Corundum-structured AlCrNbTi oxide film grown using high-energy early-arriving ion irradiation in high-power impulse magnetron sputtering

Multicomponent or high-entropy oxide films are of interest due to their remarkable structure and properties. Here, energetic ion irradiation is utilized for controlling the phase formation and structure of AlCrNbTi oxide at growth temperature of 500 ◦ C. The ion acceleration is achieved by using a high-power impulse magnetron sputtering (HiPIMS) discharge, accompanied by a 10 μ s-long synchronized substrate bias ( U sync ), to minimize the surface charging effect and accelerate early-arriving ions, mainly Al + , O + , Ar 2 + , and Al 2 + . By increasing the magnitude of U sync from (cid:0) 100 V to (cid:0) 500 V, the film structure changes from amorphous to single-phase corundum, followed by the formation of high-number-density stacking faults (or nanotwins) at U sync = (cid:0) 500 V. This approach paves the way to tailor the high-temperature-phase and defect formation of oxide films at low growth temperature, with prospects for use in protective-coating and dielectric applications.

Multicomponent or 'high-entropy' oxides (HEOs) [1] are an emerging subset of the larger class of high-entropy materials [2,3].As one of the typical HEO families, AlCr-containing high-entropy oxides in either bulk or film form have been synthesized and can exhibit self-repairing properties [4], ferrimagnetism [5], and are of interest for catalysis [6].Particular interest stems from AlCr-containing high-entropy oxide films grown in a far-from-equilibrium process, e.g., reactive magnetron sputtering.A range of structures has been reported for AlCr-containing high-entropy oxide films, including amorphous [7,8], rock salt (Fm3m) [9], spinel (Fd3m) [10,11], rutile (P42/mnm) [12], and multiphase [6], which is different from the typical crystallographic structure of the binary oxides for constituent elements.For example, formation of the amorphous structure was reported for (AlCrTaTiZr)O x films grown at 350 • C by direct current (DC) magnetron sputtering [7], while AlCrNbTaTi oxides with the same constituent elements, except for Zr being replaced by Nb, preferentially forms the rutile structure at 400 • C [12].In light of these features of AlCr-containing high-entropy oxide films, we strive to investigate the possibility of synthesizing corundum-structured (space group of R3c) films.
There is a consensus that energetic irradiation from low-mass ions 1 is of importance for the phase formation and crystallinity of grown oxide thin films.Purely rutile structured TiO 2 film could be grown under intense bombardment from O + in contrast to the formation of the anatase phase under medium bombarding energy [13,14].It was also demonstrated by Schneider et al. [15] that the crystallinity, as well as the crystal structure of Al 2 O 3 films, are highly correlated to the ion energy of the irradiating Al + and O + ions.More importantly, in addition to the effect on the surface of the growing Al 2 O 3 film, the increasing irradiation energy of Al + also show effect on the surface-adjacent layers.Thereby, subsurface processes such as phase transformation, defect generation and cluster dissociation are triggered, which opens for the growth of AlCr-containing multicomponent or high-entropy oxide films [16,17].
Inspired by these studies, energetic ion bombardment predominantly from early-arriving ions is in this study used to tune the growth of AlCrcontaining multicomponent oxides.We investigate the effects on phase formation and nanoscale structurial evolution in AlCrNbTi oxide films due to early-arriving ion irradiation.A HiPIMS discharge combined with a synchronized substrate acceleration pulse was used to provide acceleration for the positively charged ions.The constituent elements were selected based on our previous experience with the corresponding nitride system [18,19].In sputtering processes, achieving the proper ion acceleration for dielectric films is challenging due to the acceleration voltage drop during film growth [20,21].Therefore, this study employed a short (10 μs) synchronized substrate bias at the end of the main HiPIMS pulse to minimize the film surface charging effect and accelerate the dominantly early-arriving ions.
