Growth and characterization of zinc oxide thin films on flexible substrates at low temperature using pulsed laser deposition

Accepted Manuscript Growth and characterization of zinc oxide thin films on flexible substrates at low temperature using pulsed laser deposition Kun Tian, Bharati Tudu, Ashutosh Tiwari PII:

S0042-207X(16)30959-9

DOI:

10.1016/j.vacuum.2017.01.018

Reference:

VAC 7287

To appear in:

Vacuum

Received Date: 7 December 2016 Revised Date:

16 January 2017

Accepted Date: 20 January 2017

Please cite this article as: Tian K, Tudu B, Tiwari A, Growth and characterization of zinc oxide thin films on flexible substrates at low temperature using pulsed laser deposition, Vacuum (2017), doi: 10.1016/ j.vacuum.2017.01.018. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Growth and Characterization of Zinc Oxide Thin Films on Flexible Substrates at Low Temperature Using Pulsed Laser Deposition Kun Tian, Bharati Tudu†, and Ashutosh Tiwari* Department of Materials Science and Engineering

Abstract

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University of Utah, Salt Lake City, UT 84102, USA

The emerging technological demand of light weight, transparent and flexible electronic devices

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has raised the exploration of new semiconductor materials beyond silicon. ZnO has the potential to be integrated into flexible electronics matrix due to its excellent electrical and optical

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properties. Here, we have fabricated ZnO thin films on flexible polymer substrates, polyimide (Kapton) and polyethylene naphthalate (PEN), at room temperature and at 100 oC using PLD. These films grew along (002) axis of the hexagonal wurtzite lattice and show n-type semiconducting nature. Crystallinity of films on Kapton is better than that on PEN. Films grown

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at 100 oC show improved optical transmittance and lower thermal activation energy. Film deposited on Kapton at 100 oC shows highest Hall mobility and lowest resistivity values. UV photoconductivity measurements show good opto-electronic properties for these films. Films on

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Kapton show higher photocurrent value and faster response and decay time (~ 1 s). Seebeck coefficient measurement shows higher thermopower values of 82 µV/K for films on PEN. These

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characteristics make the low temperature pulse laser deposited ZnO films on Kapton and PEN attracting for electronic devices with various opto- and thermo- electrical applications. Key words: ZnO, thin films, pulsed laser deposition, flexible substrates *

Corresponding author. Tel: 801-585-1666; Email address: [email protected]

†

Permanent address: Department of Physics, Jadavpur University, 700032 Kolkata, India

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1. Introduction With the advancement of science and technology, the electronics industry is expected to gradually revolutionize with transparent, flexible and wearable devices, such as wearable

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smartphone, on-chip sensor, roll-up display, etc. [1,2]. Several efforts have been made for developing basic electronic components using flexible substrates such as polyimide (Kapton), polyethylene naphthalate (PEN), Polyethylene terephthalate (PET), and papers [3]. These

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substrates benefit in advantages such as light weight, high flexibility, low cost, and good transparency [4]. Various flexible electronic devices have already been realized in research and

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industry including TFTs, displays, LEDs, biosensors etc. which are coated with nanostructured materials such as 1-dimensional nanowires, 2-dimensional thin films or 3-dimensional nanoparticles on these flexible substrates [5–7]. However, challenges such as minimization of surface/interface defects, good homogeneity, low temperature manufacturing requirement etc. for

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fabricating electronic materials on flexible substrates are still present [6]. In addition, such fabrication needs to overcome issues such as lattice mismatch induced stress/strain, adhesion of thin films on flexible surface, and local burning problems while substrate heating during the

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deposition process. Thus, from a fundamental point of view, more studies need to be done with a focus on their material properties, especially on polymer/semiconductor heterostructures, as well

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as on the exploration of new materials and fabrication approaches on flexible substrates in order to overcome those difficulties. Among most of the semiconductor materials, ZnO is an excellent candidate for integration into flexible electronic platforms. Already being a technologically important material, ZnO has drawn significant attention in the last decade due to its attractive properties [8,9]. It has a wide direct band gap of 3.4 eV which make it transparent in visible light range [10] . It has a large

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exciton binding energy of 60 meV which endorses it for application in optoelectronic devices, e.g. UV light emitting diodes and UV laser diodes [11–13] . Its good biocompatibility makes it a perfect template for biomedical sensors [14] . It also has good piezoelectric property which

