Metal-semiconductor Junction Research Articles

Modeling the frequency response of organic metal-insulator-semiconductor capacitors

Substantial improvement of organic field effect transistors (OTFTs) toward circuit applications certainly demands stable device performance upon varying frequency and temperature. Organic metal-insulator-semiconductor (MIS) capacitor has been considered to be an efficient model system to predict the device performance of OTFT under the influence of frequency and temperature. In this study we show that the capacitance dispersion with respect to frequency and temperature in organic MIS capacitor device is an inherent property, which arises due to the low conductivity of the semiconductor along with Schottky type contact at the semiconductor-metal junction. In addition we propose an equivalent circuit model to explain the dispersion of capacitance with respect to frequency and temperature.

Elucidating the role of interfacial MoS2 layer in Cu2ZnSnS4 thin film solar cells by numerical analysis

In this study, the effects of transition metal dichalcogenide, MoS2 interfacial layer formation between the Cu2ZnSnS4 (CZTS) absorber layer and Mo back contact in a conventional CZTS thin film solar cell (TFSC) structure have been studied by numerical simulation using wxAMPS-1D software. The goal of this study is to elucidate the effects of both n and p-type MoS2 on the overall CZTS solar cell’s performance from the viewpoint of metal-semiconductor junction and heterojunction band alignment. Interestingly, CZTS device, regardless of p or n-type MoS2 largely outperforms device without any MoS2 due to lower back contact barrier value. Significant transition in efficiency is noticed when acceptor (increases efficiency) or donor (decreases efficiency) concentration has a transition from 1016 cm−3 to higher concentration of 1018 cm−3 or more. Also, effect of variable electron affinity and band gap of MoS2 has been discussed from band alignment perspective. Generally, MoS2 layer with lower electron affinity and band gap is preferred to induce desirable band alignment and subsequently result in higher efficiency. All-in all, the formation of p-type MoS2 in CZTS solar cells can be tuned to improve the cell performance mainly by doping with higher acceptor doping concentration and limiting layer thickness. However, the detrimental effect of n-MoS2 can be prevented by maintaining thinner layer in the vicinity of ∼30 nm with low to moderate donor doping (<1016 cm−3).

A method to discern voltage dependent internal photoemission component from photoconductivity content in spectral response of metal-organic semiconductor-metal devices and evaluate the interface barriers

Abstract The ability to detect and determine a typically weak internal photoemission (IPE) signal from a usually strong photoconductivity (PC) signal within a spectral response (SR) measurement can be helpful in studying the interface properties of metal-semiconductor junctions, which are often critical for proper design and analysis of semiconductor electronic devices including organic photovoltaic devices. In this report, we propose a method to discern the voltage dependent IPE component from PC content in a measured SR of organic devices. The method is based on the ratio of SR measured at different voltage bias conditions in metal-organic semiconductor-metal devices. The separated IPE component, hence, can be used to estimate the associated interface barrier(s). The differentiation between IPE and PC is probably caused due to the barrier changing at the metal and organic semiconductor interface on the application of bias which causes a modulation in IPE signal with the applied bias. In this work, a theoretical basis for the method of characterisation has been developed. Based on the analysis, as an example, the barriers between [6,6] phenyl C61 butyric acid methyl ester (PCBM) and electrodes indium tin oxide and aluminium for different bias voltages have been evaluated. This simple and elegant method of studying metal and organic semiconductor barrier, when applicable, can be helpful in device design and characterisation.

Toward barrier free contact to MoSe2/WSe2 heterojunctions using two-dimensional metal electrodes

