N-type Schottky Barrier Height Research Articles

Exploring the Structural Stability of 1T-PdO2 and the Interface Properties of the 1T-PdO2/Graphene Heterojunction.

Motivated by a recent study on the air stability of PdSe2, which also reports the metastability of the PdO2 monolayer [Hoffman A. N.. npj 2D Mater. Appl.2019, 3( (1), ), 50.], in this work, we use density functional theory (DFT) to further explore the thermal, dynamic, and mechanical stability of monolayer PdO2 and study its structural and electronic properties. We further studied its vertical heterojunction composed of 1T-PdO2 and graphene monolayers. We show that both the monolayer and the heterojunction are energetically and dynamically stable with no negative frequencies in the phonon spectrum and belong to the vdW-type. 1T-PdO2 is an indirect-band-gap semiconductor with band-gap values of 0.5 eV (GGA) and 1.54 eV (HSE06). The interface properties of the heterojunction show that the n-type Schottky barrier height (SBH) becomes negative at the vertical interface in the PdO2/graphene contact, forming an Ohmic contact and mainly suggesting the potential of graphene for efficient electrical contact with the PdO2 monolayer. However, at the same time, a negative band bending occurs at the lateral interface based on the current-in-plane model. Moreover, the optical absorption of the PdO2/graphene heterojunction under visible-light irradiation is significantly enhanced compared to the situation in their free-standing monolayers.

Interface engineering of multilayer cubic boron nitride terminated diamond (111): Rational regulation of Au/diamond Schottky barriers for ambipolar applications

Ultra-wide bandgap semiconductor diamond has become a new hope for developing high power devices owing to excellent properties. We have previously demonstrated that the strong Fermi pinning effect is a critical factor limiting its development. This paper proposes terminal engineering to improve the contact quality between Au and diamond by adjusting the electron affinity of the diamond (111) surface using first-principles calculations. Remarkable changes in interfacial polarity can be achieved by inserting different thicknesses of Cubic Boron Nitride (c-BN) with H/F terminals on the diamond (111) surface. The C-xBNH have the lowest negative electron affinity (−3.80 eV ∼ −2.75 eV), whereas C-xNBF supercells have the highest positive electron affinity (4.52 eV ∼ 8.53 eV). Positive charges accumulate at Diamond/xNB-Au and Diamond/xBNH-Au interfaces, while negative charges build up at the other interlayers. Furthermore, the tunneling probability of the Diamond/xNB-Au interfaces is close to 1, indicating an excellent transmission. The ideal p-type and n-type Schottky barrier heights can be achieved by rational design of the c-BN terminated Diamond contact with Au, and desirable ambipolar regulation can be effectively achieved (Diamond/xNB-Au (x = 2, 3) and Diamond/xNBF-Au (x = 1, 2) for n-type ohmic electrodes and Diamond/xBNF-Au (x = 2, 3) for p-type ohmic electrodes). The proposed strategy allows interface polarity optimization by modifying the surface electron affinity, which can be extended to other high-performance ambipolar diamond devices with the rational design of contact metals.

Electric Field Induced Schottky to Ohmic Contact Transition in Fe3GeTe2/TMDs Contacts

Although the two-dimensional transition metal dichalcogenides (TMDs) present excellent electrical properties, the contact resistance at the interface of metal/TMDs limits the device performance. Herein, we use 2D metallic Fe3GeTe2 (FGT) as an electrode in contact with TMDs semiconductors MX2 (M = Mo, W; X = S, Se, Te) and investigate the contact properties of FGT/MX2 based on density functional theory calculations. We demonstrated that FGT/MX2 presents n-type Schottky contacts, and their n-type Schottky barrier heights are lower than that of the most common bulk metal contacts with MX2, suggesting that FGT can be used as an efficient metallic electrode for MX2. The transitions from n-type Schottky contact to p-type Schottky contact and from Schottky contact to Ohmic contact can be achieved in FGT/MX2 under the electric field. This work not only illustrates an effective method to modulate the contact types and Schottky barrier heights of FGT/MX2 contacts but also provides a route for designing the nanodevices based on FGT/MX2 electrical contacts.

