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Structural and optical studies on Nd doped ZnO thin films
Accepted Manuscript Structural and optical studies on Nd doped ZnO thin films T. Deepa Rani, K. Tamilarasan, E. Elangovan, S. Leela, K. Ramamurthi, K. Thangaraj, C. Himcinschi, I. Trenkmann, S. SchuIze, M. Hietschold, A. Liebig, G. Salvan, D.R. T. Zahn PII: DOI: Reference:
S0749-6036(14)00358-9 http://dx.doi.org/10.1016/j.spmi.2014.10.001 YSPMI 3427
To appear in:
Superlattices and Microstructures
Received Date: Revised Date: Accepted Date:
16 November 2013 16 September 2014 2 October 2014
Please cite this article as: T. Deepa Rani, K. Tamilarasan, E. Elangovan, S. Leela, K. Ramamurthi, K. Thangaraj, C. Himcinschi, I. Trenkmann, S. SchuIze, M. Hietschold, A. Liebig, G. Salvan, D.R. T. Zahn, Structural and optical studies on Nd doped ZnO thin films, Superlattices and Microstructures (2014), doi: http://dx.doi.org/10.1016/j.spmi. 2014.10.001
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Structural and optical studies on Nd doped ZnO thin films 1
1
2
3
4
T. Deepa Rani , K.Tamilarasan , E. Elangovan , S. Leela , K. Ramamurthi , 1
5
6
6
6
K.Thangaraj , C. Himcinschi ,I. Trenkmann , S.SchuIze , M. Hietschold , A. 6
6
6
Liebig , G. Salvan , D.R. T. Zahn 1
Department of Physics, Kongu Engineering College, Erode, Tamil Nadu, India
2
i-Micro, Masdar Institute of Science and Technology, 54224 Masdar City, Abu Dhabi,
United Arab Emirates 3
Department of Physics, Ethiraj College for Women, Chennai, Tamil Nadu, India
4
Department of Physics and Nanotechnology, SRM University, Kattankulathur-603
203,Tamil Nadu, India 5
institute for Theoretical Physics, Technische Universitat Bergakademie Freiberg, 09596
Freiberg, Germany 6
institute of Physics, Chemnitz University of Technology, 09107 Chemnitz, Germany
Keywords: ZnO, Nd doping, Micro-Raman, XRD, SEM, PL Corresponding author T. Deepa Rani, Department of Physics, Kongu Engineering college, Perundurai, Erode +91 9865470707 deepaphvsics
@gmail.com
Abstract Thin films of Zn1-xNdxO were deposited by spray pyrolysis on Si(111) substrates preheated at 400 °C temperature and were studied as a function of neodymium(Nd)- doping concentration. X-ray diffraction (XRD) patterns confirmed that the deposited films possess hexagonal wurtzite ZnO structure. Further, it is observed that the doped films show a preferential orientation along the c-axis (002), which is perpendicular to the substrate. The un-doped films seem to be having a bit low-crystallinity, which is corroborated by the scanning electron microscope (SEM) analysis that showed nano-crystalline like features. Further, SEM analysis showed that the Nd doping triggers the formation bubble-like structure on top of the nano-crystalline structure. The SEM microstructures are interpreted with the micro-Raman studies. Photoluminescence (PL) and XRD characterizations indicate that above 5 at. % doping concentrations, the Nd atoms preferentially agglomerate in the large islands. 1. Introduction
Among the II-VI functional semiconductor materials, zinc oxide (ZnO) has attracted significant interest because of its wide band gap, large exciton binding energy, high chemical stability, and environmentfriendly applications [1, 2]. Due to their catalytic, optical, electrical, optoelectronic, gas sensing, piezoelectric, and photo-electrochemical properties, ZnO is not only attractive for fundamental research but also for practical applications [1, 3]. The doping by various metal atoms introduces new magnetic, optical, electronic, photo-physical, or chemical properties to this semiconductor, which has further increased their contribution in the research areas such as spintronics, optoelectronics, quantum computing,
photocatalysis,
and
luminescent
materials
[1].
In
particular,
diluted
magnetic
semiconductors, achieved by doping magnetic ions, are interesting for spintronic or light emitting applications [4]. Due to their application in light emitters, the nature of optical transitions in ZnO has become one of the relevant topics. Bulk ZnO typically exhibits a richly structured near-band edge recombination mainly due to free and bound excitons as well as donor-acceptor pair transitions. When
excited by UV light, this material displays luminescence over wide wavelength range, from UV to infrared. ZnO possess a potential to host rare earth metal ions such as Nd, Yp, Tm, etc. The influence of Nd doping on the structural and photo-luminescent properties of the ZnO films deposited by sol-gel method [5] and spray pyrolysis (SP) [1] is reported. SP is one of the most widely used techniques for the deposition of various transparent conducting oxides (TCOs) including ZnO [6-8]. Hence, we have utilized SP to deposit the films reported in the study. SP is a simple and economic technique that has potential to exploit the flexibility of the precursor chemistry and enable the fabrication of complex oxide compositions [6]. The substrate temperature is traditionally shown as the most important deposition parameters in SP as it controls the decomposition chemistry and decides the quality of the deposited films. However, other parameters such as the carrier-gas flow rate and the nozzle-substrate distance that are rarely specified in the available literature do have a decisive influence on the homogeneity and crystalline quality of the films [9,10]. Nd doped ZnO system received more attention due to their application in the luminescence devices like field emission displays (FED) since the UV excitation provides more efficient light emission than electron excitation [10]. Even a slight variation on one or more of these parameters are likely to be responsible for divergent results as can be evidenced from the literature. For example, Subramanian et al. [1] reports the formation of large Zn1-xNdxO micro-rods; whereas, the films reported in the present work that are deposited with the same precursor and at same substrate temperature show a nanocrystalline structure similar to the films deposited by sol-gel [5]. In the present work, we have concentrated on the structural and light emission properties of ZnO films doped with a wide range of Nd doping concentrations (varying from 5 at. % to 25 at. %). The formation of large agglomerations on top of film surface, at higher Nd doping concentration, is complementing the microRaman spectroscopy investigations. The enhanced optical properties of the Nd doped ZnO thin films deposited in the present study are promising for the applications in the luminescence devices such as FED. XRD patterns shown in [11] revealed that the Nd doped ZnO films deposited on silicon substrate are highly oriented c- axis oriented in comparison with those deposited on the glass substrate. It is shown that the Nd ions replace Zn ions in the host lattice due to decrease of c- axis length. The nature of the substrate plays an important role in deciding the structural properties of ZnO film. Hence, ZnO
thin films reported in the present study were deposited on Si substrate aiming their application in thinfilm Si solar cells.
