Search

We investigated the effect of via-holes in the lead-frame on the reliability of blue flip-chip light-emitting diodes (FCLEDs). For bonding, conventional Sn-Ag(3.0%)-Cu(0.5%) solder (SAC) and Ag sintered paste were adopted. Lateral LED, conventional FCLED (c-FCLED), and via-hole FCLED gave efficacies of 218.5, 221.6 and 225 lm/W at 30 mA. The lateral LED sample (bonding material: silicone), c-FCLED sample (SAC), via-hole-type sample (Ag sinter), and via-hole-type sample (SAC) exhibited total thermal resistance of 15.24, 10.19, 8.95, and 11.79 K/W, respectively. The humidity/temperature/H2S gas reliability examinations showed that unlike the via-hole-type samples, the lumen of the c-FCLED was fallen by 15.0% after exposure for 500 h. Furthermore, the c-FCLED exhibited a 19.6% higher forward voltage than the via-hole FCLED after 1,000 thermal-cycles. SEM results showed that unlike the via-hole type FCLED, the c-FCLED underwent cracking after thermal cycle of 1000 times.

Light-emitting diodes (LEDs) have been widely applied for backlight unit for displays, general lighting, and automotive illumination because they have a high light efficiency and long life time, and are environmentally friendly. These diverse applications require smaller and slim LED chip scale packages (CSPs). CSP LEDs have been developed to produce white LED chips by forming a thin phosphor film on blue LED chips. In this study, we investigated the effect of phosphor layer thickness and area on the optical and thermal properties of the CSP LEDs. The phosphor-conversion of chips is maximized by varying the thickness and area of the phosphor layer. Eight different types of phosphor-converted white CSPs with normal performance (CRI > 70, CCT ~ 5700 K) were prepared by using mixed multicolor phosphor materials. In other words, for five samples whose CSP area was fixed at 1.8 x 1.8 mm2, flip-chip area was 1.4 x 1.4 mm2, and flip-chip thickness was 250 μm, the thickness of the phosphor layers was varied from 100 to 500 μm at intervals of 100 μm. For other three samples where the phosphor layer thickness was fixed at 300 μm, the flip-chip area was 1.4 x 1.4 mm2, and flip-chip thickness was 250 μm, the CSPs had three different areas of 1.6 x 1.6, 1.8 x 1.8, and 2.0 x 2.0 mm2. First, the relative luminous flux and LED beam distribution were characterized using integrating sphere and a goniometer. It was shown that as the thickness of the phosphor layer increased, the luminous flux and the directivity angle increased. On the other hand, as the CSP area increased, the luminous flux and the directivity angle decreased. This could be attributed to the fact that CSP does not have a reflector (namely, a housing material) and so the blue light of the chip does not excite the whole area of the phosphor. This may be confirmed by the images showing light distribution and luminescence characteristics, which is currently under investigation. Furthermore, a junction temperature measurement system was used to characterize the junction temperatures of the five different samples whose phosphor layer thicknesses were varied from 100 to 500 μm. It was found that when the thickness of the phosphor layer exceeded the thickness of the chips (250 μm), the junction temperature of the samples dramatically increased. Further details regarding the phosphor layer thickness of the junction temperature characteristics will be investigated and described.

Demonstrated are visible GaN-based light-emitting diodes (LEDs) on economical large-area Si substrates using an advanced device and packaging architecture to improve optical output power, while reducing manufacturing costs. The process employs thin-film-flip-chip devices and wafer-level chip-scale packages and uses through-Si-via substrate and anisotropic conductive film for bonding. The improved curvature control region is applied in the epitaxial growth of the LED structure on a Si substrate to achieve flat wafers for epitaxial structures at room temperature, which is critical for wafer-level bonding. External quantum efficiency and light-output power at 350 mA increase by $\sim 12$ % compared with those of conventional flip-chip LEDs grown on a sapphire substrate. The devices also show a reverse-bias leakage current failure rate of

We investigated the effect of phosphor deposition methods on the correlated color temperature (CCT), luminous flux and thermal characteristics of packaged white light-emitting diodes (WLEDs) for use in mobile display products. For both the samples, the CCT decreased with increasing viewing angle. Phosphor sedimentation samples displayed much better angular color uniformity than phosphor dispersion samples. The phosphor sedimentation sample had higher luminous flux and luminous efficacy at 20 mA than the phosphor dispersion sample. The phosphor sedimentation sample displayed much better high-temperature/humidity (85 °C/85%) reliability and lower package temperatures compared with the phosphor dispersion sample.

