Characterization of Electron Emission from High Density Self-Aligned Si-Based Quantum Dots by Conducting-Probe Atomic Force Microscopy

Self-aligned Si-quantum dots (QDs) with an areal density as high as ~1011 cm-2 were formed on ~1.0 nm thick SiO2/n-Si(100). For spatially-resolved characterization of electron emission from the aligned dots in a diode structure through Au top electrodes, current images were taken using atomic force microscopy in a non-contact mode while maintaining a distance of ~200 nm from the sample surface by using an Au-coated Si cantilever with DC negative bias application to an Al back contact with respect to the grounded Au top electrode at room temperature in atmosphere. In DC bias application of over -5.0 V, non-uniform current image contrast, which is attributable to local electron emission, emerged and enhanced with increasing applied bias. From the band diagram of the self-aligned dots structures at applied biases over -7 V, which is drawn by considering the dot size and oxide thickness evaluated using high resolution transmission electron microscopy observations, the quantized levels of the dots higher than the vacuum level of the top electrode and the tip are thought to be occupied by electrons, which can be interpreted in terms in which the tunneling rate between the lower to upper dots is enhanced as the electrons acquire high kinetic energies. Therefore, the observed electron emission implies quasi-ballistic transport through the aligned-dots structure.