Enhanced performance of field effect transistors based on black

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Functional Inorganic Materials and Devices

Enhanced performance of field effect transistors based on black phosphorus channels reduced by galvanic corrosion of Al over-layers Sangik Lee, Chansoo Yoon, Ji Hye Lee, Yeon Soo Kim, Mi Jung Lee, Wondong Kim, Jaeyoon Baik, Quanxi Jia, and Bae Ho Park ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04700 • Publication Date (Web): 16 May 2018 Downloaded from http://pubs.acs.org on May 16, 2018

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ACS Applied Materials & Interfaces

Enhanced performance of field effect transistors based on black phosphorus channels reduced by galvanic corrosion of Al over-layers Sangik Lee†, Chansoo Yoon†, Ji Hye Lee†, Yeon Soo Kim†, Mi Jung Lee†, Wondong Kim‡, Jaeyoon Baik§, Quanxi Jia†,∥, and Bae Ho Park*,† †

Division of Quantum Phases & Devices, Department of Physics, Konkuk University, Seoul

05029, Korea ‡

Korea Research Institute of Standards and Science (KRISS), Daejeon 34113, Korea

§

Pohang Accelerator Laboratory, Pohang University of Science and Technology, Pohang 37673,

Korea ∥

Department of Materials Design and Innovation, University of Buffalo – The State University

of New York, Buffalo, New York 14260, USA

KEYWORDS: reduction of black phosphorus, anodic oxidation of Al over-layer, mobility, subthreshold swing, interface trap density

ACS Paragon Plus Environment

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ABSTRACT Two-dimensional (2D) layered semiconducting materials with considerable bandgaps are emerging as a new class of materials applicable to next-generation devices. Particularly, black phosphorus (BP) is considered to be very promising for next-generation 2D electrical and optical devices due to its high carrier mobility of 200−1000 cm2V-1s-1 and large on/off ratio of 104−105 in field effect transistors. However, its environmental instability in air requires fabrication processes in a glove box filled with nitrogen or argon gas followed by encapsulation, passivation, and chemical functionalization of BP. Here, we report a new method for reduction of BP-channel devices fabricated without the use of a glove box by galvanic corrosion of an Al over-layer. The reduction of BP induced by an anodic oxidation of Al over-layer is demonstrated through surface characterization of BP using atomic force microscopy, Raman spectroscopy, and X-ray photoemission spectroscopy along with electrical measurement of a BP-channel field effect transistor (FET). After the deposition of an Al over-layer, the FET device shows significantly enhanced performance, including restoration of ambipolar transport, high carrier mobility of 220 cm2V−1s−1, low sub-threshold swing of 0.73 V/decade, and low interface trap density of 7.8 × 1011 cm-2eV−1. These improvements are attributed to both the reduction of the BP channel and the formation of an Al2O3 interfacial layer resulting in a high-k screening effect. Moreover, ambipolar behavior of our BP-channel FET device combined with charge-trap behavior can be utilized for implementing reconfigurable memory and neuromorphic computing applications. Our study offers a simple device fabrication process for BP-channel FETs with high performance using galvanic oxidation of Al over-layers.


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INTRODUCTION Since the discovery of graphene1,2, two-dimensional (2D) materials have attracted much attention due to their outstanding electrical and mechanical properties3. Although graphene has shown extremely high mobility and high mechanical strength1,4,5, the absence of a bandgap makes it difficult for semiconducting and optoelectronic devices6. This has triggered research interest in other 2D materials with intrinsic bandgaps. Among the studied 2D semiconductors, transition metal dichalcogenides (TMDs) and black phosphorus (BP) show promising properties for electronic and optoelectronic applications7-14. BP is a remarkable candidate 2D semiconductor, as it has a high electrical mobility and a small direct bandgap compared to TMDs, where the electrical mobility is limited due to their large bandgap. BP is a p-type 2D semiconductor with a thickness-dependent direct bandgap ranging from 0.3 eV to 2.0 eV15. Few-layer BP has been used as a channel material in field effect transistors (FETs) due to its high on/off ratio of 104 to 105, mobility up to 1000 cm2V−1s−1 at room temperature, and ambipolar transport10,16. Its tunable bandgap and high photo-responsivity of ~ 9 × 104 A/W can be useful in future electronic as well as optical devices17-19. In addition, its electronic and optical anisotropies, originating from its geometry, provide BP with anisotropic mobility and linear dichroism and allow unique device architectures20-22. The isolation of singleand few-layer BP has been readily achieved by mechanical exfoliation, which is commonly used for fabrication of single-crystalline 2D layers. However, the chemical vulnerability of BP under ambient conditions seriously hinders the implementation of promising BP-based devices. The performance of BP electronic devices is strongly affected by the surrounding environment. It has been demonstrated that ambient water and oxygen can cause highly hydrophilic surface easily absorbing moisture, which leads to