We demonstrated the growth of crack-free high-quality GaN-based UV vertical LEDs (VLEDs) (λ = 365 nm) on 6-inch sapphire substrates by using an ex-situ sputtered AlN nucleation layer (NL) and compared their performance with that of UV VLEDs with an in situ low temperature (LT) AlGaN NL. The X-ray diffraction (XRD) results showed that the ex-situ AlN sample contained lower densities of screw-type and edge-type threading dislocations than the in situ AlGaN NL sample. The micro-Raman results revealed that the ex-situ AlN sample was under more compressive stress than the in situ AlGaN sample. As the current was increased, the electroluminescence peaks of both of the samples blue-shifted, reached a minimum wavelength at 1000 mA, and then slightly red-shifted. Packaged VLEDs with the ex-situ AlN NL yielded 6.5% higher light output power at 500 mA than that with the in situ AlGaN NL. The maximum EQEs of the VLED with the in situ AlGaN and ex-situ AlN NLs were 43.7% and 48.2%, respectively. Based on the XRD and Raman results, the improved light output power of the ex-situ AlN sample is attributed to the lower density of TDs.

We investigated the effects of in situ GaN and sputtered AlN nucleation layers on the output power of GaN-based blue (445 nm) LEDs on patterned sapphire substrate (PSS) and planar sapphire substrates. Both the PSS LEDs and planar LEDs showed the same operation voltages, but the LEDs with the AlN layer had much lower reverse leakage current at −15 V. The LEDs with the AlN layer showed higher external quantum efficiency (EQE) at low current regions than those with the GaN nucleation layer. However, the AlN nucleation layer LEDs experienced a little more severe efficiency droop than did the GaN layer LEDs. It was shown that the AlN nucleation layer samples contained fewer defects than the GaN nucleation layer samples. The Raman spectral results showed that the AlN nucleation layer samples experienced higher compressive stress than the GaN nucleation layer samples. Simulation results also showed that both the AlN nucleation layer and air voids in the GaN nucleation layer samples hardly affected the light extraction efficiency. Based on the electrical, X-ray diffraction (XRD), cathodoluminescence (CL), and Raman results, the electrical behavior of the PSS and planar LEDs are described and discussed. © The Author(s) 2015. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives 4.0 License (CC BY-NC-ND, http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial reuse, distribution, and reproduction in any medium, provided the original work is not changed in any way and is properly cited. For permission for commercial reuse, please email: oa@electrochem.org. [DOI: 10.1149/2.0011602jss] All rights reserved.

In this work, simple n-type electrode structures were used to enhance the electrical and optical performances of fully packaged commercially mass-produced vertical-geometry light-emitting diodes (VLEDs). The forward bias voltage of the VLED with a Y-pattern electrode increased less rapidly than that of VLEDs with a reference electrode. The VLEDs with the reference and Y-pattern electrodes exhibited forward voltages of 2.93±0.015 and 2.89±0.015 V at 350 mA and 3.77±0.015 and 3.53±0.015 V at 2000 mA, respectively. The VLEDs with the Y-pattern electrode resulted in a higher light output than the VLEDs with the reference electrode with increase in the drive current to 2000 mA. The emission images showed that the VLEDs with the Y-pattern electrode exhibited better current spreading behavior and lower junction temperatures than the VLEDs with the reference electrode. With increase in the current from 350 to 2000 mA, the VLEDs with the Y-pattern electrode experienced a 39.4% reduction in the wall plug efficiency, whereas the VLEDs with the reference electrode suffered from a 43.3% reduction.

The effects of the n-contact design and chip size on the electrical, optical and thermal characteristics of thin-film vertical light-emitting diodes (VLEDs) were investigated to optimize GaN-based LED performance for solid-state lighting applications. For the small (chip size: 1000×1000 µm2) and large (1450×1450 µm2) VLEDs, the forward bias voltages are decreased from 3.22 to 3.12 V at 350 mA and from 3.44 to 3.16 V at 700 mA, respectively, as the number of n-contact via holes is increased. The small LEDs give maximum output powers of 651.0–675.4 mW at a drive current of 350 mA, while the large VLEDs show the light output powers in the range 1356.7–1380.2 mW, 700 mA, With increasing drive current, the small chips go through more severe degradation in the wall-plug efficiency than the large chips. The small chips give the junction temperatures in the range 51.1–57.2 °C at 350 mA, while the large chips show the junction temperatures of 83.1–93.0 °C at 700 mA, The small LED chips exhibit lower junction temperatures when equipped with more n-contact via holes.

