Si Heterojunction

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Promoting photosensitivity and detectivity of the BiSi heterojunction photodetector by inserting a WS2 layer Jiandong Yao, Zhaoqiang Zheng, Jianmei Shao, and Guo Wei Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b08677 • Publication Date (Web): 12 Nov 2015 Downloaded from http://pubs.acs.org on November 16, 2015

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ACS Applied Materials & Interfaces

Promoting Photosensitivity and Detectivity of the Bi/Si Heterojunction Photodetector by Inserting a WS2 Layer

Jiandong Yao, Zhaoqiang Zheng, Jianmei Shao & Guowei Yang* State Key Laboratory of Optoelectronic Materials and Technologies, Nanotechnology Research Center, School of Materials Science & Engineering, School of Physics & Engineering, Sun Yat-sen University, Guangzhou 510275, Guangdong, P. R. China. *Corresponding author: [email protected]

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Abstract Layered transition metal dichalcogenides (TMDs) have been proven to be essential building blocks for the high-performance optoelectronic devices due to their favorable bandgaps, extraordinary light absorption as well as closed surface electronic structures. However, the in-depth exploration of their operating mechanism as insertion layers in heterojunction photodetectors is scarce. Here, we demonstrate that a Bi/Si heterojunction photodetector can achieve a superior performance by inserting a WS2 layer. A high photosensitivity of 1.4 × 108 cm 2 /W and an outstanding detectivity of 1.36 × 1013 cm ∗ Hz1/2 W -1 are obtained, which are comparable or even surpass those of state-of-art commercial photodetectors. The working mechanism of the Bi/WS2/Si sandwich-structured photodetector is unveiled, including the efficient passivation of the interface, enhancement of light absorption as well as selective carrier blocking. Finally, a good voltage tunability of the photo response is also demonstrated. These findings are significant to the deep understanding on the integration of layered TMDs with conventional semiconductors, and they provide an attractive methodology to develop layered TMDs in a multi-junction system.

Keywords. 2D materials, Bi, WS2, heterojunction, photodetector.

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Introduction On account of its abundant attributes including the strong build-in electric field, the short carrier diffusion path and the large active area, the vertical heterojunction configuration is among the most essential cornerstones of extensive optoelectronic applications such as the photodetection, solar cells, optical communication and light-emitting diodes (LEDs).1-4Graphene has been widely used as optical windows to form Schottky junction based devices,5 which is benefit from its high transmittance6 and easily tunable Fermi level7. However, the conductivity of graphene is quite low,8 prejudicing the efficient transport of photogenerated carriers and thus resulting in poor device performances.5 Bi is an emerging semimetallic material with an exceptionally large mobility far surpassing those of traditional materials.9 It has been known for years that an insulating bulk as well as highly conductive surfaces coexist in a Bi film.10 Recently, systematic magnetotransport measurements have demonstrated that Bi is a new kind of topological insulator (TI). Consequently, the transport of its surface is robust against nonmagnetic perturbation such as surface oxidation and impurity scattering.11-12 Such kind of materials are very suitable for efficient carrier transport and collection.13 For example, a stable and fast photodetector from ultraviolet to Terahertz is successfully achieved by constructing a TI/Si heterojunction.14 However, it suffers low photocurrent and responsivity (17 mA/W), which is still insufficient to guarantee practical needs. Generally, there are several reasons for the poor device performances of a heterojunction photodetector. Firstly, conventional semiconductors such as Si and

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GaAs typically suffer large density of surface dangling bonds due to the interruption of lattice periodicity at the boundaries.15-16 Besides, the mismatch of the lattice constants between two stacked materials could cause deformation and thus introduce additional interface states.17 These defect states act as recombination centers, which seriously limit the lifetime of the photogenerated carriers.18 Moreover, they result in notorious Fermi level pinning at the middle of a semiconductor’s bandgap.19 It will significantly lower the build-in electric field that facilitates charge separation, thus degrading the photoresponse. Finally, the undesirable tunneling effect originated from the thin depletion layer is another challenge for a heterojunction photodetector.20 The tunneling carriers can recombine with their counterparts, leading to extra recombination loss. Therefore, special strategies, such as chemical modification, dielectric passivation and carrier blocking, have to be conducted to improve the electronic quality of the interface.8, 21-23 Semiconducting transition metal dichalcogenides (TMDs), a family of layered materials consist of stacked covalently bonded X-M-X (M = Mo, W, Nb, Ta, Ti, Re and S = S, Se, Te) molecular layers held together via weak van der Vaals interaction, exhibit great potential in various fields such as piezoelectricity,24 sensing25 and valley tonics.26 In the past few years, they have also carved out significant inroad into the heterojunction photoelectrical applications. For example, benefit from the excellent light absorption and efficient carrier transport of the vertically aligned MoS2, ultrafast and high-responsiveMoS2/Si heterojunction photodetectors are achieved.27 Besides, the TMD-based Schottky junction,18 p-n junction22 as well as van der Waals

