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Fundamentals of Optical Detection
Published in Antoni Rogalski, Zbigniew Bielecki, Detection of Optical Signals, 2022
Antoni Rogalski, Zbigniew Bielecki
The light wavelength range over which the device is sensitive depends on a material used for the photoemissive cathode. Metals are characterised by a large work function in the 4–5 eV range. Only high-energy ultraviolet radiation can release electrons from metal photocathodes. Most photocathodes are made of a compound semiconductor consisting mainly of alkali metals with a low work function. By changing the photocathode materials, it is possible to achieve sensitivity in various wavelengths from soft X-rays to near IR-radiation. In order to increase the long-term limit of sensitivity, the cathode surface is covered with cesium, which has a lower work function (1.92 eV); therefore, the photocathode is more sensitive in the visible range. Gas-filled devices are more sensitive, but frequency response to a modulated illumination falls off at lower frequencies compared to the vacuum devices.
Historical introduction and survey
Published in Rolf Behling, Modern Diagnostic X-Ray Sources, 2021
Ion tubes were still improved, too. Reportedly, there were instances of cold cathode ion tubes employed as late as the 1960s. In the meantime, thermionic cathodes with tungsten emitters totally superseded ionized gas as the electron source (see Behling, 2020a). In spite of many attempts to lower the temperature of the electron emitter by material with lower work function, tungsten has remained the material of choice in the harsh environment of a sealed X-ray tube. The work function characterizes the energy supply required for electrons to leave the metal. Barium and thorium and their oxides were tried. These and materials with similar effect are in use for klystrons, switching and amplifier tubes, etc. (see Gaertner, 2012, 2020; Gaertner & Koops, 2008). (Field emission cathodes have captured some niche space for stationary anode diagnostic X-ray tubes, as discussed in Chapter 6.2.1.11.2.2.) Simplification of the workflow revolutionized professions. Before, the selection of the right ion tubes and adjustment of the technique factors required high technical skills. Radiographers were both surgeons and physicists in medicine. The inventions of Morton, Lilienfeld, and Coolidge marked the separation between radiologists, medical physicists, and engineers.
Detectors
Published in C. R. Kitchin, Astrophysical Techniques, 2020
The minimum energy of a photon, if it is to be able to produce photoemission, is known as the ‘work function’ and is the difference between the ionisation level and the top of the valence band (Figure 1.4). Its value may, however, be increased by some or all of the energy-loss mechanisms mentioned previously. In practical photoemitters pair production is particularly important at photon energies above the minimum and may reduce the expected flux considerably as the wavelength decreases. The work function is also strongly dependent upon the surface properties of the material; surface defects, oxidation products and impurities can cause it to vary widely even amongst samples of the same substance. The work function may be reduced if the material is strongly p-type and is at an elevated temperature. Vacant levels in the valence band may then be populated by thermally excited electrons, bringing them nearer to the ionisation level and so reducing the energy required to let them escape. Since most practical photoemitters are strongly p-type, this is an important process and confers sensitivity at longer wavelengths than the nominal cut-off point. The long-wave sensitivity, however, is variable because the degree to which the substance is p-type is strongly dependent upon the presence of impurities and so is sensitive to small changes in the composition of the material.
Analytical threshold voltage and subthreshold swing model for TMG FinFET
Published in International Journal of Electronics, 2019
Rajesh Saha, Brinda Bhowmick, Srimanta Baishya
The 3D TMG FinFET shown in Figure 1(a) is separated into asymmetric and symmetric TMDG MOSFET as shown in Figure 1(b). The gate consists of three metals (M1, M2 and M3) of different lengths (L1, L2 and L3) and work functions (ϕM1, ϕM2 and ϕM3). Unless otherwise mentioned, we have used ϕM1 = 4.77 eV, ϕM2 = 4.4 eV and ϕM3 = 4.1 eV. The materials cobalt (Co), tungsten (W) and aluminium (Al) have the work function values of 4.77, 4.4, and 4.1 eV, respectively. Both the n+ source and n+ drain regions are doped with arsenic with a concentration of 1020 cm−3. The channel is uniformly doped with Boron having a concentration of 1015 cm−3. SiO2 is used as gate dielectric as well as buried oxide. The other dimensions like the thickness of fin, height of fin, gate oxide and buried oxide are indicated by Tsi, Hfin, tox and toxb, respectively, and the coordinate axis is shown in Figure 1.
Detection of CNX cyanogen halides (X = F, Cl) on metal-free defective phosphorene sensor: periodic DFT calculations
Published in Molecular Physics, 2021
Mahdi Ghadiri, Mehdi Ghambarian, Mohammad Ghashghaee
Work function alterations are essential as well. Such alterations determine whether the material can be utilised as a work function sensor. The work function value was obtained from the definition ϕ = Evac − EF in which EF represents the Fermi level (eV), and Evac denotes the vacuum level (eV). The obtained data have been shown in Figure 3. The work function declined after the adsorption of the cyanogen halide molecules with the work function sensitivity sequenced as DP-CNF < DP-CNCl. This order indicates that this type of sensitivity has also been higher in the case of CNCl sensing on the DP sensor (–13.4%).
Experimental studies of electron affinity and work function from titanium on oxidised diamond (100) surfaces
Published in Functional Diamond, 2022
Fabian Fogarty, Neil A. Fox, Paul W. May
In order for an electron to be emitted from a metal surface, it must possess sufficient kinetic energy to overcome the work function (WF) situated at the surface–vacuum interface. The work function, ϕ, is defined as the energy difference between the Fermi level, EF, and that of the vacuum level. In semiconductors, electrons that reside in the valence band (VB) require additional energy equivalent to that of the band gap to first excite them into the conduction band (CB) before they can be emitted. ϕ typically has vales of a few eV for most metals and semiconductors. Therefore, high-energy ultraviolet photons or temperatures >1500 K are usually required to provide sufficient energy for electron emission. For some semiconductors and insulators, however, which include diamond [8], cubic boron nitride [9], and AlN and AlGaN [10], the work function is greatly reduced because, in these unusual cases, the CB minimum lies higher in energy than the vacuum level. This condition is known as NEA. Here, electrons located in the CB experience no emission barrier to escape the surface. Bulk electrons residing in the VB, or in mid-band-gap states because of doping, only require prior excitation (via photon absorption, thermalisation or electric fields) into the CB for emission to take place. Consequently, NEA materials are highly desirable for next-generation electron-emission applications. Because these are all wide band-gap materials, the advantages of NEA might be outweighed by the high energies needed to excite electrons from the VB into the CB. However, this problem can be reduced if the NEA is sufficiently large and negative, i.e. has a value approaching that of the band gap, and also by using suitable doping strategies, especially n-type, that raise the Fermi level and decrease the effective band gap [2].