In the present work we have studied an extension of the classical thermal rectification, arising in certain cases from the contact of two dissimilar bulk materials with different temperature dependence of the thermal conductivity, to the Si-Ge system when boundary scattering effects are taken into account. Moreover, the directionality of the in-plane heat flow in a Si plate can be achieved by tuning the thickness and the impurity concentration along the cross section of the plate. We have designed several potential structures with this function in mind and discussed the physics behind.