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makes ZnO based materials excellent for nanogenerators [15] . It is highly conductive as an ntype semiconductor and can be doped with various elements to enhance its conductivity, e.g. Aldoped ZnO is an important n-type thermoelectric material [3,16–19] , which is also a good

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transparent and conducting oxide appropriate for solar cell applications which is comparable to indium tin oxide (ITO) [20,21] . Lastly, high quality ZnO thin films have been fabricated at low

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temperatures on common stiff substrates which suggests good chances of fabricating ZnO on polymer substrates which usually have quite low glass transition and melting temperatures [22,23] .

Motivated by the versatile features of ZnO, remarkable efforts have been taken in

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depositing ZnO thin films on flexible substrates for different applications [24–29] . In literature, various fabrication approaches have been addressed for the low-temperature fabrication of good quality ZnO thin films such as solution-based methods [24,25] , sputtering [27,28,30–32] ,

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atomic layer deposition [29] , chemical vapor deposition, pulsed laser deposition, as well as new

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fabrication methods such as flame transport synthesis [33,34] etc. However, most of them have addressed limited material properties. More comprehensive studies need to be done to investigate the effect of substrate and fabrication conditions on film quality of ZnO for the actual commercial realization of flexible ZnO electronic devices. In addition, there are only few reports on the fabrication of ZnO on flexible substrates at low temperature (especially room temperature) using pulsed laser deposition (PLD) [35] . Here, we have prepared ZnO thin films on flexible substrates using PLD technique at temperatures down to 24 oC (room temperature). We have

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chosen Kapton and PEN substrates because they are among the most widely used flexible substrates and they persist relatively higher glass transition temperature (~400 oC for Kapton and 120 oC for PEN). These two substrates have different thermal expansion rates, crystallinity and

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surface roughness, so the quality of ZnO thin films grown on them is expected to be very different. This motivated us to explore their properties using various material characterization methods, including X-ray diffraction (XRD), Energy-dispersive X-ray (EDX) analysis, atomic

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force microscopy (AFM), UV-Visible spectroscopy etc. Electrical transport properties have been carried out using resistivity, Hall effect and Seebeck coefficient measurements and opto-

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electrical properties have determined using UV light photoconductivity measurement. 2. Experimental details

Flexible substrates, Kapton (Push Plastic, 50 μm thickness) and PEN (DuPont Teijin

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Teonex, 125 μm thickness) were cut into small pieces with dimension of about 5 mm × 10 mm, and were cleaned with acetone, methanol, and isopropanol in sequence. For the deposition, ZnO target was loaded in a laser deposition chamber with a KrF excimer laser (wavelength = 248 nm).

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Energy density of the pulsed laser on target surface was about 2 J/cm2 and laser repetition rate was 10 Hz. Chamber was pumped down to 10-6 Torr before the deposition. Substrates were kept

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at the desired temperature (room temperature or 100 oC). For each substrate, ZnO films were then deposited under oxygen atmosphere at 1~2 mTorr. The growth rate was about 4 Å/s and the thickness of the prepared ZnO films was approximately 250 nm. Thus, four samples were prepared on Kapton and PEN substrates at room temperature and at 100 oC which we donate as ZnO/Kapton RT, ZnO/Kapton 100 oC, ZnO/PEN RT and ZnO/PEN 100 oC, respectively.

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To characterize ZnO thin films on flexible substrates, XRD patterns were collected on the film samples using Philips X’Pert X-ray diffractometer with 2θ varying from 15o to 70o with step size of 0.02o/s. XRD spectra of substrates were also collected for comparison. EDX spectra were

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collected using an FEI Quanta 600F scanning electron microscope with an EDX x-ray detector under an accelerating voltage of 20 kV. Film roughness and morphology were characterized by AFM performed using a Bruker Dimension Icon microscope. Optical transmittance tests were

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conducted using PerkinElmer Lambda 950 UV/Vis/NIR Spectrophotometer with 1 nm resolution. Temperature dependence of electrical resistivity was recorded for all films over a temperature

