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RETRACTED: Electrical and optical properties of nanostructured indium doped zinc oxide thin films deposited by ultrasonic chemical spray technique, starting from zinc acetylacetonate and indium chloride
Materials Science in Semiconductor Processing 26 (2014) 288–293
Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Electrical and optical properties of nanostructured indium doped zinc oxide thin films deposited by ultrasonic chemical spray technique, starting from zinc acetylacetonate and indium chloride L. Castañeda a, A. Maldonado b, J. Vega Pérez a, M. de la L. Olvera b, C. Torres-Torres c,n a Escuela Superior de Ingeniería Mecánica y Eléctrica Unidad Ticomán del Instituto Politécnico Nacional, Apartado Postal 07340, D.F., Mexico b Departamento de Ingeniería Eléctrica-SEES, Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional, CINVESTAV-IPN, Apartado postal 14740, México 07000, D.F., Mexico c Sección de Estudios de Posgrado e Investigación, ESIME ZAC, Instituto Politécnico Nacional, México, D.F. 07738, Mexico
a r t i c l e in f o
Keywords: Thin films Semiconductor oxide Zinc oxide Ultrasonic spray pyrolysis
abstract The physical characteristics of indium-doped zinc oxide [ZnO:In] thin films deposited by the ultrasonic spray pyrolysis technique were studied. We report the use of zinc acetylacetonate and indium acetate as Zn and In precursors, respectively; achieving ZnO:In thin films with both fast growth rate and homogeneous morphology; although undesirable homogeneous reaction in gas phase over substrate surface, at substrate temperatures higher than 500 1C, leads to a rough and powdery film surface. The effect of both indium concentration and the substrate characteristics as temperature on the structure, morphology, optical, and electrical properties were measured. All the films, which were polycrystalline, fitted well with the hexagonal wurtzite type ZnO. A strong change in preferential growth, from (002) to (101) planes for indium doped samples was observed. ZnO films presented an electrical resistivity as low as 3.42 10 3 Ω cm and an average optical transmittance in the visible–UV region as high as 80%. The surface morphology of the films showed an evolution from hexagonal slices to triangle shaped grains when the indium concentration increases. Derived from nonlinear optical explorations, the temporary optical response of the samples was evaluated to be close to 1 femtosecond. Potential applications in regards to the resulting low electrical resistance and high optical transparency of the studied samples were contemplated. & 2014 Elsevier Ltd. All rights reserved.
1. Introduction Zinc oxide (ZnO) is a wide band gap (3.37 eV at room temperature) semiconductor with high exciton binding
n
Corresponding author. Tel.: þ52 55 5729 6000x54686. E-mail address: [email protected] (C. Torres-Torres).
http://dx.doi.org/10.1016/j.mssp.2014.05.011 1369-8001/& 2014 Elsevier Ltd. All rights reserved.
energy (60 meV), and a high transparency in thin film form. Characteristics such as adjustable electrical conductivity, high catalytic activity, piezoelectricity, and high chemical stability, make the ZnO films notably useful in both nanostructures and thin film form [1–4]. ZnO thin films processed from a wide number of physical and chemical techniques have been reported [5]. In the manufacturing of quality ZnO thin films as transparent
L. Castañeda et al. / Materials Science in Semiconductor Processing 26 (2014) 288–293
conductive oxide (TCO), a substantial effort has been focused to decrease the electrical resistivity of the films by metal doping, particularly of Group III elements of the periodic table. It is worth to mention, that sounded results have been achieved as ZnO thin films with a low resistivity based on physical deposition processes [6–8]. Among chemical deposition techniques utilized for quality ZnO thin film fabrication, the chemical spray is distinguished for its easy processing and straightforward implementation for large scale requirements [9]. While it is hard to obtain low resistivity values in undoped ZnO films, different efforts have been made to improve the conductivity by intentional doping. It has been shown that the nature of precursors played a role on the final characteristics of ZnO thin films fabricated by chemical spraying [10,11]. Differences in surface morphology of ultrasonic sprayed ZnO thin films due to the addition of solvents in the starting solution have also been reported [12]. The ultrasonic spray has shown to be an attractive alternative for the deposition of metal oxide thin films of improved properties. It is worth to note that zinc acetate has been usually employed as a zinc precursor for the deposition of ZnO films by ultrasonic chemical spray, but distinct attractive properties have been obtained when the deposition of ZnO thin films is based on zinc acetylacetonate as a zinc precursor [13–18]. Taking into account that the ultrasonic spray process resembles CVD, an organometallic precursor should be adequate for film deposition under near equilibrium conditions of mass transport and substrate temperature. It might also be an alternative for the achievement of ZnO thin films with a low resistivity, matching with those developed by complex techniques. With this motivation, in this work the deposition of ZnO: In thin films is presented on sodocalcic glass substrates, starting from zinc acetylacetonate and indium chloride as zinc and indium precursors, respectively. The effects of indium concentration and substrate temperature on the morphology, structure, electrical and optical properties of the films have been studied. 2. Experimental The deposition system used in this work included a variable frequency piezoelectric transducer operating at 1.2 MHz and 120 V. Fig. 1 shows the experimental setup used in this work for the fabrication of ZnO:In films. The ZnO thin films were deposited starting from a 0.2 M solution of zinc acetylacetonate (Merck) dissolved homogeneously into a non-aqueous mixture of acetic acid and methanol (1.25:48.75 v/v). For indium doping, different amounts of indium (III) chloride (Alfa Aesar, 99.99%) were dissolved in the previous solution to maintain the nominal [In]/[ZnþIn] atomic per cent ratio of 1.5, 3.0 and 4.0 at%. Thin films deposited with doping concentrations in the starting solution out of this range revealed higher resistivity values. Well cleaned sodocalcic glass plates of about 2.5 5.0 cm2 sizes were utilized as substrates. The selected substrate temperatures were 400, 425 and 450 1C. This is due to the fact that the thin films deposited at lower substrate temperatures yielded highly resistive values and at substrate temperatures higher than 450 1C powdery
289
Fig. 1. Schematic diagram of the ultrasonic spray pyrolysis system (USP) used to deposit the ZnO:In thin solid films.
