Hafnium-based thin oxides: Versatile insulators for microelectronics

Hafnium-based oxides are becoming more and more popular in the field of memory technology. Apart from their proved ability of reducing the leakage current in transistors and nanomemories, HfO2 and related oxides present an almost unique property: Resistive Switching, which makes them a promising candidate for building up contemporary RAM (Random Access Memory) cells. In such cells, information thereof is not recorded by storing charge, but the memory point relies on resistivity changes. A low resistive state (LRS) and high resistive state (HRS) of a conductive filament (CF) will be interpreted as logic '0' and '1', respectively. Resistive-RAM is especially interesting as it presents excellent compatibility with yet established fabrication technology and production lines, and good electrical performance. For this reason great investigative effort has been undertaken during the last years. However, in the most promising cells, resistive switching occurs at areas in the ~100nm2 range, a fact that makes necessary the use of tools with high lateral resolution. Here, we prove that CAFM can be very useful to get a deeper insight into the process of forming, set and reset. We also demonstrate that RS is a local phenomenon restricted to grain boundaries in polycrystalline layers owing to their electrically weaker behavior as a consequence of defect accumulation.