Vertical nanowire array-based light emitting diodes

Electroluminescence from a nanowire array-based light emitting diode is reported. The junction consists of a p-type GaN thin film grown by metal organic chemical vapor deposition (MOCVD) and a vertical n-type ZnO nanowire array grown epitaxially from the thin film through a simple low temperature solution method. The fabricated devices exhibit diode like current voltage behavior. Electroluminescence is visible to the human eye at a forward bias of 10 V and spectroscopy reveals that emission is dominated by acceptor to band transitions in the p-GaN thin film. It is suggested that the vertical nanowire architecture of the device leads to waveguided emission from the thin film through the nanowire array.     

Full-color InGaN/GaN dot-in-a-wire light emitting diodes on silicon

We report on the achievement of a new class of nanowire light emitting diodes (LEDs), incorporating InGaN/GaN dot-in-a-wire nanoscale heterostructures grown directly on Si(111) substrates. Strong emission across nearly the entire visible wavelength range can be realized by varying the dot composition. Moreover, we have demonstrated phosphor-free white LEDs by controlling the indium content in the dots in a single epitaxial growth step. Such devices can exhibit relatively high internal quantum efficiency (>20%) and no apparent efficiency droop for current densities up to ~ 200 A cm(-2).     

GaN-based Indium-tin-oxide light emitting diodes with nanostructured silicon upper contacts

GaN-based indium-tin-oxide (ITO) light emitting diodes (LEDs) with p-GaN, n⁺-short period superlattice (SPS) and nanostructured silicon contact layers were fabricated. It was found that surface of the ITO LED with nanostructured silicon layer was very rough. It was also found that 20 mA forward voltages measured from ITO LEDs with p-GaN, n⁺-SPS and nanostructured silicon contact layers were 6.01, 3.25 and 3.26 V, respectively. Compared with ITO LED with n⁺-SPS, it was found that output power of ITO LED with nanostructured silicon contact was 17% larger. Furthermore, it was found that ITO LED with nanostructured silicon contact was more reliable.     

Blue light emitting ZnS diodes

Low resistivity n-type ZnS single crystals of about 10³Ω cm and 10²Ω cm were achieved by firing the as-grown high resistivity ZnS single crystals using quartz ampoules and graphite crucible, respectively. Schottky diodes fabricated from sample I gave stable blue emission in reverse bias while sample II gave blue emission in forward bias. The characteristics of these diodes were studied and presented. Carrier concentrations were estimated from the capacitance-voltage measurements of the two types of diodes. Electroluminescence spectra as well as cathodoluminescence spectra of both samples are presented. The blue emission peak was ascribed to the donor-acceptor pairs transition.     

Organic light emitting diodes based on dipyridamole drug

The dipyridamole drug [DIP: 2,6-bis(diethanolamino)-4,8-dipiperidinopyrimido(5,4-d)pyrimidine] is widely used in treatment of coronary heart disease for its antiplatelet and vasodilating activities, and its high intensity photoluminescence (PL) has been widely reported. In this work, the fabrication and the characterization of a new OLED using the DIP molecule as an emitting layer is reported. The devices were assembled using a heterojunction between three organic molecular materials: the N,N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)benzidine (NPB) or the 1-(3-methylphenyl)-1,2,3,4-tetrahydroquinoline-6-carboxyaldehyde-1,1′-diphenylhydrazone (MTCD) as hole-transporting layer, the DIP layer as an emitting layer and the tris(8-hydroxyquinoline aluminum) (Alq₃) as the electron transporting layer. All the organic layers were sequentially deposited in a high vacuum by thermal evaporation onto indium tin oxide substrates and without breaking vacuum. Continuous electroluminescence emission was obtained in all configurations upon varying the applied bias voltage from 4 to 30 V, the observed wide emission band was centered at 493 nm. The luminance of the devices was about 1500 (cd)/m² with 4.5 cd/A of efficiency for the best device. The charge transport behavior in the OLED is also discussed as a function of different carrier injection levels.     

Photoluminescence characterization of InGaP/GaAs/InGaP quantum wires

We investigated the photoluminescence of InGaP/GaAs/InGaP heterostructures with the aim to prepare quantum wires by the epitaxial overgrowth of V-groove patterned substrates. Planar and V-groove patterned GaAs semiinsulating substrates were used for epitaxial growth in a low-pressure MOVPE equipment with a horizontal reactor. Low temperature photoluminescence measurements show that the composition of the InGaP ternary compound prepared on the patterned substrates is shifted to a higher InP mole fraction compared with the planar ones. On the other hand, the measurement on the V-grooved samples showed that the PL peak is shifted to higher energies (i.e. to the higher amount of Ga), which indicates a change in the ternary composition of about 5%. Crystalline quality of the overgrown structures was studied by transmission electron microscopy. Both, photoluminescence and photoluminescence polarization measurement show that quantum wires can be successfully prepared in the InGaP/GaAs/InGaP system.     

Silicon-based light emitting diodes on silicon and glass substrates using a low temperature multilayered nanocrystalline structure

Si nanocrystals have been prepared by hydrogenation and subsequent annealing of as-deposited amorphous Si layers on glass and Si substrates. The hydrogenation process has been performed at 350 °C under radio frequency hydrogen plasma. The nanocrystallites were processed by sequential reactive ion etching to allow light emission. Photoluminescence (PL) measurements demonstrate that the nanocrystallites emit light in the range of 500-570 nm. The evolution of nanocrystals has been studied using scanning electron microscopy, while atomic force microscopy and transmission electron microscopy have been utilized to examine the structure of the Si nanocrystals. Multilayer luminescent Si nanocrystals have been fabricated using alternating layers of Si nanocrystals and Si oxy-nitride. Bilayer structures have higher efficiency than a single layer structure, while multilayers with three layers of luminescent nanocrystals and above did not show a higher PL intensity. Transparent light emitting diodes have been realized based on multilayer luminescent Si nanocrystals that displayed bright emission which was visible to the naked eye in a bright room.     

