Ultra-broadband, sensitive and fast photodetection with needle-like

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Ultra-broadband, sensitive and fast photodetection with needle-like EuBiSe3 single crystal Yingxin Wang, Yingying Niu, Meng Chen, Jianguo Wen, Weidong Wu, Yingkang Jin, Dong Wu, and Ziran Zhao ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.8b01527 • Publication Date (Web): 15 Jan 2019 Downloaded from http://pubs.acs.org on January 18, 2019

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Ultra-broadband, sensitive and fast photodetection with needle-like EuBiSe3 single crystal Yingxin Wang,† Yingying Niu,† Meng Chen,† Jianguo Wen,‡ Weidong Wu,† Yingkang Jin,‡ Dong Wu,∗,¶ and Ziran Zhao∗,† †Key Laboratory of Particle & Radiation Imaging (Tsinghua University), Ministry of Education, Department of Engineering Physics, Tsinghua University, Beijing, 100084, China ‡Nuctech Company Limited, Beijing 100084, China ¶International Center for Quantum Materials, School of Physics, Peking University, Beijing 100871, China E-mail: [email protected]; [email protected]

Abstract Ultra-broadband photodetection has been a hot topic with the rapid development of materials science and the application requirements for communication, imaging and sensing. Photodetectors based on bandgap-independent bolometry are promising candidates for detection of light from the ultraviolet to the terahertz range. Here we report a photothermoelectric detector made of an alloy of EuBiSe3 single crystal. The device shows room-temperature self-powered photoresponse from ultraviolet (375 nm) to terahertz (163 µm) with nearly uniform sensitivity against wavelength and fast response speed. Thanks to the large thermoelectric power (Seebeck coefficient) of EuBiSe3 ,

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the photovoltage responsivity derived from the incident (not absorbed) power reaches as high as 1.69 V/W at 405 nm without any bias voltage, and exceeds 0.59 V/W √ even at terahertz frequencies, with noise-equivalent power below 1 nW/ Hz, which is 1–2 orders of magnitude lower than reported photothermoelectric detectors. The response time is around 200 ms, nearly two orders of magnitude faster than siliconbased heterojunction ultra-broadband photodetectors and on the same order as the millimetric-scale graphene- and carbon nanotube-based bolometric photodetectors. In addition, the as-grown EuBiSe3 crystal possesses a unique needle-like shape, intrinsically facilitating integration of the detector. Our work demonstrates that improved thermoelectric materials hold great promise for room-temperature high-performance broadband photodetection.

Keywords ultra-broadband, photodetector, sensitive, photothermoelectric, self-powered, EuBiSe3

Detection of light within an ultra-broadband range from the ultraviolet (UV) to terahertz wavelength regime of the electromagnetic spectrum in a single element with a simple structure has promising applications in communication, imaging, sensing and spectroscopy, 1,2 but it is still a challenging task owing to lack of appropriate photoactive materials with superior overall performance. Benefiting from the gapless nature, graphene, an exciting twodimensional (2D) material, can absorb light over such a wide range through the photoconductive, photovoltaic or photothermoelectric (PTE) effects. 3–5 However, the small absorption of monolayer graphene results in a low responsivity for broadband photodetection. Although considerable efforts on improving the responsivities of graphene-based photodetectors have been devoted by tailoring graphene sheet into quantum dot arrays, 6 stacking tunnel barrier separated graphene monolayers, 1 forming heterostructure with silicon nanowire array, 7

