This thesis deals with the electronic properties of a semiconductor superlattice and with electronic tunneling in a semiconductor heterostructure. Chapter 2 presents the theoretical formalism of k.p method for calculating band structures for strained-layer superlattices. A strained-layer superlattice is defined as a structure made up of alternating layers of at least two materials with different lattice constants. In this type of superlattice, a uniform strain, instead of misfit defects, accommodates the difference in the lattice constants. A strain affects the band structure since it changes the atomic position, and hence, crystal field which is the sum of all atomic potentials. The realization of strain effects in the model makes possible the understanding of physical properties of strained-layer superlattices, for example, optical properties and transport phenomena, which both are functions of the band structure. The study of ZnTe-CdTe system illustrates interesting strain effects in a strained-layer ZnTe/CdTe superlattice. The ZnTe/CdTe system has potential applications for visible-light sources and photodetectors. Because this system has a large lattice mismatch (≈ 6%), the theoretical study shows that strain plays an important role in optical properties.
Chapter 3 presents the theoretical formalism of k.p method for calculating band structures for semimagnetic semiconductor superlattices. A semimagnetic semiconductor superlattice is defined as a superlattice with one or more constituent materials containing magnetic impurities. When placed in a magnetic field, this type of superlattice exhibits interesting and possibly useful properties such as band gap reduction. These features are associated with the exchange interaction between the itinerant band electrons and localized d electrons on magnetic impurities. The exchange interaction in the theory is included within mean field approximation. Dependences of the band structure on the magnetic field and temperature follow the mean field approximation.
Chapter 4 presents the results of theoretical study of HgTe-CdTe superlattices. The HgTe-CdTe system has interesting features which make it a candidate superior to the HgCdTe alloy for infrared application. Based on the calculated band structure, the optical properties of the HgTe/CdTe superlattice are discussed. The optical absorptions in the superlattice and alloy are studied and compared. It is shown that the superlattice could have absorptions comparable to or larger than those of the alloy. The effects of strain on the optical properties and transport phenomena are discussed. It is found that the transport phenomena may be greatly affected by even a small strain in the HgTe-CdTe superlattice, where the relative difference between the lattice constants is only 0.3%. The optical properties of the HgTe-CdTe superlattice is studied for a wide range of valence band offset which is defined as the valence band edge of HgTe relative to that of CdTe and whose value is currently an unsettled issue. Both the band gap and absorptions of the superlattice are found to decrease rapidly for both negative and large positive values of offset.
Chapter 5 considers the wide-gap Cd₁₋ₓMnₓTe/Cd₁₋yMnyTe superlattice and the narrow-gap Hg₁₋ₓMnₓTe/Cd₁₋yMnyTe superlattices. Currently, the wide-gap system is of great interest because of the possibility of using it as magnetically tunable laser material. In the system spin-splitting is enhanced by the exchange interaction between the localized 3d electrons of Mn⁺⁺ and band electrons. The spin-splitting reduces the band gap opposing to the Landau level shift which enlarges the gap. However, the spin-splitting is found to dominate in the system. In consequence, the band gap decreases in a magnetic field. However, the relative change in the band gap is shown to be small. This makes suspect the idea of fabricating magnetically tunable laser out of this system. Interesting results concerning dependences of magnetic effects on temperature, magnetic field and layer thicknesses are presented. Generally speaking, temperature randomizes the spin oreintation while magnetic field aligns Mn⁺⁺ spins. In thin-layer limit, the magnetic effect in the superlattice is found to be just that of an alloy corresponding in composition to the superlattice. In contrast, the narrow-gap system is found to have larger tunability. Due to small effective mass of electrons, the Landau level shift is found to be important. Results regarding dependences of magnetic effects on temperature, magnetic field and valence band offset are shown.
Chapter 6 presents the theory and results of electronic tunneling in AlGaAs multi-barrier structures. The observation of negative differential resistance of the structure has been reported. However, basic mechanisms of current conduction in the structure have not been fully understood. We have made study of inelastic electronic tunneling due to electron-phonon coupling in a double-barrier structure. The current induced by the inelastic tunneling of electrons is calculated. The main result is that the inelastic process results in a much larger current than the elastic process at the voltage bias where no resonant tunneling occurs. Dependences of the inelastic contribution on doping level and layer thickness are discussed.
