An Atomically Resolved Schottky Barrier Height Approach for Bridging the Gap between Theory and Experiment at Metal-Semiconductor Heterojunctions

We propose an atomically resolved approach to capture the spatial variations of the Schottky barrier height (SBH) at metal-semiconductor heterojunctions. This proposed scheme, based on atom-specific partial density of states (PDOS) calculations, further enables calculation of the effective SBH that aligns with conductance measurements. We apply this approach to study the variations of SBH at MoS2@Au heterojunctions, in which MoS2 contains conducting and semiconducting grain boundaries (GBs). Our results reveal that there are significant variations in SBH at atoms in the defected heterojunctions. Of particular interest is the fact that the SBH in some areas with extended defects approaches zero, indicating Ohmic contact. One important implication of this finding is that the effective SBH should be intrinsically dependent on the defect density and character. Remarkably, the obtained effective SBH values demonstrate good agreement with existing experimental measurements. Thus, the present study addresses two long-standing challenges associated with SBH in MoS2-metal heterojunctions: the wide variation in experimentally measured SBH values at MoS2@metal heterojunctions and the large discrepancy between density-functional-theory-predicted and experimentally measured SBH values. Our proposed approach points out a valuable pathway for understanding and manipulating SBHs at metal-semiconductor heterojunctions.

Impact of synthesis temperature and precursor ratio on the crystal quality of MOCVD WSe2monolayers

Structural defects in transition metal dichalcogenide (TMDC) monolayers (ML) play a significant role in determining their (opto)electronic properties, triggering numerous efforts to control defect densities during material growth or by post-growth treatments. Various types of TMDC have been successfully deposited by MOCVD (metal-organic chemical vapor deposition), which is a wafer-scale deposition technique with excellent uniformity and controllability. However, so far there are no findings on the extent to which the incorporation of defects can be controlled by growth parameters during MOCVD processes of TMDC. In this work, we investigate the effect of growth temperature and precursor ratio during MOCVD of tungsten diselenide (WSe₂) on the growth of ML domains and their impact on the density of defects. The aim is to find parameter windows that enable the deposition of WSe₂ML with high crystal quality, i.e. a low density of defects. Our findings confirm that the growth temperature has a large influence on the crystal quality of TMDC, significantly stronger than found for the W to Se precursor ratio. Raising the growth temperatures in the range of 688 °C to 791 °C leads to an increase of the number of defects, dominating photoluminescence (PL) at low temperatures (5.6 K). In contrast, an increase of the molar precursor ratio (DiPSe/WCO) from 1000 up to 100 000 leads to less defect-related PL at low temperatures.

The effects of point defect type, location, and density on the Schottky barrier height of Au/MoS2 heterojunction: a first-principles study

Using DFT calculations, we investigate the effects of the type, location, and density of point defects in monolayer MoS₂ on electronic structures and Schottky barrier heights (SBH) of Au/MoS₂ heterojunction. Three types of point defects in monolayer MoS₂, that is, S monovacancy, S divacancy and Mo_S (Mo substitution at S site) antisite defects, are considered. The following findings are revealed: (1) The SBH for the monolayer MoS₂ with these defects is universally higher than that for its defect-free counterpart. (2) S divacancy and Mo_S antisite defects increase the SBH to a larger extent than S monovacancy. (3) A defect located in the inner sublayer of MoS₂, which is adjacent to Au substrate, increases the SBH to a larger extent than that in the outer sublayer of MoS₂. (4) An increase in defect density increases the SBH. These findings indicate a large variation of SBH with the defect type, location, and concentration. We also compare our results with previously experimentally measured SBH for Au/MoS₂ contact and postulate possible reasons for the large differences among existing experimental measurements and between experimental measurements and theoretical predictions. The findings and insights revealed here may provide practical guidelines for modulation and optimization of SBH in Au/MoS₂ and similar heterojunctions via defect engineering.

Barrier-assisted vapor phase CVD of large-area MoS2 monolayers with high spatial homogeneity

Atomically thin molybdenum disulphide (MoS2) is a direct band gap semiconductor with negatively charged trions and stable excitons in striking contrast to the wonder material graphene. While large-area growth of MoS2 can be readily achieved by gas-phase chemical vapor deposition (CVD), growth of continuous MoS2 atomic layers with good homogeneity is indeed one of the major challenges in vapor-phase CVD involving all-solid precursors. In this study, we demonstrate the growth of large-area continuous single crystal MoS2 monolayers on c-plane sapphire by carefully positioning the substrate using a facile staircase-like barrier. The barrier offered great control in mitigating the secondary and intermediate phases as well as second layer nucleation, and eventually a continuous monolayer with high surface homogeneity is realized. Both micro-Raman and high-resolution transmission electron microscopy (HRTEM) results confirmed the high structural quality of the grown MoS2 layers. Using low temperature photoluminescence spectroscopy, additional pieces of information are provided for the strong band-edge emission in the light of vacancy compensation and formation of Mo-O bonding. The monolayer MoS2 transferred to SiO2/Si exhibited a room temperature field-effect mobility of ∼1.2 cm2 V-1 s-1 in a back-gated two-terminal configuration.

