J Inorg Organomet Polym DOI 10.1007/s10904-017-0617-6

Aluminum-Doped Zinc Oxide Thin Films Deposited on Flexible Cellulose Triacetate Substrates Prepared by RF Sputtering L. G. Daza1 · E. A. Martín‑Tovar1 · R. Castro‑Rodriguez1 

Received: 27 April 2017 / Accepted: 26 June 2017 © Springer Science+Business Media, LLC 2017

Abstract  Transparent Al-doped ZnO thin films have been prepared by rf magnetron sputtering using on glass and a flexible cellulose triacetate substrates. The morphological, structural, chemical, optical and electrical properties were characterized by scanning electron microscopy, atomic force microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, ultra violet-visible spectroscopy (UV–Vis) and the Van der Pauw techniques respectively. The morphological analysis showed that the samples deposited on both substrates presented densely compacted grains with regular shapes, with roughness values of ~30  nm. The hexagonal wurtzite structure was obtained in both types of substrates with a preferred orientation (002), crystallite sizes of ~20 nm and with positive induced stress in the unit cell. The chemical analysis revealed the presence of Zn, O and Al atoms. Transmittance values of ∼80%, with an optical bandgap of ~3.50 eV, were obtained. The resistivity in both types of films was in the range ~10−3−10−2 Ω-cm with a mobility of ~3.6 cm2V−1s−1 and a carrier concentration of ~1020 cm−3. Keywords  Zinc oxide · Aluminum · Thin films · Sputtering · Cellulose triacetate · Flexible substrates

1 Introduction Among the many transparent conductive oxides (TCO’s), Al-doped ZnO (AZO) is well known In recent years * L. G. Daza [email protected] 1



Departamento de Física Aplicada, CINVESTAV-IPN, Unidad Mérida, 97310 Mérida, Yucatán, Mexico

transparent conductive oxide (TCOs) have been widely used in various technological applications due to their high visible transmittance and low resistivity, some of these devices include: liquid crystal display (LCD), organic light emitting display (OLED), [1], Solar cells, flat panel displays, optical sensors touch screens and many others [2–6]. Among the many transparent conductive oxides (TCO’s), Al-doped ZnO (AZO) is well known for having good electrical conductivity and high transparency which make it a promising candidate to replace other conventional TCO’S [7]; such as the Sn-doped I­n2O3 (ITO), which is used due to its high transmittance in the visible region and its low resistivity characteristics [8]; however, because of its toxicity and high cost, it has been necessary to seek for alternative materials to replace it [7], so AZO has aroused great interest because of its many advantages, such as its abundance, availability, large-scale applications, low cost and its thermal and chemical stability [8]. Traditionally AZO thin films are deposited on glass substrates, which are heavy, brittle and cannot be deformed easily, this makes it difficult to use them in modern technological devices, such as smart cards, electronic maps, flat screens, among others, where flexibility and lightweight in their component materials is required [9]. However, flexible substrates have some disadvantages due to their physical properties, since they could suffer severe damages at high temperature depositions due to the fact that they present low thermal and mechanical resistance in comparison to metallic and glass substrates, and considering that low substrate temperatures do not favor the obtaining of AZO thin films with good optical and electrical properties, the deposition of this material on flexible substrates becomes a difficult process [10]. Therefore finding the conditions that would allow to successful deposition of TCO thin films on flexible substrates is mandatory in

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order to develop technological devices based on the aforementioned materials. To the best knowledge of the authors there are few works where the deposition of AZO thin films in flexible is achieved [11–13]. There are many techniques used to deposit AZO thin films such as thermal evaporation [14], laser ablation (PLD) [15], chemical vapor deposition (CVD) [16], sol–gel [17], sputtering [18, 19] and Spray pyrolysis [20, 21]. However, in order to obtain AZO thin films with low resistivity values using most of the aforementioned techniques a moderate to high substrate temperature is required. On the other hand the fabrication of solar cells on several substrates needs low deposition temperatures to be achieved, especially in thin film cell technology on flexible substrates [22]. Considering all of the reasons mentioned above, radio-frequency (RF) magnetron sputtering is widely employed due the many advantages it presents, namely the possibility to deposit thin films at low temperatures, even close to room temperature (RT), with good adhesion to the substrates, high deposition rates, and good stoichiometric control [23–25]. Therefore, the purpose of this work is to investigate the influence of the sputtering deposition parameters on the structural, morphological, electrical, chemical and optical properties of AZO films deposited on cellulose triacetate and glass substrates grown by rf-sputtering.

