BREAKDOWN OF CONTINUUM ELASTICITY THEORY IN THE LIMIT OF MONATOMIC FILMS

We demonstrate that the predictions of continuum elasticity theory fail in the ultimate limit of monolayer films. We directly measure the lattice distortion of ultrathin InAs layers in GaAs by high-resolution electron microscopy. For InAs films or 3 monolayer thickness, the observed tetragonal distortion agrees with the prediction of elasticity theory. For single InAs monolayers, however, the measured strain is much higher than expected. The InAs unit cell in this case is strained such as to conserve the bulk bond length at the interface.