Low-resistivity metal-semiconductor (M-S) contact is one of the urgent challenges in the research of 2D transition metal dichalcogenides (TMDs). Here, we report a chloride molecular doping technique which greatly reduces the contact resistance (Rc) in the few-layer WS2 and MoS2. After doping, the Rc of WS2 and MoS2 have been decreased to 0.7 kΩ·μm and 0.5 kΩ·μm, respectively. The significant reduction of the Rc is attributed to the achieved high electron-doping density, thus a significant reduction of Schottky barrier width. As a proof-of-concept, high-performance few-layer WS2 field-effect transistors (FETs) are demonstrated, exhibiting a high drain current of 380 μA/μm, an on/off ratio of 4 × 10(6), and a peak field-effect mobility of 60 cm(2)/(V·s). This doping technique provides a highly viable route to diminish the Rc in TMDs, paving the way for high-performance 2D nanoelectronic devices.

HfO 2 films have been grown with two atomic layer deposition (ALD) chemistries: (a) tetrakis(ethylmethylamino)hafnium (TEMAHf)+O3 and (b) HfCl4+H2O. The resulting films were studied as a function of ALD cycle number on Si(100) surfaces prepared with chemical oxide, HF last, and NH3 annealing. TEMAHf+O3 growth is independent of surface preparation, while HfCl4+H2O shows a surface dependence. Rutherford backscattering shows that HfCl4+H2O coverage per cycle is l3% of a monolayer on chemical oxide while TEMAHf+O3 coverage per cycle is 23% of a monolayer independent of surface. Low energy ion scattering, x-ray reflectivity, and x-ray photoelectron spectroscopy were used to understand film continuity, density, and chemical bonding. TEMAHf+O3 ALD shows continuous films, density >9g∕cm3, and bulk Hf–O bonding after 15 cycles [physical thickness (Tphys)=1.2±0.2nm] even on H-terminated Si(100). Conversely, on H-terminated Si(100), HfCl4+H2O requires 50 cycles (Tphys∼3nm) for continuous films and bulk Hf–O bonding. TEMAHf+O3 ALD was implemented in HfO2∕TiN transistor gate stacks, over the range 1.2nm⩽Tphys⩽3.3nm. Electrical results are consistent with material analysis suggesting that at Tphys=1.2nm HfO2 properties begin to deviate from thick film properties. At Tphys=1.2nm, electrical thickness scaling slows, gate current density begins to deviate from scaling trendlines, and no hard dielectric breakdown occurs. Most importantly, n-channel transistors show improvement in peak and high field electron mobility as Tphys scales from 3.3 to 1.2nm. This improvement may be attributed to reduced charge trapping and Coulomb scattering in thinner films. Scaled HfO2 enables 1nm equivalent oxide thickness and 82% of universal SiO2 mobility.

Articles you may be interested inHigh capacitance density metal-insulator-metal structures based on a high-κ Hf N x O y -Si O 2 -Hf Ti O y laminate stack Appl. Phys. Lett. 92, 132902 (2008); 10.1063/1.2905273 High-temperature conduction behaviors of HfO 2 / TaN -based metal-insulator-metal capacitors J. Appl. Phys. 102, 073706 (2007); 10.1063/1.2786712 Metal-insulator-metal capacitors using atomic-layer-deposited Al 2 O 3 ∕ Hf O 2 ∕ Al 2 O 3 sandwiched dielectrics for wireless communications Physical and electrical characterization of HfO 2 metal-insulator-metal capacitors for Si analog circuit applicationsThe authors report the relationship between HfO 2 crystalline phase and the resulting electrical properties. Crystallization of amorphous HfO 2 into the monoclinic phase led to a significant increase in leakage current and formation of local defects. Admixture of 10% SiO 2 avoided formation of these defects by stabilization of the tetragonal phase, and concurrently increased the permittivity to 35. This understanding enabled fabrication of crystalline HfO 2 based metal-insulator-metal capacitors able to withstand a thermal budget of 1000°C while optimizing capacitance equivalent thickness ͑Ͻ1.3 nm͒ at low leakage ͓J͑1 V͒ Ͻ 10 −7 A/cm 2 ͔.

A parameterized, Kramers–Kronig consistent, Cody–Lorentz optical model is used to simulate the dielectric response of thin HfxSiyOz films. Optical constants are determined in the range 0.75–8.35eV. The Cody–Lorentz model has three specific differences when compared to the previously employed Tauc–Lorentz model: (1) weak exponential absorption below the band gap, (2) a modified joint density-of-states, and (3) a restriction on the ε1(∞) parameter. These three differences allow the Cody–Lorentz model to have an improved fit to experimental data. As a result of a more accurate optical model for HfxSiyOz, we were able to identify an interfacial layer with thickness in close agreement with transmission electron microscopy measurements. Use of the Tauc–Lorentz model when fitting the same experimental data could not identify an interfacial layer. Results are also discussed in which the Cody–Lorentz model shows sensitivity to varying degrees of silicate composition.

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