Disorder–Order Transition in Ga2O3 and Its Solid Solution with In2O3 upon Thermal Annealing

The local ordering in amorphous Ga2O3 and (In x Ga1−x )2O3 thin ﬁlms on Si(111) and its change on crystallization upon annealing are investigated. Synchrotron‐based extended X‐ray absorption ﬁne structure data, as probed across the Ga K‐edge in conjunction with a numerical implementation of the scattering process including multiple scattering events, provide detailed insights into the bonding character of the Ga cations. Bond lengths and coordination numbers indicate the preferential formation of GaO4 tetrahedrons in the amorphous binary and ternary solid solution at the absence of long‐range ordering. Upon annealing there is no tendency toward the formation of long‐range ordering for the ternary alloy, while the amorphous Ga2O3 ﬁlm turns into monoclinic β‐Ga2O3 crystallites with both tetra‐ and octahedral coordination.


Samples and Measurement
For this study, Ga 2 O 3 and ðIn x Ga 1Àx Þ 2 O 3 films of 250 nm thickness and a nominal concentration x ¼ 0.5 were deposited on Si(111) substrates at a comparatively low temperature of 100°C via plasma-assisted molecular beam epitaxy. The substrates are covered by a natural oxide, which prevents an epitaxial alignment of the deposited layers. In case of binary Ga 2 O 3 , a beam equivalent pressure of Φ Ga ¼ 2.04 Â 10 À7 mbar, an O-flow of 2 sccm, and a plasma cell power of 300 W were chosen. The DOI: 10.1002/pssb.202200535 The local ordering in amorphous Ga 2 O 3 and (In x Ga 1Àx ) 2 O 3 thin films on Si(111) and its change on crystallization upon annealing are investigated. Synchrotronbased extended X-ray absorption fine structure data, as probed across the Ga K-edge in conjunction with a numerical implementation of the scattering process including multiple scattering events, provide detailed insights into the bonding character of the Ga cations. Bond lengths and coordination numbers indicate the preferential formation of GaO 4 tetrahedrons in the amorphous binary and ternary solid solution at the absence of long-range ordering. Upon annealing there is no tendency toward the formation of long-range ordering for the ternary alloy, while the amorphous Ga 2 O 3 film turns into monoclinic β-Ga 2 O 3 crystallites with both tetra-and octahedral coordination.
ðIn; GaÞ 2 O 3 alloy was deposited at beam equivalent pressures of Φ Ga ¼ 1 Â 10 À7 mbar and Φ In ¼ 1 Â 10 À7 mbar while the other parameters were kept constant. The amorphous state of the layer was confirmed by in situ reflection high-energy electron diffraction. Subsequently, half of the samples underwent annealing in a rapid thermal annealing oven at 800°C for 15 min in a pure O 2 atmosphere while their counterparts remain untreated. X-ray absorption spectra on both sets, the as-deposited and annealed samples, were performed at the dedicated EXAFS beamline P65 at PETRAIII (DESY, Hamburg) across the Ga K-edge at 10 367 eV. An energy range of 10 220-11 360 eV was covered by use of the Si(111) double crystal monochromator with a resolution of ΔE=E % 10 À4 . Two plane mirrors mounted in the beam path before the detector at 2 mrad incidence angle provide for higher harmonics rejection. Because of the nearly X-ray opaque 500 μm thick Si substrate, we opted to work in fluorescence mode utilizing the passivated planarimplanted silicon detector in continuous scan mode. At various incident angles ϕ of 25°, 45°, and 65°five spectra of each specimen were acquired to treat the orientation dependence of the spectra.

Results and Discussion
Generally, a major challenge of EXAFS analytics on crystalline substrates refers to the superposition of multiple diffraction peaks superimposed on the EXAFS spectra due to an accidental Bragg diffraction. In particular, artificial contributions at higher frequencies may show up, potentially affecting the EXAFS in a region where the spectrum contains information from multiple scattering events at higher coordination shells. However, this issue can effectively be tackled since the single-crystalline substrate only contributes at very specific angles. Therefore, the spectra were first processed for each angular position ϕ separately, that is, normalization and background removal were performed. Out of this redundant data set, then a minimum function of the normalized absorption coefficient μðEÞ has been derived, which eventually yields the pure EXAFS spectrum χðkÞ in reciprocal space. To extract quantitative information, we calculated the EXAFS function jχðRÞj in real space as the Fourier transform jF½k 2 χðkÞj of the k 2 -weighted function χðkÞ. The latter is defined by the fundamental EXAFS equation.
