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Morphology and growth mechanism of aligned ZnO nanorods grown by catalyst-free MOCVD
ARTICLE IN PRESS Microelectronics Journal 40 (2009) 242–245
Contents lists available at ScienceDirect
Microelectronics Journal journal homepage: www.elsevier.com/locate/mejo
Morphology and growth mechanism of aligned ZnO nanorods grown by catalyst-free MOCVD M. Rosina a,, P. Ferret a, P.-H. Jouneau b, I.-C. Robin a, F. Levy a, G. Feuillet a, M. Lafossas a a b
CEA–LETI, Minatec, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France CEA–INAC, SP2M/LEMMA, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France
a r t i c l e in f o
a b s t r a c t
Available online 30 August 2008
We report on ZnO nanorods grown by catalyst-free metal-organic chemical vapour deposition (MOCVD) in a commercial Epigress reactor using diethylzinc and N2O as precursors. Well-aligned ZnO nanorods with uniform diameter, length and density have been grown perpendicularly to the sapphire (0 0 0 1) surface. Scanning electron microscopy (SEM) has been used to observe the morphology of the ZnO nanorods and X-ray diffraction and transmission electron microscopy (TEM) to investigate the crystalline structure of the nanorods. TEM observation as well as photoluminescence measurements confirm the very good crystalline quality of the nanorods. SEM observation on samples that have been prepared with various deposition times has been used in order to investigate the growth mechanism. Three types of ZnO morphologies have been identified: a thin two-dimensional ZnO layer formed at the sapphire surface, covered by three-dimensional hexagonal-shaped islands and hexagonal nanorods on top of them. & 2008 Elsevier Ltd. All rights reserved.
Keywords: MOCVD Zinc oxide Nanorod Nanowire Growth
1. Introduction Having a direct band gap of 3.37 eV and a large exciton binding energy of 60 meV, ZnO could be used as a material for UV light sources [1]. One-dimensional (1D) ZnO structures such as nanorods are expected to have high crystalline quality and compatibility with a large range of substrates. A better understanding of the mechanism of nucleation and growth of these nanorods is therefore of growing importance. Due to its flexibility and industrial character a metal-organic chemical vapour deposition (MOCVD) is often used among various techniques for ZnO nanorods growth. In order to avoid a source of contamination, catalyst-free MOCVD is often preferred to catalytic MOCVD. Even though catalyst-free growth of ZnO nanorods has been used for many years [2], very few studies have focused on the nucleation process [3,4]. In the present work, we report on the growth of aligned ZnO nanorods on sapphire by catalyst-free MOCVD. We analyse their growth process mechanism by studying ZnO nanorods morphology at various growth stages.
sapphire substrates were annealed at 1200 1C in oxygen for 3 h prior to the deposition. Neither substrate patterning nor buffer layer has been used in this study. Diethylzinc (DEZn) and N2O were used as Zn and oxygen precursors, and Ar was used as a carrier gas for the DEZn source. The ZnO nanorods were grown at 750–800 1C while the total pressure in the deposition chamber was maintained at 100–150 mbar. Different deposition times were used in order to investigate the ZnO growth. The morphology of the samples has been investigated by scanning electron microscopy (SEM) and by high-resolution SEM using, respectively, Hitachi 4100 and Zeiss Ultra 55 microscope. Structural properties were investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). TEM investigations have been performed at 300 kV on a FEI Titan microscope fitted with a probe Cs-corrector, either by high-resolution scanning transmission microscopy (HR-STEM) and by classical high-resolution imaging. Samples were prepared by a direct transfer of the nanorods on holey carbon films, or as classical cross-sections by a mechanical polishing followed by Ar ion thinning at low voltage. The PL spectra of the ZnO nanorods have been measured using a frequency-doubled 244 nm cw Ar laser coupled with a 0.55 m monochromator.
2. Experimental ZnO nanorods were grown using catalyst-free MOCVD with a commercial horizontal hot-wall Epigress reactor. The c-plane Corresponding author.
