Ultraefficient Ultraviolet and Visible Light Sensing and Ohmic Contacts

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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 5740−5749

Ultraefficient Ultraviolet and Visible Light Sensing and Ohmic Contacts in High-Mobility InSe Nanoflake Photodetectors Fabricated by the Focused Ion Beam Technique Hung-Wei Yang,† Ho-Feng Hsieh,‡ Ruei-San Chen,*,† Ching-Hwa Ho,† Kuei-Yi Lee,‡ and Liang-Chiun Chao‡ †

Graduate Institute of Applied Science and Technology and ‡Graduate Institute of Electro-Optical Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan S Supporting Information *

ABSTRACT: A photodetector using a two-dimensional (2D) low-direct band gap indium selenide (InSe) nanostructure fabricated by the focused ion beam (FIB) technique has been investigated. The FIB-fabricated InSe photodetectors with a low contact resistance exhibit record high responsivity and detectivity to the ultraviolet and visible lights. The optimal responsivity and detectivity up to 1.8 × 107 A W−1 and 1.1 × 1015 Jones, respectively, are much higher than those of the other 2D materialbased photoconductors and phototransistors. Moreover, the inherent photoconductivity (PC) quantified by the value of normalized gain has also been discussed and compared. By excluding the contribution of artificial parameters, the InSe nanoflakes exhibit an ultrahigh normalized gain of 3.2 cm2 V−1, which is several orders of magnitude higher than those of MoS2, GaS, and other layer material nanostructures. A high electron mobility at room temperature reaching 450 cm2 V−1 s−1 has been confirmed to be one of the major causes of the inherent superior PC in the InSe nanoflakes. The oxygen-sensitized PC mechanism that enhances carrier lifetime and carrier collection efficiency has also been proposed. This work demonstrates the devices fabricated by the FIB technique using InSe nanostructures for highly efficient broad-band optical sensing and light harvesting, which is critical for development of the 2D material-based ultrathin flexible optoelectronics. KEYWORDS: indium selenide, nanostructure, photodetector, Ohmic contact, focused ion beam, responsivity, detectivity, normalized gain

1. INTRODUCTION Indium selenide (InSe), a group III−VI compound semiconductor, has polytype stacked layer structures including twohexagonal (2H) (ε and β) and three-rhombohedral (3R) (γ) phases.1−3 Each single layer possesses honeycomb-like lattices consisting of four close-packed monoatomic sheets in the sequence of Se−In−In−Se. The adjacent quadruple layers are weakly bonded by van der Waals force when that within the layer is predominantly covalent. Transition-metal dichalcogenides (TMDs) have a similar layer structure and a big family, such as MoS2, MoSe2, WS2, WSe2, ReS2, NbS2, and TaS2.4,5 Recently, the isolated monolayer of TMD semiconductors has been found to be capable of transforming their indirect band gap to the direct band gap and enhancing quantum efficiency due to the size confinement effect.1,2,6,7 Motivated by the development of TMDs, group III−VI3 and group II−VI8 layer materials provide researchers a different direction to explore © 2018 American Chemical Society

new two-dimensional (2D) nanomaterials. In addition, different from TMDs, InSe has a low intrinsic direct band gap at ∼1.26 eV even at its bulk form, making it a superior candidate for the optoelectronic applications.9 In addition, MoS2 and partial TMDs suffer significant mobility drop in their monolayer and multilayer structures.10−12 InSe bulks have a high room-temperature (RT) mobility at 580 cm2 V−1 s−113 because of the low effective mass of electron (m*e = 0.156m0).14 Recent studies show that the few-layer InSe retains its high mobility without degradation and its RT mobility can even exceed 1000 cm2 V−1 s−1.15,16 Furthermore, the few-layer and multilayer InSe structures still keep the direct-band gap characteristic, which is different from Received: October 13, 2017 Accepted: January 15, 2018 Published: January 30, 2018 5740

DOI: 10.1021/acsami.7b15106 ACS Appl. Mater. Interfaces 2018, 10, 5740−5749

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Photograph of the InSe bulk crystals grown by the CVT approach. (b) FESEM image of InSe flakes on the dicing tape after preliminary mechanical exfoliation. (c) XRD pattern and (d) Raman spectrum of the CVT-synthesized InSe bulk crystals. (e) Schematic plots of the top-view (single-layer) and side-view (two-layer) 2H ε-phase InSe lattice.

