Reflectance-difference spectroscopy of mixed arsenic-rich phases of gallium arsenide (001)

The relationship between the reflectance difference spectra and the atomic structure of arsenic-rich reconstructions of GaAs(001) has been investigated. Scanning tunneling micrographs reveal that a roughening process occurs as the surface structure changes with decreasing arsenic coverage from 1.75 to 0.75 monolayers (ML). At 1.65 ML As, small pits, one bilayer in depth and having the same c(4 x 4) reconstruction as the top layer, form in the terraces. At the same time, gallium atoms are liberated to the surface, disrupting the c(4 x 4) ordering. At about 1.4 ML As, (2 x 4) domains nucleate and grow on top of the c(4 x 4). Further desorption of arsenic causes the underlying layer to gradually decompose into a metastable (2 x n) phase (n = 2, 3, or 4), and finally into the (2 x 4). In the reflectance difference spectra, negative peaks at 2.25 and 2.8 eV correlate with the (4 x 4)-type arsenic dimers. However, the intensity of the latter feature strongly depends on the presence of adsorbates, such as alkyl groups and gallium adatoms. By contrast, the intensity of the positive peak at 2.9 eV is directly porportional to the density of (2 x 4)-type aimers.