E-mail address: [email protected] (M. Rosina). 0026-2692/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2008.07.019
3. Results We first report on the nanorods morphology before giving a tentative analysis of the growth process.
ARTICLE IN PRESS M. Rosina et al. / Microelectronics Journal 40 (2009) 242–245
3.1. ZnO nanorods morphology and properties The SEM results have shown that the well-aligned ZnO nanorods were growing vertically on the sapphire substrate with uniform density, length and diameter (Fig. 1(a)–(c)). These nanorods characteristics vary as a function of the growth conditions but each given sample presents a high uniformity in shape and size (with a typical length of 4 mm, a diameter of 200 nm and a density of 108 cm 2 after 60 min of growth). The crystal structure and orientation of the nanorods have been investigated by XRD. Fig. 1(d) shows the XRD Y/2Y scan demonstrating the preferential orientation of the nanorods along the ZnO [0 0 0 1] direction due to minimization of surface energy. Figs. 1(b, c) and Fig. 2 show a quite complex ZnO morphology as observed by SEM. Three types of morphologies have been identified: a thin wetting ZnO layer formed on the sapphire surface, hexagonal-shaped three-dimensional (3D) islands and nanorods. The nanorods are usually present on the top of these hexagonal pyramids. Fig. 2 displays the morphology of the perfect hexagonal symmetrical morphology of the ZnO nanorods. Fig. 2(b) shows the specific morphology of the nanorod tip, shaped in a threefold symmetrical pyramid on the hexagonal base. A granularlike surface of the nanorod tip is the artefact due to the electron beam impact. In fact, an increase of granularity was observed during observation at high magnification, due to the more pronounced impact of the electron beam focused on a small area. TEM observations (Fig. 2(d)) confirm the almost perfect crystalline quality of the nanorods, with no visible dislocation and no twin on a large number of rods. Filtered convergent beam electron diffraction (CBED) patterns (not shown) recorded along a /0 11¯ 0S direction proves the Zn polarity of the nanorods, with a growth direction along the [0 0 0 1] axis. The average measured angle of the top facets with respect to the c-axis is 10974.51
243
(standard deviation), which means that all the facets have not exactly the same inclination. Photoluminescence spectroscopy has been used for the optical characterization of the ZnO nanorods [5]. The green luminescence band usually observed for ZnO nanorods [6,7] is not observed on our spectra, demonstrating the low deepdefect density. At 4 K, the main contribution to the spectrum is a donor bound exciton (D0X) emitting at 3.36 eV, probably due to unintentional aluminium doping coming from the substrate. At 300 K, the maximum intensity is found at 3.235 eV and is due to the phonon replica of the free exciton.
3.2. ZnO nanorods growth In order to explore the growth mechanism process, several samples with various deposition times ranging from 1 min up to 2 h were prepared at 750 1C and the complex morphology of asgrown samples have been investigated using SEM. Nanorod characteristics were measured on the SEM images. Surprisingly, it has been observed that the 3D islands with nanorods on their top are already present after the first minute of growth, as shown in Fig. 3(a). These nanorods are relatively long (350 nm) and 20 nm thick. This is a clear evidence for a simultaneous growth of the nanorods and 3D islands, at least since very early growth stage. Therefore, assuming a small error, we could consider as a nanorod length the total length from the bottom base of the 3D islands up to the nanorod tip, as shown in the inset in Fig. 4. Graph in Fig. 4 shows an evolution of the nanorod length and diameter as a function of the deposition time. One can clearly see that the nanorods are growing axially and radially within the investigated time range. Vertical growth rate is slowly falling down with increasing deposition time, but it did not reach any saturation value. Lateral growth rate is much slower than vertical one and is
Fig. 1. SEM images: (a) top-view, (b) 451-tilted view, (c) cross-view and (d) XRD pattern of ZnO nanorods on sapphire substrate.
