TEM study of defects in Al_xGa_(1-x)N layers with different polarity

摘要

Transmission electron microscopy (TEM) studies of defects in Al_xGa_(1-x)N layers with various Al mole fractions (x=0.2, 0.4) and polarities were carried out. The samples were grown by ammonia molecular beam epitaxy on sapphire substrates and consisted of low-temperature A1N (LT-AIN) and high-temperature A1N (HT-AIN) buffer layers, a complex AIN/AlGaN superlattice (SL) and an Al_xGa_(1-x)N layer (x=0.2, 0.4). It was observed that at the first growth stages a very high density of dislocations is introduced in both Al-polar and N-polar structures. Then, at the interface of the LT-AIN and HT-AIN layers half-loops are formed and the dislocation density considerably decreases in Al-polar structures, whereas in the N-polar structures such a behavior was not observed. The AIN/AlGaN superlattice efficiently promotes the bend and annihilation of threading dislocations and respectively the decrease of the dislocation density in the upper Al_xGa_(1-x)N layer with both polarities. The lattice relaxation of metal-polar Al_(0.8)Ga_(0.2)N was observed, while N-polar Al_(0.8)Ga_(0.2) did not relax. The dislocation densities in the N-polar Al_(0.8)Ga_(0.2)N and Al_(0.4)Ga_(0.6)N layers were 5.5 × 10~9 cm~(-2) and 9 × 10~9 cm~(-2), respectively, and in metal-polar Alo^Gao^N and Alo.4Gao.6N layers these were lxl010cm"2 and 6 × 10~9 cm~(-2), respectively. Moreover, from TEM images the presence of inversion domains (IDs) in N-polar structures has been observed. The widths of IDs varied from 10 to 30 ran. Some of the IDs widen during the growth of the A1N buffer layers. The IDs formed hills on the surface of the N-polar structures.