Query: Two-dimensional ferromagnet/semiconductor transition metal dichalcogenide contacts: p-type Schottky 

Candidate papers:

abstract: We study the ferromagnet/semiconductor contacts formed by transition metal dichalcogenide monolayers, focusing on semiconducting MoS$}_{2} and WS$}_{2} and ferromagnetic VS$}_{2} We investigate the degree of $p$ type doping and demonstrate tuning of the Schottky barrier height by vertical compressive pressure. An analytical model is presented for the barrier heights that accurately describes the numerical findings and is expected to be of general validity for all transition metal dichalcogenide metal/semiconductor contacts. Furthermore, magnetic proximity effects induce a 100% spin polarization at the Fermi level in the semiconductor where the spin splitting increases up to 0.70 eV for increasing pressure.
author_names: Liyong Gan

Qingyun Zhang

Yingchun Cheng

Udo Schwingenschlogl
title: Two dimensional ferromagnet/semiconductor transition metal dichalcogenide contacts: p type Schottky barrier and spin injection control
year: 2013

abstract: We develop a new technique for transferring metal contacts to create ultrathin solar cells from 2D materials. Two dimensional transition metal dichalcogenides are promising candidates for ultrathin optoelectronic devices due to their high absorption coefficients and intrinsically passivated surfaces. To maintain these near perfect surfaces, recent research has focused on fabricating contacts that limit Fermi level pinning at the metal semiconductor interface. Here, we develop a new, simple procedure for transferring metal contacts that does not require aligned lithography. Using this technique, we fabricate vertical Schottky junction WS2 solar cells, with Ag and Au as asymmetric work function contacts. Under laser illumination, we observe rectifying behavior and open circuit voltage above 500 mV in devices with transferred contacts, in contrast to resistive behavior and open circuit voltage below 15 mV in devices with evaporated contacts. One sun measurements and device simulation results indicate that this metal transfer process could enable high specific power vertical Schottky junction transition metal dichalcogenide photovoltaics, and we anticipate that this technique will lead to advances for two dimensional devices more broadly.
author_names: Cora M Went

Joeson Wong

Phillip Jahelka

Michael D Kelzenberg

Souvik Biswas

Matthew S Hunt

Abigail Carbone

Harry A Atwater
title: A new metal transfer process for van der Waals contacts to vertical Schottky junction transition metal dichalcogenide photovoltaics
venue: Science Advances
year: 2019

