Abigail S. Licht

GaSb Thermophotovoltaic Cells Grown on GaAs Substrate Using the Interfacial Misfit Array Method

We present gallium antimonide (GaSb) p–i–n photodiodes for use as thermophotovoltaic (TPV) cells grown on gallium arsenide (100) substrates using the interfacial misfit array method. Devices were grown using molecular beam epitaxy and fabricated using standard microfabrication processes. X-ray diffraction was used to measure the strain, and current–voltage (I–V) tests were performed to determine the photovoltaic properties of the TPV cells. Energy generation at low efficiencies was achieved, and device performance was critically analyzed.

Manipulation of magnetization states of ferromagnetic nanorings by an applied azimuthal Oersted field

We manipulate the magnetic states of ferromagnetic nanorings with an azimuthal Oersted field directed along the ring circumference. The circular field is generated by passing current through an atomic force microscope tip positioned at the center of the ring, and can directly control the chirality of the vortex state. We demonstrate switching from an onion state to a vortex state and between two vortex states, using magnetic force microscopy to image the resulting magnetic states. The understanding of the magnetization switching behavior in an azimuthal Oersted field could improve practical magnetic data storage devices.

Multiple 360° domain wall switching in thin ferromagnetic nanorings in a circular magnetic field

Micromagnetic simulations of the vortex switching process of thin ferromagnetic rings under the application of a circular field, as if created from a current-carrying wire passing through the ring center, reveal that for rings with sub-micron dimensions and thicknesses on the order of the exchange length, the vortex to vortex switching process occurs through the nucleation and annihilation of multiple 360° domain walls (DWs). The DWs can be characterized by their circulation relative to the vortex circulation; the DWs form in pairs with opposite topological indices. The DW with the same circulation annihilates first, which has a smaller energy barrier to overcome before annihilating. The contributions from both the exchange energy and demagnetization energy must be considered to predict which DW will annihilate first. Either wall could be annihilated by offsetting the current toward the wall being targeted.

Thermophotovoltaics: An Alternative to and Potential Partner with Rectenna Energy Harvesters

A technology that can be used in place of, or in addition to, rectennas is thermophotovoltaics (TPVs). The ultimate function of TPVs, like that of the rectenna, is the conversion of electromagnetic radiation to DC current. Rectennas use a rectifying diode coupled with an antenna to achieve this conversion. TPVs achieve this conversion through a single diode which both receive the radiation and converts it to a current. While rectennas are superior at longer wavelengths (greater than 5 μm), TPVs are more efficient at shorter wavelengths (less than 5 μm). Although rectennas and TPVs have been investigated independently, a hybrid technology may be possible which incorporates components from both technologies.

Study of the effect of 2D metallic photonic crystals on GaSb TPV diode performance

Thermophotovoltaics (TPVs) are a potential technology for waste-heat recovery applications and utilize IR sensitive photovoltaic diodes to convert long wavelength photons (>800nm) into electrical energy. The most common conversion regions utilize Gallium Antimonide (GaSb) as the standard semiconductor system for TPV diodes due to its high internal quantum efficiencies (close to 90%) for infrared radiation (~1700nm). However, parasitic losses prevent high conversion efficiencies from being achieved in the final device. One possible avenue to improve the conversion efficiency of these devices is to incorporate metallic photonic crystals (MPhCs) onto the front surface of the diode. In this work, we study the effect of MPhCs on GaSb TPV diodes. Simulations are presented which characterize a specific MPhC design for use with GaSb. E-field intensity vs. wavelength and depth are investigated as well as the effect of the thickness of the PhC on the interaction time between the e-field and semiconductor. It is shown that the thickness of MPhC has little effect on width of the enhancement band, and the depth the ideal p-i-n junction is between 0.6μm and 2.1μm. Additionally, simulated results demonstrate an increase of E-field/semiconductor interaction time of approximately 40% and 46% for a MPhC thickness of 350nm and 450nm respectively.

Optimization of GaSb thermophotovoltaic diodes with metallic photonic crystal front-surface filters

A promising technology for waste-heat recovery applications is thermophotovoltaics (TPVs), which use photovoltaic diodes to convert thermal energy into electricity. The most commonly used TPV diode material is gallium antimonide (GaSb). Recently, GaSb TPV diodes were fabricated with front-surface metallic photonic crystal (MPhC) filters to more optimally convert the incident spectrum. This method showed promising initial results, in part due to a shifting of the photogenerated carriers away from the front-surface and into the device. In this paper, we use the Atlas-Silvaco software package to optimize the TPV diode structure for MPhCs. We investigate the addition of an intrinsic region in the device to take advantage of the shifted photogeneration profile from the MPhCs. This design allows for a 10% improvement in internal quantum at the peak MPhC transmission wavelength.

Identification of a limiting mechanism in GaSb-rich superlattice midwave infrared detector

GaSb-rich superlattice (SL) p-i-n photodiodes grown by molecular beam epitaxy were studied theoretically and experimentally in order to understand the poor dark current characteristics typically obtained. This behavior, independent of the SL-grown material quality, is usually attributed to the presence of defects due to Ga-related bonds, limiting the SL carrier lifetime. By analyzing the photoresponse spectra of reverse-biased photodiodes at 80 K, we have highlighted the presence of an electric field, breaking the minibands into localized Wannier-Stark states. Besides the influence of defects in such GaSb-rich SL structures, this electric field induces a strong tunneling current at low bias which can be the main limiting mechanism explaining the high dark current density of the GaSb-rich SL diode.

Decreasing dark current in long wavelength InAs/GaSb thermophotovoltaics via bandgap engineering

At present, the state of the art thermophotovoltaic diode material is GaSb, with a bandgap of 0.7 eV corresponding to source temperatures greater than 1000°C. We investigate alternative bandstructure designs using the InAs/GaSb superlattice material system, which enable shorter bandgaps corresponding to lower source temperatures. For an InAs/GaSb superlattice system, we examine the effect of a monovalent barrier inserted between the p and n-doped regions. Through simulations, with the program Silvaco, we demonstrate that this barrier decreases the dark current and increases the open-circuit voltage, improving the overall power output and, thus, extending the operational wavelength of thermophotovoltaics.

Extending the operational wavelength of thermophotovoltaic devices via superlattice and barrier engineering

In this paper, we investigate extending the operational wavelength of thermophotovoltaic diodes. Our calculations demonstrate that employing a barrier structure can reduce the diffusion current by several orders of magnitude, reducing dark current and improving the overall function of the TPV diode for room temperature operation. We first investigated GaSb/InAs type–II superlattice structures with monovalent barriers targeting wavelength cut-offs of five microns. Simulations were used to optimize the band structure energy levels for superlattice materials and to align the energy bands between different layers in the device. We examine the difference in IV curves between barrier and non-barrier structures for a five micron (Eg=0.248 eV) diode with a barrier of 300 meV.