Improvement of InGaN-GaN light-emitting diodes with surface-textured indium-tin-oxide transparent ohmic contacts

Presents a surface-textured indium-tin-oxide (ITO) transparent ohmic contact layer on p-GaN to increase the optical output of nitride-based light-emitting diodes (LED) without destroying the p-GaN. The surface-textured ITO layer was prepared by lithography and dry etching, and dimensions of the regular pattern were approximately 3 /spl times/ 3 /spl mu/m. The operating voltage of the surface-textured LED was almost the same as that of the typical planar LED since the ITO layer was in ohmic contact with the p-GaN. The experimental results indicate that the surface-textured ITO layer 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.     

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.     

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.     

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.     

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.     

Flexible top-emitting warm-white organic light-emitting diodes with highly luminous performances and extremely stable chromaticity

The flexible top-emitting white organic light-emitting diode (FTEWOLED) with a very high efficiency but a significant color alteration is achieved with a blue/red/blue sandwiched tri-emission-layer. The voltage-dependent recombination region alternation and the emission mechanism are systematically investigated through a delta-doping method and the time-resolved transient photoluminescence lifetime measurement. By locating the main exciton recombination region at the 4,4′,4″-Tris(carbazol-9-yl)-triphenylamine (TCTA) and 9,9-spirobifluoren-2-yl-diphenyl-phosphine oxide (SPPO1) interface, replacing the carrier-trapping red dopant guest with an orange guest that utilizes energy transfer mechanism, and using a P–I–N structure together with the FIrpic blue guest dopant to balance the electron and hole carriers, an extremely color stable and a very high efficient FTEWOLED is fabricated, with the resulting high current and power efficiencies of 22.7 cd/A and 14.27 lm/W, and a warm white illumination with a small chromaticity variation of (−0.0087, +0.0015) over a broad luminance range of more than four orders of magnitude. In addition, the performances can be further improved to 23,340 cd/m², 24.49 cd/A and 15.39 lm/W with a slight concentration alteration of the orange emitter.     

具有纳米凹坑氧化铟锡的LED光提取效率的提高

由于低的光提取效率,氮化镓基发光二极管的应用受到了限制。氧化铟锡—氮化镓界面的光的全反射作用是造成低的光提取效率的重要原因。人们提供了多种方法来提高光提取效率。本文揭示了一种简单并且经济的方法。通过自组装和干法刻蚀的方法制作粗化的氧化铟锡薄膜。运用原子力显微镜（AFM）对表面形态和粗糙程度进行观察。测量各个样品的I-V特性、出光功率和出光辐射图并进行对比。刻蚀之后,在ITO表面形成了圆柱体和凹坑结构,它们的高度随着刻蚀时间增大。结果显示,LED的出光功率和随着刻蚀时间的增加而增加。由于圆柱体和凹坑结构的形成以及它们深度的增加,ITO-GaN界面的光的全反射减少了。因此,出光率提高。     

Performance enhancement of organic light-emitting diodes by chlorinated indium tin oxide in the presence of hydrogen peroxide

We demonstrate the enhanced performance in organic light-emitting diodes by chlorinated indium tin oxide (ITO) in the presence of hydrogen peroxide. We adopt the approach of UV light irradiation of the ITO with o-dichlorobenzene and hydrogen peroxide. Adding hydrogen peroxide in the system can accelerate the rate of UV photolysis of o-dichlorobenzene and reduce the UV illumination time, and the device shows the highest performance of current efficiency 4.15 cd/A. Under UV radiation, hydrogen peroxide would generate hydroxyl radical HO^. to attack o-dichlorobenzene, then further reducing the UV illumination time.     

