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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.

A study reports on the growth of very high-quality Al-doped n-type ZnO epilayers on sapphire substrates using a radio-frequency (rf) magnetron sputtering technique combined with a rapid-thermal annealing. Photoluminescence (PL) and Hall measurements show that both the optical and electrical properties of the ZnO layers are significantly improved with an increasing annealing temperature up to 900 degree Celsius.

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. [ABSTRACT FROM AUTHOR]

We report on the formation of high-quality p-type Al-based ohmic reflectors using Ag (3 nm)/indium tin oxide (ITO)(100 nm) interlayers for use in high-power flip-chip light-emitting diodes (LEDs). The Ag/ITO interlayers are first annealed at temperatures of 530 and 630 °C for 1 min in air, after which Al reflectors (200 nm thick) are deposited and subsequently annealed at 330 °C for 5 min in a vacuum. It is shown that the annealed Ag/ITO/Al contacts give specific contact resistances as low as ∼10-5 Ω cm-2 and reflectance of ∼85% at a wavelength of 460 nm, which are much better than those of oxidized Ni/Au schemes. LEDs fabricated with the annealed Ag/ITO/Al p-type electrodes give forward-bias voltages of 3.29–3.37 V at injection current of 20 mA. [ABSTRACT FROM AUTHOR]

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. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We report on the formation of reliable Ni–La solid solution (8 nm)/Au (8 nm) ohmic contacts to p-type GaN (N[sub a]=5×10[sup 17] cm[sup -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[sup -5] Ω cm[sup 2] 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. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have investigated a Zn–Ni solid solution/Ag scheme for use in producing high-quality ohmic contacts for flip-chip light-emitting diodes (LEDs). The as-deposited contact shows nonlinear I–V characteristics. However, oxidizing the contacts at temperatures of 350–550 °C for 1 min in air ambient results in linear I–V behaviors, yielding specific contact resistances of 10[sup -4]–10[sup -5] Ω cm[sup 2]. In addition, LEDs are fabricated with the oxidized Zn–Ni solid solution/Ag contacts and Ag single contacts. The typical I–V characteristics of the LEDs with the annealed Zn–Ni solid solution (2.5 nm)/Ag (200 nm) p-type contact layers reveal a forward-bias voltage of 3.25 V at an injection current of 20 mA, which is much better than that of the LEDs with the Ag (200 nm) contact layers. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

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). [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

We investigated the effect of step-down indium content in InGaN quantum wells (QWs) on the output efficiency of fully packaged InGaN-based light-emitting diodes (LEDs). Both the reference and step-down LED chips give maximum external quantum efficiencies (EQE) of 54.65 and 54.99% at a current density of 4.17 A/cm2, respectively. Step-down LEDs show a lower efficiency droop than reference LEDs. The step-down LEDs exhibit a 5.3% higher EQE at 83.3 A/cm2 than the reference LEDs. As the current density increases from 1.39 to 9.03 A/cm2, the electroluminescence (EL) intensity peaks of the step-down LEDs are slightly more shifted towards the larger energy side than those of the reference LEDs. The polarization field is estimated to be 1.34 and 1.41 MV/cm for the reference and step-down LEDS, respectively. The simulated internal quantum efficiency results of the reference and step-down LEDs are in agreement with the experimental results. The simulation results show that the step-down LEDs have higher hole injection efficiency than the reference LEDs. On the basis of the simulation results, the blue-shift behavior of the reference and step-down LEDs is described and discussed. [ABSTRACT FROM AUTHOR]