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strong p-type doping and structural degradation of the BP crystal and reduced FET performance23,24. Furthermore, photo-assisted oxidation processes can expedite the degradation of BP properties25. The environmental instability of BP has made it challenging to prepare and characterize BP-based electronic devices under atmospheric pressure. In order to protect BP electronic devices against oxidation-induced degradation, many efforts have been focused on minimizing the exposure of BP to oxygen-contained environment through the use of a glove box filled with dry nitrogen or argon gas environment, encapsulation of BP with air-stable overlayers such as AlOx26, graphene, h-BN27, stable native oxide layers28, etc.29-31, and chemical functionalization of BP with phosphorus–oxygen32,33 or phosphorus–carbon bonding by aryl diazonium34. However, the deposition of the encapsulation layer, such as AlOx using atomic layer deposition (ALD), can degrade the electrical properties of the BP channel due to water molecules resulting from the ALD process35,36. Moreover, no reduction methods for oxidized BP for a BP-channel FET device have been reported yet. In this paper, we report a new approach that can provide both reduction and encapsulation for an oxidized BP channel in an FET device, which is fabricated without the use of a glove box filled with dry nitrogen or argon gas, by simply introducing an Al over-layer. Typically, galvanic corrosion occurs through a series of reduction-oxidation reactions when two electrochemically dissimilar materials are electrically connected in the presence of adsorbed moisture or solid electrolyte37,38. Upon deposition of Al metal, anodic oxidation of Al and cathodic reduction of the BP oxide at the electrically contacted interface between Al and BP occur spontaneously due to the lower Gibbs free energy for the oxidation of Al (~ -1582 kJ/mole) than that of BP (~ -1355 kJ/mole) at room temperature39. In contrast, nonreactive Au with a higher Gibbs free energy for oxidation (~ 77.9 kJ/mole) than that of BP can neither reduce the BP oxide nor prevent


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continuous BP oxidation under exposure to ambient air39,40. Moreover, a naturally formed Al2O3 layer can not only encapsulate the BP channel but also suppress Coulomb scattering due to a high-k screening effect in a reduced BP-channel FET device41,42. EXPERIMENTAL SECTION Device fabrication. We mechanically exfoliated BP from a synthetic bulk crystal (~ 99.998 %, smart elements) with blue Nitto tape. Thin BP flakes were identified using optical microscope images and transferred onto SiO2 (300 nm)/Si substrates. Then, source/drain electrodes (5 nm Ti/40 nm Au) for BP-channel FET devices were patterned by e-beam lithography and deposited using an e-beam evaporator with a substrate cooling system. An Al over-layer without contact to the drain/source electrodes was also patterned by e-beam lithography and deposited using an ebeam evaporator. Al2O3 (30 nm)/HfO2 (8 nm)/Al2O3 (5 nm) gate stack was deposited using insitu atomic layer deposition (ALD). Measurement. The AFM (XE-100, Park Systems) was operated in a non-contact mode with a Pt/Ir-coated silicon cantilever to determine the BP topography and thickness. The chemical composition and bonding of oxidized and reduced BP was analyzed using an SPEM system equipped with an in situ Al metal deposition chamber in Pohang accelerator laboratory. Raman spectroscopy was performed using visible laser light (λ = 532 nm) in a WITEC system. To avoid damage of samples, all spectra were recorded at low power levels (500 µW). The electrical characteristics of all BP-channel FET devices were investigated using the source–drain current as a function of gate voltage under ambient conditions at room temperature with a semiconductor parameter analyzer (4156B, Agilent Inc.).


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RESULTS AND DISCUSSION The reduction and encapsulation of oxidized BP was characterized with atomic force microscopy (AFM) topography measurements, as shown in Figures 1a–h. The AFM topography images were obtained after Au and Al metal deposition and exposing the samples to ambient air at different periods of time. Both Au and Al were deposited using an e-beam evaporation system with substrate cooling for excluding the thermal effect for the oxidized BP surface during metal deposition. Oxidized BP samples were prepared on SiO2 (300 nm)/Si substrates by the mechanical exfoliation method under exposure to air. On the surface of air-exfoliated BP samples with thicknesses of 13 nm (Figure 1a) and 14 nm (Figure 1e), small bubbles with heights of ≤ 6 nm and ≤ 10 nm, respectively (Supporting Information, Figures S1a and S1b), are observed, which are attributed to the formation of BP oxide caused by ambient oxygen and water. After deposition of Au (10 nm) on the 13-nm-thick oxidized BP surface, the oxidation bubble density and height increase due to further oxidation during the fabrication process of the Au metal layer (Figure 1b and Supporting Information, Figure S1a). While the density of the oxidation bubbles decreases after exposure to air for 5 days, its height increases, implying that it grows continuously (Figure 1c and Supporting Information, Figure S1a). After exposure to air for 15 days, the oxidation bubbles eventually become sub-micrometer-sized with ~ 70 nm height. The characteristic Ag1, B2g, and Ag2 peaks in the BP Raman spectrum at 362.6, 439.3, and 466.3 cm−1, respectively, disappeared after exposure to air for 15 days, indicating the serious degradation of the BP crystal (Supporting Information, Figure S2a)23. Typically, the continuous increase in the volume of oxidation bubbles leads to degradation of the BP crystal by


