Memory characteristics of Al₂O₃/La₂O₃/SiO₂ multi-layer structures for charge trap flash devices

The Charge Trap Flash (CTF) memory device is a replacement candidate for the NAND Flash device. In this study, Pt/Al₂O₃/La₂O₃/SiO₂/Si multilayer structures with lanthanum oxide charge trap layers were fabricated for nonvolatile memory device applications. Aluminum oxide films were used as blocking oxides for low power consumption in program/erase operations and reduced charge transports through blocking oxide layers. The thicknesses of SiO₂ were from 30 Å to 50 Å. From the C-V measurement, the largest memory window of 1.3V was obtained in the 40 Å tunnel oxide specimen, and the 50 Å tunnel oxide specimen showed the smallest memory window. In the cycling test for reliability, the 30 Å tunnel oxide sample showed an abrupt memory window reduction due to a high electric field of 9~10MV/cm through the tunnel oxide while the other samples showed less than a 10% loss of memory window for 10⁴ cycles of program/erase operation. The I-V measurement data of the capacitor structures indicated leakage current values in the order of 10^-4 A/cm² at 1V. These values are small enough to be used in non-volatile memory devices, and the sample with tunnel oxide formed at 850°C showed superior memory characteristics compared to the sample with 750°C tunnel oxide due to higher concentration of trap sites at the interface region originating from the rough interface.