Electroluminescence Decay Time Research Articles

Bright white electroluminescence from polycrystalline dysprosium-doped yttrium gallium garnet nanofilms fabricated by atomic layer deposition on silicon.

Bright white emission is obtained under electrical excitation from dysprosium doped Y3Ga5O12 garnet (YGG:Dy) nanofilms fabricated by atomic layer deposition on silicon substrates. Electroluminescence (EL) composed of yellow (580 nm) and blue (482 and 492 nm) emission corresponds to the CIE chromaticity coordinates of (0.3568, 0.3807) and a CCT of ∼4700 K and can be used for lighting and displays. The crystallization and micro-morphology of polycrystalline YGG:Dy nanolaminates are explored by adjusting the annealing temperature, Y/Ga ratio, Ga2O3 interlayer thickness and Dy2O3 dopant cycle. The near-stoichiometric device annealed at 1000 °C presents optimal EL performance with the maximum external quantum efficiency and the optical power density reaching 6.35% and 18.13 mW cm-2, respectively. The EL decay time is estimated to be 273.05 μs, with a large excitation section of 8.33 × 10-15 cm2. The conduction mechanism is confirmed to be the Poole-Frenkel mode under operation electric fields and the impact-excitation of Dy3+ ions by energetic electrons results in emission. Bright white emission from Si-based YGG:Dy devices can provide a new route to developing integrated light sources and display applications.

The electroluminescence mechanism of Er3+ in different silicon oxide and silicon nitride environments

Rare earth doped metal-oxide-semiconductor (MOS) structures are of great interest for Si-based light emission. However, several physical limitations make it difficult to achieve the performance of light emitters based on compound semiconductors. To address this point, in this work the electroluminescence (EL) excitation and quenching mechanism of Er-implanted MOS structures with different designs of the dielectric stack are investigated. The devices usually consist of an injection layer made of SiO2 and an Er-implanted layer made of SiO2, Si-rich SiO2, silicon nitride, or Si-rich silicon nitride. All structures implanted with Er show intense EL around 1540 nm with EL power efficiencies in the order of 2 × 10−3 (for SiO2:Er) or 2 × 10−4 (all other matrices) for lower current densities. The EL is excited by the impact of hot electrons with an excitation cross section in the range of 0.5–1.5 × 10−15 cm−2. Whereas the fraction of potentially excitable Er ions in SiO2 can reach values up to 50%, five times lower values were observed for other matrices. The decrease of the EL decay time for devices with Si-rich SiO2 or Si nitride compared to SiO2 as host matrix implies an increase of the number of defects adding additional non-radiative de-excitation paths for Er3+. For all investigated devices, EL quenching cross sections in the 10−20 cm2 range and charge-to-breakdown values in the range of 1–10 C cm−2 were measured. For the present design with a SiO2 acceleration layer, thickness reduction and the use of different host matrices did not improve the EL power efficiency or the operation lifetime, but strongly lowered the operation voltage needed to achieve intense EL.

Strong electroluminescence from SiO2-Tb2O3-Al2O3 mixed layers fabricated by atomic layer deposition

We report on the bright green electroluminescence (EL) with power efficiencies up to 0.15% of SiO2-Tb2O3-mixed layers fabricated by atomic layer deposition and partly co-doped with Al2O3. The electrical, EL, and breakdown behavior is investigated as a function of the Tb and the Al concentration. Special attention has been paid to the beneficial role of Al2O3 co-doping which improves important device parameters. In detail, it increases the maximum EL power efficiency and EL decay time, it nearly doubles the fraction of excitable Tb3+ ions, it shifts the region of high EL power efficiencies to higher injection currents, and it reduces the EL quenching over the device lifetime by an approximate factor of two. It is assumed that the presence of Al2O3 interferes the formation of Tb clusters and related defects. Therefore, the system SiO2-Tb2O3-Al2O3 represents a promising alternative for integrated, Si-based light emitters.

(Invited) Rare Earth Doped Metal-Oxide-Semiconductor Structures: A Promising Material System or a Dead End of Optoelectronic Evolution?

The suitability of rare earth doped metal-oxide-semiconductor structures for optoelectronic applications is investigated. To do so, several Tb- and Er-doped devices with different designs and fabricated by different methods are compared among each other with respect to their electroluminescence (EL) properties. In detail, the investigated devices show EL power efficiencies between 2×10-4 and 2×10-3 which, taken individually for Tb and Er, have a linear dependence on the EL decay time for low and medium injection current densities. The excited fraction of Er ions is significantly higher than that of Tb ions and achieves a maximum value of 50% (with a maximum uncertainty factor of 2.25) under optimum conditions.

