Origin Of Photo Luminescence In Indium Oxide Ron Structures

APPLIED PHYSICS LETTERS 92, 171907 共2008兲

On the origin of photoluminescence in indium oxide octahedron structures Mukesh Kumar,1 V. N. Singh,1 F. Singh,2 K. V. Lakshmi,3 B. R. Mehta,1,a兲 and J. P. Singh1,a兲 1

Department of Physics, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India Inter University Accelerator Centre, Aruna Asaf Ali Marg, New Delhi 110067, India 3 Department of Chemistry and Chemical Biology, Rensselaer Polytechnic Institute, 110 8th Street, Troy, New York 12180, USA 2

共Received 4 February 2008; accepted 25 March 2008; published online 1 May 2008兲 A sixfold decrease in photoluminescence signal intensity at 590 nm with increase in deposition time from 3 to 12 h has been observed in single crystalline indium oxide octahedron structures grown by vapor-phase evaporation method. Electron paramagnetic resonance and energy dispersive x-ray analysis confirm that the concentration of oxygen vacancies increases with deposition time. These results are contrary to the previous reports where oxygen vacancies were shown to be responsible for photoluminescence in indium oxide structures. Our results indicate that indium interstitials and their associated complex defects other than oxygen vacancies are responsible for the photoluminescence in In2O3 microstructures. © 2008 American Institute of Physics. 关DOI: 10.1063/1.2910501兴 The strong photoluminescence 共PL兲 of indium oxide 共IO兲 in the visible and ultraviolet range along with its chemical inertness and heat stability makes it an important material for optical device applications. Although the PL has been reported in IO thin film and nanostructures, the mechanism is still not clear.1–6 Guha et al. reported PL emission from IO nanopyramids and nanocolumns at a wavelength of about 470 nm.1 Whereas, Cao et al.2 and Wu et al.3 have observed PL emission at 398 and 416 nm from IO nanowires and at 480 and 520 nm from IO nanoparticles.4 In some studies, In2O3 samples exhibiting strong PL have been observed to show electron paramagnetic resonance 共EPR兲 signal due to paramagnetic nature of the oxygen vacancies.1,7 This has lead to an assumption that oxygen vacancies are responsible for PL in IO nanoparticles, nanopyramids, and nanocolumns.1–7 However, no systematic study showing the effect of variation of oxygen vacancy concentration on EPR and PL signal strength in the case of In2O3 has been reported. Nevertheless, such a study is really essential to understand the role of oxygen vacancies and other defects for PL signal in IO. In this letter, a number of experimental techniques such as energy dispersive x-ray 共EDX兲 analysis, microRaman spectroscopy, along with EPR spectroscopy have been employed to understand the origin of PL in IO octahedron structures. The IO octahedron structures were grown using a horizontal tube furnace maintained at a temperature of 960 ° C and one atmosphere pressure. An alumina boat with a 1:1 mixture of IO and active carbon powder was placed at the center of the tube furnace. The p-Si共100兲 substrates were positioned exactly above the alumina boat. The reaction was carried out under constant Ar flow rate of 100 ml/ min. The deposition was carried out for 3 h 共sample A兲, 6 h 共sample B兲, and 12 h 共sample C兲. These samples are characterized by glancing angle x-ray diffraction 共GAXRD兲, scanning electron microscopy 共SEM兲 共Ziess EVO 50兲, and high resolution a兲

