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Ultrathin aligned ZnO nanorod arrays grown by a novel diffusive pulsed laser deposition method
Chemical Physics Letters 479 (2009) 125–127
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Ultrathin aligned ZnO nanorod arrays grown by a novel diffusive pulsed laser deposition method Gareth M. Fuge *, Tobias M.S. Holmes, Michael N.R. Ashfold School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, UK
a r t i c l e
i n f o
Article history: Received 12 June 2009 In final form 4 August 2009 Available online 7 August 2009
a b s t r a c t ZnO nanorod (NR) arrays were grown on Si at elevated temperatures using a two-stage diffusive pulsed laser deposition (DPLD) technique. A thin (50 nm) seed-layer was first formed by pulsed laser ablation of ZnO in O2. The sample was then turned to face away from the propagation direction of the ablation plume, and the ablation process continued. A dense array of ultrathin NRs was seen to grow from the seed-layer. These NRs are thinner (d 10 nm), and display a 20-times higher aspect ratio than those grown by traditional PLD under otherwise identical process conditions. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction ZnO is a semiconductor material of great contemporary interest, not least because of its wide direct band gap (3.37 eV) and large exciton binding energy (60 meV). Potential uses of such a material include gas sensors, UV emitting diodes and UV sensitive photocells, the efficiencies of all of which benefit from a large surface area. Reported ZnO nanostructures include nanoribbons, nanowires, nanoclusters, nanorods (NRs) [1,2] and nanotubes [3]. The growth of aligned NR arrays is of particular interest, due to their high surface area, regularity and uniformity, and because of the possible quantum confinement effects (and enhanced luminescence efficiencies) exhibited by the individual NRs. ZnO NRs can be grown in a variety of ways including, thermal vapour phase transport [4], hydrothermal methods [5,6], microwave plasma enhanced chemical vapour deposition [7] and pulsed laser deposition (PLD) [8]. PLD offers the advantage of relatively slow growth, in a dry vacuum chamber, and can yield high quality arrays of aligned, single crystal NRs with low defect densities – without the need for any catalyst (which, inevitably, constitutes a potential impurity). ZnO NRs deposited in this way display diameters, d, in the range 50–500 nm and lengths, ‘, up to a few microns. Many parameters, e.g. the incident fluence, F [9], the target to substrate distance, D, the substrate temperature, Tsub, and the background pressure, p [10], all affect the product morphology and the diameters of the resulting NRs. Growth of thin NRs, with d 50 nm, is favoured by use of low F (just above the ablation threshold of the ZnO target), larger D and/or by positioning the substrate off the centre axis of the ablation plume [11]. The net effect of such * Corresponding author. Fax: +44 (0) 117 9250612. E-mail address: [email protected] (G.M. Fuge). 0009-2614/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2009.08.008
changes is to reduce the flux and/or kinetic energies of the ablated material incident on the substrate. The present study extends this philosophy further, by demonstrating growth of ultrathin NRs on a pre-seeded substrate surface that faces away from the laser plume. Deposition under such circumstances must rely on diffusive transport of ablated material to the growing surface and, as such, has some obvious parallels with growth by thermal vapour phase transport methods. 