Muhammad Atif Khan

Electrical characterization of multilayer HfSe2 field-effect transistors on SiO2 substrate

We fabricated and characterized two-dimensional field-effect transistors (FETs) based on hafnium diselenide (HfSe2) crystalline nanoflakes. The HfSe2 FET exhibits an n-type semiconductor behavior with a high on/off current ratio exceeding 7.5 × 106. In the temperature range of 120 K–280 K, the thermally activated transport is observed at high carrier concentrations, while at low concentrations and low temperatures hopping conduction dominates the transport mechanism. We also observed the metal insulator transition at carrier density of ∼1.8 × 1012 cm−2. This initial report on the physical and electrical characterization of two dimensional HfSe2 material demonstrates the feasibility of this semiconducting material for electronic devices.

Non-degenerate n-type doping by hydrazine treatment in metal work function engineered WSe₂ field-effect transistor.

We report a facile and highly effective n-doping method using hydrazine solution to realize enhanced electron conduction in a WSe2 field-effect transistor (FET) with three different metal contacts of varying work functions-namely, Ti, Co, and Pt. Before hydrazine treatment, the Ti- and Co-contacted WSe2 FETs show weak ambipolar behaviour with electron dominant transport, whereas in the Pt-contacted WSe2 FETs, the p-type unipolar behaviour was observed with the transport dominated by holes. In the hydrazine treatment, a p-type WSe2 FET (Pt contacted) was converted to n-type with enhanced electron conduction, whereas highly n-doped properties were achieved for both Ti- and Co-contacted WSe2 FETs with on-current increasing by three orders of magnitude for Ti. All n-doped WSe2 FETs exhibited enhanced hysteresis in their transfer characteristics, which opens up the possibility of developing memories using transition metal dichalcogenides.

P-doping and efficient carrier injection induced by graphene oxide for high performing WSe2 rectification devices

In this work, we fabricated multi-layer WSe2 rectifying diodes using graphene oxide (GO) as p-doping material on one side of the contacting electrodes. This GO layer can reduce the contact resistance by forming a tunneling barrier for efficient hole injection, while it increases the contact resistance for the injection of electrons. Results of Raman shift spectra and the opto-electric response of the device confirmed the p-doping effect caused by the GO layer and the formation of a barrier, respectively. We observed a gate tunable rectification effect with a forward/reverse current ratio of 104 and low reverse bias current of 10−10 A. Applying a GO layer in the fabrication of two-dimensional transition metal dichalcogenides based devices is a very useful method in the applications in future nanotechnologies.

Reduction of persistent photoconductivity in a few-layer MoS2 field-effect transistor by graphene oxide functionalization

We functionalized two-dimensional few-layer MoS2 based FET with graphene oxide (GO) in order to improve its persistent photoconductivity and photoresponse time. Both pristine and GO functionalized devices show n-type semiconductor behavior with high on/off ratio exceeding ∼105. The photoresponse of the GO–MoS2 hybrid device shows almost complete recovery from persistent photoconductivity and a substantial decrease in response time from ∼15 s in the pristine MoS2 device to ∼1 s in the GO–MoS2 device. The reasons behind this improvement have been explored and discussed on the basis of electrostatic and photo interaction between GO and MoS2. As GO is a strong candidate for various sensing applications, therefore this intelligent hybrid system, where GO interacts electrostatically with the underlying MoS2 channel, has tremendous potential to add more functionalities to a pristine MoS2 device for realizing various smart nanoscale FET-based biochemical and gas sensors for myriad applications.

High performance MoS2-based field-effect transistor enabled by hydrazine doping.

We investigated the n-type doping effect of hydrazine on the electrical characteristics of a molybdenum disulphide (MoS2)-based field-effect transistor (FET). The threshold voltage of the MoS2 FET shifted towards more negative values (from -20 to -70 V) on treating with 100% hydrazine solution with the channel current increasing from 0.5 to 25 μA at zero gate bias. The inverse subthreshold slope decreased sharply on doping, while the ON/OFF ratio increased by a factor of 100. Gate-channel coupling improved with doping, which facilitates the reduction of channel length between the source and drain electrodes without compromising on the transistor performance, making the MoS2-based FET easily scalable.

Photodetector Based on Multilayer SnSe2 Field Effect Transistor

We demonstrate a high-performance photodetector with multilayer tin diselenide (SnSe2) exfoliated from a high-quality crystal which was synthesized by the temperature gradient growth method. This SnSe2 photodetector exhibits high photoresponsivity of 5.11 × 105 A W-1 and high specific detectivity of 2.79 × 1013 Jones under laser irradiation (λ = 450 nm). We also observed a reproducible and stable time-resolved photoresponse to the incident laser beam from this SnSe2 photodetector, which can be used as a promising material for future optoelectronic applications.

Tunable Electron and Hole Injection Enabled by Atomically Thin Tunneling Layer for Improved Contact Resistance and Dual Channel Transport in MoS2/WSe2 van der Waals Heterostructure

Two-dimensional (2D) material-based heterostructures provide a unique platform where interactions between stacked 2D layers can enhance the electrical and opto-electrical properties as well as give rise to interesting new phenomena. Here, the operation of a van der Waals heterostructure device comprising of vertically stacked bilayer MoS2 and few layered WSe2 has been demonstrated in which an atomically thin MoS2 layer has been employed as a tunneling layer to the underlying WSe2 layer. In this way, simultaneous contacts to both MoS2 and WSe2 2D layers have been established by forming a direct metal-semiconductor to MoS2 and a tunneling-based metal-insulator-semiconductor contacts to WSe2, respectively. The use of MoS2 as a dielectric tunneling layer results in an improved contact resistance (80 kΩ μm) for WSe2 contact, which is attributed to reduction in the effective Schottky barrier height and is also confirmed from the temperature-dependent measurement. Furthermore, this unique contact engineering and type-II band alignment between MoS2 and WSe2 enables a selective and independent carrier transport across the respective layers. This contact engineered dual channel heterostructure exhibits an excellent gate control and both channel current and carrier types can be modulated by the vertical electric field of the gate electrode, which is also reflected in the on/off ratio of 104 for both electron (MoS2) and hole (WSe2) channels. Moreover, the charge transfer at the heterointerface is studied quantitatively from the shift in the threshold voltage of the pristine MoS2 and the heterostructure device, which agrees with the carrier recombination-induced optical quenching as observed in the Raman spectra of the pristine and heterostructure layers. This observation of dual channel ambipolar transport enabled by the hybrid tunneling contacts and strong interlayer coupling can be utilized for high-performance opto-electrical devices and applications.

Hole dephasing caused by hole–hole interaction in a multilayered black phosphorus

We study the magnetotransport of holes in a multilayered black phosphorus in a temperature range of 1.9 to 21.5 K. We observed a negative magnetoresistance at magnetic fields up to 1.5 T. This negative magetoresistance was analyzed by weak localization theory in diffusive regime. At the lowest temperature and the highest carrier density we found a phase coherence length of 48 nm. The linear temperature dependence of the dephasing rate shows that the hole-hole scattering processes with small energy transfer are the dominant contribution in breaking the carrier phase coherence.