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spontaneous oxidation, which is in good agreement with previous studies on the ambient oxidation of BP23-27. In contrast, after Al (10 nm) deposition on the 14-nm-thick oxidized BP surface, the oxidation bubble height is reduced in comparison to that of the initial air-exfoliated BP (Figure 1f and Supporting Information, Figure S1b). This experimental observation could be attributed to the spontaneous oxidation of Al and reduction of BP oxide at the Al/BP interface. Moreover, after exposure to air for 5 days and 15 days, the oxidation bubble height on the reduced BP surface changes negligibly, which indicates that the very thin Al layer enables good resistance to ambient oxidation (Figure 1h and Supporting Information, Figure S1b). The Raman spectrum of reduced BP after exposure to ambient air for 15 days clearly shows Ag1, B2g, and Ag2 peaks supporting the reduction and encapsulation effects of the very thin Al layer (Supporting Information, Figure S2b). In order to demonstrate both cathodic reduction of BP and anodic oxidation of Al at the Al/BP interface, we performed X-ray photoemission spectroscopy (XPS) measurements for an Aldeposited BP sample. A scanning photoelectron microscope (SPEM) system was used for analyzing the surface chemical composition and bonding with core-level P 2p and Al 2p measurements through in situ Al metal deposition on the oxidized BP surface under highvacuum conditions of 10−9 Torr. SPEM equipped with scanning and focused X-ray of a 200 nm spot size is a powerful tool to investigate the local chemical structure. To avoid unintentional charge accumulation, which can be induced in the oxidized BP area or the SiO2 substrate by the highly focused X-ray beam during SPEM measurements, BP was exfoliated on a highly p-type doped Si substrate acting as a ground electrode43. Figure 2 displays the P 2p and Al 2p core-level spectra obtained from the ambient-oxidized BP surface before and after Al metal deposition. All


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spectra are calibrated using the binding energy of silicon (99.4 eV). The P 2p core-level spectrum of ambient-oxidized BP contains a doublet of 2p1/2 (P1) and 2p3/2 (P2) (spin–orbit splitting of 0.86 eV) with binding energies of 129.68 eV and 130.54 eV, respectively, and a phosphorus oxide component peak (P3) at 134.15 eV (top panel in Figure 2a)26,28,32,44. The P3 peak corresponds to POx and is located at a lower binding energy than that of the most dominant oxide P2O5 (~ 135 eV). Typically, a lower oxidation state of phosphorus oxide should be characterized by a lower binding energy35,44-46. After Al metal deposition on the ambientoxidized BP surface, the P1 and P3 peaks are shifted to higher and lower binding energies, respectively (bottom panel in Figure 2a). The shifts of the two peaks could be potentially caused by an increase in the oxygen vacancy concentration of POx and/or interfacial Al2O3 layer formation with the electron doping effect due to spontaneous reduction of oxidized BP and oxidation of Al metal by Al metal deposition. The P1 peak shifts to a higher binding energy by 0.11 eV, which is attributed to the shift of the Fermi level toward the conduction band28,47; the P3 peak shifts to a lower binding energy at 133.51 eV, which is caused by a decrease in the O concentration of POx35,45,46. Moreover, Figure 2b shows that the Al 2p core-level peak corresponding to Al2O3 appears after Al metal deposition (bottom panel in Figure 2b). This experimental observation can be understood as a result of anodic oxidation of Al metal and cathodic reduction of BP oxide at the Al/BP interface in the presence of adsorbed moisture or solid electrolyte37,38. In order to investigate the effect of BP reduction with formation of an interfacial Al2O3 layer on the FET device performance, we fabricated BP-channel FET devices with back gate structures on SiO2 (300 nm)/Si substrates (inset of Figure 3f). All sample fabrication and measurement processes were performed under ambient air. We measured the change in the transfer


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characteristics of the BP-channel FET for different oxidation states that were monitored using AFM topography images and corresponding line profiles. Figures 3a–d show AFM topography images of a BP-channel FET for as-fabricated state, after 1 day of air exposure, after Al metal deposition, and after 6 days of further exposure to air, respectively. For comparison, AFM line profiles were simultaneously obtained as shown in Figure 3e. Small oxidation bubbles are observed for the as-fabricated BP-channel FET (Figure 3a). After 1 day of air exposure, both the size and height of the oxidation bubbles increase (Figures 3b and 3e) indicating the BP channel degradation. In order to confirm the reduction effect of the oxidized BP channel, we deposited ~ 6-nm-thick Al metal on the channel. The Al metal thickness is kept below 10 nm in order to prevent electrical shorting between the source and drain electrodes. Just after the deposition of the very thin Al layer, the size and height of the oxidation bubbles significantly decrease (Figures 3c and 3e). The number and height of the oxidation bubbles, however, increase after 6 days of air exposure, which is probably due to the weak encapsulation effect of the ∼ 6-nm-thick Al metal layer. This morphologic change in AFM topography due to oxidation, reduction, and weak encapsulation is well matched with the change in the transfer characteristics of the BP-channel FET, as shown in Figure 3f. The source–drain current (Ids) measured at a source–drain voltage of Vds = 100 mV was obtained as a function of the back-gate voltage (Vbg = –30 V to 30 V), which was applied through the 300-nm-thick SiO2 back-gate dielectric, under ambient conditions. The as-fabricated BP-channel FET exhibits p-type dominant ambipolar behavior. After 1 day of air exposure (oxidation), the ambipolar behavior disappears due to the strong p-doping effect by ambient oxidation with formation of BP oxidation bubbles23-27. After Al metal deposition on the oxidized BP surface, p-type dominant ambipolar behavior is restored with a shift of the threshold voltage (Vth) to lower values supporting the suggestion that


