This numerical study examines the importance of self-consistently accounting for transport and electrostatics in the calculaiton of semiconductor/metal Schottky contact resistivity. It is shown that ignoring such self-consistency results in significant under-estimation of the contact resistivity. An explicit numerical method has also been proposed to efficiently improve contact resistivity calculations.
Current voltage and Kelvin Probe Force Microscopy (KPFM) measurements were performed on single ZnO nanowires. Measurements are shown to be strongly correlated with the contact behavior, either ohmic or Schottky. The ZnO nanowires were obtained by metallo-organic chemical vapor deposition (MOCVD) and contacted using electronic-beam lithography. Depending on the contact geometry, good quality ohmic contacts (linear I V behavior) or non-linear (diode like) Schottky contacts were obtained. Current voltage and KPFM measurements on both types of contacted ZnO nanowires were performed in order to inv…
We study the electrical detection of spin accumulation at a ferromagnet-silicon interface, which can be verified by measuring a Hanle effect in three-terminal lateral devices. The device structures used consist of a semiconducting Si channel and a high-quality Schottky tunnel contact. In a low current-bias region, the Hanle-effect curves are observed only under forward bias conditions. This can be considered that the electrical detectability of the forward-biased contact is higher than that of the reverse-biased contact. This is possible evidence for the detection of spin-polarized electrons c…
An analysis is made of the conditions for ohmic contacts realization in the case of Schottky contacts. Based on the classical notions about the mechanisms of current flow, we consider the generalized model of Schottky contact that takes into account the thermionic current of majority charge carriers and recombination current of minority charge carriers in Schottky contacts with a dielectric gap. An analysis of the results given by that model made it possible to obtain ohmicity criteria for Schottky contacts and compare the conditions for low injection level and ohmicity of Schottky contacts in…
We have studied the finite bias transport properties of a 2H-1T' MoS$_2$ lateral metal-semiconductor (M-S) junction by non-equilibrium Green's functions calculations, aimed at contacting the 2D channel in a field effect transistor. Our results indicate that (a) despite the fundamentally different electrostatics of line and planar dipoles, the Schottky barrier heights respond similarly to changes in doping and applied bias in 2D and 3D M-S junctions, (b) 2H-1T' MoS$_2$ lateral junctions are free from Fermi level pinning, (c) armchair interfaces have superior contacting properties vs.\ zigzag in…
Two-dimensional semiconductors are excellent candidates for next-generation electronics and optoelec-tronics thanks to their electrical properties and strong light-matter interaction. To fabricate devices with optimal electrical properties, it is crucial to have both high-quality semiconducting crystals and ideal con-tacts at metal-semiconductor interfaces. Thanks to the mechanical exfoliation of van der Waals crystals, atomically-thin high-quality single-crystals can easily be obtained in a laboratory. However, conventional metal deposition techniques can introduce chemical disorder and metal…
Using high-quality Fe$_{3}$Si/$n^{+}$-Ge Schottky-tunnel-barrier contacts, we study spin accumulation in an $n$-type germanium ($n$-Ge) channel. In the three- or two-terminal voltage measurements with low bias current conditions at 50 K, Hanle-effect signals are clearly detected only at a forward-biased contact. These are reliable evidence for electrical detection of the spin accumulation created in the $n$-Ge channel. The estimated spin lifetime in $n$-Ge at 50 K is one order of magnitude shorter than those in $n$-Si reported recently. The magnitude of the spin signals cannot be explained by …
Circular photocurrents (CPC), namely circular photogalvanic (CPGE) and photon drag effects, have recently been reported both in monolayer and multilayer transition metal dichalcogenide (TMD) phototransistors. However, the underlying physics for the emergence of these effects are not yet fully understood. In particular, the emergence of CPGE is not compatible with the D3h crystal symmetry of two-dimensional TMDs, and should only be possible if the symmetry of the electronic states is reduced by influences such as an external electric field or mechanical strain. Schottky contacts, nearly ubiquit…
We demonstrate electrical injection and detection of spin-polarized electrons in silicon (Si) using epitaxially grown Fe_3Si/Si Schottky-tunnel-barrier contacts. By an insertion of a delta-doped n^+-Si layer (~ 10^19 cm^-3) near the interface between a ferromagnetic Fe_3Si/Si contact and a Si channel (~ 10^15 cm^-3), we achieve a marked enhancement in the tunnel conductance for reverse-bias characteristics of the Fe_3Si/Si Schottky diodes. Using laterally fabricated four-probe geometries with the modified Fe_3Si/Si contacts, we detect nonlocal output signals which originate from the spin accum…
