Electrical Rectifying and Photosensing Property of Schottky Diode

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Functional Inorganic Materials and Devices

Electrical Rectifying and Photosensing Property of Schottky Diode Based on MoS2 Jing-Yuan Wu, Young Tea Chun, Shunpu Li, Tong Zhang, and Daping Chu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b06078 • Publication Date (Web): 04 Jul 2018 Downloaded from http://pubs.acs.org on July 8, 2018

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ACS Applied Materials & Interfaces

Electrical Rectifying and Photosensing Property of Schottky Diode Based on MoS2 Jing-Yuan Wua,b,c,‡, Young Tea Chunb,‡, Shunpu Lib, Tong Zhanga,c,d,*, and Daping Chub,* a

Joint International Research Laboratory of Information Display and Visualization, School of

Electronic Science and Engineering, Southeast University, Nanjing, 210096, P. R. China b

Centre for Photonic Devices and Sensors, University of Cambridge, Cambridge, CB3 0FA,

United Kingdom c

Suzhou Key Laboratory of Metal Nano-Optoelectronic Technology, Suzhou Research Institute

of Southeast University, Suzhou, 215123, P. R. China d

Key Laboratory of Micro-Inertial Instrument and Advanced Navigation Technology, Ministry

of Education, School of Instrument Science and Engineering, Southeast University, Nanjing, 210096, P. R. China Corresponding Author *Email: [email protected] *Email: [email protected] ‡

These authors contributed equally to this work.

KEYWORDS: molybdenum sulfide, Schottky diodes, asymmetric electrodes, dynamic electrical rectification, photosensing property ABSTRACT: Heterojunction based on two-dimensional (2D) layered materials is an emerging topic in the field of nanoelectronics and optoelectronics. Here, molybdenum sulfide (MoS2)based Schottky diodes were fabricated using the field-effect transistor configuration with asymmetric metal contact structure. Gold and chromium electrodes were employed as drain and

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source electrodes to form ohmic and Schottky contact with MoS2 respectively. The devices exhibited electrical rectifying characteristic with the current rectifying ratio exceeding 103 and an ideal factor of 1.5. A physics model of the band diagram was proposed to analyze the gatetunable rectifying behavior of the device. The dynamic rectification based on the diode circuit was further realized with the operating frequency up to 100 Hz. The devices were also demonstrated to show different sensitivities to the light under external biases in the opposite directions, with the highest photoresponsivity reaching 1.1×104 A/W and specific detectivity up to 8.3 ×1012 Jones at a forward drain bias of 10 V. This kind of 2D material based Schottky diodes have the advantage of simplicity in the design and fabrication, as well as superior electrical rectifying and photosensing characteristic, which have great potential for future integrated electronic and optoelectronic applications. 1. INTRODUCTION Two-dimensional (2D) layered materials, such as graphene, black phosphorus, molybdenum sulfide (MoS2) and other transition metal dichalcogenides (TMDCs), have emerged as promising candidates for the next generation of electronic and optoelectronic devices due to their unique properties, including mechanical flexibility, strong light-matter interaction and electronically tunable characteristic.1–5 Constructing heterojunctions, including p-n diode and Schottky diode, provide an effective approach for the practical applications of these 2D materials, especially for realizing modern semiconductor devices. The junctions consisted of 2D materials exhibit some new properties in terms of band alignment, charge transport and optical absorption compared with the single material, allowing for novel electronic and optoelectronic applications.6,7 In recent years, researchers are paying great attention to the p-n junction based on 2D materials.8–12 Different strategies, such as chemically doping one side of semiconductor,9 local electrostatic