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transmission electron microscopy 共HRTEM兲 共Tecnai G20Stwin at 200 kV兲. The PL spectrum was recorded at room temperature using a He–Cd laser with an excitation source wavelength of 325 nm. The measurement conditions were kept identical for all the three samples. The EPR spectroscopy measurements were performed on an X-band Elexsys 500 EPR spectrometer 共Bruker Instrument兲 operating at a frequency of 9.383 GHz, equipped with a TE102 cavity. The EPR measurements were carried out at room temperature to analyze the presence of paramagnetic oxygen vacancies in the IO octahedron samples. Typically, four scans were collected per spectrum under nonsaturating conditions at a microwave power of 10 mW. The spectra were acquired at a modulation frequency of 100 kHz with modulation amplitude of 1 G. The center field of the EPR scans was set to 3350 G with a sweep width of 100 G. Raman spectroscopy of the IO octahedron samples was carried out under ambient conditions by Renishaw micro-Raman setup having Ar ion laser with an excitation wavelength of 514.5 nm and a spot size of about 1.7 ␮m. GAXRD studies 共not shown here兲 of samples A, B, and C show the presence of cubic In2O3 with a lattice constant of 1.011 nm. The scanning electron micrographs of samples A and C are shown in Figs. 1共a兲 and 1共b兲. Samples A and C contain octahedron structures of about 1 and 2 ␮m sizes, respectively. The octahedrons have sharp facets with truncated tips, as shown in the inset of Figs. 1共a兲 and 1共b兲. The HRTEM images in Figs. 1共c兲 and 1共d兲 show the single crystalline nature of IO octahedrons. The lattice spacing of samples A and C with of 0.29 and 0.71 nm corresponds to 共222兲 and 共110兲 planes of cubic IO, respectively. Similar octahedrons structures with smooth surfaces and single crystalline nature have been previously reported.8 Figure 2共a兲 shows PL spectra of IO octahedron samples. The PL is centered at 590 nm for all the three samples. It is interesting to note that the intensity of PL emission decreases by a factor of about 6 as the deposition time increases from 3 h 共sample A兲 to 12 h 共sample C兲. The sample grown by 6 h deposition time 共sample B兲 has PL intensity in between samples A and C. To investigate the effect of growth time on the stoichiom-

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FIG. 1. SEM micrographs of 共a兲 sample A and 共b兲 sample C. Both samples A and C contain truncated octahedron structures. The inset show high magnified images. 共c兲 and 共d兲 show the high resolution TEM of samples A and C, respectively with their TEM micrographs in the inset.

etry of IO samples, spot EDX measurements were performed over individual IO octahedrons. The EDX analysis reveals that the sample A has an indium to oxygen 共In/ O兲 ratio of 38:60 which is in close agreement to the stoichiometric In/ O ratio of 40:60. While sample C is found to be oxygen deficient with In/ O ratio of 38:52. The spot EDX measurements were performed with a reduced beam size and low accelerating potential to eliminate any possible interference in oxygen signal from the native oxide on silicon substrate. Raman spectroscopy was also performed to investigate the change in the stoichiometry with deposition time and the results are shown in Fig. 2共b兲. Samples A and C exhibit strong Raman

