Transport properties of II-VI narrow-gap semiconducting compounds
Abstract
Transport measurements were performed on unoriented single crystals of Hg$\sb{\rm 1-x}$Fe$\sb{\rm x}$Se for 4.2K $\leq$ T $\leq$ 300K and for 0.0001 $\leq$ x $\leq$ 0.12. The data are interpreted using a band structure model which assumes the Fe impurities form a system of donors resonant with the conduction band. The electron concentration data yielded a determination of the donor energy level as a function of temperature and crystal composition. An anomalously high mobility was observed at low temperatures (T $<$ 100K) and in the range 0.0003 $\leq$ x $\leq$ 0.01. The data was interpreted as a result of a reduction in charge center scattering at these temperatures. This reduction was examined in the context of a spatial ordering in the charge state of the Fe impurity system. The analysis was found to be consistent with this hypothesis. A study of the resistance of Hg$\sb{\rm 1-x}$Fe$\sb{\rm x}$Se for 0.55K $\leq$ T $\leq$ 4.2K reveals two anomalous features. For Hg$\sb{\rm 1-x}$Fe$\sb{\rm x}$Se samples with x $<$ 0.03, the resistance decreases significantly below its value at 4.2K, not saturating until temperatures near 1K. A sudden increase in the resistivity $\rho$ is also observed for x $>$ 0.0005 in the region 4.2K $\leq$ T $\leq$ 20K, with a sharp maximum in d$\rho$/dT at at temperature of 5.5K $\pm$ 0.2K. These features are incompatible with the accepted theoretical description of electron transport in II-VI narrow-gap semiconductors which show the scattering mechanisms at low temperatures to be temperature insensitive. Transport measurements were made as a function of temperature on the Hg$\sb{\rm 1-x}$Cd$\sb{\rm x}$Te and HgTe$\sb{\rm 1-y}$Se$\sb{\rm y}$ systems containing Fe impurities. These studies were done to establish whether the unique properties of Fe in HgSe could be extended to other semiconducting compounds. The ability of Fe to "pin" the Fermi level, the stabilization of electrical properties and the enhancement of the mobility in other II-VI semiconductors by the introduction of Fe would have powerful device applications. Hg$\sb{\rm 1-x}$Cd$\sb{\rm x}$Te and HgTe$\sb{\rm 1-y}$Se$\sb{\rm y}$ compounds containing Fe were examined to establish the validity of these speculations. Stabilization of the electrical properties of Hg$\sb{\rm 1-x}$Cd$\sb{\rm x}$Te did not result in the substitution of Fe. The predicted "pinning" of the Fermi level in the HgTe$\sb{\rm 1-y}$Se$\sb{\rm y}$ system was found to be roughly valid. However, the effects of disorder scattering when significant impurity concentrations are introduced into a host lattice disrupt any possible benefits due to the introduction of Fe. This study established the limitations of other proposed complex semiconducting systems and suggests that the effect of Fe in HgSe to be unique to that system.
Degree
Ph.D.
Advisors
Reifenberger, Purdue University.
Subject Area
Condensation
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