ARTICLE IN PRESS 244
M. Rosina et al. / Microelectronics Journal 40 (2009) 242–245
Fig. 2. (a) SEM image of the ZnO nanorods, showing the morphology of the hexagonal pyramids with nanorods on their top, (b) nanorod tip, (c) top-view SEM image of a hexagonal 3D island that serves as a base for nanorod in its centre and (d) TEM image of a single nanorod showing its good crystallinity, with no visible dislocation or twin.
Fig. 3. (a) SEM cross-view image of ZnO nanorods with small islands at their bases after 1 min of growth; (b) SEM 601-tilted image of ZnO nanorods after 10 min of growth. Granular character of the ZnO layer on the sapphire surface is clearly seen as well as 3D islands with nanorods on their top.
almost constant within the whole investigated time range in our experimental conditions. The difference between vertical and lateral growth rate leads to the large aspect ratio of our nanorods.
All our samples exhibited a thin two-dimensional (2D) ZnO layer formed at the sapphire surface. Fig. 3(b) shows a granular character of this layer with the first 3D islands, some of them with
ARTICLE IN PRESS M. Rosina et al. / Microelectronics Journal 40 (2009) 242–245
245
Fig. 4. Length and diameter of the ZnO nanorods plotted as a function of the deposition time, with squares for the vertical growth and circles for the lateral growth. The length of each nanorod was determined as a total length of the nanorod together with the height of corresponding island, as shown in the inset. The lines are a guide to the eye.
nanorods on their top. Exact origin of the nucleation sites for these islands and nanorods is unknown and still under investigation. Given our experimental observation, we have deduced the following possible mechanism for the ZnO nanorod growth: first, the formation of an island with a nanorod on its top takes place. We point out that the nanorod is formed almost simultaneously with the 3D island. Second, neighbouring islands without nanorods coalesce progressively with the central one that continues to grow both vertically and laterally and tends to dominate. The formation of the large islands is favoured by an enhanced diffusion of Zn and O species on the substrate due to the high growth temperature (750 1C). Fig. 2(c) shows a result of such a process, a large ‘‘multi-legs’’ island with a hexagonal base and a nanorod located in its centre. In the later stages, the space between the islands becomes progressively filled with a thick rough 2D layer in which other small pyramidal islands are built up. On such islands further nanorods can be formed, resulting in increased density of nanorods after long deposition times. In conclusion, defect-free aligned ZnO nanorods with uniform length and diameter distribution and high optical quality were synthesized on (0 0 0 1) sapphire using MOCVD. A mechanism of
the structural evolution of the nanorods during their growth since very early growth stages was proposed.
Acknowledgements Authors greatly acknowledge fruitful discussions with Pierre Desre´ from CEA. This work was supported by the French National Research Agency (ANR) through Carnot funding. References [1] J. Bao, M.A. Zimmler, F. Capasso, X. Wang, Z.F. Ren, Nano Lett. 6 (2006) 1719. [2] W.I. Park, D.H. Kim, S.-W. Jung, Gyu-Chu Yi, Appl. Phys. Lett. 80 (2001) 4232. [3] G.W. Cong, H.Y. Wei, P.F. Zhang, W.Q. Peng, J.J. Wu, X.L. Liu, C.M. Jiao, W.G. Hu, Q.S. Zhu, Z.G. Wang, Appl. Phys. Lett. 87 (2005) 231903. [4] S.-H. Park, S.-Y. Seo, S.-H. Kim, S.-W. Han, Appl. Phys. Lett. 88 (2006) 251903. [5] I.C. Robin, B. Gauron, P. Ferret, C. Tavares, G. Feuillet, Le Si Dang, B. Gayral, J.M. Ge´rard, Appl. Phys. Lett. 91 (2007) 143120. [6] B. Xiang, P. Wang, X. Zhang, S.A. Dayeh, D.P.R. Aplin, C. Soci, D. Yu, D. Wang, Nano Lett. 7 (2007) 323. [7] K.A. Jeon, H.J. Son, C.E. Kim, J.H. Kim, S.Y. Lee, Physica E 37 (2007) 222.