where j indicates the scattering path, S 2 o is the amplitude reduction factor, R j is the half path length, N j is the coordination number, f j ðkÞ is the scattering amplitude, σ j is the Debye-Waller factor, and δ j ðkÞ the phase shift. Siah et al. [17] and Chen et al. [18] determined a constant amplitude reduction factor S 2 0 of 1 by fitting EXAFS spectra of β-Ga 2 O 3 single crystals at the Ga K-edge. Let us mention that S 2 o contains several contributions and is basically fixed here to determine the coordination numbers (i.e., NS 2 o in the EXAFS equation). Our calculations are based on an equivalent structural model [19] for β-Ga 2 O 3 and thus we use for the fitting procedure the same constant amplitude reduction factor for both the pure binary and the ternary alloy. In the pure binary β-Ga 2 O 3 , Ga atoms occupy two symmetrically inequivalent sites (cf., model in Figure 2c) with tetrahedral (Ga1) and octahedral coordination (Ga2). Moreover, the GaO 4 tetrahedrons are distorted and thus the Ga-O distance varies between 1.835 and 1.863 Å and is considerably smaller than within the (distorted) GaO 6 octahedrons with bond lengths from 1.935 to 2.074 Å. These are faint differences. Nevertheless, based on the accuracy limit of our method, a discrimination between both coordination environments becomes detectable. The calculation of amplitudes and phase shifts as well as the fitting of the residual parameters were calculated using FEFF6 as part of the Artemis software package. [20] Figure 1a depicts the experimentally derived EXAFS function from the as-deposited (amorphous) and annealed Ga 2 O 3 and ðIn x Ga 1Àx Þ 2 O 3 films. For both spectra of the ternary alloy as well as the amorphous Ga 2 O 3 layer, this data evaluation process is straight forward, whereas k 2 χðkÞ becomes disruptively superimposed by several intense Bragg peaks in case of annealed Ga 2 O 3 . Since these are due to randomly oriented crystalline grains, they cannot easily be removed from the spectrum by a filter. All our EXAFS spectra are subject to this inherent experimental artifact. Figure 1b reveals a part of the normalized absorption coefficient μðEÞ for different incident angles ϕ: the oscillations www.advancedsciencenews.com www.pss-b.com slightly vary in shape and thus imply that the annealing process induced the formation of a range of disordered crystallites. The inset depicts a normalized absorption spectrum across the Ga K-edge for an extended energy range. Figure 2a shows the magnitude of the Fourier-transformed EXAFS function jF½k 2 χðkÞj of as-deposited and annealed ðIn x Ga 1Àx Þ 2 O 3 , b) of as-deposited and annealed Ga 2 O 3 , and c) as well as model calculations. The amorphous films give rise to a single peak originating from scattering at the first coordination shell indicating the absence of long-range order. The underlying fitting parameters are summarized in Table 1. The firstshell Ga-O bond length of 1.868 Å approaches very closely the Ga-O bond length of tetragonally coordinated Ga in β-Ga 2 O 3 . This goes along with a coordination number of 4.1, which further highlights a preferential initial tetragonal coordination of Ga within amorphous ðIn x Ga 1Àx Þ 2 O 3 .
Surprisingly, the experimental jF½k 2 χðkÞj of the annealed ðIn x Ga 1Àx Þ 2 O 3 thin film in Figure 2a reveals merely one contributing frequency as well and resembles that of its as-deposited counterpart. Higher shell scattering does obviously not take place, which proves a preserved short-range order at the absence of (an anticipated trend toward) long-range ordering. However, the first-shell coordination length is shifted to a slightly larger value of 1.893 Å as well as the coordination number (4.4), see Table 1. Also, the Debye-Waller factor increases to some extent. We conclude that annealing leads to a change from a pure tetrahedral toward a more mixed tetrahedral/octahedral Ga coordination. These findings are in good agreement with recent results on ðIn 0.6 Ga 0.4 Þ 2 O 3 films annealed at 300°C for 30 min. Huang et al. [10] observed for this composition exclusively first-shell scattering at the Ga K-edge while Ga atoms preferentially occupy tetrahedral sites. The lack of experimental proof for long-range ordering might be related to a nonstatistical occupation of both lattice sites by In and Ga cations as supported by density-functional calculations. These predict a 1.1 eV higher cost for In incorporation into a tetrahedron in β-Ga 2 O 3 as compared to an octahedral site. [21] They further indicate that at exactly x ¼ 0.5, the β-phase is energetically more favorable (with all In/Ga atoms exclusively occupying octahedral/tetrahedral lattice sites) than cubic bixbyite and as stable as the trigonal phase. [14] It seems that a tendency toward such a structural decomposition at compositions x ≃ 0.5, while preferring the monoclinic β-phase, hampers the formation of long-range ordering (which relies on a periodic arrangements of octa-and tetrahedrons). At least it suppresses its weak EXAFS signature below our experimental sensitivity.
