Low-energy Bias Research Articles

How spectroscopic x-ray imaging benefits from inter-pixel communication

Spectroscopic x-ray imaging based on pixellated semiconductor detectors can be sensitive to charge sharing and K-fluorescence, depending on the sensor material used, its thickness and the pixel pitch employed. As a consequence, spectroscopic resolution is partially lost. In this paper, we study a new detector ASIC, the Medipix3RX, that offers a novel feature called charge summing, which is established by making adjacent pixels communicate with each other. Consequently, single photon interactions resulting in multiple hits are almost completely avoided. We investigate this charge summing mode with respect to those of its imaging properties that are of interest in medical physics and benchmark them against the case without charge summing. In particular, we review its influence on spectroscopic resolution and find that the low energy bias normally present when recording energy spectra is dramatically reduced. Furthermore, we show that charge summing provides a modulation transfer function which is almost independent of the energy threshold setting, which is in contrast to approaches common so far. We demonstrate that this property is directly linked to the detective quantum efficiency, which is found to increase by a factor of three or more when the energy threshold approaches the photon energy and when using charge summing. As a consequence, the contrast-to-noise ratio is found to double at elevated threshold levels and the dynamic range increases for a given counter depth. All these effects are shown to lead to an improved ability to perform material discrimination in spectroscopic CT, using iodine and gadolinium contrast agents. Hence, when compared to conventional photon counting detectors, these benefits carry the potential of substantially reducing the imaging dose a patient is exposed to during diagnostic CT examinations.

Tuning the surface plasmon resonance of silver nanoclusters by oxygen exposure and low-energy plasma annealing

Real-time surface differential reflectance spectroscopy in the visible range is used to study the optical response of silver nanoclusters, prepared by magnetron sputtering deposition, during cyclic treatments in different oxygen atmospheres and low-energy bias argon plasma. Changes in the reflectance show that the exposure to non-ionized (or partially ionized) oxygen causes a red-shift and damping (or complete vanishing) of the resonance, while bias plasma annealing induces the opposite effects, due to oxygen desorption and structural reshaping of the nanoclusters. These results open up new opportunities for developing plasmon-based devices with high tunability of the surface plasmon resonance (energy, width and amplitude) due to an interplay between morphological and chemical modifications of the nanoclusters.

Study on further reducing the epitaxial silicon temperature down to 250 °C in low-energy bias sputtering

The epitaxial silicon growth temperature has been reduced to as low as 250 °C by optimizing the ion bombardment condition in low-energy bias sputtering process. Independent and precise control of ion bombardment energy and ion flux density incident on a growing film surface is most essential to realize very low temperature epitaxy. It has been experimentally shown that the deficit in energy for epitaxial growth by reducing the substrate temperature is able to be compensated for by increasing the total energy dose on a film surface by low-energy ion bombardment. Increase in the impact energy of individual ions, however, results in the formation of high-density defects in the film. Therefore, the right direction to enhance the total energy deposition by ion bombardment is to increase the ion flux density while keeping the ion impact energy at an optimum value. As a result of such optimization, epitaxial growth of silicon has become possible at a temperature as low as 250 °C. The etch pit density in this low-temperature growth film is less than 3×103 cm−2, the detection limit of our experiments. The carbon and oxygen concentrations in a grown film as measured by secondary ion mass spectroscopy increase with the background pressure, and these impurity concentrations are correlated to the crystallinity of the film. From these observations, the profound effect of contamination on the reduction of silicon epitaxial temperature is demonstrated.

Characterisation of epitaxial CdTe/InSb heterostructures prepared by magnetron sputtering

Heteroepitaxial (100)CdTe//(100)InSb structures with chemically abrupt and morphologically smooth interfaces have been grown on (100)GaAs by a combination of metalorganic magnetron sputtering for InSb and low-energy bias sputtering for CdTe. The range of growth conditions under which high-quality heterostructures can be fabricated, however, seems to be quite restricted.

Preparation of Cross-Sectional TEM Samples of PbTe and CdxPb1−x Te on BaF2

ABSTRACTEpitaxial PbTe and CdxPb1−xTe films have been grown on single crystal (111) BaF2 by low energy bias sputtering, and have been analyzed by transmission electron microscopy (TEM) and transmission electron diffraction (TED). Preparation of suitable cross-sectional TEM samples was made difficult by the tendency of the substrate to cleave apart during dimpling, and by the epoxy forming bridges across the sample during atom milling. Suitable preparation techniques were developed employing back-polishing the BaF2 substrates to <0.2 mm thickness, using a suitable epoxy, and shielding the argon atom beam during milling to prevent milling parallel to the surface. In cases where an epoxy bridge did form across the sample, the bridge was broken manually or by atom milling, depending upon the area of sample which was being investigated. These techniques are applicable to other materials which produce similar problems during TEM sample preparation.