2 Experimental Details AZO thin films were deposited on Corning 2947 glass and cellulose triacetate substrates. A 3 inch of diameter ZnO: Al target with 2  wt% of A ­ l2O3 with a purity of 99.999% (Cathay Advance Materials Limited, China) was used to perform the deposition. Both types of substrates were cleaned using an ultrasonic bath; the glass substrates were cleaned using acetic acid (­CH3COOH), distilled water, acetone and isopropyl alcohol; lastly the cellulose triacetate substrates where sonicated with ethanol (­C2H5OH) to avoid their degradation. The target to substrate distance was 60 mm and the base pressure was 6.66 × 10−3 Pa. The sputtering deposition was performed under an rf-power of 80 W at room temperature during different deposition times which had values of 10, 20, 30 and 40 min with a sputtering Ar pressure of 1.33 Pa. Surface images were obtained by means of a scanning electron microscope (SEM) with a field emission scanning electron microscope (FESEM) JEOL 7600F instrument using a 25  kV electron source. The superficial morphology of the thin film was analyzed by atomic force microscopy (AFM) using a Park Scientific Instruments auto probe in the topography contact mode, with Si tips. Structural analysis was accomplished using X-ray diffraction (XRD) in the grazing incidence geometry with an inclination of 1° with a D5000 Siemens

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X-ray Diffractometer and CuKα radiation (λ = 1.5406  Å) operated at 40  kV and 35  mA and aperture diaphragm of 0.2 mm, the diffractograms were registered in the step scan mode with a beam incidence angle of 1° and recorded in 2θ = 0.02° steps with a step time of 10  s in a 2θ range of 30–70°. The Transmittance spectra were recorded with the help of an Agilent 8453 UV–Vis spectrophotometer with a 0.1  nm resolution, in the range of 300–900  nm, the optical band gaps were calculated only with direct transitions. Resistivity, carrier concentration and mobility values were obtained with an Ecopia HMS-5000 Van der Pauw Measurement System at 300 K. The stoichiometry and chemical state of each constituent element from the obtained AZO thin films was characterized using an X-ray photoelectron (XPS) analysis, using a K-ALPHA Thermo Scientific System. The thicknesses of the films deposited on glass substrates were measured by a surface Profilometer Dektak-Veeco; however due to the flexibility of the CTA substrates it was not possible to use the Profilometer on them, so the Swanepoel Envelope Method was used instead. The Swanepoel Envelope method allows the calculation of the refractive index (n) of transparent thin films, as well as its thickness [26]. Maximum transmittance ­(Tmax) and minimum transmittance (­Tmin) values have been recorded from optical transmittance spectra (Fig. 1). The refractive index can be calculated from the above values with the help of the following relation [27]: √ √

n=

N+

N 2 − n2s

(1)

where

N=

n2s + 1 2

+ 2ns

Tmax − Tmin Tmax Tmin

(2)

where ­ns is the refractive index of the substrate (In this case ­ns = 1.51 and 1.48 for the glass and CTA substrate respectively), ­Tmax is the maximum envelope and T ­ min is the minimum envelope. Using equations (1) and (2) the thickness of the sample is calculated as follows:

t=

𝜆1 𝜆2 2(𝜆1 n2 − 𝜆2 n1 )

(3)

where ­n1 and ­n2 are the refractive indices corresponding to the wavelengths λ1 and λ2 respectively, which correspond to two consecutive maxima or two consecutive minima in the transmittance spectrum of each film [28]. Almost every technological device where a TCO material is employed has two main requirements, namely, both good optical transparency and electrical conductivity, in most cases both parameters should be as large as possible, but their interrelationship usually excludes the

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Fig. 1  Optical transmittance spectra of AZO thin films with various deposition times on a glass and b CTA substrates

simultaneous achievement of both criteria [29]. In solar cells, the TCOs are used like front contact before the deposition of the window layer, those TCOs must have a specific electrical and optical characteristic that enhances the transmission of the solar light on the material absorbent film, in order to accurately quantify the quality of both parameters at the same time a figure of merit is required. The authors decided to employ the measurement of the solar weight of the transmission (SWT) which has proven work well for a fine comparison of transparent conductive films in solar cells [30].

