Enhancement of light output power in GaN-based light-emitting diodes using indium tin oxide films with nanoporous structures

We fabricated GaN-based light-emitting diodes (LEDs) with a transparent ohmic contact made from nanoporous indium tin oxide (ITO). The nanoporous structures are easily made and controlled using a simple wet etching technique. The transmittance, sheet resistance, and root-mean-square surface roughness of the nanoporous ITO films are correlated strongly with the etch times. On the basis of the experimental values of these parameters, we choose an optimum etch time of 50 s for the fabrication of LEDs. The wall-plug efficiency of the LEDs with nanoporous ITO is increased by 35% compared to conventional LEDs at an injection current of 20 mA. This improvement is attributed to the increase in light scattering at the nanoporous ITO film-to-air interface.     

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.     

Fabrication of 380 nm ultra violet light emitting diodes on nano-patterned n-type GaN substrate

380 nm ultraviolet (UV) light emitting diodes (LEDs) were grown on patterned n-type GaN substrate (PNS) with silicon dioxide (SiO2) nano pattern to improve the light output efficiency. Wet etched self assembled indium tin oxide (ITO) nano clusters serves as dry etching mask for converting the SiO2 layer grown on n-GaN template into SiO2 nano patterns by inductively coupled plasma etching. Three different diameter of ITO such as 200, 250 and 300 nm were used for SiO2 nano pattern fabrication. PNS is obtained by n-GaN regrowth on SiO2 nano patterns and UV LEDs were grown on PNS template by MOCVD. Enhanced light output intensity was observed by employing SiO2 nano patterns on n-GaN. Among different PNS UV LEDs, LED grown on PNS with 300 nm ITO diameter showed enhancement in light output intensity by 2.1 times compared to the reference LED without PNS.     

High-performance transparent conducting oxide nanowires

We report the growth and characterization of single-crystalline Sn-doped In2O3 (ITO) and Mo-doped In2O3 (IMO) nanowires. Epitaxial growth of vertically aligned ITO nanowire arrays was achieved on ITO/yttria-stabilized zirconia (YSZ) substrates. Optical transmittance and electrical transport measurements show that these nanowires are high-performance transparent metallic conductors with transmittance of approximately 85% in the visible range, resistivities as low as 6.29 x 10(-5) Omega x cm and failure-current densities as high as 3.1 x 10(7) A/cm2. Such nanowires will be suitable in a wide range of applications including organic light-emitting devices, solar cells, and field emitters. In addition, we demonstrate the growth of branched nanowire structures in which semiconducting In2O3 nanowire arrays with variable densities were grown epitaxially on metallic ITO nanowire backbones.     

Enhanced performance of GaN-based LEDs via electroplating of a patterned copper layer on the backside

InGaN/GaN multi-quantum well light-emitting diodes (LEDs) are conventionally grown on a sapphire substrate due to a lack of compatible substrates with a high compressive strain. This is a result of the relatively large lattice, and thermal expansion coefficient mismatches between GaN and sapphire. The compressive strain is considered to be a major obstacle to further improve next-generation high-performance GaN-based LEDs. In this paper, we have designed, electroplated, and tested an efficient substrate using a patterned copper (Cu) layer on the backside of sapphire to relax the compressive strain in a GaN epilayer. The patterned Cu layer has a significant function in that it supports the GaN/sapphire LEDs with an external tensile stress. The external tensile stress is capable of compensating for the compressive strain in the GaN/sapphire LEDs by controlling the curvature of the wafer bowing. This patterned Cu layer, when applied to the GaN/sapphire LEDs, suppresses the compressive strain by up to 0.28 GPa. The GaN-based LEDs on this innovative and effective sapphire/Cu substrate offer improved optical and electrical performance.     

