Epitaxial growth and magnetic properties of ultraviolet transparent Ga2O3/(Ga1−xFex)2O3 multilayer thin films

Multilayer thin films based on the ferromagnetic and ultraviolet transparent semiconductors may be interesting because their magnetic/electronic/photonic properties can be manipulated by the high energy photons. Herein, the Ga2O3/(Ga1−xFex)2O3 multilayer epitaxial thin films were obtained by alternating depositing of wide band gap Ga2O3 layer and Fe ultrathin layer due to inter diffusion between two layers at high temperature using the laser molecular beam epitaxy technique. The multilayer films exhibits a preferred growth orientation of crystal plane, and the crystal lattice expands as Fe replaces Ga site. Fe ions with a mixed valence of Fe2+ and Fe3+ are stratified distributed in the film and exhibit obvious agglomerated areas. The multilayer films only show a sharp absorption edge at about 250 nm, indicating a high transparency for ultraviolet light. What’s more, the Ga2O3/(Ga1−xFex)2O3 multilayer epitaxial thin films also exhibits room temperature ferromagnetism deriving from the Fe doping Ga2O3.

During the past ten years, multilayer thin films based on magnetic and nonmagnetic layers have great deal of scientific and industrial attention due to their anomalous spin dependent effects and potential applications in magnetic sensors, information storage media, thermoelectric devices and high frequency devices [1][2][3] . Particularly, the ferromagnetic and transparent semiconductors multilayer thin films are interesting because where the ferromagnetic properties can be controlled by light [4][5][6] . It integrates magnetism into optoelectronics devices, appealing materials for magnetooptical devices 7,8 . So far, there have been a few reports on transparent ferromagnetic multilayer thin films. For example, ZnO/Fe 3 O 4 and In 2 O 3 /Fe 3 O 4 multilayer structure films were fabricated and their optoelectrical and magnetic properties were studied [9][10][11] . As a typical wide band gap semiconductor material, β-Ga 2 O 3 , with a band gap of 4.5~5.3 eV and a high transparency for the visible and wide range of ultraviolet down to 280 nm [12][13][14] , is considered as one of ideal candidates to fabricate transparent multilayer magnetic-optic-electronic devices 15,16 . Ga 2 O 3 can crystallize in five different phases (α, β, γ, δ, and ε) 17,18 . Amongst them, the monoclinic β-Ga 2 O 3 (space group: C2/m) with the lattice parameters of a = 12.23 Å, b = 3.04 Å, c = 5.80 Å, and β = 103.7° is considered to be the most stable thermally in the range from room temperature up to the melting point of about 1800 °C what determines also the possibility of working at high temperatures 19 . Additionally, β-Ga 2 O 3 has great chemical stability, being unaffected even by concentrated acids such as hydrofluoric acid 20 . More importantly, β-Ga 2 O 3 is known to exhibit a strong dynamic nuclear polarization upon saturation of the magnetic resonance of conduction electrons, which is at the origin of a free electron memory referred to as bistable conduction electron spin resonance [21][22] . On the other hand, β-Ga 2 O 3 is also the host material for magnetic semiconductors, and room temperature ferromagnetism was observed in Mn-doped β-Ga 2 O 3 by our group 15 .

Results and Discussion
The Ga 2 O 3 /(Ga 1−x Fe x ) 2 O 3 multilayer thin films were fabricated by alternating depositing of Ga 2 O 3 layer and Fe ultrathin layer due to inter diffusion between two layers at high temperature using the laser molecular beam epitaxy (LMBE) technique, as is shown in Fig. 1(a). The thickness and composition of (Ga 1−x Fe x ) 2 O 3 thin layer can be controlled by solely changing the laser pulse numbers during each run for depositing the Fe layer (defined as N, N = 0, 10, 20, 30, 40, 50,100) while those for depositing Ga 2 O 3 layers in each run were fixed at 100. Herein, samples were abbreviated as Ga 2 O 3 /Fe(N) multilayer thin films for the laser pulse numbers N of depositing Fe layer. The sharp and streaky reflection high-energy electron diffraction (RHEED) patterns indicate the achievement of an atomically flat surface of Ga 2 O 3 /Fe(10) and Ga 2 O 3 /Fe(40) multilayer thin films [ Fig. 1(b)]. While the RHEED patterns of Ga 2 O 3 /Fe(100) film are fuzzy and punctiform, meaning a rough surface. Cross-section low-magnification transmission electron microscope (TEM) investigations were carried out to identify the microstructure of the thin film. The Ga 2 O 3 /Fe(50) multilayer thin film shows a thickness of about 130 nm estimated by the cross-sectional TEM bright-field image of Fig. 1(c). The average thickness of the Ga 2 O 3 /Fe(50) single thin layer is about 6.5 nm due to the layer number of 20. Figure 1(e) shows the cross-sectional high-resolution In order to further confirm the multilayer structure of the as-grown thin films, the compositions as a function of film thickness were characterized by using the secondary ion mass spectrometry (SIMS) depth profiling. The results for the representative Ga 2 O 3 /Fe(50) film were given in Fig. 2 by showing intensities of the Fe and Ga ion currents as a function of sputter depth of the film. The intensity of Ga ion remains almost constant as the sputter depth of the film. However, the intensity of Fe ion exhibits evident wavy shape with 19 peaks, corresponding to the 19 layers of (Ga 1−x Fe x ) 2 O 3 thin film in our scheming Ga 2 O 3 /(Ga 1−x Fe x ) 2 O 3 multilayer structure. Notably, the difference value between peaks and troughs of few layers near-surface is larger than that near-substrate, which is attributed to the inter diffusion between Fe ultrathin layer and Ga 2 O 3 layer at high temperature for much more time. As a result, both the TEM-EDX and SIMS results indicate that Fe ion is uneven and stratified distributed in the film.
