- Email: [email protected]
Growth and Optical Characterization of Nanogranular Sol-gel ZnO Layers
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 2 (2015) 85 – 93
5th International conference on Advanced Nano Materials
Growth and optical characterization of nanogranular sol-gel ZnO layers Mickaël Gilliota*, Aomar Hadjadja
a
LISM, Université de Reims Champagne-Ardenne, France
Abstract A study of the sol-gel/spin-coating deposition process of ZnO is presented. A particular morphology of very thin layer with nanogranular surface and good excitonic behavior is achieved. The growth and optical properties are investigated. Correlations are presented between morphology and the dielectric function evolution with thickness and with annealing temperature. These two parameters indeed have great influence on the size of grains and their packing density, which simultaneously influences the dielectric function. © 2014 The Authors. Elsevier Ltd. All rights reserved. © 2015 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of TEMA - Centre for Mechanical Technology and Automation. Selection and peer-review under responsibility of TEMA - Centre for Mechanical Technology and Automation.
Keywords:ZnO; excitons; grains; growth; ellipsometry; dielectric function
1. Introduction Zinc oxide has been known for a long time as a II-VI wide-bandgap semiconductor with large number of interesting properties. These properties include transparency in the visible range, high electron mobility, wide bandgap (3.4 eV), large excitonic binding energy (60 meV), good acousto-optic behavior, photocatalytic and gas sensing abilities. Nano-scale recent applications can be found for example in fields such as piezoelectric nanogenerators [1], UV leds [2,3] , thin film transistors [4,5], solvent or gaz detection [6,7] . Nomenclature ZnO AFM SE
zinc oxide atomic force microscopy spectroscopic ellipsometry
* Corresponding author. Tel.: +33-326-91-33-27; fax: +33-326-91-89-15. E-mail address: [email protected]
2214-7853 © 2015 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of TEMA - Centre for Mechanical Technology and Automation. doi:10.1016/j.matpr.2015.04.012
86
Mickaël Gilliot and Aomar Hadjadj / Materials Today: Proceedings 2 (2015) 85 – 93
For a large number of applications it is of primary importance to have control of morphology and properties, these two aspects often being strongly correlated. ZnO can be synthesized by a large number of elaboration methods such as rf magnetron sputtering, molecular-beam epitaxy, pulsed-laser deposition, chemical-vapor deposition, plasma methods, hydrothermal synthesis, melt growth or sol-gel. Among these techniques sol-gel synthesis of nanoparticles and their deposition by spin-coating or dip-coating allows reaching particular granular morphologies for low cost. In this work the sol-gel synthesis and spin-coating deposition techniques are used to synthesize very thin layers of ZnO (<100nm) with good optical properties presenting large excitonic features. Samples are formed with different thicknesses and they are annealed at different temperatures to obtain different textures of samples. The morphologies are investigated by atomic force microscopy and the optical properties are investigated through the dielectric function by spectroscopic ellipsometry in the UV-Visible spectral range. Interesting correlations are observed between granular morphology and the dielectric function. It is especially observed that the grain size increases and that simultaneously the dielectric function increases over the whole spectrum with thickness and with annealing temperature. In addition the dielectric function bound excitonic contribution increases with annealing temperature. 2. Experimental ZnO is prepared by sol-gel method by dissolving zinc acetate dehydrate (Zn(CH3CO2)2.2H2O) in methoxyethanol (C3H8O2) at concentration of 0.5 Mol.L−1. Monoethanolamin (C2H7NO) is used as a stabilizer with molar ratio of zinc acetate to monoethanolamin set to 1. The mix is then stirred for 30 minutes to obtain a clear and transparent ZnO precursor solution. After 24h ageing, the solution is spin-coated on 50 nm silicon dioxide layer on silicon substrate at the speed of 2500 rpm. The deposition process is repeated up to 4 times. Between two consecutive deposition operations, the sample is pre-heated in a furnace at 300 ◦C for 10 minutes so that solvents can evaporate. Finally the samples are air-annealed at different temperatures between 400°C and 550°C for 1 hour. Two sets