Semimetal contacts to monolayer semiconductor: weak metalization as an effective mechanism to Schottky barrier lowering

Recent experiment has uncovered semimetal bismuth (Bi) as an excellent electrical contact to monolayer MoS2 with ultralow contact resistance. The contact physics of the broader semimetal/monolayer-semiconductor family beyond Bi/MoS2, however, remains largely unexplored thus far. Here we perform a comprehensive first-principle density functional theory investigation on the electrical contact properties between six archetypal two-dimensional (2D) transition metal dichalcogenide (TMDC) semiconductors, i.e. MoS2, WS2, MoSe2, WSe2, MoTe2 and WTe2, and two representative types of semimetals, Bi and antimony (Sb). As Bi and Sb work functions energetically aligns well with the TMDC conduction band edge, Ohmic or nearly-Ohmic n-type contacts are prevalent. The interlayer distance of semimetal/TMDC contacts are significantly larger than that of the metal/TMDC counterparts, which results in only weak metalization of TMDC upon contact formation. Intriguingly, such weak metalization generates semimetal-induced gap states (SMIGSs) that extends below the conduction band minimum, thus offering an effective mechanism to reduce or eliminate the n-type Schottky barrier height (SBH) while still preserving the electronic structures of 2D TMDC. A modified Schottky–Mott rule that takes into account SMIGS, interface dipole potential, and Fermi level shifting is proposed, which provides an improved agreement with the density functional theory-simulated SBH. We further show that the tunneling-specific resistivity of Sb/TMDC contacts are generally lower than the Bi counterparts, thus indicating a better charge injection efficiency can be achieved through Sb contacts. Our findings reveal the promising potential of Bi and Sb as excellent companion electrode materials for advancing 2D semiconductor device technology.

Functional groups and vertical strain regulate the electronic properties of Nb2NT2/MoTe2 heterojunction

In the current effort, a novel attempt to construct heterojunctions using Nb2N two-dimensional MXene materials with fascinating property and MoTe2 transition metal disulfide compounds is proposed. The effect of vertical strain on the electronic structure and interfacial contact properties of the heterojunction is investigated by first principles calculations. The O, F, and OH functional groups prefer to be adsorbed on top of the N atoms of the Nb2N MXene material. The configuration III of heterojunction has the lowest energy and stable model, and the equilibrium layer spacing is within the range of 3.061 Å to 3.328 Å, indicating that they belong to typical vdW heterojunctions. The large work function difference between the two components induces the transfer and rearrangement of charge. 0.12 e per cell spontaneously transfer from MoTe2 to Nb2NO2 layer. In contrast, the Nb2NF2 and Nb2N(OH)2 layers transfer 0.01 e and 0.21 e to the MoTe2 layer, respectively. n-type Ohmic contacts are formed in O terminal system, while p-type and n-type Schottky contacts are formed in F and OH terminal systems, respectively. Additionally, for vertical strain, with the decrease of the layer spacing, the p-type Schottky barrier height (SBH) decreases from 0.270 eV to 0.095 eV in former system, meanwhile the n-type Schottky barrier height in the latter system also decreases from 0.260 eV to 0.156 eV. These regulations of interest lays a theoretical foundation for bandgap engineering and Schottky junction preparation and opens a window into MXene-based heterojunction electron device.

Atomic and Electronic Structure of the Al2O3/Al Interface during Oxide Propagation Probed by Ab Initio Grand Canonical Monte Carlo.

Identifying the structure of the Al2O3/Al interface is important for advancing its performance in a wide range of applications, including microelectronics, corrosion barriers, and superconducting qubits. However, beyond the study of a few select terminations of the interface using computational methods, and top-down, laterally averaged spectroscopic and microscopic analyses, the explicit structure of the interface and the initial stages of propagation of the interface into the metal are largely unresolved. In this study, we utilize ab initio grand canonical Monte Carlo to perform a physically motivated, unbiased exploration of the interfacial composition and configuration space. We find that at equilibrium, the interface is atomically sharp with aluminum vacancies and propagates in a layer-by-layer fashion, with aluminum excess in the oxide layer at the interfacial plane. Oxygen incorporation, aluminum vacancy formation, and aluminum vacancy annihilation are the building blocks of Al2O3 formation at the interface. The localized interfacial mid-gap states from under-coordinated aluminum atoms from the oxide and the immediate depletion of aluminum states near the Fermi level upon oxygen incorporation prevent oxygen dissolution ahead of the interface front and result in the layer-by-layer propagation of the interface. This is in sharp contrast to the ZrO2/Zr system, which forms interfacial sub-oxides, and also explains the favorable self-healing nature of the Al2O3/Al system. The occupied interfacial mid-gap states also increase the calculated n-type Schottky barrier heights. Additionally, we identify that interfacial aluminum core-level shifts linearly depend on the aluminum coordination number, whereas interfacial oxygen core-level shifts depend on long-range ordering at the interface. The detailed geometric and electronic insights into the interface structure and evolution expand our understanding of this fundamental interface and have important implications for the engineering and design of Al2O3/Al-based corrosion coatings with enhanced barrier properties, controllable transistor technologies, and noise-free superconducting qubits.

