Hybrid Structures of ITO-Nanowire-Embedded ITO Film for ...Research Article Hybrid Structures of...

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Research Article Hybrid Structures of ITO-Nanowire-Embedded ITO Film for the Enhanced Si Photodetectors Hyunki Kim, 1,2 Gyeong-Nam Lee, 1,2 and Joondong Kim 1,2 1 Department of Electrical Engineering, Incheon National University, 119 Academy Rd., Yeonsu, Incheon 22012, Republic of Korea 2 Photoelectric and Energy Device Application Lab (PEDAL), Multidisciplinary Core Institute for Future Energies (MCIFE), Incheon National University, 119 Academy Rd., Yeonsu, Incheon 22012, Republic of Korea Correspondence should be addressed to Joondong Kim; [email protected] Received 31 January 2018; Accepted 14 June 2018; Published 2 July 2018 Academic Editor: Yong Ding Copyright © 2018 Hyunki Kim et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. A high-performance silicon UV photodetector was achieved by using a hybrid of a lm with nanowires. Electrically conductive and optically transparent indium-tin oxide (ITO) was deposited to form an ITO lm or ITO nanowire (NW) on a Si substrate, resulting in a heterojunction. The ITO-lm device is stable with a low-leakage current. Meanwhile, the ITO NWs demonstrated an excellent capability to collect photogenerated carriers. The hybrid ITO (NWs on a lm)/Si photodetector demonstrates a fast UV reactive time of 1.6 ms among Si-based photodetectors. We may nd a means of enhancing the photoelectric performance capabilities of devices beyond the limits of conventional Si via the adoption of functional designs. Moreover, the use of a homogeneous material for the structuring of lms and nanowires would oer a remarkable advantage by reducing both the number of fabrication steps and the cost. 1. Introduction Si-based devices have been widely utilized in various applica- tions, such as solar cells, LEDs, and photodetectors [14]. Especially for photodetectors, rapid response performance is crucial for the realization of sensor networks and optical communications. In addition, high-performing UV photode- tectors have specic applications in ozone sensing, ame detection, and military surveillance [5, 6]. To improve the performance of a UV photodetector, various designs and materials have been applied, such as the GaN-based Schottky junction [6], graphene-based junction [68], GaAs/AlGaAs nanowires [9], ITO nanodomes [10], ITO nanowires with ZnO lm [11, 12], NiO/ZnO lms [5], ZnO/ZnS nanowires [13], ZnO nanorods [14], ZGO nanowires [15], and Si/ZnO nanowires [1]. When considering the complexity of elaborate photodetector designs, the fabrication steps and cost should also be considered for practical applications [2]. In the cost and fabrication aspects, the functional use of a transparent layer is highly desired to enhance the performance for short-wavelength light while passing the visible-range light. The metal oxide materials have a wide bandgap [5], and thus the highly energetic UV photons can be absorbed. However, the visible-range photons spon- taneously pass through the metal oxide layer. This feature renders strong benets for multifunctional photoelectric devices. Moreover, the active adoption of the metal oxide materials would provide the optical transparency and electri- cal excellence which are promising advantages for Si-based photoelectric applications. Recently, the signicant improve- ment of UV photodetectors has been realized by using 2D materials and advanced design schemes [1618]. Here, we report the enhanced performance of Si-based photodetectors using a functional ITO hybrid window. Electrically conductive ITO layers were formed on a Si substrate to establish a high-quality heterojunction device. In order to maximize the advantages of ITO materials, ITO nanowires were formed on the ITO lm, providing signicantly enhanced current ows with a less noisy current. This is likely the rst report of the use of homogeneous Hindawi Journal of Nanomaterials Volume 2018, Article ID 4178989, 8 pages https://doi.org/10.1155/2018/4178989

Transcript of Hybrid Structures of ITO-Nanowire-Embedded ITO Film for ...Research Article Hybrid Structures of...

