Study of the Electronic Structure of Individual Free-Standing Germanium Nanodots Using Spectroscopic Scanning Capacitance Microscopy

High spatial resolution spectroscopic scanning capacitance microscopy (SCM) measurements at room temperature based on the differential capacitance (dC/dV) versus probe tip-to-substrate bias characteristics, is directly used to characterize the electronic structure of a pyramidal and an ellipsoidal shaped germanium (Ge) nanodot (with physical sizes smaller than the Bohr exciton radius of Ge), among the arrays of Ge nanodots fabricated on a highly doped silicon substrate using an anodic alumina membrane as an evaporation mask. The ability to study the individual nanodots under the probe tip circumvents and isolates the statistical disorders encountered when studying large ensembles of size-dispersed nanodots. It is demonstrated that the spectra of the positive dC/dV peaks are reflective of the energetics of electron trapping in the quantized energy states inside the Ge nanodots. Furthermore, the spectroscopic SCM dC/dV profile is observed to be substantially influenced by the nanodot shape where the pyramidal Ge nanodot shows three distinct shells which are interpreted as the s-like ground state, the first excited p-like state and the second excited d-like state. On the other hand, the elliptical deformation of the ellipsoidal Ge nanodot breaks the shell structure which significantly affects its electronic structure. Using the spacing between the dC/dV peaks from the spectroscopic SCM measurements, an analytical expression is derived to calculate the energy separation between the different energy states in the Ge nanodots.