Improved performance of InGaN/GaN flip-chip light-emitting diodes through the use of robust Ni/Ag/TiW mirror contacts

Abstract

In this work, the authors report the incorporation of TiW alloy in InGaN/GaN-based flip-chip light-emitting diodes (LEDs). The advantages provided by the use of TiW are analyzed in detail. InGaN/GaN multiple quantum well LEDs with a Ni/Ag/TiW metal stack are found to tolerate high-temperature annealing better than those with a Ni/Ag metal stack. Highly improved current-voltage characteristics and enhanced optical output power are achieved for the devices with a TiW thin layer. These changes are ascribed to the higher reflectivity, smoother surface, and better ohmic properties of the device containing TiW after annealing. Better heat management of the device with TiW is demonstrated by comparing electroluminescence spectra of the two device structures. Overall, these factors resulted in devices with TiW exhibiting a higher external quantum efficiency than devices without TiW. Detailed x-ray photoelectron spectroscopy analyses of the reflector metal stacks reveal little intermixing of the layers after annealing in the devices with TiW. The results show that incorporation of TiW is a promising approach for the fabrication of high-performance InGaN/GaN flip-chip LEDs. V

Insights into the Silver Reflection Layer of a Vertical LED for Light Emission Optimization

“…In the context of optimization for device performance enhancement in terms of LEE and output power, factors such as nanostructural composition, selection of metallic elements at reflection, barrier or adhesion, and wafer bonding layers need to be considered in correlation with electrical resistivity, coefficient of thermal expansion (CTE) or mechanical stress, fabrication methodology, and grain morphological parameters. On the basis of our analysis and results, possible optimization suggestions are given as follows: GB engineering, that is, grain structure, SB (TBs), and texture orientation need to be well-controlled as electron or photon scattering mechanism and electromigration lifetime are highly dependent on the grain size distribution. ,, Nanotwinned metals, for example, Ag, should be promoted as these structures exhibit distinctive properties (e.g., high tensile strength, good ductility, thermal stability, and electrical conductivity) as compared to the nanocrystalline or ultrafine-grained metals. , To mitigate issues of Ag diffusion into the GaN or active region and In/Ga out-diffusion into the Ag mirror layer, high thermal treatments should be avoided on Ag and p-GaN layers to prevent metal–semiconductor diffusions. , Ni/Ag/TiW metal stack is found to tolerate high-temperature annealing To reduce thermal stress between GaN and metal–alloy-based interfaces, the tunable incorporation of diamond-like carbon (DLC) layers on the reflective layer has the distinct capability to match its CTE with GaN and hence enhances the thermal diffusion. , The GaN/Ag interface needs diffusion barrier layers such as tin–zinc oxide (TZO) and nickel–titanium (NiTi)-related alloys that can effectively block Ag diffusion .…”

“…To mitigate issues of Ag diffusion into the GaN or active region and In/Ga out-diffusion into the Ag mirror layer, high thermal treatments should be avoided on Ag and p-GaN layers to prevent metal–semiconductor diffusions. , Ni/Ag/TiW metal stack is found to tolerate high-temperature annealing …”

High-Performance Green Flip-Chip LEDs with Double-Layer Electrode and Hybrid Reflector

“…[24][25][26] Therefore, the fabrication of p-electrode with high optical reflectance is of significant importance for the realization of high-efficiency green FC-LEDs. Various approaches have been explored to mitigate the light absorption of p-electrodes, such as metal-based reflective pelectrodes [27][28][29] and distributed Bragg reflector (DBR)-based reflective p-electrodes. 30,31 Metal-based reflective p-electrodes reflect light over a broad range of frequencies incident from arbitrary angles.…”