1ST-PRINCIPLES CALCULATION OF THE ORDER-DISORDER TRANSITION IN CHALCOPYRITE SEMICONDUCTORS

We describe the polymorphic order-disorder transition in the chalcopyrite-type semiconductor Cu0.5In0.5Se through a Monte Carlo simulation of a generalized Ising Hamiltonian whose interaction energies are determined from ab initio total-energy calculations. The calculated transition temperature (T(c) = 1125 +/- 20 K) compares well with experiment (T(c) = 1083 K). Unlike the analogous ordering in isovalent III-V alloys, we find that the transition is dominated by electronic compensation between donor and acceptor states, leading to strong correlations in the disordered phase, and a decrease in the optical band gap upon disordering.