The Fourier-transformed EXAFS function jF½k 2 χðkÞj of the Ga 2 O 3 films in Figure 2b shows significant differences for the as-deposited and annealed sample. In case of untreated Ga 2 O 3 , very similar to the ternary compound, only first shell, single scattering takes place (red curve). The computed nearestneighbor distance of 1.877 Å mimics very closely the one of as-deposited ðIn x Ga 1Àx Þ 2 O 3 . While the local distancing around Ga atoms within amorphous Ga 2 O 3 is akin to the one of the ternary alloy, a decreased Ga coordination number of 3.7 indicates an environment locally even less ordered than in GaO 4 tetrahedrons. Such a reduced coordination appears in line with observations at amorphous Ga 2 O 3 on sapphire, where coordination is found to be tetrahedral or even less than that. [22] In contrast, the angle-dependent EXAFS profiles of the annealed Ga 2 O 3 film exhibit real-space frequencies due to higher shell scattering (positions indicated by vertical bars). Thus, in contrast to the ternary alloy, local pre-stages of Ga 2 O 3 crystallites, that is, building blocks consisting of GaO 4 tetrahedrons), undergo a ripening process as reflected by the established long-range order. Annealing and growth at elevated temperatures lead to the presence of higher shell features in the Ga K-edge EXAFS assigned to β-Ga 2 O 3 . [9,18] A direct comparison with a model calculation for β-Ga 2 O 3 (black curve in Figure 2c) holds an excellent agreement. Here, we have considered all possible single-and multiple-scattering events up to an overall path length of 5 Å. The black curve results from the full monoclinic structure, whereas the red and blue www.advancedsciencenews.com www.pss-b.com ones plot the disentangled contributions from tetra-and octahedral coordination geometry. As expected in the first coordination shell, the maxima of the two sub-curves are shifted against each other reflecting the different bond lengths in tetra-and octahedrons (see insets in Figure 2c). The experimental maxima slightly fluctuate near the theoretical value for different incidence angles ϕ. However, the spatial uncertainty is considerably smaller than the predicted difference between the two coordination environments, which indicates that the annealed stage contains both coordinations. Additionally, the simulation confirms the origin of the second and third maxima for the annealed Ga 2 O 3 specimen, which arise due to higher shell scattering probing Ga-Ga and Ga-O bonds.
Complementary transmission electron microscopy (TEM) studies support the statement regarding Ga 2 O 3 given so far. Figure 3 shows cross-sectional high-resolution (HR)TEM imaging of a) the as-deposited and d) annealed Ga 2 O 3 layer, as well as corresponding selected area electron diffraction (SAD) pattern. Table 1. The parameters, as given by the EXAFS equation, are the bond length R GaÀO , coordination number N, Debye-Waller factor σ 2 and the energy shift of the measured spectrum ΔE 0 . As the fits are restricted to single scattering at the first coordination shell, the chosen r-range is 0.8-2.3 Å whereas the k-range is 2.5-14 Å À1 . The R-factor is a measure of closeness of data and fit. All measurements have been performed at the Ga K-edge. The model calculation for annealed Ga 2 O 3 in Figure 2c is based on powder data [19,23,24] of β-Ga 2 O 3 . It takes into account the two different coordination environments with single-and multiple-scattering paths up to path lengths of 5 Å. The R GaÀO value of the simulation provides a weighted average over the (distorted) octa-and tetrahedrons.  www.advancedsciencenews.com www.pss-b.com