First-principles calculation of the order-disorder transition in chalcopyrite semiconductors

We describe the polymorphic order-disorder transition in the chalcopyrite-type semiconductor Cu_0.5In_0.5Se 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±10 K) compares well with experiment (T_c=1083 K). Unlike the analogous phenomena 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.