Abstract
Ferroelectric tunnel junctions (FTJs) based on ultrathin HfO2 have great potential as a fast and energy-efficient memory technology compatible with complementary metal oxide semiconductors. FTJs consist of a ferroelectric film sandwiched between two distinct electrodes, the properties of which are intricately linked to the electrical properties of the FTJs. Here we utilize a W crystallization electrode (CE) to achieve a high and reproducible remanent polarization, combined with a metal replacement process in which the W is carefully removed and replaced by another top electrode (TE). In this way we separate the ferroelectric film properties from the device design and can thereby evaluate the effect of the TE work function (WF) and conduction band electron density (ne) on the tunneling electroresistance (TER) and device reliability. We compare FTJs designed with a TiN bottom electrode and W, Cr, or Ni TE and find that the use of high electron density metals such as Ni or Cr as TE allows for an improved TER, albeit at the cost of reliability due to a large built-in electric field. To bypass this effect, a bilayer Cr/Ni TE is implemented, which allows for a high TER and minimal built-in field, leading to excellent retention and endurance beyond 108 cycles. The results presented here thus highlight a process flow for reliable design and implementation of FTJs.
Highlights
Current memory technologies cannot keep up with the exponential growth of Internet traffic and online services, requiring huge data storage and bandwidth; new solutions are required
We study W, Cr, and Ni as top electrode (TE), because of their interesting relative properties, and find that W, Cr, and Ni lead to gradually increasing built-in electric fields due to the work function (WF) difference compared to the bottom electrode (BE) (TiN) (Figure 1b)
This work describes use of a crystallization electrode (CE) and a replacement TE process to separate the ferroelectric properties in ferroelectric Hf1−xZrxO2 from the aspects of Ferroelectric tunnel junctions (FTJs) device design
Summary
Current memory technologies cannot keep up with the exponential growth of Internet traffic and online services, requiring huge data storage and bandwidth; new solutions are required. The WOx forms during the deposition step or in the annealing process, by oxygen scavenging from the Hf1−xZrxO2 to produce a thin WOx layer at the W/Hf1−xZrxO2 interface Failure to remove this oxide leads to devices with low TER and minimal influence by the choice of TE.
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