Interfacial properties of two-dimensional CdS/GO from DFT

CdS/graphene bilayers are nanocomposites that may demonstrate enhanced photocatalytic activity relative to pure CdS. In this study, we examine oxygenating the graphene layer to form graphene oxide (GO) as a means of strengthening interfacial adhesion in the CdS/graphene bilayer and tuning its electronic properties. Specifically, we investigate the effects of oxygen concentration and functional group identity on the interfacial and electronic properties, using density functional theory (DFT) calculations. We find that interfacial adhesion is weakened when the oxygen concentration is increased in epoxy-functionalized GO; however, there is potential for increased adhesion with increasing hydroxyl functional group concentration. The native electronic structure of the CdS monolayer is not significantly affected by the GO layer, preserving its optimal optoelectronic properties for photocatalytic applications. However, the band edge alignment between the layers can be tuned by varying the oxygen composition of GO—increasing the overall oxygen concentration and proportion of epoxy functional groups relative to hydroxyl groups increases the n-type Schottky barrier height, thus enhancing the efficiency of photoexcited charge transfer across the interface.

Reduced Fermi Level Pinning at Physisorptive Sites of Moire-MoS2/Metal Schottky Barriers.

Weaker Fermi level pinning (FLP) at the Schottky barriers of 2D semiconductors is electrically desirable as this would allow a minimizing of contact resistances, which presently limit device performances. Existing contacts on MoS2 have a strong FLP with a small pinning factor of only ∼0.1. Here, we show that Moire interfaces can stabilize physisorptive sites at the Schottky barriers with a much weaker interaction without significantly lengthening the bonds. This increases the pinning factor up to ∼0.37 and greatly reduces the n-type Schottky barrier height to ∼0.2 eV for certain metals such as In and Ag, which can have physisorptive sites. This then accounts for the low contact resistance of these metals as seen experimentally. Such physisorptive interfaces can be extended to similar systems to better control SBHs in highly scaled 2D devices.

Effects of electric field on Schottky barrier in graphene and hexagonal boron phosphide heterostructures

Two dimensional graphene-based van der Waals heterostructures have attracted the interest of various researchers due to their unique properties. Based on the contact between metal electrodes and two dimensional materials has a substantial impact on the performance of electronic devices. The interfacial performance and Schottky contact performance with graphene and hexagonal boron phosphide (h-BP) vdW heterostructures are investigated using density functional theory calculations. We investigated eight energy-stable stacking configurations of multilayer graphene and h-BP vdW heterostructures. The different configurations of the h-BP/graphene vdW heterostructures all exhibit n-type Schottky contacts. The band structure of the h-BP in the heterostructures can be modulated under the influence of an external electric field (E field ), thus effectively manipulating the Schottky barrier height (SBH). The n-type SBH increases with the increasing positive E field and eventually the contact transforms into an Ohmic contact, while the Schottky contact transition from n-type to p-type occurs under the negative E field . The data demonstrate that modulating the electronic properties of hexagonal boron phosphide/graphere (h-BP/Gr) vdW heterostructures with E field is a promising approach, which can control the transition from Schottky to Ohmic contacts. The results can provide theoretical support for the design of controlled Schottky nanoelectronic devices. • 2D h-BP/Gr vdW heterostructures possesses a low n-type Schottky barrier height (0.15 eV–0.18 eV). • The transformate heterostructures can be transformed from an n-type Schottky contact to a p-type Schottky contact, then to an Ohmic contact. • The SBH of h-BP/Gr vdW heterostructures can be effectively regulated by an external electric field. • As the number of graphene layers increases the external electric field required for Schottky transition decreases.

