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Stable, Fast UV-VIS-NIR Photodetector with Excellent Responsivity, Detectivity and Sensitivity Based on #-In2Te3 Films with a Direct Bandgap Jiandong Yao, Zexiang Deng, Zhaoqiang Zheng, and Guowei Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b06222 • Publication Date (Web): 26 Jul 2016 Downloaded from http://pubs.acs.org on July 30, 2016

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ACS Applied Materials & Interfaces

Stable, Fast UV-VIS-NIR Photodetector with Excellent Responsivity, Detectivity and Sensitivity Based on α-In2Te3 Films with a Direct Bandgap

Jiandong Yao †, Zexiang Deng †, Zhaoqiang Zheng & Guowei Yang * State Key Laboratory of Optoelectronic Materials and Technologies, Nanotechnology Research Center, School of Materials Science & Engineering, School of Physics, Sun Yat-sen University, Guangzhou 510275, Guangdong, P. R. China. †These authors contribute equally to this work. * Corresponding author: [email protected]

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ABSTRACT Photoelectric conversion is of great importance to extensive applications. However, thus far, photodetectors integrated with high responsivity, excellent detectivity, large photo-to-dark current ratio, fast response speed, broad spectral range and good stability are rarely achieved. Herein, we deposited large-scale and high-quality polycrystalline indium sesquitelluride (α-In2Te3) films via pulsed-laser deposition. Then, we demonstrated that the photodetectors made of the prepared α-In2Te3 films possess stable photo-switching behavior from 370 to 1064 nm and short response time better than ca. 15 ms. At a source-drain voltage of 5 V, the device achieves a high responsivity of 44 A/W, along with an outstanding detectivity of 6 × 1012 cm H

1

2

W -1

and an excellent sensitivity of 2.5 × 105 cm2/W. All of these figures-of-merit are the best among those of the reported α-In2Te3 photodetectors. In fact, they are comparable to the state-of-the-art commercial Si and Ge photodetectors. For the first time, we established the theoretical evidence that α-In2Te3 possesses a direct bandgap structure, which reasonably accounts for the superior photodetection performances above. Importantly, the device exhibits a good stability against the multiple photo-switching operation and ambient environment, along with no obvious voltage-scan hysteresis. These excellent figures-of-merit, together with the broad spectral range and good stability, underscore α-In2Te3 as a promising candidate material for next-generation photodetection. KEYWORDS: indium sesquitelluride, pulsed-laser deposition, direct bandgap, broadband, photodetection.

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INTRODUCTION The conversion of optical signal into electrical signal is essential to a variety of technological applications spanning imaging, optical communication, optoelectronic memory, energy harvesting, light polarization analyzing, night vision surveillance, flame detection, missile plume early warning, light-enhanced gas sensing, etc.1-9 In recent years, aiming to improve the device performance, a large amount of effort has been devoted to the realm of photodetection. However, thus far, photodetectors integrated with high responsivity, excellent detectivity, large photo-to-dark current ratio, fast response speed, broad spectral range and good stability are rarely reported. Therefore, it is urgent to develop a competent candidate material for high-performance photodetection application. Indium sesquitelluride (α-In2Te3), a III-VI compound semiconductor, exhibits great potential as the solution to the above predicament. Generally, α-In2Te3 holds the following advantages in photodetection considerations. First, it possesses an appropriate bandgap of ca. 1~1.2 eV, making it suitable for broad spectral photodetection covering the common UV-VIS-NIR range.10 Meanwhile, the semiconducting bandgap can efficiently suppress the thermally excited carriers and a large photo-to-dark current ratio can thus be obtained. Considering that such a bandgap is close to that of Si (1.12 eV), α-In2Te3 holds great potential as an alternative candidate material for post-silicon electronics. Second, α-In2Te3 is a stable phase in the ambient environment, which establishes the foundation for its further wide spread commercial applications. Third, α-In2Te3 possesses a high absorption coefﬁcient beyond 105 cm-1.11 This value is several times

