Labyrinth patterns of zinc oxide on porous silicon substrate

Superlattices and Microstructures 67 (2014) 72–81

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Labyrinth patterns of zinc oxide on porous silicon substrate L. Martínez a, Y. Kumar a, D. Mayorga a, N. Goswami b, V. Agarwal a,⇑ a Center for Engineering and Applied Sciences (CIICAp-UAEM), Av. Universidad 1001, Col. Chamilpa, Cuernavaca, Morelos 62209, Mexico b Department of Physics and Material Science & Engineering, Jaypee Institute of Information Technology (JIIT), (Deemed to be University under Section 3 of UGC Act) A-10, Sector-62, Noida, UP 201307, India

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Article history: Received 15 December 2013 Accepted 16 December 2013 Available online 25 December 2013 Keywords: Zinc oxide Porous silicon Morphology Characterization Nanostructure

a b s t r a c t The substrate treatment dependent formation of different micromorphologies of zinc oxide over PS substrate has been reported. Effect of substrate oxidation and annealing has been studied. Changes in the structural properties were seen in the form of labyrinth patterns developed on the surface and were studied with the help of scanning electron microscope (SEM), atomic force microscope (AFM). X-ray diffraction (XRD) along with UV–visible absorption and photoluminescence (PL) spectroscopy were performed for characterizing the zinc oxide film and the hybrid structure. A relatively flat film of nanostructured zinc oxide particles is found to form on the oxidized substrate as compared to the nanostructured labyrinth patterns formed on the un-oxidized substrate with enhanced aspect ratio. Such micromorphologies can be very promising for fabricating highly sensitive gas sensors. Ó 2013 Elsevier Ltd. All rights reserved.

1. Introduction Some of the unique properties of zinc oxide, such as direct band gap semiconductor (3.37 eV), large excitation binding energy (60 meV), near UV emission and transparent conductivity and various applications for nanodevices like varistors [1], UV light emitting devices [2], photo detectors, gas sensors and nanolasers [3], have attracted remarkable attention of the scientific community. Different morphologies of nanostructures like nanowires, nanorods, belts, tubes, nanobridges, whiskers, nanonails [4–7] have generated interest due to their potential for fundamental studies related to the effect of ⇑ Corresponding author. Tel.: +52 777 3297084. E-mail address: [email protected] (V. Agarwal). 0749-6036/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.spmi.2013.12.008