of samples are prepared: a first set of three samples consisting respectively of 2 ,3 and 4 deposited layers finally annealed at 550°C; and a second set of three samples consisting of 4 deposited layers finally annealed respectively at 400°C, 475°C and 550°C. Surface morphology of the samples is characterized by atomic force microscopy (Digital Instruments nanoscope III) in tapping mode, and thickness and optical properties are characterized by spectroscopic ellipsometry (JobinYvon Uvisel automatic phase modulated instrument) at the incident angle of 70° in the spectral range 1.5-5 eV with 0.02 eV step. SE is an indirect technique measuring the change of polarization state between incident and reflected light on the sample thanks to two ellipsometric angles Ψ and Δ related to the ratio of reflection coefficients for ppolarized (parallel to the plane of incidence) and s-polarized light (perpendicular to the plane of incidence). The ellipsometric spectra express non explicit angles and must be inverted to access the useful information representing the sample as a multilayer structure made of the silicon substrate - silicon dioxide layer - unknown ZnO layer and roughness layer. A particular method is applied to ellipsometric data in order to extract thicknesses and optical constants [8, 9]. The results of the process are thicknesses of the ZnO layer, thickness of roughness and complex refractive index (N = n − ik) over the whole considered spectral range. For all the samples roughness has been found to be less than 12 nm. Thickness in the paper is given as an equivalent addition of the ZnO layer thickness and half the roughness layer thickness. Optical constants are equivalently presented as the dielectric function (ε = ε r – i εi = N2), which is the image of possible electronic transitions between energy levels in the material. 3. Results 3.1. Effects of thickness The set of three samples corresponding to 2, 3 and 4 deposited layers of sol-gel ZnO have been analyzed by AFM and SE. Thickness gradually evolves as 43, 48 and 56 nm for the three samples. As expected thickness increases with number of deposited layers. This is a consequence of more deposited material. The evolution of thickness with
Mickaël Gilliot and Aomar Hadjadj / Materials Today: Proceedings 2 (2015) 85 – 93
number of deposited layers is not perfectly linear, because of the relatively large standard deviation of reproducibility on the order of 10 nm, and also because thickness results in the competition between the increase of amount of deposited material and some contraction mechanisms occurring with increase of number of deposited layers. AFM micrographs are shown on Error! Reference source not found. for the samples with different thicknesses. The pictures reveal the formation of very small particles homogeneously distributed on the surface. The particles size tends to increase with increasing thickness. A more detailed investigation of sizes using grain size analysis software gives reveals grain size of 37, 41 and 44 nm.
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Mickaël Gilliot and Aomar Hadjadj / Materials Today: Proceedings 2 (2015) 85 – 93
Figure 1 : 1μm * 1μm AFM micrographs of 2 (top), 3 (middle), 4 (bottom) layers ZnO sol-gel spin-coated samples annealed at 550°C.
Extracted dielectric functions are shown in Error! Reference source not found. for the samples with different thicknesses. The optical constants have a typical behavior of direct bandgap semiconductor with zero values for the imaginary part of the dielectric function related to absorption below the bandgap around 3.4 eV. The most striking observation is the increases of both real and imaginary parts of the dielectric function over the whole spectrum with increasing number of deposited layers, which shows the improvement of density with number of deposited layers. A correlation between number of deposited layers, morphology and optical properties is observed. Typical evolutions of optical properties with thickness have already been observed [10, 11, 13] with difference here that the evolution concerns thicknesses lower than 60 nm. It is the consequence of the particular growth of sol-gel very thin layers.
Mickaël Gilliot and Aomar Hadjadj / Materials Today: Proceedings 2 (2015) 85 – 93
6
2 layers 3 layers 4 layers
r
5
H
4
3 2
H
i
1
0 1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
photon energy (eV)
Figure 2 : Dielectric function of 2, 3, 4 layers ZnO sol-gel spin-coated samples annealed at 550°C.