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higher than the traditional semiconductor Si,12 indicative of its strong light-matter interaction. The above merits make α-In2Te3 stand out compared to other emerging photodetection materials such as graphene, transition metal dichalcogenides (TMD), black phosphorus (BP), perovskites, etc.13-16 Therefore, it is appealing to exploit the α-In2Te3 as the optical active material for the high-performance photodetection applications. In fact, there have been a few reports on the fabrication of α-In2Te3 photodetectors with UV-VIS-NIR spectral range.17-20 However, most of the previous works only conducted a simple investigation, a systematic characterization on the photodetection properties of α-In2Te3 is lacking. In addition, these reported devices are fabricated from the irregularly distributed nanostructures, the representative morphology for the chemical synthesis approaches.21 Thus, they can hardly be further batch-processed as integrated devices on account of their limited sizes and low maneuverability. Moreover, the previously reported synthesis processes for α-In2Te3 are time-consuming (normally tens of hours),18, 20, 22-23 which further hinder it from wide spread applications. Meanwhile, the complicated chemical synthesis processes inevitably introduce various impurities such as precursors, surfactant and by-products.19-20, 24 As a result, the figures-of-merit of these α-In2Te3 photodetectors are quite poor (R ~ 0.05 to 1 A/W, D ~ 5 × 109 to 1 × 1011 cm Hz1/2 W -1 ), which far lags the state-of-the-art commercial Si and Ge photodetectors (R ~ 0.5 to 0.85 A/W, D ~ 3 × 1011 to 3 × 1012 cm Hz1/2 W -1 ).25 Pulsed-laser deposition (PLD) is a facile technology for the growth of high-quality thin films. In the past few years, it has been successfully exploited to

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produce various materials.26-31 As shown in Fig. 1(a), in a PLD growth process, laser pulses are focused on the surface of a target. They impinge on the target and transfer their energy to the target’s surface layer. As a result, ablation plumes whose temperature is at the order of several thousand Kelvin generate. They travel downstream and settle on the relatively low-temperature substrates. Compared to other traditional growth approaches, PLD holds unique advantages. First, it is a clean physical vapor deposition approach. It doesn’t need any precursors, and there are no complicated and uncontrollable chemical reactions during the growth process. In addition, the whole growth process is conducted in high vacuum environment. Therefore, few contaminations, notably catalyst, precursors, surfactants and by-products, will be introduced. Second, the focused pulsed laser can ionize almost all condensed matter materials, making it versatile for the growth of various materials. Most importantly, it can produce large-area thin films with the lateral size of centimeter scale, the planar geometry of which is the prerequisite for compact device-integration with standard top-down lithographic fabrication techniques.29, 31-32 Given the above advantages, PLD holds great potential to overcome the predicaments encountered by the previously reported chemical vapor deposition (CVD) and solvothermal approaches for the growth of α-In2Te3. In this contribution, using PLD, we deposit the centimeter-scale and high-quality α-In2Te3 thin films, and then we fabricate the α-In2Te3 photodetectors. Interestingly, the device exhibits stable photo-switching behavior from 370 to 1064 nm and short response time better than ca. 15 ms. At a source-drain voltage of 5 V, it achieves a

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high responsivity of 44 A/W along with an outstanding detectivity of

6 × 1012 cm H

1

2

W -1 and an excellent sensitivity of 2.5 × 105 cm2/W. All these

figures-of-merit are the best among those reported for the α-In2Te3 photodetectors. In fact, they are comparable to the state-of-the-art commercial Si and Ge photodetectors, which suggest great potential for practical applications.25 By first principle calculations, we put forward the theoretical evidence that α-In2Te3 possesses a direct bandgap structure, which reasonably accounts for the above excellent device performances. Finally, the device demonstrates good stability against the multiple photo-switching operation and ambient environment, along with no obvious voltage-scan hysteresis. In general, the excellent figures-of-merit, together with the broad spectral range and good stability, strongly underscore α-In2Te3 as a promising material candidate for next-generation photodetection applications.

EXPERIMENTAL SECTION PLD was adopted to deposit the polycrystalline α-In2Te3 films. In our experiments, the deposition parameters are described in detail below. The base pressure of the growth chamber was better than 2×10-4 Pa. The target was obtained by sintering highly pure α-In2Te3 powders (99.99%), whose In: Te atomic ratio is 2: 3. Prior to loading into the growth chamber, the SiO2/Si substrates were successively washed by acetone and piranha solution (H2SO4 (98%): H2O2 (35%) = 3:1) in an ultrasonic bath environment for 30 minutes to remove the organic and inorganic contaminations, respectively. Pre-growth annealing at 500 ºC was performed for 30

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minutes to further remove the remaining contaminations from the substrates. Large-area polycrystalline α-In2Te3 films were deposited at the optimized substrate temperature of 450 ºC. Highly pure Ar2 gas (99.99%) was adopted as the working gas. The working pressure was set at 40 Pa and the flowing rate is 50 standard cubic centimeters per minute (sccm). The Coherent KrF excimer laser (248 nm) was adopted as the evaporation source. It is operated at a power fluence of ca. 1.5 W/cm2 (laser energy density on the surface of the target), with a repetition rate of 4 Hz and a total pulse number of 2000. The pulse width of the laser was ca. 20 ns. The X-ray diffraction (XRD) pattern was recorded on a Rigaku D-MAX 2200 VPC diffractometer. Cu Kα was exploited as the radiation source ( λ = 0.154 nm ). The operating voltage and current were 40 kV and 26 mA, respectively. Raman spectrum was collected in a Renishaw InVia Raman spectrometer under an excitation wavelength of 514 nm, which is generated from an argon ion laser. Surface morphology was acquired on a FEI Quanta-400 scanning electron microscope (SEM) system. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) were recorded by an FEI Tecnai G2 F30 transmission electron microscope system with a field-emission gun. The thickness profile was acquired on a Bruker Multimode 8 atomic force microscope (AFM). Au electrodes (ca. 120 nm in thickness) were deposited by a JEOL JFC-1600 auto fine coater system. The working current and time were 40 mA and 150 s, respectively. Illumination with wavelength of 370 nm was generated from the CrystaLaser Class IIIb laser, and illuminations with wavelengths of 532, 635, 808 and

