Effects of cosmic rays on single event upsets

The efforts at establishing a research program in space radiation effects are discussed. The research program has served as the basis for training several graduate students in an area of research that is of importance to NASA. In addition, technical support was provided for the Single Event Facility Group at Brookhaven National Laboratory

Simulation of Ultra-Relativistic Electrons and Positrons Channeling in Crystals with MBN Explorer

A newly developed code, implemented as a part of the \MBNExplorer package \cite{MBN_ExplorerPaper,MBN_ExplorerSite} to simulate trajectories of an ultra-relativistic projectile in a crystalline medium, is presented. The motion of a projectile is treated classically by integrating the relativistic equations of motion with account for the interaction between the projectile and crystal atoms. The probabilistic element is introduced by a random choice of transverse coordinates and velocities of the projectile at the crystal entrance as well as by accounting for the random positions of the atoms due to thermal vibrations. The simulated trajectories are used for numerical analysis of the emitted radiation. Initial approbation and verification of the code have been carried out by simulating the trajectories and calculating the radiation emitted by \E=6.7 GeV and \E=855 MeV electrons and positrons in oriented Si(110) crystal and in amorphous silicon. The calculated spectra are compared with the experimental data and with predictions of the Bethe-Heitler theory for the amorphous environment.Comment: 41 pages, 11 figures. Initially submitted on Dec 29, 2012 to Phys. Rev.

Ultrathin Amorphous Silica Membrane Enhances Proton Transfer across Solid-to-Solid Interfaces of Stacked Metal Oxide Nanolayers while Blocking Oxygen

A large jump of proton transfer rates across solid-to-solid interfaces by inserting an ultrathin amorphous silica layer into stacked metal oxide nanolayers is discovered using electrochemical impedance spectroscopy and Fourier-transform infrared reflection absorption spectroscopy (FT-IRRAS). The triple stacked nanolayers of Co3O4, SiO2, and TiO2 prepared by atomic layer deposition (ALD) enable a proton flux of 2400 ± 60 s−1 nm−2 (pH 4, room temperature), while a single TiO2 (5 nm) layer exhibits a threefold lower flux of 830 s−1 nm−2. Based on FT-IRRAS measurements, this remarkable enhancement is proposed to originate from the sandwiched silica layer forming interfacial SiOTi and SiOCo linkages to TiO2 and Co3O4 nanolayers, respectively, with the O bridges providing fast H+ hopping pathways across the solid-to-solid interfaces. Together with the complete O2 impermeability of a 2 nm ALD-grown SiO2 layer, the high flux for proton transport across multi-stack metal oxide layers opens up the integration of incompatible catalytic environments to form functional nanoscale assemblies such as artificial photosystems for CO2 reduction by H2O

Carrier Drift-Mobilities and Solar Cell Models for Amorphous and Nanocrystalline Silicon

Hole drift mobilities in hydrogenated amorphous silicon (a-Si:H) and nanocrystalline silicon (nc-Si:H) are in the range of 10-3 to 1 cm2/Vs at room-temperature. These low drift mobilities establish corresponding hole mobility limits to the power generation and useful thicknesses of the solar cells. The properties of as-deposited a-Si:H nip solar cells are quite close to their hole mobility limit, but the corresponding limit has not been examined for nc-Si:H solar cells. We explore the predictions for nc-Si:H solar cells based on parameters and values estimated from hole drift-mobility and related measurements. The indicate that the hole mobility limit for nc-Si:H cells corresponds to an optimum intrinsic-layer thickness of 2-3 2m, whereas the best nc-Si:H solar cells (10% conversion efficiency) have thicknesses around 2 2m

Theoretical Modeling of Kinetic Phenomena of Atoms and Charge Carriers in Disordered Materials

This PhD thesis deals with the computer simulation of III/V semiconductor heteroepitaxy and the description of charge transport in disordered inorganic and organic semiconductors

Charge Carrier Transport and Photogeneration at Very High Electric Fields in Amorphous Selenium

