Investigation of ternary nitride semiconductor alloys by scanning tunneling microscopy

In this thesis different lattice-matched Al$_{1-x}$In$_{x}$N/GaN heterostructures were investigated with the aim to deduce strain, compositional fluctuations, defects, and electronic properties on cross-sectional $\textit{m}$-plane Al$_{1-x}$In$_{x}$N cleavage surfaces. The electronic properties of Al$_{1-x}$In$_{x}$N($10\overline{1}$0) surfaces were investigated by cross-sectional scanning tunneling spectroscopy in combination with density functional theory calculations. The calculations revealed empty Al and/or In-derived dangling bond states at the surface, which were calculated to be within the fundamental bulk band gap for In contents smaller than 60%. For In contents of ${x}$ = 0.19 and ${x}$ = 0.20, the energy of the lowest empty In-derived surface state was extracted from tunnel spectra acquired on Al$_{1-x}$In$_{x}$N($10\overline{1}0$) cleavage surfaces to be E$_{C}$ - 1-82 $\pm$ 0.41 eV and E$_{C}$ - 1.80 $\pm$ 0.56 eV, respectively, in good agreement with the calculated energies. In addition, a polarity dependent Fermi-level pinning of the surface state was identified. Based on these results it was concluded, that under growth conditions the Fermi level is pinned by the In-derived dangling bond state for In contents smaller than about 60%. For larger In contents no Fermi level pinning is present. In order to fit simulations to the experimentally obtained tunnel spectra, an average value of the electron affinity $_{\chi AllnN}$ of 3.5 $\pm$ 0.1 eV was necessary. A thorough literature survey of theoretically and experimentally obtained values of the electron affinities of GaN, AlN, and InN revealed two issues. First, a broad range of values was reported for the electron affinities with deviations of more than 50%. Second, [...

Meandering of overgrown v-shaped defects in epitaxial GaN layers

The meandering of v-shaped defects in GaN(0001) epitaxial layers is investigated by cross-sectional scanning tunneling microscopy. The spatial position of v-shaped defects is mapped on (101¯0) cleavage planes using a dopant modulation, which traces the overgrown growth front. Strong lateral displacements of the apex of the v-shaped defects are observed. The lateral displacements are suggested to be induced by the meandering of threading dislocations present in the v-shaped defects. The meandering of the dislocation is attributed to interactions with inhomogeneous strain fields