Correlation of strain, wing tilt, dislocation density, and photoluminescence in epitaxial lateral overgrown GaN on SiC substrates

Abstract

Epitaxial lateral overgrown (ELOG) gallium nitride (GaN) on SiC is being studied as a possible substrate for blue laser diodes. A defect density below 2.2×107 cm–2 in the wings, compared to 2×109 cm–2 in the windows, was achieved. Interaction of the overgrown GaN with the SiO2 mask causes a few degree wing tilt and a transition region of high defect density between windows and wings. Diminished PL, strong tensile stress, and a defect correlated line at around 3.4 eV emerge in this up to two-micron-wide transition region. By changing the mask material from SiO2 to SiN we were able to reduce the wing tilt drastically to below 0.7°. This eliminates the defective transition region and extends the low strain and the low defect density area of the ELOG wings. The methods used to study strain, wing tilt, and threading dislocations in the ELOG samples are microphotoluminescence (µPL), transmission electron microscopy, x–ray diffraction, and scanning electron microscope. We also demonstrate the use of the first momentum of the µPL spectra as an effective means to measure strain distribution.