The AlCrNbTi oxide films were grown using a 75-mm (3-inch) Al 25 Cr 25 Nb 25 Ti 25 multicomponent target (99.95%purity, provided by Plansee AG) in an ultra-high vacuum system (VC1) with a base pressure of approximately 8×10 − 6 Pa.One-side polished silicon (001) wafers with dimensions of 10×10×0.5 mm were used as substrates.The films were grown at 500 • C in a working pressure of 0.4 Pa with a gas mixture of 26 sccm Ar and 2 sccm O 2 .The target was powered by a HiPIMS unit (HiPSTER 1, Ionautics) and the substrate was biased by a 10 μs-long synchronized pulse with a magnitude (U sync ) of − 100, − 200, − 300, − 400, and − 500 V (see Fig. 1).A reference film grown by HiPIMS with a floating substrate potential (U float ) was also prepared.Measurements of the temporal evolution of the substrate ion current density (J s ) and ion fluxes were conducted in a separate high vacuum chamber (VC2) using a substrate current probe [20] and quadrupole mass spectrometer (PSM 003, Hidden analytical Ltd.), respectively.Elemental composition was determined using an energy-dispersive X-ray spectrometer (EDS, Oxford Instruments X-Max) in a scanning electron microscope (SEM, LEO Gemini 1550, Zeiss).Time-of-flight elastic recoil detection analysis (ToF-ERDA) was utilized for oxygen content measurement for the sample grown at U sync = − 100 V and the calibration of the rest of the samples.Crystallographic structures of the films were analyzed using X-ray diffraction (XRD) in a Bragg-Brentano (θ-2θ) geometry on a high-resolution XRD diffractometer (PANalytical X'Pert PRO, Malvern Panalytical).X-ray pole figures were obtained on diffraction peaks 2θ = 36.19• and 63.73 • using an x-ray diffractometer (PANalytical Empyrean diffractometer, Malvern Panalytical).Cross-sectional and plan-view images for the film grown at U sync = − 500 were captured using an (S) TEM (FEI Tecnai G2 TF20 UT).(Detailed information for Experimental Details is referred to the Supplementary Material).
The temporal evolution of J s is shown in Fig. 2(a).It is seen that the main portion of the ion flux arrives at the substrate after the end of the main HiPIMS pulse.Since J s is proportional to the population of ions that reaches the film surface plane (if all ions are considered as singly charged), a small portion of the total ion flux, i.e., the ion flux that reaches the substrate plane during τ sync , is accelerated by the applied U sync .To identify the ion species being accelerated by U sync , the energyintegrated ion flux intensities (F i ) for both single-and double-charged ion fluxes are carried out and shown in Fig. 2(b) and (c), respectively.The evolution of F i indicates that the arriving time of the species at the substrate position is highly dependent on the ion mass, i.e., a similar peak position is observed from Ar + , Cr + , and Ti + (see Fig. 2(b)), and Cr 2+ and Ti 2+ see (Fig. 2(c)).In addition, the arrival time for Ar 2+ is earlier than the double-charged ion species which have similar ion masses.This phenomenon can be understood by the earlier ionization during the HiPIMS pulse [22] in combination with a shorter travel distance to the orifice of the mass spectrometer [23] of neutral Ar compared to the metal species from the target.From Fig. 2(a)-(c) (pink regions) we conclude that the 10 μs-long substrate bias pulse only accelerates a fraction of the total impinging ion flux and that the accelerated fraction is predominantly composed of Al + , O + , Ar 2+ , and Al 2+ ions.Note that the fraction of doubly charged Al 2+ ions is assumed to be relatively low and sometimes not detectable by mass spectrometry due to a small cross section for electron impact ionization and short plasma transit time [24,25].In our work, a weak signal from Al 2+ is seen in Fig. 2(c), which is attributed to the high peak current density (~ 1.8 A/cm 2 ).We here also show IEDFs for Al + and Ar 2+ ions (Fig. 2(d)).From this figure average primary kinetic energies of E 0 ≈ 15 eV for Al + and E 0 ≈ 10 eV for Ar 2+ are obtained.
The elemental composition of AlCrNbTi oxide films are shown in Table 1.The metallic elements in the film differ from the near-equal atomic fractions, with a substantially higher-than-nominal Al content, and correspondingly somewhat lower Ti content.The cation/anion ratio in the film, in a range of 1.68/3 -1.76/3, is slightly lower than the ratio of 2/3 for a stoichiometric Al 2 O 3 and Cr 2 O 3 compound showing excessive amount of oxygen in the film (see Table 1).