A multiband power amplifier (MBPA) with a switch-based reconfigurable output matching network is presented for long-term evolution handset applications. The MBPA is designed in a dual-path configuration to improve efficiency in two different power levels for different frequency bands of the evolved UMTS terrestrial radio access (E-UTRA). The dual-path MBPA with the switch-based reconfigurable output matching network delivered a collector efficiency of 31.8–38.5%, while satisfying the linearity specification (i.e., an adjacent channel leakage ratio less than −30 dBc) at an output power of 26.5 dBm across the five popular frequency bands of the E-UTRA. In the low power mode, the MBPA delivered a collector efficiency of 21.8–35.4% with an adjacent channel leakage ratio smaller than −32.3 dBc at an average output power of 18 dBm. © 2014 Wiley Periodicals, Inc. Microwave Opt Technol Lett 56:2881–2884, 2014

The current injection mechanisms for blue light emitting diodes (LEDs) with and without V pits were examined by controlling the bandgaps of InGaN quantum wells (QWs), which were changed by reducing the indium content. To identify the distribution of holes in the QWs, the electroluminescence of the LEDs was characterized by varying the positions of the QWs with the wider bandgap consecutively from n-cladding to p-cladding sides. For the LEDs without V pits, holes were injected through the top QWs (p-cladding side), while for the LEDs with V pits, holes were injected mainly through the bottom QWs (n-cladding side).

Demonstrated is advanced device and packaging architecture of visible GaN-based light-emitting diodes (LEDs) combining thin-film flip-chip devices and wafer-level chip-scale package with through-silicon-via (TSV) and wafer-to-wafer alignment bonding. In addition, a new interconnect technique for LEDs is introduced using an anisotropic conductive film with metal balls. Thermal rollover in light output versus current characteristics is not observed up to 700 mA. A forward voltage at 350 mA is 3.06 V. The architecture can facilitate excellent heat removal through a TSV-formed Si wafer in addition to expected benefits of easy integration of Si-based devices in lighting modules. Light-output power at 350 mA increases by 11.1% compared with that of conventional flip-chip LEDs. A Lambertian-like emission pattern is also achieved.

We investigated the effect of two different quantum well (QW) structures having different indium contents on the optical performance of fully packaged GaN-based light-emitting diodes (LEDs). Dual-spectrum QW LEDs exhibit ∼4% higher external quantum efficiency (at 350 mA) than single-spectrum QW LEDs. However, the two types of LEDs exhibit similar efficiency droop behavior. For both types of LEDs, the output power decreases with increasing junction temperature. When the junction temperature exceeds 70°C, the dual-spectrum QW LEDs exhibit lower output power than the single-spectrum QW LEDs. The wavelength dependence of the output power (at 350 mA) of single-spectrum QW LEDs shows that the LEDs with shorter wavelengths experience more rapid optical degradation than the LEDs with longer wavelengths. Based on the wavelength- and junction-temperature-dependent output power, the droop behavior of the dual-spectrum QW LEDs is described and discussed.

We have investigated the Ag (1 nm)indium tin oxide (ITO) (200 nm) contacts by scanning transmission electron microscopy (STEM), Auger electron spectroscopy (AES), and X-ray photoemission spectroscopy (XPS) to understand its ohmic mechanism. The Ag/ITO contacts exhibit ohmic behaviors, when annealed at 400-600°C. The effective Schottky barrier heights depend on the annealing temperatures. STEM and AES results reveal the formation of Ag nanodots (5-35 nm across) and Ga-Ag solid solution. Based on the STEM, AES, and XPS results, the ohmic contact formation is described in terms of the formation of the Ga-Ag solid solution and the inhomogeneous interfaces with nanodots. © 2007 The Electrochemical Society.