The low temperature electrical behavior of adjacent silicide/Si Schottky contacts with or without dopant segregation is investigated. The electrical characteristics are very well modeled by thermionic-field emission for non-segregated contacts separated by micrometer-sized gaps. Still, an excess of current occurs at low temperature for short contact separations or dopant-segregated contacts when the voltage applied to the device is sufficiently high. From two-dimensional self-consistent non-equilibrium Green's function simulations, the dependence of the Schottky barrier profile on the applied …
MoS2 is a layered two-dimensional material with strong spin-orbit coupling and long spin lifetime, which is promising for electronic and spintronic applications. However, because of its large band gap and small electron affinity, a considerable Schottky barrier exists between MoS2 and contact metal, hindering the further study of spin transport and spin injection in MoS2. Although substantial progress has been made in improving device performance, the existence of metal-semiconductor Schottky barrier has not yet been fully understood. Here, we investigate permalloy (Py) contacts to both multil…
Due to Fermi level pinning (FLP), metal-semiconductor contact interfaces result in a Schottky barrier height (SBH), which is usually difficult to tune. This makes it challenging to efficiently inject both electrons and holes using the same metal - an essential requirement for several applications, including light-emitting devices and complementary logic. Interestingly, modulating the SBH in the Schottky-Mott limit of de-pinned van der Waals (vdW) contacts becomes possible. However, accurate extraction of the SBH is essential to exploit such contacts to their full potential. In this work, we pr…
To investigate the role of the interface state on the physical properties of Schottky contacts, Co/n-Ge Schottky diodes that have undergone various cleaning methods (HF etching and in-situ thermal cleaning) were studied by Transmission Electron Microscopy (TEM), Deep-level Transient Spectroscopy (DLTS) and by a detailed analysis of the temperature dependence of the diodes characteristics. It is shown that Schottky barrier height characteristics are sensitive to the nature of the interface. The strongest Fermi level pinning and the highest spatial inhomogeneities are observed for intimate metal…
The concept of All-Back-Schottky-Contact (ABSC) thin-film photovoltaic devices is introduced and evaluated using 2D numerical simulation. The built-in field is generated by reach-through Schottky junctions with two metals of different work functions in an alternating, side-by-side pattern along the back of the device. This approach minimizes the number of interfaces through which charge transport is required while eliminating the need for transparent conductive front contacts, pn junctions, and scribing. Performance metrics are calculated for a range of design parameters and it is shown that p…
Atomically thin MoS2 has recently emerged as a very attractive material for nanoscale optoelectronic devices. While n-type transport in MoS2 devices has been demonstrated, hole conduction has been more challenging. Here we show work-function engineering to be an effective approach for controlling the polarity of MoS2 devices. Gated multi-layer MoS2 transistors with Au source/drain contacts exhibit n-type operation, while those with Pd contacts are shown to have p-type behavior. Devices with one Au and one Pd contact exhibit asymmetric ambipolar behavior and diode characteristics over a wide ra…
Making a metal contact to the two-dimensional semiconductor MoS2 without creating a Schottky barrier is a challenge. Using density functional calculations we show that, although the Schottky barrier for electrons obeys the Schottky-Mott rule for high work function ($\gtrsim 4.7$ eV) metals, the Fermi level is pinned at 0.1-0.3 eV below the conduction band edge of MoS2 for low work function metals, due to the metal-MoS2 interaction. Inserting a boron nitride (BN) monolayer between the metal and the MoS2 disrupts this interaction, and restores the MoS2 electronic structure. Moreover, a BN layer …
We report the fabrication of both n-type and p-type WSe2 field effect transistors with hexagonal boron nitride passivated channels and ionic-liquid (IL)-gated graphene contacts. Our transport measurements reveal intrinsic channel properties including a metal-insulator transition at a characteristic conductivity close to the quantum conductance e2/h, a high ON/OFF ratio of >107 at 170 K, and large electron and hole mobility of ~200 cm2V-1s-1 at 160 K. Decreasing the temperature to 77 K increases mobility of electrons to ~330 cm2V-1s-1 and that of holes to ~270 cm2V-1s-1. We attribute our abilit…
