Gr-WS2-Gr Hybrid Photodetectors with High

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Ultrafast Monolayer In/Gr-WS2-Gr Hybrid Photodetectors with High Gain Chao-Hui Yeh, Hsiang-Chieh Chen, Ho-Chun Lin, Yung-Chang Lin, Zheng-Yong Liang, Mei-Yin Chou, Kazu Suenaga, and Po-Wen Chiu ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b09032 • Publication Date (Web): 21 Feb 2019 Downloaded from http://pubs.acs.org on February 23, 2019

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Ultrafast Monolayer In/Gr-WS2-Gr Hybrid Photodetectors with High Gain

Chao-Hui Yeh1, Hsiang-Chieh Chen1, Ho-Chun Lin2,3, Yung-Chang Lin4, Zheng-Yong Liang,1 MeiYin Chou2,3, Kazu Suenaga4, and Po-Wen Chiu1,2,5* 1Department

of Electrical Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan of Atomic and Molecular Sciences, Academia Sinica, Taipei, 10617, Taiwan 3Department of Physics, National Taiwan University, Taipei 10617, Taiwan 4National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba 305-8565, Japan 5Frontier Research Center on Fundamental and Applied Science of Matters, National Tsing Hua University, Hsinchu 30013, Taiwan 2Institute

ABSTRACT One of the primary limitations of previously reported 2D photodetectors is a low frequency response ( 0 when the Fermi level of graphene crosses the Dirac point.40 Figure 6d shows the responsivity as a function of incident power. In general, R tends to decrease slowly with the increase of incident power. This behavior is in line with other TMD photodetectors and has been attributed to the lowering of the built-in field at the In/WS2 interface. The increasing number of photogenerated electrons develops an electric field opposed to the equilibrium built-in field.3,34,37 A high responsivity of 2.6 × 103 A/W can be achieved at a low illumination power density of 0.1 mW/cm2 for the In/Gr-WS2-Gr photodetectors (Figure 6d). We can calculate the device photogain using the relation of 𝐺 = 𝑅ℎ𝑐 𝑒𝜆, where h is the Planck constant, c is the light velocity, and λ is the excitation wavelength. It yields a photogain of 6.3 × 103. This value is similar to that of heterojunction phototransistors based on III-V semiconductors that have been reported in the order of 102 ― 103.41–43 If we take into account the finite absorbance (~8%) of monolayer WS2 under green light illumination, the photogain of In/Gr-WS2-Gr photodetectors can reach 7.8 × 104. The operation of photodetection can be modulated through an electrostatic field to the depletion regime, in which the dark current can be minimized to enhance the detectivity. 9

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For the In/Gr-WS2-Gr photodetectors, the maximum of light-to-dark current ratio appears at a gate voltage when the Fermi level of graphene crosses the Dirac point (Figure 6e). At 𝑉ds = 1 V, the maximum light-to-dark current ratio can reach 104, rendering the device a high-quality sensitive switch. Compared with the n-type PbS-MoS2 quantum dot photodetectors, our In/Gr-WS2-Gr photodetectors exhibit a much higher light-to-dark current ratio by four orders of magnitude.3 To determine the sensitivity of our photodetectors, we measure the noise in the dark current, as shown in Figure 6f. The detectivity, which characterizes a photodetector’s sensitivity by taking into account the electrical bandwidth, geometry, and noise of the device, can be expressed as 𝐷 ∗ = 𝑅𝐴

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𝑆𝑛, where A is the effective area of the detector and 𝑆𝑛 is the noise spectral density