3 Results and Discussions 3.1 Morphological and Structural Characterization Figure 1 shows the transmittance spectra for the AZO thin films obtained at different times for both types of substrates. The samples grown showed average transmittance values of ∼88 and ∼85% in the visible range for the glass and CTA substrates respectively, and the absorption edge of the films is found to shift to greater wavelengths with the increase in deposition time. The thickness of the AZO thin films increased from 102 to 448 nm and from 114 to 464 nm for the samples grown on the glass and CTA substrates respectively, when deposition time varied from 10 to 40 min. Since the thicknesses values obtained using 40 min of deposition time were closer to those used in multiple photoelectric applications, such as CS, CSF and/or DOETF transparent electrodes [31, 32]. a more detailed study was performed on the aforementioned samples. Figure 2 shows SEM surface micrographs for the AZO films deposited with 40  min of growth for both type of substrates. According to Fig.  2a, b, it can be seen that the grains in both films are densely compacted, having

average sizes of ~35  nm, with good uniformity throughout the surface and without the presence fractures or defects. The surface morphology was also studied using AFM micrographs as is shown in Fig. 3, which displays the 3D AFM images of the AZO thin films obtained in an area of 1  μm2 and 40  min of growth time for both types of substrates. As it can be seen, the films grown on glass substrates have more irregular grains and pointed peaks, while the samples grown on CTA have more regular grains with a flatter surface and no peaks. Furthermore, in order to calculate the root mean square (RMS) roughness of the AZO films we employed the WSXM 5.0 software [33], yielding to values of ~25 and ~28  nm for the glass and CTA substrates respectively. The roughness values did not differ much from one another, even though at plain sight the CTA surface is rougher that of the glass, therefore we could suggest that a deposition time of 40 min on a glass substrates is enough to make the roughness values for both types of substrates comparably similar. Figure 4 shows the XRD spectra for the AZO thin films deposited on glass and CTA, respectively, with 40  min of growth time. The observed diffraction peaks for all samples can be indexed as the ZnO wurtzite structure in the standard data (JCPDS, 36-1451) [34] and with preferential orientation in the (002) plane, with the c-axis oriented preferentially normal to the substrate surface [35]; all peaks are shifted to the left in their 2θ (002) peak position with respect to the standard value of 34.467° [34], which is represented by a dashed line in Fig.  4, this is evidence of tensile strain [36] and a possible expansive deformation in the crystal lattice in the direction of the c axis. Peaks of lesser intensity are also observed in the plane orientation (103). For the film grown on the CTA substrate small peaks appear in the 2θ region of 45° to 50°, those peaks correspond to the CTA substrate.

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Fig. 2  SEM micrographs of the AZO thin films deposited on a Glass and b CTA substrates using 40 min of deposition time

Fig. 3  AFM micrographs of the AZO thin films deposited on a Glass and b CTA substrates using 40 min of deposition time

Fig. 4  XRD patterns of AZO thin films prepared on a Glass and b CTA substrates using 40 min of deposition time

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In order to determine crystallite size of the AZO thin films we considered the Scherrer’s formula [35], which is given by:

D=

0.9 𝜆 𝛽 Cos𝜃

(4)

where D is the mean crystallite size, θ the diffraction angle, β is the FWHM and λ is the X-ray wavelength (0.15418  nm). The lattice constants were calculated using the following equation [37]: ) ( 1 4 h2 + k2 + hk l2 = + 2 (5) 2 2 3 d a c where h, k and l are the Miller’s indices, a and c are the lattice constants, and d is the crystalline interplanar distance, which determined using Bragg’s law [38]:

d=

n𝜆 2 sin 𝜃

(6)

Furthermore, we calculated the strain ε along the c-axis by using the equation:

ε=

C − C0 × 100% C0

(7)

where ­c0 (0.52069 nm) is the unstrained c lattice parameter of Zn. Positive strain values correspond to tensile strain and negative values to compressive strain [39].

Table 1  Structural characteristics of the AZO prepared on glass and CTA substrates using 40 min of deposition time

The obtained results for the lattice parameters, crystallite size, and unit cell volume are summarized in Table 1. The calculated crystallite sizes for both types of samples were less than 30 nm and very close to each other. By comparing the lattice parameters in both cases to the standard values for ZnO, it is observed that the films grown on a glass substrate had the greater lattice constants, while being slightly less than the standard for the samples grown on CTA. The strain (ε) along the c-axis was also calculated in both cases, being the were results 0.638 and 0.353%, for films grown on glass and CTA respectively, indicating a greater lattice relaxation for the samples grown on CTA, which agrees with the shifting of the (002) diffractions peaks mentioned before. The stoichiometry and chemical state of each constituent element from the obtained AZO thin films for both type of substrates was characterized using an X-ray photoelectron (XPS) analysis, Fig.  5 presents XPS obtained survey spectra for both types of samples, taking the films grown with 40  min of deposition time as representatives of the others. The C1s (284.6  eV) peak was employed as reference to calibrate the binding energies of all the examined elements. The typical peaks of ZnO are observed [40], besides that, the pholoelectron and auger lines in the binding energy scale identified each constituent element, so all peaks in the survey curve reveal the presence of the C, O, Zn and Al elements; no other peaks

