Design of 2D Layered PtSe2 Heterojunction for the High-Performance

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Design of 2D layered PtSe heterojunction for the highperformance room-temperature broadband infrared photodetector Di Wu, Yuange Wang, Longhui Zeng, Cheng Jia, Enping Wu, Tingting Xu, Zhifeng Shi, Yongtao Tian, Xin Jian Li, and Yuen Hong Tsang ACS Photonics, Just Accepted Manuscript • Publication Date (Web): 27 Aug 2018 Downloaded from http://pubs.acs.org on August 27, 2018

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ACS Photonics

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Design of 2D layered PtSe2 heterojunction for the

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high-performance room-temperature broadband

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infrared photodetector

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Di Wu,a Yuange Wang,a Longhui Zeng,*,b Cheng Jia,a Enping Wu,a Tingting Xu,a Zhifeng Shi,a

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Yongtao Tian,a Xinjian Lia, and Yuen Hong Tsang*,b

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a

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Education, Zhengzhou University, Zhengzhou, Henan, 450052, China

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b

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University, Hung Hom, Kowloon, Hong Kong, China

School of Physics and Engineering, and Key Laboratory of Material Physics of Ministry of

Department of Applied Physics and Materials Research Center, The Hong Kong Polytechnic

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KEYWORDS: PtSe2, 2D layered materials, heterojunction, infrared photodetector, CdTe

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ABSTRACT: The rich variety and attractive properties of two-dimensional (2D) layered

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nanomaterials provide an ideal platform for fabricating next generation of advanced

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optoelectronic devices. Recently, the newly discovered 2D layered PtSe2 thin film has exhibited

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outstanding broadband sensitivity and optoelectronic properties. In our work, a large-area 2D

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layered PtSe2 thin film was used to construct the PtSe2/CdTe heterojunction infrared

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photodetector (PD). This PD exhibited a broad detection range coverage from 200 nm to 2000

ACS Paragon Plus Environment

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nm with a high responsivity of 506.5 mA/W, a high specific detectivity of 4.2×1011 Jones, a high

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current on/off ratio of 7×106, and a fast response speed of 8.1/43.6 µs at room-temperature.

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Additionally, the PtSe2/CdTe heterojunction PD exhibits excellent repeatability and stability in

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air. The high-performance of the PtSe2/CdTe heterojunction PD demonstrated in this work

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reveals that it has great potential to be used for broadband infrared detection.

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In the past decade, two-dimensional (2D) layered materials have attracted extensive research

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interests due to their unique structure and properties, such as atomically thin, natural layer-

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dependent band-gaps, strong optoelectronic properties, broadband optical absorption, high

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conductivity and stable due to free chemical dangling bonds at the surface etc.1-3 These inherent

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advantages of 2D layered materials make them become excellent candidates for optoelectronic

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applications, especially for photodetectors (PDs), that convert optical signals to electrical signals,

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and are widely applied in imaging, sensing, optical communication, environment monitoring,

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military, and industrial automatic control etc.4-8 The PDs with broadband detection ranged from

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visible to infrared are highly demanded in the fields of military, security and industry.9-11

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Therefore, many research efforts have been explored to achieve the high-performance infrared

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broadband PDs.5-6

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Currently, most commercial available infrared photodetectors are based on some

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semiconductor materials with narrow band-gap, such as Si, Ge, InGaAs and so on.12-13 The

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detection range of such detectors is strictly restricted by the band-gaps of these materials, such as

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1.1 µm of Si and 1.7 µm of InGaAs. Moreover, these infrared PDs have several drawbacks,

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including complicated fabrication processes, low operating temperature, and large size, which


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greatly restrict their applications.5 Hence, much efforts have been devoted to overcome these

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problems to further enhance the infrared detector performance.14-15 Exploring new infrared

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sensitive materials and device structures is an effective strategy for achieving high-performance

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infrared PDs.16-23

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As a member of group-10 transition metal dichalcogenides (TMDCs), the recently discovered

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2D layered PtSe2 thin film has been demonstrated to have excellent electrical and optoelectrical

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properties with high carrier mobility and layer-dependent band-gaps.24-26 The PtSe2 has a band-

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gap of 1.2 eV for monolayer, 0.21 eV for bilayer and it will become semimetal with increasing

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layer number.27 This unique broad band-gap range of PtSe2 makes it be an excellent candidate

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for designing broadband visible to infrared PDs.28 However, the ultrathin thickness of 2D layered

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materials will result in a low absorption to incident light, leading to a small photocurrent, large

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dark current and low specific detectivity.5 Hence, 2D PtSe2 thin film heterojunction structure is

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proposed to further enhance its photodetector performance. CdTe, an important II-VI group

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semiconductor, possesses a direct band-gap (~1.5 eV), which could be mainly sensitive to near-

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infrared (NIR) light.29-30 Therefore, the construction of PtSe2 heterojunction with CdTe can

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enhance the absorption of NIR light, as well as the formation of a built-in electric field in the

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heterojunction can improve the response speed of PD. It’s expected that the PtSe2/CdTe

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heterojunction PDs will have high-performance for infrared detection.

