Control of phase formation of (Al_xGa_1 − x)₂O₃ thin films on c-plane Al₂O₃

The influence of growth parameters on the formation of κ - (Al_xGa_1 − x)₂O₃ thin films was analyzed by varying $T_\mathsf{g}$ and p(O₂) during PLD growth, utilizing the same ceramic PLD target for all prepared samples. The thin films were also examined under x-ray diffraction, revealing the crystal structure to be monoclinic, orthorhombic, or a coexistence of both. In figure 1, representative 2θ-ω scans (2θ  =  15–65^°) of (Al_xGa_1 − x)₂O₃ thin films are depicted, revealing characteristic reflection peaks, which can be assigned either to the β- or the κ-phase. Peaks corresponding to both phases are present for samples exhibiting phase separation. The growth conditions, Al-content and identified phase of these (Al_xGa_1 − x)₂O₃ thin films are presented in table 1. These samples will be referred to as samples A, B, and C below.

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Table 1. Growth temperature $T_\mathsf{g}$, oxygen pressure p(O₂), measured Al content x by EDX, and polymorph for samples A, B and C.

label	$T_\mathsf{g}$ (°C)	p(O2) (mbar)	x	polymorph
A	620	0.002	0.13	κ
B	540	0.001	0.11	phase separation
C	500	0.002	0.10	β

At the bottom of figure 1, the XRD pattern of sample C, a polycrystalline β - (Al_xGa_1 − x)₂O₃ thin film grown at 500^°C and 0.002 mbar, reveals the growth along the (-201) n lattice plane. This is indicated by the lattice plane reflection peaks observable at 2θ  =  18.82^°, 38.21^°, and 59.10^°. The growth along the (-201) lattice plane on c-plane sapphire has already been reported several times in relation to β - (Al_xGa_1 − x)₂O₃ [39, 40, 42, 43]. Aside from the (-201)$_\beta$ orientation, two additional orientations of the β-phase are visible in the scan: the (-401)$_\beta$ at 2θ  =  30.13^°_, and the (-601)$_\beta$ at 2θ  =  44.26^°. The XRD pattern of sample A (top panel of figure 1), which was grown at 620^°C and 0.002 mbar, reveals reflection peaks at 2θ  =  19.26^°, 39.03^° and 60.11^°, which can be assigned to the (002) n lattice planes of κ - (Al_xGa_1 − x)₂O₃. These reflection peaks occur at slightly higher angles compared to those of the β-thin film described above, which is typical for binary κ-Ga₂O₃ [27], as well as its ternary alloys (κ - (In,Ga,Al)₂O₃) [28–31]. The thin film B, shown in the middle of figure 1, was grown at 540^°C and 0.001 mbar. The XRD pattern for this sample contains all previously described peaks, indicating phase separation. In addition, a peak shoulder appearing at 2θ  =  58.02^° is observed for B and C (marked with an arrow), which cannot be unambiguously assigned to a lattice plane reflection. One possible explanation for this is the presence of reflections at the monoclinic (-313)$_\beta$ and the cubic (333)$_\gamma$ lattice plane, or an convolution of both. The c-plane sapphire (00.6) lattice plane reflection peak appears for all samples at 41.68^°. Small peaks at 20.4^° and 29.0^° can also be observed (e.g. for sample A), emanating from the so-called Umweganregung (x-ray double diffraction) of the substrate. Such an Umweganregung is only observable under certain φ rotations, and may therefore occur in some samples [44]. Further visible reflection peaks are due to the K$_{\beta}$ (35.2^°, 53.9^°) or the tungsten L$_{\alpha}$ (37.4^°) spectral lines.

In order to compare the crystalline quality of β- and κ - (Al,Ga)₂O₃, the FWHMs of samples A and B were determined for the (004)$_\kappa$ and (-402)$_\beta$ reflection peaks. For the (-402)$_\beta$ reflection, the FWHM amounts to 0.258^°, and for (004)$_\kappa$ it is 0.120^°, exhibiting a lower broadening, and therefore representing a higher crystalline quality. The thin film thickness does not play a role, since the thickness of the samples is similar (470 nm for sample A, and 510 nm for C). The small differences in thin film thickness are caused by the different growth temperatures and the associated desorption of volatile suboxides [36, 37].