The light output and electrical characteristics of GaN-based vertical light emitting diodes were investigated as a function of n-GaN thickness. The forward voltage increases from 3.34 to 3.42V at an injection current of 350mA as the n-GaN thickness decreases from 5.0 to 2.0@mm. Even at a high injection current of 2.0A, LEDs with 2.0@mm-thick n-GaN reveal stable forward characteristics which are comparable to those of LEDs with 5.0@mm-thick n-GaN. All the samples exhibit almost the same reverse current up to approximately -8V. The output power increases with decreasing n-GaN layer thickness. For example, LEDs with 2.0@mm-thick n-GaN yield about 12% higher light output power as compared to LEDs with 5.0@mm-thick n-GaN. Their light output power continuously increases without saturation as the injection current increases up to 1A. The n-GaN thickness dependence of the electrical characteristics is described and discussed.

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.

Hexagonal arrays of submicron polymer patterns with a high refractive index were fabricated on a vertical light-emitting diode (LED) device by means of nanoimprint lithography (NIL) to improve the light extraction efficiency. An organic-inorganic hybrid resin containing a polymeric titanium dioxide precursor was spin-coated on the n-GaN top layer of a vertical LED wafer. The coated layer was then imprinted for 10 min with an elastomeric polydimethylsiloxane stamp at 200 °C and 5 atm. The NIL process formed pillar patterns on the n-GaN layer of the vertical LED wafer. The pillar patterns have a high refractive index ( n ≈ 2.0) in the visible wavelength range; they also have a diameter of 200 nm and a pitch of 700 nm. The light output power of the patterned vertical LED device is 28% greater than that of a non-patterned vertical LED device with a driving current of 350 mA. The I–V characteristics of the vertical LED device confirm that the patterning process induces no electric degradation.

We investigated the optical and electrical properties of red AlGaInP light-emitting diodes (LEDs) as functions of chip size, p-cladding layer thickness, and the number of multi-quantum wells (MQWs). External quantum efficiency (EQE) decreased with decreasing chip size. The ideality factor gradually increased from 1.47 to 1.95 as the chip size decreased from 350 μm to 15 μm. This indicates that the smaller LEDs experienced larger carrier loss due to Shockley-Read-Hall nonradiative recombination at sidewall defects. S parameter, defined as ∂lnL/∂lnI, increased with decreasing chip size. Simulations and experimental results showed that smaller LEDs with 5 pairs of MQWs had over 30% higher IQE at 5 A/cm2 than the LED with 20 pairs of MQWs. These results show that the optimization of the number of QWs is needed to obtain maximum EQE of micro-LEDs.

GaN-based semiconductors are of great technological importance for the fabrication of optoelectronic devices, such as light-emitting diodes (LEDs) and laser diodes. The further improvement of LED performance can be achieved through the enhancement of external quantum efficiency. In this regard, high-quality p-type ohmic electrodes having low contact resistance and high transmittance (or reflectivity), along with thermal stability, must be developed because p-type ohmic contacts play a key role in the performance of LEDs. In this paper, we review recent advances in p-type ohmic-contact technology for GaN-based LEDs. A variety of methods for forming transparent and reflective ohmic contacts are introduced.

We report on the formation of low-resistance and transparent p-type ohmic contact for green GaN-based light-emitting diodes (LEDs) using Ni–Co solid solution(5 nm)/Au(5 nm) scheme. The as-deposited Ni–Co solid solution(5 nm)/Au(5 nm) contact shows nonlinear current–voltage ( I – V ) characteristics. However, annealing the sample at 550 ∘C in air results in the formation of ohmic contacts with specific contact resistance of 6.5×10−4 Ω cm2. The annealed sample shows light transmittance of 71.7% at a wavelength of 530 nm, which is better than that (64.9%) of oxidized Ni/Au contacts. Based on Auger electron spectroscopy and x-ray photoemission spectroscopy results, the annealing-induced improvement of the contacts is described and discussed.

The effect of rapid-thermal-annealing on the performance of near-UV GaN-based light-emitting diodes (LEDs) fabricated with nano-patterned p-type electrodes has been investigated. One-dimensional (1-D) nano-patterns were formed on Cu-doped indium oxide (CIO)/indium tin oxide (ITO) p-electrode by sur- face relief grating and dry etching techniques. After the nano-patterning, some of the samples are rapid-thermal-annealed at 530 and 630 °C in either air or nitrogen ambient. LEDs made with samples annealed 530 °C show much better electrical characteristics as compared to unannealed samples. In particular, LEDs with samples annealed 530 °C in air show higher output power (at 20 mA) and much reduced leakage current as compared to LEDs with unannealed samples. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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 cm 2 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.

