Si van der Waals

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Zener tunneling and photoresponse of WS/Si Van del Waals Heterojunction Changyong Lan, Chun Li, Shuai Wang, Tianying He, Tianpeng Jiao, Dapeng Wei, Wenkui Jing, Luying Li, and Yong Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b05109 • Publication Date (Web): 28 Jun 2016 Downloaded from http://pubs.acs.org on July 3, 2016

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ACS Applied Materials & Interfaces

Zener tunneling and photoresponse of WS2/Si Van del Waals Heterojunction ‡

‡

Changyong Lan†, Chun Li†, * , Shuai Wang†, Tianying He†, Tianpeng Jiao , Dapeng Wei , §

§

Wenkui Jing , Luying Li , Yong Liu† †

State Key Laboratory of Electronic Thin Films and Integrated Devices, School of

Optoelectronic Information, University of Electronic Science and Technology of China, Chengdu 610054, China ‡

Chongqing Key Laboratory of Multi-scale Manufacturing Technology, Chongqing Institute of

Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing 400714, China §

Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and

Technology, Wuhan 430074, China KEYWORDS 2D crystals; Van der Waals heterostructures; WS2; Zener diode; Photodetector

ABSTRACT

Van del Waals heterostructures built up of two-dimensional materials on a conventional semiconductor offer novel electronic and optoelectronic properties for the next generation information devices. Here, we report that by simply stacking a vapor phase synthesized

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multilayer n-type WS2 film onto a p-type Si substrate, a high-responsivity Zener photodiode can be achieved. We find that above a small reverse threshold voltage of 0.5 V, the fabricated heterojunction exhibits Zener tunneling behavior which was confirmed by its negative temperature-coefficient of the breakdown voltage. The WS2/Si heterojunction working at Zener breakdown regime shows a stable and linear photoresponse, a broadband photoresponse ranging from 340~1100 nm with a maximum photoresponsivity of 5.7 A/W at 660 nm, and a fast response speed of 670 µs. Such a high-performance can be attributed to the ultrathin depletion layer involved in the WS2/Si p-n junction, on which a strong electric field can be created even with a small reverse voltage and thereby enabling an efficient separation of the photo-generated electron-hole pairs.

INTRODUCTION

The spectacular success on graphene opens the door of two-dimensional (2D) materials

1-2

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They provide an attractive atomic-scale material platform for atomic-scale science and engineering, hold great promise for solving fundamental scientific and technological challenges311

. Among them, graphene-like layered crystals share the common feature with intra-layer

covalent bond and inter-layer van der Waals bond. The lack of surface dangling bond not only allows them to be easily exfoliated down to atomic-scale thickness but also can be readily integrated into heterostructure with atomically sharp interface even for highly lattice mismatched materials. Since heterojunction is a key building block for high-speed and photonic devices, the van del Waals heterostructure is highly expected for novel electronics and photonics with enhanced performances or unprecedented functionalities12-15.


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In the past few years, atomically thin transition metal dichalcogenides (TMDCs), an important subset of 2D crystals, have drawn a lot of attention due to their fascinating electronic, photonic and optoelectronic properties that distinct from and complementary to graphene7,

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Mechanically exfoliated monolayer semiconducting TMDCs have shown promising applications in field effect transistor18, photodetectors19-21, electroluminescent devices22, valleytronics23-25, spintronics26, memristors27, piezotronics28, and superconductors29. More recently, progress on preparing large-area few layer TMDC films by chemical vapor deposition (CVD)30-32 enables high-quality polycrystalline film with single domain size of tens of micrometers33. In addition, the sophisticated transfer techniques allow them to be easily transferred onto other substrates34. These efforts greatly facilitate the scalable implementation of heterojunction optoelectronics made by 2D material themselves or by TMDCs lie on a conventional bulk material. Among those reported MoS2 on conventional semiconductors such as MoS2/Si35-37 , MoS2/InP38, MoS2/InGaZnO39, MoS2/GaTe40, etc, MoS2/Si is the most widely investigated heterojunction considering its compatibility with the Si-integrated devices. However, to our knowledge, as a similar but a different 2D crystal, there is no report on WS2/Si heterojunctions for either electronic or optoelectronic devices. Here, we report a high-performance Zener photodiode based on a WS2/p-Si van der Waals heterojunction. The photodiode was fabricated by simply stacking a vapor phase grown multilayer WS2 film onto the surface of a p-type silicon substrate. Further systematical investigation on the electronic and photosensing properties reveals that the photodiode working at Zener tunneling regime has a photoreponsivity superior to the commercial available Si photodiode41-42 and a very stable photoresponse compared with the MoS2/Si45 avalanche photodiodes. The linear photoreponse with light illumination in the tested range (0-365.5


