Nanometer-scale capacitance spectroscopy of semiconductor donor molecules

We review recently reported scanned-probe capacitance measurements of electrons entering silicon donors in a gallium-arsenide heterostructure. Single-electron peaks were observed in the capacitance-versus-voltage curves. The precise voltage position of the peaks varied with the location of the probe, reflecting a random distribution of silicon within the donor plane. In addition, three broader capacitance peaks were observed independent of the probe location, indicating clusters of electrons entering the system at approximately the same voltages. These broad peaks are consistent with the addition energy spectrum of donor molecules, effectively formed by nearest-neighbor pairs of silicon donors.