In the design of electronic devices based on two-dimensional heterojunctions, the contact between electrodes and different surfaces of two-dimensional heterojunctions may produce different effects. Furthermore, metal–semiconductor contact plays an important role in modern devices. However, due to the Fermi level pinning effect (FLPE), it is difficult to tune the Schottky barrier height between common metals (e.g. Au, Ag, and Cu) and semiconductors. Fortunately, the FLPE becomes weak at the contact between the 2D metal and 2D semiconductor, due to the suppression of metal-induced gap states. Here, we choose monolayer NbS2 as the electrode to be in contact with the MoSe2/WSe2 bilayer. The interfacial properties as well as the stacking dependence are discussed based on the density functional theory, combined with the nonequilibrium Green’s functions. Two configurations are considered, i.e. the WSe2/MoSe2/NbS2 and MoSe2/WSe2/NbS2 stacking sequences. Our results show that barrier free contact can be formed in these 2D metal–semiconductor junctions (MSJs). In addition, the transport properties of the proposed devices are sensitive to the stacking sequence. The drain-source current versus bias voltage (I–V) curve exhibits a linear relationship for the WSe2/MoSe2/NbS2 system and its resistance is much lower than the MoSe2/WSe2/NbS2 MSJ. Detailed analysis reveals that the transport properties are governed by the electronic coupling between specific interlayer states. In WSe2/MoSe2/NbS2 configuration, large overlapping states are observed, which facilitate charge transfer and result in good ohmic contact. Our work may provide a theoretical guidance for the designing of next-generation ultrathin and flexible devices.

Room temperature toluene gas sensor based on TiO2 nanoparticles decorated 3D graphene-carbon nanotube nanostructures

This work presents a highly sensitive room-temperature gas sensor based on 3D titanium dioxide/graphene-carbon nanotube (3D TiO2/G-CNT) fabricated by chemical vapor deposition and sparking methods. Characterizations by scanning electron microscopy, X-ray photoelectron spectroscopy, Raman spectroscopy and Transmission electron microscopy confirmed the formation of 3D graphene-carbon nanotube nanostructures decorated with TiO2 nanoparticles. The toluene detection performances of 3D TiO2/G-CNT structures with varying Ti sparking times were investigated in comparison with 3D G-CNT, TiO2-CNT, graphene and CNTs at room temperature. From result, the optimal sparking time of 60 s led to an optimal sensor response of 42%–500 ppm at room temperature. In addition, the optimal 3D TiO2/G-CNT exhibited substantially higher toluene response, sensitivity and selectivity than 3D G-CNT, TiO2-CNT, graphene and CNTs over a low concentration range of 50–500 ppm. The toluene-sensing mechanisms of 3D titanium dioxide/graphene-carbon nanotube nanostructures were proposed based on the formation of Schottky metal-semiconductor junctions between metallic 3D graphene-carbon nanotube structures and n-type semiconducting titanium dioxide nanoparticles due to the adsorption of toluene molecules via low-temperature reducing reactions or direct charge transfer process.

Intrinsic Performance of Germanane Schottky Barrier Field-Effect Transistors

Germanane (GeH), a hydrogenated germanium monolayer, is a new family of 2-D semiconductors, exhibiting promising potential for electronic device applications. Here, we investigate GeH Schottky barrier (SB) field-effect transistors (FETs) using atomistic quantum transport simulations. Our simulation results reveal that the ohmic-contact device with zero SB height ( $\Phi _{\text {Bn}}$ ) exhibits ~20% lower ON-current than the metal-oxide-semiconductor (MOS) FET counterpart due to the inherent tunnel barrier at the metal–semiconductor junction. We also compare 14-nm-channel GeH and black phosphorus (BP) SBFETs with a finite SB height of $\Phi _{\text {Bn}} = 0.22$ eV for both devices. Our results show that GeH outperforms BP in the ON-state, but it can suffer from larger leakage current in the OFF-state. We further investigate the effect of barrier height in GeH SBFET by varying $\Phi _{\text {Bn}}$ from 0 eV to a half bandgap (0.78 eV). In general, as barrier height increases, both ON-current and the minimum leakage current are reduced. It is also observed that, with increasing SB height, intrinsic delay increases but the required energy per switching decreases, indicating the trade-off between the device speed and the energy dissipation. Our benchmarking of GeH SBFET against GeH and BP MOSFETs demonstrates that GeH generally outperforms BP in terms of energy-delay product (EDP). By performing careful engineering of SB height along with the device threshold voltage, we show that the minimum EDP of GeH SBFET can be as comparable as that of the MOSFET counterpart, suggesting great potential of GeH SBFETs for future switching device applications.

