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Hybrid magnetron sputtering and pulsed laser ablation for the deposition of composite ZnO-Au films
Thin Solid Films 685 (2019) 66–74
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Hybrid magnetron sputtering and pulsed laser ablation for the deposition of composite ZnO-Au films
T
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Osmary Depablos-Riveraa, , Citlali Sánchez-Akéa, Raúl Álvarez-Mendozaa,b, Tupak García-Fernándezc, Stephen Muhld, Mayo Villagrán-Muniza a
Instituto de Ciencias Aplicadas y Tecnología, Universidad Nacional Autónoma de México, Circuito Exterior s/n, Ciudad Universitaria, Ciudad de México C.P. 04510, Mexico b Facultad de Ciencias, Universidad Nacional Autónoma de México, Circuito Exterior s/n, Ciudad Universitaria, Ciudad de México C.P. 04510, Mexico c Universidad Autónoma de Ciudad de México (UACM), Prolongación San Isidro 151, Col. San Lorenzo Tezonco, Ciudad de México C.P. 09790, Mexico d Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México, Circuito Exterior s/n, Ciudad Universitaria, Ciudad de México C.P. 04510, Mexico
A R T I C LE I N FO
A B S T R A C T
Keywords: Hybrid deposition technique Magnetron sputtering Pulsed laser deposition Zinc oxide Gold Composite films Plasma diagnostics
A hybrid technique that combines magnetron sputtering (MS) of zinc oxide and pulsed laser deposition (PLD) of gold was used to synthesize ZnO-Au composite thin films. Three different laser pulse fluence values of 4.5, 13.6 and 20.9 J cm−2 were used. Films of ZnO and Au were deposited separately by MS and PLD respectively to compare the growth by the individual and hybrid techniques. For the hybrid technique, no significant changes on the Zn concentration are observed when varying the laser pulse fluence, whilst the concentrations of O decreased and Au increased. The gold was incorporated uniformly throughout the thicknesses of the films as a second phase of nanoparticles. The presence of the gold did not modify the crystal orientation of the ZnO, which was a hexagonal wurtzite-phase with the c-axis perpendicular to the substrate. The ZnO-Au films were found to be thicker than the sum of the thicknesses of the Au and ZnO produced individually by MS and PLD, indicating that the hybrid technique incremented the net deposition rate. The emission of the laser ablation plume species, specifically neutral atoms of gold, was studied by optical emission spectroscopy to compare the individual PLD and MS-PLD processes.
1. Introduction Magnetron sputtering (MS) and pulsed laser deposition (PLD) are two physical vapor deposition techniques that are commonly used to synthesize a wide array of materials. Each individual technique has its own advantages, the main ones being that MS allows the scaling of the process from the laboratory to the industry and that PLD can transfer the stoichiometry of a target directly into the deposited film. Co-deposition processes from independent targets allow the deposition and synthesis of complex films with a precise control of their composition, structure and properties. This co-deposition may be done using different targets either in PLS or MS, or by combining both deposition methods. The combination of MS and PLD is known as the MSPLD hybrid technique [1,2] and can merge the advantages of the individual methods, in addition to the benefits offered by the co-deposition method [3]. Among these advantages it is possible to obtain deposited films of high purity, and control over the composition and crystallinity of the films at low temperature as a result of the energetic
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species in the laser ablation plume. Voevodin et al. [4–7] were the first to report the use of this technique, which allowed them to prepare composite coatings of WC, TiC and chalcogenides (WS2) nanoparticles (NPs) embedded into a diamond-like carbon (DLC) matrix. Besides, they synthesized films with yttria-stabilized zirconia (YSZ) NPs embedded into a matrix of Au + amorphous-YSZ [8,9], and with YSZ and MoS2 NPs embedded into a matrix of Au + amorphous-YSZ+ DLC [10,11]. A research group incorporated NPs of silver into a TiC matrix and assessed their tribological properties and wear performance in vacuum [12]. They adjusted the power applied to the Ag target to vary the metal concentration in the films and they found that 15% was the maximum Ag content that allowed an enhancement of the mechanical properties. Most of these reports have been focused on the performance of the films, but the plasmas evaluation have not received substantial attention. The diagnostic of the plasmas allows the in-situ analysis of the deposition process, eventually leading to a deeper understanding of the growth mechanisms, structure and properties of the films, which also
Corresponding author. E-mail addresses: [email protected], [email protected] (O. Depablos-Rivera).
https://doi.org/10.1016/j.tsf.2019.06.006 Received 18 January 2019; Received in revised form 15 May 2019; Accepted 3 June 2019 Available online 04 June 2019 0040-6090/ © 2019 Elsevier B.V. All rights reserved.
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of a gold target with a purity of 99.9% in an argon atmosphere with a pressure of 0.67 Pa. An Ekspla NL300 Nd-YAG laser with a maximum pulse energy of 230 mJ was used. The wavelength, frequency and pulse width of the laser were 355 nm, 10 Hz and 8 ns, respectively. The laser pulse energy was varied using an Altechna Watt Pilot attenuator. The experimental parameters were kept constant therefore, the variation of the pulse energy meant there was a change in the fluence: 4.5, 13.6 and 20.9 J cm−2 (see Table 1). In each deposition 12,000 laser pulses were fired. The laser beam was focused on the surface of the gold target using a multiple lens system and with an incidence angle of 45° respect to the surface normal of the target. The gold target was continuously moved along the horizontal and vertical directions to avoid that the laser spot would impinge on the same surface position. The second series consisted of composite films of ZnO and Au that were deposited by the MS-PLD hybrid technique. The target geometry and deposition conditions as well as the argon pressure, deposition time, the RF power applied to the ZnO target and the laser pulse characteristics were the same as those used in the first series. The surface of the gold target was cleaned against a shutter by laser ablation prior to each deposit in order to guarantee the same surface conditions for all deposits. The samples of both series were labeled according to the deposited material as ZnO, Au-X or ZnO-Au-X, where X is the laser pulse fluence, as shown in Table 1. The substrates were 2.5 × 2.5 cm2 pieces of (100) silicon wafers positioned in front of the Au target on a heating holder that was coupled to a temperature control system. During deposition, the substrates were kept at 400 °C, after the deposition the samples were naturally cooled to room temperature. The films thicknesses were measured using a Zygo NexView optical profilometer. The analyzed area was of 3.01 × 3.01 mm2. The surface and cross-sectional morphology of the films were evaluated using a JEOL 7800F field emission - scanning electron microscope (FE-SEM) using the upper electron detector (secondary electrons) and the gentle beam method, with an acceleration voltage of 3.0 kV and a work distance of 4.7 mm. The composition of the films was determined by X-ray photoelectron spectroscopy (XPS) using a Physical Electronics spectrometer equipped with a scanning XPS microprobe PHI 5000 VersaProbe II detector. The XPS measurements were performed at a vacuum of 4 × 10−8 Pa and using an Al Kα (hν = 1486.6 eV) X-ray source. The pass energies for the recording of the low and high-resolution spectra were 117.4 eV and 11.75 eV, respectively. Before the samples surface cleaning low-resolution spectra were recorded to define as reference the C1s peak (285 eV). After the 1 kV Ar ion cleaning of the samples for 5 min, the high-resolution spectra were measured using a current density of 55.6 nA mm−2. Additionally, the elemental concentration profiles were measured using a 2 kV Ar+ erosion beam with a current density of 1 μA mm−2. The XPS data were analyzed using the Multipak© version 9.6.0.15 software. The crystalline structure of the films and the crystallite size were determined from the X-ray diffraction (XRD) data recorded using a Rigaku Ultima IV diffractometer with a thin film accessory. The measurements were done in Theta-2Theta mode with parallel beam optics and under the following conditions: step of 0.02°, scan speed of 1°min−1, 2θ angle range of 30-80°. Furthermore, the in-plane mode was used to verify the preferential growth of the ZnO crystals. The characterization of the plasma during the deposition was carried out using an OES system, see Fig. 1(a). The emitted light was captured with a lens system and an optical fiber bundle which was connected to a Spectra Pro 500i monochromator from Acton Research, with a Czerny-Turner configuration and a focal distance of 50 cm that was equipped with a Princeton Instruments Spectra Hub PD-47 photomultiplier tube (PMT). The grating used was of 1200 grooves per mm. The emission of the plasmas was collected in the perpendicular direction to the propagation axis of the laser-induced plume and scanned along the line AB shown in Fig. 1(b) at different distances, d = 5, 15, 25, 30, 35 and 45 mm from the surface of the Au target with a spatial