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etc., they suffer from the problem of complex device fabrication processes. Alternatively, other materials such as transition metal dichalcogenides, 8–11 metal monochalcogenides, 12 black phosphorus, 13 topological insulators, 14 ferroelectric single crystal, 15 polytelluride, 16 and carbon nanotubes(CNTs) 17 also exhibit the ability of broadband photoresponse, but the corresponding photodetectors either do not have high sensitivities throughout the entire waveband of concern (i.e., from UV to terahertz) or need an external bias to operate, causing consumption of energy and system size and cost increase. Therefore, novel photoactive materials and related devices remain highly desired. Considering the physical mechanisms of existing broadband photodetectors, photoconductive or photovoltaic effects are generally responsible for detection of radiation with photon energies above the band gap, whereas thermal effects (such as bolometric, PTE and pyroelectric ones) occur theoretically for any specific wavelength without the bandgap limitation. 18 Among them, PTE-based detectors can serve as good candidates for the requirement of ultra-broadband response because of their advantages of simple device geometry, zero-bias operation and low power consumption, making them suitable to be integrated into arrays. The PTE process originates from the temperature gradient across the device channel as well as the asymmetry in channel material or device structure. Typical asymmetric configurations include contacts between the active materials and the metal electrodes, 19–21 dissimilar contact metals 5 and p–n junctions, 22,23 where the first type has the simplest structure and attracts much research interest. A large thermoelectric power (Seebeck coefficient, S) of the active material for such type of photodetector would be expected to produce a high photovoltage responsivity. 24 Conventional thermoelectric materials with high figure-of-merits can fulfill this requirement, such as bulk alloy materials of bismuth and antimony chalcogenides, whose S values are on the order of several hundreds of µV/K. 25,26 Recently, alloys of rareearth element Eu and bismuth or antimony chalcogenides (i.e., ternary europium pnictogen chalcogenide compounds, EuSbSe3 and EuBiSe3 ) have been synthesized. 27,28 These two newly discovered phases possess complex crystalline structures with large unit cells, leading to

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reduction in thermal conductivity. Their Seebeck coefficients are quite attractive and approach 1000 µV/K at room temperature (RT), offering a large potential for thermoelectric applications. Moreover, EuSbSe3 and EuBiSe3 are narrow-band-gap semiconductors and show interesting electronic properties associated with the divalent state of the Eu ions. Thus they may hold great promise for PTE detection. Here we demonstrate that ultra-broadband, sensitive and fast PTE detection can be achieved at RT through light illumination at the contact between the metal electrode and the EuBiSe3 crystal with a naturally grown needle-like shape.

Experimental Section Preparation and Characterization of EuBiSe3 Crystals The EuBiSe3 samples were prepared by a pyrometallurgical procedure similar to that used in refs 24 and 25. First, the Bi2 Se3 precursor was grown by mixing and heating the bismuth and selenium elements in a vacuum quartz tube to 800–850 ◦ C, dwelling for 24 h and then quenching in water. Next, powders of europium, Bi2 Se3 and selenium samples were mixed in a 2:1:3 molar ratio and sealed in a vacuum quartz tube, which were performed in a glovebox under an argon environment. Finally, the EuBiSe3 crystals were synthesized following a temperature controlled process with a series of heating, dwelling and cooling treatments. 28 The as-prepared materials consist of irregularly shaped clusters and fragments of the target products as well as unwanted residual unreacted raw materials (see Supporting Information, Figure S1). Isolated needle-like crystal rods with smooth and shining surfaces were considered as the desired EuBiSe3 crystal and picked out for device fabrication (see Figure 1a). Observations on the crystal surface and analysis of its element composition were conducted by a field-emission scanning electron microscope (MERLIN Compact FE-SEM, Carl Zeiss, Germany). The SEM image of a representative sample is given in Figure 1b. It is clear that the sample surface is compact and has fine stripe structure along the longitudinal direction 4


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of the crystal rod. Figure 1c shows the crystal structure of EuBiSe3 viewed along c direction. This material crystallizes with the P 21 21 21 space group and its unit cell contains Bi–Se slabs separated by Eu2+ and Se2− 3 trimers. The sample element composition was identified by the energy dispersive spectroscopy (EDS) function of the SEM system (see Supporting Information, Table S1 and Figure S2), where the average Eu:Bi:Se atom ratio is about 1:1.07:2.94. To examine the crystal structure of EuBiSe3 , we measured the high-resolution transmission electron microscope (HRTEM) images and the selected area electron diffraction (SAED) pattern by a JEOL JEM-2010 microscope, as shown in Figures 1d and 1e, where the lattice fringes could be clearly observed, revealing the well crystalline nature, and the diffraction spots were indexed to the orthorhombic unit cell based on the crystallographic data provided by ref 28. Furthermore, a sufficient amount of isolated needle-like crystal rods and clusters were picked up and then grounded into a fine powder to perform the powder X-ray diffraction (XRD) measurement. The XRD pattern was collected with a Bruker D8 Advance diffractometer (Cu Kα radiation, λ = 1.5406 Å) and plotted in Figure 1f with main peaks indexed. Moreover, the lattice constants of EuBiSe3 were obtained by using a single crystal X-ray diffractometer (Supernova, Rigaku-Oxford Diffraction) with a = 33.293 Å, b = 15.568 Å, and c = 4.230 Å, in accordance with the literature values of a = 33.329 Å, b = 15.599 Å, and c = 4.224 Å. 28 These results confirm the single-crystalline nature of the as-grown EuBiSe3 sample. In order to evaluate the purity of the synthesized material, we compare the powder XRD pattern of the as-prepared EuBiSe3 sample with that measured in ref 28 and find good agreement between them (see Supporting Information, Figure S3). We also compare the XRD pattern with that of standard JCPDS cards for the precursors and possible byproducts of the synthesis process, including Bi2 Se3 (PDF#33-0214), EuSe (PDF#10-0279), Eu (PDF#02-0501) and Se (PDF#38-0768). We can see that no obvious peaks from the impurities are observed, indicating that the synthesized needle-like EuBiSe3 crystal is in a pure phase. Raman spectra of two pristine EuBiSe3 samples were also recorded on a Raman spectrometer (LabRAM HR Evolution, HORIBA Jobin Yvon) with a 532 nm