Electronic properties and tunable Schottky barrier of non-Janus MoSSe/graphene heterostructures

We investigate the stability and electronic properties of van der Waals junctions composed with graphene and non-Janus MoSSe in comparison with graphene/Janus MoSSe configuration, and furthermore explore its Schottky barrier as well as the effect of external electric field. Based on first-principles calculations, we compare the interlayer distance and binding energy between them. It is found that with the adhesion of graphene, the less-stable Janus MoSSe becomes the most stable configuration. The graphene/non-Janus MoSSe configuration has the minimum interlayer distance, and the weak vdW interactions indicates the preservation of unique electronic properties in both two materials. An n-type Schottky contact with the Schottky barrier height (SBH) of 0.38 eV is formed at the equilibrium state for graphene/non-Janus MoSSe, while the n-type SBH for two configurations of graphene/Janus MoSSe is 0.67 and 0.10 eV, respectively. With the application of an external electric field perpendicularly to the surface, not only the Schottky barrier height but also the transition of n-type and p-type Schottky contacts as well as Ohmic contacts can be modulated within a relatively small electric field strength. This property distinguishes graphene/non-Janus MoSSe from graphene/Janus MoSSe configuration, and provides great potential for applications in optoelectronic and nanoelectronic and devices.

Effects of vertical strain and electrical field on electronic properties and Schottky contact of graphene/MoSe2 heterojunction

Fabricating a low Schottky barrier is still a great challenge in electrical transport behavior of nano field effect transistors (FETs). To settle this problem, the electronic properties and Schottky contact of the graphene/MoSe2 heterojunction employing various vertical strains and external electrical fields are investigated with density functional theory (DFT) to obtain low Schottky barrier. Our calculation illustrates that in the graphene/MoSe2 heterojunction, the intrinsic electronic properties of components are well preserved owing to the weak van der Waals (vdW) mutual interaction between graphene and MoSe2 monolayer. Furthermore, there is an n-type Schottky contact formed with an n-type Schottky barrier height (SBH) of 0.56 eV at the interface of graphene/MoSe2 heterojunction. It is worth noting that the type and the height of Schottky barrier are susceptible to the external electrical field or strain employed perpendicularly to the graphene/MoSe2 heterojunction. And the SBH of the interface can be reduced by tuning the interlayer distance (d) or the electrical field intensity (E). Moreover, Schottky contact transforms from n-type to p-type at d = 3.22 Å or E = +0.08 V/Å. Additionally, the Ohmic contact occurs when the magnitude of positive or negative electrical field is larger than 0.40 V/Å, respectively. Additionally, the heterojunction maintaining high carrier mobility is inferred from the effective masses of electron and hole, namely, 0.074 m0 and 0.055 m0, respectively. Our study may put forward effective strategies to enhance the electronic performance of MoSe2-based vdW heterojunction FETs.

Tuning the Schottky barrier height in graphene/monolayer-GeI2 van der Waals heterostructure

We use first-principles simulations to investigate the structural and electronic properties of a heterostructure formed by graphene and monolayer GeI2 (m-GeI2). While graphene has been extensively studied in the last 15 years, m-GeI2 has been recently proposed to be a stable 2D semiconductor with a wide-band gap, Liu et al (2018 J. Phys. Chem. C 122 22137). By staking both structures we obtain a metal-semiconductor junction, with great potential for applications in the designing of new (opto)electronic devices. The results show that the graphene Dirac cone is preserved in the graphene/m-GeI2 heterostructure. We find that there are no chemical bonds at the graphene and m-GeI2 interface, thus the heterostructure interactions are ruled by van der Waals (vdW) forces. The interface between graphene and m-GeI2 results in a n-type Schottky contact. Furthermore, we show that a transition from n-type to p-type Schottky contact can be obtained by decreasing the interlayer distance. We also modulated the Schottky barrier heights by applying a perpendicular external electric field through the vdW heterostructure. In particular, positive values resulted in an increase of the n-type Schottky barrier height, while negative electric field values induced a transition from n-type to p-type Schottky contact. From our results, we show that m-GeI2 is an interesting material to design new electronic Schottky devices based on graphene vdW heterostructures.