Defect-Assisted Contact Property Enhancement in a Molybdenum Disulfide Monolayer

Contact engineering for two-dimensional (2D) transition metal dichalcogenides (TMDCs) is crucial for realizing high-performance 2D TMDC devices, and most studies on contact properties of 2D TMDCs have mainly focused on Fermi level unpinning. Here, we investigated electrical and photoelectrical properties of chemical vapor deposition (CVD)-grown molybdenum disulfide (MoS₂) monolayer devices depending on metal contacts, Ti/Pt, Ti/Au, Ti, and Ag, and particularly demonstrated the essential role of defects in MoS₂ in contact properties. Remarkably, MoS₂ devices with Ag contacts show a field-effect mobility of 12.2 cm² V^-1 s^-1, an on/off current ratio of 7 × 10⁷, and a photoresponsivity of 1020 A W^-1, which are outstanding compared to similar devices with other metal contacts. These improvements are attributed to a reduced Schottky barrier height, thanks to the small work function of Ag and Ag-MoS₂ orbital hybridization at the interface, which facilitates efficient charge transfer between MoS₂ and Ag. Interestingly, X-ray photoelectron spectroscopic analysis reveals that Ag₂S was formed in our defect-containing CVD-grown MoS₂ monolayer, but such orbital hybridization is not observed in a nearly defect-free exfoliated MoS₂. This distinction shows that defects existing in MoS₂ enable Ag to effectively couple to MoS₂ and correspondingly enhance multiple electrical and photoelectrical properties.

1T/2H multi-phase MoS2 heterostructures: synthesis, characterization and thermal catalysis decomposition of dihydroxylammonium 5,5′-bistetrazole-1,1′-diolate

Dependence of ln(β/T_p²) on 1/T_p for TKX-50 and mixtures with 10 wt% 2H-MoS₂ and 1T/2H-MoS₂.

Schottky barrier tuning of the single-layer MoS2 on magnetic metal substrates through vacancy defects and hydrogenation

For the practical device application of the two-dimensional semiconducting MoS₂, it is a critical issue to manipulate the electronic and magnetic properties locally at its contact to the metal electrode. For the tuning of those properties, we have proposed the vacancy-defective 1L-MoS₂ or the hydrogenated 1L-MoS₂ at the metal [Co(0001) or Ni(111)] contacts and performed first-principles electronic structure calculations. By controlling the atomic vacancy defects and the hydrogen coverages, we investigate the Schottky barrier heights and charge and spin transfers at the interface. Our findings provide a physical insight into the practical device design using the two-dimensional MoS₂.

The modulation of Schottky barriers of metal-MoS2 contacts via BN-MoS2 heterostructures

Using first-principles calculations within density functional theory, we systematically studied the effect of BN-MoS2 heterostructure on the Schottky barriers of metal-MoS2 contacts. Two types of FETs are designed according to the area of the BN-MoS2 heterostructure. Results show that the vertical and lateral Schottky barriers in all the studied contacts, irrespective of the work function of the metal, are significantly reduced or even vanish when the BN-MoS2 heterostructure substitutes the monolayer MoS2. Only the n-type lateral Schottky barrier of Au/BN-MoS2 contact relates to the area of the BN-MoS2 heterostructure. Notably, the Pt-MoS2 contact with n-type character is transformed into a p-type contact upon substituting the monolayer MoS2 by a BN-MoS2 heterostructure. These changes of the contact natures are ascribed to the variation of Fermi level pinning, work function and charge distribution. Analysis demonstrates that the Fermi level pinning effects are significantly weakened for metal/BN-MoS2 contacts because no gap states dominated by MoS2 are formed, in contrast to those of metal-MoS2 contacts. Although additional BN layers reduce the interlayer interaction and the work function of the metal, the Schottky barriers of metal/BN-MoS2 contacts still do not obey the Schottky-Mott rule. Moreover, different from metal-MoS2 contacts, the charges transfer from electrodes to the monolayer MoS2, resulting in an increment of the work function of these metals in metal/BN-MoS2 contacts. These findings may prove to be instrumental in the future design of new MoS2-based FETs with ohmic contact or p-type character.

Tunable Schottky contacts in MSe2/NbSe2 (M = Mo and W) heterostructures and promising application potential in field-effect transistors

MSe₂/NbSe₂ (M = Mo and W) heterostructures exhibit low and tunable Schottky barriers, indicating promising application potential in field-effect transistors.

