Particulate reduction in ternary-compound film growth via pulsed laser deposition from segmented binary-targets

In the PLD setup used in this work (see schematic in figure 2), pulses of ∼20 ns duration and wavelength of 248 nm (UV) produced from a KrF excimer laser operating at a repetition rate of 20 Hz, were focused into a stainless steel vacuum chamber, which was back-filled with oxygen to a pressure of 2 × 10⁻² mbar. Inside the chamber, the laser beam was focused to produce a fluence of ∼1 J cm⁻² on the surface the ceramic-disc target (50 mm in diameter and 5 mm thick), which was rotated, using a DC motor, on a cam introducing an epitroichoidal path for the incident laser beam. Such a motion produced an ablation ring of ∼12 mm in width and ∼45 mm outer diameter, after many thousands of shots, and was designed to enable greater target utilization during deposition. Targets consisting of a mixture of nominally three-eighths Y₂O₃ and five-eighths Ga₂O₃ (referred to as the mixed YGG), and a disc comprising two spatial segments of three-eighths Y₂O₃ and five-eighths Ga₂O₃ (referred to as segmented YGG), were used for the deposition of the thin films.

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The laser ablation plume was incident on the surface of a 10 mm × 10 mm × 1 mm 〈100〉-orientated YAG substrate that was polished on both faces. To ensure uniformity of the deposited film over the whole substrate, homogenous heating of the substrate was carried out using a CO₂ laser (10.6 μm wavelength, maximum output of 38 W). The CO₂ laser beam was spatially re-formatted using a tetrapism to produce a square 'top-hat' intensity profile on the rear-face of the substrate [12]. To minimize heat sinking and to maintain the homogeneity of the heating, the substrate was held using an alumina cradle with minimal surface contact. Specifically, an incident CO₂ laser power of 10.5 W–14 W, heated the substrate to ∼700 °C–760 °C, estimated from previous calibration experiments in which metal foils of known melting point were attached to the substrate [25].

The mixed YGG target was fabricated from compression and sintering of powders in the 3:5 atomic percentage ratio of Y₂O₃ to Ga₂O₃, which had an average particle size of ∼11.4 μm for Y₂O₃ and ∼1.9 μm for Ga₂O_3, from the data supplied by the target manufacturer. The segmented YGG target was formed from two binary targets, cut and shaped using a diamond tipped Dremel® rotary saw and reassembled to the normal circular 50 mm diameter format. Although it was expected that imperfect ablation would occur at the rough edge of the join between the two target pieces, this area was only ∼1/500 of the total area of the re-assembled target. Thus it was considered that the contribution to particulates from his region would be negligible. Such segmented targets have been used previously in PLD experiments, but for an alternative end goal of achieving stoichiometric growth [26, 27].

Although it would be preferable to be able to change the ablation laser pulse energy on a segment-to-segment basis in order to use the optimal ablation fluence for each target segment (i.e. the fluence that produced lowest scattering points in the grown films for each material), in our setup this was not achievable. Thus, a laser ablation fluence of ∼1 J cm⁻² was chosen for the deposition using a segmented target, since this was also the fluence used for ablation of the mixed YGG target. Based on earlier work [28], which analysed sequential PLD growth using two physically different targets in a multi-beam PLD deposition system, it was necessary to keep the maximum number of pulses for each segment below 50 (layer thickness ∼1 nm). This avoided growth of discrete multilayers of Y₂O₃ and Ga₂O₃ that can occur if sequential ablation occurs involving too many laser pulses per segment. Since the rotation speed of the target in our setup was 1 revolution per second, 20 pulses were incident on the target in one rotation, which meant that for each rotation of the segmented target, there were significantly fewer than 50 pulses per segment, thus allowing for the growth of a crystalline YGG film.

For each deposition, the corresponding target was ablated using a total of 36 000 pulses, which produced ∼2 μm thick films. The YGG films were analysed using a dark-field microscope (Nikon L200D) to determine the number of scattering points and the crystalline quality was evaluated via x-ray diffraction (XRD) using a Rigaku Smartlab x-ray diffractometer. The targets were analysed using a scanning electron microscope (SEM) (Zeiss Evo 50) to characterize their surface topology.

The XRD spectra of the film grown from the mixed YGG target is shown in figure 3 (green dotted-line), along with the XRD spectra for the YGG film grown from the segmented YGG target (green solid-line). The database values for the (400) YGG peak and the (400) YAG substrate peak, are displayed as a black-dashed line and black-dotted line, respectively. It is evident from the plot that crystalline YGG has been faithfully grown using the segmented target, and the (400) YGG peak position value from the film is an exact match to the database value, indicating the film has the correct composition of Y₃Ga₅O₁₂. However, the (400) YGG peak position value from the film deposited using the mixed YGG target is lower than that of the database value by a 2θ value of 0.1° at 28.06° indicating a reduction of the gallium content within the film [12].

