Photothermal Effect Induced Negative Photoconductivity and High

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Photothermal Effect Induced Negative Photoconductivity and High Responsivity in Flexible Black Phosphorus Transistors Jinshui Miao, Bo Song, Qing Li, Le Cai, Suoming Zhang, Weida Hu, Lixin Dong, and Chuan Wang ACS Nano, Just Accepted Manuscript • Publication Date (Web): 11 May 2017 Downloaded from http://pubs.acs.org on May 14, 2017

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Photothermal Effect Induced Negative Photoconductivity and High Responsivity in Flexible Black Phosphorus Transistors Jinshui Miao,†,§ Bo Song,†,§ Qing Li,‡ Le Cai,† Suoming Zhang,† Weida Hu,‡ Lixin Dong,† Chuan Wang†,* †

Electrical and Computer Engineering, Michigan State University, East Lansing,

Michigan 48824, United States ‡

National Lab for Infrared Physics, Shanghai Institute of Technical Physics, Chinese

Academy of Sciences, 500 Yutian Road, Shanghai 200083, China *

Corresponding author: [email protected]

(§equal contribution)

Abstract – This paper reports negative photoconductivity mechanism in flexible black phosphorus (BP) transistors built on freestanding polyimide film. Near-infrared laser (λ = 830 nm) excitation leads to significantly suppressed device on-state current with a very high responsivity of up to 53 A/W. The underlying mechanism of the negative photoconductivity is attributed to the strong photothermal effect induced by the low thermal conductivity of polyimide substrate used. The heat generated by the infrared light illumination results in enhanced phonon scattering, reduced carrier mobility, and consequently negative photocurrent. Such phenomenon was not observed in similar BP devices built on SiO2/Si substrate whose thermal conductivity is much higher. The above photothermal mechanism is also supported by temperature-dependent electrical characterization and device simulation. Such flexible BP infrared photodetector with ultrahigh responsivity may find potential applications in future wearable and bio-integrated imaging systems.

Keywords:

black

phosphorus,

infrared

photodetectors,

flexible,

negative

photoconductivity, photothermal effect 1 ACS Paragon Plus Environment

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Figure for Table of Contents

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Infrared photodetectors or imaging systems have a wide range of industrial applications such as optical imaging,1, 2 biomedical sensing,3-5 space exploration,6 and environmental monitoring.7 Over the past few decades, development of high-performance photodetection materials and chip-scale fabrication technologies has led to significant advancement of infrared imaging systems.8,

9

Despite the progress, more superior

optoelectronic components with higher speed and efficiency, broader detection wavelength range, or new form factor such as being mechanically flexible are becoming more urgently needed.10 Since the discovery of graphene,11 two-dimensional (2D) materials have attracted great research interest for optoelectronic applications, driven by their excellent electrical, optical, and mechanical properties.12 A variety of prototype optoelectronic devices based on graphene have been successfully demonstrated, such as light-emitting diodes (LED),13 ultrafast lasers,14,

15

optical modulators,16,

17

and solar

cells.18, 19 However, graphene has severe limitations in photodetection applications due to the absence of a bandgap.20,

21

Although tremendous effort has been devoted to the

development of graphene photodetectors, the high dark current inevitably leads to large shot noise.22,

23

Other 2D semiconducting materials such as transition metal

dichalcogenides (TMDs) have relatively large bandgap and strong light absorption.24 However, these materials exhibit relatively slow response and they do not work at the important telecommunication wavelengths.24,

25

The recently rediscovered 2D black

phosphorus (BP),26, 27 which has a direct bandgap ranging from ~ 0.3 eV (bulk) to ~ 2.0 eV (monolayer),26, 28 could be a very promising 2D material for visible to mid-infrared detector applications. Few-layer BP based field-effect transistors (FETs) have shown high room-temperature mobility up to 1000 cm2/V·s,26, 29 current modulation up to 105,30, 31 and good current saturation.32,

33

Compared with traditional narrow-bandgap infrared

detection materials (InAs, InGaAs, and HgGeTe), BP-based devices can be easily implemented on flexible substrates owing to its layered crystal structure and small thickness.34-36 More importantly, low-noise photodetection can be achieved using BP due 3 ACS Paragon Plus Environment