finished surfaces were obtained because of the initiation of gas phase reaction. Besides, we consider the melting point of 600 1C for sodocalcic glass. Solution flux rate was 1.0 ml/min. Nitrogen was used as the carrier gas. The ultrasonic spray pyrolysis technique is a versatile method capable for producing nanostructured powder and thin film samples. The particle/grain size in these samples can be straightforwardly controlled by varying the concentration of the source solution and the atomization parameters. The details of the deposition system have been given elsewhere [19]. Under these conditions, the ZnO thin films were grown at the rate of 100 nm/min, and the deposition was continued for 10 min to obtain a thickness about 1 μm. The obtained thin films had apparent homogeneous surface and good adhesion with the substrate. Structural analysis of the samples was performed through X-ray diffraction (XRD) measurements in θ–2θ mode utilizing the CuKα1 (λ¼1.5406 A) radiation of a Siemens Kristalloflex diffractometer. A JEOL JSM-6510LV scanning electron microscope (SEM) operating at 15 keV was employed to inspect the morphology of the films. Optical transmittance spectra of the ZnO:In thin films at normal incidence were recorded in a Shimadzu 2401 PC double-beam spectrophotometer. The sheet resistance and resistivity of the as-grown ZnO:In thin films were evaluated by a four-point probe technique, taking into account their correction geometries. Silver paint (Aldrich) was used to fabricate electrical contacts in samples of the order of 1 cm 1 cm total area. The mechanism of the third order absorptive nonlinearity in the samples was investigated by means of a high irradiance single beam transmittance experiment. A 532 nm wavelength with 1 ns pulse duration was monitored using the second harmonic of a Nd–YAG laser source. 3. Results and discussion 3.1. Structure Fig. 2 depicts the X-ray diffraction patterns of undoped and indium doped by ZnO thin films prepared with
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L. Castañeda et al. / Materials Science in Semiconductor Processing 26 (2014) 288–293
Fig. 2. XRD patterns of ZnO:In thin films containing different nominal indium concentrations deposited at Ts ¼ 450 1C.
different indium concentrations in the starting solution, at a substrate temperature of Ts ¼450 1C. Diffraction peaks of all the spectra fitted well with the hexagonal wurtzite phase of ZnO (JCPDS no.36-1451). In the case of both, the undoped and 1.5% In-doped ZnO thin films, the intensity of the peak associated with (002) planes prevail over the rest, showing preferential growth. In the case of the samples doped with an [In]/[Zn] ratio of 3 and 4 at% the growth preferences changes to (101) planes. As it could be seen from Fig. 2, neither intensity nor the position of the XRD peaks suffer any significant change on incorporating indium at a lower concentration (1.5 at%). This result pointed out that indium incorporation did not affect significantly the crystalline structure of ZnO. However, the geometry of the grains of the films would have affected by the change in preferential growth, making necessary to study the surface morphology of the films in order to evaluate such changes. Employing a low doping level of In, originated the incorporation of indium ions into the lattice but they do not importantly affect the (002) growth. Incorporation of intermediate In doping, where the solubility limit of indium in ZnO is reached, produces the effect of In ions on preferential growth; it results in the development of other planes distinct than (002). The evolution of the preferential growth is consistent with the fact that undoped ZnO thin films display a preferential (002) growth as it has been previously reported [20]. 3.2. Morphology Fig. 3 illustrates the typical SEM micrographs of the ZnO: In thin films prepared at Ts ¼450 1C, with different indium contents. As it could be seen, the deposited film from a solution containing a ratio of [In]/[Zn]¼1.5 at% revealed a well-defined morphology with stacked hexagonal slices or
Fig. 3. Typical SEM images of spray pyrolyzed ZnO:In thin films prepared with (a) 1.5, (b) 3.0, and (c) 4.0 at% nominal indium concentrations. All the samples were grown at Ts ¼450 1C.