2. Experimental Details
An in-built spray pyrolysis experimental setup was used for the deposition, which is described elsewhere [12]. The spray gun is a coaxial assembly of a corning glass tube and a capillary. One end of the glass tube is fused with the outer surface of the capillary and the other end is tapered to form the spray nozzle. The solution and the carrier gas (filtered compressed air) are passed through the inlets provided in the capillary and glass tube. A stainless steel plate kept above the resistive heater, familiarly known as hot plate, was used as substrate holder. A digital temperature controller was used to maintain the desired-set temperature.
Zinc acetate and neodymium acetate hydrate (Sigma Aldrich), 98 % purity, were used as precursors for Zn and Nd, respectively. A required amount of zinc acetate, dissolved in deionized water, was used as starting spray solution. For Nd doping, a stoichiometric amount of neodymium acetate was dissolved in deionized water separately, and mixed with the starting solution. The total concentration of the solution was 0.5 M. The doping percentage of Nd in the final spray solution is varied between 5 at. % and 25 at %. The films were deposited on Si(111) substrates covered with native oxide. Before deposition, the substrates were pre-heated at 400 °C for 5 minutes; all the films were deposited at 400 °C. A controlled compressed air was used as a carrier gas. The distance between spray nozzle and the substrate was 30 cm. The solution was sprayed on the substrates in pulses having 1 s duration/pulse at an interval of 30 s. The film thickness was varied from 100 nm to 240 nm. The spraying conditions in the present work are identical to those reported in [1], except for the shorter pulse duration and shorter waiting time.
XRD measurements were performed with a Seifert FPM XRD7 diffractometer in Bragg-Brentano geometry using Cu Ka radiation. The SEM microstructures were obtained with a NovaNano SEM from FEI. The chemical analysis was done by energy-dispersive spectroscopy (EDS), which is an attachment
to SEM tool. The Raman spectra were recorded with a 40x magnification objective in back-scattering geometry using a UV Lab ram spectrometer at an excitation wavelength of 325 nm. The optical studies were performed by UV-Vis spectroscopy (Cary 100 Scan spectrometer from Varian). PL spectra were recorded using a He-Cd laser (325nm) as light source. PL spectra of Zn1-xNdxO thin films were recorded at room temperature (RT).
3. Results and Discussion
Figure 1 shows the XRD patterns obtained from un-doped and Nd- doped ZnO films. Except for the variation in relative intensity, all the films deposited in the present study exhibit diffraction peaks from the (002), (101), (100) planes characteristic for hexagonal wurtzite ZnO structure (JCPDS card no. 89-1397). The intensities of (002) and (101) diffraction peaks of the un-doped ZnO films are similar, which is probably indicating that the films do not have any preferential growth but are randomly oriented in perpendicular to the substrate surface. However, the intensity of (002) peak is increased with the increasing Nd- doping concentration (5 at. %) and showing a preferential growth along c-axis and perpendicular to the substrate plane. The increasing Nd- doping concentration has increased the intensity of (002) peak, which is probably indicating that the crystallinity has been increased. In addition, a new peak is evidenced at 28.54° upon Nd doping which can be related to the formation of neodymium oxide [13]. The c-axis length was estimated for the (002) diffraction plane from the XRD data, by following equation found in [1]. The calculated values of the c-axis lengths were 5.192 Å and 5.189 Å for un-doped ZnO and 25 at % Nd doped ZnO, respectively. The decrease in c-axis length upon Nd doping can be attributed to Nd ions occupying the Zn sites that are tetrahedrally coordinated in the wurtzite crystal structure and thereby induce tensile stress and structural defects in the host lattice [1]. The crystallite size (d) of the samples was estimated using Scherrer formula [14] by using the X-ray wavelength, diffraction angle and the full width at half maximum (FWHM). The crystallite size is reduced from ~96 nm to ~56 nm following 25 at. % Nd doping, which is in agreement with the SEM analysis (Fig. 2). A similar trend was observed in the Nd doped films produced by Xiang et al. by sol-gel method [5].
