Photoconduction excitation spectrum

A principal obstacle to identification of defects is the difficulty of comparing the results from EPR, luminescence, absorption, and deep state experiments. Probably the least ambiguous is that between EPR and luminescence when, as for transition metal impurities, it is possible for optical Zeeman measurements of a sharp luminescence line to determine the ground state g factor. If the optical and EPR measurements give the same value, then the correlation is made (Watts, 1977). In some cases, when optical excitation enhances or quenches the EPR signal, there may be a similar response in the photoconductivity or luminescence excitation spectrum. 

The excitation energy E T ) of the triplet excitons is e.g. in the naphthalene crystal larger than a typical trap depth. An exciton is therefore energetically able to empty a charge-carrier trap and thus to increase the current. Figure 8.43 shows the excitation spectrum of photoconduction of a high-purity naphthalene crystal whose traps were previously filled at 77 K. Here, the forbidden 0,0 transition at 21208 cm" (A = 471.5 nm) and the vibronic series of the Ti So transition in the naphthalene crystal were observed [44]. 

The photogeneration of charged excitations in oligothiophene thin films was only studied for a-6T in which the threshold for photoexcitation of charges has been located at around 2.2 eV by a comparison of the one photon excitation curve for radiative recombination and the photoconduction action spectrum [257]. 

The basic instnimentation required for acquiring photoluminescence excitation (PLE) spectrum ofa given PL band is nearly the same as that for a PLsetup. However, the excitation source must be a tunable source such as a tunable laser or a broadband lamp dispersed by a monochromator. The wavelength of the excitation source is varied, and the PL spectrum or simply the intensity of a particular transition (such as the peak PL intensity) is recorded at various excitation wavelengths to obtain the excitation spectrum. The PLE spectrum is similar to the absorption spectrum with the only difference that in the case of absorption spectrum several different transitions may contribute and complicate the spectral analysis. Photoionization of a defect is an inverse process to the luminescence, and in n-type ZnO such a process involves the transition of an electron from an acceptor-like level to the conduction band or to the excited state of the defect. Note that the photoionization spectra measured by PLE, absorption, photocapacitance, and photoconductivity methods should have more or less similar features because the mechanism of the photoexcitation is the same for all these approaches. 

In systems in which the charge-transfer excitation band differs from the action spectrum of photoconductivity, the doping effect may be due to a change of recombination path that results in an enhancement of carrier liefetime (e.g. holes in merocyanines and phthalocyanines). (Details on the mechanism are given in 10,11,74).)... 

In the case of PVK, a charge transfer complex may be formed by the addition of a small amount of 2,4,7-trinitro-9-fluorenone (TNF). The PVK-TNF system has been studied widely in xerography and as a two-component material it is well understood [19]. It also has the additional advantage of a particularly low photoconductivity in the absence of light. Photoconduction due to the complex is efficient across the visible region of the spectrum, with the excited state exhibiting 80 % electron transfer from a carbazole unit of the PVK to an adjacent TNF unit. The dissociation of holes occurs in the two-component system by hole transfer to... 

Transient terahertz spectroscopy Time-resolved terahertz (THz) spectroscopy (TRTS) has been used to measure the transient photoconductivity of injected electrons in dye-sensitised titanium oxide with subpicosecond time resolution (Beard et al, 2002 Turner et al, 2002). Terahertz probes cover the far-infrared (10-600 cm or 0.3-20 THz) region of the spectrum and measure frequency-dependent photoconductivity. The sample is excited by an ultrafast optical pulse to initiate electron injection and subsequently probed with a THz pulse. In many THz detection schemes, the time-dependent electric field 6 f) of the THz probe pulse is measured by free-space electro-optic sampling (Beard et al, 2002). Both the amplitude and the phase of the electric field can be determined, from which the complex conductivity of the injected electrons can be obtained. Fitting the complex conductivity allows the determination of carrier concentration and mobility. The time evolution of these quantities can be determined by varying the delay time between the optical pump and THz probe pulses. The advantage of this technique is that it provides detailed information on the dynamics of the injected electrons in the semiconductor and complements the time-resolved fluorescence and transient absorption techniques, which often focus on the dynamics of the adsorbates. A similar technique, time-resolved microwave conductivity, has been used to study injection kinetics in dye-sensitised nanocrystalline thin films (Fessenden and Kamat, 1995). However, its time resolution is limited to longer than 1 ns. 

After several reports between 1965 and 1980, no new information has been published on the spectroscopy of donors in GaP. Odd-parity transitions from the ground to excited states associated with the lowest X band for the Si, S and Te donors have been reported in the 55-100 meV ( 440-810cm-1) spectral domain [10,39,196,223]. The spectra are superimposed on the two-phonon spectrum of GaP and the FWHMs of the absorption lines at LHeT are - ()Ai meV. LHeT photoconductivity measurements in the photoionization region of shallow impurities in GaP revealed dips due to electronic transitions accompanied by the emission of LA(X) and LO (r) phonons with energies of 404 and 254 cm 1, respectively, and they have contributed to the understanding of the donor spectra [222]. LHeT transmission spectra of GaP Si samples at LHeT showing Si donor transitions are displayed in Fig. 6.45. 

The absorption spectrum is shown in Fig. 16.126 and is characterized by the following below E there is no absorption. The first absorption peak centered on [ corresponds to the excitation of an electron from the ground state to the excited state of the imperfection. This transition does not affect the photoconductivity, however, because the electron is still localized. The next absorption centered on is due to the excitation of an electron from the ground state of the imperfection to the conduction band. This will give rise to a current. Finally, the last absorption centered on is due to the intrinsic transitions across the band gap. 

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