Fundamental absorption edge

These aspects of the optical spectra of solids are illustrated in the upper portion of Figure 1, which displays the reflectance curve (R) at room temperature for a typical semiconductor, GaAs. The fundamental absorption edge around 1.4 eV produces only a weak shoulder. Some structure is apparent in the two features around 3 eV and the large, broad peak near 5 eV. However, the dominant aspect of the line shape is the slowly varyii background. The derivative namre of Modulation Spectroscopy suppresses the uninteresting background effects in favor of sharp, deriva-... 

Fischer R (1972) Absorption and electroabsorption of trigonal selenium near the fundamental absorption edge. Phys Rev B 5 3087-3094... 

Si02 (a typical insulator) has a strong absorption rise (called the fundamental absorption edge) in the UV and it is transparent in the visible. 

Several III-V semiconductors, such as ALP, GaAs, luSb, AlAs, and InAs, show direct absorption edge transitions. The next example shows the analysis of the fundamental absorption edge for indium arsenide. 

Table 4.3 The frequency dependence expected for the fundamental absorption edge of direct- and indirect-gap materials...

It should be noted that the frequency dependence is different to those expected for direct-gap materials, given by Equations (4.33) and (4.34). This provides a convenient way of determining the direct or indirect nature of a band gap in a particular material by simply analyzing the fundamental absorption edge. Table 4.3 summarizes the frequency dependence expected for the fundamental absorption edge of direct- and indirect-gap materials. 

Figure 4.15 The absorption spectra (near the fundamental absorption edge) of sodium chloride and lithium fluoride at 300 K (reproduced with permission from Palik, 1985).

The absorption edge of (Ga,Mn)As is not sharp, as shown in fig. 20 (Kuroiwa et al. 1998 Szczytko et al. 1999b). This is probably due to impurity band formation caused by the high concentration of ionized Mn and compensating donors (Kuroiwa et al. 1998). Even below the fundamental absorption edge, the absorption coefficient is rather large due to free-carrier (Casey et al. 1975) and intra-Mn absorption. There is no report on the observation of exciton states or photoluminescence, which is probably due to non-radiative recombination, carrier screening, and the formation of an impurity band (Ando et al. 1999). 

The fundamental absorption edge in ZnO corresponds to the direct transition from the highest valence band to the lowest conduction band at the F-point of the Brillouin zone [141]. RT-data for the energy of the lowest / -point band-to-band transition are summarized in Table 3.11. 

Table 3.11. RT-data for Eo (lowest T-point band-to-band transitions) and Es (fundamental absorption edge determined from transmission measurements), and E. b of ZnO single crystal bulk samples (b) and ZnO thin films (f)...

Fig. 3.26. Energies of the fundamental absorption edge Es (open symbols) and the lowest E-point band-to-band transition Eq or Eq (filled symbols) of MgxZni xO, as determined by transmission measurements (T) and SE, respectively. Filled squares Refs. [15,16] (SE), filled circles [95] (T), filled diamonds [90] (SE), filled up-triangles [96] (T), filled down-triangles [82] (SE), open circles [11] (T), open diamonds [97] (T), open up-triangles [14] (T), open down-triangles [98] (T), open left-triangles [99] (T), open right-triangles [100] (T). The lines depict numerical approximations according to (3.31) with the parameters in Table 3.16 for Eq (wurtzite-structure Mg Zni- O) and Eq (rocksalt-structure Mg Zni- O)...

One of the main spectroscopic properties that differentiate fluoride glasses from silica-based glasses is the low multiphonon emission rate. These non-radiative relaxations that may strongly compete with radiative processes in rare-earth ions are nearly three orders of magnitude lower in ZBLAN glass than in silicate, as shown in Fig. 2. This property is directly related to the fundamental vibration modes of the host and, therefore, varies basically in the same manner as the infrared absorption edge. 

Results of photoemission studies of polyethylene have shown definite evidence for wide energy bands among deep valence orbitals ( ), but the nature of the fundamental absorption edge has not been resolved. Band structure calculations predict direct interband excitations to occur above 12.6 eV (.8) whereas the absorption threshold is at 7.2 eV and a strong peak in e occurs at 9.0 eV. The momentum dependence of the absorption threshold indicates that the threshold is of excitonic origin, i.e. the excitation is localized by the strong electron-hole or configuration inter-... 

Schoolar, R. B., and J. R. Dixon (1965). Optical constants of lead sulfide in the fundamental absorption edge region. Phys. Rev. 137, A667-70. 

The principles of photoluminescence applied to solid oxide surfaces can be most easily understood by assuming some simplifications. For example, we can start by considering the Morse potential energy curves (Fig. 1) related to an ion pair such as M-+0-, taken as a harmonic oscillator to represent an oxide, typically an alkaline earth oxide. The absorption of light close to the fundamental absorption edge of this oxide leads to the excitation of an electron in the oxide ion followed by a charge-transfer process to create an exciton (an electron-hole pair), which is essentially... 

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