Applied Science and Convergence Technology 2022; 31(5): 110-112
Published online September 30, 2022
Copyright © The Korean Vacuum Society.
aDepartment of Physics, Chungnam National University, Daejeon 34134, Republic of Korea
bInstitute of Quantum Systems, Chungnam National University, Daejeon 34134, Republic of Korea
cCenter for Supercomputing Applications, National Institute of Supercomputing and Networking, Korea Institute of Science and Technology Information, Daejeon 34141, Republic of Korea
Unintentional bubbles are formed when manufacturing devices using two-dimensional materials. Usually, these bubbles affect device performance degradation, but in the case of memory devices, an additional charge trap can be expected. We investigate the direct surface potential of bubbles formed in a hexagonal boron nitride (hBN)/multilayer graphene (MLG) heterostructure. Specifically, we study the electron transfer improvement by increasing the memory window of a MoS2/hBN/MLG heterostructure in floating gate memory owing to bubbles formed at the hBN/MLG heterointerface. This characterization of bubbles containing molecules such as water or hydrocarbon in two-dimensional material heterointerfaces can promote the understanding of charge carrier tunneling in two-dimensional material heterostructures.
Keywords: Heterostructure, Floating Gate Memory, Memory Window, Bubble
Floating gate memory (FGM)-based two-dimensional (2D) material heterostructures can achieve high performance and stability [1–9]. For instance, the transition-metal dichalcogenide (TMDC) semiconductor channel, hexagonal boron nitride (hBN) dielectric layer, and multilayer graphene (MLG) semimetal layer heterostructure have been demonstrated and widely used in FGM devices [1–9]. The program and erase steps in memory can be achieved by electron tunneling between the TMDC channel and MLG heterostructure through the hBN barrier. Hence, the tunneling condition of hBN in a TMDC/hBN/MLG heterostructure is important in FGM applications [1–9].
For preparing 2D material heterostructures, each layer of 2D material is stacked using micro-transfer systems in air, causing unintended residues in the stacked layer heterointerfaces [10–16]. Such residues are commonly referred to as bubbles. Previous studies on bubbles have considered their influence on tunneling [17–24], and we have previously reported the charge trap by bubble formation in a TMDC/hBN/ MLG heterointerface . We observed the tunneling carrier density through the hBN area that includes bubble formation and analyzed an additional charge trapping layer for electron transfer between TMDC and MLG layers in TMDC/hBN/MLG heterostructures.
In the present study, we characterized the surface potential variation on the bubble area at an hBN/MLG heterointerface. Using a MoS2/hBN/MLG heterostructure FGM device, we confirmed that the electron transfer ratio enhances the memory window (MW) for bubbles formed at the hBN/MLG heterointerface compared with a flat interface area. This characterization of electron tunneling behavior for bubbles in 2D material heterointerfaces may unveil properties of real electron tunneling in 2D material heterostructures for charge carrier tunneling devices with unintended residues.
Figure 1 shows the fabrication steps of the MoS2/hBN/MLG heterostructure on a SiO2 substrate using a micro-transfer system [25–28]. A 40-nm-thick hBN dielectric layer was stacked on MLG (22 nm in thickness), which was initially prepared on a 280-nm-thick SiO2 substrate by mechanical exfoliation. Then, multilayer MoS2 (26 nm in thickness) was transferred to hBN/MLG/SiO2, as illustrated in Figs. 1(a)-(c). During transfer, we observed the formation of bubbles, as shown in Figs. 1(b) and 1(c). To verify the surface doping variation owing to bubble formation in MoS2/hBN/MLG heterostructures, we performed scanning Kelvin probe microscopy (SKPM) using a commercial atomic force microscope (XE-100; Park Systems, Suwon, Korea) in air at room temperature. To investigate the spatial surface potentials on the surface of 2D material heterostructures, we used the feedback process (measuring contact potential difference
Figure 2 shows the spatial distribution of the contact potential difference for the MoS2/hBN/MLG surface including bubbles. Owing to the increasing influence of bubbles at the 2D heterostructure, we chose a large bubble with 60 nm in height [Figs. 2(a) and 2(b)] for analysis. To confirm bubble doping, we evaluated the normalized contact potential (△
To investigate electron tunneling related to bubble formation between hBN and MLG, we connected Cr/Au (10/100 nm in thickness) electrodes to an MoS2 channel by electron-beam lithography, as shown in Fig. 3. We chose two regions covering bubble [A1 in Fig. 3(a)] and flat [A3 in Fig. 3(a)] areas on the hBN/MLG heterostructure and transferred a MoS2 layer covering both areas. Employing the electrode marked by E2 as border, the electric transport characterization was investigated by measuring the drain-source current (
The hysteresis for transport curves (
We characterized the effects of bubbles formed at hBN/MLG interfaces on the electron tunneling ratio of a MoS2/hBN/MLG heterostructure. Evidence of charge trapping was obtained from direct observation by SKPM of the surface potential variation by doping changes on a bubble-containing area at the hBN/MLG heterointerface. Based on measurements of the MW enhancement for a MoS2/hBN/MLG FGM device with bubbles formed at the hBN/MLG interface, we confirmed the trapped charges by bubble formation leading to improved electron tunneling ratio between the MoS2 channel and MLG floating gate layer. This result may contribute to understanding the role of trapped molecules in bubbles formed on 2D material heterostructures for electron tunneling applications.
This work was supported by the research fund of Chungnam National University.
The authors declare no conflicts of interest.