Transparent and Flexible In2O3 Thin Film for Multilevel Nonvolatile

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Transparent and Flexible In2O3 Thin-Film for Multilevel Non-Volatile Photomemory Programmed by Light Sohail Abbas, Mohit Kumar, Dong-Kyun Ban, Ju-Hyung Yun, and Joondong Kim ACS Appl. Electron. Mater., Just Accepted Manuscript • DOI: 10.1021/acsaelm.8b00139 • Publication Date (Web): 01 Mar 2019 Downloaded from http://pubs.acs.org on March 7, 2019

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ACS Applied Electronic Materials

Transparent and Flexible In2O3 Thin-Film for Multilevel Non-Volatile Photomemory Programmed by Light Sohail Abbasa,b, Mohit Kumara,b, Dong-Kyun Bana,b, Ju-Hyung Yuna,b* and Joondong Kima,b,* aDepartment

of Electrical Engineering, Incheon National University, 119 Academy Rd.

Yeonsu, Incheon, 22012, Republic of Korea bPhotoelectric

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 *Author to whom correspondence should be addressed. Electronic mail: J. Kim ([email protected]) and J.H. Yun ([email protected]) Keywords: Photomemory, Multibit, Programable, Non-volatile, Transparent and Flexible, Photodetectors Abstract The optically triggered data processing and storage provides an interesting arena for developing sophisticated next–generation smart windows and computation technology. So far, transparent and flexible metal-oxides have shown phenomenal optoelectronic application. In this article, we report a photomemory of In2O3 thin-film deposited on glass and PET substrate using large-scale sputtering system. The electrical characterization of device under light and dark conditions reveals vast persistent photoconductivity (PPC) at room temperature. The PPC is systematically exploited for multibit data storage by programing with photon pulse, intensity and source-to-drain voltage. Similarly, a high degree of persistence (>104 seconds) is achieved to retain optical information. The underlying working mechanism is attributed to the trapping of photogenerated electrons by oxygen vacancies, while corresponding holes freely participate in electrical transport even after light termination. Finally, the energy band diagram is proposed using In2O3 workfunction (4.26 eV measured with KPFM) and bandgap. The functional use of transparent thin-film may provide a solution of the complex multilevel programming architecture. This flexible and light-weight device can be applied in smart transparent electronics, including memories, photodetectors, and solar cells.

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1. Introduction The concept of photomemory is based on the storage of light information as nonvolatile photocurrent within optoelectronics devices.1,2 These devices use light as an input to modulate photocurrent at multiple levels for multibit data storage, by programing via various parameters, including intensity, bias voltage and so on.3 In general, photomemory forms the foundation of many optoelectronic devices used for recording, communication and neuromorphic computation.4,5 A photomemory device works on the principle of persistent photoconductivity (PPC), in which a photocurrent persists even after the light has been terminated.6,7 Generally, PPC is attributed to the presence of active defects within an optically active material. The defects trap the photogenerated charge carriers as either electrons or holes, while allowing the corresponding carriers to freely participate in electrical transport.2 To do so, selective materials with suitable workfunction and bandgap forming a straddling band offset (that is type–I heterojunction) are needed, to facilitate the PPC by trapping/de–trapping the photogenerated charge carriers at the heterojunction interface.3 Generally, this approach involves multiple materials for heterojunction formation, requiring complex fabrication processes at high cost. Meanwhile, the introduction of single layer-based flexible and transparent device can revolutionize the cost effective wearable next-generation “invisible” electronic information technology.1,2 Transparent and flexible devices also have potential applications in military and transportation industries, including integrated see-through optoelectronics8, such as windows and displays. Mostly, metal oxides–based heterojunctions are used to realize such highly transparent electronics.9 However, realizing the PPC within sinlge layer-based device to achieve multilevel data storage is yet to be demonstrated. So far, various materials and innovative architectural designs have been used for photomemory.1–4 For example, Leydecker et al. demonstrated three therminal, non-tranparent 2 ACS Paragon Plus Environment

