Abstract
Discovering ways to increase the LED light extraction efficiency (LEE) should help create the largest performance improvement in the power of UV AlGaN LEDs. Employing surface roughening to increase the LEE of typical AlGaN UV LEDs is challenging and not well understood, yet it can be achieved easily in AlGaN LEDs grown on SiC. We fabricate thin-film UV LEDs (similar to 294-310 nm) grown on SiC-with reflective contacts and roughened emission surface-to study and optimize KOH roughening of N-face AlN on the LEE as a function of roughened AlN pyramid size and KOH solution temperature. The LEE increased the most (2X) when the average AlN pyramid base diagonals (d) were comparable to the electroluminescence (EL) wavelength in the AlN layer (d similar to lambda(EL): 42-52 pyramids/mu m(2)), but the LEE enhancement diminished when d was much larger than lambda(EL) (d -5.5 lambda(EL); 2-3 pyramids/mu m). The UV LEDs had a 10 mu p-GaN contact layer, and the forward voltage was similar to 6 V at similar to 8 A/cm(2), with a voltage efficiency (VE) of similar to 70%. The VE of the LEDs did not change after KOH roughening. This work suggests important implications to increase the LEE of AlGaN LEDs. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement