A carpet cloak for visible light

We report an invisibility carpet cloak device, which is capable of making an object undetectable by visible light. The cloak is designed using quasi conformal mapping and is fabricated in a silicon nitride waveguide on a specially developed nanoporous silicon oxide substrate with a very low refractive index (n<1.25). The spatial index variation is realized by etching holes of various sizes in the nitride layer at deep subwavelength scale creating a local effective medium index. The fabricated device demonstrates wideband invisibility throughout the visible spectrum with low loss. This silicon nitride on low index substrate can also be a general scheme for implementation of transformation optical devices at visible frequencies.     

Three-dimensional modeling and simulation of p-n junction spherical silicon solar cells

A three-dimensional numerical model is presented to simulate spherical p-n junction silicon solar cells, which is a promising new technology for photovoltaic (PV) energy conversion for terrestrial applications. Material properties imposed by the sphere formation method, geometry of the device, and the specific device structure stemming from the fabrication technology are taken into account in the optical and electrical models of the device. The spherical device is numerically simulated based on these models using finite-difference method in a spherical system of coordinates, generating the internal quantum efficiency and current-voltage (I-V) characteristics of the device. It has been shown that the efficiency of a spherical solar cell is slightly lower than a conventional device; however, the slightly inferior performance does not outweigh the cost advantage. It has been also found that subsurface diffusion length from effective impurity segregation and the depth of the denuded zone in spherical devices are parameters that mainly affect the device efficiency. Based on the simulation and analysis, design guidelines have been presented for spherical PV devices.     

Growth and Structural Characterization of Spherical Silicon Crystals Grown from Polysilicon

The growth process for spherical silicon crystals using melting and recrystallization of polysilicon powder and polysilicon granules is investigated and the structural properties of the fabricated silicon spheres are characterized. The recrystallization conditions that influence the crystallinity of the spheres are studied. It is shown by means of defect delineation that crystalline structures are obtained when the melt is solidified at temperatures above 1400°C. The results of electron backscattering confirm the formation of single-grain structures. When solidification takes place at temperatures below 1400°C, double- and multiple-grain structures start to form. The concentration of impurities during the recrystallization of the spheres, monitored by secondary-ion mass spectroscopy, show that the rate of impurity removal from the crystalline phase is much lower than predicted by segregation at the melt–solid interface, which is attributed to the nonequilibrium conditions in the solidification process.     

Properties and characterization of low-temperature amorphous PECVD silicon nitride films for solar cell passivation

Deposition conditions and some structural and electrical properties of amorphous silicon nitride (Si_xN_y:H) films deposited on Si substrates have been studied for photovoltaic applications. A plasma enhanced chemical vapor deposition (PECVD) system has been used for the study. Experiments have been performed varying the flow ratios and dilution of the reactant gases. Increased hydrogen (H₂) dilution leads to reduced deposition rate and a better controllability in the growth process. The hydrogen content in the film also decreases with increasing H₂ dilution of the reactant gases. Flow ratio of the reactant gases (SiH₄/NH₃) also influences the growth rate. There is an optimal reactant gas mix to maximize the film growth rate. However, the film stoichiometry is also modified by changing the gas mix, with higher flow ratios resulting in Si-rich films. The level of interfacial recombination of minority carriers has been studied by capacitance-voltage and effective lifetime measurements. Bombardment by the energetic species in the plasma leads to plasma damage at the interface. These interfacial defects can be annealed by a post-deposition, low temperature treatment.