translates to a better reproducibility. Jones et al. [13] documented the optical emission spectra that they recorded during the deposition of Au – YSZ films by both the hybrid MS-PLD technique and the individual techniques alone to control the deposition conditions by synchronizing the DC pulses that were being applied to the MS cathode with the intensity of the emission lines. They also analyzed the evolution of the PLD plume by performing image analysis [14]. Novotny et al. [1,15–17] detailed the characterization of the plasmas produced in a MS-PLD system during the deposition of titanium and silicon carbides. The authors [1,15] diagnosed the combined plasmas utilizing time- and spatially-resolved optical emission spectroscopy (OES), observing that the spectra profiles varied as the power applied to the MS targets (Ti and Si) and the background pressure were changed. They correlated the spectra corresponding to the MS-PLD plasma with those of the plasmas originated in the MS and PLD individual processes, and they observed the light emission from Ti+ ions and the C2 molecules only when the hybrid technique was being used, concluding that those species could influence the growth mechanisms of the carbide films. Zinc oxide (ZnO) has been extensively studied during the last decades due to its optical, electrical and magnetic properties. It is an n-type semiconductor that has a wide and direct band gap of 3.4 eV [18]. This oxide also has a large exciton binding energy (∼60 meV), a low threshold voltage [19], and it can straightforwardly be converted into a conductor through the control of its defects [20,21] and by the addition of dopants [22–27] or aggregates of noble metals such as gold or silver [28–30]. Thus, ZnO is an ideal material for its usage as a transparent conductor oxide. The combination of ZnO with gold nanoparticles (Au NPs) is an alternative way to enhance the optical and electric properties of ZnO for its use as a photocatalyst [31] and as a sensor constituent [32,33]. ZnO-Au composite films have been prepared using mainly two different ways. One of them was the deposition of bilayer films by MS, with a ZnO bottom-layer and a gold top-layer that were subsequently annealed to form Au NPs [28]. The other way was embedding Au NPs into a ZnO film, which also was obtained using two different methods. The first of these methods the embedding of the Au NPs was achieved by producing a composite film consisting of three successive layers of ZnO/Au/ZnO [34]. While in the second method, the embedding was achieved by producing Au NPs evenly distributed into a ZnO matrix, which was obtained using either chemical techniques [31] or by sputtering co-deposition using a ZnO target with gold wires on its surface [35]. The aim of the present paper is to report the use of the MS-PLD hybrid technique, which consisted in the combination of ZnO magnetron sputtering and pulsed laser deposition of gold, to obtain ZnO-Au composite films. The advantage of the MS-PLD hybrid method is the control of the concentration of gold in the films by varying the laser pulse fluence. Furthermore, we report the structure and composition of the films obtained by the hybrid and individual techniques, and the insitu temporal and spatial analysis of the plasma by Optical Emission Spectroscopy (OES) during the individual and hybrid processes. 2. Experimental details All experiments were performed inside a vacuum chamber with independent arrangements of MS and PLD, as shown in Fig. 1(a). The PLD target was located at a distance of 5.2 cm in front of the substrate holder, and the separation between the center of the MS target and the substrates was 6.8 cm. There was an angle of 67° between the surfaces of the MS target and the substrate holder, as shown in Fig. 1(b). The base pressure of the system was 4 × 10−4 Pa. Two series of films were deposited. The first series consisted of separately deposited thin film samples of pure ZnO and Au. The ZnO depositions were performed by balanced magnetron sputtering using a zinc oxide target (purity 99.9%, diameter 2 in.), a radio-frequency (RF, 13.56 MHz) power of 150 W, an argon pressure of 0.67 Pa and a deposition time of 20 min. The Au films were deposited by laser ablation 67
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Fig. 1. (a) Experimental configuration of the MS-PLD hybrid technique and the optical emission spectra measurement system. (b) Details of the geometric configuration of the targets and substrates (side view). Table 1 Notation of the samples and the values of the laser energy and fluence used. Sample
Deposition technique
Laser pulse energy (mJ)
Laser pulse fluence (J cm−2)
ZnO Au-4.5 Au-13.6 Au-20.9 ZnO-Au-4.5 ZnO-Au-13.6 ZnO-Au-20.9
MS PLD
– 49.8 ± 0.1 149.5 ± 0.3 230.8 ± 0.4 49.6 ± 0.1 150.5 ± 0.4 230.6 ± 0.5
– 4.5 ± 0.4 13.6 ± 1.2 20.9 ± 1.4 4.5 ± 0.4 13.7 ± 1.2 20.9 ± 1.9
MS-PLD
resolution about 7 mm2. The 479.26 and 405.28 nm emission lines were selected as the most suitable to detect the presence of the atomic neutral (Au I) and singly ionized (Au II) gold, respectively. The temporal evolution of the selected emission line was recorded using the PMT coupled to a Tektronix DPO 7054 digital oscilloscope and averaging 50 temporal evolution profiles. The temporal and spatial resolution of the equipment were 0.4 ns and 0.01 nm, respectively. The mean kinetic energy of the Au I and Au II species, during the PLD of gold in argon ambient, was calculated by plotting the emission intensity (PMT voltage) vs. time, also known as time of flight (TOF) distribution, using the procedure describe by Bulgakova et al. [36].
Fig. 2. Thicknesses of the ZnO, Au, and ZnO-Au films. The dashed line indicates the mean thickness of the ZnO films and the shaded area represents its error. The sum of the ZnO and Au films thicknesses is included (open symbols).
to sputter gold; 36 eV [39,40]. Therefore, it is reasonable that the excess of energy of the arriving gold species to the growing film surface could contribute to the re-sputtering of the deposited material [41,42]. Additionally, Fig. 2 shows that there was a slight decrease in the thickness of the ZnO-Au films at the highest laser fluence of 20.9 J cm−2 which was probably also due to re-sputtering of the deposit. Furthermore, we observed the ZnO-Au films prepared using the hybrid technique were 25, 42 and 36% thicker than the sum of thicknesses of the ZnO and Au films deposited by either MS or PLD. This suggests that there was either an enhancement in the deposition rate, or a decrease in the film density, with respect to the individual techniques. In fact, considering the general ideas of film deposition by PVD methods, for a constant deposition time the final film thickness is a balance between the accumulation rate, i.e. in this case the sputtering or ablation rates, the resulting arrival rates at the substrate plus the sticking coefficient of the various materials, and the removal or etch rate from the film surface. The removal phenomenon can be physical sputtering due to bombardment, chemical sputtering, thermal evaporation, etc. Now it is unlikely that the Au bombardment of the substrate from the PLD process would affect the sputtering rate, and therefore the arrival rate of the ZnO to the substrate. Similarly, it is doubtful that the Au would affect the sticking coefficient of ZnO.
