Black Phosphorus Field-Effect Transistors with Work Function Tunable Contacts Yuqiang Ma,†,# Chenfei Shen,†,# Anyi Zhang,† Liang Chen,† Yihang Liu,† Jihan Chen,† Qingzhou Liu,† Zhen Li,† Moh. R. Amer,‡,§ Tom Nilges,∥ Ahmad N. Abbas,*,‡,⊥,¶ and Chongwu Zhou*,† †
Department of Electrical Engineering, University of Southern California, 3710 McClintock Avenue, Los Angeles, California 90089-0271, United States ‡ Center of Excellence for Green Nanotechnologies, Joint Centers of Excellence Program, King Abdulaziz City for Science and Technology, P.O Box 6086, Riyadh 11442, Saudi Arabia § Department of Electrical Engineering, University of California, Los Angeles, 420 Westwood Plaza, 5412 Boelter Hall, Los Angeles, California 90095, United States ∥ Department für Chemie, Technische Universität München, Lichtenbergstrasse 4, 85747 Garching b. München, Germany ⊥ Department of Electrical and Computer Engineering, University of Jeddah, Dhahban 23881, Saudi Arabia ¶ Department of Electrical and Computer Engineering, King Abdulaziz University, Abdullah Sulayman Street, Jeddah 22254, Saudi Arabia S Supporting Information *
ABSTRACT: Black phosphorus (BP) has been recently rediscovered as an elemental two-dimensional (2D) material that shows promising results for next generation electronics and optoelectronics because of its intrinsically superior carrier mobility and small direct band gap. In various 2D field-effect transistors (FETs), the choice of metal contacts is vital to the device performance, and it is a major challenge to reach ultralow contact resistances for highly scaled 2D FETs. Here, we experimentally show the effect of a work function tunable metal contact on the device performance of BP FETs. Using palladium (Pd) as the contact material, we employed the reaction between Pd and H2 to form a Pd−H alloy that effectively increased the work function of Pd and reduced the Schottky barrier height (ΦB) in a BP FET. When the Pd-contacted BP FET was exposed to 5% hydrogen concentrated Ar, the contact resistance (Rc) improved between the Pd electrodes and BP from ∼7.10 to ∼1.05 Ω· mm, surpassing all previously reported contact resistances in the literature for BP FETs. Additionally, with exposure to 5% hydrogen, the transconductance of the Pd-contacted BP FET was doubled. The results shown in this study illustrate the significance of choosing the right contact material for high-performance BP FETs in order to realize the real prospect of BP in electronic applications. KEYWORDS: layered material, two-dimensional materials, field-effect transistor, black phosphorus, phosphorene, contact resistance
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provide large-scale and highly crystalline BP are needed for high-performance electronics. Field-effect transistors (FETs) of few-layer BP have been reported to show high charge carrier mobility, anisotropic transport behavior, high on/off ratios, and high operating frequencies.11−13 Although BP has exhibited the potential to become a promising candidate for future electronics, FETs made with single-layer or few-layer BP usually possess small Schottky barriers. The Schottky barrier causes increased contact resistances (Rc), which significantly limit the performance and scalability of BP FETs.14
lack phosphorus (BP) has been recently unveiled as a promising anisotropic direct band gap semiconducting material, with interesting optical and electrical properties.1−6 Several controlled synthesis methods of BP have been reported recently. Nilges et al. reported an ampule synthesis method to produce bulk single-crystalline BP; however, this method requires mechanical exfoliation of BP, which limits the size of BP flakes.7,8 Lau et al. reported pulsed laser deposition of ultrathin BP onto graphene/copper and Si/SiO2 substrates, but the obtained BP is amorphous, which limits its electrical performance.9 Ji et al. reported direct chemical vapor deposition of single-crystalline BP films onto Si/SiO2 substrates. Nevertheless, large size and small thickness of the BP flakes cannot be achieved simultaneously.10 Consequently, methods that can © 2017 American Chemical Society
Received: April 25, 2017 Accepted: June 19, 2017 Published: June 27, 2017 7126
DOI: 10.1021/acsnano.7b02858 ACS Nano 2017, 11, 7126−7133
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Figure 1. Effect of H2 treatment on Pd-contacted BP FETs. (a) Schematic diagram of the reaction between Pd contacts in BP FETs and H2. (b) Schematic diagram of the effect of different H2 concentrations on Schottky barrier heights. (c) Optical image of few-layer BP-based FET with Pd contacts. Inset shows an AFM height profile of the BP flake which reveals a height of ∼31 nm. (d) Typical Ids−Vds curve of few-layer BP FET before (black) and after (red) exposure to 5% H2 diluted in Ar.
the device transconductance. To determine the real value of the contact resistance, transfer line measurements (TLM) are needed.17 In terms of contact materials, most BP FET contacts are usually fabricated with traditional metals or layered semimetals like graphene.18,19 Different metal contact materials can result in n-type, p-type, or ambipolar behavior of the BP FET, which is mainly related to the work functions of the contact material. It has been reported that, with aluminum contacts, a unipolar ntype to ambipolar transition occurs as BP flake thicknesses increase from 3 to 13 nm.20 The p-type device performance has also been reported by using scandium contacts, regardless of the flake thickness.21 Ambipolar BP devices have been achieved using Ti/Au as the metal contacts for BP channel, with thicknesses of 1.5 nm/70 nm, respectively.22 The work functions of metals can give rise to Schottky barriers with various heights and depths, which lead to different transistor behaviors. Because Pd has been commonly used to achieve low contact resistances for p-type nanomaterial devices, such as carbon nanotubes (CNT) FETs,23,24 an important yet unanswered question is whether changing the work function of already high-work-function metals (such as Pd) could
Recent studies demonstrated several methods to overcome the contact problems in BP FETs. A common method to reduce the Shottky barrier height and the contact resistance is by choosing the metal with the appropriate work function, depending on the 2D semiconductor carrier type. Ye et al. reported using high work function metals, such as Pd and Ni, for BP FETs and reported contact resistances of 1.75 and 3.15 Ω·mm, respectively.15 Other studies have employed other methods to mitigate the contact resistance problem. For example, by using TiO2 and cobalt contacts, a reduced Schottky barrier of
