Influence of the Ge Dose in Ion-implanted SiO2 Layers on the Related Nanocrystal-memory Properties
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0933-G02-05
Influence of the Ge Dose in Ion-implanted SiO2 Layers on the Related Nanocrystalmemory Properties Sébastien Duguay, Jean-Jacques Grob, Abdelilah Slaoui, and Philippe Kern InESS, CNRS/ULP, 23 rue du Loess, Strasbourg, france, 67037, France
ABSTRACT Thin silicon dioxide (SiO2) on Si layers with embedded germanium nanocrystals (Gencs) were fabricated using 74Ge+-implantation at 15 keV and subsequent annealing. Transmission electron microscopy and Rutherford backscattering spectrometry have been used to study the Ge redistribution in the SiO2 films as a function of implantation dose under specific annealing conditions. At low implantation doses, Germanium is found to segregate at the Si/SiO2 interface leading to poor electrical properties. At higher doses and when the disorder limit of one displacement per atom is reached at the interface, transmission electron microscopy revealed the formation of a Ge-nc layer array located close to the Si/SiO2 interface and an another one inside the SiO2 host material. This near-interface high density (>1012 ncs/cm2) nc-layer is found to act as a floating gate embedded within the silicon dioxide. Capacitance-voltage measurements performed on metal-oxide-semiconductor structures containing such implanted SiO2 layers show significant memory properties (few volts hysteresis) at low programming voltages (4V->-3V -4V->4V->-4V
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Figure 5. C-V for 1x1016 at/cm2 (left) and 2x1016 at/cm2 (right) Ge implanted SiO2 layers showing hysteresis due to the presence of near-interface Ge-nc layers. Both implanted layers show an important memory window witnessing the charge retention within the SiO2 layer. The low voltage used to charge the layers indicates that the charge is stored close to the Si/SiO2 interface, i.e. in our case, in the near-interface nc-layer. The
memory window measured for the 2x1016 at/cm2 is much larger than for the 1x1016 at/cm2. This cannot be simply explained by the difference in nc density between both samples. Hence, we may speculate that some trap mechanisms due to the interface mixing may play a role. CONCLUSIONS To summarize, the redistribution of implanted-Ge into such thin SiO2 layers is found to be strongly dependent on the implantation dose and energy, and initial oxide thickness. The conditions required to form a single near-interface nanocrystal layer are critical. At a given SiO2 thickness, a minor change in the ion implantation energy can completely modify the Ge redistribution. A threshold situated between 0.8 and 1.5 dpa at the interface. When this threshold is reached, a single layer of Ge nanocrystals is formed near the Si/SiO2 interface. Moreover, an increase of the dose leads to the formation a bulk nc-layer in addition to the desired near-interface layer. Associated electrical characteristics show that these nanocrystal layers can be charged at low programming voltages, leading to
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