Calibration and performance of the photon sensor response of FACT -- The First G-APD Cherenkov telescope

The First G-APD Cherenkov Telescope (FACT) is the first in-operation test of the performance of silicon photo detectors in Cherenkov Astronomy. For more than two years it is operated on La Palma, Canary Islands (Spain), for the purpose of long-term monitoring of astrophysical sources. For this, the performance of the photo detectors is crucial and therefore has been studied in great detail. Special care has been taken for their temperature and voltage dependence implementing a correction method to keep their properties stable. Several measurements have been carried out to monitor the performance. The measurements and their results are shown, demonstrating the stability of the gain below the percent level. The resulting stability of the whole system is discussed, nicely demonstrating that silicon photo detectors are perfectly suited for the usage in Cherenkov telescopes, especially for long-term monitoring purpose

Polychromatic determination of spectral response of PV devices

This thesis introduces a novel spectral response (SR) measurement technique using polychromatic filters (filters with very broad spectral transmittances) to determine SR of large area PV devices. Conventionally, SR of a photovoltaic (PV) device is determined by illuminating the device under test (DUT) with a series of monochromatic beams at different wavelengths as described in the international standard IEC 60904-8, or beams of limited spectral content using narrow band pass filters or monochromator. One significant problem associated with the application of the narrow band pass filters for a large-area SR measurement is that low light intensity produced on the measurement plane particularly in certain wavelength ranges: the ultraviolet and infrared. This can produce weak signal responses from a tested PV device. In addition, the imperfection of the filter s mounting position can shift the peak wavelength of the filter s transmittance at angle of incidence greater than 10. This can cause stray light on the measurement plane. The proposed SR measurement method is called as the polychromatic SR fitting method or, in short, it is known as the polychromatic method . The advantage of this method is that higher beam intensity can be produced on the measurement plane as a result of large spectral transmittance of the polychromatic filters. This can improve the signal strength of a tested PV device. This new SR measurement method works by comparing the variations in the currents which are measured at different spectra to the currents which are calculated at the same spectral conditions using the SR model. Validations of this method for a large- and small-area SR determinations show that it is potentially feasible as a new technique for determining SR of a PV device with deviations within ±2% across the wavelength bands

Physics-based simulation of narrow and wide band gap photonic devices

Historically, infrared (IR) detector technologies are connected mainly with controlling and night-vision problems: in a first stage, applications concerned simply with detection of IR radiation, but very soon capabilities to form IR images were developed, opening the way to systems for recognition and surveillance, especially for military purposes. Since the last decade of the twentieth century, the use of IR imaging systems for civil and peaceful purposes have increased continuously: these include medical and industrial applications, detection of earth resources, earth and universe sciences, etc. As an example, IR imaging is widely used in astronomy, to study interstellar medium and first-stages of stellar evolution; in medicine, IR thermography – IR imaging of the human body – is employed to detect cancers or other trauma; IR detectors are also widely used in automotive industry, chemical process monitoring, global monitoring of environmental pollution and climate changes, etc. The discovery in 1959 by Lawson and co-workers of the wide tunability of the HgCdTe alloy allowed this compound to become one of the most important and versatile materials for detector applications over the entire IR range. A critical contribution to research is given by Technology Computer-Aided Design (TCAD), modeling and simulation. In the first part of this thesis, I present the main part of my research activity, focused on the development of abilities and methodologies for the simulation of realistic three-dimensional HgCdTe-based infrared photodetectors. The purpose is the investigation of generation-recombination (GR) mechanisms and modeling of spectral photoresponse in narrow-gap HgCdTe-based photodetectors, with one-, two and three-dimensional (1D, 2D, 3D) realistic TCAD models (Chapters 1-5). Another important topic of industrial research in semiconductor physics deals with nitride-based light-emitting diodes (LEDs). From automotive to streetlights, from lights in our houses to the displays of TVs and smartphones, LED-based technology is making its way in the market. This proliferation would have been impossible without GaN-based LEDs, whose invention by Isamu Akasaki, Hiroshi Amano and Shuji Nakamura has been rewarded with the 2014 Nobel Prize in Physics. Nevertheless, GaN-based LEDs performanceis limited by a reduction (droop) of their internal quantum efficiency (IQE) as the driving current density is increased beyond 10 A/cm2, whose physical origin is still under intense debate. In the second part of this thesis, I present a quantum model, based on condensed matter many-body theory, that allowed to obtain the electron capture time and hot-electron intraband relaxation times in a quantum well (QW)-barrier heterostructure, for longitudinal optic (LO) phonon emission, as function of carrier density. The interaction was described in the Single Plasmon Pole of the Random Phase Approximation, retaining the full density-, energy- and wavevector-dependent form of the dielectric function (Chapters 6-7)

