Large Lateral Photovoltage Observed in MoS2 Thickness

Subscriber access provided by TRENT UNIV

Article

Large lateral photovoltage observed in MoS2 thickness-modulated ITO/MoS2/p-Si heterojunctions Shuang Qiao, Bin Zhang, Kaiyu Feng, Ridong Cong, Wei Yu, Guangsheng Fu, and Shufang Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 11 May 2017 Downloaded from http://pubs.acs.org on May 13, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Large lateral photovoltage observed in MoS2 thickness-modulated ITO/MoS2/p-Si heterojunctions Shuang Qiao, Bin Zhang, Kaiyu Feng, Ridong Cong*, Wei Yu*, Guangsheng Fu, and Shufang Wang* Hebei Key Laboratory of Optic-Electronic Information and Materials, College of Physics Science and Technology, Hebei University, Baoding 071002, P. R. China. *E-mail: [email protected], [email protected] and [email protected] Abstract: Molybdenum disulfide (MoS2), as a typical two-dimensional (2D) material, has attracted extensive attention in recent years due to its fascinating optical and electric properties. However, the applications of MoS2 have been mainly in photovoltaic devices, field-effect transistors, photodetectors, and gas sensors. Here, it is demonstrated that MoS2 can be found another important application in position sensitive detector (PSD) based on lateral photovoltaic effect (LPE) in it. The ITO/MoS2(3, 5, 7, 9, 10, 20, 50, 100 nm)/p-Si heterojunctions were successfully prepared with vertically standing nanosheet structure of MoS2. Owing to the special structure and the strong light absorption of the relatively thick MoS2 film, the ITO/MoS2/p-Si heterojunction exhibits an abnormal thickness-dependent LPE, which can be ascribed to the n- to p- type transformation of MoS2. Moreover, the LPE of ITO/MoS2/p-Si structure improves greatly due to forward enhanced built-in field by type transformation in a wide spectrum response ranging from visible to near-infrared, especially the noticeable improvement in infrared region, indicating its great potential application in infrared PSDs. This work not only suggest that the ITO/MoS2/p-Si heterojunction shows great potential in LPE-based sensors, but also unveils the importance of type transformation of MoS2 in MoS2-based photoelectric devices besides strong light absorption and suitable bandgap. Keywords: ITO/MoS2/p-Si heterojunction, Schottky barrier, type transformation, broadband, position sensitive detector

ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 30

1. Introduction Graphene and other kinds of related two-dimensional (2D) materials have emerged as potential building blocks for a variety of fundamental electronic and optoelectronic devices, including field-effect transistors (FETs), nonvolatile memory devices, photonics devices, photovoltaic devices, and phototransistors.

1–11

Graphene

exhibits an ultrahigh carrier mobility over 105 cm2/V·s, but it reveals a considerable limitation in regard to real device applications due to an intrinsic difficulty caused by its incredibly small bandgap of less than 200 meV, 12, 13 so that graphene could hardly be used for digital circuits or photodetecting devices, which need clearly defined onand -off states. Very recently, MoS2, as a typical 2D transition metal dichalcogenide nanosheet material, attracted a great deal of attention due to its high photoelectric responsivity, mechanical flexibility, extraordinary on/off ratio (≈108), absence of dangling bonds and compatibility to silicon complementary metal oxide semiconductor (CMOS) processes. 14–16 More importantly, in great contrast to the zero bandgap issue of graphene, MoS2 exhibits a direct bandgap of 1.8 eV in monolayer and an indirect bandgap of about 1.3 eV for bulk or multilayer,

14, 17, 18

enabling its

light absorption span from visible to near-infrared spectral region (350–950 nm), 11, 19 the range of which is very attractive for nanoelectronics and optoelectronics applications.

20, 21

Besides, it has been reported that the MoS2-based 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 and GaAs,

22

and it shows a high carrier mobility of

about 200 cm2/V·s for monolayer and about 500 cm2/V·s for few layers.

14, 17, 18, 23

Thus, MoS2 has attracted much interest in the area of optoelectronic and photovoltaic devices. Among these optoelectronic and photovoltaic devices, one of the premises is that p-n or Schottky junction has been realized in MoS2-based materials.

