Electroluminescence Studies Research Articles

Publisher's Note: Electroluminescence and photoluminescence excitation study of asymmetric coupled GaAs/AlxGa1−xAs V-groove quantum wires [Phys. Rev. B 70, 045302 (2004)

Publisher's Note: Electroluminescence and photoluminescence excitation study of asymmetric coupled GaAs/AlxGa1−xAs V-groove quantum wires [Phys. Rev. B 70, 045302 (2004)

Electroluminescent studies of emission characteristics of InGaAsN/GaAs injection lasers in a wide temperature range

Light-current, spectral, and far-field characteristics of InGaAsN injection lasers on GaAs substrates were studied in a wide temperature range (77–300 K) at various driving current densities. The increase in indium content in InGaAsN solid solution results in a modification of the QW structure, which is manifested in the spontaneous formation of InGaAsN nanoclusters. These changes result in N-shaped temperature dependences of the threshold current density and slope efficiency.

Transient electroluminescence study of mobility balancing in organic light-emitting diodes based on poly( p-phenylenevinylene) derivatives

Transient electroluminescence (EL) measurements were carried out on organic light-emitting diodes based on poly( p-phenylenevinylene) (PPV) derivatives. As a result, in contrast to the case of a PPV derivative carrying a hole-transporting moiety, the much improved external quantum efficiencies for the device based on a PPV derivative carrying an electron-transporting moiety were explained by the enhanced EL lifetime arising from the electron–hole mobility balancing.

A comparative study of electroluminescence from Ge/SiO 2 and Si/SiO 2 films

Ge/SiO2 and Si/SiO2 films were deposited using the two-target alternation magnetron sputtering technique. The Au/Ge/SiO2/p-Si and Au/Si/SiO2/p-Si structures were fabricated and their electroluminescence (EL) characteristics were comparatively studied. Both Au/Ge/SiO2/p-Si and Au/Si/SiO2/p-Si structures have rectifying property. All the EL spectra from the two types of the structure have peak positions around 650–660 nm. The EL mechanisms of the structures are discussed.

Photo and electroluminescence studies of poly(methyl methacrylate-co-9-anthryl methyl methacrylate)

A series of poly(methyl methacrylate-co-9-anthryl methyl methacrylate) was prepared in which the anthryl groups were interspersed between each 2, 4, 10, 25 and 35 methyl methacrylate mers. The effect of the proximity of the chromophores in the photophysical properties was studied in dilute and concentrated conditions, showing that the association of anthryl groups giving rise to aggregates or excimers is strongly dependent of the chain conformations. The photophysical properties of cast-films of largely spaced copolymers are characterized by two decays: one of the isolated chromophore (longer decay rate and vibronically structured emission) and another of aggregated species (faster decay with some broadening of the red-edge of the emission band). Copolymers having the chromophores separated by two or four methyl methacrylate units presented the highest degree of association and were the only electroluminescent materials. The proximity of anthryl moieties and the conformation of the polymer chains are important factors governing excimeric-like emission and film properties of anthracene containing polymers used in technological applications.

Simultaneous Light Emission from a Mixture of Dendrimer Encapsulated Chromophores: A Model for Single-Layer Multichromophoric Organic Light-Emitting Diodes

The site isolation of two dyes capable of electronic interaction via Forster energy transfer has been studied with the two dyes coumarin 343 and pentathiophene encapsulated by dendrons containing both solubilizing and electroactive moieties. Photoluminescence studies of mixtures of the dendritic dyes show that at high dendron generation, significant site isolation is achieved with relative emission characteristics influenced by both the degree of site isolation and the emission quantum yield of the dyes. Electroluminescence studies carried out in organic light emitting diode devices confirm that color tuning may be achieved by mixing the two encapsulated dyes in a single layer. However, selective carrier trapping by one of the core component dyes can dramatically influence the effectiveness of other components in the device.

