Substrate-Au catalyst influence on the growth of ZnO nanorods

Materials Science and Engineering B 172 (2010) 225–230

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Substrate-Au catalyst influence on the growth of ZnO nanorods A. Taurino ∗ , M. Catalano, A. Cretì, M. Lomascolo, C. Martucci, F. Quaranta Institute for Microelectronics and Microsystems, CNR-Lecce, Via Monteroni, 73100 Lecce, Italy

a r t i c l e

i n f o

Article history: Received 8 February 2010 Received in revised form 23 April 2010 Accepted 22 May 2010 PACS: 81.07.−b 68.55.−a 68.37.−d 78.55.Cr Keywords: ZnO nanorods ZnO buffer Fixed incidence XRD FIB cross-sections

a b s t r a c t In this work, the evolution of the Au assisted-growth of ZnO nanorods deposited by vapour phase deposition both on sapphire and on indium–tin–oxide on glass (ITO-glass) substrates has been studied. Our investigation demonstrates that the growth proceeds first as a 3D growth, giving rise to a buffer layer, few microns thick, formed by ZnO grains with different orientation. Then a 1D transition occurs with the nucleation of a dense array of vertically aligned nanorods. A different degree of crystalline order and nanorods alignment was found between the samples grown on ITO-glass and sapphire substrates, which was ascribed to the different morphology that the Au seed layer acquires on the two different substrates. A semi-quantitative analysis of the ZnO crystalline orientation was carried out by X-ray diffraction (XRD) measurements performed at fixed incidence configuration and supported by high resolution scanning electron microscopy (HR-SEM) investigations on focused ion beam (FIB) prepared cross-sections. © 2010 Elsevier B.V. All rights reserved.

1. Introduction The recent research in the field of semiconducting materials has devoted a big effort to the study of the physical and functional properties of ZnO-based materials, as demonstrated by the huge number of scientific papers published on this topic. The excellent properties of this wide bandgap (3.37 eV) semiconductor with large exciton binding energy (60 meV), such as the high emission efficiency at UV wavelength, the sensitivity of the surface chemistry to the environment, the piezoelectricity, make it a good candidate for many possible applications ranging from light emitting diodes [1,2], lasers and photodetectors [3,4], to electron emitters [5,6] and optical and electrical sensors [7,8]. Several synthesis methods, including physical and chemical vapour deposition, laser ablation and solution synthesis [9–12], have been used to obtain nanosized structures with controlled morphological, structural and optical properties. The combined choice of the seed layer, used as catalysts, and substrate type has been used as the driving force to orient the growth towards vertically aligned nanorods with small diameters and narrow size distribution. Among these, the solid–vapour deposition techniques have been able to produce a variety of ZnO nanostructures, such as nanocombs, nanobelts, nanowhiskers, nanotetrapods, nanodisks, nanotubes, nanowires and nanorods

∗ Corresponding author. Tel.: +39 0832 422519; fax: +39 0832 422552. E-mail address: [email protected] (A. Taurino). 0921-5107/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2010.05.021

[1,13–18]. The growth of 1D ZnO nanorods is usually achieved by following the vapour–liquid–solid (VLS) approach, in which a thin metal layer with a starting thickness of few nanometer, is used as catalyst. The most commonly used catalyst for ZnO is Au. Due to the substrate heating, the Au seed layer forms liquid droplets which act as a preferential site for absorption of gas phase reactant and, when supersaturated, give raise to crystallization. The orientation, size and eventual chemical manipulation of the Au seed layer have been reported in the literature as the key parameters which influence the morphological and structural properties of ZnO nanostructures. In particular, it has been demonstrated that the crystalline orientation of the Au nanoparticles on the substrate influences the vertical alignment of ZnO nanorods [19]. Au nanoparticles, stabilized by polymethyl methacrylate (PMMA) have been used to control the density of ZnO nanowires with very uniform diameter [20]. In this paper, we have investigated the influence that the substrate and Au catalyst layers play on the structural and morphological properties of ZnO nanorods, grown by vapour phase deposition on sapphire and ITO on glass substrates. In particular, our investigation demonstrated that (i) a buffer layer of polycrystalline grains, few microns thick, forms during the early stage of the growth; (ii) this buffer layer consists of crystalline grains whose size and orientation are related to the structure and morphology that the Au catalyst layer acquires on the two different substrates; (iii) a 3D–1D transition, with the appearance of vertically aligned nanorods, occurs when the growth velocity along the 0 0 2 crystalline direction prevails. Differences in the size of the polycrystalline grains, nanorod diameters and nanorod ver-

