Investigation GaN(100) surface (1x4)-(1x1) phase transition

In this thesis, we study the critical behaviour of the order disorder phase transition for a GaN(100) surface using cellular automaton (CA) simulations on spin-1 Ising model. As is known, GaN (100) exhibits (1x4), c(2x2), (2x2) and ms(2x2) reconstructions. Also, the total energy calculations for these surfaces shows that (1x4) reconstruction is favorable than other phases. Therefore, firstly, the spin-1 Ising model Hamiltoinan for (1x4) ground state has been obtained using the results of total energies on 4x4 super cells. Ising model simulations show that GaN(100) surface exhibits a second order phase transition to other surfaces from (1x4) surface with temperature as expected. The phase transition temperature (Tc) to other surfaces from (1x4) surface has been obtained from the peaks of susceptibilty and specific heat on finite rectangular lattice for linear dimensions Lx=60, 90, 120 and Ly=Lx, 2Lx,?,10Lx with periodic boundary condition. The obtained surface images from simulations indicated that phase transition on GaN (100) surface is to 1x1 from 1x4.Key Words : GaN(100), Density Functional Theory, Ising Model