N-type Contact Research Articles

The modulation of Schottky contacts of p-type graphene-GeC/GeS heterointerface

n-Type contact of Schottky barriers at two-dimensional (2D) materials/metal interfaces is a usual formalization in the modern FETs applications. It is common to modulate it from n- to p-type through some specific methods. In this work, we came up with two new intrinsic p-type contacts of graphene-GeC/GeS and further tune them from p-type to n-type by external electric fields. It proved that the electronic properties of graphene and GeC/GeS can be roughly preserved for the weak van der Waals (vdW) interaction. p-Type contacts with relatively small barriers are formed at g-GeC/GeS heterointerfaces. After external electric field applied, the Schottky barrier can be effectively tuned by different external electric and the p-type contact further turns into n-type. Variation of the Schottky barriers indicated a partial pinning for interfaces of g-GeC/GeS. This is because the interfacial states between graphene and GeC/GeS hardly exists. The barrier height of g-GeC/GeS and the corresponding contact type can be flexibly tuned, which is of great importance in the design of novel transistors-based 2D materials. Searching for novel nanoscale electronic equipment based on 2D materials is a hot topic in the current study. This work would provide meaningful guidelines for nanoscale devices.

Tuning the band alignment of p-type graphene-AsSb Schottky contact by electric field

By combining the electronic structures of graphene and monolayer AsSb via van der Waals force interaction, the intrinsic p-type Schottky contact can be obtained. Here, a series of theoretic calculations are performed to survey the effects of interlayer coupling and the band realignment of graphene-AsSb heterointerface. It reveals that intrinsic p-type Schottky barriers of 0.184 and 0.381 eV are formed for the two types of configurations. Besides, the intrinsic electronic properties of graphene and AsSb are roughly preserved. When the external electric field is applied, the Schottky barrier can be effectively tuned up by changing the external electric field intensity and further convert the p-type contact into the n-type contact. A variation of the Schottky barriers indicates a partial Fermi level pinning at the interfaces of AsSb. It results from the low density of interfacial states between graphene and AsSb. The barrier height of AsSb and the corresponding contact type can be flexibly tuned, which is of great importance in the design of novel transistors based two-dimensional materials and they provide meaningful guidelines.

Modulation of electronic properties and Schottky barrier in the graphene/GaS heterostructure by electric gating

In this work, the structure, electronic properties, and the Schottky barrier of the van der Waals heterostructure (vdWH) based on graphene and gallium sulfide (GaS) have been theoretically considered using density functional theory. We found that the graphene/GaS vdWH keeps the extraordinary intrinsic properties of both the graphene and GaS monolayer. Moreover, an n-type Schottky contact with a small Schottky barrier of 0.51 eV was formed in the ground state of the heterostructure. Especially, our results demonstrated that applying an electric gating can tune effectively the Schottky barrier and contact types. The transformations from the n-type Schottky contact to the p-type one and from the Schottky to the Ohmic contacts were observed in the vdWH under electric gating. These results propose a great potential for the van der Waals heterostructure in future nanoelectronic and optoelectronic devices.

Layered graphene/GaS van der Waals heterostructure: Controlling the electronic properties and Schottky barrier by vertical strain

In this work, we construct an ultrathin graphene/GaS heterostructure and investigate its electronic properties as well as the effect of vertical strain using density functional theory. The calculated results of the equilibrium interlayer spacing (3.356 Å) and the binding energy show that the intrinsic properties of isolated graphene and GaS monolayers can be preserved and the weak van der Waals interactions are dominated in the heterostructures. The van der Waals heterostructure (vdWH) forms an n-type Schottky contact with a small Schottky barrier height of 0.51 eV. This small Schottky barrier height can also be tuned by applying vertical strain. Furthermore, we find that the n-type Schottky contact of the vdWH can be changed to p-type when the interlayer spacing is decreased and exceeded to 2.60 Å. These findings show the great potential application of the graphene/GaS vdWH for designing next generation devices.

The electronic and transport properties of edge contact borophane-MoSe2 heterojunction: A first principles study

First principles calculations are carried out to study the effect of contact configurations on the electronic and transport properties of borophane-MoSe2 heterojunction. Four different contact configurations between borophane and MoSe2 are designed and the most stable configurations are determined. Calculations show that metallic behavior is appeared in semiconductor MoSe2 as a result of junction formation with borophane. The metallic states are mainly located at the contact interface and produced by d states of Mo atoms, p states of B and Se atoms. The calculated band alignments indicate that n-type Schottky contacts are formed between borophane and MoSe2. The different current-voltage behaviors of two represented configurations indicate the contact configurations are important for the electronic transport properties of borophane-MoSe2 heterojunction.

