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.     

Improved Light Output of GaN-Based Light-Emitting Diodes by Using AgNi Reflective Contacts

We investigated the light output power of blue light-emitting diodes (LEDs) fabricated with AgNi contacts as a function of the Ni content. Annealing the AgNi contacts at 400°C in air significantly improved their electrical characteristics. The AgNi samples with 10.0 wt.% Ni showed reflectance of 80.9% at 460 nm, whereas the Ag-only contacts gave 71.1%. After annealing at 400°C, the AgNi contacts exhibited better thermal stability than did the Ag-only contacts. Their current–voltage relationships showed that blue LEDs fabricated with Ag-only contacts gave a forward voltage of 3.33 V at 20 mA, whereas those fabricated with AgNi contacts with 10.0 wt.% Ni produced 3.03 V. LEDs fabricated with the AgNi contacts exhibited output power higher by 5.9% to 19.1% than those with Ag-only contacts. Based on scanning electron microscopy and x-ray photoemission spectroscopy results, the improved thermal and electrical behaviors are described and discussed.     

Highly Reliable High-Brightness GaN-Based Flip Chip LEDs

The properties of indium-tin-oxide (ITO)/Ni films as transparent ohmic contacts of nitride-based flip chip (FC) light emitting diodes (LEDs) were studied. It was found that 300degC rapid thermal annealed (RTA) ITO(15 nm)/Ni(1 nm) could provide good electrical and optical properties for FC LED applications. It was also found that 20-mA operation voltage and output power of the 465-nm FC LEDs with ITO/Ni/Ag reflective mirror were 3.16 V and 21 mW, respectively. Furthermore, it was found that output intensity of the proposed LED only decayed by 5% after 1200 h under 30-mA current injection at room temperature.     

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.     

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.     

Enhanced Light Output Power of GaN-Based Vertical Light-Emitting Diodes by Using Highly Reflective ITO–Ag–Pt Reflectors

Highly reflective and thermally stable indium–tin–oxide (ITO)–Ag–Pt p-type reflectors for use in high-performance GaN-based light-emitting diodes (LEDs) have been investigated. The specific contact resistance of the ITO–Ag–Pt contacts was found to be $7.2 times 10 ^{-5} Omegacdothbox{cm}^{2}$ . The ITO–Ag–Pt contacts showed a higher reflectance after thermal annealing (82% at 460 nm), while the reflectance of the ITO–Ag contacts was reduced from 81% to 65%. In addition, surface agglomeration was drastically decreased, indicating that the Pt layer efficiently prevents the surface agglomeration of the Ag layer. The vertical LEDs (VLEDs) fabricated with the ITO–Ag–Pt contacts had a 17% higher output power (at 20 mA) than the VLEDs fabricated with the ITO–Ag contacts.      

Nitride-based flip-chip LEDs with transparent Ohmic contacts and reflective mirrors

Nitride-based flip-chip light-emitting diodes (LEDs) with various transparent ohmic contacts and reflective mirrors were fabricated. At 470 nm, it was found that Ni could provide 92% transmittance while Ag could provide 92.4% reflectively. It was also found that the 20-mA forward voltages measured from LEDs with Ni+Ag, Ni+Al, and Ni+Pt were 3.15, 3.29, and 3.18 V while the output powers were 16, 13.3, and 11.6 mW, respectively. Furthermore, it was found that lifetimes of the fabricated flip-chip LEDs were good.     

Highly reliable nitride-based LEDs with SPS+ITO upper contacts

Nitride-based blue light emitting diodes (LEDs) with an n/sup +/-short period superlattice (SPS) tunnel contact layer and an indium tin oxide (ITO) transparent contact were fabricated. Compared with conventional nitride-based LEDs with Ni/Au upper contacts, it was found that we could achieve a 60% increase in electroluminescence (EL) intensity by using ITO upper contacts. However, it was also found that the lifetime of ITO LEDs were much shorter. Furthermore, it was found that we could achieve a longer lifetime and a smaller reverse leakage current (I/sub R/) by the deposition of a SiO/sub 2/ layer on top of the ITO LEDs.     

