Surface preparation of freestanding GaN substrates for homoepitaxial GaN growth by rf-plasma MBE

The authors have investigated different methods for preparing the surfaces of freestanding, Ga-polar, hydride vapor-phase epitaxy grown GaN substrates to be used for homoepitaxial GaN growth by plasma-assisted molecular beam epitaxy (MBE). Cross-sectional transmission electron microscopy and secondary ion mass spectroscopy, respectively, were used to characterize the microstructure and to measure the concentrations of impurities unintentionally incorporated in the MBE-grown homoepitaxial GaN layers. Heating Ga-polar substrates to similar to 1100 degrees C is as effective as a wet chemical clean for reducing impurity concentrations of oxygen, silicon, and carbon. The combination of an aggressive ex situ wet chemical clean with in situ Ga deposition and thermal desorption results in homoepitaxial GaN layer growth with very low residual impurity concentrations and without generating additional threading dislocations.