3. Results and discussion Despite the relative position of the cathode and the substrate implied a gradient in the film thickness, there was a zone of about 1 × 1 cm2 in which the thickness was homogenous. This zone was organized to be the same as where the laser ablation plume arrived on the substrates. All of the film characterization was carried out in this homogeneous zone. Fig. 2 shows the average thickness of the MS deposited ZnO, PLD deposited Au, and the MS-PLD deposited ZnO-Au films in the homogenous zone. The sum of the thicknesses of the individual Au and ZnO films is also shown. It can be seen that there was a reduction in the thicknesses of the gold films as the laser pulse energy was increased. It has been reported that an increase in the laser fluence normally increases the gold deposition rate [37,38]. Possibly our results can be explained by the fact that as the laser fluence is increased the kinetic energy of the emitted ions also increases [38], and therefore, since the final film thickness is a balance between the deposition rate and the removal rate by re-sputtering, an increase in the re-sputtering results in a decrease in the deposited thickness. The reported kinetic energy values of Au species in the PLD plume, under similar conditions to ours, are higher than the threshold energy 68
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Consequently, the accumulation rate of ZnO on the substrate is probably independent of the laser fluence. The bombardment by Au species may have increased the removal rate, but such an affect cannot help to explain why the hybrid film thickness was greater than the sum of the MS-ZnO and PLD-Au film thicknesses. However, if we consider the deposition of Au the situation is somewhat different. Given the geometry of the MS and PLD targets it is probable that the arrival rate of Au to the substrate was independent of the presence of ZnO. Nevertheless, the sticking coefficient of Au to the Si substrate, or the previously deposited Au, would not be the same as the sticking coefficient of Au to the ZnO. It is not clear which sticking coefficient is larger, but the accumulation rate of Au was probably not the same with and without the presence of ZnO. Furthermore, the removal rates with and without ZnO are almost certainly different. Without ZnO the re-sputtering of Au by high energy bombardment by Au species would be quite efficient because the mass of the projectile and sputtered particle was the same. However, in the hybrid process the deposited Au atoms are partially covered and surrounded by ZnO molecules, and it is reasonable to expect that the removal rate of Au was significantly reduced. In summary, the PLD deposited Au suffered substantial removal due to bombardment by energetic Au species and the final thickness of the Au deposit was lower. MS deposited ZnO suffered some removal of Zn at the highest laser fluence due to the strong Au bombardment (see the reduction in the thickness of the ZnO-Au at the highest laser fluence, Fig. 2 and the Zn and O at.% versus the laser fluence presented below). During the hybrid deposition the removal rate of Au was reduced, therefore the final thickness of the films was closer to the sum of the accumulation rates of ZnO and Au, and greater than the sum of the individual deposition rates. Fig. 3 show the integrated optical emission intensity emitted by the atomic neutral Au I (479.26 nm) as a function of the distance from the PLD target surface for the three laser pulse fluence values (4.5, 13.6 and 20.9 J cm−2), during both (a) the PLD and (b) the MS-PLD hybrid technique deposition. For the highest laser fluence there was an increase in the Au I emission at d = 45 mm, i.e. 7 mm in front of the substrates, and we consider that this supports the idea of re-sputtering, since the emission is probably from Au atoms sputtered from the surface of the growing film. This behavior was not observed during the MS-PLD process; this could be interpreted as the inhibition of re-sputtering as a consequence of the presence of the MS plasma. The data for Fig. 3 was extracted from the temporal profiles of the Au I and Au II emission lines curves measured by OES. Two of these TOF distributions obtained for Au I and Au II during the PLD process in argon are shown in Fig. 4. We only carried out the TOF analysis for the strongest signals of Au
Fig. 4. TOF distribution of neutral atoms and ions of gold, Au I 479.26 nm and Au II 405.28 nm, measured at 45 and 25 mm from the surface of the PLD target, respectively, during laser ablation of gold in argon atmosphere (0.67 Pa) and irradiating with the laser fluence of 20.9 J cm−2. The thicker lines correspond to the data smoothed using the Savitzky-Golay method. Time zero corresponds to the laser pulse onset.
atoms and ions to calculate their average kinetic energy at distances nearer to the substrates, d = 45 mm for Au I and d = 25 mm for Au II emission lines. The signal of gold ions was almost non-existent at distances longer than 25 mm from the gold target surface probably due to the recombination of the ions. The average kinetic energy was calculated using the Eq. (1), extracted from the reference [36]; where m was the species mass, L the target to probe distance, and I (t) was the OES signal intensity as a function of time. ∞
〈EK 〉 =
mL2 ∫0 t −2I (t ) dt ∞
2 ∫0 I (t ) dt
(1)
The uncertainty of the TOF analysis of the weak distributions was too large to allow us to determine the mean kinetic energy for all of the deposition conditions. These weak distributions are corresponding to the OES data recorded at distances d between 25 and 35 mm or longer than 25 mm for the neutral atoms and ions of gold, respectively, in combination with the lowest laser fluence. Furthermore, calculations of kinetic energy from the TOF emission distributions of the different
Fig. 3. Integrated intensity of the temporal evolution profiles of the Au I emission line (479.26 nm) as a function of the distance from the target d, recorded during the (a) Au and (b) ZnO-Au films deposition process when laser pulse fluence values of 4.5, 13.6 and 20.9 J cm-2 were used. The insets correspond to the dot-dashed square zone where an abnormal behavior of the Au I emission was observed. 69
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Fig. 5. SEM micrographs of the surface morphology of (a) ZnO film, and ZnO-Au films deposited using laser pulse fluence values of (b) 4.5, (c) 13.6 and (d) 20.9 J cm. Inset: cross-sectional images of the ZnO-Au films.
2
species involved in the MS-PLD hybrid process would have uncertainty because the emission is produced by collisions between species of both plasmas, thus it cannot be interpreted directly as kinetic data. The analysis of the temporal profiles or TOF distributions for Au I and Au II emission lines allowed to calculate the average kinetic energy of the neutral gold atoms and gold ions, which were 130 and 700 eV, respectively. A comparison of the surface morphology of the ZnO and ZnO-Au films is provided by the micrographs shown in Fig. 5(a) – (d); the morphology of the ZnO-Au films can be seen from the cross-sectional images. The cross-sectional micrographs show compact films. Additionally, the differences between the surface morphology of the films of ZnO deposited by MS and the ZnO-Au films deposited by MS-PLD hybrid technique can be observed. The morphology of ZnO films was granular, while the morphology of the other films looked smooth. Dark spots were observed in Fig. 5(c – d), which were probably due to surface contamination in the films since they were not observed along the entire films. We consider that the composite films had small droplets on the smooth surface as a consequence of the typical splashing from the laser ablation process. This has been reported in some studies of the preparation of films by MS-PLD [43,44]. The change of the morphology was probably related to an increase in the surface mobility of the species on the surface of the growing film due to bombardment from the gold ions and fast neutrals, and the increment of the re-sputtering effect with the laser fluence. The gold species not only have a larger atomic mass than the ZnO species but also arrive with significant kinetic energy and therefore the effect of bombardment on the morphology of the deposits was large [45,46]. Fig. 6 shows the atomic composition of the ZnO and ZnO-Au films, determined by XPS by analysis of the Au 4d5/2, Zn 2p3/2 and O 1s orbitals. The concentration of Zn did not vary with the laser fluence. The oxygen concentration decrease with the laser fluence and therefore Zn/ O ratio increased from 1.09 to 1.39 for the highest laser fluence, as shown in Table 2. This loss of oxygen, the lightest atom in the deposits, is probably due to the bombardment of the deposit by energetic gold species as mention above. The gold atomic percentages in the ZnO-Au films were 5.7 ± 0.3, 9.3 ± 0.5 and 15.9 ± 0.8 at.% for the corresponding fluence values of 4.5, 13.6 and 20.9 J cm−2. Thus, the gold incorporation into the ZnO-Au films was proportional to the laser pulse
Fig. 6. Elemental composition of the ZnO and ZnO-Au films as function of the laser pulse fluence. Table 2 Summary of data extracted from XPS and XRD results. Sample
ZnO Au-4.5 Au-13.6 Au-20.9 ZnO-Au-4.5 ZnO-Au-13.6 ZnO-Au-20.9
Zn/O ratio
1.09
1.19 1.38 1.39
Size of the crystal (nm) ZnOa
Aub
67 – – – 47 41 36
– 77 66 57 9 7 7
The crystallite size was calculated using the Scherrer's equation. a For the peaks corresponding to the ZnO (002) plane. b For the peaks corresponding to the Au (111) plane.
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Fig. 7. (a) Concentration profiles of the ZnO-Au-13.6 film, and the XPS high-resolution spectra of the (b) Au 4d, (c) O 1 s and (d) Zn 2p orbitals of the ZnO and ZnOAu films. The open symbols represent the experimental data, the thicker lines on the symbols represents the fitted total spectra, and the thinner lines represent the peaks deconvolution. The Shirley background was used for the fittings.
fluence and Fig. 6(a) shows that the atomic concentrations throughout the ZnO-Au-13.6 film was approximately constant. Fig. 7(b–d) show the XPS high-resolution spectra of the (b) Au 4d, (c) O 1s and (d) Zn 2p orbitals for the ZnO and ZnO-Au films. The form of the Au 4d5/2 peak at 335.15 eV corresponds to the metallic state of gold (Au0) and is the same for all films [47]. Two peaks were seen in the O1s high resolution spectra, Fig. 7(c), instead of the single peak corresponding to ZnO. One peak was at 531.8 eV and is attributed to bonding between O and residual (O1sC) [32]. The second peak at 529.7 eV was related the O2– ions in the wurtzite-phase ZnO (O1sZnO) [48]. For the ZnO-Au films a small but detectable shift of the O1s peak was observed toward higher binding energy (~530 eV). This change is probably associated with the presence of gold in these samples The peak attributed to the Zn2p3/2 orbital was located at 1021.3 eV in all of the films and the difference of the binding energy between the Zn2p3/2 and Zn2p1/2 orbitals was 23 eV, which agrees with the literature [47]. However, even though the Zn/O ratio (see Table 2) indicated an excess of Zn in the films no peak related to the Zn metallic state was detected.
