Patterned laser lift-off simplifies LED fabrication
Taiwanese researchers from National Cheng Kung University and We-Fung Institute of Technology have fabricated vertical-structured GaN-based LEDs using nickel electroplating and patterned laser lift-off.
These devices, with an output power over twice that of a standard LED, will help the development of high-brightness sources for cost-effective solid state lighting.
The team published their results in the journal Applied Physics Letters. In their paper they say that sapphire substrates, which are a common choice for GaN LED production, have poor thermal and electrical conductivity, which prevents efficient heat dissipation.
In addition conventional GaN LEDs have their positive and negative electrodes on the same side of the wafer, which causes severe current crowding and reduces the emitter's internal quantum efficiency.
The team addressed both of these issues by using a patterned laser lift-off technique to simultaneously define the device area and separate the GaN epilayer from the sapphire substrate. Nickel is subsequently deposited onto the p-type GaN, followed by layers of silver, titanium and aluminum, to form a metallic substrate that reduces current crowding and improves the device's heat dissipation properties.
Using laser lift-off to enhance GaN LED performance has been demonstrated before. However, according to the Taiwanese team, previous efforts have required complicated fabrication processes that could impact production yields.
The GaN LEDs, which contained five InGaN quantum wells, were grown by MOCVD on 2 inch sapphire substrates. Copper masks were used to define the device's shape, with 25 ns pulses with a wavelength of 248 nm separating the GaN structures from the sapphire substrates. Metallic mirrors were then formed by electron-beam evaporation.
The team compared the vertical-structured LED performance with that of conventional GaN LEDs. At an operating current of 20 mA, the output power of the vertical-structured LED was typically 2.3 times higher, and the forward voltage drop 10% lower.