(Invited) Multi-Bit Transistor Memories Made from Mechanically Exfoliated WSe2 and Plasma-Doped MoS2
Authors: Liang Xiaogan , Chen Mikai 
Published: February 2020

It is still desirable to explore new simple nano/microfabrication approaches that can create low-cost solid-state drives (SSDs) based on field-effect transistor (FET) memories. 2D semiconducting layered transition metal dichalcogenides (TMDCs, e.g., MoS2 and WSe2) are anticipated to provide new opportunities in this research area[1][2]. However, most recent efforts have been focused on the direct application of TMDC FET channels into the conventional FET-based memory structure consisting of regular floating gates, tunnel barriers, or charge trapping layers.[3][4] Such memory devices still have quite complicated architectures, and such works have not effectively reduced the complexity and cost of solid-state memory manufacturing. Recently, we found that an exfoliated multilayer MoS2 flake (Fig. 1a), after a plasma doping process, has a rough (or rippled) top layer (Fig. 1b). When incorporated into a FET structure (Fig. 1c), this rough layer can serve as a charge-trapping layer interfacing with the underlying pristine layers that serve as the FET channel. This device structure can function as a FET memory and enable multi-bit data storage (i.e., year-scale binary or 2-bit data storage; day-scale 3-bit data storage (see Fig. 1d)).2 This work has preliminarily leveraged the unique layered structure of TMDCs for making ultra-low-cost memory devices.
Here, we further report that a mechanically exfoliated multilayer WSe2 flake can directly serve as a FET memory channel with no need of plasma doping and enable multi-bit data storage functionality. This finding could further simplify the fabrication steps for producing SSDs.
The newly observed memory states in multilayer WSe2 are tentatively attributed to the vulnerability of WSe2 layers to mechanical exfoliation. This means that the mechanical exfoliation of a multilayer WSe2 flake from a bulk WSe2 stamp could directly result in a rough (or rippled) top layer on the exfoliated flake, as illustrated in Fig. 2a. Such a rough WSe2 top layer, similar to the plasma-doped MoS2 top layer illustrated in Fig. 1b, could serve as a charge-trapping layer for the FET memory and enable multi-bit data storage (Fig. 2b). Fig. 2c shows the retention characteristics of a set of 2-bit (i.e., 4 data levels) memory states measured from a representative WSe2 FET memory, which are comparable to the memory states obtained from a plasma-doped MoS2 FET memory (Fig. 1d). To support our hypothesis, we further characterized as-exfoliated WSe2 and MoS2 flakes using atomic force microscopy (AFM) (Fig. 3). The AFM results show that as-exfoliated multilayer WSe2 flakes exhibit a much larger surface roughness in comparison with as-exfoliated multilayer MoS2 flakes. Furthermore, the transmission electron micrograph (TEM) of exfoliated WSe2 layers exhibit clear Moire patterns, which implies the formation of rippled layers on top of mechanically exfoliated WSe2 flakes. These characterization results strongly support our hypothesis on the exfoliation-induced memory states in multilayer WSe2. Additional work will be performed to understand the physical origin of the vulnerability of WSe2 layers to mechanical exfoliation and evaluate the reliability/uniformity of multilayer WSe2 FET memories.
In addition to advance the fabrication technology for producing low-cost memories, this work also provides important scientific insights for creating new nanoelectronic devices through mechanically processing emerging layered materials.

[1] Sangwan, Vinod K, et al. Nature nanotechnology 10.5 (2015): 403-406.
[2] Chen, Mikai, et al.ACS nano 8.4 (2014): 4023-4032.
[3] Choi, Min Sup, et al. Nature communications 4 (2013): 1624.
[4] Jariwala, Deep, et al. ACS nano 8.2 (2014): 1102-1120.

Figure Captions:

Figure 1:

Illustrations of (a) an as-exfoliated multilayer MoS2 flake with a smooth top layer, (b) a plasma-doped MoS2 flake with a rough (or rippled) top layer, and (c) a FET memory with the plasma-doped MoS2 flake as the FET channel. (d) displays the retention characteristics of 2-bit data storage states in a typical FET memory made from a plasma-doped multilayer MoS2 flake.

Figure 2: Illustrations of (a) an as-exfoliated multilayer WSe2 flake with a rough (or rippled) top layer, and (b) a FET memory with the exfoliated WSe2 flake as the FET channel. (c) displays the retention characteristics of 2-bit data storage states in a typical FET memory made from an as-exfoliated multilayer WSe2 flake.

Figure 3:

AFM images of the top surfaces of (a) an as-exfoliated multilayer WSe2 flake and (b) an as-exfoliated multilayer MoS2 flake. The WSe2 flake exhibits a much larger top surface roughness (RMS value: 5.6 nm) in comparison with the MoS2 flake (RMS value: 2.7 nm).

Figure 4:

TEM image of the top surfaces of an as-exfoliated multilayer WSe2 flake, which exhibits clear Moire patterns.

Figure 1