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1064 nm were generated from the Viasho lasers. The transport characteristics of the α-In2Te3 photodetector were evaluated using a Keithley 4200-SCS semiconductor parameter analyzer equipped with a probe station. All measurements were performed at room temperature under ambient condition. First principle calculations were performed based on density functional theory implemented in Quantum Espresso package to study the electronic structure of α-In2Te3.

The

exchange-correlation

functional

was

treated

using

the

Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA). An energy cutoff of 50 Ry was used in all calculations. The structure optimization was carried out until the forces on the atoms fall below 0.01 eV. The electronic convergence tolerance was set to 10-6 eV. Monkhorst-Pack Γ-centered k grid of 5 × 5 × 3 and 7 × 7 × 5 were used in optimization calculations. The optimized crystal lattice is a = 6.17 Å, b = 6.17 Å, c = 12.44 Å, respectively.

RESULTS AND DISCUSSION PLD was adopted to produce the α-In2Te3 thin films, the detailed information of which is depicted in the experimental section. Fig.1 (b) presents the crystal structure of α-In2Te3. It has a zinc blende type structure, in which one-third of the cation sites are occupied by ordered vacancies. To investigate the crystalline quality of the PLD-grown α-In2Te3 films, XRD and Raman measurements were carried out. Fig. 1(c) presents the 2θ-ω XRD pattern, along with a representative digital photograph of the α-In2Te3 sample in the inset, demonstrating a centimeter scale α-In2Te3 film. The peak

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at 25° is attributed to the (511) family lattice planes of α-In2Te3 (JCPDS 33-1488), and the peak at 33° is attributed to the (201) family lattice planes of the Si substrate (JCPDS 40-0932). Unlike the XRD patterns of previously reported CVD and solvothermal-grown α-In2Te3, no additional diffraction peak for other materials is observed, indicative of negligible contaminations introduced by the PLD growth process.19-20 In addition, the strong and sharp (511) diffraction peak reveals that the PLD-grown α-In2Te3 film possesses a quite high crystalline quality. Then, the grain size of the α-In2Te3 film is calculated according to the Scherrer formula D =

Kλ , B cos θ

where D is the grain size along the normal direction of the corresponding lattice plane of the diffraction peak, K is the Scherrer constant (0.89), λ is the wavelength of the X-ray (0.154 nm), B is the full width at half maximum (FWHM) of the diffraction peak (0.28 degrees). The grain size is calculated to be ca. 30.9 nm, which is comparable to the thickness of the film measured below (Fig. 3(b)). Therefore, there are little grain boundaries along the vertical direction of the α-In2Te3 film, indicative of its high crystalline quality. It is to be noted that the calculated grain size is slightly larger than the thickness of the film, which seems to be counterintuitive. However, it is actually reasonable considering the surface undulation of the film and the limitation of the accuracy the Scherrer formula. Fig. 1(d) presents the Raman spectrum under a 514-nm excitation laser. The position of the vibration peaks are also highly consistent with the previous report for α-In2Te3.33 To investigate the microscopic surface morphology and structure of the PLD-grown α-In2Te3 film, SEM, AFM and TEM measurements are performed. Fig.

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2(a) presents the low-magnification SEM image. A continuous morphology without pinholes is demonstrated. Such pinhole-free morphology is of great significant for efficient and homogeneous in-plane carrier transport. A high-magnification SEM image is presented in Fig. 2(b), which clearly reveals that the film is consisted of a large number of compact grains. Fig. 2(c) shows a representative AFM image. The room mean square (RMS) of the film is extracted to be ca. 10 nm. Fig. 2(d) and (e) presents a typical low and high-resolution TEM (HRTEM) image, respectively. As marked in the figure, the lattice fringe analysis reveals a interplane spacing of 3.54 Å. It can be assigned to the (511) planes of α-In2Te3. Fig. 2(f) presents the SAED pattern. The sharp diffraction rings evidently indicate the high-quality polycrystalline nature of the film. The intensity of the diffraction rings changes successively from strong to weak from the inside to outside. They correspond to the lattice planes with interplane spacing of 0.355, 0.217 and 0.185 nm. Therefore, they can be denoted as the (511), (822) and (933) lattice planes of α-In2Te3, respectively, as marked in the figure. Obviously, the HRTEM and SAED results are highly consistent with the XRD patterns in Fig. 1(c). In summary, all the characterization results suggest that the PLD approach is a promising technology to produce high-quality polycrystalline α-In2Te3 film, which provide an attractive material platform to unlock its potential in next-generation optoelectronic applications. A prototype photoresistor, the three-dimensional (3D) schematic diagram of which is presented in Fig. 3(a), was constructed to investigate the photodetection properties of the PLD-grown α-In2Te3 film. A Si wafer with 300-nm compact SiO2

