High-performance black phosphorus field-effect transistors with long

3 days ago - Two-dimensional layered materials (2DLMs) are of considerable interest for high-performance electronic devices for their unique electroni...
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High-performance black phosphorus fieldeffect transistors with long-term air-stability Daowei He, Yiliu Wang, Yu Huang, Yi Shi, Xinran Wang, and Xiangfeng Duan Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.8b03940 • Publication Date (Web): 04 Dec 2018 Downloaded from http://pubs.acs.org on December 4, 2018

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Nano Letters

High-performance black phosphorus field-effect transistors with long-term air-stability Daowei He,﹠,﹟ Yiliu Wang,﹠ Yu Huang,§ Yi Shi,﹟ Xinran Wang,*,﹟ Xiangfeng Duan*,﹠ ﹠Department

of Chemistry and Biochemistry, University of California, Los Angeles, California

90095, USA ﹟National

Laboratory of Solid State Microstructures，Collaborative Innovation Center of

Advanced Microstructures, School of Electronic Science and Engineering, Nanjing University, Nanjing 210093, China §Department

of Materials Science and Engineering, University of California, Los Angeles,

California 90095, USA * Correspondence to X. D. ([email protected]), X. W. ([email protected]).

TOC graphic Abstract Two-dimensional layered materials (2DLMs) are of considerable interest for high-performance electronic devices for their unique electronic properties and atomically thin geometry. However, 1

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the atomically thin geometry makes their electronic properties highly susceptible to the environment changes. In particular, some 2DLMs (e.g., black phosphorus (BP) and SnSe2) are unstable and could rapidly degrade over time when exposed to ambient conditions. Therefore, the development of proper passivation schemes that can preserve the intrinsic properties and enhance their lifetime represents a key challenge for these atomically thin electronic materials. Herein we introduce a simple, non-disruptive and scalable van der Waals passivation approach by using organic thin films to simultaneously improve the performance and air stability of BP field-effect transistors (FETs). We show that dioctylbenzothienobenzothiophene (C8-BTBT) thin films can be readily deposited on BP via van der Waals epitaxy approach to protect BP against oxidation in ambient conditions over 20 days. Importantly, the non-covalent van der Waals interface between C8-BTBT and BP effectively preserves the intrinsic properties of BP, allowing us to demonstrate high-performance BP FETs with a record-high current density of 920 µA/um, hole drift velocity over 1 ⅹ 107 cm/s, and on/off ratio of 104~107 at room temperature. This approach is generally applicable to other unstable two-dimensional (2D) materials, defining a unique pathway to modulate their electronic properties and realize high-performance devices through hybrid heterojunctions. KEYWORDS: two-dimensional materials, black phosphorus, passivation, field effect transistors, saturation current density, saturation velocity Two-dimensional layered materials (2DLMs) are of considerable interest for high-performance electronic devices for their unique electronic properties and atomically thin geometry. In particular, black phosphorus (BP), with high carrier mobility and a tunable band gap, bridges the gap between zero band-gap graphene and low carrier mobility transition metal dichalcogenides (e.g., MoS2), represents an attractive 2D semiconductor for field-effect transistors (FETs) with high on-off ratio and high-speed operation.1 However, the atomically thin geometry makes 2DLMs highly susceptible to the environment changes. In particular, black phosphorus (BP) are unstable and could rapidly degrade over time when exposed to ambient 2

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conditions. Therefore, it is of great significance to develop proper passivation schemes that can preserve the intrinsic properties and sustain the long-term environment stability.1-3 The lone pairs of phosphorus atoms are attractive to oxygen chemisorption, leading to the formation of oxygenated defects on BP surface and enhancing its surface hydrophilicity, ultimately resulting in the formation of phosphoric acid. This transformation process (also called BP degradation or oxidization) can also be significantly accelerated under light conditions.4,5 So far, several strategies have been explored for BP surface passivation,6-8 including encapsulation by hexagonal boron nitride (hBN), polymethylmethacrylate (PMMA) or oxide layer.5 These passivated layers have led to considerable improvement on BP stability from tens of hours to several days in term of electrical feature. For example, hBN encapsulated few-layer BP transistors have been demonstrated with the hole mobility of 5200 cm2/Vs and the drain current density as high as 430 µA/µm at room temperature,9,10 comparable to that of silicon devices. Recently, several works reported a great improvement on electron doped BP FETs that the electron mobility over 1000 cm2/Vs by hBN encapsulation.11,12 However, the encapsulation with exfoliated h-BN flake is time consuming and intrinsically unscalable. Other encapsulation approaches can often lead to partial damage of the underlying BP flakes, leading to transistors with lower current density (< 100 µA/µm) and lower mobility (< 300 cm2/Vs),6,13,14 which is far the intrinsic potential of the BP devices.10,15 Overall, the reported passivation methods to date are either not scalable, or covalent in nature, which could introduce additional surface scattering source to degrade device performance. Here, we report a unique scalable passivation approach for BP by using van der Waals epitaxial grown organic C8-BTBT thin films as the passivation layer and investigate the electrical performance and long-term air stability the C8-BTBT-passivated BP FETs. Compared with the previous appraoches,6-8 our vdW epitaxial organic thin film encapsulated BP features the non-covalent passivation, scalability and additional doping to ensure excellent electronic properties and long-term air stability. Furthermore, we use a transfer contact approach to ensure 3

