Majid Gharghi

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.

Heterojunction Silicon Microwire Solar Cells

We report radial heterojunction solar cells of amorphous silicon on crystalline silicon microwires with high surface passivation. While the shortened collection path is exploited to increase the photocurrent, proper choice of the wire radius and the highly passivated surface prevent drastic decrease in the voltage due to high surface-to-volume ratio. The heterojunction is formed by depositing a ∼12-16 nm of amorphous silicon on crystalline silicon wires of radius approximately equal to minority carrier diffusion length (∼10 μm). In spite of very short carrier lifetime (<1 μs), the microwire array devices generate photocurrent of ∼30 mA/cm(2), and the same time, voltages close to 600 mV are achieved, leading to efficiency in excess of 12% in extremely short carrier lifetime silicon. We also find that formation of nanocrystallites of silicon in the deposited film results in loss of the expected passivation.

Exciton-dominant electroluminescence from a diode of monolayer MoS2

We studied the microscopic origin of the electroluminescence from monolayer MoS2 fabricated on a heavily p-type doped silicon substrate. Auger recombination of the exciton-exciton annihilation of bound exciton emission is observed.

Overcoming the bandgap limitation on solar cell materials

The thermodynamic efficiency of a single junction solar cell is bounded by the Shockley-Queisser detailed balance limit at ∼30% [W. Shockley and H. J. Queisser, J. Appl. Phys. 32, 510 (1961)]. This maximal efficiency is considered achievable using a semiconductor within a restricted bandgap range of 1.1-1.5 eV. This work upends this assumption by demonstrating that the optimal material bandgap can be shifted to lower energies by placing selective reflectors around the solar cell. This technique opens new possibilities for lower bandgap materials to achieve the thermodynamic limit and to be effective in high efficiency solar cells.

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.

Spherical silicon photovoltaic devices with surface-passivated shallow emitters

The design, fabrication and photovoltaic properties of surface-passivated, shallow emitter silicon devices with a spherical geometry are presented. It is shown that, for the specific metallization technique employed, deep-diffused emitters (>1 µm) are necessary to avoid device shunting. A wet-etching technique has been employed to thin down the spherical emitter in the off-metal regions to enhance the photocurrent. Also, a plasma deposition process over the spherical surface to passivate the surface states of the emitter has been developed to enhance the quantum efficiency and open circuit voltage. The beneficial effects of the passivation film are pronounced only in shallow emitter devices.

Minority carrier recombination in spherical silicon crystals: measurement and process-induced effects

Minority carrier recombination kinetics in defective, sphere-shaped silicon crystals is studied and a methodology is developed to evaluate the carrier lifetime. A theoretical model has been developed for the conductivity transient taking into account the spherical crystal geometry. The recombination rates are extracted using the experimentally obtained conductivity transient through analytical consideration of the different decay components specific to the spherical crystals. The methodology and analytical interpretation of the recombination kinetics provide an efficient tool to demonstrate the sensitivity of the recombination process to changes in bulk and interfacial electronic properties. We also show that the method is very useful in monitoring process-induced effects in various stages of crystal formation (oxygen denuding, annealing, diffusion) and in defect deactivation (gettering, precipitation, passivation) processes.

Interpretation of Photo-Conductivity Decay Lifetime in Silicon Spheres

A novel modeling approach for extracting and interpreting the photoconductivity decay pattern in silicon spheres after an impulse laser excitation is presented. It has been shown that a single exponential fit to conductivity decay will lead to inaccurate data interpretation and would only be possible if radial laser excitation were applied. A double-exponential kinetic has been fit to the experimental results to extract both bulk recombination lifetime and surface recombination velocity

Carpet Cloak Device for Visible Light

We report a cloak device that makes objects undetectable by visible light. It is designed using conformal mapping and is fabricated in silicon nitride waveguide on nano-porous silicon oxide substrate with very low refractive index.

Process development for tailoring the emitter on sphere shaped silicon structures

A dry process is developed for the etch-back of spherical silicon surfaces, intended for thinning of the emitter in spherical pn photodiode. The process is based on reactive ion etching, and enjoys higher reliability and controllability compared to wet etching. The process conditions are controlled to obtain an angle-dependent selective etch rate across the spherical surface.