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surface oxide layer was exploited as the substrate. Two 120-nm Au films were deposited as electrodes. As is well known, the thickness of the semiconducting channel is a key parameter affecting the photodetection performances. Fig. 3(b) presents the laser pulse dependent photocurrent and photo-to-dark current ratio of the α-In2Te3 photodetector. The photocurrent increases first and then saturates as the laser pulse number increases. In contrast, the photo-to-dark current ratio increases and then decreases as the laser pulse number increases. Therefore, a pulse number of 2000 is demonstrated to be superior for photodetection application. Generally，the above trend can be explained as follows. On one hand, the film can’t adequately absorb the incident light if it is too thin. In this case, as the thickness of the film increases, the number of the absorbed photons increases and thus the number of photogenerated carriers increases. As a result, the photocurrent increases as the thickness of the film increases. On the other hand, if the film is too thick, little incident light can penetrate to the bottom layer. Therefore, the bottom layer contributes much less to the photocurrent than the upper layer, while they contribute approximately equally to the dark current. As a result, in this case, the photo-to-dark current ratio decreases as the thickness of the film increases. Fig. 3(c) presents the AFM thickness profile of the film corresponding to a laser pulse number of 2000, which possesses the superior photodetection properties. The scan area is shown in the inset. The thickness is deduced to be ca. 22 nm. Fig. 3(d) presents the representative photo-switching curves under illumination with different wavelengths including 370, 532, 635, 808 and 1064 nm. In all cases,

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the device exhibits definite photo-switching behavior. In addition, the photocurrent reaches the maximum value at a 635-nm illumination, with a slight decrease as the wavelength deviates from it. This phenomenon can be qualitatively explained by the tradeoff between the kinetic energy and the number of the photogenerated carriers under illumination with constant incident power but different wavelengths.34 When an incident photon is absorbed by an electron, part of its energy is exploited to excite the electron from the conduction band to the valence band, and the rest is transformed into the kinetic energy of the electron. For incident photons with higher energy (i.e., shorter wavelength), the photogenerated carriers possesses larger kinetic energy. Therefore, they have a larger possibility to overcome the ubiquitous energy barriers, typically the exciton binding, energy band bending as well as defect-level trapping, and finally contribute to the photocurrent. However, the increase of photon energy is at the cost of the photon number for an incident light with constant power fluence. As a result, the decrease in the number of incident photons can compensate the increase in the possibility for the excited electron-hole pairs to become effective photogenerated carriers in photocurrent consideration. At a certain wavelength where the tradeoff is optimized, 635 nm in this experiment, the photocurrent reaches its maximum value. And it decreases when the tradeoff is less efficient as the wavelength deviates from 635 nm. Response time is an important figure-of-merit determining the ultimate operation frequency of a photodetector. The majority of previous studies only investigated the response time to a certain wavelength. However, photodetectors may exhibit obvious

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wavelength dependent response time.17, 35-36 For example, the response time of the α-In2Te3 nanowire photodetector under VIS to NIR illumination is fast (~ 70 ms), while it is three order of magnitudes longer (~ 100 s) under UV illumination.17 In addition, the response time of the graphene oxide photodetector under UV to VIS illumination is fast (~ 130 ms), while it is three order of magnitudes longer (~ 150 s) under NIR illumination.35 Therefore, it is essential to investigate the response time under illumination with different wavelengths for a broad spectral photodetector to demonstrate their extensive applicability. Fig. 3(e) presents the temporal photo-switching curves of our α-In2Te3 photodetector under 370, 532, 635, 808 and 1064-nm illuminations. In all cases, there are no sampling points at the rise and decay edges. Therefore, the response time of our device over a UV-VIS-NIR range is shorter than ca. 15 ms, the limited sampling interval of our Keithley 4200-SCS measurement system. In addition, the abrupt rise and decay of the photocurrent are manifestation of the high crystalline quality of the PLD-grown α-In2Te3 films. Consequently, the wavelength dependent response time reported by Wang et al. seems not to be the intrinsic property of α-In2Te3.17 In fact, it is probably caused by the defects, adsorbates and chemical residues coming from the chemical synthesis processes, which is reminiscent of the superiority of our clean PLD growth technology. Then the voltage dependent photoresponse is systematically investigated. Fig. 4(a) presents the photo-switching curves under source-drain voltages of 0.5, 1, 2, 4 V, respectively. In all cases, the device exhibits definite photo-switching behavior. Fig. 4(b) presents the voltage dependent channel current under illumination with different

