Occurrence of cubic GaN and strain relaxation in GaN buffer layers grown by low-pressure metalorganic vapor phase epitaxy on (0001) sapphire substrates

Investigations on GaN buffer layers grown by low-pressure metalorganic vapo r phase epitaxy on (0001) sapphire substrates indicated that the mechanisms by way of which GaN buffer layers relax stresses introduced by the lattice mismatch and thermal expansion coefficient difference between GaN epilayer and sapphire substrate are related to both the crystallographic structure of GaN and thickness of the buffer layers. Beside forming misfit dislocatio ns, mismatch-induced stresses can also be relaxed by forming stacking fault s and microtwin boundaries parallel to (11-1) of GaN near the interface bet ween GaN and sapphire substrate in cubic GaN buffer layers. It was found th at, in cubic GaN buffer layers, there exists a critical thickness within wh ich the stacking faults and/or microtwin boundaries parallel to (11-1) of G aN can be formed. This critical value is determined to be 50 nm. (C) 1999 A merican Institute of Physics. [S0003-6951(99)02105-1].