The XRD patterns of the AlCrNbTi oxide films grown at different U sync are shown in Fig. 3 where the identical peak positions for this compound also shifts to lower angles as compared with corundum-Al 2 O 3 due to the increased lattice constant [27].For the films grown with U float and U sync = − 100 V, low peak intensities corresponding to the growth of (110) and (300) planes is seen, revealing low film crystallinity.In contrast, as U sync increases, film growth transforms from the (110) to the (300) plane, and the absolute intensity of the 300 peak increases with increasing U sync .It is also seen that the position of the 300 peak shifts towards lower angles as U sync increases, indicating an expansion of the out-of-plane lattice.A possible explanation for this phenomenon is related to an increasing defect number density within grains induced by the elevated ion irradiation energy [28,29].In general, it can be concluded that the increasing ion irradiation energy significantly increases film crystallinity and influences the preferred orientation of the film, i.e., the growth towards the low-energy (300) plane during high-energy ion irradiation, which is consistent with studies of α-Cr 2 O 3 and α-Al 2 O 3 [30][31][32].
A low magnification cross-sectional STEM image of AlCrNbTi oxide films grown at U sync = − 500 V is shown in Fig. 4(a).The film exhibits a columnar structure.The corresponding EDS elemental maps, shown in Fig. 4(b), were collected from the boxed region b in Fig. 4(a), and show a homogenous film.The selected area electron diffraction (SAED) pattern in Fig. 4(c) acquired close to the surface of the film is attributed to the corundum (R3c) structure.The pulse length of the main HiPIMS pulse and the synchronized substrate bias is marked as τ HiPIMS and τ sync , respectively.

Table. 1
Elemental composition and cation/anion ratio for AlCrNbTi oxide films.The elemental compositions of the as-deposited films are measured by EDS and calibrated by using the oxygen content of the sample grown at U sync = − 100 V as determined by ToF-ERDA.The oxygen concentration is normalized to 3 and the following concentration for the metallic element is calculated by using the cation/anion ratio determined by film composition.and the corresponding FFT patterns (Fig. 4(i) and (j)).The FFT patterned from a single grain is shown in Fig. 4(j).Indexing this pattern is complicated by the presence of stacking fault defects, resulting in the presence of additional spots.A close match, however, is found to the pattern obtained along the [110] direction.The FFT from several adjacent grains show well-defined geometric patterns.This indicates that in the in-plane orientation the grains are not randomly orientated, but rather have preferred orientations.The strengthing benefits from stacking faults or nanotwinned structures are assumed to overcome the hardness-toughness trade-off for materials [33,34].The elevated ion irradiation energy shows that it is indeed possible to overcome the extremely high stacking fault formation threshold of oxides, e.g., the stacking fault energy in α-Al 2 O 3 is 1.2 -4.6 J/m 2 for {0001} planes and 0.3 -0.9 J/m 2 for {1010} and {1120} planes [35].Our results open up for new processes for ceramic film deposition, as stacking faults are precursors for nanotwin structures [36].We are interested in whether we can draw conclusions on the effects of ion irradiation on defects and phase transformation in early-arriving ions.During ion-assisted film growth, the film structure can be rearranged due to thermal spikes created by the irradiating ions [37].This can induce thermal hopping inside the expanding heat pulses and result in surface diffusion, local heating, and recrystallization, leading to film densification and changes in growth, such as preferred orientation and crystallite size [38].Note that a thermal spike-induced temperature increase predominantly depends on the surface migration energy, rather than the ion flux density and ion energy [39].As a result, low-energy ions tend to induce thermal spikes leading to higher adatom mobility and the growth of grains with the lowest surface energy planes.Differently, the high-energy ions penetrate grains and generate defects according the channeling-related mechanism, and thereby induce a film growth towards more open-channeling planes [39].Due to the high energy and low mass of the irradiating ions in this study, we do not expect a predominately thermal spike effect, but instead, the channeling effect is assumed to be more important.As the TiAlCrNb oxide film grown at the floating potential prefer to grow in an amorphous structure, the high-energy irradiation from low-mass ions drives the film towards a state of minimum volume free-energy density, with major crystallographic channeling directions aligned parallel to the incident ion direction.For crystals with an hcp structure, the strongest channeling direction corresponds to <2110> directions, perpendicular to the (3030) plane [40].Therefore, high-energy ion irradiation promotes both higher crystallinity and growth of TiAlCrNb oxide film towards the more open-channeling (300), i.e., (3030), plane (see Fig. 3), i.e., different from preferred (1014) orientation [41,42].Due to the fiber texture feature of the grown TiAlCrNb oxide films (see Fig. 3(c)), defects such as stacking faults and nanowins can be generated in grains with out-of-plane growth directions not parallel to the channeling direction (see Fig. 4).The fraction of ions incident parallel to channeling directions and subsequently channeled increases with increasing ion energy and/or decreasing ion mass [39], giving rise to the increasing (300) peak intensities with increasing U sync in the present work (see Fig. 3).