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The broader full width at half maximum (FWHM) of the (400) YGG peak from the film grown using a segmented YGG target (0.11°), compared with the FWHM of the (400) YGG peak from the film grown from the mixed YGG target (0.03°) is interesting and warrants some further comment. Unlike the case for ablation from the mixed target, use of a segmented target leads to a sequential or stepwise deposition of elemental Y and Ga (or their oxides) in the growing YGG film. As discussed in section 2.2, previous work suggests that deposition using more than 50 pulses for sequential ablation of two separate targets in a multi-beam chamber can lead to layering of the two target compounds [28]. However, in this work, since a thickness of ∼0.05 nm of material is deposited per pulse, on average, 12.5 sequential ablation pulses of the Ga₂O₃ segment per target rotation potentially deposits ∼0.625 nm of Ga₂O₃ oxide, and so layers of Ga₂O₃ are potentially grown in addition to the formation of Y₃Ga₅O₁₂, that can be formed following the deposition of the necessary Y₂O₃. Likewise, since, on average, 7.5 ablation pulses hit the Y₂O₃ segment per target rotation, a potential deposition of ∼0.375 nm of Y₂O₃ may occur, and so layers of Y₂O₃ could also be produced in addition to the formation of Y₃Ga₅O₁₂. Such minority phases of Ga₂O₃ and Y₂O₃ can indeed be observed in figure 3(b), which presents the XRD spectra of film growth from a segmented-target for a 2θ scan range from 42° to 66°, and shows the growth of (202) Ga₂O₃ at a 2θ peak value of 44.1° and a growth of (721) Y₂O₃ at a 2θ peak value of 64.25°. Since the height of these alternative peaks are relatively small compared with the (400) YGG peak (0.3% and 0.1% for (202) Ga₂O₃ and (721) Y₂O₃, respectively, normalised to the peak height of the intended Y₃Ga₅O₁₂ film growth), they only appear to cause broadening of the (400) YGG peak, as the (400) YGG peak value matches the database value.

Dark-field microscope images of the film deposited from the mixed YGG target and segmented YGG target, taken using a 100-x magnification objective, are shown in figure 4. The white features in the figure are regions of the film from which light has been scattered. Using a peak-intensity detection algorithm, 566 peaks were determined to be present in the image of the thin film grown using a mixed YGG target, while only 5 peaks were detected in the image of the thin film grown using a segmented YGG target.

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SEM images of the surfaces of the film, displayed in figure 5, show that the scattering points on the surface do appear to be particulates, rather than phase growth, owing to their non-cubic shape.

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To further aid in understanding the differences between the numbers of scattering points within the film grown using the mixed YGG target and the film grown using the segmented YGG target, the surfaces of the targets were analysed before and after ablation. Figure 6 displays SEM secondary-electron images of the surface of the targets before ablation (a)–(c) and after ablation (d)–(f). Before ablation, the surface of all the targets look similar, with the structure of the constituent powder present in all. After ablation, the surfaces of Y₂O₃ and Ga₂O₃ show no signs of structure that was present before ablation and no signs of conical structures. However, as seen in figure 6(e), irregular cone-like features of ∼10 μm diameter are present on the surface of the mixed YGG target.

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This cone structure is also likely to contribute to the difference in stoichiometry between the two YGG films, indicated in figure 3, since the stoichiometry and geometry of these cones can be different to that of non-ablated regions [29–31]. In this instance, this means less Ga is potentially being transferred from the target to the substrate. Since the mixed YGG target was formed from a sintered hot-pressed combination of two materials Y₂O₃ and Ga₂O₃, the surface of the target will inevitably possess non-uniformity of composition that results from the intrinsic particle size of the grains from which the target is formed.

Within the mixed YGG target, the different ablation thresholds of each of the binary compounds, which results from their differing values of optical absorption, thermal conductivity, melting and boiling points (e.g. Ga₂O₃ and Y₂O₃ melting points are 1900 °C and 2425 °C, respectively), combined with the non-homogenous initial distribution of the compounds within the target, all point to an explanation for the origin of the cone structure on the surface of the mixed YGG target. The higher percentage of porosity of the mixed YGG target, 32.1%, compared with the segmented targets, which were 19.4% and 16.6%, for Y₂O₃ and Ga₂O₃, respectively, potentially contributed to the higher particulates present in the films. Although the cones can progressively reduce the efficiency of ablation, in this work, the relatively few pulses-per-site, (∼240 pulses-per-site) was found to not decrease the deposition rate significantly. In work by others, it has been found for the PLD of YBa₂Cu₃O_7−x, by 2000 pulses-per-site, ablation rate of 10% initial rate can be reached [18].

The correlation between the presence of cones on the target and a significantly higher (by a factor of ∼100) number of scattering points when using the mixed target compared to a segmented target suggests an obvious way forward for future PLD growth strategies, if reduced particulate counts are an important factor for the particular PLD research direction. The effect of target surface quality with variation in deposition parameters, such as fluence, has been explored by O'Brien et al [32], in which higher fluence increased the angle of the cone slant height relative to the surface of the target and increased the particle density in the grown film. However, future work should involve analysis of the chemical composition of the particulates in the film to clarify any potential variation in the origin of the target particulates over the surface of the film.