Photonic-crystal light-emitting diodes on p-type GaAs substrates for optical communications

Oxide-confined photonic-crystal (PhC) light-emitting diodes (LEDs) on p-type GaAs substrate in the 830 nm range are reported. The device consists of a bottom distributed Bragg reflector (DBR), quantum wells (QWs), and a top DBR, with a photonic-crystal structure formed within the n-type ohmic contact ring for light extraction. The etching depth of the PhC holes is 17-pair out of the 22-pair top DBR being etched off. The internally reflected spontaneous light emission can be extracted out of PhC holes because of lower reflectance within those areas. High-resolution micrographic imaging studies indicate that the device emits light mainly through the photonic-crystal holes and it is suitable for optical communications.     

Full wafer scale nanoimprint lithography for GaN-based light-emitting diodes

A UV-imprinting process for a full wafer was developed to enhance the light extraction of GaN-based green light-emitting diodes (LEDs). A polyvinyl chloride flexible stamp was used in the imprinting process to compensate for the poor flatness of the LED wafer. Two-dimensional photonic crystal patterns with pitches ranging from 600 to 900 nm were formed on the p-GaN top cladding layer of a 2 inch diameter wafer using nanoimprint and reactive ion etching processes. As a result, the optical output power of the patterned LED device was increased by up to 44% at a driving current of 20 mA by suppressing the total internal reflection and enhancing the irregular scattering of photons at the patterned p-GaN surface.     

InGaN-based nano-pillar light emitting diodes fabricated by self-assembled ITO nano-dots

InGaN/GaN based nano-pillar light emitting diodes (LEDs) with a diameter of 200-300 nm and a height of 500 nm are fabricated by inductively coupled plasma etching using self-assembled ITO nano-dots as etching mask, which were produced by wet etching of the as-deposited ITO films. The peak PL intensity of the nano-pillar LEDs was significantly higher than that of the as-grown planar LEDs, which can be attributed to the improvement of external quantum efficiency of the nano-pillar LEDs due to the large sidewall of the nano-pillars. We have also demonstrated electrical pumping of the InGaN/GaN based nano-pillar LEDs with a self-aligned TiO2 layer as a passivation of sidewall of the nano-pillars.     

Nanoscale conjugated-polymer light-emitting diodes

We use e-beam lithography to pattern an indium tin oxide (ITO) electrode to create arrays of conjugated-polymer LEDs, each of which has a hole-injecting contact limited to 100 nm in diameter. Using optical microscopy, we estimate that the electroluminescence from a 100 nm diameter LED comes from a region characterized by a diameter of approximately 170 nm. This apparent broadening occurs due to current spreading within a PEDOT:PSS layer which was included to aid hole injection.     

Controlling electron overflow in phosphor-free InGaN/GaN nanowire white light-emitting diodes

We have investigated for the first time the impact of electron overflow on the performance of nanowire light-emitting diodes (LEDs) operating in the entire visible spectral range, wherein intrinsic white light emission is achieved from self-organized InGaN quantum dots embedded in defect-free GaN nanowires on a single chip. Through detailed temperature-dependent electroluminescence and simulation studies, it is revealed that electron leakage out of the device active region is primarily responsible for efficiency degradation in such nanowire devices, which in conjunction with the presence of nonradiative surface recombination largely determines the unique emission characteristics of nanowire light-emitting diodes. We have further demonstrated that electron overflow in nanowire LEDs can be effectively prevented with the incorporation of a p-doped AlGaN electron blocking layer, leading to the achievement of phosphor-free white light-emitting diodes that can exhibit for the first time virtually zero efficiency droop for injection currents up to ~2200 A/cm(2). This study also provides unambiguous evidence that Auger recombination is not the primary mechanism responsible for efficiency droop in GaN-based nanowire light-emitting diodes.     

Evaluation of metal/indium-tin-oxide for transparent low-resistance contacts to p-type GaN

In search of a better transparent contact to p-GaN, we analyze various metal/indium-tin-oxide (ITO) (Ag/ITO, AgCu/ITO, Ni/ITO, and NiZn/ITO) contact schemes and compare to Ni/Au, NiZn/Ag, and ITO. The metal layer boosts conductivity while the ITO thickness can be adjusted to constructive transmission interference on GaN that exceeds extraction from bare GaN. We find a best compromise for an Ag/ITO (3 nm/67 nm) ohmic contact with a relative transmittance of 97% of the bare GaN near 530?nm and a specific contact resistance of 0.03 Ω·cm². The contact proves suitable for green light-emitting diodes in epi-up geometry.     

