Research Article www.acsami.org
MoS2−InGaZnO Heterojunction Phototransistors with Broad Spectral Responsivity Jaehyun Yang,†,‡,∇ Hyena Kwak,†,∇ Youngbin Lee,§ Yu-Seon Kang,∥ Mann-Ho Cho,∥ Jeong Ho Cho,‡,§,⊥ Yong-Hoon Kim,†,§ Seong-Jun Jeong,# Seongjun Park,# Hoo-Jeong Lee,†,§ and Hyoungsub Kim*,†,‡ †
School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea Samsung-SKKU Graphene/2D Center, Sungkyunkwan University, Suwon 440-746, Republic of Korea § SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, Suwon 440-746, Republic of Korea ∥ Institute of Physics and Applied Physics, Yonsei University, Seoul 120-749, Republic of Korea ⊥ School of Chemical Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Korea # Device Laboratory, Device and System Research Center, Samsung Advanced Institute of Technology, Samsung Electronics Co., Suwon 443-803, Republic of Korea ‡
S Supporting Information *
ABSTRACT: We introduce an amorphous indium−gallium− zinc-oxide (a-IGZO) heterostructure phototransistor consisting of solution-based synthetic molybdenum disulfide (few-layered MoS2, with a band gap of ∼1.7 eV) and sputter-deposited aIGZO (with a band gap of ∼3.0 eV) films as a novel sensing element with a broad spectral responsivity. The MoS2 and aIGZO films serve as a visible light-absorbing layer and a high mobility channel layer, respectively. Spectroscopic measurements reveal that appropriate band alignment at the heterojunction provides effective transfer of the visible lightinduced electrons generated in the few-layered MoS2 film to the underlying a-IGZO channel layer with a high carrier mobility. The photoresponse characteristics of the a-IGZO transistor are extended to cover most of the visible range by forming a heterojunction phototransistor that harnesses a visible light responding MoS2 film with a small band gap prepared through a large-area synthetic route. The MoS2−IGZO heterojunction phototransistors exhibit a photoresponsivity of approximately 1.7 A/W at a wavelength of 520 nm (an optical power of 1 μW) with excellent time-dependent photoresponse dynamics. KEYWORDS: molybdenum disulfide, solution-based synthesis, InGaZnO, heterojunction, phototransistor structures. In these structures, the a-IGZO film is capped with a visible light-absorbing layer, such as graphene dots in a polymer matrix8 or organic films including poly(3-hexylthiophene) (P3HT)9 and copper phthalocyanine (CuPc)10 to generate and deliver photoinduced charge carriers. Although these approaches are quite meaningful, the use of dispersed graphene dots8 may require complex procedures for controlling the areal density/size distribution of the dots and the uniformity of the films over a large scale. In the case of the organic light-absorbing films, their relatively high thickness (100−120 nm)9,10 may reduce the light absorption crosssection, and devices may display time-dependent instabilities as a result of their susceptibility to ambient molecules. In addition,
1. INTRODUCTION For many years, amorphous indium−gallium−zinc-oxide (aIGZO) has attracted widespread interest for use as a highperformance channel material in thin film transistors of integrated circuit systems on display panels due to its high field-effect carrier mobility, large-area applicability, low-temperature processing features, and excellent stability.1−3 Because aIGZO itself has a large optical band gap of more than 3 eV,1 it has been successfully used in commercially available display backplanes that require a large on−off ratio with a low subthreshold swing and visible light transparency. However, despite its broad utility, the large band gap of a-IGZO has limited its applicability as a photodetector (photosensor) to ultraviolet (UV) detection.4−6 The response range of the a-IGZO film to visible light in active-matrix imaging arrays (photosensors)3 and touch screen panels7 has been extended by developing heterojunction © XXXX American Chemical Society
Received: December 2, 2015 Accepted: March 18, 2016
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DOI: 10.1021/acsami.5b11709 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 1. (a) Schematic illustration of the heterostructured MoS2/a-IGZO phototransistor fabricated on a SiO2/Si substrate. The inset shows an optical microscopy image of an array of phototransistors. (b, left) STEM-HAADF and (right) HR TEM images of the heterostructured MoS2/aIGZO stack. (c) Room-temperature PL spectra of the a-IGZO, MoS2, and MoS2/a-IGZO films. (c, inset) Raman spectra obtained from the a-IGZO and MoS2/a-IGZO films. (d) Schematic illustrations showing the energy band alignment and the photoinduced charge transfer mechanism of the heterostructured MoS2/a-IGZO stack without (left) or with (right) an applied positive gate bias. (e) A series of ID−VD characteristics for the MoS2/ a-IGZO phototransistor without an applied gate bias under illumination with a 520 nm laser at different optical powers. (e, inset) Optical microscopy image of a heterostructured phototransistor under light illumination during electrical characterization.