its monolayer structure transforming into indirect band gap.17 On the basis of these advantages, InSe nanostructures with a thickness beyond the quantum confinement scale have been demonstrated to be the building blocks for the next-generation high-speed flexible electronic devices such as field-effect transistors (FETs)15,18,19 and photodetectors.20−24 High-performance photodetectors based on different layer materials such as GaS25 and GaSe nanosheets,26 GaTe multilayers,27 In2S3 nanoflakes,28 In2Se3 nanosheets,29 MoS2 monolayers,30 surface-modified MoS2 and WSe2,31 graphene nanoribbons,32 and their related heterostructures33−35 have been reported. When most studies focus on the device performance such as responsivity, gain, and detectivity, the inherent photoconduction property of these 2D materials has been less investigated and compared. Moreover, contact resistance at the metal−layer semiconductor interface could lower down the carrier collection efficiency and degrade the device performance in photodetectors, which has become a critical issue. Even when using an Ohmic metal to avoid Schottky contact formation, the presence of surface contaminants and native oxides on the semiconductor and uncontrollable interface states could still contribute to the contact resistance in the devices.16,19 In this report, a two-terminal conductor-type photodetector based on InSe nanoflakes with definably low contact resistance has been fabricated by the focused ion beam (FIB) approach. The FIB-fabricated InSe detector exhibits a record high responsivity (1.8 × 107 A W−1) and detectivity (1.1 × 1015 Jones) among the 2D layered materials. The inherent photocarrier collection efficiency defined by the normalized gain in the InSe nanostructures has also been investigated and compared. The high-mobility and long-lifetime characteristics of the InSe nanoflakes have been confirmed to be the origins of the ultrahigh normalized gain and superior photodetector performance.

2. RESULTS AND DISCUSSION Figure 1a depicts a photograph of the pristine InSe bulk crystals with a length of a few millimeters grown by the chemical vapor transport (CVT) method36 used for this study. A field-emission scanning electron microscopy (FESEM) image of the InSe flakes after the preliminary mechanical exfoliation using a dicing tape is shown in Figure 1b. The stripped flakes on the dicing tape have irregular shapes, and their lengths (5−20 μm) are much smaller than those of the millimeter-sized bulk crystals. The X-ray diffraction (XRD) measurement of the InSe layer crystals, Figure 1c, shows five diffraction peaks centered at 10.6°, 21.3°, 32.2°, 43.5°, and 67.5°, which are indexed as the (002), (004), (006), (008), and (0012) diffraction planes, respectively (JCPDS #34-1431). The sole ⟨001⟩ out-of-plane orientation indicates the hexagonal (2H) structure and singlecrystalline quality along the c-axis of InSe. Figure 1d depicts the Raman spectrum for the InSe crystal. Two major peaks obtained at 176 and 225 cm−1 are consistent with the E″ and A′1 modes for the 2H phase InSe, respectively.37,38 Two minor features observed at 198 and 237 ± 3 cm−1 are attributed to the A2″(LO) and second-order A1′ modes, respectively. By the curve fitting, the full width at half-maximum values of the two major Raman peaks are 10.2 (E″) and 8.2 (A′1) cm−1. The narrow line width shows the excellent single-crystalline quality of the CVTgrown InSe. Figure 1e illustrates the schematic plots of topview and side-view 2H ε-phase InSe lattice. Figure 2a depicts the measured current versus applied voltage (I−V) curves for the two-terminal devices of InSe nanoflakes with different thicknesses. The symmetric and linear I−V curves indicate that the FIB-fabricated nanoflake devices all share a good electric contact condition. The chip template with patterned Ti/Au multiple electrodes used for the InSe nanoflake photodetector fabrication by FIB is also shown in the inset of Figure 2a.39 The length (l) and width (w) values of the nanoflakes were obtained using top-view FESEM images. The thickness (t) values of the nanoflakes on the chip 5741


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lower bound of Rsc of a good Ohmic contact. Compared with the previous reports on the 2D materials, the optimal Rsc at near 1000 Ω μm of the FIB-fabricated InSe devices is also better than those of most metal contacts on MoS2, such as Ti on monolayer, few-layer, and multilayer MoS2 (Rsc = 800−740 000 Ω μm),42−44 Ni/Au on few-layer MoS2 (Rsc = 18 000 Ω μm),45 and Mo on few-layer MoS2 (Rsc ≈ 2000 Ω μm).46 Ohmic contact fabrication using FIB has been investigated for MoSe2 multilayers.47 Our study quantitatively provides the evidence of low contact resistance of the Pt/InSe interface for the first time. In addition, electrode fabrication using the FIB approach is relatively simple because pretreatment of the semiconductor surface and post annealing are not necessary. The metal deposition area is selective, and an Ohmic contact is formed simultaneously with metal deposition through FIB processing. A conducting amorphous alloy layer has been confirmed to be embedded in between the Pt and layer semiconductor interface.47 The conducting alloy layer at the interface and ion bombardment on the semiconductor surface can prevent the formation of the Schottky barrier and eliminate the surface contaminants and native oxides on the semiconductors. Minimizing the potential origins of contact resistance is probably the reason why the FIB processing can produce a good Ohmic contact. Photocurrent responses under the illumination of UV (λ = 325 nm), green (λ = 532 nm), and red (λ = 633 nm) lights at a bias of 1 V and at different laser powers for an InSe nanoflake (t = 38 nm) are shown in Figure 3. A constant background (dark) current (∼22 μA) has been subtracted from the response curves to reveal the photocurrent values. Photocurrent is defined as the increased current induced by light illumination. The results show clear photoresponses to the different excitation powers, and the photocurrent increases with an increase in power for the three different wavelengths. The photocurrent (ip) versus light intensity (I) curves for the different wavelengths are summarized in Figure 4a. The result shows that the overall photocurrent at 325 nm excitation is higher than those at 532 and 633 nm. The ip−I curves follow a nonlinear relationship with intensity in common. The responsivity (R) is a measure of photocurrent generation efficiency of a photodetector and is defined as the photocurrent generated by the optical power incident on an effective area of a photoconductor (P). Responsivity is written as