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laser and the spectral features are almost unchanged for different samples (see Supporting Information, Figure S4).

a

(3 5 0)

5 1/nm 10

20

30 40 2Theta (degree)

(11 7 0) (10 2 2)

(5 4 0)

(6 3 0) (3 4 0) (5 4 0) (7 1 1) (2 5 0) (8 1 1)

(8 1 1)

(9 6 1)

0.3 nm

(4 5 0)

(f)

(e)

5 nm

Bi Eu Se

100 µm

1 mm

(d)

b

(c)

(b)

(10 3 1) (14 2 0) (11 5 0) (15 2 0)

(a)

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

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50

60

Figure 1: (a) Photograph and (b) SEM image of the as-grown needle-like EuBiSe3 single crystal. (c) Crystal structure of EuBiSe3 viewed along c direction. (d) HRTEM image and (e) SAED pattern of the EuBiSe3 sample. (f) XRD pattern of the EuBiSe3 powder sample.

Fabrication and Characterization of EuBiSe3 –Metal Contact Devices The crystal rods with diameters of around 100–200 µm were chosen to fabricate the EuBiSe3 – metal contact device. Two gold electrodes were deposited on the ends of each rod by thermal evaporation to enable a good ohmic contact and then connected to the conducting wires via the silver paint. A glass substrate was used to support the sample. Figure 2a shows the schematic of the device structure and its inset displays the optical microscopy image of one representative device whose channel has 2.8 mm length, 220 µm width, and 100 µm thickness. Different photodetectors with similar channel dimensions had been fabricated for compar6


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ison and no obvious difference in the response performance was observed. Consequently, in the following, we provide only the experimental results of this representative device for demonstration. Several types of continuous-wave lasers served as the excitation light sources with different wavelengths, including UV (375 and 405 nm), visible (532 and 635 nm) and near infrared (NIR, 1064 and 1550 nm) semiconductor lasers, a mid-infrared (MIR, 10.6 µm) CO2 laser, and a far-infrared gas laser generating terahertz waves (96.5, 118.8 and 163 µm, FIRL 100, Edinburgh Instruments Ltd.). A total of ten wavelengths were tested at RT. The incident power at each wavelength was monitored by a calibrated power meter operating in the corresponding waveband. The light beam was focused onto the device with the beam spot size being evaluated by the scanning knife-edge method. 20 To avoid the spot size effect on the response time in our experiments, the incident beam diameters from different lasers were kept on the same level (about 1.5∼2 mm). For terahertz illumination, we also fabricated a device using high-resistivity silicon (HRSi) as the substrate, which is transparent to terahertz waves, to evaluate the substrate absorption effect. All experimental results refer to the device with glass substrate unless otherwise mentioned. The electrical characteristics of the photodetector under dark and light illumination conditions were measured by using a sourcemeter (Keithley 2602B). Vacuum and cryogenic measurement environments were created in a continuous flow optical cryostat system (ST-100, Janis Research Co., Inc.).

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( a)

EuBi Se3 1mm

1 Dark Negative end Positive end

(b)

Current (mA)

0.5 Voc 0

Voc

−0.5

−1 −10

−5

0 Voltage (mV)

3

5

10

532 nm 635 nm 1064 nm 1550 nm 118.8 µm

(c) Photovoltage (a.u.)

