Quenching of electroluminescence and charge trapping in high-efficiency Ge-implanted MOS light-emitting silicon diodes

Combined measurements of charge trapping and electroluminescence intensity as a function of injected charge and current have been carried out with the aim of clarifying the mechanisms of electroluminescence (EL) quenching in Ge-implanted ITO-SiO₂-Si light-emitting silicon diodes. Good correlation between the negative charge capture in traps of small effective capture cross-sections (σ_t1 ^e=1.7×10^-19 cm² and σ_t2 ^e=4.8×10^-20 cm²) located in SiO₂, and the quenching of the asymmetrical EL line with a maximum intensity at 400 nm has been observed. Similar correlation between the electron capture in traps with extremely small effective capture cross-section (σ_t3 ^e=5×10^-21 cm²) and the quenching of the EL line at 637 nm has been established. A quantitative model for the EL quenching has been developed, which takes into account the modification of the luminescent centers with subsequent electron capture at the newly generated traps. The model shows good agreement between simulation and experimental data. It also demonstrates that small effective capture cross-sections for electron charging during the EL quenching are determined by the probability of the luminescence centers (LCs) being disrupted, and enables one to estimate the Ge concentration associated with the EL at 400 nm. PACS 72.20.Jv; 73.40.Qv; 73.50.Gr     

Characterization of the edge emission in Na doped ZnSe

Analysis of edge emission spectra of Na-diffused ZnSe is compared with theoretical predictions of donor-acceptor transition behavior. It is found that the R₀ series fits the criteria for D—A transitions. Mass analysis of the samples studied tends to confirm Na as the responsible acceptor. Properties of R₀ and the I^Y₁ excitonic line are discussed.     

Discrete electroluminescence lines in sub-micron p-i-n resonant tunnelling diodes

We have fabricated GaAs/AlAs p-i-n double barrier resonant tunnelling diodes with active lateral dimensions down to 0.25μn² using optical lithography and wet etching. Many devices have been investigated and systematic variations in the quantum well emission have been observed as the device size is decreased. We observe a red shift of the quantum well recombination lines. In addition a new line is observed at lower energy in the spectra of the smallest devices. The quantum well luminescence efficiency is found to be constant down to the smallest device size.     

Study on electroluminescence processes in dye-doped organic light-emitting diodes

Electroluminescence (EL) mechanism of dye-doped organic light-emitting diodes (OLEDs) was investigated by using three familiar fluorescent dyes, i.e., 5,12-Dihydro-5,12-dimethylquino [2,3-b]acridine-7,14-dione (DMQA), 4-(dicyanomethylene)-2-t-butyl-6(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB), and 5,6,11,12-tetraphenylnaphthacene (Rubrene). EL spectra of the doped devices with structure of indium tin oxide (ITO)/N,N′-bis-(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′- diamine (NPB) (40 nm)/tris-(8-hydroxyquinolate)-aluminum (Alq₃) (x nm, x=0–40 nm)/dye: Alq₃ (weight ratio≈1%, 2 nm)/Alq₃ (48−x nm)/MgAg indicated that direct carrier trapping (DCT) process dominated light emission of devices. As a result, investigation of carrier-recombination site via doping, which is conventionally applied in OLEDs, is questionable since the doping site and the dopant itself may significantly influence the carrier-recombination process in the doped devices.     

Infrared electroluminescence from a Si MOS structure with Ge in the oxide

We report on room-temperature infrared electroluminescence (EL) from metal-oxide-semiconductor devices made from Si. We compare the luminescence from RF sputtered oxide films containing SiO₂ with and without Ge by using a composite target and luminescence from a SiO₂ layer made by rapid thermal oxidation. The sputtered films were annealed in the temperature range 600-900 °C. This densifies the films and is likely to reduce the concentration of defects. A luminescence peak located around 1150-1170 nm is observed at current densities as low as 0.1 A/cm². The corresponding photon energy is close to that of the Si band gap. In addition, we observe several broad luminescence bands in the range 1000-1750 nm. These bands get stronger with Ge in the SiO₂ film. Some of these bands have previously been suggested and are directly associated with Ge. Since we observe that the intensity is correlated with the presence of Ge while the mere presence of the bands is not, we discuss the EL bands being due to defects which concentration is influenced by Ge in the oxide.     

Photo and electroluminescence of ZnSe: Sn and ZnSe:(Sn,Pr) phosphors

We have prepared ZnSe (luminescent grade) phosphor doped with Sn and (Sn,Pr) with varying concentration in an inert atmosphere in a silica tubular furnace at temperature of (780 ± 20) °C for 1 hr to obtain ZnSe:Sn and ZnSe: (Sn,Pr) phosphors. The photo luminescence (PL) and electroluminescence (EL) spectra of these phosphors have been studied at room temperature and results were discussed in the light of existing models. Dependence of EL emission on the voltage frequency has also been carried out. It is found that the plot between the integrated light intensity versus 1/√V_rms is a straight line suggesting the existence of Mott–Schottky type barrier on the metal semiconductor interface.     