This study examined systematically the mechanism of light interaction in the sapphire/MgF2/Al triple-layer omnidirectional reflectors (ODR) and its effects on the light output power in near ultraviolet light emitting diodes (NUV-LEDs) with the ODR. The light output power of NUV-LEDs with the triple-layer ODR structure increased with decreasing surface roughness of the sapphire backside in the ODR. Theoretical modeling of the roughened surface suggests that the dependence of the reflectance of the triple-layer ODR structure on the surface roughness can be attributed mainly to light absorption by the Al nano-structures and the trapping of scattered light in the MgF2 layer. Furthermore, the ray tracing simulation based upon the theoretical modeling showed good agreement with the measured reflectance of the ODR structure in diffuse mode.

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 Ωcm2 when annealed at temperatures of 330–530 °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.

MgZnO(MZO) interlayers are introduced to form high-quality Ag-based ohmic contacts to p-type GaN ( N a = 4 × 10 17 cm −3 ) for GaN-based flip-chip light-emitting diodes (LEDs). The MZO (2.5 nm)/Ag (250 nm) contacts become ohmic with specific contact resistance of 3.83 × 10 −4 Ω cm 2 and reflectance of ∼83% at a wavelength of 405 nm when annealed at 530 °C for 1 min in air. It is shown that LEDs made with the MZO interlayers yield reverse leakage currents three orders of magnitude lower than that of LEDs without the interlayers. In addition, the LEDs fabricated with the MZO interlayers show higher output power as compared with the LEDs without the interlayers.

We report on the enhancement of the light output of near-UV (298 nm) GaN-based light-emitting diodes (LEDs) by using nanopatterned indium tin oxide (ITO) p-type contact layers. One-dimensional (1D) and two-dimensional (2D) nanopatterns are defined using a TiO2 nano-mask, fabricated by means of a surface relief grating technique. The LEDs fabricated with the 1D and 2D nanopatterned p-type electrodes produce higher output powers by 33–48% (at 20 mA) as compared to those fabricated with the unpatterned contacts. The pattern-induced improvement of the output power is described in terms of the formation of the sidewalls of p-type electrodes.

We have investigated the addition effect of an Ag interlayer (2 nm) at the Pd and GaN interface on the ohmic behaviour of a single Pd contact (110 nm). The Ag layer is broken up into nano-dots (11–22 nm in size) when annealed at temperatures of 330–530 °C. It is shown that the use of the Ag interlayer is effective in widening the temperature range for the ohmic formation of the Pd contact and improving the adhesion of the Pd contact to GaN. The improved ohmic behaviours are attributed to the reduction of Schottky barrier heights due to the shift of the surface Fermi level towards the valence-band edge and the formation of Ag nano-dots at the Pd/GaN interfaces.

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.

We have investigated indium-doped ZnO(IZO)/Ag Ohmic contacts to p-type GaN for use in UV-light-emitting diodes (LEDs). The as-deposited sample shows rectifying behaviour. However, the samples become Ohmic when annealed at temperature of 330–530 °C for 1 min in air. For example, the contact produces specific contact resistance of 5.51 × 10−4 Ω cm2 upon annealing at 530 °C. In addition, the 530 °C annealed IZO/Ag contacts give reflectance of 82.3% at a wavelength of 405 nm, which is better than that (70.6%) of the single Ag contacts. Near-UV LEDs fabricated with the IZO/Ag contacts give forward voltage of 3.24 V at an injection current of 20 mA, which is better than that with the single Ag contacts. On the basis of x-ray photoemission spectroscopy and scanning transmission electron microscopy results, possible Ohmic formation mechanisms are described.

We have investigated highly low resistance and reflective Cu-Ni solid solution/Ag ohmic contacts to p-GaN for flip-chip light emitting diodes (LEDs) for solid-state lighting applications. The Cu-Ni solid solution/Ag contacts produce specific contact resistances in the range of 10 -5 -10 -6 Ω cm 2 when annealed at temperatures of 450 and 550 °C for 1 min in air ambient. Measurements show that the reflectance of the annealed Cu-Ni solid solution/Ag contacts is 82.6% at a wavelength of 460 nm. Blue LEDs are fabricated with the annealed Cu-Ni solid solution/Ag contacts and single Ag contacts. The typical I-V characteristic of the LEDs made with the annealed Cu-Ni solid solution/Ag contact layer gives a forward-bias voltage of 3.08 V at 20 mA.