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tical orientation were observed between the two substrates and related to the different morphology of the Au seed layer. Moreover, a quantitative distinction of the structural order was derived from X-ray investigations performed at fixed incidence angle, in the range 2.5–25◦ , and related to the results of the cross-sectional investigations. PL measurements at low temperature evidenced the high emission efficiency of the samples and confirmed the different structural quality of the films in relation to the substrate type. 2. Experimental The ZnO samples investigated in the present paper were grown by vapour phase deposition. The growth apparatus consisted of a horizontal quartz tube (25 mm inner diameter) furnace [21]. 5 N pure Zn granular source was placed in a tungsten crucible and inserted in the furnace at room temperature. The substrate was placed in front of the crucible, at a distance of 10 mm. 5 N Ar flow was used as carrier gas with a flux of 1 N l/min. The furnace was heated up to 600 ◦ C in 10 min, maintained at this temperature for 60 min and then slowly cooled down to room temperature. The deposition was carried out on ITO-glass and sapphire substrates, preliminarily coated with a 3 nm gold film deposited by e-beam evaporation. The thickness of the ITO layer was 150 nm. Morphological analyses were performed by using a Jeol JEM 6500F scanning electron microscope. A Zeiss 1540 ESB Cross-beam FIB operating with 30 keV gallium ions was used for the preparation of the cross-sections and for the in situ SEM analysis of the samples in cross-sectional geometry. X-ray analyses were carried out by the Cu K␣ line delivered by a Rigaku diffractometer. The measurements were performed in the conventional theta–2theta configuration and in a non conventional approach by using a fixed X-ray incidence in order to analyze the degree of misorientation of a given set of lattice planes of the ZnO nanostructures; in particular, different X-ray diffraction spectra were acquired by varying the incidence angle between 2.5◦ and 25◦ . PL measurements were performed at 7 K by using the 325 nm line delivered by an He–Cd laser and dispersed by a 0.3 m focal length monochromator, equipped with a cooled GaAs photomultiplier, operating in photon counting mode. 3. Results and discussion A preliminary analysis of the morphological and structural quality of the samples was obtained by SEM investigations in plan-view geometry, PL measurements at low temperature and X-ray diffraction in the theta–2theta geometry. Fig. 1 reports the low magnification images of the ZnO samples on sapphire (a) and ITO-glass (b) substrate, respectively. The images show that the samples consist of a high density of nanorods on both substrates; the nanorods on the ITO-glass substrate have a smaller diameter, ranging between 100 and 150 nm, and a narrower size dispersion; in the case of the sapphire substrate, the diameter varies between 250 and 500 nm. Furthermore, the nanorods exhibit an hexagonal section with a tapered tip; the faceting of the tip of single nanorods is evident in the insets; in particular, in the case of the ITO sample, the rod tips exhibit 114 facets whereas, in the case of the sapphire sample, the faceting is more complex. 116, 214 Facets initially form and terminate with 0 0 2 facets forming the narrow needle at the topmost part of the tip. The different shape of the tips has been attributed in the literature to the position of the substrate relative to the carrier gas flow [22]. In our experiments, we believe that the different faceting is probably the result of the progressive tapering of the tip due to the relative lack of material during the transient phase before the growth interruption. This

Fig. 1. Morphology of the ZnO samples grown on sapphire (a) and ITO (b) substrates. The insets show the high magnification images of single nanorods, evidencing the crystal facets at the rod tips.