PtSe2/graphene hetero-multilayer: gate-tunable Schottky barrier height and contact type

Graphene-based two-dimensional hybrid materials are attracting significant attention because they can preserve novel characteristics of Dirac cone. Here, based on first-principles methods, we focus on the electronic characteristics of PtSe2/graphene hetero-multilayer. The negative binding energies indicate that the hybrid materials can be fabricated easily in practice. Also, the n-type Schottky contact is formed and its barrier height is robust to the number of graphene layer. Moreover, the gate-voltage can effectively induce the Schottky barrier transformation from n-type to p-type and contact type transformation from Schottky to Ohmic in the PtSe2/graphene hetero-multilayer. Thus, the work demonstrates that the graphene stacking configuration and gate-voltage will tune the electronic characteristics of PtSe2/graphene-based nanodevices.

First principles study of the electronic properties and Schottky barrier in vertically stacked graphene on the Janus MoSeS under electric field

In this paper, we design novel ultra-thin graphene/MoSeS and graphene/MoSSe heterostructures and investigate systematically their structural and electronic properties as well as the effect due to perpendicularly applied electric field on the heterostructure. Our results show that the electronic properties of both the graphene (Gr) and Janus MoSeS monolayer are well kept in the Gr/MoSeS and Gr/MoSSe heterostructures due to weak interaction between them. The interlayer distance between the Gr and Janus MoSeS monolayer is derived to be 3.34 Å, whereas the binding energy in the heterostructure is found to be -3 meV per carbon atom, indicating the weak interactions between the Gr and Janus MoSeS layers. We find that in both Gr/MoSeS and Gr/MoSSe heterostructures, the Gr becomes a semiconductor with a tiny band gap of about 3 meV, forming between the π and π∗ bands at the high symmetry K point. The appearance of the fundamental band gap in the Gr makes it suitable for application in electronics and optoelectronics like as field effect transistors. Furthermore, the Gr/MoSeS heterostructure forms an n-type Schottky contact with the Schottky barrier height of 0.53 eV at the equilibrium state. Our results also indicate that the electric field applied perpendicularly to the heterostructure could control not only the Schottky barrier height, but also the Schottky contact type from the n-type to p-type. Based on these extraordinary electronic properties of ultra-thin Gr/MoSeS heterostructures, which are expected to be with applications in nanoelectronic and optoelectronic devices in the future experiments.

First principles calculations of the geometric structures and electronic properties of van der Waals heterostructure based on graphene, hexagonal boron nitride and molybdenum diselenide

In this work, by means of density functional theory, we systematically investigate the geometric structure and electronic properties of the vertical heterostructure by stacking graphene on top of single-layered hexagonal boron nitride and molybdenum diselenide (Gr/h-BN/MoSe2). Our results show that the interlayer coupling in the heterostructure is mainly governed by the weak van der Waals interactions. We find that in the heterostructure, a tiny band gap of 82 meV is opened around the Dirac K point of graphene due to sublattice symmetry breaking, and a minigap of 39 meV is opened between the π band of graphene and the vertical orbitals of MoSe2 due to their hybridization. This band gap opening in graphene makes it suitable for application in novel high-performance nanoeclectronic devices. Furthermore, the Gr/h-BN/MoSe2 heterostructure with inserting insulated h-BN layer forms an n-type Schottky contact with a small Schottky barrier height of 0.1 eV. These findings could provide helpful information for designing novel nanoelectronic devices by stacking graphene on top of single-layered h-BN and MoSe2.

First principles study on the electronic properties and Schottky barrier of Graphene/InSe heterostructure

Graphene-based van der Waals heterostructures by stacking graphene on other two-dimensional materials have recently attracted much attention due to their extraordinary properties and greatly extend the applications of the parent materials. By means of the density functional theory from first-principles calculations, in this work, the electronic properties and Schottky contact of the Graphene/InSe heterostructure, together with the effect of strain, are investigated systematically. Our results show that in the graphene/InSe heterostructure, graphene is very weakly bound to the InSe monolayer. Furthermore, we find that due to the sublattice symmetry breaking, a tiny band gap of 5 meV is opened in the graphene/InSe heterostructure, making it suitable for applications in electronic and optoelectronic devices. Moreover, we also find that the n-type Schottky contact is formed in the graphene/InSe heterostructure with a very small Schottky barrier height of 0.05 eV. The Schottky barrier height as well as Schottky contact types in the graphene/InSe heterostructure could be controlled by vertical strain applied perpendicularly to the heterostructure. When the interlayer distance between graphene and the topmost InSe monolayer is smaller than 2.40 Å, one can observe a transformation of the Schottky contact of the graphene/InSe heterostructure. Our results may provide helpful information for designing novel high-performance graphene-based van der Waals heterostructures and explore their potential applications in future nanoelectronic and optoelectronic devices.

