Ghazanfar Nazir

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.

Enhanced photoresponse of ZnO quantum dot-decorated MoS2 thin films

Transition metal dichalcogenides (TMDs) have been attracting attention because of their applications in optoelectronics and photo-detection. A widely used TMD semiconductor is molybdenum disulfide (MoS2), which has tremendous applications because of its tunable bandgap and high luminescence quantum efficiency. This paper reports on high photo responsivity (Rλ ∼ 1913 A W−1) of MoS2 photodetector by decorating a thin layer of zinc oxide (ZnO) quantum dots (ZnO-QDs) on MoS2. Results show that Rλ increases dramatically to 2267 A W−1 at Vbg = 30 V. The high response of ZnO-QDs/MoS2 heterostructures is attributed to a number of factors, such as effective charge transfer between ZnO-QDs and MoS2 surface and re-absorption of light photon resulting in production of electron–hole pairs.

Two- and four-probe field-effect and Hall mobilities in transition metal dichalcogenide field-effect transistors

We have fabricated WS2 and MoS2 multilayer field-effect transistors (FETs) to compare two-probe and four-probe field-effect and Hall mobility measurements. Hall mobility provides accurate information and shows the largest value, whereas field-effect mobility shows small values. The influence of contact resistance is not negligible in the two-probe field-effect mobility measurement. The current–voltage characteristics of Cr/Au (8/80 nm) contacts exhibit ohmic behavior in the WS2 multilayer FET and non-linear behavior in the MoS2 multilayer FET. Regardless of the electrical characteristics of the contacts, the field-effect mobility is much lower than the Hall mobility. Electrical contacts in the WS2 or MoS2-based FETs produce a non-discountable influence on the field-effect mobility estimation in the two-probe configuration. When the carrier concentration is not linearly dependent on gate voltage, the equivalence of field-effect and Hall mobilities does not hold. In this case, field-effect mobility provides only a rough estimate of Hall mobility.

A facile route to a high-quality graphene/MoS2 vertical field-effect transistor with gate-modulated photocurrent response

Two-dimensional layered materials, such as graphene (Gr) and molybdenum disulfide (MoS2), have become fascinating and exciting candidates for next-generation electronic device materials. Their vertical combinations have led to novel electronic and photonic devices. We fabricated vertical field-effect transistors (FETs) with h-BN/Gr/MoS2/Mo multi-heterostructures. A facile route was followed to design high-quality vertical FETs with improved performance. MoS2 was directly transferred to SiO2/h-BN/Gr without using any polymer, which produced a clean interface between Gr and MoS2. A high current ON–OFF ratio of ∼106 was demonstrated with a high current density of ∼105 A cm−2. Our results were attributed to the high-quality bottom Gr on h-BN, the top-most molybdenum metal contact, and the clean interface between Gr and MoS2. The photoresponse of vertical FETs was also investigated under deep ultraviolet irradiation. The current density and photocurrent response of these vertical devices were strongly dependent on the back-gate voltage.

Gate Modulation of the Spin-orbit Interaction in Bilayer Graphene Encapsulated by WS 2 films

Graphene has gigantic potential in the development of advanced spintronic devices. The interfacial interactions of graphene with semiconducting transition metal dichalcogenides improve the electronic properties drastically, making it an intriguing candidate for spintronic applications. Here, we fabricated bilayer graphene encapsulated by WS2 layers to exploit the interface-induced spin-orbit interaction (SOI). We designed a dual gated device, where the SOI is tuned by gate voltages. The strength of induced SOI in the bilayer graphene is dramatically elevated, which leads to a strong weak antilocalization (WAL) effect at low temperature. The quantitative analysis of WAL demonstrates that the spin relaxation time is 10 times smaller than in bilayer graphene on conventional substrates. To support these results, we also examined Shubnikov-de Haas (SdH) oscillations, which give unambiguous evidence of the zero-field spin-splitting in our bilayer graphene. The spin-orbit coupling constants estimated by two different measurements (i.e., the WAL effect and SdH oscillations) show close values as a function of gate voltage, supporting the self-consistency of this study’s experimental results. The gate modulation of the SOI in bilayer graphene encapsulated by WS2 films establishes a novel way to explore the manipulation of spin-dependent transport through an electric field.

Van der Waals heterojunction diode composed of WS2 flake placed on p-type Si substrate

P-N junctions represent the fundamental building blocks of most semiconductors for optoelectronic functions. This work demonstrates a technique for forming a WS2/Si van der Waals junction based on mechanical exfoliation. Multilayered WS2 nanoflakes were exfoliated on the surface of bulk p-type Si substrates using a polydimethylsiloxane stamp. We found that the fabricated WS2/Si p-n junctions exhibited rectifying characteristics. We studied the effect of annealing processes on the performance of the WS2/Si van der Waals p-n junction and demonstrated that annealing improved its electrical characteristics. However, devices with vacuum annealing have an enhanced forward-bias current compared to those annealed in a gaseous environment. We also studied the top-gate-tunable rectification characteristics across the p-n junction interface in experiments as well as density functional theory calculations. Under various temperatures, Zener breakdown occurred at low reverse-bias voltages, and its breakdown voltage exhibited a negative coefficient of temperature. Another breakdown voltage was observed, which increased with temperature, suggesting a positive coefficient of temperature. Therefore, such a breakdown can be assigned to avalanche breakdown. This work demonstrates a promising application of two-dimensional materials placed directly on conventional bulk Si substrates.

