Iman Khodadad

Highly ordered vertical GaAs nanowire arrays with dry etching and their optical properties

We report fabrication methods, including metal masks and dry etching, and demonstrate highly ordered vertical gallium arsenide nanowire arrays. The etching process created high aspect ratio, vertical nanowires with insignificant undercutting from the mask, allowing us to vary the diameter from 30 nm to 400 nm with a pitch from 250 nm to 1100 nm and length up to 2.2 μm. A diameter to pitch ratio of ∼68% was achieved. We also measured the reflectance from the nanowire arrays and show experimentally diameter-dependent strong absorption peaks resulting from resonant optical mode excitations within these nanowires. The reflectance curves match very well with simulations. The work done here paves the way towards achieving high efficiency solar cells and tunable photodetectors using III-V nanowires.

Multi-spectral optical absorption in substrate-free nanowire arrays

A method is presented of fabricating gallium arsenide (GaAs) nanowire arrays of controlled diameter and period by reactive ion etching of a GaAs substrate containing an indium gallium arsenide (InGaP) etch stop layer, allowing the precise nanowire length to be controlled. The substrate is subsequently removed by selective etching, using the same InGaP etch stop layer, to create a substrate-free GaAs nanowire array. The optical absorptance of the nanowire array was then directly measured without absorption from a substrate. We directly observe absorptance spectra that can be tuned by the nanowire diameter, as explained with rigorous coupled wave analysis. These results illustrate strong optical absorption suitable for nanowire-based solar cells and multi-spectral absorption for wavelength discriminating photodetectors. The solar-weighted absorptance above the bandgap of GaAs was 94% for a nanowire surface coverage of only 15%.

Adjustable optical response of amorphous silicon nanowires integrated with thin films.

We experimentally demonstrate a new optical platform by integrating hydrogenated amorphous silicon nanowire arrays with thin films deposited on transparent substrates like glass. A 535 nm thick thin film is anisotropically etched to fabricate vertical nanowire arrays of 100 nm diameter arranged in a square lattice. Adjusting the nanowire length, and consequently the thin film thickness permits the optical properties of this configuration to be tuned for either transmission filter response or enhanced broadband absorption. Vivid structural colors are also achieved in reflection and transmission. The optical properties of the platform are investigated for three different etch depths. Transmission filter response is achieved for a configuration with nanowires on glass without any thin film. Alternatively, integrating thin film with nanowires increases the absorption efficiency by ∼97% compared to the thin film starting layer and by ∼78% over nanowires on glass. The ability to tune the optical response of this material in this fashion makes it a promising platform for high performance photovoltaics, photodetectors and sensors.

Virtual spectral multiplexing for applications in in-situ imaging microscopy of transient phenomena

Multispectral sensing is specifically designed to provide quantitative spectral information about various materials or scenes. Using spectral information, various properties of objects can be measured and analysed. Microscopy, the observing and imaging of objects at the micron- or nano-scale, is one application where multispectral sensing can be advantageous, as many fields of science and research that use microscopy would benefit from observing a specimen in multiple wavelengths. Multispectral microscopy is available, but often requires the operator of the device to switch filters which is a labor intensive process. Furthermore, the need for filter switching makes such systems particularly limiting in cases where the sample contains live species that are constantly moving or exhibit transient phenomena. Direct methods for capturing multispectral data of a live sample simultaneously can also be challenging for microscopy applications as it requires an elaborate optical systems design which uses beamsplitters and a number of detectors proportional to the number of bands sought after. Such devices can therefore be quite costly to build and difficult to maintain, particularly for microscopy. In this paper, we present the concept of virtual spectral demultiplexing imaging (VSDI) microscopy for low-cost in-situ multispectral microscopy of transient phenomena. In VSDI microscopy, the spectral response of a color detector in the microscope is characterized and virtual spectral demultiplexing is performed on the simultaneously-acquired broadband detector measurements based on the developed spectral characterization model to produce microscopic imagery at multiple wavelengths. The proposed VSDI microscope was used to observe colorful nanowire arrays at various wavelengths simultaneously to illustrate its efficacy.

Photon-Induced Negative Capacitance in Metal Oxide Semiconductor Structures

Design and fabrication of photon-induced negative capacitance is presented. The capacitor is implemented using Silicon on Insulator Metal Oxide Semiconductor platform, where the gate dielectric is made of a nonferroelectric material. Operating at room temperature, when the device is illuminated, in depletion mode the total capacitance grows in magnitude to values larger than the geometrical capacitance. We believe this is caused by the trap states existing at the interface of dielectric and semiconductor layers, and present the supporting modeling results. Using our model, we investigate the role of the trap density and light intensity, as well as the device geometry such as gate-ground position and the thickness of the silicon layer. Our model shows the depletion capacitance can grow to values more than three times larger than the geometrical capacitance.

