Nanoparticle embedded p-type electrodes for GaN-based flip-chip light emitting diodes

We have investigated high-quality ohmic contacts for flip-chip light emitting diodes using Zn-Ni nanoparticles/Ag schemes. The Zn-Ni nanoparticles/Ag contacts produce specific contact resistances of 10(-5)-10(-6) omegacm2 when annealed at temperatures of 330-530 degrees C for 1 min in air ambient, which are much better than those obtained from the Ag contacts. It is shown that blue InGaN/GaN multi-quantum well light emitting diodes fabricated with the annealed Zn-Ni nanoparticles/Ag contacts give much lower forward-bias voltages at 20 mA compared with those of the multi-quantum well light emitting diodes made with the as-deposited Ag contacts. It is further presented that the multi-quantum well light emitting diodes made with the Zn-Ni nanoparticles/Ag contacts show similar output power compared to those fabricated with the Ag contact layers.     

Impact of layer thickness and light transmission of ZnO nanomaterials on GaN-based light emitting diodes

Highly transparent ZnO nanomaterials have been successfully dispersed in the form of nanoparticles and nanorods on InGaN/GaN-based surface mounted light emitting diodes (SM-LEDs). An effortless spin-coating technique is employed to disperse the ZnO nanoparticle layers, and a well-known hydrothermal technique is used for growing the ZnO nanorods. The layer thickness and the light transmission at a specific wavelength are the major factors in improving the light output power of the devices. Field emission scanning electron microscope (FESEM) images are used to confirm the uniform dispersion of the ZnO nanostructures on the top of the SM-LEDs. The layer thickness and the level of light transmission at 460 nm are examined from the cross-sectional FESEM images and UV absorption spectra, respectively.     

Monolithic GaAs/InGaP nanowire light emitting diodes on silicon

Vertical light emitting diodes (LEDs) based on GaAs/InGaP core/shell nanowires, epitaxially grown on GaP and Si substrates, have been fabricated. The devices can be fabricated over large areas and can be precisely positioned on the substrates, by the use of standard lithography techniques, enabling applications such as on-chip optical communication. LED functionality was established on both kinds of substrate, and the devices were evaluated in terms of temperature-dependent photoluminescence and electroluminescence.     

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).     

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.     

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.     

Light extraction efficiency enhancement of GaN-based light emitting diodes on n-GaN layer using a SiO2 photonic quasi-crystal overgrowth

In this paper, GaN-based LEDs with a SiO2 photonic quasi-crystal (PQC) pattern on an n-GaN layer by nano-imprint lithography (NIL) are fabricated and investigated. At a driving current of 20 mA on Transistor Outline (TO)-can package, the better light output power of LED III (d = 1.2 microm) was enhanced by a factor of 1.20. After 1000 h life test (55 degrees C/50 mA) condition, Normalized output power of LED with a SiO2 PQC pattern (LED III (d = 1.2 microm)) on an n-GaN layer only decreased by 5%. This results offer promising potential to enhance the light output power of commercial light-emitting devices using the technique of nano-imprint lithography.     

Blue fluorescent organic light emitting diodes with multilayered graphene anode

As an innovative anode for organic light emitting devices (OLEDs), we have investigated graphene films. Graphene has importance due to its huge potential in flexible OLED applications. In this work, graphene films have been catalytically grown and transferred to the glass substrate for OLED fabrications. We have successfully fabricated 2 mm × 2 mm device area blue fluorescent OLEDs with graphene anodes which showed 2.1% of external quantum efficiency at 1000 cd/m². This is the highest value reported among fluorescent OLEDs using graphene anodes. Oxygen plasma treatment on graphene has been found to improve hole injections in low voltage regime, which has been interpreted as oxygen plasma induced work function modification. However, plasma treatment also increases the sheet resistance of graphene, limiting the maximum luminance. In summary, our works demonstrate the practical possibility of graphene as an anode material for OLEDs and suggest a processing route which can be applied to various graphene related devices.     

Recombination zone in white organic light emitting diodes with blue and orange emitting layers

White fluorescent OLED devices with a 10 nm thick blue-emitting layer and a 31 nm thick orange-emitting layer have been fabricated, where the blue-emitting layer is stacked on a hole transport layer. An interlayer was inserted between the two emitting layers. The thickness of the interlayer was changed among 0.3, 0.4, and 1.0 nm. White emission with CIE coordinates close to (0.33, 0.33) was observed from all the OLEDs. OLED with 0.3 nm thick interlayer gives the highest maximum luminous efficiency (11 cd/A), power efficiency (9 lm/W), and external quantum efficiency (5.02%). The external quantum efficiency becomes low with increasing the interlayer thickness from 0 nm to 1.0 nm. When the location of the blue- and orange-emitting layers is reversed, white emission was not obtained because of too weak blue emission. It is suggested that the electron–hole recombination zone decreases nearly exponentially with a distance from the hole transport layer.     

Top emitting organic light emitting diodes with opaque metal grid electrodes by transfer technique

The opaque metal grid electrodes are introduced to fabricate top emitting organic light emitting diodes (TOLEDs) through metal transfer technique. To transmit the lights, micrometer-sized patterns of aluminum (Al) were utilized as top cathodes in OLEDs and Al mirrors were also deposited at the other side of transparent substrates to reflect the lights emitted at the bottom sides. Although the only 50% of brightness compared to bottom emitting OLEDs (BOLEDs) could be achieved theoretically, the actual devices showed more than 70% based on the compressive effects during the metal transfer process. Since the resolution of human eyes recognizes these micrometer-sized grid structures as one pixel, TOLEDs can be simply fabricated without significant loss of efficiency.     

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.     

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.     

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.     

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.     

GaN-based p-i-n sensors with ITO contacts

Nitride-based p-i-n sensors with indium-tin-oxide electrodes on Mg-doped AlGaN/GaN strain layer superlattice structure were fabricated and characterized. It was found that the fabricated sensors exhibit small dark current and large reverse breakdown voltage. With an incident wavelength of 355 nm, we achieved a peak responsivity of 0.17 A/W which corresponds to 59% external quantum efficiency for sensors with 500/spl deg/C annealed ITO(70 nm) p-contacts.