Effect of Environmental Conditions on the Electrical Properties of MoTe2 Monolayers with hBN Cap Layer
Authors: Yoshimura Taku , Shigeno Hiroshi , Watanabe Kenji , Taniguchi Takashi , Hoshi Yusuke 
Published: December 2024

We investigate effects of vacuum level on electrical properties of hBN-capped monolayer MoTe2 (1L-MoTe2) channel FETs. The 1L-MoTe2 transistors show ambipolar transfer characteristics, which enable both p- and n-type operation by gate bias tuning. The conductivity minimum point is shifted toward negative values by reducing vacuum level, demonstrating that charge transfer occurs by adsorption of some molecules in an atmosphere even for the device structures with hexagonal boron nitride (hBN) cap layer.

A semiconducting molybdenum ditelluride (MoTe2) monolayer have attracted great research interest owing to the photon emission in the near-infrared wavelength and a strong spin-orbit coupling to result in the longer spin and valley relaxation, providing a pathway to novel optoelectronic device applications. Recently, it has been reported that electrostatic carrier density modulation by two separated local gate enables lateral PN junction formation. On the other hand, adsorption of atmospheric gases leads to charge transfer to the 1L-MoTe2, and the resulting carrier density changes lead to loss of the controllability and reproducibility of the lateral PN junction formation. The formation of hBN cap layer is reported to suppress degradation of optical properties of the 1L-MoTe2. However, there are few reports for the effect of adsorption of atmospheric gases on electrical properties of 1L-MoTe2 channel FETs with hBN cap layer. In this study, we focus on effects of vacuum level on current-voltage curves obtained from 1L-MoTe2 channel FETs with hBN cap layer.

Firstly, 1L-MoTe2 was fabricated through the Au-mediated exfoliation process. Gold was deposited onto a bulk MoTe2 crystal by thermal evaporation. The gold film together with the topmost layer of the MoTe2 crystals 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 onto a 290-nm-thick SiO2/Si substrate, and subsequently, the surface gold film was selectively etched with a KI/I2 mixed solution. Graphite and hBN flakes mechanically exfoliated were prepared on a polydimethylsiloxane (PDMS), and these flakes were transferred 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.

Figure (a) shows the current-voltage (I
d–V
g) curves obtained from the fabricated 1L-MoTe2 channel FET under various vacuum levels. 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, demonstrating the ambipolar operation of the FET. Figure (b) represents the conductivity minimum point V
min plotted as a function of the pressure in the chamber. On decreasing the pressure, the V
min is gradually shifted to the negative values, which correspond to the change of n-type behavior. It is found that the V
min shifts by about -30 V at the pressure of 6.9×10-4 Pa. As previously reported, the V
min shift should result from the charge transfer between the 1L-MoTe2 and some adsorbed molecules under the atmospheric conditions. Considering the existence of the hBN cap layer on the 1L-MoTe2 channel, some molecules physically absorbed in the interfaces between 1L-MoTe2 and the SiO2 without atomically flat surface may be desorbed by reducing the pressure, and thus the p-doping effect by adsorption of atmospheric gases disappears. It could be concluded that the changes of environmental conditions affect electrical properties of 1L-MoTe2 FETs even for the device structures with the hBN cap layer.

Figure 1