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heterojunction28 have been demonstrated as promising platforms for photovoltaic applications. Though tremendous progresses have been achieved, the exploitation of a TMD film as an insertion layer in a heterojunction photodetector and the in-depth exploration of its operating mechanism are scarce to date. In this contribution, we construct a Bi/WS2/Si heterojunction by successively depositing polycrystalline WS2 and Bi films on the p-Si substrate with pulsed-laser deposition (PLD), a versatile and efficient technology for the growth of various films.29-31 We demonstrate that the photoresponse of the Bi/Si heterojunction is enhanced by inserting a polycrystalline WS2layer. The Bi/WS2/Si sandwich-structured photodetector achieves a high photosensitivity of 1.4 × 108 cm 2 /W as well as an outstanding detectivity of 1.36 × 1013 cm ∗ Hz1/2 W -1 , which are comparable or even surpass those of state-of-art commercial photodetectors. Based on the qualitative analysis of the band structure of the system, the reasons for the superior device performance of the Bi/WS2/Si heterojunction photodetector are unveiled, including the efficient passivation of the interface, enhancement of light absorption as well as selective carrier blocking. Meanwhile, a good voltage tunability of the photoresponse is demonstrated, suggesting great potential for multi-functional optoelectronic applications. Therefore, these results are not only significant to the in-depth understanding on the integration of layered TMDs with conventional semiconductors, but also provided an attractive methodology to develop potential opportunities of layered TMDs in a multi-junction system. Firstly, a well-defined optically active window was opened on the (100)

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p-typeSiO2/Si ( 0.01 ~ 0.05 Ω cm -1 ) substrate (Supporting Information S1). Prior to loading into the growth chamber, the substrate was washed following a standard cleaning procedure to obtain an intact surface.32 After quickly dried by a hairdryer, the substrate was immediately loaded into the growth chamber to avoid the growth of native oxide. In both deposition processes, the base pressure of the growth chamber was better than 10-4 Pa. The polycrystalline WS2 film was deposited onto the exposed window of the SiO2/Si substrate by PLD at the optimized substrate temperature of 500 °C. Pre-growth annealing at 500 °C for 30 minutes was performed to further remove the native contaminations from the substrate. The WS2target was consist of highly pure and uniform W (99.99%) and S (99.99%) element with the W:S atomic ratio of 1:2.And the working pressure was set at50 Pa with flowing Ar2 as the working gas at the rate of 50 sccm.33 The KrF excimer laser was operated at a power fluence of c.a. 4 J/cm2with a repetition rate of4 Hz and a total pulse number of 10000.Note that the power fluence was the laser power density at the surface of the target. Then, the sample was cooled down to 30 °C and the Bi film was subsequently deposited onto the WS2 film. The Bi target was made of highly pure Bi (99.999%) element. In this case, the KrF excimer laser was operated at a power fluence of c.a. 3.2 J/cm2with a repetition rate of 2 Hz and a total pulse number of 2500. And the working pressure was set at 30 Pa with flowing Ar2 as the working gas at the rate of 50 sccm.32 It is to be noted that the whole deposition process can be done in situ, which is of great significant for reducing the external environment’s pollution to the interface of the heterojunction.