Contents lists available at ScienceDirect
Microelectronics Journal journal homepage: www.elsevier.com/locate/mejo
Morphology and growth mechanism of aligned ZnO nanorods grown by catalyst-free MOCVD M. Rosina a,, P. Ferret a, P.-H. Jouneau b, I.-C. Robin a, F. Levy a, G. Feuillet a, M. Lafossas a a b
CEA–LETI, Minatec, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France CEA–INAC, SP2M/LEMMA, 17 rue des Martyrs, 38054 Grenoble Cedex 9, France
a r t i c l e in f o
a b s t r a c t
Available online 30 August 2008
We report on ZnO nanorods grown by catalyst-free metal-organic chemical vapour deposition (MOCVD) in a commercial Epigress reactor using diethylzinc and N2O as precursors. Well-aligned ZnO nanorods with uniform diameter, length and density have been grown perpendicularly to the sapphire (0 0 0 1) surface. Scanning electron microscopy (SEM) has been used to observe the morphology of the ZnO nanorods and X-ray diffraction and transmission electron microscopy (TEM) to investigate the crystalline structure of the nanorods. TEM observation as well as photoluminescence measurements confirm the very good crystalline quality of the nanorods. SEM observation on samples that have been prepared with various deposition times has been used in order to investigate the growth mechanism. Three types of ZnO morphologies have been identified: a thin two-dimensional ZnO layer formed at the sapphire surface, covered by three-dimensional hexagonal-shaped islands and hexagonal nanorods on top of them. & 2008 Elsevier Ltd. All rights reserved.
Keywords: MOCVD Zinc oxide Nanorod Nanowire Growth
1. Introduction Having a direct band gap of 3.37 eV and a large exciton binding energy of 60 meV, ZnO could be used as a material for UV light sources [1]. One-dimensional (1D) ZnO structures such as nanorods are expected to have high crystalline quality and compatibility with a large range of substrates. A better understanding of the mechanism of nucleation and growth of these nanorods is therefore of growing importance. Due to its flexibility and industrial character a metal-organic chemical vapour deposition (MOCVD) is often used among various techniques for ZnO nanorods growth. In order to avoid a source of contamination, catalyst-free MOCVD is often preferred to catalytic MOCVD. Even though catalyst-free growth of ZnO nanorods has been used for many years [2], very few studies have focused on the nucleation process [3,4]. In the present work, we report on the growth of aligned ZnO nanorods on sapphire by catalyst-free MOCVD. We analyse their growth process mechanism by studying ZnO nanorods morphology at various growth stages.
sapphire substrates were annealed at 1200 1C in oxygen for 3 h prior to the deposition. Neither substrate patterning nor buffer layer has been used in this study. Diethylzinc (DEZn) and N2O were used as Zn and oxygen precursors, and Ar was used as a carrier gas for the DEZn source. The ZnO nanorods were grown at 750–800 1C while the total pressure in the deposition chamber was maintained at 100–150 mbar. Different deposition times were used in order to investigate the ZnO growth. The morphology of the samples has been investigated by scanning electron microscopy (SEM) and by high-resolution SEM using, respectively, Hitachi 4100 and Zeiss Ultra 55 microscope. Structural properties were investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). TEM investigations have been performed at 300 kV on a FEI Titan microscope fitted with a probe Cs-corrector, either by high-resolution scanning transmission microscopy (HR-STEM) and by classical high-resolution imaging. Samples were prepared by a direct transfer of the nanorods on holey carbon films, or as classical cross-sections by a mechanical polishing followed by Ar ion thinning at low voltage. The PL spectra of the ZnO nanorods have been measured using a frequency-doubled 244 nm cw Ar laser coupled with a 0.55 m monochromator.
2. Experimental ZnO nanorods were grown using catalyst-free MOCVD with a commercial horizontal hot-wall Epigress reactor. The c-plane Corresponding author.