plate-like structures of 250–700 nm diameters with a width that could be estimated, at naked eye from the micrograph, of a few nm. However, further increase in the [In]/[Zn] ratio changes the morphology of the films to irregular rectangular grains with embedded pyramidal structures (300–800 nm) as in the case of films deposited from a solution containing a ratio of [In]/[Zn]¼3.0 at%. An increase of indium concentration in the starting solution, to 4.0 at% modifies also the surface morphology of the films as triangular shaped laminar structures of about 400 nm height and 10 nm thicknesses at the surface which were formed. We estimated that the substrate temperature together with the doping process, are the main responsible for this triangular morphology. As the In concentration increases in the experiment, a change in the morphology of the films can be observed. From Fig. 3a, it is worth noting that in the case of the films prepared with a nominal In concentration of 1.5%, grains with pyramidal-shape of about 500 nm average can be
L. Castañeda et al. / Materials Science in Semiconductor Processing 26 (2014) 288–293
obtained. The sample prepared with a nominal In concentration of 3.0% gives origin to grains with rounded geometry. The film prepared with a nominal In concentration of 4% shows compact surfaces covered by smaller grains with similar rounded geometries. These results clearly show that textured ZnO:In thin solid films can be obtained by the ultrasonic spray pyrolysis process. 3.3. Electrical characteristics Electrical conductivity in all samples was measured by the four-point probe method considering the geometry of electrical contacts located over them. It was observed that, as the substrate temperature increase, the resistivity of the ZnO:In thin films decreases, reaching a minimum value, for a fixed [In]/[Zn] ratio in the starting solution. This trend of the resistivity of the films as a function of the substrate temperature had been explained on the basis of stoichiometry deviations, as in the regime of low substrate temperatures. It was considered that the synthesis reaction of ZnO was not completed. The substrate temperature led to an increase in oxygen vacancies and/or zinc interstitial. Moreover, the corresponding resistivity decreases. Doped Indium enhanced conductivity transport, as it was considered that a fraction of Zn2 þ ions were replaced by In3 þ ions, releasing one electron into the lattice which in turn increases the carrier concentration. In the high substrate temperature regime (4450 1C), exo-diffusion of alkaline ions coming from the substrate is not expected [21], but undesirable gas phase reaction takes place, degrading the smooth and uniform surface finish. Regarding the doping effect, there was an optimum value, around [In]/[Zn]¼3 at% relation with the solubility limit of In into the ZnO lattice, that is associated with the optimal incorporation of In ions into Zn sites, increasing donor concentration and contributing to a decrease in the resistivity. From the measured electrical conductivity of the samples presented in Table 1, it was clear to see that a substrate temperature of 450 1C and a ratio of [In]/[Zn] ratio ¼3.0 at % in the starting solution were optimum parameters to obtain ZnO:In thin films of minimum resistivity. Thin films were deposited at those optimum conditions manifest a resistivity as low as 3.42 10 3 Ω-cm. It was almost one order higher than the lowest resistivity achieved in ZnO thin films so far [6]. Low resistivity of the ZnO:In films
291
containing 3.0 at% nominal indium at Ts ¼450 1C might be associated with their compact surface morphology and bigger crystallite sizes. Comparatively, an undoped ZnO and also the case of doped ZnO with In concentration 10 at% deposited at 450 1C were measured; the corresponding values are 1 10 2 Ω-cm and 5 10 2 Ω-cm, respectively. Moreover, in respect to other starting solutions for preparing ZnO:In films, zinc acetylacetonate as the Zn source material seems to allow to reduce the resistivity [22]. 3.4. Optical measurements Fig. 4 shows the optical transmittance spectra of the studied ZnO:In samples deposited at different [In]/[Zn] ratios for a fixed substrate temperature of 450 1C. All the samples revealed a sharp decrease in transmittance in ultra-violet region (around 375 nm) with a high transmittance ( 80%) in the visible spectral range (500–850 nm). The modulation observed for visible wavelengths is due to multi reflection interference effect in the thin film sample. This effect can be considered as a Fabry–Perot interferometer for modulating the transmittance of optical wavelengths. The absorption peaks change uniformly because of the different resulting thicknesses of the studied samples. Considering their direct optical transition, band gap energy (Eg) of the samples was calculated from the 2 ðα h νÞ vs. hν plots presented in Fig. 5, where α was the optical absorption coefficient and hν was the energy of the incident photons. The band gap energy values were estimated from the intersections of the linear fits to the 2 ðα h νÞ values to the energy (hν) axis. The band gap energy values changed from 3.44 to 3.56 eV with the variation of nominal indium contents in the precursor solution. 3.5. Nonlinear optical response Similar behaviors associated to a dominant saturable absorption were exhibited by all the studied thin solid films during the high intensity transmittance experiments. The numerical fitting was performed using the expression for the transmitted irradiance I(L) in the presence of
Table 1 Band gap energy and room temperature electrical resistivity of ZnO:In thin films fabricated at different growth conditions. Nominal indium concentration (at%)
Substrate temperature (1C)
Resistivity (Ω-cm)
1.5
400 425 450
6.82 10 2 7.40 10 3 6.52 10 3
3.0
400 425 450
1.49 10 2 3.42 10 3 3.73 10 3
4.0
400 425 450
2.02 10 2 4.24 10 3 3.60 10 3
Fig. 4. Optical transmittance spectra of ZnO:In thin films deposited at different [In]/[Zn] ratios in the starting solution, for a fixed substrate temperature of 450 1C.