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range of 10 K to 300 K using four probe technique. Hall effect measurements were carried out by standard van der pauw method for calculating the hall mobility and carrier concentration. Seebeck coefficient measurements were performed under ambient condition by placing a heater at one end of the thin film and measuring voltage and temperature across it. The UV photo-

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conductivity measurements were performed using a UV lamp with broad light wavelength of 400 nm to 510 nm and a power density of 500 mW/cm2. The sample was mounted below the UV light source at a height of 1 cm and two silver contacts on the film were made for applying

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voltage and measuring the photo generated current. The time dependent photocurrent

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measurements were conducted in the dark at ambient condition. 3. Results and Discussions 3.1 Crystal structure

The XRD patterns of ZnO films deposited on Kapton and PEN substrates at RT and 100 oC are shown in Fig. 1. The Substrate XRD patterns are shown in Fig. S1, which indicates semicrystalline nature of both Kapton and PEN. From Fig.1, it can be seen that ZnO films exhibit a preferred growth orientation along (002) axis of hexagonal wurtzite structure. It can be inferred 5

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that the crystallinity of ZnO film is better on Kapton than that on PEN since the (002) peak intensity with respect to that of substrate peak is relatively higher in case of Kapton. The (002) peak positions corresponding to ZnO/Kapton RT, ZnO/Kapton 100 oC, ZnO/PEN RT, and

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ZnO/PEN 100 oC, are found at 2θ = 34.14o, 34.04o, 34.11o, and 33.93o, respectively. On comparing them with the (002) peak position of ZnO target which is 34.40o (see Fig. S2), it is found that the peak shifts towards a smaller angle. This is as expected because films grown on

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polymer substrates undergo high stress resulting in lattice deformation [36] .

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The estimated grain size found from the full width at half maximum (FWHM) corresponding to (002) peak (using Scherrer’s equation) is 34.2 nm for ZnO/Kapton RT and 57.5 nm for ZnO/Kapton 100 oC. Thus, at higher substrate temperature, grain size increases which improves the crystallinity of film on Kapton. While in the case of films on PEN substrate, the grain size is 66.6 nm for ZnO/PEN RT and 23.1 nm for ZnO/PEN 100 oC. This implies a

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decrease in grain size as deposition temperature increases, which could be due to the deposition temperature approaching the glass transition temperature of PEN.

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3.2. Surface Morphology and Chemical Composition The atomic force microscopy (AFM) images (3µm x 3µm) for the ZnO films deposited on

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polymer substrates, are shown in Fig. 2. AFM images of the substrates were also recorded which are shown in Fig. S3. ZnO films on Kapton show parallel-chain kind of texture which could be due to the presence of similar kind of surface morphology of the Kapton substrate, as shown in Fig. S3 (a) and (b). Similar surface pattern was observed for ZnO film grown on PET substrate [20]. At higher substrate temperature, root-mean-square roughness (RRMS) value decreases from 2.0 to 1.6 nm. However, the films grown on PEN exhibit an entirely different morphology due to a different surface structure of the substrate. Pit-like texture can be seen for the film grown on 6

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PEN at RT which reveals a RRMS value of 6.2 nm. While the corresponding film at higher substrate temperature, interestingly shows wrinkle-shaped texture around the pits that may be due to surface energy minimization, which increases its RRMS value to 11.2 nm. This study

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indicates that the ZnO/Kapton films are much smoother than ZnO/PEN films.

The elemental composition of the ZnO films on polymer substrates examined by EDX analysis is shown in Fig. 3. All films confirm the presence of Zn, C, and O and no other

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impurities were found.

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3.3. Optical properties

The prepared ZnO films on Kapton and PEN appeared highly transparent. For an instant comparative study of the film transparency, all the samples and substrates were placed over the colored letters R, G and B which were printed in red, green and blue respectively. Fig. 4 (a)

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shows photograph of the samples placed over those colored letters. PEN films showed a good transparency over red, green and blue letters compared to the films on Kapton. In addition, higher deposition temperature (100 oC) increases the transparency of ZnO films on both the

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substrates. For detailed analysis, optical transmittance measurement over a wide wavelength range of 850 nm to 300 nm was carried out on ZnO films as well as on pure substrates which is

