Thermoelastic strain and plastic yielding in aluminum nitride on sapphire

The stress state of AlN/Al2O3 heteroepitaxial structures was probed by curvature measurements. The structures were found to be concave at the deposition temperature and to become convex or remain concave-depending on film thickness-on cooling to room temperature. The conclusion is drawn that film deposition is accompanied by plastic deformation. The upper layers of the film may be unstressed. Cooling from the deposition temperature gives rise to elastic strains of opposite sign. An interplay between various contributions determines the magnitude and sign of structure curvature. The calculated variation of strain with film thickness differs from that predicted by Stoney's equation and is at variance with the present experimental data. (C) 1999 American Institute of Physics. [S0021-8979(99)01703-X].