Laser-Induced Liftoff and Laser Patterning of Large Free-Standing GaN Substrates
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Laser-Induced Liftoff and Laser Patterning of Large Free-Standing GaN Substrates
0. Ambacher1 , M. K. Kelly 2, C. R. Miskys, L. H6ppel, C. Nebel, and M. Stutzmann Walter Schottky Institute, Technical University of Munich Am Coulombwall, D-85748 Garching, Germany,
ABSTRACT Free-standing GaN crystals are produced from 200-300 prn 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 5x106 cm 2 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. ItNTRODUCTION To fully explore the large application potential of group-lIl-nitrides, it is a big advantage to achieve epitaxial growth on lattice-matched substrates. Although relatively small bulk GaN crystals can be reproducibly grown [I], large bulk single crystals of GaN or AIN for the fabrication of epi-ready substrates are not yet commercially available. Therefore all GaN-based devices realized up to date are deposited by heteroepitaxy, mostly on sapphire (A12 0 3) or silicon carbide (SiC) substrates. A12 0 3 features a relatively low cost and is the most commonly used substrate material for optoelectronic devices because of its large bandgap, thermal stability and epi-ready surface quality [2-4]. SiC is of special interest for high power devices like field effect transistors, hetero-bipolar devices and microwave amplifiers because of its high thermal I e-mail: [email protected], 2 [email protected]
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conductivity and the easy implementation of back contacts on n- or p-type doped substrates. However, the mismatch in lattice constants and thermal expansion coefficients between epitaxial layers of group-lII-nitrides and the available substrates gives rise to a high density of dislocations (between 108 and 1010 cm-2) and strain, limiting e.g the electron mobility, doping efficiency and lifetime of devices [5, 6]. The capability to produce superior material quality by homoepitaxial growth of GaN on GaN bulk crystals or bulk-like substrates has been demonstrated before [7]. At present, the largest GaN crystals are grown using the high-pressure method [8]. Such crystals (flat hex
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