Figure 2. (a) Two-terminal I−V measurements for InSe nanoflakes with different thicknesses. The inset depicts a chip template with patterned Ti/Au multiple electrodes used for the InSe nanoflake photodetector fabrication by FIB. (b) Height profile and its image (the inset) of the AFM measurements for the InSe nanoflake with a thickness of 42 nm. (c) TLM plot for the FIB-fabricated InSe flake devices.

templates were defined using atomic force microscopy (AFM) measurements. Figure 2b depicts a typical AFM image and its height profile for an InSe nanoflake with a thickness of 42 nm. To examine the contact resistance, the transmission line method (TLM) model was adopted to analyze the devices of InSe nanoflakes with different thicknesses. The TLM model is based on a simple concept that the total resistance (R) is equal to the sum of two contact resistances (Rc) and sample resistance (Rs) for a two-terminal device, that is, R = 2Rc + Rs = 2Rc + ρl/wt, where ρ is the resistivity of the measured sample.40 According to the equation, R is linearly dependent on the dimension-related variable of l/wt. Rc and ρ values can be obtained by fitting the intercept and slope of the R versus l/wt curve, respectively. Figure 2c shows the TLM plot for four InSe nanoflake samples. The fitting result shows the Rc and ρ values at 1100 Ω and 0.15 Ω cm, respectively. The total contact resistance (2Rc) is pretty low and is only 1.5−8.2% of the total resistance (R = 13 400−73 000 Ω). The analysis confirms the excellent metal−semiconductor contact of the FIB-fabricated InSe nanostructure devices. The specific contact resistance (Rsc) defined for the 2D material system is given by Rsc = Rc × w.41 The w values of our flake devices are 0.9−4.0 μm. The corresponding Rsc values are 990−4400 Ω μm. Usually, the Rsc value for a reasonably good Ohmic contact is approximately in the range of 5000−30 000 Ω μm.16 These values of the InSe devices are even lower than the

R=

ip P

(1)

where P = IA = Iwl, where A is the projected area of the photoconductor (A = wl), w is the width, and l is the length of the conducting channel.48 Figure 4b depicts the responsivity as a function of light intensity for the InSe nanoflake detector. The responsivity exhibits a substantial decrease with an increase in the light intensity. The maximal responsivity can reach 1.8 × 107 A W−1 for UV light (325 nm) and 2.3 × 106/6.4 × 105 A W−1 for green/red light (532 nm/633 nm). Note that the FIB-fabricated InSe nanoflake device exhibits a record high responsivity compared to those of the few-layer InSe photoconductors (R ≈ 0.035 A W−1),21 few-layer InSe phototransistors (R = 12.3 A W−1),20 InSe nanosheet phototransistors (R ≈ 104 A W−1),22 graphene-contacted InSe nanoflake planar/vertical photoconductors (R = 4 × 103/105 A W−1),24 In2S3 nanoflake photoconductors (R ≈ 1.4 × 102 A W−1),28 and In2Se3 nanosheet photoconductors (R ≈ 4 × 102 A W−1)29 in the previous reports. The responsivity up to 5742


