Defect formation mechanism and quality improvement of InAlN epilayers grown by metal-organic chemical vapor deposition

The effect of growth pressure on defect formation in InAlN epilayers grown on GaN/sapphire templates by metal-organic chemical vapor deposition was systematically investigated in this study. From X-ray diffraction measurements, it was found that a serious phase separation occurred in the InAlN epilayers grown at 1 x 10(4) Pa (100 mbar). The inhomogeneity of the In composition was observed at the beginning of the InAlN growth as examined by transmission electron microscopy. The initial In composition inhomogeneity close to the InAlN/GaN interface was confirmed to play an important role in the formation of V-shaped defects and the phase separation. When the growth pressure increased from 1 x 10(4) Pa (100 mbar) to 5 x 10(4) Pa (500 mbar), the phase separation diminished over 3 x 10(4) Pa (300 mbar), and the In content continuously increased from 6.0 to 25%. However, in spite of the fact that there was no phase separation in the InAlN layer grown at 3 x 10(4) Pa (300 mbar), the inhomogeneity of the In composition still existed near the surface instead of the InAlN/GaN interface. This was caused by the fact that the In adatoms preferred to accumulate at the V-shaped defects which were induced by the low surface mobility and parasitic reaction of Al adatoms. Two distinct formation mechanisms of the V-shaped defects at the low and high growth pressures were confirmed. To explore the effect of thickness on the epilayer quality, a series of InAlN samples (In = similar to 20%) with various thicknesses ranging from 5 to 125 nm were investigated. The InAlN epilayer with a thickness of 10 nm showed the optimum crystallinity and minimum surface roughness. A higher growth pressure (>= 3 x 10(4) Pa (300 mbar)) and a thinner thickness (<= 10 nm) favored the In composition homogeneity and suppressed the formation of V-shaped defects. Both key growth parameters were demonstrated in detail to achieve a high-quality InAlN epilayer for device applications.