Laser-induced liftoff and laser patterning of large free-standing GaN substrates

Free-standing GaN crystals are produced from 200-300 μm thick GaN films grown on 2 inch sapphire substrates by hydride vapor phase epitaxy. The GaN films are separated from the growth substrate by laser-induced liftoff, using a pulsed laser to thermally decompose a thin layer of GaN close to the film-substrate interface. The free-standing films are polished and used for the homoepitaxial growth of high quality GaN layers by metalorganic chemical vapor deposition. The structural and optical properties of the homoepitaxial films in comparison to layers grown on sapphire are significantly improved, mainly because of lower dislocation density and surface roughness as low as 5×10⁶ cm² and 0.2 nm, respectively. Laser-induced thermal decomposition is also applied to achieve etching of GaN. At exposures of 500 mJ/cm² with 355 nm light, etch rates of up to 90 nm for one pulse are obtained. Illumination with an interference grating is used to produce trenches as narrow as 100 nm or sinusoidal surface patterns with a period of 260 nm. Such surface morphologies are very useful for the processing of anti-reflection coatings or distributed Bragg reflectors.