Purely electronic nanometallic resistance switching random-access memory
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Turning insulating thin films into nanoconductors and resistance switching random-access memory A nanometallic memory is a purely electronic two-terminal device that offers fast switching speeds, small switching voltage, long retention time, and good durability. It is built on thin films of amorphous insulators and semiconductors, with a three-dimensional (3D) conducting network or channel embedded within and gated by one or more nearby trapped charge.1–3 This trapped-charge memory is put into the high resistance state (HRS) when the trapped electrons block the channel via long-range Coulomb repulsion. Conversely, it enters the low resistance state (LRS) when the traps empty and the channel reopens. An as-fabricated nanometallic resistance switching random-access memory (ReRAM) is in the LRS and nominally a conductor. But this only holds for thin films, typically less than ∼10-nm thick.2,3 Thicker films, like their bulk counterparts, tend to remain insulators and semiconductors, which do not switch unless they instead form filaments by ion migration as in most other ReRAM. Thus, nanometallic ReRAM is distinctly a nanodevice. Trapped-charge memory usually suffers from the voltage– time dilemma, which suggests that it is improbable for the memory to satisfy the three fundamental specifications for a memory—a small switching voltage, a fast switching time
(speed), and long retention time. The argument is based on a simple theoretical consideration—an energy barrier that separates two memory states has only two independent characteristics—barrier height and width, so it cannot possibly satisfy three specifications. Often, a trapped electron occupies a higher energy state, so the escape barrier is lower than the capture barrier. However, nanometallic ReRAM is exempt from the dilemma because it has smart gates that autonomously reprogram the barrier to prevent detrapping.2,3 As will become clear, the reprogrammable barrier along with innate nanoconductivity and the size effect of this smart ReRAM are all rooted in the unique electron physics of disordered insulators.
Negative-U centers Nanometallic memory uses negative-U centers, described next, where U is energy, to trap electrons.3 As shown in the inset in Figure 1, after an electron is trapped at such a center, the energy of the trapped-electron state is lowered, so the escape barrier becomes higher, possibly even exceeding the capture barrier. Since there are now three independent characteristics, two barrier heights and one width, it is possible to satisfy three specifications. This explains why nanometallic ReRAM can resolve the voltage–time dilemma. The idea of negative-U centers was first proposed to explain the absence of unpaired electrons in amorphous chalcogenide semiconductors.4
Yang Lu, University of Pennsylvania, USA; [email protected] Jung Ho Yoon, University of Massachusetts Amherst, USA; [email protected] Yanhao Dong, Massachusetts Institute of Technology, USA; [email protected] I.-Wei Chen, University of Pennsylvania, USA; [email protected]
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