Muhammad Farooq Khan

Improving the electrical properties of graphene layers by chemical doping

Abstract Although the electronic properties of graphene layers can be modulated by various doping techniques, most of doping methods cost degradation of structural uniqueness or electrical mobility. It is matter of huge concern to develop a technique to improve the electrical properties of graphene while sustaining its superior properties. Here, we report the modification of electrical properties of single- bi- and trilayer graphene by chemical reaction with potassium nitrate (KNO3) solution. Raman spectroscopy and electrical transport measurements showed the n-doping effect of graphene by KNO3. The effect was most dominant in single layer graphene, and the mobility of single layer graphene was improved by the factor of more than 3. The chemical doping by using KNO3 provides a facile approach to improve the electrical properties of graphene layers sustaining their unique characteristics.

Tuning the electrical properties of exfoliated graphene layers using deep ultraviolet irradiation

It is possible to tune the electrical properties of graphene layers in graphene-based nanoelectronic and optoelectronic devices while maintaining their unique band structure and electrical properties. We report here the use of deep ultraviolet irradiation (DUV) to tune the electronic properties of mechanically exfoliated single-layer graphene (SLG), bilayer graphene (BLG), and trilayer graphene (TLG). Raman spectroscopy and electrical transport measurements showed that DUV imposes p-doping on SLG, BLG, and TLG devices. The shift in the G and 2D peak wave numbers and intensity ratios of SLG, BLG, and TLG devices were examined as a function of irradiation time. Analysis of the shift in the Dirac points as a function of irradiation time indicated the p-type doping effect for all SLG, BLG, and TLG devices. This investigation has shown that DUV irradiation is a non-destructive approach which can be used to tailor the electrical properties of SLG, BLG, and TLG while maintaining their important structural and electrical characteristics.

Highly Stable and Tunable Chemical Doping of Multilayer WS2 Field Effect Transistor: Reduction in Contact Resistance

The development of low resistance contacts to 2D transition-metal dichalcogenides (TMDs) is still a big challenge for the future generation field effect transistors (FETs) and optoelectronic devices. Here, we report a chemical doping technique to achieve low contact resistance by keeping the intrinsic properties of few layers WS2. The transfer length method has been used to investigate the effect of chemical doping on contact resistance. After doping, the contact resistance (Rc) of multilayer (ML) WS2 has been reduced to 0.9 kΩ·μm. The significant reduction of the Rc is mainly due to the high electron doping density, thus a reduction in Schottky barrier height, which limits the device performance. The threshold voltage of ML-WS2 FETs confirms a negative shift upon the chemical doping, as further confirmed from the positions of E(1)2g and A1g peaks in Raman spectra. The n-doped samples possess a high drain current of 65 μA/μm, with an on/off ratio of 1.05 × 10(6) and a field effect mobility of 34.7 cm(2)/(V·s) at room temperature. Furthermore, the photoelectric properties of doped WS2 flakes were also measured under deep ultraviolet light. The potential of using LiF doping in contact engineering of TMDs opens new ways to improve the device performance.

Photocurrent Response of MoS2 Field-Effect Transistor by Deep Ultraviolet Light in Atmospheric and N2 Gas Environments

Molybdenum disulfide (MoS2), which is one of the representative transition metal dichalcogenides, can be made as an atomically thin layer while preserving its semiconducting characteristics. We fabricated single-, bi-, and multilayer MoS2 field-effect transistor (FET) by the mechanical exfoliation method and studied the effect of deep ultraviolet (DUV) light illumination. The thickness of the MoS2 layers was determined using an optical microscope and further confirmed by Raman spectroscopy and atomic force microscopy. The MoS2 FETs with different number of layers were assessed for DUV-sensitive performances in various environments. The photocurrent response to DUV light becomes larger with increasing numbers of MoS2 layers and is significantly enhanced in N2 gas environment compared with that in atmospheric environment.

Room temperature spin valve effect in NiFe/WS2/Co junctions

The two-dimensional (2D) layered electronic materials of transition metal dichalcogenides (TMDCs) have been recently proposed as an emerging canddiate for spintronic applications. Here, we report the exfoliated single layer WS2-intelayer based spin valve effect in NiFe/WS2/Co junction from room temperature to 4.2 K. The ratio of relative magnetoresistance in spin valve effect increases from 0.18% at room temperature to 0.47% at 4.2 K. We observed that the junction resistance decreases monotonically as temperature is lowered. These results revealed that semiconducting WS2 thin film works as a metallic conducting interlayer between NiFe and Co electrodes.

Large-area, continuous and high electrical performances of bilayer to few layers MoS 2 fabricated by RF sputtering via post-deposition annealing method

We report a simple and mass-scalable approach for thin MoS2 films via RF sputtering combined with the post-deposition annealing process. We have prepared as-sputtered film using a MoS2 target in the sputtering system. The as-sputtered film was subjected to post-deposition annealing to improve crystalline quality at 700 °C in a sulfur and argon environment. The analysis confirmed the growth of continuous bilayer to few-layer MoS2 film. The mobility value of ~29 cm2/Vs and current on/off ratio on the order of ~104 were obtained for bilayer MoS2. The mobility increased up to ~173–181 cm2/Vs, respectively, for few-layer MoS2. The mobility of our bilayer MoS2 FETs is larger than any previously reported values of single to bilayer MoS2 grown on SiO2/Si substrate with a SiO2 gate oxide. Moreover, our few-layer MoS2 FETs exhibited the highest mobility value ever reported for any MoS2 FETs with a SiO2 gate oxide. It is presumed that the high mobility behavior of our film could be attributed to low charged impurities of our film and dielectric screening effect by an interfacial MoOxSiy layer. The combined preparation route of RF sputtering and post-deposition annealing process opens up the novel possibility of mass and batch production of MoS2 film.