Optical characterization of radiative deep centres in 6H–SiC junction field effect transistors

Electroluminescence studies were carried out as a function of bias and temperature to understand the nature of deep centres in 6H–SiC n-channel junction field-effect transistors (FETS) for high-power applications. Vanadium impurities in different configurations were suggested to be responsible for the levels found. The contributions of hot and cold carriers were distinguished by biasing the devices both in the transistor-like and in the diode-like configurations. This allowed us to spatially localize the deep centres inside the device structure. In the transistor-like configuration, two broad emission bands (1.1–1.7 eV and 2.5–3 eV), superimposed to the contribution arising from hot carrier intraband transitions, were detected. The A band at 1.17 eV (observed only in the transistor-like configuration) was attributed to a radiative transition between an excited level and its ground state of a vanadium centre located in the n-type conductive channel. The B (1.32 eV) and C (1.62 eV) bands originated from the space–charge region of the p–n junctions in both the configurations and, contrary to the A band, they were characterized by a thermal capture barrier (180 meV) which inhibits the luminescence in the diode-like configuration at temperatures below 200 K.

Tuning of the intersubband emission below the longitudinal optical phonon energy in GaAs/AlGaAs quantum cascade emitters

We present a study of far-infrared intersubband electroluminescence from a number of GaAs–Al0.15Ga0.85As quantum-cascade emitters with emission energies below the LO phonon energy. A range of samples with emission energies between 13 and 21 meV were investigated. A systematic decrease in the normalized emission intensity with increasing intersubband separation was observed. The possible mechanisms such as the grating coupling efficiency and the role of scattering processes are discussed.

Synthesis, structural characterization and electroluminescence study of alkylgallium derivatives of thiobenzhydrazones

Six dimethylgallium complexes of type Me 2GaL [L= N-benzylidenethiobenzahydrazonato ( 1), N-(4-methoxy)benzylidenethiobenzahydrazonato ( 2), N-(3,4-dimethoxy)benzylidenethiobenzahydrazonato ( 3), N-(4- N, N-dimethylamino)benzylidenethiobenzahydrazonato ( 4), N-(2-naphthyl)methylenethiobenzahydrazonato ( 5) and N-(9-anthryl)methylenethiobenzahydrazonato ( 6)] have been synthesized by reaction of trimethylgallium with appropriate N-arylmethylenethiobenzahydrazones. The complexes obtained have been characterized by elemental analysis, 1H-NMR, IR and mass spectroscopy. Structure of 2 has been determined by X-ray single-crystal analysis, in which Ga atom is four-coordinated. Compounds 2– 4 emit blue colors at λ max=432–479 nm when irradiated by UV light. The electroluminescent (EL) properties of 2, 3 and 4 were examined by fabricating EL devices using 2, 3 and 4 as emitter, respectively. The EL bands are located in the blue region (451, 454 and 479 nm for complexes 2, 3 and 4, respectively).

Optoelectronic properties of thick SiGe layers grown as small mesas by low pressure chemical vapor deposition

Arrays of Si0.80Ge0.20/Si(001) square mesas were epitaxially grown by low pressure chemical vapor deposition to optimize the light emission in the near infrared range. To study the influence of mesa size on light emission the current–voltage characteristics, the spectral photocurrent, and the electroluminescence of p-i-n structures were measured. While the plastic relaxation has a strong influence on the electroluminescence spectra, the current–voltage characteristics are only slightly changed. At low temperatures, a tunneling current was observed and its possible location is discussed. Due to the high SiGe thickness, both the contributions of the no-phonon and transversal optical phonon-assisted transitions to the photocurrent spectra could be observed. Direct evidence of the higher band gap of relaxed SiGe was obtained from electroluminescence studies.

Electroluminescence and photoluminescence studies on carrier radiative and nonradiative recombinations in metal-oxide-silicon tunneling diodes

Electroluminescence (EL) and photoluminescence (PL) measurements at silicon band gap energy from metal-oxide-semiconductor tunneling diodes on silicon have been carried out to study the carrier radiative recombination and nonradiative recombination near the silicon/silicon dioxide (Si/SiO2) interface. The temporal EL response indicates that the radiative recombination coefficient involved is as much as ten times greater than that of the bulk silicon. However, the nonradiative recombination is still the dominant carrier recombination process. Voltage-dependent PL shows that PL intensity increases with the bias voltage. This voltage dependence is attributed to the variations of nonradiative recombination rates due to the change of Fermi level with the bias voltage. The intensity of EL is found to be less temperature dependent than that of PL. The near-band-edge Fermi level for EL leads to the reduced and less thermal-active nonradiative recombination as compared to PL. This study shows that Fermi level near the Si/SiO2 interface strongly influences the nonradiative recombination rates and the resulting EL and PL intensities.