the relatively large band gap of P3HT (2.1 eV)11 and the negligible absorption of CuPc in the blue light region (400− 500 nm)12 may also limit the broad spectral responsivity of heterojunction photosensors. In a different approach, Jeon et al.3 introduced an inorganic indium−zinc-oxide (IZO) film as a light-absorbing layer in an IGZO-sandwiched structure to fabricate high-performance transparent photosensor arrays. However, their photoresponsive range was narrow (with an absorption band that extended to the green light region) due to the relatively large band gap of the IZO film (2.9 eV).3 In view of the drawbacks of various heterojunction a-IGZO phototransistors, there remains a need for novel visible lightabsorbing capping layers with a small band gap (less than 2 eV), ultrathin layer thickness (nanometer range), and excellent stability/robustness (inorganic material). Ideally, such capping layers should be synthesizable via large-area and uniform deposition methods. In recent years, atomically thin two-dimensional (2D) layered transition metal dichalcogenide (TMD) semiconductors have been widely considered for use in future nano/ optoelectronic devices.13 Among these materials, molybdenum disulfide (MoS2) has been extensively employed in visible light photodetector applications14−22 because it has a band gap of 1.3−1.9 eV that can be adjusted by varying the number of atomic stacked layers.23 This system offers a high and broad responsivity with a controllable response range. Although mechanically exfoliated MoS2 flakes have been used directly in visible light photodetectors,14−19 they cannot be applied in large-area devices, and methods of fabricating these flakes are incompatible with conventional processes. As a result, a variety of studies have been conducted to develop large-area synthesis methods for producing high-quality MoS2 films with a controllable layer thickness.24−29 Recently, we reported the
development of a solution-based synthesis process (spincoating method) for fabricating uniform MoS2 films on the scale of a two-inch wafer (on both Si and plastic substrates).28 This approach offers a simple and cost-effective method for obtaining large-area few-layered MoS2 films that are suitable for use in phototransistor applications. In this work, we introduce a heterostructured a-IGZO phototransistor comprising a MoS2/a-IGZO stack in which the few-layered MoS2 serves as a visible light-absorbing layer with broad responsiveness (due to its small bandgap). As large-area applicable fabrication approaches, few-layered MoS2 films were synthesized using a solution-based synthesis process,28 and aIGZO films were deposited using a sputtering method. Various spectroscopic analyses revealed that the photogenerated electrons in the overlying MoS2 layer were efficiently transferred to the high-mobility a-IGZO channel due to appropriate band alignment. The heterostructured a-IGZO phototransistors showed excellent performance with a broad response range from the UV throughout most of the visible region.
2. EXPERIMENTAL SECTION A 10 nm thick a-IGZO film was deposited onto a p++-type Si wafer (resistivity of 10−6 A) over most of the visible range, demonstrating that a large number of electrons had been generated within the MoS2 layer by the visible light and were then successfully transferred to the underlying a-IGZO layer, where they contributed to the drain current. A similar mechanism was attributed to phototransistors prepared using rhodamine 6G-treated MoS218 and MoS2−graphene hybrid films.35,36 As the illumination light energy approaches 3 eV (corresponding to an optical band gap of a-IGZO), the Iph of the heterostructured devices approaches the value of the aIGZO device (Figure 2b). These results indicate that the number of electrons that are directly photoexcited from the valence band of a-IGZO overwhelms the number of electrons transferred from the conduction band of MoS2. The photoresponse of the a-IGZO transistor, therefore, prevails in the heterostructured devices. Considering that similar values of Iph were acquired from the a-IGZO and MoS2/a-IGZO devices at a wavelength of 405 nm (corresponding to the UV range), the atomically thin MoS2 film does not appear to obstruct light absorption by the underlying a-IGZO film. An excellent response across the visible range can, therefore, be added to the UV-sensitive, high mobility a-IGZO phototransistors by forming a heterojunction with the MoS2 film. The rather poor electrical quality of the solution-synthesized MoS2 film (low mobility and on-current) can be largely addressed by the highly functioning a-IGZO transistor channel (high mobility and oncurrent), which provides outstanding phototransistor characteristics (specifically, a high Iph with a large mobility) and excellent spectral responsiveness across the entire visible light range and extending into the UV region. External photoresponsivity (R) is a figure of merit for photodetectors that can be calculated as R = Iph/P,37 where P is the total optical power of the illumination. Figure 2c shows the calculated R values for all phototransistors. Among devices that responded across the visible spectral range (i.e., MoS2-only and MoS2/a-IGZO phototransistors), the heterostructured phototransistor exhibited much higher R values (∼55 mA/W) compared to those of the MoS2-only phototransistor under illumination at 520 nm, which is roughly midwavelength within