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shown in Fig. 4 (b). Both Kapton and PEN substrate absorb all the light with wavelength below 380 nm, so the absorbance band of ZnO (~365 nm) is not observed. Films on Kapton grown at RT shows transparency of over 60%, while the one grown at 100 oC is almost as transparent as the substrate (over 80% transparency). On the other hand, film on PEN deposited at RT shows a better transparency (over 70%) compared to Kapton for the wavelength range of 550 nm to 850 nm. Similar to film on Kapton, ZnO/PEN 100 oC shows almost the same transparency as the pure PEN substrate. It suggests that ZnO films deposited at 100 oC have significantly improved 7

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transparency than films deposited at RT for both the substrates. With these results, it can be concluded that these highly transparent ZnO films on flexible substrates can be good candidates

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for transparent conductive oxide (TCO) layers for solar cell and photovoltaics applications. 3.4. Electrical properties

Most studies on ZnO based flexible substrates, only addressed room temperature resistivity

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values, and only few temperature dependent resistivity studies have been reported for films deposited on polymer substrates so far [16,20,25–27] . Thus, to characterize the electrical

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transport properties at broader temperature range, resistivity measurements were performed over the temperature range of 10 K to 300 K for all the ZnO films on Kapton and PEN. The variation of resistivity with temperature for these films is shown in Fig. 5. It can be seen that the resistivity values of every sample increase as temperature decreases which exhibit typical semiconductor behavior. The room temperature resistivity values are listed in the first column of Table 1. Films

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grown at 100 oC are found to have lower resistivity compared to films deposited at RT for both the substrates. Comparing films on different substrates at the same deposition temperature, the

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resistivity is found to be lower in film grown on Kapton. This could be due to the higher crystallinity of the films grown on Kapton as discussed earlier. It has been reported that

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formation of point defects, such as oxygen vacancies (Vo), during a controlled deposition process can strongly affect the electronic and optoelectronic properties of ZnO [37–39] . From the aspect of Vo in ZnO, which contributes to electron donors in n-type ZnO [11,26,40] , it is also possible that higher density of Vo is present in ZnO/Kapton films which contributes to the higher conductivity. The lowest resistivity at room temperature among the four samples was found to be 0.06 Ω·cm, which is comparable to the value reported in literature for ZnO grown on flexible substrates [16,26] . However, these values are not so impressive compared to ZnO deposited on 8

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stiff substrates [30,31,41] , which may be due to a better crystallinity of thin films grown on common stiff substrates. In addition, the thermal activation energy, Ea was also calculated by

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fitting the curve of ln (σ) vs 1/T using the following equation [42] , (1)

where σ is the conductivity at temperature T; σ0 is a pre-exponential constant; and kB is the

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Boltzmann constant. The values of Ea for the films have been calculated for temperature range

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300 K ≥ T ≥ 50 K and 50 K ≥ T ≥ 10 K which are listed in columns 2 and 3 of Table 1. For films on both the substrates, Ea value decreases with deposition temperature which implies that at higher deposition temperature the barrier of electron ionization energy is lowered resulting in improved conductivity.

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Hall effect measurements were carried out for calculating the carrier concentrations and Hall mobilities which are listed in first two columns of Table 2. The carrier concentration values for all the films are in the order of 1017 to 1018 cm-3 and the hall mobility values in the range of

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101 to 102 cm2 V-1s-1. Of these, films grown on Kapton shows higher mobility than those on PEN. From the point of view of deposition temperature, significant improvement in film mobility is

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found for films grown on each substrate. With higher deposition temperature, the film quality improves which significantly reduces the electron scattering sites such as point defects and grain boundaries resulting in higher mobility. These values are above average compared to the reported values for pure ZnO films deposited on polymer substrates [26,43] , which indicates good quality ZnO films for flexible electronics can be fabricated by PLD method. To characterize the thermoelectric property of these films, Seebeck coefficient measurements were carried out at room temperature. The Seebeck coefficient values are shown in the third column of 9

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Table 2. All samples show negative Seebeck coefficient, which confirmed their n-type nature. Seebeck coefficient values are higher (around 80 µV/K) in case of films grown on PEN than

substrates for fabricating flexible thermoelectric modules. 3.5. UV photo-response

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those grown on Kapton which signifies that further studies can be done on ZnO films on polymer

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ZnO is very sensitive to UV illumination which generates remarkable photocurrent [44– 51] . To investigate the optoelectronic properties of ZnO films on polymer substrates, photo

