The use of semiconductor nanowires for photovoltaic applications is advantageous for several reasons: 1) it permits
interpenetrating networks of materials for semiconductor heterojunctions at the nanoscale, allowing efficient carrier
extraction following light absorption, 2) long absorption paths are possible while maintaining short distances for carrier
collection, even in imperfect materials, 3) single crystal materials can be grown in relatively thin films with little
material, 4) strong light trapping is possible due to the geometry of the nanowires, and 5) manipulation of materials
properties is possible by varying the size of the nanostructures. These advantages must be traded off against the
difficulties of fabricating devices (particularly planarization of structures), issues with recombination centers at
interfaces, and the requirement of making ohmic contacts with relatively low temperature processes. The optical and
electronic properties of semiconductor nanowires, nanowire arrays, and heterojunction interfaces are discussed. Recent
results for photovoltaic cells based on semiconductor nanorods and nanowires are summarized, and opportunities for
improvement of device characteristics are presented.
Diatoms are single-celled algae which possess silica shells called "frustules" that contain periodic submicron scale
features. A diatom cell culture process was used to fabricate a two-dimensional photonic crystal slab of Ge-doped
biosilica that possessed 120 nm holes, 330 nm lattice constant, and dielectric constant of 8.5. This material was
integrated into an electroluminescent (EL) device by spin coating of the frustules onto indium tin oxide, followed by
atomic layer deposition of 400 nm hafnium silicate. No photonic band gap was predicted. However, the EL spectrum
possessed resonant UV line emissions that were consistent with photonic band calculations. An EL band gap between
500-640 nm was also observed between blue and red EL line emissions. These EL characteristics have not been observed
previously, and are unique to the diatom photonic crystal. This study represents a first step towards the realization of
optoelectronic devices which utilize nanoscale components fabricated through cell culture.
A model for projection laser crystallization of thin silicon films has been developed. The model is capable of simulating stochastic nucleation and grain growth to predict the extent of lateral growth (LG) in the film and the details of the final microstructure. This model was used to simulate irradiation schemes involving multiple pulses, designed to increase the lateral growth length (LGL) in the irradiated domain. For an irradiation scheme involving two pulses, with an adjustable time delay, our simulation predicted a maximum increase in LGL of about 50% (from 2μm to 3μm) with a maximum film temperature of ~2700 K. For a three-pulse irradiation scheme (without time delay) a 50% increase in LGL was also predicted, but with a maximum film temperature of ~2200 K. These simulations show the efficacy and the relative merit of each of the examined schemes, as well as, their associated process window.
We have developed excimer-laser-annealing modeling capability by broadening the computational ability of a standard finite-element based computational-fluid-dynamics software package to adopt to the specific demands of very rapid heating of thin a-Si films. This was achieved by the incorporation of a subroutine employing a phase function and a set of rules for determining latent heat absorption or release. Wit this enhancement the model was able to correctly calculate the degree of superheating/undercooling in the film and track the melt-solid interface velocity. The model also provided reasonable estimates of the expected poly-Si lateral growth length as a function of the laser irradiation scenario. The model in its current form is a useful tool for first order calculations and for supporting relevant experimental studies.
In this work we have investigated the crystallization of PECVD as-deposited amorphous silicon films by excimer laser anneal. A lambda-physik XeCl excimer laser was used to produce thin polysilicon films under a variety of operating conditions. The effect of process parameters, such as laser energy density, substrate temperature and annealing ambient was investigated with respect to the grain size and surface roughness of the crystallized films. It was found that annealing in rough vacuum, at a substrate temperature of 450 degrees C and with an energy density of 270mJ/cm2 resulted in films with an average grain size of 0.5micrometers and surface roughness of 6nm. It was shown that by introducing a two-step anneal, the distribution of the grain size could be improved with a small compromise in the average grain size. The annealing ambient was shown to significantly affect the surface roughness of the films, with O2-rich environments generally promoting the development of roughness. Incorporation of a barrier layer under the annealed film was shown to increase the grain size and, tat the same time, improve the resistance of the substrate to laser-induced roughening.
Laser-assisted epitaxial growth of III-V semiconductors has been achieved using both pyrolytic and photolytic reactions. A focused beam from an argon laser operating at 514.5 nm was used to 'direct-write' epitaxial microstructures using the pyrolytic process, whereas an excimer laser was utilized to examine the photolytic process. Dependence of the film properties on the laser parameters is investigated. This discussion is limited to homo- and heteroepitaxy of GaP.
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