Atomic structure of surfaces and ultrathin epitaxial films.

摘要

Two stepped surfaces of copper with small interlayer spacings are studied with quantitative low-energy electron diffraction (QLEED): Cu{lcub}320{rcub} and Cu{lcub}331{rcub}. It is found that Cu{lcub}320{rcub}, which has an interlayer spacing of 0.501 Å—one of the shortest interlayer spacing studied to date, has the following multilayer relaxation sequence: Δd₁₂ = −0.12 ± 0.03 Å = (−24 ± 6)%, Δd₂₃ = −0.08 ± 0.06 Å = (−16 ± 12)% and Δd₃₄ = +0.05 ± 0.03 Å = (+10 ± 6)% (Δd_ij is the change in the distance between layers i and j). For Cu{lcub}331{rcub}, which has an interlayer spacing of 0.829 Å,{09}the multilayer sequences are: Δd₁₂ = −0.114 ± 0.03 Å = (−13.8 ± 4)%, Δd₂₃ = +0.003 ± 0.03 Å = (+0.4 ± 4)%, Δd₃₄ = +0.03 ± 0.03 Å = (+4 ± 4)% and Δd₄₅ = −0.036 ± 0.03 Å = (−4.3 ± 4)%, in agreement with published theoretical predictions within the experimental error.; A study of vicinal surfaces of Ge{lcub}111{rcub} with scanning tunneling microscopy (STM) shows the relationship between the tilt directions and the shape of the steps on the surface. Vicinal surfaces tilted toward [1¯21¯] and [12¯1] are more likely to form straight and continuous steps than vicinal surfaces tilted toward [01¯1]. Vicinal surfaces tilted toward [1¯21¯] produce more continuous steps than those tilted toward [12¯1]. The atomic structure on the terraces shows the well-known reconstruction of the Ge{lcub}111{rcub} surface, i.e., the c(2 x 8) reconstruction.; Epitaxial growth of ultrathin films is used to stabilize and characterize metastable phases. The stable phase of vanadium metal at room temperature is body-centered-cubic (bcc), hence the axial ratio c/ a is equal to 1. But the epitaxial Bain path (EBP) of vanadium reveals the existence of a metastable body-centered-tetragonal (bct) phase with c/a = 1.78. Aiming at stabilizing this phase, ultrathin vanadium films are grown epitaxially on a Ni{lcub}001{rcub} and a Cu{lcub}001{rcub} substrate, respectively, and their structures are determined by QLEED. It is found that vanadium films on Ni{lcub}001{rcub} have a bct structure with a = 2.49 Å and c between 4.32 and 4.42 Å, while vanadium films on Cu{lcub}001{rcub} have a bct structure with a = 2.556 Å and c = 4.40 Å. The EBP shows unambiguously that the films' structures are strained states of the metastable equilibrium bct phase at c/a = 1.78, rather than strained states of the bcc ground state at c/a = 1.