2. Experimental ZnO NR arrays were grown on Si(1 0 0) substrates using a twostage diffusive pulsed laser deposition (DPLD) method involving growth of an initial seed-layer using the traditional PLD geometry, then reversing the substrate and continuing growth on the seedlayer that is now facing away from the target and the plume propagation direction. The substrate was first cleaned by immersing in acetone in an ultrasonic bath and then washed with ethanol, and then mounted in the vacuum chamber as shown in Fig. 1a at D = 50 mm from the front surface of the ZnO target (Testbourne, 99.99% purity). The chamber was then evacuated to a base pressure of 1 10 6 Torr and subsequently back-filled with a steady (10 sccm) flow of oxygen, so as to maintain pO2 = 10 mTorr. A 250 W tungsten halogen quartz bulb served as an efficient and contaminant-free substrate heater, capable of maintaining Tsub to within 1 °C of any user-selected value for the duration of the deposition. The seed-layer was deposited using an ArF excimer laser (193 nm, Lambda-Physik Compex 201, operating at a repetition rate of 10 Hz), the output of which was steered and focused onto the ZnO target for, typically, t1 = 10 min at F1 4 J cm 2. The ZnO seed-layer coated substrate was then allowed to cool from Tsub,1 to room temperature in pO2 10 mTorr, before the
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Fig. 2a shows a FE–SEM image of a ZnO NR array grown by traditional PLD using the following conditions: Tsub = 600 °C, F 4 J cm 2, D = 50 mm, pO2 10 mTorr and t1 = 55 min. Fig. 2b–d, in contrast, show ZnO NR arrays grown by DPLD for t2 = 45 min using the above values for Tsub,2, F2, D and pO2, after pre-depositing
a ZnO seed-layer for t1 = 10 min at Tsub,1 values of, respectively, 25, 300 and 600 °C. Henceforth, the Tsub, F, D and pO2 conditions used to form the arrays shown in Fig. 2a and d will be termed the base conditions. All four images were taken at a 35° tilt to the normal to illustrate the NR alignments and thicknesses more clearly. The insets in each panel are FE–SEM cross-sectional images of the corresponding NR arrays indicating the height of individual nanorods within the array. NRs grown by traditional PLD under base conditions are shorter and thicker (d 50 nm) than those grown by DPLD. This can be seen clearly by comparing the FE–SEM images shown in Fig. 2a and d. These samples were both grown using Tsub = 600 °C throughout, but with one major difference: the latter sample was produced by DPLD on a seed-layer facing away from the ablation plume. NRs grown on a seed-layer deposited at Tsub,1 = 25 °C show no obvious alignment (Fig. 2b), but are still obviously ultrathin (with d as small as 10 nm). NRs grown by DPLD on a seed-layer formed at Tsub,1 to 300 °C show similar diameters, but are now aligned with the substrate surface normal (Fig. 2c). Increasing Tsub,1 to 600 °C (Fig. 2d) gives no obvious further improvement in alignment or aspect ratio. The NRs shown in Fig. 2d have ‘ 600 nm and d P 10 nm, corresponding to an aspect ratio ‘=d60:1 (some twenty times larger than that of the sample grown by traditional PLD methods shown in Fig. 2a). Fig. 3a and b show TEM images of a single NR, with d = 13 nm, broken from an array described in Fig. 2d. The HR–TEM image (Fig. 3b) exhibits parallel lattice planes aligned normal to the growth direction, which extend to the NR tip, and are separated by 0.26 nm – characteristic of wurtzite ZnO(0 0 0 2). Such observations are entirely consistent with previous findings that these NRs are single crystals that have grown along the [0 0 0 1] direction via a vapour–solid (V–S) mechanism [12]. The improved alignment of NRs grown on seed-layers deposited at higher Tsub,1 (recall Fig. 2b– d) is assumed to reflect improved alignment within the nanocrystalline seed-layer, with higher Tsub,1 enabling more species migration and improved epitaxy with the underlying substrate.
Fig. 2. FE–SEM images of ZnO NR arrays deposited on Si(1 0 0) substrates by: (a) traditional PLD at Tsub = 600 °C; and (b)–(d) by DPLD at Tsub,2 = 600 °C on top of a ZnO seed-layer grown by PLD at, respectively, at Tsub,1 = 25, 300 and 600 °C. Crosssectional FE–SEM images of the NR arrays are shown in the corresponding insets.
Fig. 3. (a) Low resolution and (b) high-resolution TEM images of a single NR broken from a NR array deposited using Tsub,1 = Tsub,2 = 600 °C shown in Fig. 2d. (c) and (d) Show SEM images (overview and detail, respectively) illustrating selective area growth of ZnO NRs by using a TEM grid as a shadow mask during seed-layer deposition.
