  We report measurements of the rectification of microwave radiation (0.7-20
GHz) at the boundary between two-dimensional electron systems separated by a
narrow gap on a silicon surface for different temperatures, electron densities
and microwave power. For frequencies above 4 GHz and different temperatures,
the rectified voltage V_{dc} as a function of microwave power P can be scaled
onto a single universal curve V*_{dc}=f*(P*). The scaled voltage is a linear
function of power, V*_{dc} ~ P* for small power and proportional to P*^{1/2} at
higher power. A theory is proposed that attributes the rectification to the
thermoelectric response due to strong local overheating by the microwave
radiation at the boundary between two dissimilar 2D metals. Excellent agreement
is obtained between theory and experiment.