Since the oxygen pressures during growth of the three samples were nearly the same (0.001 or 0.002 mbar, respectively), the difference in growth temperature is accountable for the crystallization of the different phases seen here. Moreover, the growth temperature influences the incorporation of Al into the layers, which increases with increasing $T_\mathsf{g}$. Note that the Al content present in the target is approximately 8.8 at.%. In the layers, values of 10 to 13 at.% were detected (see table 1). A detailed description of this phenomenon maycan be found in the section Phase Control. In the next section we want to focus on an atomically resolved description of the orthorhombic thin film (sample A) and the thin film exhibiting phase separation (sample B).

TEM investigations were performed for samples A and B and the cross-sectional TEM bright field images are shown in figure 2(a) and (b), respectively. In both layers, two phases of different morphologies can be clearly distinguished: a speckle-contrast region starting from the interface, indicating a highly granular structure, and on top of that a phase with vertically aligned features, indicating columnar-like domains. Selected area electron diffraction (SAED) was performed for each of these regions to identify the crystal structure and the growth orientations. This measurement was performed for two zone axis orientations of the sapphire substrate, [11$\bar{2}$0] and [1$\bar{1}$00], for which the data are presented in figure 2(c). With regard to electron diffraction simulations [45], the red circled region is identified as a coexistence of the monoclinic β- and the cubic γ-phase. The γ-Ga₂O₃ phase has a cation-deficient spinel structure ($Fd\bar{3}m$) described in detail by Playford et al [46], and similar to that of γ-Al₂O₃. The blue circled region can be identified as the orthorhombic κ-structure in both samples, as shown in figure 2(c). The SAED patterns of the κ- and β + γ-regions of sample A look the same as those presented for sample B. The domain boundaries between the β + γ-part and the κ-phase extend diagonally through the layer. This suggests that the κ-phase starts to nucleate as islands in certain positions, probably due to Sn accumulations, then grows at a higher growth rate than the β + γ-phase until the latter is overgrown. The composition in both phases is the same, as confirmed by STEM-EDXS measurements of the TEM sample. A STEM-EDXS spectrum can be found in the supplemental material (stacks.iop.org/JPD/53/485105/mmedia).

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The different phases are investigated in greater detail via Scanning TEM (STEM) imaging. Figure 3(a) shows a STEM image, taken close to the interface in sample B, and including the sapphire substrate and regions with β + γ- and κ-phase. The κ-phase is shown at higher magnification in figure 3(b), where we see that it is growing in narrow columnar domains of a few nanometers in width. The structure exhibits vertical lines of defects, extending through the structure (positions indicated by the white arrows), which are indicative of the vertical domain boundaries. The atomically resolved structure of the κ-phase is displayed in figure 3(c)II, and fits well to the atomic model superimposed on the experimental image. Ga columns appear bright in these images, while oxygen atoms are too light to produce a visible contrast. The domains are 60^° rotated in plane with respect to each other, and the epitaxial relationships are (001)$_\kappa ||$(00.1)$_\mathsf{s}$ and [010]$_\kappa$, [310]$_\kappa ||$[1$\bar{1}$00]$_\mathsf{s}$ and [100]$_\kappa$, [110]$_\kappa||$[11$\bar{2}$0]$_\mathsf{s}$.

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A high magnification image of the interface region (shown in figure 3(c) III) actually reveals the presence of a fourth phase, namely α. It grows pseudomorphically for 3-4 monolayers on the sapphire substrate, with the same crystal structure. This phase is not detectable in XRD, because the layer is buried and too thin. The formation of this α-interlayer has already been shown to be typical for Ga₂O₃ layer growth on sapphire [47].