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the following expression [23]: β¼
αo Is
ð2Þ
On the other hand, we took into account that the saturation irradiance Is occurring during electronic transitions could be expressed as, I s ¼ ħω=sτ;
ð3Þ
where ħω was the photon energy, s was the absorption cross-section of the transition and τ was the corresponding lifetime of the excited state. At this point, the response time given by τ could be estimated in the sample if we considered a two level model where, αo ¼ sN0 ;
ð4Þ
Fig. 6. Representative nonlinear optical transmittance for the ZnO:In samples deposited at Ts ¼ 450 1C.
where N0 is the density number and αo is the measured value of the linear absorption coefficient for the wavelength λ of the optical beam in propagation through the sample. By means of our statistical data for the density of ZnO nanostructures and the optical absorption measurements, we approximated that for a λ¼532 nm wavelength, we could get s ¼5.1923 10 17 m 2 for the ZnO:In 1.5% and s ¼1.4290 10 16 m 2 for the ZnO:In 4%. Because in our case, ħω¼3.7365 10–19 J, we substituted these values in Eq. (5), and then it results that the different samples seem to exhibit a lifetime, τ¼9.9Ε 15 s and τ¼3.6Ε 15 s; respectively. Apparently τ was more than twice times faster in the thin sample when a higher concentration of indium was employed for the preparation of the ZnO doped film. The estimated temporal response for these samples were in good agreement with previous results reported in ZnO:In samples studied in the femtosecond regime [24]. The saturated optical absorption exploration gives information about the lifetime of optical interactions exhibited by the sample. The results indicate that the estimated optical lifetime in nanostructured ZnO allows to contemplate the development of nonlinear optical applications as it has been previously suggested [25]. Potential applications for developing all-optical systems in regards to the resulting ultrafast optical response could be considered.
nonlinear optical absorption:
4. Conclusions
Fig. 5. (αhv)2 vs. hν plots for the ZnO:In samples deposited at different temperatures.
Transmitance
0.78
Theory Experiment
0.76 0.74 0.72 0.7 0.4
IðLÞ ¼
I o expð αo LÞ ; 1 þ βI o Lef f
0.6 0.8 1 1.2 Input irradiance [GW/cm2]
1.4
ð1Þ
where β represents the nonlinear absorption coefficient; Io is the total irradiance of the incident beam, Leff is the effective length given by Lef f ¼ ð1 e αo L Þ=αo , with L the sample length, αo the linear absorption coefficient and β the nonlinear absorption coeffcient. If we compare the experimental data with numerical simulations performed with Eq. (1), we can get the nonlinear optical absorption β¼ 9.3 10 9 m/W. Fig. 6 shows the representative experimental transmittance of the beam for different incident irradiances. An error bar of 73% was estimated for the experimental irradiance data. It was considered as the first approximation, that a saturable absorption coefficient β could be estimated by
Indium-doped zinc oxide thin films with nonlinear optical response, low electrical resistivity and high optical transmittance in the visible spectral region were successfully deposited over cheap glass substrates by the ultrasonic spray technique. Starting from zinc acetylacetonate as the zinc precursor, ZnO:In thin films, deposited at Ts ¼ 450 1C with a nominal [In]/[Zn]¼3.0 at% indium concentration in the starting solution revealed electrical resistivity as low as 3.42 10 3 Ω cm. The substrate temperature played a significant role in controlling the crystallite size of ZnO:In films while the concentration of indium ions in starting solution affected their crystallite orientation and room temperature electrical resistivity. A strong nonlinear optical absorption effect was measured in all the films, and an ultrafast optical response close to 1 femtosecond was estimated.
L. Castañeda et al. / Materials Science in Semiconductor Processing 26 (2014) 288–293
Acknowledgments The authors wish to thanks M. A. Luna, A. Tavira and A. Palafox for technical assistance. Special thanks to María de Lourdes-Rojas, from Laboratorios Centrales-CINVESTAVIPN for SEM analysis. Also gratefully acknowledge for the financial support from the Instituto Politécnico Nacional under Project SIP-20140273 and from CONACyT under 155996 contract. L. Castañeda gratefully acknowledges the financial support from the Escuela Superior de Ingeniería Mecánica y Eléctrica Unidad Ticomán, Instituto Politécnico Nacional, through Project no. SIP-20140729. References [1] A. Janotti, C.G. Van de Walle, Rep. Prog. Phys. 72 (2009) 126501. [2] S. Yun, J. Lee, J. Chung, S. Lim, J. Phys. Chem. Solids 71 (12) (2010) 1724–1731. [3] K.L. Chopra, S. Major, D.K. Pandya, Thin Solid Films 102 (1983) 1–46. [4] A. Stadler, Materials. 5 (2012) 661–683. [5] P.M. Ratheesh Kumar, C. Sudha Kartha, K.P. Vijayakumar, T. Abe, Y. Kashiwaba, F. Singh, D.K. Avasthi, Semicond. Sci. Technol. 20 (2005) 120–126. [6] K. Ellmer, J. Phys. D: Appl. Phys. 34 (2001) 3097–3108. [7] T.V. Vimalkumar, N. Poornima, K.B. Jinesh, C. Sudha Kartha, K.P. Vijayakumar, Appl. Surf. Sci. 257 (2011) (2011) 8334–8340. [8] T.V. Vimalkumar, N. Poornima, C. Sudha Kartha, K.P. Vijayakumar, Appl. Surf. Sci. 256 (2010) 6025–6028. [9] P.S. Patil, Mater. Chem. Phys. 59 (1999) 185–198.