Appl. Phys. Lett. 92, 171907 共2008兲

shifts at 133.3, 307.9, 366.9, and 496.9 cm−1. The Raman peak observed at 307.9 cm−1 is symmetric in sample A whereas as this peak is asymmetric in sample C. This asymmetric peak in sample C is deconvoluted and the lower intensity Raman peak at 321.2 cm−1 can be ascribed to quantum confinement effect or the nonstoichiometry in the sample.9,10 Zuo et al. have reported an additional Raman peak due to quantum confinement in tin oxide nanoparticles9 while, in an another report, Mcguire et al. pointed out nonstoichiometry responsible for the same.10 It is important to notice that the size of the observed IO octahedron structures 共typical Bohr radius of In2O3 is 2.14 nm兲 are too big to observe any significant quantum confinement effects. The Raman peak at 307.9 cm−1 corresponds to the stretching mode of the IO and is very sensitive to presence of oxygen vacancies.11 Thus, the observed asymmetry of Raman mode at 307.9 cm−1 in sample C has been attributed to the oxygen vacancies. The change in stoichiometry in IO octahedron samples with deposition time is explained as follows. The precursor IO powder is carbothermally reduced into In and InxO 共x = 1,2兲 vapor at 960 ° C. The vapor species are driven by argon gas and get deposited on the silicon substrate.3 With the passage of deposition time, IO powder in the source boat and thus the InxO in vapor phase become oxygen deficient. Therefore, continuous deposition for 12 h results in oxygen deficiency in sample C compared to sample A which was deposited for 3 h. Hao et al. reported that lengthening in deposition time leads to the oxygen deficiency during the growth process.8 To probe the defects responsible for PL emission in IO octahedron samples, EPR spectroscopy was performed on samples A and C at room temperature. The results are shown in Fig. 2共c兲. It is evident that no EPR signal is observed in sample A while sample C showed a strong EPR signal. The absence of EPR signal in sample A indicates a complete absence of paramagnetic centers. In contrast, the strong EPR signal in sample C indicates the presence of paramagnetic · 兲, incenters such as singly charged oxygen vacancies 共VO 2+ dium interstitial in doublet charged state 关Ini 兴, and other complex defects. In the earlier reports, the presence of EPR signal was universally taken as a signature of oxygen vacancies, which was attributed to be the reason for the observation of PL in IO.1,7 Besides the oxygen vacancies, the presence of other defects has also been reported in IO.12,13 Gurlo et al. reported three types of paramagnetic centers in IO films.13 Also, the impurities and point defects could form different complexes and associates. Thus, the EPR signal may arise due to the presence of several types of defects enumerated above and oxygen vacancies is one of these. It should be mentioned that the PL intensity is observed to be significantly lower in sample C, having larger number of oxygen vacancies in comparison to sample A, as confirmed by EDX and EPR results. Since increase in the deposition time leads to an increase in the oxygen vacancies, the PL intensity results do not seem to follow the common trend and so our results are in contradiction with the previous reports where oxygen vacancies were shown to be responsible for PL emission.1,7 There are other defects like oxygen antisite 共OIn兲, indium interstitial 共Ini兲, oxygen interstitial 共Oi兲, and indium vacancies 共VIn兲 which may be responsible for PL emission in IO.14 The large diameter of oxygen atom 共1.38 Å兲 with respect to the cell volume reduces the possibility of forming the Oi.15 The indium interstitial is found to

FIG. 2. 共Color online兲 共a兲 PL spectra of IO samples deposited for different time. Sample A has six times higher PL intensity than the sample C without any peak shift. 共b兲 Raman spectra of IO samples. Raman shift at 307.95 cm−1 is sensitive to the oxygen vacancies and more asymmetric in the sample C. Inset: representative Raman spectra of sample C. 共c兲 The EPR spectra of samples A and C. Sample A does not show any EPR signal while sample C shows a strong EPR signal. 共d兲 PL spectra of sample C heated at 700 ° C for 30 min in presence of a constant electric field of 200 V / cm. The PL intensity is found to increase at cathode side 共curve 1兲 while it decreases at anode side 共curve 2兲 with respect to the pristine sample A 共curve A兲. Inset shows the schematic of sample annealed in presence of an electric field. The A, B, and C in figures denoted the samples A, B, and C, respectively. Downloaded 04 May 2008 to 220.227.156.148. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

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be a dominant defect in IO and Ini stays in the triplet charged 12,16 10 It is interesting to notice that In3+ state 共In3+ i 兲. i 共关Kr兴4d 兲 is nonparamagnetic in nature and hence will not give an EPR signal. In order to confirm the hypothesis of In3+ i induced PL emission in IO, samples A and C were heated at 700 ° C for 30 min in presence of a constant electric field of 200 V / cm. The Ar gas flow was maintained during the experiment. The samples were thereafter cooled down to room temperature. The room temperature PL measurements were performed on the IO sample surface at positions near cathode and anode electrodes and results are shown in Fig. 2共d兲. The PL intensity on sample A increases by 1.3 times near cathode side 共curve 1兲 while PL intensity decreases by 3.5 times near anode side 共curve 2兲 compared to the PL intensity measured over the pristine sample A 共curve A兲. For pristine samples the PL intensity was found to be same measured over different positions on the sample surface. The variation of PL intensity from the sample surface adjacent to the cathode and anode electrodes is due to the electric field induced change in the defect density. Similar experiments on ZnO samples heated in the presence of electric field were performed to investigate the nature of defects associated with the PL emission from ZnO samples.17,18 It was concluded that the Zn interstitials were responsible for PL emission from ZnO. Bylander reported that the electronic transitions from interstitial metal to metal vacancies were responsible for the luminescence properties.19 The mobility of In3+ i increases during heating of the IO sample at 700 ° C and the presence of electric field provide a drift to In3+ i towards the cathode side. This results defect density on the IO sample surface in the higher In3+ i regions near cathode than the anode electrodes. The PL intensity is also found to be higher on the cathode side of the sample than the anode side. These experimental findings provide a direct evidence that In+3 i and their associated complex defects are responsible for PL emission in IO octahedron structures. The PL signal has not shown any appreciable change in the intensity on sample C measured at cathode and anode sides. The lengthening of the deposition time of 12 h for IO sample C leads to the oxygen deficiency during the growth process.8 The higher oxygen vacancy concentration

