The Ge(001) surface is known to undergo phase transitions from c(4 x 2) to (2 x 1) at low temperature and from (2 x 1) to (I x 1) at high temperature. In the former case the phase transition is shown to occur at 240 K and to fall into the 2D Ising universality class. The high temperature phase transition takes place above 900 K and is characterized by (2 x 1) domain wall proliferation, thus indicating an order-disorder character. We have also observed an increase in the density of steps, but these are shown to be only partially involved in the disordering of the (2 x 1) phase.
