Outline

1. Synthesis and Optical Properties of Zinc Oxide Nanoparticles grown on Sn-coated Silicon Substrate by Thermal Evaporation Method

2. Outline • Introduction • ZnO Vs. GaN • Experimental details • Results and discussion • Surface morphology • Crystalline structure • Optical properties • Photoluminescence spectrum • Raman spectrum • Conclusion • Future work • Applications (ICCESD-2013)

3. 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 Nanocageshave 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 (ICCESD-2013)

4. Different types of ZnO Nanostructure (a) (b) (d) (c) 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)

5. ZnO vs. GaN Table. 1 Comparison of the basic semiconductor properties of ZnO compared to those of GaN • 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

6. Experimental Details (ICCESD-2013)

7. 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 (ICCESD-2013)

8. Schematic representation Fig. 2 Schematic process flow for synthesis of ZnO nanoparticles on Si substrate (ICCESD-2013)

9. Results and Discussions (ICCESD-2013)

10. 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

11. 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

12. Continue…………. • It is observed from the Fig 4(a), that ZnO thin film directly deposited on bare n-Si 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 (ICCESD-2013)

13. X-Ray diffraction pattern Fig. 5 Typical X-ray diffraction pattern of ZnO nanoparticles on Sn-Coated Silicon substrate (ICCESD-2013)

14. 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 • The crystallite size of ZnO nanoparticles is found to be 42.92 nm. This is fairly matched with SEM results. (ICCESD-2013)

15. Photoluminescence spectrum Fig. 6 Room temperature photoluminescence spectrum of ZnO nanoparticles (ICCESD-2013)

16. 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.2have 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.3have 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).

17. Raman spectrum Fig.7 Room temperature Raman spectrum of ZnO nanoparticles grown by thermal evaporation method (ICCESD-2013)

18. 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−1correspond to the A1T and E1L modes of the ZnO4, 5. • The appearance of E1Lmode 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−1in 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). (ICCESD-2013)

19. 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 seed-layer (ICCESD-2013)

20. Future Work Pd/ZnO NPs schottky Ultraviolet photodetector

21. 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 by plasma-enhanced chemical vapor deposition," J. Appl. Phys. 95(6), 3141-3147 (2004). • 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). (ICCESD-2013)

22. (ICCESD-2013)