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2

1

0 Au −1

−2

Au −1 0 1 Illumination position (mm)

2

Figure 2: (a) Schematic of the EuBiSe3 –metal contact device; the inset shows the optical microscopy image of the device. (b) I − V characteristics of the photodetector in the dark (black curve) and under 635-nm light illumination on the positive end (red curve) and negative end (blue curve). (c) Scanning photovoltage measurement results under visible (532 and 635 nm), infrared (1064 and 1550 nm) and terahertz (118.8 µm) light illuminations; the shaded regions indicate the Au electrodes; the curves are vertically offset relative to the corresponding dashed curves for visual clarity.

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Results and discussion Photoresponse of EuBiSe3 –Metal Contact Devices Figure 2b plots the current–voltage (I − V ) characteristic curves under dark condition and steady-state 635-nm light illumination at the positive and negative ends of the device at RT, respectively. The positive end is defined as the one connected to the positive channel of the sourcemeter, and vice versa. Only the data between −10 mV and 10 mV bias voltages are given here for clarity, and that within a wider range from −1 V to 1 V can be seen in the Supporting Information (Figure S5). Linear behavior of the I − V relationship within ±0.5 V indicates that the channel material has an ohmic contact with the electrodes. The slight deviation from the linear dependence out of this range may be a result of electrical resistance increase caused by Joule heating of the sample at a large bias voltage. The total resistance of the sample extracted from these measurements is about 13 Ω. Obviously, the I −V curves are rigidly shifted upon light illumination and a net photocurrent is generated even at zero bias, which are typical PTE signatures. 29 Take the open-circuit voltage as the photovoltage (VOC ) induced by light excitation. We can observe that the photovoltages produced at two ends of the device have opposite signs, which is more pronounced in the scanning photovoltage measurements, as seen from Figure 2c. These measurements were performed by mechanically moving the device step by step along the EuBiSe3 channel to change the THz beam position between the two electrodes. The photoresponse curves have peak amplitudes at the left and right EuBiSe3 –metal contacts and change sign across the center of the channel. For all examined wavelengths, the scanning photovoltage measurement results exhibit similar behaviors and only five of them (532 nm, 635 nm, 1064 nm, 1550 nm, and 118.8 µm) are shown here. Such phenomenon could be well explained by the PTE theory. It states that the photo-induced local temperature rise in the channel material drives the carriers to diffuse toward the cold region and a net current will be formed if the temperature distribution is asymmetric with respect to the two electrodes. Thus a maximum photoresponse occurs when

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the focused beam illuminates the EuBiSe3 –metal interface, corresponding to the strongest asymmetry either for the left or right interface with only the photovoltage polarity being opposite due to the different carrier transport direction that is always from the hot to the cold end. In addition to the PTE mechanism, bolometric effect may also present in the channel material which originates from the change in conductance induced by heating associated with incident photons. 4 Instead of direct photocurrent generation, this effect will emerge when applying an external bias. Comparing the slopes of the I − V curves between the case of light illuminating the channel center and that of dark state (taking the data within ±0.5 V linear region, see Supporting Information, Figure S5), we find that the channel resistance increases only about 1.6% under 5 mW incident power. One key figure of merit to evaluate the bolometric property is known as the temperature coefficient of resistance (TCR), defined by TCR =

d(ln R) , dT

(1)

where R is the resistance of the sample and T is the absolute temperature. We measured the temperature dependence of the resistance of a EuBiSe3 sample from 200 K to 350 K (see Supporting Information, Figure S6) and obtained a positive TCR of 0.1%/K at 300 K. This level is much lower than other thermosensitive materials with good bolometric performance such as vanadium oxide thin film 30 and CNTs, 31 indicating that the bolometric response of EuBiSe3 is relatively weak and could be ignored in our device. Indeed, the bolometric contribution may play a role only in the presence of a bias voltage and it should be taken into account only in the photocurrent measurement mode for PTE experiments since our photodetector operates at zero bias. Fortunately, based on the above results, we do not need to consider the contribution from the bolometric response at all. On the other hand, the positive TCR means that the electrical conductivity of EuBiSe3 at RT has a metal-like behavior, coinciding with the above observation that Joule heating causes an increase of resistance at a large bias voltage. 10


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The responsivity RV is one of the key figures of merit needed to be evaluated for photodetectors, which quantifies the detector sensitivity and is defined as

