FORMATION AND MORPHOLOGY OF INAS/GAAS HETEROINTERFACES

The synthesis of perfect InAs/GaAs heterostructures by conventional solid-source molecular-beam epitaxy is achieved by a growth approach that employs the modulation of growth temperature during the interface formation. Details of the evolution of the growth front during interface formation are probed by in situ reflection high-energy electron diffraction. The nucleation of InAs on GaAs occurs layer by layer for the first two monolayers. At this stage, the InAs film wets the underlying GaAs; i.e., a continuous coverage of the substrate is thermodynamically preferred. Thus, allowing the growth front to relax towards the equilibrium state minimizes the step density on the surface. During overgrowth of the InAs film with GaAs, a fraction of the deposited In segregates on the growing surface. This In floating layer is desorbed prior to overgrowth by a heating cycle, which prevents its incorporation into the GaAs capping layer. The resulting exceptional degree of crystalline perfection of the InAs/GaAs heterostructures is confirmed by double-crystal x-ray diffraction patterns of multilayer structures which are essentially indistinguishable from the theoretical pattern predicted by the dynamical diffraction theory. The atomic configuration at the InAs/GaAs interfaces is studied by high-resolution electron microscopy. Extensive image simulations provide reliable quantitative information on layer thickness and interface configuration. Lattice images of InAs films having thickness in excess of two monolayers reveal that both interfaces are structurally equivalent. Fluctuations of the interfaces are restricted to monoatomic steps. The atomic-scale morphology of these highly strained interfaces compares favorably with that of GaAs/AlxGa1-xAs heterointerfaces. The crucial point for obtaining this structural perfection is the flashoff of the In floating layer prior to GaAs overgrowth.