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Contents lists available at ScienceDirect
Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Electrical and optical properties of nanostructured indium doped zinc oxide thin films deposited by ultrasonic chemical spray technique, starting from zinc acetylacetonate and indium chloride L. Castañeda a, A. Maldonado b, J. Vega Pérez a, M. de la L. Olvera b, C. Torres-Torres c,n a Escuela Superior de Ingeniería Mecánica y Eléctrica Unidad Ticomán del Instituto Politécnico Nacional, Apartado Postal 07340, D.F., Mexico b Departamento de Ingeniería Eléctrica-SEES, Centro de Investigación y de Estudios Avanzados del Instituto Politécnico Nacional, CINVESTAV-IPN, Apartado postal 14740, México 07000, D.F., Mexico c Sección de Estudios de Posgrado e Investigación, ESIME ZAC, Instituto Politécnico Nacional, México, D.F. 07738, Mexico
a r t i c l e in f o
Keywords: Thin films Semiconductor oxide Zinc oxide Ultrasonic spray pyrolysis
abstract The physical characteristics of indium-doped zinc oxide [ZnO:In] thin films deposited by the ultrasonic spray pyrolysis technique were studied. We report the use of zinc acetylacetonate and indium acetate as Zn and In precursors, respectively; achieving ZnO:In thin films with both fast growth rate and homogeneous morphology; although undesirable homogeneous reaction in gas phase over substrate surface, at substrate temperatures higher than 500 1C, leads to a rough and powdery film surface. The effect of both indium concentration and the substrate characteristics as temperature on the structure, morphology, optical, and electrical properties were measured. All the films, which were polycrystalline, fitted well with the hexagonal wurtzite type ZnO. A strong change in preferential growth, from (002) to (101) planes for indium doped samples was observed. ZnO films presented an electrical resistivity as low as 3.42 10 3 Ω cm and an average optical transmittance in the visible–UV region as high as 80%. The surface morphology of the films showed an evolution from hexagonal slices to triangle shaped grains when the indium concentration increases. Derived from nonlinear optical explorations, the temporary optical response of the samples was evaluated to be close to 1 femtosecond. Potential applications in regards to the resulting low electrical resistance and high optical transparency of the studied samples were contemplated. & 2014 Elsevier Ltd. All rights reserved.
1. Introduction Zinc oxide (ZnO) is a wide band gap (3.37 eV at room temperature) semiconductor with high exciton binding
n
Corresponding author. Tel.: þ52 55 5729 6000x54686. E-mail address: [email protected] (C. Torres-Torres).
http://dx.doi.org/10.1016/j.mssp.2014.05.011 1369-8001/& 2014 Elsevier Ltd. All rights reserved.
energy (60 meV), and a high transparency in thin film form. Characteristics such as adjustable electrical conductivity, high catalytic activity, piezoelectricity, and high chemical stability, make the ZnO films notably useful in both nanostructures and thin film form [1–4]. ZnO thin films processed from a wide number of physical and chemical techniques have been reported [5]. In the manufacturing of quality ZnO thin films as transparent
L. Castañeda et al. / Materials Science in Semiconductor Processing 26 (2014) 288–293
conductive oxide (TCO), a substantial effort has been focused to decrease the electrical resistivity of the films by metal doping, particularly of Group III elements of the periodic table. It is worth to mention, that sounded results have been achieved as ZnO thin films with a low resistivity based on physical deposition processes [6–8]. Among chemical deposition techniques utilized for quality ZnO thin film fabrication, the chemical spray is distinguished for its easy processing and straightforward implementation for large scale requirements [9]. While it is hard to obtain low resistivity values in undoped ZnO films, different efforts have been made to improve the conductivity by intentional doping. It has been shown that the nature of precursors played a role on the final characteristics of ZnO thin films fabricated by chemical spraying [10,11]. Differences in surface morphology of ultrasonic sprayed ZnO thin films due to the addition of solvents in the starting solution have also been reported [12]. The ultrasonic spray has shown to be an attractive alternative for the deposition of metal oxide thin films of improved properties. It is worth to note that zinc acetate has been usually employed as a zinc precursor for the deposition of ZnO films by ultrasonic chemical spray, but distinct attractive properties have been obtained when the deposition of ZnO thin films is based on zinc acetylacetonate as a zinc precursor [13–18]. Taking into account that the ultrasonic spray process resembles CVD, an organometallic precursor should be adequate for film deposition under near equilibrium conditions of mass transport and substrate temperature. It might also be an alternative for the achievement of ZnO thin films with a low resistivity, matching with those developed by complex techniques. With this motivation, in this work the deposition of ZnO: In thin films is presented on sodocalcic glass substrates, starting from zinc acetylacetonate and indium chloride as zinc and indium precursors, respectively. The effects of indium concentration and substrate temperature on the morphology, structure, electrical and optical properties of the films have been studied. 2. Experimental The deposition system used in this work included a variable frequency piezoelectric transducer operating at 1.2 MHz and 120 V. Fig. 1 shows the experimental setup used in this work for the fabrication of ZnO:In films. The ZnO thin films were deposited starting from a 0.2 M solution of zinc acetylacetonate (Merck) dissolved homogeneously into a non-aqueous mixture of acetic acid and methanol (1.25:48.75 v/v). For indium doping, different amounts of indium (III) chloride (Alfa Aesar, 99.99%) were dissolved in the previous solution to maintain the nominal [In]/[ZnþIn] atomic per cent ratio of 1.5, 3.0 and 4.0 at%. Thin films deposited with doping concentrations in the starting solution out of this range revealed higher resistivity values. Well cleaned sodocalcic glass plates of about 2.5 5.0 cm2 sizes were utilized as substrates. The selected substrate temperatures were 400, 425 and 450 1C. This is due to the fact that the thin films deposited at lower substrate temperatures yielded highly resistive values and at substrate temperatures higher than 450 1C powdery
289
Fig. 1. Schematic diagram of the ultrasonic spray pyrolysis system (USP) used to deposit the ZnO:In thin solid films.