Controlling blue-violet electroluminescence of Ge-rich Er-doped SiO2 layers by millisecond annealing using flash lamps

Systematic evolution of the 400 nm electroluminescence (EL) with increasing flash lamp annealing (FLA) temperature from 800 to 1100 °C in an Er-doped Ge-rich metal-oxide semiconductor structure is presented. No significant change in the 1535 nm Er EL is observed with increasing FLA temperature. Enhancement of the 400 nm EL decay time with rising FLA temperature is found to be associated with recrystallization of the damaged Ge clusters in the absence of Ge outdiffusion. The 400 nm EL quenching with continuous charge injection process is also discussed within the device operation time.

Anomalous wear-out phenomena of europium-implanted light emitters based on a metal-oxide-semiconductor structure

The anomalous wear-out phenomena of Eu-implanted metal-oxide-semiconductor devices were investigated. It will be shown that in contrast to other rare earth elements the electroluminescence (EL) intensity of Eu-implanted SiO2 layers can rise under constant current injection before the known EL quenching will start. Under certain circumstances, this rise may amount up to two orders of magnitude. The EL behavior will be correlated with the microstructural and electrical properties of the devices. Transmission electron microscopy and Rutherford backscattering spectroscopy were applied to trace the development of Eu/Eu oxide clusters and the diffusion of Eu to the interfaces of the gate oxide layer. The hydrogen profile within the SiO2–SiON interface region was determined by nuclear reaction analysis. Current-voltage characteristics, EL decay times, and the progression of the voltage and the EL spectrum with increasing charge injection were measured to study charge and trapping phenomena in the oxide layer to reveal details of the EL excitation mechanism. A first qualitative model for the anomalous life time behavior is proposed.

Red electroluminescent process excited by hot holes in SrGa 2S 4:Ce, Mn thin film

This paper reports the first observation of red electroluminescence (EL) in SrGa 2S 4:Ce, Mn thin film. The EL spectrum consists of single broad emission band having a peak wavelength of 665 nm. The dominant EL decay time was 31 μs. The relationship between the applied voltage and the EL waveform was measured in single insulating thin film electroluminescent (TFEL) devices. An asymmetric EL waveform was observed in SrGa 2S 4:Ce, Mn TFEL devices under a rectangular applied voltage. The polarity of the EL waveform in these devices was different from the waveform in manganese-activated zinc sulfide ZnS:Mn devices. This indicates that hot holes excite the Mn 2+ ions to cause the red EL.

Estimation of internal quantum efficiency in InGaN‐based light emitting diodes using electroluminescence decay times

AbstractWe investigate the internal quantum efficiency of InGaN‐based Light Emitting Diodes (LEDs) from the semiconductor rate equation of pulse current injection. A method is presented for estimating the internal quantum efficiency based on data of electroluminescence decay times measured as a function of current in the pulse injection. For the screening of built‐in electric field, the pulse current was injected with bias voltage. For blue LED, the internal quantum efficiency was measured to be about 70% in high injection condition. It is found that the internal quantum efficiency increase with injection current, whereas the external quantum efficiency decreases at high injection. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Millisecond radiative recombination in poly(phenylene vinylene)-based light-emitting diodes from transient electroluminescence

The current and electroluminescence transient responses of standard poly(phenylene vinylene)-based light-emitting devices have been investigated. The electroluminescence time response is longer (milliseconds scale) than the current switch-off time by more than one order of magnitude, in the case of small area devices (<0.1cm2). For large area devices (∼6cm2) the electroluminescence decay time decreases from 1.45msto∼100μs with increasing bias voltage. The fast current decay limits the electroluminescence decay at higher voltages. Several approaches are discussed to interpret the observed slow decrease of electroluminescence after turning off the bias. One relies upon the Langevin-type bimolecular recombination kinetics which is governed by the minority carriers (electrons), and another focuses on the slow release of trapped electrons as possible explanations. Additionally, we show that the device current density is mainly determined by the transport of the fastest carriers (holes).

Electroluminescence properties of the Gd3+ ultraviolet luminescent centers in SiO2 gate oxide layers

Electroluminescence (EL) properties in the ultraviolet (UV) range were studied on Gd-implanted indium tin oxide/SiO2:Gd∕Si metal-oxide-semiconductor light emitting devices. The efficient UV line at 316nm from Gd3+ centers shows a maximum power density of 2mW∕cm2 and a quantum efficiency above 5%. The Gd3+ luminescent center has an excitation cross section above 7.4×10−15cm2 with an EL decay time around 1.6ms at a Gd concentration of 3%. A decrease of the EL efficiency is observed by a cross relaxation at a high Gd concentration and by clustering of Gd atoms at an annealing temperature of 1000°C. A strong quenching of the UV EL due to electron trapping around optically active Gd3+ centers is observed during the injection of hot electrons.