Transparent conductors with an ultrathin nickel layer for high-performance photoelectric device applications

A thin nickel (Ni) layer of thickness 5 nm was inserted in between the indium tin oxide (ITO) layers of thickness 50 nm each so as to increase the conductivity of ITO without affecting much of its transmittance nature. ITO layers with and without Ni film were prepared by reactive DC sputtering on both Si and glass substrates. The influence of Ni layer on the optical and electrical properties of prepared devices was investigated. Due to the insertion of thin Ni layer, the resistivity of ITO/Ni/ITO sample (3.2×10⁻⁴ Ω cm) was reduced 10 times lesser than that of ordinary ITO layer (38.6×10⁻⁴ Ω cm); consequently it increased the mobility of ITO/Ni/ITO device. The external and internal quantum efficiencies (EQE and IQE) of ITO/Ni/ITO device exhibited better performance when compared to ITO layer that has no Ni film. At wavelengths of 350 and 600 nm, the photoresponses of ITO/Ni/ITO device were predominant than that of reference ITO device. This highly conductive and photoresponsive Ni inserting ITO layers would be a promising device for various photoelectric applications.     

用表面粗化ITO的欧姆接触提高GaN基LED性能

应用ICP干法刻蚀工艺和自然光刻技术,制备了ITO表面粗化的GaN基LED芯片。聚苯乙烯纳米颗粒在干法刻蚀中作为刻蚀掩膜。通过扫描电镜(SEM)观察ITO薄膜的粗糙度,并且报道了优化的粗化工艺参数。结果表明,ITO表面粗化的GaN基LED芯片同传统的表面光滑的芯片相比在20 mA的驱动电流下,发光强度提高了70%。     

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.     

Reflecting p-contact based on thin ITO films for AlGaInN flip-chip LEDs

A reflecting contact to a p-GaN layer, used in fabrication of blue flip-chip light-emitting diodes, has been produced by deposition of thin indium tin oxide (ITO) films by electron-beam evaporation. The high reflectance of the contact, which exceeds that of a Ni/Ag contact, provides a 15–20% increase in the external quantum efficiency of light-emitting crystals. The forward voltage drops for crystals with an ITO(5 nm)/Ag(220 nm) contact are comparable with the corresponding values for crystals with a Ni(1.5 nm)/Ag(220 nm) contact. The specific resistance of the contact with an ITO layer is 3.7 × 10^?3 Ω cm². It is shown that, for ITO films produced by the given method, the optimal thicknesses providing the best electrical and optical characteristics of the crystals are in the range 2.5–5.0 nm.

     

Large area roll-to-roll sputtering of transparent ITO/Ag/ITO cathodes for flexible inverted organic solar cell modules

We fabricated solution-processed flexible inverted organic solar cell (IOSC) modules (10 cm × 10 cm) on roll-to-roll (RTR) sputtered ITO/Ag/ITO multilayer cathodes. By using a pilot-scale RTR sputtering system equipped with mid-range frequency power for dual ITO targets and direct current power for the Ag target, we were able to continuously deposit a high-quality ITO/Ag/ITO multilayer on PET substrate with a width of 700 mm and length of 20,000 mm as a function of Ag thickness. At the Ag thickness of 12 nm, the ITO/Ag/ITO multilayer had a very low sheet resistance of 3.03 Ohm/square and high transmittance of 88.17%, which are better values than those of amorphous ITO film. A strip-type ITO/Ag/ITO cathode was successfully patterned using a RTR wet etching process. Successful operation of flexible IOSC modules on RTR sputtered ITO/Ag/ITO cathodes indicate that the RTR sputtering technique is a promising coating process for fabrication of high-quality transparent and flexible cathodes and can advance the commercialization of cost-efficient flexible IOSCs.     

