Variation in the Properties of Superlattices with Band Offsets
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Variation in the Properties of Superlattices with Band Offsets T. C. McGill, R. H. Miles and G. Y. Wu T. J. Watson, Sr., Laboratory of Applied Physics California Institute of Technology Pasadena, California 91125
Abstract The implications for recent reports of large valence band offsets at the HgTe-CdTe heterojunction are examined. The variation of the band gap and effective mass for transport normal to the layers in the superlattice is examined in detail. Introduction Alloys of HgTe and CdTe are used extensively in the infrared.1 These alloys have the property that the band gap can be adjusted from zero up to the value of the CdTe band gap (•1.6 eV). In recent years it has been predicted that a number of advantages result from layering the HgTe and CdTe into a superlattice structure like that shown in 23 Fig.1 instead of intermixing the Hg and Cd to make an alloy. , In the alloy the band gap is controlled by the relative composition of Hg to Cd, while in the superlattice, the band 3 gap is controlled by the thickness of the layers making up the superlattice. Studies of the growth of alloys and superlattices by molecular beam epitaxy indicate that it will be 4 much easier to control the band gap of the superlattice than the band gap of the alloy. In alloys, the effective mass is strongly coupled with the band gap. The effective mass is proportional to the gap. Hence, small band gap implies small effective mass. This has 3 lead to serious difficulties with leakage currents in p-n-junction-device structures. In the 3 degree. decoupled to some band gap can be the effective mass and superlattices,
Mat. Res. Soc. Symp. Proc. Vol. 90.
1987 Materials Research Society
144
CdTe
BUFFER
SUBSTRATf
_.
Fig. 1. A schematic diagram of a HgTe-CdTe superlattice. CdTe, CdZnTe and GaAs have all been used as substrates. The superlattices are characterized by the thicknesses of the HgTe and CdTe layers.
The properties of superlattices have been studied extensively both experimentally 5 and theoretically in the last couple of years. , 6 The basic situation is that superlattices 7 have been successfully fabricated by on the order of ten groups. The optical properties of 0 8 the superlattice including infrared absorption, , 9 photoluminescence, and photoconductivity 9 have been measured on a rather small number of superlattices. A number of other 6 experimental studies have been carried out. BAND OFFSETS AT HETEROJUNCTIONS Conduction Band Edge AiE CdTe
HgTe
AE Valence Band Edge
Fig. 2. Schematic diagram illustrating band offsets in the valence band AE, and conduction band AEr at a heterojunction between HgTe and CdTe.
One of the major unknowns comparing theory with experiment is the basic property 5 of the electronic states at the heterojunction between the HgTe and CdTe, the band offset. As shown in Fig. 2, the band offsets are the parameters which give the relative location of the valence and conduction band edges in the HgTe and CdTe as one goes across the heterojunction.
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In this manuscript, we will concentrate on
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