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conductance under UV light was measured. At a bias voltage of 2 mV, time-dependent photocurrent measurements were performed for all the samples through two silver contacts on the films by periodically switching on the UV illumination for 10 s and switching it off for 30 s. Photocurrent, which is the difference between the current at “light-on” state (Ion) and “light-off”

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state (Ioff), plotted against time is shown in Fig. 6. For all the four samples, it can be seen that the photocurrent value changes to a high level or a low level when the UV light switches to “on” or “off” states. For each cycle, the photocurrent increased within few seconds when the light was

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turned on which stayed stable for 10 s before decaying exponentially when the light turned off. From previous studies [46,47] , it is known that two processes are involved in the photoresponse

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of ZnO. One is the photo generation-recombination of electron-hole pairs under light illumination, and the other one is the adsorption-desorption of oxygen on the surface of ZnO [44–47] . Considering adsorption of free oxygen on the surface of ZnO at light “off” state, each adsorbed O2 molecules capture one electron from the n-type ZnO forming a depletion layer with low conductivity: (2)

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Upon exposure to light, electron-hole pairs are generated in the ZnO films: (3)

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The generated electron-hole pairs are separated under the bias voltage and additional electrons are excited from the valance band to conduction band which contributes to the photo-conduction. In the meantime, holes migrate to the surface and get trapped by the surface adsorbed O2- ions

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which enhances the photoconductivity of ZnO:

(4)

When the light is switched off again, excess holes recombine with the electrons available in the system through recombination centers:

(5)

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At the same time, oxygen molecules get adsorbed on ZnO surface capturing more electrons thereby decreasing the conductivity. The magnitude of photocurrent of a sample relies on various aspects under the same illumination condition, such as mobility of electrons and holes in the

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system; surface roughness of the film as rougher films provide more surface area for oxygen adsorption; and density of defects which controls the rate of recombination of excess holes [46] .

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For the ZnO films deposited on PEN and Kapton, relatively higher photocurrent (~100 nA) is found for the film ZnO/Kapton 100 oC which can be attributed to its higher electron conductivity and mobility as revealed by electrical resistivity and Hall effect measurements. It is also found that films deposited at higher temperature have higher photocurrent than those deposited at room temperature.

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To find the photocurrent response time, τR, and decay time, τD, photocurrent measured during one such illumination period is plotted as shown in Fig. 7. The rising and decaying

(6)

(7)

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photocurrent curves were fitted using the functions:

where I (I0) is the measured (maximum) photocurrent and t is the time. In simple words, the

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response time represents the time needed to reach 63% (≈ 1- e-1) of I0 under illumination condition and the decay time is the time measured from the moment the light was turned off to the moment I decays to 37% (≈ e-1) of I0 [50] . The calculated τR and τD values of the prepared

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ZnO films are listed in Table 3. It can be seen that ZnO films on Kapton show faster response and decay time compared to those on PEN, which favors their applications in fast switching photoconductive devices, e.g. photodetector. The decay time also represents the mean

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recombination time or carrier life time in a system, which is the time it takes for the excess holes generated from photoexcitation to recombine with the electrons. This carrier lifetime of ZnO can

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be determined by the density of crystal defects (e.g. Vo) present in the system which provides available recombination centers and dictates the rate of recombination. The amount of O2- ions on the surface also prolongs the carrier life time of a system by capturing the holes before their recombination with electrons. A longer carrier life time (1.04 s for ZnO/Kapton and 6.68 s for ZnO/PEN) is observed for films grown on both substrates when the deposition temperature was 100 oC compared to the films fabricated at RT (0.6 s for ZnO/Kapton and 5.25 s for ZnO/PEN).

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This indicates that films grown at 100 oC preserve a longer carrier lifetime than films grown at RT.

