Improvement of the luminous intensity of light-emitting diodes by using highly transparent Ag-indium tin oxide p-type ohmic contacts

We have investigated Ag-indium tin oxide (ITO) scheme for obtaining high-quality p-type ohmic contacts for GaN-based light-emitting diodes (LEDs). The Ag(1 nm)-ITO(200 nm) contacts exhibit greatly improved electrical characteristics when annealed at temperatures in the range 400/spl deg/C-600/spl deg/C for 1 min in air, yielding specific contact resistances of /spl sim/10/sup -4/ /spl Omega//spl middot/cm/sup 2/. In addition, the contacts give transmittance of about 96% at 460 nm, which is far better than that of the conventionally used oxidized Ni-Au contacts. It is shown that the luminous intensity of blue LEDs fabricated with the Ag-ITO contacts is about three times higher than that of LEDs with oxidized Ni-Au contacts. This result strongly indicates that the Ag-ITO scheme can serve as a highly promising p-type ohmic contact for the realization of high brightness near ultraviolet LEDs.     

Enhanced output power of InGaN-GaN light-emitting diodes with high-transparency nickel-oxide-indium-tin-oxide Ohmic contacts

This study develops a highly transparent nickel-oxide (NiO/sub x/)-indium-tin-oxide (ITO) transparent Ohmic contact with excellent current spreading for p-GaN to increase the optical output power of nitride-based light-emitting diodes (LEDs). The NiO/sub x/-ITO transparent Ohmic contact layer was prepared by electron beam in-situ evaporation without postdeposition annealing. Notably, the transmittance of the NiO/sub x/-ITO exceeds 90% throughout the visible region of the spectrum and approaches 98% at 470 nm. Moreover, GaN LED chips with dimensions of 300 /spl times/ 300 /spl mu/m fabricated with the NiO/sub x/-ITO transparent Ohmic contact were developed and produced a low forward voltage of 3.4 V under a nominal forward current of 20 mA and a high optical output power of 6.6 mW. The experimental results indicate that NiO/sub x/-ITO bilayer Ohmic contact with excellent current spreading and high transparency is suitable for fabricating high-brightness GaN-based light-emitting devices.     

Nitride-based light-emitting diodes with Ni/ITO p-type ohmic contacts

The optical and electrical properties of Ni(5 nm)-Au(5 nm) and Ni(3.5 nm)-indium tin oxide (ITO) (60 nm) films were studied. It was found that the normalized transmittance of Ni/ITO film could reach 87% at 470 nm, which was much larger than that of the Ni-Au film. It was also found that the specific contact resistance was 5 /spl times/ 10/sup -4/ /spl Omega/ /spl middot/ cm/sup 2/ and 1 /spl times/ 10/sup -3/ /spl Omega/ /spl middot/ cm/sup 2/, respectively, for Ni-Au and Ni/ITO on p-GaN. Furthermore, it was found that the 20 mA output power of light-emitting diode (LED) with Ni-Au p-contact layer was 5.26 mW. In contrast, the output power could reach 6.59 mW for the LED with Ni/ITO p-contact layer.     

High-brightness inverted InGaN-GaN multiple-quantum-well light-emitting diodes without a transparent conductive layer

Unlike the conventional layer structure of an InGaN-GaN multiple-quantum-well light-emitting diode (LED), an LED with reversed p-type and n-type layer sequence, and an n+/p+ tunnel junction has been investigated. When operated at 20 mA, the output power of the inverted LED is almost twice that of the conventional LED. Since the structures of these two LEDs are alike when analyzed by X-ray diffraction, the improvement in the light intensity could be attributed to the elimination of the absorption/reflection by the transparent conductive layer and/or some quality improvement of p-type GaN in the inverted LED.     

Improvement of GaN-based light-emitting diode by indium-tin-oxide transparent electrode and vertical electrode

A sapphire-etched vertical-electrode nitride semiconductor (SEVENS) light-emitting diode (LED) is fabricated by means of a selective chemical wet-etching technique. The SEVENS-LED formed on sapphire substrate exhibits excellent device performance compared to a conventional NiAu lateral-electrode (LE) GaN-based LED formed on the same sapphire substrate. The integral light-output power of SEVENS-LED is /spl sim/7 mW, which is 1.75 times stronger than that of the conventional NiAu LE-LED (/spl sim/4 mW). The external quantum efficiency of SEVENS-LED is estimated to be approximately 13%.     