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In this work, the PtSe2/CdTe heterojunction was constructed for infrared photodetection, which

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demonstrated a wide spectrum response range from deep ultraviolet (DUV, 200 nm) to NIR

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(2000 nm). Further investigation reveals that the PtSe2/CdTe heterojunction PD has high

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sensitivity to infrared signals. It also exhibits a high responsivity, specific detectivity, current

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on/off ratio (Ion/Ioff) and fast response speed at room temperature (RT) in the infrared range. It


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suggests that 2D layered PtSe2 thin film has great potential for high-performance broadband

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infrared PDs.

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Experimental Section

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The 2D layered PtSe2 thin films were prepared by selenization of sputtered Pt films in a

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chemical vapor deposition (CVD) system. The experimental details have been presented in our

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previous work.31 To construct a PtSe2/CdTe heterojunction device, the PMMA supported PtSe2

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film by wet-etching from SiO2/Si substrate was transferred onto a CdTe crystal (5×3×0.5 mm).

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The crystal has a resistivity of ~1×106 Ω cm-1 and it is purchased from MTI Corporation. Then,

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two electrodes (Au) were defined on PtSe2 thin film and CdTe crystal by a thermal evaporation.

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The silver wires were connected to the device via high-purity silver paste for electrical and

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optoelectrical measurements.

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The as-synthesized PtSe2 films were analyzed by Raman spectrometer (LabRAM HR

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Evolution, HORIBA), X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250), and

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energy-dispersive spectroscope (EDS, Oxford instruments). The thickness of the PtSe2 film was

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obtained by an atomic force microscope (AFM, Dimension Icon, Bruker).

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Electronic and optoelectronic properties of the heterojunction device were measured at RT

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with an optoelectronic characterization system combining a Keithley 4200-SCS (Tektronix,

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USA), a digital SourceMeter (Keithley 2636B, Tektronix), a monochromator (Omni-λ300, Zolix),

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light sources, an oscilloscope (DPO2012B, Tektronix) and a waveform generator (SDG1032X,

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Siglent).

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Results and Discussions


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PtSe2 crystal is a typical 1T-type hexagonal crystal structure.28 The thickness of as-synthesized

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2D layered PtSe2 films was confirmed to be ~22 nm according to AFM measurement in Figure

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1a. The Raman spectrum of PtSe2 samples was depicted in Figure 1b, in which two Raman active

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modes were observed at 176.6 cm-1 and 206.8 cm-1 corresponding to E2g1 and A1g1 modes,

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respectively. The components and binding energies of PtSe2 films were investigated by XPS, as

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shown in Figure 1c-d. The peaks at 71.25 eV and 74.55 eV are corresponding to the Pt 4f7/2 and

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4f5/2, respectively. The XPS data confirms the presence of Pt4+. Similarly, another two peaks at

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54.6 eV and 55.4 eV are related to Se 3d5/2 and Se 3d3/2 orbitals, respectively. The atomic ratio of

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Pt/Se was obtained to be 1:2.08, which is close to the stoichiometric ratio of PtSe2. The XRD

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result (Figure S1) shows the peaks at 16.98°, 27.86°, 32.9°44.34° 48.98°, and 52.38° can be

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assigned to the (001), (100), (101), (102), (110) and (111) crystal planes of PtSe2, respectively.

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The crystal structure and element mapping of the PtSe2 samples was investigated by

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transmission electron microscopy (TEM), as shown in Figure 2. From the TEM images, the

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polycrystalline PtSe2 layer with a vertically standing layered structure can be observed, which

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can enhance the absorption of incident light.31-32 The high-resolution TEM (HRTEM) image

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(inset of Figure 2b) reveals that a layer distance along (001) is approximately 0.52 nm. From the

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results of EDS element mapping as shown in Figure 2c-e, both Pt and Se are uniformly

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distributed in the observed zone, suggesting that high-quality PtSe2 film was obtained.

