Mechanisms for Si Dopant Migration in MBE Al sub x Ga sub x-1 As

Because of well-known surface segregation effects in MBE growth, Si dopant atoms deposited as thin layers in Al sub x Ga sub 1-x As typically become distributed over many atomic layers. We have measured the Si depth distributions in (100) and (311)A samples grown at temperatures between 400 degrees C and 635 degrees C, with Al fraction x=0, 0.1 and 0.32. The surface migration decay length LAMBDA for a Si atom on a growing (100) surface is strongly temperature dependent but nearly independent of x, with LAMBDA ~ 8 nm at 635 degrees C. The x=0 measurements show evidence for a minimum value LAMBDA ~ 0.6 nm at low temperatures and a maximum value LAMBDA ~ 8.5 nm at high temperatures. The data are in accord with a thermally activated surface segregation process with activation energy (1.8 +- 0.4) eV acting in parallel with a temperature independent surface segregation mechanism. The (311)A surface shows LAMBDA = (3.3 +- 0.1) nm virtually independent of temperature for x = 0. The Si decay length for the (311)A surface strongly increases with x, and for x=0.32 there is no significant differences in LAMBDA for the (100) and (311)A surfaces.