It is not so much a question of strength as of intimacy. An intimate contact is necessary for electron or hole transfer across a semiconductor interface. An intimate contact means that the semiconductor surface should be within a tunnelling distance of the substrate surface.
it is question of metal/semi-conductor interface mechanisms such as Contact Métal/SC: diode Schottky or Tunnel contacts. more information in http://ecee.colorado.edu/~bart/book/book/chapter3/ch3_5.htm