RV =

VOC , Pin

(2)

where VOC is the photoinduced voltage and Pin is the incident power, i.e., the actually received portion of the active area of the detector normalized to the light beam spot size. In order to obtain this quantity, we measured the steady-state photovoltage under different incident power levels for each wavelength at RT. Figure 3a gives the power dependence of the photovoltage at four wavelengths (532 nm, 635 nm, 1064 nm, and 118.8 µm) and they all exhibit good linear behaviors, ensuring the linearity property of our photodetector when acting as a photodetector. By linear fitting the data, we can obtain the responsivity. Figure 3b displays the responsivity as a function of wavelength, nearly uniform against wavelength, revealing the broadband nature of the EuBiSe3 –metal contact structure for photodetection, which is also a typical feature of PTE effect. The responsivities are around the level of 1 V/W and range from 0.59 V/W (163 µm) to 1.69 V/W (405 nm). These values are much higher than that of previously reported nanomaterial–metal contact PTE detectors without enhancement by means of structure design (such as using an antenna or plasmonic resonance), where the latter is usually on the level of mV/W, 20,32,33 and are also higher than other selfpowered broadband photodetectors based on different effects, e.g., photoconductive 14 and pyroelectric. 15 Table 1 gives the responsivities at various wavelengths, along with the noiseequivalent powers (NEPs). Because our device generates photocurrent at zero bias, the dark √ noise is limited by the Johnson noise 24 and its spectrum is given by 4kB T R, where kB is Boltzmann‘s constant, T is the room temperature, and R is the channel resistance. Thus the NEP can be calculated as 24,34 √ 4kB T R NEP = . RV

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√ The values of NEP for all examined wavelengths are below 1 nW/ Hz, which is 1–2 orders of magnitude lower than previously reported PTE detectors based on graphene, CNT films and thermoelectric nanostructures. 5,18,23,24 Such high sensitivity of our photodetector results from the broadband and strong light absorption of EuBiSe3 material as well as its large Seebeck coefficient. In other words, this kind of material has a relatively high efficiency to convert light energy into thermal energy and then to an electrical signal. Certainly, the practical value of NEP is definitely larger than the theoretically estimated one. The noise voltage spectral density of our photodetector was measured by a network signal analyzer (SR780, Stanford Research Systems) and is illustrated in the Support Information (Figure S7). Taking a responsivity of 1 V/W, the practical NEP was obtained to be about 20 √ nW/ Hz at 5 Hz frequency. In Figure S7, we also show the background noise of the signal analyzer with its input short circuited and it is on the same level as that of the photodetector. This means that measuring the practical NEP is limited by the background noise of the √ measurement system and the real NEP of our detector may not exceed 20 nW/ Hz above 5 Hz. Another important parameter for evaluating the capability of photodetectors to detect √ a weak light signal is the specific detectivity, defined as D∗ = A/NEP with a unit of Jones √ (cm Hz/W), where A is the active area of the detector. We calculate the detectivities at various wavelengths and give them in Table 1. The detectivity values are larger than 108 Jones for all examined wavelengths, very competitive to other broadband photodetectors. Note that the EuBiSe3 sample rod has a unique, long and thin shape, implying its specific crystal growth direction and anisotropic property. The lattice parameter c of EuBiSe3 is much smaller than parameters a and b, 27 thus the crystal grows generally along the caxis and forms a needle-like shape. To explore the intrinsic anisotropy in EuBiSe3 , we measured the polarization-dependent photoresponse with a polarizer and a half-wave plate placed successively in the beam path behind the 635 nm laser source. Figure 3c shows the photovoltage VOC generated when illuminating the device channel near the EuBiSe3 –metal interface versus the polarization rotation angle of the incident beam. The focused light spot

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(a)

Photovoltage (mV)

15

10

532 nm 635 nm 1064 nm 118.8 µm

5

0 0

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10 Power (mW)

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Responsivity (V/W)

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0.5

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1

1

10 Wavelength (µm)

100

(c)

0.9

0.8

0

90 180 270 Polarization angle (deg)

360

Figure 3: (a) Power dependence of the photovoltage under visible (532 and 635 nm), infrared (1064 nm) and terahertz (118.8 µm) light illuminations. (b) Photovoltage responsivities at ten examined wavelengths measured in air at RT. (c) Polarization dependence of the photovoltage (635-nm wavelength, 0 degree corresponds to the polarization parallel to the long axis of the EuBiSe3 sample); the dashed curve is a sine fit of the experimental data.

