Electronic structures of Mn-induced phases on GaN(0001)

We investigate the atomic and electronic structures of Mn-incorporated GaN(0001) surface by scanning tunneling microscopy and first-principles calculations. The incorporation of Mn at room temperature initially leads to a disordered phase, and subsequently an ordered $\left(3\times 3\right)$ reconstruction at $\sim 0.5$ ML Mn coverage. The $\left(3\times 3\right)$ exhibits a honeycomb structure for sample bias below $-0.5\text{V}$, and appears as a closed-packed structure above $-1.2\text{V}$, which is attributed to antiferromagnetic ordering induced by Mn atoms. Incorporation of the same amount of Mn at an elevated substrate temperature of $300°\text{C}$ produces an additional phase with local $\left(5\times 5\right)$ periodicity, consisting of two types of trimers: one with three spots of equal intensity and the other with uneven intensities. This disparity in intensity can be resolved using a Fe-coated W tip, which is attributed to enhanced spin-dependent tunneling between the Mn-induced minority states and the Fe-induced states of the functionalized W tip.

Figure 1

(Color online) STM images of structures observed after Mn incorporation on the GaN(0001)-“$1\times 1$” at room temperature. (a) At the beginning of Mn incorporation, the surface is converted to a disordered phase (sample bias: $-0.5\text{V}$, tunneling current: 0.3 nA, and image size: $20\times 20{\text{nm}}^2$). (b) With increasing Mn coverage, the disordered phase covers the majority of the surface (sample bias: $-1.3\text{V}$, tunneling current: 1.0 nA, and image size: $94\times 94{\text{nm}}^2$). (c) At 0.5 ML Mn, an ordered $\left(3\times 3\right)$ phase is formed (sample bias: $-1.4\text{V}$, tunneling current: 1.0 nA, and image size: $50\times 50{\text{nm}}^2$). (d) Reconstruction upon Mn deposition at $300°\text{C}$. Both $\left(3\times 3\right)$ and $\left(5\times 5\right)$ are observed (sample bias: $-0.15\text{V}$, tunneling current: 2.0 nA, and image size: $50\times 50{\text{nm}}^2$).

Figure 2

(Color online) (a) Atomic resolution STM image of the $\left(3\times 3\right)$ structure (sample bias: $-0.3\text{V}$, tunneling current: 1.0 nA, and image size: $5\times 5{\text{nm}}^2$); (b) The topography changes from honeycomb to closed-packed structure as the bias voltage is switched from $-0.5$ to $-1.2\text{V}$; dotted line marks the position of the change (tunneling current: 1.0 nA and image size: $9.5\times 9.5{\text{nm}}^2$). The parallelograms mark the unit cells in both cases. (c) Profile of line AB marked in Fig. 2b showing the height difference between neighboring protrusions for the honeycomb and closed-packed structure.

Figure 3

(Color online) (a) STM image of the local $\left(5\times 5\right)$ obtained with sample bias at $-0.7\text{V}$, tunneling current at 3.0 nA (image size: $22\times 22{\text{nm}}^2$). Variant I and II are marked by dashed and solid outlines, respectively. (b) STM image of the same location as in (a) but with sample bias at 0.7 V, tunneling current at 3.0 nA. (c) Closed-up view of the triangular features of variant I. (d) STM image of variant II taken using a Fe/W tip. [Sample bias: $-0.4\text{V}$, tunneling current: 0.5 nA, and image size: $9\times 9{\text{nm}}^2$ for (c) and $5\times 5{\text{nm}}^2$ for (d)].

Figure 4

(Color online) (a) The addition of Mn to the “$1\times 1$” structure causes a structural change into a tetrahedral Ga bilayer with a Ga adatom. (b) The resulting $\left(3\times 3\right)$ structure consists of AFM-coupled Mn atoms and Ga adatoms. (c) Calculated line profiles (solid lines: local majority spin and dashed lines: minority spin) along a line through Mn and its neighboring Ga adatoms for two biases for various constant values of the energy-integrated (position-dependent) density of states. (d) Calculated spin-resolved local density of states for Mn.