In situx-ray standing-wave analysis of electrodeposited Cu monolayers on GaAs(001)

Copper was electrodeposited onto n- and p-type GaAs(001) from mMol solutions of CuSO₄ in 0.5 Mol sulfuric acid and the registration of the Cu adsorbate was analyzed with respect to the GaAs lattice in situ with x-ray standing waves, recording the Cu-K{α} fluorescence radiation from the sample surface while scanning the GaAs(004) Bragg reflection. For coverages below 1 ML, the determined coherent position PCu⁰⁰⁴≈0.0 is in agreement with a substitutional site of the Cu. However, the coherent fraction F⁰⁰⁴<~0.4 indicates that the Cu is not well ordered or occupies other sites. The measurements also show that part of the Cu diffuses a few nm into the bulk in an amount that is larger for n type (≈0.5 ML) than for p type (0.05 ML). If thick Cu layers are stripped at anodic potentials, the Cu desorption starts to significantly slow down at Cu coverages of about 10 ML while the anodic current stays almost constant, which is explained by the fact that the Cu film is no longer continuous. At coverages 1 ML the stripping becomes extremely slow and Cu stays at the GaAs(001) interface even while the GaAs surface dissolves, exhibiting a ``reversed surfactant'' behavior.