222 research outputs found
The Garret Near the Sky
No full textDreaming of a cottage by the sea while in a garret in London\u27s polluted skyhttps://egrove.olemiss.edu/kgbsides_uk/1339/thumbnail.jp
Avoiding erroneous analysis of MIM diode current-voltage characteristics through exponential fitting
No full textAccurate fitting of measured current-voltage [I(V)I(V)] data is crucial to the correct analysis and understanding of metal-insulator–metal (MIM) diodes, especially for optical rectennas. With the commonly used polynomial fitting of the I(V)I(V) data, the order of the fit can drastically affect the diode performance metrics such as resistance, responsivity, and asymmetry. Additionally, the resulting fitting coefficients provide no useful parameters. An exponential-based equation can fit the I(V)I(V) data well, can avoid artifacts from the choice of order of the polynomial, and allows for the accurate calculation of diode performance metrics directly from the fitting coefficients. Connecting the performance metrics to fitting coefficients shows a correspondence between zero-bias responsivity and asymmetry at any given voltage.The authors would like to thank David Doroski and Brad Herner for their assistance in fabricating the MIM devices and Miena Armanious, Ayendra Weerakkody and John Stearns for their helpful discussions. This work was carried out in part under contract from RedWave Energy Inc. and funded in part by the Advanced Research Projects Agency - Energy (ARPA-E), U.S. Department of Energy, under Award Number DE-AR0000676. Also, research reported in this publication was supported by competitive research funding from King Abdullah University of Science and Technology (KAUST). G. Moddel holds stock in RedWave Energy, Inc
A True and perfect Inventory of all and singular the good and chattles, rights, and credits, of Garret J. Newkirk late of the township of Bergen, in the County of Bergen Deceas'd made by us whose names are hereunto Subscribed, the twenty sixth day of December in the year of our lord one thousand eight hundred and nineteen.
No full textInventory of the estate of Garret Newkirk, November 16, 1818, including three slaves as well as tools and products of a farm estate. An example of the possessions of a well-off farm, and the relative value and prices."Taken Nov 6th 1818
III-V Bismides as a New Heterojunction Material System for Electronic Devices
No full textIncorporating bismuth into epitaxially grown GaAs layers produces the alloy GaAs1-xBix. This new material system shifts the band gap of GaAs down by approximately 88 meV/Bi%, while maintaining a small lattice mismatch of less than 0.25 % for a 200 meV to 300 meV band gap shift. This material has many potential applications in optical and electron devices. In this work the material is studied for use in device applications, specifically heterojunction bipolar transistors with a narrow band gap GaAsBi base layer. The performance of this device is simulated to find its maximum potential gain and frequency of operation in the X-band at devices sizes of 0.5 μm for < 2.5 % bismuth alloying. P-N and HBT devices are fabricated to characterize material quality and HBT performance. Loss mechanisms are studied to improve future devices in the GaAsBi material system.</p
Graphene Geometric Diodes for Optical Rectennas
No full textOptical rectennas, which are micro-antennas to convert optical-frequency radiation to alternating current combined with ultrahigh-speed diodes to rectify the current, can in principle provide high conversion efficiency solar cells and sensitive detectors. Currently investigated optical rectennas using metal/insulator/metal (MIM) diodes are limited in their RC response time and poor impedance matching between diodes and antennas. A new rectifier, the geometric diode, can overcome these limitations. The thesis work has been to develop geometric diode rectennas, along with improving fabrication processes for MIM diode rectennas. The geometric diode consists of a conducting thin-film, currently graphene, patterned into a geometry that leads to diode behavior. In contrast with MIM diodes that have parallel plate electrodes, the planar structure of the geometric diode provides a low RC time constant, on the order of 10-15 s, which permits operation at optical frequencies. Fabricated geometric diodes exhibit asymmetric DC current-voltage characteristics that match well with Monte Carlo simulations based on the Drude model. The measured diode responsivity at DC and zero drain-source bias is 0.012 A/W. Since changing the gate voltage changes the graphene charge carrier concentration and can switch the majority charge type, the rectification polarity of the diode can be reversed. Furthermore, the optical rectification at 28 THz has been measured from rectennas formed by coupling geometric diodes with graphene and metal bowtie antennas. The performance of the rectenna IR detector is among the best reported uncooled IR detectors. The noise equivalent power (NEP) of the rectenna detector using geometric diode was measured to be 2.3 nW Hz-1/2. Further improvement in the diode and antenna design is expected to increase the detector performance by at least a factor of two. Applications for geometric diodes and graphene bowtie antennas include detection of terahertz and optical waves, ultra-high speed electronics, and optical power conversion.</p
The Local Bonding Environment of Amorphous In-Zn-O Films Studied by X-ray Absorption Fine Structure and Total X-ray Scattering Using Synchrotron Radiation
No full textAmorphous transparent conducting oxides (a-TCOs) are a class of materials becoming increasingly important for their use in transparent electronic devices. Depending on the application, these materials can be tuned to behave as conductors, semiconductors, or even insulators. These materials have been gaining interest because they exhibit several desirable properties that are limiting factors in more common crystalline TCOs. The amorphous nature of these films means that they have no grain boundaries. This is beneficial because grain boundaries act as defects, decreasing the mobility of charge carriers in the material. Grain boundaries also provide a path for contaminants from the surrounding environment, such as water vapor, to transport through the films. This can be particularly detrimental to the performance of photovoltaics. Typically, these materials are very smooth and are mechanically robust, making them well suited for use in the emerging field of flexible electronics.