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gating10 and vertically stacking different ultrathin 2D materials,13 have been used to form 2D materials based p-n junction and diode, applying to integrated electronics11 and photodetection.14 However, the research focused on the metal-semiconductor Schottky junction fabrication and application based on TMDCs is lacking. The Schottky diode is promising for various practical applications, for instance sensors, integrated circuits and photovoltaic devices,15–17 thanks to its simplicity in the design and fabrication compared with p-n diodes. The barrier at the metal-semiconductor interface is critical for the performance of Schottky diode.18 In term of early work on 1D nanostructure based Schottky diode, such as carbon nanotubes and ZnO nanowires, the Schottky barrier induced by the difference between the work function of metal and the Fermi level of semiconductor is considered to be the key factor which determines the performance of the diode.19–21 N-type semiconductor would form Schottky junction with metal of high work function and form ohmic junction with metal of low work function respectively, while p-type semiconductor displays the opposite characteristics. Therefore, by choosing the metal with suitable work function, the Schottky diode based on 1D nanomaterial could be well designed. However, for the TMDCs-metal contact, the interface condition is more complex. Because 2D layered material has no dangling bond on the surface, additional tunnel barrier induced by the weak van der Waals interaction between the deposited metal and semiconductor should also be taken into consideration when analyzing the metalsemiconductor contact.22,23 On the basis of understanding the contact interface and charge carrier transport between MoS2 and different metals, the Schottky junction based functional devices could be better designed with purposes. Here, we fabricated MoS2 field-effect transistor (FET) with asymmetric metal contacts (AuCr), investigating the metal-semiconductor interfaces with different charge carrier transport


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properties. Taking advantage of the tunable carrier transport and sufficient optical absorption characteristic of multilayer MoS2, we demonstrated the outstanding electrical rectifying and photosensing performance of the Schottky diode based on MoS2. As the Au electrode forms good ohmic contact while the Cr forms Schottky junction with MoS2 channel, the Schottky diode based on the asymmetric electrodes showed remarkable current rectification characteristic with rectifying ratio exceeding 103 by controlling the applied gate bias. We further demonstrated the dynamic rectifying application of the Schottky diode with operation frequency reaching 100 Hz for the first time. Besides, we showed that the device displayed high specific detectivity under forward biases due to the reduced dark current, with the highest detectivity reaching 8.3 ×1012 Jones. These results suggest a novel and effective approach to the application of MoS2 and other 2D materials towards electronics and optoelectronics, including logic rectifiers and photosensors, using Schottky junctions. 2. RESULT AND DISCUSSION Figure 1a shows the schematic diagram of the MoS2 transistor structure with asymmetric electrodes (Au-Cr). We also fabricated transistors with symmetric electrodes for performance comparison. The optical microscopic (OM) images of the transistor with different electrode configurations are shown in Figure 1b and 1c respectively. Here, we chose multilayer MoS2 as the channel of the transistors because of its better manufacturability and higher carrier mobility compared with single-layer counterpart device.24,25 The Raman spectrum of multilayer MoS2 and the OM images of another MoS2 transistor in contact with only one electrode and two electrodes are shown in the supporting information (Figure S1). We further measured the electrical performance of the devices with Au-Cr and Au-Au electrode configurations. In terms of the transfer curves, both devices exhibited n-type characteristics with on/off ratio reaching 106


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(Figure 1d, f). However, the device with Au-Cr contacts exhibited almost ten times smaller current than that with Au-Au contacts due to the larger barrier at the interface between MoS2 and Cr. The carrier mobility was then calculated according to the expression: µ =

dIDS dVG

×

L WCOX VDS

(1)

Figure 1. (a) Schematic 3D view of the transistor based on MoS2 channel with asymmetric electrodes (Au-Cr). Optical microscopic images of the device with (b) asymmetric electrodes (Au-Cr) and (c) symmetric electrodes (Au-Au). The bar is 5 µm. Transfer curve at VDS = 0.1 V