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incident power densities. There are mainly two features. First, the channel current under illumination exhibits an obvious super-linear dependence on the source-drain voltage, which exists under illumination with different incident power densities. Second, the threshold voltage for the sudden increase of the channel current decreases as the incident power density increases. The channel current under illumination ( I light ) can be expressed as

I light = I dark + I P ,

(1)

where I dark is the dark current, and I P is the photocurrent. I P can be expressed as

I P ∝ µV ,

(2)

where µ and V are the mobility and source-drain voltage, respectively.37 If IP

I dark , which is the case of our experiment, I light should exhibit approximately

linear dependence on the source-drain voltage. Obviously, it is not the case for our experiment. In previous works, Liao et al. and Zhou et al. observed similar super-linear dependent photoresponse to ours at photodetectors with two non-ohmic contacts connected back-to-back.38-39 Liao et al. attributed the phenomenon to the tunneling effect at the metal/semiconductor (M/S) interface.39 In brief, the increase of the electrical filed increases the tunneling rate of the channel carriers through the M/S Schottky junction. Fig. 4(c) presents the representative I-V curve in dark of the α-In2Te3 photodetector. The slightly curving shape suggests the existence of only a weak Schottky barrier between the Au electrodes and the α-In2Te3 channel. In addition, the blocking effect of the M/S Schottky barrier is valid to both the photogenerated and

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non-photogenerated carriers. The increase of the electric field can increase the tunneling rate through the M/S Schottky junction for both of them. Therefore, the

I light / I dark

ratio in photodetectors with two non-ohmic contacts connected

back-to-back should have little dependence on the source-drain voltage.38-39 Fig. 4(d) presents the voltage dependence I light / I dark ratio under an illumination density of

19.6 mW/cm2. Obviously, it increases dramatically with the increase of source-drain voltage. Therefore, the super-linear voltage dependent current is not originated from the voltage-driven tunneling effect. Instead, we attribute the super-linear dependent channel current under illumination to the voltage-driven carrier multiplication effect. On light illumination, electron-hole pairs generate. On one hand, some of these photogenerated electrons and holes are accelerated by the external electric field along opposite directions. They cycle along the α-In2Te3 channel and contribute to the photocurrent. On the other hand, other photogenerated electrons and holes may be bounded by various factors such as exciton binding and defect traps. These electrons (holes) either recombine or become free carriers again, depending on the external perturbations. As shown in Fig. 4(e), under the bias of a relatively low source-drain voltage, the velocity of the transporting carriers ( v ) is low according to v ∝ µV . Therefore, they can hardly ionize the localized carriers by impacting them (step I). As a result, the bounded photogenerated electron-hole pairs finally recombine and don’t contribute to the photocurrent. As shown in Fig. 4(f), under the bias of a relatively large source-drain voltage, the velocity of the transporting carriers is greatly increased. These energetic carriers

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cause a consecutive impact ionization of the bounded photogenerated carriers. As a result, the number of free photogenerated carriers is multiplied and photocurrent is thus greatly enhanced. Note that the stronger the incident light density, the more the photogenerated electron-hole pairs. Thus, the cycling carriers have a larger possibility to encounter and ionize them. As a result, the threshold voltage for the sudden increase of the photocurrent decreases as the incident light density increases. Then the power dependent photoresponse is systematically investigated. Fig. 5(a) presents the photo-switching curves under periodic 635-nm illumination with different incident power densities. In all cases, the device exhibits definite photo-switching behavior. Fig. 5(b) summarizes the power dependent photocurrent under illumination with different wavelengths. The photocurrent exhibits positive dependence on the incident power density, providing good tunability for multifunctional applications. Responsivity ( R ) is an important figure-of-merit defined as the photocurrent generated per unit power of the incident light on the eﬀective area. It can be calculated according to the following equation

R=

IP , P* A

(3)

where P, A are incident power density and active area, respectively. Detectivity ( D * ) is another important figure-of-merit representing the ability of a photodetector to detect weak optical signals. It can be calculated according to the following equation 1

1

D* = A 2 R / (2qI dark ) 2 ,

(4)

where q, Id are elementary charge and dark current, respectively. Fig. 5(c) presents the power dependent responsivity and detectivity under illumination with different 16