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2 × 107 A W−1 is also several orders of magnitude higher than the optimal values of graphene nanoribbons (R = 1 A W−1),32 GaS (R = 4.2−19.2 A W−1)25 and GaSe (R ≈ 2.8 A W−1)26 nanosheets, GaTe multilayers (R ≈ 104 A W−1),27 ReSe2 nanoflakes (R ≈ 3.0 A W−1),49 MoS2 monolayers (R = 880 A W−1),30 surface-modified MoS2 and WSe2 (R = 5750−14 500 A W−1),31 p-GaSe/n-GaSb heterostructures (R ≈ 0.12 A W−1),34 p-GaTeSe/n-Si heterostructures (R ≈ 0.21 A W−1),35 and pGaSe/n-InSe heterostructures (R = 350 A W−1)33 and is comparable with those of graphene/quantum dot (R = 0.12 A W−1)50 and MoS2/graphene (R = 1−5 × 108 A W−1)51 hybrid phototransistors. Responsivity depends on the photocurrent but is independent on the dark current. For practical applications, the dark current and the associated noise are the drawbacks of a photodetector and have to be considered. Detectivity takes both the photocurrent and the noise current into account, which can provide a more objective view on the overall photodetector performance. Considering that the dark current, id, is the major contribution to the total noise current and usually the reference bandwidth is taken as 1 Hz, the detectivity (D*) is given by48 D* =

R A 2eid

(2)

Figure 4c depicts the detectivity as a function of light intensity for the InSe flake detector. Overall, the detectivity follows similar intensity and wavelength dependences of the responsivity. The detectivity goes up to a maximum of 1.1 × 1015 Jones (cm Hz1/2 W−1) for a wavelength of 325 nm and at a low intensity of 0.08 W m−2. For 532 and 633 nm excitations, the maxima are 1.4 × 1014 and 4.2 × 1013 Jones, respectively. Compared with other photodetectors based on 2D materials, the optimal detectivity of the InSe nanoflake is also higher than the best reported values of the MoS2 monolayer phototransistors (D* = 2 × 1012 to 1.7 × 1014 Jones),30 InSe nanosheet phototransistors (D* = 1013 Jones),20 InSe nanoflake

Figure 3. (a) Photoresponse curves under different excitation intensities at wavelengths (λ) of (a) 325, (b) 532, and (c) 633 nm measured in air ambience for the InSe nanoflake with a thickness of 32 nm. A dark current (∼22 μA) has been subtracted from the photoresponse curves to reveal the photocurrent values.

Figure 4. (a) Photocurrent, (b) responsivity, and (c) detectivity as a function of light intensity at the wavelengths (λ) of 325, 532, and 633 nm for the InSe nanoflake (t = 32 nm). (d) Responsivity as a function of applied bias at λ = 325 nm and I = 400 W m−2 for the InSe nanoflake. 5743


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Table 1. Comparison of Responsivity (R), Detectivity (D*), and Normalized Gain (Γn) Values for the Different 2D MaterialBased Photodetectorsa material InSe nanoflake GaS nanosheet GaSe nanosheet GaTe multilayer InSe nanosheet InSe nanoflake (planar/vertical) In2S3 nanoflake In2Se3 nanosheet GaSe/InSe p−n heterostructure GaSe/GaSb p−n heterostructure GaTeSe/Si p−n heterostructure MoS2 monolayer surface-modified MoS2 & WSe2 graphene nanoribbon graphene/quantum dots MoS2/graphene hybrid a

λ (nm) 325 532 254 254 532 254 633 450 300 410 680 520 561 655 1550 600 635

V (V)

l (μm)

1

2.1

2 5 5 5 2 1 5 −2

20 20 8.57 18.3 2/

0 8 5 2 5 0.1

0.24

0.85 5 115 4.4 14.3

R (A W−1) 1.8 × 10 2.4 × 106 19.2 2.8 104 104 4000/105 137 400 350 0.12 0.21 880 5750 & 14 500 1 2 × 107 1 × 108 7

D* (Jones) 1.1 × 10 1.4 × 1014 1014 2.36 × 1011 15

3.2 0.25 1.9 × 10−4 1.1 × 10−5 3.42 × 10−3 3.3 × 10−2 1.6 × 10−4/

1013 1012/1015 4.74 × 1010 2.26 × 1012 3.7 × 1012 2.2 × 1012

1.9 × 10−7

1.73 × 1014 4.47 × 109 & 5.3 × 1010 7 × 1013

Γn (cm2V−1)

1.75 × 10−6 5.43 × 10−4 & 1.37 × 10−3 2.1 × 10−7 4

refs this work 25 26 27 22 24 28 29 33 34 35 30 31 32 50 51

λ is the excitation wavelength, V is the applied bias, and l is the channel length.