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µW/cm2), broadband photoresponse from 340 to 1100 nm, and fast response time of 670 µs imply its potential application for high-performance photodetectors. RESULTS AND DISCUSSION Before device fabrication, Raman and TEM were employed to characterize the as-synthesized WS2 film. The Raman spectrum of the film is shown in figure 1a. Peaks belonging to E21 g and A1g phonon modes of WS2 can be observed, which are located at 353.6 and 418.5 cm-1,

respectively. The difference between the two modes is 64.9 cm-1, indicating a multilayer of WS2. For further investigation of the film crystallinity, TEM measurements were carried out. Figure 1b shows a typical TEM image of the WS2 film, the film has a uniform contrast except the folded area, which suggests its uniformity. A typical high-resolution TEM (HRTEM) image of the WS2 film is shown in figure 1c. The presence of Moiré pattern indicates different layers rotated with each other, which is always observed in multilayer WS2 made by sulfurization of WO3 or W film21, 43. The ring shaped selected area electron diffraction (SAED) pattern shown in the inset of figure 1c suggests the polycrystalline nature of the WS2 film. A typical HRTEM image given in figure 1d showing the edge of the WS2 film confirms that the film thickness is about 4~5 layers. Next, we studied the electronic properties of the WS2/Si heterojunctions. The current-voltage (I-V) curve of the device (see the schematic of the device in figure 2a) in dark state is shown in figure 2b. Under forward bias, the device shows a typical diode-like rectify behavior. When applied a small reverse bias larger than the breakdown (onset) voltage of 0.5 V, the reverse current increases dramatically as shown in the inset of figure 2b. Generally, if a breakdown voltage is less than 4Eg/q, the breakdown can be assigned to Zener tunneling44. Here, Eg is the bandgap of the semiconductor and q is the charge of electron. Considering the bandgaps of WS2 (Eg=1.4~2.0 eV) and Si (Eg=1.1 eV), the value of 4Eg/q for WS2 and Si is 5.6~8 and 4.4 V,


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respectively. Therefore, such a room-temperature reverse breakdown can be attributed to Zener tunneling. It is also known that the Zener breakdown usually occurs in a p-n junction diode with a heavy doping or thin junction44. In our case, the depletion layer width is sufficiently thin so that the field on the depletion layer is very strong, leading to band-to-band tunneling process44. We believe the ultrathin thickness of the WS2 should be the reason for the Zener breakdown. To further confirm the breakdown mechanism, I-V curves with varied temperatures were measured and the typical I-V curves are depicted in figure 3a. With a forward bias, the device shows typical rectify behavior for all the tested temperatures. When applying a reverse bias at the temperature below 213 K, we find that except for the Zener tunneling in the small voltage range between (-4 to -0.5V), another abrupt current-increases occur at large voltage range between -14 to -8V associated a clear temperature-dependent breakdown voltage variation (figure 3b). These breakdown voltages are apparently much larger than the above-mentioned breakdown voltage criteria of Zener tunneling (4Eg/q). We can divide the voltage region into to two regimes, as shown in the enlarge drawings of the typical I-V curves in figure 3b and 3c, respectively. For each regime, the breakdown voltage as a function of temperature is displayed in figure 3d. For the regime with high breakdown voltage (-14 to -8V), we find that the breakdown voltage increases as the increase of temperatures (figure 3b and 3d), suggesting a positive temperature coefficient. Therefore, such kind of breakdown has can be assigned to avalanche breakdown44. In contrast, for the regime with small breakdown voltage (-4 to -0.5V), the breakdown voltage decreases as the increase of temperature (figure 3c and 3d), and the breakdown becomes more obvious at an elevated temperatures as indicated in the inset of figure 3d. This means the breakdown happened in this small voltage region is a Zener breakdown since the breakdown originated from Zener tunneling has a negative temperature-coefficient44, which