The crystal structure and film orientation of the Ga 2 O 3 /(Ga 1−x Fe x ) 2 O 3 multilayer thin films were determined from θ -2θ scans of XRD, as is shown in Fig. 3(a). In order to facilitate compared with the others, the diffraction intensities of the Ga 2 O 3 /Fe(0) and Ga 2 O 3 /Fe(100) films were multiplied by 3 and 10 times respectively. Except Ga 2 O 3 /Fe(100) film, only (201) and higher order peaks of Ga 2 O 3 monoclinic β phase appear for the other as-grown Ga 2 O 3 /(Ga 1−x Fe x ) 2 O 3 multilayer films besides those from the substrate, indicating single phase and a preferred (201) plane orientation of the films. Furthermore, as shown in Fig. 3(b) with the enlarged view of θ -2θ XRD patterns around 38°, the peaks of (402) are located at 38.36°, 38.31°, 38.29°, 38.26°, 38.24°, and 38.19° for N = 0, 10, 20, 30, 40, and 50, respectively, indicating that the peak gradually shifts to smaller 2θ with the increase of Fe layer thickness. The lower angles shift of (201) and higher order diffraction peaks indicates an increase of the lattice constants, which means that Fe ion has incorporated into Ga ion site and possesses a larger radius than Ga ion [23][24][25] . However, for the Ga 2 O 3 /Fe(100) film, extra diffraction peaks of (110) and (113) of β phase Ga 2 O 3 appear and the diffraction intensity of the family of (201) crystal planes decreases, featuring a polycrystalline nature. No impurity peaks related to Fe metal clusters and Fe oxides were observed.
The chemical compositions and chemical states of Fe ions in the as-grown films were characterized by using X-ray photoelectron spectroscopy (XPS), presented in Fig. 4. The elements present in the Ga 2 O 3 /Fe(50) multilayer film are Fe, Ga, O, and C (not shown). The reason of C found in the samples was asserted to be due to surface contamination. The charge-shift spectrum was calibrated using the fortuitous C 1s peak at 284.8 eV. The energy peak for Ga 3d is centered at 20.4 eV [ Fig. 4(a)], which is attributable to the presence of Ga-O bond 26 . The high-resolution XPS spectrum of Fe 2p core level shows a spin-orbit doublet (j = 3/2, 1/2) [ Fig. 4(b)]. Of the two peaks Fe 2p 3/2 peak is narrower and stronger than Fe 2p 1/2 and the area of Fe 2p 3/2 peak is greater than that of Fe 2p 1/2 because Fe 2p 3/2 has degeneracy of four states whilst Fe 2p 1/2 has only two. For Fe 2+ cations, the peak position of Fe 2p 3/2 and Fe 2p 1/2 is located at about 709.0 and 722.6 eV, and the satellite peak of Fe 2p 3/2 is located approximately 6 eV higher than the main Fe 2p 3/2 peak; While for Fe 3+ cations, the peak position of Fe 2p 3/2 and Fe 2p 1/2 is located at about 711.0 and 724.6 eV, and the satellite peak of Fe 2p 3/2 is located approximately 8 eV higher than the main Fe 2p 3/2 peak 27,28 . In our Ga 2 O 3 /(Ga 1−x Fe x ) 2 O 3 multilayer thin films, the peak positions of Fe 2p 3/2 and Fe 2p 1/2 are, respectively, 710.4 and 723.5 eV. They are located between the values for Fe 2+ and Fe 3+ cations, which can be deconvoluted into the Fe 2+ and Fe 3+ peaks, indicating that both Fe 2+ and Fe 3+ cations are contained. In addition, the satellite peaks at the high binding energy side of the main peaks are further demonstrated the coexistence of Fe 2+ and Fe 3+ . Figure 5 shows the ultraviolet-visible (UV-Vis) absorbance of the Ga 2 O 3 /(Ga 1−x Fe x ) 2 O 3 multilayer thin films. All the samples exhibit a sharp absorption edge at about 250 nm, corresponding to the intrinsic absorption of β-Ga 2 O 3 13,14 . The absorption onset shows no obvious shift with the increase of Fe ultrathin layer thickness. The band gap is derived by fitting the