Crystalline Structure and Optical Properties of Cobalt Nickel Oxide Thin Films Deposited with a Pulsed Hollow-Cathode Discharge in an Ar+O 2 Gas Mixture

: Cobalt nickel oxide films are deposited on Si(111) or fluorine-doped tin-oxide-coated (FTO) glass substrates employing a pulsed hollow-cathode discharge. The hollow cathode is operated with argon gas flowing through the nozzle and with O 2 gas admitted to the vacuum chamber. Three different cathode compositions (Co20Ni80, Co50Ni50, and Co80Ni20) are investigated. Deposited and annealed thin films are characterized by X-ray diffraction, infrared (Raman) spectroscopy, and ellipsometry. As-deposited films consist of a single mixed cobalt nickel oxide phase. Upon annealing at 600 ◦ C, the mixed cobalt nickel oxide phase separates into two cystalline sub-phases which consist of cubic NiO and cubic Co 3 O 4 . Annealed films are investigated by spectroscopic ellipsometry and the optical bandgaps are determined.

In the present paper, we utilize a pulsed hollow-cathode (PHC) discharge with a cobalt nickel cathode to deposit mixed cobalt nickel oxide films.The potential of PHC discharges for deposition of thin films has not been fully explored yet.In particular, rather few investigations regarding the deposition of multi-component films have been reported yet.In this context, PHC discharges using multi-component cathodes are particularly useful.Here, we employ mixed CoNi cathodes with different stoichiometric compositions, e.g., Co20Ni80, Co50Ni50, and Co80Ni20.Films are deposited on Si(111) wafers and on soda lime glass coated with fluorine-doped tin oxide (FTO).The structural, morphological, and optical properties of the deposited films are studied with the help of scanning electron microscopy, X-ray diffraction, Raman spectroscopy, and ellipsometry.

Experiment
The experimental set-up has been described elsewhere [27,28].It consists of a cylindrical hollow cathode (HC) inside a vacuum chamber.The HC is made from cobalt nickel alloy with a purity of 99.95% [28][29][30][31].Several cathodes with different compositions have been employed for film deposition, e.g., Co20Ni80, Co50Ni50, and Co80Ni20, where the numbers indicate the atomic percentage of the cathode material.We use argon and oxygen as working gases at gas flow rates of 200 sccm and 100 sccm, repsectively.
The pulsed hollow cathode is operated with a discharge current of 0.25 A at a repetition frequency of 5 kHz and a pulse length of 100 µs.
Films are deposited on Si(111) or FTO substrates at two different gas pressures (2.8 Pa and 50 Pa) in the vacuum chamber.More details may be found elsewhere [27].Typical deposition times are 90-120 min.
As-deposited and annealed films are characterized by X-ray diffraction, Raman spectroscopy, and ellipsometry.Annealing is carried out in ambient air using temperatures of 410 • C and 600 • C.
Energy dispersive X-ray spectroscopy (EDX) is employed for the elemental composition analysis of the deposited thin films.The TESCAN Ferra 3 scanning electron microscope (SEM) equipped with an EDAX Octane super 60 mm 2 is employed.Chemical composition analysis is performed with an energy of 15 kV at a working distance of 9 mm.Thickness measurements are performed with an energy of 5 kV at the same distance.
The crystallographic structure of the samples is investigated with the help of the X-ray diffraction (XRD) technique.The Grazing Incidence (GIXRD) geometry with Cu Kα radiation (λ = 0.154 nm) is employed [32].Raman spectroscopy is carried out at room temperature using a Renishaw Raman Microscope RM 1000.
The optical transmission is measured with a UV-VIS spectrophotometer (LAMBDA 1050 UV/Vis/NIR, Perkin-Elmer).Spectroscopic ellipsometry is conducted at room temperature (300 K) in a dry nitrogen atmosphere with a variable angle ellipsometer (J.A. Woollam VUV-VASE) in the photon energy range 0.8-7.5 eV at three angles of incidence (60 • , 65 • , and 70 • ).Refraction index n, extinction coefficient k, and absorption coefficient α are obtained with the help of commercial WVASE software.
The photoelectrochemical (PEC) activity is investigated in an electrochemical cell in a three-electrode configuration.Details may be found elesewhere [27].