finished surfaces were obtained because of the initiation of gas phase reaction. Besides, we consider the melting point of 600 1C for sodocalcic glass. Solution flux rate was 1.0 ml/min. Nitrogen was used as the carrier gas. The ultrasonic spray pyrolysis technique is a versatile method capable for producing nanostructured powder and thin film samples. The particle/grain size in these samples can be straightforwardly controlled by varying the concentration of the source solution and the atomization parameters. The details of the deposition system have been given elsewhere [19]. Under these conditions, the ZnO thin films were grown at the rate of 100 nm/min, and the deposition was continued for 10 min to obtain a thickness about 1 μm. The obtained thin films had apparent homogeneous surface and good adhesion with the substrate. Structural analysis of the samples was performed through X-ray diffraction (XRD) measurements in θ–2θ mode utilizing the CuKα1 (λ¼1.5406 A) radiation of a Siemens Kristalloflex diffractometer. A JEOL JSM-6510LV scanning electron microscope (SEM) operating at 15 keV was employed to inspect the morphology of the films. Optical transmittance spectra of the ZnO:In thin films at normal incidence were recorded in a Shimadzu 2401 PC double-beam spectrophotometer. The sheet resistance and resistivity of the as-grown ZnO:In thin films were evaluated by a four-point probe technique, taking into account their correction geometries. Silver paint (Aldrich) was used to fabricate electrical contacts in samples of the order of 1 cm 1 cm total area. The mechanism of the third order absorptive nonlinearity in the samples was investigated by means of a high irradiance single beam transmittance experiment. A 532 nm wavelength with 1 ns pulse duration was monitored using the second harmonic of a Nd–YAG laser source. 3. Results and discussion 3.1. Structure Fig. 2 depicts the X-ray diffraction patterns of undoped and indium doped by ZnO thin films prepared with
290
L. Castañeda et al. / Materials Science in Semiconductor Processing 26 (2014) 288–293
Fig. 2. XRD patterns of ZnO:In thin films containing different nominal indium concentrations deposited at Ts ¼ 450 1C.
different indium concentrations in the starting solution, at a substrate temperature of Ts ¼450 1C. Diffraction peaks of all the spectra fitted well with the hexagonal wurtzite phase of ZnO (JCPDS no.36-1451). In the case of both, the undoped and 1.5% In-doped ZnO thin films, the intensity of the peak associated with (002) planes prevail over the rest, showing preferential growth. In the case of the samples doped with an [In]/[Zn] ratio of 3 and 4 at% the growth preferences changes to (101) planes. As it could be seen from Fig. 2, neither intensity nor the position of the XRD peaks suffer any significant change on incorporating indium at a lower concentration (1.5 at%). This result pointed out that indium incorporation did not affect significantly the crystalline structure of ZnO. However, the geometry of the grains of the films would have affected by the change in preferential growth, making necessary to study the surface morphology of the films in order to evaluate such changes. Employing a low doping level of In, originated the incorporation of indium ions into the lattice but they do not importantly affect the (002) growth. Incorporation of intermediate In doping, where the solubility limit of indium in ZnO is reached, produces the effect of In ions on preferential growth; it results in the development of other planes distinct than (002). The evolution of the preferential growth is consistent with the fact that undoped ZnO thin films display a preferential (002) growth as it has been previously reported [20]. 3.2. Morphology Fig. 3 illustrates the typical SEM micrographs of the ZnO: In thin films prepared at Ts ¼450 1C, with different indium contents. As it could be seen, the deposited film from a solution containing a ratio of [In]/[Zn]¼1.5 at% revealed a well-defined morphology with stacked hexagonal slices or
Fig. 3. Typical SEM images of spray pyrolyzed ZnO:In thin films prepared with (a) 1.5, (b) 3.0, and (c) 4.0 at% nominal indium concentrations. All the samples were grown at Ts ¼450 1C.