Terahertz rectifier exploiting electric field-induced hot-carrier effect in asymmetric nano-electrode

Rectifiers have been used to detect electromagnetic waves with very low photon energies. In these rectifying devices, different methods have been utilized, such as adjusting the bandgap and the doping profile, or utilizing the contact potential of the metal–semiconductor junction to produce current flow depending on the direction of the electric field. In this paper, it is shown that the asymmetric application of nano-electrodes to a metal–semiconductor–metal (MSM) structure can produce such rectification characteristics, and a terahertz (THz) wave detector based on the nano-MSM structure is proposed. Integrated with a receiving antenna, the fabricated device detects THz radiation up to a frequency of 1.5 THz with responsivity and noise equivalent power of 10.8 V/W and respectively, estimated at 0.3 THz. The unidirectional current flow is attributed to the thermionic emission of hot carriers accelerated by the locally enhanced THz field at the sharp end of the nano-electrode. This work not only demonstrates a new type of THz detector but also proposes a method for manipulating ultrafast charge-carrier dynamics through the field enhancement of the nano-electrode, which can be applied to ultrafast photonic and electronic devices.

Highly Responsive, Polarization Sensitive, Self-Biased Single GeO2-Ge Nanowire Device for Broadband and Low Power Photodetectors

Highly uniform and dense GeO2 nanowires with Ge as the core were grown on Si (001) substrates by vapor–liquid–solid technique with an aim to utilize GeO2 as the active material for photodetectors. X-ray photoelectron spectroscopy was carried out to investigate the composition and interfaces of the resultant Ge/GeO2 NWs. The observed broad and visible photoluminescence emission from as-grown core–shell NW heterostructure is attributed to oxygen-related defect states in the GeO2 shells. Single Ge-GeO2 NW based metal–semiconductor–metal photodetectors were fabricated using nanolithography techniques. Self-driven (zero bias) detectors have been found to be responsive to a broadband spectrum from 350 to 900 nm with a peak responsivity (∼0.6 × 104 A/W) and detectivity (∼3.8 × 1012 Jones). The high gain in photocurrent has been explained using a back-to-back metal–semiconductor junction model in single NW. The device also shows sensitivity to polarization-dependence of light source. Finite element method (FEM) b...

Performance analysis of Fe-doped calcium copper titanate quadruple perovskite in optoelectronic device

We report here the synthesis of nanosized (50–55 nm) Fe-doped calcium copper titanate CaCu2.5Ti3.5FeO12 quadruple perovskite semiconductor by a two-step synthetic protocol involving citrate combustion for precursor synthesis followed by its calcination at different temperatures for the synthesis of the target materials. Materials were applied in thin film metal-semiconductor (MS) junction based Schottky Barrier Diodes (SBDs). The non-linear current-voltage characteristics indicate successful application of the materials in optoelectronic device. The device performance of the fabricated SBDs are analyzed by employing the thermionic emission (TE) theory and the space charge limited current (SCLC) theory followed by a schematic band structure.

Barrier height control in metal/silicon contacts with atomically thin MoS2 and WS2 interfacial layers

As complementary metal-oxide-semiconductor technology nodes are scaled down, lowering the contact resistance has become a critical problem for continued scaling. In this study, we suggested the reduction method of the Schottky barrier height, one of the main causes of contact resistance, by insertion of atomically thin two-dimensional (2D) materials between the metal and Si interface. Also, we found that the inserted 2D materials could modulate the work function of the metal and mitigate the Fermi level pinning, leading to reduced barrier height and, hence, reduced contact resistance of the metal–semiconductor junction. With the insertion of MoS2 and WS2 materials a two-layer thick, we achieved 160 meV reductions in the Schottky barrier height and increased the current density by 14 times for titanium contact to the n-type silicon. Finally, we suggested a modified band diagram of Ti/n-Si contacts with the 2D interfacial layer. Our results showed that employing 2D materials can be an alternative route for overcoming the contact resistance challenges in modern transistors.