E-mail address: [email protected] (M. Rosina). 0026-2692/$ - see front matter & 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.mejo.2008.07.019
3. Results We first report on the nanorods morphology before giving a tentative analysis of the growth process.
ARTICLE IN PRESS M. Rosina et al. / Microelectronics Journal 40 (2009) 242–245
3.1. ZnO nanorods morphology and properties The SEM results have shown that the well-aligned ZnO nanorods were growing vertically on the sapphire substrate with uniform density, length and diameter (Fig. 1(a)–(c)). These nanorods characteristics vary as a function of the growth conditions but each given sample presents a high uniformity in shape and size (with a typical length of 4 mm, a diameter of 200 nm and a density of 108 cm 2 after 60 min of growth). The crystal structure and orientation of the nanorods have been investigated by XRD. Fig. 1(d) shows the XRD Y/2Y scan demonstrating the preferential orientation of the nanorods along the ZnO [0 0 0 1] direction due to minimization of surface energy. Figs. 1(b, c) and Fig. 2 show a quite complex ZnO morphology as observed by SEM. Three types of morphologies have been identified: a thin wetting ZnO layer formed on the sapphire surface, hexagonal-shaped three-dimensional (3D) islands and nanorods. The nanorods are usually present on the top of these hexagonal pyramids. Fig. 2 displays the morphology of the perfect hexagonal symmetrical morphology of the ZnO nanorods. Fig. 2(b) shows the specific morphology of the nanorod tip, shaped in a threefold symmetrical pyramid on the hexagonal base. A granularlike surface of the nanorod tip is the artefact due to the electron beam impact. In fact, an increase of granularity was observed during observation at high magnification, due to the more pronounced impact of the electron beam focused on a small area. TEM observations (Fig. 2(d)) confirm the almost perfect crystalline quality of the nanorods, with no visible dislocation and no twin on a large number of rods. Filtered convergent beam electron diffraction (CBED) patterns (not shown) recorded along a /0 11¯ 0S direction proves the Zn polarity of the nanorods, with a growth direction along the [0 0 0 1] axis. The average measured angle of the top facets with respect to the c-axis is 10974.51
243
(standard deviation), which means that all the facets have not exactly the same inclination. Photoluminescence spectroscopy has been used for the optical characterization of the ZnO nanorods [5]. The green luminescence band usually observed for ZnO nanorods [6,7] is not observed on our spectra, demonstrating the low deepdefect density. At 4 K, the main contribution to the spectrum is a donor bound exciton (D0X) emitting at 3.36 eV, probably due to unintentional aluminium doping coming from the substrate. At 300 K, the maximum intensity is found at 3.235 eV and is due to the phonon replica of the free exciton.
3.2. ZnO nanorods growth In order to explore the growth mechanism process, several samples with various deposition times ranging from 1 min up to 2 h were prepared at 750 1C and the complex morphology of asgrown samples have been investigated using SEM. Nanorod characteristics were measured on the SEM images. Surprisingly, it has been observed that the 3D islands with nanorods on their top are already present after the first minute of growth, as shown in Fig. 3(a). These nanorods are relatively long (350 nm) and 20 nm thick. This is a clear evidence for a simultaneous growth of the nanorods and 3D islands, at least since very early growth stage. Therefore, assuming a small error, we could consider as a nanorod length the total length from the bottom base of the 3D islands up to the nanorod tip, as shown in the inset in Fig. 4. Graph in Fig. 4 shows an evolution of the nanorod length and diameter as a function of the deposition time. One can clearly see that the nanorods are growing axially and radially within the investigated time range. Vertical growth rate is slowly falling down with increasing deposition time, but it did not reach any saturation value. Lateral growth rate is much slower than vertical one and is
Fig. 1. SEM images: (a) top-view, (b) 451-tilted view, (c) cross-view and (d) XRD pattern of ZnO nanorods on sapphire substrate.