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highly clean contact interface16,17 with a low specific contact resistance of 0.75 Ω·mm. Together, we demonstrate high-performance BP FETs with a record-high current density of 920 µA/um, hole drift velocity over 1

ⅹ 107 cm/s, and on/off ratio of 104~107 at room temperature. This

low-temperature vdW organic thin film passivation approach can also be applicable to other unstable 2D materials, thus defining a general pathway to modulate their electronic properties and realize high-performance devices through hybrid heterojunctions. Results and discussion We exfoliated few-layer BP on 300nm SiO2/Si substrate. For bare BP without passivation, many noticeable bumps appeared on the BP surface after one-day exposure in ambient conditions (Figure 1a, b), consistent with the BP oxidization as reported in previous studies.5,18,19 After then, the bumps continued to grow until BP flakes were completely oxidized (Figure S1). In order to protect against BP oxidization, we deposited C8-BTBT thin film on BP surface (as shown in Figure 1c and Figure S2) using a low temperature vapor transport deposition approach (see Methods). The C8-BTBT covered BP apparently shows considerably improved stability when exposed in ambient conditions. Figure 1d-h show the sequential AFM snapshots of C8-BTBT encapsulated BP during 31-day characterization. The atomically surface smoothness was maintained over 20 days (Figure 1d-f, l). Until 27 days, some small bumps appeared which continually enlarge over time, suggesting the beginning BP degradation due to oxidization (Figure 1g, h).

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Figure 1. Stability characterization of the C8-BTBT encapsulated BP. (a, b) AFM images of unencapsulated BP at 0.25 hour and 20 hours, respectively. The thickness of the BP flake is about 9nm. (c) Microscopic image of C8-BTBT thin film encapsulated BP. (d-h) Sequential AFM snapshots of C8-BTBT thin film encapsulated BP shown in Figure 1c at 0, 10, 20, 27, 31 days, respectively. The dashed line in c and d represents the outline of BP. (i) Raman spectrum of the C8-BTBT encapsulated BP at different time. Three Raman modes of BP are marked. The normalized peak intensity (j) and band shift (k) of Ag2 as a function of time. (l) The evolution of surface roughness of the C8-BTBT encapsulated BP. The roughness is extracted from the white dashed square in Figure 1d at different time. We further carried out the in-situ Raman characterization on the passivated BP to monitor the degradation process (Figure 1i-k). The intensity and peak position of three Raman modes of BP remained essentially unchanged for 20 days, indicating that the BP was well protected with little degradation. However, in the next 10 days the peak intensity of A2g was reduced by 50% and the A2g band shifted by 1.2 cm-1 (Figure 1i-k). Though the excitation laser polarization direction can significant effect on the peak intensity of BP Raman modes due to its highly anisotropic optical responses, we have kept a constant orientation of the BP flake vs. the laser polarization angle to ensure the change A2g peak intensity is indeed originated from degradation 5

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induced change rather than polarization induced intensity variation. So, these changes suggest the initiation of the oxidation of BP and continued oxidation afterwards.5,18,19 In contrast, a non-encapsulated BP samples show a fast noticeable change in Raman spectrum (Figure S3), where the A2g peak intensity decreased by 60% and A2g band shifted by 1.1 cm-1 within two days of exposure in ambient conditions (Figure 1j, k), consistent with the AFM characterization. Together, the AFM and Raman spectroscopy studies clearly demonstrate that the vdW grown organic thin film can effectively isolate BP from oxygen and water, and significantly slows down the degradation process. The oxidation of BP after ~20 days was presumably attributed to the existence of cracks in C8-BTBT thin film (Figure S2).

