Sreetosh Goswami1 Hariom Jani1 Soumya Sarkar1 Sreebrata Goswami2 Thirumalai Venkatesan1 Jens Martin1

1, National Univ of Singapore, Singapore, , Singapore
2, Indian Association for the Cultivation of Science, Kolkata, , India

Charge transport through resistive memory devices has been an arena of intense research. Here we explore temperature dependent charge transport in a memristive film of a Ru-complex of azo-aromatic ligand. In-situ Raman and UV-Vis spectroscopic measurements establish that the switching in film conductance is controlled by the ligand redox states of the film molecules, while the counter ions account for the hysteresis[1]. At temperature values lower than 145K, the hysteresis in J(V) gradually decays till ~5K where it completely quenches. In an intermediate temperature range of 135K to 110K, we observe well-resolved conductance plateaus at different applied bias with a sharp transition in between them. The number of plateaus N in the J(V) follows the empirical rule of N= d[nm]/5 where d is the film thickness in nm. The conductance plateaus correspond to a correlation of [J0]N, with J0 = current of the first plateau. Each of the plateaus is characterized by in-situ Raman spectroscopy as well as photoluminescence (PL) measurement, both of which exhibit sharp transitions corresponding to the switching observed in J(V). PL and Raman peak intensities scale as N x I0, where I0 is the PL/Raman intensity of the first plateau. These observations indicate a layer by layer electron doping in the film where layers of around 5 nm get doped sequentially. The intermediate temperature range provides an optimal activation energy for counterion motion in the film leading to the formation of electronic phases with sharp boundaries. By reducing the van der Wall radius of the counterion this step can be reduced to a single layer ~ 1.5nm! This is an unprecedented result in an amorphous film. This multi-step memory has the potential for neuromorphic computing.

[1] Goswami, Sreetosh, et al. "Robust resistive memory devices using solution-processable metal-coordinated azo aromatics." Nature Materials (2017), DOI: 10.1038/NMAT5009