Paper TF2-MoA9
Current Fluctuations in Au/HfO₂/PT Resistive Switching Memories

Monday, November 9, 2009, 4:40 pm, Room B3

In the last years, the semiconductor industry has shown a growing interest for the fabrication of nonvolatile memories based on a resistance switching mechanism to store information: Resistive Random Acces Memories (RRAM). A basic RRAM cell consists in a Metal / Insulating oxide / Metal (MIM) structure where the oxide is a metal oxide (NiO, TiO₂, ZrO₂, Cu_xO…), a perovskite (BaTiO₃, SrTiO₃…) or a manganite (Pr_1‑xCa_xMnO₃, La_1-xSr_xMnO₃…). A commonly admitted model to explain resistance switching is the formation of conducting filaments upon field-enhanced migration of cations or anions across the film's thickness. In this respect, oxygen vacancies seem to play an important role.

In this work, HfO₂ MIM RRAM cells are elaborated and tested in two electrical modes: voltage sweep mode and constant voltage stress (CVS) mode. The HfO₂ choice is based on the fact that hafnia-based dielectrics are the most promising materials for the manufacturing of CMOS gates below the 45 nm technology node. Therefore, as far as integration and process compatibility are concerned, HfO₂ would be an interesting candidate for the RRAM technology. HfO₂ films (10 nm) were grown at 350°C by atomic layer deposition (ALD) using alternate cycles of H₂O and HfCl₄ precursors (1 Torr) on Pt (100 nm ) / Ti (10 nm) / Si wafers. The films are crystallized in the monoclinic phase and their gap is around 5.5 eV. Angle-resolved X-ray photoelectron spectroscopy (ARXPS) performed on thinner films (2 nm) reveals a homogeneous composition across the films thickness with a sharp HfO₂/Pt interface (absence of PtO oxides). Top gold electrodes (100 nm thick, 2 mm in diameter) were deposited on the HfO₂ films by dc sputtering.

The voltage sweep mode shows that HfO₂ RRAM is bipolar and that the switching mechanism is probably due to the oxygen vacancies migration and accumulation through the electrical field to the Pt/HfO₂ interface. These positively charged vacancies may act as dopant and form a locally conducive channel in the dielectric. In the CVS mode the device's resistance was observed to be unstable, displaying fluctuations between the high resistance state and the low resistance state which are typical of random telegraph signals. The role played by oxygen vacancies on these fluctuations will be discussed and a mechanism based on a competition between the oxygen vacancies alignment by the electric field and their thermal redistribution by Joule heating will be proposed.