SEM analysis was performed on the un-doped ZnO and on the Zn1-xNdxO samples containing 5 at. % and 25 at. % Nd. All samples have a compact granular structure with nano-crystalline grains (Fig. 2), which is in agreement with the literature on ZnO films (un-doped and Sn- doped) deposited by spray pyrolysis [15] from the same precursor at 350 °C substrate temperature. However, Subramanian et al. [1] obtained long, randomly oriented, micro-rods on top of an uniform nano-crystalline layer both for the ZnO and for the Nd doped ZnO films [1] deposited by spray-pyrolysis for the conditions similar to the present work. However, Duoyar et al. [16] have obtained polycrystalline films while using spray pyrolysis method; wherein, they have observed that the doped Nd ions have occupied the proper ZnO lattice positions until 1 at. % Nd doping concentration. Though the dopant concentration in the present work is increased to until and above 5 at %, the dopant ions still seem to occupy proper lattice positions. This enhancement in the occupation capacity is probably due to the different chemical composition of the film that could have influenced the grain size of the film. During the quantum confinement process, the grain size of the film is decreased and it leads to an increase in the band gap. Zheng et al. [17] have reported the sol-gel prepared nano-rods having enhanced material properties. Further, they have showed that the intensity of (002) diffraction peak is decreased with the increasing Nd- doping, which contradicts the observation in the present work. The foregoing discussion clearly indicates the effect of precursor and deposition technique on the material properties.
The higher Nd- doping concentration (5 at. % and 25 at. %) form micrometer-large structures on top of the nano-crystalline layer (Figs. 2c-2f) similar to Bilgin et al. found in Sn- doped ZnO [15]. In contrast, Subramanian et al. report an increase in size and density of the micro-rods upon Nd- doping [1]. This is probably inferring that the pulse time and pulse interval (time between two successive pulses) adopted in the present study has contributed to the enhanced crystallintiy. Longer pulse time (3 s) and pulse interval (1 min) appear to favor the formation of large needle-like microcrystals on top of a uniform nanocrystalline layer [1]. On the other hand, short pulse time and pulse interval appear to favor the formation of small nano-crystalline grains. Apparently, the grain size of the nano-crystalline layers (Fig. 2) varies from 50 nm to 76 nm, depending on the dopant concentration. With the increasing Nd concentration, the densely packed spherical shaped grains get elongated to become needle-like grains with sub-
micrometer dimensions. The structural changes between these microstructures (SEM analysis) were further confirmed by micro-Raman spectroscopy. Exemplary Raman spectra in the spectral range of the longitudinal optical (LO) phonon are shown in Fig. 3. The approximate size and position of the laser spot are identified on the corresponding SEM microstructures on the right hand side.
Raman spectroscopy is a powerful tool to identify specific materials in complex structures and for extracting useful information on the nano-scaled objects. For the ZnO film, spectra recorded at various positions show the same peak positions and line shape of the A1(LO) phonon (Fig. 3, top). For the Zn75Nd25O film, spectra recorded in the "flat" nano-crystalline regions have similar shape and peak position as the un-doped ZnO spectra. On top of the large bubble-like islands the LO phonon is slightly red-shifted and broadened, which is an indication for the presence of crystalline defects in the ZnO lattice (Fig. 3, bottom). These islands are probably richer in Nd2O3 as compared to the "flat" layer considering the XRD patterns, which show both the preservation of the ZnO crystalline structure and the appearance of an additional crystalline phase.
Optical transmission spectra were recorded on Zn1-xNdxO films deposited on glass substrates. For direct-allowed band gap semiconductors, the optical band gap energy can be determined from the Tauc 2
plot {(ahv) vs. hv, refer Fig. 4} based on the equation found in [18]. The intercepts of these graphs with the X-axis (at Y = 0) yield the values of the band gap. With the increasing Nd doping concentration, band gap is slightly decreased first from 3.19 eV (± 0.01) for un-doped ZnO to 3.15 eV (± 0.01) for Nddoped (5 at %) ZnO; and then increased again to 3.18 eV (± 0.01) for Nd- doped (25 at %) ZnO. A decrease in the band gap upon doping has been observed earlier for other dopants in ZnO, for example Cr [19]; there is also report for the increasing band gap for Cr- doping. The band gap values are bit low than the reported values [20] due to that there may be some defects such as improper decomposition of the sprayed precursors, which is probably increased the optical absorption. However, based on low temperature PL measurements, Yilmaz et al. have recently suggested that such variations in the band gap actually indicate a change in excitonic properties; which affects the absorption spectral bandwidth rather than a change in band gap [21].
In order to verify this proposed correlation between excitonic effects and absorption, PL spectra were recorded at room temperature using a He-Cd laser (325 nm) as light source. Selected spectra of Zn1xNd xO
films (for x= 0 at. %, 5 at. %, and 25 at. %) in the region of the bound exciton transitions
normalized to the peak height are shown in Fig. 5. The position of band edge shifts towards higher wavelength and the bandwidth increases for Nd- concentration of 5 at. % (from 376 nm for ZnO to 380 nm for Zn95Nd5O). The shift attributed in a mixing between free exciton, bound exciton and bound exciton with phonon emission of the doping with host material. A red shift was also observed for Nd- doping of TiO2 films [22] and also in Zn1-xNdxO micro-rods [1]. In both cases it was attributed to the introduction of new unoccupied states by Nd 4f electrons that are located closer to the lower edge of conduction band of the TiO2. Further, increase in the Nd- doping concentration shifts the PL peak again to lower wavelengths (377 nm for 25 at %) and decreases the peak-width. This may be correlated to the agglomeration of the Nd- atoms in the bubbles formed on top of the nano-crystalline films, which is in agreement with the Raman spectroscopy analysis. When comparing the absorption (Fig. 4) and the PL spectra (Fig. 5), the strong shift of the absorption onset for 5 at. % Nd- doping concentration used for the band gap determination in the Tauc plot can be a consequence of the decrease in the excitonic transition bandwidth.