Fig. 1. Schematic of the substrate arrangement for (a) PLD of the seed-layer and (b) the subsequent DPLD of ZnO NRs. The ArF laser propagation direction is indicated.
deposition chamber was brought up to air and the substrate reoriented as shown in Fig. 1b such that the distance separating the target from the growing face was maintained at D = 50 mm. The chamber was then re-evacuated, pO2 re-established at 10 mTorr, Tsub,2 set to the required value and ablation continued at a fluence F2 for a further period t2. Deposited material was analysed and characterized by scanning electron microscopy (FEG–SEM, using a JEOL JSM 6330F instrument) and high-resolution transmission electron microscopy (HRTEM, JEOL JEM 2010). Photoluminescence (PL) spectra were measured at room temperature using a He–Cd laser (325 nm, output power 3 mW) as the excitation source. 3. Results and discussion
G.M. Fuge et al. / Chemical Physics Letters 479 (2009) 125–127
Fig. 4. Wavelength dispersed PL spectra from 325 nm excitation of ZnO NR arrays deposited on Si (1 0 0) by: (a) traditional PLD at Tsub = 600 °C; and (b)–(d) by DPLD at Tsub,2 = 600 °C on top of a ZnO seed-layer grown by PLD at, respectively, Tsub,1 = 25, 300 and 600 °C.
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in PLD of ZnO serves to constrain the plume expansion, and ensures that the ablated species undergo multiple collisions en route to the substrate surface. Nonetheless, the incident fluxes and KEs accompanying traditional PLD under the present base conditions are still sufficient to nucleate a clean Si surface and faster, unselective deposition results. DPLD requires that the growth species diffuse around the substrate and then impact the seed-layer. The incident KEs under such circumstances will be close to thermal, few if any charged species will survive to impact on the substrate surface, and the conditions for successful nucleation and NR growth will be much more stringent – as reflected by the striking SAD illustrated in Fig. 3c and d, and the high aspect ratios of the resulting NRs. In this regard, DPLD has some parallels with thermal vapour phase transport [4] – wherein growth species are delivered with low KE in a slow flow of Ar and nucleation and growth encouraged by use of high (though rarely well characterised) Tsub values. The DPLD method introduced here offers opportunities for greater process control at lower Tsub and with only very localised heating requirements, and for selective area deposition. Simpler, one-stage DPLD strategies can be readily envisaged – e.g. the off-axis, ‘eclipsed’ PLD strategy reported by Mendelsberg et al. [16] or translating a beam-block into the plume to prevent direct deposition after growth of the initial seed-layer. These will be explored in future work. 4. Conclusion
The potential of DPLD as a selective area deposition (SAD) technique was investigated also. ZnO was deposited through a 200 mesh copper TEM grid to form a patterned seed-layer on the substrate. The mask was then removed, the substrate reversed, and a ZnO NR array grown by DPLD. Fig. 3c shows an SEM image of the patterned array grown by DPLD for t1 = 10 min and t2 = 45 min under base conditions. Clearly, DPLD follows the pattern of the seed-layer (which corresponds to the open areas of the TEM grid) faithfully. The higher magnification image (Fig. 3d) highlights the contrast between growth and no-growth regions; NRs have only deposited on the seeded areas, not on the regions of Si surface that were masked during the seed-layer growth stage. Such SAD was not achievable by traditional PLD which, under the specified base conditions, yielded thicker NRs (as in Fig. 2a) on both seeded and unseeded areas of the Si substrate. Indeed, traditional PLD at progressively higher F results in yet thicker NRs