The Schottky barrier height ($E_{SBH}$) is a crucial factor in determining the transport properties of semiconductor materials as it directly regulates the carrier mobility in opto-electronics devices. In principle, van der Waals (vdW) Janus heterostructures offer an appealing avenue to controlling the ESBH. However, the underlying atomistic mechanisms are far from understood conclusively, which prompts for further research in the topic. To this end, here, we carry out an extensive first principles study of the electronic properties and $E_{SBH}$ of several vdW Janus MXY/Graphene (M=Mo, W; X, …
Recently demonstrated metal-semiconductor heterojunctions with few-atom thickness show their promise as 2D Schottky contacts for future integrated circuits and nanoelectronics. The theory for 3D Schottky contacts, however, fails on these low-dimensional systems. Here, we propose a new model that yields carrier distribution and potential profile across the 2D metal-semiconductor heterojunction under the equilibrium condition, based on the input from first-principle calculations. Our calculation also suggests that, at the same forward bias, the current density of a stack of 2D graphene-phosphore…
The segregation of As+ ions implanted into thin Er films deposited on n-Si substrates is studied after ErSi2-x formation. The same lowering of the effective Schottky barrier height (SBH) below 0.12 eV is obtained at moderate annealing temperatures, regardless of the redistribution of As dopants at the ErSi2-x/Si interface. On the other hand, if the implanted dose is slightly enhanced, the annealing temperature required to reach sub-0.12-eV effective SBH can be further reduced. This process enables the formation of very low effective SBH ErSi2-x/n-Si contacts with a low thermal budget.
Nonlocal Electrostatic Origin of Schottky-Barrier Variability in 2D Contacts
Institute of High Performance Computing (IHPC), Agency for Science, Technology；Research (A STAR), 1 Fusionopolis Way, \#16-16 Connexis, Singapore, 138632, Republic of Singapore
Armchair nanotubes of MoS2 and WS2 offer a sizeable band gap, with the advantage of a one dimensional (1D) electronic material, but free from edge roughness and thermodynamic instability of nanoribbons. Use of such semiconducting MX2 (MoS2, WS2) armchair nanotubes (NTs) in conjunction with metallic carbon nanotubes (CNT) could prove useful for nanoelectronics and photonics applications. In this work atomistic simulations of MoS2 NT-CNT and WS2 NT-CNT junctions are carried out to study the physics of such junctions. With density functional theory (DFT) we study the carrier density distribution,…
Intertwined bulk photocarrier and interfacial barrier dynamics in van der Waals point-contact Schottky junctions
UNISOKU Co., Ltd.（UNISOKU有限公司）；Faculty of Pure and Applied Sciences, University of Tsukuba（筑波大学理学院）；Department of Fundamental Biosciences (Physics), Shiga University of Medical Science（滋贺医科大学基础生命科学系（物理））；Institute for Molecular Science (IMS), National Institutes of Natural Sciences（国立自然科学研究所分子科学研究所）
Ultrathin two-dimensional semiconductors obtained from layered transition-metal dichalcogenides such as molybdenum disulfide (MoS2) are promising for ultimately scaled transistors beyond Si. Although the shortening of the semiconductor channel is widely studied, the narrowing of the channel, which should also be important for scaling down the transistor, has been examined to a lesser degree thus far. In this study, the impact of narrowing on mechanically exfoliated MoS2 flakes was investigated according to the channel-width-dependent Schottky barrier heights at Cr/Au contacts. Narrower channel…
The 2D ferromagnets, such as CrX3 (X=Cl, Br and I), have been attracting extensive attentions since they provide novel platforms to fundamental physics and device applications. Integrating CrX3 with other electrodes and substrates is an essential step to their device realization. Therefore, it is important to understand the interfacial properties between CrX3 and other 2D materials. As an illustrative example, we have investigated the heterostructures between CrX3 and graphene (CrX3/Gr) from firstprinciples. We find unique Schottky contacts type with strongly spin-dependent barriers in CrX3/Gr…
Emergent phenomena at interfaces between oxides and metals can appear due to charge transfer and mass transport that modify the bulk properties. By coating the metallic oxide LaNiO$_3$ by aluminium, we fabricated a junction exhibiting a diode-like behaviour. At the equilibrium, the interface is insulating. The metallic behaviour can be recovered by applying a voltage drop across the junction in one polarity only. The electrical properties in direct and reverse bias are investigated. The observed electro-resistive effect rises up to $10^5$ \% and can be interpreted in terms of (i) a spontaneous…