linear region of the plot (αhν) 2 versus hν. The inset of Fig. 5 shows the Ga 2 O 3 /Fe(50) multilayer film has a band gap of about 4.95 eV. On the other hand, it is observed that all the Ga 2 O 3 /(Ga 1−x Fe x ) 2 O 3 multilayer films has no obvious absorbance to the wavelength until 250 nm, indicating a high transparency for ultraviolet light. Figure 6 shows the magnetization versus magnetic field (M-H) curves of the Ga 2 O 3 /Fe(50) multilayer thin film at room temperature with that of pure β-Ga 2 O 3 thin film for comparing. The diamagnetic contribution from the α-Al 2 O 3 substrate was subtracted from the data. The pure β-Ga 2 O 3 thin film [Ga 2 O 3 /Fe(0)] displays paramagnetic behavior while Ga 2 O 3 /Fe(50) film show hysteresis loops indicative of ferromagnetism when the applied magnetic field is parallel to the films. Furthermore, the Ga 2 O 3 /Fe(50) multilayer thin film exhibits a magnetic anisotropy while applying the magnetic field parallel and perpendicular to the film surface. As seen in the enlarged image of M-H loops in the inset of Fig. 6, the coercivity and magnetic remanence (M r ) are ~73 Oe and 4.99 emu/cm 3 for the magnetic field paralleling to the film, while they are ~91 Oe and 3.68 emu/cm 3 respectively for the perpendicular   at the moment. The anisotropic behavior cannot be explained by the presence of randomly oriented ferromagnetic particles. In addition, the XRD and XPS measurements have confirmed the successful substitution of Fe for Ga and ruled out the possible secondary phases of Fe metal cluster and Fe-based oxides. It is reasonably to conclude that the room temperature ferromagnetism in the Ga 2 O 3 /(Ga 1−x Fe x ) 2 O 3 multilayer films is intrinsic. The perpendicular & parallel magnetic moments of Fe cation in the Ga 2 O 3 /Fe(50) multilayer thin film at 2 T are 3.74 and 3.89 μ B /Fe cation respectively, which are very close to the magnetic moment of Fe cation reported by others 29,30 .
In conclusion, the multilayer epitaxial thin films based on wide band gap of Ga 2 O 3 and magnetic semiconductor of (Ga 1−x Fe x ) 2

Methods
The epitaxial thin films were prepared on 10 × 10 mm α-Al 2 O 3 (0001) substrates by the LMBE technique at a repetition frequency of 1 Hz and with a fluence of ~5 J/cm 2 . The thin film deposition was grown in a vacuum environment of 1 × 10 −6 Pa and at a substrate temperature of 900 °C. Alternating depositions of Ga 2 O 3 layer and Fe ultrathin layer were performed for 20 times to prepare the Ga 2 O 3 /(Ga 1−x Fe x ) 2 O 3 multilayer epitaxial thin films. The (Ga 1−x Fe x ) 2 O 3 layer (that is Fe doping Ga 2 O 3 layer) was obtained due to inter diffusion between Fe and Ga 2 O 3 layers at high temperature. The Fe doping concentration of the Ga 2 O 3 /Fe(50) multilayer thin film was determined as 2.44 at.% by the X-ray energy dispersive spectroscopy. The targets were kept inside the chamber, so that deposition of all the layers could be done without breaking vacuum. This is essential to avoid any contaminations of interfaces. RHEED was utilized in-situ to monitor the whole epitaxial growth process. The orientation and crystallinity of the as-grown thin films were investigated by the XRD at θ -2θ scan. The thickness and microstructure of thin films were obtained by the TEM. Cross-section TEM specimens were prepared by a standard procedure which includes mechanical grinding, polishing, precision dimpling, and ion milling. The valences of Mn ions and elements distribution were analyzed by XPS and SIMS. Magnetic properties of the films were measured in a commercial superconducting quantum interference device (SQUID), Quantum design.