Tuning Schottky barrier in graphene/InSe van der Waals heterostructures by electric field

The contacts between semiconductor and metal are vital in the fabrication of nano electronic and optoelectronic devices. The contact type has a great influence on the function realization and performance of the device. In order to prepare multifunctional devices with high performance, it is necessary to modulate the barrier height and contact type at the interface. First-principles calculations based on the density functional theory (DFT) are implemented in the VASP package. The generalized gradient approximation of Perdew, Burke, and Ernzerhof (GGA-PBE) with van der Waals (vdW) correction proposed by Grimme (DFT-D3) is chosen due to its good description of long-range vdW interactions. It is demonstrated that weak vdW interactions dominate between graphene and InSe with their intrinsic electronic properties preserved. We find that the n-type ohmic contact is formed at the graphene/InSe interface with the Fermi level through the conduction band of InSe (<i>Φ</i><sub>Bn</sub> < 0). The Fermi level of graphene/InSe heterostructure moves down to below the Dirac point of graphene layer, which results in p-type (hole) doping in graphene. Moreover, the external electric field is effective to tune the Schottky barrier, which can control not only the Schottky barrier height but also the type of contact. With the negative external electric field varying from 0 to –1 V/nm, the conduction band minimum of InSe below the Fermi level declines gradually but the n-type ohmic contact is still preserved. Nevertheless, with the positive external electric field varying from 0 to 0.8 V/nm, the conduction band minimum of InSe shifts upward and across the Fermi level, the conduction band minimum of InSe is closer to the Fermi level than the valence band maximum, which indicates that the n-type Schottky contact is formed. The Fermi level moves from the the conduction band minimum to the valence band maximum of InSe when the positive external electric field increases from 0.8 V/nm to 2 V/nm. The n-type Schottky barrier height exceeds the p-type Schottky barrier height gradually, which demonstrates that the positive external electric field transforms the n-type Schottky contact into the p-type Schottky contact at the graphene/InSe interface. When the positive external electric field exceeds 2 V/nm, the valence band of InSe moves upward and cross the Fermi level (<i>Φ</i><sub>Bp</sub> < 0), the ohmic contact is obtained again. Meanwhile, p-type (hole) doping in graphene is enhanced under negative external electric field and a large positive external electric field is required to achieve n-type (electron) doping in graphene. The external electric field can control not only the amount of charge transfer but also the direction of charge transfer at the graphene/InSe interface.

Schottky barrier height of epitaxial lattice-matched TiN/Al0.72Sc0.28N metal/semiconductor superlattice interfaces for thermionic energy conversion

Since the initial development of semiconductor heterostructures in the 1960s, researchers exploring the potential of artificially structured materials for applications in quantum electronic, optoelectronic, and energy conversion devices have sought a combination of metals and semiconductors, which could be integrated at the nanoscale with atomically sharp interfaces. Initial demonstration of such metal/semiconductor heterostructures employed elemental polycrystalline metal and amorphous semiconductors that demonstrated electronic tunneling devices, and more recently, such heterostructures were utilized to demonstrate several exotic optical phenomena. However, these metal/semiconductor multilayers are not amenable to atomic-scale control of interfaces, and defects limit their device efficiencies and hinder the possibilities of superlattice growth. Epitaxial single-crystalline TiN/Al0.72Sc0.28N metal/semiconductor superlattices have been developed recently and are actively researched for thermionic emission-based waste heat to electrical energy conversion, optical hyperbolic metamaterial, and hot-electron solar-to-electrical energy conversion devices. Most of these applications require controlled Schottky barrier heights that determine current flow along the cross-plane directions. In this Letter, the electronic band alignments and Schottky barrier heights in TiN/Al0.72Sc0.28N superlattice interfaces are determined by a combination of spectroscopic and first-principles density functional theory analyses. The experimental EF(TiN)-EVBM(Al0.72Sc0.28N) at the interfaces was measured to be 1.8 ± 0.2 eV, which is a bit smaller than that of the first-principles calculation of 2.5 eV. Based on the valence band offset and the bandgap of cubic-Al0.72Sc0.28N, an n-type Schottky barrier height of 1.7 ± 0.2 eV is measured for the TiN/Al0.72Sc0.28N interfaces. These results are important and useful for designing TiN/Al0.72Sc0.28N metal/semiconductor superlattice based thermionic and other energy conversion devices.