Theoretical evaluation of contact stack for high efficiency IBC-SHJ solar cells

In this work we present a theoretical analysis of charge carriers transport mechanisms in IBC-SHJ solar cells. The concepts of contact and transport selectivity are correlated through the band bending at c-Si interface and are used to identify thin-film silicon parameters affecting fill factor (FF) and open-circuit voltage (VOC). Additionally, the transport of carriers is associated to energy barriers at the conduction band for electrons and at the valence band for holes. In case of p-type contact, the transport of holes is mainly affected by activation energy and band gap of the p-type layer and work function of the TCO. In case of n-type contact, the activation energy and work function of the doped layer impact the most on transport of electrons. Selective transport is improved by maximizing the collection of majority carrier in each doped contact stack while blocking minority carriers. In particular, low activation energy values of doped layers are crucial to minimize energy barriers for majority carriers and increase the band bending at c-Si interface. Simulation results based on TCAD Sentaurus reveal that the FF increases as the activation energy of the doped layers is reduced. Also, for the p-type contact, the bandgap of p-type layer strongly affects the band bending at c-Si interface. Particularly, widening the bandgap of p-type layer enhances passivation and transport in terms of VOC and FF but work function mismatch between the p-type layer and the related transparent conductive oxide (TCO) strongly increases as bandgap increases. This possibly makes the device less performant because it is more sensitive to activation energy of the p-layer in combination with the choice of the proper TCO. Considering realistic deposited layers, a wide bandgap p-type layer, in combination with low activation energy, potentially improves hole collection leading to maximal simulated FF = 86.8% and VOC = 754 mV for a conversion efficiency η = 27.2%.

A theoretical study on the metal contacts of monolayer gallium nitride (GaN)

Low-Schottky barrier height (SBH) metal contacts to 2D materials is indispensable for achieving high performance in atomic layer 2D materials channel based optoelectronic devices. In this study, we systematically investigate the detailed face contact properties of monolayer (ML) hexagonal gallium nitride (GaN) with six different commonly used metals (Au, Ag, Pd, Pt, Ti, and Ni) in field-effect transistors (FETs) utilizing the first principles electronic structure calculations based on density functional theory (DFT). It is found that no tunnelling barriers (TB) exist in all the ML GaN-metal face contact systems by calculating and analyzing the average effective potentials (Veff). Moreover, ML GaN undergoes a vanishing of Schottky barriers in the vertical interfaces by analyzing the binding energy, electron localization function (ELF), and projected state density (PDOS) of six contact combinations. In terms of the energy band calculation, ML GaN forms n-type Schottky contacts with Au, Pd, Pt and Ti electrodes, and an n-type ohmic contacts with Ag electrode, while a p-type ohmic contact with Ni electrode is obtained in the lateral interfaces. The results would provide an insight into the ML GaN-metal interfaces, and be beneficial for developing low dimensional GaN-based devices with high performance.

Atomic-layer deposited Nb2O5 as transparent passivating electron contact for c-Si solar cells

Passivating contacts based on metal oxides have proven to enable high energy conversion efficiencies for crystalline silicon (c-Si) solar cells at low processing complexity. In this work, the potential of atomic-layer deposited (ALD) Nb2O5 as novel electron-selective passivating contact is explored in terms of recombination parameter J0 and contact resistivity ρc. It is shown that after forming gas annealing, ALD Nb2O5 can provide adequate surface passivation with J0 values down to 25–30 fA/cm2. On HF-treated c-Si surfaces a minimum film thickness of ~ 3 nm is required to achieve this high level of passivation, whereas on surfaces with a wet-chemical SiO2 interlayer the high passivation level is persistent down to film thicknesses of only 1 nm. Ohmic n-type contacts have been achieved using Al as contacting metal, where annealing the samples after Al contacting proved crucial for obtaining good contact properties. Low contact resistivity values of 70 and 124 mΩ cm2 for 1 and 2 nm Nb2O5 films, respectively, have been achieved on c-Si substrates that received an HF treatment prior to Nb2O5 deposition. Transmission electron microscopy imaging shows that on such surfaces the annealing treatment leads to the formation of a (1.7 ± 0.2) nm interfacial oxide in between the c-Si substrate and the Nb2O5 film. The presented results demonstrate the potential of ALD Nb2O5 as electron-selective passivating contact and directions for future research are outlined.

