(Invited) Fundamental Differences Between the Traditional III-V Compounds and Nitride Semiconductors

Abstract

In this talk I will address the fundamental question as to why the performance of emitters based on nitride semiconductors, grown heteroepitaxially on foreign substrates, is insensitive to high concentration of extended defects, which is not the case for traditional III-V compounds. I will discuss the following important differences between these two families of materials, which contribute to this finding: (a) the growth of Nitride semiconductors occurs at temperatures much lower than the half of their melting temperature, where the brittle to ductile transition occurs. In traditional III-V compounds the growth temperature is generally higher than ½ TM; (b) the chemical bonding in nitrides is strongly ionic while is mostly covalent in traditional III-V compounds. This leads to the bunching of the intrinsic surface states as well as the states associated with dangling bonds in edge dislocations near the band edges where they act as traps rather than recombination centers. Furthermore, the surface states have less effect on the surface Fermi level position; (c) the nitrides can exist in the wurtzite structure (equilibrium) and the cubic structure (metastable) and the enthalpy of formation of the two allotropic forms differs by only a few meV. Thus, conversion between the two phases occurs easily by creation of stacking faults along the close-packed (0001) and (111) planes, which due to the smaller energy gap of the cubic phase leads to strong band structure potential fluctuations in the wurtzite matrix; (d) additional band structure potential fluctuations exist in the nitride alloys due to various forms of partial alloy ordering or to compositional inhomogeneities introduced by phase separation or the growth mode, These potential fluctuations contribute to exciton localization and thus efficient radiative recombination.