Strain Relief of Heteroepitaxial bcc-Fe(001) Films

The strain relief of heteroepitaxial bcc-Fe(001) films, deposited at 520–570 K onto MgO(001), has been investigated by scanning tunneling microscopy. In accordance with real-time stress measurements, the tensile misfit strain is relieved during coalescence of flat, mainly 2–3 monolayers (ML) high Fe islands at the high thickness of $\sim 20\mathrm{M}\mathrm{L}$. To accommodate the misfit between merging strain-relaxed islands, a network of $\frac{1}{2}[111]$ screw dislocations is formed. A strong barrier for dislocation glide—which is typical for bcc metals—is most likely responsible for the big delay in strain relief of $\mathrm{F}\mathrm{e}/\mathrm{M}\mathrm{g}\mathrm{O}(001)$, since only the elastic energy of the uppermost layer(s) is available for the formation of an energy-costly intermediate layer.

Figure 1

Film forces (i.e., integral forces in films of unit width) as a function of the mean film thickness measured in real time during the deposition of Fe onto MgO(001) at 520 K.

Figure 2

Strain relaxation in $\mathrm{F}\mathrm{e}/\mathrm{M}\mathrm{g}\mathrm{O}(001)$: (a) $600\times 600{\mathrm{n}\mathrm{m}}^2$ STM image of a 7.5 nm Fe film deposited at 570 K; the entire film surface is patterned by a dense array of square islands with heights of about 1 nm (cf. line scan) which sit on top of a 6 nm thick continuous layer; each island contains at least one, frequently two (right- and left-handed), screw dislocations. Inset: $180\times 180{\mathrm{n}\mathrm{m}}^2$ zoom displaying the screw dislocations in more detail; bias voltage $U_T=80\mathrm{m}\mathrm{V}$; tunneling current $I_T=1\mathrm{n}\mathrm{A}$. (b) $36\times 26{\mathrm{n}\mathrm{m}}^2$ STM image of a 5 nm Fe film deposited at 520 K illustrating the formation of screw dislocations (arrow) during island coalescence; for details see text; $U_T=250\mathrm{m}\mathrm{V}$; $I_T=0.35\mathrm{n}\mathrm{A}$. (c) Respective $19\times 12{\mathrm{n}\mathrm{m}}^2$ STM image showing an island with borders raised by $\sim 0.03\mathrm{n}\mathrm{m}$ because of lattice contraction (cf. Fig. 3); single scan is along dashed line in the top view; $U_T=250\mathrm{m}\mathrm{V}$; $I_T=0.35\mathrm{n}\mathrm{A}$.

Figure 3

Sphere model (top view) illustrating the mechanism for strain relaxation in $\mathrm{F}\mathrm{e}/\mathrm{M}\mathrm{g}\mathrm{O}(001)$: The bottom layer (dark spheres) represents the uppermost (001) plane of the expanded and thus highly strained continuous Fe(001) layer. For reasons of presentation the misfit is increased to $\sim 11\%$ in the sphere model, i.e., requiring an additional row every nine rows for complete strain relief. Upon coalescence of the two strain-relaxed 2D islands (bright spheres) a screw dislocation is formed in the contact region (for details see text).