During the past few years remarkable progress has been made in the development of optical and electronic devices based on the group-III nitrides. Light emitting diodes, laser diodes, ultraviolet detectors, Bragg reflectors, high-power transistors, etc were fabricated employing III-nitrides and their ternary alloys layers.
In contrast to the cubic III-V arsenides and phosphides where ordering was previously observed, the III-nitrides have the hexagonal wurtzite structure. The first observation of atomic long range ordering in AlGaN and InGaN thin films grown by electron cyclotron resonance assisted molecular beam epitaxy on sapphire and 6H-SiC substrates was reported recently. The observed superlattice exists in a form of ideal, stochiometric, perfectly ordered alloy.
The electronic band structure of ternary III-nitrides random alloys and ordered structures was investigated using model empirical pseudopotential dependent on local strain. 32-atom 2x2x2 wurtzite supercell with periodic boundary conditions was adopted to describe different atomic configurations and bond length relaxations.
Imaginary and real part of the dielectric function and reflectivity of the disordered and ordered III-nitride alloys were calculated. The effect of substrate was investigated using the biaxial strain. It is shown that the atomic long range order changes the band gap and energy positions of the main peaks of ε2(hν) much more stronger than in conventional III-V ternary alloys.