Evaluation of ohmic contacts to p-type 6H-SiC created by C and Alcoimplantation

A comparative study of specific contact resistance and sheet resistivity of p-type 6H-SiC created by Al implantation only and by C and Al coimplantation into n^--6H-SiC epilayer grown on n⁺-6H-SiC has been performed to address the challenging issue of ohmic contacts to the anode of SiC thyristors and other thyristor-based advanced devices. Direct experimental evidence has been obtained which shows the obvious advantage of C and Al coimplantation in terms of contact resistance and sheet resistivity. Under our experimental conditions, it is found that the specific contact resistance can be reduced by three orders of magnitude and the sheet resistivity can be improved by a factor of 6 when C and Al are coimplanted into 6H-SiC     

Development of InxGa1−xAs-based ohmic contacts for p-type GaAs by radio-frequency sputtering

In_xGa_1−xAs-based ohmic contacts which showed excellent contact properties for n-GaAs were demonstrated to be applicable to p-GaAs ohmic contacts. These contacts, prepared by radio-frequency sputtering, provided low contact resistance (0.2 Ω-mm), excellent thermal stability, smooth surface, and good reproducibility. The contact resistances had a weak dependence on the annealing temperatures, which was desirable in a manufacturing view point. This weak temperature dependence was explained to be due to a unique Schottky barrier height at the metal/p-In_xGa_1−xAs interface which does not depend on the In concentration in the In_xGa_1−xAs layer. The present experiment showed the possibility of simultaneous preparation of ohmic contacts for both n and p-GaAs using the same contact materials.     

Preparation of ZnSe light-emitting diodes by metalorganic chemical vapor deposition using trisdimethylaminoarsine as a p-type doping source

Trisdimethylaminoarsine was used in atmospheric-pressure metalorganic chemical vapor deposition growth of ZnSe on GaAs. The metalorganic precursors employed for ZnSe growth were diethylzinc and diethylselenide, and ethyliodide was used as then-type dopant. P-on-n light emitting diode (LED) structures were prepared, and molecular beam epitaxially deposited HgTe layers were used as ohmic contacts to the p-type ZnSe. Blue LEDs were fabricated on p-on-n samples. Preliminary LED data and the material characterization data are presented.     

Transition resistances of ohmic contacts to p-type cdte and their time-dependent variation

Ohmic contacts to p-type CdTe are important for the development of solar cells based on this semiconductor, as for instance CdS/CdTe or ITO/ CdTe solar cells. Ohmic contacts to CdTe Bridgman crystals, doped with phosphorus, have been examined with respect to their resistivity dependence and their variation as a function of time. The ‘specific’ contact resistance r shows a linear dependence on the bulk resistivity; in addition, it is affected by the oxygen content of the CdTe. The lowest r obtained was 0.07Ω cm. With one exception, ali the contacts with nickel, gold and platinum deposited on different crystals show a more or less pronounced increase of r as a function of time.     

High temperature stability of chromium boride ohmic contacts to p-type 6H-SiC

Ohmic contacts have been fabricated on p-type 6H-SiC using CrB₂. Two hundred nanometer thick films were sputter-deposited on substrates of doping concentration 1.3×10¹⁹ cm⁻³ in a system with a base pressure of 3×10⁻⁷ Torr. Specific contact resistances were measured using the linear transmission line method, and the physical properties of the contacts were examined using Rutherford backscattering spectrometry, x-ray photoelectron spectroscopy, and transmission electron microscopy. The as-deposited CrB₂ contacts exhibited rectifying characteristics and contained oxygen as a major contaminant. Ohmic behavior with linear current-voltage characteristics was observed following short anneals at 1100°C for 2 min at a pressure of 5×10⁻⁷ Torr. The oxygen in the CrB₂ films was removed by the annealing process, and the lowest value of the specific contact resistance (r_c) measured at room temperature was 8.2×10⁻⁵ Ω-cm². Longer anneals at 1100°C for 3.5 h and 1200°C for 2 h reduced the room temperature values of r to 1.4×10⁻⁵ Ω-cm². A thin reaction region has been identified at the CrB₂/SiC interface; however, the interface remains essentially stable. Thermal stressing at 300°C in vacuum for over 2200 h produced only a slight increase in the specific contact resistance. The low value of the specific contact resistance and the excellent high temperature stability of the CrB₂/SiC interface make this contact a candidate for high power/high temperature SiC device applications.     

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.     