3.2. Effects of annealing temperature The set of three samples corresponding to 4 deposited layers of sol-gel ZnO annealed at 400°C, 475°C and 550°C have been analyzed by AFM and SE. Thickness varies as 58, 65 and 56 nm for the three samples. There is no real evolution of thickness with annealing temperature but only little differences related to the relatively large standard deviation of reproducibility on the order of 10 nm. AFM micrographs are shown on Error! Reference source not found. for the samples annealed at different temperatures. The pictures reveal the formation of very small particles homogeneously distributed on the surface. The particles size tends to increase with increasing annealing temperature. A more detailed investigation of sizes using grain size analysis software gives reveals grain size of 31, 37 and 44 nm.
89
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Mickaël Gilliot and Aomar Hadjadj / Materials Today: Proceedings 2 (2015) 85 – 93
Mickaël Gilliot and Aomar Hadjadj / Materials Today: Proceedings 2 (2015) 85 – 93
Figure 3 : 1μm * 1μm AFM micrographs of four layers ZnO sol-gel spin-coated samples annealed at 400°C (top), 475°C (middle), 550°C (bottom).
Extracted dielectric functions are shown in Error! Reference source not found. for the samples annealed at different temperatures. The optical constants have a typical behavior of direct bandgap semiconductor with zero values for the imaginary part of the dielectric function related to absorption below the bandgap around 3.4 eV. The most striking observation is the gradual increase of the peak of the imaginary part of the dielectric function at 3.4 eV. This peak is the bound exciton absorption of photons with energy slightly below the bandgap to create interacting electron-hole pairs, which shows the improvement of crystal quality. A correlation between annealing temperature, morphology and optical properties is observed. Increase of annealing temperature is associated to increase of particles size and to a large evolution of the excitonic peak. Dielectric functions of ZnO can present excitonic peaks with very sharp shapes for crystalline epitaxial ZnO [13, 14], or wider shape [15] and also non-existing peak [16]. The presence of the excitonic peak reveals a good crystallinity of the particles.
91
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Mickaël Gilliot and Aomar Hadjadj / Materials Today: Proceedings 2 (2015) 85 – 93
6
400°C 475°C 550°C
Hr
5
4
3 2
Hi
1
0 1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
photon energy (eV) Figure 4 : Dielectric function of four layers ZnO sol-gel spin-coated samples annealed at 400°C, 475°C, 550°C.
4. Discussion The different observations reveal an evolution of grain size with thickness and evolution of excitonic contribution with annealing temperature. These two points can be explained with simple growth scheme. Sol-gel is a well known method of colloidal particles formation [17, 18]. The formation of the dense ZnO layer is obtained after successive deposition of particles in suspension, then spreading it into a very thin layer, and finally preheating to evaporate the solvents. The formation of grains is the result of two contributions: aggregation of particles at the successive preheating stages and aggregation and reorganization at the final stage of annealing. The final size of the particles is a competition between temperature which increases the mobility of the particles and quantity of available deposited particles which can be a limiting factor. Temperature allows the aggregation of small grains to form larger grains. The size of the final grains is determined by the temperature: the higher the temperature is, the higher the mobility of particles is and the aggregation process is more efficient. However for very low thickness of the layers the agglomeration process is aborted by the too low amount of material, and the particles can not reach their final size. When the size of very thin layers is increased, the size of the particles is increased thanks to more available particles. The aggregation process is initiated by centers randomly distributed on the surface which act as centers of ripening for other smaller particle to agglomerate to form grains. The very low thickness constraints the aggregation