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the BP channel is reduced with electron doping by the oxidation of Al metal leading to formation of an interfacial Al2O3 layer. Moreover, the BP-channel FET device with a ~ 6-nm-thick Al layer (Figure 3f) shows better stability than that with a ~ 3-nm-thick Al layer after 6 days of air exposure (Supporting Information, Figure S3). This is in agreement with recent studies on Al2O3-thickness-dependent encapsulation of BP-channel FET devices31. In contrast, an oxidized BP-channel FET device with an Au layer does not recover the ambipolar behavior but shows a strong p-doping effect due to degradation of the device by spontaneous oxidation of BP (Supporting Information, Figure S4a). To determine the effect of the formed interfacial Al2O3 layer on the BP-channel FET, we extracted the device parameters such as hole field effect mobility ( ), subthreshold swing (SS), and interface trap density (Dit) using the Ids–Vbg curve in Figure 3f. The value of can be extracted in the linear region of the conductance (G) using the following equation16,27:

=

(1)

Here, L and W are the length and width of the channel, respectively, and is the gate capacitance. By fitting the linear region of the G versus Vbg curve using Eq. (1), values of 23.4, 11.9, and 21.1 cm2V−1s−1 were extracted for the as-fabricated, after 1-day oxidation, and after Al metal deposition BP-channel FETs, respectively (Supporting Information, Figure S5a). In addition, the value of Dit is normally extracted from the following relationship between SS and Cit48,49: SS =

( )

=

!"

#

ln 10 (

) * ) +,

-

(2)

where . is the electronic charge, /0 is the Boltzmann constant, T is the temperature in K, and 1 and 23 are the semiconductor and interface-trap capacitances, respectively. 23 is equal to .423 , and 1 is negligible due to the relatively thin BP layer48,49. Based on the SS values of 5.1, 6, and


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3.2 V/decade, respectively (Supporting Information, Figure S5b), extracted from the linear region of the Vgb versus log (Id) curve, the values of Dit are calculated to be 6.1 × 1012, 7.1 × 1012, and 3.8 × 1012 cm-2eV−1 for the as-fabricated, after 1-day oxidation, and after Al metal deposition BP-channel FETs, respectively. The , SS, and Dit values could be affected by charge trap and/or scattering centers originating from oxygen, water, and oxides on the BP surface48,49. Therefore, we expect that the decrease in the SS and Dit values with the restored p-type dominant ambipolar behavior after Al metal deposition is attributed to the reduction of BP and charge screening effect of the formed interfacial Al2O3 layer. It has been reported that the positive fixed charge in an Al2O3 layer can induce energy band bending near the electrode/BP contact edge and lead to modulating the type of operation and contact resistance in a few-layer BP FET device50. Therefore, our BP-channel FET after Al deposition shows a decrease in the conductance and hole mobility compared to the as-fabricated device, which is attributed to the increased contact resistance along with restoration of the ambipolar behavior. In order to exclude the effect of positive fixed charge in Al2O3 on the Schottky barrier near the electrode/BP interface, we fabricated a BP-channel FET device on a SiO2 (300 nm)/Si substrate without contact between the Al over-layer and drain/source electrodes. A ~ 70-nm-thick Al overlayer with 1 µm length was deposited on the BP channel with 2 µm length between the source and drain electrodes. In this BP-channel FET device, an exposed BP region existing between the Al metal and drain/source electrodes is expected to be also reduced by galvanic corrosion of the electrically connected Al metal in the presence of adsorbed moisture or solid electrolyte37,38. The cross-section of the device is schematically illustrated in Figure 4a. Figure 4b exhibits electrical characteristics of the FET device before and after deposition of the Al over-layer, which are obtained at Vds = 100 mV as a function of Vbg = −30 V to 30 V. Before the Al over-layer


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deposition, the FET device exhibits p-type behavior due to ambient air oxidation. After the Al over-layer deposition, p-type ambipolar behavior is also restored with a shift of Vth to low voltages, which is due to the reduction of oxidized BP channel along with the formation of an interfacial Al2O3 layer. It is noteworthy that the extracted device parameters of , SS, and Dit are remarkably enhanced from 78.8 to 220 cm2V−1s−1, from 12.6 to 0.73 V/decade, and from 1.2 × 1013 to 7.8 × 1011 cm-2eV−1, respectively, after the Al over-layer deposition (Supporting Information, Figures S5c and S5d). In a previously reported BP-channel FET device fabricated in a glove box filled with dry nitrogen or argon gas environment and encapsulated with Al2O3 and h-BN, SS value has been reported to be 1.5 and 0.85 V/decade respectively48,51. In general, SS value of a BP-channel FET device was attributed to the quality of interface between BP and a gate insulator or an encapsulation layer. By comparing Figure 3f to Figure 4b, we can observe the much higher hole mobility and remarkably decreased subthreshold swing of the FET device without contact between the Al over-layer and drain/source electrodes. This may originate from different factors such as the decreased interface trap density, absence of the Schottky barrier modulation, and suppression of Coulomb scattering due to the high-k screening effect of the interfacial Al2O3 layer. Typically, top-gated BP-channel FETs are fabricated in a glove box and require protection layers, such as Al2O3, deposited using ALD process. Conventional Al2O3 protection layer does not induce the reduction of BP channel, but rather water molecules resulting from the ALD process degrade the electrical properties of the BP-channel FETs35,36. In contrast, we deposited the Al over-layer directly on the BP channel between source and drain electrodes without the aid of a glove box. We achieved lower SS value of 0.73 V/decade than those of the BP-channel FETs encapsulated with Al2O3 (1.5 V/decade) and h-BN (0.85 V/decade) using a conventional


