Memory and electroluminescence properties of silicon nanocrystal MOS-FETs

One of the main goals of materials research using ion beams is to synthesize nanostructures, for example semiconducting or metallic nanocrystals (NCs) in insulating films. A great effort is currently devoted to NC fabrication for micro- and optoelectronics by ion beam synthesis (IBS), because this method is compatible with modern CMOS technology. The present contribution addresses the Si NC formation by a non-conventional IBS approach of ion beam mixing of SiO2/Si interfaces in thin gate oxides [1], with special emphasis on well-controlled size and position tailoring as well as their application in non-volatile nanocrystal memories and in light emitting field-effect transistors (LEFET). Compared to conventional Si NC synthesis by Si+ ion implantation into the gate oxide [2], we take advantage of this self-alignment process, i.e., the Si NCs are formed in SiO2 at a well-controlled small distance of ca. 2 nm from the Si/SiO2 interfaces.
In order to implement our novel technique in CMOS technology, ion irradiation through a MOS-FET stack of 50nm poly-Si/15nm SiO2/Si substrate was performed with 50 keV Si+ ions. The ion beam mixing of the upper poly-Si/SiO2 interface and the lower SiO2/(001)Si interface leads to Si excess in the gate oxide. Subsequent rapid thermal annealing reforms sharp interfaces and separates the excess Si from SiO2. Adjacent to the recovered interfaces, 3-4 nm thick SiO2 zones denuded completely of excess Si have been found, whereas the more distant tails of excess Si form well-aligned narrow layers of NCs with 2-3 nm diameter. The self-alignment of the NC layers with the SiO2/Si interfaces allows to control shorter (direct) tunneling distances between the NCs and the Si electrodes with the potential of faster devices operating at reduced voltages.
The Si NC MOSFETs with an active gate area of 20x20 µm2 were fabricated as nMOSFET devices in a standard 0.6 µm CMOS process line. Their electrical characteristics have been evaluated in terms of write/erase voltage, duration of the programming time, endurance and retention for different ion irradiation and annealing conditions. For the investigation of the light-emitting characteristics of the same nMOSFETs, an AC voltage was applied to the gate in order to inject charges of both polarities into the NCs. AC voltage and frequency dependent electroluminescence spectra in the wavelength region of 400-1000 nm were recorded for different annealing conditions. The performance of the Si NC memories and LEFETs with further possibilities of optimization of efficient charge storage and light emission properties will be discussed.