Refractometric Sensing Using High-Order Diffraction Spots From Ordered Vertical Silicon Nanowire Arrays

We propose to use high-order diffraction spots from 2-D silicon nanowire (NW) arrays for refractive index sensing based on spatial changes in the diffractive spots position. The NW arrays act both as a refractive index sensor and as dispersive elements, eliminating the need for external spectrometers for the measurement of refractive index changes. The setup uses a simple laser diode source and a low-cost camera and results in higher sensitivity to environmental refractive index changes, as compared with previously demonstrated colorimetric sensors. The sensitivity is greater for higher order diffraction spots, as compared with the lower order ones due to a larger dispersion angle change at higher orders. We also demonstrate that the observed diffraction angle and efficiency of the diffractive orders depend on a number of factors, such as excitation wavelength, NW diameters, pitch, and surrounding medium index. The simple solution of using diffraction spot displacements on a 2-D detector array would provide a novel means of sensing refractive index changes in the surrounding medium of NWs without the burden of complicated spectral analysis.

OPTIMIZATION OF A MICRO POWER UNIT

In this work, we report on the optimization of a micro power unit. The power unit is comprised of an electromagnetic micro power generator (MPG), power conditioning circuits, and an energy storage component. First, we minimize the mechanical impedance of the MPG by optimizing the size of the seismic mass and support beam as well as re-designing the seismic mass. Second, we investigate the use of off-the-shelf dc-dc converters and custom voltage multipliers and transformers to rectify and step-up the output voltage. We find that transformers were best suited for electromagnetic MPG delivering a rectification efficiency of 75%.Copyright

Electron beam lithography using fixed beam moving stage

Large area patterns with small submicron features are difficult to write using conventional electron beam lithography (EBL) methods. This would be more challenging especially if the patterns have large lateral aspect ratios such as waveguide tapers. Conventionally, the patterning area is divided into smaller write fields and the stage moves in between various write fields. Precise stage movement is necessary to reduce stitching errors. However, even the most accurate laser interferometer based control systems are prone to stochastic thermal drifts. In this paper, new methods of EBL patterning are explored using the stitch free method of writing and overcoming the conventional time constraints for writing large area patterns. Further, the methods presented are suited for writing structures with micron and nanosized features in the same pattern.

Deep Tissue Sequencing Using Hypodermoscopy and Augmented Intelligence to Analyze Atypical Pigmented Lesions

Background: Over the past decade, new technologies, devices, and methods have been developed to assist in the diagnosis of cutaneous melanocytic lesions. Objective: Our objective was to evaluate the performance of an augmented intelligence system in the assessment of atypical pigmented lesions. Methods: Nine atypical pigmented lesions on 8 patients were evaluated prior to surgical removal. No lesions had received previous treatment other than a diagnostic biopsy. Prior to surgical removal, each lesion was evaluated by an Augmented Intelligence Dermal Imager (AID) and the assessment parameters reviewed in light of the final histopathological diagnosis. Results: The AID was used to evaluate a limited set of atypical pigmented lesions and showed sensitivity and specificity of 82% and 61%, respectively, based on its internal risk assessment algorithms. Limitations: These cases represent early assessments of the AID in a clinical setting, all prior assessments having been carried out on digital images. The information received from these evaluations requires further validation and analysis to be able to extrapolate its clinical usefulness. Conclusion: The AID combines dermoscopy, hypodermoscopy, and a trained augmented algorithm to produce a diffusion map representing the features of each lesion compared to the learned characteristics from a database of known dermoscopy images of lesions with definitive prior diagnosis. The information gathered from the diffusion map might be used to calculate a malignancy risk factor for the lesion compared to known melanoma features. This malignancy risk factor could be helpful in providing information to justify the biopsy of an atypical pigmented lesion.

Photon induced negative capacitance in metal oxide semiconductor structures

Design and fabrication of photon induced, negative capacitance in a Metal Oxide Semiconductor (MOS) capacitor is presented, where a non-ferroelectric material serves as the gate dielectric. The underlying device physics involves states at the semiconductor-oxide interface, coupled with the injection of photo-generated electrons. This forces the otherwise increasing surface potential to decrease in response to an increase in the gate voltage; and causes the total capacitance in depletion mode to be greater than the geometrical capacitance at room temperature. We find the occurrence and value of the negative capacitance to depend on the interface trap density, and light intensity.