GaN基双波长发光二极管电致发光谱特性研究

通过同时调节同一有源区内不同阱层和垒层的In组分，制备了GaN基单有源区蓝、绿光双波长发光二极管(LED).实现了20mA下蓝、绿光同时发射.实验发现随注入电流由10mA增大到60mA，电致发光(EL)谱中绿光峰强度相对于蓝光峰强度不断增强，峰值波长蓝移也更加明显.同时考虑极化效应和载流子不均匀分布的影响，通过对一维薛定谔方程、稳态速率方程和泊松方程的联立自洽求解.分析了测试电流下蓝、绿光EL谱峰值波长和功率的变化情况.发现理论结果与实验结果有很好地符合. 关键词： 极化 载流子不均匀分布 双波长     

Optical absorption edge in doped and undoped ZnSe crystals

The optical absorption near the fundamental edge has been investigated for different types of doped and undoped cubic ZnSe crystals. Crystals grown from the vapour phase, with and without doping, and crystals grown by the chemical transport technique are compared.Results of absorption coefficient measurements made from 77 to 313 LK are discussed and it is shown that for undoped samples, the absorption tail is dominated by electrons-phonons interactions and Urbach's rule is nearly verified, whereas for doped samples the influence of defects is preponderant.     

Room-temperature electroluminescence in the mid-infrared (2-3 microm) from bulk chromium-doped ZnSe

Electroluminescence associated with impact excitation or ionization of deep Cr(2+) impurity centers in bulk ZnSe is reported. A broad signal of mid-infrared luminescence between 2 and 3 microm is observed once the biased bulk ZnSe device runs into a nonlinear conduction regime. Optical powers in the nanowatt range have been measured at room temperature. The different mechanisms involved in this intracenter infrared light emission are discussed.     

Modeling of transient electroluminescence overshoot in bilayer organic light-emitting diodes using rate equations

When a voltage pulse is applied under forward biased condition to a spin-coated bilayer organic light-emitting diode (OLED), then initially the electroluminescence (EL) intensity appearing after a delay time, increases with time and later on it attains a saturation value. At the end of the voltage pulse, the EL intensity decreases with time, attains a minimum intensity and then it again increases with time, attains a peak value and later on it decreases with time. For the OLEDs, in which the lifetime of trapped carriers is less than the decay time of the EL occurring prior to the onset of overshoot, the EL overshoot begins just after the end of voltage pulse. The overshoot in spin-coated bilayer OLEDs is caused by the presence of an interfacial layer of finite thickness between hole and electron transporting layers in which both transport molecules coexist, whereby the interfacial energy barrier impedes both hole and electron passage. When a voltage pulse is applied to a bilayer OLED, positive and negative space charges are established at the opposite faces of the interfacial layer. Subsequently, the charge recombination occurs with the incoming flux of injected carriers of opposite polarity. When the voltage is turned off, the interfacial charges recombine under the action of their mutual electric field. Thus, after switching off the external voltage the electrons stored in the interface next to the anode cell compartment experience an electric field directed from cathode to anode, and therefore, the electrons move towards the cathode, that is, towards the positive space charge, whereby electron–hole recombination gives rise to luminescence. The EL prior to onset of overshoot is caused by the movement of electrons in the electron transporting states, however, the EL in the overshoot region is caused by the movement of detrapped electrons. On the basis of the rate equations for the detrapping and recombination of charge carriers accumulated at the interface expressions are derived for the transient EL intensity I, time t_m and intensity I_m corresponding to the peak of EL overshoot, total EL intensity I_t and decay of the intensity of EL overshoot. In fact, the decay prior to the onset of EL overshoot is the decay of number of electrons moving in the electron transporting states. The ratio I_m/I_s decreases with increasing value of the applied pulse voltage because I_m increases linearly with the amplitude of applied voltage pulse and I_s increases nonlinearly and rapidly with the increasing amplitude of applied voltage pulse. The lifetime τ_t of electrons at the interface decreases with increasing temperature whereby the dependence of τ_t on temperature follows Arrhenius plot. This fact indicates that the detrapping involves thermally-assisted tunneling of electrons. Using the EL overshoot in bilayer OLEDs, the lifetime of the charge carriers at the interface, recombination time of charge carriers, decay time of the EL prior to onset of overshoot, and the time delay between the voltage pulse and onset time of the EL overshoot can be determined. The intense EL overshoot of nanosecond or shorter time duration may be useful in digital communication, and moreover, the EL overshoot gives important information about the processes involving injection, transport and recombination of charge carriers. The criteria for appearance of EL overshoot in bilayer OLEDs are explored. A good agreement is found between the theoretical and experimental results.     

