Searching for refractory plasmonic materials: the structural and optical properties of Au$_{3}$Zr intermetallic thin films

Optical properties of refractory intermetallic thin films of Au$_{3}$Zr were experimentally investigated for the first time, which show distinctive plasmonic properties in the visible and near infrared region. The films were fabricated through DC magnetron sputtering at various deposition temperature ranging from room temperature to 427$^{o}$C and annealed at different vacuum levels. Both the structural and optical properties are found to be critically dependent on deposition temperature and anneal conditions. Films deposited between 205-320$^{o}$C are shown to exhibit lower negative permittivity and better thermal stability, which could be linked to specific crystalline orientations. The films are stable when annealed at 10$^{-8}$ Torr, but are partially oxidized when annealed at 10$^{-6}$ Torr, suggesting oxidization could be a restricting issue for high-temperature applications in ambient environment.

region. However, the softness of noble metals and the reduction of melting points at the nanoscale make them vulnerable for applications in high-temperature environment, such as in heat-assisted magnetic recording (HAMR), where the working temperature is above 400°C [9]. This causes Au-based near-field transducer to deform, which is one of the major obstacles establishing the reliability of HAMR technology [10]. Apart from HAMR, many other high-temperature technologies and applications also have strong demand of the development of refractory plasmonic materials, such as thermoelectrics [11], thermophotovoltaics [12], water splitting [13] and nonlinear optics [14]. This has prompted great interest in the search for refractory plasmonic materials that can work in harsh environment of elevated temperature [15,16].
Previous work has looked at various options from metallic silicides, nitrides, conducting oxides, and intermetallic alloys [17][18][19]. Among these, intermetallic alloys represent an interesting new paradigm of refractory plasmonic material. It is predicted that many intermetallic alloys could exhibit promising plasmonic properties [20,21]. However, so far very little experimental work has been demonstrated. In this investigation we alloyed Au with Zr, one of the refractory metals of high melting temperature, which is also highly resistant to corrosion. We focus on the Au3Zr binary alloy, which has a melting temperature up to 1610°C [22]. Keast et al. performed density-of-states calculations of the imaginary part of permittivity of Au-Zr compound (87.5at%Au-12.5at%Zr), which shows a strong peak around 2.4 eV just below the interband transition of Au [20]. However, to date the optical properties of Au-Zr intermetallic alloys have not been established. The results presented here provide the first detailed experimental study of the optical properties of Au-Zr intermetallic compound.

II. MATERIALS AND METHODS
Thin films of Au3Zr, close to 100 nm thick, were co-sputtered at an argon process pressure of 0.8 mTorr using DC magnetrons from elemental gold and zirconium targets of purity better than 99.99% and 99.5% respectively in a cryo-pumped UHV deposition chamber with base pressure below 10 -8 Torr. A set of films were fabricated at different substrate deposition temperature ranging from room temperature (RT) to 427°C. The thin films were deposited onto pre-cleaned Si wafers, with 300 nm oxide layer. There were two exceptions. Films deposited at 320°C and 415°C were fabricated on transparent SiO2 wafers. This was intended to identify any changes on the undersides of these films during annealing processes. No adhesion layers were applied between the substrate and the deposited film.
The samples were left in the process chamber to anneal in-situ at 450°C for 1 hour, immediately after deposition, to relieve thermal stress and promote grain growth. Comparative samples, deposited at RT and 439°C were also fabricated, without the in-situ anneal. Structural properties were characterised by XRR and XRD on a Bruker D8 Discover X-ray diffractometer (Cu Ka, l = 1.5418 Å) using Bruker Leptos and EVA software for data analysis. Au3Zr forms an orthorhombic structure, belonging to the Cu3Ti prototype [23] and the space group of Pmmn (59) with lattice parameters of a = 6.062 Å, b = 4.865 Å, c = 4.785 Å [20]. The major peaks in the samples were identified by peak fitting using these lattice parameters and space group.
The resistivity of the films was determined by using a Jandel 4-point probe. The optical properties of the structures were determined by the use of a J A Woollam Co. Inc, EC-400 spectroscopic ellipsometer, with an M-2000VI light source, operating with a spectral range from 370 nm to 1690 nm. The reflection spectra were measured using an Olympus BX51 optical microscope with a 5× objective (NA=0.1). The theoretical reflection spectra of the films were calculated from Fresnel's equation of reflectance at normal incidence, which is a close approximation of the experimental condition. To establish the thermal stability the films were annealed for a second time in a vacuum oven that had been purged with argon gas. The temperature was raised to 497°C at a rate of 3°C/min, and dwelled at this peak for 1 hour, at a chamber pressure of 1.5×10 -6 Torr.