Ohmic contacts between monolayer WSe2 and two-dimensional titanium carbides

Formation of Ohmic contact is crucial for achieving high device performance in two-dimensional (2D) materials based transistors. By using ab initio electronic structure calculations and quantum transport simulations, we reveal the great potential of 2D titanium carbides as the electrode materials for monolayer (ML) WSe2 device. In the vertical direction, both tunnel barrier and Schottky barrier are absent in all the three hybrid heterostructures of ML WSe2/Ti2C and ML WSe2/Ti2CY2 (Y = F and OH), implying a high vertical carrier injection efficiency. In the lateral direction, although pristine Ti2C and ML WSe2 form a Schottky contact with electron barrier of 0.40 eV, Ti2CF2/ML WSe2 and Ti2C(OH)2/ML WSe2 form p-type quasi-Ohmic and n-type Ohmic contacts, respectively. Therefore, 2D titanium carbides can be promising candidates as electrode materials for the ML WSe2 transistors by proper controlling of the surface functional group.

Van der Waals graphene/g-GaSe heterostructure: Tuning the electronic properties and Schottky barrier by interlayer coupling, biaxial strain, and electric gating

Graphene-based van der Waals heterostructures are expected recently to design and fabricate many novel electronic and optoelectronic devices. The combination of the electronic structures of graphene and graphene-like GaSe monolayer (g-GaSe) in an ultrathin heterostructure has been realized experimentally, such as graphene/g-GaSe field effect transistor and dual Schottky diode device. In the present work, we investigate the electronic properties of the graphene/g-GaSe heterostructures under the applied electric field, in-plane strains, and interlayer coupling. Our results show that the electronic properties of the graphene/g-GaSe heterostructures are well preserved owing to a weak vdW interaction. Especially, a tiny band gap of 13 meV has opened in the presence of the g-GaSe monolayer. We found that the n-type Schottky contact is formed in the graphene/g-GaSe heterostructure with a Schottky barrier height of 0.86 eV, which can be efficiently modulated by applying the electric field, in-plane strains, and interlayer coupling. Furthermore, a transformation from the n-type to p-type Schottky contact is observed when the applied electric field is larger than 0.1 V/Å or the interlayer distance is smaller than 3.2 Å. Our results may provide helpful information to design and fabricate the future graphene-based vdW heterostructures, such as graphene/g-GaSe heterostructure and understand the physics mechanism in the graphene-based 2D vdW heterostructures.

Delta Doped $\beta$ -Ga2O3 Field Effect Transistors With Regrown Ohmic Contacts

We report silicon delta-doped $\beta $ -Ga2O3 metal semiconductor field effect transistors (MESFETs) with source–drain ohmic contacts formed by patterned regrowth of n-type Ga2O3. We show that regrown n-type contacts can enable a lateral low-resistance contact to the two-dimensional electron gas channel, with contact resistance lower than $1.5~\Omega $ -mm. The fabricated MESFET has a peak drain current ( ${I} _{D,\text{MAX}}$ ) of 140 mA/mm, transconductance ( ${g} _{m}$ ) of 34 mS/mm, and 3-terminal off-state breakdown voltage of 170 V. The proposed device structure could provide a promising path towards vertically scaled $\beta $ -Ga2O3 field effect transistors.

4H-SiC monolithic Darlington transistors with slight current gain drop at high collector current density

Profit from high current gain features, 4H-SiC power Darlington transistor has the capacity for handling high current transmission. In this paper, monolithic Darlington transistors were fabricated using a simultaneous formation process for both n-type (emitter) and p-type (base) ohmic contact. The isolated device shows current gain of 1061 and 823 with collector current density ( J C) increasing from 200 to 800 A/cm2, exhibiting a slight current gain drop at high J C. By extracting the interface state density ( D it) between SiO2 and p-type 4H-SiC, it is found that this advantage owes to the improvement of the shallow bulk minority carrier lifetime in base region. Furthermore, ISE-TCAD (technology computer aided design) simulation was carried out to study the relationship between base minority lifetime and the current gain, from which the total base minority lifetime is estimated to be 48 ns. The open base breakdown voltage ( BV CEO) is 850 V at a leakage current of 2 μA due to the electric filed crowding at the isolation bottom between drive bipolar junction transistor (BJT) and output BJT. To solve this, non-isolated devices were also fabricated with improved BV CEO of 2370 V, indicating the superior potential of 4H-SiC monolithic Darlington transistors for high power application, while the current gain is deceased to 420, which needs further improvement.