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further confirms that the device indeed works as a Zener diode at room-temperature under reverse bias below 10 V. In addition, the device working in the Zener regime has a more stable current than that in the avalanche regime for all the tested temperatures (figure 3b and 3c). This is mainly because the avalanche multiplication original from collisional ionization of charge carriers, while Zener effect is essentially resulted from tunneling mechanism. Therefore, Zener tunneling is more suitable for the photodetectors which require a stable photocurrent. It should be noted that the breakdown voltage of avalanche is not very stable compared with the Zener breakdown, especially at low temperatures as shown in figure 3d. Such instability of the avalanche breakdown may be attributed to the low possibility of impact ionization at low temperature. It also should be noticed that since the avalanche breakdown has a positive temperature coefficient, higher avalanche breakdown voltage can be observed if we enlarge the sweeping voltage range. But a larger applied voltage may lead to an unrecoverable breakdown of the p-n diode. It is known that Zener diode usually functions as a voltage regulator while seldom is used as a photodetector. However, interestingly, we find that our Zener tunneling diode also exhibits decent photodetecting performance. Under a forward bias, the device shows almost no photoresponse (figure 4a), which indicates the photoconductive effect can be ignored in our device. While under a small reverse bias larger than the breakdown voltage, the device shows strong response to light illumination (figure 4b). As the external field increases, the rate of electron-hole pair separation and the quantum efficiency increases until all the photo-generated charge carriers contribute to photocurrent before recombination, and consequently saturation occurs. The photocurrent, defined as the current difference between dark state and light illumination, shows linear increase with the increase of illumination-light intensity (figure 4c),


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which means the photoresponsivity of the heterojuction is almost constant for the monochromatic light (5.5 A/W for 650 nm light). This linear relationship between photocurrent and light intensity also indicates that the recombination loss of photo-generated carriers in the depletion layer is negligible45. To further investigate the response behavior of the device, the time-dependent current under chopped light illumination was also measured as shown in figure 4d. The switching behavior of the current with light on and off can be clearly seen. The steep rise and fall edges indicate a fast response speed of the device. To further investigate the photoresponse stability, we measured the current under constant light illumination with different bias voltage as shown in figure 4e. We find that the current is very stable for all the bias voltages, which is different from the avalanche diode made from MoS2/p-Si46, Such a good reproducibility of the photocurrent suggests the high stability of the device, indicating the low noise feature of our device. In order to determine wavelength response characteristic, the spectral response in the range of 340~1100 nm was measured as shown in figure 4f. The device shows wide response to the light ranging from near violet to near infrared with the largest responsivity of 5.7 A/W around 660 nm. This broadband spectral response of WS2/Si photodiode is similar to that of MoS2/p-Si due to the similarity between WS2 and MoS2.36,

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Note that such a photoresponsivity is already one order of

magnitude larger than that of typical commercial Si photodiode (0.3~0.8 A/W)41-42 Because the responsivity can be expressed as R = η G ⋅

qλ , where η is the external quantum efficiency, G is hc

the photo-gain, q is the electronic charge, and λ is the wavelength of incident light, h is the Planck’s constant, c is the velocity of light in vacuum. Clearly, the maximum gain is located at 660 nm with the value of 10.65 assuming the external quantum efficiency is unity. We note that photodiodes with gain have been observed in many Van der Waals heterostructures, such as


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MoS2/Si35,

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, GaTe/MoS248, and the reason may be attributed to the increased lifetime of

photogenerated carriers that are separated by the built-in electric field 48. As one of the key figures of merit for a photodetector, response speed of WS2/Si heterojuction photodetector was measured by using an optical chopper to generate pulsed light with varied frequencies. The schematic illustration of the measurement is shown in figure 5a. The voltage variation on the resistance, which was monitored by an oscilloscope, can be used to represent the variation of photocurrent in the photodiode. Figure 5b-5d show the time-dependent normalized photocurrent with different frequency pulsed light illumination. The photodiode exhibits excellent stability and reproducibility at different frequency. However, as the increase of frequency, the photocurrent does not drop to zero as shown in figure 5d, indicating the device has a high-frequency limit. In order to determine the high-frequency limit of the photodiode, the relative balance was measured (see figure 5e), which is defined as (Imax-Imin)/Ip-max. Imax is the largest current, Imin is the minimum current, and Ip-max is the photocurrent under continuous light illumination. The relative balance decreases by 25% when the frequency increases to 600 Hz. This means the 3dB bandwith of the device is 600 Hz. In time domain, the speed of a photodiode is often characterized by the rise time trise and decay time tdecay. The time needed for the current to increase from 10% to 90% of the peak value or vice versa is defined as the rise time and decay time, respectively. As shown in the figure 4c and 5f, the rise time and decay time is 670 and 998 µs, respectively. Because the response time of a diode is related to the capacitance of the p-n junction, the smaller capacitance, the shorter response time will be. Since the capacitance is proportional to the junction area, we expect that the WS2/Si heterojunction with a smaller junction size would have a shorter response time, implying potential application for high-speed photodetectors.