Improvement of near-ultraviolet InGaN-GaN light-emitting diodes through higher pressure grown underlying GaN layers

The 410-nm near-ultraviolet (near-UV) InGaN-GaN multiple quantum-wells light-emitting diodes (LEDs) with low-pressure-grown (200 mbar) and high-pressure-grown (400 mbar) Si-doped GaN underlying layers were grown on c-face sapphire substrates by metal-organic vapor phase epitaxy. Increasing the growth pressure during the initial growth of the underlying n-type GaN epilayers of the near-UV InGaN-GaN LEDs was found to reduce the amount of threading dislocations that originated from the GaN-sapphire interfaces. The electroluminescence intensity of LEDs with underlying GaN layers grown at a higher pressure was nearly five times larger than that of LED with layers grown at lower pressure. Additionally, two-order reduction of leakage current was also induced for the LEDs grown at a higher pressure.     

Improvement of near-ultraviolet InGaN-GaN light-emitting diodes with an AlGaN electron-blocking layer grown at low temperature

The 400-nm near-ultraviolet InGaN-GaN multiple quantum well light-emitting diodes (LEDs) with Mg-doped AlGaN electron-blocking (EB) layers of various configurations and grown under various conditions, were grown on sapphire substrates by metal-organic vapor phase epitaxy system. LEDs with AlGaN EB layers grown at low temperature (LT) were found more effectively to prevent electron overflow than conventional LEDs with an AlGaN one grown at high temperature (HT). The electroluminescent intensity of LEDs with an LT-grown AlGaN layer was nearly three times greater than that of LEDs with an HT-grown AlGaN. Additionally, the LEDs with an LT-grown AlGaN layer in H/sub 2/ ambient were found to increase the leakage current by three orders of magnitude and reduce the efficiency of emission.     

Low resistance and reflective Mg-doped indium oxide-Ag ohmic contacts for flip-chip light-emitting diodes

We have investigated an Mg-doped In/sub x/O/sub y/(MIO)-Ag scheme for the formation of high-quality ohmic contacts to p-type GaN for flip-chip light-emitting diodes (LEDs). The as-deposited sample shows nonlinear current-voltage (I--V) characteristics. However, annealing the contacts at temperatures of 330/spl deg/C-530/spl deg/C for 1 min in air ambient results in linear I--V behaviors, producing specific contact resistances of 10/sup -4/--10/sup -5/ /spl Omega//spl middot/cm/sup 2/. In addition, blue LEDs fabricated with the MIO-Ag contact layers give forward-bias voltages of 3.13-3.15 V at an injection current of 20 mA. It is further shown that LEDs made with the MIO-Ag contact layers give higher output power compared with that with the Ag contact layer. This result strongly indicates that the MIO-Ag can be a promising scheme for the realization of high brightness LEDs for solid-state lighting application.     

具有微米/纳米复合粗化ITO的GaN基LED

Three types of textured indium-tin-oxide （ITO） surface, including nano-texturing and hybrid micro/nano-texturing with micro-holes （concave-hybrid-pattem） or micro-pillars （convex-hybrid-pattern）, were applied to GaN-based light-emitting diodes （LEDs）. The nano-texturing was realized by maskless wet-etching, and the micro-texturing was achieved by standard photolithography and wet-etching. Compared to LED chips with flat ITO surface, those with nano-pattern, concave-hybrid-pattern, and convex-hybrid-pattern exhibit enhancement of 11.3%, 15.8%, and 17.9%, respectively, for the light-output powers at 20 mA. The electrical performance has no degradation. Moreover, the convex-hybrid-pattern show higher light-output efficiency under small injection current, while the concave-hybrid-pattern exhibit better light-output efficiency at large injection current. The light- extraction efficiency is simulated by use of two-dimensional finite difference time domain method, and the numer- ical results are consistent with the experiments.     