Mickaël Gilliot and Aomar Hadjadj / Materials Today: Proceedings 2 (2015) 85 – 93
process to the formation of monolayers of grains. The formation of larger particles would indeed require the formation of lager holes between the particles until some free substrate surface appear, which would not be energetically favorable. As a consequence, for the considered very low thickness, the thickness of the layer is a limitation of the aggregation process and when the thickness increases the size of the particles can increase. When the annealing temperature is increased, the size of the particles is increased thanks to larger mobility of the particles. In fact the increase of annealing temperature allows the movement of larger particles and the system can reorganize in a different way to form lager grains in a more compact arrangement. When the annealing temperature is increased, the bound exciton contribution is also increased. The formation of large grains as well as their orientation is necessary for the observation of large excitonic effect. As grains are made of agglomerated smaller grains the orientation of the grains is basically not well defined, but a higher annealing temperature allows a reorganization of the domains inside the grains to form a large crystallite. Consequently the excitonic effect increases with annealing temperature. 5. Conclusion Nano-granular very thin layers of ZnO have been formed by sol-gel and spin coating with different morphologies. The investigation of the optical properties has been carried out by spectroscopic ellipsometry. Effects of thickness and annealing temperature have been investigated. The morphology and dielectric function can simply be changed by changing the number of deposited layers and annealing temperature. The increase of thickness increases the size of the grains and simultaneously increases the dielectric function over the whole spectrum. The increase of annealing temperature increases the size of the grains and simultaneously increases the bound exciton contribution of the dielectric function. These observations are the results of the sol-gel spin coating deposition mechanisms based on grains agglomeration and packing.
References [1] [2] [3] [4]
Z.L. Wang and J. Song, Science 14 (2006) 242–246. J. H. Lim, C. K. Kang, K. K. Kim, I. K. Park, D. K. Hwang and S. J. Park, Adv. Mater. 18 (2006) 2720–2724. Yong-Seok Choi, Jang-Won Kang, Dae-Kue Hwang and Seong-Ju Park, IEEE Trans. Electron Dev. 57 (2010) 26–41. E.M.C. Fortunato, P.M.C. Barquinha, A.C.M.B.G. Pimentel, A.M.F. Gonçalves, A.J.S. Marques, L.M.N. Pereira and R.F.P. Martins, Adv Mater. 17 (2005) 590–594. [5] G. Xiong, G. A. C. Jones, R. Rungsawan, and D. Anderson,Thin Solid Films 518 (2010) 4019–4023. [6] K. Mirabbaszadeh and M. Mehrabian, Phys. Scripta 85 (2012) 35701–35709. [7] P.R. Ohodnicki Jr., C. Wang and M. Andio, Thin Solid Films 539 (2013) 327–336. [8] M. Gilliot, Thin Solid Films 520 (2012) 5568–5574. [9] M. Gilliot, Thin Solid Films 542 (2013) 300–305. [10] L. Xu, X. Li, Y. Chen and F. Xu, Appl. Surf. Sci. 257 (2011) 4031–4037. [11] X. D. Li, T. P. Chen, P. Liu, Y. Liu, Z. Liu and K. C. Leong, J. Appl. Phys. 115 (2014) 103512. [12] L. Cui, G.-G. Wang, H.-Y. Zhang, R. Sun, X.-P. Kuang and J.-C. Han, Ceram. Int. 39 (2013) 3261–3268. [13] P. L.Washington, H. C. Ong, J. Y. Dai and R. P. H. Chang, Appl. Phys. Lett. 72 (1998) 3261–3263. [14] G. E. Jellison and L. A. Boatner, Phys. Rev. B 58 (1998) 3586–3589. [15] H. Yoshikawa and S. Adachi, Japanese J. Appl. Phys. 36 (1997) 6237–6243. [16] E. Dumont, B. Dugnoille and S. Bienfait, Thin Solid Films 353 (1999) 93–99. [17] L. Znaidi, G. S. Illia, S. Benyahia, C. Sanchez and A. Kanaev, Thin Solid Films 428 (2003) 257–262. [18] L. Xu, G. Zheng, J. Miao and F. Xian, Appl. Surf. Sci. 258 (2012) 7760–7765.