abstract: Two dimensional transition metal dichalcogenides are promising candidates for low cost and high specific power solar cells due to their high absorption coefficients and intrinsically passivated surfaces. Here, we demonstrate dramatically improved photovoltaic performance in vertical Schottky junction WS2 solar cells with transferred metal contacts rather than evaporated metal contacts. We use 16 nm thick WS2 as the photovoltaic absorber, with Ag and Au as asymmetric work function contacts, where the Au top contact is transferred to avoid Fermi level pinning. For simpler fabrication, we develop a new procedure for transferring metal contacts that has high yield and does not require an aligned lithography step. Under one sun illumination, we measure an opencircuit voltage of 256 mV, a short circuit current density of 4.10 mA/cm2, and a fill factor of 0.44. Simulations of further optimized devices indicate that such a vertical transferred contact Schottkyjunction device geometry is promising for high efficiency transition metal dichalcogenide photovoltaics with scalable photoactive areas. As silicon solar cells continue to decrease in cost, researchers are seeking new materials and devices for beyond Si only photovoltaics, and also for applications beyond commercial utilityscale solar, such as efficient building integrated, aerospace and vehicle integrated solar. New ultrathin and processable materials may have advantages in these applications that demand high specific power (i.e. power per unit weight) and superior thermal performance1. Two dimensional (2D) semiconducting transition metal dichalcogenides (TMDs) including MoS2, WS2, MoSe2, and WSe2, are particularly promising for high specific power photovoltaics. With absorption coefficients 1 2 orders of magnitude higher than conventional semiconductors, monolayer <1 nm thick) TMDs can absorb as much sunlight as about 15 nm of GaAs or 50 nm of Si2. Due to their layered structure and out of plane van der Waals bonding, TMDs have intrinsically passivated surfaces with no dangling bonds and can form heterostructures without the constraint of lattice matching. In their monolayer form, TMDs have direct bandgaps at ideal energies for tandem solar cells with Si, and high photoluminescence quantum yields, especially upon passivation3 5. In their multilayer form, TMDs have indirect bandgaps and lower photoluminescence quantum yields, but also smaller exciton binding energies and ideal bandgaps for single junction solar cells approaching the Shockley Queisser efficiency limit6. Both multilayer and monolayer TMDs can achieve near unity broadband absorption in the visible range7,8. The theoretical maximum power conversion efficiency achievable for multilayer and monolayer single junction solar cells is similar, due to the tradeoff between bandgap energy and photoluminescence quantum yield9. Different carrier collection techniques have been explored in multilayer and monolayer TMD photovoltaics. Monolayer TMD solar cells usually collect carriers laterally, as tunneling dominates transport in vertical monolayer devices. The carrier separating junctions formed in monolayer TMD solar cells include p n junctions defined by split gate electrodes10 12, p n junctions defined by type II band alignment between two TMDs13 16, p n junctions defined by chemical doping17, and Schottky junctions18,19. In multilayer TMD solar cells, both lateral and vertical devices have been fabricated. Internal quantum efficiency (IQE) >70% has been demonstrated in 3 nm MoS2/9 nm WSe2 vertical p n junctions20. AM1.5G power conversion efficiencies of 0.4% and 2.8% have been achieved in ultrathin (11 nm)21 and thicker (120 nm)22 chemically doped vertical p n junctions of MoS2, respectively, and AM1.5G power conversion efficiency 0.7% has been achieved in lateral Schottky junctions with a 3 nm thick TMD23. Typically, Schottky junctions are among the least efficient of carrier separating junctions, as there is infinite surface recombination velocity at the metal semiconductor interface, the junctions occur near the contacts, and Fermi level pinning limits the achievable surface bandbending24. However, recent work by Liu et al shows that transferring rather than evaporating metal contacts onto TMDs can yield interfaces with no Fermi level pinning, where the Schottky barrier height can be predicted by the ideal Schottky Mott rule18. They fabricate gated monolayer and multilayer lateral Schottky junction devices and demonstrate high VOC values (1.02 V for monolayer and 0.76 V for multilayer) under laser illumination18. To date, Schottky junctions with transferred metal contacts have not been explored in vertical and ungated architectures, which are necessary for photovoltaic applications. Further, a simple procedure for transferring metal contacts without an aligned lithography step has yet to be developed. Here, we demonstrate vertical Schottky junction solar cells with multilayer TMD absorber layers and transferred metal contacts. We explore multilayer vertical devices that possess a scalable photoactive area, and are therefore aligned with conventional large area photovoltaics. Ultrathin (16 nm) WS2 forms the absorber layer, while Au and Ag form the asymmetric workfunction contacts. To facilitate device fabrication, we develop a simple, scalable procedure for transferring metal contacts. Devices made with transferred metal contacts show diode like I V behavior and high VOC, while similar devices made with evaporated metal contacts show ohmic I V behavior and near zero VOC. We demonstrate peak external quantum efficiency (EQE) of >40% and peak IQE of >60% in transferred contact devices. Using a solar simulator, we measure a photovoltaic power conversion efficiency of 0.46% Finally, we explore the potential of this device architecture by simulating the performance of further optimized devices. Results Fabrication of vertical WS2 Schottky junction solar cells. We prepare vertical WS2 Schottkyjunction solar cells made from 16 nm thick WS2 absorber layers, with Ag (fAg 4.3 eV) and Au (fAu 5.1 eV) as asymmetric work function contacts (Fig. 1a)18. Template stripped Ag, which exhibits an RMS roughness <0.5 nm, forms both the electron collecting bottom contact and back reflector for all devices25. We mechanically exfoliate WS2 directly onto the Ag substrate. The subwavelength thick WS2 achieves broadband, angle insensitive absorption on top of the highly reflective Ag, giving the WS2 a deep purple color as illustrated in Fig. 1b7,26. For transferredcontact devices, we transfer thin Au disks from a thermally oxidized Si donor substrate to form the semi transparent hole collecting top contact, using the process described in the following section. Both the top surface of the template stripped Ag and the bottom surface of the transferred Au inherit the smoothness of the SiO2/Si donor substrate, leading to near atomically sharp metalWS2 interfaces18,25. For comparison, we also fabricate devices by direct evaporation of thin Au disks onto the WS2 using standard photolithography techniques. The ideal band diagram of this Schottky junction solar cell is shown in Fig. 1c. We assume a doping concentration of 1014 cm 3 for WS2, as provided by the bulk crystal vendor. Since the length of the depletion region at a Schottky junction between bulk WS2 and either Au or Ag is on the order of 1 mm, the device is fully depleted. We measure the final thicknesses of the WS2 and the Au to be 16 nm and 19 nm, respectively, using atomic force microscopy (Fig. 1d) Metal transfer process. We develop a new, simple process for transferring metal contacts onto TMDs (Fig. 2) This process relies on a self assembled monolayer (SAM) to reduce the adhesion between the Au and the SiO2/Si donor substrate27, a thermoplastic polymer to preferentially pick up or drop down the metal28, and a variable peeling rate to tune the velocity dependent adhesion between a metal and a viscoelastic stamp27. Briefly, we create a SAM on clean thermally oxidized Si chips in a vacuum desiccator27. We then deposit 20 nm of Au via electron beam evaporation. Using photolithography, we define the contact areas with positive photoresist and a positive photomask. We etch the Au outside the masked contact areas, then dissolve the remaining photoresist in acetone, leaving Au disks on the SAM coated SiO2/Si substrates. We prepare a polydimethylsiloxane (PDMS) stamp coated with the thermoplastic polymer polypropylene carbonate (PPC) on a glass slide28. In a 2D transfer setup, we align and slowly lower the stamp onto a contact at 60oC. We set the temperature to 40oC, and once the stage reaches that temperature, we raise the transfer arm rapidly to peel the stamp and pick up the contact. We align the contact with the target TMD and slowly lower the stamp down at 60oC, and then slowly peel it away immediately after contact at the same temperature. The contact delaminates from the PDMS/PPC stamp and sticks to the TMD. Further details of the procedure are provided in Supplementary Note 1. This metal transfer technique has worked in 15 out of 16 devices fabricated thus far (94% yield) It works with both 20 nm and 100 nm of Au, and can likely be extended to other metals. A substantial advantage of this technique is that, whereas prior metal transfer techniques require a final aligned electron beam lithography step to expose the contact area18, this technique only utilizes unaligned photolithography to define the initial contacts on the SiO2/Si donor substrate. Comparison of transferred and evaporated metal contacts. We measure I V curves under illumination with a 633 nm laser focused to a ~1 mm2 spot in a confocal microscope at room temperature. In devices with transferred metal contacts, we observe rectifying I V curves and a pronounced photovoltaic effect (Fig. 3a) We measure a VOC of 510 mV under the maximum laser excitation. Short circuit current follows a power law as a f
author_names: Cora M Went