Electrical characteristics of In/ITO p-type ohmic contacts for high-performance GaN-based light-emitting diodes

We have investigated the annealing-induced improved electrical properties of In(10 nm)/ITO(200 nm) contacts with p-type GaN. The contacts become ohmic with a specific contact resistance of 2.75×10^–3 Ω cm² upon annealing at 650 °C in air. X-ray photoemission spectroscopy (XPS) Ga 2p core levels obtained from the interface regions before and after annealing indicate a large band-bending of p-GaN, resulting in an increase in the Schottky barrier height. STEM/energy dispersive X-ray (EDX) profiling results exhibit the formation of interfacial In-Ga-Sn-oxide. Based on the STEM and XPS results, the ohmic formation mechanisms are described and discussed. It is also shown that patterning by nano-imprint lithography improves the light output power of blue LEDs by 18–28% as compared to that of LEDs fabricated with unpatterned In/ITO contacts.     

InP mixer diodes with etched via ohmic contacts

InP mixer diodes processed with Ag/TiW/Au Schottky diodes have exhibited a noise figure of 6.5?7.0 dB at 94 GHz. InP surface preparation is shown to be critical in diode performance. An indium-stabilised surface has resulted in a barrier height of 0.45 eV.     

Omnidirectional reflective contacts for light-emitting diodes

An electrically conductive omnidirectional reflector (ODR) is demonstrated in an AlGaInP light-emitting diode (LED). The ODR serves as p-type contact and comprises the semiconductor, a metal layer and an intermediate low-refractive index dielectric layer. The dielectric layer is perforated by an array of AuZn microcontacts thus enabling electrical conductivity. It is shown that the ODR significantly increases light extraction from an AlGaInP LED as compared to a reference LED employing a distributed Bragg reflector (DBR). External quantum efficiencies of 18% and 11% are obtained for the ODR- and the DBR-LED, respectively.     

Improvement of InGaN-GaN light-emitting diode performance with a nano-roughened p-GaN surface

This investigation describes the development of InGaN-GaN light-emitting diode (LED) with a nano-roughened top p-GaN surface which uses Ni nano-mask and wet etching. The light output of the InGaN-GaN LED with a nano-roughened top p-GaN surface is 1.4 times that of a conventional LED, and wall-plug efficiency is 45% higher. The operating voltage of InGaN-GaN LED was reduced from 3.65 to 3.5 V at 20 mA and the series resistance was reduced by 20%. The light output is increased by the nano-roughening of the top p-GaN surface. The reduction in the series resistance can be attributed to the increase in the contact area of nano-roughened surface.     

Electrical and thermal stability of Ag ohmic contacts for GaN-based flip-chip light-emitting diodes by using an AgAl alloy capping layer

We have investigated Ag(200 nm)/AgAl(100 nm) ohmic contacts to p-type GaN for near-UV (405 nm) flip-chip light-emitting diodes (LEDs). It is shown that the use of an AgAl alloy capping layer (with 8 at% Al) results in better electrical and optical properties as compared to single Ag contacts when annealed at 430 °C. For example, Ag/AgAl (8 at% Al) contacts give specific contact resistance of 4.6×10^–4 Ω cm² and reflectance of 90% at a wavelength of 405 nm. However, use of an AgAl (with 50 at% Al) layer is not effective. LEDs fabricated with the Ag/AgAl (8 at% Al) reflectors produce higher light output as compared with the ones with single Ag reflectors. Ohmic mechanisms of the Ag/AgAl (8 at% Al) contacts are described and discussed.     

Formation of low-resistance and transparent indium tin oxide ohmic contact for high-brightness GaN-based light-emitting diodes using a Sn–Ag interlayer

We report on the formation of low-resistance and highly transparent indium tin oxide (ITO) ohmic contacts to p-GaN using a Sn–Ag alloy interlayer. Although the as-deposited Sn–Ag(6 nm)/ITO(200 nm) contacts show non-ohmic behaviors, the scheme becomes ohmic with specific contact resistance of 4.72×10⁻⁴ Ω cm² and produce transmittance of ∼91% at wavelength of 460 nm when annealed at 530 °C. Blue light-emitting diodes (LEDs) fabricated with the Sn–Ag/ITO contacts give forward-bias voltage of 3.31 V at injection current of 20 mA. LEDs with the Sn–Ag/ITO contacts show the improvement of the output power by 62% (at 20 mA) compared with LEDs with Ni/Au contacts.     

