Electrostatically Tunable PN Junction in 1L-MoTe2 Formed By Gold-Mediated Exfoliation
Authors: Taku Yoshimura , Shigeno Hiroshi , Ishikawa Ryousuke , Watanabe Kenji , Taniguchi Takashi , Hoshi Yusuke 
Published: December 2024

Atomically thin layered materials have attracted great research interests owing to exciton formation at room temperature based on extraordinarily large binding energy. In particular, a semiconducting molybdenum ditelluride (MoTe2) emits photons in the near-infrared region around 1.1 eV, which is the photon energy applicable to optical telecommunication. So far, various optoelectronic devices such as circularly polarized light emitting diodes and lasers are developed by utilizing Mo-based transition metal dichalcogenides. Recently, it has been reported that gold-mediated exfoliation technique enables to form large-area MoTe2 monolayer. In this study, we focus on electrostatic carrier density modulation in the large-area 1L-MoTe2 formed by gold-mediated exfoliation technique and demonstrate that the fabricated large-area 1L-MoTe2 has a high potential for developing optoelectronic devices such as light emitting diodes.

Schematic diagram of the device structure is shown in Fig. 1(a). First, MoTe2 multilayers were mechanically exfoliated by scotch tape, and gold was deposited onto the MoTe2 multilayers by thermal evaporation. Then, the gold film together with the topmost layer of the MoTe2 multilayers was peeled off by using a thermal release tape owing to the stronger interaction of the topmost layer with the evaporated gold compared to the van der Waals interactions in the MoTe2 bulk crystals. The 1L-MoTe2/gold structures on the thermal release tape were transferred to a 290-nm-thick SiO2/Si substrate, and selective wet etching of the surface gold film was performed with a KI/I2 mixed solution. Graphite and hBN flakes were prepared on a polydimethylsiloxane (PDMS), and these flakes were successively stacked on the 1L-MoTe2 using the standard dry transfer process for utilization as electrodes to measure the electrical properties and as a cap layer, respectively. Finally, we deposited Ni/Au top gate electrodes on the hBN/1L-MoTe2 by a lift-off process, which are separated by 8 μm.

Figure 1(b) shows the current-voltage (I
D–V
BG) curves obtained by applying top gate voltages of V
TG1 = V
TG2 = 0 V. An increase in drain current I
D is seen by increasing back-gate voltage V
g for both positive and negative polarities in the 1L-MoTe2 channel, showing the ambipolar behavior. Figure 1(c) plots the value of drain current I
D as a function of top gate voltages V
TG1 and V
TG2 at fixed voltages of V
BG = 20 V and V
D = 2 V. Considering that the positive voltage of 20 V is applied to the Si back gate electrode, the carrier type of the 1L-MoTe2 area between top gate electrodes becomes n-type. Furthermore, we can tune the local carrier densities in the 1L-MoTe2 areas below the top gate electrodes by the gate bias tuning, and thus the four corners of the figure correspond to P/N/N+, N+/N/N+, N+/N/P, and P/N/P configurations. For P/N/N+ and N+/N/P configurations, the I
D-V
D curves show rectification behavior with the on/off ratio around 103. Additionally, we found the typical current amplification by modulating the Si back gate voltage for the P/N/P configuration. These results demonstrate the versatility of the 1L-MoTe2 device to functionality as a PN junction and an electrostatically controlled transistor.

Figure 1