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was kept slightly away from the metallic electrode edge to avoid its influence on the light polarization. VOC is normalized to its maximum value corresponding to the polarization parallel to the c-axis or longitudinal direction of the EuBiSe3 sample, defined as 0◦ for the polarization rotation angle. The photovoltage ratio between parallel and perpendicular polarizations is about 1.3. Although this value is not high, the optical anisotropy of EuBiSe3 crystal can be proved to a certain extent. Another factor associated with the photodetector performance is the response time which can be measured by periodically switching the incident light on/off and monitoring the dynamic change of the photocurrent or photovoltage signal. Figure 4 shows the time-resolved photovoltage of the photodetector in response to light on/off for six wavelengths at RT. By exponential fitting of the curve rising edge, we can extract the response times (defined as the time constant of the exponential function), as listed in Table 1. For terahertz range, double-exponential fitting was used to fit the curve with a high accuracy and two time constants (342 ms and 2.72 s) were obtained (see the Supporting Information, Figure S8a). We analyze that the longer one results from the glass substrate absorption to terahertz waves and such an absorption interaction leads to a gradual heating of the substrate as well as a slow thermal equilibrium process for the PTE response of the EuBiSe3 photodetector. This can be confirmed from previous studies where the response times of PTE detectors with glass as the substrate and macroscopical CNT film or graphene stripe as the channel material were found to be on the order of several seconds. 32 Also, we tested the other device with HRSi as the substrate and measured the terahertz temporal photoresponse, as given by the black curve in Figure 4. The response time was fitted to be about 236 ms by using only a single exponential function (see the Supporting Information, Figure S8b), suggesting that the slow terahertz response of the device fabricated on glass substrate is indeed attributed to the heating of the substrate. For other wavelengths, a single exponential fitting was enough to extract the time constants, which are all around 200 ms. This value is nearly 2 orders of magnitude faster than reported silicon-based heterojunction broadband photodetectors 7,13

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and on the same order of magnitude as the millimetric-scale graphene and carbon nanotube bolometric detectors. 17,35 The response speed on this level may be acceptable for detection applications.

2 Normalized photovoltage (a.u.)

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405 nm

532 nm

635 nm

1064 nm

1550 nm

118.8 µm

118.8 µm (HRSi substrate)

1.5

1

0.5

0 0

5

10

15 Time (s)

20

25

30

Figure 4: Temporal photovoltage curves with three light on–off cycles measured in air at RT, normalized with the same incident power of UV (405 nm), visible (532 and 635 nm), infrared (1064 and 1550 nm) and terahertz (118.8 µm) illuminations. Table 1: Response parameters of the device with glass substrate under different illumination wavelengths in air at RT. Wavelength (nm)

118800

1550 1064

635

Responsivity (V/W)

532

0.69

1.19

1.02

0.89 1.25 1.69

Response time (ms)

342, 2720

210

191

213

√ NEP (nW/ Hz)

0.67

0.39

0.45

0.52 0.37 0.27

Detectivity (108 Jones)

1.17

2.01

1.74

1.51 2.12 2.91

184

405

207

Furthermore, we perform thermoelectric measurements by monitoring the temperature distribution of the sample under laser illumination using an infrared (IR) camera (Dali Technology Co. LTD, DL700). A 635 nm laser was used as the light source and focused by a 50× objective lens with a numerical aperture (NA) of 0.55 to produce a point-like heat source as 15


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much as possible so as to exclude the spot size impact on the temperature profile measurement. The focal spot size was determined to be about 30 µm with a CCD camera. First, the temperature rise at the EuBiSe3 –electrode contact induced by laser heating was extracted from the IR camera image, as seen from Figure 5a and its maximum value was found to reach 5.5 K. Taking the emissivity of EuBiSe3 material to be 0.4, 36 the actual temperature rise ∆T is thus about 13.8 K. According to the definition of the Seebeck coefficient S of a material and using the photovoltage value VOC (−4.6 mV) measured under this illumination condition, we can roughly estimate that S = VOC /∆T = −333 µV/K for EuBiSe3 at RT, much larger than that reported for its precursor Bi2 Se3 , which generally does not exceed −100 µV/K. 37–39 Such large S value is responsible for the high responsivity of our detector, as mentioned above. Note that S has a negative sign and electron diffusion may dominate the PTE process in the EuBiSe3 material. This means that EuBiSe3 is a kind of n-type material. 40 Another important parameter involved in the PTE detection is the thermal decay length λ of optical heating, 41,42 which describes the exponential decay behavior of local heating and temperature distribution in the sample. To avoid the influence of the electrode, we measured λ by recording the IR image for light illuminating at the channel center not the contact region, as shown in Figure 5b. Fitting the temperature profile along the horizontal central axis of the sample (see Figure 5c) by the following expression 41

