First-principles calculation of the order-disorder transition in chalcopyrite semiconductors (original) (raw)
We describe the polymorphic order-disorder transition in the chalcopyrite-type semiconductor Cuo. slnosSe 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, =1 l25+'20 K) compares well with experiment (T, =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. Recent theory' and observations of spontaneous long-range order in isovalent III-V semiconductor alloys created interest in the theoretical implications on selforganization in random systems and in the technologically attractive possibility of changing the optical band gaps of random alloys at frxed composition through ordering. ' The relatively weak interactions between the isovalent atoms in such III-V alloys lead, however, to a small enthalpy difference between the disordered and ordered bulk phases (bH (0.5 kcal/mole), so the driving force for the transition is dominated by surface energetics. This leads to imperfect ordering and irreversibility that complicates the study of the transitions. There is, however, a large class of tetrahedrally bonded semiconductorsthe A 'B "'C '2 chalcopyrites"where the stronger interactions between the nonisovalent A'-B"' atoms (reflected in much larger latent heat BH of 2-3 kcal/mole) leads to reversible order-disorder transitions observable at conveniently higher temperatures even in bulk crystals. These ternary A'B"'C"'2 chalcopyrites (e.g., CuInSe2) undergo as a function of temperature a first-order phase transition between the high-temperature disordered zinc-blende-like (ZB) phase and the ordered chalcopyrite (CH) structure. Depending on the system, the disordering transition occurs in the temperature range" ' T,-800-1300 K, is accompanied by an abrupt disappearance of the zinc-blende-forbidden x-raydiffraction peaks, " a large (0.1-0.5 eV) reduction in the semiconducting band gaps' ' and marked changes in the short-range order seen in NMR studies. ' Since the disordered phase contains cross substitutions between nonisovalent A '-B"' atoms, it manifests donor-acceptor
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