in Figure 1d) will be generated and then dominate the photocurrent flow in the high-mobility a-IGZO channel. Figure 1e shows representative drain current−drain voltage (ID−VD) characteristics of the heterostructured MoS2/a-IGZO phototransistor operated without gate biasing (VG), as measured both under dark and illuminated (λ = 520 nm) conditions using laser light at various powers (1 μW−2 mW). The linear ID−VD behavior, which is independent of the illumination conditions, indicates good Ohmic contact between the channel layer and the S/D electrodes, and the contact resistance minimally affects the measured transistor characteristics.17 At a fixed VD, an increase in the laser power led to a systematic increase in ID (Iilluminated), starting from a dark current (Idark), due to an increase in the number of electrons generated and subsequently transferred to the channel layer. These results indicate that the fabricated MoS2/a-IGZO phototransistor successfully functions as expected. A clearer picture of the laser power-dependent photocurrent increase, Iph (Iph = Iilluminated − Idark), is evident from Figure S4, where the y axis (ID) is plotted on a logarithmic scale. Figure 2a shows the transfer characteristics (ID−VG at VD = 10 V) of the a-IGZO, MoS2, and MoS2/a-IGZO photo-
Figure 2. (a) Transfer characteristics (VD = 10 V) of a-IGZO, MoS2, and MoS2/a-IGZO phototransistors as a function of the illuminating light wavelength at a fixed optical power (0.5 mW). Wavelengthdependent (b) photocurrent and (c) photoresponsivity of the a-IGZO, MoS2, and MoS2/a-IGZO phototransistors at VG = −10 V.
transistors measured under dark and illuminated conditions. The light was directed onto the active region of the phototransistor using focused laser sources of different wavelengths (405, 520, 655, 785, 850, and 980 nm) at a power of 0.5 mW. Although the field-effect mobility of the aIGZO channel under the dark conditions degraded somewhat from ∼13.5 to ∼6.8 cm2/(V s) after introducing the MoS2 film, D
DOI: 10.1021/acsami.5b11709 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 3. (a) Photoinduced transfer characteristics (VD = 10 V) of the MoS2 and MoS2/a-IGZO phototransistors as a function of the optical power at a wavelength of 520 nm. Optical power dependencies of the (b) photocurrent and (c) photoresponsivity of the MoS2 and MoS2/a-IGZO phototransistors at VG = −10 V.
Figure 4. Time-dependent photoresponse characteristics of the heterostructured MoS2/a-IGZO phototransistor under alternating dark and light illumination conditions (a) without and (b) with cyclic application of a reset voltage pulse (VG = 40 V) to the gate electrode, showing both linear and logarithmic y-scale plots. The upper figures in panels a and b depict the time-dependent cyclic laser and gate pulse schedules. (c) Transfer characteristics of the heterostructured MoS2/a-IGZO phototransistor during the first cycle, which overlaps with the sequence of the drain current at VD = 10 V.
the MoS2-only phototransistor, and the maximum R value was around 1.7 A/W. Furthermore, the R value extrapolated from the measured data points is expected to exceed 103 A/W at an incident optical power of 1 pW. The time-dependent photoresponse properties of the heterostructured MoS2/a-IGZO phototransistor were measured by cyclically switching the illumination of the laser at a fixed wavelength of 520 nm and an optical power of 0.5 mW (Figure 4a). ID was measured at VG = −10 V and VD = 10 V while the illuminating laser was repeatedly switched on and off for 60 and 120 s, respectively. When the laser was first turned on, the ID measured at VG = −10 V from the pristine device increased rapidly from Idark (corresponding to position 1 in Figure 4c) to Iilluminated (corresponding to position 2 in Figure 4c) with an Iilluminated/Idark ratio exceeding 105. Turning off the laser resulted in an abrupt decrease in ID on a short time scale, followed by a slow decay. This behavior prevented the device from reaching its initially low Idark value and yielded a significantly low I illuminated /I dark ratio of less than 10 2 (corresponding to position 3 in Figure 4c). The low
the visible range. As the incident light entered the UV regime at 405 nm, the R value for the MoS2/a-IGZO phototransistor reached a maximum of ∼135 mA/W and approached the value obtained for the a-IGZO-only phototransistor, as observed in the Iph data (Figure 2b). The transfer characteristics of the MoS2-only and MoS2/aIGZO phototransistors depended on the optical power when illuminated using light at a fixed excitation wavelength of 520 nm, as shown in Figure 3a. The optical power dependencies of the Iph and R values are plotted in Figure 3b,c. As shown here, the Iph and R values of both phototransistors varied linearly with the optical power in a log−log scaled plot. These plots followed the power-law relations of Iph ∝ Pα and R ∝ Pα−1 (α ≤ 1), respectively.20 A linear fit revealed α values of 0.5−0.6 for both curves, which indicates the presence of defect-mediated charge recombination in the MoS2 film.20 Across most of the power range, the heterostructured phototransistor displayed much higher Iph and R values than the MoS2-only device. For example, at an incident optical power of 1 μW, Iph was more than 4 orders of magnitude higher than the value obtained from E
