Nb-induced two-dimensional electron gas on $n-\mathrm{InAs}\mathrm{}\left(110\right):$ Anomalous coverage dependence

Angle-resolved photoelectron spectroscopy was used to determine the coverage dependence of the Nb-induced Fermi-level shift and the formation of a two-dimensional electron gas (2DEG) on n-type InAs(110). The maximum Fermi-level shift of 300 meV was achieved at a Nb coverage of 20%, which is a factor of 70 higher than expected from the surface doping model. Scanning tunneling microscopy images reveal the formation of Nb clusters (1–4 atoms) at room temperature, however, the resulting reduced Nb-Nb distance cannot explain the dramatically reduced ionization probability of the Nb atoms. We propose that hybridization of the donor levels in Nb clusters has to be taken into account to understand the coverage dependence. The coverage dependence of the 2DEG peak exhibits good agreement with a one-dimensional model of the confinement energy. A Nb coverage of 0.1% yields a 2DEG with only $1\times {10}^{11}$ $\mathrm{electrons}/{\mathrm{cm}}^2$ giving a filling factor of one in magnetic fields as low as 2 T.