Metastable surface oxide on CoGa(100): Structure and stability

We investigated the structure and formation of a surface oxide and bulk $\beta {\text{-Ga}}_2{\text{O}}_3$ on CoGa(100) from ultrahigh vacuum to 1 bar oxygen pressure in a temperature range from 300 to 1040 K. We combined in situ surface x-ray diffraction with scanning tunneling microscopy, atomic force microscopy, and density-functional theory calculations. We find that the two-dimensional epitaxial surface oxide layer exhibits a $p2mm$ symmetry with an additional mirror plane as compared to the bulk oxide. The surface oxide layer is found to form under metastable conditions at an oxygen chemical potential $\sim 1.6\text{eV}$ above the stability limit for bulk $\beta {\text{-Ga}}_2{\text{O}}_3$. The formation of the bulk oxide is kinetically hindered by the presence of the oxygen-terminated surface oxide, which most likely hampers dissociative oxygen chemisorption. We observe that below 620 K, the surface oxide is surprisingly stable at 1 bar oxygen pressure. Substrate faceting accompanies the bulk oxide formation at temperatures higher than 1020 K.

Figure 1

(Color online) Schematic representation of the CoGa(100) substrate and possible arrangement of the ${\text{Ga}}_2{\text{O}}_3$ bulk structure on it. (a) Side view of two ${\text{Ga}}_2{\text{O}}_3$ mirror domains. (b) Top view of two rotational domains. The bulk structure is made up by a stacking of Ga ions in octahedral (oct) and tetrahedral (tet) coordination within the distorted fcc oxygen sublattice. The lattice constants of $\beta {\text{-Ga}}_2{\text{O}}_3$ are at 273.2 K: $a_{\text{M}}=12.214\text{Å}$, $b_{\text{M}}=3.037\text{Å}$, and $c_{\text{M}}=5.798\text{Å}$ and $\alpha =90°$, $\beta =103.83°$, and $\gamma =90°$ (Ref. 17). [The “M” subscripts indicate the monoclinic coordinates]. (c) Out-of-plane reciprocal $\left(H,0,L\right)$ plane of CoGa (black open circles). Superimposed are the reflections arising from the two mirror domains of $\beta {\text{-Ga}}_2{\text{O}}_3$, indicated by the dark gray (blue) and light gray circles.

Figure 2

(Color online) (a) Structure factor amplitude calculated for two surface rods—(0.5, 0) and (0.5, 1)—corresponding to one ${\text{Ga}}_2{\text{O}}_3$ unit cell (dashed, blue line), for a five (solid, red line), and a three layers slab (dotted, green line) as highlighted in Fig. 1a. The experimental structure factors as a function of the reciprocal-lattice coordinate, $L$, perpendicular to the surface for different in-plane momentum transfers $\left(H,K\right)$ are indicated by the black open circles. (b) Experimental (black symbols) and calculated (solid, red curves) structure factors for the best-fit structure as a function of the reciprocal-lattice coordinate, $L$. Out of the complete data set, two surface rods and two CTRs are plotted. For comparison, the structure factors calculated for two other DFT-relaxed structures (dashed (blue) and dotted (green) lines) are plotted (see text for details).

Figure 3

(Color online) (a) Side view, (b) detailed bottom view of the bonding configuration, and (c) top view of the best-fit surface oxide model [see Tables ]. The oxygen atoms located close to the interface are represented by the light gray (blue) spheres for clarity.

Figure 4

(Color online) (a) STM image of the surface oxide on CoGa(100) obtained at a sample voltage of $-49\text{mV}$ and a tunnel current of 0.3 nA imaging occupied states. (b) Simulated STM image (upper panel), structural model for the topmost O-Ga ion layer and the Ga ion layer below the surface (middle panel), and zoom in the experimental STM image in (a) (lower panel). Small spheres correspond to Ga ions, large spheres to oxygen ions. (c) STM image obtained under similar tunneling conditions exhibiting a much stronger $\left(2\times 1\right)$ contrast.

Figure 5

Experimental $\left(p\text{−}T\right)$ stability diagram of gallium oxide formation on CoGa(100). The open circles and rectangles indicate the onset of surface and the bulk oxide formation, respectively, whereas the crosses mark the conditions at which the surface oxide was found to be stable. The solid line represents a constant oxygen chemical potential of $\Delta {\mu }_{\text{O}}=-1.92\text{eV}$. Note that the $\Delta \mu$ line calculated for the stability limit of the bulk oxide lies about 20 orders of magnitude in pressure below. The dotted line is a guide to the eye for the transition from the surface oxide to the bulk oxide.

Figure 6

(Color online) (a) Diffracted intensity along the (0.5, 0) rod for the as-prepared surface oxide layer (open circles) and after exposure to 1 bar of oxygen at room temperature (filled squares). (b) Diffracted intensity along the $\left(0.5,0,L\right)$ reciprocal-lattice direction after bulk oxide growth at 723 K (open (black) circles) and 773 K [open (red) squares].

Figure 7

(a) AFM image $\left(10\times 10\mu \text{m}\right)$ of the CoGa(100) surface after oxidation-induced faceting. (b) Line scan along the white line in (a). The slope of the (102) and $\left(10\overline{2}\right)$ planes is indicated by dashed lines.

Figure 8

(Color online) (a) Line scans performed in the $H$ direction at $K=1$ and different $L$ values. The contribution from the facets is given by the side peaks (gray), whereas the CTR signal is represented by the middle peak. (b) Schematic representation of the (001)/{102} planes of the CoGa viewed along the [010] direction.

Figure 9

(Color online) Surface phase diagram for the ultrathin Ga oxide layer on CoGa(100). The adhesion energy $\Delta \gamma$ of the ultrathin surface oxide as calculated from DFT is plotted as a function of the oxygen chemical potential $\Delta {\mu }_{\text{O}}$ (solid, red line). The chemical potential for bulk oxide formation is marked as a solid vertical line and the experimentally determined chemical potential for the surface oxide formation as a dashed vertical line.