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Finally, we note that compared to the reported MoS2/p-Si36, MoS2/p-InP38, no distinguishable photovoltaic effect was observed in our WS2/p-Si diode. To illustrate the photoresponse mechanism of our device, the band alignments of the heterojunction under different bias voltages are depicted in figure 6. First, we note that the van der Waals gap can act as a tunneling barrier for charge carriers49. Considering the wet transfer technique involved, a thin natural oxide layer on the surface of Si may also exist. Therefore, a small barrier may exist between WS2 and p-Si as shown in figure 6. Since the depletion layer for a diode locates at the junction, this small tunneling barrier should be in the depletion layer. With zero bias, an build-in potential exists in the depletion layer. Under light illumination, electron-hole pairs are generated, and they can be separated by this internal electric field in the depletion layer. However, due to the existence of above-motioned small tunneling barrier, both electrons and holes can hardly go across the barrier (see figure 6a), and most of the generated electron-hole pairs recombine. As a result, no photocurrent generated by photovoltaic effect is observed with zero bias. For forward bias, because of the upward bending of conductance band and downward bending of valance band (see figure 6b), the photo-generated electron-hole pairs can not be separated. Therefore, no photoresponse can be also observed in the forward bias. Also due to the existence of the tunneling barrier, the turn-on voltage is relatively high (>5 V). In contrast, under reverse bias, the electric field in the depletion layer is enhanced and the band offset between WS2 and p-Si increases as shown in figure 6c. The photo-generated electron-hole pairs in the depletion layer can be readily separated by the enhanced electric field, cross the barrier, and be collected by electrodes, leading to obvious increase of current under illumination. Since the depletion layer is very thin, even a small reverse bias can create a significantly strong electric field in the depletion layer. Consequently, the photo-generated electron-hole pairs can be efficiently separated, which


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in turn results in the linear dependence of photocurrent on light intensity (figure 4c). Therefore, the high responsivity of the heterojunction above reverse Zener breakdown voltage can be rationally attributed to the very efficient separation of photo-generated electron-hole pairs in the ultrathin depletion layer, which is rooted in the ultrathin thickness of multilayer WS2 and also unique charge carrier concentration of the both two semiconductor components.

CONCLUSIONS In summary, we report that by simply stacking a vapor phase grown multilayer planar WS2 film onto a p-type silicon substrate, a photodiode based on WS2/p-Si van der Waals heterojunction can be achieved. The photodiode working at Zener breakdown regime shows high photodetecting performance with a broadband photoresponse ranging from near ultraviolet to near infrared light (340-1100 nm), a maximum photoresponsivity of 5.7 A/W at 660 nm, and a fast response speed of 670 µs. Such a high performance can be attributed to the thin depletion layer in the photodiode, which leads to large electric field and thereby facilitate the photo-carrier transport. The Zener tunneling WS2/Si heterojunction photodiode shows great potential application for high-sensitivity Si-integrated photonic devices. EXPERIMENTAL DETAILS Multilayer WS2 was prepared by sulfurization of a W film at a high temperature. Briefly, a thin layer of 2 nm W film was deposited on SiO2/Si substrate by a radio frequency magnetron sputtering technique. After that, the W film was inserted into the center of the heating zone of a horizontal tube furnace. S powder (200 mg) was loaded into the position of upper gas flow stream in furnace. Before heating, the tube was pumped down to 0.1 Pa by a mechanical pump. Then, high purity Ar gas was introduced into the tube with a flow rate of 50 sccm. The pressure


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inside the tube was kept at 200 Pa during the reaction. The S powder was heated by a heating belt with a temperature of 120 oC. The furnace was heated to 600 oC in 30 min and kept at that temperature for 1 h. After sulfurization, the film was transferred to a SiO2 (300 nm)/p-type Si (100) substrate with a square window (2×2 mm2) where the SiO2 layer was etched