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(d) and output curves (e) of FET with Au-Cr electrodes. Transfer curve at VDS = 0.1 V (f) and output curves (g) of FET with Au-Au electrodes. where L, W and COX are the channel length, channel width and the capacitance per unit area to the gate dielectrics, respectively. The field effect mobility of the device with Au-Cr electrode configuration was calculated to be 4.1 cm2/V·s while the mobility of the device with Au-Au electrodes reached 39.8 cm2/V·s both at a drain voltage of VDS = 0.1 V. The Semi-logarithmic scale output curves further reflect the difference of carrier transport of the devices. As shown in Figure 1e, the output curves of the device with asymmetric electrodes showed clear rectifying behavior, especially at low gate biases. This is due to the large barrier between Cr and MoS2. With the increase of gate bias, the forward current gradually increased due to the thinning of the barrier. In comparison, the output curves of the device with symmetric Au electrodes kept symmetric at different gate biases, illustrating the good ohmic contact between Au and MoS2. We also measured the transfer curve and output curves of the transistor with symmetric Cr electrodes (Supporting information Figure S2). Compared with the electrode configuration of Au-Au and Au-Cr, it shows the smallest current due to the largest barrier existed at the interface between MoS2 and both two electrodes. The mobility of the device with the symmetric Cr-Cr electrodes was calculated to be 0.56 cm2/V·s.

Figure 2. (a) The drain current change as a function of gate voltage under forward and reverse


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bias of 5 V, respectively. (b) The rectifying ratio as a function of the gate voltage calculated from (a). (c) The linear and semi-logarithmic scale IDS-VDS characteristic in source-drain electrode configuration with back gate disconnected. We further investigated the performance of the Schottky diode. Figure 2a shows the gate bias-tuned current under forward and reverse source-drain bias, which values are extracted from Figure 1e. The IDS displayed different changing trend with gate bias at VDS = -5 V and 5 V, respectively. Under forward bias, the device displayed the behavior of n-type transistor, while it turned to be non-semiconducting under reverse biases, which is opposite to the p-type carbon nanotube based Schottky diode.19 We calculated two important parameters, including current rectification ratio and ideality factor, to characterize the performance of the Schottky diode device. The current rectification ratio is defined as the ratio of reverse and forward current at the same source-drain bias magnitude. 26 The rectifying ratio could be tuned from 7 to 2000 by changing the gate bias (Figure 2b). Then we measured the I-V curve of the device with gate disconnected. The result showed that the device is still working as a diode in a two-electrode configuration with the ideality factor at about 1.5 (Figure 2c). The large rectification ratio and small ideality factor reflect the good diode characteristic of our devices, making them promising for the application of nanoelectronic integrated circuits. We also conducted the temperature-dependent transport measurement to experimentally extract the Schottky barrier height (SBH) of Cr and Au with MoS2, respectively. Figure 3a shows the transport characteristic of MoS2 with symmetric Cr electrodes in the temperature range of 100 K - 340 K. We noted that when the gate bias is larger than about 18 V, the resistance reduces with decreasing temperature. The result is consistent with the previously reported metal-insulator transition (MIT) of the channel material.4,27,28 According to the traditional thermionic theory, we


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used the equation27,29,30 IDS =Adevice A* T2 exp -

φB T

q kB

VDS n

(2)

where Adevice is the area of the detector, A* is the Richardson’s constant, q is the elementary electron charge, kB is the Boltzmann constant, T is the temperature, VDS is the applied sour-drain bias and n is the ideality factor. Figure 3b shows the Arrhenius plot of the device at various gate overdrive voltages. From the slope of lnIDS /T2 and 1000/T (Figure 3b) and based on the flatband condition, we calculated the SBH of Cr and MoS2 was about 535 meV. Similarly, we extracted the SBH of Au was around 90 meV. Therefore, we chose Au and Cr to construct different interfaces with MoS2 to realize the Schottky diode with rectifying function.

