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Electrical and photovoltaic characteristics of MoS2/Si p-n junctions Lanzhong Hao, Yunjie Liu, Wei Gao, Zhide Han, Qingzhong Xue, Huizhong Zeng, Zhipeng Wu, Jun Zhu, and Wanli Zhang Citation: Journal of Applied Physics 117, 114502 (2015); doi: 10.1063/1.4915951 View online: http://dx.doi.org/10.1063/1.4915951 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/117/11?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Impact of carbon-fluorine doped titanium dioxide in the performance of an electrochemical sensing of dopamine and rosebengal sensitized solar cells AIP Advances 5, 017149 (2015); 10.1063/1.4907168 Scalable synthesis of layer-controlled WS2 and MoS2 sheets by sulfurization of thin metal films Appl. Phys. Lett. 105, 083112 (2014); 10.1063/1.4893978 Reconfigurable p-n junction diodes and the photovoltaic effect in exfoliated MoS2 films Appl. Phys. Lett. 104, 122104 (2014); 10.1063/1.4870067 p-type doping of MoS2 thin films using Nb Appl. Phys. Lett. 104, 092104 (2014); 10.1063/1.4867197 Electrical performance of monolayer MoS2 field-effect transistors prepared by chemical vapor deposition Appl. Phys. Lett. 102, 193107 (2013); 10.1063/1.4804546

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JOURNAL OF APPLIED PHYSICS 117, 114502 (2015)

Electrical and photovoltaic characteristics of MoS2/Si p-n junctions Lanzhong Hao,1,a) Yunjie Liu,1,b) Wei Gao,1 Zhide Han,1 Qingzhong Xue,1 Huizhong Zeng,2 Zhipeng Wu,2 Jun Zhu,2 and Wanli Zhang2 1

College of Science, China University of Petroleum, Qingdao, Shandong 266580, People’s Republic of China State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 610054, People’s Republic of China 2

(Received 10 January 2015; accepted 11 March 2015; published online 20 March 2015) Bulk-like molybdenum disulfide (MoS2) thin films were deposited on the surface of p-type Si substrates using dc magnetron sputtering technique and MoS2/Si p-n junctions were formed. The vibrating modes of E12g and A1g were observed from the Raman spectrum of the MoS2 films. The current density versus voltage (J-V) characteristics of the junction were investigated. A typical J-V rectifying effect with a turn-on voltage of 0.2 V was shown. In different voltage range, the electrical transporting of the junction was dominated by diffusion current and recombination current, respectively. Under the light illumination of 15 mW cm2, the p-n junction exhibited obvious photovoltaic characteristics with a short-circuit current density of 3.2 mA cm2 and open-circuit voltage of 0.14 V. The fill factor and energy conversion efficiency were 42.4% and 1.3%, respectively. According to the determination of the Fermi-energy level (4.65 eV) and energy-band gap (1.45 eV) of the MoS2 films by capacitance-voltage curve and ultraviolet-visible transmission spectra, the mechanisms of the electrical and photovoltaic characteristics were discussed in terms of the energy-band structure of the MoS2/Si p-n junctions. The results hold the promise for the integration of MoS2 thin films with commercially available Si-based electronics in high-efficient phoC 2015 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4915951] tovoltaic devices. V I. INTRODUCTION

Structurally, molybdenum disulfide (MoS2) has a crystal structure consisting of sandwich layers of S-Mo-S, where a Mo-atom layer is enclosed within two S-atom layers and the atoms in layers are hexagonally packed, as shown in Fig. 1. These layers are held together by van der Waals interaction. The weak van der Waals interaction make it easy for MoS2based materials to be exfoliated into monolayers.1 Due to its good electrical, mechanical, and optical properties, monolayer MoS2 has become one of good candidates to develop next-generation microelectronic devices and optoelectronic devices.2–4 Unlike graphene, monolayer MoS2 is a n-type semiconductor with a direct band gap of 1.9 eV and the band gap decreases with increasing the layer numbers.5 The presence of a band gap allows fabrication of MoS2 transistors with an on/off ratio exceeding 108 and photodetectors with high responsivity.6,7 It has been reported that the MoS2based materials can absorb up to 5%–10% of incident sunlight in a thickness of less than 1.0 nm and exhibit one order of magnitude higher sunlight absorption than the most commonly used solar absorbers such as Si.8 Thus, MoS2 has attracted much interest in the area of solar cells. Most solar cell designs comprise a junction between adjoining materials at which the separation of the photogenerated carriers occurs. Recently, p-n and Schottky junctions have been realized in MoS2-based materials.9–13 MoS2/WSe2 van der Waals heterojunctions were fabricated by Furchi et al. and the devices showed an obvious photovoltaic effect.9 Further, Wi et al. a)

