Photovoltaic retinal prosthesis: implant fabrication and performance.

The objective of this work is to develop and test a photovoltaic retinal prosthesis for restoring sight to patients blinded by degenerative retinal diseases. A silicon photodiode array for subretinal stimulation has been fabricated by a silicon-integrated-circuit/MEMS process. Each pixel in the two-dimensional array contains three series-connected photodiodes, which photovoltaically convert pulsed near-infrared light into bi-phasic current to stimulate nearby retinal neurons without wired power connections. The device thickness is chosen to be 30 µm to absorb a significant portion of light while still being thin enough for subretinal implantation. Active and return electrodes confine current near each pixel and are sputter coated with iridium oxide to enhance charge injection levels and provide a stable neural interface. Pixels are separated by 5 µm wide trenches to electrically isolate them and to allow nutrient diffusion through the device. Three sizes of pixels (280, 140 and 70 µm) with active electrodes of 80, 40 and 20 µm diameter were fabricated. The turn-on voltages of the one-diode, two-series-connected diode and three-series-connected diode structures are approximately 0.6, 1.2 and 1.8 V, respectively. The measured photo-responsivity per diode at 880 nm wavelength is ∼0.36 A W(-1), at zero voltage bias and scales with the exposed silicon area. For all three pixel sizes, the reverse-bias dark current is sufficiently low (<100 pA) for our application. Pixels of all three sizes reliably elicit retinal responses at safe near-infrared light irradiances, with good acceptance of the photodiode array in the subretinal space. The fabricated device delivers efficient retinal stimulation at safe near-infrared light irradiances without any wired power connections, which greatly simplifies the implantation procedure. Presence of the return electrodes in each pixel helps to localize the current, and thereby improves resolution.

Evolution of thermodynamic potentials in closed and open nanocrystalline systems: Ge-Si:Si(001) islands.

An open (closed) system, in which matter is (not) exchanged through surface diffusion, was realized via growth kinetics. Epitaxially grown Si-Ge:Si (001) islands were annealed in different environments affecting the diffusivity of Si adatoms selectively. The evolution of the driving forces for intermixing while approaching the equilibrium was inferred from Synchrotron x-ray measurements of composition and strain. For the open system, intermixing due to the Si inflow from the wetting layer (reservoir) caused a decrease in the Ge content, leading to a lowering of the elastic energy and an increase in the mixing entropy. In contrast, for the closed system, while keeping the average Ge composition constant, atom rearrangement within the islands led to an increase in both elastic and entropic contributions. The Gibbs free energy decreased in both cases, despite the different evolution paths for the composition profiles.

Alloying mechanisms for epitaxial nanocrystals.

The different mechanisms involved in the alloying of epitaxial nanocrystals are reported in this Letter. Intermixing during growth, surface diffusion, and intraisland diffusion were investigated by varying the growth conditions and annealing environments during chemical vapor deposition. The relative importance of each mechanism was evaluated in determining a particular composition profile for dome-shaped Ge:Si (001) islands. For samples grown at a faster rate, intermixing during growth was reduced. Si surface diffusion dominates during H2 annealing, whereas Ge surface diffusion and intraisland diffusion prevail during annealing in a PH3 environment.

3D composition of epitaxial nanocrystals by anomalous X-ray diffraction: observation of a Si-rich core in Ge domes on Si(100).

Three-dimensional composition maps of nominally pure Ge domes grown on Si(001) at 600 degrees C were obtained from grazing incidence anomalous x-ray scattering data at the Ge K edge. The data were analyzed in terms of a stack of layers with laterally varying concentration. The results demonstrated that the domes contained a Si-rich core covered by a Ge-rich shell and were independently supported by selective etch experiments. The composition profile resulted from substrate Si alloying into the Ge during growth to partially relax the stress in and under the domes.

Thermodynamics of the size and shape of nanocrystals: epitaxial Ge on Si(001).

The growth and evolution of strained epitaxial Ge on a Si(001) surface provides a rich system for exploring the behavior of strongly interacting nanocrystals. In the temperature regime above 500 degrees C, there are two different (metastable) shapes of defect-free nanocrystals, termed pyramids and domes, that dominate the system depending on the temperature of the substrate during growth and the amount of Ge deposited. In contrast to the usual case considered in nucleation theory, the relaxation of the strain energy at the surface of the nanocrystals makes those surfaces stabilizing, i.e. the surface contribution to the free energy of the Ge nanocrystals is negative. Given that the edges of the nanocrystals are destabilizing (positive free energy), the interaction of the surfaces and edges of the nanocrystals in an ensemble renders an internal free energy for the system that has a local minimum with respect to the size (volume) of the nanocrystal. At finite temperatures, this free energy yields a size distribution with a characteristic centroid, width, and skewness for each nanocrystal shape. The smaller pyramids transform into domes when they grow to the point where they can surmount a kinetic energy barrier between the two structures. However, the Ge nanocrystals also effectively repel one another strongly via the strain fields that are produced in the Si substrate. This repulsive interaction makes the ensemble of Ge nanocrystals a highly nonideal thermodynamic system and, in turn, makes the free energies of the nanocrystals a function of their number density, or equivalently a function of the amount of Ge deposited. The interplay of the stabilizing effect of the nanocrystal surfaces and the destabilizing influence of their repulsive interactions yields a complex behavior for the nanocrystal-size distributions that can nonetheless be modeled using simple thermodynamic expressions.