8, 10, 11, 24-32

Undoubtedly, Si semiconductors are dominating the commercial optoelectronic and photovoltaic markets due to the high abundance and mature processing technology. The integration of MoS2 on Si could lower largely the cost of the optoelectronic and photovoltaic devices and multifunctional devices would be realized. Thus, recently,


Page 3 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


MoS2/Si p-n junctions have attracted more and more attention and have been studied in solar cells and photodetectors (PDs).

8, 10, 11, 24, 27-30, 32

For solar cells, researchers

mainly focus on power conversion efficiency and are trying their best to improve it, while for PDs, high response speed and photoresponsivity are very important parameters. However, besides these photovoltaic or optoelectronic devices, position sensitivity detector (PSD), a key component for optoelectronic systems, shows great practical application in biomedical applications, robotics, process controls, position information systems, and other systems requiring precision measurements based on lateral photovoltaic effects (LPE) in MoS2/Si p-n heterojunctions. Especially, a device, which is not suitable for photovoltaic devices due to undesirable efficiency, may exhibit excellent LPE response due to their different carrier transport mechanism 33. The LPE was first discovered by Schottky in 1930,

34

and later expanded upon

by Wallmark in 1957, 35 who successfully applied it to produce PSDs. Since then, the LPE has attracted much attention and been found in many different systems, such as a-Si:H p-i-n structures,

36-38

metal/semiconductor or metal/insulator/semiconductor

structures, 39-41 oxide/semiconductor structures, semiconductor polymers, structure

46

42, 43

perovskite materials, 44, 45 organic

nano- or micro-structured films,

47, 48

and GaAs/AlGaAs

49

. However, till now, the related studies on the LPEs in MoS2/Si p-n

junctions are absent, and the electrical transporting and the photovoltaic mechanisms in them are still unclear.More importantly, MoS2, as a new emerging two dimensional material, is still in research stage, its photoelectric conversion efficiency or responsivity is very low, and even some properties and its potential applications are still unknown. While LPE would be used as an excellent method to explore new physical phenomena or measure other physical properties

50, 51

. Our research of LPE

in MoS2/Si junctions will provide some important information of type transformation in MoS2-based photoelectric devices. 2. EXPERIMENTAL SECTION Preparation of ITO/MoS2/p-Si heterojunctions: In this paper, a series of MoS2 bulk-like thin films with thickness of 3, 5, 7, 9, 10, 20, 50, 100 nm were deposited on



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 30

(100)-oriented p-type Si substrates using the DC magnetron sputtering technique. The thickness of the Si wafer was 0.3 mm, and the resistivity was in the range of 1-10 Ω-cm at room temperature. Before the deposition, the wafer was cut into 25 mm × 25 mm slices and carefully cleaned by standard Radio Corporation of America (RCA) cleaning method to eliminate the organic and ionic impurities and dipped into 10% hydrogen fluoride solution for 30 seconds to remove the native SiO2 from the silicon surface. After cleaning, the wafers were immediately transferred to magnetron sputtering vacuum chamber. High purity MoS2 target (99.99%) was used to fabricate the MoS2 films and the chamber was vacuated to a base pressure of 1×10-4 Pa. The sputtering power, Ar gas pressure and substrate temperature were maintained to be 70 W, 2 Pa and 400 °C, respectively during sputtering. The deposition rate was approximately 3 nm/min, and the films of different thicknesses were prepared by controlling the deposition time. Finally, for well LPE measurement, a 80 nm thickness indium tin oxide (ITO), which is usually used as a transparent conductive layer for collecting separated carriers in photovoltaic or photoelectric devices,

6, 7, 27

was

deposited on the MoS2 layer by RF sputtering for 10min at 150 °C. Characterization of MoS2 films: The structure and morphology of the as-prepared MoS2 films were identified by X-ray diffraction (XRD, Bruker D8 Advance), high-resolution transmission electron microscopy (HRTEM, FEI, Tecnai G2), scanning electron microscopy (SEM, FEINova NanoSE M450) and energy dispersive X-ray spectroscopy (EDS, EDAX OCTANE PLUS). The optical properties were measured by using Raman spectroscopy (Horiba JobinYvon, LabRAM HR Evolution), and ultraviolet-visible spectrophotometer (UV, Hitachi U-4100). Lateral photovoltaic effect (LPE) measurements: The current-voltage (I-V) curves were measured with a Keithley 2400 SourceMeter. The lateral photovoltage (LPV) was done using a Keithley 2700 voltmeter and three dimensional electric motorized stage with a continuous wave laser of different wavelengths (405 nm, 532 nm, 671 nm, and 980 nm) as the illumination source. 3. RESULTS AND DISCUSSION