plate-like structures of 250–700 nm diameters with a width that could be estimated, at naked eye from the micrograph, of a few nm. However, further increase in the [In]/[Zn] ratio changes the morphology of the films to irregular rectangular grains with embedded pyramidal structures (300–800 nm) as in the case of films deposited from a solution containing a ratio of [In]/[Zn]¼3.0 at%. An increase of indium concentration in the starting solution, to 4.0 at% modifies also the surface morphology of the films as triangular shaped laminar structures of about 400 nm height and 10 nm thicknesses at the surface which were formed. We estimated that the substrate temperature together with the doping process, are the main responsible for this triangular morphology. As the In concentration increases in the experiment, a change in the morphology of the films can be observed. From Fig. 3a, it is worth noting that in the case of the films prepared with a nominal In concentration of 1.5%, grains with pyramidal-shape of about 500 nm average can be
L. Castañeda et al. / Materials Science in Semiconductor Processing 26 (2014) 288–293
obtained. The sample prepared with a nominal In concentration of 3.0% gives origin to grains with rounded geometry. The film prepared with a nominal In concentration of 4% shows compact surfaces covered by smaller grains with similar rounded geometries. These results clearly show that textured ZnO:In thin solid films can be obtained by the ultrasonic spray pyrolysis process. 3.3. Electrical characteristics Electrical conductivity in all samples was measured by the four-point probe method considering the geometry of electrical contacts located over them. It was observed that, as the substrate temperature increase, the resistivity of the ZnO:In thin films decreases, reaching a minimum value, for a fixed [In]/[Zn] ratio in the starting solution. This trend of the resistivity of the films as a function of the substrate temperature had been explained on the basis of stoichiometry deviations, as in the regime of low substrate temperatures. It was considered that the synthesis reaction of ZnO was not completed. The substrate temperature led to an increase in oxygen vacancies and/or zinc interstitial. Moreover, the corresponding resistivity decreases. Doped Indium enhanced conductivity transport, as it was considered that a fraction of Zn2 þ ions were replaced by In3 þ ions, releasing one electron into the lattice which in turn increases the carrier concentration. In the high substrate temperature regime (4450 1C), exo-diffusion of alkaline ions coming from the substrate is not expected [21], but undesirable gas phase reaction takes place, degrading the smooth and uniform surface finish. Regarding the doping effect, there was an optimum value, around [In]/[Zn]¼3 at% relation with the solubility limit of In into the ZnO lattice, that is associated with the optimal incorporation of In ions into Zn sites, increasing donor concentration and contributing to a decrease in the resistivity. From the measured electrical conductivity of the samples presented in Table 1, it was clear to see that a substrate temperature of 450 1C and a ratio of [In]/[Zn] ratio ¼3.0 at % in the starting solution were optimum parameters to obtain ZnO:In thin films of minimum resistivity. Thin films were deposited at those optimum conditions manifest a resistivity as low as 3.42 10 3 Ω-cm. It was almost one order higher than the lowest resistivity achieved in ZnO thin films so far [6]. Low resistivity of the ZnO:In films
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containing 3.0 at% nominal indium at Ts ¼450 1C might be associated with their compact surface morphology and bigger crystallite sizes. Comparatively, an undoped ZnO and also the case of doped ZnO with In concentration 10 at% deposited at 450 1C were measured; the corresponding values are 1 10 2 Ω-cm and 5 10 2 Ω-cm, respectively. Moreover, in respect to other starting solutions for preparing ZnO:In films, zinc acetylacetonate as the Zn source material seems to allow to reduce the resistivity [22]. 3.4. Optical measurements Fig. 4 shows the optical transmittance spectra of the studied ZnO:In samples deposited at different [In]/[Zn] ratios for a fixed substrate temperature of 450 1C. All the samples revealed a sharp decrease in transmittance in ultra-violet region (around 375 nm) with a high transmittance ( 80%) in the visible spectral range (500–850 nm). The modulation observed for visible wavelengths is due to multi reflection interference effect in the thin film sample. This effect can be considered as a Fabry–Perot interferometer for modulating the transmittance of optical wavelengths. The absorption peaks change uniformly because of the different resulting thicknesses of the studied samples. Considering their direct optical transition, band gap energy (Eg) of the samples was calculated from the 2 ðα h νÞ vs. hν plots presented in Fig. 5, where α was the optical absorption coefficient and hν was the energy of the incident photons. The band gap energy values were estimated from the intersections of the linear fits to the 2 ðα h νÞ values to the energy (hν) axis. The band gap energy values changed from 3.44 to 3.56 eV with the variation of nominal indium contents in the precursor solution. 3.5. Nonlinear optical response Similar behaviors associated to a dominant saturable absorption were exhibited by all the studied thin solid films during the high intensity transmittance experiments. The numerical fitting was performed using the expression for the transmitted irradiance I(L) in the presence of
Table 1 Band gap energy and room temperature electrical resistivity of ZnO:In thin films fabricated at different growth conditions. Nominal indium concentration (at%)
Substrate temperature (1C)
Resistivity (Ω-cm)
1.5
400 425 450
6.82 10 2 7.40 10 3 6.52 10 3
3.0
400 425 450
1.49 10 2 3.42 10 3 3.73 10 3
4.0
400 425 450
2.02 10 2 4.24 10 3 3.60 10 3
Fig. 4. Optical transmittance spectra of ZnO:In thin films deposited at different [In]/[Zn] ratios in the starting solution, for a fixed substrate temperature of 450 1C.