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ScienceDirect Materials Today: Proceedings 2 (2015) 85 – 93
5th International conference on Advanced Nano Materials
Growth and optical characterization of nanogranular sol-gel ZnO layers Mickaël Gilliota*, Aomar Hadjadja
a
LISM, Université de Reims Champagne-Ardenne, France
Abstract A study of the sol-gel/spin-coating deposition process of ZnO is presented. A particular morphology of very thin layer with nanogranular surface and good excitonic behavior is achieved. The growth and optical properties are investigated. Correlations are presented between morphology and the dielectric function evolution with thickness and with annealing temperature. These two parameters indeed have great influence on the size of grains and their packing density, which simultaneously influences the dielectric function. © 2014 The Authors. Elsevier Ltd. All rights reserved. © 2015 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of TEMA - Centre for Mechanical Technology and Automation. Selection and peer-review under responsibility of TEMA - Centre for Mechanical Technology and Automation.
Keywords:ZnO; excitons; grains; growth; ellipsometry; dielectric function
1. Introduction Zinc oxide has been known for a long time as a II-VI wide-bandgap semiconductor with large number of interesting properties. These properties include transparency in the visible range, high electron mobility, wide bandgap (3.4 eV), large excitonic binding energy (60 meV), good acousto-optic behavior, photocatalytic and gas sensing abilities. Nano-scale recent applications can be found for example in fields such as piezoelectric nanogenerators [1], UV leds [2,3] , thin film transistors [4,5], solvent or gaz detection [6,7] . Nomenclature ZnO AFM SE
zinc oxide atomic force microscopy spectroscopic ellipsometry
* Corresponding author. Tel.: +33-326-91-33-27; fax: +33-326-91-89-15. E-mail address: [email protected]
2214-7853 © 2015 Elsevier Ltd. All rights reserved. Selection and peer-review under responsibility of TEMA - Centre for Mechanical Technology and Automation. doi:10.1016/j.matpr.2015.04.012
86
Mickaël Gilliot and Aomar Hadjadj / Materials Today: Proceedings 2 (2015) 85 – 93
For a large number of applications it is of primary importance to have control of morphology and properties, these two aspects often being strongly correlated. ZnO can be synthesized by a large number of elaboration methods such as rf magnetron sputtering, molecular-beam epitaxy, pulsed-laser deposition, chemical-vapor deposition, plasma methods, hydrothermal synthesis, melt growth or sol-gel. Among these techniques sol-gel synthesis of nanoparticles and their deposition by spin-coating or dip-coating allows reaching particular granular morphologies for low cost. In this work the sol-gel synthesis and spin-coating deposition techniques are used to synthesize very thin layers of ZnO (<100nm) with good optical properties presenting large excitonic features. Samples are formed with different thicknesses and they are annealed at different temperatures to obtain different textures of samples. The morphologies are investigated by atomic force microscopy and the optical properties are investigated through the dielectric function by spectroscopic ellipsometry in the UV-Visible spectral range. Interesting correlations are observed between granular morphology and the dielectric function. It is especially observed that the grain size increases and that simultaneously the dielectric function increases over the whole spectrum with thickness and with annealing temperature. In addition the dielectric function bound excitonic contribution increases with annealing temperature. 2. Experimental ZnO is prepared by sol-gel method by dissolving zinc acetate dehydrate (Zn(CH3CO2)2.2H2O) in methoxyethanol (C3H8O2) at concentration of 0.5 Mol.L−1. Monoethanolamin (C2H7NO) is used as a stabilizer with molar ratio of zinc acetate to monoethanolamin set to 1. The mix is then stirred for 30 minutes to obtain a clear and transparent ZnO precursor solution. After 24h ageing, the solution is spin-coated on 50 nm silicon dioxide layer on silicon substrate at the speed of 2500 rpm. The deposition process is repeated up to 4 times. Between two consecutive deposition operations, the sample is pre-heated in a furnace at 300 ◦C for 10 minutes so that solvents can evaporate. Finally the samples are air-annealed at different temperatures between 400°C