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to its moderate direct bandgap. Nevertheless, recent studies on few-layer BP photodetectors only report modest responsivity of ~ 4-150 mA/W in both visible light and infrared regime.37-42 Although one group reported much higher responsivity of ~ 82 A/W, the device responsivity decreases drastically from ~ 82 A/W to ~ 0.2 A/W with increasing light intensity.43 In this paper, we demonstrate high-performance flexible near-infrared photodetectors using few-layer BP with ultrahigh responsivity of ~ 53 A/W at a laser intensity of 16.5 W/cm2. The device was built on an ultrathin polyimide substrate, whose low thermal conductivity (~ 0.2 Wm-1K-1) 44 leads to strong photothermal effect and substrate heating upon light illumination. The photo-generated heat leads to enhanced phonon scattering and mobility degradation, which generate significant negative photocurrent of ~ -20 to -30 µA at a light intensity of 16.5 W/cm2. Temperature-dependent study of the device electrical characteristics and device simulation using Sentaurus TCAD further confirm the heating is the underlying mechanism governing the negative photoconductivity in flexible BP FETs. The photothermal effect-based flexible BP photodetector may find interesting

potential

applications

in

high-sensitivity

broadband

wearable

and

bio-integrated photodetectors or imaging systems.

RESULTS AND DISCUSSION Figure 1a illustrates the fabrication process flow for a flexible BP FET on the polyimide substrate. Specifically, liquid polyimide was spun coated on a SiO2/Si handling wafer followed by annealing at 300 °C to form an ultrathin (~ 10 µm) flexible polyimide film. The circular-shaped device bottom gate (Ti/Au 1 nm/40 nm) with a diameter of ~ 150 µm was thermally evaporated onto the polyimide film through a shadow mask. Then, a 15-nm-thick Al2O3 gate dielectric layer was deposited on top of the bottom gate using atomic layer deposition (ALD). Next, few-layer BP flakes (~ 10 nm thick) were mechanically exfoliated from bulk BP crystals onto the Al2O3 capped bottom gate 4 ACS Paragon Plus Environment

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electrodes. A 50-nm-thick Au source/drain (S/D) electrodes were subsequently patterned onto the BP flake by e-beam lithography (EBL), metal evaporation, and lift-off processes. Another 10-nm-thick Al2O3 dielectric was grown by ALD to fully encapsulate the BP FET. Once the device fabrication was completed, the polyimide film was delaminated from the SiO2/Si handling wafer to obtain the flexible BP FETs on polyimide film. Figure 1b shows the optical microscope and atomic force microscopy (AFM) images of a representative BP FET fabricated on polyimide substrate. The Raman spectra collected from BP flakes on polyimide (red) and SiO2/Si (blue) substrates are presented in Figure 1c. Three distinct peaks can be observed at ~ 360, ~ 436, and ~ 463 cm-1, corresponding to the Ag1, B2g, and Ag2 phonon modes of few-layer BP flake.29, 45 The Raman spectra baseline for BP flake on polyimide film was slightly tilted compared with the one on SiO2/Si substrate. Moreover, we found it was very easy to get burned for the BP flakes on polyimide film during the Raman measurement (Supporting Information Figure S1), which is likely due to the significantly lower thermal conductivity of polyimide (~ 0.2 Wm-1K-1) compared with silicon (~ 150 Wm-1K-1). 48 The heat generated by the laser (λ=532 nm and intensity ≈ 258 W/cm2) of the Raman system was difficult to be quickly dissipated through the polyimide film, causing the BP flakes to be burned. So, we reduced the laser intensity during Raman measurement. Figure 1d shows the photograph of flexible BP FETs attached onto a transparent PET film being bent. The PET film is used as a handling substrate for the flexible BP device characterization and bending test. To fully elucidate the effect of the substrate, we compare the photoresponse of a BP device fabricated on the polyimide film before and after being delaminated from its SiO2/Si handling wafer. A near-infrared laser diode with a wavelength of λ = 830 nm was used. Figure 2a shows the transfer characteristics (IDS - VGS) of a BP FET before being delaminated from SiO2/Si handing wafer measured in the dark (blue) and under illumination (red). Under illumination, the device exhibits photoresponse with slight current decrease in the on-state (VGS = -6 V) and large current increase in its off-state 5 ACS Paragon Plus Environment