and, in the limit, an apparently continuous film [13]. Fig. 4 shows wavelength dispersed room temperature PL spectra of the ZnO NR arrays displayed in Fig. 2, obtained following 325 nm excitation with an incident power on the sample of 7 lW. The intense peak centred at 380 nm in each spectrum is the characteristic ZnO band-edge emission. The comparative narrowness of these features in all four spectra (12 nm full width half maximum), and the lack of visible luminescence in the 450– 600 nm range, both serve to illustrate the quality and crystallinity of the NR arrays. The visible emission is traditionally attributed to oxygen vacancies and zinc interstitials [14]; its absence points to the low-defect density within the as-grown material. There is an apparent increase in the peak intensity of the band-edge emission as Tsub,1 increases from 25 °C (Fig. 4b) to 600 °C (Fig. 4d) which can be attributed to an improvement in crystalline quality of the predeposited seed-layer. The present observations highlight the way that the flux and kinetic energies (KEs) of the ablated species control the eventual film morphology. Previous studies have shown that species arising in the pulsed laser ablation of ZnO in vacuum can have velocities >30 km s 1 along the target surface normal [15]. The low pO2 used
Uniform arrays of ultrathin (d 10 nm), vertically-aligned ZnO NRs have been synthesized on Si(1 0 0) substrates by a diffusive pulsed laser deposition method, that also enables highly selective area deposition and patterned growth. The PL spectrum resulting from 325 nm excitation of NR arrays grown by DPLD under base conditions displays sharp band-edge emission but negligible visible emission – reflecting the crystallinity and low-defect density of the as-grown material. Acknowledgements The authors are most grateful to: EPSRC, for financial support; K.N. Rosser, Dr. J.A. Smith, B.S. Truscott, and J. Jones for their contributions to the described work; and Prof. S.R.P. Silva and Dr. S.J. Henley (University of Surrey) for very helpful discussions. References [1] B.D. Yao, Y.F. Chan, N. Wang, Appl. Phys. Lett. 81 (2002) 757. [2] J.G. Wen, J.Y. Lao, D.Z. Wang, T.M. Kyaw, Y.L. Foo, Z.F. Ren, Chem. Phys. Lett. 372 (2003) 717. [3] H.D. Yu, Z.P. Zhang, M.Y. Han, X.T. Hao, F.R. Zhu, J. Am. Chem. Soc. 127 (2005) 2378. [4] M.H. Huang, Y. Wu, H. Feick, N. Tran, E. Weber, P. Yang, Adv. Mater. 13 (2001) 113. [5] S.J. Henley, M.N.R. Ashfold, D.P. Nicholls, P. Wheatley, D. Cherns, Appl. Phys. A. -Mater. Sci. Proc. 79 (2004) 1169. [6] B. Liu, H.C. Zeng, J. Am. Chem. Soc. 125 (2003) 4430. [7] J.-J. Wu, S.-C. Liu, Adv. Mater. 14 (2002) 215. [8] Y. Sun, G.M. Fuge, M.N.R. Ashfold, Chem. Phys. Lett. 396 (2004) 21. [9] Y. Sun, R.P. Doherty, J.L. Warren, M.N.R. Ashfold, Chem. Phys. Lett. 447 (2007) 257. [10] Z.W. Liu, C.K. Ong, T. Yu, Z.X. Shen, Appl. Phys. Lett. 88 (2006) 053110. [11] R. Nishimura, T. Sakano, T. Okato, T. Saiki, M. Obara, Jpn. J. Appl. Phys. 47 (2008) 4799. [12] Y. Sun, G.M. Fuge, M.N.R. Ashfold, Superlattices Microstruct. 39 (2006) 33. [13] Y. Sun, M.N.R. Ashfold, Nanotechnology 18 (2007) 245701. [14] X. Liu, X. Wu, H. Cao, R.P.H. Chang, J. Appl. Phys. 95 (2004) 3141. [15] F. Claeyssens, A. Cheesman, S.J. Henley, M.N.R. Ashfold, J. Appl. Phys. 92 (2002) 6886. [16] R.J. Mendelsberg, M. Kerler, S.M. Durbin, R.J. Reeves, Superlattices Microstruct. 43 (2008) 594.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Ultrathin aligned ZnO nanorod arrays grown by a novel diffusive pulsed laser deposition method Gareth M. Fuge *, Tobias M.S. Holmes, Michael N.R. Ashfold School of Chemistry, University of Bristol, Cantock’s Close, Bristol BS8 1TS, UK
a r t i c l e
i n f o
Article history: Received 12 June 2009 In final form 4 August 2009 Available online 7 August 2009