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Illumination with the wavelengths of 370, 635, 808 and 1064 nm was generated from the semiconductor lasers. The transport characteristics of the devices were evaluated using a Keithley 4200-SCS semiconductor parameter analyzer equipped with a probe station. During the measurements, the two-terminal configuration was exploited, with one probe connecting to the Bi side and another probe connecting to the Si side. The forward bias was defined as applying a positive bias on the Si side. All measurements were conducted at room temperature (25 °C) under ambient condition. The schematic diagrams of the separated individual parts of the Bi/WS2/Si heterojunction photodetector as well as its cross-sectional view are depicted in Fig. 1(a) and (b), respectively. The device is composed of the PLD-grown polycrystalline Bi and WS2 films as well as the Si substrate, which are sandwiched between the Au top and Al bottom electrodes. The Au and Al electrodes were deposited by ion sputtering at room temperature. The thickness of the Au and Al electrodes is c.a. 100 nm and c.a. 200 nm, respectively. Fig. 1(c) presents are AFM images of the polycrystalline WS2 film. The RMS of the WS2 film is c.a. 1.5 nm, indicative of a flat surface. In addition, there are no observable pinholes, the existence of which will result in serious leakage current. Such smoothness and compactness are of great significance to the following deposition of the polycrystalline Bi film. The AFM thickness profiles of the polycrystalline Bi and WS2filmsare presented in Fig. 1(d). The thickness of the polycrystalline Bi film is c.a. 47 nm. On the other hand, that of the polycrystalline WS2 layeris c.a. 15 nm, which is a suitable value for acting as an

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insertion layer.34 Firstly, it is thick enough for the initially-grownWS2 islands to connect into a continuous and compactness film, as is shown in Fig. 1(c). Therefore, it can act as an efficient passivation and blocking layer. On the other hand, it is thin enough for the Bi and Si to maintain strong interaction and thus to form strong build-in electric field, which is discussed later in this article. Then, XRD and Raman spectroscopy were conducted to assess the crystal quality and structure of the PLD-grown polycrystalline Bi and WS2 films. The XRD pattern of the Bi/WS2 heterojunction is presented in Fig. 1(e). There are two sets of peaks, the peaks from Bi (red) and the peak from WS2 (black). Therefore, the maintenance of the good crystalline quality of each material in a PLD-grown Bi/WS2 heterojunction is demonstrated. It is to be noted that there is only the (002) peak for the polycrystalline WS2 film, indicative of its highly c-axis oriented nature. This is of great significance because the surface atoms of the c-axis oriented WS2film is saturated, which helps a lot in the passivation of the underneath Si surface.35 To further assess the crystal structure, Raman scattering measurement with the 514 nm excitation laser was conducted, as shown in Fig. 1(f). There are also two sets of peaks. The peaks at 69.7 and 96.8 cm-1 (red) are assigned as the Eg and A1g vibration modes of Bi, while the peaks at 175.2, 352.6 and 418.2 cm-1 (black) are assigned respectively as the first 1 order Raman modes LA(M), E2 g and A1 g of WS2.The rest peaks corresponds to

the second order Raman modes of WS2, which are marked in the labels of the graph. Generally, both of the characterizations suggest that the PLD-grown Bi/WS2/Si heterojunction is of high quality and provides an attractive material platform to

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develop potential opportunities of such multi-junction system. Fig. 2(a) shows the dark current vs voltage (I-V) characteristic curve of the Bi/WS2/Si heterojunction, exhibiting an obvious rectifying behavior. Considering that the I-V curve of the WS2/Si heterojunction exhibits quite good linear behavior (Supporting Information S2), the rectifying characteristic of the Bi/WS2/Si heterojunction can thus be exclusively attributed to the interaction between Bi and Si, with the WS2 layer acting as just an insertion layer. For comparison, the dark I-V characteristic of the Bi/Si heterojunction is presented in the inset of Fig. 2(a). As expected, the Bi/Si heterojunction also exhibit similar rectifying effect to the Bi/WS2/Si heterojunction. However, the rectification ratio of the Bi/Si heterojunction (c.a. 3) is a little smaller than that of the Bi/WS2/Si heterojunction (c.a. 12.5), indicative of the superior junction property of the latter. Fig. 2(b) presents the photovoltaic characteristic of the Bi/WS2/Si heterojunction under 635 nm light illumination. Obvious photovoltaic effect is observed, demonstrating its great potential as a self-powered photodetector. To have a clear understanding on the physical mechanism underneath the different I-V characteristics between the Bi/Si and Bi/WS2/Si heterojunctions, the band diagrams of them are qualitatively investigated. Fig. 2(c) illustrates the band structure of the Bi/Si heterojunction. The original Fermi levels of elemental Bi and p-Si are c.a. 4.4 eV36 and c.a. 5.0 eV, respectively. Since the band gap of Bi is quite small (14 meV),37 it can be treated as a metal. In general, once the Bi film is deposited onto the p-type Si, electrons of Bi tend to flow to the Si side to keep the Fermi levels