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(VGS = -3 V). After the polyimide substrate was delaminated from the SiO2/Si handling wafer, the same BP FET shows much more predominant current decrease in its on-state (VGS = -6 V) as shown in Figure 2b. The photoresponse in the output characteristics (IDS VDS) of the same device measured at VGS = -5 V before and after polyimide delamination are shown in Figure 2c and d, respectively. Above experiment results further demonstrate that the flexible BP FET on freestanding polyimide shows much larger conductance decrease in its on-state (current decreases from 38.7 to 32.0 µA/µm under a laser intensity of 14.9 W/cm2 before peel off and 50.5 to 21.2 µA/µm after peel off) under laser illumination. Figure 2e and f present the photoswitching characteristics of a flexible BP FET on freestanding polyimide substrate under different gate biases. The device shows current increase under illumination when biased in the off-state (VGS = -3 V), as shown in Figure 2e. On the other hand, when biased in the on-state (VGS = -5 V), the device current decreases under illumination, as illustrated in Figure 2f. As a comparison, we also studied the photoresponse in BP FETs fabricated directly on the SiO2/Si substrate using p+Si and 300-nm-thick SiO2 as the gate electrode and gate dielectric (Supporting Information Figure S2). The device transfer characteristics (Figure S2a) exhibited a monotonic current increase under laser illumination with gate voltage varying from -45 V to +45 V. No current decrease was observed at any gate bias even with laser intensity up to 18.2 W/cm2. The calculated responsivity is ~ 1.3 A/W at a laser intensity of 4.69 W/cm2, which is comparable with reported works.38, 43 Therefore, we can conclude that the substrate plays an important role in the photoresponse of BP FETs. A possible explanation of negative photoconductivity in flexible BP FETs built on polyimide film, the photo-generated heat cannot be effectively dissipated through the substrate due to the low thermal conductivity of polyimide. In such scenario, the photothermal induced bolometric effect becomes dominated, which leads to enhanced phonon scattering, reduced carrier mobility, and consequently decrease in current. In contrast, for BP devices fabricated directly on the SiO2/Si substrate whose thermal 6 ACS Paragon Plus Environment

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conductivity is much larger, the photothermal effect becomes much smaller, resulting in no observable current decrease under illumination. To further investigate the photoresponse in the flexible BP FET on freestanding polyimide film, we examined the light intensity dependent I-V characteristics. Figure 3a shows the transfer characteristics of a delaminated flexible BP FET under illumination with increasing laser intensities from 0 to 16.5 W/cm2. The device current increases with increasing laser intensity when it is in the off-state (VGS = -1 V). On the other hand, the incident laser results in significant current decrease when device is in the on-state (VGS = -4 V). The output characteristics measured at VGS = -4 V (Figure 3b) further demonstrate the decrease of on-state current under illumination. Figure 3c plots the device conductance (G = I/V) as a function of laser intensity indicating a drastic decrease with increasing laser intensity. The negative photoconductivity in flexible BP FETs is attributed to photothermal effect - a rise in device temperature upon light irradiation and it has already been demonstrated by published work, in which the photoinduced bolometric effect dominates graphene or BP photoresponse in the n-type and p-type doping regimes.49,50 According to semiconductor physics,46 temperature rise would generate more free carriers in semiconductors due to thermal excitation, while also suppress the carrier mobility due to enhanced phonon scattering. These two opposite factors in combination determine the device conductivity. When the flexible BP FETs are biased in the on-state, the carrier concentration is very high and the device conductivity will be dominated by the factor of the reduced carrier mobility at elevated temperatures. In such a scenario, the device current decreases under illumination due to the photoinduced bolometric effect which is consistent with reported graphene and BP works.49,50 Unlike the thick BP flake (~ 100 nm) used in the reported BP phototransistor paper,50 the thickness of the BP flake used in our study is only around 10 nm. The device channel can be fully depleted when it is biased in the off-state, resulting in very low carrier concentration. As a result, the increase in carrier concentration at elevated 7 ACS Paragon Plus Environment