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transistor configuration featuring non-voltile (>500 days) and multilevel (~256) optical memory based on poly(3-hexylthiophene) and photochromic diarylethene flexible thin film.1 Similarly, Yao et al. designed a flexible and broadband optoelectronic memory utlizing perisistent photoconductivity at In2O3/In2(TeO3)3 bulk heterojunction prepared with an innovative post annealing process.2 Qian et al. exploited vanadyl-phthalocyanine (VOPc) on para-sexiphenyl (p-6P) thin films to realize a type-I band aligment to trap and modulate the charge carriers for functional use of photomemory using externally bias voltage and light intentsity.3 This photomemory device has a promising on and off ratio (photocurrent/dark current) of 1.5×105, while storing light information for ~5000 seconds but with a decrease of 20%. Wang et al. designed a non-volatile (104 seconds) photomemory using 2D MoS2/PbS vand der walls heterostuctures.4 This non-flexible and non-tranparent phtomemory device was particularly programmed with infrared radations such as 850, 1310 and 1550nm. On the other hand, In2O3 is considered a promising candidate for designing transparent photomemory devices, due to its excellent optoelectronic properties.10 Ruprecht for the first time explored the transparent and conductive nature of polycrystalline In2O3 grown via the thermal oxidation of indium in air at high temperature.11–13. Generally, it shows the n-type conductivity with an energy bandgap of ~3.6 eV.13 However, the conductivity majorly rely on the stoichiometry of grown layer under oxygen regulation, which allowed it to be exploited as either a semiconductor or insulator.13 The semiconductor nature is harnessed by growing under controlled oxygen regulation, which also generates the deep level intrinsic defect states known as oxygen vacancies. These vacancies play a vital role in the trapping/de-trapping of charge carriers, to design photomemory.14,15 However, the utilization of In2O3 to design highly transparent and flexible device is yet to be demonstrated. It is also worth to mention here that conventional memory storage involves electrical and magnetic manipulation.16 The current memory technology is highly dependent on complex fabrication steps and expensive processes. This will become an even more serious issue as 3 ACS Paragon Plus Environment


multiple storage devices by deep scaling-down.1,16 In addition, it has the promise to be applied into the next-generation transparent and flexible electronics by freeing from the dependency of a rigid substrate.17 In this aspect, we consider an optically controlled device for photomemory. By using a flexible and transparent substrate, a light signal can manipulate the multiple current levels to program the multibit data management.1,2 For example, to design a 2-bits storage system (2n, where n is the number of levels, then 2 bits need 4 levels) we need a switching ratio (ratio of current on and off) of 104 while keeping 1-degree difference from each level.1 For a 4-bits storage system (16-levels), we need an extremely high switching frequency of 1016, which is an extremely challenging issue. Instead, the degree of difference between the on and off can be reduced to a level where the current maximum and minimum is still differentiable.1 In this way, we can easily program even an 8-bits storage system (256 levels) with a compromise switching ratio of 103 by modulating the photon pulse, intensity or bias voltage. In this report, we have designed a photomemory by growing an In2O3 thin-film via largescale sputtering method. Initially, X-ray diffraction (XRD) revealed the highly crystalline nature of the In2O3 layer. SEM analysis further illustrated the formation of the smooth crystalline surface. Corresponding EDS analysis confirmed the presence of indium and oxygen. In addition, XPS results indicated the high density of oxygen vacancies in the asprepared indium oxide. These vacancies facilitates the PPC by trapping and de-trapping the photogenerated charge carriers. The PPC is further used to store the optical information. The underlying working mechanism is finally explained using an energy band diagram, which validates the results obtained from the electrical and material characterization. The transparent and flexible In2O3 thin-film devices were fabricated on PET and glass substrates to demonstrate the cost-effective and easy process for light-induced photomemory of multilevel programming.


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2. Result and Discussion 2.1 Schematic and material characterization Figure 1a illustrates a schematic diagram of the transparent thin-film In2O3 photomemory device on a glass substrate. The two silver dots (dia.~1 mm), which worked as source and drain electrodes, were deposited on the In2O3 layer. The in-between gap is utilized as the active part for light illumination. The crystalline quality of the deposited In2O3 layer was explored using XRD analysis, as shown in Figure 1b. The highest peaks at 2θ=30.4o and 61.60o are corresponding to the (222) and (622) planes, respectively. Similarly, smaller peaks corresponding to other planes of In2O3 with miller indices of (211), (332), (431), (440) and (026) were obtained at 21.22o, 41.68o, 45.56o, 50.76o and 60.16o, respectively. All the observed peaks matched well to the cubic structure of In2O3 and no any impurity peaks were observed, thus confirming the phase purity of the deposited indium oxide (JCPDS No. 06041).18 In addition, crystalalline size (D) of ~29 nm for the (222) plane was calculated using the Scherrer formula: , D=0.94λ/(B • Cosθ), where B represents full width at halfmaximum (FWHM) in radians, λ represents wavelength of x-ray in nm and θ represents the Braggs’ angle in degrees.8,19 The planar morphology of the deposited indium oxide was observed using scanning electron microscopy (SEM) imaging. Interestingly, we found an smooth layer as shown in the Figure 1c, where the nanocrystals were arranged in a uniform thin-film. In addition, EDS mapping was employed for elemental investigation of the corresponding layer. This confirmed the presence of indium (In) and oxygen (O), as shown by differentiating colors in the Figure 1d. XPS was also employed to further explore the chemical composition and oxidation states of the as-grown thin-film. In the binding energy range from 435 to 460 eV, the two distinct peaks at 444.3 and 451.8 eV confirm the presence of the 3d5/2 and 3d3/2 orbitals of the trivalent indium ion (In3+), see Figure 1e.20 The 7.5 eV difference between two peaks is 5 ACS Paragon Plus Environment