Tem, Raman, and Langmuir Probe Studies of the Heteroepitaxial Growth of Metastable Cd(x)pb (1–X) Te Alloys Deposited on BaF2 by Low-Energy Bias Sputtering

AbstractMetastable Cd(x)Pb(1–x)Te films with x values from 0 to 0.47, well past the range of bulk thermodynamic solubility, have been grown on single crystal (lll) BaF2 by co-deposition from CdTe and PbTe r.f. magnetron sputter targets. Cross-sectional transmission electron microscopy and transmission electron diffraction revealed epitaxial growth across the interface. However, the lattice of the deposited epilayers was observed to be typically rotated 180°C about the surface normal <111> axis of the substrate. Raman spectra of the alloys showed no evidence of segregation. Langmuir probe diagnostics were employed to estimate the energy of the ions incident on the substrate during growth which promote extended miscibility in the alloy epilayers.

Raman and Secondary Ion Mass Spectroscopy of Epitaxial CdTe/InSb Interfaces Grown by Low-Energy Bias Sputtering

ABSTRACTHeteroepitaxial (100)CdTe || (100)InSb structures have been fabricated by growing CdTe epilayers, at growth temperatures below 200°C, on single crystal InSb substrates by low-energy bias sputtering. Controlled low-energy ion bombardment at the substrate was employed to clean the growth surface in-situ just prior to film deposition and to modify the growth kinetics and enhance adatom mobility during deposition. Raman spectroscopy of the interface revealed no evidence of In2Te3 and secondary ion mass spectroscopy showed the interface to be chemically abrupt.

Formation of device-grade epitaxial silicon films at extremely low temperatures by low-energy bias sputtering

Device-grade epitaxial single silicon layers have been formed at extremely low temperatures of about 300 °C by low-energy bias sputtering in conjunction with in situ substrate surface cleaning and an ultraclean processing environment. Dopant impurities in the target material are fully incorporated into the sputter-deposited silicon film and these impurities are 100% electrically activated without any additional heat cycles. An epitaxial silicon film having a resistivity as low as 0.0014 Ω cm has been obtained using a heavily arsenic-doped silicon block as a target material. A p-n junction diode formed by directly depositing a phosphorus-doped n-type epilayer on a p-type substrate indicates a reverse current level as low as 1.88×10−9 A/cm2 at a reverse bias voltage of 5 V, thus verifying that the in situ substrate surface cleaning by low-energy Ar ion bombardment is very effective and damage free. The electrical characteristics of a grown film have shown a good correlation to the crystal structure of the film, which is primarily determined by the energy of ions concurrently bombarding the growing film surface.

Low-temperature silicon epitaxy by low-energy bias sputtering

Bias sputtering at low energies, i.e., comparable to typical crystal interatomic binding energies, has been utilized to control the kinetics of thin-film growth. It was found that the crystallographic structures of sputter-deposited silicon films are drastically changed by the energy of ions incident at the substrate. As a result, formation of high quality epitaxial silicon films on (100) silicon substrates has been realized at such low temperatures as 320–350 °C. At the same time, low-temperature impurity doping of the epitaxial layer has been also demonstrated. Furthermore, the low-energy bias sputtering process has made it possible to perform very effective substrate surface cleaning at extremely low temperatures without introducing any damage to the substrate.

Coping with Plastic Scintillators in Nuclear Safeguards

Plastic scintillators offer several advantages for nuclear safeguards research and technology to those who design, assemble, encapsulate, and calibrate detectors from raw materials that are commercially available. These large, inexpensive detectors have good spatial uniformity and good high-energy gamma-ray response. Uniform light collection is obtained with a light pipe attached to a polished scintillator wrapped with aluminum foil. Best low-energy response is obtained by applying a variance analyzer to select the low-energy bias level.

Direct measurement of 14CO 2 in a liquid scintillation counter

Samples of CO 2 are dissolved in a liquid scintillator solution for the measurement of 14C activity with counting efficiencies of greather than 90% above an appropriate low-energy bias. With an integral counting technique, the counting efficiency of (100 ± 2)% was obtained. As much as 5 ml (STP) of CO 2 per ml of scintillator solution produced no measurable quenching.