Progress on Charge Distribution in Multiply-Stacked Si Quantum Dots/SiO2 Structure as Evaluated by AFM/KFM

Abstract

Multiply-stacked structures of Si quantum dots (Si-QDs) in gate oxide are attracting much attention because of their potential importance to improve retention characteristics in a high density charge storage. In this work, we have fabricated 6-fold stacked Si-QDs with 2nm-thick SiO2 interlayers, whose areal dot density and average dot size were 5.7×1011cm-2 in each dot layer and -5nm in height, and studied progress on electron distribution in 6-fold stacked Si-QDs with 2nm-thick SiO2 interlayers from the measurements of temporal changes in the surface potential after electron charging and discharging locally at room temperature using an AFM/Kelvin probe technique in clean room air. First, by scanning an area of 2×2μm2 with the AFM tip biased at +3V with respect to the substrate in a tapping mode, the area was negatively charged due to electron injection from the substrate to the dot through the bottom tunnel oxide and subsequently, the central part of 100×100nm2 in the pre-charged area was scanned with the tip biased at -3V to emit the electrons from the Si-QDs to the substrate. As a result, the negative charging level was markedly reduced in the central part in comparison to its peripheral region. And then, the surface potential of the negatively-charged peripheral region was decay monotonously with time as a result of progressive electron tunneling to the substrate. In contrast to this, the temporal change in the surface potential of the central part shows that the electron charging proceeds with time until the surface potential becomes almost the same as that in the peripheral region. This result can be interpreted in terms of lateral spreading of electrons stored in the Si-QDs layer due to the potential difference between the central part and its peripheral region more negatively charged.