Sputter deposition of ZnO–AlN pseudo-binary amorphous alloys with tunable band gaps in the deep ultraviolet region

ZnO–AlN pseudo-binary amorphous alloys (a-ZAON hereinafter) with tunable band gaps in the deep ultraviolet (DUV) region have been synthesized using magnetron sputtering. The miscibility gap between ZnO and AlN has been overcome using room-temperature sputtering deposition, leveraging the rapid quenching abilities of sputtered particles to fabricate metastable but single-phase alloys. X-ray diffraction patterns and optical transmittance spectra revealed that the synthesized films with chemical composition ratios of [Zn]/([Zn] + [Al]) = 0.24–0.79 likely manifested as single-phase of a-ZAON films. Despite their amorphous structures, these films presented direct band gaps of 3.4–5.8 eV and thus high optical absorption coefficients (105 cm−1). Notably, the observed values adhered to Vegard’s law for crystalline ZnO–AlN systems, implying that the a-ZAON films were solid solution alloys with atomic-level mixing. Furthermore, atomic force microscopy analyses revealed smooth film surfaces with root-mean-square roughness of 0.8–0.9 nm. Overall, the wide-ranging band gap tunability, high absorption coefficients, amorphous structures, surface smoothness, and low synthesis temperatures of a-ZAON films position them as promising materials for use in DUV optoelectronic devices and power devices fabricated using large-scale glass and flexible substrates.


Introduction
Ultra-wide band-gap (UWBG) semiconductors have gained considerable attention due to their versatile applications across diverse domains, including power devices [1,2], solar-blind photodetectors [3][4][5][6], and deep ultraviolet (DUV) light emitting diodes [7][8][9].Al x Ga 1-x N materials, among typical UWBG material systems, exhibit band gap (E g ) values ranging from 3.4 eV for wurtzite GaN to 6.2 eV for wurtzite AlN [10,11].Tunable wide band gaps and large breakdown fields [12], coupled with high electron mobilities [13], render these materials highly promising for the fabrication of next-generation power devices and DUV-absorbing/emitting devices.AlGaN films are, however, conventionally grown via metal-organic chemical vapor deposition on epitaxial substrates including SiC or freestanding AlN at temperatures as high as 1000 °C, potentially hindering their large-scale production.As a potential alternative growth technique, sputtering growth of AlGaN has been explored in some studies [14][15][16], capitalizing on its scalability, high material usage efficiency, and the ability of energetic sputtered species to enhance adatom migration even at low temperatures.For instance, Neuhaus et al successfully demonstrated the sputtering deposition of epitaxial AlN films with good homogeneity and crystalline quality on 8″ substrates under improved vacuum conditions [17].Nonetheless, films fabricated using conventional chambers can undergo oxidization in the presence of residual gases such as O 2 or H 2 O vapors.In this context, amorphous GaO x (a-GaO x ), with a band gap of approximately 4.1 eV, has attracted increasing attention owing to its feasible fabrication through room-temperature (RT) sputtering and its suitability to flexible devices.However, when fabricating a-GaO x films, carrier doping presents significant challenges.Nonetheless, Kim et al successfully fabricated a-GaO x films with carrier density of 2 ´10 14 cm −3 on glass substrates at RT [18].Despite this, the non-tunable band gaps of gallium oxides may still present drawbacks, limiting wavelength choices for their applications in DUV-absorbing/emitting devices.
Here we report a promising material system, ZnAlON (referred to as ZAON hereafter), demonstrating potential for resolving the tradeoff between band gap tunability and manufacturing costs.Given that ZAON can be fabricated as a pseudo-binary system of ZnO (E g : 3.4 eV) [19] and AlN, which share wurtzite structures, it could offer wide ranging composition tunability and, consequently, a broad spectrum of band gap tunability in the UV region, alike to AlGaN.While one may anticipate challenges in fabricating stoichiometric films in such multiple-anion systems, we expect that precise control over the O and N atomic fluxes onto substrates can facilitate the synthesis of stoichiometric ZAON films.In fact, in a previous study, we have already accomplished the sputtering synthesis of (ZnO) x (InN) 1-x , a pseudo-binary system of ZnO and InN, with stoichiometric compositions of , by meticulously controlling the O and N atomic fluxes onto the substrates.This rigorous control was facilitated through the monitoring of atom densities in the sputtering atmosphere using vacuum UV absorption spectroscopy [20].In this study, we synthesize ZAON films via magnetron sputtering at RT and 450 °C on c-plane sapphire substrates, subsequently investigating their structural and optical properties.We further delve into the relationship between their optical band gaps and chemical compositions, drawing comparisons with the empirical Vegard's law, which suggests a linear correlation between physical properties and alloy compositions [21].

