Metal Organic Vapor Phase Deposition of AlN, GaN, and Their Alloys

Chemical surface treatments were conducted on mechanically polished (MP) and chemomechanically polished (CMP) (0001)-oriented single crystalline aluminum nitride (AlN) substrates to determine a surface preparation procedure for the homoepitaxial deposition of AlN epitaxial layers by metal organic vapor phase deposition (MOCVD). MP AlN substrates characterized by atomic force microscopy exhibited 0.5 nm RMS roughness and polishing scratches, while CMP AlN substrates exhibited 0.1 nm RMS roughness and were scratch-free. X-ray photoelectron spectroscopy analysis of MP and CMP AlN substrates indicated the presence of a surface hydroxide layer composed of mixed aluminum oxide hydroxide and aluminum trihydroxide. Wet etching with sulfuric and phosphoric acid mixtures reduced the amount of surface hydroxide. Ammonia annealing at 1250 &;AlxGa1-xN/GaN heterostructures (0 ≤ x ≤ 1) were deposited on (0001)-sapphire by low-pressure (20 Torr) MOCVD utilizing a range of group-III precursor and NH3 flow rates and a deposition temperature of 1050°C. The Al-mole fraction of AlxGa1-xN epitaxial layers was controlled by variations in the molar flows of triethylgallium (fTEG) and trimethylaluminum (fTMA). Characterization by x-ray diffraction confirmed that increasing the fTMA/f TEG ratio during deposition resulted in increased Al-mole fraction of AlxGa1-xN epitaxial layers. A linear relationship between the experimental Al-mole fraction of AlxGa1-xN epitaxial layers and the group-III precursor flows was obtained if a correction factor gamma was introduced such that x = gammafTMA / (gammafTMA + fTEG). This correction factor was found to be independent of group-III precursor flows but dependent on NH3 partial pressure. The value of gamma for these experiments decreased from 1.4 to 1.0 when the NH3 partial pressure was increased from 0.8 to 16.0 Torr during AlxGa1-xN deposition. The growth rate of AlxGa 1-xN epitaxial layers was found to be a linear combination of the independent AlN and GaN growth rates when equivalent fTMA and fTEG were utilized under the same deposition conditions.