Normalized gain (Γn) defined as the product of η, τ, and μ was adopted to determine the inherent PC of the InSe nanostructures.54−56 Because the τμ product is an intrinsic quantity determining the excess carrier transport efficiency,57 for a constant η, Γn has the same physical meaning as τμ, and its intrinsic property can rule out the contributions of V and l. Γn taking η into account could also reflect the realistic efficiency for optical to electrical energy conversion of different materials. Accordingly, Γn can be the material figure of merit determining the inherent PC efficiency. Γn can be estimated by the equation54−56

planar/vertical photoconductors (D* = 1012/1015 Jones),24 In2Se3 nanosheet photoconductors (D* = 2.3 × 1012 Jones),29 GaTe nanosheet phototransistors (D* = 1012 Jones),27 p-GaSe/ n-GaSb heterostructures (D* = 2.2 × 1012 Jones),34 p-GaSe/nInSe heterostructures (D* = 3.7 × 1012 Jones),33 and MoS2 quantum dot/p-Si heterojunctions (D* = 8 × 1011 Jones).52 Table 1 summarizes the comparison of responsivity and detectivity values for the different 2D material-based photodetectors. The aforementioned maximal responsivity and detectivity are obtained at a channel bias of 1 V. The value can be further improved by simply increasing the applied bias. Figure 4d depicts the bias-dependent responsivity at an illumination of λ = 325 nm and I = 400 W m−2. The responsivity values have been normalized by the value at 1 V. The result shows that the responsivity increases linearly with an increase in the applied voltage. The maximal responsivity can be achieved at a bias of 8 V in this linear region, which is eight times higher than that operated at 1 V. In addition to the device performance, the inherent properties resulting in the superior photocurrent generation efficiency of the InSe nanoflakes have also been investigated. Photocurrent generation is determined by two processes including light absorption and carrier collection. On the basis of this concept, the photocurrent (ip) is linearly dependent on two physical parameters: quantum efficiency (η) and photoe conductive gain (Γ). ip can be written as i p = E Pη Γ , where E is the photon energy. Γ is physically defined as the ratio of carrier lifetime (τ) to transit time (τt) between two electrodes, that is, V Γ = τ/τt, and can also be written as Γ = 2 τμ.48,53 Accordingly, l Γ value can be manipulated by controlling the electrode interdistance, l, and applied voltage, V. A high gain in photodetectors can be achieved by decreasing l and increasing V, as illustrated in Figure 4d. However, to understand the inherent differences between the InSe nanosheet and other materials that result in the difference of photoconductivity (PC) efficiency, the artificial effects of V and l have to be excluded.

Γn = ητμ = η

Γ E ip l 2 = e PV (V / l 2 )

(3)

Figure 5 depicts Γn versus I for the InSe nanoflakes under the illumination at λ = 532 and 325 nm. Γn increases from 0.008 to

Figure 5. Normalized gain as a function of light intensity at the wavelengths (λ) of 325 and 532 nm for the InSe nanoflake (t = 32 nm). Normalized gain values of a MoS2 nanoflake (t = 45 nm) and a GaS nanoflake (t = 150 nm) fabricated by our FIB processing are also plotted for comparison.

3.2 cm2 V−1 for λ = 325 nm and increases from 0.0022 to 0.25 cm2 V−1 for λ = 325 nm when the intensity decreases from 320 to 0.08 W m−2. For comparison, the Γn values of a MoS2 nanoflake (t = 45 nm) and a GaS nanoflake (t = 150 nm) fabricated by our FIB processing are also plotted in Figure 5. For the same intensity of 40 W m−2, Γn of the InSe nanoflake is 5744


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substantially from 14 to 510 s and reaches a saturation level when the intensity decreases from 320 to 0.08 W m−2 for λ = 325 nm. An inverse power law can also been found for the τ−I relationship before the saturation, that is, τ ∝ I−κ, where κ = 0.48. However, the τ−I curve for λ = 532 does not exhibit a similar intensity dependence. The τ value is not sensitive to the visible light intensity and follows τ ∝ I−0.13. The different intensity-dependent behaviors of carrier lifetime could imply different PC mechanisms for the wavelengths of 325 and 532 nm. Because the UV light (325 nm) has a lower penetration depth in the semiconductor compared to the visible light (532 nm), the surface-controlled PC in the InSe nanoflakes will be discussed in the later section. Nevertheless, at low-intensity conditions, the maximal (saturation) τ values are at a similar level of 450 ± 50 s for λ = 325 and 532 nm. The maximal carrier lifetime is over 2 orders of magnitude longer than those of MoS2 monolayers (τ = 4 s)30 and few-layer InSe phototransistors (τ = 50 ms),20 few-layer ε/ γ-InSe photoconductors (τ = 488 μs),21 and β-InSe nanosheet photoconductors (τ = 5−8 ms).22 According to eq 3, the photocurrent or Γn is linearly dependent on the carrier lifetime because long lifetime means excess carriers with a low recombination rate in the transport channel, which enhances the carrier collection efficiency and thus the photocurrent or Γn. The comparison indicates that the high Γn in the InSe nanoflakes partially benefits from the carrier lifetime. In addition to the carrier lifetime, carrier mobility, μ, was also obtained by the back-gate FET measurement. Figure 7 depicts the drain-to-source current (Ids) versus gate voltage (Vg) curves for the InSe nanoflake FET. The Ids−Vg curve shows an nchannel transistor behavior for the InSe flake. The linear Ids versus Vds (drain-to-source voltage) curve at 0 gate voltage excludes the probable field effect induced by the Schottky