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wavelengths. They both decrease as the power density increases. Under a 635-nm illumination with a low power density of 7 µW/cm 2 , the device achieves a high responsivity

of

6 × 1012 cm2 Hz

1

44 2

A/W,

corresponding

to

an

excellent

detectivity

of

W -1 . These excellent figures-of-merit are at least an order of

magnitude higher than those of the previously reported α-In2Te3 photodetectors (R ~ 0.05 to 1 A/W, D ~ 5 × 109 to 1 × 1011 cm Hz1/2 W -1 ).17-20 In fact, they are comparable to those of the state-of-the-art commercial Si and Ge photodetectors (R ~ 0.5 to 0.85 A/W,

D*

~

cm Hz1/2 W -1 ).25

3 × 1011 to 3 × 1012

As

emphasized

before,

extraordinary responsivity and detectivity can’t guarantee a definite photo-switching behavior.15 Therefore, another figure-of-merit should be introduced to evaluate the photo-switching characteristic. Considering that the photo-to-dark current ratio is dependent on the incident light power density, the photosensitivity describing the photo-switching behavior should be defined as the photo-to-dark current ratio per unit power.40 Therefore, it can be expressed as

S=

IP , I dark * P

(5)

Fig. 5(d) presents the corresponding power dependent sensitivity. It also decreases as the power density increases. A high sensitivity of 2.5 × 105 is achieved under a 635-nm illumination with a weak power density of 7 µW/cm 2 . For comparison,

Table I summarizes important figures-of-merit for recently reported photodetectors based on various emerging materials. In general, our device stands out considering the overall performances, which reveals the superiority of the α-In2Te3 in the photodetection applications. 17



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

To unravel the reason for the excellent photodetection properties of α-In2Te3, its band structure is studied by the first-principle calculation. Fig. 6 presents the calculated result. Obviously, the conduction band minimum (CBM) and valence band maximum (VBM) appear at the same momentum point, revealing that the α-In2Te3 is a direct bandgap semiconductor. Considering that the absorption of photons for direct bandgap materials doesn’t need the assistance of phonons. Therefore, the absorption coefficient is high and the absorption dynamic process is fast, which result in the high responsivity and short response time in this work, respectively.11 In addition, the bandgap of α-In2Te3 deduced from the calculation result is ca. 0.6 eV. It is to be noted that this value is a little smaller than the previously reported experimental value, which suggests that more details need to be considered to fully understand the electronic structure of α-In2Te3 theoretically.10 Finally, the stability of the α-In2Te3 photodetector is systematically investigated. Fig. 7(a) presents a consecutive photo-switching curve spanning a long period. After operating for an hour, which includes 60 photo-switching cycles, the device still maintain definite photo-switching behavior. In addition, there is no obvious drifting for both the dark current and photocurrent, which is superior to the previously reported GaTe photodetectors.36, 41 Fig. 7(b) presents the photo-switching curves of the device after storing it under ambient environment for 10, 17 and 24 days. The photo-switching curve of the as-fabricated device is also shown for comparison. Obviously, the device still retains definite photo-switching behavior, with only a slight decrease in the photocurrent. Fig. 7(c) presents the I-V dual scan of the device in dark

18


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and under illumination after storing under ambient environment for 24 days. The hysteresis between the forward and backward scan is small in both cases, which is much superior to the previous reported perovskite photodetectors.42 Fig. 7(d) presents the temporal photo-switching curve after storing under ambient environment for 24 days. Remarkably, the device still maintains the short response time of ca. 15 ms. In general, the above results reveal excellent stability of the α-In2Te3 photodetector, which suggest great potential for wide spread applications.

CONCLUSION In summary, we have deposited large-scale and high-quality polycrystalline α-In2Te3 thin films via PLD, and we have demonstrated that the resulting α-In2Te3 photodetectors possess stable photoresponse from 370 to 1064 nm and short response time better than ca. 15 ms. At a source-drain voltage of 5 V, the device achieves a high responsivity of 44 A/W, along with an outstanding detectivity of 6 × 1012 cm H

1

2

W-1

and an excellent sensitivity of 2.5 × 105 cm2/W. Further, we have put forward the theoretical evidence that the electronic structures of α-In2Te3 is a direct bandgap, which reasonably accounts for the above excellent device performances. Finally, the device has demonstrated good stability against the multiple photo-switching operation and ambient environment, along with no obvious voltage-sweeping hysteresis. In general, the excellent figures-of-merit, along with the good stability, strongly underscore α-In2Te3 as a promising material candidate for next-generation photodetection applications.

19



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

ACKNOWLEDGEMENTS The National Natural Science Foundation of China (91233203) and State Key Laboratory of Optoelectronic Materials and Technologies supported this work.

