Tin-Assisted Synthesis of $\epsilon \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ by Molecular Beam Epitaxy

The synthesis of $\epsilon \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ and $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ by plasma-assisted molecular beam epitaxy on $(001){\mathrm{Al}}_2{\mathrm{O}}_3$ substrates is studied. The growth window of $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ in the Ga-rich regime, usually limited by the formation of volatile gallium suboxide, is expanded due to the presence of tin during the growth process, which stabilizes the formation of gallium oxides. X-ray diffraction, transmission electron microscopy, time-of-flight secondary-ion mass spectrometry, Raman spectroscopy, and atomic force microscopy are used to analyze the influence of tin on the layer formation. We demonstrate that it allows the synthesis of phase-pure $\epsilon \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$. A growth model based on the oxidation of gallium suboxide by reduction of an intermediate sacrificial tin oxide is suggested.

Figure 1

Comparison of the growth rates of series B and A (with and without tin) as a function of ${\mathrm{BEP}}_{\mathrm{Ga}}$. An attenuation of etching and an expansion of the growth window with the presence of tin are observed.

Figure 2

Growth rate at a ${\mathrm{BEP}}_{\mathrm{Ga}}$ of $1.77\times {10}^{-7}\mathrm{mbar}$ as a function of ${\mathrm{BEP}}_{\mathrm{Sn}}$ (series C). Only above a critical ${\mathrm{BEP}}_{\mathrm{Sn}}$ of $4\times {10}^{-12}\mathrm{mbar}$ growth is observed.

Figure 3

$\omega \text{−}2\theta$ scans of the samples grown at a ${\mathrm{BEP}}_{\mathrm{Ga}}$ of $1.77\times {10}^{-7}\mathrm{mbar}$ and at different ${\mathrm{BEP}}_{\mathrm{Sn}}$ values (series C). The presence of tin stabilizes $\epsilon \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$.

Figure 4

The $\omega \text{−}2\theta$ scans of samples of series B and an additional sample with a higher plasma power (350 W) are shown. Oxygen-rich conditions lead to the formation of $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$, whereas gallium-rich conditions lead to the formation of $\epsilon \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$. This holds for both the variation of ${\mathrm{BEP}}_{\mathrm{Ga}}$ and the oxygen plasma power, as shown in the top curve.

Figure 5

Results of the TOF-SIMS analysis for samples with different ${\mathrm{BEP}}_{\mathrm{Sn}}$ (series C). (Inset) Linear dependence of the tin content on the ${\mathrm{BEP}}_{\mathrm{Sn}}$.

Figure 6

Bright-field TEM images of (a) a pure $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$, (b) a pure $\epsilon \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$, and (c) a mixed $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3/\epsilon \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ sample. In contrast to the image of the pure $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ sample, the image of the pure $\epsilon \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ sample shows columnar contrasts starting directly at the interface to the substrate. The image of the mixed sample also shows columnar contrasts but exhibits an intermediate layer at the interface to the substrate. (d) A magnified image [the rectangle in (c)] of the mixed $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3/\epsilon \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ sample at the interface.

Figure 7

The AFM micrograph of a $\epsilon \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ sample shows the surface structure of approximately 50-nm-diameter crystallites.

Figure 8

$\varphi$ scans of a $\epsilon \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ sample performed for different sample orientations. The black line shows the $\varphi$ scan for the (122) reflex of orthorhombic $\epsilon \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$. The red and green lines display the $\varphi$ scan for the $(012){\mathrm{Al}}_2{\mathrm{O}}_3$ and the (211) reflex of orthorhombic $\epsilon \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$, respectively. (Inset) Schematic of the relative in-plane orientation of the unit cells with three different rotation domains.

Figure 9

(Top panel) Raman spectra of a sapphire substrate and a $\epsilon \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ epitaxial thin film grown on a sapphire substrate. (Bottom panel) Difference spectrum revealing one-phonon Raman signals of $\epsilon \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$ (indicated by arrows). Dotted, dotted-dashed, and dashed lines correspond to the frequencies of the one-phonon Raman signals of $\alpha$-, $\beta$-, and $\gamma \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3$, respectively, and are shown for comparison.

Figure 10

Model for tin-assisted growth in metal-rich conditions.