Photo-catalytic deactivation of hazardous sulfate reducing bacteria using palladium nanoparticles decorated silicon carbide: A comparative study with pure silicon carbide nanoparticles

Sulfate reducing bacteria (SRB) wreaks havoc in the oil industry by being an agent for oil souring and building corrosion in pipelines and thereby degrade the quality of crude oil. Palladium nanoparticles decorated silicon carbide nanomaterial (Pd-SiC) was synthesized by a facile single step synthesis process and this nanomaterial as a photo-catalyst, as compared to pure silicon carbide (SiC) nanoparticles showed a significant enhancement in the photo-catalytic efficiency in the process of the photo-catalytic deactivation of hazardous SRB. The morphological characterization studies such as Field emission scanning electron microscopy, Transmission electron microscopy, X-ray diffraction, and Fourier transform infrared spectroscopy unambiguously revealed that SiC surface was successfully decorated with palladium (Pd) nanoparticles. Also the Pd nanoparticles decorated on the SiC surface was further substantiated by EDX and elemental mapping analysis, which clearly revealed the presence of Pd along with Si and O. The band gap energy estimated using Diffuse reflectance spectra (DRS) showed the reduction of band gap energy from 2.82 eV to 2.72 eV. The observed enhancement in the efficiency of photo-catalytic deactivation of SRB with Pd-SiC can be attributed to the significant reduction in the recombination of photo-generated charge carriers, characteristically resulting from the metal (Pd) semiconductor (SiC) junction established by this synthesis process.

Electron – Phonon interaction to tune metal – Semiconductor junction characteristics: Ultralow potential barrier and less non-thermionic emission

We present a two-step facile method to prepare Ni2O3 coral–like and flower–like nanostructures first time followed by structural, optical characterizations by XRD, FESEM, HRTEM, Raman, luminescence spectroscopy etc. We also report rectifying I–V characteristics of Ni2O3 nanostructures/Al based metal–semiconductor junction with ultralow turn-on voltages (0.36 V), potential barrier (0.33 eV), very ideal thermionic current (η = 1.11). Photo-responsive character illustrates that the junction devices could be a promising material for light sensing application. Parameters like series resistance (111.4 Ω), electron mobility (16.73 × 10−10 m2V1s−1), diffusion length (5.13 × 10−7 m), density of states (3.09 × 1040 eVm−3) etc. have been evaluated and it is discussed that the defect (Ni3+ vacancy) induced electron–phonon interaction within the active semiconducting layer plays the crucial role to determine these parameters. Most importantly, it has been identified that the charge-transport across the junction follows non-adiabatic mechanism. Our results suggest a new insight into current transport mechanism that may be generalized to understand microstructural, defect dependence MS junctions.

Electrochemical Study of Photocatalytic Activity Enhanced By Plasmon Resonance

Titanium oxide has been proven the most promising material for water treatment via Advance oxidation process (AOP) due to its low recombination rate and high capacity to generate hydroxyl radicals, its most important downside is restricted photocatalytic activity to the UV range. Enhancing photocatalytic activity under visible light has been the objective of a lot of researchers around the world. Plasmon resonance in conjunction with traditional semiconductor photocatalyst provides an opportunity to understand and improve on the already useful properties of titanium oxide. The present study is focused on the interaction of an anodized titanium surface (ATS) and silver nanoparticles (Ag-NP) deposited on the ATS by photo electrochemical electroless reduction. Photocatalytic activity enhanced by plasmon resonance is expected to improve the material activity in two fronts, via the metal semiconductor junction where the electron hole (e/h) separation and fast lane charge transfer are induced, and a second front where localized surface plasmon resonance (LSPR) benefit photocatalysis by enabling visible light response, enhancing Visible/UV absorption, local electric fields reducing (e/h) diffusion length, generation of LSPR powered (e/h), local heating effects, lastly molecule polarization. ATS was form by anodic oxidation of a titanium sheet at 30 V for 120 minutes in an organic electrolyte compose of ethylenglicol, NH4F and water at 5%. Subsequently Ag-NP were deposited by photoreduction of 0.1 M AgNO3, The material was characterized by cyclic voltammetry and scanning electron microscopy. The photo response to visible and UV light was evaluated by zero-current chronopotenciometry. The ATS/ Ag-NP system was successfully fabricated showing a porous titanium oxide surface, decorated by Ag-NP with a uniform size of around 60 nm. Results shows an increase in the visible light response provoked by the silver deposit, the way the photocatalytc response is affected appears to be dependent on the amount of silver deposited, and the size of the particles. Further studies on the effect of the nanostructures over the photocatalytic activity are being conducted. Figure 1

Two-Dimensional Materials Inserted at the Metal/Semiconductor Interface: Attractive Candidates for Semiconductor Device Contacts.