Layer dependent magnetoresistance of vertical MoS2 magnetic tunnel junctions

Spin polarization of electrons through transition metal dichalcogenides (TMDs) from ferromagnetic metals (FMs) is a fascinating phenomenon in condensed matter physics. The spin polarized current makes high- and low-resistance states in FM/TMDs/FM junctions depending on magnetization alignment of FM electrodes. We have manifested vertical spin valve junctions by incorporating MoS2 layers of different thicknesses by an ultraclean fabrication method. The current-voltage (I-V) characteristics show the ohmic contact behavior, indicating that mono-, bi-, and tri-layer MoS2 work as conducting thin film. In contrast, FM/multilayer MoS2/FM junction shows non-linear I-V characteristics and the junction resistance increases as the temperature is lowered, indicating that multilayer MoS2 provides a tunneling barrier between FM electrodes. We have found that the magnetoresistance (MR) ratio increases gradually as the thickness of the MoS2 layer is increased. Our investigation will provide a guide to make an optimal choice in the development of magnetic tunnel junctions with two-dimensional layered TMDs.

Temperature-Dependent and Gate-Tunable Rectification in a Black Phosphorus/WS2 van der Waals Heterojunction Diode

Heterostructures comprising two-dimensional (2D) semiconductors fabricated by individual stacking exhibit interesting characteristics owing to their 2D nature and atomically sharp interface. As an emerging 2D material, black phosphorus (BP) nanosheets have drawn much attention because of their small band gap semiconductor characteristics along with high mobility. Stacking structures composed of p-type BP and n-type transition metal dichalcogenides can produce an atomically sharp interface with van der Waals interaction which leads to p-n diode functionality. In this study, for the first time, we fabricated a heterojunction p-n diode composed of BP and WS2. The rectification effects are examined for monolayer, bilayer, trilayer, and multilayer WS2 flakes in our BP/WS2 van der Waals heterojunction diodes and also verified by density function theory calculations. We report superior functionalities as compared to other van der Waals heterojunction, such as efficient gate-dependent static rectification of 2.6 × 104, temperature dependence, thickness dependence of rectification, and ideality factor of the device. The temperature dependence of Zener breakdown voltage and avalanche breakdown voltage were analyzed in the same device. Additionally, superior optoelectronic characteristics such as photoresponsivity of 500 mA/W and external quantum efficiency of 103% are achieved in the BP/WS2 van der Waals p-n diode, which is unprecedented for BP/transition metal dichalcogenides heterostructures. The BP/WS2 van der Waals p-n diodes have a profound potential to fabricate rectifiers, solar cells, and photovoltaic diodes in 2D semiconductor electronics and optoelectronics.

Comparison of Electrical and Photoelectrical Properties of ReS2 Field-Effect Transistors on Different Dielectric Substrates

As one of the newly discovered transition-metal dichalcogenides (TMDs), rhenium disulfide (ReS2) has been investigated mostly because of its unique characteristics such as the direct band gap nature even in bulk form, which is not prominent in other TMDs (e.g., MoS2, WSe2, etc.). However, this material possesses a low mobility and an on/off ratio, which restrict its usage in high-speed and fast switching applications. Low mobilities or on/off ratios can also be caused by substrate scattering as well as environmental effects. In this study, we used few-layer ReS2 (FL-ReS2) as a channel material to investigate the substrate-dependent mobility, current on/off ratio, Schottky barrier height (SBH), and trap density of states of different dielectric substrates. The hexagonal boron nitride (h-BN)/FL-ReS2/h-BN structure was observed to exhibit a high mobility of 45 cm2 V-1 s-1, current on/off ratio of about 107, the lowest SBH of about 12 mV at a zero back-gate voltage ( Vbg), and a low trap density of states of about 5 × 1013 cm-3. These quantities are reasonably superior compared to the FL-ReS2 devices on SiO2 substrates. We also observed a nearly 5-fold improvement in the photoresponsivity and external quantum efficiency values for the FL-ReS2 devices on h-BN substrates. We believe that the photonic characteristics of TMDs can be improved by using h-BN as the substrate and capping layer.

Gate Tunable Transport in Graphene/MoS2/(Cr/Au) Vertical Field-Effect Transistors

Two-dimensional materials based vertical field-effect transistors have been widely studied due to their useful applications in industry. In the present study, we fabricate graphene/MoS2/(Cr/Au) vertical transistor based on the mechanical exfoliation and dry transfer method. Since the bottom electrode was made of monolayer graphene (Gr), the electrical transport in our Gr/MoS2/(Cr/Au) vertical transistors can be significantly modified by using back-gate voltage. Schottky barrier height at the interface between Gr and MoS2 can be modified by back-gate voltage and the current bias. Vertical resistance (Rvert) of a Gr/MoS2/(Cr/Au) transistor is compared with planar resistance (Rplanar) of a conventional lateral MoS2 field-effect transistor. We have also studied electrical properties for various thicknesses of MoS2 channels in both vertical and lateral transistors. As the thickness of MoS2 increases, Rvert increases, but Rplanar decreases. The increase of Rvert in the thicker MoS2 film is attributed to the interlayer resistance in the vertical direction. However, Rplanar shows a lower value for a thicker MoS2 film because of an excess of charge carriers available in upper layers connected directly to source/drain contacts that limits the conduction through layers closed to source/drain electrodes. Hence, interlayer resistance associated with these layers contributes to planer resistance in contrast to vertical devices in which all layers contribute interlayer resistance.