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temperatures induced by photothermal effect would have a more predominant contribution to the conductivity than the reduction in carrier mobility. The temperature-dependent study presented below in Figure 4 further demonstrates that the thermal excitation of free carriers is the main mechanism behind the positive photoresponse when the device is biased in its off-state regime. Furthermore, since the near-infrared laser spot (~ 104 µm2) used in our work is significantly larger than the BP device (~ 10 µm2). There is almost no temperature gradient dT/dx across the BP channel. Therefore, the thermoelectric effect is likely negligible in our flexible BP device, and with an applied drain bias, the bolometric current can further overwhelm the thermoelectric contribution. Figure 3d

and

e present the photocurrent

and

photoconductive gain of flexible BP FET with gate voltage of VGS = -4 (on-state) and -1 V (off-state), respectively. The device shows distinct negative photocurrent and photoconductive gain of ~ -17 µA/µm (Figure 3d and Figure S3) and ~ 8,000% in its on-state (VGS = -4 V). However, in its off-state (VGS = -1 V), the device exhibits very small positive photocurrent and photoconductive gain of ~ 0.13 µA/µm (Figure 3e and Figure S3) and ~ 57%. The responsivity of a photodetector can be defined as R = Iph/(P·A), where Iph is the net photocurrent, P is the incident laser power, and A is the effective area of the detector. Figure 3f shows the comparison of responsivity when the device is biased at VGS = -4 and -1 V, respectively. The device on-state responsivity (red cycle) induced by photothermal effect is orders of magnitude higher than the off-state one (blue cycle) induced by photo-generated carriers. More importantly, the responsivity is also significantly higher than other reported work on BP photodetectors.37-42 Flexible BP FETs on freestanding polyimide film exhibit significant conductance decrease under near-infrared laser illumination. We attributed such photoresponse with negative photocurrent to the photothermal effect induced mobility reduction. To further support that the photothermal induced bolometric effect is indeed the underlying mechanism, we study the transfer and output characteristics when the device is heated to 8 ACS Paragon Plus Environment

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various temperatures using a Peltier heater. The BP device used here for the temperature study is the exact same device used in Figure 3 for the photoresponse measurements. The temperature study in Figure 4 was carried out before the photoresponse measurements in Figure 3 so the device on polyimide was still attached on the SiO2/Si handling wafer (before delamination) and can be easily attached onto the Peltier heater during heating. As the substrate temperature changes from 293 K to 373 K with a step of 20 K, the device shows similar on-state current decrease (VGS = -4 V) with increasing substrate temperature, as shown in Figure 4a and b. Compared with Figure 3a and b, the variation in both transfer and output characteristics induced by temperature is very similar to the ones induced by laser illumination. We can extract the bolometric coefficient β = ∆σ/∆T, which describes the sensitivity of the electrical conductivity σ to changes in temperature T. The calculated bolometric coefficient is β ~ -2.53 µSK-1, which is larger than the reported BP and graphene photodetectors.49,50 We attribute the larger bolometric coefficient β in our flexible BP photodetectors to the low thermal conductivity of polyimide substrate used. Figure 4c shows the field-effect mobility in BP FET as a function of substrate temperature. The mobility (µ) is extracted from the transfer characteristics in Figure 4a in the VGS range from -2 to -4 V, using the expression µ = [dIDS/dVGS] × [L/(WCoxVDS)],47 where L and W are the device channel length and width and Cox is unit area capacitance of the Al2O3 gate dielectric. An increase in temperature will result in mobility degradation in BP FETs because the phonon scattering becomes the dominant scattering mechanism at high temperatures. In the phonon-limited high temperature regime, the mobility is expected to follow a µ ~ T-γ temperature dependency.46 We fit the curve in Figure 4c using the generic temperature dependency of the mobility µ ~ T-γ, where the exponent depends on the phonon scattering mechanism. From the fitted curve we find the value of γ to be ≈ 1.56. We also examine the temperature dependency of device conductance (G) using similar equation G ~ T-γ, as shown in Figure 4d. The fitted curve (red line, Figure 4d) gives a γ value of ≈ 3.3. As can 9 ACS Paragon Plus Environment