Summary
The effect of Nd- doping on the structural and optical properties of ZnO films deposited by spray pyrolysis onto Si (111) substrates was investigated. All the samples consist of compact granular structure with nano-crystalline grains (SEM) having a preferential orientation along the c-axis (002) perpendicular to the substrate plane (XRD). The increasing Nd- doping concentration triggers the formation of large bubbles on top of nano-crystalline features observed in un-doped films; similar observation is made earlier on Sn doped ZnO films prepared by ultrasonic spray pyrolysis. The increased pulse time and pulse interval are presumed to have significant influence on the crystallinity of the deposited films. Based on the obtained experimental data, we have made an attempt to identify the origin of the observed
phonon frequency shifts in each of the nanostructure samples. It has been understood that the optical -1
phonon confinement results in phonon frequency shifts of only few cm . Direct-allowed band gap of the deposited Zn1-xNd xO films were estimated from the absorption coefficient calculated from the transmittance data. The estimated band gap was found varying between 3.15 eV and 3.19 eV depending on the variation in Nd doping concentration. The optical properties are further complimented by microRaman and PS studies. Such variations in the band gap are probably indicating a change in excitonic properties, which affects the absorption spectral bandwidth.Micro-Raman spectroscopy showed that the bubbles on top of nano-crystalline layer exhibit a broadening of the LO phonons compared to the underlying film, which is presumably due to the incorporation of Nd atoms. This conclusion is reinforced by the fact that the broadening of the bound exciton band probed by PL at RT, is initially increased (at 5 at. % Nd) but then decreased later (at 25 at. % Nd).
Acknowledgments The authors gratefully acknowledge the German Federal Ministry of Education and Research (BMBF) in the frame of the nanett -nano systems integration- network of excellence for financial support (grant number: 03IS2011A and 03IS2011B). The German Research Foundation (DFG) is gratefully acknowledged for the financial support in the frames of the research unit DFG-FOR1154 "Towards Molecular Spintronics".
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M. Subramanian, P. Thakur, S. Gautam, K H Chae, M. Tanemura, T Hihara, S.Vijayalakshmi, T. Soga, S. Kim, K. Asokan, R. Jayavel, J.Phys.D:Appl.Phys. 42 (2009) 105410.
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C. Cheng, R. Xin, Y. Leng, D.Yu, N. Wong, Inorg. Chem.47, 7868 (2008).
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Z.L. Wang, J.H. Song, Science 312 (2006) 242.
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T.Monterio,A.J.Neves,M.C.Carmo,M.J.Soares,M.Peres,J.Wang,E.Alves,E.Rita,U.Wahl, J.Appl.Phys. 98, 013502 (2005).
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F. Xiang, X. Li, Optics & LaserTechnology 45 (2013) 508.
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G.J. Exarhos, X.-D. Zhou, Thin Solid Films 515 (2007) 7025.
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T.Prasad Rao,M.C.Santhoshkumar, Applied Surface Science 255 (2009) 7212-7215.
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T.Dedova,M.Krunks,M.Grossberg,O.Volobujeva,I.Oja Acik Superlattices and Microstructures 42 (2007) 444-450.
[9]
M.Krunks,T.Dedova,E.Karber,V.Mikli,I.Oja Acik,M.Grossberg,A.Mere,Physica B 404 (2009) 44224425.
[10] T.Prasad Rao,M.C.Santhoshkumar, Applied Surface Science 255 (2009) 4579-4584. [11] Preetham Singh, Ashvani Kumar, Deepak, Davinder Kaur Journal of Crystal Growth 306 (2007) 303310. [12] E. Elangovan, K. Ramamurthi, J. Opt. Adv. Mater. 5 (2003) 45. [13] B. Shahmoradi, K. Soga, S. Ananda, R. Somashekar, K. Byrappa, Nanoscale, 2 (2010) 1160. nd
[14] B.D. Cullity, Elements of X-ray diffraction, Addison- Wesley Series on Metallurgy and Materials, 2 Edition, Addison-Wesley Publishing Company, Inc., 1978, p. 102, Ch. 3. [15] [9] V. Bilgin, S. Kose, F. Atay, I. Akyuz, J. Mat. Sci. 40 (2005) 1909.
[16] A. Douayar, P. Prieto, G. Schmerber, K. Nouneh, R. Diaz, I. Chaki, S. Colis, A. El Fakir, N. Hassanain, A. Belayachi, Z. Sekkat, A. Slaoui, A. Dinia, and M. Abd-Lefdil, Eur. Phys. J. Appl. Phys. 61 (2013) 10304. [17] J.H. Zheng, J. L. Song, Z. Zhao, Q. Jiang, and J. S. Lian Cryst. Res. Techmol. 47, No. 7, 713 (2012). [18] T. Prasada Rao, M.C. Santhoshkumar, Appl. Surf. Sci. 255 (2009) 7212. [19] Y.M. Hu, Y.T. Chen, Z.X. Zhong, C.C. Yu, G.J. Chen, P.Z. Huang, W.Y. Chou, J. Chang, C.R. Wang, Appl. Surf. Sci. 254 (2008) 3873. [20] L. Li, W. Wang, H. Liu, X. Liu, Q. Song, S. Ren, J. Phys. Chem. C 113 (2009) 8460. [21] S. Yilmaz, M. Parlak, S. Ozcan, M. Altunbas^ E. McGlynn, E. Bacaksiz, Appl. Surf. Sci. 257 (2011) 9293. [22] W. Li, Y. Wang, H. Lin, S. Ismat Shah, C. P. Huang, D. J. Doren, S. A. Rykov, J. G. Chen, M. A. Barteau, Appl. Phys. Lett. 83 (2003) 4143.
