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Simplifying blue RCLED fabrication Optimised laser-lift-off eases the fabrication of high-quality,blue RCLEDs


CHINESE researchers have built high- quality, blue-emitting, resonant-cavity (RC) LEDs with a relatively simple two-step substrate transfer technique.


This team’s efforts shows how to avoid the complex processing techniques that have previously hampered fabrication of blue RCLEDs, which are promising sources for printers and displays.


“RCLEDs offer a number of advantages over conventional LEDs, such as improved directionality, spectral purity, spectral stability and enhanced output power,” explains corresponding author Bao-ping Zhang from Xiamen University. For example, the superior directionality of the emitted light from the RCLED enables an increase in the dots-per-inch capability of printers and scanners.


Fabricating blue RCLEDs with the approach used to make their GaAs-based, red- emitting cousins is very tricky, because in this case the pairing of materials needed


to make the mirrors – GaN and AlGaN – differ very little in refractive index, but have substantially different lattice constants.


Back in 2000 a team from Brown University and Agilent Technologies reported how to overcome this issue with two dielectric mirrors – these can be added with a series of processing steps involving electrodeposition, bonding and laser lift-off. However, processing is complex, due to difficulties associated with bonding, the need to dry etch the silicon-doped GaN layer to form the n- contact, and realising a sufficiently smooth GaN surface by polishing to allow high-quality deposition of the top mirror.


Through the development of a high- quality laser lift-off process, the Chinese team avoids many of these issues. They have devoted tremendous effort to finding a high-strength adhesive with a low curing temperature and worked hard to realise a uniform energy distribution of the spot size through optimisation of the energy


density of the laser, its spot size and the scanning speed. “AFM scans indicate a root-mean-square surface roughness of about a few nanometers,” says Zhang.


The 461 nm RCLED produced by the team has a turn-on voltage of 3.3 V and produces a 0.3 nm emission line width at a current density of 2 kA cm-2


. Zhang says that the


performance of this device compares favourably to that produced by the US team more than a decade ago: “Our device exhibits a lower voltage due to the good ohmic contact between indium tin oxide and p-GaN, and a higher ‘Q-factor’ was obtained.”


Improving RCLED efficiency is the team’s next goal. “A green RCLED will also be fabricated because it is an excellent light source for PMMA-based plastic optical fibre communication. However, to fabricate a blue VCSEL is our main target.”


X. -L. Hu et. al. Electron. Lett. 47 986 (2011)


“Now offering Germanium Reclaim”


October 2011 www.compoundsemiconductor.net 47


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