Laser direct writing of GaN-based light-emitting diodes—The suitable laser source for mesa definition

The development of a process chain allowing for rapid prototyping of GaN-based light-emitting diodes (LEDs) is presented, which does not rely on photolithography. Structuring of the epitaxial layers is realized by direct-writing laser ablation, allowing a flexible chip layout that can be changed rapidly and at low cost. Besides contact metallization and trench formation, mesa definition is the most critical processing step. For mesa formation and to expose the n-GaN contact layer, the epitaxial grown p-GaN layer together with the active region has to be removed completely without forming cracks or crystal defects in the n-GaN layer or the mesa sidewalls, which would cause sidewall leakage currents. In developing an appropriate laser ablation process that meets these requirements, three different laser systems have been employed in a comparative study. These are a frequency-tripled picosecond (ps) Nd:YVO₄ laser emitting at a wavelength of 355 nm and a pulse length of 10 ps and two 20 nanosecond (ns) pulse length laser systems, operating at a wavelength of 248 nm (Excimer laser) and 355 nm (frequency-tripled Nd:YVO₄ laser), respectively. First, the laser sources are compared regarding the morphological properties of the resulting laser trenches. Due to band filling effects resulting in optical bleaching of the GaN material when irradiating with ps-laser pulses at 355 nm, the resulting ablation process suffers from cracking. Laser ablation using ns-pulses at both 355 nm and 248 nm leads to crack-free material removal up to a well-defined depth. To keep reverse-bias leakage currents at a level comparable to that of conventional dry-etched mesa-LEDs, subsequent wet etching is essential to remove residues in the mesa-trenches irrespective of the laser source used. Besides wet etching, an additional annealing step has to be applied to mesa-trenches fabricated using ns- and ps-laser pulses at a wavelength of 355 nm. Due to the larger penetration depth at 355 nm, defects causing an increased leakage current are generated in the quantum well region during laser irradiation, which manifest themselves by a spectrally broad defect luminescence. To separate and quantify the contributions from the mesa sidewall leakage and from the areal leakage through the p-n-junction, a series of LEDs have been fabricated and analyzed for which the mesa perimeter length was varied while keeping the mesa area constant. In this way, it is shown that laser ablation with ns-pulses at 248 nm results in the lowest sidewall leakage current as well as forward bias voltage-current and output power-current characteristics very similar to those of conventionally dry etched reference LEDs.