INFLUENCE OF IN-SITU SAPPHIRE SURFACE PREPARATION AND CARRIER GAS ON THE GROWTH MODE OF GAN IN MOVPE

A comparative study of GaN layers grown on sapphire (0 0 0 1) by metal organic vapor phase epitaxy with different sapphire surface preparatio ns and carrier gas combinations is reported. Observation of the growth modes at several stages of the growth process is carried out by in si tu laser reflectometry measurements and es situ transmission electron microscopy. It is shown that the combination involving both an in situ sapphire surface preparation consisting in the deposition of a thin S ixN1-x film and a H-2-N-2 mixture as carrier gas, induces a three dime nsional mode in the first stage of the growth of GaN epilayer. The gro wth occurs then at high temperature by coalescence of GaN islands limi ted by (0 0 0 1) top surface and {1 0 (1) over bar 1} lateral facets. This island formation is initiated by the modification of the morpholo gy of the GaN buffer layer, due to the dual action of a ''morphactant' ' effect of H-2 and a modification of the surface energy of sapphire b y the SixN1-x coating. This three-dimensional growth mode yields a red uction of the total number of extended defects from above 10(10) cm(-2 ) usually obtained down to 7 x 10(8) cm(-2) for the best samples. (C) 1998 Elsevier Science B.V. All rights reserved.