Letter pubs.acs.org/NanoLett
Ultrafast Broadband Photodetectors Based on Three-Dimensional Dirac Semimetal Cd3As2 Qinsheng Wang,† Cai-Zhen Li,‡ Shaofeng Ge,† Jin-Guang Li,‡ Wei Lu,† Jiawei Lai,† Xuefeng Liu,† Junchao Ma,† Da-Peng Yu,‡,§ Zhi-Min Liao,*,‡,§ and Dong Sun*,†,§ †
International Center for Quantum Materials, School of Physics and ‡State Key Laboratory for Mesoscopic Physics, School of Physics, Peking University, Beijing 100871, P. R. China § Collaborative Innovation Center of Quantum Matter, Beijing 100871, P. R. China S Supporting Information *
ABSTRACT: Photodetection with extreme performances in terms of ultrafast response time, broad detection wavelength range, and high sensitivity has a wide range of optoelectronic and photonic applications, such as optical communications, interconnects, imaging, and remote sensing. Graphene, a typical two-dimensional Dirac semimetal, has shown excellent potential toward a highperformance photodetector with high operation speed, broadband response, and efficient carrier multiplications benefiting from its linear dispersion band structure with a high carrier mobility and zero bandgap. As the three-dimensional analogues of graphene, Dirac semimetal Cd3As2 processes all advantages of graphene as a photosensitive material but potentially has stronger interaction with light as a bulk material and thus enhanced responsivity. In this work, we report the realization of an ultrafast broadband photodetector based on Cd3As2. The prototype metal−Cd3As2−metal photodetector exhibits a responsivity of 5.9 mA/W with a response time of about 6.9 ps without any special device optimization. Broadband responses from 532 nm to 10.6 μm are achieved with a potential detection range extendable to far-infrared and terahertz. Systematical studies indicate that the photothermoelectric effect plays an important role in photocurrent generation. Our results suggest this emerging class of exotic quantum materials can be harnessed for photodetection with a high sensitivity and high speed (∼145 GHz) over a broad wavelength range. KEYWORDS: Dirac semimetal, photodetector, ultrafast, photothermoelectric effect, topological materials
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time11 is promising for a high speed response approaching terahertz operation speed.12,13 The gapless bandstructure allows for challenging low energy photon detection14 down to THz frequency,15−17 and the efficient carrier multiplications can enhance the internal quantum efficiency.18,19 Consequently, the emergency of stable 3D Dirac semimetal Cd3As2 provides outstanding opportunity as a new class of material platform for optoelectronics. Experimental studies on Cd3As2 so far mainly focus on the tranport and angle-resolved photoemission spectroscopy (ARPES) measurements to confirm the 3D Dirac semimetal
hree-dimensional (3D) Dirac semimetal Cd3As21,2 is a stable compound with an ultrahigh carrier mobility up to 9 × 106 cm2 V−1 s−1 (refs 3−5), as a result of suppressed backscattering of high Fermi velocity 3D Dirac fermions.3 The high carrier mobility of Cd3As2 surpasses suspended graphene and any bulk semiconductors, making Cd3As2 promising for new-generation electronics and optoelectronics with supreme performances.6,7 Compared to the two-dimensional graphene6−9 and surface states of topological insulator,10 3D Dirac semimetals are more robust against environmental defects or excess conductive bulk states.5 On the other hand, 3D Dirac semimetals possess all advantages of 2D Dirac semimetals as photosensitive materials, which is inherent from the gapless linear dispersion relation of massless Dirac fermions. The extremely high mobility3 and ultrafast transient © XXXX American Chemical Society
Received: September 29, 2016 Revised: January 17, 2017 Published: January 18, 2017 A
DOI: 10.1021/acs.nanolett.6b04084 Nano Lett. XXXX, XXX, XXX−XXX
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Nano Letters
Figure 1. Schematic of photocurrent measurement and Cd3As2 sample characterization. (a) Schematic of scanning photocurrent measurement of Cd3As2 nanowire and (b) nanoplate devices. (c) SEM image of Cd3As2 nanowires. (d) Optical image of a typical Cd3As2 nanowire device. (e) Measured I−V curve and (f) photocurrent mapping of a Cd3As2 nanowire device. The photoexcitation power is 10 μW at a wavelength of 532 nm. (g) SEM image of Cd3As2 nanoplates. (h) Optical image of a typical Cd3As2 nanoplate device. (i) Measured I−V curve and (j) photocurrent mapping of a Cd3As2 nanoplate device. The photoexcitation power is 25 μW at 800 nm. Both the I−V curves and the photocurrent responses are measured at room temperature. The red solid rectangulars in d and h indicate the area where the photocurrent is measured, and the black and white dashed lines in f and j indicate the outline of the electrode and sample, respectively.
