Single domain structure of 2H-AlN films on Si(001) substrates

Abstract

Aluminum nitride (2H-AlN) films were grown by plasma-assisted molecular beam epitaxy (MBE) on Si(001). By conventional (CTEM) and high resolution transmission electron microscopy (HRTEM) investigations the influence of the off-axis angle of the substrate surface on the film structure was studied. Three types of Si(001) substrates were used: on-axis, ∼1°, and ∼5° off-axis. The 2H-AlN layer on an exact oriented Si(001) substrates consists of 3 AlN film domains: two main film domains, AlN _I and AlN _II, and a small domain AlN _III at substrate surface defects. Their c-axis orientations are parallel to the c-axis of the substrate: [0001]AlN _{I, II, III} ∥ [001]Si. The a-axes of AlN _I and AlN _II rotated by 30° to each other: [11 20]AlN _II∥[01 10]AlN _II ∥ [1 1 0]Si. The orientation of AlN _III is [01 10]AlN _III ∥ [100]Si. In 2H-AlN films grown on off-axis Si(001) substrates (∼1° and ∼5°) the ratio between the AlN _I and AlN _II film domains changes dramatically as far as a single domain film structure consisting of mainly AlN _I is reached. The AlN c-axes of all domains on the off-axis substrates are not parallel to the Si c-axis but tilted by the off-axis angle of the Si(001) substrate (∼1° respectively ∼5°), i.e. [0001]AlN is parallel to the Si(001) substrate surface orientation. Determination of the AlN _I domain / Si(001) interface structure by HRTEM illuminates the origin of the preference of this domain in the 2H-AlN film by using off-axis Si(001) substrates. On the on-axis substrate a regular array of misfit dislocations causes a 5:4 fit between the (1 1 00)AlN and ( 1 1 0)Si lattice planes. The off-axis Si(001) leads to a rotation of the AlN lattice in respect of the Si lattice. An array of misfit dislocations with a 4:3 fit between (1 1 01)AlN and ( 1 1 1)Si lattice planes decreases the residual lattice misfit from -1.6% to -0.8%.