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be seen, the exponent (γ ≈ 3.3) for the temperature dependency of conductance is larger than the one for mobility. The main reason is that besides mobility, the device conductance is also dependent on carrier concentration, which is also temperature dependent, and other factors such as Au/BP contacts, charged impurities, and surface roughness. The relationship between the device substrate temperature and incident laser intensity can also be obtained if we compare Figure 4d with Figure 3c. The inset in Figure 4d shows the extracted device substrate temperature as a function of laser intensity, indicating an almost linear relationship. One can find that the temperature of the polyimide substrate increases from 310 K to 358 K as the laser intensity is increased from 11.9 W/cm2 to 16.5 W/cm2. The magnitude of relative change in conductance |∆G/G0| (where ∆G = G – G0, G is the instantaneous conductance of the device and G0 is the conductance of the device at room temperature or dark condition) plotted as a function of laser intensity is shown in Supporting Information Figure S4. The laser illumination and temperature exert almost the same effect on the device conductance. To further confirm that the substrate heating is the main mechanism governing the negative photoconductivity in flexible BP FETs, we also carried out detailed device simulation using Sentaurus TCAD. The BP flake used in the simulation has a thickness of 10 nm. Figure 5a shows the simulated transfer characteristics of the BP FET with substrate temperature varying from 293 K to 383 K. The simulated BP device shows an ambipolar behavior with room-temperature on-state current of ~ 158 µA at VGS = -4 V and VDS = 0.3 V. The device also shows on-off ratio of more than 102 at room temperature, which is comparable to the device from experiments. Increase in substrate temperature leads to decrease in on-state current, which is also consistent with the experimental results. In the simulation, the room-temperature mobility is set to be µ = 186.7 cm2/V·s which is obtained from the experiment (Figure 4a). The mobility is set to follow a µ ~ T-γ phonon-limited temperature dependency with γ = 2.5. In this scenario, the simulated 10 ACS Paragon Plus Environment

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relative conductance change ∆G/G0 fully supports our experimental results. Figure 5b presents the simulated conductance plotted as a function of substrate temperature. The room-temperature (293 K) conductance of ~ 528 µS (at VGS = -4 V and VDS = 0.3 V) decreases to ~ 259 µS at a substrate temperature of 383 K. We also fit the curve in Figure 5b using the temperature dependency of conductance G ~ T-γ. From the fitted curve, the value of γ is found to be ≈ 2.7, which is again consistent with experimental result (γ ≈ 3.3). From the temperature study and device simulation, we can safely conclude that the negative photoconductivity in flexible BP FETs on freestanding polyimide substrate is caused by the photothermal effect induced by the incident light. BP based electronic and optoelectronic devices also show superior mechanical flexibility due to its layered crystal structure and small thickness. Figure 6 shows the systematic bending tests conducted on flexible BP devices. The device transfer and output characteristics show negligible variations when bent down to a small curvature radius of 5.9 mm (Supporting Information Figure S5a and b) and remain essentially unchanged throughout the process of up to 1000 bending cycles (Supporting Information Figure S5c and d). The device electrical performance metrics, including carrier mobility and on-off ratio are plotted as functions of curvature radius and bending cycle are shown in Figure 6a and b. The flexible BP devices shows stable carrier mobility of ~ 150 cm2/V·s and on-off ratio of ~ 6 × 103 when the polyimide substrate was bent down to 5.9 mm curvature radius. Another flexible BP device also exhibits negligible variations in mobility (~ 130 cm2/V·s) and on-off ratio (~ 2 × 103) throughout 1000 bending cycles. The excellent stability of our flexible BP FET can be attributed to the mechanical robustness of ultrathin BP flake and ultrathin polyimide substrate used. Next, the device photoresponse during the bending tests was also studied. Because it is very difficult to conduct the near-infrared laser alignment, we only measured the device photoresponse before and after 1000 bending cycles with a curvature radius of ~ 6 mm. The dashed lines in Figure 6c and d are the device transfer and output curves measured in dark and under 11 ACS Paragon Plus Environment

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laser illumination after 1000 bending cycles. From the results, one can find that the device also exhibits very minor variations in its photoresponse. The slight difference in photoresponse before (solid line) and after (dashed line) 1000 bending cycles is most likely caused by the infrared laser alignment. Such flexible BP photodetectors with superior robustness under mechanical bending tests associated with superior responsivity can potentially be used as sensing components in future wearable and bio-integrated imaging systems.