a b s t r a c t ZnO nanorod (NR) arrays were grown on Si at elevated temperatures using a two-stage diffusive pulsed laser deposition (DPLD) technique. A thin (50 nm) seed-layer was first formed by pulsed laser ablation of ZnO in O2. The sample was then turned to face away from the propagation direction of the ablation plume, and the ablation process continued. A dense array of ultrathin NRs was seen to grow from the seed-layer. These NRs are thinner (d 10 nm), and display a 20-times higher aspect ratio than those grown by traditional PLD under otherwise identical process conditions. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction ZnO is a semiconductor material of great contemporary interest, not least because of its wide direct band gap (3.37 eV) and large exciton binding energy (60 meV). Potential uses of such a material include gas sensors, UV emitting diodes and UV sensitive photocells, the efficiencies of all of which benefit from a large surface area. Reported ZnO nanostructures include nanoribbons, nanowires, nanoclusters, nanorods (NRs) [1,2] and nanotubes [3]. The growth of aligned NR arrays is of particular interest, due to their high surface area, regularity and uniformity, and because of the possible quantum confinement effects (and enhanced luminescence efficiencies) exhibited by the individual NRs. ZnO NRs can be grown in a variety of ways including, thermal vapour phase transport [4], hydrothermal methods [5,6], microwave plasma enhanced chemical vapour deposition [7] and pulsed laser deposition (PLD) [8]. PLD offers the advantage of relatively slow growth, in a dry vacuum chamber, and can yield high quality arrays of aligned, single crystal NRs with low defect densities – without the need for any catalyst (which, inevitably, constitutes a potential impurity). ZnO NRs deposited in this way display diameters, d, in the range 50–500 nm and lengths, ‘, up to a few microns. Many parameters, e.g. the incident fluence, F [9], the target to substrate distance, D, the substrate temperature, Tsub, and the background pressure, p [10], all affect the product morphology and the diameters of the resulting NRs. Growth of thin NRs, with d 50 nm, is favoured by use of low F (just above the ablation threshold of the ZnO target), larger D and/or by positioning the substrate off the centre axis of the ablation plume [11]. The net effect of such * Corresponding author. Fax: +44 (0) 117 9250612. E-mail address: [email protected] (G.M. Fuge). 0009-2614/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2009.08.008
changes is to reduce the flux and/or kinetic energies of the ablated material incident on the substrate. The present study extends this philosophy further, by demonstrating growth of ultrathin NRs on a pre-seeded substrate surface that faces away from the laser plume. Deposition under such circumstances must rely on diffusive transport of ablated material to the growing surface and, as such, has some obvious parallels with growth by thermal vapour phase transport methods. 2. Experimental ZnO NR arrays were grown on Si(1 0 0) substrates using a twostage diffusive pulsed laser deposition (DPLD) method involving growth of an initial seed-layer using the traditional PLD geometry, then reversing the substrate and continuing growth on the seedlayer that is now facing away from the target and the plume propagation direction. The substrate was first cleaned by immersing in acetone in an ultrasonic bath and then washed with ethanol, and then mounted in the vacuum chamber as shown in Fig. 1a at D = 50 mm from the front surface of the ZnO target (Testbourne, 99.99% purity). The chamber was then evacuated to a base pressure of 1 10 6 Torr and subsequently back-filled with a steady (10 sccm) flow of oxygen, so as to maintain pO2 = 10 mTorr. A 250 W tungsten halogen quartz bulb served as an efficient and contaminant-free substrate heater, capable of maintaining Tsub to within 1 °C of any user-selected value for the duration of the deposition. The seed-layer was deposited using an ArF excimer laser (193 nm, Lambda-Physik Compex 201, operating at a repetition rate of 10 Hz), the output of which was steered and focused onto the ZnO target for, typically, t1 = 10 min at F1 4 J cm 2. The ZnO seed-layer coated substrate was then allowed to cool from Tsub,1 to room temperature in pO2 10 mTorr, before the