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Page 28 of 39

Table I. Summary of important figures-of-merit for photodetectors based on various materials. Material

In2Te3 In2Te3 In2Te3 In2Te3 In2Te3 MoS2

Structure

film nanowire nanosheet nanowire nanowire nanosheet

Responsivity (A/W)

44 0.3 0.094 0.16 0.05 0.007

1/2

Spectral range (nm)

Ref

15/ 15 ms 70/ 110 ms

370 ~ 1064 350 ~ 1090

This work

94

200/ 200 ms

633

18

314 ND

ND ND

633 350 ~ 1000

19

1.3*10

9

0.01

50/ 50 ms

450 ~ 700

43

7

0.1

60/ 190 ms

532

14

30

5 ms/ non

400 ~ 900

15

0.3/ 6.8 s

473

41

625

40/ 4000 ms

450 ~ 800

44

1100

24/ 62 ms

365

42

6*10 2.7*1010 5.3*10

nanosheet

0.018

1.4*10

nanosheet

106

1014

800

InSe

nanosheet

7

CH3NH3PbCl3

crystal

0.0469

2.4*10

10

13

1.07*10

Sensitivity (cm /W)

2.5*10 670

9

1.24*10 ND

BP

nanosheet

2

Rise/decay time

12

WS2 GaTe

-1

Detectivity (cmHz W )

1.6*10

11

1.2*1010

5

4

ND: No data.

28


17

20

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Figure Captions

Figure 1. Structural characterizations of the PLD-grown α-In2Te3 film. Schematic diagrams of the (a) PLD growth process and (b) crystal structure of α-In2Te3. Blue and yellow balls in (b) represent indium (In) and tellurium (Te) atoms, respectively. (c) 2θ-ω XRD pattern. Inset: digital photograph of a centimeter scale α-In2Te3 film on a SiO2/Si substrate. Scale bar: 1 cm. (d) Raman spectrum under a 514-nm excitation laser.

Figure 2. Morphology characterizations of the PLD-grown α-In2Te3 film. (a) Low magnification SEM image. Scale bar: 2 µm . (b) High magnification SEM image. Scale bar: 500 nm. (c) AFM image. (d) Low resolution TEM image. Scale bar: 100 nm. (e) HRTEM image. Scale bar: 2 nm. (f) SAED pattern. Scale bar: 5 1/nm.

Figure 3. Structure of the α-In2Te3 photoresistor and investigation of its photo-switching characteristic. (a) 3D schematic diagram of the α-In2Te3 photodetector. The width and length of the active area are 15 µm and 2 mm, respectively. (b) Laser pulse dependent photocurrent (dark) and photo-to-dark current ratio (blue). Source-drain voltage: 5 V. Power density: 19.6 mW/cm2. (c) AFM thickness profile of the PLD-grown α-In2Te3 film. Inset: AFM image of the scan area. (d) Broad spectral photo-switching curves. Source-drain bias: 5 V. Power density: 23 mW/cm2. (e) Temporal photo-switching curves under illumination with different

29



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

wavelengths. The curves are normalized for clarity.

Figure 4. Super-linear voltage dependent photoresponse and its mechanism. (a) Photo-switching curves under periodic 635-nm illumination with different source-drain bias. Power density: 23 mW/cm2. (b) Voltage dependent channel current under 635-nm illumination with different power densities. (c) I-V curve of the device in dark. (d) Voltage dependent photo-to-dark current ratio. Power density: 19.6 mW/cm2. Schematic illustrations of the energy band diagram under (e) small and (f) large source-drain bias.

Figure 5. Power dependent photoresponse. (a) Photo-switching curves under periodic 635-nm illumination with different power densities. Power dependent (b) photocurrent and the corresponding (c) responsivity, detectivity as well as (d) sensitivity under illumination with different wavelengths. The source-drain bias for the above measurements is 5 V.

Figure 6. The energy band diagram of α-In2Te3 obtained from the first-principle calculations.

Figure 7. Stability of the α-In2Te3 photodetectors. (a) Long-term photo-switching curve under periodic illumination (3600 s, 60 cycles). (b) Photoswitching curves after exposing to ambient environment for 10, 17 and 24 days. (c) Dual scan I-V curves

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and (d) normalized temporal photo-switching curve after exposing to ambient environment for 24 days. The source-drain voltage, power intensity and wavelength for the above measurements are 5 V, 23 mW/cm2 and 635 nm, respectively.

31



Figure 1

(a)

(b)

(c)

In2Te3 (511)

In2Te3

(d)

9k

1 cm

6k 3k Si (201)

Excitation wavelength: 514 nm

142.3

40k 104.9 20k 73.9

0 10

125.6

60k Intensity (a.u.)

Intensity (a.u.)