Film Composition
Deposited films have been analyzed with the help of scanning electron microscopy (SEM).The film thickness is obtained from the cross sectional view of the sample which is cut at the center.The extracted film thickness is 1.98 µm, 3.68 µm, and 1.56 µm for the films deposited with Co20Ni80, Co50Ni50, and Co80Ni20 cathodes, respectively.The film composition of annealed films is measured at two positions (center and near the edge) by EDX.The two results agree well with each other and only the average results are shown in Table 1.The measured film composition and, in particular, the extracted Co/(Co+Ni) ratios agree reasonably with the nominal composition of the considered cathodes.Figure 1 shows GIXRD results for as-deposited and annealed (at 410 • C and 600 • C) films on Si(111) substrates deposited with the Co50Ni50 cathode at gas pressures of 2.8 Pa and 50 Pa.These films, hence, should contain an approximately equal amount of Co and Ni.The X-ray diffractograms of the as-deposited films show strongly broadened reflections.For the as-deposited films, it was not possible to properly distinguish between pure nickel oxide, cobalt oxide, and/or cobalt nickel oxide phases.The annealed films (410 • C) contain a single crystalline phase which is assigned to cubic nickel oxide (c-NiO) with space group (SG) 225 (ICSD CIF 9866 [33]).We mention in passing that this does not rule out the existence of a mixed Ni 1−x Co x O phase with a c-NiO structure [34] or of an amorphous cobalt-rich phase.In fact, mixed Ni 1−x Co x O phases with a c-NiO lattice structure and x as large as 0.4 (40 at%) have been reported before [34].Annealing at 600 • C results in a phase separation into c-NiO (SG 225) and spinel-type c-Co 3 O 4 (SG 227, ICSD CIF 28158 [33]) crystalline phases and leads to an increase in the mean grain size D. Here, grain reflects the structural homogenous part in the direction of the diffraction vector and its size is given as a length unit (nm).The mean grain size was evaluated using the Rietveld refinement method (TOPAS software, Bruker).A separation of crystalline phases during annealing has been observed before for CuNiO films [27].The crystalline phase fractions f of 50% each reflect well the initial composition of the employed Co50Ni50 cathode.No evidence of crystalline CoO or mixed NiCo 2 O 4 phases was found.Table 2 summarizes the results of our analyses.

Co20Ni80 Cathode
Figure 2 shows the GIXRD results for the nickel-rich films deposited with the Co20Ni80 cathode.Diffractograms of the as-deposited film and of the film annealed at 410 • C show reflections from c-NiO but not from cobalt oxide.Hence, there is no indication of a second crystalline phase.The film deposited at 2.8 Pa has an almost perfect NiO(200) preferred orientation whereas the film deposited at 50 Pa does not show this preference.The reference value for the lattice parameter of pure c-NiO is 0.4178 nm [33].The slightly smaller lattice parameters (Table 2) can be explained by a partial incorporation of Co 2+ (and, eventually, Co 3+ ) ions into the Ni 2+ lattice sites and the smaller ion radii of Co ions compared to Ni ions [35].

Film
Crystalline Phase Lattice Parameter (nm)  Annealing at 600 • C leads to a complete separation into cobalt oxide and nickel oxide phases.The results of a quantitative phase analysis with the Rietveld method correspond well with the composition of the employed cathode.

Co80Ni20 Cathode
Figure 3a shows the GIXRD results for the cobalt-rich film deposited at 2.8 Pa with the Co80Ni20 cathode.The diffractograms of the as-deposited film and of the film annealed at 410 • C show the typical reflections from a c-Co 3 O 4 phase modification with space group 216 (ICSD CIF 9362).There is no indication of a second crystalline phase.The X-ray diffractogram of the as-deposited film deposited at 50 Pa shows a single reflection in the angular range 36.8-38.1 • , which can be attributed to c-NiO, c-Co 3 O 4 , and/or c-NiCo 2 O 4 (Figure 3b).It was not possible to distinguish between these crystalline phases due to the strongly broadened line profile, however.Upon annealing at 410 • C, the crystalline phase changes to c-Co 3 O 4 (SG 216, ICSD CIF 9362).Mixed Co 3−x Ni x O 4 phases with a c-CO 3 O 4 lattice structure and x as large as 0.8 (80 at %) have been reported before [34].Further annealing at 600 • C again leads to a phase separation into c-NiO (SG 225) and c-Co 3 O 4 (SG 227, ICSD CIF 28158) and gives rise to large grain sizes of about 200 nm.The extracted Ni/Co ratio of 25/75 is in reasonable agreement to the measured composition (Table 1) and close to the one expected for a Co80Ni20 cathode.

Raman Spectroscopy
The GIXRD results are supported by Raman spectroscopy.The as-deposited film from the nickel-rich Co20Ni80 cathode shows a pronounced broad peak at about 545 cm −1 and a smaller peak at 1080 cm −1 (Figure 4).The peaks are frequently assigned to onephonon longitudinal optical (LO) mode and two-phonon vibrational modes associated with NiO [15,36,37].In particular, the one-phonon LO mode at 545 cm −1 mode is strongly suppressed in bulk NiO, while it is enhanced in nano-crystalline NiO [38,39].Similar structures were observed for copper nickel oxide (Cu50Ni50) films [27].The center wave number of the 545 cm −1 peak broadens and shifts to 565 cm −1 if the Co50Ni50 cathode is employed.The appearance of two weak shoulders at 480 cm −1 and 660 cm −1 is indicated.The film deposited with the cobalt-rich Co80Ni20 cathode in addition displays several narrow peaks at 196 cm −1 , 480 cm −1 , and 680 cm −1 which are attributed to Co 3 O 4 [27].
The Raman spectra of the annealed films display several narrow peaks at 193 cm −1 , 480 cm −1 , 525 cm −1 , and 680 cm −1 , which are attributed to Co 3 O 4 (Figure 4) [40,41].The Raman spectrum obtained with the nickel-rich Co20Ni80 cathode additionally shows a pronounced shoulder at 560 cm −1 and a weak peak at 1080 cm −1 indicating the presence of NiO [15,36,37].The pronounced peak at 680 cm −1 is frequently assigned to the A 1g mode of Co 3 O 4 [29].The weak band at 193 cm −1 is assigned to tetrahedral sites of Co 3 O 4 [15].