CONCLUSION In summary, we have demonstrated flexible BP infrared photodetectors with high responsivity of ~ 53 A/W and gain of ~ 8,000% at room temperature. Such high responsivity is achieved due to the use of polyimide substrate with low thermal conductivity, which results in strong photothermal induced bolometric effect and substrate heating upon light illumination. The photo-generated heat leads to enhanced phonon scattering and mobility degradation, which generate significant negative photocurrent of ~ -20 to -30 µA under infrared laser illumination (16.5 W/cm2). To further elucidate the underlying mechanism governing the negative photoconductivity in flexible BP FETs, temperature study and detailed device simulation using Sentaurus TCAD were performed. The results confirm that the increase in substrate temperatures leads to almost the same device conductance decreases, which further demonstrate that the negative photoconductivity in flexible BP FETs is caused by heating effect. This works provides insights on understanding photothermal effect in photodetectors built on low thermal conductivity substrate and may also lead to high-sensitivity broadband wearable and bio-integrated photodetectors or imagers.

METHODS Device fabrication: Liquid polyimide (HD MicroSystems, PI-2525) was spun coated 12 ACS Paragon Plus Environment

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(2000 rpm for 1 min) on SiO2/Si handling wafer followed by annealing at 300 °C for 1 hour to form an ultrathin (~ 10 µm) flexible film. Circular-shaped metal electrodes (Ti/Au 1 nm/40 nm) with diameter of ~ 150 µm serving as the bottom gate were deposited onto the polyimide film using a thermal evaporator (Edward Auto 306) through shadow masks. The vacuum during evaporation was around 6 × 10-7 Torr. A 15-nm-thick Al2O3 dielectric layer was deposited onto the metal back gate using atomic layer deposition (precursor: trimethylaluminum (TMA) and water; deposition temperature = 150 °C). After that, the few-layer BP flakes were mechanically exfoliated from bulk crystals (Smart-element, Austria) and transferred on top of the metal backgate. Prior to device fabrication, the samples were dipped into warm acetone for 1 hour to remove the tape residues. To fabricated BP devices, bilayer PMMA e-beam resists (MicroChem 495 A6 and 950 C2) were spun coated (5000 rpm for 1 min) onto the substrates followed by baking on hot plate at 175 °C for 5 min. Electron-beam lithography was subsequently used to define the source/drain patterns, and a 50-nm-thick Au film was deposited using the same thermal evaporator followed by lift-off process to form the source/drain electrodes. Finally, the polyimide film was delaminated from the SiO2/Si handling wafer to get flexible BP transistors. Device characterization: The electrical characteristics of flexible BP transistors were measured using a Signatone probe station and a semiconductor parameter analyzer (Agilent B1500A). The device photoresponse was measured using near-infrared laser diode with a wavelength of 830 nm. The Peltier heater and Thorlabs temperature controller were used for heating up the BP device in the temperature dependent study. Device simulation: Fully self-consistent numerical simulation was carried out using Synopsys Sentaurus TCAD. The device structure design and finite element calculation were performed by using software packages of SDE and SDEVICE, respectively. The device structure in the simulation is based on experimentally fabricated BP device, where the BP flake is 10-nm-thick, channel length is 2 µm, channel width is 1 µm in default 13 ACS Paragon Plus Environment

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(2-dimensional simulation), and gate dielectric is 15-nm-thick with dielectric constant of 8.5. The bandgap and density-of-states were summarized in the intrinsic density equation: =

, where are the density-of-states, and Eg is the

bandgap which was set to be 0.4 eV at 300 K. Carrier transport models for the device , simulation were governed by continuity equation: ∇ ∙ , = , where J is the current

density, n and p are the electron and hole density.

Conflict of Interest: The authors declare no competing financial interests.

Acknowledgements: This research project was funded by Michigan State University. The flexible BP device fabrication was done in the Keck Microfabrication Facility (KMF) of Michigan State University. The authors want to thank Dr. Baokang Bi for helpful discussions regarding the fabrication process.