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of each other in alignment, leading to the accumulation of holes and electrons in the boundaries of Bi and Si, respectively. In thermal equilibrium, a build-in-electric field pointing from the Bi side to the Si side is formed, leading to a downward bending of the band structure at the Si boundary. However, since the Si substrate is heavily doped ( 0.01 ~ 0.05 Ω cm -1 ), the dissipation layer is relatively thin. Consequently, the holes in Bi can easily tunnel through the depletion layer (step I in Fig. 2(c)), resulting in a considerable diffusion current.20 As a result, the reverse leakage current of the Bi/Si heterojunction is quite large. For the Bi/WS2/Si heterojunction, the situation differs quite a lot. The energy band diagram is depicted in detail in Fig. 2(d).36, 38 First of all, the insertion of a c-axis oriented WS2 film between the Bi film and Si substrate greatly broadens the blocking barrier, which plays a similar role to the insulating layer of a p/i/n configuration. Besides, the small value (c.a. 5.6 eV) of the lowest unoccupied molecular orbital (LUMO) of WS2can also heighten the blocking potential barrier. As a result, the holes of Bi can hardly tunnel through the depletion layer to reach the Si side (Step I in Fig. 2(d)).Therefore, the reverse diffusion movement is greatly suppressed in a Bi/WS2/Si heterojunction, leading to a smaller reverse leakage current. Then, the feasibility of the Bi/WS2/Si heterojunction for a self-powered photodetector is developed. Fig. 3(a) presents the switching characteristics of the Bi/Si and Bi/WS2/Si heterojunctions under the same 635 nm light illumination. It reveals that both devices can follow the fast-varying external optical signals with good stability and reproducibility. In addition, they both yield significant photocurrent

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and excellent on/off ratio, which is attributed to the efficient carrier transport and collection by the Bi film.9 Most importantly, it is obvious that the Bi/WS2/Si heterojunction yields a higher photocurrent than that of the Bi/Si heterojunction, suggesting the WS2 film plays an important role in enhancing the photoresponse. Then, further measurements demonstrate that the Bi/WS2/Si heterojunction can work in a wide spectrum range from ultraviolet (370 nm) to near-infrared (1064 nm) (Supporting Information S3). To evaluate the response speed of the Bi/WS2/Si heterojunction, a temporal switching cycle is presented in Fig. 3(b). There is no sampling points at the rise and decay edges，indicating that the response time of the device is better than 100 ms, the fastest sampling interval of our Keithley 4200-SCS measurement system. Such fast response time can probably be attributed to the synergistic effect of the short carrier transport distance for a vertical heterojunction configuration, quick separation of the photogenerated carriers by the build-in electric field and fast carrier transport by the Bi film. Fig. 3(c) presents the power dependent photocurrent. As expected, it increases with the increase of the incident light intensity on account of the increase of photogenerated carriers. Besides, the power dependent photosensitivity is also presented in Fig. 3(c). Benefit from the alliance of the low dark current and high photocurrent, the device exhibits an excellent photosensitivity in the range of 107 ~ 108 cm 2 / W . Fig. 3(d) summarizes the corresponding power dependent responsivity and detectivity, respectively. They increase with the decrease of the incident light intensity. The responsivity of the device reaches a decent value of 0.42 A/W under an

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illumination intensity of 6 mW/cm2, with the corresponding detectivity of 1.36 × 1013 cm ∗ Hz1/2W −1 . For comparison, the parameters of several reported

photodetectors based on layered materials and state-of-art commercial photodetector are summarized in Supporting Information S4. In general, our device stands out considering all indexes. To have a clear understanding on the superior device performance of the Bi/WS2/Si heterojunction compared to the Bi/Si heterojunction, the band diagrams of their operating mechanisms are qualitatively investigated. For the Bi/Si heterojunction, on light illumination, a large number of electron-hole pairs generates in the Si. The photogenerated carriers within one mean free path will drift to the depletion layer and be quickly separated by the build-in electric field. As a result, the electrons flow to the Bi side and the holes flow to the Si side (Step I in Fig. 3(e)), contributing to a net photocurrent pointing from Bi to Si. However, the Bi and Si forms a type I heterojunction, in which the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the heterojunction lie in the same material. Consequently, holes and electrons tend to gather in the same material to get the lowest energy state. What’s worse, the dissipation layer of the Bi/Si heterojunction is relatively thin on account of the quite heavily doping nature of Si ( 0.01 ~ 0.05 Ω cm -1 ). Accordingly, holes in Si possess a great possibility to tunnel through the thin build-in potential barrier and drift to the Bi side (Step II in Fig. 3(e)), which thus have a great trend to recombine with the electrons. In addition, there are a large number of dangling bonds at the surface of Si due to the sudden break of

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periodic

arrangement.