Fig. 8 shows the results of the XRD analysis of the deposits. Fig. 8(a) shows the XRD patterns of the Au films deposited at the different laser pulse fluence values. The identification and indexation of the Au peaks were performed using the ICDD JCPDS file 03-065-8601. The spectra indicates that the films were polycrystalline preferentially orientated in the [111] direction. A similar preferential orientation has been observed previously in Au films deposited by PLD [39]. The XRD patterns of the Au films show that the intensity of the peaks decreased as the laser fluence increased even though the experimental measurement parameters and the area of the sample were the same. There are two possible explanations for this observation, that the thickness of the deposit was less because of re-sputtering, or that less of the sample was crystalline. In the inset of the Fig. 8(a), it can be seen that the background signal between the diffraction peaks is the same for the three samples. Thus, it is probable that the amount of any amorphous phase of Au was the same, and therefore the first explanation is correct in agreement with the profilometry and OES results (Figs. 2 and 3(b)). Fig. 8(b) presents the diffraction patterns of the ZnO and ZnO-Au 71
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Fig. 8. XRD patterns of the (a) Au films and (b) ZnO and ZnO-Au films, deposited under irradiation laser pulse fluence values of 4.5, 13.6 and 20.9 J cm-2 and measured using the Theta-2Theta mode. (c) XRD patterns of the ZnO and ZnO-Au films measured by the in-plane mode.
films The zinc oxide crystallized in the hexagonal wurtzite-type structure with a preferred orientation perpendicular to the (002) basal plane, identified using the ICDD JCPDS file 01-075-0576. This orientation of the hexagonal ZnO has been reported for films grown by MS [49,50]. In the case of the ZnO-Au films patterns, Fig. 8(b) clearly shows that the broader peak located around 2θ = 38.3° observed in all the patterns of the ZnO-Au films does not correspond to any ZnO crystalline structure, this matches with the plane (111) of the gold. This suggests that both materials are in the films as separated phases. According to Wang et al. [35], the maximum limit of the solid solubility of gold in ZnO films, prepared by RF magnetron sputtering and annealed at 350 °C, is 1.00 at. %, and the samples of our work have more than that value. Thus it is expected that gold does not substitute the Zn2+ ions in the lattice of ZnO. The ZnO remained the c-axis orientation in the ZnO-Au films, while the gold apparently had a preferred orientation along the [111] crystalline direction. In addition, an increment of the relative intensity
of the (111) peak of gold with respect to the ZnO main peak is seen when the laser pulse fluence was increased. This is in agreement with the elemental concentration analysis (Fig. 6). The ZnO and ZnO-Au-20.9 films were also measured by the in-plane mode to verify the wurtzite-type structure and the preferential orientation of the zinc oxide crystal growth in both films, as shown in Fig. 8(c). The indexed planes in these XRD patterns were (100), (110) and (200), these peaks belong to the hexagonal phase and were perpendicular to the basal planes indicating that the crystals are completely oriented. The results of these measurements for the ZnO-Au20.9 films show that there was an additional weak peak indexed as (101) plane. This suggests a slight disruption of the ZnO crystalline growth by the presence of Au. The ZnO crystal size was calculated using the Scherrer's equation by considering the (002) peak. For Au the (111) plane was considered for calculation. Table 2 shows the results of these calculations for the 72
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corresponding films. It can be seen that the introduction of gold nanocrystals produced a reduction in the size of the ZnO crystals almost inversely proportional to the gold concentration in the film.
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Martins, High mobility and low threshold voltage transparent thin film transistors based on amorphous indium zinc oxide semiconductors, Solid State Electron. 52 (2008) 443–448. [20] T.K. Roy, D. Sanyal, D. Bhowmick, A. Chakrabarti, Temperature dependent resistivity study on zinc oxide and the role of defects, Mater. Sci. Semicond. Process. 16 (2013) 332–336. [21] B.J. Jin, S.H. Bae, S.Y. Lee, S. Im, Effects of native defects on optical and electrical properties of ZnO prepared by pulsed laser deposition, Mater. Sci. Eng. B 71 (2000) 301–305. [22] D.C. Look, Doping and defects in ZnO, in: C. Jagadish, S. Pearton (Eds.), Zinc Oxide Bulk, Thin Films and Nanostructures, Elsevier Science Ltd, Oxford, 2006, pp. 21–42. [23] F.J. Sheini, M.A. More, S.R. Jadkar, K.R. Patil, V.K. Pillai, D.S. Joag, Observation of photoconductivity in Sn-doped ZnO nanowires and their photoenhanced field emission behavior, J. Phys. Chem. C 114 (2010) 3843–3849. [24] Y.-S. Kim, W.-P. 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4. Conclusions The hybrid technique combining MS of ZnO and PLD of Au can be used to produce composite films. No notable changes of the Zn concentration were observed as a function of the laser pulse fluence, while the concentrations of the O decrease and Au increase as the fluence increased. The gold was incorporated as a second nanocrystalline phase and homogeneously distributed throughout the thickness of the deposits. The combination of the plasmas allowed the growth of thicker films than those deposited with the individual techniques. We conclude that bombardment of the growing film by energetic Au species, during the PLD of Au, produced substantial removal of atoms and the final thickness of the Au deposit was low. However, during the hybrid deposition, the deposited films also suffer some removal of material as the laser fluence is incremented. Therefore, the final thickness of the films was closer to the sum of the accumulation rates of ZnO and Au, and greater than the sum of the individual deposition rates. Acknowledgement This work was supported by DGAPA-UNAM-IG100418-PAPIIT, CONACyT-INFR 280635 and FONCICYT-CONACyT-CNR-278094 projects. Osmary Depablos-Rivera wishes to acknowledge the DGAPA-CICUNAM for her postdoctoral fellowship. We also wish to thank Dr. Karen Volke for her collaboration, and the academic technicians, Diego Quiterio-Vargas, Carlos Magaña, Samuel Tehuacanero Cuapa and Manuel Aguilar Ramírez, from the Laboratory of Electronic Microscopy of the Institute of Physics-UNAM; and Adriana Tejeda and Lázaro Huerta, from the Institute of Materials Research – UNAM, for the characterization of the samples by XRD and XPS. References [1] M. Novotny, J. Bulir, J. Lancok, M. Jelinek, A comparison of plasma in laser and hybrid laser-magnetron SiC deposition systems, Plasma Process. Polym. 4 (2007) S1017–S1021. [2] D. Benetti, R. Nouar, R. Nechache, H. Pepin, A. Sarkissian, F. Rosei, J.M. MacLeod, Combined magnetron sputtering and pulsed laser deposition of TiO2 and BFCO thin films, Sci. Rep. 7 (2017) 2503. [3] M. Jelinek, J. Lancok, J. Bulir, M. Novotny, Hybrid laser deposition techniques: overview and experiences, Laser Phys. 12 (2002) 306–309. [4] A.A. Voevodin, S.V. Prasad, J.S. Zabinski, Nanocrystalline carbide amorphous carbon composites, J. Appl. Phys. 82 (1997) 855–858. [5] A.A. Voevodin, J.P. O'Neill, S.V. Prasad, J.S. Zabinski, Nanocrystalline WC and WC/ a-C composite coatings produced from intersected plasma fluxes at low deposition temperatures, J. Vac. Sci. Technol. A 17 (1999) 986–992. [6] A.A. Voevodin, M.A. Capano, A.J. Safriet, M.S. Donley, J.S. Zabinski, Combined magnetron sputtering and pulsed laser deposition of carbides and diamond-like carbon films, Appl. Phys. Lett. 69 (1996) 188–190. [7] A.A. Voevodin, J.P. O'Neill, J.S. Zabinski, Nanocomposite tribological coatings for aerospace applications, Surf. Coat. Technol. 116 (1999) 36–45. [8] A.A. Voevodin, J.J. Hu, T.A. Fitz, J.S. Zabinski, Tribological properties of adaptive nanocomposite coatings made of yttria stabilized zirconia and gold, Surf. Coat. Technol. 146 (2001) 351–356. [9] A.A. Voevodin, J.J. Hu, J.G. Jones, T.A. Fitz, J.S. Zabinski, Growth and structural characterization of yttria-stabilized zirconia–gold nanocomposite films with improved toughness, Thin Solid Films 401 (2001) 187–195. [10] A.A. Voevodin, T.A. Fitz, J.J. Hu, J.S. Zabinski, Nanocomposite tribological coatings with “chameleon” surface adaptation, J. Vac. Sci. Technol. A 20 (2002) 1434–1444. [11] C.C. Baker, R.R. Chromik, K.J. Wahl, J.J. Hu, A.A. Voevodin, Preparation of chameleon coatings for space and ambient environments, Thin Solid Films 515 (2007) 6737–6743. [12] J.L. Endrino, J.J. Nainaparampil, J.E. Krzanowski, Microstructure and vacuum tribology of TiC–Ag composite coatings deposited by magnetron sputtering-pulsed laser deposition, Surf. Coat. Technol. 157 (2002) 95–101. [13] J.G. Jones, A.A. Voevodin, Magnetron sputter pulsed laser deposition: technique and process control developments, Surf. Coat. Technol. 184 (2004) 1–5. [14] J.G. Jones, C. Muratore, A.R. Waite, A.A. Voevodin, Plasma diagnostics of hybrid magnetron sputtering and pulsed laser deposition, Surf. Coat. Technol. 201 (2006) 4040–4045.