and 550°C for 1 hour. Two sets of samples are prepared: a first set of three samples consisting respectively of 2 ,3 and 4 deposited layers finally annealed at 550°C; and a second set of three samples consisting of 4 deposited layers finally annealed respectively at 400°C, 475°C and 550°C. Surface morphology of the samples is characterized by atomic force microscopy (Digital Instruments nanoscope III) in tapping mode, and thickness and optical properties are characterized by spectroscopic ellipsometry (JobinYvon Uvisel automatic phase modulated instrument) at the incident angle of 70° in the spectral range 1.5-5 eV with 0.02 eV step. SE is an indirect technique measuring the change of polarization state between incident and reflected light on the sample thanks to two ellipsometric angles Ψ and Δ related to the ratio of reflection coefficients for ppolarized (parallel to the plane of incidence) and s-polarized light (perpendicular to the plane of incidence). The ellipsometric spectra express non explicit angles and must be inverted to access the useful information representing the sample as a multilayer structure made of the silicon substrate - silicon dioxide layer - unknown ZnO layer and roughness layer. A particular method is applied to ellipsometric data in order to extract thicknesses and optical constants [8, 9]. The results of the process are thicknesses of the ZnO layer, thickness of roughness and complex refractive index (N = n − ik) over the whole considered spectral range. For all the samples roughness has been found to be less than 12 nm. Thickness in the paper is given as an equivalent addition of the ZnO layer thickness and half the roughness layer thickness. Optical constants are equivalently presented as the dielectric function (ε = ε r – i εi = N2), which is the image of possible electronic transitions between energy levels in the material. 3. Results 3.1. Effects of thickness The set of three samples corresponding to 2, 3 and 4 deposited layers of sol-gel ZnO have been analyzed by AFM and SE. Thickness gradually evolves as 43, 48 and 56 nm for the three samples. As expected thickness increases with number of deposited layers. This is a consequence of more deposited material. The evolution of thickness with
Mickaël Gilliot and Aomar Hadjadj / Materials Today: Proceedings 2 (2015) 85 – 93
number of deposited layers is not perfectly linear, because of the relatively large standard deviation of reproducibility on the order of 10 nm, and also because thickness results in the competition between the increase of amount of deposited material and some contraction mechanisms occurring with increase of number of deposited layers. AFM micrographs are shown on Error! Reference source not found. for the samples with different thicknesses. The pictures reveal the formation of very small particles homogeneously distributed on the surface. The particles size tends to increase with increasing thickness. A more detailed investigation of sizes using grain size analysis software gives reveals grain size of 37, 41 and 44 nm.
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Figure 1 : 1μm * 1μm AFM micrographs of 2 (top), 3 (middle), 4 (bottom) layers ZnO sol-gel spin-coated samples annealed at 550°C.
Extracted dielectric functions are shown in Error! Reference source not found. for the samples with different thicknesses. The optical constants have a typical behavior of direct bandgap semiconductor with zero values for the imaginary part of the dielectric function related to absorption below the bandgap around 3.4 eV. The most striking observation is the increases of both real and imaginary parts of the dielectric function over the whole spectrum with increasing number of deposited layers, which shows the improvement of density with number of deposited layers. A correlation between number of deposited layers, morphology and optical properties is observed. Typical evolutions of optical properties with thickness have already been observed [10, 11, 13] with difference here that the evolution concerns thicknesses lower than 60 nm. It is the consequence of the particular growth of sol-gel very thin layers.
Mickaël Gilliot and Aomar Hadjadj / Materials Today: Proceedings 2 (2015) 85 – 93
6
2 layers 3 layers 4 layers
r
5
H
4
3 2
H
i
1
0 1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
photon energy (eV)
Figure 2 : Dielectric function of 2, 3, 4 layers ZnO sol-gel spin-coated samples annealed at 550°C.