T (x − x0 ) = T0 e−|x−x0 |/λ ,

(4)

where T (x) is the position-dependent temperature and x0 is the position of the laser spot, we p can get λ ≈ 270 µm. Theoretically, the thermal decay length is given by λ = κh/G , where κ is the thermal conductivity of EuBiSe3 , h is the sample thickness, and G is the thermal conductance between the sample and the glass substrate. According to ref 39 and using the values κCNT = 60 W/m·K, 23 κglass = 1 W/m·K, and hCNT = 2 µm, we estimate G = 1.6×103 W/m2 ·K. Thus the thermal conductivity κ of EuBiSe3 can be calculated to be 1.2 W/m·K, in

16


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good agreement with the result measured by a T-type method. 43 This thermal conductivity value is lower than that of Bi2 Se3 (typically on the order of 4 W/m·K for bulk material 44 ). Both the relatively large Seebeck coefficient and small thermal conductivity result in the enhancement of thermoelectric as well as PTE properties of EuBiSe3 via introducing the Eu element into Bi2 Se3 .

307 307 KK

1 mm

303 303 KK 309 K 309 K

1 mm 305 K 305 K

312 312 Temperature (K)

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

ACS Photonics

310 308 306 304 −1.5

−1

−0.5

0 0.5 Position (mm)

1

1.5

Figure 5: Temperature distribution over the photodetector illuminated with a focused light beam (5 mW, 635 nm) at (a) the EuBiSe3 –metal contact and (b) the center of the EuBiSe3 channel. The EuBiSe3 rod is marked with a green dashed square. (c) Temperature profile extracted along the horizontal axis of the EuBiSe3 rod in (b). Square dots are the measured data, and the dashed line is a fit to Eq. 4. For PTE effect, environments may also affect the detector photoresponse. Consequently, the measurements were then performed under vacuum (∼ 10−2 Torr) and cryogenic (liquid nitrogen temperature, 77 K) conditions, and the temporal photovoltage curves for different environments but the same excitation conditions (635 nm laser, constant incident power) are illustrated in Figure 6. We can find that the responsivity increases more than 5 times but the response speed decreases 5 times from air to vacuum, yielding NEP values to be smaller than √ 100 pW/ Hz. This is because that, in air environment, the heat generated in the sample 17


ACS Photonics

can dissipate via the surrounding air quickly, leading to a lower but faster temperature rise. While in vacuum, more heat accumulation is allowed and the thermal equilibrium time is longer, leading to a higher but slower response. When the environment temperature goes down, the responsivity decreases, because the light induced temperature rise of the sample is smaller under a cryogenic environment. These results agree well with the PTE mechanism. 20 Air, 296 K Vacuum, 296 K Vacuum, 77 K 15 Photovoltage (mV)

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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10

5

0 0

10

20 Time (s)

30

40

Figure 6: Comparison of the photoresponse to illumination at 635 nm under air (black curve), vacuum (red curve) and cryogenic (blue curve) environments.

Discussion on the photoresponse mechanism and performance improvement All the above measurement results provide the evidence that the PTE effect is responsible for the photoresponse of EuBiSe3 –metal contact structures. First, the high responsivity and nearly constant response time throughout the ultra-broadband spectrum from the UV to terahertz range as well as their dependences on the vacuum degree and environment temperature suggest a thermal origin of the photoresponse. This can be understood as a photo-thermal conversion process, in which electrons receive energy from the incident photons by optical absorption and heat is then produced via electron–phonon interactions in the material. Second, the maximum self-powered (zero-bias) photovoltages with opposite 18