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

Page 32 of 39

Si 192.7 273.9

0 61.6 20

30 40 2θ (deg)

50

60

100

200 300 400 -1 Raman shift (cm )

32


500

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Figure 2

(a)

(b)

(c)

(d)

(e)

(f)

(511) (822) (933)

d(511) = 0.35 Å

2 nm

33



(b)

(a)

photocurrent photo-to-dark current ratio

Photocurrent (µA)

5

80

4 3

60

2 40

1 500

(c) 30

(d) In2Te3

370 nm 635 nm

SiO2

1E-5

532 nm 808 nm

1064 nm

20

10

0 0.2

Current (A)

Height (nm)

1000 1500 2000 2500 3000 Laser pulse number

22 nm

0.4 d (µm)

0.6

1E-6

1E-7

1E-8

0

50

100 150 Time (s)

200

(e) 1.2

370 nm

532 nm

635 nm

808 nm

1064 nm

0.9 0.6 0.3 0.0

-0.3 0.0 0.1 1.0 1.1 0.0 Time (s)

0.1 1.0 1.1 0.0 0.1 0.2 1.2 1.3 0.0 0.1 0.2 0.9 1.0 0.0 Time (s) Time (s) Time (s)

34


0.1 1.1 Time (s)

1.2

Photo-to-dark current ratio

Figure 3

Photocurrent (a.u.)

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

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Figure 4

(a)

(b)

1E-5 4V 2V 1E-7 1V 1E-8

0.5 V

2 1

Current (µA)

Current (µA)

Current (A)

dark 2 20 µW/cm 2 130 µW/cm 2 2.6 mW/cm 2 4 mW/cm

4 3

1E-6

2

8.8 mW/cm 2 13 mW/cm 2 16.5 mW/cm 2 19.6 mW/cm

0.2 0.1 0.0 0

2 4 Voltage (V)

0 1E-9

0

30

60 90 Time (s)

120

0

(c) 60

Photo-to-dark current ratio

20 0 -20 -40 -60 -6

(e)

1

2 3 Voltage (V)

4

5

4

5

(d)

40 Current (nA)

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


-4

-2

0 2 Voltage (V)

4

6

635 nm 60

30

0 0

1

(f)

35


2 3 Voltage (V)


Figure 5

(a)

(b)

Photocurrent (A)

Current (A)

7 µW/cm 2 20 µW/cm 2 8.8 mW/cm 2 19.6 mW/cm

1E-6

370 nm 532 nm 635 nm 808 nm 1064 nm

1E-5

2

1E-5

1E-7

1E-6 1E-7 1E-8

200 300 Time (s)

400

1E-6

500 1k (d)

1E-5 1E-4 1E-3 0.01 2 Power density (W/cm )

0.1

100k

1

0.1

10

100

2

1/2

10

370 nm 532 nm 635 nm 808 nm 1064 nm

10

1

1E-5 1E-3 2 Power density (W/cm )

0.1

Sensitivity (cm /W)

-1

(c) 100

100

Detectivity (10 cmHz W )

0

Responsivity (A/W)

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

Page 36 of 39

370 nm 532 nm 635 nm 808 nm 1064 nm 1E-6

1E-5 1E-4 1E-3 0.01 2 Power density (W/cm )

36


10k

1k

0.1

Page 37 of 39

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


Figure 6

37



Figure 7

(a)

Current (A)

The 60th cycle

635 nm

1E-5

Light On

1E-6

1E-7 Light Off 1E-8 0

100

200

300

400

3100 Time (s)

3200

3300

3400

3500

3600

(b)

Current (A)

1E-5

as-fabricated

24 days

17 days

10 days

1E-6

1E-7

1E-8 0

200

400

Photocurrent (a.u.)

dark light

1E-6 1E-7 1E-8 1E-9 1E-10

Time (s)

600

800

1000

(d) 1.5

1E-5

(c)

Current (A)

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

Page 38 of 39

0

1

2 3 Voltage (V)

4

1.0 0.5 0.0 -0.5 0.0

5

635 nm

0.2

1.0 Time (s)

38


1.2

Page 39 of 39

TOC

We have demonstrated that the photodetectors made of the prepared α -In 2Te3 films possess the stable photo-switching behavior from 370 to 1064 nm and the response time shorter than 15 ms. The device achieves a high responsivity of 44 A/W at a source-drain voltage

of

5 V,

along with an outstanding detectivity of

1

6 × 1012 cmH 2 W-1 and an excellent sensitivity of 2.5 × 105 cm2/W.

100

1k

1/2

-1

Detectivity (10 cmHz W )

10

100

10

Responsivity (A/W)

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


1

0.1

370 nm 532 nm 635 nm 808 nm 1064 nm

10

1 1E-5

1E-3 2 Power density (W/cm )

39


0.1

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