Transcript Synthesis and optical properties of Zinc Oxide

Synthesis and Optical Properties of Zinc Oxide
Nanoparticles grown on Sn-coated Silicon Substrate by
Thermal Evaporation Method
Outline
Introduction
ZnO Vs. GaN
Experimental details
Results and discussion
Surface morphology
Crystalline structure
Optical properties
Photoluminescence spectrum
Raman spectrum
Conclusion
Future work
Applications
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Introduction
Semiconductor nanostructures are ideal system for exploring a large number of
novel phenomena at the nanoscale
Nanostructure represents a system or object with at least one dimension in the
order of one-hundred nanometer or less
Among all the semiconductors, Zinc oxide (ZnO) is a unique material that
exhibits semiconducting, piezoelectric, and pyroelectric multiple properties
Using different processing techniques, various nanostructures of ZnO such as
Nanowires, Nanorods, Nanocombs, Nanorings, Nanobows, Nanoparticles,
Nanobelts, and Nanocages have been synthesized under specific growth
conditions
These unique nanostructures unambiguously demonstrate that ZnO is probably
the richest family of nanostructures among all known materials, both in
structures and properties
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Different types of ZnO Nanostructure
(a)
(c)
(b)
(d)
Fig.1 SEM image of different nanostructure (a) Nanoparticles (b) Nanowires (c) Nanotubes (d) Nanorods
Therefore, accurate knowledge of the structural and optical properties of the ZnO
nanostructures is essential for exploring the various possible applications of the material.
1. Wang, J. L., "Nanostructures of ZnO," Mater Today 7(6), 26-33 (2004)
ZnO vs. GaN
Table. 1 Comparison of the basic semiconductor properties of ZnO compared to those of GaN
Semiconductor
ZnO
GaN
Band gap (300K)
3.37eV
3.40V
Crystal structure
wurtzite
wurtzite
Lattice constants
a = 0.3250 nm
c = 0.5205 nm
a = 0.3189 nm
c = 0.5185 nm
Electron mobility (cm2V-1s-1)
200
1000
Hole mobility (cm2V-1s-1)
10
5-50
Exciton binding energy
60 meV
25meV
Advantages of ZnO over GaN
Availability of wet chemical etching.*
The ability to grow high quality bulk single crystal substrates.*
Simpler, lower temperature growth of thin films.*
* Gyu-Chul Yi, Chunrui Wang and Won Il Park, Semicond. Sci. Technol. 20 (2005) S22–S34
7/20/2015
Experimental Details
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Experimental
Initially, n-Si substrate have been cleaned by using standard cleaning
procedure.
Cleaned Si substrate immediately put in the vacuum coating unit (model
12A4D of HINDVAC, India) for deposition of thin film of Sn metal (thickness =
50 nm ) which is working as seed layer on the substrate surface
The seed layer of Sn metal provides excellent nucleation sites for growth of
ZnO nanostructures on n-Si substrate
Finally, ZnO film of thickness ~ 300 nm deposited on Sn coated Si substrate by
thermal evaporation method
To improve crystallinity of ZnO thin film, annealing treatment is performed in
N2 gas atmosphere at 550 °C for duration of 30 minutes respectively.
Another film of ZnO on bare n-Si substrate also deposited to analyze the effect
of seed layer on morphology of ZnO thin film
Further, samples cool down to room temperature for further characterization
purpose
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Schematic representation
Fig. 2 Schematic process flow for synthesis of ZnO nanoparticles on Si substrate
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Results and Discussions
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Influence of Sn seed layer
Fig. 3 Typical SEM image of ZnO thin film (a) without (b) with Sn seed layer on n-Si substrate
by thermal evaporation method
Scanning electron microscope analysis
Fig 4. Different magnification Scanning electron microscope images (top view) of well crystallized ZnO
nanoparticles grown on Sn coated silicon substrate prepared by thermal evaporation method
Continue………….
It is observed from the Fig 4(a), that ZnO thin film directly deposited on bare nSi substrate produces ZnO nanocrystalline structure whereas, evaporation of
ZnO on Sn seed layer coated silicon substrate reveals nanoparticles structure
It could be mentioned here that the pre deposited metallic Sn seed layer
influences the morphology of as-grown ZnO nanostructures drastically
It provides nucleation seeds for growth of nanoparticles and also reduces
mismatching between lattice parameters of Si and ZnO
The as-grown ZnO nanoparticles are well crystallized, uniformly distributed,
with very high density relatively perpendicular to the substrate surface
The diameter of ZnO nanoparticles varies from 30 to 70 nm
Some of the ZnO nanoparticles are accumulated together to form bigger
particle
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X-Ray diffraction pattern
Fig. 5 Typical X-ray diffraction pattern of ZnO nanoparticles on Sn-Coated Silicon substrate
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Continue………….