3 orders of magnitude higher than those of the MoS2 and GaS nanoflakes. Estimating the Γn values of the other highly efficient photodetectors based on 2D materials from the literature shows that the InSe nanoflake still exhibits optimal Γn compared to the graphene nanoribbon (Γn = 2.1 × 10−7 cm2 V−1),32 GaS nanosheets (Γn = 1.9 × 10−4 cm2 V−1),25 GaSe nanosheets (Γn = 1.1 × 10−5 cm2 V−1),26 MoS2 monolayers (Γn = 1.8 × 10−6 cm2 V−1),30 surface-modified MoS2 and WSe2 (Γn = 5.4 × 10−4/1.4 × 10−3 cm2 V−1),31 GaTe multilayers (Γn = 3.4 × 10−3 cm2 V−1),27 In2Se3 nanosheets (Γn = 1.9 × 10−7 cm2 V−1),29 InSe nanosheets (Γn = 0.033 cm2 V−1),22 and graphenecontacted InSe nanoflakes (Γn = 1.6 × 10−4 cm2 V−1),24 which is comparable with those of graphene/quantum dot hybrid system (Γn = 4 cm2 V−1).50 The comparison of Γn values is also summarized in Table 1. To understand the physical origin of the ultrahigh Γn in the InSe nanoflakes, carrier lifetime, τ, was investigated by the timeresolved PC measurement. Figure 6a,b depicts the normalized

Figure 6. Normalized photocurrent rise curves under different excitation intensities at the wavelengths (λ) of (a) 325 and (b) 532 nm measured in air ambience for the InSe nanoflake. (c) Carrier lifetime vs light intensity curves at the wavelengths of 325 and 532 nm for the InSe nanoflake.

photoresponse curves at different intensities for the excitation wavelengths of 325 and 532 nm, respectively. Because the response time can be regarded as the carrier lifetime (the detailed theoretical background can be found in the Supporting Information), by curve fitting using the stretched exponential growth function ip(t) = ip0[1 − exp(−t/τ)γ], where ip0 is the saturation photocurrent and γ is a fitting parameter, the τ value can be obtained. Figure 6c shows that the τ value increases

Figure 7. (a) Schematic of an InSe nanoflake FET device with a bottom-gate configuration. (b) Ids−Vg curves for the InSe nanoflake operated at Vds = 5 V. The dimensions of the nanoflake are l = 10 μm, w = 11 μm, and t = 100 nm. The inset shows the linear Ids−Vds curve. 5745


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ACS Applied Materials & Interfaces contact at the source and drain electrodes. The mobility value can be calculated according to the equation58

μ=

dIds l dVds wC iVds

(4)

where l = 10 μm and w = 11 μm are, respectively, the length and width of the conducting channel and Ci = 1.15 × 10−8 F cm−2 is the capacitance between the channel and the back gate per unit area and can be calculated by Ci = ε0εr/d, where ε0 = 8.85 × 10−14 F cm−1 is the permittivity of free space, ε0 = 3.9 is the dielectric constant of SiO2, and d = 300 nm is the thickness of bottom-gate oxide. Vds is fixed at 5 V for the FET operation. The estimation indicates the InSe nanoflake with a high mobility reaching 450 cm2 V−1 s−1. The value is close to the optimal reported values of the few-layer InSe (μ = 1000−1250 cm2 V−1 s−1)15,16 and the InSe bulks (μ = 580 cm2 V−1 s−1).13 Compared with other 2D material-based photodetectors, the value is much higher than those of GaS nanosheets (μ = 0.1 cm2 V−1 s−1),25 GaTe multilayers (μ = 0.2 cm2 V−1 s−1),27 and MoS2 monolayers (μ = 4 cm2 V−1 s−1).30 The mobility of our InSe flakes is also over 1 order of magnitude higher than that of the InSe nanosheet (μ ≈ 33 cm2 V−1 s−1) reported by Feng et al.22 The higher mobility in this study could be correlated with the better Ohmic contact quality of the source and drain electrodes fabricated by the FIB method. The presence of the interface states in the metal−InSe contact has been proposed to be the cause of contact resistance. The FIB-fabricated Pt electrodes with a conducting alloy layer embedded in between the Pt metal and InSe can prevent the formation of interface states and minimize the influence of surface contaminants on semiconductors.37,47 According to the aforementioned discussion, the ultrahigh normalized gain in InSe nanoflakes is mainly contributed by both the high mobility and the long lifetime of carriers. The high mobility has been understood as a result of the low effective mass of electron (m*e = 0.156m0) in the intrinsic ntype InSe.14 However, the long excess carrier lifetime is inferred to be dominated by the surface-controlled PC mechanism. Figure 8a illustrates the photoresponses measured in air and vacuum ambiences under 325 and 532 nm excitations at the same intensity of 400 W m−2 for the InSe flake with a thickness of 180 nm. The result shows that the photocurrent and response time both are enhanced in vacuum compared to those in air (see Figure S1 in the Supporting Information for the fitting results of the response time). A similar result can also be found for the device with t = 38 nm (Figure S2 in the Supporting Information). The phenomenon is consistent with the oxygen-sensitized photoconduction (OSPC) mechanism.59,60 According to the OSPC model, in thermal equilibrium, an oxygen molecule acts as an electron trap when adsorbs on the material surface [O2(g) + e− → O2−(ad)]. Negatively charged surface states originate from the adsorbed oxygen ions and partial native surface defects generate upward surface band bending (SBB) (step 1). When electron−hole pairs (EHPs) are created by photoexcitation (step 2), the excess electrons and holes are subsequently separated by the SBB (step 3). With the driving force of the surface built-in field, holes migrate to the surface and recombine with the oxygen ions and the neutralized oxygen molecules are released to the ambience [h+ + O2−(ad) → O2(g)] (step 4). Because excess holes are mostly consumed at the surface, unpaired electrons have a long lifetime (as