3.2. Effects of annealing temperature The set of three samples corresponding to 4 deposited layers of sol-gel ZnO annealed at 400°C, 475°C and 550°C have been analyzed by AFM and SE. Thickness varies as 58, 65 and 56 nm for the three samples. There is no real evolution of thickness with annealing temperature but only little differences related to the relatively large standard deviation of reproducibility on the order of 10 nm. AFM micrographs are shown on Error! Reference source not found. for the samples annealed at different temperatures. The pictures reveal the formation of very small particles homogeneously distributed on the surface. The particles size tends to increase with increasing annealing temperature. A more detailed investigation of sizes using grain size analysis software gives reveals grain size of 31, 37 and 44 nm.
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Mickaël Gilliot and Aomar Hadjadj / Materials Today: Proceedings 2 (2015) 85 – 93
Mickaël Gilliot and Aomar Hadjadj / Materials Today: Proceedings 2 (2015) 85 – 93
Figure 3 : 1μm * 1μm AFM micrographs of four layers ZnO sol-gel spin-coated samples annealed at 400°C (top), 475°C (middle), 550°C (bottom).
Extracted dielectric functions are shown in Error! Reference source not found. for the samples annealed at different temperatures. The optical constants have a typical behavior of direct bandgap semiconductor with zero values for the imaginary part of the dielectric function related to absorption below the bandgap around 3.4 eV. The most striking observation is the gradual increase of the peak of the imaginary part of the dielectric function at 3.4 eV. This peak is the bound exciton absorption of photons with energy slightly below the bandgap to create interacting electron-hole pairs, which shows the improvement of crystal quality. A correlation between annealing temperature, morphology and optical properties is observed. Increase of annealing temperature is associated to increase of particles size and to a large evolution of the excitonic peak. Dielectric functions of ZnO can present excitonic peaks with very sharp shapes for crystalline epitaxial ZnO [13, 14], or wider shape [15] and also non-existing peak [16]. The presence of the excitonic peak reveals a good crystallinity of the particles.
91
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Mickaël Gilliot and Aomar Hadjadj / Materials Today: Proceedings 2 (2015) 85 – 93
6
400°C 475°C 550°C
Hr
5
4
3 2
Hi
1
0 1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
photon energy (eV) Figure 4 : Dielectric function of four layers ZnO sol-gel spin-coated samples annealed at 400°C, 475°C, 550°C.
4. Discussion The different observations reveal an evolution of grain size with thickness and evolution of excitonic contribution with annealing temperature. These two points can be explained with simple growth scheme. Sol-gel is a well known method of colloidal particles formation [17, 18]. The formation of the dense ZnO layer is obtained after successive deposition of particles in suspension, then spreading it into a very thin layer, and finally preheating to evaporate the solvents. The formation of grains is the result of two contributions: aggregation of particles at the successive preheating stages and aggregation and reorganization at the final stage of annealing. The final size of the particles is a competition between temperature which increases the mobility of the particles and quantity of available deposited particles which can be a limiting factor. Temperature allows the aggregation of small grains to form larger grains. The size of the final grains is determined by the temperature: the higher the temperature is, the higher the mobility of particles is and the aggregation process is more efficient. However for very low thickness of the layers the agglomeration process is aborted by the too low amount of material, and the particles can not reach their final size. When the size of very thin layers is increased, the size of the particles is increased thanks to more available particles. The aggregation process is initiated by centers randomly distributed on the surface which act as centers of ripening for other smaller particle to agglomerate to form grains. The very low thickness constraints the aggregation
