Ag Reflector Research Articles

Enhancement in output power of blue gallium nitride-based light-emitting diodes with omnidirectional metal reflector under electrode pads

In this study, we demonstrate a GaN-based light-emitting diode (LED) with nonalloyed metal contacts onto the n+-GaN surface and transparent contact layer (indium tin oxide) to serve as the n-type electrode (cathode) and the p-type electrode pad (anode), respectively. Comparing with the conventional LEDs, which the electrode pads and/or Ohmic contacts form through conventional Cr∕Au metal contacts, the nonalloyed metal contacts (Ag∕Cr∕Au or Al∕Cr∕Au) used in the present experimental blue LEDs also play the role of reflector to prevent the emitted light from absorption by the opaque electrode pads with low reflectivity (Cr∕Au). With an injection current of 20mA, the enhancement in the light output power has approximately a 14% magnitude compared to the GaN-based LEDs without Ag or Al reflectors under the Cr∕Au electrode pads.

Enhanced light extraction efficiency in flip-chip GaN light-emitting diodes with diffuse Ag reflector on nanotextured indium-tin oxide

We investigated a flip-chip light emitting diode (FCLED) with a diffuse reflector fabricated by depositing a Ag film on a nanotextured indium-tin oxide (ITO) layer. The FCLED with a diffuse Ag reflector showed remarkably good adhesion and high reflectance than that with a specular Ag reflector deposited on the planar ITO layer. The optical output power of FCLED with the diffuse Ag reflector was enhanced by 161.3% at 300mA compared to that with the specular Ag reflector.

Thermally Stable and Highly Reflective ITO/Ag-Based Ohmic Contacts to p-GaN

Thermal stability of indium tin oxide (ITO)/Ag-based reflective contacts to p-GaN was investigated for their application to vertical structure GaN-based light-emitting diodes (LEDs). The ITO/Ag-based p-contacts showed good ohmic property with low specific contact resistance of . In order to study the thermal degradation of the ITO/Ag-based p-contacts, they were annealed at for in air. The reflectance of ITO/Ag and was significantly reduced due to Ag degradation on annealing. However, the reflectance of and was not degraded on annealing, but instead it was slightly increased to 81.4% and 83.7% at , respectively, because the Ag reflector was protected with or encapsulation layers. Moreover, on annealing of the ITO/Ag-based p-contacts, the vertical structure GaN-based LEDs with thermally stable and p-contacts showed higher output power of and at , respectively, compared to those with thermally unstable ITO/Ag and p-contacts.

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 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.

High-Reflectance and Thermally Stable AgCu Alloy p-Type Reflectors for GaN-Based Light-Emitting Diodes

We report on the formation of high-quality AgCu alloy p-type reflectors for GaN-based light-emitting diodes (LEDs). Compared with Ag contacts, the AgCu alloy reflectors produce lower specific contact resistance (7.5times10-5 Omegamiddotcm2), higher light reflectance (89.5% at 400 nm), and better thermal stability (absence of interfacial voids), when annealed at 400 degC in N2 : O2(=1:1) ambient. LEDs fabricated with the AgCu reflectors show light output power better than that of LEDs with the Ag reflectors. The ohmic mechanism for the AgCu alloy reflectors is explained in terms of the formation of Ag-Ga solid solution and the presence of Cu-oxide nano-particles at the contact/GaN interface

Enhanced Light Extraction in GaInN Light-Emitting Diode With Pyramid Reflector

GaInN light-emitting diodes (LEDs) that employ a reflector consisting of an array of three-dimensional (3-D) SiO2 pyramids and a Ag layer are demonstrated to have enhanced light extraction compared with GaInN LEDs with planar Ag reflector. Ray tracing simulations reveal that the pyramid reflector provides 14.1% enhancement in extraction efficiency. Consistent with the simulation, it is experimentally demonstrated that GaInN LEDs with the pyramid reflector show 13.9% higher light output than LEDs with a planar Ag reflector. The enhancement is attributed to the appearance of an additional escape cone for light extraction enabled by the 3-D pyramid reflector