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.     

Characteristics of dielectrophoretically aligned UV-blue GaN nanowire LEDs

This is a report on the characteristics of UV-blue light emitting diodes (LEDs) containing homojunction gallium nitride (GaN) nanowires (NWs). These LEDs were prepared by the dielectrophoresis assisted assembly deposition (DAAD) method. The incorporation of an additional silicon dioxide (SiO2) and a Au capping metal layer was found to improve the electrical and optical properties of the DAAD-prepared GaN NW LEDs. These LEDs exhibited a parasitic series resistance of 120-180 komega with a sharp turn-on forward voltage of 3.4-4.0 V and had as low as approximately approximately 7 x 10(-7) A of leakage current for a reverse bias of -10 V. Typical electroluminescence (EL) spectra, observed from these LEDs under a forward bias, had a broad ultraviolet (UV)-blue emission with a wavelength of 388-422 nm. These LEDs could be seen with the naked eye. We concluded that the high-brightness EL spectra resulted from the enhancement of the carrier injection due to the size effect of the n-GaN nanowires on the p-GaN substrate.     

High efficiency GaN-based light emitting diode with nano-patterned ZnO surface fabricated by wet process

The improvement of the optical output power of GaN-based light emitting diodes (LEDs) was achieved by a novel bi-layer transparent top electrode scheme. The proposed bi-layer structure is composed of a Ga-doped ZnO layer with nano-patterns obtained solely by wet etching process and an Indium Tin Oxide p-type transparent conducting electrode layer. We employed various wet-etching conditions to maximize light extraction efficiency and it was observed that the crystal morphologies of nano-patterns and optoelectronic properties are dependent on etching duration. Because of ITO under GZO layer, the current spreading was not affected even after formation of nano-patterned surface on the GZO layer by wet etching. Consequently, an enhancement of as high as 43.1% in optical output power at an injection current of 100 mA for the LED with nano-patterns wet-etched by 0.025% HCl for 30 seconds was realized without significant degradation in electrical property when compared to a reference LED.     

Light-emitting characteristics of organic light-emitting diodes with the MoOx-doped NPB and C60/LiF layer

The hole ohmic properties of the MoO_x-doped NPB layer have been investigated by analyzing the current density-voltage properties of hole-only devices and by assigning the energy levels of ultraviolet photoemission spectra. The result showed that the performance of organic light-emitting diodes (OLEDs) is markedly improved by optimizing both the thickness and the doping concentration of a hole-injecting layer (HIL) of N, N′-diphenyl-N, N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (NPB) doped with molybdenum oxide (MoO_x) which was inserted between indium tin oxide (ITO) and NPB. For the doping concentration of above 25%, the device composed of a glass/ITO/MoO_x-doped NPB (100 nm)/Al structure showed the excellent hole ohmic property. The investigation of the valence band structure revealed that the p-type doping effects in the HTL layer and the hole concentration increased at the anode interfaces cause the hole-injecting barrier lowering. With both MoO_x-doped NPB as a hole ohmic contact and C₆₀/LiF as an electron ohmic contact, the device, which is composed of glass/ITO/MoO_x-doped NPB (25%, 5 nm)/NPB (63 nm)/Alq₃ (37 nm)/C₆₀ (5 nm)/LiF (1 nm)/Al (100 nm), showed the luminance of about 58,300 cd/m² at the low bias voltage of 7.2 V.     

