Thermoelectric power and dc electrical conductivity measurements are made on MoTe, single crystals in a wide temperature range (77 to 770 K). The results are analyzed on the basis of impurity conduction. It is shown that three processes contribute to the total conductivity: hopping conduction between impurity sites, scattering by optical modes, and, in the intrinsic domain, scattering by impurities. The thermal energy gap (0.99 eV) is approximately the same as the optical one (1.03 eV). The effective mass of electrons in the conduction band is not very different from the mass of the free electron (m* == 0.76rn), and the dielectric constant of the medium is K = 34.The agreement between theoretical and experimental results is quite good. A comparison with previous results performed on compact polycrystalline samples is given.Des mesures de pouvoir thermoklectrique et de conductivitk klectrique sont effectukes entre 77 e t 770 K sur des kchantillons monocristallins MoTe,. Les rksultats expkrimentaux sont analyshs en terme de conduction par impuretks. On montre que la conductivith fait intervenir trois mkcanismes: conduction par sauts cntre sites d'impuretks, interaction avec 1es phonons optiques et, dans le domaine intrinsAque, interaction avec les impuretks. Les riesures de la bande d'knergie interdite thermique e t optique sont peu diffkrentes (0,99 e t 1.03 eV). La masse effective des electrons dans la bande de conduction est proche de celle de I'klectron libre (m* = 0,76m) e t la constante diBlectrique du milieu est K 34. L'accord entre rksultats thhoriques e t expkrimentaux est tr6s satisfaisant. Les rhsultats sur les kchantillons monocristallins sont compares & ceux obtenus prkckdemment sur des irchantillons polycristallins.
In the past, amorphous layers have been described as n or p type according to their majority carriers. The majority carrier type varies from one study to another, even with films of the same composition, which is a very serious problem. In this paper we explain this discrepancy by demonstrating that microcrystallite inhomogeneities are present in the amorphous . While x-ray diffraction patterns are typical of amorphous samples, the selected-area diffraction obtained using a transmission electron microscope depends on the area studied. It is shown that the layers are constituted of microcrystallites embedded in an amorphous matrix. Therefore the majority carrier type changes when the p-type nature of the amorphous matrix is masked by the n-type nature of the crystallites at the percolation threshold. Of course this percolation threshold depends not only on the composition of the layer but also on the deposition process, which explains the different majority carrier types found for identical composition. The two critical temperatures measured from the conductivity curves can be attributed to the phase transition of the microcrystallites (the first one) and the overall crystallization of the layer (the second one). The typical optical absorption and differential thermal analysis properties are explained in the same way.
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