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4. Conclusion In summary, we have obtained good quality ZnO thin films using PLD on polymer substrates: Kapton and PEN, at low temperatures. XRD characterization shows films have grown

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with a preferred orientation along (002) axis, and crystallinity of films on Kapton is better than those on PEN. AFM result shows that the films grown on different substrates at different

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deposition temperatures exhibit different texture and surface morphology. All the films are found to be highly transparent. Slightly higher deposition temperature further improves their optical transmittance. Low temperature electrical resistivity measurements show lowering of thermal activation energy of these films at 100 oC. Highest Hall mobility value is found for the film

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deposited on Kapton at 100 oC which also shows the lowest resistivity. Seebeck coefficient measurement confirms their n-type semiconducting nature. The maximum Seebeck coefficient value of 82 µV/K paves the road for future studies on optimization and fabrication of flexible

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thermoelectric modules using ZnO films on polymer substrates. UV photoconductivity measurements show good opto-electronic properties for the films grown on Kapton which have

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higher photocurrent value and faster response and decay time (~ 1s). With the above discussion, we conclude that good quality conductive and transparent ZnO films can be prepared at low temperature by PLD on Kapton and PEN polymer substrates. This study opens the door for more extensive research using these films which can be potential candidates for practical realization of flexible electronic devices. Future work should be focused on studying mechanical properties, e.g. anchoring strength and bending, nanotribological properties, and on further improvement of electrical properties (to achieve higher conductivity) 13

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of ZnO films grown on these substrates to make flexible electronics based on these materials feasible.

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Acknowledgement The authors gratefully acknowledge the financial support from US National Science Foundation through grant# 1407650 and 1234338. B.T. thanks UGC, Government of India for Raman Post-

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doctoral fellowship. Reference

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35. Matsumura, M. & Camata, R. P. Pulsed laser deposition and photoluminescence measurements of ZnO thin films on flexible polyimide substrates. Thin Solid Films 476, 317–321 (2005). 36. Shabannia, R. Synthesis and characterization of Cu-doped ZnO nanorods chemically grown on flexible substrate. J. Mol. Struct. 1118, 157–160 (2016). 37. Zeng, H., Liu, P., Cai, W., Cao, X. & Yang, S. Aging-Induced Self-Assembly of Zn/ZnO Treelike Nanostructures from Nanoparticles and Enhanced Visible Emission. Cryst. Growth Des. 7, 1092–1097 (2007).

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38. Zeng, H., et al. Blue Luminescence of ZnO Nanoparticles Based on Non-Equilibrium Processes: Defect Origins and Emission Controls. Adv. Funct. Mater. 20, 561-572 (2010). 39. Zeng, H., et al. Microstructure Control of Zn/ZnO Core/Shell Nanoparticles and Their Temperature-Dependent Blue Emissions. J. Phys. Chem. B 111, 14311-14317 (2007).

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40. Bhosle, V., Tiwari, A. & Narayan, J. Electrical properties of transparent and conducting Ga doped ZnO. J. Appl. Phys. 100, 33713 (2006).

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41. Kumar, M., Wen, L., Sahu, B. B. & Han, J. G. Simultaneous enhancement of carrier mobility and concentration via tailoring of Al-chemical states in Al-ZnO thin films. Appl. Phys. Lett. 106, 241903 (2015). 42. Sawalha, A., Abu-Abdeen, M. & Sedky, A. Electrical conductivity study in pure and doped ZnO ceramic system. Phys. B Condens. Matter 404, 1316–1320 (2009). 43. Mofor, A. C. et al. Growth of ZnO layers for transparent and flexible electronics. Thin Solid Films 516, 1401–1404 (2008). 44. Liu, B. et al. ZnO-nanoparticle-assembled cloth for flexible photodetectors and recyclable photocatalysts. J. Mater. Chem. 22, 9379 (2012). 45. Wang, Z. et al. A flexible UV nanosensor based on reduced graphene oxide decorated ZnO nanostructures. Nanoscale 4, 2678 (2012). 46. Zhu, Q., Xie, C., Li, H. & Yang, Q. Comparative study of ZnO nanorod array and nanoparticle film in photoelectric response and charge storage. J. Alloys Compd. 585, 267–276 (2014). 16

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47. Zhu, Q. et al. Selectively enhanced UV and NIR photoluminescence from a degenerate ZnO nanorod array film. J. Mater. Chem. C 2, 4566 (2014). 48. Tiginyanu, I. et al. Strong light scattering and broadband (UV to IR) photoabsorption in stretchable 3D hybrid architectures based on Aerographite decorated by ZnO nanocrystallites. Sci. Rep. 6, 32913 (2016).

