Metal-Exchange Catalysis in the Growth of Sesquioxides: Towards Heterostructures of Transparent Oxide Semiconductors

We observe that the growth rate of ${\mathrm{Ga}}_2{\mathrm{O}}_3$ in plasma-assisted molecular beam epitaxy can be drastically enhanced by an additional In supply. This enhancement is shown to result from a catalytic effect, namely, the rapid formation of ${\mathrm{In}}_2{\mathrm{O}}_3$, immediately followed by a transformation of ${\mathrm{In}}_2{\mathrm{O}}_3$ to ${\mathrm{Ga}}_2{\mathrm{O}}_3$ due to an In-Ga interatomic exchange. We derive a simple model that quantitatively describes this process as well as its consequences on the formation rate of ${\mathrm{Ga}}_2{\mathrm{O}}_3$. Moreover, we demonstrate that the catalytic action of ${\mathrm{In}}_2{\mathrm{O}}_3$ allows the synthesis of the metastable hexagonal phase of ${\mathrm{Ga}}_2{\mathrm{O}}_3$. Since the ${\mathrm{Ga}}_2{\mathrm{O}}_3(0001)/{\mathrm{In}}_2{\mathrm{O}}_3(111)$ interface is closely lattice matched, this novel growth mode opens a new path for the fabrication of sesquioxide heterostructures.

Figure 1

Desorption of ${\mathrm{Ga}}_2\mathrm{O}$ and In from a $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3(\overline{2}01)$ surface at $T_G=700°\mathrm{C}$, ${\varphi }_{\mathrm{Ga}}=6.5{\mathrm{nm}}^{-2}{\mathrm{s}}^{-1}$, and ${\varphi }_{\mathrm{In}}=4.0{\mathrm{nm}}^{-2}{\mathrm{s}}^{-1}$. The metals supplied in the time intervals (i)–(v) are indicated by the bars at the top.

Figure 2

(a)–(e) $T_G$ dependence of ${\mathrm{\Gamma }}_{{\mathrm{Ga}}_2{\mathrm{O}}_3}$. ${\varphi }_{\mathrm{In}}$, supplied for each set of experiments, is indicated in the figures in units of ${\mathrm{nm}}^{-2}{\mathrm{s}}^{-1}$. The symbols represent experimental data and the solid lines are fits to Eq. (6). The dashed line is a guide for the eye. (f) Activation energy of In desorption as a function of ${\varphi }_{\mathrm{In}}$. The symbols represent experimental data and the solid line is a fit of the data to Eq. (8).

Figure 3

Dependence of ${\mathrm{\Gamma }}_{{\mathrm{Ga}}_2{\mathrm{O}}_3}$ on ${\varphi }_{\mathrm{In}}$ for different $T_G$’s, as indicated in the figures. The symbols represent experimental data and the solid lines are model predictions by Eqs. (6)–(8).

Figure 4

(a) Longitudinal XRD scan recorded from an ${\mathrm{In}}_2{\mathrm{O}}_3$-catalyzed ${\mathrm{Ga}}_2{\mathrm{O}}_3$ film grown on a $\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3(\overline{2}01)/\alpha \text{−}{\mathrm{Al}}_2{\mathrm{O}}_3(0001)$ template at $T_G=700°\mathrm{C}$, ${\varphi }_{\mathrm{Ga}}=6.5{\mathrm{nm}}^{-2}{\mathrm{s}}^{-1}$, and ${\varphi }_{\mathrm{In}}=5.4{\mathrm{nm}}^{-2}{\mathrm{s}}^{-1}$. Reflections labeled “sub.” and “nl” stem from the substrate [$\alpha \text{−}{\mathrm{Al}}_2{\mathrm{O}}_3(0006)$] and the nucleation layer [$\beta \text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3(\overline{2}01)$ and ($\overline{4}02$)], respectively. The reflections from the ${\mathrm{Ga}}_2{\mathrm{O}}_3$ film are identified to originate from the metastable $ϵ$ phase, as indicated in the figure. (Inset) Transverse scans across the $ϵ\text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3(0004)$ (solid black line) and $ϵ\text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3(10\overline{1}4)$ (dashed-dotted blue line) reflections. The full width at half maxima of the reflections are indicated in the figure. (b), (c) Side view of the $ϵ\text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3(0001)$ and ${\mathrm{In}}_2{\mathrm{O}}_3(111)$ planes, respectively. The dashed-dotted lines indicate the bulk-terminated surfaces. The Ga, In, and O atoms are depicted in gold, red, and blue, respectively. (d) Top view of $ϵ\text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3(0001)$ and ${\mathrm{In}}_2{\mathrm{O}}_3(111)$. The dashed lines illustrate a $5:4$ coincidence lattice of O-terminated $ϵ\text{−}{\mathrm{Ga}}_2{\mathrm{O}}_3(0001)$ relative to In-terminated ${\mathrm{In}}_2{\mathrm{O}}_3(111)$.