ARTICLE IN PRESS 244
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Fig. 2. (a) SEM image of the ZnO nanorods, showing the morphology of the hexagonal pyramids with nanorods on their top, (b) nanorod tip, (c) top-view SEM image of a hexagonal 3D island that serves as a base for nanorod in its centre and (d) TEM image of a single nanorod showing its good crystallinity, with no visible dislocation or twin.
Fig. 3. (a) SEM cross-view image of ZnO nanorods with small islands at their bases after 1 min of growth; (b) SEM 601-tilted image of ZnO nanorods after 10 min of growth. Granular character of the ZnO layer on the sapphire surface is clearly seen as well as 3D islands with nanorods on their top.
almost constant within the whole investigated time range in our experimental conditions. The difference between vertical and lateral growth rate leads to the large aspect ratio of our nanorods.
All our samples exhibited a thin two-dimensional (2D) ZnO layer formed at the sapphire surface. Fig. 3(b) shows a granular character of this layer with the first 3D islands, some of them with
ARTICLE IN PRESS M. Rosina et al. / Microelectronics Journal 40 (2009) 242–245
245
Fig. 4. Length and diameter of the ZnO nanorods plotted as a function of the deposition time, with squares for the vertical growth and circles for the lateral growth. The length of each nanorod was determined as a total length of the nanorod together with the height of corresponding island, as shown in the inset. The lines are a guide to the eye.
nanorods on their top. Exact origin of the nucleation sites for these islands and nanorods is unknown and still under investigation. Given our experimental observation, we have deduced the following possible mechanism for the ZnO nanorod growth: first, the formation of an island with a nanorod on its top takes place. We point out that the nanorod is formed almost simultaneously with the 3D island. Second, neighbouring islands without nanorods coalesce progressively with the central one that continues to grow both vertically and laterally and tends to dominate. The formation of the large islands is favoured by an enhanced diffusion of Zn and O species on the substrate due to the high growth temperature (750 1C). Fig. 2(c) shows a result of such a process, a large ‘‘multi-legs’’ island with a hexagonal base and a nanorod located in its centre. In the later stages, the space between the islands becomes progressively filled with a thick rough 2D layer in which other small pyramidal islands are built up. On such islands further nanorods can be formed, resulting in increased density of nanorods after long deposition times. In conclusion, defect-free aligned ZnO nanorods with uniform length and diameter distribution and high optical quality were synthesized on (0 0 0 1) sapphire using MOCVD. A mechanism of
the structural evolution of the nanorods during their growth since very early growth stages was proposed.
Acknowledgements Authors greatly acknowledge fruitful discussions with Pierre Desre´ from CEA. This work was supported by the French National Research Agency (ANR) through Carnot funding. References [1] J. Bao, M.A. Zimmler, F. Capasso, X. Wang, Z.F. Ren, Nano Lett. 6 (2006) 1719. [2] W.I. Park, D.H. Kim, S.-W. Jung, Gyu-Chu Yi, Appl. Phys. Lett. 80 (2001) 4232. [3] G.W. Cong, H.Y. Wei, P.F. Zhang, W.Q. Peng, J.J. Wu, X.L. Liu, C.M. Jiao, W.G. Hu, Q.S. Zhu, Z.G. Wang, Appl. Phys. Lett. 87 (2005) 231903. [4] S.-H. Park, S.-Y. Seo, S.-H. Kim, S.-W. Han, Appl. Phys. Lett. 88 (2006) 251903. [5] I.C. Robin, B. Gauron, P. Ferret, C. Tavares, G. Feuillet, Le Si Dang, B. Gayral, J.M. Ge´rard, Appl. Phys. Lett. 91 (2007) 143120. [6] B. Xiang, P. Wang, X. Zhang, S.A. Dayeh, D.P.R. Aplin, C. Soci, D. Yu, D. Wang, Nano Lett. 7 (2007) 323. [7] K.A. Jeon, H.J. Son, C.E. Kim, J.H. Kim, S.Y. Lee, Physica E 37 (2007) 222.