  • Research ArticleHybrid Structures of ITO-Nanowire-Embedded ITO Film for theEnhanced Si Photodetectors

    Hyunki Kim,1,2 Gyeong-Nam Lee,1,2 and Joondong Kim 1,2

    1Department of Electrical Engineering, Incheon National University, 119 Academy Rd., Yeonsu, Incheon 22012, Republic of Korea2Photoelectric and Energy Device Application Lab (PEDAL), Multidisciplinary Core Institute for Future Energies (MCIFE), IncheonNational University, 119 Academy Rd., Yeonsu, Incheon 22012, Republic of Korea

    Correspondence should be addressed to Joondong Kim; [email protected]

    Received 31 January 2018; Accepted 14 June 2018; Published 2 July 2018

    Academic Editor: Yong Ding

    Copyright © 2018 Hyunki Kim et al. This is an open access article distributed under the Creative Commons Attribution License,which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

    A high-performance silicon UV photodetector was achieved by using a hybrid of a film with nanowires. Electrically conductive andoptically transparent indium-tin oxide (ITO) was deposited to form an ITO film or ITO nanowire (NW) on a Si substrate, resultingin a heterojunction. The ITO-film device is stable with a low-leakage current. Meanwhile, the ITO NWs demonstrated an excellentcapability to collect photogenerated carriers. The hybrid ITO (NWs on a film)/Si photodetector demonstrates a fast UV reactivetime of 1.6ms among Si-based photodetectors. We may find a means of enhancing the photoelectric performance capabilities ofdevices beyond the limits of conventional Si via the adoption of functional designs. Moreover, the use of a homogeneousmaterial for the structuring of films and nanowires would offer a remarkable advantage by reducing both the number offabrication steps and the cost.

    1. Introduction

    Si-based devices have been widely utilized in various applica-tions, such as solar cells, LEDs, and photodetectors [1–4].Especially for photodetectors, rapid response performanceis crucial for the realization of sensor networks and opticalcommunications. In addition, high-performing UV photode-tectors have specific applications in ozone sensing, flamedetection, and military surveillance [5, 6]. To improve theperformance of a UV photodetector, various designs andmaterials have been applied, such as the GaN-based Schottkyjunction [6], graphene-based junction [6–8], GaAs/AlGaAsnanowires [9], ITO nanodomes [10], ITO nanowires withZnO film [11, 12], NiO/ZnO films [5], ZnO/ZnS nanowires[13], ZnO nanorods [14], ZGO nanowires [15], and Si/ZnOnanowires [1]. When considering the complexity of elaboratephotodetector designs, the fabrication steps and cost shouldalso be considered for practical applications [2].

    In the cost and fabrication aspects, the functional useof a transparent layer is highly desired to enhance the

    performance for short-wavelength light while passing thevisible-range light. The metal oxide materials have a widebandgap [5], and thus the highly energetic UV photonscan be absorbed. However, the visible-range photons spon-taneously pass through the metal oxide layer. This featurerenders strong benefits for multifunctional photoelectricdevices. Moreover, the active adoption of the metal oxidematerials would provide the optical transparency and electri-cal excellence which are promising advantages for Si-basedphotoelectric applications. Recently, the significant improve-ment of UV photodetectors has been realized by using 2Dmaterials and advanced design schemes [16–18].

    Here, we report the enhanced performance of Si-basedphotodetectors using a functional ITO hybrid window.Electrically conductive ITO layers were formed on a Sisubstrate to establish a high-quality heterojunction device.In order to maximize the advantages of ITO materials,ITO nanowires were formed on the ITO film, providingsignificantly enhanced current flows with a less noisy current.This is likely the first report of the use of homogeneous

    HindawiJournal of NanomaterialsVolume 2018, Article ID 4178989, 8 pageshttps://doi.org/10.1155/2018/4178989

    http://orcid.org/0000-0002-9159-0733https://doi.org/10.1155/2018/4178989

  • materials for the nanowire-embedded film structure appliedto a photodetector.

