZnO nanonails array.

High-performance quantum cascade lasers with wide electroluminescence (∼600 cm−1), operating in continuous-wave above 100 °C

The authors report high temperature continuous-wave (cw) operations of broad-gain quantum cascade lasers based on the anticrossed dual-upper-state to multiple-lower-state design. The devices exhibit extremely wide electroluminescence (>600 cm−1) and subthreshold amplified spontaneous emission (∼570 cm−1) spectra at room temperature. Despite showing such broad electroluminescence spectra, the high-reflection coated, buried heterostructure lasers operating at 6.8 μm demonstrate a low threshold current density of ∼1.5 kA/cm2 and a high power of >500 mW with a high slope efficiency of ∼1.6 W/A in cw mode at 300 K. The maximum cw operating temperature of above 100 °C is achieved.

Broadband light sources using InAs quantum dots with InGaAs strain‐reducing layers

AbstractWe fabricated broadband superluminescent diodes (SLDs) for optical coherence tomography (OCT). We used three kinds of quantum dot (QD) layers with different emission peak wavelengths in the active region of SLD. The emission wavelength was controlled by reducing the strain in QDs; by using the In0.1Ga0.9 As strain‐reducing layer, the peak wavelength shifted toward the longer‐wavelength side, and the photoluminescence peak intensity becomes strong in contrast to QDs on GaAs.By stacking these strain‐controlled QD layers, the SLD device shows a broad electroluminescence spectrum with the center wavelength of 1130 nm and the spectral linewidth of approximately 240 nm at the injection of 1A caused by the increased emission intensity from the excited states. This corresponds to an resolution of 2.3 μm in OCT. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Control of Electroluminescence Spectra Using Hole-Blocking Layer for White Organic Light-Emitting Diodes

We reviewed white organic light-emitting diodes (WOLEDs) briefly in this paper and suggested the significance of a hole-blocking material for the fabrication of WOLEDs. And we studied the electroluminescence (EL) spectra of two kinds of WOLEDs using each different hole-blocking material. The WOLED with blue-emitting aluminum quinoline (BAlq) had a broad EL spectrum. The broad spectra were caused by overlapping three emissions of 3 kinds of emission materials due to wide recombination zone. The WOLED with bathocuproine (BCP) showed two separated emissions. The difference between two specimens was concluded by strong hole-blocking property of BCP. The introduction of BCP produced a recombination zone due to hole accumulation at the BCP interface.

Electroluminescence from thin SiO 2 layers after Si- and C-coimplantation

In this paper we explore the electroluminescence (EL) properties of thermally grown 350 nm thick SiO 2 layers co-implanted with Si + and C + ions. The implanted fluences were chosen in such a way that peak concentrations of excess Si and C of 5– 10 at% were achieved. The devices show a broad photoluminescence (PL) between 2.0 and 3.2 eV with a main peak around 2.7 eV . The broad EL spectra show additional peaks around 3.3 eV and between 2.1 and 2.5 eV which are decreased with increasing Si/C concentration. The shape of the EL spectra does not change with increasing injection currents which implies that various types of defects occur for the different concentrations. The device stability is improved in comparison to Ge or Sn implanted oxide layers.

Electrical characterization of visible emitting electroluminescent Schottky diodes based on n-type porous silicon and on highly doped n-type porous polysilicon

The electrical behaviour of electroluminescent Schottky diodes fabricated on the base of aluminum and n-type porous silicon is reported. Porous silicon was formed by electrochemical etching in HF aqueous solutions of n-type monocrystalline silicon and of degenerate n +-type polysilicon. The polysilicon layer was formed on monocrystalline silicon substrate by low pressure chemical vapour deposition (LPCVD). The electroluminescence (EL) starting voltage was in the range 4–20 Volt, depending on the doping level of Si substrate; polysilicon samples showed a higher starting voltage. Developed devices showed broad EL spectra, covering the whole visible range. Electrical measurements include current-voltage characteristics and capacitance-voltage characteristics. Time response and stability of the light emitting devices was also measured, showing excellent speed and reliability characteristics.

Electroluminescence from p-type silicon during anodic oxidation in ethylene glycol solution

Electroluminescence (EL) from p-type Si during anodic oxidation under galvanostatic conditions in ethylene glycol solutions of KNO 3 with and without different concentrations of chloride ions was measured to investigate the relation between the EL behavior and the defective structure of the silica film formed on Si by anodic oxidation. The measured broad EL spectra had two peaks at 370 nm and 670 nm. The EL peak intensity of the 670 nm peak, rather than that of the 370 nm peak, was enhanced significantly by the addition of chloride ions to the solution, from which it is deduced that the EL source of 670 nm is associated with point defects which are increased by the incorporation of chloride ions in the film. Comparison between the film thickness measured with ellipsometry and the total electric charge required for anodic oxidation revealed that the current efficiency for film growth is only 1% and the majority of the anodic current is electronic, due to electrons injected into the conduction band of the silica film from solution. The relation between EL peak intensities and electrode potential suggested that the EL is operative in an impact ionization—electron avalanche mechanism, where electrons accelerated by a high electric field (2 × 10 7 V cm −1) in the film come into collision with the EL source. The origin of the EL peaks of 670 nm and 370 nm is discussed on the basis of the EL mechanism using an electronic band model of the Si/SiO 2/solution system. The breakdown of the silica film due to chloride ions was accompanied by abrupt decreases in the EL peak intensities as well as a rapid potential drop, indicating that EL can be used to monitor breakdown of the film. (1) The broad EL spectra observed during the anodic oxidation of p-type Si have two peaks at 370 nm and 670 nm, the intensities of which increase with increasing electrode potential above 40 V(SHE). (2) The current efficiency for silica film growth is only 1%. The majority of the anodic current is electronic, due to electrons injected into the conduction band of the silica film from solution. (3) The EL occurs by an impact ionization—electron avalanche mechanism, where electrons accelerated by a high electric field in the film come into collision with the EL source. (4) The addition of Cl − ions to the solution increases the EL intensities. In particular, the increase in EL peak intensity at 670 nm rather than at 370 nm is significant. (5) The EL source of 670 nm is situated in an energy level 1.9 eV below the conduction band edge of silica film and is associated with some kind of point defect enhanced by the incorporation of Cl − ions in the film. The EL source of 370 nm is associated with an electron transition between the conduction bands of silica film and the Si substrate. (6) The breakdown of the silica film due to Cl − ions is accompanied not only by a rapid potential drop but also by an abrupt decrease in EL peak intensities. The breakdown of the silica film is different from that of passive films on metals which leads to pitting corrosion. The many circular swellings with different sizes and the burst swellings observed on the specimen surfaces by scanning electron microscopy indicate that the evolution of gas (Cl 2 or O 2) at the Si|SiO 2 interface induces the breakdown.

Anomalous Luminescence near the InGaAsP–InP Heterojunction Interface

Electroluminescence and photoluminescence of InGaAsP in InGaAsP–InP double heterostructure wafers were investigated in connection with the distance from the heteroepitaxial interface. A sharp electroluminescence spectrum was obtained when the p-n junction was located at the InGaAsP/InP interface. On the other hand, broad emission was observed for the p-n junction located at the InP/InGaAsP interface. In the photoluminescence of InGaAsP measured at 4.2 K, strong, deep level emission was observed in the vicinity of the InP/InGaAsP interface. This phenomenon seems to be responsible for the broad electroluminescence spectrum.