Optimization of Subthreshold Swing and Hysteresis in Hf0.5Zr0.5O2-Based MoS2 Negative Capacitance Field-Effect Transistors by Modulating Capacitance Matching.

Negative capacitance field effect transistors made of Hf0.5Zr0.5O2 (HZO) are one of the most promising candidates for low-power-density devices because of the extremely steep subthreshold swing and high open-state currents resulting from the addition of ferroelectric materials in the gate dielectric layer. In this paper, HZO thin films were prepared by magnetron sputtering combined with rapid thermal annealing. Their ferroelectric properties were adjusted by changing the annealing temperature and the thickness of HZO. Two-dimensional MoS2 back-gate negative capacitance field-effect transistors (NCFETs) based on HZO were prepared as well. Different annealing temperatures, thicknesses of HZO thin films, and Al2O3 thicknesses were studied to achieve optimal capacitance matching, aiming to reduce both the subthreshold swing of the transistor and the hysteresis of the NCFET. The NCFET exhibits a minimum subthreshold swing as low as 27.9 mV/decade, negligible hysteresis (∼20 mV), and the ION/IOFF of up to 1.58 × 107. Moreover, a negative drain-induced barrier lowering effect and a negative differential resistance effect have been observed. This steep-slope transistor is compatible with standard CMOS manufacturing processes and attractive for 2D logic and sensor applications as well as future energy-efficient nanoelectronic devices with scaled power supplies.

Scalable Synthesis of Highly Crystalline MoSe2 and Its Ambipolar Behavior.

Atomically thin, two-dimensional material molybdenum diselenide (MoSe2) has been shown to exhibit significant potential for diverse applications. The intrinsic band gap of MoSe2 allows it to overcome the shortcomings of the zero-band-gap graphene, while its higher electron mobilities when compared to molybdenum disulfide (MoS2) make it more appropriate for practical devices in electronics and optoelectronics. However, its controlled growth has been an ongoing challenge for investigations and practical applications of the material. Here, we present an atmospheric pressure chemical vapor deposition (CVD) method to achieve highly crystalline, single- and few-layered MoSe2 using a SiO2/Si substrate. Our findings suggested that careful optimization of the flow rate can result in the controlled growth of large-area MoSe2 with desired layer numbers due to the adjustment of gaseous MoSe2 partial pressure and nucleation density. The FETs fabricated on such as-synthesized MoSe2 displayed different transport behaviors depending on the layer numbers, which can be attributed to the formation of Se vacancies generated during low flow rates. Monolayer MoSe2 showed n-type characteristics with an Ion/Ioff ratio of ∼106 and a carrier mobility of ∼19 cm2 V-1 s-1, whereas bilayer MoSe2 showed n-type-dominant ambipolar behavior with an Ion/Ioff ratio of ∼105 and a higher mobility of ∼65 cm2 V-1 s-1 for electrons as well as ∼9 cm2 V-1 s-1 for holes. Our results provide a foundation for property-controlled synthesis of MoSe2 and offer insight on the potential applications of our synthesized MoSe2 in electronics and optoelectronics.

Synthesis of Large-Area Highly Crystalline Monolayer Molybdenum Disulfide with Tunable Grain Size in a H2 Atmosphere.

Large-area and highly crystalline monolayer molybdenum disulfide (MoS2) with a tunable grain size was synthesized in a H2 atmosphere. The influence of introduced H2 on MoS2 growth and grain size, as well as the corresponding mechanism, was tentatively explored by controlling the H2 flow rate. The as-grown monolayer MoS2 displays excellent uniformity and high crystallinity evidenced by Raman and high-resolution transmission electron microscopy. The Raman results also give an indication that the quality of the monolayer MoS2 synthesized in a H2 atmosphere is comparable to that synthesized by using seed or mechanical exfoliation. In addition, the electronic properties and dielectric inhomogeneity of MoS2 monolayers were also detected in situ via scanning microwave microscopy, with measurements on impedance and differential capacitance (dC/dV). Back-gated field-effect transistors based on highly crystalline monolayer MoS2 shows a field-effect mobility of ∼13.07 cm2 V(-1) s(-1) and an Ion/Ioff ratio of ∼1.1×10(7), indicating that the synthesis of large-area and high-quality monolayer MoS2 with H2 is a viable method for electronic and optoelectronic applications.