Electronic mail: [email protected] Electronic mail: [email protected]

b)

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utilized the plasma-induced p-doping approach to form p-n junctions in MoS2 layers.10 The results demonstrated that the fabricated p-n junctions showed good photovoltaic characteristics with high short-circuit photocurrent density up to 20.9 mA cm2 and good power-conversion efficiencies up to 2.8%. Recently, Tsai et al. realized the enhancement of the power conversion efficiency of Si-based Schottky-type solar devices from 4.64% to 5.23% by the incorporation of monolayer MoS2 into the Al/p-Si interface.11 All these results fully prove that MoS2 has large potential application in the area of solar cells. However, due to the difficulties in fabricating large-scale continuous uniform monolayer MoS2, the development of monolayer MoS2 solar devices is limited. Comparatively, from the point of view of fabrication, application, and physics, bulk-like MoS2 thin films have three advantages in the area of solar cells. First, it is easier to form large-scale uniform MoS2 thin films on different kinds of substrates using magnetron sputtering technique and pulse laser deposition.14–16 Second, bulk-like MoS2 thin films have a much smaller energy-band gap (1.2 eV) than the value of the monolayer (1.9 eV). This can enhance the sunlight absorption of the fabricated devices. Especially, the excellent electrical and optical properties of bulk-like MoS2 thin films can be integrated easily with Si materials. Undoubtedly, Si semiconductors are dominating the commercial photovoltaic market due to the high abundance and mature processing technology. The integration of MoS2 on Si could lower largely the cost of the solar cells and multifunctional devices would be realized. Thus, it is important to form the heterojunction composed of bulk-like MoS2 thin films and Si, not only due to its large potential applications in solar cells but also due to the rich scientific topics, such as the interface

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FIG. 2. Schematic illustration of the electrical measurement of the MoS2/Si solar cell device.

FIG. 1. (a) Structure of bulk MoS2 showing layered structure of material. (b) Top view of MoS2 materials. Atom color code: light blue-green, Mo; yellow, S.

effect in nanoscale and evolution of electrical transporting mechanisms. However, the related studies on the fabrication and photovoltaic characteristics of the heterojunctions composed of bulk-like MoS2 thin films and Si are absent. Based on the above analysis, MoS2 thin films were grown on p-type Si substrates using magnetron sputtering technique and the MoS2/Si p-n junctions were formed in this study. The electrical characteristics of the fabricated junction were investigated. The results demonstrated that the fabricated junctions exhibited good rectifying and obvious photovoltaic characteristics. According to the experimental results, the mechanisms of the electrical and photovoltaic characteristics of the p-n junction were proposed in terms of the energy-band alignments at the MoS2/Si interface. II. EXPERIMENT

MoS2 thin films were grown on (100)-oriented Si substrates using dc magnetron sputtering technique. The MoS2 targets (purity, 99.9%) were cold-pressed. The Si substrates used in this work are p-type semiconductors with the resistivity in the range of 1.2–1.8 X cm. Before the deposition, the substrates were ultrasonically cleaned in sequence by using alcohol, acetone, and de-ionized water. Then, the substrate was dipped into HF solution (5.0%) for 60 s to remove the amorphous SiO2 layer from the silicon surface. Subsequently, the Si substrate was immediately moved into the vacuum chamber. Then, about 40-nm-thickness MoS2 thin films were deposited. The background of vacuum chamber was 4 104 Pa. During the deposition, the working pressure of argon gas was kept at 0.3 Pa. The deposition temperature and sputtering power were about 380.0 C and 120.0 W, respectively. After the deposition, the top 30-nmthickness palladium (Pd) and backside indium (In) electrodes were fabricated on MoS2 thin films and Si, respectively. The schematic illustration of the p-n junction is shown in Fig. 2. In this work, forward bias voltage is defined as a positive dc voltage applied on In electrode. In order to obtain the

