Hall measurements

One problem with methods that produce polycrystalline or nanocrystalline material is that it is not feasible to characterize electrically dopants in such materials by the traditional four-point-probe contacts needed for Hall measurements. Other characterization methods such as optical absorption, photoluminescence (PL), Raman, X-ray and electron diffraction, X-ray rocking-curve widths to assess crystalline quality, secondary ion mass spectrometry (SIMS), scanning or transmission electron microscopy (SEM and TEM), cathodolumi-nescence (CL), and wet-chemical etching provide valuable information, but do not directly yield carrier concentrations. 

Fig. 9. Hole mobility versus temperature for an InP Zn epilayer grown by OMVPE. a) before any hydrogenation, b) after hydrogen plasma for 6 hours at 200°C followed by a 275°C, 5 min. annealing. After hydrogenation, the InP layer is highly resistive and Hall measurements are difficult. The slight annealing is performed to partially reactivate the zinc acceptors to a value of 1.3 x 1016 cm-3, which makes possible Hall measurements. J. Cheval-lier et al., Materials Science Forum, 38-41, 991 (1989). Trans. Tech. Publications Semicond. Sci. Technol. 4, 87 (1989). IOP Publishing Ltd.

Measurements of mobility in PS suffer from the fact that the number of free charge carriers is usually small and very sensitive to illumination, temperature and PS surface condition. Hall measurements of meso PS formed on a highly doped substrate (1018 cm3, bulk electron mobility 310 cm2 V-1 s-1) indicated an electron mobility of 30 cm2 V 1 s 1 and a free electron density of about 1013 cm-3 [Si2]. Values reported for effective mobility of electron and hole space charges in micro PS are about five orders of magnitude smaller (10-3 to 10 4 cm2 V 1 s ) [PelO]. The latter values are much smaller than expected from theoretical investigations of square silicon nanowires [Sa9]. For in-depth information about carrier mobility in PS see [Si6]. 

This same equation is, of course, also used to rationalise the general electronic behaviour of metals, semiconductors and insulators. The quantitative application of Eqn (2.1) is handicapped for ionic conductors by the great difficulty in obtaining independent estimates of c,- and u,-. Hall effect measurements can be used with electronic conductors to provide a means of separating c, and u,- but the Hall voltages associated with ionic conduction are at the nanovolt level and are generally too small to measure with any confidence. Furthermore, the validity of Hall measurements on hopping conductors is in doubt. 

In Ref. 54, XRD showed the deposit to be hexagonal CuSe. Analysis of the absorption spectrum gave a direct bandgap of 2.02 eV. As commonly seen for these compounds, there was still strong absorption at lower energies (e.g., at 1.9 eV, the absorption coefficient was >7 X 10" cm ), possibly due to an indirect transition but likely due, at least in part, to free-carrier absorption. From Hall measurements, the doping (acceptor) density was ca. 10 cm (heavily degenerate) and the mobility ca. 1 cm V sec The dependence of film thickness and deposition rate on the deposition parameters has been studied in a separate paper [62]. 

We depart briefly from our discussion of SI GaAs to consider an example that better illustrates some of the features of temperature-dependent Hall measurements. This example (Look et al., 1982a) involves bulk GaAs samples that have sc — F — 0-15 eV. We suppose, initially, that the impurity or defect controlling the Fermi level is a donor. Then any acceptors or donors above this energy (by a few kT more) are unoccupied and any below are occupied. Also, p n for kT eG. From Eq. (B34), Appendix B, we get... 

Automated computer-controlled measurement techniques for Hall effect, photoconductivity, and photo-Hall measurements in semi-insulating... 

Morrison (31), using sintered zinc oxide, applied a different technique to study the conductivity effects in the range between room temperature and 500°C. He studied the variation in conductance as a function of time with the temperature held constant. Figure 3 shows one such conductivity-time experiment. The sample used was a slab of zinc oxide cut from a pill which had been compressed and sintered in air for eighteen hours at 1000°C. The sample was immersed in oil (the oil does not penetrate into the pores of the sample) at the start of the run. The sample container was immersed in boiling water, the temperature reaching 100 C in the order of one-half of a minute. The conductance was recorded as a function of time while the sample was held at 100°C. The results are shown in Fig. 3. The inverse of the Hall voltage is also plotted as a function of time. An interpretation of the Hall measurement is discussed in Section III. 

Withx beyond the dopant s Mott transition but short of collapsing the host. Hall measurements actually reveal that, however many of the Sm sites may then stand instantaneously converted to the d configuration, these same sites are con-... 

Electrical conductivity (a.c. and d.c.), Hall measurements, and determination of Seebeck voltage. 

Fig. 11.12. The results of Hall measurements of mobility are shown in Table 11.3. The Mott-Schottky plot showed a flatband potential of -0.23 V on the NHS. Some electrode kinetic measurements (Miller, 1992) are shown in Fig. 11.13.

Fig. 7.16. Growth temperature-dependent reduction of carrier concentration (300 K) of ZnO films on sapphire due to introduction of Ce02 buffer layers. The growth temperature is about 50° C lower as the given heater temperature. Hall measurements by H. von Wenckstern...

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