Formation and Characterization of Silicon- Quantum-Dots/Metal-Silicide-Nanodots Hybrid Stack and Its Application to Floating Gate Functional Devices

The use of high density nanodots (NDs) as a floating gate (FG) in Metal-Oxide-Semiconductor (MOS) memories has been attracting much attention because of its potential advantages over conventional planar FG memories. Semiconducting nanodots can show well-discrete charged states due to quantum confinement and Coulomb blockade effect, which leads to multi-valued memory capability [1]. On the other hand, metallic NDs with an appropriate work function can provide a deep potential well in charge storage, which results in practically large memory window [2]. To satisfy both multiple valued capability and charge storage capacity for a sufficient memory window and to open up novel functionality, we have proposed and fabricated a hybrid nanodots FG in which Si quantum dots (QDs) and metal silicide nanodots are stacked with a very thin SiO2 interlayer [3]. In this paper, our recent achievement on functional hybrid NDs FG consisting Si quantum dots (Si-QDs) and silicide NDs has been reviewed, in which not only charge injection characteristics of FG MOS devices but also their infrared optical response are reported. Hemispherical Si-QDs were grown firstly on an ultrathin SiO2 layer by controlling the early stages of LPCVD of pure SiH4 at 580°C. The average dot height and the areal dot density, which were evaluated by AFM observations, were typically ~5nm and ~3.5x10cm, respectively. High density silicide-NDs was prepared by full-silicidation of pre-grown Si-QDs, in which the Si dot surface was covered with either an ultrathin Ni layer by electron beam evaporation or an ultrathin Pt layer by Ar sputtering, and followed by a remote H2 plasma exposure without external heating. The silicidation of Si-QDs promoted by such a remote H2 plasma treatment was confirmed from chemical shifts in photoemission spectra of core lines and distinct changes in valence band spectrum, and the work function values of the NiSiand PtSi-NDs were determined to be 4.53±0.05eV and 5.11±0.05eV, respectively, from the cut-off energy in photoemissions. Also, the electrical separation among the silicide-NDs as well as Si-QDs was confirmed from the surface potential changes due to electron injection into and emission from the NDs by scanning an electrically-biased AFM tip in a tapping mode. For the Si-QDs/silcide-NDs hybrid stack, a process step to form a ultrathin SiO2 interlayer between Si-QDs and silicide-NDs was added, where thermal oxidation was performed on Si-QDs surface and remote plasma CVD on silicide-NDs. After the formation of NDs hybrid stack structure, a ~20nm SiO2 layer as a control oxide was formed at 350°C by inductivelycoupled remote plasma CVD with SiH4 and excited O2/Ar. Al top electrodes with 1mm in diameter for MOS capacitors and n poly-Si gates with a size of 0.5 x 10μm for MOSFETs were fabricated. In electron charging and discharging characteristics measured with application of pulsed gate biases to MOS capacitors with a NDs FG of a hybrid stack structure consisting of Ni-silicide nanodots and Si-QDs, stepwise changes in the rates for electron injection and emission were revealed with increasing pulse width at room temperature. In addition, nMOSFETs with a hybrid NDs FG, in which Pt-silicide NDs were stacked on Si-QDs covered with an ultrathin SiO2 interlayer, show unique hysteresis with stepwise changes in the drain current gate voltage characteristics as seen in Fig. 1. These observed characteristics of MOS capacitors and FETs with such hybrid NDs FGs are attributable to the electron injection and storage into silicide-NDs through Si-QDs. Namely, the discrete energy states of the Si-QDs [5] limit the change in the electron number stored in the silicide-NDs. For MOS capacitors with a triple-stacked hybrid NDs FG by adding another Si-QDs [6], with 1310nm (~0.95eV) light irradiation, a distinct infrared optical response in capacitance voltage characteristics was detected, which can be interpreted in terms of the shift of charge centroid in the hybrid FG stack due to transfer of photoexcited electrons from silicide-NDs to the Si-QDs.