Study of conduction mechanism and electroluminescence in CdSe/ZnS quantum dot composites

Charge transport and electroluminesence properties of colloidally synthesized CdSe/ZnS core–shell nanocrystal quantum dots (QDs) were studied. Nanocrystals were prepared using the conventional technique of pyrolisis of organometallic reagents in a hot coordinating solvent medium. Thin film diodes were produced by depositing a layer of QDs on top of a layer of conducting polymer poly (3,4-ethylenedioxythiophene): polystyrenesulfonate followed by the deposition of a metal electrode. For all the metals used in this study only a small rectification could be observed upon reversing the bias. The efficiency of the light generation showed strong dependence on the work function and bias of the metal electrodes, indicating that light generation in these devices is a result of recombination of injected holes and electrons. Assuming that the charge transport is due mainly to electrons, the voltage dependence of the current in QD composites could be explained by space charge limited current (SCLC) in the presence of defects. At sufficiently high voltages the traps could be filled and a quadratic dependence of current on voltage, characteristic of trap free SCLC, could be observed. The mobility and the trap density were estimated to be μn=1.5×10−10 m2/V s and Nt=8×1023 m−3, respectively, and the characteristic trap depth was estimated to be around 0.15 eV.

Optical and electrical transport mechanisms in Si-nanocrystal-based LEDs

In this work, we present photoluminescence (PL) and electroluminescence studies on thermal SiO 2 Si-implanted light-emitting devices (LEDs) containing crystallites of size 3 nm . The PL measurements reveal a bulk-type behavior of nc-Si-associated PL confirming that the PL obtained at ∼1.6 eV is closely related to Q-confined nc-Si. Analysis of electrical transport mechanisms confirms that the current is related to a tunneling process as expected and not to Fowler–Nordheim regime. Finally, threshold electric field for light emission obtained was as low as 5 MV/cm , confirming the real potential of a such Si-implanted material for LEDs based on Si.

Study of electroluminescence in porous silicon for sensing applications

Electroluminescence (EL) of porous silicon (PS) samples was studied at anodic polarisation using neutral electrolyte contact. SEM and AFM morphology study of PS revealed its complex structure comprising features with micrometre and tens of nanometre dimensions. The observed features of EL, including its degradation during anodisation, were discussed in terms of a phenomenological model of a chemically modified silicon surface. The adsorption of polyelectrolytes polyallylamine (PAA) and polysterylsulphonate (PSS) demonstrated sufficient improvement of EL stability. The effect of adsorption of protein bovine serum albumin (BSA) on EL quenching was studied.

Time‐Resolved Electroluminescence Study of Green Light‐Emitting Diode

We have performed time-resolved photoluminescence and electroluminescence measurements for an InGaN quantum well at 300 K to investigate the carrier dynamics. Electroluminescence measurement yields more direct information of the carrier dynamics of device operation, because carriers are generated by current injection in optoelectronic devices. The time evolution of EL spectra indicates that spatial indium fluctuations become significant with increasing In content in nitride-based optoelectronic applications.

Electroluminescence studies under forward and reverse bias conditions of a nitride-rich GaN 1− xP x SQW structure LED grown by laser-assisted metal-organic chemical vapor deposition

We have fabricated a nitride-rich GaN 1− x P x single quantum well structure by laser-assisted metal-organic chemical vapor deposition. In the forward bias condition, the electroluminescence of the LED had two main peaks, of which the energy positions around 2.9 eV were called the blue-band and that around 2.2 eV were called the yellow-band. Further, the LED emitted in the yellow-band even during a reverse-bias condition. The peak energy of these emissions shifted toward higher energy as the drive current increased. The blue-band shift of increasing current may be explained by a band-filling mechanism, as reported for InGaN LEDs. The I– V characteristics of the LED are not very similar to those of the conventional InGaN LED. The component of current flow consists of an ohmic ingredient and an exponential ingredient. The equivalent ideal factor ( n) of this LED is over 200; also in the reverse bias region, the I– V curve has good linearity rather than an exponential component. These results suggest that blue-band emission relates to the exponential ingredient and the yellow-band emission relates to the ohmic ingredient.

