Surface structure of phosphorus-terminated $\mathrm{Ga}\mathrm{P}\left(001\right)\text{−}(2\times 1)$

The surface structure of phosphorus-terminated $\mathrm{Ga}\mathrm{P}\left(001\right)\text{−}(2\times 1)$ has been studied by low-energy electron diffraction (LEED), high-resolution electron energy loss spectroscopy (HREELS), scanning tunneling microscopy (STM), and synchrotron radiation photoemission spectroscopy. HREELS spectra indicate that hydrogen is adsorbed on the surface, leading to formation of a $\mathrm{P}\mathrm{H}$ bond. The intensity of the stretching vibration of the $\mathrm{P}\mathrm{H}$ remains constant for the $(2\times 1)$ surface annealed at $300–600\mathrm{K}$, where $(2\times 1)$ LEED patterns also remain. The vibration mode and patterns disappeared simultaneously upon annealing at $700\mathrm{K}$. STM filled state images show zigzag chain structures. On the other hand, straight rows along the [110] direction are seen in the empty state images. Surface core-level shifts are found: $+0.60\mathrm{eV}$ for $\mathrm{Ga}3d$ and $-0.69$ and $+0.39\mathrm{eV}$ for $\mathrm{P}2p$. These results can well explain a theoretical model of buckled P-dimers with hydrogen adsorbed in an alternating sequence.

Figure 1

A $(2\times 1)∕(2\times 2)$ structure model of the $\mathrm{Ga}\mathrm{P}\left(001\right)\text{−}(2\times 1)∕(2\times 2)$ surface. Top (a) and side (b) views are shown. Shaded, filled, and open circles correspond to hydrogen, phosphorus, and gallium atoms, respectively.

Figure 2

HREELS spectra for the $\mathrm{Ga}\mathrm{P}\left(001\right)\text{−}(2\times 1)$ surface annealed at $300–700\mathrm{K}$. The spectrum of the $\mathrm{Ga}\mathrm{P}\left(001\right)\text{−}(2\times 4)∕c(2\times 8)$ surface is also shown. The intensity ratio of the $\mathrm{P}\mathrm{H}$ vibration mode to the elastic peak remains almost constant up to $600\mathrm{K}$.

Figure 3

STM filled (a) and empty (b) states images measured at bias voltages of $-3.5$ and $+3.0\mathrm{V}$ (tunneling currents, $0.20\mathrm{nA}$), respectively. The image sizes are $4\mathrm{nm}\times 4\mathrm{nm}$. The $n$- and $p\text{−}\mathrm{Ga}\mathrm{P}\left(001\right)\text{−}(2\times 1)$ surfaces are used for the measurements of the filled and empty states images, respectively. The $(2\times 2)$ and $c(4\times 2)$ unit cells are shown.

Figure 4

Core-level spectra of $\mathrm{Ga}3d$ for the $\mathrm{Ga}\mathrm{P}\left(001\right)\text{−}(2\times 1)$ surface. The detection angle is 0° (a) and 80° (b) off normal to the surface. Photon energy of $100\mathrm{eV}$ was employed. The spectra are separated into two components: B (bulk) and S1 (surface). The measured and fitted spectra are shown by small open circles and solid lines, respectively. Differences between the measured and fitted spectra were shown below the spectra.

Figure 5

Core-level spectra of $\mathrm{P}2p$ for the $\mathrm{Ga}\mathrm{P}\left(001\right)\text{−}(2\times 1)$ surface. The detection angle is 0° (a) and 80° (b) off normal to the surface. Photon energy of $170\mathrm{eV}$ was employed. The spectra are separated into one bulk (B) and three surface (S1, S2, and S3) components. The measured and fitted spectra are shown by small open circles and solid lines, respectively. Differences between the measured and fitted spectra are shown below the spectra.