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Hybrid magnetron sputtering and pulsed laser ablation for the deposition of composite ZnO-Au films
T
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Osmary Depablos-Riveraa, , Citlali Sánchez-Akéa, Raúl Álvarez-Mendozaa,b, Tupak García-Fernándezc, Stephen Muhld, Mayo Villagrán-Muniza a
Instituto de Ciencias Aplicadas y Tecnología, Universidad Nacional Autónoma de México, Circuito Exterior s/n, Ciudad Universitaria, Ciudad de México C.P. 04510, Mexico b Facultad de Ciencias, Universidad Nacional Autónoma de México, Circuito Exterior s/n, Ciudad Universitaria, Ciudad de México C.P. 04510, Mexico c Universidad Autónoma de Ciudad de México (UACM), Prolongación San Isidro 151, Col. San Lorenzo Tezonco, Ciudad de México C.P. 09790, Mexico d Instituto de Investigaciones en Materiales, Universidad Nacional Autónoma de México, Circuito Exterior s/n, Ciudad Universitaria, Ciudad de México C.P. 04510, Mexico
A R T I C LE I N FO
A B S T R A C T
Keywords: Hybrid deposition technique Magnetron sputtering Pulsed laser deposition Zinc oxide Gold Composite films Plasma diagnostics
A hybrid technique that combines magnetron sputtering (MS) of zinc oxide and pulsed laser deposition (PLD) of gold was used to synthesize ZnO-Au composite thin films. Three different laser pulse fluence values of 4.5, 13.6 and 20.9 J cm−2 were used. Films of ZnO and Au were deposited separately by MS and PLD respectively to compare the growth by the individual and hybrid techniques. For the hybrid technique, no significant changes on the Zn concentration are observed when varying the laser pulse fluence, whilst the concentrations of O decreased and Au increased. The gold was incorporated uniformly throughout the thicknesses of the films as a second phase of nanoparticles. The presence of the gold did not modify the crystal orientation of the ZnO, which was a hexagonal wurtzite-phase with the c-axis perpendicular to the substrate. The ZnO-Au films were found to be thicker than the sum of the thicknesses of the Au and ZnO produced individually by MS and PLD, indicating that the hybrid technique incremented the net deposition rate. The emission of the laser ablation plume species, specifically neutral atoms of gold, was studied by optical emission spectroscopy to compare the individual PLD and MS-PLD processes.
1. Introduction Magnetron sputtering (MS) and pulsed laser deposition (PLD) are two physical vapor deposition techniques that are commonly used to synthesize a wide array of materials. Each individual technique has its own advantages, the main ones being that MS allows the scaling of the process from the laboratory to the industry and that PLD can transfer the stoichiometry of a target directly into the deposited film. Co-deposition processes from independent targets allow the deposition and synthesis of complex films with a precise control of their composition, structure and properties. This co-deposition may be done using different targets either in PLS or MS, or by combining both deposition methods. The combination of MS and PLD is known as the MSPLD hybrid technique [1,2] and can merge the advantages of the individual methods, in addition to the benefits offered by the co-deposition method [3]. Among these advantages it is possible to obtain deposited films of high purity, and control over the composition and crystallinity of the films at low temperature as a result of the energetic
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species in the laser ablation plume. Voevodin et al. [4–7] were the first to report the use of this technique, which allowed them to prepare composite coatings of WC, TiC and chalcogenides (WS2) nanoparticles (NPs) embedded into a diamond-like carbon (DLC) matrix. Besides, they synthesized films with yttria-stabilized zirconia (YSZ) NPs embedded into a matrix of Au + amorphous-YSZ [8,9], and with YSZ and MoS2 NPs embedded into a matrix of Au + amorphous-YSZ+ DLC [10,11]. A research group incorporated NPs of silver into a TiC matrix and assessed their tribological properties and wear performance in vacuum [12]. They adjusted the power applied to the Ag target to vary the metal concentration in the films and they found that 15% was the maximum Ag content that allowed an enhancement of the mechanical properties. Most of these reports have been focused on the performance of the films, but the plasmas evaluation have not received substantial attention. The diagnostic of the plasmas allows the in-situ analysis of the deposition process, eventually leading to a deeper understanding of the growth mechanisms, structure and properties of the films, which also
Corresponding author. E-mail addresses: [email protected], [email protected] (O. Depablos-Rivera).
https://doi.org/10.1016/j.tsf.2019.06.006 Received 18 January 2019; Received in revised form 15 May 2019; Accepted 3 June 2019 Available online 04 June 2019 0040-6090/ © 2019 Elsevier B.V. All rights reserved.
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of a gold target with a purity of 99.9% in an argon atmosphere with a pressure of 0.67 Pa. An Ekspla NL300 Nd-YAG laser with a maximum pulse energy of 230 mJ was used. The wavelength, frequency and pulse width of the laser were 355 nm, 10 Hz and 8 ns, respectively. The laser pulse energy was varied using an Altechna Watt Pilot attenuator. The experimental parameters were kept constant therefore, the variation of the pulse energy meant there was a change in the fluence: 4.5, 13.6 and 20.9 J cm−2 (see Table 1). In each deposition 12,000 laser pulses were fired. The laser beam was focused on the surface of the gold target using a multiple lens system and with an incidence angle of 45° respect to the surface normal of the target. The gold target was continuously moved along the horizontal and vertical directions to avoid that the laser spot would impinge on the same surface position. The second series consisted of composite films of ZnO and Au that were deposited by the MS-PLD hybrid technique. The target geometry and deposition conditions as well as the argon pressure, deposition time, the RF power applied to the ZnO target and the laser pulse characteristics were the same as those used in the first series. The surface of the gold target was cleaned against a shutter by laser ablation prior to each deposit in order to guarantee the same surface conditions for all deposits. The samples of both series were labeled according to the deposited material as ZnO, Au-X or ZnO-Au-X, where X is the laser pulse fluence, as shown in Table 1. The substrates were 2.5 × 2.5 cm2 pieces of (100) silicon wafers positioned in front of the Au target on a heating holder that was coupled to a temperature control system. During deposition, the substrates were kept at 400 °C, after the deposition the samples were naturally cooled to room temperature. The films thicknesses were measured using a Zygo NexView optical profilometer. The analyzed area was of 3.01 × 3.01 mm2. The surface and cross-sectional morphology of the films were evaluated using a JEOL 7800F field emission - scanning electron microscope (FE-SEM) using the upper electron detector (secondary electrons) and the gentle beam method, with an acceleration voltage of 3.0 kV and a work distance of 4.7 mm. The composition of the films was determined by X-ray photoelectron spectroscopy (XPS) using a Physical Electronics spectrometer equipped with a scanning XPS microprobe PHI 5000 VersaProbe II detector. The XPS measurements were performed at a vacuum of 4 × 10−8 Pa and using an Al Kα (hν = 1486.6 eV) X-ray source. The pass energies for the recording of the low and high-resolution spectra were 117.4 eV and 11.75 eV, respectively. Before the samples surface cleaning low-resolution spectra were recorded to define as reference the C1s peak (285 eV). After the 1 kV Ar ion cleaning of the samples for 5 min, the high-resolution spectra were measured using a current density of 55.6 nA mm−2. Additionally, the elemental concentration profiles were measured using a 2 kV Ar+ erosion beam with a current density of 1 μA mm−2. The XPS data were analyzed using the Multipak© version 9.6.0.15 software. The crystalline structure of the films and the crystallite size were determined from the X-ray diffraction (XRD) data recorded using a Rigaku Ultima IV diffractometer with a thin film accessory. The measurements were done in Theta-2Theta mode with parallel beam optics and under the following conditions: step of 0.02°, scan speed of 1°min−1, 2θ angle range of 30-80°. Furthermore, the in-plane mode was used to verify the preferential growth of the ZnO crystals. The characterization of the plasma during the deposition was carried out using an OES system, see Fig. 1(a). The emitted light was captured with a lens system and an optical fiber bundle which was connected to a Spectra Pro 500i monochromator from Acton Research, with a Czerny-Turner configuration and a focal distance of 50 cm that was equipped with a Princeton Instruments Spectra Hub PD-47 photomultiplier tube (PMT). The grating used was of 1200 grooves per mm. The emission of the plasmas was collected in the perpendicular direction to the propagation axis of the laser-induced plume and scanned along the line AB shown in Fig. 1(b) at different distances, d = 5, 15, 25, 30, 35 and 45 mm from the surface of the Au target with a spatial