Structural properties
The thickness of the films was between 90-95 nm. Fig.1 shows the XRD spectra (the intensity is normalised to the maximal peak between 32 o and 42 o ) of the films.
From Fig.1 it can be seen that the films are preferentially aligned along one primary crystalline orientation, with a small proportion of other textures, a trend to be expected given the lack of lattice matching between the deposition surface and the deposited film, which has been suggested for polycrystalline samples to result in higher resistivities as a result of grain-boundary effects [24,25]. The most prominent peaks for the majority of films are the Au3Zr (002) and Au3Zr (121) orientations, with the lower temperature films exhibiting greater preference for the former, and the higher temperature films for the latter. The transition from the dominance of Au3Zr (002) orientation to the Au3Zr (121) orientation was observed in the films of 205 and 320 (for simplicity, hereafter we will simply represent the films as the number of deposition temperature, omitting the °C unit), identifiable by the greater relative proportion of the latter orientation compared to the films deposited below 205°C.  Figure 1 shows that both peak position and broadness change with deposition temperature, which is caused by combined effects of strain and grain size. The effects can be assessed by applying the Williamson-Hall analysis on the XRD spectra [26,27], is the width of the diffraction peak in radian, ℇ is the strain, is the average size of grain, is the wavelength of X-ray, is the Bragg diffraction angle, and is the Scherrer constant in the order of unity.
Though the XRD spectra are dominated by one primary peak, a weak secondary peak is identifiable in each spectrum. We chose the two most significant peaks from each diffraction spectrum in Fig.1. The results (see Supplementary Information Fig.S1 for details) are plotted as cos vs 4sin in Fig.2a. The results of grain size (assuming Scherrer constant =0.9) and strain (the gradients of lines in Fig.2a are negative, indicating compressive strains) are displayed in Fig.2b. The strain decreases and the grain size increases with deposition temperature, which are consistent with general trends observed in thermally deposited thin films [28]. Films deposited at temperatures below 320°C have an average grain size of about 4 nm.
However, the 415 and 427 films have significantly larger grain sizes of 24.4 nm and 26.7 nm, respectively. It could be possible that different crystalline orientations lead to the growth of different sizes of grains. For these two films, the dominant orientation is Au3Zr (121), while those deposited at lower temperatures are predominantly Au3Zr (002) orientation. We further measured the resistivity of the films, which are shown in Fig.2b (squares). It is seen that the resistivity decreases with deposition temperature, consistent with the trend of increasing grain size. Larger grains reduce the scattering loss of electrons at grain boundaries, thus have smaller resistivity.

Optical properties
We characterized the optical properties of the thin films through spectroscopic ellipsometry. The results are displayed in Fig.3, including both the real and imaginary components of the refractive index (Fig.3a, b) and (b) (a) permittivity (Fig.3c, d). The optical properties of the films can be approximately segregated into three groups: RT and 137, 205 and 320, 415 and 427. Each pair has similar optical properties, which is largely in correspondence with the textures revealed by the XRD spectra shown in Fig.1. This suggests that the optical properties of thin films are significantly impacted by crystal orientations. Most films exhibit negative real part permittivity in the visible range between 450-800 nm. In particular, the films of 205 and 320 show substantial negative real part permittivity across the entire visible and near infrared region, which are comparable to that of titanium nitride [17], one of the most promising refractory plasmonic materials. The imaginary parts of the permittivity increase almost linearly with wavelength, which are also impacted by deposition temperature, but are not as dramatic as the real parts. To further characterize the optical properties of the thin films, we measured the reflectance spectra of the films using a 5× low magnification objective and compare the spectra with theoretically calculated ones based on the Fresnel equations of normal incidence, using the refractive indices measured from the ellipsometry. The results are shown in Fig. 4. There is good agreement between experimental and theoretical curves. The maximum reflectance of these films is between 60-70% (@1000 nm), lower than that of Au which is above 90% in the near infrared. We went on to investigate how anneal affects the films' structural and optical properties. Fig.5a compares RT films with and without the in-situ anneal. Very little change was noted in the XRD spectra. Fig.5b compares the XRD results of 426 films with and without the in-situ anneal (the deposition temperature of the sample without in-situ anneal was slightly higher at 439°C). The XRD spectra remain similar with and without in-situ anneal.
However, more pronounced changes were noted on the two small peaks Au3Zr (021) and Au3Zr (002), which became weaker after the in-situ anneal with regard to the dominant Au3Zr (121) peak.  .6a) and the other two deposited at a higher temperature of 426°C and 439 o C (Fig.6b). We can see that insitu anneal has minimal impacts on the optical properties, which are almost identical to those without the anneal.
The small difference in optical properties with in-situ anneal possibly reflects a coalesce of small grains into larger ones. To further test the stability of the films, we annealed the films in an oven of a lower vacuum level of 1.5×10 -6 Torr at 497°C for one hour, after the films had been annealed in-situ within the sputtering chamber. The XRD spectra of the films after the second anneal are shown in Fig.7. One striking change is that the Au3Zr (121)

CONFLICTS OF INTEREST
The authors declare no conflicts of interest.