These

defect

states are

recombination centers for

photogenerated electron-hole pairs (Step III in Fig. 3(e)), leading to serious recombination loss. Consequently, the alliance of the above unfavorable factors results in the reduction of the number of effective photogenerated carriers. As a result, the photocurrent is seriously suppressed. For the Bi/WS2/Si heterojunction, the situation differs quiet a lot. The energy band diagram is depicted in Fig. 3(f). Since the insertion of a WS2 film significantly heightens and broadens the blocking barrier, the excited holes in Si can hardly tunnel to the Bi side. They are thus forced back to the Al side (Step I in Fig. 3(f)), degrading the recombination loss. Additionally, the WS2 film is highly c-axis oriented, whose surfaces are saturated and free of dangling bonds. The surface of Si can thus be effectively passivated by it.35 Consequently, the surface states at the interface become much less. On the other hand, the Fermi level pinning at the interface of the heterojunction is also greatly relieved, as demonstrated by the smaller open-circuit voltage (0.23V) of a Bi/Si heterojunction (Supporting Information S5) compared to that of the Bi/WS2/Si heterojunction (0.31 V). As a result, the alliance of these two factors leads to the efficient separation of the photogenerated electron-hole pairs. Moreover, extraordinary light absorption ten times surpassing that of traditional semiconducting Si has been demonstrated for the layered TMD semiconductors.39 Therefore, the insertion of a WS2 thin film can efficiently enhanced the absorbance of the incident light and results in more electron-hole pairs, contributing to the increase of the photocurrent.40 On the other hand, the WS2 and Si forms a type II

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heterojunction, in which the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the heterojunction lie in different materials. This is also favorable for reducing the recombination loss of photogenerated electron-hole pairs because the holes and electrons tend to move to different materials to get the lowest energy state (Step II in Fig. 3(f)). All in all, the WS2 layer plays multiple roles in enhancing the performance of the Bi/Si heterojunction photodetector. Finally, the effect of the external voltage on the device performances is investigated. Firstly, the switching behavior of the Bi/WS2/Si heterojunction with a source-drain bias of -2 V is investigated. As is shown in Fig. 4(a), the device also exhibits definite on/off states with good stability and reproducibility. Fig. 4(b) presents the voltage dependent photocurrent and photosensitivity. The photocurrent exhibits a nearly linear dependence on the voltage on account of the enhanced efficiency of carrier separation by the external voltage, suggesting good tunability for multifunctional applications. However, due to the increase of dark current, the photosensitivity of the device decreases with the increase of voltage. Fig. 4(c) summarizes the corresponding voltage dependent responsivity and detectivity, respectively. The responsivity of the device exhibits the same trend to the photocurrent. On the other hand, the detectivity of the device seems to be a constant value of 1010 cm * Hz1/2W −1 , probably attributed to the balanced tradeoff between the increase of both photocurrent and dark current. Conclusion

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In summary, we have constructed a Bi/WS2/Si heterojunction by successively depositing polycrystalline WS2 and Bi films onto the p-Si substrate with PLD. It is demonstrated that the photoresponse of the Bi/Si heterojunction was enhanced by inserting the WS2 film. The Bi/WS2/Si heterojunction photodetector achieves a high photosensitivity of 1.4 × 108 cm 2 /W as well as an outstanding detectivity of 1.36 × 1013 cm ∗ Hz1/2 W -1 , which are comparable or even surpass those of state-of-art

commercial photodetector. The decent device performance is attributed to the efficient passivation of the interface, enhancement of light absorption as well as selective carrier blocking. Moreover, a good voltage tunability of the photoresponse is demonstrated. These findings are useful for the deep-understanding on the integration of layered TMDs with conventional semiconductors. Meanwhile, they provide an attractive methodology to develop layered TMD in a multi-junction system.

Supporting Information. The etching process of the SiO2/Si substrate, I-V curve of the WS2/Si heterojunction, photovoltaic effect of the Bi/Si heterojunction, broadband photodetection of the Bi/WS2/Si heterojunction and a summary of reported photodetectors are provided.

Acknowledgements. State Key Laboratory of Optoelectronic Materials and Technologies of Sun Yat-sen University supported this work.

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Si Heterojunction

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