attributed to the spin–orbit splitting.21 Further, fitting and deconvolution of 3d5/3 revealed a metallic indium peak and its specie in indium oxide.22 In addition, the oxygen peak evaluation led us to an interesting finding of excessive oxygen vacancies along with lattice oxygen (In– O–In), as shown in Figure 1f. The possible source of the oxygen vacancies could be the crystalline boundaries. One can observe that the oxygen vacancy and lattice peak at 531 and 529 eV binding energy have an approximately equal normalized intensity.21

2.2 Optical and Electrical characteristics The optical characteristics of the In2O3 thin-film device are shown in Figure 2a. The device was highly transparent (see the inset picture) with an average transmittance higher than 85% in the visible region (400–800 nm).23 Interestingly, transmittance of 80 and 81% was recorded at 507 (for scotopic vision) and 555 nm (for photopic vision), respectively. The transmittance values at these specific wavelengths for scotopic and photopic vision actually illustrate the feasibility of the proposed device in transparent electronics, since they are suitable for proper human eye sensitivity at low and high light illumination, respectively.24 In addition, the absorbance spectra further clarifies that the grown layer is absorbing the lower wavelength of light, which has higher energy.25 Therefore, we can observe an absorption edge in the UV region which matches well to the larger bandgap of In2O3.26 Figure 2b shows the I–V characteristics of the device under dark condition. In the range of ±1 Volt, the device exhibits linear behavior, which confirms that the silver electrode forms an Ohmic contact with the In2O3 layer. Similarly, under light illumination the charge carriers are generating and no deviation from the Ohmic nature is noticed in electrical transport, as shown in Figure 2c. In traditional photodetectors, only a specific photocurrent curve is obtained for each bias voltage for all I-V cycles,17 however, in our device, we obatained multiple current levels for each voltage with different input cycles, as shown in Figure 2c. This is due to prominent hysteresis effect, in which the device current line keep 6 ACS Paragon Plus Environment

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increasing based on the history of trapped charge carries.27 These current levels can be used to store information in the form of optically triggered signals, which is illustrated by an inset figure with five programmed current levels.1–3 The semi–log representation of the dark and light I–V curves further illustrates the current level setting shown in Figure 2d, with an excellent on/off ratio of 103. The proposed In2O3 thin-film photomemory was also grown on a PET substrate to illustrate its feasibility for flexible electronics.17,28,29 A photograph of the device and its transmittance spectra are shown in Figure 2e, which demonstrates that the device is highly transparent and flexible in nature. Further, it was tested to explore its electrical characteristics as shown in Figure 2f, which confirms that it can store optical information with each light pulse and sustain it for a long period of time (>104 seconds, which is termed as degree of persistance).

2.3 Non-Volatile, Programmable and Multilevel Characteristics The transient photoresponse with distinct increasing current under light, and corresponding persistent photoconductivity under the dark condition, are shown in Figure 3a. These results indicate two important parameters, i.e., the optical window and degree of persistence. Both of these are necessary to categorize the non-volatile photomemory.2,3 The former is the opening between lines representing current under light and dark condition (on/off ratio), which should be very high to efficiently store multibit information via optical trigged signal.1 Similarly, the latter is also the most sought–after parameter, defined in terms of non-volatility, for sustaining information for a long period of time.1,3,4,6 Interestingly, our thin-film memory device ranks very high for non-volatile data storage in both parameters with a huge optical window (103, which is the on/off ratio) and giant degree of persistence (>104 seconds, depicted in Figure 2f).