Well‐Defined Ruthenium‐Coordinated Block Copolymers of 2‐Vinylpyridine with 2‐(N‐Carbazolyl)ethyl Methacrylate and Their Optical Properties

AbstractIn order to study the energy‐transfer phenomenon in ruthenium‐coordinated polymers, novel block copolymers from carbazolylethyl methacrylate and 2‐vinylpyridine were prepared by living anionic polymerization; the copolymers possessed varying repeating units and had the desired molecular weight and a narrow‐molecular‐weight distribution. The ruthenium‐coordinated block copolymers were formed by reacting [RuII(tpy)(dmbpy)Cl] with the polymers, in which the pyridine units acted as ligands. Photoluminescence and optical absorption measurements of the coordinated block copolymers were carried out to observe the energy transfer in the complexes. The energy transfer from the carbazole blocks to the ruthenium‐coordinated blocks might take place either by an intra‐ and/or an intermolecular energy‐transfer mechanism. From the optical absorption, photoluminescence, and electroluminescence studies, it was observed that the ease of the energy transfer increased with an increasing number of metal‐complex units. Normalized electroluminescence spectra of the device (ITO/PEDOT/RuIIL2bpolym/TAZ/Alq3/Ag).magnified imageNormalized electroluminescence spectra of the device (ITO/PEDOT/RuIIL2bpolym/TAZ/Alq3/Ag).

Electrical and optical studies of organic light emitting devices using SWCNTs-polymer nanocomposites

Abstract The novel organic light emitting diodes with the structure of [ITO-coated glass/PEDOT:PSS/SWCNTs-PVK (polyvinylcarbazole) nanocomposites/(4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM)-doped) Alq3/Li:Al] were fabricated and characterized. Single-walled carbon nanotubes (SWCNTs) were synthesized by the laser ablation method and dispersed with the hole conducting polymer PVK in toluene. SWCNTs-PVK nanocomposite was spin-coated on the PEDOT:PSS/ITO-coated glass and then DCM-doped Alq3 was deposited by the cluster beam deposition methods. Studies of photoluminescence, electroluminescence (EL) and device characteristics demonstrate that whereas the EL of the devices with SWCNTs shows the same characteristic emission compared to those without SWCNTs, the device qualities such as external quantum efficiency were improved by a factor of 2–3 for SWCNTs concentrations up to 0.2 wt%.

Relaxation dynamics in light-emitting devices based on a poly(3-alkylthiophene) derivative: transient capacitance and transient electroluminescence studies

Relaxation dynamics of light-emitting devices (LEDs) based on a poly(3-alkylthiophene) derivative have been studied from transient electroluminescence (EL) and transient capacitance measurements. Transient EL has been studied by applying a sequence of two voltage pulses. During the time delay between the two voltage pulses, the devices relaxed which has been monitored by measuring the EL during the second pulse. The width and amplitude of the voltage pulse have been varied, so that different initial state of the device can be reached before the device was allowed to relax. The time delay between the two pulses has been varied for different cases to study relaxation processes in the device. Transient capacitance technique was also used to monitor the relaxation processes. The time constants for dielectric relaxation and charge redistribution in the devices obtained from the two studies have been compared.

Enhanced electroluminescence using polystyrene as a matrix

Poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH–PPV) blends with polystyrene (PS) were used as emitting layers in polymer light-emitting diodes. Studies of photoluminescence and electroluminescence (EL) of the blends indicate that interchain interactions were tremendously suppressed due to the dilution effect. The device of MEH–PPV/PS (50/50) shows much higher EL efficiency compared to pure MEH–PPV devices. Since there is neither energy transfer nor charge transfer involved in MEH–PPV/PS blends, the observed efficiency enhancement is mainly attributed to the suppressed interchain species, which are responsible for the low photoluminescence yields. In addition, the addition of PS into MEH–PPV improves the thermal stability of polymer thin films and reduces the sensitivity of device performance to processing conditions.