Mechanisms for si dopant migration in molecular beam epitaxy Al_xGa_1-xxAs

Because of well-known surface segregation effects in molecular beam epitaxy growth, Si dopant atoms deposited as thin layers in Al_xGa_1-xAs typically become distributed over many atomic layers. We have measured the Si depth distributions in (100) and (311)A samples grown at temperatures between 420 and 655 °C, with Al fraction x=0, 0.1, and 0.32. The surface migration decay length A for a Si atom on a growing (100) surface is strongly temperature dependent but nearly independent of x, with Λ≈8 nm at 655 °C. The x=0(100) measurements show evidence for a minimum value Λ≈0.6nm at low temperatures and a maximum value Λ≈8.5nm 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 Λ=(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 difference in Λ for the (100) and (311)A surfaces.