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Fig. 2a shows a FE–SEM image of a ZnO NR array grown by traditional PLD using the following conditions: Tsub = 600 °C, F 4 J cm 2, D = 50 mm, pO2 10 mTorr and t1 = 55 min. Fig. 2b–d, in contrast, show ZnO NR arrays grown by DPLD for t2 = 45 min using the above values for Tsub,2, F2, D and pO2, after pre-depositing
a ZnO seed-layer for t1 = 10 min at Tsub,1 values of, respectively, 25, 300 and 600 °C. Henceforth, the Tsub, F, D and pO2 conditions used to form the arrays shown in Fig. 2a and d will be termed the base conditions. All four images were taken at a 35° tilt to the normal to illustrate the NR alignments and thicknesses more clearly. The insets in each panel are FE–SEM cross-sectional images of the corresponding NR arrays indicating the height of individual nanorods within the array. NRs grown by traditional PLD under base conditions are shorter and thicker (d 50 nm) than those grown by DPLD. This can be seen clearly by comparing the FE–SEM images shown in Fig. 2a and d. These samples were both grown using Tsub = 600 °C throughout, but with one major difference: the latter sample was produced by DPLD on a seed-layer facing away from the ablation plume. NRs grown on a seed-layer deposited at Tsub,1 = 25 °C show no obvious alignment (Fig. 2b), but are still obviously ultrathin (with d as small as 10 nm). NRs grown by DPLD on a seed-layer formed at Tsub,1 to 300 °C show similar diameters, but are now aligned with the substrate surface normal (Fig. 2c). Increasing Tsub,1 to 600 °C (Fig. 2d) gives no obvious further improvement in alignment or aspect ratio. The NRs shown in Fig. 2d have ‘ 600 nm and d P 10 nm, corresponding to an aspect ratio ‘=d60:1 (some twenty times larger than that of the sample grown by traditional PLD methods shown in Fig. 2a). Fig. 3a and b show TEM images of a single NR, with d = 13 nm, broken from an array described in Fig. 2d. The HR–TEM image (Fig. 3b) exhibits parallel lattice planes aligned normal to the growth direction, which extend to the NR tip, and are separated by 0.26 nm – characteristic of wurtzite ZnO(0 0 0 2). Such observations are entirely consistent with previous findings that these NRs are single crystals that have grown along the [0 0 0 1] direction via a vapour–solid (V–S) mechanism [12]. The improved alignment of NRs grown on seed-layers deposited at higher Tsub,1 (recall Fig. 2b– d) is assumed to reflect improved alignment within the nanocrystalline seed-layer, with higher Tsub,1 enabling more species migration and improved epitaxy with the underlying substrate.
Fig. 2. FE–SEM images of ZnO NR arrays deposited on Si(1 0 0) substrates by: (a) traditional PLD at Tsub = 600 °C; and (b)–(d) by DPLD at Tsub,2 = 600 °C on top of a ZnO seed-layer grown by PLD at, respectively, at Tsub,1 = 25, 300 and 600 °C. Crosssectional FE–SEM images of the NR arrays are shown in the corresponding insets.
Fig. 3. (a) Low resolution and (b) high-resolution TEM images of a single NR broken from a NR array deposited using Tsub,1 = Tsub,2 = 600 °C shown in Fig. 2d. (c) and (d) Show SEM images (overview and detail, respectively) illustrating selective area growth of ZnO NRs by using a TEM grid as a shadow mask during seed-layer deposition.