Schottky barrier engineering via adsorbing gases at the sulfur vacancies in the metal-MoS2 interface

Sulfur vacancies (S-vacancies) are common in monolayer MoS₂ (mMoS₂). Finding an effective way to control rather than abolish the effect of S-vacancies on contact properties is vital for the application of mMoS₂. Here, we propose the adsorption of gases to passivate the S-vacancies in Pt-mMoS₂ interfaces. Results demonstrate that gases are stably and preferentially adsorbed at S-vacancies. The n-type Schottky barriers of Pt-mMoS₂ interfaces are reduced significantly upon the adsorption electron-donor gases, especially Cl₂. The n-type transport character of the Pt-mMoS₂ interface can be changed to p-type by the adsorption of electron-acceptor gases. As the adsorption concentration increases, both n- and p-type Schottky barriers are further reduced, and the lowest n- and p-type Schottky barriers are 0.36 and 0 eV, respectively. Note that the variations in Schottky barriers are independent of the oxidizing ability of gases but relative to the average number of valence electrons per gas atom. Analysis demonstrates that although gases at S-vacancies cannot cause gap states to vanish, and can even enhance Fermi level pinning, they modulate charge redistribution and the potential step at the interface region. Moreover, with increasing adsorption concentration, the valence band maximum of mMoS₂ shows the opposite variation tendency to that of the potential step. Our results suggest that adsorption of gases is an effective way to passivate S-vacancies to modulate the transport properties of Pt-mMoS₂ interfaces.

Fermi Level Pinning at Electrical Metal Contacts of Monolayer Molybdenum Dichalcogenides

Electrical metal contacts to two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDCs) are found to be the key bottleneck to the realization of high device performance due to strong Fermi level pinning and high contact resistances (R_c). Until now, Fermi level pinning of monolayer TMDCs has been reported only theoretically, although that of bulk TMDCs has been reported experimentally. Here, we report the experimental study on Fermi level pinning of monolayer MoS₂ and MoTe₂ by interpreting the thermionic emission results. We also quantitatively compared our results with the theoretical simulation results of the monolayer structure as well as the experimental results of the bulk structure. We measured the pinning factor S to be 0.11 and -0.07 for monolayer MoS₂ and MoTe₂, respectively, suggesting a much stronger Fermi level pinning effect, a Schottky barrier height (SBH) lower than that by theoretical prediction, and interestingly similar pinning energy levels between monolayer and bulk MoS₂. Our results further imply that metal work functions have very little influence on contact properties of 2D-material-based devices. Moreover, we found that R_c is exponentially proportional to SBH, and these processing parameters can be controlled sensitively upon chemical doping into the 2D materials. These findings provide a practical guideline for depinning Fermi level at the 2D interfaces so that polarity control of TMDC-based semiconductors can be achieved efficiently.

A study on the electronic and interfacial structures of monolayer ReS2–metal contacts

In this paper, we perform a systematic and rigorous study to evaluate the Ohmic nature of the top-contact formed by the monolayer ReS₂(mReS₂) and metals (gold, silver, platinum, nickel, titanium, and scandium) by means of first-principles density functional theory calculations.

Controlling the magnetic and optical responses of a MoS2 monolayer by lanthanide substitutional doping: a first-principles study

The absorption spectrum and TDOS of lanthanide doped MoS₂ for the E-field parallel and perpendicular to the xy-plane.

Performance of field-effect transistors based on Nb(x)W(1-x)S2 monolayers

The Schottky barrier has been detected in many field-effect transistors (FETs) based on transition metal dichalcogenide (TMD) semiconductors and has seriously affected the electronic properties of the devices. In order to decrease the Schottky barrier in WS2 FETs, novel Nb doping in WS2 monolayers has been performed and p-FETs based on Nb-doped WS2 (Nb(x)W(1-x)S2) monolayers as the active channel have been fabricated for the first time. The monolayer Nb0.15W0.85S2 p-FET has a drain current of 330 μA μm(-1), an impressive I(ON)/I(OFF) of 10(7), and a high effective hole mobility of ∼146 cm(2) V(-1) s(-1). The novel Nb doping in monolayer WS2 has eliminated the ambipolar behavior and reduced the Schottky barrier in WS2 FETs. The reduction of the Schottky barrier is ascribed to the hybridization between W 5d, Nb 4d and S 3p states near the EF and to the enhancement of the metallization of the contact between the Pd metal and monolayer Nb(x)W(1-x)S2 after Nb doping.