An archetypical a-TCO is amorphous indium zinc oxide (a-IZO). This material can be produced with a wide range of compositions and conductivities while not displaying any evidence of crystallization. The electrical and optical properties of a-IZO have been well characterized under a variety of different growth methods and conditions. Despite being well characterized from the standpoint of device performance, the structure of a-IZO has not, until recently, been thoroughly investigated. The native oxide of indium (In) is bixbyite, a cubic structure, while zinc (Zn) naturally crystallizes as wurtzite, a hexagonal structure. These two crystal structures are incompatible. It is believed that the inclusion of Zn atoms in IZO “frustrates” the crystal structure, hindering the creation of any long-range periodicity. This is because, when deposited at low temperatures, both Zn and In try to form their respective native oxides. The structural description of sputtered a-IZO thin films is of interest for this work.
The amorphous nature of these materials means that any structural description will be statistical in nature, and traditional methods to measure structure, such as X-ray diffraction, will not be particularly illuminating. Despite the lack of long-range periodicity, a-TCOs still exhibit short-range ordering that closely resembles what would be expected in a crystalline material. Two methods well suited for structural determinations of amorphous materials are the Pair-Distribution Function (PDF), which is derived from the total scattering spectrum, and X-ray Absorption Fine Structure spectroscopy (XAFS). XAFS is capable of probing the structure of materials on very short (≲ 5 Å) length scales surrounding a specific element of interest. The PDF method is capable of probing the structure over longer distances than XAFS. Both techniques have strengths and weaknesses that will be discussed.
Both XAFS and PDF methods require highly monochromatic X-rays with widely tunable energy ranges and high fluxes. These requirements necessitate the use of synchrotron-based X-ray sources. Although synchrotron based measurements are becoming more routine in the scientific community, these methods still present significant difficulties in the determination and interpretation of results. The Stanford Synchrotron Radiation Lightsource and the Advanced Photon Source were both used in the collection of experimental data for this work.
Previous XAFS results from similar systems has shown that the oxygen coordination around each metal site is close to that of the native oxide, however current literature for XAFS analysis of aIZO is lacking. The PDF results confirm that the arrangement of nearest and next-nearest neighbors in a-IZO is similar to what would be expected of crystalline In2O3. From the XAFS results, it has been concluded that InO6 octahedra and ZnO4 tetrahedra form edge-sharing linkages in a-IZO. Supporting this conclusion, EXAFS analysis provided a measure of the local cation (Zn/In) ratio which was consistent with the measurement of the bulk cation ratio.</p
The Local Bonding Environment of Amorphous In-Zn-O Films Studied by X-ray Absorption Fine Structure and Total X-ray Scattering Using Synchrotron Radiation
No full textAmorphous transparent conducting oxides (a-TCOs) are a class of materials becoming increasingly important for their use in transparent electronic devices. Depending on the application, these materials can be tuned to behave as conductors, semiconductors, or even insulators. These materials have been gaining interest because they exhibit several desirable properties that are limiting factors in more common crystalline TCOs. The amorphous nature of these films means that they have no grain boundaries. This is beneficial because grain boundaries act as defects, decreasing the mobility of charge carriers in the material. Grain boundaries also provide a path for contaminants from the surrounding environment, such as water vapor, to transport through the films. This can be particularly detrimental to the performance of photovoltaics. Typically, these materials are very smooth and are mechanically robust, making them well suited for use in the emerging field of flexible electronics.