ultraviolet-visible (UV) transmission spectra of the MoS2 films and the electrical characteristics of Pd/MoS2, the films were also deposited on sapphire and transparent conducting oxide (ITO) substrates under the same conditions with the above. Samples were characterized using Raman spectroscopy (Renishaw, 514 nm laser). The surface morphology was characterized by atomic forced microscope (AFM, SPM300HV, SEIKO). The current density versus voltage (J-V) curves were measured using Keithley2400 source meter. The capacitance-voltage (C-V) curves were measured using HP4284 source meter. The transmission spectra were measured by Shimadzu UV-3150 spectrophotometer.

III. RESULTS AND DISCUSSION

Figure 3 shows the Raman spectrum of the bulk-like MoS2 films on Si substrates. Besides the Raman peak of the Si substrate at 520 cm1, two significant active modes of the as-grown MoS2 film were identified at 380 cm1 and 405 cm1 which correspond to E12g mode and A1g mode, respectively. In the E12g mode, both sulfur and molybdenum atoms vibrate along in-plane direction, whereas the sulfur atoms vibrate in the perpendicular-to-plane direction in the A1g mode, as shown in the insets. Our Raman results are consistent with the reported results in the literature.16

FIG. 3. Raman spectrum of the MoS2 films on Si substrates. The insets show the schematic illustration the vibrating mode of E12g and A1g. Atom color code: light blue-green, Mo; yellow, S.


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FIG. 4. AFM surface morphology of the MoS2 thin film grown on the Si substrate.

Figure 4 shows AFM topographic image of the MoS2 thin film grown on the Si substrate. As shown in the figure, outgrowths are evident. The surface of the film is composed of dense cone-like grains. According to the measurements, the root-mean-square roughness (RMS) of the film is about 1.3 nm, and the average size of grains is about 79.4 nm in diameter. The rough surface is important for the solar cell devices because it can effectively decrease the reflection to sunlight and improve the efficiency of photogenerated carrier separation.17 Figure 5 shows the J-V curve of the Pd/MoS2/Si/In junction at room temperature. From the figure, obvious rectifying behavior is observed. The rectifying ratio (Iþ/I) measured at 60.5 V is about 42. A turn-on voltage (Von) of 0.2 V, where the current starts to increase quickly, can be obtained. Besides the MoS2/p-Si junction, Schottky junctions are likely to be formed at the Pd/MoS2 and In/Si interfaces in heterostructure. The inset in Fig. 5(a) shows the J-V curves of Pd/MoS2 and In/Si. From the figure, the linear characteristics of the J-V curves for the Pd/MoS2 and In/Si are observed. Therefore, we can conclude that the rectifying J-V characteristic in the device structure is attributed to the MoS2/p-Si junction. The forward J-V characteristic of a heterojunction can be described by the exponential relation18 J / exp ðqV=nkTÞ;

FIG. 5. (a) J-V curve of the Pd/MoS2/Si/In junction. (b) Replot of the J-V curve of the heterojunction in the forward voltage range using semilogarithmic mode.

estimated to be 8.0. This might be due to the series resistance of the MoS2 film and the carrier trapping at the lattice defects caused by the large lattice difference between MoS2 and Si at the interface.20 Figure 6 shows the J-V characteristics of the fabricated solar cell device under the dark and light illumination of 15 mW cm2. The current density at a given voltage of the device under illumination is larger than that in the dark. This indicates that light absorption in MoS2 and Si generates a photocurrent due to the production of electron-hole pairs.