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dimensionality, morphology and size on their physical and chemical properties along with their application in optoelectronic devices [8–11]. Moreover, recent reports on lasers from nanowire arrays [12] have caused great interest in the studies related to various morphologies of nanostructures. These nanostructures have been successfully prepared by various physical and chemical methods [13]. Apart from that, various kinds of porous substrates, such as porous alumina [14] and porous silicon [15] have also been employed to design different morphologies and have possible sensing applications. In particular, porous silicon (PS) is one of the important porous materials since Canham in 1990 [16] presented the first observation of efficient photoluminescence from PS at room temperature. Its open structure, tunable pore dimensions, large surface area, convenient surface chemistry, compatibility with the silicon IC technology [17], combined with the unique optical and electrical properties, make PS a good candidate for templates [18–21] and an alternative material for gas sensing, operating at relatively low temperatures [22–24]. Possibility to further increase the surface area and hence, the optical and electrical properties of such porous substrates, has invoked the interest of many researchers to investigate the growth of different microstructures of semiconducting oxides on porous silicon substrate. Recently, Hashim et al. [25] demonstrated fabrication of nanorods on porous silicon by thermal evaporation method. In the present work, ZnO films were spin coated over freshly etched and oxidized low porosity nanostructured mesoporous silicon followed with annealing and laser treatment to obtain to microstructures such as nanoribbons, and granular labyrinth patterns. 2. Experimental details PS samples were fabricated by wet electrochemical etching of p++-type Si, (100) orientation wafers with a resistivity of 0.002–0.005 X cm. The samples were prepared with 1:1 concentration of hydrofluoric acid (48 wt.% HF) and ethanol (99.9%). Multilayered porous structure was fabricated with anodization time and current density as A/B and 7.4/118 mA/cm2 for high/low porosity layer respectively. After the fabrication, the samples were rinsed by ethanol and dried in pentane [26]. ZnO was synthesized via wet chemical route i.e. sol–gel method and thin films were deposited onto the PS substrates using spin coating. In order to see the effect of substrate oxidation, on the morphological properties of zinc oxide films, PS substrates were oxidized with 99.9% of oxygen at 600 °C for a duration of 30 min. Preparation of ZnO via sol–gel technique consisted of dissolving zinc acetate dehydrate [Zn(CH3COO)22H2O] in ethanol along with monoethanolamine (MEA), which is highly water soluble, non-ionic and a sol stabilizer. A homogeneous transparent solution with a concentration of 0.2 M zinc acetate and a 1:1 M ratio of MEA/zinc acetate dehydrate was kept for an hour in ultrasonic bath at 50 °C and later left at room temperature for 48 h (hydrolysis). The above solution was spin coated at 3000 rpm for 25 s. After each deposition, the film was dried at 200 °C for 3 min and the process was repeated 10 times to get the desired film thickness. The morphology and the topographical characteristics were studied before (ZB2A) and after annealing (ZB2B) the hybrid structures at 500 °C for 30 min. in nitrogen environment. Similarly, hybrid structures formed on the oxidized substrate, without (OZB1A) and with annealing treatment (OZB1B) were studied. The structural properties were analyzed using high resolution field emission scanning electron microscope (Quanta 3D FEG) and atomic force microscopy (AFM) (Vecco Nanoscope V model). The orientation and crystallinity of the ZnO crystallites were analyzed by XRD spectrometer (Xpert’PRO) equipped with Cu anode X ray tube (with Ka radiation wavelength of 1.54 Å) from the angle 2 ranged from 20° to 60°. The steady state photoluminescence properties were studied using Varian Fluorescence spectrometer (Cary Eclipse) under the excitation wavelength of 325 nm using a Xenon lamp and the reflectance was measured using Perkin–Elmer UV–Vis–NIR spectrometer (Lambda 950). For localized heat treatment, 532 nm laser with single-frequency green output, generated from a compact solid-state diode-pumped frequency-doubled Nd:Vanadate (Nd:YVO4), Verdi V-8 coherent high intensity laser system (1W) was used. 3. Results and discussion Fig. 1 shows the cross-sectional view of a typical hybrid structure with thin ZnO layer deposited over nanostructured PS substrate (ZB2B). The cross sectional image shows the approximate thickness

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Fig. 1. The SEM image of ZnO-PS cross-sectional view showing porous multilayer over the silicon substrate followed by ZnO layer at the top after annealing at 500 °C for 30 min. The average thickness of ZnO film, measured by FESEM, is 190 nm.

of ZnOx layer as 187 nm. A 1125 nm thick, periodic multilayered porous substrate can be seen in the form of low and high porosity layers with the top most layer of low porosity. Fig. 2 shows the typical XRD pattern (corresponding to the sample ZB2B) of the ZnO-PS structures after annealing the hybrid structure at 500 °C. All the samples are shown to exhibit dominant peaks at 2h = 32.17°, 34.52°, 36.39° corresponding to (1 0 0), (0 0 2) and (1 0 1) planes of ZnO, respectively. The XRD pattern of ZnO reveals a hexagonal Wurtzite structure and polycrystalline nature (JCDPS card number: 36-1451). The films are oriented perpendicular to the substrate surface in c-axis and with respect to the ZnO powder. The c-axis orientation can be understood due to the fact that c-plane of the zinc oxide crystallites corresponds to the densest packed plane. The XRD peak around 2h = 56° corresponds to porous silicon and observed in all the samples. Band gap determination: In order to determine band gap of ZnO film, the absorption coefficient (a) is obtained from transmittance data, using the following equation:

Fig. 2. Typical XRD pattern of ZnO-PS hybrid structure (a) as deposited (red line) (ZB2A) and (b) after annealing at 500 °C for 30 min (ZB2B). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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1 d

a ¼  ln T; where d is the thickness of the film and T is the optical transmittance. For direct semiconductors, the band gap can be estimated using the equation (ahv)2 = A(hvEg), where h is Planck constant, m is the frequency of incident photon and A is a constant. Therefore, from Fig. 3, the estimated optical band gap of the ZnO film is 3.3 eV. This signifies that similar to bulk ZnO, direct electronic transitions are observed for prepared ZnO film [27,28]. Moreover, the data reveals the good quality and impurity free nature of the prepared films. It is well established that PL emission is primarily due to the recombination of free carriers. The PL emission spectra provide significant information regarding the trapping, immigration and transfer of charge carriers in semiconductors. Emission of light from ZnO has been the focus of several investigations. In order to investigate the luminescent properties of the hybrid structures, room temperature PL studies were performed in the visible region, before and after annealing (shown in Fig. 4(a and b)).