Page 5 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The crystal structure and phase purity of the MoS2 thin films (3-100 nm) were characterized by X-ray diffraction (XRD) using a high-resolution four circles diffractometer, as shown in figure 1a. Only the (002) diffraction peak was observed in all films. In order to further identify the crystal phase, the X-ray photoelectron spectroscopy (XPS) was measured, as shown in figure 2. It is indicated that the as-prepared MoS2 films can be indexed to the hexagonal molybdenite phase with no detectable impurities of other phases 52-54. The diffraction peak was very low for films within 10 nm, which can be ascribed to both the weak diffraction signal for thinner films and the formation of armorphous phase as a result of the large lattice parameter mismatch between the Si substrate and the MoS2 film. In order to confirm the crystallinity of the MoS2 film, a typical MoS2(20 nm)/Si structure was investigated with cross-section high-resolution transmission electron microscopy (HRTEM), as shown in figure 1b. We can observe that some ordered regions (the dotted line circles) distributed in the measured area, indicating that the MoS2 film is partly crystalline, which is in accordance with our XRD results. Figure 3 shows the Raman spectrum of the MoS2 films on the Si substrate. The films exhibit two characteristic MoS2 Raman peaks, the E12g mode at ∼379 cm−1 and A1g mode at ∼415 cm−1. The E12g mode corresponds to the in-plane vibration of Mo and S atoms, and the A1g mode corresponds to the out-of-plane vibration of S atoms. 10, 30, 55

The frequency separation between the two Raman peaks of the films (∆ ≈36

cm−1), is much larger than that in monolayer orseveral-layer MoS2,

8, 31, 55, 56

but is

consistent with that in bulk-like MoS2 films 10, 30, 55. Scanning electron microscopy (SEM) was performed to characterize the morphology of those samples. The SEM images of the cross-section structures of MoS2(50 nm)/Si and MoS2(100 nm)/Si are shown in figure 4a and 4b, respectively. Interestingly, the MoS2 film is a distinct vertically standing nanosheet structure. Figure 4c, 4d, and 4e gives the surface morphology of MoS2 (10 nm)/Si, MoS2 (50 nm)/Si, and MoS2 (100 nm)/Si, respectively. We can obtain that the nanosheet appears at about 10 nm, which can also be observed in the cross-section images (marked by red arrows) of HRTEM in figure 1b, and then comes into being an uniform vertically



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 30

standing layered structure gradually with increasing the MoS2 thickness, as shown in Figure S1. Layered materials usually have a higher in-plane conductivity

57

and a

much smaller out-of-plane conductivity because of the large layer distanceand weak van der Waals force between adjacent layers

58,59

. Therefore, this distinct vertically

standing nanosheet structure of the MoS2 film may greatly facilitate carrier transport from the junction interface to the ITO conductive layer. Transmittance

spectrum

was

also

measured

with

ultraviolet-visible

spectrophotometer in order to characterize the absorption of MoS2 films, which were prepared on quartz glass substrates, as shown in figure 5. We can observe that the transmittance decreases with increasing MoS2 thickness quickly in visible and ultraviolet range, suggesting the strong optical absorption in thicker MoS2 films. However, the transmittance also decreases a little with increasing MoS2 thickness in infrared range, which can be ascribed to the absorption by defects or impurities in the MoS2 films. Figure 6a shows the typical longitudinal current-voltage (I-V) curve of the ITO/MoS2(20 nm)/Si junction at room temperature. Two indium electrodes with a diameter of about 0.5 mm were prepared on the surface of the ITO film and the back of Si substrate, respectively, as shown in the inset of figure 6a. It is clear that the longitudinal I-V curve exhibits a good backward diode-like rectifying behavior, suggesting a Schottky barrier (SB) is produced at the interface of the ITO/MoS2/Si heterostructure. Therefore, when a light beam is scanned away from electrode A to electrode B on the surface of ITO, a LPV as a function of laser position would be observed. For LPE measurement, two electrodes were prepared on the surface of each sample with a constant distance of 1.0 mm, and the films were scanned spatially with a laser (ranging from visible to near-infrared) focusing on a roughly 100 µm diameter spot at the surface. The sketch of LPE measurement and a typical laser position-dependent LPV are shown in figure 6b. Firstly, we measured the ITO/MoS2/Si heterostructures under illumination of a 532 nm laser with a constant laser power of 10 mW, as shown in figure 7a. The LPE shows a significant dependence on the thickness of the MoS2 film, which first


