TEM and PL characterisation of MBE-grown epitaxial GaN/GaAs

MBE-grown epitaxial GaN deposited at 700 degrees C on {001}, {111}A and {(1) over bar (1) over bar (1) over bar}B GaAs has been characterised using the combined techniques of transmission electron microscopy (TEM) and photoluminescence (PL). On both {111}A and {(1) over bar (1) over bar (1) over bar}B GaAs substrates, single crystal wurtzite GaN was formed, but with very high densities of threading defects. Best epitaxy occurred on {(1) over bar (1) over bar (1) over bar}B GaAs in accordance with PL measurements. An amorphous phase was identified at the GaN/{111}A GaAs interface and the GaN epilayer evolved in this instance with the same N-terminated growth surface as for the case of growth on {(1) over bar (1) over bar (1) over bar}B GaAs, as determined by convergent beam electron diffraction (CBED). Growth on (001) GaAs produced highly faulted columnar grains of zincblende GaN. Conversely, growth on {001} GaAs under an additional arsenic flux at 700 degrees C resulted in the deposition of single crystal zincblende GaN with a high density of stacking faults and microtwins. Thus, the microstructure of epitaxial GaN depends very much on the detailed growth conditions and substrate orientations used.