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Figure 3a shows a schematic diagram of device structure. Figure 3b exhibits the I-V

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characteristics of the PtSe2/CdTe heterojunction PD in dark and under light illuminations with

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different wavelengths. From the I-V curve in dark, a typical rectifying curve with a rectifying

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ratio of 2.2×102 within ±20 V was obtained, indicating that the heterojunction formed between

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PtSe2 and CdTe, because Au electrodes can provide good Ohmic contacts with PtSe2 and CdTe


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(Figure S3). When the PD is under light illumination, the reverse current of the PD has a

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remarkable increase. The current at -20 V greatly increased from 46 pA in dark to 1.95 nA at

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1550 nm (6.37 mW/cm2), 22.5 nA at 265 nm (2.4 mW/cm2), 3.02 µA at 980 nm (58.2 mW/cm2)

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and 277 µA at 780 nm (54.7 mW/cm2), respectively, resulting in a high Ion/Ioff. Therefore, the

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real-time photoresponse to the light of 780 nm under varied voltage biases were investigated, as

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shown in Figure 3c, which exhibited stable and repeatable photoresponse to the switching of 780

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nm light with Ion/Ioff of 7×106, 5.3×106, 3.3×106 and 8.9×105 under bias voltages of -20 V, -15 V,

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-10 V and -5 V, respectively. It's worth noting that CdTe crystal and PtSe2 film have

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photoresponse to 780 nm with Ion/Ioff of 4.5 and 1.05, respectively (Figure S4). Moreover, the

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repeatability of PtSe2/CdTe heterojunction PD was investigated. A laser diode of 780 nm (54.7

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mW/cm2) driven by a signal generator provided an incident light at a frequency of 500 Hz. The

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PD was applied a voltage bias of -20 V, and an oscilloscope monitored the photocurrent of PD

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with time. From Figure 3d, the PtSe2/CdTe heterojunction PD can maintain its photoresponse

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property with no degradation after five thousand consecutive response cycles, suggesting an

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excellent performance stability of the PtSe2/CdTe heterojunction PD.

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The I-V curves under light illumination with different wavelengths have revealed the

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PtSe2/CdTe heterojunction PD has a broad photoresponse spectrum from DUV to NIR. Hence,

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the response spectrum range of this PD was carried out, as shown in Figure 4a. This PD exhibits

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multiband photoresponse in the range of 200-2000 nm with the highest sensitivity around 825

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nm, which is directly associated with inherent band-gap of CdTe (Figure S2). Furthermore, the

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photoresponse properties of PtSe2/CdTe heterojunction PD to incident light with different

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wavelengths were investigated, as shown in Figure 4b-e, in which this PD demonstrated

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pronounced, stable and repeatable photoresponse to light signals of 200 nm (2.2 µW/cm2), 980


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ACS Photonics

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nm (58.2 mW/cm2), 1550 nm (6.37 mW/cm2) and 2000 nm (20.1 µW/cm2), respectively,

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confirming that PtSe2/CdTe heterojunction PD can response light signals with a wide spectrum

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range from DUV to NIR. Significantly, a remarkable photoresponse to the light of 200 nm with

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an Ion/Ioff ratio of 10 can be observed at a weak light intensity of 2.2 µW/cm2, representing this

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PD is capable of detecting weak signals. The above photoresponse of the PtSe2/CdTe

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heterojunction PD can be illuminated from the energy-band diagram, as shown in Figure 4f. The

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multilayer 2D PtSe2 film could be considered as a semimetal.24 Once PtSe2 and CdTe are

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contacted with each other, electrons would spread to CdTe from PtSe2, whereas holes would

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spread toward opposite directions, eventually leading to the formation of a built-in electric field

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between PtSe2 and CdTe. When the heterojunction was illuminated, the absorption of photons

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will generate excitons. Afterward, the excitons will be rapidly separated by the built-in electric

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field and collected by the Au electrodes, leading to an increase of photocurrent. Moreover, when

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the device is under a reverse bias, the external electric field can effectively enhance the

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generation and separation efficiency of the photogenerated electron-hole pairs, resulting in a rise

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of photocurrent and fast response speed.