Enhanced light-extraction in GaInN near-ultraviolet light-emitting diode with Al-based omnidirectional reflector having NiZn∕Ag microcontacts

Enhancement of light extraction in a GaInN near-ultraviolet light-emitting diode (LED) employing an Al-based omnidirectional reflector (ODR) consisting of GaN, a SiO2 low-refractive-index layer perforated by an array of NiZn∕Ag microcontacts, and an Al layer is presented. A theoretical calculation reveals that a SiO2∕Al ODR has much higher reflectivity than both a SiO2∕Ag ODR and a Ag reflector at a wavelength of 400nm. It is experimentally shown that GaInN near-ultraviolet LEDs with GaN∕SiO2∕Al ODR have 16% and 38% higher light output than LEDs with SiO2∕Ag ODR and Ag reflector, respectively. The higher light output is attributed to enhanced reflectivity of the Al-based ODR in the near-ultraviolet wavelength range.

GaInN light-emitting diode with conductive omnidirectional reflector having a low-refractive-index indium-tin oxide layer

Enhancement of light extraction in a GaInN light-emitting diode (LED) employing a conductive omnidirectional reflector (ODR) consisting of GaN, an indium-tin oxide (ITO) nanorod low-refractive-index layer, and an Ag layer is presented. An array of ITO nanorods is deposited on p-type GaN by oblique-angle electron-beam deposition. The refractive index of the nanorod ITO layer is 1.34 at 461nm, significantly lower than that of dense ITO layer, which is n=2.06. The GaInN LEDs with GaN∕low-n ITO/Ag ODR show a lower forward voltage and a 31.6% higher light-extraction efficiency than LEDs with Ag reflector. This is attributed to enhanced reflectivity of the ODR that employs the low-n ITO layer.

GaInN light-emitting diodes with RuO2/SiO2/Ag omni-directional reflector

A GaInN light-emitting diode (LED) employing an omni-directional reflector (ODR) is presented. The ODR consists of a RuO2 ohmic contact to p-type GaN, a quarter-wave thick SiO2 low-index layer perforated by an array of micro-contacts, and an Ag layer. Calculations predict a 98% angle-averaged reflectivity at λ=450 nm for an GaN/SiO2/Ag ODR, much higher than that for a 20 period Al0.25Ga0.75N/GaN distributed Bragg reflector (49%) and an Ag reflector (94%). It is shown that the RuO2/SiO2/Ag ODR has higher reflectivity than Ni/Au and even Ag reflectors, leading to a higher light extraction efficiency of GaInN LEDs with ODR. The electrical properties of the ODR-LED are comparable to those LEDs with a conventional Ni/Au contact.

Magneto-optical Kerr effect of amorphous TbFeCoTa film with a quadrilayer structure

The magneto-optical Kerr effect (MOKE) of a multilayered system has been described by using the characteristic matrix method of film optics. By using this method, the MOKE and read-out performance of an amorphous TbFeCoTa film with the quadrilayer structure of SiC/TbFeCoTa/SiC/metallic reflector/glass substrate, has been investigated in detail. Different from the MO/metallic bilayer structure, whose Kerr rotation can be strongly enhanced by the metallic reflector with low values of the optical constants n and κ, the Kerr rotation and the read-out figure of merit of the quadrilayer structure are strongly enhanced at wavelengths where the reflector layer has a high reflectivity, due to multiple reflection and optical interference. Among the Al, Ag, Cu, Cr and Au reflectors, Al and Ag reflectors result in the largest two increments in the Kerr rotation and the read-out figure of merit at short wavelengths, indicating that Al and Ag are the best reflector materials for the TbFeCoTa MO disk for short wavelength recording. The thickness dependencies of each layer on the MOKE, reflectivity and the figure of merit of the quadrilayer structure have been investigated. By employing this method, the optimum structure for the TbFeCoTa MO disk can be determined.