Thermoelastic strain and plastic yielding in aluminum nitride on sapphire

The stress state of AlN/Al2O3 heteroepitaxial structures was probed by curv ature measurements. The structures were found to be concave at the depositi on temperature and to become convex or remain concave-depending on film thi ckness-on cooling to room temperature. The conclusion is drawn that film de position is accompanied by plastic deformation. The upper layers of the fil m may be unstressed. Cooling from the deposition temperature gives rise to elastic strains of opposite sign. An interplay between various contribution s determines the magnitude and sign of structure curvature. The calculated variation of strain with film thickness differs from that predicted by Ston ey's equation and is at variance with the present experimental data. (C) 19 99 American Institute of Physics. [S0021-8979(99)01703-X].