Electrical contacts to monolayer black phosphorus: A first-principles investigation

Kui Gong, Lei Zhang, Wei Ji, and Hong Guo

Phys. Rev. B 90, 125441 – Published 23 September 2014

Abstract

We report first-principles theoretical investigations of possible metal contacts to monolayer black phosphorus (BP). By analyzing lattice geometry, five metal surfaces are found to have minimal lattice mismatch with BP: Cu(111), Zn(0001), In(110), Ta(110), and Nb(110). Further studies indicate Ta and Nb bond strongly with monolayer BP causing substantial bond distortions, but the combined Ta-BP and Nb-BP form good metal surfaces to contact a second layer BP. By analyzing the geometry, bonding, electronic structure, charge transfer, potential, and band bending, it is concluded that Cu(111) is the best candidate to form excellent Ohmic contact to monolayer BP. The other four metal surfaces or combined surfaces also provide viable structures to form metal/BP contacts, but they have Schottky character. Finally, the band bending property in the current-in-plane (CIP) structure where metal/BP is connected to a freestanding monolayer BP, is investigated. By both work function estimates and direct calculations of the two-probe CIP structure, we find that the freestanding BP channel is $n$ type.

Received 14 May 2014
Revised 31 July 2014

DOI:https://doi.org/10.1103/PhysRevB.90.125441

Authors & Affiliations

Kui Gong^1,2, Lei Zhang^2,*, Wei Ji³, and Hong Guo²

¹School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing, 100083, People's Republic of China
²Department of Physics, McGill University, Montreal, H3A 2T8, Canada
³Department of Physics and Beijing Key Laboratory of Optoelectronic Functional Materials and Micro-nano Devices, Renmin University of China, Beijing 100872, China

^*zhanglei@physics.mcgill.ca

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Issue

Vol. 90, Iss. 12 — 15 September 2014

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Images

Figure 1
(a) Top and side view of monolayer BP. The shadowed area with four P atoms is a unit cell of monolayer BP. In the side view, atoms in monolayer BP are classified as A/B sublayers. (b) The calculated band structure of monolayer BP and its Brillouin zone with high symmetry points.
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Figure 2
(a) Schematic plot of the monolayer BP and metal contact region. (b) Side view of monolayer BP absorbed on Cu(111), Zn(0001), and In(110) metal surfaces at the contact region. (c) Side view of monolayer BP absorbed on TaP or NbP electrode at the contact region. The dashed line in panels (b) and (c) indicates the supercell in the $x z$ plane.
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Figure 3
PDOS of P atoms, in (a) the monolayer BP, (b) the Cu/BP system, (c) the Zn/BP system, and (d) the TaP/BP system (the P atoms in the top layer). The blue solid line, red dashed line, and black dotted line represent the $3 p_{z}$ orbital, $3 p_{x} + 3 p_{y}$ orbital, and $3 s$ orbital of top layer P atoms as indicated by the legend in (a). The vertical green dashed line indicates the Fermi level.
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Figure 4
Electronic structure at the interface of the contact region: (a)–(d) for Cu substrate, (e)–(h) for Zn substrate, and (i)–(l) for TaP substrate. The left side contour plots represent charge density $ρ$ which is integrated in the $y$ direction and normalized to one BP unit cell. The unit is number of electrons per area (denoted as $el . / δ^{2}$ ) The middle panels (b), (f), and (j) are charge density difference $Δ ρ (z)$ , which is integrated over the $x$ and $y directions$ and normalized to one BP unit cell. The middle panels (c), (g), and (k) are contour plots of the electron localization function (ELF) in a slice along the $y$ axis that crosses both P atoms and metal atoms. The panels (d), (h), and (l) are average local potential $V$ . The average potential barriers at the BP and metal interface are also indicated. $δ$ is the length of the real-space grid, equal to 0.065 Å. The green dashed lines represent the position of each average P atom layer and the pink dashed lines represent the position of the average metal surface after structure relaxation.
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Figure 5
(a) Schematic plot of a CIP device where the metal/BP contact is connected to a freestanding BP channel extending to the right. $W$ and $W_{BP}$ denote work functions of the metal/BP and the freestanding BP, respectively. The black lines qualitatively indicate the band bending. (b) and (c) plot the calculated local density of states (LDOS) in color coding for CIP devices with Cu/BP and Zn/BP electrodes, respectively. The red line indicates the boundary of metal/BP and the freestanding monolayer BP. In the LDOS plot, only the contributions from BP atoms are plotted in the metal/BP region for clarity purposes.
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