Novel Ni–Mg solid solution (5 nm)/Au (5 nm) schemes are investigated for the development of ohmic contacts to p-type GaN. It is shown that the samples annealed at 550 °C for 1 min in air ambient produce a specific contact resistivity of 3.0 × 10−5 Ω cm2. The light transmittance of the annealed samples is about 95% at a wavelength of 470 nm. GaN-based light-emitting diodes (LEDs) made with the annealed Ni–Mg solid solution/Au p-contact layers are fabricated. The typical I–V characteristics of the green LEDs with the annealed p-type contact layers reveal a forward-bias voltage of 3.32 V at an injection current of 20 mA, which is much better than that of the LEDs with Ni/Au contact layers.

Thermally stable ohmic contact to moderately doped p-GaN:Mg (1.13 × 10 17 cm −3 ) have been investigated using Pt (20 nm)/Re (30 nm)/Au (80 nm) metallization scheme. It is shown that annealing the samples at 600 °C for 1 min in a N 2 ambient improves the I – V characteristics of the as-deposited contact. The 600 °C contact produces a specific contact resistance of 1.4 × 10 −3 Ω cm 2 . However, annealing at 800 °C results in the degradation of the I – V characteristics. It is further shown that the surface morphology of the contact annealed at 600 °C is very smooth with a root-mean-square roughness of 1.4 nm and that the contact resistivity of the sample annealed at 600 °C for 30 min is comparable to that of the 1 min annealed sample. Glancing angle X-ray diffraction and Auger electron microscopy are used to characterise the interfacial reactions between the Pt/Re/Au layers and the p-GaN and hence to understand the annealing dependence of ohmic behaviour.

The correlations between the electrical behaviors of Ni contacts on p-GaN and the insertion of metal nano-dots at the Ni/GaN interfaces have been investigated. The Pt, Au, and Ti nano-dots directly deposited on p-GaN using the anodic porous alumina membrane mask by electron beam evaporation. It is shown that the samples with the inserted nano-dots exhibit better electrical behaviors compared with those without the nano-dots. The improvement of the electrical behaviors is explained in terms of the difference of the Schottky barrier heights between the Ni film and the metal nano-dots, and the enhanced electrical field at the Ni/GaN interfaces due to the presence of the nano-dots. Based on experimental and theoretical results, the possible mechanisms for the improved electrical behaviors is discussed. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We have investigated Ni–Zn alloy(5 nm)/Pd(5 nm) metallization schemes for use in forming high-quality ohmic contacts to p-GaN (5 × 1017 cm−3). The as-deposited Ni–Zn alloy/Pd contact exhibits nonlinear current–voltage (I–V) characteristics. However, annealing the contacts at 530 °C for 1 min in either air or N2 ambient improves I–V behaviour drastically. It is shown that the contacts produce specific contact resistance of 6.95 × 10−5 and 4.8 × 10−5 Ω cm2 when annealed in N2 and air ambient, respectively. It is further shown that multi-quantum-well light-emitting diodes fabricated with the annealed Ni–Zn alloy/Pd contact layers give a forward-bias voltage of 3.17 and 3.28 V at an injection current of 20 mA when annealed in air and N2 ambient, respectively.

We investigated the electrical properties of Cr(30 nm)/Al(200 nm) contacts to N-polar n-type GaN for high-performance vertical light-emitting diodes and compare them with those of Ti(30 nm)/Al(200 nm) contacts. Before annealing, both the samples show ohmic behaviors with a contact resistivity of 1.9–2.3 × 10−4 Ωcm2. Upon annealing at 250 °C for 1 min in N2 ambient, the Ti/Al contacts become non-ohmic, while the Cr/Al contacts remain ohmic with a contact resistivity of 1.4 × 10−3 Ωcm2. Based on X-ray photoemission spectroscopy and secondary ion mass spectrometry results, ohmic formation and degradation mechanisms are briefly described and discussed.