The photowriting is programmed via external bias voltage, varying light intensity and the pulsed frequency of light. Initially, the bias voltage is exploited to program the photomemory under a fixed light intensity (4 mW cm-2), as shown in Figure 3b. We can see that the increase in bias voltage from 0.1 to 1 volt facilitates an increase in the optical window. This is due to the efficient separation of photogenerated charge carriers under the increasing external bias voltage. The multiple level tuning is possible by varying the bias voltage level, to permit the data program by triggering signal.1 For example, when we need to store a small amount of information, a small bias voltage (~0.1) may be applied. This small operating voltage assists in the miniaturization of next generation photonics by reducing electrical and heating losses.16 Similarly, varying the intensity also manipulates the the photoactive In2O3 thin-film under a fixed applied bias, as shown in Figure 3c. The increase in intensity from 1 to 4 mW cm-2 systemically widens the optical window (on/off ratio) due to the excessive number of photogenerated charge carriers. This enables multiple options for extensive data storage.2 The efficient conversion of photon energy to electrical signal is imperative for a robust photomemory device. It can be evaluated by the photodetector figure of merits (FOM), including responsivity, detectivity, noise equivalent power and etc.8,17,30. A high value of responsivity (R=Ip/(A×Pin), where Ip is photocurrent, Pin is input power per area and A is the effective area) and detectivity (D=R(A/2qId)1/2, where q is the electronic charge, and Id is the dark current) are considered an efficient optoelectronic device.17,31 Interestingly, our In2O3 thin-film device showed increasing behavior with decreasing light intensity for both responsivity and detectivity, as shown in Figure 3d. These parameters are also directly proportional to the input bias voltage, which actually modulates the separation of the photogenerated charge carriers, resulting in the modulation of photocurrent. In addition, a noise equivalent power of 7.16×10-11 W Hz-1/2 was also calculated under similar conditions. This particularly illustrates the device’s excellency to operate under a very low light intensity, i.e., above a picowatt.30 8 ACS Paragon Plus Environment

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Finally, optical data storage by pulsating light in the form of photocurrent levels is investigated in Figure 3f. Initially a single current level for a one-bit data storage is shown in Figure 3e. We intentionally set a single current level using a small optical signal. This level will store only one bit of data obtained through an optical trigger in the form of current.1 In addition, the device also shows a very sharp rise time of 782 μs, which is a prerequisite for state-of-the-art ultra-fast computing technology.4,16 Multibit data storage by programing multilevel current states with pulsating light is shown in Figure 3f. We can see that the proposed design can set many current levels and even go beyond 400 levels, with higher pulsating light, which is enough to integrate even a modern 8-bit (256 levels) storage system.1,3 In the inset, a clear demonstration of 5 bits of optical information managment is established by 5 times discrete light pulsation.

2.4 Working Mechanism An energy band diagram is proposed to illustrate the underlying photowriting and storing mechanism of the In2O3 (bandgap of 3.6 eV) thin-film device. Initially the workfunction (the energy required to remove an electron from the fermi-level to vacuum-level) of the In2O3 (qϕIn2O3=4.26 eV) was measured using KPFM (see Figure 4a), which is smaller than that of the metallic workfunction of a silver (Ag) electrode (qϕAg= 4.46 eV). This is illustrated by an energy band diagram before contact in Figure 4b. After contact, the Ag and n-type In2O3 form a blocking band alignment due to the higher workfunction of the Ag with respect to In2O3. Generally, this type of alignment (qϕAg-qϕIn2O3>0) exhibits Ohmic behavior under electrical measurement, as depicted in the electrical characterization paragraph, see Figure 2b.32 Under light illumination, an electron hole (e–h) pair is generated in the In2O3 thin-film as shown in Figure 4c.17 However, In2O3 thin-film possess excessive oxygen vacancies which are considered as usual trapping sites for electrons. These trapped states in fact totally change the transport dynamics.2 The hole from photogenerated e–h pair would drift freely into the 9 ACS Paragon Plus Environment


external circuit and participate in electrical transport, while the counter electron (from the e–h pair) is trap via oxygen vacancies, see band alingment in Figure 4c. Consequently, the excess holes (from the e–h pairs) participate in electrical transportation until corresponding electron jumps via trap-to-trap for relaxation.30 This facilitates a photocurrent even after light termination known as peristent photoconductivity. It is worth to mention here that to confirm the role of oxygen vacancies, the In2O3 was annealed at 500 °C in oxygen ambient. Interestingly, the deice based on annealed In2O3 did not show PPC, which further authenticate our proposition. Similar kind of mechanism-based on oxygen vacancies has been proposed and demonstrated previously.2 This is the basic and important mechanism of the light-induced photowriting mechanism to control the photogenerated carriers for quick transport or trap-totrap relaxation. Multilevel memory states can be established by light pulsation time and intensity and/or bias voltage.

