Strain Relief in Cu-Pd Heteroepitaxy

We present experimental and theoretical studies of $\mathrm{Pd}/\mathrm{Cu}(100)$ and $\mathrm{Cu}/\mathrm{Pd}(100)$ heterostructures in order to explore their structure and misfit strain relaxation. Ultrathin Pd and Cu films are grown by pulsed laser deposition at room temperature. For $\mathrm{Pd}/\mathrm{Cu}$, compressive strain is released by networks of misfit dislocations running in the [100] and [010] directions, which appear after a few monolayers (ML) already. In striking contrast, for $\mathrm{Cu}/\mathrm{Pd}$ the tensile overlayer remains coherent up to about 9 ML, after which multilayer growth occurs. The strong asymmetry between tensile and compressive cases is in contradiction with continuum elasticity theory and is also evident in the structural parameters of the strained films. Molecular dynamics calculations based on classical many-body potentials confirm the pronounced tensile-compressive asymmetry and are in good agreement with the experimental data.

Figure 1

STM images for (a) 0.5 ML Pd ($200\times 200{\mathrm{nm}}^2$), (b) 1.6 ML Pd ($200\times 200{\mathrm{nm}}^2$), (c) 4.8 ML Pd ($100\times 100{\mathrm{nm}}^2$), and (d) 6.0 ML Pd ($100\times 100{\mathrm{nm}}^2$ deposited at 300 K.

Figure 2

STM images for (a) clean Pd(100) substrate ($500\times 500{\mathrm{nm}}^2$), (b) 0.85 ML Cu ($100\times 100{\mathrm{nm}}^2$), (c) 2.1 ML Cu ($100\times 100{\mathrm{nm}}^2$), (d) 3.05 ML Cu ($100\times 100{\mathrm{nm}}^2$), (e) 6.0 ML Cu ($100\times 100{\mathrm{nm}}^2$), and (f) 9.0 ML Cu ($100\times 100{\mathrm{nm}}^2$) deposited at 300 K.

Figure 3

Perpendicular interlayer distance calculated based on a kinetic model from LEED intensity of the (00) diffraction beam vs energy curves. The change of the interlayer distance occurs at about 3–4 ML for $\mathrm{Pd}/\mathrm{Cu}(100)$ films and at about 10 ML for $\mathrm{Cu}/\mathrm{Pd}(100)$ films. Assuming a pseudomorphic growth up to 3 ML for Pd on Cu(100) and 10 ML for Cu on Pd(100), PLD films can be identified as fcc Pd and face-centered tetragonal (fct) Cu structures. The results from experimental and theoretical work at 300 K are indicated by open and solid symbols, respectively. The lines are guides to the eye.

Figure 4

In-plane lattice strain derived from the separation of the first order rods in the RHEED pattern as a function of Pd or Cu coverages. Our results from experimental and theoretical work at 300 K are indicated by open and solid symbols, respectively. See the text for details.

Figure 5

Typical configuration of stacking faults for 7 ML Pd on Cu(100) based on molecular dynamics annealing. The lateral size is $20\times 20{\mathrm{\text{atom}}}^2$. The misfit dislocations orientate along the $⟨100⟩$ direction of the substrate.