Impact of silicon incorporation on the formation of structural defects in AlN

Abstract

The impact of Si impurities on the structural properties of AlN, grown by plasma-assisted molecular-beam epitaxy on c-plane sapphire is studied. Under nitrogen-rich growth conditions silicon can be homogeneously incorporated up to Si concentrations of [Si]=5.2×1021 cm−3. The presence of silicon on the surface during the growth process is demonstrated to be beneficial for the surface morphology and the structural properties of the AlN films. For [Si] up to (5±3)×1020 cm−3, this surfactant behavior results in a decrease of the surface roughness from 8 nm for undoped layers grown in a nitrogen-rich regime to less than 1 nm. In addition, high resolution x-ray diffraction studies reveal an increase of the average lateral crystal size from 300 nm to more than 1 μm and a simultaneous decrease of the screw dislocation density from 3.8×108 cm−2 for (comparably) weakly doped samples to 2×107 cm−2. At the same [Si] the heterogeneous stress shows a minimum of less than 50 MPa and drastically increases for higher [Si]. The analysis of edge dislocations as a function of [Si] reveals that their density is directly related to Si-induced compressive biaxial stress which increases up to (2.0±0.15) GPa, independently determined by x-ray diffraction and Raman spectroscopy. While edge dislocations are found to be strongly correlated with the release of stress, screw dislocations are formed due to the coalescence of AlN islands with different stacking order, as their density is decreasing with increasing lateral crystal size. For AlN films with [Si]<1.2×1021 cm−3, a Poisson ratio ν=0.525±0.022 is determined.