    2. Experimental Procedures

    The 200μm-thick FZ p-type Si wafers (100) having resistivityvalues of 1–3Ω∙cm were used as substrates and cleaned byacetone, methanol, and DI water under an ultrasonicationcleaning process. A rapid thermal annealing process wasapplied to clean the Si surface and form a thin oxide layer.The thin oxide layer is effective for reducing the reverse sat-uration current and thus improves the device property [19].Silicon was used with a thin oxide layer, which is the samefor all the devices [20]. Therefore, the observed changes areonly due to the top ITO layer. In order to form a heterojunc-tion, a transparent ITO layer was deposited onto the thinoxide-coated p-Si substrate by using a sputter system(SNTEK, Korea). To tailor the ITO layers, various ITO depo-sition processes were applied to grow the ITO film, ITOnanowires (NWs), and hybrid ITO structure (ITO-NW-embedded ITO film). To deposit the ITO film, 100W ofDC power was applied to an ITO target (In2O2 containing10wt.% SnO2) for 20min at 600

    °C.In order to grow ITO NWs, a thin Ni film (5 nm) was

    deposited before the sputtering of the ITO [11, 21, 22]. Pre-annealed Ni dots serve as seeds to grow ITO nanowires bythe sputtered ITO nanoparticles. The ITO sputtering isalmost identical to that of the ITO film growth.

    For the hybrid structure of the ITO-NW-embedded ITOfilm, the ITO film was initially deposited under typical sput-tering conditions for 20min, and the ITO nanowire growthcondition was then applied for 8min. Subsequently, Al metalcontacts were formed on the front and back of the photo-detector using the same sputtering system. The generalconfiguration of the photodetector is as follows: Al—front/ITO—window/p-Si/Al—back contact.

    A field emission transmission microscope (FETEM,JEOL, JEM-2100F) was used to observe the ITO structuresand the interface with the Si substrates. All ITO structureswere also formed on glass substrate to monitor the transmit-tance profiles. Optical transparency was obtained by using aUV-visible spectrophotometer (Shimadzu, UV-1800) forthe various ITO structures in the range of 300–1100 nm.Room temperature dark I −V measurements were performedusing a source meter unit (Keithley, 2400). Photoresponsesof the ITO-embedded Si photodetectors were performedunder UV-light illumination from a 400nm monochro-matic LED lamp. The LED source was calibrated with a powermeter (Kusam-Meco, KM-SPM-11). A field emission trans-mission electron microscope (FETEM, JEOL, JEM-2100F)was used to observe the ITO structures and the interface toSi substrates.

    3. Results and Discussions

    Various types of ITO structures were applied to Si substratesto realize the transparent window-embedded Si photodetec-tors. In this case, n-type ITO layers spontaneously form aheterojunction by coming into contact with the thin oxide-

    coated p-type Si substrates. Figure 1(a) shows a schematicof the ITO-embedded Si photodetector. Three different typesof ITO structures were applied. A typical ITO film wasapplied as a reference device. The ITO NW device was pre-pared by controlling the surficial morphology. The hybridstructure has ITO nanowires grown onto the ITO film. Here-after, each sample is referred to as the ITO film, ITO NW,and hybrid-ITO. Figure 1(b) shows photograph images ofthe three devices.

    In order to investigate the transmittance, glass sampleswere also prepared under identical ITO deposition condi-tions. The transmittance profiles were obtained in thewavelength (λ) range of 300 and 1100 nm, as shown inFigure 1(c). The ITO film has the highest average trans-mittance of 86.72%. The ITO film has an oscillation fea-ture due to the light interference of the ITO-film layer.Meanwhile, no significant oscillation profiles were observedfrom the ITO-NW structure or hybrid-ITO window sam-ples due to the existing nanoscale entities to relieve thelight interference.