translates to a better reproducibility. Jones et al. [13] documented the optical emission spectra that they recorded during the deposition of Au – YSZ films by both the hybrid MS-PLD technique and the individual techniques alone to control the deposition conditions by synchronizing the DC pulses that were being applied to the MS cathode with the intensity of the emission lines. They also analyzed the evolution of the PLD plume by performing image analysis [14]. Novotny et al. [1,15–17] detailed the characterization of the plasmas produced in a MS-PLD system during the deposition of titanium and silicon carbides. The authors [1,15] diagnosed the combined plasmas utilizing time- and spatially-resolved optical emission spectroscopy (OES), observing that the spectra profiles varied as the power applied to the MS targets (Ti and Si) and the background pressure were changed. They correlated the spectra corresponding to the MS-PLD plasma with those of the plasmas originated in the MS and PLD individual processes, and they observed the light emission from Ti+ ions and the C2 molecules only when the hybrid technique was being used, concluding that those species could influence the growth mechanisms of the carbide films. Zinc oxide (ZnO) has been extensively studied during the last decades due to its optical, electrical and magnetic properties. It is an n-type semiconductor that has a wide and direct band gap of 3.4 eV [18]. This oxide also has a large exciton binding energy (∼60 meV), a low threshold voltage [19], and it can straightforwardly be converted into a conductor through the control of its defects [20,21] and by the addition of dopants [22–27] or aggregates of noble metals such as gold or silver [28–30]. Thus, ZnO is an ideal material for its usage as a transparent conductor oxide. The combination of ZnO with gold nanoparticles (Au NPs) is an alternative way to enhance the optical and electric properties of ZnO for its use as a photocatalyst [31] and as a sensor constituent [32,33]. ZnO-Au composite films have been prepared using mainly two different ways. One of them was the deposition of bilayer films by MS, with a ZnO bottom-layer and a gold top-layer that were subsequently annealed to form Au NPs [28]. The other way was embedding Au NPs into a ZnO film, which also was obtained using two different methods. The first of these methods the embedding of the Au NPs was achieved by producing a composite film consisting of three successive layers of ZnO/Au/ZnO [34]. While in the second method, the embedding was achieved by producing Au NPs evenly distributed into a ZnO matrix, which was obtained using either chemical techniques [31] or by sputtering co-deposition using a ZnO target with gold wires on its surface [35]. The aim of the present paper is to report the use of the MS-PLD hybrid technique, which consisted in the combination of ZnO magnetron sputtering and pulsed laser deposition of gold, to obtain ZnO-Au composite films. The advantage of the MS-PLD hybrid method is the control of the concentration of gold in the films by varying the laser pulse fluence. Furthermore, we report the structure and composition of the films obtained by the hybrid and individual techniques, and the insitu temporal and spatial analysis of the plasma by Optical Emission Spectroscopy (OES) during the individual and hybrid processes. 2. Experimental details All experiments were performed inside a vacuum chamber with independent arrangements of MS and PLD, as shown in Fig. 1(a). The PLD target was located at a distance of 5.2 cm in front of the substrate holder, and the separation between the center of the MS target and the substrates was 6.8 cm. There was an angle of 67° between the surfaces of the MS target and the substrate holder, as shown in Fig. 1(b). The base pressure of the system was 4 × 10−4 Pa. Two series of films were deposited. The first series consisted of separately deposited thin film samples of pure ZnO and Au. The ZnO depositions were performed by balanced magnetron sputtering using a zinc oxide target (purity 99.9%, diameter 2 in.), a radio-frequency (RF, 13.56 MHz) power of 150 W, an argon pressure of 0.67 Pa and a deposition time of 20 min. The Au films were deposited by laser ablation 67
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Fig. 1. (a) Experimental configuration of the MS-PLD hybrid technique and the optical emission spectra measurement system. (b) Details of the geometric configuration of the targets and substrates (side view). Table 1 Notation of the samples and the values of the laser energy and fluence used. Sample
Deposition technique
Laser pulse energy (mJ)
Laser pulse fluence (J cm−2)
ZnO Au-4.5 Au-13.6 Au-20.9 ZnO-Au-4.5 ZnO-Au-13.6 ZnO-Au-20.9
MS PLD
– 49.8 ± 0.1 149.5 ± 0.3 230.8 ± 0.4 49.6 ± 0.1 150.5 ± 0.4 230.6 ± 0.5
– 4.5 ± 0.4 13.6 ± 1.2 20.9 ± 1.4 4.5 ± 0.4 13.7 ± 1.2 20.9 ± 1.9
MS-PLD
resolution about 7 mm2. The 479.26 and 405.28 nm emission lines were selected as the most suitable to detect the presence of the atomic neutral (Au I) and singly ionized (Au II) gold, respectively. The temporal evolution of the selected emission line was recorded using the PMT coupled to a Tektronix DPO 7054 digital oscilloscope and averaging 50 temporal evolution profiles. The temporal and spatial resolution of the equipment were 0.4 ns and 0.01 nm, respectively. The mean kinetic energy of the Au I and Au II species, during the PLD of gold in argon ambient, was calculated by plotting the emission intensity (PMT voltage) vs. time, also known as time of flight (TOF) distribution, using the procedure describe by Bulgakova et al. [36].
Fig. 2. Thicknesses of the ZnO, Au, and ZnO-Au films. The dashed line indicates the mean thickness of the ZnO films and the shaded area represents its error. The sum of the ZnO and Au films thicknesses is included (open symbols).
to sputter gold; 36 eV [39,40]. Therefore, it is reasonable that the excess of energy of the arriving gold species to the growing film surface could contribute to the re-sputtering of the deposited material [41,42]. Additionally, Fig. 2 shows that there was a slight decrease in the thickness of the ZnO-Au films at the highest laser fluence of 20.9 J cm−2 which was probably also due to re-sputtering of the deposit. Furthermore, we observed the ZnO-Au films prepared using the hybrid technique were 25, 42 and 36% thicker than the sum of thicknesses of the ZnO and Au films deposited by either MS or PLD. This suggests that there was either an enhancement in the deposition rate, or a decrease in the film density, with respect to the individual techniques. In fact, considering the general ideas of film deposition by PVD methods, for a constant deposition time the final film thickness is a balance between the accumulation rate, i.e. in this case the sputtering or ablation rates, the resulting arrival rates at the substrate plus the sticking coefficient of the various materials, and the removal or etch rate from the film surface. The removal phenomenon can be physical sputtering due to bombardment, chemical sputtering, thermal evaporation, etc. Now it is unlikely that the Au bombardment of the substrate from the PLD process would affect the sputtering rate, and therefore the arrival rate of the ZnO to the substrate. Similarly, it is doubtful that the Au would affect the sticking coefficient of ZnO.