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method in a glove box48,51. The Al over-layer induces both the reduction of the BP channel and the formation of an Al2O3 interfacial insulator with high-k screening effect resulting in the enhanced performances of the BP-channel FET. Compared to the traditional fabrication process for BP-channel FETs in a glove box, the introduction of Al over-layer can provide a simple fabrication process to achieve high performance BP-channel FETs without the aid of a glove box. Our simple method for fabricating high performance BP-channel FET with ambipolar behavior can be utilized for implementing an ambipolar charge trap memory device applicable to reconfigurable memory circuit and neuromorphic computing. Conventional charge trap memory device typically works with a fixed channel carrier polarity and device characteristics52,53. Dualgated charge trap memory with ambipolar channel can tune the memory characteristics by systematically shifting the Fermi level, which leads to a dynamically reconfigurable ambipolar memory device54. Our BP-channel FET device with Al over-layer shows good ambipolar behavior while the BP-channel FET device without Al over-layer exhibits p-type behavior. Thus, we fabricated a BP-channel charge trap memory with Al2O3/HfO2/Al2O3 gate stack and ~2 nm Al over-layer on BP channel. The device clearly exhibited both ambipolar behavior under back gate bias sweep and hysteresis behavior induced by charge trap/detrap under top gate bias sweep (Supporting Information, Figures S6), which offered a potential for dynamically reconfigurable ambipolar memory.

CONCLUSION We have demonstrated that an ambient-oxidized BP channel can be reduced by anodic oxidation of an Al over-layer by performing AFM and XPS analyses. After deposition of an Al


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over-layer, the BP-channel FET device restores the p-type ambipolar behavior with a lower threshold voltage and shows remarkably enhanced performance, including a high hole mobility of 220 cm2V−1s−1, low sub-threshold swing of 0.73 V/decade, and low interface trap density of 7.8 × 1011 cm-2eV−1 compared to those before the deposition of the Al over-layer. This enhanced performance is attributed to both the reduction of the BP channel and the formation of an Al2O3 interfacial layer with a high-k screening effect. Moreover, ambipolar behavior of our BP-channel FET device combined with hysteresis behavior induced by charge trap can be utilized for implementing reconfigurable memory and neuromorphic computing applications. Our study offers a simple device structure and a fabrication method of a BP-channel FET with enhanced performance and provides a better understanding of the role of an Al over-layer formed on the BP-channel FET.


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FIGURES

Figure 1. AFM topography of oxidized BP after metal deposition and exposure to air for different periods of time. AFM topography images of BP samples with thicknesses of ∼ 13 nm (a–d) and ∼ 14 nm (e–h), which were exfoliated on 300-nm-thick SiO2 under ambient conditions: as-fabricated (a and e), after deposition of Au (b) and Al (f) over-layers with 10 nm thickness, and after exposure to air for 5 days (c and g) and 15 days (d and h).


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Figure 2. Chemical identification of BP. P 2p (a) and Al 2p (b) core-level XPS spectra obtained before (top panels) and after (bottom panels) in situ deposition of Al on the ambient-oxidized BP surface.


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Figure 3. AFM topography and electrical characteristics of an oxidized BP-channel FET device. AFM topography images of an as-fabricated (a), after 1 day of air exposure (b), after ~ 6 nm Al deposition (c), and after 6 days of air exposure (d) BP channel FET devices. Change in AFM topography line profiles (e) and electrical characteristics (f) of the device. In (e) and (f), black, red, blue, and green lines denote data obtained from the as-fabricated, after 1 day of air exposure, after ~ 6 nm Al deposition, and after 6 days of air exposure devices, respectively.


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Figure 4. Electrical characteristics of a BP-channel FET device without contact between the Al over-layer and drain/source electrodes. Schematic (a) and electrical characteristics (b) of a BPchannel FET device without contact between the Al over-layer and drain/source electrodes. In (b), black and blue lines denote data obtained from the device before and after the deposition of the Al over-layer.


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ASSOCIATED CONTENT Supporting Information. The following files are available free of charge.

AFM topography line profiles and Raman spectra of BP, AFM topography, electrical characteristics and extracted device parameters of BP-channel FET devices (PDF). AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions S.L. and B.H.P. planned the project and designed the experiments; S.L. performed sample fabrication, measurement, and analysis of transport properties, atomic force microscopy measurement, and X-ray photoemission spectroscopy measurement; C.Y. assisted in measurement and analysis of transport properties and X-ray photoemission spectroscopy measurement; J.H.L. and Y.S.K. assisted in data analysis and atomic force microscopy measurement; M.J.L. assisted in sample fabrication; J.B. and W.K. performed X-ray photoemission spectroscopy measurement and data analysis; S.L., Q.X.J., and B.H.P. interpreted the results; all authors participated in discussions and writing the manuscript. Notes The authors declare no competing financial interest.


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ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea (NRF) grants funded by the

Korea

government

(MSIP)

(No.

2013R1A3A2042120

and

2011-0030229)

and

Nano·Material Technology Development Program through the NRF funded by the MSIP (No. 2016M3A7B4909668).

