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 investigate d. Scanning tunneling micrographs reveal that a roughening process occurs a s 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 an d having the same c(4 x 4) reconstruction as the top layer, form in the ter races. At the same time, gallium atoms are liberated to the surface, disrup ting the c(4 x 4) ordering. At about 1.4 ML As, (2 x 4) domains nucleate an d grow on top of the c(4 x 4). Further desorption of arsenic causes the und erlying 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 spe ctra, negative peaks at 2.25 and 2.8 eV correlate with the (4 x 4)-type ars enic 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 porport ional to the density of (2 x 4)-type aimers.