  We investigate the mechanical properties of freely suspended nanostrings
fabricated from tensile-stressed, crystalline In$_{1-x}$Ga$_{x}$P. The
intrinsic strain is a consequence of the epitaxial growth given by the lattice
mismatch between the thin film and the substrate which is confirmed by x-ray
diffraction measurements. The flexural eigenfrequencies of the nanomechanical
string resonators reveal an orientation dependent stress with a maximum value
of 650 MPa. The angular dependence is explained by a combination of anisotropic
Young's modulus and a change of elastic properties caused by defects. As a
function of the crystal orientation a stress variation of up to 50 % is
observed. This enables fine tuning of the tensile stress for any given Ga
content $x$, which implies interesting prospects for the study of high Q
nanomechanical systems.
