Photoconduction pulsed

Free-electron lasers have long enabled the generation of extremely intense, sub-picosecond TFlz pulses that have been used to characterize a wide variety of materials and ultrafast processes [43]. Due to their massive size and great expense, however, only a few research groups have been able to operate them. Other approaches to the generation of sub-picosecond TFlz pulses have therefore been sought, and one of the earliest and most successfid involved semiconducting materials. In a photoconductive semiconductor, carriers (for n-type material, electrons)... 

Traditionally, charge transfer mechanisms have been studied by such methods as conductivity, the Hall effect, and thermoelectric effect. Details of these applications may be found in Experimental Methods of Physics, Vol. 6, Pt. b (12), the article on ionic conductivity by Lidiard (70), and in many of the original papers quoted. More recently, techniques such as electron spin resonance (13), dielectric loss and pulsed photoconductivity methods (5—8) have been used to study semiconduction in organic materials. 

IET serves as a theoretical basis not only for fluorescence and photochemistry but also for photoconductivity and for electrochemiluminescence initiated by charge injection from electrodes. These and other related phenomena are considered. The kinetics of luminescence induced by pulse and stationary excitation is elucidated as well as the light intensity dependence of the fluorescence and photocurrent. The variety and complexity of applications proves that IET is a universal key for multichannel reactions in solutions, most of which are inaccessible to conventional (Markovian) chemical kinetics. 

The photoinduced ionization of benzophenon in acetonitrile has also been reported to proceed via triplet-triplet annihilation at very low laser pulse intensities [272], The biexcitonic ionization in this system has been studied by applying the transient photoconductivity technique and described with the conventional (Markovian) rate equations, with the time-independent rate constants [273], Such equations can be represented as follows... 

Terahertz imaging approaches have typically used either short-pulsed laser or continuous wave (CW) THz generation and detection. The short-pulsed method usually involves the generation and detection of sub-picosecond THz pulses using either photoconductive antenna structures or optical rectification in a non-linear crystal. Pulsed sources seem to be more favorable (in particular for close proximity applications) because they can be used for acquiring depth information. Spectral information is retrieved by a Fourier transform of the time-domain data to the frequency domain. 

When a light pattern illuminates the front face (Fig. 8.17(b)), the light passes through the transparent electrode to interact with the photoconductive layer, raising its conductivity. Simultaneously a voltage pulse (about 200 Y) is applied across the electrodes, the major part of which, in the illuminated regions, drops across the ferroelectric. As a result the polarization is switched from the strain-biased configuration to a direction approximately normal to the plate. In this... 

The ceramic plate, ITO electrodes and photoconductive layer are arranged as in the previous example. In its unpoled state the PLZT scatters light, probably at the 71° and 109° domain boundaries associated with the rhombohedral structure. When a voltage pulse is applied simultaneously with a light pattern incident on the plate, the polarization is switched to a direction normal to the plane of the plate and the scattering is reduced. 

The photoconductivity increases when the a-Si H is lightly doped with phosphorus (Anderson and Spear, 1977). However, phosphorus doping causes very slow decay of photoresponse. The photoresponse characteristic for the phototconductive sensor using undoped a-Si H is shown in Fig. 3. The illumination is the modulated light from a GaP LED. The modulation ratio is defined as M = (it — i2)/i2, where is the peak photocurrent and i2 is the bottom current just prior to the next pulse. Figure 4 shows the modulation ratio of a-Si H versus the pulse width T, compared to that of the CdS-CdSe photoconductive sensor. The CdS-CdSe sensor modulation ratio decreases as the repetition time becomes shorter. On the other hand, in the a-Si H photoconductive sensor, the modulation ratio does not decrease... 

Fig. 4. Modulation ratio for a-Si H photoconductive sensor [under 200- (O) and 50- (A) fiW cm-2 illumination] and CdS photoconductive sensor ( ) versus LED driving pulse width. [From Kagawa et al. (1982).]...

The depletion layer profile on undoped a-Si H can be obtained by transient photoconductivity. A pulse of light excites electron-hole pairs very near the contact. As in the time-of-fiight experiment, holes are immediately collected by the contact and electrons drift down the internal field of the depletion layer, giving transient conductivity of... 

Fig. 4.8 Diagram, of a pulsed photoconductivity experiment. The inset diagram shows the form of the oscilloscope trace.

Pulsed photoconductivity provides a powerful means of measuring carrier mobilities. The usual experimental arrangement shown in Fig. 4.8 (Kepler,1960) uses a sandwich cell with a transparent front electrode. If the coefficient of... 

Recent measurements of fast transient photoconductivity (11) in trans-fCm have demonstrated that the photogenerated soiitons are mobile and contribute to the electrical conductivity. Figure 2 shows the transient photoconductivity following a 1 pJ pulse at 2.1 eV with a bias voltage of 300 V. The charge carriers are produced within picoseconds of optical excitation. The fast rise is foiiowed by (approximateiy exponentiai) decay with a time constant of - 300 ps. The magnitude and time decay of Oph(t) are temperature independent... 

Since it is important to address this issue at the earliest times following photoexcitation, measurements of transient photoconductivity in the picosecond to nanosecond regime were carried out [145,146,201,202], In response to an ultrafast light pulse (duration 25 ps), there is an initial fast photocurrent response with decay time of about 100 ps followed by a slower component with... 

The ultra violet laser-pulse having such a short pulsewidth Tw as Ins and such a short wavelength as 337.1nm irradiates the silicon surface to excite excess carriers as shown in the bottom left figure of Fig. l.The generated carriers are confined within the subsurface since extremely short penetration depth of 200A. An amplitude (or intensity) of the photoconductivity... 

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