Reconfigurable carrier type and photodetection of MoTe2 of various thicknesses by deep ultraviolet light illumination.

Tuning of the Fermi level in transition metal dichalcogenides (TMDCs) leads to devices with excellent electrical and optical properties. In this study, we controlled the Fermi level of MoTe2 by deep ultraviolet (DUV) light illumination in different gaseous environments. Specifically, we investigated the reconfigurable carrier type of an intrinsic p-MoTe2 flake that gradually transformed into n-MoTe2 after illumination with DUV light for 30, 60, 90, 120, 160, 250, 500, 900, and 1200 s in a nitrogen (N2) gas environment. Subsequently, we illuminated this n-MoTe2 sample with DUV light in oxygen (O2) gas and reversed its carrier polarity toward p-MoTe2. However, using this doping scheme to reveal the effect of DUV light on various layers (3-30 nm) of MoTe2 is challenging. The DUV + N2 treatment significantly altered the polarity of MoTe2 of different thicknesses from p-type to n-type under the DUV + N2 treatment, but the DUV + O2 treatment did not completely alter the polarity of thicker n-MoTe2 flakes to p-type. In addition, we investigated the photoresponse of MoTe2 after DUV light treatment in N2 and O2 gas environments. From the time-resolved photoresponsivity at different polarity states of MoTe2, we have shown that the response time of the DUV + O2 treated p-MoTe2 is faster than that of the pristine and doped n-MoTe2 films. These carrier polarity modulations and photoresponse paves the way for wider applications of MoTe2 in optoelectronic devices.

The non-volatile electrostatic doping effect in MoTe2 field-effect transistors controlled by hexagonal boron nitride and a metal gate.

The electrical and optical properties of transition metal dichalcogenides (TMDs) can be effectively modulated by tuning their Fermi levels. To develop a carrier-selectable optoelectronic device, we investigated intrinsically p-type MoTe2, which can be changed to n-type by charging a hexagonal boron nitride (h-BN) substrate through the application of a writing voltage using a metal gate under deep ultraviolet light. The n-type part of MoTe2 can be obtained locally using the metal gate pattern, whereas the other parts remain p-type. Furthermore, we can control the transition rate to n-type by applying a different writing voltage (i.e., - 2 to - 10 V), where the n-type characteristics become saturated beyond a certain writing voltage. Thus, MoTe2 was electrostatically doped by a charged h-BN substrate, and it was found that a thicker h-BN substrate was more efficiently photocharged than a thinner one. We also fabricated a p-n diode using a 0.8 nm-thick MoTe2 flake on a 167 nm-thick h-BN substrate, which showed a high rectification ratio of ~ 10-4. Our observations pave the way for expanding the application of TMD-based FETs to diode rectification devices, along with optoelectronic applications.

Black Phosphorus-IGZO van der Waals Diode with Low-Resistivity Metal Contacts.

There have been a few studies of heterojunctions composed of two-dimensional transition-metal dichalcogenides (TMDs) and an oxide layer, but such studies of high-performance electric and optoelectronic devices are essential. Such heterojunctions with low-resistivity metal contacts are needed by the electronics industry to fabricate efficient diodes and photovoltaic devices. Here, a van der Waals heterojunction composed of p-type black phosphorus (p-BP) and n-type indium-gallium-zinc oxide (n-IGZO) films with low-resistivity metal contacts is reported, and it demonstrates high rectification. The low off-state leakage current in the thick IGZO film accounts for the high rectification ratio in a sharp interface of p-BP/n-IGZO devices. For electrostatic gate control, an ionic liquid is introduced to achieve a high rectification ratio of 9.1 × 104. The photovoltaic measurements of p-BP/n-IGZO show fast rise and decay times, significant open-circuit voltage and short-circuit current, and a high photoresponsivity of 418 mA/W with a substantial external quantum efficiency of 12.1%. The electric and optoelectronic characteristics of TMDs/oxide layer van der Waals heterojunctions are attractive for industrial applications in the near future.

Observation of giant spin-orbit interaction in graphene and heavy metal heterostructures.

Graphene is a promising material demonstrating some interesting phenomena such as the spin Hall effect, bipolar transistor effect, and non-trivial topological states. However, graphene has an intrinsically small spin-orbit interaction (SOI), making it difficult to apply in spintronic devices. The electronic band structure of graphene makes it possible to develop a systematic method to enhance SOI extrinsically. In this study, we designed a graphene field-effect transistor with a Pb layer intercalated between graphene (Gr) and Au layers and studied the effect on the strength of the SOI. The SOI in our system was significantly increased to 80 meV, which led to a giant non-local signal (∼180 Ω) at room temperature due to the spin Hall effect. Further, we extract key parameters of spin transport from the length and width dependence of non-local measurement. To support these findings, we also measured the temperature and gate-dependent weak localization (WL) effect. We obtained the magnitude of the SOI and spin relaxation time of Gr via quantitative analysis of WL. The SOI magnitudes estimated from the non-local signal and the WL effect are close in value. The enhancement of the SOI of Gr at room temperature is a potential simple manipulation method to explore the use of this material for spin-based applications.