PVK/Alq3 organic light emitting diodes obtained by evaporation

Poly(N-vinycarbazole) (PVK)/tris(8-hydroxy)-quinoline-aluminum (Alq₃) bilayer diodes were deposited by vacuum evaporation onto SnO₂ coated glass substrates. After deposition of an aluminum upper electrode, the diodes were studied by current–voltage (I–V) and electroluminescence–voltage (EL–V) measurements. It is shown that the luminescence of the structures should be attributed to Alq₃. However the presence of evaporated PVK allows us to decrease the barrier height at the interface SnO₂/PVK because of the localized states induced in the PVK band gap during the evaporation which allows multistep tunneling effect. The optimum Alq₃ thickness is shown to be about 75 nm.     

Dislocation engineered silicon light emitting devices

The influence of boron-induced dislocation loops on the luminescence efficiency of silicon-based light-emitting diodes is investigated. Luminescence measurements and transmission-electron-microscopy images from devices fabricated by boron implantation into crystalline silicon, and subsequently processed under different conditions to form dislocation loops of different size and densities, were compared. Light emitting devices were also fabricated in an otherwise identical but a pre-amorphized substrate, to prevent boron-induced loop formation. The results demonstrate a strong correlation between the dislocation loop density and areal coverage, and the light emission efficiency. The devices produced in the pre-amorphized substrate, without dislocation loops, show strongly quenched light emission.     

Graphene/silicon nanowire Schottky junction for enhanced light harvesting

Schottky junction solar cells are assembled by directly coating graphene films on n-type silicon nanowire (SiNW) arrays. The graphene/SiNW junction shows enhanced light trapping and faster carrier transport compared to the graphene/planar Si structure. With chemical doping, the SiNW-based solar cells showed energy conversion efficiencies of up to 2.86% at AM1.5 condition, opening a possibility of using graphene/semiconductor nanostructures in photovoltaic application.     

The optical linewidth of InGaN light emitting diodes

A comparative study of the optical linewidths of high-quality InGaN epilayers and commercial single quantum well light emitting diode structures was undertaken using photo- and electroluminescence. Optical linewidths show a linear increase with increasing indium concentration in both cases. We assess the contribution of three mechanisms to the luminescence linewidth: alloy fluctuations, well width fluctuations and strain effects. It is found that the broadening of the emission line is an intrinsic property of InGaN alloys. The piezoelectric effect in wurtzite semiconductors is proposed as a novel line-broadening mechanism.     

Nanoscale ITO/ZnO layer-texturing for high-efficiency InGaN/GaN light emitting diodes

We propose a simple technique to improve the light extraction efficiency of GaN-based light emitting diodes (LEDs) by using nanoscale ITO/ZnO layer-texturing. The surface texturing of the ITO and ZnO layers was performed using a wet chemical etching and a spin-coating process, respectively. It has been found that the light extraction efficiency of the ITO-/ZnO-textured LED was 34.5% greater than that of a conventional LED with a planar ITO, at 20 mA of current injection. A high level of multiple light scattering at the textured surface promoted a high-efficiency in the InGaN/GaN LEDs. In addition, the individual performance of the ITO and ZnO texturing on the LED surface was also investigated. The lowered forward voltage of the ITO/ZnO layer-textured LED indicated this could be a damage-free approach for device fabrication.     

Electroluminescence of organolanthanide based organic light emitting diodes

Recent advances in organolanthanide based organic light emitting diodes have lead to the demonstration of infra-red emitting devices. A silicon based organic light emitting diode exhibiting 1.53 μm electroluminescence at room temperature has also recently been reported. Furthermore, recent work has led to a clearer understanding of the quenching mechanisms in these organolanthanide based devices and suggests that the efficiencies obtained to date can be greatly improved.     

Temperature-dependent nonradiative recombination processes in GaN-based nanowire white-light-emitting diodes on silicon

In this paper, we have performed a detailed investigation of the temperature- and current-dependent emission characteristics of nanowire light-emitting diodes, wherein InGaN/GaN dot-in-a-wire nanoscale heterostructures and a p-doped AlGaN electron blocking layer are incorporated in the device's active region to achieve white-light emission and to prevent electron overflow, respectively. Through these studies, the Auger coefficient is estimated to be in the range of ～10(-34) cm(6) s(-1) or less, which is nearly four orders of magnitude smaller than the commonly reported values of planar InGaN/GaN heterostructures, suggesting Auger recombination plays an essentially negligible role in the performance of GaN-based nanowire light-emitting diodes. It is observed, however, that the performance of such nanowire LEDs suffers severely from Shockley-Read-Hall recombination, which can account for nearly 40% of the total carrier recombination under moderate injection conditions (～100 A cm(-2)) at room temperature. The Shockley-Read-Hall nonradiative lifetime is estimated to be in the range of a few nanoseconds at room temperature, which correlates well with the surface recombination velocity of GaN and the wire diameters used in this experiment.     

树形分子有机电致发光材料研究

简要介绍了一类新型大分子-树形分子的结构、特性及其合成方法;综述了树形分子材料作为发光材料和载流子传输材料所具有的特点、类型及其在有机电致发光领域的研究进展;展望了此类材料的应用前景.