tapering leads to a different faceting probably due to many factors, such as the different diameter of the nanorods, and their different crystalline quality and average orientation relative to the 0 0 2 direction, as it will be discussed afterwards. Fig. 2 compares the PL spectra detected at 7 K, from ZnO nanorods on sapphire (line S) and ITO-glass (line I) substrates. In particular, Fig. 2(a) reports the full PL spectra plotted on a logarithmic scale. The luminescence signal extends from the green/orange spectral range to the UV-region in both spectra. The strong UV emission, due to band-edge recombination, is 4 orders of magnitude higher than the defect emission band, centred at about 2.5 eV, demonstrating the high structural quality of the samples. Fig. 2(b) shows in details the excitonic region of the PL spectra. All the band-edge transitions visible in the spectrum of the ITO sample are red-shifted with respect to those on the sapphire sample, for both samples, the main peaks of this band are attributed to a neutral donor-bound exciton (denoted as D0 X) [23], located at 3.377 and 3.381 eV, in the case of sapphire sample, and at 3.365 and 3.370 eV, in the case of the ITO substrate. Both spectra also exhibit three additional peaks D0 X-nLO at lower energy, corresponding to the donor bound phonon replicas [24,25]. The main difference between the two samples is represented by the presence of the free exciton transitions (XA and XB at 3.392 and 3.401 eV [26]), their phonon replicas (FXAB -nLO) and the relevant first-excited states (XAB band at 3.429 and at 3.438 eV) only in the spectrum of the nanowires grown on sapphire substrate. This is a clear indication of the higher structural quality of this sample. No evidence for free exciton is observed for the sample grown on ITO substrate. Nevertheless free exciton transition phonon repli-

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Fig. 2. PL spectra at low temperature (7 K) of ZnO samples on sapphire (line S) and ITO (line I) substrates (a) and zoom of excitonic region of the two spectra (b).

cas (FX-nLO) can be observed in this spectrum and the free exciton peak position can be estimated at 3.383 eV. Fig. 3(a) and (b) shows the theta–2theta X-ray diffractions of the ZnO nanorods on sapphire and ITO-glass substrates respectively. The peaks in the diffractions are all ascribable to ZnO in the wurtzite structure (JCPDS-ICDD 36-1451 card; crystal system: Hexagonal, space group P63mc, a = 0.325 nm and c = 0.521 nm). In particular, a very sharp and intense 0 0 2 peak is evident in Fig. 3(a), indicating a preferential ordering of the ZnO nanostructures, with the c-axis of the wurtzite structure perpendicular to the substrate plane. Very faint secondary peaks (100, 101, 102, 103, etc.) are also present in the spectrum (not visible for scale reasons) whose intensity is lower than 1% with respect to the main peak height. In Fig. 3(b), the intensity of the 0 0 2 peak is much smaller in comparison with the secondary peaks. These results would suggest a better structural quality for the sample on sapphire. In order to understand the origin of the structural and morphological differences between the ZnO nanorods grown on ITO-glass and sapphire substrates, cross-sectional investigations were performed by high resolution SEM on FIB prepared cross-sections. The cross-sections were prepared by using the following procedure: a tungsten strip, 20 ␮m × 3 ␮m, was deposited on top of the ZnO nanorods in order to protect them from the ion bombardment, but also to obtain a flat surface in order to prevent from the curtain effect [27]. Then a rectangular box was dug perpendicularly to the strip, down to a depth overcoming the substrate/film interface. The final surface of the section was accurately polished at very low ion current; the manual nudge control of the ion beam was used in order to repetitively clean the surface of the section to reduce residual curtain effects due to the strong topography of the surface of the ZnO film.

Fig. 3. XRD spectra obtained in the theta–2theta configuration from the ZnO samples on sapphire (a) and ITO (b). The insets show a restricted range of the spectra in logarithmic scale, to enhance the visibility of the (1 1 1) Au peak.

