In this paper, a near-ideal subthreshold swing MoS 2 back-gate transistor with an optimized ultrathin HfO 2 dielectric layer is reported with detailed physical and electrical characteristics analyses. Ultrathin (10 nm) HfO 2 films created by atomic-layer deposition (ALD) at a low temperature with rapid-thermal annealing (RTA) at different temperatures from 200 °C to 800 °C have a great effect on the electrical characteristics, such as the subthreshold swing (SS), on-to-off current (I ON /I OFF ) ratio, etc, of the MoS 2 devices. Physical examinations are performed, including x-ray diffraction, atomic force microscopy, and electrical experiments of metal-oxidesemiconductor capacitance-voltage. The results demonstrate a strong correlation between the HfO 2 dielectric RTA temperature and the film characteristics, such as film density, crystallization degree, grain size and surface states, inducing a variation in the electrical parameters, such as the leakage, D it , equivalent oxide thickness, SS, and I ON , as well as I ON /I OFF of the MoS 2 field effect transistors with the same channel materials and fabrication methods. With a balance between the crystallization degree and the surface state, the ultrathin (10 nm) HfO 2 gate dielectric RTA at 500 °C is demonstrated to have the best performance with a field effect mobility of 40 cm 2 V −1 s −1 and the lowest SS of 77.6 mV −1 decade, which are superior to those of the control samples at other temperatures. The excellent transistor results with an optimized industry-based HfO 2 ALD and RTA process provide a promising approach for MoS 2 applications into the scaling of the nanoscale CMOS process.
In this article, multilayer MoS 2 manufactured from a multiple-transfer process of chemical vapor deposition (CVD)-grown monolayer MoS 2 is studied. Because of the lattice mismatch and larger distance between adjacent MoS 2 layers, the interlayer interaction is weakened and the band structure transition from direct to indirect as well as band gap shrinkage effect in multilayer is suppressed, as indicated by Raman and photoluminescence spectra. These structural differences from that of the exfoliated MoS 2 make stacked MoS 2 layers a better configuration for fabricating high-performance MoS 2 field effect transistor (FET). Here, back-gate MoS 2 FETs with different number of layers were fabricated. As the number of layers increases from 1 to 3, the devices' mobility and on/off ratio show an enhancement from 2 to 62 cm 2 /s•V and 10 6 to 10 8 , respectively. Metal-to-insulator transition (MIT) phenomena are also observed in bilayer MoS 2 FET. A distributed resistance-based model is proposed to study the conductivity of weakly coupled MoS 2 layers. Combining the resistance model with temperature dependence characteristics, it is demonstrated that the electron mobility in monolayer MoS 2 is limited by the hopping transport mechanism, whereas the electron in the bilayer can be excited to band-like transport mode because of the immunity of the influence from the charge traps at the substrate, which explains the enhancement of mobility and MIT phenomena. This study is universally valid for other twodimensional materials, paving way to fabricate high-performance nanoelectronics for integrated circuits.
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