Organic/metal hybrid cathode for transparent organic light-emitting diodes

We report a highly transparent organic/metal hybrid cathode of a Cs-doped electron transport layer (Cs-ETL)/Ag for transparent organic light-emitting diode (TOLED) applications. Particular attention is paid to the surface morphology on the Ag film and its influence on the optical transparency and electrical conductivity. With the use of Cs-ETL, a smooth and continuous surface morphology of Ag film was achieved, leading to a high transmittance of ～75% in TOLED with a low sheet resistance of 4.5 Ω/Sq in cathode film. We successfully applied our Cs-ETL/Ag transparent cathode to fabricate highly transparent OLEDs. Our approach suggests a new electrode structure for transparent OLED applications.     

Improvement of thermal management of high-power GaN-based light-emitting diodes

The thermal management of high-power light-emitting-diode (LED) devices employing various die-attach materials is analyzed. Three types of die-attach materials are tested, including silver paste, Sn–3 wt.% Ag–0.5 wt.% Cu (SAC305) solder, and SAC305 solder added with a small amount of carbon nanotubes (CNTs). The analysis of thermal management is performed by comparing the temperatures of the LED chips in use and the total thermal resistances of the LED devices obtained respectively from the thermal infrared images and thermal transient analysis. Due to the high thermal conductivity of CNT, the addition of CNTs into the SAC305 solder reduces the total thermal resistance and chip temperature of the LED device, and the thermal management of the LED devices is improved accordingly.     

Improvement of carrier distribution in dual wavelength light-emitting diodes

The effect of different barriers between green and blue light regions in dual wavelength light emitting diodes was studied.Compared with a traditional sample,electroluminescence and photoluminescence spectra of the newly designed samples showed peak intensity improvements and smaller blue-shifts with increasing injection current level,and the bottom quantum-wells light emitting is enhanced.All these phenomena can be ascribed to reduced barrier thickness and indium doping in the quantum-barrier influencing electric fields and more holes injecting into the bottom QWs.     

A tunable lighting system integrated by inorganic and transparent organic light-emitting diodes

A tunable surface-emitting integrated lighting system is constructed using a combination of inorganic light-emitting diodes （LEDs） and transparent organic LEDs （OLEDs）. An RB two-color LED is used to supply red and blue light emission, and a green organic LED is used to supply green light emission. Currents of the LED and OLED are tuned to produce a white color, showing different Commission Internationale d＇Eclairage （CIE） chromaticity coordinates and correlated color temperatures with a wide adjustable range. Such an integration can compensate for the lack of the LED＇s luminance uniformity and the transparent OLED＇s luminance intensity.     

Phosphorescent transparent organic light-emitting diodes with enhanced outcoupling efficiency: Reduction of surface plasmon losses

In this work, we fabricated nanostructured transparent organic light-emitting diodes (TrOLEDs) using phosphorescent materials and a WO₃ layer with various periods of perforation, to improve light extraction. Using these nanostructured TrOLEDs, higher external quantum efficiency (EQE) values were achieved, of 7.8% (bottom emission), and 2.0% (top emission) at 100 mA/cm². Compared to conventional TrOLEDs, these were 28% and 33% higher for bottom and top emission, respectively. In addition, by varying the periods of the nanostructures, we found that the extraction of the trapped surface plasmon mode was mainly responsible for enhancing outcoupling efficiency. When adopting light extraction methods in TrOLEDs, one should consider the influence of the optical clarity of devices. The nanostructured TrOLEDs in this study showed good optical clarity as the total transmittance was consistent with direct transmittance. Photographs of the TrOLEDs also showed neither optical blur nor haziness. Lastly, the total transmittance of the nanostructured TrOLEDs was similar to that of a conventional TrOLED except for two points where light coupling to the surface plasmon mode and waveguide mode occurred.