A study of the electrical characteristics of various metals on p-type GaN for ohmic contacts

We study the electrical characteristics (current vs voltage, I/V) of Co, In, Mg, Mn, Ni, and Zn each with an Au overlayer to determine their usefulness as ohmic contact metals to p-type GaN. For all the metals, none of the I/V relationships are completely linear even after annealing. At a fixed voltage of 3V Co, In, Ni, and Zn have comparable current levels, whereas Mg and Mn are almost an order of magnitude less. Due to the various mechanisms by which the metals may form ohmic contacts, we further examine the metals in multilayer combinations in an attempt to reduce contact resistance. Three p-type GaN wafers with carrier concentrations of 1.2 × 10¹⁷, 1.5 × 10¹⁷ and 4.7 × 10¹⁷ cm⁻³ are used with Ni/Au metallizations as a common standard for comparison. The lowest average specific contact resistance obtained in this study is with Co/Au at 0.0081 ohm-cm². In addition to comparing magnitudes of contact resistances, thermal aging studies of the metal contacts are performed from 300 to 700°C for 6 h periods to determine its effect on their electrical stability. In this test, Ni/Au is found to be the most electrically invariant with thermal aging prior to failure. However, the temperature at which it fails occurred sooner than that for many of the other metallizations examined (e.g., Co/Au, In/Au, and Zn/Au). The temperature for failure is arbitrarily defined to be the temperature that the contact resistance degrades to twice its pre-thermal-aging contact resistance.     

Effects of p-type GaN thickness on optical properties of GaN-based light-emitting diodes

The influence of p-type Ga N(p Ga N) thickness on the light output power(LOP) and internal quantum efficiency(IQE) of light emitting diode(LED) was studied by experiments and simulations. The LOP of Ga N-based LED increases as the thickness of p Ga N layer decreases from 300 nm to 100 nm, and then decreases as the thickness decreases to 50 nm. The LOP of LED with 100-nm-thick pG a N increases by 30.9% compared with that of the conventional LED with 300-nm-thick p Ga N. The variation trend of IQE is similar to that of LOP as the decrease of Ga N thickness. The simulation results demonstrate that the higher light efficiency of LED with 100-nm-thick p Ga N is ascribed to the improvements of the carrier concentrations and recombination rates.     

Electrical characteristics and structural properties of ohmic contacts to p-type 4H-SiC epitaxial layers

Epitaxial films grown by low-temperature liquid phase epitaxy on p-type 4H-SiC were used as strongly doped subcontact layers for making low-resistance contacts to the p-type material. The layers had a bulk resistivity of ∼0.02 Ω · cm and an aluminum atom concentration of ∼1.5×10²⁰ cm⁻³. The absence of polytype inclusions and the distinct crystalline quality of the strongly doped subcontact layers was confirmed by x-ray diffraction methods. Ohmic contacts with resistivities less than 10⁻⁴ Ω · cm² were prepared by depositing and then annealing multilayer metal mixtures containing Al and Ti. The structural properties and energy characteristics of the resulting ohmic contacts are discussed. Fiz. Tekh. Poluprovodn. 33, 1334–1339 (November 1999)     

Improvement of the electrical performance of near UV GaN-based light-emitting diodes using Ni nanodots

We report on the improvement of the electrical characteristics of GaN-based light-emitting diodes (LEDs) fabricated with Ag (1 nm)/indium tin oxide (ITO) (250 nm) p-type contacts by using Ni nanodots. As-deposited Ag/ITO contacts with and without the Ni nanodots give non-linear electrical behaviour. However, annealing the samples at 530–630 °C for 1 min in air results in ohmic behaviour. The reverse leakage current characteristics of near UV LEDs are much improved when the Ni nanodots are used. The output power (at 20 mA) of LEDs fabricated with the Ni nanodots is enhanced by a factor of 1.34 as compared with that of LEDs made without the Ni nanodots.     

Design and fabrication of highly efficient GaN-based light-emitting diodes

A promising fabrication method and an innovative geometrical design for highly efficient GaN-based light-emitting diodes (LEDs) were investigated based on the current spreading phenomenon. Based on theoretical considerations, it was possible to determine the critical transparent-electrode thickness, which resulted in significant improvements in the electrical and optical characteristics of LEDs. In addition, we were able to define conditions for an ideal geometrical design and the resulting product exhibited significant improvements in characteristics in spite of the fact that a transparent electrode, acting as a p-type current spreader, was not used. Considering the simple fabrication process and good device performance, the proposed fabrication methods, as well as the innovative geometrical design, have considerable promise for use in practical applications.