Figure 8. (a) Photocurrent responses to the excitation wavelengths of 532 and 325 nm measured in air and vacuum ambiences for the InSe nanoflake. (b) Dark current measured on repeated exposure to air and vacuum ambiences for a typical InSe nanoflake with a thickness of 150 nm. A high background current at 63.7 μA (applied bias = 1 V) has been subtracted to reveal the current response to different ambiences. (c) Schematic of the OSPC mechanism in InSe.

observed in Figure 6) and dominate the photocurrent. More devices have been examined to confirm that the long lifetime is a general phenomenon in our InSe flakes (Figure S3 in the Supporting Information). Under these conditions, only oxygen readsorption can capture electrons (step 5), and thus the excess electron lifetime is decided by the oxygen adsorption rate. A lower oxygen adsorption rate in vacuum ambience results in a longer carrier lifetime (response time) and higher photocurrent in comparison with the carrier lifetime and photocurrent in air ambience. The prediction of the OSPC model is consistent with the observation of the ambience-dependent photoresponse measurement for the InSe nanoflakes shown in Figure 8a. To further verify the model, the dark current measurement on repeated exposure to air and vacuum ambiences was also conducted. Figure 8b shows that the dark current level in vacuum is higher than that in air. The result confirms that the adsorption of oxygen/water on the InSe surface captures electrons and thus decreases the dark current in air ambience. The OSPC mechanism is also schematically drawn in Figure 8c. Theoretically, photocurrent (or normalized gain) is linearly proportional to the product of quantum efficiency, carrier lifetime, and mobility, that is, ip ∝ ητμ (can be derived from eq 3). However, in our case, the 325 nm excitation condition has a higher photocurrent (Figure 3) and a higher normalized gain 5746


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required to optimize the 2D InSe devices fabricated by the FIB approach.

