Spectroscopic scanning capacitance microscopy as a nano-characterization technique for obtaining the electronic structure in an individual Ge nanodot

Introduction

The electronic structure of a nanostructure can be used to explain many of its electrical and optical properties and forms the foundation of understanding its device-related application. Scanning capacitance microscopy (SCM) is a nano-characterization metrology technique with high spatial resolution of the order of 10 nm. It utilizes a conducting cantilever with a small radius of curvature (typically < 20 nm) where the tunnelling current through the tip can be used to probe the local electronic structure of the underlying germanium (Ge) nanodot with sizes less than the Bohr radius.

Discussion

Prior measurements on Ge nanodots with sizes larger than the Bohr radius of Ge (~ 24.3 nm) did not result in any observable periodicities of the measured differential capacitance (dC/dV) spectra which would indicate that the electronic structure of the nanodot was probed. A decrease of the scan rate during measurements for these larger Ge nanodots only resulted in the reduction of the peak dC/dV magnitude as well as the hysteresis between the forward and reverse dC/dV sweep [Jpn. J. Appl. Phys. 48, 085002 (2009)]. This is due to the increased electron trapping in the larger sized Ge nanodot as a lower scan rate of the probe tip bias will lead to greater electron charging in both the nanodot and the defects in the Ge native oxide layer. Similarly, a decrease in the nonequilibrium charging between the applied dc bias sweep and the occupancy (charging) of the traps in the Ge native oxide layer at a lower scan rate could result in lesser hysteresis between the forward and reverse dC/dV sweep.

Conversely, the resulting periodicities of the measured dC/dV spectra with nearly constant peak heights was observed on a Ge nanodot (with size smaller than the Bohr radius of Ge) and was independent of the differences in the scan rate of the dc probe tip bias. The discrete charging states are interpreted by considering the electronic states in the Ge nanodot. In addition, the periodicities of the dC/dV spectra is strongly dependent on the shape of the nanodot where the periodic structure in the dC/dV spectra are interpreted as the s-like ground state, the first excited p-like state and the second excited d-like state [Figs. 4 (a) and 4 (b) of Jpn. J. Appl. Phys. 48, 085002 (2009)] for a pyramidal-shaped Ge nanodot. Importantly, a deformation of the nanodot (for an ellipsoidal-shaped nanodot) may lead to the deformation of the spatial confinement potential inside the nanodot, thus leading to the symmetry breaking or splitting of the degeneracies of the energy levels.

The corresponding filling of the energy levels/states in the Ge nanodot take place between the two oxide layers (GeO_x and SiO₂) which act as high potential barriers for electron confinement in the conduction band of the Ge nanodot as the electrons are injected into the nanodot from the negative biased platinum probe tip. When the dc probe tip bias, V_tip is varied, the Fermi level is shifted with respect to the quantized energy levels in the Ge nanodot which leads to the sequential filling of the different energy levels inside the nanodot and starts from the lowest energy level in the nanodot [Fig. 4 (c) of Jpn. J. Appl. Phys. 48, 085002 (2009)]. The electron population in the nanodot increases as the probe tip negative bias increases.

Since V_tip corresponding to the dC/dV peak positions are associated with the charging of the different energy states in the Ge nanodot, hence the voltage bias scale in the dC/dV spectra can be converted into an energy scale (eV) which represents the different energy levels/states in the Ge nanodot. With an appropriate analytical expression which considers the size, shape of the probe tip and the nanodot, the spacing between the dC/dV peaks in the spectroscopic SCM measurements can be converted into the separation between the different energy states in the Ge nanodots. For further details and discussions on the results as well as the in depth descriptions of the experimental procedures, the reader is referred to Jpn. J. Appl. Phys. 48, 085002 (2009). The results in the article demonstrate that the high spatial resolution capability of the SCM technique could be suitably utilized as a viable nano-characterization tool which could provide useful information on the electronic structure in an individual Ge nanodot.

(a) Forward sweep (in blue) and reverse sweep (in red) of the SCM spectroscopic dC/dV spectra of the pyramidal-shaped Ge nanodot at the scan rate of 0.1 V/s. (b) Forward sweep (in blue) and reverse sweep (in red) of the SCM spectroscopic dC/dV spectra of the ellipsoidal-shaped Ge nanodot at the scan rate of 0.1 V/s. (c) Electronic structure of the two germanium nanodot with different shapes [K. M. Wong, Jpn. J. Appl. Phys. 48, 085002 (2009).]

Reference

Wong, Kin Mun. Study of the Electronic Structure of Individual Free-Standing Germanium Nanodots Using Spectroscopic Scanning Capacitance Microscopy Japanese Journal of Applied Physics, 48 (8), 085002 (2009). DOI: 10.1143/JJAP.48.085002