Close to the interface, small grains (measuring only a few nm) of β- and γ-phase are mixed randomly, while at larger thicknesses the grains grow larger and the β-phase is dominant. Figure 3(c) shows area scans for γ- and a β-phase grains where the structures can be fitted to the stick-and-ball models. Despite these small grains, both β- and γ-phases exhibit an epitaxial relationship with respect to the sapphire. For the out-of-plane orientation, (-201)$_\beta ||$(00.1)$_\mathsf{s}$ and (111)$_\gamma ||$ (00.1)$_\mathsf{s}$ was found. The spacings of these planes are very close to each other in value (d((-201)$_\beta$)  =  4.70 Å and d((111)$_\gamma$)  =  4.75 Å), therefore the phases can hardly be distinguished in the XRD patterns. The in-plane relationship for the γ-phase is [1$\bar{1}$0]$_\gamma ||$[1$\bar{1}$00]$_\mathsf{s}$. The β-grains grow in different orientations, rotated in-plane by 60^° with respect to each other, according to [010]$_\beta$ or [132]$_\beta ||$[1$\bar{1}$00]$_\mathsf{s}$. Since this polycrystalline interlayer is only a few 10 nm thick in sample A, it implies that these grains are even smaller, which could explain the x-ray amorphous behavior of this layer. The formation of the approximately 20–30 nm small and wavy interlayer can be explained by the PLD approach employed here. During the first laser pulses an insufficient amount of tin is ablated, and the required liquid tin layer cannot form instantaneously. Only when this tin layer has formed can the growth of the orthorhombic phase begin.

Investigations of the surface morphology of samples A and C show similar results to comparable samples published previously [29, 31]. Therefore, only the surface morphology of sample B is presented below. AFM measurements show a surface consisting of 3D islands, which has a root mean square surface roughnesses of 8.6 nm. Figure 4(a) shows a 5 × 5 µm² surface scan, depicting 3D islands, 200–400 nm in diameter. By considering the cross-sectional view, presented in figure 4(b), it can be established that the higher parts of the thin films can be allocated to the orthorhombic phase. The higher growth rates of the κ-phase, compared with those of the β-phase, leads to an overgrowth of the β + γ-part. If the thin film growth were to continue, e.g. by a higher pulse number from the PLD, it may be assumed that the κ-phase proportion would grow further in lateral size and finally overgrow the β + γ-part completely. Such complete overgrowth is visible in figure 2(a) for the κ - (Al,Ga)₂O₃ thin film.

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In addition to the aforementioned samples, thin films were deposited under several growth conditions (various combinations of p(O₂) and $T_\mathsf{g}$) and investigated via XRD to determine their crystal structure, by EDX to define their cation composition, and by spectroscopic ellipsometry to examine their growth rates. Note that the ceramic target employed here consists of Ga₂O₃ doped with 8.8 at.% Al₂O₃. Although TEM images show that the phase distribution is more complicated, below, we will focus only on phases with a high volume fraction, as identified by XRD.

The growth conditions, such as oxygen pressure and substrate temperature, were selected in the ranges of 3× 10⁻⁴ mbar ≤ p(O₂) ≤ 0.024 mbar, and 450^°C ≤ $T_\mathsf{g}$ ≤ 670^°C. Every sample was measured using XRD to determine the respective polymorph. The sample series for p(O₂) = 0.000 3 mbar is depicted in figure 5, showing a change in the crystal structure, dependent on $T_\mathsf{g}$. For temperatures from 670 to 580^°C, the κ-phase forms, for 540^°C we obtain a mixture of the β + γ- and κ-phases, and for 500-450^°C, the β-phase forms. The XRD spectra for the remaining oxygen pressures and growth temperatures can be found in the supplemental material. The evaluation of these detected crystal structures revealed defined growth conditions, among them the κ-modification forms presented in figure 6(a). These specific growth conditions are required for the given amount of tin in the target, due to the surfactant-mediated growth, where a liquid tin layer forms on top of the thin film, inducing the κ-growth below. The tin will not be incorporated into the layer, and desorbs, or rather accumulates at the top, as demonstrated for κ-(Al,Ga)₂O₃ [31].

