Nonvolatile memory characteristics in metal-oxide-semiconductor containing quantum dots fabricated by unique laser irradiation method

Abstract

Quantum dots (QDs) floating gate memory is considered a potential candidate for the replacement of conventional continuous floating gate memory since the latter has reached tunneling limits through continuous downscaling. Many kinds of QDs memory devices, such as Si, Ge, and metals have demonstrated improved performance, such as higher programming/erasing speed or longer retention times. Among these materials, it is known that metal QDs have the advantages of a higher density of states around the Fermi level, a wide range of available work functions, and smaller energy perturbation compared to their semiconductor counterparts. However, the drawback of using metal QDs is the metal/oxide reaction or metal diffusion which can occur during high-temperature processes in device integration, such as source/drain/gate dopant activation. To solve this issue, materials like TaN or silicide, which have both good metal characteristics and good thermal ability of the QDs, have been reported. Among them, cobalt silicide has merits of metal characteristics and shows little reactivity with metal/oxide interfaces. This letter present the performance of cobalt silicide (CoSi₂) QDs as discrete charge traps in metal oxide semiconductor (MOS) capacitor devices embedded in thin HfO2 tunneling and control oxides layers deposited ALD and sputter. The CoSi₂QDs were synthesized without a post-annealing process by exposure of Co/Si/HfO₂ tunneling oxide/Si stacks to an external UV laser. The thicknesses of the Co and Si layers were intentionally controlled to obtain ideal CoSi₂QDs. Transmission electron microscopy clearly confirms formation of CoSi₂QDs. And observations from x-ray photoelectron spectroscopy showed that silicide’s values correspond to the Co-Si bonding energies that are shifted 0.3 eV from original values. These CoSi₂QDs in MOS devices exhibited a memory window of 3.4 V under +11 V/ -9 V in capacitance-voltage curves as well as efficient programming/erasing speeds and good retention and endurance times, thereby demonstrating that they show promise for nonvolatile QDs memory applications.