Reduction of dislocation density in GaN films on sapphire using AIN interlayers

Abstract

GaN (00.1) thin films, of thickness 1.25 to 2.25 μm grown on sapphire substrate (11.0) by metallo organic chemical vapor phase deposition (MOCVD) with different number of AlN interlayers, were characterized by triple crystal diffractometry and synchrotron white beam x-ray topography (SWBXT). The full width at half maximum (FWHM) of x-ray rocking curves from symmetric and asymmetric reflections was used to estimate the dislocation density in GaN films. It has been found that the edge dislocation density decreased from 1.63 × 10¹⁰ cm⁻² to 1.23 × 10¹⁰ cm⁻² and the screw dislocation density decreased from 2.0 × 10⁸ cm⁻² to 1.1 × 10⁸ cm⁻² when one AlN interlayer was inserted between the high temperature GaN layer. The dislocation density decreased further with the increase in number of interlayers. On the other hand the compressive stress in the GaN film increased from −0.29 GPa to −0.86 GPa. The compressive stress further increased as the number of interlayers increased but no cracking in the GaN film was observed. This could be due to better adhesion between the film and substrate due to interlayers. SWBXT in transmission from a GaN(00.1)/Al₂O₃(11.0) sample confirms the orientation of GaN and indicates that it is a single crystal with high dislocation density. SWBXT from the Al₂O₃ substrate shows cellular structure of dislocations.