Fig. 1. Schematic of the substrate arrangement for (a) PLD of the seed-layer and (b) the subsequent DPLD of ZnO NRs. The ArF laser propagation direction is indicated.
deposition chamber was brought up to air and the substrate reoriented as shown in Fig. 1b such that the distance separating the target from the growing face was maintained at D = 50 mm. The chamber was then re-evacuated, pO2 re-established at 10 mTorr, Tsub,2 set to the required value and ablation continued at a fluence F2 for a further period t2. Deposited material was analysed and characterized by scanning electron microscopy (FEG–SEM, using a JEOL JSM 6330F instrument) and high-resolution transmission electron microscopy (HRTEM, JEOL JEM 2010). Photoluminescence (PL) spectra were measured at room temperature using a He–Cd laser (325 nm, output power 3 mW) as the excitation source. 3. Results and discussion
G.M. Fuge et al. / Chemical Physics Letters 479 (2009) 125–127
Fig. 4. Wavelength dispersed PL spectra from 325 nm excitation of ZnO NR arrays deposited on Si (1 0 0) by: (a) traditional PLD at Tsub = 600 °C; and (b)–(d) by DPLD at Tsub,2 = 600 °C on top of a ZnO seed-layer grown by PLD at, respectively, Tsub,1 = 25, 300 and 600 °C.
127
in PLD of ZnO serves to constrain the plume expansion, and ensures that the ablated species undergo multiple collisions en route to the substrate surface. Nonetheless, the incident fluxes and KEs accompanying traditional PLD under the present base conditions are still sufficient to nucleate a clean Si surface and faster, unselective deposition results. DPLD requires that the growth species diffuse around the substrate and then impact the seed-layer. The incident KEs under such circumstances will be close to thermal, few if any charged species will survive to impact on the substrate surface, and the conditions for successful nucleation and NR growth will be much more stringent – as reflected by the striking SAD illustrated in Fig. 3c and d, and the high aspect ratios of the resulting NRs. In this regard, DPLD has some parallels with thermal vapour phase transport [4] – wherein growth species are delivered with low KE in a slow flow of Ar and nucleation and growth encouraged by use of high (though rarely well characterised) Tsub values. The DPLD method introduced here offers opportunities for greater process control at lower Tsub and with only very localised heating requirements, and for selective area deposition. Simpler, one-stage DPLD strategies can be readily envisaged – e.g. the off-axis, ‘eclipsed’ PLD strategy reported by Mendelsberg et al. [16] or translating a beam-block into the plume to prevent direct deposition after growth of the initial seed-layer. These will be explored in future work. 4. Conclusion
The potential of DPLD as a selective area deposition (SAD) technique was investigated also. ZnO was deposited through a 200 mesh copper TEM grid to form a patterned seed-layer on the substrate. The mask was then removed, the substrate reversed, and a ZnO NR array grown by DPLD. Fig. 3c shows an SEM image of the patterned array grown by DPLD for t1 = 10 min and t2 = 45 min under base conditions. Clearly, DPLD follows the pattern of the seed-layer (which corresponds to the open areas of the TEM grid) faithfully. The higher magnification image (Fig. 3d) highlights the contrast between growth and no-growth regions; NRs have only deposited on the seeded areas, not on the regions of Si surface that were masked during the seed-layer growth stage. Such SAD was not achievable by traditional PLD which, under the specified