The flat-panel digital X-ray detectors (e.g. amorphous selenium, a-Se, based detectors) are replacing the film-based technology in various diagnostic medical imaging modalities such as mammography and chest radiography. Whereas, there is a huge demand for lowering the irradiation dose in various medical imaging modalities, the present flat-panel digital X-ray imaging technology is severely challenged under low dose conditions. To date, amorphous selenium (a-Se) is one of the most highly developed photoconductors used in digital X-ray imaging, which exhibits impact ionization and usuable carrier multiplication. The viability of avalanche multiplication can increase the signal strength and improve the signal to noise ratio for application in low dose medical X-ray imaging detectors. In spite of the interesting outlook of a-Se, some of its fundamental properties are still not fully understood. Specifically, an understanding of carrier transport at extremely high field in a-Se is in a very premature state. Therefore, an extensive research work is vital to clearly understand the fundamental underlying physics of carrier generation, multiplication, and transport mechanisms in a-Se. In this dissertation, a physics-based model is developed to investigate the mechanisms of the electric field and temperature dependent effective drift mobility of holes and electrons and also the impact ionization in a-Se. The models consider the density of states distribution near the band edges, field enhancement release rate from the shallow traps, and carrier heating. The lucky-drift model for a-Se is developed based on the observed field dependent microscopic mobility. The validation of the developed models via comparison with the experimental data verifies the mechanisms behind the electric field and temperature dependent behaviours of impact ionization coefficient in a-Se. The density of state function near the band edges, consisting of an exponential tail and a Gaussian peak, successfully described the electric field and temperature-dependent effective drift mobility characteristics in a-Se. The photogeneration efficiency in a-Se under optical excitation strongly depends on photon wavelength and electric field. A physics-based model is proposed to investigate the physical mechanism of charge carrier photogeneration in a-Se under high electric fields. The exact extension of Onsager theory can explain the photogeneration efficiency in a-Se at extremely high electric field. The mechanism of carrier recombination following X-ray excitation and hence the evaluation of electric field and X-ray photon energy dependent electron-hole pair (EHP) creation energy (amount of energy needed to produce a detectable free EHP upon the absorption of an X-ray photon) in a-Se have been topics of a very vital debate over the last two decades. These issues are addressed in this thesis. Towards this end, a physics-based analytical model is developed via incorporating a few valid assumptions to study the initial recombination mechanisms of X-ray generated EHPs in a-Se. The analytical model is later verified by a full phase numerical model, considering three-dimensional coupled continuity equations of electrons and holes under carrier drift, diffusion and bimolecular recombination. The corresponsding calculations of EHP creation energy with wide variations of X-ray energy, electric field and temperature are verified with respect to the available published experimental data. According to this, it is found that the columnar recombination model is capable of describing the electric field, temperature and photon energy dependent EHP creation energy in a-Se for high-energy photons. The theoretical work of this thesis unveil the physics of the charge carrier transport and photogeneration mechanisms in a-Se at very high electric fields, which is vital to optimum design of avalanche a-Se detectors. This work will also provide a guideline for further improvement of the radiation imaging detectors

Growth and characterization of polysilicon films deposited by reactive plasma beam epitaxy

Polycrystalline silicon was deposited at low temperatures (400-550°C) by using a new technique, reactive plasma beam epitaxy. The technique consists of using an intense flux of hydrogen radicals, generated by an electron cyclotron resonance (ECR) source to promote nucleation and crystallinity at low temperatures. The film structure could be smoothly changed from polysilicon to amorphous by changing the flux of incoming H radicals on the surface of the sample. The flux of hydrogen radicals can be controlled by changing the deposition pressure, with lower pressures leading to a higher H radical flux and greater degree of crystallinity. The effect of H radicals on the growth of polysilicon films was studied by using two different plasma characterizations techniques, namely Langmuir probe and optical emission spectroscopy. Both n and p type films were deposited by using a phosphine and diborane mixture in hydrogen. The growth rate is found to be independent of the doping or of the deposition temperature. The crystalline nature of the films was verified by using Raman spectroscopy. The crystallographic orientations of the films were examined by x-ray diffraction. The grain size of the polysilicon films was estimated by using the full width at half maximum of x-ray diffraction spectra, and were of the order of 200A. There was no significant change in either the grain size or the orientation of the films as the deposition temperature was increased from 400 to 550°C. Surface morphology of the films was studied by using scanning electron microscope and ultraviolet reflectance measurements. The electronic properties of the polysilicon films were examined by conductivity, activation energy and Hall mobility measurements. The results of these characterization techniques indicate that the as deposited films are slightly n type, and they have high mobilities (40 cm[superscript]2/v.s)

Nature of intrinsic and extrinsic electron trapping in SiO 2

Using classical and ab initio calculations we demonstrate that extra electrons can be trapped in pure crystalline and amorphous SiO2 (a-SiO2) in deep band gap states. The structure of trapped electron sites in pure a-SiO2 is similar to that of Ge electron centers and so-called [SiO4/Li]0 centers in α quartz. Classical potentials were used to generate amorphous silica models and density functional theory to characterize the geometrical and electronic structures of trapped electrons in crystalline and amorphous silica. The calculations demonstrate that an extra electron can be trapped at a Ge impurity in α quartz in six different configurations. An electron in the [SiO4/Li]0 center is trapped on a regular Si ion with the Li ion residing nearby. Extra electrons can trap spontaneously on pre-existing structural precursors in amorphous SiO2, while the electron self-trapping in α quartz requires overcoming a barrier of about 0.6 eV. The precursors for electron trapping in amorphous SiO2 comprise wide (≥132∘) O–Si–O angles and elongated Si–O bonds at the tails of corresponding distributions. Using this criterion, we estimate the concentration of these electron trapping sites at ≈4×1019 cm−3