Figure 3. (a) Temperature-dependent measurement of MoS2 transistor with symmetric Cr electrodes. (b) Arrhenius plot for the device at various gate overdrive voltages, VG-Vth. (c) Extracted SBHs of the device with symmetric Cr electrodes (black points) and Au electrodes (blue points), respectively. To understand the current rectifying mechanism of FET with asymmetric electrodes, combining with the measured SBHs, we proposed a physics model of the band diagrams of the Au/MoS2/Cr structure as shown in Figure 4. Figure 4a shows the reported work function of MoS2, Cr and Au before contact. In fact, Au has a large work function around 5.1 eV, but it has been reported that the interface between MoS2 and Au is strongly affected by the Fermi level


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pinning closed to the conduction band of MoS231,32 (Figure 4b). Our experimentally extracted SBH of 90 meV between Au and MoS2 also matched well with this phenomenon. Therefore, Au is widely used as the metal electrode contacted with MoS2 channel to obtain the low-resistance contact in reported transistors.33,34 While for Cr, in addition to the Schottky barrier, the influence of tunnel barrier on the contact should also be considered. Because 2D MoS2 has no dangling bond on the surface, the van der Waals interaction between MoS2 and metal electrode could contribute to a physical separation in the interface between the semiconductor and electrode, which is called tunnel barrier (TB) that has been demonstrated by the ab initio density-functional theory calculation.22,23,35 The TB is partly affected by the lattice difference between metal and semiconductor. It has been reported that Cr has a large lattice mismatch with MoS2, which induces a weak orbital overlap and a large TB.22 In addition, Cr is easily oxidized during the fabrication process, which also makes the TB at the Cr-MoS2 interface larger. While compared with Cr, Au is more stable and has a smaller lattice mismatch with MoS2, contributing to a narrower TB.


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Figure 4. Band diagram of metals and MoS2 before (a) and after (b) forming a contact. Band diagrams for the diode under (c) forward biases and (d) reverse biases. To further illustrate the gate tunable rectification behavior, we plotted the band alignment diagram at different external biases (Figure 4c, d). Because the SBH of Au with MoS2 is much smaller than that of Cr, here we supposed MoS2 and Au form ohmic contact and mainly discussed the barrier characteristic between MoS2 and Cr at different gate biases. When MoS2 and Cr are brought into contact, the conduction band (Ec) and valence band (Ev) of MoS2 will bend to keep the equilibrium between the Fermi level of Cr and MoS2. Meanwhile, the TB exists between Cr and MoS2. The barrier characteristic could be effectively modulated by the gate voltage. Under the forward drain bias, electrons are blocked by the barrier at low gate voltages, leading to a small current. With the increase of gate voltage, the barrier becomes thinned due to the tunneling transport mechanism, which promotes the current flow. In comparison, under the reverse drain bias, because the electrons are arising from the reverse bias, the gate bias has little effect on the carrier transport. As a result, the device displayed non-rectifying characteristic.17,19 The above band alignment analysis could explain the observed experimental phenomenon with high rectification ratio at forward bias and low rectification ratio at reverse bias. Employing the large current rectification ratio as well as good ideality factor of the Schottky diode, we further demonstrated the dynamic rectifying application of the device. According to the previously measured I-V curve, we found that the diode exhibited a current rectification ratio about 100 without applying gate biases. Therefore, here we measured the dynamic rectifying characteristic of the diode in two-electrode configuration, which is convenient for the practical application. Figure 5a shows the Schottky diode circuit scheme with the measurement system, where a function generator was used to apply an input AC voltage (sine waveform or square


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waveform with different frequencies) displayed on a digital oscilloscope, while the input signal went through the Schottky diode by the probe station. By connecting a load resistor of 1 MΩ, we measured the output voltage signal which was also displayed on the oscilloscope. As shown in Figure 5b-e, we observed obvious rectified output signal with the input sine and square waveform of Vin = ± 5V at 1 Hz. The highest operation frequency reached 100 Hz, which is the highest value for MoS2 Schottky diode with asymmetric electrodes to the best of our knowledge.


