Surface diffusion is a critical parameter for non-equilibrium growth techniques such as molecular beam epitaxy. However, very little is known about diffusion rates of individual cations in a mixed cation material. Using droplet epitaxy as the growth technique, we isolate the diffusivity prefactor (D0) and activation energy (EA) of indium on the surface of In0.53Ga0.47As/InP(100). We report two regimes of indium diffusivity under As2-rich conditions: above and below the droplet deposition temperature of 300 °C, corresponding to a change in surface reconstruction. We also discuss methods of extracting the indium diffusion parameters on metal-rich surfaces using droplet epitaxy and nucleation theory. The obtained diffusion parameters are compared to previous work in the literature and could be employed to optimize growth conditions for non-equilibrium crystal growth.

a b s t r a c tWe demonstrate a planar organic solar cell with a four-layer cascade architecture that exhibits an opencircuit voltage (Voc) greater than the offset in energy between the highest occupied molecular orbital (HOMO) of the outermost donor and the lowest unoccupied molecular orbital (LUMO) of the outermost acceptor. The device consists of a subphthalocyanine (SubPc)/fullerene (C60) heterojunction that is modified by inserting one or two additional donor layers between SubPc and the anode. We find that two-, three-and four-layer structures yield similar Voc (1.0 V, 0.91 V and 0.94 V, respectively), even though the outermost HOMO-LUMO offset decreases from 1.4 eV to 1.10 eV, and to 0.9 eV, respectively. Analysis of the turn-on voltage in dark provides further evidence that open-circuit voltage is not limited by the outermost HOMO-LUMO offset.

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GaAs 1Àx Bi x has been grown by solid-source molecular beam epitaxy using varying substrate rotation rates. Changes in local bismuth saturation were studied by varying the Bi/Ga pressure ratio across the wafer. Films were grown on both GaAs and InGaAs buffer layers with varying indium content to change the strain conditions of the bismide layer and the out-of-plane growth rate. All samples demonstrated vertical composition modulations with a period of $ 4 nm that tracked with the rate of growth per substrate rotation cycle. The thermal stability of these composition modulations was shown to behave similarly to bulk GaAsBi. Bismide composition modulations are attributed to the low growth temperature and the varying Bi/Ga pressure ratio across the sample rather than the varying V/III ratio.