Near-Ultraviolet Light-Emitting Devices Using Vertical ZnO Nanorod Arrays

Reports on electroluminescence (EL) in solid-state, nanomaterial-based devices emitting in the lower wavelength range of the visible spectrum are limited, and the emission stability of these devices is rarely reported. We have fabricated light-emitting devices (LEDs) based on integration of n-ZnO nanorods and p-GaN films, which emit in the violet to near-ultraviolet (NUV) region. We also present data on the stability of EL in fabricated devices. Vertical arrays on ZnO nanorods, with estimated ZnO nanorod density ~10⁸ cm⁻², were grown on p-GaN films with typical length of ~4 μm and width of ~120 nm. The NUV LEDs show low turn-on voltage (~3.0 V), small reverse saturation current (~10 μA), and more than two orders of magnitude rectification ratio, all of which indicates a good-quality p–n junction at the p-GaN/n-ZnO nanorod interface. The EL spectra of LEDs present an emission band centered at ~403 nm. Gaussian fitting of the EL peak revealed three emission peaks at 378 nm, 405 nm, and 431 nm with dominant emission in the NUV region. Significantly, the fabricated NUV LEDs present stable and repeatable EL characteristics, as revealed by bias-stress stability tests. The good electrical properties and stable EL performance make these nanostructure-based NUV LEDs potential candidates for mass production of next-generation lighting devices.     

Vertical InGaN light-emitting diodes with Ag paste as bonding layer

Vertical InGaN-based light-emitting diodes (LEDs) were fabricated with a Si substrate using Ag paste as bonding layer. Vertical LEDs with Ag paste bonding layer were bonded with Si substrate at a low temperature of 140 °C. In addition to the low-temperature bonding process, the soft property of Ag paste could better alleviate thermal stress compared with conventional eutectic metal bonding layer such as Au–Sn. Under the same test conditions, these two LEDs showed similar optical and electrical properties and reliability. However, LEDs with Ag-paste bonding layer were fabricated through a low-temperature bonding process. The characteristic of soft solder enables a relatively wider process window, such as bonding pressure and temperature, and a higher yield as compared with the vertical LEDs with Au–Sn eutectic bonding layer.     

High brightness InGaN green LEDs with an ITO on n/sup ++/-SPS upper contact

Indium tin oxide (ITO) (260 nm) and Ni (5 nm)/Au (10 nm) films were deposited onto glass substrates, p-GaN layers, n/sup +/-InGaN/GaN short-period-superlattice (SPS), n/sup ++/-SPS and nitride-based green light-emitting diodes (LEDs). It was found that ITO could provide us an extremely high transparency (i.e., 95% at 520 nm). It was also found that the 1.03/spl times/10/sup -3/ /spl Omega/cm/sup 2/ specific contact resistance of ITO on n/sup ++/-SPS was reasonably small. Although the forward voltage of the LED with ITO on n/sup ++/-SPS upper contacts was slightly higher than that of the LED with Ni/Au on n/sup ++/-SPS upper contacts, the 20 mA output power and external quantum efficiency of the former could reach 4.98 mW and 8.2%, respectively, which were much larger than the values observed from the latter. The reliability of ITO on n/sup ++/-SPS upper contacts was also found to be reasonably good.     

LED用石墨烯复合透明电极厚度组合的仿真与优化

为降低石墨烯(Gr)透明电极与p-GaN之间的肖特基势垒与接触电阻,进行了将银、金、镍和铂四种金属或氧化镍作为中间层引入它们两者之间的尝试。使用有限元方法模拟研究了Gr与金属或氧化镍的不同厚度组合对LED的光、热和电特性的影响。发现:透明导电层的透光率和LED芯片的表面温度均随石墨烯和金属或氧化镍厚度的增加而降低;1.5nm的Ag、Ni、Pt,1nm Au或1nm的NiO_x分别与3层(3L)Gr复合时为优化厚度组合,其中,1.5nm Ni/3L Gr为最佳Gr/金属复合透明电极。     

Electrical and optical properties of Ni-assisted grown single crystalline and transparent indium-tin-oxide nanowires

Transparent and single crystalline indium-tin-oxide (ITO) nanowires (NWs) were grown by sputtering method. A thin Ni film of 5 nm was deposited before ITO sputtering. Thermal treatment forms Ni nanoparticles, which act as templates to diffuse Ni into the sputtered ITO layer to grow single crystalline ITO NWs. This Ni diffusion through an ITO NW was investigated by transmission electron microscope to observe the Ni-tip sitting on a single crystalline ITO NW. Meanwhile, a single crystalline ITO structure was found at bottom and body part of a single ITO NW without remaining of Ni atoms. This indicates the Ni atoms diffuse through the oxygen vacancies of ITO structure.Rapid thermal process (RTP) applied to generate an initial stage of a formation of Ni nanoparticles with variation in time periods to demonstrate the existence of an optimum condition to initiate ITO NW growth. Modulation in ITO sputtering condition was applied to verify the ITO NW growth or the ITO film growth. The Ni-assisted grown ITO layer has an improved electrical conductivity while maintaining a similar transmittance value to that of a single ITO layer. Electrically conductive and optically transparent ITO nanowire-coated surface morphology would provide a great opportunity for various photoelectric devices.     