Type of sample

Lattice parameter a (Å)

Lattice parameter b (Å)

Volume (Å3)

Strain of the c-axis ε (%)

Crystallite size (nm)

Standard ZnO Glass CTA

3.2495 3.2504 3.2483

5.2069 5.2401 5.2253

47.625 47.947 47.748

0 0.638 0.353

0 ∼24 ∼23

Fig. 5  XPS survey spectrum of the AZO thin films grown a Glass and b CTA substrates using 40 min of deposition time

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corresponding to additional elements were observed. The obtained atomic concentrations in at.% of O, Zn, C and Al for the samples deposited in glass substrates were 45.51, 45.59, 7.07 and 1.93, respectively; and for the samples deposited on CTA substrates the concentrations Zn 2p3/2 1021.68 eV

Intensity (a.u.)

Intensity (a.u.)

(b)

Zn 2p1/2 1044.58 eV

1030

1040

Zn 2p1/2 1044.46 eV

1050

1020

(d) 530.1 eV

Intensity (a.u.)

531.6 eV

525

530

535

540

525

530

74.2 eV

73.5 eV

540

76

Binding energy (eV)

78

80

Al 2p CTA

74.2 eV

73.5 eV

Intensity (a.u)

Intensity (a.u)

(f)

Al 2p Glass

74

535

Binding energy (eV)

(e)

72

1050

531.5 eV

Biding energy (eV)

70

1040

O 1s CTA

Intensity (a.u)

O 1s Glass

530.2 eV

1030

Binding Energy (eV)

Binding Energy (eV)

(c)

CTA

Zn 2p3/2 1021.37 eV

Glass

(a)

1020

in at.% for the same elements in the same order were 46.51, 49.65, 2.28 and 1.57 respectively. Figure  6a, b present the Zn 2p core level XPS scan, using a higher resolution and a smaller energy window, for the AZO thin films grown in glass and CTA substrates

70

72

74

76

78

80

Biding energy (eV)

Fig. 6  XPS spectrum of the AZO thin films deposited on glass and CTA substrates. Binding energy spectrum of Zn 2p (a, b), O 1s (c, d) Al 2p e , f)

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respectively. The Zn 2p core level XPS spectrum shows the presence of two sharp peaks at 1021.7 eV (Zn 2p3/2) and 1044.6 eV (Zn 2p1/2) for the glass substrate and for the CTA substrate the peaks were present at 1021.4  eV (Zn 2p3/2) and 1044.5  eV (Zn 2p1/2) and correspond to zinc present as ZnO [41]. Figure 6c, d show the O 1s core level scan for both types of samples. The O1s peaks have a shoulder at higher binding energy; these two peaks were fitted into two peaks by a Gaussian distribution adjustment. The peak with low binding energy (530.2 and 530.1  eV for the glass and CTA substrate respectively) corresponds to ­O2− on normal wurtzite structure of ZnO single crystal [42]. Another peak centered at 531.6 and 531.5  eV for the glass and CTA substrates respectively, is attributed to O ­ 2− in the oxygen deficient regions within the matrix of ZnO [43]. The Al 2p spectra of the AZO thin films for both the glass and CTA substrates are shown in Fig.  6e, f respectively. The Al 2p spectrum reveals the presence of two peaks which were fitted using in two Lorentzian functions, the aforementioned peaks are located at binding energies of 73.5 and 74.2  eV for both types of substrates, both peaks that belong to binding energies regions which are characteristic of A ­ l 2O 3 and seem to indicate that A ­ l2O3 is oxidized from the Al in the AZO film [44]. The SWT, which is the ratio of the usable photons transmitted to the total usable photons, can be estimated by normalizing the reflectance spectra with the solar spectral photon flux integrated over a wavelength range of 350–900 nm. Its formula is given by following relation:

SWT(𝜆) =

900nm S(𝜆)T(𝜆)d𝜆 ∫350nm 900nm S(𝜆)d𝜆 ∫350nm

(8)

where S (λ) is the photon spectral flux, T (λ) is the transmittance spectra of a particular TCO [30]; in this case The S (λ) spectrum used was the air mass 1.5 global tilt (AM 1.5G) solar spectrum. Consequently, from the transmittance spectrum for the AZO thin films deposited during 40 min of deposition time on glass and CTA substrates (Fig.  1) a solar optical transmittance (SWT) estimation was performed, obtaining values of 82.2 and 78.8%, for glass and CTA respectively; which represents a difference of ~3.4%. This suggests that there is a slight decrease in optical quality when a AZO thin film is deposited in a CTA substrate. Figure  7 shows the transmittance spectra of the AZO thin films grown in glass and CTA with 40 min of deposition, where it is observed that the transmittance spectra for the CTA deposited films has smaller transmittance values in the 350–600  nm wavelength, being ∼73% for the CTA samples and 88% for the glass sample. The

Fig. 7  Optical transmittance of AZO thin films grown glass and CTA substrates using 40 min of deposition time

optical energy gap was calculated using the Tauc equations for all samples considering AZO a direct band gap n-type semiconductor obtained from the (αhν)2 versus hν plot by extrapolating the linear portion of the curve to (hν = 0) [45]. The calculation is carried out by prolonging the linear band edge region up to where it intercepts the energy axis, and it is shown in the inset of Fig.  7. The bandgap values were ~3.48 and 3.51 eV for the samples grown in glass and CTA substrates respectively. The obtained bandgap values presented increased values with respect of the standard ZnO, which is in the range 3.37 eV [5]. The higher band gap of the present AZO thin films compared with the bulk ZnO film could be attributed to the Burstein Moss effect [46]. Two main factors are responsible for the conduction mechanisms in ZnO: The electrons generated from oxygen vacancies and the interstitial Zn Atoms [47]. Due to the incorporation of ­Al3+ on substitutional sites of Zn + 2 or Al atoms into interstitial sites, as well as oxygen vacancies and Zn interstitial atoms, the electrical conductivity in Al doped ZnO thin films is expected to be higher than in pure ZnO films [48]. The electrical properties of the deposited AZO deposited in both types of substrates are summarized in Table 2. As can be observed resistivity values for both type of AZO thin films were in the range of ∼10−3–10−2  Ω  cm, with mobility (∼1020  cm−3) and carrier concentrations also within the characteristic range of the TCOs based on AZO [49]. The Hall coefficients obtained were negative, which indicates the n-type nature of AZO films. We suggest that the electrical behavior of the obtained AZO thin films can be attributed to carrier scattering by grain boundaries [50].

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Table 2  Electrical characteristics of the AZO prepared on glass and CTA substrates using 40 min of deposition time

J Inorg Organomet Polym Type of sample

Resistivity (Ω cm)

Mobility ­(cm2V−1s−1)

Carrier concentration ­(cm−3)

Hall coefficient

AZO/Glass AZO/CTA

8.52 × 10−3 1.47 × 10−2

4.19 3.06

1.75 × 1020 1.39 × 1020

−3.57 × 10−2 −4.50 × 10−2

4 Conclusions In summary, transparent conductive ZnO:Al (AZO) films have been deposited successfully by RF magnetron sputtering onto flexible cellulose triacetate and glass substrates. The structural, optical, morphological, chemical and electrical properties of AZO films deposited on CTA substrates were studied and compared to those of the AZO thin films obtained in a glass substrate. We applied several techniques such as scanning electron microscopy, X-ray diffraction, UV–Vis, XPS and the Van der Pauw technique to analyze the properties of the AZO thin films. Structural properties showed that all samples had grown at a preferential orientation in the (002) plane direction with a wurtzite structure. Morphology results obtained with SEM and AFM images showed that surfaces of the AZO thin films were densely compacted, but that the CTA deposited samples had have more regular grains with a flatter surface and no peaks. Both type of deposited samples films presented a transmittance higher than 70% in the visible range, being the transmittance value of the CTA (∼73%) deposited sample less than that of the glass deposited sample (∼88%), and the estimated optical energy gaps were in the range of 3.48–3.51  eV for the former and the latter. The XPS results showed the successful deposition of the constituent elements of AZO, namely the incorporation of Al into ZnO thin films. The resistivity obtained in both types of films was in the range ∼10−3–10−2  Ω-cm with a average mobility of ∼3.6 cm2V−1s−1 and a carrier concentration of ∼1020 cm−3. Acknowledgements  The authors acknowledge Oswaldo Gomez, Mario Herrera, Dora Huerta, Mauricio Romero, Wilian Cauich and Daniel Aguilar for technical support and Lourdes Pinelo for secretarial assistance. This work has been supported by the Project No. CB/2012/178748 CONACYT/México.

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