Experimental procedure
ZAON films were fabricated on c-plane sapphire substrates by radio frequency (RF) magnetron sputtering.The substrate temperatures were RT and 450 °C.ZnO and Al targets (2 inch diameter, 99.99% purity) were utilized, and RF power ranging from 5-50 W was supplied.Before the actual sputtering, pre-sputtering was performed for 30 min to clean the targets.Ar, N 2 , and O 2 gases were utilized with flow rates were 30-42, 11-24, and 1.8 sccm, respectively.The total pressure was maintained at 0.50 Pa.The chemical compositions of the ZAON films were regulated by adjusting the RF power supplied to each target and the gas flow rate ratio.No postdeposition annealing was performed.

Results and discussion
3.1.Structural and optical properties of the ZAON films fabricated at 450 °C First, we elucidate the structural and optical properties of the ZAON films fabricated at 450 °C, also highlighting the difficulty in fabricating pseudo-binary ZnO and AlN alloys.Figure 1 depicts the XRD patterns of 2θ-ω scans of the ZAON films fabricated at 450 °C.Diffraction peaks appeared at 34.4°, corresponding to the (002) reflections of ZnO [22].Apart from these, no other peaks, including those corresponding to AlN reflections, were evident.Given that the AlN content of the films can reach 0.81, we posit that the films decompose into crystalline ZnO and amorphous Al(O)N phases.This hypothesis is corroborated by the results of optical measurements.Furthermore, at high [Zn]/([Zn] + [Al]) ratios, the ZnO (002) diffraction angle slightly shifted toward higher values.The corresponding angle significantly exceeded that of bulk ZnO, suggesting that the shift in the diffraction angleresulted from the substitution of Zn 2+ , with a larger ionic radius of 72 pm, by Al 3+ , with a relatively smaller ionic radius of 53 pm, in the ZnO crystalline region [23].
Figure 2 displays the transmittance spectra and optical band gaps of the ZAON films fabricated at 450 °C.The latter were determined using Tauc's plot [(αhν) 2 versus hν; where α, h, ν, and hν denote the absorption coefficient, Planck's constant, the incident light frequency, and the incident photon energy, respectively].Meanwhile, the absorption coefficient, α, was calculated using the equation T = exp(−αd), where T denotes the transmittance, and d represents the film thickness.The dashed line in figure 2(b) depict the band gaps of the pseudo-binary ZnO-AlN system composed of ZnO (3.4 eV) and AlN (6.2 eV), derived from Vegard's law using the following equation:

Structural and optical properties of ZAON films fabricated at RT
Next, we discuss the structural and optical properties of the ZAON films fabricated at RT. Notably, this RT fabrication was aimed at synthesizing pseudo-binary amorphous ZnO-AlN (hereafter a-ZAON) alloys.
Regarding alloy formation in immiscible systems, rapid cooling of liquid alloy solutions is known to be a powerful method for producing metastable and single-phase alloys even in a system with large miscibility gaps.The resulting alloys generally exhibit frozen nonequilibrium structures, where atomic arrangement is dictated by both thermodynamics and kinetics.Meanwhile, rapid quenching can be achieved through vacuum evaporation onto cold substrates.For instance, Mader et al successfully fabricated amorphous alloys in a Cu-Ag system (known for its immiscibility) at 80 K through vacuum evaporation, observing that the films retained their amorphous structures even after RT annealing [25].Similarly, He et al successfully synthesized amorphous alloys in an Ag-Ni system,which is immiscible even in the liquid state, utilizing direct current (DC) sputtering and liquid-nitrogen-cooled Si wafers [26].Additionally, Kwon et al reported alloy formation in immiscible systems at high temperatures of RT, demonstrating the synthesis of amorphous Cu-Ta alloys through DC magnetron sputtering [27].
Here we employed RT sputtering deposition to synthesize our ZnO-AlN alloys, leveraging the rapid quenching capabilities of energetic sputtered particles on substrates at RT. Figures 3(a  evident even in the wide-range spectra, implying the amorphous natures of these ZAON films.Additionally, no XRD peaks corresponding to AlN were observed.These results are in agreement with the AFM results. Figure 4 shows the surface AFM images of the ZAON films fabricated at RT.At a composition ratio [Zn]/ ([Zn] + [Al]) of 0.86, the films presented distinct grain boundaries, with a large root-mean-square (RMS) roughness of 1.4 nm.Conversely, films with [Zn]/([Zn] + [Al]) 0.68 did not present any distinct boundaries, and their surfaces were significantly flat with RMS roughness of 0.8-0.9nm, indicative of their amorphous nature.
The structures of the fabricated films were further investigated through TEM analysis.Figure 5 presents the TEM images of the ZAON films with a chemical composition ratio of [Zn]/([Zn] + [Al]) = 0.24.Here, figures 5(a) and (b) display the TEM images of the ZAON films fabricated at RT and 450 °C, respectively.As evident, the structures of the films were clearly distinct.Specifically, the film fabricated at RT exhibit a homogeneous and smooth structure, whereas that fabricated at 450 °C demonstrated severe phase separation, resulting in an inhomogeneous and rough structure.These findings are consistent with the XRD and AFM results, suggesting the formation of pseudo-binary amorphous ZnO-AlN alloys at RT.More detailed nanoscale analysis of the ordered structures of the ZAON films fabricated at RT will be reported elsewhere in the near future.
Subsequently, we evaluated the optical properties of the ZAON films fabricated at RT.In addition, we observed high absorption coefficients of 10 5 cm −1 for the films, comparable to those of crystalline ZnO and AlN films, suggesting the presence of direct bandgaps in the a-ZAON films.Furthermore, Tauc's plots presented a linear relationship between photon energy and (αhν)

