Divya Somvanshi

Bio: Divya Somvanshi is an academic researcher from Jadavpur University. The author has contributed to research in topics: Schottky diode & Materials science. The author has an hindex of 11, co-authored 32 publications receiving 375 citations. Previous affiliations of Divya Somvanshi include Georgia State University & Banaras Hindu University.

Nature of carrier injection in metal/2D-semiconductor interface and its implications for the limits of contact resistance

Abstract: Monolayers of transition metal dichalcogenides (TMDCs) exhibit excellent electronic and optical properties. However, the performance of these two-dimensional (2D) devices are often limited by the large resistance offered by the metal contact interface. To date, the carrier injection mechanism from metal to 2D TMDC layers remains unclear, with widely varying reports of Schottky barrier height (SBH) and contact resistance (${R}_{\mathrm{c}}$), particularly in the monolayer limit. In this paper, we use a combination of theory and experiments in Au and Ni contacted monolayer $\mathrm{Mo}{\mathrm{S}}_{2}$ device to elucidate the following points: (i) the carriers are injected at the source contact through a cascade of two potential barriers---the barrier heights being determined by the degree of interaction between the metal and the TMDC layer; (ii) the conventional Richardson equation becomes invalid due to the multidimensional nature of the injection barriers, and using Bardeen-Tersoff theory, we derive the appropriate form of the Richardson equation that describes such a composite barrier; (iii) we propose a novel transfer length method (TLM) based SBH extraction methodology, to reliably extract SBH by eliminating any confounding effect of temperature dependent channel resistance variation; (iv) we derive the Landauer limit of the contact resistance achievable in such devices. A comparison of the limits with the experimentally achieved contact resistance reveals plenty of room for technological improvements.

Ultraviolet Detection Properties of p-Si/n-TiO 2 Heterojunction Photodiodes Grown by Electron-Beam Evaporation and Sol–Gel Methods: A Comparative Study

Abstract: This paper reports a comparative study of the ultraviolet (UV) detection properties of n-TiO2/p-Si heterojunction devices fabricated using two different deposition techniques namely the electron-beam evaporation (EBE) and sol–gel (SG) methods. A systematic study has also been carried out to investigate the structural, electrical, and optical properties of the as deposited TiO2 thin films on p-Si substrates by the EBE and SG methods. The electrical parameters of both the n-TiO2/p-Si heterojunction photodiodes have been measured and compared under dark and UV illumination conditions. The SG based n-TiO2/p-Si heterojunction photodiodes are observed with an excellent contrast ratio of ∼83911 at −5.2 V bias voltage, which is ∼6445 times higher than the EBE-based device. The measured responsivities of the EBE and SG based devices are ∼0.69 and ∼1.25 A/W at a bias voltage of −10 V ( P opt = 650 μW and λ = 365 nm), respectively. Thus, the n-TiO2/p-Si heterojunction diodes with SG derived TiO2 films are considered to be a better choice over the EBE-based n-TiO2/p-Si diodes for UV detection applications.

Mean Barrier Height and Richardson Constant for Pd/ZnO Thin Film-Based Schottky Diodes Grown on n-Si Substrates by Thermal Evaporation Method

Abstract: This letter reports the temperature-dependent electrical parameters of Pd/n-ZnO thin film-based Schottky diodes grown on n-Si substrates by thermal evaporation method. The parameters have been investigated by considering a Gaussian distributed barrier height across the Pd/n-ZnO interface with a standard deviation σ0 around a mean barrier height qφB,m. As compared with the reported results, the estimated values of the Richardson constant (~19.54Acm-2K-2) and mean barrier height (~1.41 eV) are much closer to their theoretically predicted values of 32Acm-2K-2 (for me*=0.27 m0) and 1.42 eV (for work function of Pd = 5.12 eV and electron affinity of ZnO = 3.7 eV), respectively.