respectively. The Cd3As2 nanowires (Figure 1c) and nanoplates (Figure 1g) were synthesized by chemical vapor deposition (CVD) method.22,29 The lateral metal−Cd3As2−metal devices based on individual Cd3As2 nanowires (Figure 1d) and nanoplates (Figure 1h) were fabricated for photodetection. The diameters of the nanowires range from 100 to 200 nm, and the channel length of a typical device shown in Figure 1d is about 7−8 μm. Transmission electron microscopy (TEM) image shows the nanowire surface has 2−5 nm amorphous oxide layer.22 The nanoplate sample also has an oxide layer, but it is relatively sparser compared to nanowire. The linear current−voltage curve shown in Figure 1e indicates the resistance of the nanowire device is ∼5.37 kΩ. The nanoplates are typically 200 nm thick with a length of 5−15 μm and a width of 3−4 μm. A typical nanoplate device shown in Figure 1h has a source−drain resistance of 606 Ω (Figure 1i), an order smaller than that of nanowire device. Both the nanoplates and the nanowires are grown along the (112) direction according to the selected area electron diffraction patterns (Supplementary Figure S1). At room temperature, the transport of nanowire is electron dominant with electron mobility μe = 2.7 × 102 cm2/ (V s); while at low temperatures, the mobilities for electrons and holes are μe = 1.8 × 103 cm2/(V s) and μh = 2.2 × 102 cm2/ (V s), respectively.29 The carrier mobility of nanoplate is about
phase and its related electronic behaviors near the Fermi level, such as giant magnetoresistance (MR), nontrivial quantum oscillations, and Landau level splitting under a high magnetic field.4,20−22 Although the exceptional properties are theoretically expected, the photonic and optoelectronic response of 3D Dirac semimetal is largely unexplored experimentally.23 Here, we report the photodetectors based on the metal−Cd3As2− metal structure and characterize the performance of the photodetectors with spatially resolved scanning photocurrent microscopy (SPCM) and time-resolved photocurrent spectroscopy (TRPC). An ultrafast (∼6.9 ps) photocurrent (PC) response is observed in Cd3As2 device through time-resolved PC measurement, which is ascribed to the fast transient relaxation time of photoexcited carriers.24,25 The fast response time facilitates the hot carriers’ dominant photothermoelectric effect in PC generation similar to that in graphene.13,26 Combining with their broad spectrum response and reasonable responsivity, these photodetectors may find a wide range of photonic applications including high-speed optical communications, interconnects, imaging, remote sensing, surveillance, and spectroscopy.27,28 Figure 1 shows the schematic of PC measurement and sample characterization. The schematics of Cd3As2 nanowire and nanoplate device are shown in Figure 1a and b, B
DOI: 10.1021/acs.nanolett.6b04084 Nano Lett. XXXX, XXX, XXX−XXX
Letter
Nano Letters
Figure 2. Photoresponse of Cd3As2 nanowire based photodetectors. (a) Optical micrograph of a Cd3As2 nanowire device, where the red box indicates the scanning area of photocurrent measurement. (b) Scanning reflection micrograph of the Cd3As2 nanowire device. Dashed line indicates the edge of electrodes and Cd3As2. (c−f) SPM images of the Cd3As2 nanowire device with excitation powers of 5, 60, 70, and 500 μW at 532 nm, respectively. Dashed lines mark the profile of the device. (g) Line cut of photocurrent response along the centerline of the nanowire under different excitation powers. Dashed lines mark the contact edges. The scale bar is 5 μm in all figures.
an order higher than that of the nanowire, that is, μe = 0.38 × 104 cm2/(V s) at room temperature, while at low temperatures, the mobilities for electrons and holes are μe = 1.3 × 104 cm2/(V s) and μh = 9 × 102 cm2/(V s), respectively.30 Figure 1f and j shows the room temperature SPCM images of a nanowire and nanoplate devices with a relatively low excitation power, respectively. While the photoresponse of the nanowire device is along the whole nanowire, the photocurrent of the nanoplate device is mainly at the interface of the Cd3As2−metal junctions. To study the PC generation mechanism of Cd3As2 photodetectors, excitation powerdependent measurements were performed on the nanowire (Figure 2) and nanoplate devices (Supplementary Figure S2). For nanowire devices, the power dependence of PC responses show different patterns along the nanowire under two different excitation power ranges: under low excitation power (