Tuning the electronic properties and Schottky barrier height of the vertical graphene/MoS2 heterostructure by an electric gating

In this paper, the electronic properties and Schottky contact in graphene/MoS2 (G/MoS2) heterostructure under an applied electric field are investigated by means of the density functional theory. It can be seen that the electronic properties of the G/MoS2 heterostructure are preserved upon contacting owing to the weak van der Waals interaction. We found that the n-type Schottky contact is formed in the G/MoS2 heterostructure with the Schottky barrier height of 0.49 eV. Furthermore, both Schottky contact and Schottky barrier height in the G/MoS2 heterostructure could be controlled by the applied electric field. If a positive electric field of 4 V/nm is applied to the system, a transformation from the n-type Schottky contact to the p-type one was observed, whereas the system keeps an n-type Schottky contact when a negative electric field is applied. Our results may provide helpful information to design, fabricate, and understand the physics mechanism in the graphene-based two-dimensional van der Waals heterostructures like as G/MoS2 heterostructure.

Tunable Schottky barrier and electronic properties in borophene/g-C2N van der Waals heterostructures

By stacking different layers of two dimensional (2D) monolayer materials, the electronic properties of the 2D van der Waals (vdW) heterostructures can be tailored. However, the Schottky barrier formed between 2D semiconductor and metallic electrode has greatly limited the application of 2D semiconductor in nanoelectronic and optoelectronic devices. Herewith, we investigate the electronic properties of borophene/g-C2N vdW heterostructures by first-principles calculations. The results indicate that electronic structures of borophene and g-C2N are preserved in borophene/g-C2N vdW heterostructures. Meanwhile, upon the external electric field, a transition from the n-type Schottky contact to Ohmic contact is induced, and the carrier concentration between the borophene and g-C2N interfaces can be tuned. These results are expected to provide useful insight in the nanoelectronic and optoelectronic devices based on the borophene/g-C2N vdW heterostructures.

Schottky barrier tuning of the graphene/SnS2 van der Waals heterostructures through electric field

Combining the electronic structures of two-dimensional monolayers in ultrathin hybrid nanocomposites is expected to display new properties beyond their single components. The effects of external electric field (Eext) on the electronic structures of monolayer SnS2 with graphene hybrid heterobilayers are studied by using the first-principle calculations. It is demonstrated that the intrinsic electronic properties of SnS2 and graphene are quite well preserved due to the weak van der Waals (vdW) interactions. We find that the n-type Schottky contacts with the significantly small Schottky barrier are formed at the graphene/SnS2 interface. In the graphene/SnS2 heterostructure, the vertical Eext can control not only the Schottky barriers (n-type and p-type) but also contact types (Schottky contact or Ohmic contact) at the interface. The present study would open a new avenue for application of ultrathin graphene/SnS2 heterostructures in future nano- and optoelectronics.

N-Type Ohmic contact and p-type Schottky contact of monolayer InSe transistors.

Owing to their few lateral dangling bonds and enhanced gate electrostatics, two-dimensional semiconductors have attracted much attention for the fabrication of channels in next-generation field-effect transistors (FETs). Herein, combining first-principle band structure calculations with more precise quantum transport simulations, we systematically explore the interface properties between monolayer (ML) indium selenide (InSe) and a sequence of common electrodes in an FET. The ML InSe band structure is damaged by Sc, Au, Cr, Pt, and Pd electrodes but identifiable in contact with Ag, Cu, In, graphene and ML O-terminated Cr2C electrodes. A lateral n-type Schottky contact is generated with Sc, Au, Cr, Pt, Pd, and ML graphene electrodes owing to Fermi level pinning originating from the metal-induced gap states, which feature a pinning factor of 0.32. Luckily, a highly desirable lateral n-type Ohmic contact is generated with the Ag, Cu, and In electrodes. The calculated contact polarity is in agreement with the available experimental results using Au, Cr, ML graphene, Ag, and In as electrodes. Remarkably, a lateral p-type Schottky contact is generated with ML O-terminated Cr2C despite the very high work function of ML InSe. Therefore, this study offers a deeper understanding of ML InSe device interfaces and instructions for the design of ML InSe transistors.