is confirmed by EPR and EDX results. This may cause a relatively much lower concentration of In3+ i defects in sample C than in sample A and hence a difference in the PL intensity at cathode and anode sides was not observed. In summary, a strong PL signal was observed from the sample A deposited for 3 h having lack of oxygen vacancies. This PL emission intensity was found to decrease by a factor of 6 with increase in deposition time from 3 to 12 h and hence increase in the oxygen vacancies. Our results indicate that In3+ i playing a significant role and control the optical properties of IO octahedron samples. P. Guha, S. Kar, and S. Choudhari, Appl. Phys. Lett. 85, 3851 共2004兲. H. Cao, X. Qiu, Y. Liang, Q. Zhu, and M. Zhao, Appl. Phys. Lett. 83, 761 共2003兲. 3 X. C. Wu, J. M. Hong, Z. J. Han, and Y. R. Tao, Chem. Phys. Lett. 373, 28 共2003兲. 4 H. Zhou, W. Cai, and L. Zhang, Appl. Phys. Lett. 75, 495 共1999兲. 5 W. S. Seo, H. H. Jo, K. Lee, and J. T. Park, Adv. Mater. 共Weinheim, Ger.兲 15, 795 共2003兲. 6 A. Murali, A. Barve, V. J. Leppert, S. H. Risbud, I. M. Kennedy, and H. W. H. Lee, Nano Lett. 1, 287 共2001兲. 7 M. J. Zheng, L. D. Zhang, G. H. Li, X. Y. Zhang, and X. F. Wang, Appl. Phys. Lett. 79, 839 共2001兲. 8 Y. F. Hao, G. W. Meng, C. H. Ye, and L. D. Zhang, Cryst. Growth Des. 5, 1617 共2005兲. 9 J. Zuo, C. Xu, X. Liu, C. Wang, C. Wang, Y. Hu, and Y. Qian, J. Appl. Phys. 75, 1835 共1994兲. 10 K. Mcguire, Z. W. Pan, D. Milkie, J. Menéndez, and A. M. Rao, J. Nanosci. Nanotechnol. 2, 499 共2002兲. 11 W. B. White and V. G. Keramidas, Spectrochim. Acta, Part A 28, 501 共1972兲. 12 J. H. W. De Wit, J. Solid State Chem. 8, 142 共1973兲. 13 A. Gurlo, M. Ivanovskaya, A. Pfau, U. Weimar, and W. Gopel, Thin Solid Films 307, 288 共1997兲. 14 M. Mazzera, M. Zha, D. Calestani, A. Zappettini, G. Salviati, and L. Zanotti, Nanotechnology 18, 355707 共2007兲. 15 A. A. Kaminskii, Laser Crystals 共Springer, Berlin, 1981兲, p. 457. 16 S. H. Lee, J. H. Lee, K. H. Kim, and J. H. Jun, Bull. Korean Chem. Soc. 10, 418 共1989兲. 17 N. O. Korsunska, L. V. Borkovska, B. M. Bulakh, L. Yu. Khomenkova, and V. I. Markevich, J. Lumin. 102-103, 733 共2003兲. 18 M. Liu, A. H. Kitia, and P. Mascher, J. Lumin. 54, 35 共1992兲. 19 E. G. Bylander, J. Appl. Phys. 49, 1188 共1978兲. 1 2

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