3. Results and discussion Despite the relative position of the cathode and the substrate implied a gradient in the film thickness, there was a zone of about 1 × 1 cm2 in which the thickness was homogenous. This zone was organized to be the same as where the laser ablation plume arrived on the substrates. All of the film characterization was carried out in this homogeneous zone. Fig. 2 shows the average thickness of the MS deposited ZnO, PLD deposited Au, and the MS-PLD deposited ZnO-Au films in the homogenous zone. The sum of the thicknesses of the individual Au and ZnO films is also shown. It can be seen that there was a reduction in the thicknesses of the gold films as the laser pulse energy was increased. It has been reported that an increase in the laser fluence normally increases the gold deposition rate [37,38]. Possibly our results can be explained by the fact that as the laser fluence is increased the kinetic energy of the emitted ions also increases [38], and therefore, since the final film thickness is a balance between the deposition rate and the removal rate by re-sputtering, an increase in the re-sputtering results in a decrease in the deposited thickness. The reported kinetic energy values of Au species in the PLD plume, under similar conditions to ours, are higher than the threshold energy 68
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Consequently, the accumulation rate of ZnO on the substrate is probably independent of the laser fluence. The bombardment by Au species may have increased the removal rate, but such an affect cannot help to explain why the hybrid film thickness was greater than the sum of the MS-ZnO and PLD-Au film thicknesses. However, if we consider the deposition of Au the situation is somewhat different. Given the geometry of the MS and PLD targets it is probable that the arrival rate of Au to the substrate was independent of the presence of ZnO. Nevertheless, the sticking coefficient of Au to the Si substrate, or the previously deposited Au, would not be the same as the sticking coefficient of Au to the ZnO. It is not clear which sticking coefficient is larger, but the accumulation rate of Au was probably not the same with and without the presence of ZnO. Furthermore, the removal rates with and without ZnO are almost certainly different. Without ZnO the re-sputtering of Au by high energy bombardment by Au species would be quite efficient because the mass of the projectile and sputtered particle was the same. However, in the hybrid process the deposited Au atoms are partially covered and surrounded by ZnO molecules, and it is reasonable to expect that the removal rate of Au was significantly reduced. In summary, the PLD deposited Au suffered substantial removal due to bombardment by energetic Au species and the final thickness of the Au deposit was lower. MS deposited ZnO suffered some removal of Zn at the highest laser fluence due to the strong Au bombardment (see the reduction in the thickness of the ZnO-Au at the highest laser fluence, Fig. 2 and the Zn and O at.% versus the laser fluence presented below). During the hybrid deposition the removal rate of Au was reduced, therefore the final thickness of the films was closer to the sum of the accumulation rates of ZnO and Au, and greater than the sum of the individual deposition rates. Fig. 3 show the integrated optical emission intensity emitted by the atomic neutral Au I (479.26 nm) as a function of the distance from the PLD target surface for the three laser pulse fluence values (4.5, 13.6 and 20.9 J cm−2), during both (a) the PLD and (b) the MS-PLD hybrid technique deposition. For the highest laser fluence there was an increase in the Au I emission at d = 45 mm, i.e. 7 mm in front of the substrates, and we consider that this supports the idea of re-sputtering, since the emission is probably from Au atoms sputtered from the surface of the growing film. This behavior was not observed during the MS-PLD process; this could be interpreted as the inhibition of re-sputtering as a consequence of the presence of the MS plasma. The data for Fig. 3 was extracted from the temporal profiles of the Au I and Au II emission lines curves measured by OES. Two of these TOF distributions obtained for Au I and Au II during the PLD process in argon are shown in Fig. 4. We only carried out the TOF analysis for the strongest signals of Au
Fig. 4. TOF distribution of neutral atoms and ions of gold, Au I 479.26 nm and Au II 405.28 nm, measured at 45 and 25 mm from the surface of the PLD target, respectively, during laser ablation of gold in argon atmosphere (0.67 Pa) and irradiating with the laser fluence of 20.9 J cm−2. The thicker lines correspond to the data smoothed using the Savitzky-Golay method. Time zero corresponds to the laser pulse onset.
atoms and ions to calculate their average kinetic energy at distances nearer to the substrates, d = 45 mm for Au I and d = 25 mm for Au II emission lines. The signal of gold ions was almost non-existent at distances longer than 25 mm from the gold target surface probably due to the recombination of the ions. The average kinetic energy was calculated using the Eq. (1), extracted from the reference [36]; where m was the species mass, L the target to probe distance, and I (t) was the OES signal intensity as a function of time. ∞
〈EK 〉 =
mL2 ∫0 t −2I (t ) dt ∞
2 ∫0 I (t ) dt
(1)
The uncertainty of the TOF analysis of the weak distributions was too large to allow us to determine the mean kinetic energy for all of the deposition conditions. These weak distributions are corresponding to the OES data recorded at distances d between 25 and 35 mm or longer than 25 mm for the neutral atoms and ions of gold, respectively, in combination with the lowest laser fluence. Furthermore, calculations of kinetic energy from the TOF emission distributions of the different
Fig. 3. Integrated intensity of the temporal evolution profiles of the Au I emission line (479.26 nm) as a function of the distance from the target d, recorded during the (a) Au and (b) ZnO-Au films deposition process when laser pulse fluence values of 4.5, 13.6 and 20.9 J cm-2 were used. The insets correspond to the dot-dashed square zone where an abnormal behavior of the Au I emission was observed. 69
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Fig. 5. SEM micrographs of the surface morphology of (a) ZnO film, and ZnO-Au films deposited using laser pulse fluence values of (b) 4.5, (c) 13.6 and (d) 20.9 J cm. Inset: cross-sectional images of the ZnO-Au films.
2
species involved in the MS-PLD hybrid process would have uncertainty because the emission is produced by collisions between species of both plasmas, thus it cannot be interpreted directly as kinetic data. The analysis of the temporal profiles or TOF distributions for Au I and Au II emission lines allowed to calculate the average kinetic energy of the neutral gold atoms and gold ions, which were 130 and 700 eV, respectively. A comparison of the surface morphology of the ZnO and ZnO-Au films is provided by the micrographs shown in Fig. 5(a) – (d); the morphology of the ZnO-Au films can be seen from the cross-sectional images. The cross-sectional micrographs show compact films. Additionally, the differences between the surface morphology of the films of ZnO deposited by MS and the ZnO-Au films deposited by MS-PLD hybrid technique can be observed. The morphology of ZnO films was granular, while the morphology of the other films looked smooth. Dark spots were observed in Fig. 5(c – d), which were probably due to surface contamination in the films since they were not observed along the entire films. We consider that the composite films had small droplets on the smooth surface as a consequence of the typical splashing from the laser ablation process. This has been reported in some studies of the preparation of films by MS-PLD [43,44]. The change of the morphology was probably related to an increase in the surface mobility of the species on the surface of the growing film due to bombardment from the gold ions and fast neutrals, and the increment of the re-sputtering effect with the laser fluence. The gold species not only have a larger atomic mass than the ZnO species but also arrive with significant kinetic energy and therefore the effect of bombardment on the morphology of the deposits was large [45,46]. Fig. 6 shows the atomic composition of the ZnO and ZnO-Au films, determined by XPS by analysis of the Au 4d5/2, Zn 2p3/2 and O 1s orbitals. The concentration of Zn did not vary with the laser fluence. The oxygen concentration decrease with the laser fluence and therefore Zn/ O ratio increased from 1.09 to 1.39 for the highest laser fluence, as shown in Table 2. This loss of oxygen, the lightest atom in the deposits, is probably due to the bombardment of the deposit by energetic gold species as mention above. The gold atomic percentages in the ZnO-Au films were 5.7 ± 0.3, 9.3 ± 0.5 and 15.9 ± 0.8 at.% for the corresponding fluence values of 4.5, 13.6 and 20.9 J cm−2. Thus, the gold incorporation into the ZnO-Au films was proportional to the laser pulse
Fig. 6. Elemental composition of the ZnO and ZnO-Au films as function of the laser pulse fluence. Table 2 Summary of data extracted from XPS and XRD results. Sample
ZnO Au-4.5 Au-13.6 Au-20.9 ZnO-Au-4.5 ZnO-Au-13.6 ZnO-Au-20.9
Zn/O ratio
1.09
1.19 1.38 1.39
Size of the crystal (nm) ZnOa
Aub
67 – – – 47 41 36
– 77 66 57 9 7 7
The crystallite size was calculated using the Scherrer's equation. a For the peaks corresponding to the ZnO (002) plane. b For the peaks corresponding to the Au (111) plane.
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Fig. 7. (a) Concentration profiles of the ZnO-Au-13.6 film, and the XPS high-resolution spectra of the (b) Au 4d, (c) O 1 s and (d) Zn 2p orbitals of the ZnO and ZnOAu films. The open symbols represent the experimental data, the thicker lines on the symbols represents the fitted total spectra, and the thinner lines represent the peaks deconvolution. The Shirley background was used for the fittings.
fluence and Fig. 6(a) shows that the atomic concentrations throughout the ZnO-Au-13.6 film was approximately constant. Fig. 7(b–d) show the XPS high-resolution spectra of the (b) Au 4d, (c) O 1s and (d) Zn 2p orbitals for the ZnO and ZnO-Au films. The form of the Au 4d5/2 peak at 335.15 eV corresponds to the metallic state of gold (Au0) and is the same for all films [47]. Two peaks were seen in the O1s high resolution spectra, Fig. 7(c), instead of the single peak corresponding to ZnO. One peak was at 531.8 eV and is attributed to bonding between O and residual (O1sC) [32]. The second peak at 529.7 eV was related the O2– ions in the wurtzite-phase ZnO (O1sZnO) [48]. For the ZnO-Au films a small but detectable shift of the O1s peak was observed toward higher binding energy (~530 eV). This change is probably associated with the presence of gold in these samples The peak attributed to the Zn2p3/2 orbital was located at 1021.3 eV in all of the films and the difference of the binding energy between the Zn2p3/2 and Zn2p1/2 orbitals was 23 eV, which agrees with the literature [47]. However, even though the Zn/O ratio (see Table 2) indicated an excess of Zn in the films no peak related to the Zn metallic state was detected.