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Figure 5. Dynamic rectifying demonstration of the Schottky diode. (a) The measurement system diagram for the Schottky diode circuit. Output voltage signal was obtained by applying the input sine waveform at (b) 1 Hz, (c) 100 Hz and square waveform at (d) 1 Hz, (e) 100 Hz of Vin = ± 5V. We then compared the rectifying behavior of our device with other reported dynamic rectifiers based on MoS2 p-n junction.36 We found that the value of output voltage signal of our device under reverse bias was almost ten times higher than the reported one with the same input signal amplitude and external resistance, which demonstrates that our device has superior rectifying performance and the output signal is easier to detect with no need for adding higher external resistance. Moreover, compared with the p-n diode, the Schottky diode exhibits the advantage of simplified fabrication technique and reduced cost, making them more promising for applying in the field of nanoelectronic circuits at radio frequency. However, we noted that when operating reaching 100 Hz, the output current has a small displacement. It may result from the parasitic capacitance induced by the large overlapping area between electrodes and the substrate of heavily doped Si.36,37 To further increase the operation speed, fabricating top-gate structure to reduce the overlapping area, or replacing the Si with the material without back charging as the substrate, for example, glass,11 can be adopted.

Figure 6. The linear output curves of the device with Au-Cr electrode in dark and under light


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illumination at (a) forward VDS and (b) reverse VDS. In addition to the electrical rectifying characteristic, we also investigated the photosensing application of the Schottky diode. Figure 6 shows the linear output curve of the device with asymmetric electrodes in dark and under light illumination at forward and reverse biases respectively. The incident light originates from the green light emitting diode (LED) with the center wavelength at about 519 nm. We observed that the device was more sensitive to the light at forward bias compared with reverse bias condition. We analyzed the reason for the phenomenon in terms of the band diagram. As discussed earlier in Figure 4c and 4d, the Schottky diode with Cr-Au asymmetric electrodes will naturally develop a potential gradient in MoS2. The applied source-drain bias will further change the potential gradient, leading to different efficiencies of the electron-hole separation and carrier injection. We further plotted the band diagrams of metal and MoS2 under light illumination (Supporting information, Figure S3). At forward bias, the dark current is small due to large contact resistance between Cr and MoS2. Upon the illumination, the photo-excited electron-hole pairs are generated in MoS2 due to the band-to-band transition.38 The electron-hole pairs separate and then the photo-generated electrons inject effectively from the conduction band of MoS2 to Au under the forward bias. Therefore, the photocurrent dominates the channel current, contributing to a sensitive photoresponse. However, under the reverse bias applied, the efficiency of the photo-generated carrier separation and injection is lower. The dominating current are thermionic and tunneling current, especially when the device is in the on state. As a result, the photocurrent is almost negligible compared with the channel current in this case. We further calculated the photoresponsivity of the devices according to R=

Iph P

=

|Iillumination - Idark | Pin

·

Aspot Adevice

(3)


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Where Iph is the photocurrent, which is defined as the absolute change in the source-drain current of the device under illumination. P is the effective incident optical power, which can be calculated by P= Pin·Adevice/ Aspot. For the Schottky diode, we plotted the photocurrent change with VDS at different VGS (Supporting information, Figure S4). We found that the photocurrent is proportional to VDS. The largest photoresponsivity is calculated to be 1.1×104 A/W with the VDS of 10 V and Pin of 1.4 mW/cm2. The photoresponsivity at negative VDS under the same incident light illumination condition is about four times smaller than that at positive VDS, with the value of 2.5×103 A/W at VDS = -10 V. We also measured the photosensing performance of the symmetric Au-Au electrode contacted device for comparison. The device with symmetric Au-Au electrode configuration showed similar sensitivity to the light at forward and reverse biases (Supporting information, Figure S5). Then we calculated the photoresponsivity of the Au-Au symmetrical contacted device at the same measurement condition, which value is 2.9×104 A/W. In addition to the photoresponsivity, another figure of merit used to access the detector performance is specific detectivity which is given by D* = R Adevice 1/2/(2qIDS) 1/2

(4)

Where R is the responsivity, Adevice is the area of the detector, q is the elementary electron charge and IDS is the source-drain dark current. We calculated that the specific detectivity of the Cr-Au contacted device reached 8.3 ×1012 Jones, which is higher than that of Au-Au contacted device of 5.8 ×1012 Jones at the same measurement condition (VDS = 10 V, Pin = 1.4 mW/cm2). The higher specific detectivity of the Cr-Au contacted device is owed to the reduced dark current through the larger barrier between Cr and MoS2. Therefore, the MoS2 based Schottky diode device working at suitable biases is promising for realizing practical photo-sensing devices due to its high specific detectivity and sensitivity.


