Sensing sub-surface strain in GaAsBi(001) surfaces by reflectance anisotropy spectroscopy

Reflectance anisotropy spectroscopy (RAS) is applied to study the reconstructed GaAsBi(001) surfaces at room temperature. Arsenic-capped GaAsBi samples with 7% Bi concentration are grown by molecular beam epitaxy (MBE) in nearly matched conditions on a proper buffer layer and annealed in ultra-high vacuum (UHV). Low energy electron diffraction (LEED) shows that, following the As decapping, a 2 x 3/1 x 3 phase (Bi-rich) is obtained after annealing the sample at 400 degrees C, while subsequent annealing at 450 degrees C yields a deterioration of the surface order. RAS spectra measured in situ allow to definitely confirm that the characteristic Bi-dependent anisotropy measured below 2.5 eV has not a true surface origin, although being connected to the surface: it is related to the strain of the directional bonds between Bi atoms existing at the surface and below the surface. This result has a twofold significance: it recommends that previous attributions to the surface of RAS anisotropy features in III-V semiconductors should be in some cases revisited; for the future, it shows that RAS is suitable to characterize 2D-layered materials, and to investigate the consequences of strain in the electronic properties of low-dimensional systems.