of the detector. To obtain a high 𝐷 ∗ , photodetectors are usually operated in the depletion mode where the noise current is low. However, this will cause a large reduction of responsivity by orders of magnitudes in the WS2-based photodetectors. To retain the high responsivity while keeping the noise level low, the photodetectors can be operated at high frequencies in view of the detectors’ fast response. For example, under a biasing condition of 𝑉ds = 1 V and 𝑉gs = 2 V with a light power density of 0.1 mW/cm2, where the Fermi level is near the Dirac point of graphene and the In/Gr-WS2-Gr photodetector possesses the maximum light-to-dark current ratio of 104, the R and 𝑆𝑛 are 2.5 × 102 A/W and 5 × 10 ―14A Hz at a frequency of 1 kHz, respectively. The 𝐷 ∗ can reach 2.2 × 1012 Jones. The photoconductivity measurements shown above demonstrate the synergistic role played by the In adatoms and graphene, which form a fundamental basis for sensitive photodetection with gain. Figure 7 schematically illustrates the band structures of the three types of WS2 photodetectors. It has been well documented that the metal contact to WS2 layers forms a Schottky barrier. The presence of trap states at the contact interface and modification of the metal work function by the interface dipole cause the pinning of Fermi level down below the conduction band minimum of WS2 (Figure 7a).4,44 A barrier height of hundreds of meV usually exists at the contact interface.45 Contact engineering by graphene has been reported as an effective means to manipulate the Schottky barrier.23 Through an electrostatic gating, the finite density of states near the charge neutrality point of graphene renders the modification of graphene’s work function feasible (Figure 7b).

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Increasing 𝑉gs causes electrons injection into graphene, lowering graphene’s work function and the barrier height accordingly. This explains the monotonic increase of responsivity as the gate voltage increases in the In/Gr-WS2-Gr photodetectors. Upon light illumination, the In adatoms adsorb photons and generate massive electron-hole pairs. The photogenerated electrons transfer from the In adatoms to the WS2 channel due to the internal built-in potential, while the holes remain trapped in the In adatoms. By applying 𝑉ds, the transferred electrons are drifted to the drain within a timescale of 𝜏𝑡𝑟𝑎𝑛𝑠𝑖𝑡. We can calculate this transit 2

time by 𝜏𝑡𝑟𝑎𝑛𝑠𝑖𝑡 = 𝐿𝑐ℎ 𝜇𝑉𝑑𝑠, where 𝐿𝑐ℎ is the channel length (𝐿𝑐ℎ = 1 μm) and 𝜇 is the mobility. This yields a transit time of 6.8 ns for 𝑉ds = 0.1 V and 𝑉gs = 2.5 V (𝜇 = 14.6 cm2 Vs) and 64 ns for 𝑉gs = 0 (𝜇 = 1.6 cm2 Vs). The trapped holes act as a gate and modulate the channel resistance. The recirculation of channel carriers during the residence of trapped charges in adatoms yields the photoconductive gain, as schematically shown in Figure 7c. The fast photoresponse is the most important figure of merit for our In/Gr-WS2-Gr photodetectors. Figure 8a shows the schematic of the experimental setup for the measurements of time-resolved photoresponse. The repeated ON/OFF switches of light illumination are defined by the frequency of an optical chopper. An oscilloscope is used to monitor the time dependence of the photocurrent. The stability of time-resolved photoresponse under a pulsed illumination with a chopper frequency of 450 Hz is shown in Figure 8b. At a fixed bias of 𝑉ds = 0.5 V and 𝑉gs = 0 V, alternating high and low impedance states follow the frequency of the pulsed light. No clear persistent photocurrent is seen on the OFF-state. The time constant of rise and decay responses, defined as the time required to change the signal amplitude between 10% and 90%, are 47 and 62 μs for the illumination wavelength of 550 nm, respectively (Figure 8c). Other wavelength shows similar timescale for rising and falling edges [700 nm: 53 s (rise); 65 s (fall); 940 nm: 54 s (rise); 63 s (fall)]. The temporal response of our In/Gr-WS2-Gr photodetectors surpasses the reported TMD-based photodetectors with metal electrodes.14,16,46,47 From the decay time (𝜏𝑙𝑖𝑓𝑒 = 62 μs) and the transit time (𝜏𝑡𝑟𝑎𝑛𝑠𝑖𝑡 = 64 ns) calculated previously for the illumination wavelength of 550 nm, we can deduce the gain 𝐺 = 𝜏𝑙𝑖𝑓𝑒/𝜏𝑡𝑟𝑎𝑛𝑠𝑖𝑡 = 9.7 ×

