Comparison between the electronic dielectric functions of a GaAs/AlAs superlattice and its bulk components by spectroscopic ellipsometry using core levels

Unveiling Variations in Electronic and Atomic Structures Due to Nanoscale Wurtzite and Zinc Blende Phase Separation in GaAs Nanowires

Phase separation is an intriguing phenomenon often found in III-V nanostructures, but its effect on the atomic and electronic structures of III-V nanomaterials is still not fully understood. Here we study the variations in atomic arrangement and band structure due to the coexistence of wurtzite (WZ) and zinc blende (ZB) phases in single GaAs nanowires by using scanning transmission electron microscopy and monochromated electron energy loss spectroscopy. The WZ lattice distances are found to be larger (by ∼1%), along both the nanowire length direction and the perpendicular direction, than the ZB lattice. The band gap of the WZ phase is ∼20 meV smaller than that of the ZB phase. A shift of ∼70 meV in the conduction band edge between the two phases is also found. The direct and local measurements in single GaAs nanowires reveal important effects of phase separation on the properties of individual III-V nanostructures.

Elemental mapping using the Ga 3d and In 4d transitions in the ε2 absorption spectra derived from EELS

It is proposed that by using the valence-band states in electron energy loss spectroscopy, high-spatial resolution maps of quantitative elemental composition may be acquired with high acquisition rates. Further, it is shown that by using the epsilon(2) spectrum instead of single scattering data, the noise in the observed transitions and associated maps is significantly reduced. The epsilon(2) spectra are derived through a Kramers-Kronig transformation from electron energy loss spectra obtained in a scanning transmission electron microscope. Using transitions that occur in the epsilon(2) absorption spectrum (<40eV), quantitative elemental maps for III-V device structures have been produced. An example is provided using the Ga 3d transition to map a GaInNAs/GaAs laser structure. Weaker transitions such as In 4d have also been used to verify the Ga elemental distribution.

Ab Initio calculations of the anisotropic dielectric tensor of GaAs/AlAs superlattices

The static dielectric properties of (001)(GaAs)(p)/(AlAs)(p) superlattices have been calculated as a function of their period p for 1< or = p < or =12, starting from density-functional theory. The interplay between quantum confinement and local field effects is shown to be crucial. For light polarized in the growth direction it leads to the otherwise surprising justification of the use of a classical effective medium theory, even for the smallest periods. Only the inclusion of both contributions allows in ab initio and in semiempirical calculations to reproduce the experimentally observed birefringence.