Fig. 4(a) and (b) shows the backscattered electron images obtained from ZnO nanorods grown on sapphire and ITO-glass substrates, respectively. In both cases, a buffer layer with randomly oriented crystalline grains can be observed. At the interface between the ZnO buffer layer and the substrates, the Au layer is visible. In the case of the sapphire substrate, this layer consists of wellseparated spherical droplets with a diameter of 26 ± 4 nm, whereas, in the case of the ITO-glass substrate, the nanoparticles, with an average diameter of 33 ± 4 nm, appear immersed in a layer of comparable contrast, as it can be observed in Fig. 4(b). This layer can be probably the result of intermixing between Au and ITO which is due to the interdiffusion of Au atoms into the ITO layer and vice versa. This mechanism is promoted by the high temperature at which the substrate is heated up. The presence of the Au nanoparticle layer at the bottom of the deposited ZnO layer suggests that in our case the ZnO growth cannot be explained with the wellknown catalyst-assisted vapor–liquid–solid mechanism (VLS) [28] but can instead be ascribed to vapour–solid (VS) growth [29] on Au particles, which act as seeds for the crystalline orientation of the deposited material, as it will be demonstrated afterwards. The VS growth mechanism has been widely used in the literature to explain the Au-assisted growth of ZnO nanorods where no Au nanodroplets could be observed on the tip of the nanorods [19–31]. Nevertheless, in those cases, the evolution of the growth, starting from the Au seed layer to the formation of the ZnO nanorods has

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Fig. 4. SEM cross-sectional images obtained by FIB cutting and polishing from the sample on sapphire (a) and ITO (b).

not been demonstrated through a cross-sectional investigations of the samples. The novelty of our investigations is that they demonstrate that the growth of the ZnO layer on the Au nanoparticles proceeds first as a 3D growth, giving rise to a buffer layer, few microns thick, formed by ZnO grains with different orientation. The size of the grains increases as the growth proceeds. Then a 3D–1D transition can be observed. ZnO nanorods grow on the buffer layer below with an average length of about 10 ␮m. In the case of the ITO-glass substrate, the buffer layer consists of smaller grains with a more random orientation, as suggested by the variation of the channelling contrast in the SEM image. This can be due to the random crystalline orientation of the Au nanoparticles which is in turn related to the amorphous nature of the substrate. In the case of the sapphire substrate, the Au nanoparticles have an average crystalline orientation with the {1 1 1} planes parallel to the substrate, as demonstrated by peak at 38.1◦ , observed in the inset of Fig. 2(a), which reports the range of interest of the XRD spectrum in logarithmic scale. No evident Au peaks were observed in the case of the ZnO on ITO-glass substrate, as evident in the inset of Fig. 2(b). Therefore, the presence of well oriented Au nanoparticles on the sapphire sample is likely to favour the growth of ZnO grains with [0 0 2] orientation, which gradually increase in size until a 3D–1D transition occurs, with the appearance of nearly vertically aligned nanorods. The 3D–1D transition is due to the competition between the growth velocity on different crystallographic planes; therefore after the nucleation of preferentially oriented ZnO crystals, subsequent growth takes place predominantly along the [0 0 1] direction, as the sticking coefficient of Zn atoms is the highest [32], thus promoting the formation of 1D structures. In the case of the ITO-glass substrate, the ZnO grains, at the beginning with random orientation, gradually approach the [0 0 2] orientation, which is more favourable for the growth; nevertheless, when the 3D–1D transition occurs, a higher degree of misalignment of the ZnO grains with respect to the sapphire substrate is expected, which produces ZnO nanorods exhibiting a worse vertical alignment, as demonstrated by the X-ray results; furthermore, the smaller diameter of the nanorods on ITO-glass is likely to be related to the smaller size of the grains from which they nucleate. In order to obtain a more quantitative evaluation of the structural order of the nanorods in the two samples, X-ray mea-

Fig. 5. 0 0 2 Peak profiles obtained from the sapphire sample at 4◦ , 8◦ , 15◦ , 16◦ , 17.2◦ and 23◦ of X-ray incidence angle (a) and 0 0 2 peak height vs incidence angle (b).