An archetypical a-TCO is amorphous indium zinc oxide (a-IZO). This material can be produced with a wide range of compositions and conductivities while not displaying any evidence of crystallization. The electrical and optical properties of a-IZO have been well characterized under a variety of different growth methods and conditions. Despite being well characterized from the standpoint of device performance, the structure of a-IZO has not, until recently, been thoroughly investigated. The native oxide of indium (In) is bixbyite, a cubic structure, while zinc (Zn) naturally crystallizes as wurtzite, a hexagonal structure. These two crystal structures are incompatible. It is believed that the inclusion of Zn atoms in IZO “frustrates” the crystal structure, hindering the creation of any long-range periodicity. This is because, when deposited at low temperatures, both Zn and In try to form their respective native oxides. The structural description of sputtered a-IZO thin films is of interest for this work.
The amorphous nature of these materials means that any structural description will be statistical in nature, and traditional methods to measure structure, such as X-ray diffraction, will not be particularly illuminating. Despite the lack of long-range periodicity, a-TCOs still exhibit short-range ordering that closely resembles what would be expected in a crystalline material. Two methods well suited for structural determinations of amorphous materials are the Pair-Distribution Function (PDF), which is derived from the total scattering spectrum, and X-ray Absorption Fine Structure spectroscopy (XAFS). XAFS is capable of probing the structure of materials on very short (≲ 5 Å) length scales surrounding a specific element of interest. The PDF method is capable of probing the structure over longer distances than XAFS. Both techniques have strengths and weaknesses that will be discussed.
Both XAFS and PDF methods require highly monochromatic X-rays with widely tunable energy ranges and high fluxes. These requirements necessitate the use of synchrotron-based X-ray sources. Although synchrotron based measurements are becoming more routine in the scientific community, these methods still present significant difficulties in the determination and interpretation of results. The Stanford Synchrotron Radiation Lightsource and the Advanced Photon Source were both used in the collection of experimental data for this work.
Previous XAFS results from similar systems has shown that the oxygen coordination around each metal site is close to that of the native oxide, however current literature for XAFS analysis of aIZO is lacking. The PDF results confirm that the arrangement of nearest and next-nearest neighbors in a-IZO is similar to what would be expected of crystalline In2O3. From the XAFS results, it has been concluded that InO6 octahedra and ZnO4 tetrahedra form edge-sharing linkages in a-IZO. Supporting this conclusion, EXAFS analysis provided a measure of the local cation (Zn/In) ratio which was consistent with the measurement of the bulk cation ratio.</p
The Influence of Surface Plasmons on Excited State Dynamics in PTCDA
No full textOrganic thin film solar cells can be paired with nanostructured substrates to overcome the issue of narrow spectral absorption in a thin-film configuration. The nanostructured surface acts not only as an effective scattering back reflector to increase the light path within the absorbing thin film but also affords plasmonic activity. The interface between the metal and the absorbing chromophore supports surface plasmon modes. The associated strong electromagnetic field can potentially couple with excitations of the chromophore, altering its exciton dynamics. Such a plasmon-exciton coupling can lead to control over excitation processes, namely singlet fission. Singlet fission is a sharing of excited state energy between chromophores that may regulate instances of multi-exciton generation, allowing the solar cell efficiency to exceed the thermodynamical Shockley-Queisser limit.
The current investigation focuses on hybridization of the plasmon and molecular exciton. We coat an organic semiconductor, 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA), on the nanogratings consisting of lines of Ag on a substrate coated with a thick Ag backing. A dielectric spacer layer is included between the organic and the metal in some samples to eliminate any reaction between the two. The SP resonance of the grating is tuned through a PTCDA exciton line by sweeping the incident wave vector. Successful anticrossing between the plasmon and the exciton peaks would be observed in steady-state reflectance data as a function of angle. Though a detailed analysis of reflectance spectra has not been completed, the potential for plasmon-exciton hybdridization is demonstrated
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