Fig. 3. (ahm)2 vs. hm (photon energy) plot of zinc oxide film deposited over corning glass substrate.

Fig. 4. Photoluminescence of PS-ZnO composites (a) formed with oxidized substrate before annealing (red curve OZB1A) and after annealing at 500 °C (blue curve OZB1B) and non-oxidized substrate (b) before annealing (red curve ZB2A) and after annealing at 500 °C (blue curve ZB2B). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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For understanding the mechanism involved in the emission processes, PL Spectra can be de-convoluted into various PL emission bands (380, 520, 620 and 720 nm emissions from ZnO films over PS). The PL emission in the UV region around 380 nm could be attributed to the near band edge excitonic recombinations [29] in ZnO film. Green emission centered around 520 nm (2.38 eV) has been reported as the most common yet controversial band in ZnO [30]. Diverse hypotheses have been proposed to elucidate the green emission but hitherto, no consensus [31]. More recently, the green luminescence around 550 nm is ascribed to the deep level defects [32]. The Zn vacancies [33], and oxygen antisite [34] could possibly be the source of green emission. The singly ionized oxygen vacancy remains the most commonly cited hypothesis for the origin of green emission in ZnO [35]. In our case, these defects arise probably due to the oxygen vacancies in ZnO lattice [35]. The yellow emission around 620 nm, arise from the native deep level defects, namely oxygen interstitials in ZnO [36,37]. In this regard, it has been proposed recently that presence of Zn(OH)2 group on the surface of ZnO nanorods, could be one of the reasons for deep level yellow emission [38]. On the other hand, PL emissions

Fig. 5. SEM image shows the top view of ZnO-PS composite ZB2A formed on non-oxidized PS substrate (un-annealed) revealing the presence of labyrinth patterns.

Fig. 6. SEM image of annealed ZnO-PS hybrid structure of sample ZB2B formed on non-oxidized PS substrate (a) top view revealing the regular labyrinth pattern. (b) Amplified image which shows the formation of nanostructured, granular zinc oxide labyrinth patterns (bottom scale measures 500 nm).

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observed in the higher wavelength region 695–800 nm are due to porous silicon substrate, used as the template in our case. After explaining the fundamental PL emissions, observed commonly in all ZnO films, we now expatiate the influence of annealing over oxidized/un-oxidized substrates on the PL emissions in

Fig. 7. SEM image shows the top view of (a) ribbon-like structures formed after the laser treatment (around the irradiated region) of ZnO-OPS composite of sample OZB1A formed on the oxidized PS substrate (b) OZB1B formed with the deposition of ZnO over OPS, after thermal annealing reveals the flattened, nanostructured zinc oxide film (bottom scale measures 400 nm).

Fig. 8. Atomic force microscopy images (5 lm  5 lm) of Zinc oxide layer deposited on different porous substrates. (a) Unoxidized substrate (ZB2A) without annealing treatment (RMS value of roughness = 85 nm). (b) Un-oxidized substrate (ZB2B) after annealing treatment (RMS value of roughness = 101 nm). (c) Oxidized substrate (OZB1A) with localized laser treatment (RMS value of roughness = 25 nm). (d) Oxidized substrate (OZB1B) with annealing treatment (RMS value of roughness = 83 nm).

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prepared ZnO films. As it can be seen through Fig. 4(a) (red line OZB1A) that PL of PS-ZnO composites on oxidized substrate before annealing, centered around 520 nm, in the form of a narrow peak in the range of 450–650 nm. After annealing, (Fig. 4(a); blue line OZB1B) this peak remains narrow however, with a relative decrease in the peak intensity accompanied with a blue shift as compared to the PSZnO composites (red line ZB2A; Fig. 4(b) before annealing formed with non-oxidized substrate, where in the later case, a broad PL band around 550 nm, in the emission range 400–800 nm, is observed. In the later case, after annealing (blue line ZB2B; Fig. 4(b) treatment, PL retains its original characteristic broad band, with slight blue shift in the peak position. Fig. 5 shows the surface morphologies of ZnO-PS composites formed on non-oxidized PS. SEM micrographs clearly show the formation of regular. nanometric labyrinth patterns with the feature size (width) ranging from 200 to 270 nm. The results suggest that initially ZnO film covers the porous silicon surface without penetrating. Annealing of the above mentioned samples resulted in the formation of granular Labyrinth patterns (Figs. 6a and b). Fig. 7(a) shows the top view of the ribbon shaped ZnO structures formed over the the oxidized PS (OPS) substrate after laser treatment. The localized agglomeration of zinc oxide can be attributed to the redistribution of matter (zinc oxide crystallites in this case), accompanied by a smaller pore dimensions of oxidized porous template preventing any possible infiltration of ZnO (illustrated in the latter part of the manuscript).Hence, a direct consequence of localized heating through laser irradiation of ZnOx thin film deposited over a porous template with small pore dimensions (with no