(Figure 5) but a lower carrier lifetime (Figure 6) compared to those at 532 nm. To explain this inconsistency, we probably need to consider the surface-dominant nature of the OSPC mechanism in InSe. It is expected that the effective area that produces long-lifetime excess carriers is only limited in the surface depletion region (SDR) (i.e., the region with SBB). The width of the SDR is roughly a few tens of nanometers for a general case. The 325 nm wavelength has a shorter penetration depth than 532 nm. This means that the 325 nm excitation can generate more EHPs with spatial separation in the SDR as photons are mostly absorbed at the near-surface region. Because only the spatially separated holes can involve in the interaction with the charged surface states as the description in the OSPC model, more “effective holes” generated by 325 nm illumination indicate a higher “effective quantum efficiency”. On the other hand, 532 nm has a longer penetration depth and generates partial EHPs in the neutral region (bulk region) and less EHPs in the SDR (surface region). The lifetime of the carriers in the neutral region (due to the direct recombination) is several orders of magnitude shorter than the lifetime of the carriers in the SDR (due to the spatial separation). We can reasonably assume that the carriers in the neutral region only contribute very low photocurrent. Accordingly, though 532 nm excitation has a longer carrier lifetime, the higher effective quantum efficiency at 325 nm condition could compensate its shorter carrier lifetime and produce a higher photocurrent. Actually, more optical absorption in the SDR under 325 nm illumination is also expected to have a stronger band flattening due to the photovoltaic effect. Higher SBB results in a longer electron lifetime because the unpaired electrons need to overcome the potential barrier to reach the surface states to recombine (or be captured) if the oxygen capture rate is higher than the thermionic emission rate of electrons in air ambience. Therefore, the lower SBB at higher light intensity conditions could result in a shorter carrier lifetime. This statement explains why the carrier lifetime exhibits more sensitive intensity dependence under 325 nm illumination (τ ∝ I−0.48) compared to that under 532 nm condition (τ ∝ I−0.13) (Figure 6c). Because the quasi-2D materials have a high surface-tovolume ratio, the influence of foreign molecules is obvious as expected.61 Oxygen and water are well-known oxidizing agents in air ambience and intend to capture electrons to become negatively charged states when adsorption occurs. Any solid surface provides free electrons, such as n-type semiconductors, can easily interact with oxygen and water. This explains why the ambience-dependent photoresponse induced by the OSPC mechanism was frequently observed in the different unintentionally doped n-type semiconductors such as Ge, CdS, CdSe, InN, GaN, MoS2, NbSe2, and InSe in addition to the metal oxides. A long carrier lifetime enhances the carrier collection and photocurrent, but the photoresponse speed to reach the steady state could be slow because of the OSPC mechanism in our InSe devices. Recently, graphene has been confirmed to be a good Ohmic contact material on InSe23,24 and the related heterostructures.33 Though their responsivity is not as high as our FIB-fabricated devices, the graphene-contacted photodetectors can reach a much faster response speed for the InSe flakes (≤100 μs)23,24 and p-GaSe/n-InSe heterostructures (∼2 μs).33 To realize the high-photocurrent and high-frequency operation for photodetection, further research studies are still

3. CONCLUSIONS A photodetector based on InSe 2D materials fabricated by the FIB method has been demonstrated. Both the excellent Ohmic contact of devices and the extremely high inherent photoconduction efficiency of the InSe nanostructures have been confirmed to be the causes of the record high responsivity and detectivity for UV and visible light sensing. The inherent property of the material that determines the photocarrier collection efficiency has also been defined and compared. The InSe nanoflakes with high electron mobility and long lifetime demonstrate the potential to be the building blocks for highly efficient broad-band photodetection, light harvesting, and flexible optoelectronic devices. 4. EXPERIMENTAL SECTION The high-quality InSe layer crystals used for this study were synthesized by the CVT method using ICl3 as the transport agent.36 The InSe nanoflakes were produced by the mechanical exfoliation method. InSe bulk crystals were stripped using a dicing tape to make millimeter-sized layer crystals into micrometer-long and nanometerthick small flakes. An insulating SiO2(300 nm)/n-Si chip with a prepatterned Ti/Au electrode layout was used as the template for the nanoflake device fabrication (see Figure 2a, inset). The purpose of the Ti/Au electrodes is to function as an interconnection between the millimeter-sized bonded wire and the microelectrode fabricated using FIB. The InSe flakes were dispersed randomly on the template by turning the dicing tape (with InSe flakes on the sticky top surface) upside down on the chip template and then lightly taping the tape on the back side.39 Two platinum, Pt, contact electrodes were subsequently deposited on the selected InSe nanoflakes using the FIB technique. The voltage and current values of the ion beam for the Pt precursor decomposition were 30 kV and 100 pA, respectively. The InSe crystals were characterized using FESEM, XRD, and Raman spectroscopy. The thicknesses of the InSe nanoflakes were defined using AFM. Electrical characterization was performed at an ultralowcurrent leakage cryogenic probe station (LakeShore Cryotronics TTP4). A semiconductor characterization system (Keithley 4200SCS) was used as the current and voltage source measure units for the two-terminal current versus voltage and three-terminal FET measurements. A He−Cd gas laser (325 nm), a Nd:YAG laser (532 nm), and a diode laser (633 nm) were used as the excitation sources for the PC measurements. A UV holographic diffuser was used to broaden the laser beam size (∼25 mm2) for minimizing errors in light intensity calculations. The incident laser power was measured by a calibrated power meter (Ophir Nova II) with a silicon photodiode head (Ophir PD300-UV).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b15106. Photoresponse curves measured in air and vacuum at the wavelengths of 532 and 325 nm for different InSe devices and theoretical background of regarding the photoresponse time as the carrier lifetime in InSe (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Ruei-San Chen: 0000-0003-0042-2521 Ching-Hwa Ho: 0000-0002-7195-208X 5747


Research Article

ACS Applied Materials & Interfaces Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS R.-S.C. thanks the support of the Ministry of Science and Technology (MOST) of Taiwan under the projects MOST 105-2112-M-011-001-MY3 and MOST 104-2923-M-011-001MY3.

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Ultraefficient Ultraviolet and Visible Light Sensing and Ohmic Contacts

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