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In the oxygen rich regime of 0.024 mbar, and at growth temperatures between 540^°C and 580^°C, the tin atoms completely oxidize, a liquid tin layer does not form, and hence the formation of the κ-phase is suppressed. To determine the point of transition from the β- to the κ-phase more accurately, thin films were grown between 540^°C and 620^°C instead of at an oxygen pressure of 0.01 mbar at 0.016 mbar. For temperatures below 500^°C, surface-mediated growth cannot be observed, such that the monoclinic polymorph forms across the whole examined pressure range. In a narrow growth window of $T_\mathsf{g}$  =  540^°C and 3× 10⁻⁴ mbar ≤ p(O₂) ≤ 0.016 mbar, as well as for 620^°C and 0.024 mbar, both phases coexist, as described in the previous section. Conceivably, the creation of the liquid tin layer is insufficient and/or small in this growth regime. The cation composition was investigated by means of EDX measurements, and is depicted as a function of the Al content x in figure 6(b). As reported for β - (Al_xGa_1 − x)₂O₃ thin films [40], x increases with decreasing oxygen pressure and/or increasing growth temperature up to 2.25 times the amount of material actually offered in the target. In monoclinic thin films, volatile Ga₂O suboxides form at the layer surface, and subsequently desorb. As a result, the Ga₂O species is preferentially formed, due to the lower dissociation energy of the Ga-O bond compared to the Al-O bond. The higher the oxygen deficit (low p(O₂)) and/or the higher $T_\mathsf{g}$, the more suboxides form and desorb, leading to a higher Al concentration in the thin film. Since for the κ-polymorph grown by PLD, surface-mediated growth was reported [27, 31], the desorption process should be influenced by the existence of the liquid tin layer on the thin film's surface. The exact growth and desorption mechanism should be the focus of future research projects. However, the Al incorporation for the κ-, β- and the samples exhibiting phase separation is presented in figure 6(b) in relation to its dependence on p(O₂) and $T_\mathsf{g}$. Regarding the $T_\mathsf{g}$ series, it is observable that the desorption process diminishes for decreasing $T_\mathsf{g}$. The growth pressure, where a stoichiometric target-to-layer cation transfer can be monitored, shifts continuously to lower p(O₂). The stoichiometric cation transfer is x  =  0.088, and is marked in figure 6(b). For $T_\mathsf{g}$  =  670^°C, this point is reached at p(O₂)  =  0.01 mbar, for 580^°C at around 0.006 mbar, for 540^°C and 500^°C at 0.003 mbar, and for 450^°C a stoichiometric cation transfer was observed between 3 × 10⁻⁴ and 0.002 mbar. Since the Al content of the samples grown at 500^°C and 450^°C shows only a small change or no change in x, no additional thin films were grown. In an oxygen rich regime, p(O₂) ≥ 0.016 mbar, the cation transfer of Al is x  =  0.07, because the lighter Al ions are more widely scattered than the heavier Ga ions. For high p(O₂), x remains constant and increases are no longer as visible at 620^°C, 580^°C, and 500^°C, indicating that the formation of volatile Ga₂O is suppressed [36, 37]. Nevertheless, the surface-mediated growth reduces the desorption process compared to the monoclinic phase. To demonstrate this, the κ - (Al_xGa_1 − x)₂O₃ thin films grown at 670^°C can be compared with β - (Al_xGa_1 − x)₂O₃ thin films published before [40]. The target used for both sample series consists of an admixture of 8.8 at.% Al₂O₃, and the samples are grown at 670^°C and at various oxygen pressures. The monoclinic thin films show systematically higher x for all oxygen pressures. Another indication of the lower incorporation of Ga into the layer due to desorption of volatile Ga₂O is a decrease in the growth rate r, which was determined by spectroscopic ellipsometry and presented in figure 6(c). The graph shows a dependency on the growth conditions. With decreasing p(O₂) and/or increasing $T_\mathsf{g}$, the growth rate also decreases. The behaviour of r is in accordance with an increased desorption of volatile Ga₂O, which is indicated by the inset of figure 6(c), where r is plotted against x: r decreases approximately linearly with increasing x. The lowest r is about 19 pm/pulse for the lowest investigated oxygen pressure (3 × 10⁻⁴ mbar), the highest temperature (670^°C) and the lowest Ga content (1-x  =  0.8). For the highest investigated oxygen pressures (0.016 mbar and 0.024 mbar), the growth rates seem to saturate for all investigated $T_\mathsf{g}$ between 45 - 46.5 pm/pulse. Interestingly, this is the growth region where fewer Al atoms are present, as they were incorporated into the thin film, suggesting that deposition kinematics, such as scattering, play a minor role compared to the forming and desorption of volatile suboxides.