DIFFERENTIAL ION DAMAGE AND ITS ANNEALING BEHAVIOR IN ALAS/GAAS HETEROSTRUCTURES

The nature of differential ion damage in A1As-GaAs epitaxial heterostructures is explored using conventional and high-resolution transmission electron microscopy, together with Rutherford backscattering spectrometry. By use of Si+ ion implanation, the rapid buildup of lattice damage in GaAs, and the relative resistance of A1As to structure breakdown is highlighted. The ion dose levels required for bulk amorphization of the two materials differ by at least two orders of magnitude. The way in which lattice disorder changes near the AlAs/GaAs interface is studied in detail and it is demonstrated that the AlAs layer, which remains crystalline up to high ion doses, promotes in situ annealing of narrow zones of GaAs crystal adjacent to both of its interfaces. These crystalline GaAs zones show substantially enhanced resistance to ion damage accumulation but they contain planar defects and are finally rendered amorphous after extended ion bombardment. During this process, defects propagate into the edges of the AlAs layer which are then progressively amorphized in an apparently heterogeneous (boundary-dependent) manner. In addition, it is shown that GaAs in other regions of the sample is amorphized by a mechanism which at first leaves nanometer-scale blocks of crystal isolated within the newly formed amorphous material, although these blocks are then rapidly broken down by further bombardment. The way in which the implantation-damaged layers restructure during annealing treatment is also described. Up to approximately 320 nm of amorphous GaAs beneath the AlAs layer can be regrown as a single crystal by the motion of two opposing growth interfaces during annealing at 800-degrees-C.