Riphah International University, Lahore 2017.03. 15 High-mobility and air-stable single-layer WS2 field-effect transistors sandwiched between chemical vapor deposition-grown hexagonal BN films Dr. Muhammad Waqas Iqbal Assistant Professor, Head of Department Physics Riphah International University, Lahore

What is Nanotechnology The study of the controlling of matter on an atomic and molecular scale. Generally nanotechnology deals with structures sized between 1 to 100 nanometer in at least one dimension. Making small stuff do big things.

Two-dimensional layered materials

Graphene as a Material Graphene as a material is completely new –not only the thinnest ever but also the strongest. It is the one-atom thick planar sheet of carbon atoms, which makes it the thinnest material ever discovered. Carbon atoms in the sheet are densely packed in a two-dimensional(2D) honeycomb crystal lattice. The carbon-carbon bond length in graphene is about 0.142nm. Graphene is the basic structural element of some carbon allotropes including graphite, charcoal, carbon nanotubes & fullerenes. Graphene is highly conductive-conducting both heat & electricity better than any other material, copper & stronger than diamond. It is almost completely transparent, yet so dense that not even helium can pass through it.

Graphene as a Material High electron mobility- upto 25000cm² per volt-second . The theoretical mobility of electrons is 200 times that of silicon. The corresponding resistivity of the graphene sheet would be 10⁻⁶ Ω·cm. Among strongest bonds in nature Highly transparent, Large current carrying capability Large Young’s modulus, High thermal conductance The below is the some allotropic structures of the carbon It can be wrapped up into 0D fullerences, rolled into 1D nanotubes or stacked into 3D graphite.

Introduction With all advantages graphene cannot be used as promising material for active channel in field effect transistors (FETs) due to absence of bandgap. Thickness-dependent electronic band structure, ML-WS2(1.4 eV),SL-WS2 (2.1 eV) Chemically stable ,weak impurity band High on/off output current ratio in FET devices High thermal stability, no dangling bonds High mobility comparable to that of silicon WS2 a new competitor to graphene Low-power FETs, optoelectronic devices, memory devices and chemical sensors.

Problem Statement WS2 based devices suffer degradation in intrinsic properties because of environmental effects Interfacial charged impurities, surface roughness on Si/SiO2 substrates Suspending geometry Critically influenced by contact resistance at the metal/semiconductor interfaces

Schematic diagram and Optical images of devices SL-WS2 on h-BN b 5 µm c h-BN/SL-WS2/h-BN   5 µm Schematic of a h-BN/SL-WS2/h-BN field-effect transistor. Optical image of the mechanically exfoliated single-layer WS2 film on CVD-grown h-BN film.

Atomic force microscopy Atomic force microscopy (AFM) of CVD-grown h-BN film on SiO2 substrate. Thickness profile of CVD-grown h-BN film on SiO2 substrate along the green line in AFM image. The 6.8 nm thickness indicates nine layers of CVD-grown h-BN. AFM image of single-layer WS2 flake on h-BN film. Height profile of the single-layer WS2 along the green line in AFM image. The 0.77 nm thickness indicates one layer of WS2. 0.0 nm 20.0 nm a Lower h-BN layer Upper h-BN layer   5.0 µm 0.5 µm 0.0 nm 2.0 nm SL-WS2 on h-BN c

Transport characteristics of SL-WS2 FET on SiO2 Substrate. Transfer characteristics (Ids–Vbg) of SL-WS2 FET on SiO2 substrate with Cr/Au contact. The ON/OFF ratio of the device is ~106. Output characteristics (Ids–Vds) of SL-WS2 FET at different back-gate voltages ranging from –30 V to +40 V in steps of 10 V.

Transport properties of SL-WS2 FETs on SiO2 substrate with Al contacts Output characteristics (Ids–Vds) of SL-WS2 FET at different back-gate voltages ranging from –30 V to +40 V in steps of 10 V. Transfer characteristics (Ids–Vbg) of the SL-WS2 FET on SiO2 substrate with Al/Au contacts. ON/OFF ratio of the device is ~107 at room temperature.

Transport properties of SL-WS2 FETs on CVD h-BN film with Al contacts. Transfer characteristics (Ids–Vbg) of the mechanically exfoliated SL-WS2 FET on CVD-grown h-BN film at 300 K. Transfer characteristics (Ids–Vbg) of the mechanically exfoliated SL-WS2 FET enclosed by h-BN at 300 K. ON/OFF ratio of the device is ~107. Transfer characteristics (Ids–Vbg) of the mechanically exfoliated SL-WS2 FET enclosed by h-BN (device #2) at 300 K. Transfer characteristics (Ids–Vbg) of h-BN/SL-WS2/h-BN after exposure to DUV + O2 treatment for a certain time.

Temperature-dependent electrical transport properties of SL-WS2 FETs Transfer characteristics (Ids–Vbg) of the mechanically exfoliated SL-WS2 FET enclosed by h-BN films at different temperatures. Output current as function of temperature for different values of the back-gate voltage. Electron field-effect mobility of SL-WS2 FETs on different substrates at various temperatures.

Hysteresis in transfer characteristics of SL-WS2 FETs on different substrates. Transfer characteristics (Ids–Vbg) of SL-WS2 FET on SiO2 substrate, in which the back-gate voltage was swept continuously from −70 V to +40 V and from +40 V to −70 V. Transfer characteristics (Ids–Vbg) of the mechanically exfoliated SL-WS2 FET enclosed by h-BN films. Measurement was performed under vacuum at room temperature.

Contact resistance measurement by transfer length method Cr/Au Al/Au SL-WS2 a Optical image of SL-WS2 device with Cr/Au (10/80 nm) and Al/Au (60/40 nm) contacts to measure the contact resistance by transfer length method. Specific contact resistant (rc = RcW) for the SL-WS2 device with Al/Au contact was 1.25 kW.mm, whereas it was 6.55 kW.µm for Cr/Au contact

Raman spectra of WS2 films on different substrates Raman spectra for SL-WS2 on SiO2 and h-BN/SL-WS2/h-BN. Lorentzian fitting for E2g and 2LA(M) peaks. Statistical distribution of the Raman intensity ratio (I2LA(M)/IA1g).

Summary Low SBH was achieved by using low work function metals SL-WS2 FET of unprecedented high quality has been achieved by CVD-grown h-BN films as substrate and capping layer SL-WS2 FET on h-BN film exhibited high-mobility and transfer characteristics that are free of charged impurities in comparison with SL-WS2 FET on SiO2. Semiconductor-to-metal transition was also observed when Vbg was increased over 10 V h-BN/SL-WS2/h-BN structure offered considerable advantages in fabricating stable WS2 electronic devices

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