Time-resolved electroluminescence studies of III-nitride ultraviolet photonic-crystal light-emitting diodes

Transient responses of III-nitride photonic-crystal (PC) ultraviolet (UV) light-emitting diodes (LEDs) were measured by picosecond time-resolved electroluminescence (EL) spectroscopy. Triangular arrays of PCs with different diameters/periodicities were fabricated on 333nmUV LEDs for enhancing light extraction efficiency using electron-beam lithography and inductively coupled-plasma dry etching. With the incorporation of PCs on LEDs, the EL decay time constant τ decreases systematically with the increase of the etched sidewall area indicating the strong effect of the surface recombination. The surface recombination velocities on the p-type epitaxial surface and on the sidewall of etched holes on LEDs were determined to be 1.73×104cm∕s and 1.48×105cm∕s, respectively. The angular distribution of light emission from LEDs with PCs shows slight narrowing in far-field pattern. Because of the increased transient response along with enhanced light extraction, the incorporation of PCs in UV LEDs provide an effective method to control the modulation speed of UV LEDs, which could be very useful for many applications.

Electroluminescence efficiency of silicon diodes

The intrinsic electroluminescence (EL) of a silicon light-emitting diode with a forward-biased p-n junction is studied. The substantial enhancement of the integrated EL intensity observed with the temperature increasing from the liquid-nitrogen to room-temperature level, which is paralleled by an increase in the EL decay time when the current through the diode is terminated, indicates thermal suppression of the nonradiative recombination channel associated with deep traps. A simple model developed by us for the radiative processes occurring in a p-n junction offers an interpretation for all the experimental data obtained. It is shown that the internal quantum efficiency of the EL may reach a level of a few percent under optimal doping of the diode p and n regions.

Excitation transfer processes in a phosphor-doped poly(p-phenylene vinylene) light-emitting diode

We present experimental measurements and theoretical calculations of the electrical and optical properties of phosphor-doped poly (p-phenylene vinylene) light-emitting diodes to determine the excitation processes that lead to radiative recombination from the phosphor molecule. Three possible phosphor excitation processes are considered: (1) sequential electron and hole capture by the phosphor, (2) energy transfer from the polymer triplet exciton (Dexter transfer), and (3) energy transfer from the polymer singlet exciton (Forster transfer). The properties of the doped polymer are investigated for doping levels up to about 20 wt %, At the highest doping density, all radiative recombination occurs in the phosphor molecule and the observed electroluminescence decay time increases significantly compared to the undoped polymer, Built-in potential and current-voltage measurements indicate that the electron and hole energy levels of the phosphor are outside the energy gap of the polymer, and that the phosphor molecule does not capture either individual electrons or holes, Measurements of triplet optical absorption show that the triplet population in the polymer is not affected by the presence of the phosphor, indicating that Dexter transfer processes are weak. Calculations of the triplet optical-absorption cross section combined with the measurements of the triplet optical absorption determine the triplet exciton density in the device. In an analogous chemically substituted polymer, no significant excitation transfer occurs when there is no overlap between the emission spectrum of the polymer and the absorption spectrum of the phosphor. These results demonstrate that the dominant excitation transfer path from the polymer to the phosphor is dipole-dipole (Forster) coupling. Calculations of the charged and neutral electronic excitation energies of the polymer and phosphor are performed using hybrid and time-dependent, density-functional theory. The results of these calculations show why Forster transfer is strong in this system, and why the other two transfer processes do not take place.

Luminescence lifetime of the 1.5-μm emission of β-FeSi 2 precipitate layers in silicon

Semiconducting iron disilicide precipitates in silicon were fabricated by ion beam synthesis and characterised by photoluminescence and TEM. We have measured the decay time of the 1.55-μm line, which was found to be 10 μs at 10 K. No evidence for a fast, direct transition could be found from the time resolved photoluminescence measurements. Furthermore, we have measured the electroluminescence decay time of a demonstrator FeSi 2–Si light emitting device. This shows a fast time response of below 30 ns.

Electroluminescence from triplet excited states of benzophenone

We report the electroluminescent characteristics of organic multilayer light-emitting diodes (LEDs) with a benzophenone (BP) dispersed poly(methylmethacrylate) (PMMA) film as an emitting layer. The electroluminescence (EL) intensity of these LEDs increases with decreasing temperature from 273 to 100 K when they are operated at the same voltages or the same current densities. The EL spectrum of the LEDs, which peaks at around 450 nm, is identical to the phosphorescence spectrum of BP in PMMA. In addition, the EL decay time was determined as 46.8 μs at 100 K by applying a rectangular voltage pulse. These results indicate that the EL of the LED originates from the triplet excited states of BP.

