Precise Layer Control of MoTe2 by Ozone Treatment

Air Annealing Process for Threshold Voltage Tuning of MoTe2 FET

A stable doping technique for modifying the conduction behaviour of two-dimensional (2D) nanomaterial-based transistors is imperative for applications based on low-power complementary oxide thin-film transistors. Achieving an ambipolar feature with a controlled threshold voltage in both the p- and n-regimes is crucial for applying MoTe2-based devices as electronic devices because their native doping states are unipolar. In this study, a simple method to tune the threshold voltage of MoTe2 field-effect transistors (FETs) was investigated in order to realise an enhancement-mode MoTe2 thin-film transistor by implementing a facile method to modulate the carrier polarity based on the oxidative properties of MoTe2 FETs. Annealing in air induced a continuous p-doping effect in the devices without significant electrical degradation. Through a precise control of the duration and temperature of the post-annealing process, the tailoring technique induces hole doping, which results in a remarkable shift in transfer characteristics, thus leading to a charge neutrality point of the devices at zero gate bias. This study demonstrates the considerable potential of air heating as a reliable and economical post-processing method for precisely modifying the threshold voltage and further controlling the doping states of MoTe2-based FETs for use in logic inverters with 2D semiconductors.

Modulation of electrical properties in MoTe2 by XeF2-mediated surface oxidation

Transition metal dichalcogenides (TMDs) are promising candidates for the semiconductor industry owing to their superior electrical properties. Their surface oxidation is of interest because their electrical properties can be easily modulated by an oxidized layer on top of them. Here, we demonstrate the XeF₂-mediated surface oxidation of 2H-MoTe₂ (alpha phase MoTe₂). MoTe₂ exposed to XeF₂ gas forms a thin and uniform oxidized layer (∼2.5 nm-thick MoO _x ) on MoTe₂ regardless of the exposure time (within ∼120 s) due to the passivation effect and simultaneous etching. We used the oxidized layer for contacts between the metal and MoTe₂, which help reduce the contact resistance by overcoming the Fermi level pinning effect by the direct metal deposition process. The MoTe₂ field-effect transistors (FETs) with a MoO _x interlayer exhibited two orders of magnitude higher field-effect hole mobility of 6.31 cm² V^-1 s^-1 with a high on/off current ratio of ∼10⁵ than that of the MoTe₂ device with conventional metal contacts (0.07 cm² V^-1 s^-1). Our work shows a straightforward and effective method for forming a thin oxide layer for MoTe₂ devices, applicable for 2D electronics.

Nonlinear Optical Imaging, Precise Layer Thinning, and Phase Engineering in MoTe2 with Femtosecond Laser

The control of layer thickness and phase structure in two-dimensional transition metal dichalcogenides (2D TMDCs) like MoTe₂ has recently gained much attention due to their broad applications in nanoelectronics and nanophotonics. Continuous-wave laser-based thermal treatment has been demonstrated to realize layer thinning and phase engineering in MoTe₂, but requires long heating time and is largely influenced by the thermal dissipation of the substrate. The ultrafast laser produces a different response but is yet to be explored. In this work, we report the nonlinear optical interactions between MoTe₂ crystals and femtosecond (fs) laser, where we have realized the nonlinear optical characterization, precise layer thinning, and phase transition in MoTe₂ using a single fs laser platform. By using the fs laser with a low fluence as an excitation light source, we observe the strong nonlinear optical signals of second-harmonic generation and four-wave mixing in MoTe₂, which can be used to identify the odd-even layers and layer numbers, respectively. With increasing the laser fluence to the ablation threshold (F_th), we achieve layer-by-layer removal of MoTe₂, while 2H-to-1T' phase transition occurs with a higher laser fluence (2F_th to 3F_th). Moreover, we obtain highly ordered subwavelength nanoripples on both the thick and few-layer MoTe₂ with a controlled fluence, which can be attributed to the fs laser-induced reorganization of the molten plasma. Our study provides a simple and efficient ultrafast laser-based approach capable of characterizing the structures and modifying the physical properties of 2D TMDCs.

Homogeneous 2D MoTe2 CMOS Inverters and p-n Junctions Formed by Laser-Irradiation-Induced p-Type Doping

Among all typical transition-metal dichalcogenides (TMDs), the bandgap of α-MoTe₂ is smallest and is close to that of conventional 3D Si. The properties of α-MoTe₂ make it a favorable candidate for future electronic devices. Even though there are a few reports regarding fabrication of complementary metal-oxide-semiconductor (CMOS) inverters or p-n junction by controlling the charge-carrier polarity of TMDs, the fabrication process is complicated. Here, a straightforward selective doping technique is demonstrated to fabricate a 2D p-n junction diode and CMOS inverter on a single α-MoTe₂ nanoflake. The n-doped channel of a single α-MoTe₂ nanoflake is selectively converted to a p-doped region via laser-irradiation-induced MoO_x doping. The homogeneous 2D MoTe₂ CMOS inverter has a high DC voltage gain of 28, desirable noise margin (NM_H = 0.52 V_DD , NM_L = 0.40 V_DD ), and an AC gain of 4 at 10 kHz. The results show that the doping technique by laser scan can be potentially used for future larger-scale MoTe₂ CMOS circuits.

Self-Terminated Surface Monolayer Oxidation Induced Robust Degenerate Doping in MoTe2 for Low Contact Resistance

We introduce an effective method to degenerately dope MoTe₂ by oxidizing its surface into the p-dopant MoO_x in oxygen plasma. As a self-terminated process, the oxidation is restricted only in the very top layer, therefore offering us an easy and efficient control. The degenerate p-doping with the hole concentration of 2.5 × 10¹³ cm^-2 can be obtained by applying a ∼300 s O₂ plasma treatment. Using the degenerately doped MoTe₂, we demonstrate a record low contact resistance of 0.6 kΩ μm for MoTe₂. Our measurement highlights an excellent stability for the plasma-doped MoTe₂. The doped characteristics are robust with no significant degradation even after a one-year exposure to the air. The oxygen plasma doping technique is compatible with the conventional semiconductor processes, which can be utilized to realize high-performance MoTe₂ field-effect transistors (FETs) or tunnel FETs in the future.