surements were performed at fixed incident angle, in the range 2.5–25◦ , which allowed us to study the degree of misorientation of the 0 0 2 crystallographic planes with respect to the substrate plane. If the X-rays hit the sample with a fixed incidence angle ϕi and  B is the Bragg angle for the h k l planes, a rotation of these planes by the angle | B − ϕi | with respect to the direction parallel to the sample surface is required in order to observe the h k l diffraction maximum. This fact can be used to evaluate the misalignment of the grains in a polycrystalline material by observing the intensity change of a given X-ray peak with the change of the incidence angle; for each incidence angle, only the fraction of grains with the h k l planes suitably oriented will contribute to the intensity of the h k l diffraction peak. Fig. 5(a) shows the 0 0 2 peak profiles obtained from the sample on sapphire for 6 different values of incidence angle: a considerable variation of the peak height can be noted, with the variation of the incidence angle. Fig. 5(b) plots the 0 0 2 peak height vs the incidence angle, for all the measured values in the range 2.5–25◦ . The experimental points can be fitted with a Lorentzian curve centered at 17.37 ± 0.05◦ with a full width at half maximum (FWHM) of 5.3 ± 0.2◦ . This indicates that the maximum occurs exactly at the condition ϕi =  B for which the c-axis of the ZnO crystals is perpendicular to the substrate with a small degree of misorientation, as suggested by the small value of the FWHM. A similar trend was derived for the sample on ITO-glass substrate, but in this case a 47% higher FWHM was obtained, indicating a larger misalignment of the nanorods. A further quantitative analysis of the structural order in the two samples can be inferred from Fig. 6 where the comparison between the XRD spectra obtained at 2.5◦ and 17.2◦ is reported. Important differences can be noted: in particular, for the ITOglass substrate, the 0 0 2 peak height at 2.5◦ (Fig. 6(a)) is 37% of the peak at 17.2◦ (Fig. 6(b)), whereas, in the case of the sapphire

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Fig. 6. X-ray spectra obtained from ITO (a and b) and Sapphire (c and d) samples at 2.5◦ (a and c) and 17.2◦ (b and d) incidence angles.

substrate the 0 0 2 peak intensity at 2.5◦ (Fig. 6(c)) is only 4% of the peak at 17.2◦ (Fig. 6(d)). Moreover, at 2.5◦ , the XRD spectrum of the ITO sample shows 1 0 0 and 1 0 1 peaks whose intensity is respectively 5.2% and 11.5% of the 0 0 2 peak, whereas in the case of the sapphire the 1 0 0 and 1 0 1 peaks are respectively 2.5% and 4% of the 0 0 2 peak. These peaks are mainly ascribable to the ZnO buffer layer and their relative heights indicate the more random orientation of the buffer grains in the case of the ITO-glass substrate. Finally, the sapphire sample exhibits much more intense secondary peaks with high l-index (1 0 2 and 1 0 3 peaks) which indicate the tendency of the ZnO grains to orient themselves towards the c-axis. At 17.2◦ only the ITO-glass sample shows evident secondary peaks together with the main 0 0 2 peak. All these results indicate a more preferential orientation of the ZnO crystals along the c-axis in the case of the sample on sapphire.

4. Conclusions In this paper we have investigated the Au-assisted growth evolution of ZnO nanorods deposited by vapour phase deposition on ITO-glass and sapphire substrates. High resolution SEM crosssectional analyses demonstrated that a polycrystalline buffer layer grows below the nanorods, then a 3D–1D transition occurs giving rise to a dense array of nanorods. The structural properties of the buffer layer, which are in turn affected by the structure and morphology of the Au catalyst layer in relation to the substrate type, influenced the overall structural and morphological properties of the ZnO nanorods. In particular, a different degree of crystalline order and vertical alignment was found between the nanorods deposited on sapphire and ITO-glass substrates. A semi-quantitative comparison of the crystalline order and verti-

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