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102, in reasonable agreement with the experimental value of 6.0 × 102 under the same biasing conditions. CONCLUSIONS We have demonstrated the fabrication of monolayer TMD photodetectors, in which the active channel WS2 is grown by CVD from the edges of graphene electrodes, forming a Gr-WS2-Gr junction. The WS2 and graphene regions are sensitized and doped with indium adatoms spontaneously in the CVD process. The indium adatoms on WS2 function as a n-type doping source as well as a photogate, effectively lowering the Schottky barrier at the Gr/WS2 contact and boosting the photoresponse upon light illumination. The photoresponsivity of the hybrid In/Gr-WS2-Gr photodetectors increases monotonically with the increase of gate voltage and reaches a record-high value of 2.6 × 103 A/W at 𝑉gs = 2 V for monolayer WS2, with a corresponding gain of 6.3 × 103 electrons per photon and a time constant (rise and decay time) in the range of 40–60 μs. The ultrafast temporal response, especially for the decay time, is attributed to the strong coupling with In adatoms, allowing for rapid transfer of charge carriers between WS2 and In adatoms upon the switch of light. In combination with the graphene electrodes in the 2D photodetectors, the critical contact barrier becomes gate-tunable and light-sensitive, rendering the highly sensitive and ultrafast photodetection feasible. Based-on this device architecture, search for surface adsorbates that also provide a broadband spectrum coverage could hold great promise for future optoelectronic applications that require simultaneously high speed and high sensitivity. METHODS Growth of graphene and WS2 We grow the graphene sheets on polycrystalline Cu foils by means of atmospheric pressure CVD. Diluted methane (80 ppm CH4 in Ar) is used as the carbon source. Prior to the growth, we clean the Cu foils using acetic acid to remove the surface oxides. Then, the Cu foils are loaded into the tube furnace with a steady flow of 15-sccm hydrogen. The temperature at the furnace center is ramped up to 1020 oC in 60 min, followed by an annealing for 30 min at this temperature. In the CVD process, the diluted methane (200 sccm) mixed with argon

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(230 sccm) and hydrogen (15 sccm) were fed into the reaction chamber. The growth lasts for 20 min. After the growth, the Cu foils are cooled down to room temperature under a protective atmosphere. For other details of the graphene growth and clean transfer can be referred to the previously reported literatures.25,38,48–52 The monolayer WS2 sheets are grown in a horizontal clam-shell tube furnace. Tungsten(VI) oxide (WO3, Sigma Aldrich) and sulfur powder (Sigma Aldrich) are used as solid precursors for W and S, with purity of 99.8% and 99.98%, respectively. 1-mg KI (99.5% in purity) is used to promote the growth. To adsorb In atoms on WS2, In2O3 powder (12 mg) are mixed with the WO3 precursor (400 mg) and placed in a partially capped quartz boat near the furnace center. The sapphire substrates, patterned with pre-transferred graphene sheets, are located close to the WO3/In2O3 mixture. The KI grains and sulfur powders are set up at the upstream of the gas flow. The growth is carried out at 850 °C under a base pressure of 0.1 Torr, with a constant gas flow of 200 sccm argon and 15 sccm hydrogen. The temperature at the sulfur and KI quartz boats are set at 165 °C and 600 °C, respectively. After the growth, the heating power is shut down and the clam-shell keeps closing until the temperature is below 50 °C. Device Fabrication The devices are fabricated directly on a sapphire substrate with CVD grown Gr-WS2-Gr junctions. A top gate structure is made on top of the Gr-WS2-Gr junctions. A bilayer stack of polymethyl methacrylate (PMMA) is used as an e-beam resist: 996k PMMA for the bottom layer and a copolymer of methyl methacrylate and methacrylic acid P(MMA-MAA) for the top layer. A T-shape undercut can be created after e-beam patterning. A thin layer of AlOx, with a thickness of 5 nm, is deposited, followed with a deposition of a 100-nmthick Al gate. The devices are sealed in a clean chamber filled with ultrahigh purity oxygen (> 99.999%) at a pressure of 2 atm. A high-quality oxide layer can be formed surrounding the Al gate. Afterwards, a standard e-beam lithography process is applied to define the selfaligned drain/source contacts and external electrode pads. Thermal evaporation is applied for the self-aligned drain/source metallization with Cr/Au (0.5 nm/40 nm). The important gate parameter here is the resulting AlOx capacitance which is determined by a dual-gate