REFERENCES (1) Novoselov, K.; Geim, A.; Morozov, S.; Jiang, D.; Zhang, Y.; Dubonos, S.; Grigorieva, I.; Firsov, A. Electric Field Effect in Atomically Thin Carbon Films. Science 2004, 306, 666-669. (2) Novoselov, K.; Geim, A.; Morozov, S.; Jiang, D.; Grigorieva, M. I. K. I. V.; Dubonos, S.; Firsov, A. Two-Dimensional Gas of Massless Dirac Fermions in Graphene. Nature 2005, 438, 197-200. (3) Wang, Q. H.; Kalantar-Zadeh, K.; Kis, A.; Coleman, J. N.; Strano, M. S. Electronics and Optoelectronics of Two-Dimensional Transition Metal Dichalcogenides. Nat. Nanotechnol. 2012, 7, 699-712. (4) Bolotin, K. I.; Sikes, K. J.; Jiang, Z.; Klima, M.; Fudenberg, G.; Hone,J.; Kim, P.; Stormer, H. L. Ultrahigh Electron Mobility in Suspended Graphene. Solid State Commun. 2008, 146, 351355. (5) Lee, C.; Wei, X.; Kysar, J. W.; Hone J. Measurement of the Elastic Properties and Intrinsic Strength of Monolayer Graphene. Science 2008, 321, 385-388. (6) Schwierz, F. Graphene Transistors. Nat. Nanotechnol. 2010, 5, 487-496. (7) Podzorov, V.; Gershenson, M. E.; Kloc, C.; Zeis, R.; Bucher, E. High-Mobility Field-Effect Transistors Based on Transition Metal Dichalcogenides. Appl. Phys. Lett. 2004, 84, 3301-3303.


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Page 21 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(8) Radisavljevic, B.; Radenovic, A.; Brivio, J.; Giacometti, V.; Kis, A. Single-Layer MoS2 Transistors. Nat. Nanotechnol. 2011, 6, 699-712. (9) Ovchinnikov, D.; Allain, A.; Huang, Y-S.; Dumcenco, D.; Kis, A. Electrical Transport Properties of Single-Layer WS2. ACS Nano 2014, 8, 8174-8181. (10) Li, L.; Yu, Y.; Ye, G. J.; Ge, Q.; Wu, H.; Feng, D.; Chen, X. H.; Zhang, Y. Black Phosphorus Field-Effect Transistors. Nat. Nanotechnol. 2014, 9, 372-377. (11) Lopez-Sanchez, O.; Lembke, D.; Kayci, M.; Radenovic, A.; Kis, A. Ultrasensitive Photodetectors Based on Monolayer MoS2. Nat. Nanotechnol. 2013, 8, 497-501. (12) Ross, J. S.; Klement, P.; Jones, A. M.; Ghimire, N. J.; Yan, J.; Mandrus, D. G.; Taniguchi, T.; Watanabe, K.; Kitamura, K.; Yao, W.; Cobden, D. H.; Xu, X. Electrically Tunable Excitonic Light-Emitting Diodes Based on Monolayer WSe2 p–n Junctions. Nat. Nanotechnol. 2014, 9, 268-272. (13) Perea-López, N.; Elías, A. L.; Berkdemir, A.; Castro-Beltran, A.; Gutiérrez, H. R.; Feng, S.; Lv, R.; Hayashi, T.; López-Urías, F.; Ghosh, S.; Muchharla, B.; Talapatra, S.; Terrones, H.; Terrones, M. Photosensor Device Based on Few-Layered WS2 Films. Adv. Func. Mater. 2013, 23, 5511-5517. (14) Engel, M.; Steiner, M.; Avouris, P. Black Phosphorus Photodetector for Multispectral, HighResolution Imaging. Nano Lett. 2014, 14, 6414-6417. (15) Tran, V.; Soklaski, R.; Liang, Y.; Yang, L. Layer-Controlled Band Gap and Anisotropic Excitons in Few-Layer Black Phosphorus. Phys. Rev. B 2014, 89, 235319. (16) Koenig, S. P.; Doganov, R. A.; Schmidt, H.; Castro Neto, A. H.; Özyilmaz, B. Electric Field Effect in Ultrathin Black Phosphorus. Appl. Phys. Lett. 2014, 104, 103106.


21


Page 22 of 27

(17) Wu, J.; Koon, G. K.; Xiang, D.; Han, C.; Toh, C. T.; Kulkarni, E. S.; Verzhbitskiy, I.; Carvalho, A.; Rodin, A. S.; Koenig, S. P; Eda, G.; Chen, W.; Castro Neto, A. H.; Özyilmaz, B. Colossal Ultraviolet Photoresponsivity of Few-Layer Black Phosphorus. ACS Nano 2015, 9, 8070-8077. (18) Buscema, M.; Groenendijk, D. J.; Blanter, S. I.; Steele, G. A.; van der Zant, H. S. J.; Castellanos-Gomez, A. Fast and Broadband Photoresponse of Few-Layer Black Phosphorus Field-Effect Transistors. Nano Lett. 2014, 14, 3347-3352. (19) Yuan, H.; Liu, X.; Afshinmanesh, F.; Li, W.; Xu, G.; Sun, J.; Lian, B.; Curto, A. G.; Ye, G.; Hikita, Y.; Shen, Z.; Zhang, S-C.; Chen, X.; Brongersma, M.; Hwang, H. Y.; Cui, Y. PolarizationSensitive Broadband Photodetector Using a Black Phosphorus Vertical p–n Junction. Nat. Nanotechnol. 2015, 10, 707-713. (20) Qiao, J.; Kong, X.; Hu1, Z-X.; Yang, F.; Ji, W. High-Mobility Transport Anisotropy and Linear Dichroism in Few-Layer Black Phosphorus. Nat. Comm. 2014, 5, 4475. (21) Liu, H.; Neal, A. T.; Zhu, Z.; Luo, Z.; Xu, X.; Tománek, D.; Ye, P. D. Phosphorene: An Unexplored 2D Semiconductor with a High Hole Mobility. ACS Nano 2014, 8, 4033-4041. (22) Tian, H.; Guo, Q.; Xie, Y.; Zhao, H.; Li, C.; Cha, J. J.; Xia, F.; Wang, H. Anisotropic Black Phosphorus Synaptic Device for Neuromorphic Applications. Adv. Mater. 2016, 2, 4991-4997. (23) Castellanos-Gomez, A.; Vicarelli, L.; Prada, E.; Island, J. O.; Narasimha-Acharya, K. L.; Blanter, S. I.; Groenendijk, D. J.; Buscema, M.; Steele, G. A.; Alvarez, J. V.; Zandbergen, H. W.; Palacios, J. J.; van der Zant, H. S. J. Isolation and Characterization of Few-Layer Black Phosphorus. 2D Mater. 2014, 1, 025001. (24) Island, J. O.; Steele, G. A.; van der Zant, H. S. J.; Castellanos-Gomez, A. Environmental Instability of Few-Layer Black Phosphorus. 2D Mater. 2015, 2, 011002.