Metal-semiconductor junctions are indispensable in semiconductor devices, but they have recently become a major limiting factor precluding device performance improvement. Here, we report the modification of a metal/n-type Si Schottky contact barrier by the introduction of two-dimensional (2D) materials of either graphene or hexagonal boron nitride (h-BN) at the interface. We realized the lowest specific contact resistivities (ρc) of 3.30 nΩ cm2 (lightly doped n-type Si, ∼ 1015/cm3) and 1.47 nΩ cm2 (heavily doped n-type Si, ∼ 1021/cm3) via 2D material insertion are approaching the theoretical limit of 1.3 nΩ cm2. We demonstrated the role of the 2D materials at the interface in achieving a low ρc value by the following mechanisms: (a) 2D materials effectively form dipoles at the metal-2D material (M/2D) interface, thereby reducing the metal work function and changing the pinning point, and (b) the fully metalized M/2D system shifts the pinning point toward the Si conduction band, thus decreasing the Schottky barrier. As a result, the fully metalized M/2D system using atomically thin and well-defined 2D materials shows a significantly reduced ρc. The proposed 2D material insertion technique can be used to obtain extremely low contact resistivities in metal/n-type Si systems and will help to achieve major performance improvements in semiconductor technologies.

Charge Carrier Dynamics in Photocatalytic Hybrid Semiconductor-Metal Nanorods: Crossover from Auger Recombination to Charge Transfer.

Hybrid semiconductor-metal nanoparticles (HNPs) manifest unique, synergistic electronic and optical properties as a result of combining semiconductor and metal physics via a controlled interface. These structures can exhibit spatial charge separation across the semiconductor-metal junction upon light absorption, enabling their use as photocatalysts. The combination of the photocatalytic activity of the metal domain with the ability to generate and accommodate multiple excitons in the semiconducting domain can lead to improved photocatalytic performance because injecting multiple charge carriers into the active catalytic sites can increase the quantum yield. Herein, we show a significant metal domain size dependence of the charge carrier dynamics as well as the photocatalytic hydrogen generation efficiencies under nonlinear excitation conditions. An understanding of this size dependence allows one to control the charge carrier dynamics following the absorption of light. Using a model hybrid semiconductor-metal CdS-Au nanorod system and combining transient absorption and hydrogen evolution kinetics, we reveal faster and more efficient charge separation and transfer under multiexciton excitation conditions for large metal domains compared to small ones. Theoretical modeling uncovers a competition between the kinetics of Auger recombination and charge separation. A crossover in the dominant process from Auger recombination to charge separation as the metal domain size increases allows for effective multiexciton dissociation and harvesting in large metal domain HNPs. This was also found to lead to relative improvement of their photocatalytic activity under nonlinear excitation conditions.

Highly Efficient Visible-Light-Driven Schottky Catalyst MoN/2D g-C3N4 for Hydrogen Production and Organic Pollutants Degradation

Charge separation efficiency is vital both in photocatalytic hydrogen production and pollutants degradation, which can be enhanced by loading cocatalysts. Unfortunately, the vast majority of high active and stable cocatalysts is noble-metal (such as platinum), which greatly impedes the commercialization of photocatalysis technology. In this work, we reported a non-noble-metal Schottky catalyst MoN/2D g-C3N4 based on metal–semiconductor junction principles. MoN can serve as the acceptor and transporter of photogenerated electrons. For photocatalytic performance, the best one achieved much higher efficiency for hydrogen (H2) generation (265.1 times) and Rh B degradation (1.4 times) over bare g-C3N4 due to the improved charge separation and transportation. The advantages of MoN can be summarized as (i) non-noble-metal; (ii) superior conductivity; and (iii) abundant adsorption and active sites.

Photoelectric Detectors Based on Inorganic p-Type Semiconductor Materials.

