Firing voltage reduction in thermally annealed Ge–As–Te thin film with ovonic threshold switching

Abstract

Recently, chalcogenide materials have exhibited ovonic threshold switching characteristics, improving their suitability as selector devices to effectively depress the sneak current in the cross-point array (CPA) structures. However, chalcogenides must be subjected to a firing process before they can exhibit the threshold switching behavior. The firing process causes operation problems with respect to the memory operation process in the case of the CPA structure. Although the firing process is expected to be related to Joule heating with a high electric current, the physical nature of the changes during the firing process remains unclear. In this study, selector devices are fabricated by sandwiching amorphous Ge–As–Te thin films between the TiN and W layers. Furthermore, the authors examine the microstructure of the Ge–As–Te thin films before and after the electric firing process using transmittance electron microscopy (TEM, JEOL JEM-F200). The TEM analysis of the pristine Ge–As–Te thin films denotes the uniform contrast of the image, which implies the uniform chemical composition of the film. However, the Ge–As–Te thin films exhibit nonuniform contrast due to the effects of Joule heating after the electric firing process. The Ge–As–Te thin films were thermally annealed at 150 and 250 °C for 1 min via the rapid thermal annealing process to verify the effects of thermal treatment on the firing process in chalcogenide thin films. The effect of thermal annealing on the threshold switching behavior was also investigated by studying the compositional stability. Their results showed that the thermal annealing process caused the uniform compositions on the pristine Ge–As–Te films to be fluctuated with decreasing firing voltage for threshold switching.