22

Page 23 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(25) Favron, A.; Gaufrès, E.; Fossard, F.; Phaneuf-L’Heureux, A-L.; Tang, N. Y-W.; Lévesque, P. L.; Loiseau, A.; Leonelli, R.; Francoeur, S.; Martel, R. Photooxidation and Quantum Confinement Effects in Exfoliated Black Phosphorus. Nat. Mater. 2015, 14, 826-832. (26) Wood, J. D.; Wells, S. A.; Jariwala, D.; Chen, K-S.; Cho, E. K.; Sangwan, V. K.; Liu, X.; Lauhon, L. J.; Marks, T. J.; Hersam, M. C. Effective Passivation of Exfoliated Black Phosphorus Transistors Against Ambient Degradation. Nano Lett. 2014, 14, 6964-6970. (27) Doganov, R. A.; O’Farrell, E. C. T.; Koenig, S. P.; Yeo, Y.; Ziletti, A.; Carvalho, A.; Campbell, D. K.; Coker, D. F.; Watanabe, K.; Taniguchi, T.; Castro Neto, A. H.; Özyilmaz, B. Transport Properties of Pristine Few-Layer Black Phosphorus by Van der Waals Passivation in an Inert Atmosphere. Nat. Comm. 2015, 6, 6647. (28) Edmonds, M. T.; Tadich, A.; Carvalho, A.; Ziletti, A.; O’Donnell, K. M.; Koenig, S. P.; Coker, D. F.; Özyilmaz, B.; Castro Neto, A. H.; Fuhrer, M. S. Creating a Stable Oxide at the Surface of Black Phosphorus. ACS Appl. Mater. Inter. 2015, 7, 14557-14562. (29) Wan, B.; Yang, B.; Wang, Y.; Zhang, J.; Zeng, Z.; Liu, Z.; Wang, W. Enhanced Stability of Black Phosphorus Field-Effect Transistors with SiO2 Passivation. Nanotechnology 2015, 26, 435702. (30) Artel, V.; Guo, Q.; Cohen, H.; Gasper, R.; Ramasubramaniam, A.; Xia, F.; Naveh, D. Protective Molecular Passivation of Black Phosphorous. npj 2D Mater. Appl. 2017, 1, 6. (31) Kim, J. S.; Liu, Y.; Zhu, W.; Kim, S.; Wu, D.; Tao, L.; Dodabalapur, A.; Lai, K.; Akinwande, D. Toward Air-Stable Multilayer Phosphorene Thin-Films and Transistors. Sci. Rep. 2015, 5, 8989.


23


Page 24 of 27

(32) Kwon, H.; Seo, S. W.; Kim, T. G.; Lee, E. S.; Lanh, P. T.; Yang, S.; Ryu, S.; Kim, J. W. Ultrathin and Flat Layer Black Phosphorus Fabricated by Reactive Oxygen and Water Rinse. ACS Nano 2016, 10, 8723-8731. (33) Pei, J.; Gai, X.; Yang, J.; Wang, X.; Yu, Z.; Choi, D-Y.; Luther-Davies, B.; Lu, Y. Producing Air-Stable Monolayers of Phosphorene and Their Defect Engineering. Nat. Comm. 2016, 7, 10450. (34) Ryder, C. R.; Wood, J. D.; Wells, S. A.; Yang, Y.; Jariwala, D.; Marks, T. J.; Schatz, G. C.; Hersam, M. C. Covalent Functionalization and Passivation of Exfoliated Black Phosphorus via Aryl Diazonium Chemistry. Nat. Chem. 2016, 8, 597-602. (35) Luo, W.; Zemlyanov, D. Y.; Milligan, C. A.; Du, Y.; Yang, L.; Wu1, Y.; Ye, P. D. Surface Chemistry of Black Phosphorus under a Controlled Oxidative Environment. Nanotechnology 2016, 27, 434002. (36) Li, L.; Engel, M.; Farmer, D. B.; Han, S-J.; Wong, H. S. P. High Performance p-Type Black Phosphorus Transistor with Scandium Contact. ACS Nano 2016, 10, 4672−4677. (37) Osenbach, J. W. Corrosion-Induced Degradation of Microelectronic Devices. Semicond. Sci. Technol. 1996, 11, 155-162. (38) Kuo, C. C.; Chen, I. C.; Shih, C. C.; Chang, K. C.; Huang, C. H.; Chen, P. H.; Chang, T-C.; Tsai, T. M.; Chang, J. S.; Huang, J. C. Galvanic Effect of Au-Ag Electrodes for Conductive Bridging Resistive Switching Memory. IEEE Electron Device Lett. 2015, 36, 1321-1324. (39) Barin, I. Thermochemical Data of Pure Substances; VCH: Weinheim, 1989. (40) Liao, Z. L.; Wang, Z. Z.; Meng, Y.; Liu, Z. Y.; Gao, P.; Gang, J. L.; Zhao, H. W.; Liang, X. J.; Bai, X. D.; Chen, D. M. Categorization of Resistive Switching of Metal-Pr0.7Ca0.3MnO3-Metal Devices. Appl. Phys. Lett. 2009, 94, 253503.