Insight into the Al/N-GaN barrier property to realize high quality n-type Ohmic contact

Controlling the Schottky barrier at the interface between metal and the N-face n-GaN (N-GaN) is greatly vital to realize the high quality Ohmic contact. The effects of different defects on the n-type Schottky properties of Al(111) and N-GaN (Al/N-GaN) interface are studied using first-principles calculations. Our calculated results show that the n-type Schottky barrier height of Al/N-GaN interfaces is reduced by decorating defects like the nitrogen vacancy (VN) and the oxygen dopant at the nitrogen site (ON), and enhanced by introducing the gallium vacancy (VGa) and the ON−VGa complex. We demonstrate that the interface configuration and carrier transport properties play critical roles in controlling the Ohmic characteristics of metal-semiconductor contacts, based on which a schematic diagram for the formation mechanism of the n-type Schottky barrier height is established. Moreover, it is found that introducing donor defects at the Al/N-GaN interface with the ‘short’ Al–N bond results in the thinning of the depletion region and then enhances the tunneling, which is identified to be a better solution for improving the Ohmic contact of GaN-based LEDs. Our theoretical work enriches the fundamental understanding of the contact characteristics of Al/N-GaN and provides an important guide for designing the low-resistance N-GaN and metal contact devices.

Graphene/GeTe van der Waals heterostructure: Functional Schottky device with modulated Schottky barriers via external strain and electric field

Vertical heterogeneous combinations of two-dimensional (2D) materials have been evident as an effective strategy to design novel photovoltaic and optoelectronic devices. Herein, we perform the first-principles calculations about electronic and optical properties of graphene/GeTe van der Waals heterostructures (vdWHs), accompanied by the efficient modulations about Schottky barriers via an external strain and electric field. A slightly opened band gap of 2.0 meV is obtained for graphene/GeTe vdWH in the equilibrium configuration, and the Dirac point is well preserved at K point. The built-in electric field caused by the charge transfer is devoted to its visible contribution to the efficient hindrance for photoexcited electron-hole pair recombination, consequently increasing the number of carriers and extending their lifetimes. The graphene/GeTe vdWH possesses enhanced optical absorption in the visible region in contrast to the separate GeTe monolayer, indicating its significant potential applications in optoelectronic devices. Besides, the graphene/GeTe vdWH at the equilibrium state exhibits an n-type Schottky contact with n-type Schottky barrier height (SBH) of 0.684 eV. By applying suitable external vertical strains or electric fields, the SBHs can be efficiently tuned, making it of significant possibility to realize the transportations between p-type Schottky contact and n-type Schottky contact, and between Schottky contact and Ohmic contact at the graphene/GeTe interface. These findings together predict the graphene/GeTe vdWH nanocomposite as a competitive candidate in next-generation optoelectronics and Schottky devices.

Graphene/g-GeC bilayer heterostructure: Modulated electronic properties and interface contact via external vertical strains and electric fileds

Using DFT calculations, we perform the modulated electronic properties and interface contact in the graphene/GeC heterostructure by tuning the interlayer spacing, along with the application of an external electric field. The graphene/GeC interface is examined to be dominated by the van der Waals (vdW) force with equilibrium interlayer spacing of 3.413 Å and binding energy per C atom of approximately −50 meV. This indicates graphene/GeC nanostructure a type of vdW heterostructure (vdWH). A direct band gap up to 6 meV is opened at the Dirac point, with the Dirac point well preserved, suggesting its significant application as a suitable candidate in nano-electronic and optoelectronic devices. Moreover, the graphene/GeC vdWH forms a p-type Schottky contact at the equilibrium state with a Schottky barrier height (SBH) of 0.14 eV. A transition for the interface contact from Schottky to Ohmic can be achieved by modifying the interlayer spacing smaller than 3.20 Å or applying a positive electric field of 0.1–0.7 V Å−1. Interestingly, the p-type SBH (1.00 eV) can be tailored extensively approaching to the n-type SBH (1.09 eV) when negative electric field strengthened to 0.63 V Å−1, demonstrating it substantial potential for the transition of Schottky contact from p-type to n-type. The findings are crucial for designing new nano-electronic devices comprising graphene-based vdWHs, which ascribes to the feasibility for application of tunable vertical strain and electric field in industrial applications.

