Structural and optical characterization of Ge nanocrystals showing large nonvolatile memories in metal-oxide-semiconductor structures

The structural and the optical properties of Ge nanocrystals (NCs) showing large capacitance-voltage hysteresis have been studied by using infrared photoluminescence (PL), high-resolution transmission electron microscopy (HRTEM), high-resolution X-ray diffraction (HRXRD). The memory effect strongly depends on the implant dose, the oxide thickness, and the annealing temperature in metal-oxide-semiconductor devices containing Ge NCs. Well-defined C-V characteristics with large hysteresis are found only for annealing temperatures ≥950 °C where Ge NCs are known to form. HRTEM demonstrates the existence of Ge NCs which are almost aligned at an average distance of about 6.7 nm from the SiO₂/Si interface. This suggests that the memory effect can be enhanced by accurately controlling the distribution of Ge NCs with respect to the Si/SiO₂ interface, although it is also influenced by implantation-induced deep-level defects and SiGe complexes formed at the interface, as confirmed by PL and HRXRD.