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the following expression [23]: β¼
αo Is
ð2Þ
On the other hand, we took into account that the saturation irradiance Is occurring during electronic transitions could be expressed as, I s ¼ ħω=sτ;
ð3Þ
where ħω was the photon energy, s was the absorption cross-section of the transition and τ was the corresponding lifetime of the excited state. At this point, the response time given by τ could be estimated in the sample if we considered a two level model where, αo ¼ sN0 ;
ð4Þ
Fig. 6. Representative nonlinear optical transmittance for the ZnO:In samples deposited at Ts ¼ 450 1C.
where N0 is the density number and αo is the measured value of the linear absorption coefficient for the wavelength λ of the optical beam in propagation through the sample. By means of our statistical data for the density of ZnO nanostructures and the optical absorption measurements, we approximated that for a λ¼532 nm wavelength, we could get s ¼5.1923 10 17 m 2 for the ZnO:In 1.5% and s ¼1.4290 10 16 m 2 for the ZnO:In 4%. Because in our case, ħω¼3.7365 10–19 J, we substituted these values in Eq. (5), and then it results that the different samples seem to exhibit a lifetime, τ¼9.9Ε 15 s and τ¼3.6Ε 15 s; respectively. Apparently τ was more than twice times faster in the thin sample when a higher concentration of indium was employed for the preparation of the ZnO doped film. The estimated temporal response for these samples were in good agreement with previous results reported in ZnO:In samples studied in the femtosecond regime [24]. The saturated optical absorption exploration gives information about the lifetime of optical interactions exhibited by the sample. The results indicate that the estimated optical lifetime in nanostructured ZnO allows to contemplate the development of nonlinear optical applications as it has been previously suggested [25]. Potential applications for developing all-optical systems in regards to the resulting ultrafast optical response could be considered.
nonlinear optical absorption:
4. Conclusions
Fig. 5. (αhv)2 vs. hν plots for the ZnO:In samples deposited at different temperatures.
Transmitance
0.78
Theory Experiment
0.76 0.74 0.72 0.7 0.4
IðLÞ ¼
I o expð αo LÞ ; 1 þ βI o Lef f
0.6 0.8 1 1.2 Input irradiance [GW/cm2]
1.4
ð1Þ
where β represents the nonlinear absorption coefficient; Io is the total irradiance of the incident beam, Leff is the effective length given by Lef f ¼ ð1 e αo L Þ=αo , with L the sample length, αo the linear absorption coefficient and β the nonlinear absorption coeffcient. If we compare the experimental data with numerical simulations performed with Eq. (1), we can get the nonlinear optical absorption β¼ 9.3 10 9 m/W. Fig. 6 shows the representative experimental transmittance of the beam for different incident irradiances. An error bar of 73% was estimated for the experimental irradiance data. It was considered as the first approximation, that a saturable absorption coefficient β could be estimated by
Indium-doped zinc oxide thin films with nonlinear optical response, low electrical resistivity and high optical transmittance in the visible spectral region were successfully deposited over cheap glass substrates by the ultrasonic spray technique. Starting from zinc acetylacetonate as the zinc precursor, ZnO:In thin films, deposited at Ts ¼ 450 1C with a nominal [In]/[Zn]¼3.0 at% indium concentration in the starting solution revealed electrical resistivity as low as 3.42 10 3 Ω cm. The substrate temperature played a significant role in controlling the crystallite size of ZnO:In films while the concentration of indium ions in starting solution affected their crystallite orientation and room temperature electrical resistivity. A strong nonlinear optical absorption effect was measured in all the films, and an ultrafast optical response close to 1 femtosecond was estimated.
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Acknowledgments The authors wish to thanks M. A. Luna, A. Tavira and A. Palafox for technical assistance. Special thanks to María de Lourdes-Rojas, from Laboratorios Centrales-CINVESTAVIPN for SEM analysis. Also gratefully acknowledge for the financial support from the Instituto Politécnico Nacional under Project SIP-20140273 and from CONACyT under 155996 contract. L. Castañeda gratefully acknowledges the financial support from the Escuela Superior de Ingeniería Mecánica y Eléctrica Unidad Ticomán, Instituto Politécnico Nacional, through Project no. SIP-20140729. References [1] A. Janotti, C.G. Van de Walle, Rep. Prog. Phys. 72 (2009) 126501. [2] S. Yun, J. Lee, J. Chung, S. Lim, J. Phys. Chem. Solids 71 (12) (2010) 1724–1731. [3] K.L. Chopra, S. Major, D.K. Pandya, Thin Solid Films 102 (1983) 1–46. [4] A. Stadler, Materials. 5 (2012) 661–683. [5] P.M. Ratheesh Kumar, C. Sudha Kartha, K.P. Vijayakumar, T. Abe, Y. Kashiwaba, F. Singh, D.K. Avasthi, Semicond. Sci. Technol. 20 (2005) 120–126. [6] K. Ellmer, J. Phys. D: Appl. Phys. 34 (2001) 3097–3108. [7] T.V. Vimalkumar, N. Poornima, K.B. Jinesh, C. Sudha Kartha, K.P. Vijayakumar, Appl. Surf. Sci. 257 (2011) (2011) 8334–8340. [8] T.V. Vimalkumar, N. Poornima, C. Sudha Kartha, K.P. Vijayakumar, Appl. Surf. Sci. 256 (2010) 6025–6028. [9] P.S. Patil, Mater. Chem. Phys. 59 (1999) 185–198.
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