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conductivity of the p-type Si is about 5 Ω·cm. Finally, the silver paste served as electrode was coated onto the surface of WS2 film. The WS2 was characterized by a Raman (Andor, SR-5001-A-R) spectrometer excited by 532 nm laser and a transmission electron microscopy (TEM, FEI Titan G2 60-300) technique. The photo-sensing behavior of the device was measured by a Agilent B2902A source unit. For the measurement of spectrum photoresponse, monochromatic illumination was provided by a Zolix Omni-300 monochrometer with a Xe light source (150 W). For the light intensity response measurement, a 650 nm laser diode was employed as light source. An attenuator was used in order to obtain light with different intensity. The illumination power was measured using a Newport 1935C power meter. In order to measure the time response, a bias voltage of -9 V was applied and a resistor with a resistance of 95 kΩ was connected in series with the device. An oscilloscope (Tektronix, TDS 220) was used to monitor the variation of voltage on the resistor, which can be transformed into current in the circuit. The laser beam was mechanically chopped using an optical chopper (Stanford Research, SR540) to investigate the time-dependent photoresponse. All the photo-sensing properties were measured in a home-made vacuum probe station.


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FIGURES

Figure 1. (a) Raman spectrum of WS2. (b) TEM image of the WS2. (c) and (d) HRTEM image of WS2. The inset of (c) shows the SAED of WS2 film.


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Figure 2. (a) Schematic of the device. (b) I-V curve of the diode in dark state at roomtemperature under a wide bias range (from -15 to 15 V) . Inset of (b) shows enlarged plotting of I-V curve with the applied voltage ranging from -12 to 12 V, from which the current rapid increase can be clearly seen once the revise bias larger than the threshold breakdown voltage of 0.5 V.

Figure 3. (a) I-V curves at different temperatures. (b) Enlarged I-V curves showing the avalanche breakdown regime and (c) the Zener breakdown regime, respectively. (d) Avalance and Zener breakdown voltages as a function of temperature. The inset of (d) shows the I-V curves at elevated temperatures.


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Figure 4. (a) Forward I-V curves of the device under 650 nm light illumination with different light intensities. (b) Reverse I-V curves of the device under 650 nm light illumination with different light intensities. (c) Photocurrent as a function of light intensity with a bias voltage of 5 V. (d) I-t curve with bias voltage of -5 V under chopped light illumination (650 nm) with light intensity of 365.5 µW/cm2. (e) I-t curve with different bias voltage under 650 nm light illumination with light intensity of 292.4 µW/cm2. (f) Responsivity as a function of wavelength.


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Figure 5. (a) Schematic illustration for the photoresponse speed measurement Time response characteristics of the device under light illumination (650 nm, 365.5 µW/cm2) with different chopping frequencies. (b) 50 Hz, (c) 200 Hz, and (d) 800 Hz. (e) Frequency dependent of relative balance value ((Imax-Imin)/Ip-max). (f) Magnified part of (c), showing the rise time and fall time of the device. The photocurrent has been normalized.

Figure 6. Band alignments of the WS2/p-Si heterojunction under different bias voltages. (a) Under zero bias, a small potential barrier for electrons and holes is located in the depletion layer as displayed in the figures, which is originated from the van del Walls contact and natural silicon oxide layer in the contact interface, the photo-generated electron-hole pairs can not be separated. (b) Under forward bias, the photo-generated electron-hole pairs also can not be separated


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because of the upward bending of conductance band and downward bending of valance band. Therefore, almost no photovoltaic effect was observed. (c) Under reverse bias, when the bias voltage is large enough, photo-generated electron-hole pairs can be separated efficiently, electrons and holes will drift to WS2 and Si, respectively, and collect by electrodes, leading to photocurrent.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. Acknowledgement This work was supported by the Natural Science Foundation of China (No. 61421002, 61106040 and 61475030), the Program for New Century Excellent Talents in University (No. NCET-13-0092), the State Key Laboratory of Electronic Thin Film and Integrated Device Program (No. KFJJ201408), and the Central University Basic Scientific Research Business Expenses (No. ZYGX2015Z001). REFERENCES AND NOTES


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Table of Contents Graphic


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Si van der Waals

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