MoS2-Titanium Contact Interface Reactions

The formation of the Ti-MoS2 interface, which is heavily utilized in nanoelectronic device research, is studied by X-ray photoelectron spectroscopy. It is found that, if deposition under high vacuum (∼1 × 10(-6) mbar) as opposed to ultrahigh vacuum (∼1 × 10(-9) mbar) conditions are used, TiO2 forms at the interface rather than Ti. The high vacuum deposition results in an interface free of any detectable reaction between the semiconductor and the deposited contact. In contrast, when metallic titanium is successfully deposited by carrying out depositions in ultrahigh vacuum, the titanium reacts with MoS2 forming Ti(x)S(y) and metallic Mo at the interface. These results have far reaching implications as many prior studies assuming Ti contacts may have actually used TiO2 due to the nature of the deposition tools used.

Designing high performance metal–mMoS2 interfaces by two-dimensional insertions with suitable thickness

The contact properties of metal–MoS₂ interfaces are improved by 2D insertions, but they deteriorate with the increasing thickness of insertions.

First-principles investigation of the Schottky contact for the two-dimensional MoS2 and graphene heterostructure

The electronic properties of an MoS₂ and graphene heterostructure are investigated by density functional calculations.

Investigation of Single-Wall MoS2 Monolayer Flakes Grown by Chemical Vapor Deposition

Recently, two-dimensional monolayer molybdenum disulfide (MoS2), a transition metal dichalcogenide, has received considerable attention due to its direct bandgap, which does not exist in its bulk form, enabling applications in optoelectronics and also thanks to its enhanced catalytic activity which allows it to be used for energy harvesting. However, growth of controllable and high-quality monolayers is still a matter of research and the parameters determining growth mechanism are not completely clear. In this work, chemical vapor deposition is utilized to grow monolayer MoS2 flakes while deposition duration and temperature effect have been systematically varied to develop a better understanding of the MoS2 film formation and the influence of these parameters on the quality of the monolayer flakes. Different from previous studies, SEM results show that single-layer MoS2 flakes do not necessarily grow flat on the surface, but rather they can stay erect and inclined at different angles on the surface, indicating possible gas-phase reactions allowing for monolayer film formation. We have also revealed that process duration influences the amount of MoO3/MoO2 within the film network. The homogeneity and the number of layers depend on the change in the desorption-adsorption of radicals together with sulfurization rates, and, inasmuch, a careful optimization of parameters is crucial. Therefore, distinct from the general trend of MoS2 monolayer formation, our films are rough and heterogeneous with monolayer MoS2 nanowalls. Despite this roughness and the heterogeneity, we observe a strong photoluminescence located around 675 nm.

Electrical and Optical Characterization of MoS2 with Sulfur Vacancy Passivation by Treatment with Alkanethiol Molecules

We investigated the physical properties of molybdenum disulfide (MoS2) atomic crystals with a sulfur vacancy passivation after treatment with alkanethiol molecules including their electrical, Raman, and photoluminescence (PL) characteristics. MoS2, one of the transition metal dichalcogenide materials, is a promising two-dimensional semiconductor material with good physical properties. It is known that sulfur vacancies exist in MoS2, resulting in the n-type behavior of MoS2. The sulfur vacancies on the MoS2 surface tend to form covalent bonds with sulfur-containing groups. In this study, we deposited alkanethiol molecules on MoS2 field effect transistors (FETs) and then characterized the electrical properties of the devices before and after the alkanethiol treatment. We observed that the electrical characteristics of MoS2 FETs dramatically changed after the alkanethiol treatment. We also observed that the Raman and PL spectra of MoS2 films changed after the alkanethiol treatment. These effects are attributed to the thiol (-SH) end groups in alkanethiols bonding at sulfur vacancy sites, thus altering the physical properties of the MoS2. This study will help us better understand the electrical and optical properties of MoS2 and suggest a way of tailoring the properties of MoS2 by passivating a sulfur vacancy with thiol molecules.

Tuning the threshold voltage of MoS2 field-effect transistors via surface treatment

Controlling the threshold voltage (Vth) of a field-effect transistor is important for realizing robust logic circuits. Here, we report a facile approach to achieve bidirectional Vth tuning of molybdenum disulfide (MoS2) field-effect transistors. By increasing and decreasing the amount of sulfur vacancies in the MoS2 surface, the Vth of MoS2 transistors can be left- and right-shifted, respectively. Transistors fabricated on perfect MoS2 flakes are found to exhibit a two-fold enhancement in mobility and a very positive Vth (18.5 ± 7.5 V). More importantly, our elegant hydrogen treatment is able to tune the large Vth to a small value (∼0 V) without any performance degradation simply by reducing the atomic ratio of S : Mo slightly; in other words, it creates a certain amount of sulfur vacancies in the MoS2 surface, which generate defect states in the band gap of MoS2 that mediates conduction of a MoS2 transistor in the subthreshold regime. First-principles calculations further indicate that the defect band's edge and width can be tuned according to the vacancy density. This work not only demonstrates for the first time the ease of tuning the Vth of MoS2 transistors, but also offers a process technology solution that is critical for further development of MoS2 as a mainstream electronic material.