Transport Properties of Narrow Gap II-VI Superlattices
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TRANSPORT PROPERTIES OF NARROW GAP II-VI SUPERLATTICES C. A. HOFFMAN, J. R. MEYER, AND F. J. BARTOLI Naval Research Laboratory, Washington, D. C. 20375
ABSTRACT The present understanding of the carrier transport properties in Hg-based superlattices such as the HgTe-CdTe system is reviewed. Novel features in the calculated superlattice band structures and their implications for macroscopic transport properties are discussed. Nearly all of the main experimental results are qualitatively consistent with a large valence band offset, but are difficult to explain if the offset is small. The valence band offset controversy thus appears to have been largely resolved.
INTRODUCTION Impressive advances have been made in the fabrication of narrow-gap II-VI heterostructures since the first successful MBE growth of a HgTe-CdTe superlattice in 1982.[11 Significant strides have been made experimentally and theoretically toward the development of a fundamental understanding of their properties.[2, 3] It is now apparent that the band structures of these superlattices exhibit unique features that are distinct from those found in either Hg-based alloys or in wide-gap superlattices. These have led to the observation of a number of novel phenomena. Only two years ago there was no clear picture of the relation between the predictions of band structure theory and the results of experimental studies. Several intriguing observations, such as extremely high superlattice hole mobilities,[4] were unexplained except for the vague suggestion that they may be related to some special property of the Type-III interface states. Early magneto-optical investigations[5] reported a valence band offset A of 40 meV, in sharp disagreement with x-ray photoemission experiments[6, 7] which yielded A ; 350 meV. Several a priori calculations also tended to support a larger value for A.[8] It will be seen in the following sections that most of the principle experimental results can now be understood qualitatively in terms of theoretical dispersion relations which are sensitive to A. It will be argued that the long-standing controversy concerning the valence band offset has effectively been resolved in favor of a large value. The unique nature of narrow gap HgTe-CdTe superlattice band structures makes them attractive for a variety of electronic and electro-optical applications. Indeed, promising infrared applications of these superlattices were discussed theoretically as early as 1979.[9] However, to assess the ultimate potential, a thorough understanding of their fundamental properties is required. In this paper, we review recent advances in the understanding of HgTe-CdTe and related superlattices. Section 2 summarizes theoretical results for the superlattice band structure, including its strong dependence on valence band offset. In Section 3, recent magneto-transport data are reviewed and correlated with the predictions of band structure theory.
BAND STRUCTURE THEORY Theoretical investigations of the HgTe-CdTe superlattice band structure have been discussed
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