Joeson Wong

Phillip Jahelka

Michael D Kelzenberg

Souvik Biswas

Harry A Atwater
title: Transferred metal contacts for vertical Schottky junction transition metal dichalcogenide photovoltaics
year: 2019

abstract: Two dimensional (2D) metal semiconductor transition metal dichalcogenide (TMDC) vertical heterostructures play a crucial role in device engineering and contact tuning fields, while their direct integration still challenging. Herein, a robust epitaxial growth method is designed to construct multiple lattice matched 2D metal semiconductor TMDC vertical stacks (VSe2/MX2, M: Mo, W; X: S, Se) by a two step chemical vapor deposition method. Intriguingly, the metallic VSe2 preferred to nucleate and extend from the energy favorable edge site of the semiconducting MX2 underlayer to form VSe2/MX2 vertical heterostructures. This growth behavior was also confirmed by density functional theory calculations of the initial adsorption of VSe2 adatoms. In particular, the formation of Schottky diode or Ohmic contact type band alignments was detected for the stacks between VSe2 and p type WSe2 or n type MoSe2, respectively. This work hereby provides insights into the direct integration, band alignment engineering, and potential applications of such 2D metal semiconductor stacks in next generation electronics, optoelectronic devices, and energy related fields.
author_names: Zhepeng Zhang

