Study of Electron Transport Characteristics through Self-Aligned Si-Based Quantum Dots

The charge storage in and conduction through nanometer-scaled Si-based quantum dots (QDs) have attracted considerable attention because of their importance in practical applications such as multi-valued memories and functional nano-devices with quantum transport. In this work, we focused on the electron transport properties through self-aligned Si-based QDs by means of atomic force microscopy (AFM) with conductive cantilever. For self-aligned Si-based QDs structures with ultra-thin oxide interlayer spontaneously formed on thermally-grown ~1nm-thick SiO2/Si(100) formed by LPCVD, the current bump and the negative differential conductance were clearly observable around -1.8 V when the substrate bias was swept from 0 to -4 V with the sweep rate of 0.3 V/sec. The peak to valley current ratio is as high as 100 although the second peak expected to appear at -3.4 V is not clearly visible. From the similarity between the measured and calculated current based on transfer matrix method, the observed current bump and negative differential conductance can be interpreted in terms of resonant tunneling mediated by quantized energy levels of the dots. As for the second peak, it is likely that an increase in the leakage current through the ~1.0 nm-thick SiO2 barrier smears out the current bump of the second peak. It is interesting to note that distinct current oscillation with a voltage period of ~18mV is observed in the first resonance current peak. The origin of this periodic current could be associated with electron phonon interaction in the quantum dot because the transverse acoustic (TA) phonon energy in crystalline Si at the edge of phonon Brillouin zone is 16meV. This implies that the emission and absorption of TA phonon in the self-aligned QDs can be responsible for the energy and momentum conservation in resonant tunneling.