base conditions, yielded thicker NRs (as in Fig. 2a) on both seeded and unseeded areas of the Si substrate. Indeed, traditional PLD at progressively higher F results in yet thicker NRs and, in the limit, an apparently continuous film [13]. Fig. 4 shows wavelength dispersed room temperature PL spectra of the ZnO NR arrays displayed in Fig. 2, obtained following 325 nm excitation with an incident power on the sample of 7 lW. The intense peak centred at 380 nm in each spectrum is the characteristic ZnO band-edge emission. The comparative narrowness of these features in all four spectra (12 nm full width half maximum), and the lack of visible luminescence in the 450– 600 nm range, both serve to illustrate the quality and crystallinity of the NR arrays. The visible emission is traditionally attributed to oxygen vacancies and zinc interstitials [14]; its absence points to the low-defect density within the as-grown material. There is an apparent increase in the peak intensity of the band-edge emission as Tsub,1 increases from 25 °C (Fig. 4b) to 600 °C (Fig. 4d) which can be attributed to an improvement in crystalline quality of the predeposited seed-layer. The present observations highlight the way that the flux and kinetic energies (KEs) of the ablated species control the eventual film morphology. Previous studies have shown that species arising in the pulsed laser ablation of ZnO in vacuum can have velocities >30 km s 1 along the target surface normal [15]. The low pO2 used
Uniform arrays of ultrathin (d 10 nm), vertically-aligned ZnO NRs have been synthesized on Si(1 0 0) substrates by a diffusive pulsed laser deposition method, that also enables highly selective area deposition and patterned growth. The PL spectrum resulting from 325 nm excitation of NR arrays grown by DPLD under base conditions displays sharp band-edge emission but negligible visible emission – reflecting the crystallinity and low-defect density of the as-grown material. Acknowledgements The authors are most grateful to: EPSRC, for financial support; K.N. Rosser, Dr. J.A. Smith, B.S. Truscott, and J. Jones for their contributions to the described work; and Prof. S.R.P. Silva and Dr. S.J. Henley (University of Surrey) for very helpful discussions. References [1] B.D. Yao, Y.F. Chan, N. Wang, Appl. Phys. Lett. 81 (2002) 757. [2] J.G. Wen, J.Y. Lao, D.Z. Wang, T.M. Kyaw, Y.L. Foo, Z.F. Ren, Chem. Phys. Lett. 372 (2003) 717. [3] H.D. Yu, Z.P. Zhang, M.Y. Han, X.T. Hao, F.R. Zhu, J. Am. Chem. Soc. 127 (2005) 2378. [4] M.H. Huang, Y. Wu, H. Feick, N. Tran, E. Weber, P. Yang, Adv. Mater. 13 (2001) 113. [5] S.J. Henley, M.N.R. Ashfold, D.P. Nicholls, P. Wheatley, D. Cherns, Appl. Phys. A. -Mater. Sci. Proc. 79 (2004) 1169. [6] B. Liu, H.C. Zeng, J. Am. Chem. Soc. 125 (2003) 4430. [7] J.-J. Wu, S.-C. Liu, Adv. Mater. 14 (2002) 215. [8] Y. Sun, G.M. Fuge, M.N.R. Ashfold, Chem. Phys. Lett. 396 (2004) 21. [9] Y. Sun, R.P. Doherty, J.L. Warren, M.N.R. Ashfold, Chem. Phys. Lett. 447 (2007) 257. [10] Z.W. Liu, C.K. Ong, T. Yu, Z.X. Shen, Appl. Phys. Lett. 88 (2006) 053110. [11] R. Nishimura, T. Sakano, T. Okato, T. Saiki, M. Obara, Jpn. J. Appl. Phys. 47 (2008) 4799. [12] Y. Sun, G.M. Fuge, M.N.R. Ashfold, Superlattices Microstruct. 39 (2006) 33. [13] Y. Sun, M.N.R. Ashfold, Nanotechnology 18 (2007) 245701. [14] X. Liu, X. Wu, H. Cao, R.P.H. Chang, J. Appl. Phys. 95 (2004) 3141. [15] F. Claeyssens, A. Cheesman, S.J. Henley, M.N.R. Ashfold, J. Appl. Phys. 92 (2002) 6886. [16] R.J. Mendelsberg, M. Kerler, S.M. Durbin, R.J. Reeves, Superlattices Microstruct. 43 (2008) 594.