Drift mobility transitions

Li Y, Nakano C, Imaeda K, Inokuchi H, Maruyama Y, Iwasawa N, Saito G (1990) Charge-carrier drift mobilities and phase transition in tetrakis(octylthio)tetrathiafulvalene, TTCg-TTF. Bull Chem Soc Jpn 63 1857-1859... 

The study of the dispersion of photoinjected charge-carrier packets in conventional TOP measurements can provide important information about the electronic and ionic charge transport mechanism in disordered semiconductors [5]. In several materials—among which polysilicon, a-Si H, and amorphous Se films are typical examples—it has been observed that following photoexcitation, the TOP photocurrent reaches the plateau region, within which the photocurrent is constant, and then exhibits considerable spread around the transit time. Because the photocurrent remains constant at times shorter than the transit time and, further, because the drift mobility determined from tt does not depend on the applied electric field, the sample thickness carrier thermalization effects cannot be responsible for the transit time dispersion observed in these experiments. 

The assignment of ti, t2, and t- (see inflection points in Fig. 4.17) to transit times in the top, middle, and bottom layers is supported by the fact that the drift mobility of charge carriers for the three layers were calculated to be similar to the corresponding single layers. The general features of current waveforms described earlier are common to both hole and electron response. 

Let us first consider the carrier drift in pure amorphous selenium. Both the electron and hole drift mobility can be measured in a-Se by the TOF technique outhned earlier. At temperatures above 200 K, a well-defined transit pulse is observed. The transient... 

For positive lit electrodes one can register the drift of holes, and for negative ones- the drift of the electrons. The photosensitizer (for example Se) may be used for carrier photoinjection in the polymer materials if the polymer has poor photosensitivity itself. The analysis of the electrical pulse shape permits direct measurement of the effective drift mobility and photogeneration efficiency. The transit time is defined when the carriers reach the opposite electrode and the photocurrent becomes zero. The condition RC < tlr and tr > t,r should be obeyed for correct transit time measurement. Here R - the load resistance, Tr -dielectric relaxation time. Usually ttras 0, 1-100 ms, RC < 0.1 ms and rr > 1 s. Effective drift mobility may be calculated from Eq. (4). The quantum yield (photogenerated charge carriers per absorbed photon) may be obtained from the photocurrent pulse shape analysis. 

The drift mobility has the expected power law decrease with time. To obtain Pu from the transit time in the time-of-flight experiment, the transit time is defined, somewhat arbitrarily, as the time when the average carrier is half way across the sample. 

The drift mobility ( x) of a photoinjected carrier is its mean velocity per unit field ( ) and may itself be field dependent. The transit time (t J and mobility are related to specimen thickness (L) according to equation 1 ... 

Laser Raman spectroscopy has been used as a tool to elucidate the molecular structure of crystals, liquids, and amorphous alloys in the As-S-Se-Te system. Characteristic monomer and polymer structures have been identified, and their relative abundances have been estimated as a function of temperature and atomic composition. These spectroscopic estimates are supported by calculations based on the equilibrium polymerization theories of Tobolsky and Eisenberg (1,2) and of Tobolsky and Owen (3). Correlations between the molecular structure of the amorphous alloys and physicochemical properties such as the electron drift mobility and the glass transition temperature are presented. 

The transit time, which is clearly evident as a knee in Fig. 6, was measured as a function of applied bias V. The resulting drift mobility defined by = Lyvt, has a power-law field dependence, as shown in Fig. 7. In the theory of dispersive transport worked out by Scher and Montroll (1975) and others, the field dependence of the transit time is related to the time dependence of the current decay through a dispersion parameter a. In the theory, the current decay at short times (f < fx) the form and at long times (t > tx) the form t Similarly the transit time tx is proprotional to (L/Fy . Note that the data in Figs. 6 and 7 are consistent with these predictions of the theory with a = 0.51 at 160°K. 

The power-law decay in Eq. (3) can be regarded as a decay of free carrier density or as a decay of the drift mobility. The tatter interpretation and the usual definition of the transit time tj lead to... 

The drift mobility obtained from Eq. (5.34) is usually not equal to the conductivity mobility ju of Eqs. (5.6) or (5.7) because the drifting carriers may repeatedly be trapped by and thermally released from shallow traps during transit. Shallow traps are those with which the carriers can retain local thermal equilibrium. The demarcation line between deep and shallow traps depends therefore on T. The relative time each carrier spends immobilized in shallow traps and freely drifting in the band, r, is... 

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