One-dimensional carbon nanostructures for terahertz electron-beam radiation

One-dimensional carbon nanostructures such as nanotubes and nanoribbons can feature near-ballistic electronic transport over micron-scale distances even at room temperature. As a result, these materials provide a uniquely suited solid-state platform for radiation mechanisms that so far have been the exclusive domain of electron beams in vacuum. Here we consider the generation of terahertz light based on two such mechanisms, namely, the emission of cyclotronlike radiation in a sinusoidally corrugated nanowire (where periodic angular motion is produced by the mechanical corrugation rather than an externally applied magnetic field), and the Smith-Purcell effect in a rectilinear nanowire over a dielectric grating. In both cases, the radiation properties of the individual charge carriers are investigated via full-wave electrodynamic simulations, including dephasing effects caused by carrier collisions. The overall light output is then computed with a standard model of charge transport for two particularly suitable types of carbon nanostructures, i.e., zigzag graphene nanoribbons and armchair single-wall nanotubes. Relatively sharp emission peaks at geometrically tunable terahertz frequencies are obtained in each case. The corresponding output powers are experimentally accessible even with individual nanowires, and can be scaled to technologically significant levels using array configurations. These radiation mechanisms therefore represent a promising paradigm for light emission in condensed matter, which may find important applications in nanoelectronics and terahertz photonics.DMR-1308659/National Science Foundationhttp://ultra.bu.edu/papers/Tantiwanichapan-2016-PRB-CNT-THz.pd

Wide-field Magnetic Field and Temperature Imaging using Nanoscale Quantum Sensors

The simultaneous imaging of magnetic fields and temperature (MT) is important in a range of applications, including studies of carrier transport, solid-state material dynamics, and semiconductor device characterization. Techniques exist for separately measuring temperature (e.g., infrared (IR) microscopy, micro-Raman spectroscopy, and thermo-reflectance microscopy) and magnetic fields (e.g., scanning probe magnetic force microscopy and superconducting quantum interference devices). However, these techniques cannot measure magnetic fields and temperature simultaneously. Here, we use the exceptional temperature and magnetic field sensitivity of nitrogen vacancy (NV) spins in conformally-coated nanodiamonds to realize simultaneous wide-field MT imaging. Our "quantum conformally-attached thermo-magnetic" (Q-CAT) imaging enables (i) wide-field, high-frame-rate imaging (100 - 1000 Hz); (ii) high sensitivity; and (iii) compatibility with standard microscopes. We apply this technique to study the industrially important problem of characterizing multifinger gallium nitride high-electron-mobility transistors (GaN HEMTs). We spatially and temporally resolve the electric current distribution and resulting temperature rise, elucidating functional device behavior at the microscopic level. The general applicability of Q-CAT imaging serves as an important tool for understanding complex MT phenomena in material science, device physics, and related fields

Spatial mapping of band bending in semiconductor devices using in-situ quantum sensors

Band bending is a central concept in solid-state physics that arises from local variations in charge distribution especially near semiconductor interfaces and surfaces. Its precision measurement is vital in a variety of contexts from the optimisation of field effect transistors to the engineering of qubit devices with enhanced stability and coherence. Existing methods are surface sensitive and are unable to probe band bending at depth from surface or bulk charges related to crystal defects. Here we propose an in-situ method for probing band bending in a semiconductor device by imaging an array of atomic-sized quantum sensing defects to report on the local electric field. We implement the concept using the nitrogen-vacancy centre in diamond, and map the electric field at different depths under various surface terminations. We then fabricate a two-terminal device based on the conductive two-dimensional hole gas formed at a hydrogen-terminated diamond surface, and observe an unexpected spatial modulation of the electric field attributed to a complex interplay between charge injection and photo-ionisation effects. Our method opens the way to three-dimensional mapping of band bending in diamond and other semiconductors hosting suitable quantum sensors, combined with simultaneous imaging of charge transport in complex operating devices.Comment: This is a pre-print of an article published in Nature Electronics. The final authenticated version is available online at https://dx.doi.org/10.1038/s41928-018-0130-