Characterization of ITO/CdO/glass thin films evaporated by electron beam technique

Abstract

A thin buffer layer of cadmium oxide (CdO) was used to enhance the optical and electrical properties of indium tin oxide (ITO) films prepared by an electron-beam evaporation technique. The effects of the thickness and heat treatment of the CdO layer on the structural, optical and electrical properties of ITO films were carried out. It was found that the CdO layer with a thickness of 25 nm results in an optimum transmittance of 70% in the visible region and an optimum resistivity of 5.1×10^?3 Ω cm at room temperature. The effect of heat treatment on the CdO buffer layer with a thickness of 25 nm was considered to improve the optoelectronic properties of the formed ITO films. With increasing annealing temperature, the crystallinity of ITO films seemed to improve, enhancing some physical properties, such as film transmittance and conductivity. ITO films deposited onto a CdO buffer layer heated at 450 °C showed a maximum transmittance of 91% in the visible and near-infrared regions of the spectrum associated with the highest optical energy gap of 3.61 eV and electrical resistivity of 4.45×10^?4 Ω cm at room temperature. Other optical parameters, such as refractive index, extinction coefficient, dielectric constant, dispersion energy, single effective oscillator energy, packing density and free carrier concentration, were also studied.     

Enhanced light extraction from GaN-based LEDs with a bottom-up assembled photonic crystal

Photonic crystal (PhC) structure is an efficient tool for light extraction from light-emitting diodes (LEDs). The fabrication of a large area PhC structure on the light output surface of LEDs often involves sophisticated equipments such as nanoimprint lithography machine. In this study a monolayer of polystyrene (PS) microspheres was employed as a template to fabricate a noninvasive photonic crystal of indium tin oxide (ITO) on the surface of GaN-based LED. PS spheres can help to form periodic arrangement of bowl-like holes, a photonic crystal with gradually changed fill factors. Importantly, the electroluminescence intensity of LED with a photonic crystal was significantly enhanced by 1.5 times compared to that of the conventional one under various forward injection currents.     

Characterization of ITO/CdO/glass thin films evaporated by electron beam technique

A thin buffer layer of cadmium oxide (CdO) was used to enhance the optical and electrical properties of indium tin oxide (ITO) films prepared by an electron-beam evaporation technique. The effects of the thickness and heat treatment of the CdO layer on the structural, optical and electrical properties of ITO films were carried out. It was found that the CdO layer with a thickness of 25 nm results in an optimum transmittance of 70% in the visible region and an optimum resistivity of 5.1×10⁻³ Ω cm at room temperature. The effect of heat treatment on the CdO buffer layer with a thickness of 25 nm was considered to improve the optoelectronic properties of the formed ITO films. With increasing annealing temperature, the crystallinity of ITO films seemed to improve, enhancing some physical properties, such as film transmittance and conductivity. ITO films deposited onto a CdO buffer layer heated at 450 °C showed a maximum transmittance of 91% in the visible and near-infrared regions of the spectrum associated with the highest optical energy gap of 3.61 eV and electrical resistivity of 4.45×10⁻⁴ Ω cm at room temperature. Other optical parameters, such as refractive index, extinction coefficient, dielectric constant, dispersion energy, single effective oscillator energy, packing density and free carrier concentration, were also studied.     

Diode-Pumped Violet Energy Upconversion in BaF(2):Er(3+)

Under 805-nm diode-laser excitation we detected intense upconversion signals at 410, 380, and 275 nm in BaF(2):Er(3+). Energy upconversion schemes and efficiencies are discussed in detail. Intensity parameters of Er(3+) in BaF(2) were derived as Omega(2) = 1.048 ? 0.117 x 10(-20) cm(2), Omega(4) = 1.478 ? 0.180 x 10(-20) cm(2), and Omega(6) = 1.009 ? 0.127 x 10(-20) cm(2).     

Electrodeposition of Ni-Si Schottky barriers

Electrodeposition is being used to fabricate magnetic microstructures directly on patterned n-type Si wafers of various substrate resistivities. The Ni-Si Schottky barrier is characterized and found to be of high quality for relatively low Si resistivities (1-2 /spl Omega//spl middot/cm), with extremely low reverse leakage. It is shown that a direct correlation exists among the electrodeposition potential, the roughness, and the coercivity of the films. A conductive seed layer or a back contact is not compulsory for electrodeposition on Si with resistivities up to 15 /spl Omega//spl middot/cm. This shows that electrodeposition of magnetic materials on Si might be a viable fabrication technique for magnetoresistance and spintronics applications.