    Meanwhile, the ITO-NW sample has a lower transmit-tance value of 73.40% due to light scattering by the nanowires[15]. For the hybrid-ITO sample, the combination of ITOfilm and ITO nanowires reduced the transmittance value fur-ther by 69.88%. According to the transmittance profiles,nanowire-embedded surfaces have an overall lower transmit-tance value for short wavelengths, which clearly indicates thestrong interaction between nanowires and short λ photons.

    SEM observation was employed to show the structuralchanges of the samples. Figure 2(a) shows the top-view imageof the ITO thin film deposited on a Si substrate, whichconfirms the uniform growth. The film thickness is approxi-mately 180nm, as shown in Figure 2(d). Meanwhile, the sam-ple grown with ITO nanowires (ITO NWs) has a relativelythin ITO base film of approximately 130nm (Figure 2(e)).A single ITO NW has a Ni tip on the top of the ITO NW,which is an intrinsic feature of the ITO NW grown by thesputtering method [21].

    Figure 2(b) depicts regular ITO nanowires on anITO base film. The hybrid-ITO sample is presented inFigure 2(c), showing the combined structures of ITO nano-wires and the ITO film. Due to the combined processes ofthe ITO-film formation step and the ITO-nanowire growthstep, the ITO base film has a thickness of 227nm, as shownin Figure 2(f). Short nanowires and nanoscale clusters canclearly be observed in Figure 2(c).

    The working mechanism of the heterojunction device isaccording to the energy band diagram of Figure 3(c). Thetype II junction is suitable for drifting photogenerated car-riers to external electrodes. In order to determine the workfunction (WF) of ITO, Kelvin probe force microscopy(KPFM) measurement was performed. The topography andthe WF map of ITO are shown in Figures 3(a) and 3(b),respectively. The topography FESEM image is shown inFigure 2 of the ITO surface and the corresponding root meansquare (RMS) profile was calculated to be 1.76 nm. Mean-while, the WF of ITO was obtained as 4.71 eV.

    To characterize the device performances, current-voltageprofiles were obtained for all three devices under a dark

    2 Journal of Nanomaterials

  • condition, as shown in Figure 4. In order to evaluate the rec-tifying characteristics, the rectification ratios were obtainedby the ratio of the current value of 1.5V to the current valueof −1.5V. The ITO-film device provides a good rectificationratio of 161.37. Meanwhile, the ITO-nanowire device has alower value of 7.167. This is mainly caused by the reverse sat-uration current density of 9.21 (mA/cm2), which is 35 timeshigher than 0.256 (mA/cm2) of the ITO-film device. Inter-estingly, a significantly enhanced rectification value of 891.69was obtained from the hybrid-ITO structures. This improve-ment was induced due mainly to the improved forwardcurrent density, which is more than double to that of theITO-film device (Table 1). Moreover, the reverse saturationcurrent was also reduced, which clearly reflects the inherentbenefit of the ITO film.

    To qualify the diode junction, the ideality factor (n) wasobtained from the following equation:

    n = qkT

    ∂V∂ ln I =

    qkT

    ∂V2 3 × ∂ log I , 1

    where q, kT , and I are the electron charge, thermal energy(eV), and current, respectively. The ITO-film device has a fairvalue of 1.73, compared to the ITO-NW device which has avalue of 1.92. On the other hand, an improved value (1.25)was achieved from the hybrid-ITO device, which confirmsa better-quality junction compared to that of the ITO-filmdevice (Table 1). The ITO-NW surface effectively providesa carrier transport route from Si to the Al-front contact toresolve the current spreading effect from the conventional

    ITO film

    Al fron

    t electro

    de

    ITO nanowire

    Light

    P-Si sub

    strate

    Al back

    electrod

    e

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    100

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    smitt

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    (%)

    20

    0400 600 800 1000

    Wavelength (nm)

    ITO filmITO NWsHybrid-ITO (film + NWs)

    (c)

    Figure 1: (a) Schematic of the hybrid-ITO (ITO-nanowire-embedded ITO film) photodetector, (b) photographic images of Si photodetectorsand ITO-coated glasses, and (c) transmittance profiles of the ITO-film, ITO-NW, and hybrid-ITO window samples.