Efficient visible electroluminescence from porous silicon diodes with low driven voltage

By using n-butylamine as carbon resource, diamond-like carbon film (DLCF) was deposited on the p-n porous silicon (PS) surface by means of a radio-frequency glow discharge plasma system. Electroluminescent (EL) spectra show that EL intensity of the passivated PS diodes increases by 4.5 times and 30-nm blue-shift of EL peak occurs compared with the diodes without treatment and both of them are stable while the passivated diodes are exposed to the air indoor. The current-voltage (Ⅰ-Ⅴ) characteristics exhibit that the passivated diodes have a smaller series resistance and a lower onset voltage. The EL intensityvoltage (ⅠEL-Ⅴ) relations of the PS devices with different DLCF thicknesses show that only medium DLCF thickness is optimum. These experimental phenomena have been explained based on Raman spectra and IR spectra of the diamond-like carbon films and IR spectra of the passivated PS samples.     

Influence of DCM dye doping on the magnetic field dependent electroluminescence in organic light emitting diodes

The DCM dye doped organic electroluminescence devices with structure of ITO/NPB/Alq 3 : DCM/Alq 3 /LiF/Al were fabricated. From 15 K to room temperature, the magnetic field dependent of electroluminescence (MEL) of devices was investigated. Our observations indicated that the MEL is composed of two effects in different regimes: a low field (0≤B≤40 mT) effect and a high field (B 40 mT) effect. For undoped devices, the low field effect exhibits a rapid rising with the increasing field, and the high field effect shows a slow increase and gradually saturates at room temperature. For doped devices, the low field rapid increase is also present, whereas the high field effect displays a decrease with the increasing field. The larger the injection current is, the more apparent the high field decrease is. In addition, the doped device demonstrates less temperature dependence of the high field effect than undoped device, although the undoped devices also present high field decrease of electroluminescence at low temperatures (T≤150 K). Based on the energy level trapping effect due to dye doping and magnetic field modulated triplet exciton annihilation, the experimental results are carefully explained.     

A new emission band in the near band edge region in ZnSe single crystal

An emission band with FWHM of 2.7 meV has been observed at 2.8002 eV. Its emission intensity is proportional to the 1.2 power of the excitation intensity. Excitation spectrum and selective excitation spectrum do not show any additional spectral structure. It is proposed that the origin of this emission is the scattering of excitons by other free particles at a structural defect with slightly lower potential for free particles and free excitons.     

Injection electroluminescence from CdTe p-n junctions prepared by LPE

CdTep-n junction diodes were prepared by LPE using CdCl₂ as a solvent. Excess cadmium was added to the CdCl₂-CdTe solution. Capacitance-voltage characteristics show that the diode structure is ofp-i-n type. Injection electroluminescence spectra reveal that radiative transitions occur mainly in thep-type region; relevant recombination centers are discussed in connection with those in a previous paper on the photoluminescence of CdTe:P crystals. Temperature dependences of the electroluminescence spectra were explained taking into account a change in sites where electrons radiatively recombine.     

Observation of a donor-acceptor pair recombination in the edge emission of ZnSe crystal by electro-luminescence

An injection type electro-luminescence in ZnSe crystal has been studied by using ZnSe-SnO₂ hetero-junction at 20°K. In the emission peak observed around 2·70 eV, a clear energy shift toward the higher energy side with increasing injection current density has been found at low temperatures, which could be associated with the D-A pair (donor-acceptor pair) recombination process. The energy sum of the donor and acceptor activation is estimated to be larger than 137 meV. In the higher temperature region, this emission line turns out due to the ‘free-to-bound’ recombination, and the related acceptor ionization energy is considered to be ～120 meV. By taking into consideration the energies of bound exciton emission, the exciton localization energies and the related donor and acceptor ionization energies are evaluated.     

Enhanced ESD properties of GaN-based light-emitting diodes with various MOS capacitor designs

High electrostatic discharge (ESD) protection of GaN-based light-emitting diodes (LEDs) has been developed using a metal–oxide semiconductor (MOS) capacitor. This structure is realized by adopting various metal electrode patterns. The MOS capacitor can be implemented by extending the metal line directly from the p-type electrode to the top surface of an SiO₂-capped n-GaN layer near the vicinity of the n-type electrode. By connecting a MOS capacitor in parallel with the GaN-based LED, the negative ESD strike could be significantly increased from 385 to 1075 V of human body mode (HBM).