Stability, electronic and vibrational properties of GaAlN wurtzoid molecules and nanocrystals: A DFT study

Electronic structure and vibrational characteristics of GaAlN wurtzoids at the molecular-nanoscale limit are investigated. The investigated properties include energy gap, vibrational properties, and phase stability against transition to GaAlN diamondoids. At the molecular-nanoscale limit, wurtzite GaAlN nanocrystals are represented by wurtzoids. The properties of GaAlN molecules and nanocrystals as a function of Ga and Al contents are shown. The results show that the energy gap depends on size, shape and surface conditions. Molecules with hydrogen passivated surface have wide energy gap while bare molecules have smaller energy gaps. On the other hand, vibrational longitudinal optical mode of bare molecules experiences a redshift after surface hydrogen passivation with smaller vibrational reduced masses and force constant at a given frequency. Differences between GaAlN two limits (GaN and AlN) are explained by the existence of d orbitals in Ga atoms. Calculated Gibbs free energy of atomization show the stability of wurtzoids against transition to the diamondoids at the molecular-nanoscale limit for both bare and hydrogen passivated cases.