Figure captions
Figure 1: XRD patterns from un-doped and Nd- doped ZnO films deposited by spray pyrolysis. Figure 2: SEM microstructures at different scan-lengths obtained from: ZnO (a, b) Zn95Nd5O (c, d), and Zn75Nd25O (e, f) films deposited by spray pyrolysis onto Si(111) substrates. Figure 3: Micro-Raman spectra (left column) of the A1(LO) phonons obtained from un-doped ZnO sample (top-left) and for a Nd-doped (25 at. %) ZnO film (bottom-left) recorded at different positions
on
the sample. These representative
positions
are
marked
on
the
corresponding SEM microstructures (right column). The Raman spectra recorded at an excitation energy of 3.82 eV are normalized to the height of the ZnO LO phonon scattering peak. 2
Figure 4: Plot of (Ahv) vs. (hv) as a function of Nd- doping in ZnO thin films. Figure 5: PL spectra recorded at RT with excitation energy of 3.28 eV for un-doped and Nd doped ZnO films (The spectra are normalized with respect to the height of the PL peak for a better understanding).
a
b
1 µm
c
5 µm
d
1 µm
e
f
1 µm
20 µm
5 µm
Highlights u
grain size of the Nd doped ZnO layers varies from 50 nm to 76 nm by SEM
u
From Raman, the LO phonon is slightly red-shifted and broadened.
u
Presence of crystalline defects in the ZnO lattice confirmed by Raman studies
u
A red shift was also observed for Nd doping of ZnO thin films confirmed by PL
S0749-6036(14)00358-9 http://dx.doi.org/10.1016/j.spmi.2014.10.001 YSPMI 3427
To appear in:
Superlattices and Microstructures
Received Date: Revised Date: Accepted Date:
16 November 2013 16 September 2014 2 October 2014
Please cite this article as: T. Deepa Rani, K. Tamilarasan, E. Elangovan, S. Leela, K. Ramamurthi, K. Thangaraj, C. Himcinschi, I. Trenkmann, S. SchuIze, M. Hietschold, A. Liebig, G. Salvan, D.R. T. Zahn, Structural and optical studies on Nd doped ZnO thin films, Superlattices and Microstructures (2014), doi: http://dx.doi.org/10.1016/j.spmi. 2014.10.001
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Structural and optical studies on Nd doped ZnO thin films 1
1
2
3
4
T. Deepa Rani , K.Tamilarasan , E. Elangovan , S. Leela , K. Ramamurthi , 1
5
6
6
6
K.Thangaraj , C. Himcinschi ,I. Trenkmann , S.SchuIze , M. Hietschold , A. 6
6
6
Liebig , G. Salvan , D.R. T. Zahn 1
Department of Physics, Kongu Engineering College, Erode, Tamil Nadu, India
2
i-Micro, Masdar Institute of Science and Technology, 54224 Masdar City, Abu Dhabi,
United Arab Emirates 3
Department of Physics, Ethiraj College for Women, Chennai, Tamil Nadu, India
4
Department of Physics and Nanotechnology, SRM University, Kattankulathur-603
203,Tamil Nadu, India 5
institute for Theoretical Physics, Technische Universitat Bergakademie Freiberg, 09596
Freiberg, Germany 6
institute of Physics, Chemnitz University of Technology, 09107 Chemnitz, Germany
Keywords: ZnO, Nd doping, Micro-Raman, XRD, SEM, PL Corresponding author T. Deepa Rani, Department of Physics, Kongu Engineering college, Perundurai, Erode +91 9865470707 deepaphvsics
@gmail.com
Abstract Thin films of Zn1-xNdxO were deposited by spray pyrolysis on Si(111) substrates preheated at 400 °C temperature and were studied as a function of neodymium(Nd)- doping concentration. X-ray diffraction (XRD) patterns confirmed that the deposited films possess hexagonal wurtzite ZnO structure. Further, it is observed that the doped films show a preferential orientation along the c-axis (002), which is perpendicular to the substrate. The un-doped films seem to be having a bit low-crystallinity, which is corroborated by the scanning electron microscope (SEM) analysis that showed nano-crystalline like features. Further, SEM analysis showed that the Nd doping triggers the formation bubble-like structure on top of the nano-crystalline structure. The SEM microstructures are interpreted with the micro-Raman studies. Photoluminescence (PL) and XRD characterizations indicate that above 5 at. % doping concentrations, the Nd atoms preferentially agglomerate in the large islands. 1. Introduction
Among the II-VI functional semiconductor materials, zinc oxide (ZnO) has attracted significant interest because of its wide band gap, large exciton binding energy, high chemical stability, and environmentfriendly applications [1, 2]. Due to their catalytic, optical, electrical, optoelectronic, gas sensing, piezoelectric, and photo-electrochemical properties, ZnO is not only attractive for fundamental research but also for practical applications [1, 3]. The doping by various metal atoms introduces new magnetic, optical, electronic, photo-physical, or chemical properties to this semiconductor, which has further increased their contribution in the research areas such as spintronics, optoelectronics, quantum computing,
photocatalysis,
and
luminescent
materials
[1].