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Page 25 of 27 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(41) Chen, F.; Xia, J.; Ferry, D. K.; Tao, N. Dielectric Screening Enhanced Performance in Graphene FET. Nano Lett. 2009, 9, 2571-2574. (42) Das, S.; Chen, H-Y,; Penumatcha, A. V.; Appenzeller, J. N. High Performance Multilayer MoS2 Transistors with Scandium Contacts. Nano Lett. 2012, 13, 100-105. (43) Park, W.; Baik, J.; Kim , T-Y,; Cho, K.; Hong, W-K.; Shin, H-J.; Lee, T. Photoelectron Spectroscopic Imaging and Device Applications of Large-Area Patternable Single-Layer MoS2 Synthesized by Chemical Vapor Deposition. ACS Nano 2014, 8, 4961-4968. (44) Kang, J.; Wood, J. D.; Wells, S. A.; Lee, J-H.; Liu, X.; Chen, K-S.; Hersam, M. C. Solvent Exfoliation of Electronic-Grade, Two-Dimensional Black Phosphorus. ACS Nano 2015, 9, 35963604. (45) Ziletti, A.; Carvalho, A.; Campbell, D. K.; Coker, D. F.; Castro Neto, A. H. Oxygen Defects in Phosphorene. Phys. Rev. Lett. 2015, 114, 046801. (46) Ziletti, A.; Carvalho, A.; Trevisanutto, P. E.; Campbell, D. K.; Coker, D. F.; Castro Neto, A. H. Phosphorene Oxides: Bandgap Engineering of Phosphorene by Oxidation. Phys. Rev. B 2015, 91, 085407. (47) Xiang, D.; Han, C.; Wu, J.; Zhong, S.; Liu, Y.; Lin, J.; Zhang, X-A.; Hu, W. P.; Özyilmaz, B.; Castro Neto, A. H.; Wee, A. T. S.; Chen, W. Surface Transfer Doping Induced Effective Modulation on Ambipolar Characteristics of Few-Layer Black Phosphorus. Nat. Comm. 2015, 6, 6485. (48) Na, J.; Lee, Y. T.; Lim, J. A.; Hwang, D. K.; Kim, G-T.; Choi, W. K.; Song, Y-W. FewLayer Black Phosphorus Field-Effect Transistors with Reduced Current Fluctuation. ACS Nano 2014, 8, 11753-11762.


25


Page 26 of 27

(49) Liu, X.; Ang, K-W.; Yu, W.; He, J.; Feng, X.; Liu, Q.; Jiang, H.; Tang, D.; Wen, J.; Lu, Y.; Liu, W.; Cao, P.; Han, S.; Wu, J.; Liu, W.; Wang, X.; Zhu, D.; He, Z. Black Phosphorus Based Field Effect Transistors with Simultaneously Achieved Near Ideal Subthreshold Swing and High Hole Mobility at Room Temperature. Sci. Rep. 2016, 6, 24920. (50) Liu, H.; Neal, A. T.; Si, M.; Du, Y.; Ye, P. D. The Effect of Dielectric Capping on Few-Layer Phosphorene Transistors: Tuning the Schottky Barrier Heights. IEEE Electron Device Lett. 2014, 35, 795-797. (51) Ra, H-S.; Lee, A-Y.; Kwak, D-H.; Jeong, M-H.; Lee, J-S. Dual-Gate Black Phosphorus Field-Effect Transistors with Hexagonal Boron Nitride as Dielectric and Passivation Layers. ACS Appl. Mater. Inter. 2018, 10, 925-932. (52) Zhang, E.; Wang, W.; Zhang, C.; Jin, Y.; Zhu, G.; Sun, Q.; Zhang, D. W.; Zhou, P.; Xiu, F. Tunable Charge-Trap Memory Based on Few-Layer MoS2. ACS Nano 2015, 9, 612-619. (53) Feng, Q.; Yan, F.; Luo, W.; Wang, K. Charge Trap Memory Based on Few-Layer Black Phosphorus. Nanoscale 2016, 8, 2686-2692. (54) Tian, H.; Deng, B.; Chin, M. L.; Yan, X.; Jiang, H.; Han, S-J.; Sun, V.; Xia, Q.; Dubey, M.; Xia, F.; Wang, H. A Dynamically Reconfigurable Ambipolar Black Phosphorus Memory Device. ACS Nano 2016, 10, 10428-10435.


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SYNOPSIS TOC.


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Enhanced performance of field effect transistors based on black

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