Adsorption of tetracyanoquinodimethane and tetrathiafulvalene on aluminium (100) surface – a first principle study of structural and electronic properties

ABSTRACTPossible adsorption configurations and electronic properties, such as charge analysis, density of states, work function and Schottky barrier height of tetracyanoquinodimethane (TCNQ) and tetrathiafulvalene (TTF) on Aluminium (100) surface is studied by using density functional theory methods with local density approximation (LDA), generalised gradient approximation (GGA), PBE and PBE-D2 methods. TCNQ is strongly adsorbed on Al(100) with adsorption energy of −3.66 eV. The charge is transferred from Al(100) to TCNQ and charge transfer occurs mainly through cyano group of TCNQ. Adsorption on Al(100) surface leads to downshift in energy difference between highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of TCNQ by 2.5 eV as result of hybridisation of p orbital of carbon and nitrogen atoms of TCNQ and p orbital of surface Al atoms. Compared to TCNQ adsorption, TTF adsorption on Al(100) surface is having less adsorption energy and type of interaction is physisorption. The charge transfer from TTF to Al(100) surface leads to decrease in work function of Al by 0.24 eV. The n-type Schottky barrier height of TCNQ/Al(100) and p-type Schottky barrier height of TTF/Al(100) is 0.68eV and 1.97 eV, respectively showing that TCNQ and Al(100) are suitable for organic photovoltaic and electrochemical applications.

1T phase as an efficient hole injection layer to TMDs transistors: a universal approach to achieve p-type contacts

Recently, the fabricated MoS2 field effect transistors (FETs) with 1T-MoS2 electrodes exhibit excellent performance with rather low contact resistance, as compared with those with metals deposited directly on 2H-MoS2 (Kappera et al 2014 Nat. Mater. 13 1128), but the reason for that remains elusive. By means of density functional theory calculations, we investigated the carrier injection at the 1T/2H MoS2 interface and found that although the Schottky barrier height (SBH) values of 1T/2H MoS2 interfaces can be tuned by controlling the stacking patterns, the p-type SBH values of 1T/2H MoS2 interfaces with different stackings are lower than their corresponding n-type SBH values, which demonstrated that the metallic 1T phase can be used as an efficient hole injection layer for 2H-MoS2. In addition, as compared to the n-type Au/MoS2 and Pd/MoS2 contacts, the p-type SBH values of 1T/2H MoS2 interfaces are much lower, which stem from the efficient hole injection between 1T-MoS2 and 2H-MoS2. This can explain the low contact resistance in the MoS2 FETs with 1T-MoS2 electrodes. Notably, the SBH values can be effectively modulated by an external electric field, and a significantly low p-type SBH value can be achieved under an appropriate electric field. We also demonstrated that this approach is also valid for WS2, WSe2 and MoSe2 systems, which indicates that the method can most likely be extended to other TMDs, and thus may open new promising avenues of contact engineering in these materials.

Promising Approach for High-Performance MoS2 Nanodevice: Doping the BN Buffer Layer to Eliminate the Schottky Barriers.

Reducing the Schottky barrier height (SBH) of metal-MoS2 interface with no deteriorating the intrinsic properties of MoS2 channel layer is crucial to realize the high-performance MoS2 nanodevice. To realize this expectation, a promising approach is present in this study by doping the boron nitride (BN) buffer layer between metal electrode and MoS2 channel layer. Results demonstrate that no matter the types of concentrations and dopants the intrinsic electronic structure, low electron effective mass of MoS2 channel layer, and the weak Fermi level pinning effects of metal/BN-MoS2 interfaces are preserved and not deteriorated. More importantly, the n- and p-type SBHs of metal/BN-MoS2 interfaces are significantly reduced by the electron-poor and -rich dopants, respectively, when the doped BN buffer layer spreads all over the nanodevice, which is in contrast to the traditional doping rule. Moreover, both the n- and p-type SBHs are further decreased and even eliminated when the concentrations of dopants increase. The n-type SBH of doped Au/BxN-MoS2 interface and the p-type SBH of doped Pt/BNx-MoS2 interface can be reduced to -0.21 and -0.61 eV by doping with high concentrations of Li and O, respectively. This theoretical work provides an effective and promising method to realize high-performance MoS2 nanodevices with negligible SBHs.