Smoothening of ultrathin Fe films grown on GaAs(100) observed by scanning tunneling microscopy and Brillouin light scattering

We report on an in situ scanning tunneling microscopy study of the growth of Fe on the GaAs(100)-pseudo-$4\times 6$ reconstructed surface in the thickness range of 1–4 ML (monolayers). We observe that at 1 ML, the Fe islands have nucleated on the As dimer rows, and that subsequent growth is primarily directed along the $\left[0\overline{1}1\right]$ direction. Initially, the islands are well separated, but as more Fe is deposited, the islands start to coalesce in both the [011] and $\left[0\overline{1}1\right]$ directions. Percolation probably occurs between 2 and 3 ML. Scanning tunneling microscopy studies of a nominally 2 ML thick film over a period of $30\mathrm{h}$ reveal that the islands relax primarily growing in the $\left[0\overline{1}1\right]$ direction, reducing the roughness of the film. The smoothening of the film is also corroborated by Brillouin light scattering measurements on slightly thicker films of 4.25 and 6.2 ML as evidenced by an increase in the spin-wave frequency and a decrease of the linewidth over a period of $11\mathrm{h}$.

Figure 1

(Color online) $50\times 30{\mathrm{nm}}^2$ filled state STM images of films with nominal Fe thicknesses of (a) 1 ML, (b) 2 ML, (c) 3 ML, and (d) 4 ML. These were taken within $5\mathrm{h}$ of deposition, with the exception of the 2 ML sample (b) which was taken $30\mathrm{h}$ after deposition, but the inset of (b) shows a $20\times 13{\mathrm{nm}}^2$ image of the as-deposited surface. (e) and (f) show line profiles from the lines along the (e) [011] and $\left[0\overline{1}1\right]$ directions indicated in (d). Several of the images show more than one GaAs terrace, which are separated by the expected $0.15\mathrm{nm}$.

Figure 2

Island size along (a) [011], (b) $\left[0\overline{1}1\right]$, (c) area, (d) height, (e) density, and (f) surface coverage against nominal thickness for 0.3–4 ML of Fe. The black squares denote data taken within $5\mathrm{h}$ of deposition; the gray squares denote data taken after the 2 ML film had been allowed to relax. The dashed lines show a fit to curves of the form $x\sim t_{\mathrm{Fe}}^n$ using the data taken before the film was allowed to relax.

Figure 3

(Color online) Histograms of island sizes along (a) [011] and (b) $\left[0\overline{1}1\right]$. Both histograms show data for two different times after deposition of a 2 ML Fe film: black circles indicate results from less than $5\mathrm{h}$ after deposition and red triangles indicate results from approximately $30\mathrm{h}$ after deposition. The lines are guides for the eye.

Figure 4

(Color online) Histogram of island heights, as in Fig. 3. Black circles indicate results from less than $5\mathrm{h}$ after deposition and red triangles indicate results from approximately $30\mathrm{h}$ after deposition. The lines are guides for the eye.

Figure 5

(Color online) Normalized MOKE loops from a 3.4 ML taken immediately after deposition (thin line) and after $18\mathrm{h}$ (dashed line); the data taken immediately after deposition were fitted to the Langevin function shown (thick line). No changes are seen within experimental errors during the course of $19\mathrm{h}$.

Figure 6

(a) BLS spectra for a 4.25 ML Fe film, taken immediately (solid line) and $11\mathrm{h}$ (dotted line) after deposition with the magnetic field applied along the easy axis ([011]). The curves are offset for clarity. (b) The change in frequency $\nu$ with time for films with thicknesses 4.25 ML (●) and 6.2 ML (◻) and (c) the change in linewidth $\Delta \nu$ for the same thicknesses.