Yue Gong

Xiaolong Zou

Porun Liu

Pengfei Yang

Jianping Shi

Liyun Zhao

Qing Zhang

Lin Gu

Yanfeng Zhang
title: Epitaxial Growth of Two Dimensional Metal Semiconductor Transition Metal Dichalcogenide Vertical Stacks (VSe2/MX2) and Their Band Alignments.
venue: ACS nano
year: 2019

abstract: We demonstrated n and p type carrier injections into a transition metal dichalcogenide (TMD) WSe2 using van der Waals (vdW) contacts of two dimensional (2D) materials: graphite for an n type contact and NbSe2 for a p type contact. Instead of conventional methods such as the evaporation of metals on TMD, 2D metals were transferred onto WSe2 in order to form van der Waals contacts. With these contacts, we demonstrated a small Schottky barrier height for both carrier polarities. Our finding reveals the potential of a high performance vdW metal/semiconductor contact for use in electronics applications.
author_names: Yohta Sata

Rai Moriya

Satoru Masubuchi

Kenji Watanabe

Takashi Taniguchi

Tomoki Machida
title: N and p type carrier injections into WSe2 with van der Waals contacts of two dimensional materials
year: 2017

abstract: It is a big challenge to make a Schottky barrier (SB) free hole contact to MoS2 with a high ionization of ~6.0 eV. Here, using first principles calculations, in a recently discovered large family of two dimensional transition metal carbides or nitrides (MXenes) we have found six materials (V2CO2, Cr2CO2, Mo2CO2, V4C3O2, Cr2NO2, and V2NO2) that can be used as metal contacts to monolayer MoS2 with vanishing p type Schottky barriers at contacting interfaces, resulting in highly efficient hole injection into MoS2. We reveal that the successful achievements of the SB free hole contacts at these MoS2/MXene interfaces depend on not only the high work functions of the MXenes but also the absence of the formation of interfacial gap states that usually result in strong Fermi level pinning in the midgap of a semiconductor. We further propose that efficient charge injection into MoS2 facilitated by SB free contact could also increase the hydrogen evolution reaction (HER) activity of the 2H MoS2 basal plane by improv.
author_names: Jinxuan You

Chen Si

Jian Zhou

Zhimei Sun
title: Contacting MoS2 to MXene: Vanishing p Type Schottky Barrier and Enhanced Hydrogen Evolution Catalysis
year: 2019

abstract: In the active and growing explorations of the rapidly emerging two dimensional (2D) electronic and optoelectronic devices based upon atomically thin semiconductors and their heterostructures, developing better understanding of electrostatic gating is very important, especially for realizing logic and switching devices by employing 2D field effect transistors (FETs) with low subthreshold swing (SS) and high on off ratio (I_{\mathrm{O}\mathrm{n}/I_\text{Off} In this study, we propose and demonstrate a method that includes a combination of two probe and four probe $I$ $V$ measurements on 2D transition metal dichalcogenide (TMDC) FETs to uncover the evolution of resistance of channel and contacts separately, upon change of gate voltage during switching between On and Off states. In 2D TMDC FETs with Schottky barrier (SB) contacts, we demonstrate that switching between On and Off states is primarily attained by modulating SBs due to change of electric field via electrostatic gating. Therefore, transistor characteristics is mostly determined by the contact resistance change upon the application of gate voltage. We present our method for 2D TMDC (MoS2, MoTe2) FETs. In addition, we also perform C V measurements to investigate the presence of interface trap states and quantum capacitance in TMDC FETs.
author_names: Arnob Islam

Xia Liu

Bradley Odhner

Mary Anne Tupta

Philip X -L Feng
title: Investigation of Electrostatic Gating in Two Dimensional Transitional Metal Dichalcogenide (TMDC) Field Effect Transistors (FETs)
venue: 2018 IEEE 13th Nanotechnology Materials and Devices Conference (NMDC)
year: 2018