Fig. 9. Comparison of typical depth profile obtained from Atomic force microscopy images (5 lm  5 lm) of Zinc oxide layer deposited on different porous substrates. (a) Un-oxidized substrate, without annealing treatment ZB2A. (b) Un-oxidized substrate with annealing treatment ZB2B. (c) Oxidized substrate, after localized laser annealing treatment OZB1A. (d) Oxidized substrate with thermal annealing treatment OZB1B.

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penetration possibility) is revealed in the form of ribbon shaped ZnO agglomerations in the regions localized around the laser treated spot. On the other hand, complete annealing of ZnO-OPS hybrid structures (Fig. 7(b)) resulted in the formation of planer granular structures. Formation of similar kind of nanostructures has been recently shown by Wen et al. in 2012 [39] via solution combustion synthesis method, followed by a subsequent calcination in air and were found to demonstrate superior gas sensing performance. The formation of the planer morphologies on the oxidized substrate can be attributed to the decrease in pore dimensions due to oxidation of low porosity PS substrate at 600 °C making it an equivalent example of zinc oxide deposition on glass substrates. The oxidation of the silicon to SiOx is known to result in an increase of volume [40] and consequently a decrease of pore dimensions for porous silicon substrate. Therefore, in both the above mentioned cases, ZnO film remains on the top of the oxidized substrate like on flat glass substrate. Fig. 8 shows the topographical information taken through AFM images in 5  5 lm2 area of ZB2A, ZB2B, OZB1A and OZB1B sample. Similar to the observed differences in the morphologies, viewed through scanning electron microscope (in terms of pattern formation), AFM scans also confirm the formation of ZnO nanocrystallites with different grain sizes, depending upon the surface and annealing treatment of the porous substrate and the hybrid structure. The ZnO film deposited on the un-oxidized substrate without annealing treatment is found to have regular labyrinth patterns with the root meansquare (rms) value of roughness as 85 nm as compared with patterns formed on the un-oxidized surface with annealing treatment (Fig. 8b), where the root mean-square roughness 101 nm. Fig. 8b reveals granular labyrinth pattern, similar to the one obtain through SEM image (ref. to Fig. 6b). An increase in the roughness or a decrease in the crystallite size at nanometer scale can have possible applications in enhancing the sensing response of zinc oxide [39,41]. A relationship between morphology and inter-electrode gap has also been established by another group to demonstrate the relationship between the sensing response and the number of grains present between inter-electrode gap [42]. Some similar comparison of the patterns formed on the oxidized substrate with localized laser treatment and annealing gives the rms value of roughness as 25 nm and 83 nm respectively. For qualitative analysis of the above mentioned assertion, a comparison of typical depth profiles (obtained from AFM images) is shown in Fig. 9. The line scan of sample ZB2B (surface of the hybrid structure formed with the un-oxidized substrate after annealing) reveals a topography with a larger

Fig. 10. Proposed growth mechanism of different micromorphologies of zinc oxide over the porous silicon substrate. (a–c) Correspond to the steps involved in the deposition and the development of the granular labyrinth pattern formation on nonoxidized substrate; (d–f) correspond to the steps involved in the deposition of ZnO on the oxidized substrate with relatively small pores.