(1)

where q is the electron charge, n is the ideality factor, k is the Boltzmann constant, and T is the temperature. In general p-n junctions, the forward dark-current density constitutes the diffusion current density caused by the diffusion of carriers in neutral regions and the recombination current density inside space-charge regions near the interface.19 Fig. 5(b) shows the replot of the J-V curve of the heterojunction in the forward voltage range using semi-logarithmic mode. Three distinctly different regions with different n (labeled I, II, and III) can be observed. In the lower voltage range (0–0.04 V), n is 1.0; the electrical transporting mechanism is dominated by diffusion current in this range. When 0.05 V 0.28 V, n is 2.0; this shows that the recombination current dominates the conduction in this range. When the voltage is larger than 0.28 V, the obtained ideality factor is unusually high,

FIG. 6. Photovoltaic characteristics of the MoS2/p-Si heterojunction.


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FIG. 7. Time dependence of (a) JSC and (b) VOC of the solar device with light on and off. (c) and (d) Rising and falling edges of VOC in the first period, respectively.

According to the measured results, the fabricated device has an open-circuit voltage (VOC) of 0.14 V and a short-circuit current density (JSC) of 3.2 mA cm2. The fill factor and power conversion efficiency were calculated to be 42.4% and 1.3%, respectively. Time dependence of the short-circuit current density of the fabricated solar device with light on and off is shown in Fig. 7(a). When the light-on and light-off conditions are switched alternately, two distinct current states for the device are shown, the ‘‘low’’ current state in the dark and the ‘‘high’’ current state in the light. The open-circuit voltage shows similar dependence with the short-circuit current density, as shown Fig. 7(b). Figs. 7(c) and 7(d) show the rising and falling edges of the open circuit voltage in a period, respectively. From the figures, we can see that the “ON” and “OFF” states are steady, and the variation between two states is fast (40.0 ms) and well reversible. These characteristics are similar with the reported results about MoS2/a-Si heterojunction.21 This make it possible for the device to act as a photosensitive resistor. Using the data from the UV spectra of the MoS2 film, the (ah)2 was plotted as a function of photon energy h, as shown in Fig. 8(a), wherein h is the Planck constant and is photon frequency. The a is the absorption coefficient, calculated by ad ¼ ln(1/T), d and T are thickness and transmittance of films, respectively.22 The band gap (Eg) of the film can be obtained by intercept of the line on h axis. From the figure, Eg ¼ 1.45 eV. The obtained energy-band value for the film is little larger than MoS2 bulk (1.2 eV) and much smaller than the monolayer (1.9 eV). According the measured C-V curve in the reverse voltage range, the plot of C3 versus reverse voltages is shown in Fig. 8(b). The linear C3 versus V dependence demonstrates that the fabricated MoS2/Si junction is graded.23 According to the intercept on voltage axis, the built-in electrical field (Vbi) can be obtained, about

0.35 V. According to schematic illustration of the electrical measurement, it can be determined that the built-in electrical field points from the MoS2 film to Si.

FIG. 8. (a) Plot of (ah)2 versus h for the MoS2 film. (b) C3 versus reverse voltages curve.


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FIG. 9. (a) Energy band diagram of isolated MoS2 and p-Si. (b) Energy band diagram of MoS2/p-Si heterojunction. W is work function, w is electron affinity, Eg is energy band gap, Ef is Fermi-energy level, Ei is the energy level of the middle of the energy band gap, EC is the bottom of conduction band, EV is the top of valence band, DEC and DEV are conduction band offset and valence band offset near the interface, respectively.