Sol-Gel-Based Highly Sensitive Pd/n-ZnO Thin Film/n-Si Schottky Ultraviolet Photodiodes

Abstract: High-performance ultraviolet (UV) Schottky photodiodes obtained by growing Pd Schottky contacts on the sol-gel-derived n-ZnO thin films deposited on n-Si substrates have been reported in this paper. The current-voltage ( $I$ – $V$ ) measurements of the as-fabricated Schottky photodiodes show an excellent room temperature contrast ratio (i.e., the ratio of the current under UV illumination to the dark current) of $\sim 5.332\times 10^{3}$ and responsivity (i.e., the parameter characterizing the sensitivity of the device to the UV light) of $\sim 8.39$ A/W at −5 V reverse bias voltage, respectively; when the device is illuminated by an UV source of $\sim 650~\mu \text{W}$ output power at $\sim 365$ nm. The measured room temperature contrast ratio and responsivity are believed to be the highest among the reported values in the literature for ZnO thin film-based Schottky photodiodes using sol-gel method.

Analysis of Temperature-Dependent Electrical Characteristics of n-ZnO Nanowires (NWs)/p-Si Heterojunction Diodes

Abstract: This paper presents the electrical characteristics of n-zinc oxide (ZnO) nanowires (NWs)/p-Si (100) heterojunction diodes fabricated by the oxidation of thermally deposited metallic Zn on Al:ZnO-coated p-Si 〈1 0 0〉 substrates. The electrical parameters of the n-ZnO NWs/p-Si diodes have been estimated by using the room temperature capacitance-voltage (C-V) and temperature-dependent current-voltage (I-V) characteristics of the heterojunction. The carrier concentration of the ZnO NW film and the barrier height of the diode estimated from the C-V characteristics at room temperature are 1.54 × 10 15 cm -3 and 0.75 eV, respectively. The thermionic emission model was used to analyze the temperature-dependent measured I-V characteristics to estimate the parameters of the diode. The estimated values of the barrier height and ideality factor at room temperature were 0.715 eV and 2.13, respectively. The spatial barrier inhomogeneity was included in the aforementioned analysis by assuming a Gaussian distribution for the barrier height at the n-ZnO NWs/p-Si heterojunction. The Richardson constant A* of ZnO was found to be increased from a relatively low value of 9.75 ×10 - 8 A ·cm - 2 ·K - 2 to a more realistic value of 49A ·cm - 2 ·K - 2 after incorporating the barrier inhomogeneity phenomenon in the aforementioned analysis.

Two-dimensional transition metal dichalcogenides: interface and defect engineering

Abstract: Two-dimensional (2D) transition metal dichalcogenides (TMDCs) have been considered as promising candidates for next generation nanoelectronics. Because of their atomically-thin structure and high surface to volume ratio, the interfaces involved in TMDC-based devices play a predominant role in determining the device performance, such as charge injection/collection at the metal/TMDC interface, and charge carrier trapping at the dielectric/TMDC interface. On the other hand, the crystalline structures of TMDCs are enriched by a variety of intrinsic defects, including vacancies, adatoms, grain boundaries, and substitutional impurities. Customized design and engineering of the interfaces and defects provides an effective way to modulate the properties of TMDCs and finally enhance the device performance. Herein, we summarize and highlight recent advances and state-of-the-art investigations on the interface and defect engineering of TMDCs and their corresponding applications in electronic and optoelectronic devices. Various interface engineering approaches for TMDCs are overviewed, including surface charge transfer doping, TMDC/metal contact engineering, and TMDC/dielectric interface engineering. Subsequently, different types of structural defects in TMDCs are introduced. Defect engineering strategies utilized to modulate the optical and electronic properties of TMDCs, as well as the developed high-performance and functional devices are summarized. Finally, we highlight the challenges and opportunities for interface and defect engineering in TMDC materials for electronics and optoelectronics.