In
particular,
diluted
magnetic
semiconductors, achieved by doping magnetic ions, are interesting for spintronic or light emitting applications [4]. Due to their application in light emitters, the nature of optical transitions in ZnO has become one of the relevant topics. Bulk ZnO typically exhibits a richly structured near-band edge recombination mainly due to free and bound excitons as well as donor-acceptor pair transitions. When
excited by UV light, this material displays luminescence over wide wavelength range, from UV to infrared. ZnO possess a potential to host rare earth metal ions such as Nd, Yp, Tm, etc. The influence of Nd doping on the structural and photo-luminescent properties of the ZnO films deposited by sol-gel method [5] and spray pyrolysis (SP) [1] is reported. SP is one of the most widely used techniques for the deposition of various transparent conducting oxides (TCOs) including ZnO [6-8]. Hence, we have utilized SP to deposit the films reported in the study. SP is a simple and economic technique that has potential to exploit the flexibility of the precursor chemistry and enable the fabrication of complex oxide compositions [6]. The substrate temperature is traditionally shown as the most important deposition parameters in SP as it controls the decomposition chemistry and decides the quality of the deposited films. However, other parameters such as the carrier-gas flow rate and the nozzle-substrate distance that are rarely specified in the available literature do have a decisive influence on the homogeneity and crystalline quality of the films [9,10]. Nd doped ZnO system received more attention due to their application in the luminescence devices like field emission displays (FED) since the UV excitation provides more efficient light emission than electron excitation [10]. Even a slight variation on one or more of these parameters are likely to be responsible for divergent results as can be evidenced from the literature. For example, Subramanian et al. [1] reports the formation of large Zn1-xNdxO micro-rods; whereas, the films reported in the present work that are deposited with the same precursor and at same substrate temperature show a nanocrystalline structure similar to the films deposited by sol-gel [5]. In the present work, we have concentrated on the structural and light emission properties of ZnO films doped with a wide range of Nd doping concentrations (varying from 5 at. % to 25 at. %). The formation of large agglomerations on top of film surface, at higher Nd doping concentration, is complementing the microRaman spectroscopy investigations. The enhanced optical properties of the Nd doped ZnO thin films deposited in the present study are promising for the applications in the luminescence devices such as FED. XRD patterns shown in [11] revealed that the Nd doped ZnO films deposited on silicon substrate are highly oriented c- axis oriented in comparison with those deposited on the glass substrate. It is shown that the Nd ions replace Zn ions in the host lattice due to decrease of c- axis length. The nature of the substrate plays an important role in deciding the structural properties of ZnO film. Hence, ZnO
thin films reported in the present study were deposited on Si substrate aiming their application in thinfilm Si solar cells.
2. Experimental Details
An in-built spray pyrolysis experimental setup was used for the deposition, which is described elsewhere [12]. The spray gun is a coaxial assembly of a corning glass tube and a capillary. One end of the glass tube is fused with the outer surface of the capillary and the other end is tapered to form the spray nozzle. The solution and the carrier gas (filtered compressed air) are passed through the inlets provided in the capillary and glass tube. A stainless steel plate kept above the resistive heater, familiarly known as hot plate, was used as substrate holder. A digital temperature controller was used to maintain the desired-set temperature.
Zinc acetate and neodymium acetate hydrate (Sigma Aldrich), 98 % purity, were used as precursors for Zn and Nd, respectively. A required amount of zinc acetate, dissolved in deionized water, was used as starting spray solution. For Nd doping, a stoichiometric amount of neodymium acetate was dissolved in deionized water separately, and mixed with the starting solution. The total concentration of the solution was 0.5 M. The doping percentage of Nd in the final spray solution is varied between 5 at. % and 25 at %. The films were deposited on Si(111) substrates covered with native oxide. Before deposition, the substrates were pre-heated at 400 °C for 5 minutes; all the films were deposited at 400 °C. A controlled compressed air was used as a carrier gas. The distance between spray nozzle and the substrate was 30 cm. The solution was sprayed on the substrates in pulses having 1 s duration/pulse at an interval of 30 s. The film thickness was varied from 100 nm to 240 nm. The spraying conditions in the present work are identical to those reported in [1], except for the shorter pulse duration and shorter waiting time.
XRD measurements were performed with a Seifert FPM XRD7 diffractometer in Bragg-Brentano geometry using Cu Ka radiation. The SEM microstructures were obtained with a NovaNano SEM from FEI. The chemical analysis was done by energy-dispersive spectroscopy (EDS), which is an attachment
to SEM tool. The Raman spectra were recorded with a 40x magnification objective in back-scattering geometry using a UV Lab ram spectrometer at an excitation wavelength of 325 nm. The optical studies were performed by UV-Vis spectroscopy (Cary 100 Scan spectrometer from Varian). PL spectra were recorded using a He-Cd laser (325nm) as light source. PL spectra of Zn1-xNdxO thin films were recorded at room temperature (RT).