Fabrication of Ag nanoparticle/ZnO thin films using dual-plasma-enhanced metal-organic chemical vapor deposition (DPEMOCVD) system incorporated with photoreduction method and its application

We report a novel method to grow silver nanoparticle/zinc oxide (Ag NP/ZnO) thin films using a dual-plasma-enhanced metal-organic chemical vapor deposition (DPEMOCVD) system incorporated with a photoreduction method. The crystalline quality, optical properties, and electrical characteristics of Ag NP/ZnO thin films depend on the AgNO₃ concentration or Ag content and annealing temperature. Optimal Ag NP/ZnO thin films have been grown with a AgNO₃ concentration of 0.12 M or 2.54 at%- Ag content and 500 °C- rapid thermal annealing (RTA); these films show orientation peaks of hexagonal-wurtzite-structured ZnO (002) and face-center-cubic-crystalline Ag (111), respectively. The transmittance and resistivity for optimal Ag NP/ZnO thin films are 85% and 6.9×10⁻⁴ Ω cm. Some Ag NP/ZnO transparent conducting oxide (TCO) films were applied to InGaN/GaN LEDs as transparent conductive layers. The InGaN/GaN LEDs with optimal Ag NP/ZnO TCO films showed electric and optical performance levels similar to those of devices fabricated with indium tin oxide.     

Effects of surface treatment of ITO anode layer patterned with shadow mask technology on characteristics of organic light-emitting diodes

We investigated the effects of various surface treatments of indium tin oxide (ITO) on the electrical and optical characteristics of organic light-emitting diodes (OLEDs). A 150-nm-thick ITO anode layer was patterned directly with a shadow mask during the sputtering process without the use of a conventional photolithography patterning method. The sputtered ITO layer was subjected to thermal and oxygen plasma treatments to reduce the sheet resistance and improve surface roughness. The thermal treatment was performed for 1 h at temperatures of 250 and 380 °C, which were chosen so that the glass substrates would not deform from thermal damage. The measured sheet resistance decreased from 30.86 Ω/sq for the as-sputtered samples to 8.76 Ω/sq for the samples thermally treated at 380 °C for 1 h followed by oxygen plasma treatment. The root-mean-square surface roughness measured by atomic force microscopy considerably decreased to 3.88 nm with oxygen plasma treatment. The thermal treatment considerably decreased the sheet resistance of the ITO anode layer patterned with the shadow mask. The spike-like structures that are often formed and observed in shadow mask-patterned ITO anode layers were almost all removed by the oxygen plasma treatment. Therefore, a smooth surface for shadow mask-patterned ITO layers with low sheet resistance can be obtained by combining thermal and oxygen plasma treatments. A smooth surface and low sheet resistance improves the electrical and optical characteristics of OLEDs. The surface-treated ITO layer was used to fabricate and characterize green phosphorescent OLED devices. The typical characteristics of OLED devices based on surface-treated shadow mask-patterned ITO layers were compared with those fabricated on untreated and photolithography-patterned ITO layers to investigate the surface treatment effects. The OLED devices fabricated by thermal treatment at 380 °C for 1 h followed by oxygen plasma treatment for 180 s showed the highest luminance and current density. Furthermore, the leakage current that might be induced by the rough ITO surface was dramatically reduced to 0.112 mA/cm². Our study showed that the shadow mask-patterned ITO anode layer treated by heat and plasma and having a low sheet resistance and surface roughness yielded excellent electrical and optical properties for OLEDs compared to those based on an untreated ITO layer. The fabricated OLED devices using the surface-treated shadow mask-patterned ITO layer exhibited comparable characteristics to those obtained from a conventional photolithography-patterned ITO anode.