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To quantify the effect of light intensity on the device performance, the I-V characteristics of

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PtSe2/CdTe heterojunction PD were measured under light illumination of 780 nm with power

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intensities ranging from dark to 54.7 mW/cm2. The results are shown in Figure 5a, which shows

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the current increased quickly with the increased light intensity. The photocurrent (Iph) as a

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function of light intensity was plotted in Figure 5b, which is consistent with the power law

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(Iph=APθ) with good linearity. Hence, the responsivity (R) and specific detectivity (D*) of this

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PD can be obtained based on the formulas as follow:


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R( A / W ) =

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D *( Jones) =

I ph

Popt ⋅ S

A⋅R 2eI D

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(1)

(2)

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where Popt, S, A, e and ID are light intensity, irradiation area, device area, unit charge and dark

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current, respectively. The responsivity and specific detectivity as a function of light intensities

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(780 nm) were presented in Figure 5c. Both responsivity and specific detectivity increase with

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the increasing light intensities, and reach 506.5 mA/W and 4.2×1011 Jones (1 Jones =1 cm Hz1/2

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W-1) under a light intensity of 54.7 mW cm-2, respectively. The photoresponse under varying

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light intensities was also explored, as depicted in Figure 5d. The reversible and stable

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photoresponse with tunable Ion/Ioff ratios from 1.2×102 (25 µW/cm2) to 7×106 (54.7 mW cm-2)

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could be obtained.

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The response speeds and frequency response were investigated to further study the

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performances of PtSe2/CdTe heterojunction PD via a measurement system, as shown in Figure

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6a. A laser diode of 780 nm driven by a signal generator can provide incident light with tunable

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frequency. And an oscilloscope monitors the photocurrent of PD with time. The frequency

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response of 1 kHz, 10 kHz and 50 kHz were exhibited in Figure 6b-d, revealing stable and

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reversible response to fast varying light signals with a wide frequency range. According to the

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relative balance [(Imax-Imin)/Imax] shown in Figure 6e, this PtSe2/CdTe heterojunction PD can

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response the signal with frequency over 105 Hz, suggesting that such PtSe2/CdTe heterojunction

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PD can detect ultrafast optical signals. Moreover, a high 3 dB frequency (f3dB) of 6.2 kHz can be

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obtained. The response speed of this PD was evaluated by analyzing the rising and falling edges


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of individual response cycle, as shown in Figure 6f and Figure S5, from which the response

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speeds were obtained to be 8.1/43.6 µs at 10 kHz and 10/156.4 µs at 1 kHz. Significantly, the

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stability of PtSe2/CdTe heterojunction PD was further investigated with results shown in Figure

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6b and Figure S6. This PD can maintain its initial device performance even after storage in the

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air over three months. The current of the PD at a bias voltage of -20 V in dark and under light

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illumination of 780 nm can keep stable over 2×104 s. These results confirmed that such

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PtSe2/CdTe heterojunction PD is highly-air-stable.

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To further explore the capability of this PD to response the ultrafast varying signal, a 266 nm

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pulse laser with a pulse width of 1 ns and pulse energy of 12 µJ at 1 kHz was used as light source.

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The response properties of PtSe2/CdTe heterojunction PD to 266 nm pulse laser were

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demonstrated in Figure 7. It can be found that PtSe2/CdTe heterojunction PD can quickly

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response each pulse with an ultrafast rising time of 160 ns. The performances of PtSe2/CdTe

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heterojunction PD demonstrated in this work are superior to the reported 2D-based NIR PDs, as

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summarized in Table 1.

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Conclusion

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In summary, the large-area 2D layered PtSe2 thin films were synthesized, and the PtSe2/CdTe

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heterostructure was constructed for broadband infrared detection. This heterojunction device has

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demonstrated the multiband detection ranging from deep UV (200 nm) to near infrared (2000 nm)

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with a high responsivity of 506.5 mA W-1, specific detectivity of 4.2×1011 Jones, current on/off

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ratio of 7×106, as well as fast response speeds of 8.1/43.6 µs at RT. Meanwhile, the as-fabricated

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PD has exhibited excellent repeatability, stability and the capability of detecting weak signals.

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Furthermore, this infrared PD can response well to the pulse signal with a pulse width of 1 ns.


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The results demonstrated in this work suggest that 2D layered PtSe2 thin films will have great

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potential applications in high-performance broadband RT infrared PDs.

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ACS Photonics

FIGURES

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Figure 1. (a) AFM image of PtSe2 and its corresponding height profile. (b) Raman spectrum of

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the synthesized PtSe2 film. The XPS spectra show the binding energies of (c) Pt and (d) Se of the

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PtSe2 film.


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Figure 2. (a)-(b) TEM images of PtSe2 film under different magnifications. (c) STEM image of

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the PtSe2 film. (d)-(e) The EDS element mapping of PtSe2 film.