Supporting Information Available: Optical microscope images of BP flakes on SiO2/Si and polyimide substrates before and after Raman measurements (S1); Photoresponse of BP device fabricated on SiO2/Si substrate (S2); Gate bias-dependent photocurrent and responsivity in flexible BP photodetectors (S3); Relative change in device conductance (∆G/G0) induced by incident laser or substrate heating (S4); Bending test of flexible BP transistors (S5). This material is available free of charge via the Internet at http://pubs.acs.org.

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Figure 1

Figure 1. Flexible BP FET on polyimide substrate. (a) Schematic flow chart illustrating the fabrication process for a flexible BP FET on polyimide substrate. (b) Optical microscope and AFM images of a representative BP FET on polyimide film. The BP flakes used in the work have a thickness of ~ 10 nm. The scale bar is 5 and 1 µm for the optical microscope and AFM images, respectively. (c) Raman spectra taken from BP flakes on polyimide (red) and SiO2/Si (blue) substrates. (d) Photograph of flexible BP FETs attached on a transparent PET handling substrate in its bending state. The schematic in the right inset shows the exploded view of the flexible BP FET structure.


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Figure 2

Figure 2. Photoresponse of the BP FET built on polyimide substrate. (a, b) Transfer characteristics of a BP FET fabricated on polyimide film before (a) and after (b) being delaminated from the SiO2/Si handling wafer measured in dark and under laser illumination (λ = 830 nm). The inset shows the cross section of the device structure. (c, d) Output characteristics of the same device before (c) and after (d) being delaminated from the SiO2/Si handling wafer measured in dark and under illumination at a VG of -5 V. (e) Photoswitching behavior of the delaminated flexible BP FET (measured at VDS = 0.2 V and VGS = -3 V) showing positive photocurrent. (f) Photoswitching behavior of the same delaminated flexible BP FET measured at VDS = 0.2 V and VGS = -5 V showing negative photocurrent. 21 ACS Paragon Plus Environment

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Figure 3

Figure 3. Light intensity dependent photoresponse in BP FETs on freestanding polyimide substrate. (a, b) Transfer (a) and output (b) characteristics of a BP FET after being delaminated from SiO2/Si handling wafer measured in dark and under laser illumination with various intensities. (c) Device conductance extracted from panel a plotted as a function of laser intensity. (d, e) Device photocurrent and photoconductive gain plotted as a function of laser intensity when the gate bias is set to be VGS = -4 V (d) and VGS = -1 V (e). Note that for panel (d), the device photocurrent is actually negative and the absolute value was presented instead. (f) Comparison of device responsivity when it is biased at VGS = -4 V and -1 V.


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Figure 4

Figure 4. Temperature-dependent electrical characteristics of the flexible BP FETs. (a, b) Transfer (a) and output (b) characteristics of the same BP FET shown in Figure 3 before being delaminated from the SiO2/Si handling substrate measured at different substrate temperatures. (c) Device mobility at different substrate temperatures (293 K, 313 K, 333 K, 353 K, and 373 K). (d) Device conductance extracted from panel a plotted as a function of substrate temperature. The inset shows the deduced relationship between the incident laser intensity and device substrate temperature.


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Figure 5

Figure 5. Device simulation results using Sentaurus TCAD. (a) Simulated transfer characteristics of a BP FET plotted as a function of substrate temperatures ranging from 293 K to 383 K in 10 K step. (b) Device conductance extracted from the transfer curves at VGS = -4 V and VDS = 0.3 V plotted as a function of substrate temperature. The inset shows the temperature-dependent mobility, where the exponent depends on the dominant scattering mechanism.


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Figure 6

Figure 6. Bending test of flexible BP infrared photodetectors. (a, b) FET performance metrics including field-effect mobility and on/off ratio plotted as functions of curvature radius (a) and bending cycle (b). (c, d) Transfer (c) and output (d) characteristics of a flexible BP FET measured in dark and under laser illumination before and after 1000 bending cycles. The solid lines represent the pristine device characteristics and the dashed lines represent the device characteristics after 1000 bending cycles.


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