Band-gap radiation

The role of surface or near-surface electrons was demonstrated in the work of Ying and Ho for the band-gap radiation induced desorption of NO from Si(lll). Piltered Xe arc lamp light was used to tune the photodesorption wavelength across the first direct band gap of Si(l 11), around 400 nm. The loss of adsorbate was monitored both by mass spectroscopic detection of the... 

Changes in intensity of semiconductor PL or EL can be used to detect molecular adsorption onto semiconductor surfaces [1,3]. PL occurs most efficiently when ultra-band-gap radiation excites electrons from the valence band to the conduction band of a direct-band-gap semiconductor and the electrons recombine radiatively with the holes left behind in the valence band. 

In the compensation process, there is only a change in the charge state of the impurity or dopant atom and it is temporarily reversible, for instance by illumination of the crystal with band-gap or above-band-gap radiation, which produces electrons and holes that are trapped by the ionized centres. This is a non-equilibrium condition, which exists only during illumination. 

When a semiconductor is illuminated with the band-gap radiation, excess electrons and holes are photo-created. They can form free excitons or be trapped by ionized impurities, but their ultimate fate is their annihilation by thermal or radiative recombination. The formation of free excitons will be discussed in Sect. 3.3.2, but in direct band-gap semiconductors, electron-hole radiative recombination can also occur at an energy close to Eg if the pumping beam is kept at a low level. This can provide an accurate determination of Eg [87],... 

Illuminating the sample with a band gap radiation allows detection of lines of neutralized minority impurities as negative peaks. 

Fig. 6.9. Absorption spectrum between 5 and 11.2 me V of a Ge sample with [Sb] = 8 x 1014 cm-3 at a temperature between 1.7 and 4.2 K. The spectral resolution is 0.14 cm-1 ( 17peV). The lines with indexes (1) and (3) originate from the Is (Ai) and Is (T2) levels, respectively. Band gap radiation reaching the sample explains the sharpness of the lines and the observation of line D of the B acceptor. [15] Copyright 1997, with permission from World Scientific Publishing Co. Pte. Ltd, Singapore...

Fig. 7.1. (a) Absorption spectrum at LHeT of 3/2(6) between 234 and 363 cm-1 in a natSi sample with [B] = 8.5 x 1014 cm-3. P donor lines are also observed because band-gap radiation reaches the sample, (b) Lines 1, 2 and 4, 5, 6 of B on an expanded scale in the same sample with a reduced thickness [82]. Numbering 1 is given in parentheses. For the attributions, see Table 7.2. Copyright 1981 by the American Physical Society... 

Silicon is closer to carbon, and germanium is closer to lead, in the periodic table. We expect the band gap in silicon to be larger than that in germanium, so it makes sense that lower-energy radiation can generate electrical conductivity in germanium than in silicon. 

Colloids of a-FejOj are made by hydrolysis of FeClj and subsequent dialysis of the sol. Polyvinyl alcohol is often used as a stabilizing agent. The band gap in Fe203 is 2.2 eV. In some of the studies on colloidal Fc203 free radicals were generated by ionizing radiation and electron transfer reactions with the colloidal particles investigated. Buxton et al. observed a cathodic dissolution of a-FCjOj in acidic... 

The band-gap excitation of semiconductor electrodes brings two practical problems for photoelectrochemical solar energy conversion (1) Most of the useful semiconductors have relatively wide band gaps, hence they can be excited only by ultraviolet radiation, whose proportion in the solar spectrum is rather low. (2) the photogenerated minority charge carriers in these semiconductors possess a high oxidative or reductive power to cause a rapid photocorrosion. 

In addition to the generation of platelets, hydrogenation of silicon also induces electronic deep levels in the band gap. As in the case of platelet formation, these defects are considered to be unrelated to either plasma or radiation damage because they can be introduced with a remote hydrogen plasma. Comparison of depth distributions and annealing kinetics of the platelets and gap states has been used to a limited extent to probe the relationship among these manifestations of H-induced defects. 

In Chapter 5.4, optical ultraviolet radiation sensors are described, including UV-enhanced silicon-based pn diodes, detectors made from other wide band gap materials in crystalline or polycrystalline form, the latter being a new, less costly alternative. Other domestic applications are personal UV exposure dosimetry, surveillance of sun beds, flame scanning in gas and oil burners, fire alarm monitors and water sterilization equipment surveillance. 

For low-intensity applications, a concentrator lens can be used to focus the radiation onto the detector surface. The detector area can be kept small, which is advantageous for the use of expensive wide-band gap UV-photodiodes. The lens must be made of quartz or another UV-transmitting glass and is often used as the entrance window of a standard TO housing. A lens limits the angle of acceptance to usually less than 10 degrees. 

Chapter 4, presents details of the absorption and reflectivity spectra of pure crystals. The first part of this chapter coimects the optical magnimdes that can be measured by spectrophotometers with the dielectric constant. We then consider how the valence electrons of the solid units (atoms or ions) respond to the electromagnetic field of the optical radiation. This establishes a frequency dependence of the dielectric constant, so that the absorption and reflectivity spectrum (the transparency) of a solid can be predicted. The last part of this chapter focuses on the main features of the spectra associated with metals, insulators, and semiconductors. The absorption edge and excitonic structure of band gap (semiconductors or insulator) materials are also treated. 

Figure 2, Plots of the efficiencies tje, rjY, rfp, and -qc as a function of the wavelength Xg corresponding to the band gap E. The distributions have been calculated for AM 1.2 solar radiation (taken from distribution T/S of Ref. 6). Curves, E, Y, P, and C are plots of -qEy my VPy rjc as defined in Equations 3,8,12, and 16, respectively. Tfc has been calculated for 0.6, 0.8, and 1.0 eV energy loss, respectively, as...

In research laboratories, different types of light sources are used instead of solar radiation. In most cases the simulated spectrums have considerable deviation from the solar spectrum. Based on equation (3.6.9) Murphy et al [109] analyzed the maximum possible efficiencies for different materials according to their band gap in the case of solar global AM 1.5 illumination and xenon arc lamp, see Fig. 3.21. For example, anatase titania with a bandgap of 3.2 eV has a maximum possible efficiency of 1.3% under AM 1.5 illumination, and 1.7% using Xe lamp without any filter. For rutile titania these values are 2.2% and 2.3% respectively. 

There have been many investigations of photoinduced effects in -Si H films linked to material parameters. Changes have been observed in the carrier diffusion length, unpaired spin density, density of states in the gap, and infrared transmission. The transition from state A to B seems to be induced by any process that creates free carriers, including x-ray radiation and injection (double) from the electrodes. Because degradation in a solar cell is accentuated at the open-circuit voltage conditions, the A to B transition occurs upon recombination of excess free carriers in which the eneigy involved is less than the band gap. It has been pointed out that this transition is a relatively inefficient one and the increase in spin density takes place at a rate of 10-8 spins per absorbed photon. 

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