Electrolyte-Dependent Modification of Resistive Switching in Anodic Hafnia

Abstract

Anodic HfO2 memristors grown in phosphate, borate, or citrate electrolytes and formed on sputtered Hf with Pt top electrodes are characterized at fundamental and device levels. The incorporation of electrolyte species deep into anodic memristors concomitant with HfO2 crystalline structure conservation is demonstrated by elemental analysis and atomic scale imaging. Upon electroforming, retention and endurance tests are performed on memristors. The use of borate results in the weakest memristive performance while the citrate demonstrates clear superior memristive properties with multilevel switching capabilities and high read/write cycling in the range of 106. Low temperature heating applied to memristors shows a direct influence on their behavior mainly due to surface release of water. Citrate-based memristors show remarkable properties independent on device operation temperatures up to 100 °C. The switching dynamic of anodic HfO2 memristors is discussed by analyzing high resolution transmission electron microscope images. Full and partial conductive filaments are visualized, and apart from their modeling, a concurrency of filaments is additionally observed. This is responsible for the multilevel switching mechanism in HfO2 and is related to device failure mechanisms.

Structure and Electrical Behavior of Hafnium-Praseodymium Oxide Thin Films Grown by Atomic Layer Deposition

“…Comparison of our results with literature data confirmed the excellent performance of Pr-doped HfO 2 studied in this work compared to that of many other HfO 2 -based undoped and doped RS media. Typical I LR /I HR values reported [ 16 , 70 , 71 , 72 , 73 , 74 , 75 ] have ranged from one to two orders of magnitude being well comparable to the corresponding values obtained in our work. However, I LR /I HR values as high as three orders of magnitude have also been reported for samples with optimized electrode structure [ 76 ].…”

“…However, I LR /I HR values as high as three orders of magnitude have also been reported for samples with optimized electrode structure [ 76 ]. In most cases, the endurance tests, if performed, have been limited to 1000 switching cycles [ 70 , 71 , 73 , 75 ] but reports about endurance tests up to 10 5 [ 72 , 74 ] and 10 8 [ 16 ] switching cycles have also been published. Comparing the results described in the latter papers with those obtained in our experiments one can still conclude that after optimization of the device processing procedures, Pr-doped HfO 2 can also become a promising candidate for application in resistive switching random access memories.…”

Composite Memristors by Nanoscale Modification of Hf/Ta Anodic Oxides

“…In contrast, the predominantly polycrystalline structure is specific to the zone III oxide due to the high amount of HfO 2 . 16 This may also explain the unstable memristive effect of zones I and III, in which HfO 2 is believed to play the role of a barrier blocking CFs formation through the whole oxide later. Zone II shows a mixture of amorphous and crystalline oxide regions, with visible amorphous Ta 2 O 5 “fingers” delimited by crystalline HfO 2 areas, as roughly indicated in the HRTEM image by white dashed lines using EDX elemental map as a reference ( Figure 3 ).…”

“…The different gradients for Hf and Ta were defined aiming for a thickness of the final oxide layer below 20 nm, as recommended for memristor formation. 16 , 34 …”

“…Previous studies on pure Hf anodic memristors have confirmed concurrent competing CFs formation. 16 Thus, one could assume that the switching mechanism can be conditioned by the formation of oxides with such structures. The selected Hf/Ta ratio corresponding to those devices produced in zone II could be an excellent choice for improved memristors fabrication.…”