Submonolayer growth of Fe on a $\mathrm{Ga}\mathrm{As}\left(100\right)\text{−}2\times 6$ reconstructed surface

We present the results of in situ STM measurements of the submonolayer growth of Fe on the Ga-rich and As-terminated $(2\times 6)$ reconstruction of the GaAs(100) surface. In the beginning of the nucleation regime (0.1 ML), the surface reconstruction influences the nucleation sites, so that almost round islands are observed solely on the top of the As rows. In the growth regime from 0.3 to 0.6 ML the islands become elliptically elongated along the [011] direction due to faster diffusion of Fe atoms along the As rows. Throughout our study, Fe atom loss is observed, becoming more pronounced from 0.6 ML onwards, due to the penetration of Fe into the substrate. This penetration leads to a trapping mechanism, which not only changes the diffusion properties of Fe clusters but also supplies extra energy to adatoms atop the Fe islands to surmount the Schwoebel barrier, resulting in a two-dimensional island nucleation from 0.3 ML onwards. A structural nucleation model is presented which provides insight into the interface structure and intermixing in the submonolayer regime.

Figure 1

LEED pattern of a “pseudo” $(4\times 6)$ reconstructed GaAs (100) surface, $E=120\mathrm{eV}$. The “pseudo” $(4\times 6)$ reconstruction pattern is clearly visible. In addition to the $4\times$ spots in the $\left[0\overline{1}1\right]$ direction some disorder lines, the so-called $2\times ∕3\times$ intermediate streaks, are also visible.

Figure 2

(Color online) (a) The atomic model for the $(2\times 6)$ reconstruction as suggested by Biegelsen et al. (Ref. 32). The dashed line represents the elementary unit cell of the $(2\times 6)$ reconstruction. (b) An empty state $50\times 90{\mathrm{nm}}^2$ STM image of the “pseudo” $(4\times 6)$ reconstructed GaAs (100) surface. The $(2\times 6)$ phase (vertical) and the $(4\times 2)$ phase (horizontal) are clearly distinguishable. The bright spots show disordered Ga clusters surrounding the Ga-rich $(4\times 2)$ reconstruction. The image was acquired at room temperature with a constant current of $0.5\mathrm{nA}$ and a sample bias of $1.8\mathrm{V}$.

Figure 3

(Color online) A $33\times 50{\mathrm{nm}}^2$ STM empty state image of 0.1 ML $\mathrm{Fe}∕\mathrm{Ga}\mathrm{As}\left(100\right)\text{−}2\times 6$. The image shows two terraces with the $(2\times 6)$ reconstruction. The Fe starts to cluster on the top of the As rows. The image was captured with a current of $2.7\mathrm{nA}$ and a bias of $2.8\mathrm{V}$.

Figure 4

(Color online) (a) A $16.5\times 25{\mathrm{nm}}^2$ STM image of 0.3 ML $\mathrm{Fe}∕\mathrm{Ga}\mathrm{As}\left(100\right)\text{−}2\times 6$. On the picture two terraces with a $(2\times 6)$ reconstruction are visible. Most of the Fe islands tend to cluster on the top of the As rows, a few of them are in-between the rows. On the upper terrace on the upper right of the image some clusters have already coalesced and formed a bridge between two rows. The image was captured with a current of $2.6\mathrm{nA}$ and a bias of $2.8\mathrm{V}$. The graph (b) displays the cross-section profile of an Fe cluster which was taken where indicated by the white line in (a).

Figure 5

(Color online) (a) A $33\times 50{\mathrm{nm}}^2$ empty state STM image of 0.6 ML $\mathrm{Fe}∕\mathrm{Ga}\mathrm{As}\left(100\right)\text{−}2\times 6$. On the picture two terraces with a $\left(2\times 6\right)$ reconstruction are visible. A large number of Fe islands are already covering the surface, but the reconstruction is still visible. The image was captured with a current of $1.2\mathrm{nA}$ and a bias of $1.7\mathrm{V}$. (b) A $33\times 50{\mathrm{nm}}^2$ image of the same coverage which was captured with a current of $2\mathrm{nA}$ and a bias of $-1.1\mathrm{V}$.

Figure 6

(Color online) A $33\times 50{\mathrm{nm}}^2$ STM image of 1.0 ML $\mathrm{Fe}∕\mathrm{Ga}\mathrm{As}\left(100\right)\text{−}2\times 6$. The surface is covered by 2D Fe islands. The $\left(2\times 6\right)$ reconstruction can be hardly seen and is only apparent due to the regular island arrangement. The image was acquired with a current of $0.8\mathrm{nA}$ and a bias of $-0.6\mathrm{V}$.

Figure 7

(Color online) The resulting size (a) and area (b) of the Fe islands during the growth process. The lines are guides to the eye. The height of the Fe islands versus the nominal Fe thickness is presented in (c). At $\approx 0.4$ ML the slope of the height becomes nearly constant. The Fe island density per ${\mathrm{nm}}^2$ is presented in (d). The island density remains almost constant from 0.3 to 0.6 ML. From this thickness on the coalescence of islands reduces the density by increasing the island size.

Figure 8

(Color online) The difference between nominal and real atomic coverage. The discrepancy can neither be explained by the error calculation nor by an assumed miscalibration of the evaporation source. Therefore a reasonable assumption is that Fe adatoms diffuse into the substrate as proposed and calculated by Erwin et al. (Ref. 50) and Mirbt et al. (Ref. 51).

Figure 9

(Color online) (a) Top view of our proposed structural model for the Fe island nucleation observed for 0.3 ML. The dashed line represents the elementary unit cell of the initial $\left(2\times 6\right)$ reconstruction. The dotted line shows the boundary of the average cluster. Triangular bonds inside the clusters indicate an out of plane bonding, whereas thick lines represent in-plane bonds in the model. It is assumed that the adsorbed Fe atoms are inside and 1 ML above the top As layer. The islands are always separated along the $\left[0\overline{1}1\right]$ direction by at least one missing As dimer. The bottom image (b) displays a side view of our model with Fe substituting on Ga sites and Ga on an interstitial position (i).

Figure 10

(Color online.) (a) A 3D topview of our suggested modelled island and environment at a nominal Fe coverage of 0.3 ML. (b) A 3D side-view of our modelled island.

Figure 11

(Color online.) The tetragonal structure of ${\mathrm{Fe}}_2\mathrm{As}$ with dark gray (gray) spheres representing Fe and light gray (cyan) ones As.