The crystallinity of the ZnO nanoparticles grown on silicon substrate have been
investigated by X-ray diffraction (XRD) analysis as shown in Fig. 5
All the observed diffraction peaks (002), (100) and (101) of ZnO are well
matched with JCPDS data card no- 36-1451
In addition, the (220) peak belongs to Sn metal (JCPDS card no.05-0390) are
also observed in the XRD pattern due to presence of Sn metal in the seed layer
The crystalline size of ZnO nanoparticles is calculated from the full width at
half maximum (FWHM) value of the (002) peak by using the Debye-Scherer
formula
D
0.9λ
β cosθ
The crystallite size of ZnO nanoparticles is found to be 42.92 nm. This is fairly
matched with SEM results.
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Photoluminescence spectrum
Fig. 6 Room temperature photoluminescence spectrum of ZnO nanoparticles
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Continue….
The leading region in the PL spectrum consists of intense UV emission at the
wavelength of 355 nm (~3.49 eV)
In this PL spectrum, the broadening from ~ 371 to ~ 550 nm are also observed
which is attributed to the near band edge (NBE) emission due to presence of
different defect states such as zinc interstitial (Zni), oxygen vacancies (VO) etc in
the band gap of ZnO
Fei Li et al.2 have reported in their work, that the blue peaks at 448 and 461
nm come from radiative recombination of an electron occupying shallow
donor level and a hole in the top of valence band
Vanheusden et al.3 have concluded that, the singly ionized oxygen vacancy is
responsible for the green emission in ZnO and this emission is resulted from
the recombination of a photogenerated hole with the singly ionized charge
state of this defect
2. Li. F, Li, Z., and Jin F., Physica B 403, 664–669 (2008)
3. Vanheusden, K., Warren, W. L., Seager, C. H., Tallant, D. R., Voigt, J. A., and Gnade, B., E., J. Appl.
Phys. 79 (10), 7983-7990 (1996).
Raman spectrum
Fig.7 Room temperature Raman spectrum of ZnO nanoparticles grown by thermal evaporation method
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Continue…..
The presence of a strong, dominated and sharp optical phonon mode E2 at 437
cm−1 (as compared to other peaks except silicon substrate (~517 cm-1) confirms
that the formed nanoparticles are the pure hexagonal phase of ZnO.
In addition, two observed weak peaks at 380.4 and 581.9 cm−1 correspond to
the A1T and E1L modes of the ZnO4, 5.
The appearance of E1L mode is due the presence of some structural defects in
the band gap of ZnO4, 5.
The higher intensity and narrower spectral width of the dominant E2 mode at
437 cm−1 in the spectrum also indicate that the grown ZnO nanoparticles are
wurtzite hexagonal phase with very good crystal quality
4. Djurisˇic´, A. B. and Leung, Y. H. Small 2(8), 944-961 (2006).
5. Talam, S., Karumuri, S. R. and Gunnam N., ISRN Nanotechnology 372505, 6, (2012).
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Conclusion
Successful synthesis of ZnO nanoparticles on Sn coated n-Si substrate by low cost
thermal evaporation method is presented in this paper
Detailed structural investigation confirms that as-grown ZnO nanoparticles
exhibit high crystallinity with hexagonal wurtzite phase of ZnO with preferential
growth direction along (002)
The Photoluminescence (PL) spectrum shows a strong UV emission at
wavelength of 355 nm due to excitonic transition between valance bands and
conduction bands
The Raman spectroscopy measurement has been carried out for ZnO
nanoparticles to show the presence of E2 mode at 437 cm-1 of ZnO
The results demonstrate that the simple and low-cost thermal evaporation
technique can be well explored for the growth of high-quality uniformly
distributed ZnO nanoparticles on Si substrates with a pre-coated thin Sn seedlayer
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Future Work
Pd/ZnO NPs schottky Ultraviolet photodetector
References
 Wang, J. L., "Nanostructures of ZnO," Mater Today 7(6), 26-33 (2004).
 Li. F, Li, Z., and Jin F., “Fabrication and characterization of ZnO micro and nanostructures
prepared by thermal evaporation,” Physica B 403, 664–669 (2008).

Vanheusden, K., Warren, W. L., Seager, C. H., Tallant, D. R., Voigt, J. A., and Gnade, B., E.,”
Mechanisms behind green photoluminescence in ZnO phosphor powders,” J. Appl. Phys. 79
(10), 7983-7990 (1996).
 Djurisˇic´, A. B. and Leung, Y. H., "Optical Properties of ZnO nanostructures," Small 2(8), 944-961
(2006).
 Talam, S., Karumuri, S. R. and Gunnam N., "Synthesis, characterization and spectroscopic
properties of ZnO nanoparticles," ISRN Nanotechnology 372505, 6, (2012).
 Mende, L. S. and Judith, L. M.-D., "ZnO-nanostructures, defects and devices," Mater Today
10(5), 40-48 (2007).
 Liu, X., Wu, X., Cao, H. and Chang, R. P. H., "Growth mechanism and properties of ZnO nanorods
synthesized
(2004).
by plasma-enhanced chemical vapor deposition," J. Appl. Phys. 95(6), 3141-3147
 Shen, W. J., Wang, J., Wang, Q. Y., Duan, Y., and Zeng, Y. P., "Structural and optical properties of
ZnO films on Si substrates using a γ-Al2O3 buffer layer," J. Phys. D: Appl. Phys. 39 269-273
(2006).
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