DOI: 10.1021/acsami.5b11709 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
ACS Applied Materials & Interfaces
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Iilluminated/Idark ratio was maintained as the cycle continued, and the overall Iilluminated value exhibited a slowly increasing trend. These behaviors mainly resulted from the large relaxation time for recombination of the photogenerated electron−hole pairs relative to the switching speed. These behaviors could also be attributed to the persistent photoconductance (PPC) originating from the slow recombination process in deep trap sites, as observed in the synthesized MoS220 and oxide semiconductor phototransistors prepared with a-IGZO.3,20,38−41 The measured rising and falling curves were used to calculate the time constants (τ) using a stretched exponential function (Figure S5),20,40−42 which yielded time constants of 2.6 and 1.7 s, respectively. Because rapid and near-complete discharge of the trapped photoinduced electrons can be facilitated by applying a short positive gate pulse to a three-terminal device,3,17,40,43 we applied a short gate pulse (VG) of 40 V after turning off the light (duration time of 5 s) at a constant VD = 10 V. As shown in Figure 4b, ID abruptly decreased to ∼10−8 A (corresponding to position 4 in Figure 4c) after the light was turned off and a short gate pulse was applied, yielding a maximum Iilluminated/Idark ratio of nearly 103 with more stable time-dependent photoresponse characteristics. Although significantly improved recombination characteristics were achieved, the new Idark value after the gate pulse discharge did not reach the initial low value. This result appears to be caused by deep carrier traps at the defect sites created during the solution-based MoS2 synthesis (a large number of grain/domain boundaries formed) and also by the ex situ transfer process (which contaminated the MoS2/a-IGZO interface). Both of these processes might have also led to a rather smaller R value than that of the similarly heterostructured a-IGZO phototransistor.8 Nevertheless, the characteristics of the suggested phototransistor may be further improved by introducing gate dielectric materials with a high dielectric constant (for low-voltage operation) and by optimizing the MoS2 synthesis/transfer processes.
Research Article
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.5b11709. EDS spectrum of the a-IGZO film, REELS spectra of the solution-synthesized MoS2 and sputter-deposited aIGZO films, XPS spectra used for the energy band diagram analyses, ID−VD characteristics of the MoS2/aIGZO phototransistor, and time-dependent photoresponse characteristics of the heterostructured MoS2/aIGZO phototransistor. (PDF)
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AUTHOR INFORMATION
Corresponding Author
* E-mail:
[email protected]. Author Contributions ∇
These authors contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by Basic Science Research programs (Grant nos. NRF-2012R1A1A2042548 and NRF2014R1A4A1008474) through the National Research Foundation of Korea funded by the Ministry of Education and the Ministry of Science, ICT & Future Planning.
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REFERENCES
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4. CONCLUSION In summary, we introduced a heterostructured a-IGZO phototransistor consisting of a solution-based synthetic fewlayered MoS2 film (with a band gap of ∼1.7 eV) and a sputterdeposited a-IGZO film (with a band gap of ∼3.0 eV). This phototransistor is a highly sensitive element with a broad spectral responsivity (up to a wavelength corresponding to the band gap of MoS2) and large-area applicability. The electrons generated by visible light in the upper MoS2 layer were effectively transferred to the underlying high mobility a-IGZO film, providing an a-IGZO phototransistor with visible light responsiveness while maintaining excellent transistor characteristics. The heterostructured phototransistor showed excellent photoresponse characteristics, with a responsivity of ∼1.7 A/W at a wavelength of 520 nm and an optical power of 1 μW. The time-resolved photoresponse measurements indicated that the PPC could be suppressed with a high Iilluminated/Idark ratio of nearly 103 by applying a short positive gate pulse. Although further optimization is required, we believe that the MoS2/aIGZO heterojunction phototransistor prepared here and enabled by cost-effective wide-area deposition techniques (including vacuum-based deposition and solution-based deposition of MoS2) represents a significant step forward for fabricating future nanoscale optoelectronic devices. F
DOI: 10.1021/acsami.5b11709 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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DOI: 10.1021/acsami.5b11709 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