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49. Gedamu, D. et al. Rapid Fabrication Technique for Interpenetrated ZnO Nanotetrapod Networks for Fast UV Sensors. Adv. Mater. 26, 1541–1550 (2014). 50. Liu, Y. et al. Ag nanoparticles@ZnO nanowire composite arrays: an absorption enhanced UV photodetector. Opt. Express 22, 30148 (2014).

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51. Mishra, Y. K. et al. Crystal growth behaviour in Au-ZnO nanocomposite under different annealing environments and photoswitchability. J. Appl. Phys. 112, 64308 (2012).

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Figure Caption: Fig. 1: XRD patterns of ZnO films deposited on polymer substrates at different temperatures.

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The diffraction peaks from the substrates are indicated by * (for Kapton) and ∇ (for PEN). Fig. 2: AFM images (3D image on the left and 2D image on the right) of (a) ZnO/Kapton RT; (b) ZnO/Kapton 100 oC; (c) ZnO/PEN RT; and (d) ZnO/PEN 100 oC.

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Fig. 3: EDX spectra of different ZnO films on Kapton and PEN substrates.

Fig. 4: (a) Photographs of different samples placed above the printed letters of red, green, and

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blue color. The left box shows the films deposited at room temperature and the right box shows films deposited at 100 oC. For each box, the left column shows the films deposited on Kapton (Kapton substrates are yellow in color). The right column shows the films on PEN. (b) UV-Vis transmittance spectra of pure substrates and ZnO films.

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Fig. 5: Temperature versus resistivity curves for the ZnO films on polymer substrates, Kapton and PEN grown at RT and 100 oC.

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Fig. 6: Time dependent photocurrent measured for different ZnO films on Kapton and PEN under a periodic UV illumination: 10 s (on), 30 s (off).

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Fig. 7: Time dependent photocurrent values of the ZnO films for a single illumination period.

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Table 1 Values of the resistivity measured at RT and the calculated activation energy for different samples at temperature range (300 K ≥ T ≥ 50 K) and (50 K ≥ T ≥ 10 K)

(Ω · cm)

Activation Energy

(300 K ≥ T ≥ 50 K)

(50 K ≥ T ≥ 10 K)

(meV)

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Resistivity at RT

Sample

Activation Energy

(meV)

0.09

5.46

ZnO/PEN RT

1.55

1.54

ZnO/Kapton 100 oC

0.06

1.60

0.06

ZnO/PEN 100 oC

0.12

1.16

0.06

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ZnO/Kapton RT

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0.10

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Carrier concentration at RT (cm-3)

Hall mobility

ZnO/Kapton RT

1.6×1018

46

ZnO/PEN RT

2.8×1017

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ZnO/Kapton 100 oC

7.7×1017

124

ZnO/PEN 100 oC

6.2×1017

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Table 2 Values of the calculated carrier concentration, Hall mobility and Seebeck coefficient for different samples.

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Seebeck Coefficient at RT (µV/K)

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(cm V s )

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-59 -82

-22

-78

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Table 3 Photocurrent response time and decay time for the ZnO films Response time, τR (s)

Decay time, τD (s)

ZnO/Kapton RT

0.97

0.60

ZnO/PEN RT

3.59

ZnO/Kapton 100 oC

1.35

ZnO/PEN 100 oC

1.69

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Sample

5.25

1.04

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6.68

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Fig. 1

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Supplementary Materials Growth and Characterization of Zinc Oxide Thin Films on Flexible Substrates at Low Temperature Using Pulsed Laser Deposition

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Kun Tian, Bharati Tudu, and Ashutosh Tiwari*

Fig. S1 XRD patterns of the substrates Kapton and PEN.

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Fig. S2 XRD pattern of ZnO PLD target which corresponds to polycrystalline wurtzite ZnO structure (Zincite, JCPDS 5-0664).

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Fig. S3 AFM images of surface morphology of (a) Kapton without any heat treatment; (b) Kapton after heating at 100 oC; (c) PEN without any heat treatment; (d) PEN after heating at 100 oC.

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ZnO thin films were deposited on polymer substrates by pulsed laser deposition.

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Film qualities were characterized by XRD, AFM, EDAX, and UV-Visible measurements.

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Resistivity measurements were performed over temperature range from 10 K to 300 K.

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ZnO films on polymer substrates showed fast photo-response to UV-light.

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