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  • ITO-film conductor [10]. The conventional ITO conductorhas a disadvantage for electron movements due to the filmshape for the current spreading effect. Meanwhile, the sharp

    geometric feature of ITO NWs spontaneously provides thepoint contact to the Al-front metal to induce an efficientflow of electrons. The hybrid structure simultaneously has

    (a) (b)

    (c) (d)

    (e) (f)

    Figure 2: Top-view and cross-sectional-view of FESEM images of (a,d) ITO-film, (b,e) ITO-NW, and (c,f) the hybrid-ITO (ITO-NW-embedded ITO film) samples.

    10 nm

    0 nm

    (a)

    4.9 eV

    4.6 eV

    (b)

    Vaccum level

    4.01 eV 4.71 eV 4.2 eV

    AlAl

    ITO

    p-Si

    h

    h

    e

    e e

    h

    (c)

    Figure 3: ITO (a) topography, (b) work function map, and (c) energy band diagram of the Al/ITO/P-Si/Al device.

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  • advantages for the quality junction formation in the meritof the ITO film and effective collection formation due tothe ITO NWs.

    In order to investigate the photoelectric performances,photoresponse measurements were obtained from all threesamples. The photoresponse performances were obtainedby turning on and off the UV light illumination. The UVLED source has a wavelength of 400nm with a light intensityof 3mW/cm2. By illuminating the UV light, significantresponses were clearly obtained from all three samples, asshown in Figure 5. To estimate the photoreactive perfor-mances, the photoresponse ratio (PR) was obtained by thefollowing relation:

    Photoresponse ratio = IUV‐onIUV‐of f

    2

    The PR is a function of the light-induced current over thedark current. The ITO-NW device has the lowest value of1.194, which is attributed to the high dark current (IUV−Of f )value of −7.07μA, compared to the light current (IUV−On)value of −8.44μA. This serious dark current is induced fromthe poor interface between Si and ITO NWs. Meanwhile, theITO-film device has an improved PR value of 4.85 by the sup-pressed IUV−Of f value (−1.50μA) and a much enhancedIUV−On value (−7.27μA). A remarkable PR value of 7.97was obtained from the hybrid-ITO device. This result sig-nifies the combined advantages of the ITO-film device andthe ITO-NW device. The ITO nanowires have a tapered fea-ture with Ni tips, which induce the efficient flows of thephotogenerated carriers, resulting in a remarkably improvedIUV−On value (−9.86μA). In addition, the ITO film provides ahigh-quality interface to Si and effectively controls theIUV−Of f value (−1.23μA).

    The photodetection performances of the three deviceswere evaluated by the response speed. The UV reactive time(τUV−On) and UV recovery time (τUV−Of f ) were estimated.The change of current Ichange was obtained by the followingequation:

    Ichange = IUV‐On − IUV‐Of f 3

    The τUV−On is defined as the time taken for the increase ofthe current value from 10% to 90% of the change of current[11]. The τUV−Of f is the time for the decrease of the currentvalue from 90% to 10% of the change of current. Due to theabrupt heterojunction of the ITO and Si contact, theτUV−Of f values are excellent (0.3–0.4ms) for all three devices,as presented in Figure 4(b). In the aspects for UV reaction,different huge performances appeared by ITO structures.The ITO-NW device shows a fast UV response time of2.15ms. Compared to this, the ITO film has the longer

    200

    150

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    50

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    −1.5 −1.0 −0.5 0.0Voltage (V)

    0.5 1.0 1.5

    Curr

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    m2 )

    ITO filmITO NWsHybrid-ITO (film + NWs)

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    ITO filmITO NWsHybrid-ITO (film + NWs)

    (b)

    Figure 4: I −V characteristics of the ITO-film device, ITO-NW device, and the hybrid-ITO (ITO-nanowire-embedded ITO film) device. (a)Linear plot and (b) semilog plot.