A comprehensive current-voltage (I-V) characterization is performed for three different Schottky contacts; Pt, Ni and Ti, to unintentionally doped (UID) \{beta}-(Al0.19Ga0.81)2O3 grown by molecular beam epitaxy (MBE) on \{beta}-Ga2O3 for temperatures ranging between 25C -300C. Reciprocal space mapping shows the (Al0.19Ga0.81)2O3 films are strained and lattice matched to the substrate. Schottky Barrier Height (SBH), ideality factor (n), and series resistance (Rs) are extracted from the I-V characteristics for the three types of metals and temperatures. Room temperature capacitance-voltage (C-V)…
Although monolayer black phosphorus (BP) or phosphorene has been successfully exfoliated and its optical properties have been explored, most of electrical performance of the devices is demonstrated on few-layer phosphorene and ultra-thin BP films. In this paper, we study the channel length scaling of ultra-thin BP field-effect transistors (FETs), and discuss a scheme for using various contact metals to change transistor characteristics. Through studying transistor behaviors with various channel lengths, the contact resistance can be extracted from the transfer length method (TLM). With differe…
The development of thermoelectric devices faces not only the challenge of optimizing the Seebeck coefficient, the electrical and thermal conductivity of the active material, but also further bottlenecks when going from the thermoelectric material to an actual device, e.g., the dopant diffusion at the hot contact. We show that for large bandgap thermoelectrics another aspect can dramatically reduce the efficiency of the device: the formation of Schottky barriers. Understanding the effect, it can then be fixed rather cheaply by a two-metals contact solution.
Schottky barrier contact has been fabricated by thermal deposition of Al on (100) Ge (impurity concentration~1010/cm3 at 80K) that shows extrinsic p-type to intrinsic n-type transition near 180K. Both p and n-type Ge exhibits ideal Schottky behaviour with low reverse current and near unity ideality factors obtained from the linear form of temperature dependent current-voltage (I-V) characteristics. The diode current at various temperatures change its direction at non-zero applied bias that reflects a shift in position of charge neutrality level (CNL) from the Fermi level of Ge. With the rise i…
An atomistic insight into potential barrier formation and band bending at the interface between a metal and an n-type semiconductor is achieved by ab initio simulations and model analysis of a prototype Schottky diode, i.e., niobium doped rutile titania in contact with gold (Au/Nb:TiO$_2$). The local Schottky barrier height is found to vary between 0 and 1.26 eV depending on the position of the dopant. The band bending is caused by a dopant induced dipole field between the interface and the dopant site, whereas the pristine Au/TiO$_2$ interface does not show any band bending. These findings op…
Metal-Ferroelectric-Metal structures with Schottky contacts: II. Analysis of the experimental current-voltage and capacitance-voltage characteristics of Pb(Zr,Ti)O3 thin films
The density of states (DOS) of graphene underneath a metal is estimated through a quantum capacitance measurement of the metal/graphene/SiO2/n+-Si contact structure fabricated by a resist-free metal deposition process. Graphene underneath Au maintains a linear DOS - energy relationship except near the Dirac point, whereas the DOS of graphene underneath Ni is broken and largely enhanced around the Dirac point, resulting in only a slight modulation of the Fermi energy. Moreover, the DOS of graphene in the contact structure is correlated with the contact resistivity measured using devices fabrica…
Two-dimensional MoS2 has emerged as promising material for nanoelectronics and spintronics due to its exotic properties. However, high contact resistance at metal semiconductor MoS2 interface still remains an open issue. Here, we report electronic properties of field effect transistor devices using monolayer MoS2 channels and permalloy (Py) as ferromagnetic (FM) metal contacts. Monolayer MoS2 channels were directly grown on SiO2/Si substrate via chemical vapor deposition technique. The increase in current with back gate voltage shows the tunability of FET characteristics. The Schottky barrier …
Over 3 kV and Ultra-Low leakage Vertical (011) \b{eta}-Ga2O3 Power Diodes with Engineered Schottky Contact and High-permittivity Dielectric Field Plate
The contact properties between metal and graphene were examined. The electrical measurement on a multiprobe device with different contact areas revealed that the current flow preferentially entered graphene at the edge of the contact metal. The analysis using the cross-bridge Kelvin structure (CBK) suggested that a transition from the edge conduction to area conduction occurred for a contact length shorter than the transfer length of ~1 micron. The contact resistivity for Ni was measured as ~5*10-6 Ohmcm2 using the CBK. A simple calculation suggests that a contact resistivity less than 10-9 Oh…