Conclusion
ZAON films with varying chemical compositions have been synthesized using RF magnetron sputtering.Due to large immiscibility caps, all ZAON films fabricated at 450 °C ehibit mixed crystalline ZnO and amorphous Al(O) N phases, with band gaps around 3.4 eV.Here, the ZnO portions primarily influenced the optical absorption edge.Meanwhile, the films grown at RT with [Zn]/([Zn] + [Al]) ratios 0.79 did not exhibit any crystalline phases.Optical measurements revealed continuous shifts in the optical absorption edges of the a-ZAON films toward shorter wavelength with decreasing [Zn]/([Zn] + [Al]) ratios.The band gap energies, determined using Tauc's plots, varied from 3.4 to 5.8 eV and obeyed Vegard's law for the pseudo-binary crystalline ZnO-AlN system.With absorption coefficients reaching 10 5 cm −1 , comparable to those of crystalline ZnO and AlN films, the a-ZAON films fabricated at RT with [Zn]/([Zn] + [Al]) ratios 0.79 manifested as solid-solution alloys with atomic-level mixing.Overall, the synthesized a-ZAON films offer wide ranging band gap tunability in the UV region, demonstrate high absorption coefficients, display amorphous structures with surface smoothness, and allow low-temperature synthesis, making them promising materials for use in optoelectronic devices.
x denotes the mole fraction of ZnO in the system.As depicted in figure 2(a), the absorption edges appeared somewhat broad and independent of the chemical composition ratio, with all films exhibiting band gaps approximately 3.4 eV.Here, the significant deviation from the prediction of Vegard's law likely originate from phase separation into crystalline ZnO and amorphous Al(O)N phases.The ZnO portions of the fabricated films, with a band gap of 3.4 eV, predominantly influence the behaviours of optical absorption edges, as the Al(O)N portions are anticipated to exhibit wider band-gaps exceeding 6 eV[24].These findings indicate the presence of a significant miscibility gap between ZnO and AlN at 450 °C, likely attribute to differences in interatomic spacing between ZnO and AlN, which constrain the equilibrium AlN mole fraction in ZnO at this temperature.

Figure 6 (
a) shows the transmittance spectra of the ZAON films, with chemical composition ratios [Zn]/([Zn] + [Al]) ranging from 0.24-0.92.The optical absorption edges were relatively sharp and shifted continuously toward shorter wavelengths with decreasing [Zn]/([Zn] + [Al]) ratios.Meanwhile, the band gap energies, derived from Tauc's plot, changed from 3.4 to 5.8 eV (figure 6(b)).These findings demonstrate the wide-ranging tunability of the band gap of the ZAON films in the UV region.Interestingly, despite the amorphous nature of the films, their band gaps align with those for the pseudo-binary crystalline ZnO (3.4 eV)-AlN (6.2 eV) system derived from Vegard's law.This indicates that the a-ZAON films manifested as almost perfect solid-solution alloys with atomic-scale mixing, as adherence to Vegard's law suggests overlapping between the Zn s-orbitals and Al s-orbitals constituting the conduction band minimum of ZAON films.
2 at [Zn]/([Zn] + [Al]) ratios of 0.54 and 0.46 (see Supplementary data online), where the film structures are x-ray amorphous, corroborating our findings of direct band gaps in the a-ZAON films.Conversely, at a [Zn]/([Zn] + [Al]) ratio of 0.87, Tauc's plots did not show a similar linear trend.Based on the XRD results indicating the presence of crystalline ZnO portions at this composition ratio, we attributed this deviation to phase separation into crystalline ZnO phase and an amorphous Al(O)N phases, where ZnO portions with a band gap of 3.4 eV primarily influence the optical absorption edge.All the results of XRD, AFM, TEM, and optical analyses indicate that the a-ZAON films fabricated at RT have high likelihood of being pseudo-binary amorphous ZnO-AlN alloys.