3. Results and Discussion
Figure 1 shows the XRD patterns obtained from un-doped and Nd- doped ZnO films. Except for the variation in relative intensity, all the films deposited in the present study exhibit diffraction peaks from the (002), (101), (100) planes characteristic for hexagonal wurtzite ZnO structure (JCPDS card no. 89-1397). The intensities of (002) and (101) diffraction peaks of the un-doped ZnO films are similar, which is probably indicating that the films do not have any preferential growth but are randomly oriented in perpendicular to the substrate surface. However, the intensity of (002) peak is increased with the increasing Nd- doping concentration (5 at. %) and showing a preferential growth along c-axis and perpendicular to the substrate plane. The increasing Nd- doping concentration has increased the intensity of (002) peak, which is probably indicating that the crystallinity has been increased. In addition, a new peak is evidenced at 28.54° upon Nd doping which can be related to the formation of neodymium oxide [13]. The c-axis length was estimated for the (002) diffraction plane from the XRD data, by following equation found in [1]. The calculated values of the c-axis lengths were 5.192 Å and 5.189 Å for un-doped ZnO and 25 at % Nd doped ZnO, respectively. The decrease in c-axis length upon Nd doping can be attributed to Nd ions occupying the Zn sites that are tetrahedrally coordinated in the wurtzite crystal structure and thereby induce tensile stress and structural defects in the host lattice [1]. The crystallite size (d) of the samples was estimated using Scherrer formula [14] by using the X-ray wavelength, diffraction angle and the full width at half maximum (FWHM). The crystallite size is reduced from ~96 nm to ~56 nm following 25 at. % Nd doping, which is in agreement with the SEM analysis (Fig. 2). A similar trend was observed in the Nd doped films produced by Xiang et al. by sol-gel method [5].
SEM analysis was performed on the un-doped ZnO and on the Zn1-xNdxO samples containing 5 at. % and 25 at. % Nd. All samples have a compact granular structure with nano-crystalline grains (Fig. 2), which is in agreement with the literature on ZnO films (un-doped and Sn- doped) deposited by spray pyrolysis [15] from the same precursor at 350 °C substrate temperature. However, Subramanian et al. [1] obtained long, randomly oriented, micro-rods on top of an uniform nano-crystalline layer both for the ZnO and for the Nd doped ZnO films [1] deposited by spray-pyrolysis for the conditions similar to the present work. However, Duoyar et al. [16] have obtained polycrystalline films while using spray pyrolysis method; wherein, they have observed that the doped Nd ions have occupied the proper ZnO lattice positions until 1 at. % Nd doping concentration. Though the dopant concentration in the present work is increased to until and above 5 at %, the dopant ions still seem to occupy proper lattice positions. This enhancement in the occupation capacity is probably due to the different chemical composition of the film that could have influenced the grain size of the film. During the quantum confinement process, the grain size of the film is decreased and it leads to an increase in the band gap. Zheng et al. [17] have reported the sol-gel prepared nano-rods having enhanced material properties. Further, they have showed that the intensity of (002) diffraction peak is decreased with the increasing Nd- doping, which contradicts the observation in the present work. The foregoing discussion clearly indicates the effect of precursor and deposition technique on the material properties.
The higher Nd- doping concentration (5 at. % and 25 at. %) form micrometer-large structures on top of the nano-crystalline layer (Figs. 2c-2f) similar to Bilgin et al. found in Sn- doped ZnO [15]. In contrast, Subramanian et al. report an increase in size and density of the micro-rods upon Nd- doping [1]. This is probably inferring that the pulse time and pulse interval (time between two successive pulses) adopted in the present study has contributed to the enhanced crystallintiy. Longer pulse time (3 s) and pulse interval (1 min) appear to favor the formation of large needle-like microcrystals on top of a uniform nanocrystalline layer [1]. On the other hand, short pulse time and pulse interval appear to favor the formation of small nano-crystalline grains. Apparently, the grain size of the nano-crystalline layers (Fig. 2) varies from 50 nm to 76 nm, depending on the dopant concentration. With the increasing Nd concentration, the densely packed spherical shaped grains get elongated to become needle-like grains with sub-
micrometer dimensions. The structural changes between these microstructures (SEM analysis) were further confirmed by micro-Raman spectroscopy. Exemplary Raman spectra in the spectral range of the longitudinal optical (LO) phonon are shown in Fig. 3. The approximate size and position of the laser spot are identified on the corresponding SEM microstructures on the right hand side.
Raman spectroscopy is a powerful tool to identify specific materials in complex structures and for extracting useful information on the nano-scaled objects. For the ZnO film, spectra recorded at various positions show the same peak positions and line shape of the A1(LO) phonon (Fig. 3, top). For the Zn75Nd25O film, spectra recorded in the "flat" nano-crystalline regions have similar shape and peak position as the un-doped ZnO spectra. On top of the large bubble-like islands the LO phonon is slightly red-shifted and broadened, which is an indication for the presence of crystalline defects in the ZnO lattice (Fig. 3, bottom). These islands are probably richer in Nd2O3 as compared to the "flat" layer considering the XRD patterns, which show both the preservation of the ZnO crystalline structure and the appearance of an additional crystalline phase.
Optical transmission spectra were recorded on Zn1-xNdxO films deposited on glass substrates. For direct-allowed band gap semiconductors, the optical band gap energy can be determined from the Tauc 2
plot {(ahv) vs. hv, refer Fig. 4} based on the equation found in [18]. The intercepts of these graphs with the X-axis (at Y = 0) yield the values of the band gap. With the increasing Nd doping concentration, band gap is slightly decreased first from 3.19 eV (± 0.01) for un-doped ZnO to 3.15 eV (± 0.01) for Nddoped (5 at %) ZnO; and then increased again to 3.18 eV (± 0.01) for Nd- doped (25 at %) ZnO. A decrease in the band gap upon doping has been observed earlier for other dopants in ZnO, for example Cr [19]; there is also report for the increasing band gap for Cr- doping. The band gap values are bit low than the reported values [20] due to that there may be some defects such as improper decomposition of the sprayed precursors, which is probably increased the optical absorption. However, based on low temperature PL measurements, Yilmaz et al. have recently suggested that such variations in the band gap actually indicate a change in excitonic properties; which affects the absorption spectral bandwidth rather than a change in band gap [21].