The effect of contact architecture, graphene defect density and metal-semiconductor work function difference on resistivity of metal-graphene contacts have been investigated. An architecture with metal on the bottom of graphene is found to yield resistivities that are lower, by a factor of 4, and most consistent as compared to metal on top of graphene. Growth defects in graphene film were found to further reduce resistivity by a factor of 2. Using a combination of method and metal used, the contact resistivity of graphene has been decreased by a factor of 10 to 1200 +- 250 Ohm-um using Palladi…
Contact resistivity rc of InP and GaAs based ohmic contacts was measured in the 4.2/300 K temperature range. Nonmonotonic dependences rc(T), with a minimum at temperature 50 K (150 K) for InP (GaAs) based contacts were obtained. The results can be explained within the framework of the mechanism of current flow through metal shunts (associated with dislocations) penetrating into the semiconductor bulk, with allowance being made for electron freeze-out at helium temperatures. Contact ohmicity in the 4.2/30K temperature range is due to accumulation band bending near shunt ends at the metal/semico…
Specific contact resistivity measurements have conventionally been heavy in both fabrication and simulation/calculation in order to account for complicated geometries and other effects such as parasitic resistance. We propose a simpler geometry to deliver current, and the use of a scanning voltage probe to sense the potential variation along the sample surface, from which the specific contact resistivity can be straightforwardly deduced. We demonstrate an analytical example in the case where both materials are thin films. Experimental data with a scanning Kelvin probe measurement on graphene f…
V-telluride superlattice thin films have shown promising performance for on-chip cooling devices. Recent experimental studies have indicated that device performance is limited by the metal/semiconductor electrical contacts. One challenge in realizing a low resistivity contacts is the absence of fundamental knowledge of the physical and chemical properties of interfaces between metal and V-telluride materials. Here we present a combination of experimental and theoretical efforts to understand, design and harness low resistivity contacts to V-tellurides. Ab initio calculations are used to explor…
We study temperature evolution of spin accumulation signals obtained by the three-terminal Hanle effect measurements in a nondegenerated silicon channel with a Schottky-tunnel-barrier contact. We find the clear difference in the temperature-dependent spin signals between spin-extraction and spin-injection conditions. In a spin-injection condition with a low bias current, the magnitude of spin signals can be enhanced despite the rise of temperature. For the interpretation of the temperature-dependent spin signals, it is important to consider the sensitivity of the spin detection at the Schottky…
We demonsrtate electrical spin injection and detection in $n$-type Ge ($n$-Ge) at room temperature using four-terminal nonlocal spin-valve and Hanle-effect measurements in lateral spin-valve (LSV) devices with Heusler-alloy Schottky tunnel contacts. The spin diffusion length ($λ$$_{\rm Ge}$) of the Ge layer used ($n \sim$ 1 $\times$ 10$^{19}$ cm$^{-3}$) at 296 K is estimated to be $\sim$ 0.44 $\pm$ 0.02 $μ$m. Room-temperature spin signals can be observed reproducibly at the low bias voltage range ($\le$ 0.7 V) for LSVs with relatively low resistance-area product ($RA$) values ($\le$ 1 k$Ω$$μ$m…
Half-Heusler(HH) alloys have shown promising thermoelectric properties in the medium and high temperature range. To harness these material properties for thermoelectric applications, it is important to realize electrical contacts with low electrical contact resistivity. However, little is known about the detailed structural and electronic properties of such contacts, and the expected values of contact resistivity. Here, we employ atomistic ab initio calculations to study electrical contacts in a subclass of HH alloys consisting of the compounds HfCoSb, HfZrCoSb, and HfZrNiSn. By using Ag as a …
The effect of realistic metal electronic structure on the lower limit of resistivity in [100] oriented n-Si is investigated using full band Density Functional Theory and Semi-Empirical Tight Binding (TB) calculations. Using simulation unit cells guided by the interface chemistry of epitaxial CoSi2 on [100] oriented Si observed experimentally, it is shown that the 'ideal metal' assumption fails in some situations and consequently underestimates the lower limit of contact resistivity in n-Si by at least an order of magnitude at high doping concentrations. The mismatch in transverse momentum spac…