abstract: Here we propose a design to optimise measurement of the transverse spin voltage resulting from the spin Hall effect (SHE) generated by shining circularly polarised light on a transition metal dichalcogenide (TMD) monolayer. There is currently a lot of interest in trying to create ohmic contacts to single layer TMDs due to the inherent Schottky barriers present at the metal to semiconductor interface, but the more successful methods either cover the entire flake or rely on a temporary doping with a low half life. We propose using graphene as an intermediary contact with top gates only over the contacted regions leaving a central area of the TMD flake exposed so that the laser can reach it unimpeded. To maximise the SHE, the strength of the spin orbit coupling of the atmospherically stable TMDs have been reviewed. Armed with this information, and given that WSe2 is an intrinsically p type TMD, it was found to be the best candidate for investigating the SHE in such two dimensional materials. We have successfully both exfoliated and used Raman spectroscopy to characterise graphene and TMD flakes, using photoluminescence (PL) spectroscopy for the characterisation of the latter as well: whereas Raman spectroscopy has proven to be very effective in determining the layer number of the graphene and WSe2 flakes, in the case of MoS2, PL spectra can be more definitive in distinguishing monolayer from multilayer flakes. We unexpectedly, as WSe2 is thought to be atmospherically stable, observed a change in the visibility of WSe2 over a matter of four weeks, even when stored in a vacuum. The corresponding PL spectrum was also found to be heavily suppressed. Here we also report out attempts at stacking graphene onto TMD flakes to produce ohmic contacts.
author_names: K Conder
title: Optimisation of transition metal dichalcogenide devices for measurement of spin Hall voltages generated by optical spin orientation
year: 2018

abstract: Author(s) Zahin, Adiba Advisor(s) Lake, Roger K Abstract: ABSTRACT OF THE DISSERTATIONSchottky Barrier Heights at Two Dimensional Metallic and SemiconductingTransition Metal Dichalcogenide InterfacesbyAdiba ZahinMaster, Graduate Program in Electrical and Computer EngineeringUniversity of California, Riverside, September 2017Dr. Roger K. Lake, ChairpersonSeveral advances have been made in the realization of electronic devices that utilizeatomically thin two dimensional (2D) materials. The semiconducting transition metaldichalcogenides in particular have been used to demonstrate a wide range of deviceswhich include steep tunnel field effect transistors [1,2] photodetectors [3,4] field effecttransistors [5, 6] and chemical sensors [7, 8] A variety of experimental [9, 10] and theoretical [11, 12] studies have been devoted to understand the interface formed betweenthe bulk metals that are deposited on the surface of the 2D transition metal dichalcognides. There is growing evidence that the Schottky like transport behavior observedin TMDC metal contacts is a consequence of strong Fermi level pinning (FLP) Theorigin of the Fermi level pinning in metal TMDC interfaces has been attributed to theformation of interface dipoles [11] defects at the metal TMDC interface and the existence of metal induced gap states (MIGS) which arise from the exponential decay ofthe wavefunction of the metal Fermi level into the TMDC band gap [13, 14] One approach to minimize the effect of Fermi level pinning would be achieving an epitaxiallyclean interface between the metal and the semiconducting TMDC. Prior experimentalstudies of the contact resistance between the 2H/1T polytypes of MoS2 succeeded inidemonstrating record low contact resistance [15] Indeed recent study shows that, FLP
author_names: Adiba Zahin
title: Schottky Barrier Heights at Two Dimensional Metallic and Semiconducting Transition Metal Dichalcogenide Interfaces
year: 2017

abstract: Transition metal dichalcogenides are a promising avenue for fabricating high performance silicon in a simplified manner, by exploiting their two dimensional properties. This paper looks at simulating monolayers of four such materials on silicon, namely molybdenum disulfide and diselenide, and tungsten disulfide and diselenide. Materials parameters determined by density functional theory were used in a semiconductor device simulator, TCAD, to assess the potential performances for solar cells using these layers as a front surface contact layer. Results are presented for molybdenum disulfide on both n and p type silicon, showing a kink in the current voltage curve already reported in the literature. A potential efficiency of 19.2% is identified for an n type molybdenum disulfide on p type silicon device.
author_names: Jing Zhao

Yuanfeng Xu

Fajun Ma

Anita W Y Ho-baillie

Hao Zhang

Stephen P Bremner
title: Simulation of Solar Cells Employing 2 Dimensional Transition Metal Dichalcogenide Silicon Front Surfaces
venue: 2018 IEEE 7th World Conference on Photovoltaic Energy Conversion (WCPEC) (A Joint Conference of 45th IEEE PVSC, 28th PVSEC 34th EU PVSEC)
year: 2018