Acceptors optical ionization energy

Beinikhes et al. [18], with a calculated energy of 2.71 meV. The average calculated value of the energy of the excited state of line 14 (2.69 meV) is added to its measured position to obtain the optical ionization energy E-10 of the group-III acceptors in silicon. The excited state of line 11, identified as 7T8-, has been used before as the reference level to obtain Em [132], but in most boron spectra, this line is rather close to the 3p- -i(P) line of residual P while line 14 is well isolated and more intense. 

Table 7.15. Low-resolution positions (meV) of lines of the neutral groupll acceptors and of Cu° in germanium near 7K. When given, the positions of the thermalized lines are the upper ones. The optical ionization energy reported for Au° is 0.21 eV [89]...

Scott and Schmit [8.78] have reported a careful study of the infrared properties of thallium-doped Si, doped as highly as 5 x 10 atoms/cm thallium acts as an acceptor. They measured an infrared ionization energy of0.246 eV, corresponding to a long-wavelength cutoff = 5.0 pm, well suited for 3-5 pm detection. They also estimated a peak optical cross section of 2.6 x 10 cm for thallium in Si. Later work by Brotherton and Gill [8.79] verified the 0.24 eV ionization energy of thallium by means of thermal emission rate measurements. 

Chemical properties of deposited monolayers have been studied in various ways. The degree of ionization of a substituted coumarin film deposited on quartz was determined as a function of the pH of a solution in contact with the film, from which comparison with Gouy-Chapman theory (see Section V-2) could be made [151]. Several studies have been made of the UV-induced polymerization of monolayers (as well as of multilayers) of diacetylene amphiphiles (see Refs. 168, 169). Excitation energy transfer has been observed in a mixed monolayer of donor and acceptor molecules in stearic acid [170]. Electrical properties have been of interest, particularly the possibility that a suitably asymmetric film might be a unidirectional conductor, that is, a rectifier (see Refs. 171, 172). Optical properties of interest include the ability to make planar optical waveguides of thick LB films [173, 174]. 

An expression for the internal space-charge field can be obtained through the Kukhtarev model [38] that was developed to describe photorefractivity in most inorganic materials. In this model, the photorefractive material is described by a band model. As for a traditional semiconductor, the material consists of a conduction and a valence bands separated by a band gap as shown in Fig. 13. The model describes the transport of single carrier species and the band gap of the material contains localized energy levels that can be excited optically promoting either holes in the valence band (VB) or electrons in the conduction band (CB). In the model that we adopt here, we assume that the dopant is a donor with an energy level located in the band gap with concentration N. Furthermore, the crystal contains Nj acceptors with that are all ionized and that have accepted a... 

In order to facilitate efficient device action, donor and acceptor polymers must possess compatible optical, electronic, and physical properties. Efficient exciton dissociation requires sufficient differences in the electron affinity and ionization potential of the paired polymers. As a rule of thumb, a 0.4 eV difference in the lowest unoccupied molecular orbitals (LUMOs) is typically required to drive dissociation of excitons generated in the donor phase via electron transfer (see Figure 14.2 for a schematic energy level diagram). A similar offset in the highest occupied molecular orbitals (HOMOs) is required to drive dissociation of excitons generated in the acceptor phase via hole transfer. Note that the assignment of donor and... 

In semiconductors doped with donor or acceptor atoms, optical transitions between discrete levels of these atoms and conduction or valence band are possible. Since these levels are within the band gap, close to the conduction band or valence band, respectively, this extrinsic photoexcitation is already possilbe at low photon energies (Fig.4.77b). Because of these low excitation energies the donor or acceptor levels can easily be ionized by thermal excitation which would decrease the population density and therefore the absorption coefficient. Such extrinsic photoconductors which are sensitive in the infrared up to wavelengths of about 30 ym, have to be operated at low temperatures to prevent thermal ionization. The absorption coef-... 

Why is the photoinduced electron transfer so strongly inhibited in the polydiacetylenes The energetics of the donor and acceptor components and the relative positions of the various energy levels are important parameters of the photoinduced electron transfer mechanism. The ionization potentials of the polydiacetylenes are around 5.5 eV [170], nearly identical to the ionization potentials of polythiophenes (5.2 eV [171]) and very close to those of the PPVs (5.11 eV [171]). The optical absorption gaps are comparable. Thus, the extraordinary difference— ultrafast photoinduced electron transfer in the PPVs and P3ATs and the complete inhibition of photoinduced electron transfer in the PDAs— must have its origin in the photophysics of the polydiacetylene. 

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