Effects of Sb Doping and Preferred Crystal Orientation on the Performance of Electro-Plated Cu2o Resistive Random Access Memory
Authors: Lin Hsueh-Pin , Chang Ching-Hsiang , Perng Dung-Ching 
Published: May 2020

Effects of antimony doping and film preferred crystal orientation on the performance and stability of electro-plated cuprous oxide (Cu2O) resistive random access memory (RRAM) devices were studied. Previous study indicated that electro-plated Cu2O with a small amount of antimony doping improves film uniformity, surface roughness1. The Sb doped film (Cu2O:Sb) also has a smaller grain size as compared to that of an un-doped cuprous oxide (u-Cu2O) film at same PH plating solution. SEM image of the grain boundaries of the Cu2O:Sb film also indicated that the film is more vertically oriented as compare to that of the u-Cu2O film. As some literatures reported, the conductive path (or charge hopping distance) of the RRAM is likely shorter if electrons hopping take place along the vertical-like grain boundaries. It will also reduce the voltage to set the conductive path formation (or set up of the charge hopping path). Moreover, uniform film and smoother surface are likely to have a stable set/reset performance because more uniform charge hopping distance and/or hopping site distribution.

Our studies show that the forming voltages of a 700nm electroplated Cu2O:Sb (~2 at. %) and u-Cu2O (strongly (111) oriented) films are about 4V and 7V, respectively. Some of Cu2O:Sb RRAMs are forming free. The set voltages (Vset) of Cu2O:Sb and u-Cu2O RRAMs are ranging from 1.5V to 2.6V and from 1.2V to 9V, respectively. The reset voltages (Vreset) are 1.2V ~ 1.5V and 1V ~ 2V, respectively. The ratio of high/low resistance state (HRS/LRS) for both of the RRAMs is about two orders of magnitude, and no decay is observed for retention test of 5000 seconds at 0.1V. These results indicate that Sb doping does reduce forming (likely forming free) and significantly stabilize its set voltage.

Surface morphology and texture of a (200)-preferred Cu2O film is very different from (111)-preferred film2
−
4. For a (200)-preferred film, its grains’ boundaries are more perpendicular to the plating surface (or electrode). This kind of film texture is also likely can help forming shorter conductive paths and/or stabilize the charge hopping resistance. It is similar to our Cu2O:Sb film texture as previous described. For Cu2O:Sb film, the XRD intensity ratio (R) of (111) to (200) phase is ~1. While the u-Cu2O is strongly (111)-preferred film with R= 12. Preferred crystal orientation change will give rise to varying in vacancy, defects, and trap charge distribution along the hopping paths. These variations can affect charge hopping resistance and its characteristics. Charge hopping in film with vertical-like grain boundaries is believed to be a shorter and easier path.

Performance of preferred crystal orientation dependence of the u-Cu2O RRAM is investigated. A (200)-preferred u-Cu2O film with R= 0.185 was used. Its RRAM performance improved in forming voltage of ~5V and in HRS/LRS ratio with three orders of magnitude.

In short, Cu2O:Sb film can be a good candidate for RRAM application because of its low cost, potential forming free, low set/reset voltages and stability. Sb doping may have helped forming shorter conductive paths due to its vertical like gains boundaries, less charge hopping resistance due to more uniformly distributed Sb induced hopping sites. Improved performance of (200)-preferred u-Cu2O RRAM may result from vacancy, defects and trap charge distribution change along the hopping paths.

Reference

1. Baek, S. K., Kwon, Y. H., Shin, J. H., Lee, H. S., & Cho, H. K. “Low‐Temperature Processable High‐Performance Electrochemically Deposited p‐Type Cuprous Oxides Achieved by Incorporating a Small Amount of Antimony” Advanced Functional Materials, 25, (2015) 5214

2. J. Siegfried and K.S. Choi, “Electrochemical Crystallization of Cuprous Oxide with Systemic Shape Evolution” Advanced Materials, 16 (2004) 1743

3. E. Rakhshani and J. Varghese “Surface texture in electrodeposited films of cuprous oxide” J. Materials Science 23 (1988) 3847

4. H. Lee, I.C. Leu, C.L. Liao, K.Z. Fung “The structural evolution and electrochemical properties of the textured Cu2O thin films” J. Alloy Compounds 436 (2007) 241