Figure 2. Electrical characteristics of BP FET before and after encapsulation by C8-BTBT thin film. The output characteristics of BP FET before (a) and after (b) encapsulation by C8-BTBT thin film under the same coordinate scale. The channel length and width are 2.6μm and 0.9μm, respectively. (c) Transfer characteristics of BP FET. Inset shows the band alignment between BP and C8-BTBT. (d) The change of current density under the condition of BP FET encapsulated and unencapsulated at Vg=-60V and Vds=4V. Unencapsulated and encapsulated are abbreviated as U and E. It was previously shown that PMMA and AlOx encapsulation could also passivate BP. However, these approaches could introduce additional surface defects, resulting in relatively low device performance (on-current, mobility or current on/off ratio) far from the highest 6

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reported value.6,13,14 To this end, the encapsulation by vdW grown C8-BTBT could be advantageous by retaining the dangling bond free surface. To further prove that the passivation by C8-BTBT was non-destructive, we built backgated BP FETs by non-destructive transfer electrode process and studied the effect of passivation by C8-BTBT (see Figure S4 and methods for the detailed fabrication process).16,17 Compared with bare BP FETs, we repeatedly observed much higher output current density after C8-BTBT deposition (Figure 2a, b), accompanied by positive shift of the threshold voltage (Figure 2c). This indicated that the C8-BTBT molecule had p-type doping effect on BP, consistent with the type-I band alignment where the holes were transferred from the highest occupied molecular orbital (HOMO) of C8-BTBT to valance band of BP (Figure 2c, inset). Furthermore, the apparent field-effect mobility increased by 60% after encapsulation, due to the suppression of the oxidation and the decrease of surface scattering, and partly reduced contact resistance due to C8-BTBT doping effect on BP. When removing the C8-BTBT film by thermal annealing, the doping effect disappeared and the output current density of BP FETs decreased down to the value of before passivation (Figure 2d). Interestingly, the increased doping and current could be repeated many times by C8-BTBT deposition and annealing (Figure 2d). To rule out the current conduction by organic thin film, we have also built a control device by directly depositing C8-BTBT thin film between source/drain electrodes. The current density was four orders of magnitude lower than BP FETs (Figure S5). The improved current and mobility were very encouraging for constructing high-performance BP FETs to explore their performance limit. To this end, we passivated BP FETs by thick C8-BTBT (60nm) immediately after transferring the source/drain electrodes. Figures 3a and b present the room-temperature transfer (Ids-Vg) and output (Ids-Vds) characteristics of a representative C8-BTBT encapsulated BP FET. The inset in Figure 3a shows the scanning electron microscope image of the device. The device showed an on-state current of 590µA/µm and on/off ratio of 1 ⅹ 105 under Vds=2V. In addition, the high current could be sustained over 8 days without much degradation (the red open circle in Figure 3c) with 7

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little change in the transfer curves (Figure S6). The long-term stability test of another device shows similar behavior (the black open circle in Figure 3c). This is in stark contrast to the non-encapsulated BP FETs, in which the current rapidly decayed more than four orders of magnitude within one day (the blue open circle in Figure 3c). In Table 1, we compared the BP device performance and stability using different passivation schemes, including on/off ratio, mobility, current density and stability. These comparisons clearly show our approach features several apparent advantages including device performance, long term stability, and potential scalability.6,9,13,20,21

Figure 3. Electrical stability measurement of C8-BTBT encapsulated BP FETs. (a, b) A representative room temperature transfer characteristics (Ids-Vg) and output characteristics (Ids-Vds) of the C8-BTBT encapsulated BP FET. Inset in Figure 3a shows scanning electron microscope (SEM) image of the C8-BTBT encapsulated BP FET, the dashed line represents the outline of BP. (c) The saturation current density of the C8-BTBT encapsulated BP FETs as a function of time at Vds=2.5V and Vg=-60V (red and black open circle). The red open circle was extracted from Figure 3b in the saturation regime, but this device was burnt down in the further measurement. The blue open circle was from the unencapsulated BP FET at Vds=2V and Vg=-40V. (d) Histogram of the room temperature two-terminal extrinsic mobility of C8-BTBT encapsulated BP FETs.

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Table 1. Comparison of the BP device electrical performance and stability using different passivation schemes. Reference number

On/off ratio

FET mobility (cm2/Vs)

Current density (μA/μm)

Stability (days)

Sample thickness (nm)

Passivation method

This work

2.2E5

592

590

8

< 8 nm

vdW encapsulation

1.7E5

600

480

12

N.A.

N.A.

N.A.

31 (Raman)

Ref. 6

~1E3

~50

~ 60

14