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surface area than rest of the three profiles. On comparing the first two profiles corresponding to the un-oxidized substrate (ZB2A and ZB2B), larger variation on the y-axis scale of ZB2B reveals a larger surface area to volume ratio in the latter case with possible applications in superior gas sensing properties. Micromorphologies formed on the oxidized substrate(OZB1A and OZB2B) reveals relatively less surface area. The possible growth mechanism of zinc oxide labyrinth patterns discussed in this work is schematically visualized in Fig. 10. The first figure in both the rows shows the two different kind of substrates without (Fig. 10a) and with (Fig. 10d) oxidation. The possible qualitative explanation of the labyrinth pattern formation is illustrated in the first row. The deposited zinc oxide films act as a transparent capping on the top of the porous structure (Fig. 10a). The air present in these pores and ‘voids’ of PS is sealed up, and its expansion or escaping outwards during the heating of substrate at 200 °C (Fig. 10b) to get the desired film thickness of zinc oxide [43] results in the formation of crests and troughs to form the labyrinth patterns. Further annealing of the hybrid structure (Fig. 10c) at 500 °C leads to the formation of relatively smaller grain size within the labyrinth pattern (as shown in Fig. 6a and b). In case of oxidized porous substrate, a relatively flat zinc oxide film with small particle size (after annealing) can be attributed to the decrease in the pore dimensions of porous silicon after oxidation (Fig. 10d–f), which in turn is equivalent to a flat glass substrate. Such morphological changes in the PS oxidized substrate with respect to the un-oxidized samples have been demonstrated previously by several groups, e.g. Jarvis et al. [44] showed a decrease in the specific surface area, mesopore volume and diameter due to the increase in the molecular volume of the oxidized silicon layer within the pores. 4. Conclusions Substrate and post deposition treatment dependent growth of ZnO labyrinth and ribbon shaped patterns on PS substrates, by simple sol–gel spin coating technique, has been demonstrated. Effect of oxidizing the low porosity substrate results in a decrease of pore volume exhibiting the planer morphology of the hybrid structure as compared to the ZnO labyrinth patterns formed on the un-oxidized substrate. Annealing the hybrid structure, results in an increase in the crystalline nature and a granular structure with relatively small grain size. A large surface area to volume ratio of ZnO micro-morphologies is promising for enhancing its gas sensing properties. Optical characterization reveals the characteristic parameters of zinc oxide. Acknowledgements This work has been supported by CONACyT under the Project No. 128953 and Bilateral Indo-Mex Project. We acknowledge the SEM and AFM (Dr. Juan Mendez) facilities provided by CNyMN-IPN. References [1] S.C. Pillai, J.M. Kelly, D.E. McCormack, R. Rhagavendra, J. Mater. Chem. 14 (2004) 1572–1578. [2] H. Kim, C.M. Gilmore, J.S. Horwitz, A. Piqué, H. Murata, G.P. Kushto, R. Schlaf, Z.H. Kafafi, D.B. Chrisey, Appl. Phys. Lett. 76 (2000) 259–261. [3] G. Visimberga, E.E. Yakimov, A.N. Redkin, A.N. Gruzintsev, V.T. Volkov, S. Romanov, G.A. Emelchenko, Phys. Status Solidi (c). 7 (2010) 1668–1671. [4] W. Wu, S. Feng Zhang, X. Xiao, C. Jiang, Adv. Mat. Lett. 4 (8) (2013) 610–614. [5] Z.L. Wang, X.Y. Kong, Y. Ding, P.X. Gao, W.L. Hughes, R. Yang, Y. Zhang, Adv. Funct. Mater. 14 (2004) 943–956; R. Majithia, J. Speich, K.E. Meissner, Materials 6 (2013) 2497–2507. [6] G.C. Yi, C.R. Wang, W.I. Park, Semiconduct. Sci. Technol. 20 (2005) S22–S34. [7] Z. Zhu, T.L. Chen, Y. Gu, J. Warren, R.M. Osgood, Chem. Mater. 17 (2005) 4227–4234. [8] J.T. Hu, T.W. Odom, C.M. Lieber, Acc. Chem. Res. 32 (1999) 435–445. [9] Y. Xia, P. Yang, Y. Sun, Y. Wu, B. Mayers, B. Gates, Y. Yin, F. Kim, H. Yan, Adv. Mater. 15 (2003) 353–389. [10] C.M. Lieber, Solid State Commun. 107 (1998) 607–616. [11] X.H. Kong, Y.D. Li, Chem. Lett. 32 (2003) 1062–1063. [12] L.E. Greene, M. Law, J. Goldberger, F. Kim, J.C. Johnson, Y. Zhang, R.J. Saykally, P. Yang, Angew. Chem. Int. 42 (2003) 3031– 3034. [13] L. Vayssieres, K. Keis, S.E. Lindquist, A. Hagfeldt, J. Phys. Chem. 105 (2001) 3350–3352.

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