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measurement of a WS2 FET with the known capacitance of silicon back gate with 300 nm surface dry oxide. Optoelectronic Measurements For the optoelectronic measurements, we use a solid-state laser as the light source. The laser spot size is ~1 mm in diameter, which covers the whole area of the devices. The output power is adjustable between 100 µW to 80 mW. The light source is coupled with an optical chopper to define the switching ON/OFF intervals. For 𝐼 ― 𝑉 measurements, the 𝑉ds and 𝑉gs biases are provided through Keithley 2400 source meters, and the generated photocurrent is acquired via a Keithley 2000 multimeter connected to a current preamplifier. The 400 MHz oscilloscope (LeCroy WaveSurfer 44Xs-A) coupling with 1 MΩ AC-input impedance are provided for time-resolved shot/base signal evaluation. The 500 ms timebase was set to record the voltage response via the periodically visual radiation controlled by programming chopper system. Notably, generated photocurrent was converted into voltage by connecting one resistance (1-10 MΩ) in series so as to obtain appropriate resolution scale of corresponding voltage drop via the oscilloscope system, where the alternative voltage change of resistance can be probed using conducting wires with the co-axial cables and careful ground terminal design. Flicker noise measurement was conducted by 1/f noise analyzer (Celestry 9812B) and dynamic signal analyzer (Agilent 35670A) in the shielding box to isolate the interference of exterior noise. STEM characterization and simulation JEOL ARM-200F, which is equipped with a dodecaple Delta corrector, is used to acquire the STEM images presented in this work. The cold field emission gun is operated at 60 kV to avoid damage of graphene and WS2. The probe current is approximately 28-32 pA. The convergence semi-angle is 37 mrad and the inner acquisition semi-angle is 53 mrad. Image acquisition time is 38.5 μs for 1024 x 1024 pixels imaging. Computational method First-principles calculations are performed using the Vienna Ab initio simulation package (VASP)53–55 with the projector augmented wave (PAW) method.56,57 A plane-wave energy cut-off of 400 eV and a 6 × 6 × 1 k-mesh were employed for the structure optimization, 14


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and the density of states (DOS) calculation used a 12 × 12 × 1 k-mesh. The resulting lattice constant was 3.18 Å for the 2H WS2 monolayer, which was fixed in adsorption calculations using a 4 × 4 supercell. Because of the size of the supercell, spin-orbit interaction is not included. We expect that the main conclusion will not be changed, because the major In-induced features are near the conduction band edge, while the spinorbit splitting of the conduction band minimum is only about 30 meV.58 We used the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) functional59 for the structural optimization with a 17Å vacuum separating the atomic layers. For In adsorption, we have considered two different sites: above the metal (m-site) and above the center of the W-S hexagon (h-site). The optimized vertical distance between In and the S layer in WS2 is 2.34 Å and 2.45 Å for M-site and H-site, respectively. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Details of graphene and WS2 characterization using SEM, STEM, EELS, and PL spectroscopy analyses; device characterizations for gate capacitance; electronic band structures of pristine WS2 and In/WS2 complex; TEM Z-contrast analysis of the In adatoms on WS2.

AUTHOR INFORMATION Corresponding Authors E-mail: [email protected] ORCID Po-Wen Chiu: 0000-0003-4909-0310 Kazu Suenaga: 0000-0002-6107-1123 Mei-Yin Chou: 0000-0003-0113-7191 Yung-Chang Lin: 0000-0002-3968-7239 Chao-Hui Yeh: 0000-0002-9437-055X

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Funding This work in Taiwan was supported by the Ministry of Science and Technology of Taiwan under grant No. MOST 107-2119-M-007-011-MY2, MOST 106-2119-M-007-008-MY3, and MOST 106-2628-M-007-003-MY3 as well as from Academia Sinica under AS-TP106-A07. KS and YCL acknowledge the support from JSPS-KAKENHI (JP16H06333) and (18K14119). REFERENCES (1)

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Figure 1. Device schematic of a WS2 phototransistor. (a) A 3D sketch of a top-gated WS2 phototransistor with the WS2 channel grown from the edges of graphene electrodes, forming a Gr-WS2-Gr junction. A T-shape gate is fabricated on the Gr-WS2-Gr junction for the deposition of self-aligned drain/source Au contacts on graphene regions. (b) A cross-sectional view of the device schematic. The indium adatoms adsorbed on the Gr-WS2-Gr junction is used to sensitize and dope the WS2 channel and graphene electrodes. The WS2 channel length and width are, respectively, 1 m and 20 m. The graphene width (between drain/source electrode and WS2) is 2 m. The light is illuminated from the back side of the device.