Current Dependent Electroluminescence Decay and Degradation Behaviour in Green and Yellow LED's

The injection current dependent electroluminescence decay time of green and yellow emitting diodes produced by Zn diffusion into VPE-GaP:N, Te and VPE-GaAs0.11P0.89:N, Te, respectively, is compared with the photoluminescence decay time measured in the as-grown layers independence on the excitation intensity. As it is possible to distinguish in this way between the nonradiative recombination at point defect centres and at dislocations the influence of these two types of defects can be investigated. This is confirmed applying this method to LED's prepared by different dicing methods. The degradation behaviour of the samples investigated can be understood in terms of decreasing minority carrier lifetime only in the n-region of the LED caused by increasing dislocation density.

Nonradiative recombination in GaAlAs proton-bombarded stripe-geometry lasers

Nonradiative recombination processes in GaAlAs double-heterostructure lasers are examined using luminescence time-response measurements. Photoluminescence time-decay data for proton-bombarded (PB) isotype (p-type) heterostructures give an effective recombination velocity for the PB region of (7±1) ×105 cm/sec. Electroluminescence decay times of laser devices indicate that the interfacial recombination velocity of the anisotype interface (p-GaAs/N-GaAlAs) is 1500±500 cm/sec compared with 400±100 cm/sec for isotype (p-type) interfaces. The contribution of each important recombination process to the laser threshold current is calculated. For devices in which the PB region penetrates into the active layer, recombination in the PB region and interfacial recombination are the major nonradiative paths.

Field Dependence of Quantum Efficiency in CdS-Resin Layers Due to Injective Contact and Blocking Contact

Field Dependence of Photoconductivity in Anthracene Crystals Hiroyuki Mitsudo Field Dependence of Magnetoresistance in No-binder Soft Carbons at 77°K Yoshihiro Hishiyama, Akio Ono and Masaaki Hashimoto Temperature and Magnetic Field Dependence of Several Commercial Si-Diodes and Their Applicability as Thermometers Koshichi Noto and R. P. Huebener Electroluminescence Decay Time and Quantum Efficiency of GaP Green-Emitting Diodes Tatsuro Beppu, Masami Iwamoto, Tetsuo Sekiwa et al. Electric Field Dependence of the Capacitance of a Twisted Nematic Liquid Crystal Cell Hiroyoshi Onnagawa and Kazuo Miyashita Wavelength Demultiplexing Photodiode with Very High Isolation Ratio Shiro Sakai and Masayoshi Umeno Analysis of InGaN High-Brightness Light-Emitting Diodes Song Jae Lee Field Dependence of Quantum Efficiency in CdS-Resin Layers Due to Injective Contact and Blocking Contact

Luminescent Properties Variation toward Growth Direction in Nitrogen Doped GaP n-LPE Layer

Variations of luminescent properties toward the growth direction were investigated at room temperature for nitrogen-doped n-type GaP liquid-phase epitaxial (LPE) layers grown on n-type liquid encapsulated Czochralski (LEC) substrates. Photoluminescence (PL) measurements of an angle-lapped epitaxial layer showed that green emission intensity increases rapidly toward the growth direction from the growth boundary near the LEC substrate. Properties of light-emitting diodes formed by growing a p-type epitaxial layer on this angle-lapped surface were also investigated. The green emission efficiency of the diodes is proportional to the green PL efficiency and inversely proportional to the red emission efficiency of the diodes. This red omission is found to be due to Zn–O pairs. The measured nitrogen concentration was nearly constant in the epitaxial layer. This result and a measurement of green electroluminescence decay time led to the conclusion that variation in green emission efficiency is mainly caused by a change in the lifetime of minority carriers.

Electroluminescence in GaAsxP1−x, InxGa1−xP, and AlxGa1−xP Junctions with x≲0.01

The addition of low concentrations (≲1%) of As, In, or Al to the p side of GaP junctions, where exciton recombination at N or Zn, O centers generates green and red electroluminescence (EL), respectively, drastically decreases the quantum efficiency η of the junctions. The decrease in η is due to increased nonradiative recombination at deep centers in the junction space charge, while recombination outside the space charge is unchanged according to measurements of the EL decay time. Low-temperature photoluminescence efficiency measurements confirm that recombination in the bulk semiconductor is unaffected by the presence of these low concentrations of As, In, and Al. The junctions were formed using liquid epitaxial techniques. Using InxGa1−xP layers, where the alloy composition was accurately determined using radioactive tracer techniques, the composition dependence of the transition energies of the N and S bound excitons was measured by low-temperature absorption and luminescence studies.