GALLIUM DESORPTION FROM (AL,GA)AS GROWN BY MOLECULAR-BEAM EPITAXY AT HIGH-TEMPERATURES

We report on a detailed study by cross-sectional transmission electron microscopy (XTEM) of gallium desorption from (Al,Ga)As structures grown by molecular beam epitaxy (MBE) at substrate temperatures in the range 680-730-degrees-C. The Ga desorption rate (D(r)) depends only on substrate temperature, with an activation energy, E(a) for re-evaporation of 2.56 eV, comparable to E(a) for Ga evaporation from liquid gallium. The presence of aluminium has no measurable influence on D(r) except where the desorbing gallium flux exceeds the incident flux (D(r) >> G(r)), when a few monolayers of residual GaAs can be detected on an AlAs surface. No As4 overpressure dependence has been observed. In practice, therefore, multilayer structures of (Al,Ga)As with controlled thickness and compositions can be grown with As4 in the temperature regime investigated by making a constant allowance for D(r), irrespective of the compositional fraction of the (Al,Ga)As.