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ACS Photonics

signs occur when light illuminates the two ends of the EuBiSe3 channel, corresponding to the strongest asymmetry in temperature distribution across the channel, which implies that a thermoelectric conversion follows the photo-thermal process. Hence, we conclude that the photoresponse mechanism of our device is attributed to the PTE effect. To further evaluate the performance of our EuBiSe3 –metal contact device, we compare it with reported ultra-broadband photodetectors based on various photoactive materials and summarize the results in Table 2. Although some parameters may be not the most superior, our detector has comparable overall performance with some excellent devices, and stands out especially for self-powered photodetectors. According to the physical principle of the PTE effect, optimizing the detector responsivity and speed may be achieved by designing the device configuration to maximize the temperature difference ∆T between the two electrodes and decrease the carrier transport and heat transfer time. Generally, suspending the channel to prevent heat dissipation through the substrate, 45 coupling light with antenna, 46,47 plasmonic structure 48,49 or metasurface 50 to enhance the optical absorption, and scaling down the channel to speed up the thermal equilibrium process 51 would be useful strategies. Their implementations are beyond the scope of the present work and will be considered in future research.

CONCLUSIONS The naturally grown needle-like EuBiSe3 single crystals were prepared for serving as the sensing element of the photodetector. Devices consisting of EuBiSe3 –metal contacts were fabricated and their photoresponse properties over an ultra-broadband spectral range from UV (375 nm) to terahertz (163 µm) were systematically investigated. High self-powered responsivities up to around 1 V/W with fast response times around 200 ms had been observed throughout the entire examined spectral range. Experiments on scanning photovoltage measurements, the thermoelectric characterization, and the working environment influence all

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Table 2: Performance of ultra-broadband photodetectors reported in the literatures. Active

Spectral Mechanism

material

Response

Bias

Responsivity range

ref time

voltage

∼200 ms

0

0.59–1.69 V/W EuBiSe3

PTE

UV–THz

This

(0.05–0.13 A/W) PCb ,

a

RGO –Si

Visc –THz 9 mA/W (10.6 µm)

work ∼10 s

1V

7

bolometric 1T–TaS2

PC

Vis–THz

0.2–4 A/W

∼ns

0.72 V

9

WSe2

PC

UV–NIR

0.92 A/W

0.9 s

10 V

11

∼100 ms

–5 V

14

PC, Bi2 Te3 –Si

1 A/W (635 nm), UV–THz

bolometric

1.7 mA/W (THz)

PMN–PTd

PEe

UV–THz

0.09–0.5 µA/W

∼ms

0

15

EuSbTe3

PC

UV–THz

1–8 A/W

8 ms

1.2 V

16

CNT

Bolometric

UV–THz

0.08–0.58 A/W

∼150 µs

0.2 V

17

RGO

Bolometric

UV–THz

0.3–1.3 mA/W

∼150 ms

1V

35

a

Reduced graphene oxide; b Photoconductive; c Visible; d 0.72Pb(Mg1/3 Nb2/3 )O3 –0.28PbTiO3 ; e Pyroelectric.

20


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ACS Photonics

confirm the PTE mechanism of the zero-bias photoresponse. The high responsivity originates from the large Seebeck coefficient of EuBiSe3 material. Moreover, our prototype device exhibits optical anisotropy owing to the intrinsic growth direction of EuBiSe3 crystal. The unique needle-like shape of this kind of crystal is particularly favorable for fabrication of extremely compact photodetector array. Our work opens a new way of using thermoelectric material-based integrated optoelectronic devices with low power consumption for ultra-broadband, sensitive and RT photodetection.

Acknowledgement The authors gratefully acknowledge financial support from National Natural Science Foundation of China (No. U1730246, 61731007) and the Ministry of Science and Technology of China (2017YFC0803601).

Supporting Information Available Optical microscopy, EDS analysis, XRD pattern, Raman spectra of the EuBiSe3 materials, I-V characteristics, TCR measurements, noise voltage spectral density measurements, and temporal response fitting for terahertz illumination.

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ACS Photonics

For Table of Contents Use Only Title Ultra-broadband, sensitive and fast photodetection with needle-like EuBiSe3 single crystal

Names of authors Yingxin Wang, Yingying Niu, Meng Chen, Jianguo Wen, Weidong Wu, Yingkang Jin, Dong Wu, and Ziran Zhao.

Brief synopsis Needle-like EuBiSe3 single crystal exhibits sensitive, fast and self-powered photoresponse from ultraviolet to terahertz range. Such unique features originate from the large thermoelectric power and broadband light absorption of this material, holding great promise for room-temperature high-performance ultra-broadband photodetection. 2

RV (V/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

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1

0

UV Visible Near IR

Mid IR

Hot

28

Terahertz

EuBiSe3

Cold


Ultra-broadband, sensitive and fast photodetection with needle-like

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