Structural properties and electronic states of carbon delta-layers in GaAs

We present a cross-sectional STM study of delta-doped GaAs on the nanoscale. GaAs (001) layers frown by MBE with a series of embedded carbon delta-layers having nominal sheet densities in the range 3x10(11)/cm(2) to 1X10(14)/cm(2) were investigated. The dopant concentrations were checked with SIMS. The carbon concentration profiles in growth direction were determined, and found to be atomically sharp and symmetric up to the highest concentrations, with a FWHM of 0.85nm. This shows that even at the elevated growth temperature of 860K and up to 1x10(14)/cm(2) density neither segregation nor diffusion plays a significant role in carbon delta-doping of GaAs. The observed residual spreading of the profile is attributed to the generic surface roughness during epitaxial growth. In a second step we analyzed in detail the spectra in the delta-layer regions. In the vicinity of near surface acceptors we found a nonvanishing conductivity throughout the band gap energy range. This is markedly different from the discrete LDOS peak found in the band gap for isolated acceptors in bulk samples, and it indicates metallic conductivity. Inhomogeneities of the metallic conductance are found to be strongly correlated with the arrangement of dopant atoms in the delta-layer.