Quasi-One-Dimensional Metal-Insulator Transitions in Compound Semiconductor Surfaces

Existing examples of Peierls-type 1D systems on surfaces involve depositing metallic overlayers on semiconducting substrates, in particular, at step edges. Here we propose a new class of Peierls system on the $(10\overline{1}0)$ surface of metal-anion wurtzite semiconductors. When the anions are bonded to hydrogen or lithium atoms, we obtain rows of threefold coordinated metal atoms that act as one-atom-wide metallic structures. First-principles calculations show that the surface is metallic, and below a certain critical temperature the surface will condense to a semiconducting state. The idea of surface scaffolding is introduced in which the rows are constrained to move along simple up-down and/or sideways displacements, mirroring the paradigm envisioned in Peierls’s description. We predict that this type of insulating state should be visible in the partially hydrogenated $(10\overline{1}0)$ surface of many wurtzite compounds.

Figure 1

The structure of GaN-1H and GaN-1Li slabs: Side view (a) of a 12-layer symmetric slab of GaN-1H. Surface Ga forms continuous chains along $[11\overline{2}0]$. (b) Top view of the $\mathrm{GaN}(10\overline{1}0)\text{−}1\mathrm{H}$ and (c) $\mathrm{GaN}(10\overline{1}0)\text{−}\mathrm{Li}$ surface, in which the surface atoms are shown as spheres. The $p(1\times 2)$ and the $p(2\times 2)$ cells are marked by blue and red lines, respectively. The color code to label the atoms is indicated in the figure.

Figure 2

(a) Phonon dispersion curves of a $p(1\times 1)$ 12-layer slab of $\mathrm{GaN}(10\overline{1}0)\text{−}1\mathrm{H}$. The phonon frequencies are in units of THz, while the negative values refer to unstable phonon modes. (b) The 2D Fermi surface of the $p(1\times 1)$ 12-layer slab of $\mathrm{GaN}(10\overline{1}0)\text{−}1\mathrm{H}$. The origin is marked at the center of the BZ and the nesting vectors are highlighted.

Figure 3

Band structures of the different phases of $\mathrm{GaN}(10\overline{1}0)\text{−}1\mathrm{H}$ in (a) $p(1\times 1)$, (b) $p(2\times 2)$, and (c) $p(1\times 2)$. The Fermi energy is set to zero. The band structures of the $p(1\times 2)$ and $p(2\times 2)$ phase are calculated in a $p(2\times 2)$ cell. Majority and minority spin bands are drawn in different colors in panel (a). The DOS of the three phases are reported in panel (d), the behavior of charge just below the Fermi level suggests that the $p(1\times 1)$ to $p(1\times 2)$ transition is of the Peierls type.