Optical Properties
The optical properties of annealed films are investigated by UV-VIS measurements and by ellipsometry.The transmittance of annealed cobalt nickel oxide films deposited on FTOcoated glass using different cathodes obtained from the UV-VIS measurements is displayed in Figure 5a.No corrections regarding the different film thicknesses have been applied.It is evident that films are opaque at small wavelengths below 400 nm and become increasingly transparent at larger wavelengths.This result is further confirmed by our ellipsometry measurements; see below.The transmittance of non-stoichiometric Co x Ni 1−x O films was investigated by Roffi et al. [42].Accordingly, the measured transmittance decreases with increasing cobalt content x, while the absorption edge moves to longer wavelengths (smaller photon energies).These observations are in fair agreement with our results (Figure 5).The optical properties of films deposited at 50 Pa on FTO-coated glass and annealed at 600 • C are further obtained from ellipsometry measurements.Figure 5b displays the refractive index n, which increases (decreases) with wavelength λ (photon energy E ph ) from about n ≈ 1.2 at λ = 180 nm (E ph = 6.9 eV) to n ≈ 1.8 at λ = 1000 nm (E ph = 1.24 eV).Only minor differences are observed for the three samples deposited with different cathode compositions (Co20Ni80, Co50Ni50, and Co80Ni80).The absorption coefficient for these films is displayed in Figure 5c as function of wavelength.Again, the present ellipsometry results show that all films become opaque at small wavelengths and increasingly transparent at larger wavelengths.A similar behavior of the absorbance was observed before [43].The absorption behavior is rather complicated, however, showing more than one absorption edge corresponding to several optical bandgaps [44][45][46].
A more thorough analysis of the absorption behavior employing a so-called Tauc plot making use of (α is shown in Figure 6, where α is the absorption coefficient and E ph is the photon energy [47].
A direct optical bandgap as in the case of CoO, Co 3 O 4 , and NiO is assumed [48][49][50].From this plot we derive optical bandgaps of 4. Smaller optical bandgaps are reported for cobalt oxide [21,22].In general, cobalt oxide shows several optical bandgaps which depend on the composition and on the morphology.An expanded view of the photon energy region below 3.5 eV is displayed in the inset of Figure 6.Direct optical bandgaps of 2.10 eV, 2.23 eV, and 2.19 eV for the Co20Ni80, Co50Ni50, and Co80Ni20 cathodes, respectively, are extracted from this graph.The extracted bandgaps agree well with the reported optical bandgap of about 2.17 eV for Co 3 O 4 [23][24][25][26]55].The result is further supported by our XRD analysis.
Finally, we mention in passing that films deposited on FTO-coated glass are also tested for photoelectrochemical activity using the same set-up as for CuNiO films [27].Na 2 SO 4 and NaOH are employed as electrolytes.No photoelectrochemical activity of the present copper nickel oxide has been observed, however.

Conclusions
Results are reported for mixed cobalt nickel oxide films deposited with the help of a pulsed hollow-cathode (PHC) discharge operated at 5 kHz.Deposited films are composed of a mixed metal oxide (MMO) phase.During annealing at 600 • C, the MMO phase separates into two crystalline cubic Co 3 O 4 and cubic NiO subphases.The results are confirmed by Raman spectroscopy.The refraction index and absorption coefficient are derived from ellipsometry measurements.The refractive index increases from 1.2 at 200 nm to about 1.8 at 1000 nm.Annealed films are opaque at small wavelengths and become increasingly more transparent at longer wavelengths.Optical bandgaps of about 4.15 eV are linked to the NiO phase of the annealed films, while a second optical bandgap at about 2.17 eV is linked to the Co 3 O 4 phase.Photoelectrochemical activity was not observed.

Figure 2 .
Figure 2. GIXRD of as-deposited and annealed (410 • C, 600 • C) CoNiO films deposited with a Co20Ni80 cathode at (a) 2.8 Pa and (b) 50 Pa.Typical crystallite (grain) sizes of the as-deposited and annealed (410 • C) samples are in the range of 11-17 nm.Both diffraction patterns from the as-deposited and annealed (410 • C) sample deposited at 2.8 Pa differ from the 50 Pa sample only in the peak intensities.