The observed electrical and photovoltaic characteristics can be explained by considering the energy-band diagram of the MoS2/Si junction. Fig. 9(a) shows the energy band diagram of isolated MoS2 and p-Si. As shown in Fig. 8(a), the energy-band gap of the MoS2 thin film (Eg1) is 1.45 eV. The Hall Effect measurements show that the as-grown MoS2 films is a quasi-intrinsic semiconductor with a very small Hall coefficient (0). Due to the narrow band gap nature and the existence of thermal emitted electrons, it is reasonable to assume that the as-grown MoS2 thin films are n-type semiconductors. Thus, the Fermi energy level of the MoS2 film (EF1) is close to (but above) the middle of the energy band gap. For the p-Si used in our experiments, the Fermi energy level [EF2 ¼ 5.0 eV] and energy band gap [Eg2 ¼ 1.12 eV] are taken to construct the band structure and the difference (EF2-EV2) between the Fermi energy level and the top of the valence band is about 0.2 eV.24 When n-MoS2 films are deposited on the surface of p-Si, the electrons will flow from MoS2 into Si at the interface due to the higher Fermi energy level of the MoS2. The flowing process stops when Fermi levels are equal and a MoS2/Si p-n junction is fabricated, as shown in Fig. 9(b). Thus, asymmetric characteristics and obvious rectifying effect can be observed from the J-V curve in Fig. 5(a). As a result of electrons flowing, a built-in electrical field is formed near the interface, as illustrated in Fig. 8(b). Under illumination, the incident photons generate the electron-hole (e-h) pairs in the MoS2 film and the Si. The built-in electric field can effectively facilitate the separation of photo-generated e-h pairs, transporting separated electrons from Si to MoS2 and holes towards Si. The processes of photo-excitation and carrier transport in the MoS2/Si p-n junction are demonstrated in Fig. 9(b). Therefore, obvious photovoltaic characteristics are resulted in the p-n junction. From Fig. 6, the open-circuit voltage is 0.14 V which is smaller than the reported values.9,10,25 As is well known that the VOC depends on the build-in field near the interface.26 The Vbi of 0.35 V in the MoS2/Si p-n junction is smaller than the reported Au/MoS2 Schottky junction and monolayer MoS2 p-n junction. In order to enhance the VOC and improve the photovoltaic properties, some effective routes should be employed in future, such as incorporation of interfacial layers with high work function.27

Based on the extracted Vbi, the Fermi energy level of the MoS2 film (EF1) can be determined by Vbi ¼ EF2 EF1,19,24 about 4.65 eV. This value is in accord with the reported results.25,28 Then, the values of electron affinity (w) for the MoS2 film and p-Si can be calculated, w1 ¼ 3.92 eV and w2 ¼ 4.08 eV. Furthermore, the conduction band offset is DEC ¼ w1 w2 ¼ 0.16 eV and the valence band offset is DEV ¼ [w1 þ Eg1] [w2 þ Eg2] ¼ 0.17 eV. The barrier heights for electrons and holes in the p-n heterojunction should take account of the band offsets. Therefore, the barrier heights for electrons and holes are 0.19 and 0.52 eV, respectively. Typically, if the barrier height for holes is 0.2 eV larger than that for electrons, the electron current will be approximately a factor of 104 larger than the hole current.29 Thus, in our case, the current is dominated by the electrons. Therefore, the ideal turn-on voltage should be 0.2 V. Within the precision level of the measurement, this value is almost same with the value evaluated from the J-V curve of the p-n junction. IV. CONCLUSION

Bulk-like MoS2 thin films were deposited on the surface of p-type Si substrates using dc magnetron sputtering technique and MoS2/Si p-n junctions were formed. The p-n junction exhibited obvious rectifying characteristics with a turn-on voltage of 0.2 V. In different voltage range, the electrical transporting of the junction was dominated by diffusion current and recombination current, respectively. Under the light illumination of 15 mW cm2, the p-n junction exhibited obvious photovoltaic characteristics, JSC ¼ 3.2 mA cm2, VOC ¼ 0.14 V, FF ¼ 42.4%, and g ¼ 1.3%. The mechanisms of the electrical and photovoltaic characteristics were proposed in terms of the energy-band structure of the MoS2/Si p-n junctions. The results demonstrated that MoS2/Si p-n junctions have large potential application in the area of photovoltaic devices. ACKNOWLEDGMENTS

The authors acknowledge the financial support by the Fundamental Research Funds for the Central Universities (14CX05038A and 13CX02018A) and the National Natural Science Foundation of China (Grant No. 51102284 and 51372030).


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