We report on the formation of highly reliable Ti/Al-based Ohmic contacts to N-polar n-GaN for vertical light-emitting diodes via laser-annealing. All as-deposited samples are Ohmic with specific contact resistances of 1.1 - 4.3 × 10-4 Ω cm2. After annealing at 250°C, unlike the untreated sample, the laser-annealed samples remain Ohmic with specific contact resistances of 2.6- 3.9 × 10-4 Ω cm2. The laser-annealed samples remain electrically stable up to 60 min at 300°C. Laser-annealing causes the formation of interfacial TiN/β -AlN phases with rock salt structure. Based on X-ray photoemission spectroscopy and scanning transmission electron microscopy results, possible Ohmic mechanisms are described.

We first fabricated GaN-based vertical light-emitting diodes (LEDs) with electrostatic discharge (ESD) protection internal diode. Despite the reduced emitting area due to the internal diode, the LEDs with the internal diode give a forward voltage of 3.42 V at 350 mA similar to that (3.40) of LEDs without the internal diode. It is shown that the output power of the LEDs with the internal diode is reduced by about 6% at 350 mA compared to reference LEDs without the internal diode. It is, however, further shown that the LEDs without the internal diode give a yield of 0% at the reverse voltages above 400 V, while the LEDs with the internal diode show a yield of ~ 90% at reverse voltages of 2-4 kV.

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.

A high-brightness GaN-based vertical light-emitting diode (VLED) was demonstrated by introducing a large-area low-cost direct printing process. A hexagonally close-packed micrometer convex array was fabricated on the n-GaN top layer of the VLED by direct printing using a hydrogen silsesquioxane solution and subsequent inductively coupled plasma etching. To confirm that the enhancement of light extraction by this structure, a conventional wet-chemical-etched structure was also fabricated on the n-GaN top layer of the VLED, yielding randomly oriented pyramid structures on the layer. Both VLEDs showed much stronger electroluminescence emission than an unpatterned VLED. However, the micrometer convex array improved the light extraction significantly more than the random pyramid structure owing to its greater ability to enlarge the light escape cone, attributed to its 50°-tapered profile and large extraction area. After chip packaging with silicone encapsulation, the light output power of the micropatterned VLED was 11.4% and 106% greater than those of the wet-etched and unpatterned VLEDs, respectively, under a 350-mA drive current.

We report on the formation of highly transparent and low-resistance Ag(3nm)∕Sb-doped SnO2 (ATO) (200nm) ohmic contacts to p-GaN (5×1017cm−3). It is shown that the samples become ohmic with a specific contact resistance of 8.7×10−5Ωcm2 upon annealing at 530°C for 1min in air. The oxidized contacts produce an extremely high light transmittance of 99% at a wavelength of 400nm. The light-emitting diodes (LEDs) fabricated with the annealed Ag/ATO p-type contact layers give a forward-bias voltage of 3.42V at injection current of 20mA, which is better than that of LEDs with the most common oxidized Ni(5nm)∕Au(5nm) contact layers. Based on scanning electron microscopy and x-ray photoemission spectroscopy results, the ohmic formation mechanisms are discussed.

High-quality Zn–Ni solid solution(8 nm)/Au(8 nm) ohmic contacts on p-GaN (5×1017 cm−3) have been investigated by means of current–voltage (I–V) measurements and x-ray photoemission spectroscopy. The as-deposited contact shows nonlinear I–V characteristics. However, oxidizing the contacts at 530 °C for 1 min in air ambient gives rise to considerable improvement in their I–V behaviors, yielding a specific contact resistance of 5.2×10−5 Ω cm2. The light transmittance of the annealed Zn–Ni solid solution/Au contacts is measured to be better than 74% at a wavelength of 470 nm. Green light-emitting diodes (LEDs) fabricated with the annealed Zn–Ni solid solution/Au contacts give a forward-bias voltage of 3.39 V at an injection current of 20 mA, which is better than that of the LEDs with the oxidized Ni/Au contacts.

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.

We report on the formation of reliable Ni–La solid solution (8 nm)/Au (8 nm) ohmic contacts to p-type GaN (Na=5×1017 cm−3). The as-deposited contact reveals nonlinear current–voltage (I–V) characteristics. However, the contacts show considerably improved I–V behavior when annealed at temperatures of 350–550 °C for 1 min in air ambient. For example, the specific contact resistance as low as 7.2×10−5 Ω cm2 is obtained from the samples annealed at 550 °C, whose transmittance is measured to be 82.5% at a wavelength of 470 nm. Based on Auger electron spectroscopy and x-ray photoemission spectroscopy results, possible ohmic formation mechanisms for the annealed contacts are described and discussed.