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Figure 3. (a) A schematic diagram of device structure. (b) Current-voltage characteristics of the

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PD in dark and under a light of 265 nm, 780 nm, 980 nm and 1550 nm, respectively. (c) Time-

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dependent photoresponse of the PtSe2/CdTe heterojunction PD under different bias voltages. (d)

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Five thousand photoresponse cycles of the PtSe2/CdTe heterojunction PD.

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Figure 4. (a) Spectral response of the PtSe2/CdTe heterojunction PD. The inset shows a

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photograph of the PD. Time-dependent photoresponse of PtSe2/CdTe heterojunction PD to light

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of (b) 200 nm, (c) 980 nm, (d) 1550 nm and (e) 2000 nm, respectively. (f) The energy band

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schematic of the PtSe2/CdTe heterojunction PD under light illumination.

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Figure 5. (a) I-V characteristics of the PtSe2⁄CdTe heterojunction PD at varying light intensities.

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(b) The photocurrent as a function of light intensity. (c) The responsivity and specific detectivity

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of the PtSe2⁄CdTe heterojunction PD as a function of light intensity. (d) Time-dependent

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photoresponse of the PD at different light intensities.

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Figure 6. (a) The schematic diagram of the configuration for frequency response measurement.

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The photoresponse properties of the PtSe2/CdTe heterojunction PD to the light of 780 nm with

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frequencies of (b) 1 kHz, (c) 10 kHz and (d) 50 kHz, respectively. (e) The relative balance [(Imax-

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Imin)/Imax] as a function of switching frequency. (f) The enlarged single response for estimating

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the response speeds at 10 kHz.

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Figure 7. (a) The photoresponse of PtSe2/CdTe heterojunction to 266 nm pulse laser at 1 kHz

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with a pulse width of 1 ns and pulse energy of 12 µJ. (b) Falling and rising edges of one

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response.

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Table 1. A summary of the device performances of PtSe2/CdTe heterojunction PD and some

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reported NIR PDs. Devices

λ [nm]

Ion/Ioff

Responsivity [mA W-1]

D* [Jones]

Rise/fall time [µs]

Ref.

PtSe2/CdTe

200-2000

7×106

506.5

4.2×1011

8.1/43.6

This work

PtSe2/GaAs

200-1200

104

262

1012

5.5/6.5

33

MoS2/CdTe

200-1700

3×104

36.6

6.1×1010

43.7/82.1

21

MoS2/Si

200-1100

60

11.9 A/W

2.1×1010

30.5/71.6

34

Graphene/Ge

1200-1600

104

51.8

1010

23/108

35

WS2/Si

340-1100

10

5.7 A/W

/

670/998

36

8 9 10

ASSOCIATED CONTENT Supporting Information


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The following files are available free of charge.

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The XRD patterns of the synthesized PtSe2 film, absorption spectrum of CdTe, electrical

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contacts and photoresponse of CdTe and PtSe2, response speed of PtSe2/CdTe heterojunction PD

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at 1 kHz, and stability of PtSe2/CdTe heterojunction PD (PDF).

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AUTHOR INFORMATION

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Corresponding Author

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*Longhui Zeng, E-mail: [email protected]

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*Yuen Hong Tsang, E-mail: [email protected]

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Notes

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The authors declare no competing financial interest.

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ACKNOWLEDGMENT

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This work was financially supported by the National Natural Science Foundation of China (Nos.

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61605174, 61774136 and 11604302), the Key Projects of Higher Education in Henan Province

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(No. 17A140012) and Research Grants Council, University Grants Committee (RGC, UGC)

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(GRF 152109/16E).

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REFERENCES

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(36) Lan, C.; Li, C.; Wang, S.; He, T.; Jiao, T.; Wei, D.; Jing, W.; Li, L.; Liu, Y. Zener Tunneling and Photoresponse of a WS2/Si van der Waals Heterojunction. ACS Appl. Mater. Interfaces 2016, 8, 18375-18382.

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Table of Contents

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The PtSe2/CdTe heterojunction was designed to achieve high-performance infrared

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photodetector. This photodetector exhibited a broadband detection range coverage from 200 nm

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to 2000 nm with a high responsivity of 506.5 mA/W, a high specific detectivity of 4.2×1011

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Jones, a high current on/off ratio of 7×106, as well as a fast response speed of 8.1/43.6 µs at

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room-temperature.


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Design of 2D Layered PtSe2 Heterojunction for the High-Performance

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