Mickaël Gilliot and Aomar Hadjadj / Materials Today: Proceedings 2 (2015) 85 – 93
process to the formation of monolayers of grains. The formation of larger particles would indeed require the formation of lager holes between the particles until some free substrate surface appear, which would not be energetically favorable. As a consequence, for the considered very low thickness, the thickness of the layer is a limitation of the aggregation process and when the thickness increases the size of the particles can increase. When the annealing temperature is increased, the size of the particles is increased thanks to larger mobility of the particles. In fact the increase of annealing temperature allows the movement of larger particles and the system can reorganize in a different way to form lager grains in a more compact arrangement. When the annealing temperature is increased, the bound exciton contribution is also increased. The formation of large grains as well as their orientation is necessary for the observation of large excitonic effect. As grains are made of agglomerated smaller grains the orientation of the grains is basically not well defined, but a higher annealing temperature allows a reorganization of the domains inside the grains to form a large crystallite. Consequently the excitonic effect increases with annealing temperature. 5. Conclusion Nano-granular very thin layers of ZnO have been formed by sol-gel and spin coating with different morphologies. The investigation of the optical properties has been carried out by spectroscopic ellipsometry. Effects of thickness and annealing temperature have been investigated. The morphology and dielectric function can simply be changed by changing the number of deposited layers and annealing temperature. The increase of thickness increases the size of the grains and simultaneously increases the dielectric function over the whole spectrum. The increase of annealing temperature increases the size of the grains and simultaneously increases the bound exciton contribution of the dielectric function. These observations are the results of the sol-gel spin coating deposition mechanisms based on grains agglomeration and packing.
References [1] [2] [3] [4]
Z.L. Wang and J. Song, Science 14 (2006) 242–246. J. H. Lim, C. K. Kang, K. K. Kim, I. K. Park, D. K. Hwang and S. J. Park, Adv. Mater. 18 (2006) 2720–2724. Yong-Seok Choi, Jang-Won Kang, Dae-Kue Hwang and Seong-Ju Park, IEEE Trans. Electron Dev. 57 (2010) 26–41. E.M.C. Fortunato, P.M.C. Barquinha, A.C.M.B.G. Pimentel, A.M.F. Gonçalves, A.J.S. Marques, L.M.N. Pereira and R.F.P. Martins, Adv Mater. 17 (2005) 590–594. [5] G. Xiong, G. A. C. Jones, R. Rungsawan, and D. Anderson,Thin Solid Films 518 (2010) 4019–4023. [6] K. Mirabbaszadeh and M. Mehrabian, Phys. Scripta 85 (2012) 35701–35709. [7] P.R. Ohodnicki Jr., C. Wang and M. Andio, Thin Solid Films 539 (2013) 327–336. [8] M. Gilliot, Thin Solid Films 520 (2012) 5568–5574. [9] M. Gilliot, Thin Solid Films 542 (2013) 300–305. [10] L. Xu, X. Li, Y. Chen and F. Xu, Appl. Surf. Sci. 257 (2011) 4031–4037. [11] X. D. Li, T. P. Chen, P. Liu, Y. Liu, Z. Liu and K. C. Leong, J. Appl. Phys. 115 (2014) 103512. [12] L. Cui, G.-G. Wang, H.-Y. Zhang, R. Sun, X.-P. Kuang and J.-C. Han, Ceram. Int. 39 (2013) 3261–3268. [13] P. L.Washington, H. C. Ong, J. Y. Dai and R. P. H. Chang, Appl. Phys. Lett. 72 (1998) 3261–3263. [14] G. E. Jellison and L. A. Boatner, Phys. Rev. B 58 (1998) 3586–3589. [15] H. Yoshikawa and S. Adachi, Japanese J. Appl. Phys. 36 (1997) 6237–6243. [16] E. Dumont, B. Dugnoille and S. Bienfait, Thin Solid Films 353 (1999) 93–99. [17] L. Znaidi, G. S. Illia, S. Benyahia, C. Sanchez and A. Kanaev, Thin Solid Films 428 (2003) 257–262. [18] L. Xu, G. Zheng, J. Miao and F. Xian, Appl. Surf. Sci. 258 (2012) 7760–7765.
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