We report on the formation of Ni–Mg solid solution/Au ohmic contacts on p-GaN (5×1017 cm−3). The as-deposited Ni–Mg solid solution (8 nm)/Au (8 nm) contact shows near-linear I–V characteristics. However, oxidizing the contacts at 450 and 550 °C for 1 min in air results in a dramatic improvement in their I–V behaviors, producing specific contact resistance of ∼10−6 Ω cm2, which is much better than the conventional oxidized Ni/Au contacts. The light transmittance of the Ni–Mg solid solution/Au contacts annealed at 550 °C is measured to be better than 79% at a wavelength of 460 nm. Based on the I–V measurements, Auger electron spectroscopy, and x-ray photoemission spectroscopy results, possible ohmic formation mechanisms are described.

We report on a promising Rh-based scheme for high-quality ohmic contacts to surface-treated p-GaN:Mg (4×1017 cm−3). It is shown that the two-step surface-treated Rh contacts (10 nm) produce a specific contact resistance of 1.7×10−5 Ω cm2. It is also shown that the two-step treated Rh/Ni (5/5 nm) and Rh/Au (5/5 nm) contacts yield 6.0×10−5 and 9.3×10−6 Ω cm2, respectively. Based on the current–voltage measurement, x-ray photoemission spectroscopy, and Auger electron spectroscopy results, the mechanisms for the formation of the nonalloyed Rh-based ohmic contacts is described and discussed.

We report on a promising Ni (5 nm)/Al-doped ZnO (AZO) (450 nm) metallization scheme for low resistance and transparent ohmic contacts to p-GaN (5×1017 cm−3). It is shown that the as-deposited Ni/AZO contact shows a nonohmic characteristic due to the insulating nature of the as-deposited AZO. However, annealing the contacts at 450 and 550 °C for 2 min in air ambient results in linear current–voltage characteristics, giving a specific contact resistance of 1.01×10−5 and 8.46×10−6 Ω cm2, respectively. It is further shown that annealing the contact at 550 °C for 5 min produces a specific contact resistance of 6.23×10−6 Ω cm2. The light transmittance of the contacts annealed at 550 °C for 2 min is measured to be higher than 76% at wavelengths in the range of 400–550 nm. It is shown that the Ni/AZO contact could be a suitable scheme for high-performance optical devices.

We investigate the effect of CH3CSNH2 solution treatment on Ti/Al ohmic contacts to n-GaN:Si (3×1018 cm−3). It is shown that the sulfide treatment results in a drastic increase in the photoluminescence intensity, compared with that of the untreated sample. Current-voltage (I–V) results show that the sulfide treatment significantly improves the specific contact resistance. The annealing of the sulfide-treated sample (at 700 °C) results in a specific contact resistance of 3.1×10−6 Ω cm2. X-ray photoemission spectroscopy (XPS) results show that the Ga 2p core levels shift toward the higher-binding energy side by 0.2 eV for the sulfide-treated sample and by 0.4 eV for the annealed (sulfide-treated) sample, compared with that of the untreated one. It is further shown that the intensity of O 1s core level decreases with the sulfide treatment. Based on the I–V and XPS results, the sulfide and annealing treatment dependence of the specific contact resistance is discussed.

The electrical properties of GaN-based light emitting diodes (LEDs) fabricated with indium–tin-oxide (ITO) p-contacts were investigated as functions of the thickness of the (In)GaN contact layers and the In content. The LEDs with the GaN contact layers showed lower forward voltages (in the range of 3.48–3.03 V) than the LEDs with ITO-only contacts (4.2 V); the forward voltages of the LEDs decreased with increasing contact layer thickness (from 1–4 nm) and increasing In content. However, the output power linearly decreased with increasing In content, whereas that of the GaN contact layer LEDs became saturated at a thickness of 2 nm. The X-ray photoemission spectroscopy (XPS) Ga 2p core level for the samples with the contact layers was shifted toward lower binding energies by 0.11–0.22 eV compared with that of the sample without the contact layer. However, the energy shift decreased with increasing In content. Unlike the contact-layer samples, the sample without the contact layer experienced outdiffusion of N atoms. Based on the XPS and atomic force microscopy results, the contact-layer-induced electrical improvement was described and discussed.