A comprehensive review of one-dimensional metal-oxide nanostructure photodetectors

Abstract: One-dimensional (1D) metal-oxide nanostructures are ideal systems for exploring a large number of novel phenomena at the nanoscale and investigating size and dimensionality dependence of nanostructure properties for potential applications The construction and integration of photodetectors or optical switches based on such nanostructures with tailored geometries have rapidly advanced in recent years Active 1D nanostructure photodetector elements can be configured either as resistors whose conductions are altered by a charge-transfer process or as field-effect transistors (FET) whose properties can be controlled by applying appropriate potentials onto the gates Functionalizing the structure surfaces offers another avenue for expanding the sensor capabilities This article provides a comprehensive review on the state-of-the-art research activities in the photodetector field It mainly focuses on the metal oxide 1D nanostructures such as ZnO, SnO(2), Cu(2)O, Ga(2)O(3), Fe(2)O(3), In(2)O(3), CdO, CeO(2), and their photoresponses The review begins with a survey of quasi 1D metal-oxide semiconductor nanostructures and the photodetector principle, then shows the recent progresses on several kinds of important metal-oxide nanostructures and their photoresponses and briefly presents some additional prospective metal-oxide 1D nanomaterials Finally, the review is concluded with some perspectives and outlook on the future developments in this area

Tuning the Electronic and Chemical Properties of Monolayer MoS$_2$ Adsorbed on Transition Metal Substrates

Abstract: Using first-principles calculations within density functional theory, we investigate the electronic and chemical properties of a single-layer MoS(2) adsorbed on Ir(111), Pd(111), or Ru(0001), three representative transition metal substrates having varying work functions but each with minimal lattice mismatch with the MoS(2) overlayer. We find that, for each of the metal substrates, the contact nature is of Schottky-barrier type, and the dependence of the barrier height on the work function exhibits a partial Fermi-level pinning picture. Using hydrogen adsorption as a testing example, we further demonstrate that the introduction of a metal substrate can substantially alter the chemical reactivity of the adsorbed MoS(2) layer. The enhanced binding of hydrogen, by as much as ~0.4 eV, is attributed in part to a stronger H-S coupling enabled by the transferred charge from the substrate to the MoS(2) overlayer, and in part to a stronger MoS(2)-metal interface by the hydrogen adsorption. These findings may prove to be instrumental in future design of MoS(2)-based electronics, as well as in exploring novel catalysts for hydrogen production and related chemical processes.

Universal Scaling Laws in Schottky Heterostructures Based on Two-Dimensional Materials

Abstract: We identify a new universality in the carrier transport of two-dimensional (2D) material-based Schottky heterostructures. We show that the reversed saturation current (J) scales universally with temperature (T) as log(J/T^{β})∝-1/T, with β=3/2 for lateral Schottky heterostructures and β=1 for vertical Schottky heterostructures, over a wide range of 2D systems including nonrelativistic electron gas, Rashba spintronic systems, single- and few-layer graphene, transition metal dichalcogenides, and thin films of topological solids. Such universalities originate from the strong coupling between the thermionic process and the in-plane carrier dynamics. Our model resolves some of the conflicting results from prior works and is in agreement with recent experiments. The universal scaling laws signal the breakdown of β=2 scaling in the classic diode equation widely used over the past sixty years. Our findings shall provide a simple analytical scaling for the extraction of the Schottky barrier height in 2D material-based heterostructures, thus paving the way for both a fundamental understanding of nanoscale interface physics and applied device engineering.

Chloride Molecular Doping Technique on 2D Materials: WS2 and MoS2

Abstract: 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 kohm*um and 0.5 kohm*um, respectively. The significant reduction of the Rc is attributed to the achieved high electron doping density thus significant reduction of Schottky barrier width. As a proof-ofconcept, high-performance few-layer WS2 field-effect transistors (FETs) are demonstrated, exhibiting a high drain current of 380 uA/um, an on/off ratio of 4*106, and a peak field-effect mobility of 60 cm2/V*s. This doping technique provides a highly viable route to diminish the Rc in TMDs, paving the way for high-performance 2D nano-electronic devices.