Amorphous Hall coefficient

Hall effect is another important transport phenomenon and has been extensively studied in amorphous semiconductors. The Hall effect studies also assumed importance because of an anomaly observed between the sign of the charge carriers indicated by Hall coefficient and S in amorphous semiconductors. The Hall coefficient Rh is given by. 

The measurement of the Hall coefficient, one of the most useful tools for the study of transport phenomena in crystalline semiconductors, proved to be of little value in amorphous semiconductors in the past. Not only was the experiment exceedingly difficult to perform, because of the small value of the Hall mobility, but the Hall coefficient could not be related to the carrier concentration in the usual manner. The observation that the Hall coefficient was always negative regardless of the sign of the thermopower served as a warning that something was wrong. 

Table 5.2 summarizes the values of the obtained so far except those shown in Figure 5.26. The sign represents that of the Hall coefficient. A positive Hall coefficient was reported by Komfeld and Sochava (1959). These measurements were recently verified by Nagels etal. (1970) who took special care to ascertain that the Hall coefficient was a genuine property of the amorphous phase and not caused by crystalline inclusions. This exception is remarkable because of the close chemical similarity of all the systems investigated. It wiU be remembered that this small gap material had a thermopower which was difficult to interpret. 

In two recent papers Allgaier (1969 1970) has emphasised the dangers in extrapolating ideas particular to ordered solids into the liquid and amorphous states. For example, suppose that a liquid exists in which it is permissible to use an energy-momentum description of the electron state. Since the E-k relationship is the same in all directions, the Fermi surface is a sphere enclosing all the valence (free) electrons. Within this framework, the Hall coefficient is given by... 

As described above, the electrical resistivity at room temperature for the Al-R amorphous alloys increases significantly from 48 to 109p,Qcm with increasing R content fiom 8 to 12at%. Furthermore, it has previously been reported by Pont et al. (1989) that the Hall coefficient (/ h) at room temperature for the Al-Y amorphous alloys is negative and its magnitude increases from 22x 10 to 32x 10 m /As with increasing... 

The atomic disorder associated with the amorphous state leads to completely different transport properties from those encountered in the crystalline state. Experimental results of the electrical resistivity, the Hall effect, magnetoresistance, thermoelectric power and the occurrence of. superconductivity are discussed in section 8. The main emphasis is placed on the electrical resistivity. The occurrence of negative temperature coefficients of the resistivity is related to models based on the extended Ziman theory. In the low temperature regime the resistivity often shows a In T... 

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