Fig. 8 shows the results of the XRD analysis of the deposits. Fig. 8(a) shows the XRD patterns of the Au films deposited at the different laser pulse fluence values. The identification and indexation of the Au peaks were performed using the ICDD JCPDS file 03-065-8601. The spectra indicates that the films were polycrystalline preferentially orientated in the [111] direction. A similar preferential orientation has been observed previously in Au films deposited by PLD [39]. The XRD patterns of the Au films show that the intensity of the peaks decreased as the laser fluence increased even though the experimental measurement parameters and the area of the sample were the same. There are two possible explanations for this observation, that the thickness of the deposit was less because of re-sputtering, or that less of the sample was crystalline. In the inset of the Fig. 8(a), it can be seen that the background signal between the diffraction peaks is the same for the three samples. Thus, it is probable that the amount of any amorphous phase of Au was the same, and therefore the first explanation is correct in agreement with the profilometry and OES results (Figs. 2 and 3(b)). Fig. 8(b) presents the diffraction patterns of the ZnO and ZnO-Au 71
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Fig. 8. XRD patterns of the (a) Au films and (b) ZnO and ZnO-Au films, deposited under irradiation laser pulse fluence values of 4.5, 13.6 and 20.9 J cm-2 and measured using the Theta-2Theta mode. (c) XRD patterns of the ZnO and ZnO-Au films measured by the in-plane mode.
films The zinc oxide crystallized in the hexagonal wurtzite-type structure with a preferred orientation perpendicular to the (002) basal plane, identified using the ICDD JCPDS file 01-075-0576. This orientation of the hexagonal ZnO has been reported for films grown by MS [49,50]. In the case of the ZnO-Au films patterns, Fig. 8(b) clearly shows that the broader peak located around 2θ = 38.3° observed in all the patterns of the ZnO-Au films does not correspond to any ZnO crystalline structure, this matches with the plane (111) of the gold. This suggests that both materials are in the films as separated phases. According to Wang et al. [35], the maximum limit of the solid solubility of gold in ZnO films, prepared by RF magnetron sputtering and annealed at 350 °C, is 1.00 at. %, and the samples of our work have more than that value. Thus it is expected that gold does not substitute the Zn2+ ions in the lattice of ZnO. The ZnO remained the c-axis orientation in the ZnO-Au films, while the gold apparently had a preferred orientation along the [111] crystalline direction. In addition, an increment of the relative intensity
of the (111) peak of gold with respect to the ZnO main peak is seen when the laser pulse fluence was increased. This is in agreement with the elemental concentration analysis (Fig. 6). The ZnO and ZnO-Au-20.9 films were also measured by the in-plane mode to verify the wurtzite-type structure and the preferential orientation of the zinc oxide crystal growth in both films, as shown in Fig. 8(c). The indexed planes in these XRD patterns were (100), (110) and (200), these peaks belong to the hexagonal phase and were perpendicular to the basal planes indicating that the crystals are completely oriented. The results of these measurements for the ZnO-Au20.9 films show that there was an additional weak peak indexed as (101) plane. This suggests a slight disruption of the ZnO crystalline growth by the presence of Au. The ZnO crystal size was calculated using the Scherrer's equation by considering the (002) peak. For Au the (111) plane was considered for calculation. Table 2 shows the results of these calculations for the 72
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corresponding films. It can be seen that the introduction of gold nanocrystals produced a reduction in the size of the ZnO crystals almost inversely proportional to the gold concentration in the film.
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4. Conclusions The hybrid technique combining MS of ZnO and PLD of Au can be used to produce composite films. No notable changes of the Zn concentration were observed as a function of the laser pulse fluence, while the concentrations of the O decrease and Au increase as the fluence increased. The gold was incorporated as a second nanocrystalline phase and homogeneously distributed throughout the thickness of the deposits. The combination of the plasmas allowed the growth of thicker films than those deposited with the individual techniques. We conclude that bombardment of the growing film by energetic Au species, during the PLD of Au, produced substantial removal of atoms and the final thickness of the Au deposit was low. However, during the hybrid deposition, the deposited films also suffer some removal of material as the laser fluence is incremented. Therefore, the final thickness of the films was closer to the sum of the accumulation rates of ZnO and Au, and greater than the sum of the individual deposition rates. Acknowledgement This work was supported by DGAPA-UNAM-IG100418-PAPIIT, CONACyT-INFR 280635 and FONCICYT-CONACyT-CNR-278094 projects. Osmary Depablos-Rivera wishes to acknowledge the DGAPA-CICUNAM for her postdoctoral fellowship. We also wish to thank Dr. Karen Volke for her collaboration, and the academic technicians, Diego Quiterio-Vargas, Carlos Magaña, Samuel Tehuacanero Cuapa and Manuel Aguilar Ramírez, from the Laboratory of Electronic Microscopy of the Institute of Physics-UNAM; and Adriana Tejeda and Lázaro Huerta, from the Institute of Materials Research – UNAM, for the characterization of the samples by XRD and XPS. References [1] M. Novotny, J. Bulir, J. Lancok, M. Jelinek, A comparison of plasma in laser and hybrid laser-magnetron SiC deposition systems, Plasma Process. Polym. 4 (2007) S1017–S1021. [2] D. Benetti, R. Nouar, R. Nechache, H. Pepin, A. Sarkissian, F. Rosei, J.M. MacLeod, Combined magnetron sputtering and pulsed laser deposition of TiO2 and BFCO thin films, Sci. Rep. 7 (2017) 2503. [3] M. Jelinek, J. Lancok, J. Bulir, M. Novotny, Hybrid laser deposition techniques: overview and experiences, Laser Phys. 12 (2002) 306–309. [4] A.A. Voevodin, S.V. Prasad, J.S. Zabinski, Nanocrystalline carbide amorphous carbon composites, J. Appl. Phys. 82 (1997) 855–858. [5] A.A. Voevodin, J.P. O'Neill, S.V. Prasad, J.S. Zabinski, Nanocrystalline WC and WC/ a-C composite coatings produced from intersected plasma fluxes at low deposition temperatures, J. Vac. Sci. Technol. A 17 (1999) 986–992. [6] A.A. Voevodin, M.A. Capano, A.J. Safriet, M.S. Donley, J.S. Zabinski, Combined magnetron sputtering and pulsed laser deposition of carbides and diamond-like carbon films, Appl. Phys. Lett. 69 (1996) 188–190. [7] A.A. Voevodin, J.P. O'Neill, J.S. Zabinski, Nanocomposite tribological coatings for aerospace applications, Surf. Coat. Technol. 116 (1999) 36–45. [8] A.A. Voevodin, J.J. Hu, T.A. Fitz, J.S. Zabinski, Tribological properties of adaptive nanocomposite coatings made of yttria stabilized zirconia and gold, Surf. Coat. Technol. 146 (2001) 351–356. [9] A.A. Voevodin, J.J. Hu, J.G. Jones, T.A. Fitz, J.S. Zabinski, Growth and structural characterization of yttria-stabilized zirconia–gold nanocomposite films with improved toughness, Thin Solid Films 401 (2001) 187–195. [10] A.A. Voevodin, T.A. Fitz, J.J. Hu, J.S. Zabinski, Nanocomposite tribological coatings with “chameleon” surface adaptation, J. Vac. Sci. Technol. A 20 (2002) 1434–1444. [11] C.C. Baker, R.R. Chromik, K.J. Wahl, J.J. Hu, A.A. Voevodin, Preparation of chameleon coatings for space and ambient environments, Thin Solid Films 515 (2007) 6737–6743. [12] J.L. Endrino, J.J. Nainaparampil, J.E. Krzanowski, Microstructure and vacuum tribology of TiC–Ag composite coatings deposited by magnetron sputtering-pulsed laser deposition, Surf. Coat. Technol. 157 (2002) 95–101. [13] J.G. Jones, A.A. Voevodin, Magnetron sputter pulsed laser deposition: technique and process control developments, Surf. Coat. Technol. 184 (2004) 1–5. [14] J.G. Jones, C. Muratore, A.R. Waite, A.A. Voevodin, Plasma diagnostics of hybrid magnetron sputtering and pulsed laser deposition, Surf. Coat. Technol. 201 (2006) 4040–4045.
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