Electrical characteristics of SiO2/ZrO2 hybrid tunnel barrier for charge trap flash memory

Abstract

In this paper, we investigate the electrical characteristics of SiO2/ZrO2 hybrid tunnel oxide in metal–Al2O3–SiO2–Si3N4–SiO2–silicon (MAONOS) structure in an effort to improve program and erase speed as well as retention characteristics. Inserting ZrO2 into the conventional MAONOS structure increased the programmed Vth variation to 6.8 V, and increased the erased Vth variation to −3.7 V at 17 MV/cm. The results can be understood in terms of reducing the Fowler–Nordheim (F/N) tunneling barrier due to high-k ZrO2 in the tunneling oxide. In addition, Zr diffusion in SiO2 caused the formation of ZrxSi1−xO2 at the interface region, which reduced the energy band gap of SiO2. The retention property of the hybrid tunnel oxide varied depending on the thickness of SiO2. For thin SiO2 less than 30 Å, the retention properties of the tunneling oxides were poor compared with those of the SiO2 only tunneling oxides. However, the hybrid tunneling oxides with SiO2 thickness thicker than 40 Å yielded improved retention behavior compared with those of the SiO2-only tunneling oxides. The detailed analysis in charge density of ZrO2 was carried out by ISPP test. The obtained charge density was quite small compared to that of the total charge density, which indicates that the inserted ZrO2 layer serves as a tunneling material rather than charge storage dielectric.