Hall mobility, measurement Impurities

Boron Phosphide (BP). Boron phosphide is extrinsic at room tenqterature, and the transport is limited by impurity scattering. Figure 4.1-60 shows the temperature dependence of the conductivity, carrier concentration, and Hall mobility of several samples. In n-doped single crystals, electron mobilities of the order of 30-40 cm /V s have been found at 300 K in p-doped single crystals, mobilities around 500cm /Vs have been measured at 300 K. 

Hall effect is the most widely used technique to measure the transport properties and assess the quality of epitaxial layers. For semiconductor materials, it yields the carrier concentration, its type, and carrier mobility. More specifically, experimental data on Hall measurements over a wide temperature range (4.2-300 K) provide quantitative information on impurities, imperfections, uniformity, scattering mechanisms, and so on. The Hall coefficient and resistivity (p) are experimentally determined and then related to the electrical parameters through (for n-type conduction) ffn = fulne and M-h = f n/P. where n is the free electron concentration, e is the unit electronic charge, Ph is the Hall mobility, and Th is the Hall scattering factor that depends on the particular scattering mechanism. The drift mobility is the average velocity per unit electric field in the limit of zero electric field and is related to the Hall mobility... 

There is one other means of determining lifetime, available if both photo-Hall (PH) and absorption experiments can be carried out. This possibility is simply illustrated by Eq. (35). Here the PH measurement gives An, the absorption measurement gives ocB, and f0 can be easily measured with a calibrated light detector. An obvious caveat here, of course, is that we must assume that a = aB, i.e., that all of the light absorption is due to electronic transitions. For above-band-gap light this assumption will almost certainly be true. It was seen before that absorption measurements can be useful in determining impurity concentrations. Thus, a combination of PH and absorption data may yield both N and rB. If the carrier mobility can... 

The carrier concentration n = Nt, — and the compensation ratio 6 which were obtained by measurements of the Hall coefficient and carrier mobility, respectively, were found to be functions of Fas or Pas- The compensation ratio 0 exhibits a minimum and the carrier concentration n a maximum, at the optimum temperature 617 °C, shown in Fig. 3.47. From the compensation ratio and the carrier concentration, the concentrations of the ionized donors, A, are calculated as a function of Tas, as shown in Fig. 3.48.At the optimum temperature 617 °C, Aq shows a maximum and Aa a minimum. The total concentration of ionized impurities, N, = N/ + N, remains essentially constant in the measured Tas range. This result suggests that vacancy-related mechanisms are associated with the formation of dislocations during growth and the compensation process. Thus, high quality crystal GaAs with a low density of dislocations has been grown by precise... 

Hall, and C-V, four-point probe and spreading resistance measurements to some extent provide a measure of the net impurity concentration of dopants in SiC ([ND - NA] or (Na - Nd]). In addition, Hall measurements provide a method for obtaining the mobility of the net carriers. These measurements have been applied to both n- and p-type SiC and its various polytypes. In this Datareview, we will report on the mobilities for most of the SiC polytypes under various growth conditions. 

According to studies of the temperature dependence of the electrical conductivity and the Hall effect of n-type samples in system 3, a weak variation in these properties with tenq>era-ture has been established, which indicates degeneracy of the electron gas and ionization of impurities, Similar measurements on p-type samples in the same system showed that there is a reversal of the sign of the Hall coefficient (at about 500 K), and a high ratio of the electron mobility to the hole mobility (B = 60-80) [7],... 

One can easily see that if n p, Eh reduces to -1/ne. Hall measurements are easiest to interpret in doped materials when either n 3> p orn < p. Otherwise one is faced with four unknowns, which require other measurements to resolve. For example, except for the difference between electron and hole mobilities, the Hall effect would be zero for intrinsic materials. One can also see that doing Hall measurements as a fimction of temperature offers a means of determining the occupancy number and energy levels of the various impurity states in the freeze-out region through Equation 20.22. 

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