The remaining question is whether the formation of the α-phase in TiAlCrNb oxide is related to its constituent elements.It has been suggested that the incorporation of elements such as Cr and Fe can lower the growth temperature of α-Al 2 O 3 .For example, a corundum-structured Al-Cr-Fe-O film can be grown using cathodic arc evaporation at a deposition temperature of 550 • C [42,43].It has also been found that the growth of the corundum phase at a lower deposition temperature (500 • C) can be achieved at relatively high oxygen partial pressure using a Cr 0.3 Al 0.7 target [44].In general, the mechanism of low-temperature synthesis of high-temperature phases with the assistance of a dopant lies in the lower formation temperature of α-Cr 2 O 3 and α-Fe 2 O 3 , which stabilizes the formation of α-Al 2 O 3 in the same space group at low temperature.However, for the TiAlCrNb oxide investigated in this study, there is no clear indication of corundum forming at the growth temperature of 500 • C. It has been shown that (AlCrTaTiZr)O x [7] and AlCrNbTaTi oxide [12] prefer to grow in amorphous and rutile structures, respectively, at 350 ~ 400 • C, accompanied by low ion acceleration conditions.Furthermore, the formation of a cubic structure of Al x CoCrCuFeNi oxide [11] as well as (Al 0.31 Cr 0.20 Fe 0.14 Ni 0.35 )O [45] films and an amorphous (TiVAlCrZr)O film [46], are suggested, where more elements are able to form Me 2 O 3 -type oxides at low temperature with the R3c space group.In particular, the Al x CoCrCuFeNi oxide shows no phase transformation even after being annealed at 500 • C for 5 h [11].Therefore, we speculate that the formation of the here-observed corundum structure of TiAlCrNb oxide mainly stem from the increasing energy of ion irradiation.
In conclusion, this study clearly demonstrates that increasing ion irradiation energy from early-arriving ions at a constant growth temperature results in the elevated crystallinity and the formation of a single-phase corundum structure at the expense of the amorphous matrix.Additionally, the study discovered the generation of high-density stacking faults (or nanotwin) structure in the U sync = − 500 V film.These findings hold great potential for growing crystalline high-temperature phases and for tuning the nanoscale structure of oxide films using energetic ion irradiation, with the possibility of applications in hardcoating and dielectric fields.

Fig. 1 .
Fig. 1.Target voltage (U HiPIMS ) and synchronized substrate voltage (U sync ) waveforms.The pulse length of the main HiPIMS pulse and the synchronized substrate bias are marked as τ HiPIMS and τ sync , respectively.

Fig. 4 (
d) shows the high-resolution TEM image.The corresponding Fast Fourier Transformation (FFT) pattern (Fig. 4(e)) closely matches the simulated diffraction from α-Al 2 O 3 viewed along the [001] direction (Fig. 4(f)).Thus, we here can conclude that the crystalline α-Al 2 O 3 type corundum phase is obtained at U sync = − 500 V.As seen both in the plan-view TEM image in Fig. 4(g) and the additional spots in the SAED pattern (Fig. 4(c)), high-number-density stacking faults are identified in individual film grains, which can be simultaneously confirmed by the high-resolution TEM images (Fig. 4(h))

Fig. 4 .
Fig. 4. (a) Cross-sectional low-resolution STEM image of AlCrNbTi oxide films grown at U sync = − 500 V.(b) EDS mapping acquired from region b in (a) for Al, Cr, Nb, Ti, O, and Si.(c) the SAED pattern is taken from the top area c of the film in (a).(d) High-resolution TEM image of AlCrNbTi oxide films grown at U sync = − 500 V and its magnified image (bottom right).(e) Fast Fourier Transformation (FFT) pattern of (d), in comparison with (f) the simulated diffraction on α-Al 2 O 3 viewed in the [001] direction.(g) low-resolution and (h) high-resolution plan-view STEM images of AlCrNbTi oxide films grown at U sync = − 500 V. (i) and (j) are FFT patterns at area i and j in (h), respectively.