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Figure 2. CVD growth of the In/Gr-WS2-Gr junction and their optical characterizations. (a) A graphene sheet grown on a Cu foil is transferred onto a sapphire substrate, followed by an oxygen plasma etching to define the channel region. The substrate is loaded into another CVD system to grow WS2 in the channel region, with In adsorption atop. (b) From the left to the right: the first two panels show the optical images of patterned graphene gap and filling of the gap with the CVD-grown WS2. The other three panels sequentially show the spatial mapping of the graphene Raman G mode, WS2 Raman 𝐸12𝑔 mode, and PL of the In/Gr-WS2-Gr junction. (c) Raman spectra of graphene before and after the growth of WS2 channel. (d) and (e) are, respectively, the Raman and PL spectra of the WS2 channel with In adatoms.

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Figure 3. Structure characterizations of the Gr/WS2 heterostructure. (a) TEM image of the Gr/WS2 heterostructure interface. The inset shows the corresponding FFT image where the two sets of diffraction spots indicate the structure of WS2 (yellow circles) and graphene (red circles). (b) STEM image of the Gr/WS2 heterostructure interface with In adatoms. (c) STEM image of the In/WS2 complex. The In adatoms, which are highlighted by the green circles, are randomly dispersed on the WS2 lattice with a dopant concentration of ~0.45 at%. (d)-(f) ADF images of In adatoms (indicated by the green arrows) are located on top of W, S, and H (hollow center) sites, respectively. (g) EELS spectrum of a single In adatom on WS2.

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Figure 4. (a)-(c) XPS spectra of WS2 grown along the edge of graphene, with In adatoms adsorbed on surface. (d) Side and top views of two chemisorption configurations for the indium atoms on WS2. (e) Projected DOS for the pristine WS2, in comparison with the M-site and H-site In-adsorbed WS2.

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Figure 5. 𝑰 ― 𝑽 characteristics of the field-effect transistors. (a) and (c) are the 𝐼 ― 𝑉ds curves for the Gr-WS2-Gr and In/Gr-WS2-Gr FETs, respectively. (b) and (d) are the 𝐼 ― 𝑉gs curves for the Gr-WS2-Gr and In/Gr-WS2-Gr FETs, respectively.

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Figure 6. Photoinduced current and dark current of the Au-WS2-Au, Gr-WS2-Gr, In/ Gr-WS2-Gr photodetectors. (a) The 𝐼 ― 𝑉ds curves in the dark and illuminated states. (b) The 𝐼 ― 𝑉ds curves under illumination with different wavelength for the In/Gr-WS2-Gr photodetector. (c) Responsivity as a function of top gate voltage in a semi-logarithmic scale. (d) Responsivity as a function of light power density. (e) Light-to-dark current ON/OFF ratio as a function of gate voltage for the In/Gr-WS2-Gr photodetector. (f) Flicker noise current spectral at a fixed drain bias and various of gate voltages. All the measured flicker noise current densities show a linear dependence on the frequency in the double logarithmic plot, proportional to the slope of 1/f.

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Figure 7. Energy band diagrams of WS2 photodetector with different device configurations under light illumination. (a) The band diagram of the device with Au metal contacts. EF, EC, EV, and B stand for the Fermi level energy, conduction band minimum, valence band maximum, and Schottky barrier, respectively. (b) The band diagram of the device with graphene contacts. (c) The band diagram of the device with graphene contacts and weakly chemisorbed indium adatoms. The band diagrams schematically illustrate the In-induced photogating effect.

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Figure 8. Temporal response time of the In/Gr-WS2-Gr photodetectors. (a) The experimental setup for the measurement of time-resolved photoresponse under a pulsed illumination. (b) The time-resolved photoresponse of the device in ambient condition at a fixed bias of 𝑉ds = 0.5 V and 𝑉gs = 0 V for the wavelength of 550, 700, and 940 nm. The power density of light is 𝑃 = 10

mW/cm2. The photoresponse for the wavelength of 700 nm and 940 nm are equally upshifted by 0.2 nA for clarity. (c) A close-up look of a single On/Off switch of the device for the wavelength of 550 nm.

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Table of Contents Graphic

Top gate AlOx / Al

Drain

Source

h+ e⎻

e⎻

h+

In atom

WS2 Sapphire

Light


Graphene

Gr-WS2-Gr Hybrid Photodetectors with High

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