3. Conclusion We report the development and evaluation of a metal–oxide–based highly transparent and flexible photomemory device. It exhibited a huge optical window (103) along with a gigantic degree of persistence and was able to store the information for a long period of time (>104 sec) under the ambient condition. The information can be stored in the form of current states by input light pulse. It can be easily programmed using an external bias voltage, varying intensity and the chopping frequency of light for a non-volatile multilevel storage system. The device demonstrated a window layer, even with a small operating voltage of 100 mV, which is beneficial for the miniaturization of nanophononics. Moreover, the proposed photomemory device facilitates extensive current states (~400 levels), which is why it is suitable for integration in a state–of–the-art 8-bits storage system (that is, 256 levels). Finally, the underlying working mechanism was elaborated using an energy band diagram, where the electrons trap in oxygen vacancies while allowing the corresponding holes to freely 10 ACS Paragon Plus Environment

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participate in the electrical transport. This design and approach provide a basis for revolutionizing transparent and flexible photonics, particularly advanced optical memory devices.

4. Experimental Section Device Fabrication. Initially, the glass was cleaned with acetone, methanol and DI water in an ultrasonic machine (Power sonic 505). Nitrogen gas was subsequently blown on the glass to remove the remaining contaminants. Afterwards, the sample was placed in a sputtering machine (SNTEK, Korea) with an Indium target (99.99% purity). Then an oxygen and argon ratio of 1 to 3 sccm was supplied in the chamber to maintain a working pressure of 5 m.Torr. Finally, the DC power of 100 Watt was supplied for deposition of a 50 nm In2O3 layer on glass. A PET substrate was similarly used instead of glass to make a flexible device. Material and Electrical Characterization. The chemical characterization of indium oxide was done by X–ray photoelectron spectroscopy (XPS-PHI 5000 Versa Probe II). Scanning electron microscopy (SEM–JSM7800F, Jeol) was used to analyze the surface morphology, and energy dispersive spectroscopy (EDS-JSM7800F, Jeol) was employed for elemental mapping. X–ray diffraction (XRD, Rigaku, D/Max 2500) was employed to explore the crystalline quality of the deposited oxide. The optical analysis was performed with UV– Visible-NIR spectroscopy (Shimadzu, UV–2600) with air as baseline. The current voltage (I– V)

characteristics

were

measured

using

the

cyclic

voltometery

function

of

a

Potentiostat/Galvanostat (Zive SP1, ZIVELAB) and its chronoamperometry function was utilized to check the dynamic photoresponse with an LEDs light (365 ± 10nm, LEDENGIN) powered from a functional generator (MFG–3013A). The varying intensity of the light was measured via photometer (TES-1333 solar power meter). The workfunction was measured by KPFM (Kelvin Probe Force Microscopy).



Notes The authors declare no competing ﬁnancial interest.

Acknowledgements The authors acknowledge the financial support of the Incheon National University, Republic of Korea (2017-0371) and the Basic Science Research Program through the National Research Foundation (NRF) of Korea by the Ministry of Education (NRF2018R1D1A1B07049871). S. Abbas and M. Kumar equally contributed to this work.

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Figure 1. The In2O3 thin-film photomemory device. a) Schematic diagram of the device, b) XRD spectra, c) Plane-view SEM image, d) EDS elemental mapping of indium (In) in the top panel and oxygen (O) in the bottom panel, with a scale bar of 1 μm, respectively. e) XPS spectra of trivalent indium (In3+) and f) oxygen lattice and vacancies, respectively.


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Figure 2. Thin-film In2O3/Glass device performance. a) Transmittance and absorbance. The inset demonstrates the transparency with complete visible background under device, b) I–V curve under dark condition, showing low current even at ±1V, c) I–V curve under an illumination of 4 mW cm-2 with the inset showing the five allocated photocurrent memory levels and d) the semi–log representation of dark and light curves with a high switching ratio of 103. Thin-film In2O3/PET device e) transmittance with inset showing the flexible nature of the device and f) its persistence photoconductivity for a very long time under dark condition.



Figure 3. Transient current response under light (shown by a yellow strip) and dark (shown by the silver-gray colored area), a) illustrating persistent photoconductivity. Programming photomemory with b) voltage and c) intensity, respectively. d) Responsivity and detectivity, e) storing optical memory in a current level and f) multilevel current states for multibit storage with pulsating light. The inset shows five magnified storage current levels.


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Figure 4. a) Workfunction(WF) map of In2O3 under dark condition. Band diagram of In2O3 with Ag b) before contact and c) after contact and under illumination.



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Transparent and Flexible In2O3 Thin Film for Multilevel Nonvolatile

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