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3. CONCLUSIONS In summary, we have realized a MoS2 based Schottky diode by constructing asymmetric metal contacts (Au-Cr) structure. The electrical measurement indicated that Au forms good ohmic contact with MoS2, while Cr and MoS2 form Schottky contact. According to the Arrhenius plots through temperature-dependent transport measurement, we extracted the SBH value for Cr and Au to MoS2 are 535 meV and 90 meV, respectively. The Schottky diode showed excellent rectifying characteristic with the rectifying ratio varying from 7 to 2000 by changing gate bias, along with the ideality factor of around 1.5. Utilizing the large current rectification ratio, we further demonstrated the dynamic rectifying of the diode with the highest operating frequency reaching 100 Hz. Besides, the output voltage under reverse bias was almost 70% of the input signal, which illustrates that the rectifying signal is easy to detect. We also showed that our device could act as light sensors. Due to the different barrier characteristics at the metal interface, the photoresponsivity at forward bias was about four times higher than that of reverse bias condition. The highest specific detectivity reached 8.3 ×1012 Jones with the VDS of 10 V and effective incident optical power of 0.6 nW because of the reduced dark current. These results indicate that this kind of Schottky diode based on 2D material is a promising candidate as a highperformance diode for future electronics and optoelectronics. 4. EXPERIMENTAL METHODS Schottky Diode Device Fabrication: MoS2 nanoflakes were prepared by the mechanical exfoliation using scotch tape from the crystal (SPI), and transferred on a heavily p doped Si substrate capped with 300 nm SiO2. Then the source electrode (Cr) was patterned by photolithography (Karl Suss MJB4 Mask Aligner), electron beam evaporation (Kurt J. Lesker ebeam evaporator) and acetone lift-off procedures sequentially. The thickness of the electrode was


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set to be 70 nm. We employed oxygen plasma to treat the sample to remove the residue during the processes. After that, we patterned the drain electrode (Au) with the identical procedures of patterning Cr. Finally, the devices were annealed at 250 °C in vacuum for 2 hours to reduce the contact resistance between MoS2 and electrodes. Electrical and Photosensing Measurement: The optical microscopic images were taken with the Nikon Eclipse LV100. The Raman spectrum measurement was carried out using a Raman spectroscopy (Horiba T64000). The electrical characterizations were performed using an Agilent 4156C precision Semiconductor Parameter Analyzer connected with Wentworth probe station. Electrical dynamic rectifying characteristics of the Schottky diode were measured by using a function waveform generator (Aglient 33220A) and an oscilloscope (LeCroy LC584AL). For the photosensing measurement, the green LED with the center wavelength at about 519 nm was used as the incident light source. ASSOCIATED CONTENT Supporting Information This section includes Raman spectrum of MoS2; the optical images of FET with one electrode (Au) and two electrodes (Au-Cr); the transfer curve and output curves of the device with symmetric Cr electrodes; the band diagram of the device with Au-Cr electrode configuration under light illumination; the photocurrent change with drain bias of the device with asymmetric electrodes; the output curve of the device with symmetric Au electrodes in dark and under light illumination. ACKNOWLEDGMENT This work is supported by MOST under Grant Number 2017YFA0205800, NSFC under grant numbers 11734005, 61307066 and 61450110442, the Fundamental Research Funds for the


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Electrical Rectifying and Photosensing Property of Schottky Diode

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