Photoelectric detectors are the central part of modern photodetection systems with numerous commercial and scientific applications. p-Type semiconductor materials play important roles in optoelectronic devices. Photodetectors based on p-type semiconductor materials have attracted a great deal of attention in recent years because of their unique properties. Here, a comprehensive summary of the recent progress mainly on photodetectors based on inorganic p-type semiconductor materials is presented. Various structures, including photoconductors, phototransistors, homojunctions, heterojunctions, p-i-n junctions, and metal-semiconductor junctions of photodetectors based on inorganic p-type semiconductor materials, are discussed and summarized. Perspectives and an outlook, highlighting the promising future directions of this research field, are also given.

Canonical Schottky barrier heights of transition metal dichalcogenide monolayers in contact with a metal

The transition metal dichalcogenide ($MX_{2}$, where $M$=Mo, W and $X$=S, Se, Te) monolayers are of high interest for semiconducting applications at the nanoscale level; this interest is due to both their direct band gaps and high charge mobilities. In this regard, an in-depth understating of the related Schottky barrier heights, associated with the incorporation of $MX_{2}$ sheets into novel low-dimensional metal-semiconductor junctions, is of crucial importance. Herein, we generate and provide analysis of the Schottky barrier heights behavior to account for the metal-induced gap states concept as its explanation. In particular, the present investigations concentrate on the estimation of the charge neutrality levels directly by employing the primary theoretical model, $i.e.$ the cell-averaged Green's function formalism combined the complex band structure technique. The results presented herein place charge neutrality levels in the vicinity of the mid-gap; this is in agreement with previous reports and analogous to the behavior of three-dimensional semiconductors. The calculated canonical Schottky barrier heights are also found to be in agreement with other computational and experimental values in cases where the difference between electronegativities of the semiconductor and metal contact is small. Moreover, the influence of the spin-orbit effects is herein considered and supports that Schottky barrier heights have metal-induced gap state-derived character, regardless whether spin-orbit coupling interactions are considered. The results presented within this report constitute a direct and vital verification of the importance of metal-induced gap states in explaining the behavior of observed Schottky barrier heights at $MX_{2}$-metal junctions.

ZnO nanorod based highly selective visible blind ultra-violet photodetector and highly sensitive NO2 gas sensor

By using 1D ZnO nanorod, obtained from a simple two step chemical method as the sensor material, we report the fabrication of highly selective metal-semiconductor-metal (MSM) ultraviolet (UV) photodetector and NO2 gas sensor. The fabricated MSM UV photodetector and gas sensor possesses simplicity in material synthesis and sensor design. The effect of morphological evolution and structural intactness on the sensing properties of UV photodetector and NO2 gas sensor are deeply studied. The well aligned 1 dimensional (1D) ZnO nanorod provides high surface area for the sensing mechanism. The UV photodetector possesses high responsivity in the UV A region (364.81 A/W) with fast photoswitching characteristics at 5 V bias. It shows formation of good ohmic contact between metal semiconductor junctions. NO2 sensing measurement was carried out at relatively lower temperature of about 175 °C. The ZnO nanorods are highly selectivity towards NO2 gas and maximum gas response observed is 35 at 40 ppm of NO2. The NO2 gas sensor shows good repeatability at lower gas concentration of 2 ppm. The present synthesis technique provides cost effective route for obtaining highly ordered ZnO nanorods with simple sensor technology for multifunctional ZnO nanorod sensor.

Isotope Effect of Hot Electrons Generated on Pt Nanoparticle Surfaces Under H2 and D2 Oxidation

Hot electrons are generated when an exothermic chemical reaction takes place on the surface of a metal catalyst. Detection of these electrons using a catalytic nanodiode based on a metal-semiconductor Schottky junction can shed light on the mechanisms for energy transfer between the reacting molecules and the catalyst. Here, we present a study on the isotope effect of hot electron generation during the catalytic water formation reaction on platinum nanoparticles. To elucidate the isotope effect of hot electrons and to distinguish the reaction steps responsible for the creation of hot electrons, we carried out H2 and D2 oxidation reactions. We also considered the dependence of hot electron flux across the nanodiode on the temperature and geometry of the catalyst. Based on these results, we conclude that the observed effect of hot electron creation is mainly associated with energy released during the surface reaction of adsorbed hydrogen atoms and hydroxyl radicals, i.e. \({\text{H}}+{\text{OH}} \to {{\text{H}}_2}{\text{O,}}\) at high gas pressure.