    Table 1: Performance of the ITO-film device, ITO-NW device, andthe hybrid-ITO (ITO-nanowire-embedded ITO film) device.

    ITO film ITO NWs Hybrid-ITO

    Current at +1.5 V (mA/cm2) 0.0779 0.0762 0.188

    Current at −1.5V (mA/cm2) −0.256 −9.21 0.211Rectifying ratio 161.37 7.167 891.69

    Ideality factor 1.73 1.92 1.25

    Response ratio (IUV−On/IUV−Of f )

    4.85 1.194 7.972

    IUV−On (μA) −7.27 −8.44 −9.84IUV−Of f (μA) −1.50 −7.07 −1.23

    UV-reactive time (τUV−On) (s) 0.00895 0.00215 0.0016

    Recovery time (τUV−Of f ) (s) 0.0004 0.0004 0.0003

    5Journal of Nanomaterials

  • τUV−On time of 8.95ms, which signifies the delay of the col-lection time for the photogenerated carriers. Due to the filmtype, the photogenerated carriers readily spread through theITO film. This is due to the similar resistance values betweenthe ITO film and the Al-metal contact [10]. A significantlyimproved result was obtained from the hybrid-ITO devicefor the τUV−On value of 1.6ms, which outperforms otherSi-based photodetectors [4, 11, 23, 24] and is a furtherenhancement over organic devices [25].

    These results strongly suggest that the photoelectricdevice performances can be improved by doing structuralengineering [26]. In this work, the hybrid-ITO structure(ITO-NW-embedded ITO film) demonstrated significantlyenhanced device performances. This is a promising advan-tage with regard to device fabrication using homogenousmaterials with consecutive processes, in contrast to heteroge-neous devices which require multiple and complex processes.

    We may find a route by which to realize efficient photoelec-tric devices by adopting the advantages of the nanostructuresand films described here.

    4. Conclusion

    Optically transparent and electrically conductive ITO wasapplied to form a quality ITO film or ITO nanowires forhigh-performance Si UV photodetectors. Due to the utiliza-tion of a homogeneous material for the ITO film and ITOnanowires, an identical sputtering method can be applied totailor the ITO front surface onto a Si substrate, which is anaffordable device fabrication approach which does not inter-rupt the device processes or lead to complex issues. Thehybrid-ITO (nanowire-embedded ITO film) device showedgreatly enhanced UV photoresponses compared to the filmconsisting of only ITO or the ITO nanowire device. A

    2

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    s)

    ITO film ITO NWs

    Response ratioUV reactive time

    Hybrid-ITO

    (b)

    Figure 5: Photoresponses of the ITO-film device, ITO-NW device, and the hybrid-ITO (ITO-nanowire-embedded ITO film) device. (a)Current-time profiles. (b) Response ratio and UV-reactive time plots.

    6 Journal of Nanomaterials

  • remarkably fast UV reactive time of 1.6ms was obtained dueto the combined merits of the nanowires and film.

    Homogeneous material-based designs are rarely reportedfor photoelectric applications. However, the effect is substan-tial with affordable processing steps. In the future, we mayfind a strong design approach for functional photoelectricdevices with unexpected but substantial enhancements.

    Data Availability

    The data used to support the findings of this study areavailable from the corresponding author upon request.

    Conflicts of Interest

    The authors declare that they have no conflicts of interest.

    Authors’ Contributions

    Hyunki Kim and Gyeong-Nam Lee contributed equally tothis work.

    Acknowledgments

    This research was supported by Basic Science ResearchProgram through the National Research Foundation(NRF) of Korea by the Ministry of Education (NRF-2018R1D1A1B07045336) and the Korea Institute ofEnergy Technology Evaluation and Planning by the Minis-try of Knowledge Economy (KETEP-20168520011370).

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    8 Journal of Nanomaterials

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