Page 7 of 30

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


increases from 5.31 mV/mm to 8.44 mV/mm with increasing MoS2 thickness from 3 nm to 5 nm, then decreases to a minimum of 2.47 mV/mm for ITO/MoS2(9 nm)/Si structure, and finally increases quickly with increasing the MoS2 thickness (3.33 mV/mm @ 10 nm, 18.86 mV/mm @ 20 nm, and 26.81 mV/mm @ 50 nm), suggesting that theMoS2 not only has an important effect on the light absorption, but also on the interface region of the built-in field, which plays a crucial role in the transport of the photo-excited carriers. The bandgaps of Si and MoS2 are about 1.12 and 1.3 eV

10, 11

, respectively, so

that the photons of 532 nm laser can excite electron-hole pairs in both of them. The excited electron-hole pairs in MoS2 or Si are separated by the built-in field in the interface of MoS2/Si, and transmitted to the ITO layer, then the excessive carriers in the ITO side will diffuse laterally along the surface layer away from the illuminated spot toward around and thus generate a gradient laterally between the illuminated and the non-illuminated zones. Based on the carrier diffuse equation, LPV can be described as: 29, 60 LPV = KN 0 [exp( −

x−L l

) − exp( −

x+L l

)]

(1)

Where K is the proportional coefficient, L is the half distance between two electrodes, N0 is the number of the separated electron-hole pairs per second at position x , and l is the electron diffusion length in ITO. The LPV curve in figure 6b can be well fitted by formula (1) with K ≈ 0.9998 , N 0 ≈ 30 , l ≈ 1 , L = 0.5 , indicating that the LPE can be well explained by the carrier diffusion theory. Considering the nearly linear region between two contact electrodes, l should be much larger than L , thus formula (1) can be simplified as:

LPV =

2 KN 0 L exp(− ) x (− L ≤ x ≤ L) l l

(2)

It is clear that the LPV is linearly proportional to laser position, which is an unique characteristic of LPE using for PSD devices, and then position sensitivity, as one of important parameters for judging PSD 60,can be deduced:

Position Sensitivity =

LPV 2 KN 0 L = exp(− ) (− L ≤ x ≤ L) x l l


(3)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 30

The position sensitivity is only related to N0 and K under a constant contact distance for ITO layer. To better understand the LPE, we investigated the influence of laser power on the LPE. The output laser power was changed from 0 to 70 mW through an optical attenuation and measured with a photo power meter (Coherent Field MaxII-TOP). Figure 7b shows the extracted position sensitivity as a function of laser power in the ITO/MoS2/Si heterojunctions with different MoS2 thicknesses. It is clearly obtained that the position sensitivity increases considerably from 0.61 mV/mm to 30.25 mV/mm with increasing laser power from 0.1 mW to 20 mW (typically for 50 nm sample), and then increases slowly (from 32.35 mV/mm to 42.27 mV/mm) until it saturates with increasing laser power gradually. The saturation can be attributed to the rapidly increasing recombination rate of the carriers in the region of irradiation with 49

increasing laser intensity

. In formula (3), K is dependent on the longitudinal

diffusion lengthin MoS2, the field strength of interface, and especially the power-dependent carrier recombination rate, which can be described as (1-δ γτ P /N0 ) [where γ is the proportional coefficient, δ is the recombination rate, τ is the carrier lifetime, and P is the laser power],

61, 62

and N0 is proportional to the

amount of photons, which can be expressed as N0 =κ (

P β ) [where h is the Planck hν

constant, ν is the laser frequency, and κ and β are the proportional coefficients with 0

Large Lateral Photovoltage Observed in MoS2 Thickness

Recommend Documents