Electrical and photo-electrical properties of MoS2 nanosheets with and without an Al2O3 capping layer under various environmental conditions

Abstract The electrical and photo-electrical properties of exfoliated MoS2 were investigated in the dark and in the presence of deep ultraviolet (DUV) light under various environmental conditions (vacuum, N2 gas, air, and O2 gas). We examined the effects of environmental gases on MoS2 flakes in the dark and after DUV illumination through Raman spectroscopy and found that DUV light induced red and blue shifts of peaks (E12 g and A1 g) position in the presence of N2 and O2 gases, respectively. In the dark, the threshold voltage in the transfer characteristics of few-layer (FL) MoS2 field-effect transistors (FETs) remained almost the same in vacuum and N2 gas but shifted toward positive gate voltages in air or O2 gas because of the adsorption of oxygen atoms/molecules on the MoS2 surface. We analyzed light detection parameters such as responsivity, detectivity, external quantum efficiency, linear dynamic range, and relaxation time to characterize the photoresponse behavior of FL-MoS2 FETs under various environmental conditions. All parameters were improved in their performances in N2 gas, but deteriorated in O2 gas environment. The photocurrent decayed with a large time constant in N2 gas, but decayed with a small time constant in O2 gas. We also investigated the characteristics of the devices after passivating by Al2O3 film on the MoS2 surface. The devices became almost hysteresis-free in the transfer characteristics and stable with improved mobility. Given its outstanding performance under DUV light, the passivated device may be potentially used for applications in MoS2-based integrated optoelectronic circuits, light sensing devices, and solar cells.

Tailoring the electrical and photo-electrical properties of a WS2 field effect transistor by selective n-type chemical doping

Here, we demonstrate a doping technique which remarkably improves the electrical and photoelectric characteristics of a WS2 field effect transistor (FET) by chemical doping. The shift of the threshold voltage towards a negative gate voltage and the red shift of the E12g and A1g peaks in the Raman spectra confirm the n-type doping effect in WS2 FETs. WS2 films show an unprecedented high mobility of 255 cm2 V−1 s−1 at room temperature. The on/off ratio of the output current is ∼108 at room temperature. The mobility of a multilayer ML-WS2 FET was found to be 425 cm2 V−1 s−1 at 5 K. Semiconductor-to-metal transitions were also observed at Vbg > 30 V. A decrease in contact and sheet resistance was observed after potassium iodide (KI) doping. The photocurrent in WS2 FETs was also enhanced after n-type doping. Chemical doping exhibited a very stable, effective, and easy-to-apply method to enhance the performance of a WS2 FET.

A progressive route for tailoring electrical transport in MoS2

Typically, molybdenum disulfide (MoS2) synthesized by chemical vapor deposition (CVD) is polycrystalline; as a result, the scattering of charge carriers at grain boundaries can lead to performances lower than those observed in exfoliated single-crystal MoS2. Until now, the electrical properties of grain boundaries have been indirectly studied without accurate knowledge of their location. Here, we present a technique to measure the electrical behavior of individual grain boundaries in CVD-grown MoS2, imaged with the help of aligned liquid crystals. Unexpectedly, the electrical conductance decreased by three orders of magnitude for the grain boundaries with the lowest on/off ratio. Our study provides a useful technique to fabricate devices on a single-crystal area, using optimized growth conditions and device geometry. The photoresponse, studied within a MoS2 single grain, showed that the device responsivity was comparable with that of the exfoliated MoS2-based photodetectors.

Layer-modulated, wafer scale and continuous ultra-thin WS2 films grown by RF sputtering via post-deposition annealing

Tungsten disulfide (WS2) is a layered semiconducting material with a tunable bandgap that is promising in next generation nanoelectronics as well as energy harvesting devices. In this study, we presented a continuous and wafer-scale uniform WS2 layer preparation technique through sulfurization of a RF-sputtered WO3 film. Various characterization techniques were employed in order to investigate the structural and physical properties of the WS2 films. It was observed that the thickness of WS2 films could be controlled by tuning the sputtering time. The fabricated WS2 transistors exhibited high mobility values of ∼17 and 37–38 cm2 V−1 s−1 and on/off ratios in the range of ∼104 and 104–105 for 80–100 s-sputter time and 120–140 sputter time, respectively, which is in the maximum range for CVD-grown WS2 FETs with an SiO2 gate oxide. Photoresponse was also studied for a few layers of WS2 on a transparent quartz substrate and it was observed that the photosensitivity was linearly dependent on bias voltage. The proposed growth technique is attractive for next-generation transparent and nanoelectronic devices, as well as for other potential applications.