In order to verify this proposed correlation between excitonic effects and absorption, PL spectra were recorded at room temperature using a He-Cd laser (325 nm) as light source. Selected spectra of Zn1xNd xO
films (for x= 0 at. %, 5 at. %, and 25 at. %) in the region of the bound exciton transitions
normalized to the peak height are shown in Fig. 5. The position of band edge shifts towards higher wavelength and the bandwidth increases for Nd- concentration of 5 at. % (from 376 nm for ZnO to 380 nm for Zn95Nd5O). The shift attributed in a mixing between free exciton, bound exciton and bound exciton with phonon emission of the doping with host material. A red shift was also observed for Nd- doping of TiO2 films [22] and also in Zn1-xNdxO micro-rods [1]. In both cases it was attributed to the introduction of new unoccupied states by Nd 4f electrons that are located closer to the lower edge of conduction band of the TiO2. Further, increase in the Nd- doping concentration shifts the PL peak again to lower wavelengths (377 nm for 25 at %) and decreases the peak-width. This may be correlated to the agglomeration of the Nd- atoms in the bubbles formed on top of the nano-crystalline films, which is in agreement with the Raman spectroscopy analysis. When comparing the absorption (Fig. 4) and the PL spectra (Fig. 5), the strong shift of the absorption onset for 5 at. % Nd- doping concentration used for the band gap determination in the Tauc plot can be a consequence of the decrease in the excitonic transition bandwidth.
Summary
The effect of Nd- doping on the structural and optical properties of ZnO films deposited by spray pyrolysis onto Si (111) substrates was investigated. All the samples consist of compact granular structure with nano-crystalline grains (SEM) having a preferential orientation along the c-axis (002) perpendicular to the substrate plane (XRD). The increasing Nd- doping concentration triggers the formation of large bubbles on top of nano-crystalline features observed in un-doped films; similar observation is made earlier on Sn doped ZnO films prepared by ultrasonic spray pyrolysis. The increased pulse time and pulse interval are presumed to have significant influence on the crystallinity of the deposited films. Based on the obtained experimental data, we have made an attempt to identify the origin of the observed
phonon frequency shifts in each of the nanostructure samples. It has been understood that the optical -1
phonon confinement results in phonon frequency shifts of only few cm . Direct-allowed band gap of the deposited Zn1-xNd xO films were estimated from the absorption coefficient calculated from the transmittance data. The estimated band gap was found varying between 3.15 eV and 3.19 eV depending on the variation in Nd doping concentration. The optical properties are further complimented by microRaman and PS studies. Such variations in the band gap are probably indicating a change in excitonic properties, which affects the absorption spectral bandwidth.Micro-Raman spectroscopy showed that the bubbles on top of nano-crystalline layer exhibit a broadening of the LO phonons compared to the underlying film, which is presumably due to the incorporation of Nd atoms. This conclusion is reinforced by the fact that the broadening of the bound exciton band probed by PL at RT, is initially increased (at 5 at. % Nd) but then decreased later (at 25 at. % Nd).
Acknowledgments The authors gratefully acknowledge the German Federal Ministry of Education and Research (BMBF) in the frame of the nanett -nano systems integration- network of excellence for financial support (grant number: 03IS2011A and 03IS2011B). The German Research Foundation (DFG) is gratefully acknowledged for the financial support in the frames of the research unit DFG-FOR1154 "Towards Molecular Spintronics".
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Figure captions
Figure 1: XRD patterns from un-doped and Nd- doped ZnO films deposited by spray pyrolysis. Figure 2: SEM microstructures at different scan-lengths obtained from: ZnO (a, b) Zn95Nd5O (c, d), and Zn75Nd25O (e, f) films deposited by spray pyrolysis onto Si(111) substrates. Figure 3: Micro-Raman spectra (left column) of the A1(LO) phonons obtained from un-doped ZnO sample (top-left) and for a Nd-doped (25 at. %) ZnO film (bottom-left) recorded at different positions
on
the sample. These representative
positions
are
marked
on
the
corresponding SEM microstructures (right column). The Raman spectra recorded at an excitation energy of 3.82 eV are normalized to the height of the ZnO LO phonon scattering peak. 2
Figure 4: Plot of (Ahv) vs. (hv) as a function of Nd- doping in ZnO thin films. Figure 5: PL spectra recorded at RT with excitation energy of 3.28 eV for un-doped and Nd doped ZnO films (The spectra are normalized with respect to the height of the PL peak for a better understanding).
a
b
1 µm
c
5 µm
d
1 µm
e
f
1 µm
20 µm
5 µm
Highlights u
grain size of the Nd doped ZnO layers varies from 50 nm to 76 nm by SEM
u
From Raman, the LO phonon is slightly red-shifted and broadened.
u
Presence of crystalline defects in the ZnO lattice confirmed by Raman studies
u
A red shift was also observed for Nd doping of ZnO thin films confirmed by PL