Electronic Structure of AlAs/GaAs and CdTe/HgTe Superlattices and Interfaces

Author: Schulman, Joel Nathan

Year: 1979

Degree: Dissertation (Ph.D.)

Advisors: McGill, Thomas C.; Smith, Darryl L.

Committee Member: Unknown, Unknown

Option: Physics

DOI: 10.7907/arnr-m290

Abstract

This thesis is concerned with reporting the results of a theoretical study of two semiconductor superlattices constructed parallel to (001) zincblende planes. One consists of alternating slabs of AlAs and GaAs, and the other of CdTe and HgTe, A tightbinding band structure calculation was done in order to find the energy levels and wave functions of these systems. Two aspects of the superlattice band structure were of particular interest - first, the nature and value of the band gap as a function of the thicknesses of the superlattice slabs and, second, the electronic structure of the interfaces between the constituent materials. These features were analyzed by referencing the superlattice results to relevant features of the constituent bulk band structures of AlAs, GaAs, CdTe, and HgTe. Through this procedure novel superlattice and interface quantum effects were identified and studied.

AlAs/GaAs Superlattice

The AlAs/GaAs superlattice is discussed first. The superlattice is found to have a semiconducting band gap for all ranges of AlAs and GaAs repeated slab thicknesses. The values of the gap are found to vary widely as the slab thicknesses are changed. For large slabs the gaps approach a value characteristic of bulk GaAs. They differ substantially from those of the random AlxGa1-xAs random alloy with the same aluminum concentrations. The calculated band gaps are compared with the available experimental data and good agreement is found.

Whether the bond gap is direct (conduction band minimum and valence band maximum occurring at the same point ink space) or indirect is determined. Again, behavior differing from the random alloy is noted. We find that certain superlattices can have direct band gaps with aluminum concentrations such that the corresponding random alloy would be indirect. This occurs due to the concentration of GaAs in slabs in the superlattice case. This effect is studied in detail for several ratios of AlAs to GaAs slab thicknesses.

The electronic states at the valence band maximum and the conduction band minimum are found to be affected greatly by the periodic superlattice potential. The AlAs slabs act as potential barriers to electrons and holes in the GaAs slabs at the band edges. The states therefore resemble the lowest energy states of a particle in a finite well. States with characteristic single and double peaked structures, like well states, are reported.

The AlAs/GaAs interface is investigated in the limit in which the superlattice slabs become large. Interface states are found near the boundaries of the Brillouin zone, but not at energies in the fundamental gap. The interface state density is too small to be observable as a separate peak in the calculated energy density of states. A layer density of states calculation shows that the superlattice is substantially bulk-like except for the layers directly adjacent to the interface.

CdTe/HgTe Superlattice

The CdTe/HgTe superlattice is reported on next. It is also found to be semiconducting. The values of the superlattice band gap decrease monotonically with increasing HgTe slab thickness for superlattices with specified cadmium concentrations. The band gaps of superlattices with the thinnest alternating slabs are close to the random Hg1-x CdxTe values. The gaps approach the zero band gap value of HgTe as the HgTe slab thickness increases. The material is always direct, in contrast with the AlAs/GaAs superlattice.

The syrrmetry of the state at the valence band maximum undergoes a transition as the HgTe slab thickness is increased. This is explained in terms of unusual features of the band structure of bulk HgTe. Well-like confinement of the conduction band minimum state, to a lesser degree than that found in the AlAs/GaAs superlattice, occurs in the HgTe slabs of the CdTe/HgTe superlattice. No hole confinement exists at the valence band maximum, unlike in the AlAs/GaAs case. These features are related to the band discontinuities between the two semiconductors making up the superlattice and their band structures.

The CdTe/HgTe interface is also investigated. Interface states are found, again near the Brillouin zone boundaries. In this case there is a state whose energy is the highest valence band energy at that point k space, but it is still below the k=O valence band maximum energy.

Parts of this thesis have been or will be published under the following titles:

1. Band Structure of AlAs-GaAs (100) Superlattices, J, N. Schulman and T. C. McGill, Physical Review Letters 39, 1680 (1977).

2. Tight-binding Calculation for the AlAs-GaAs (100) Interface, J. N. Schulman and T. C. McGill, Journal of Vacuum Science and Technology 15, 1456 (1978).

3. The CdTe/HgTe Superlattice: Proposal for a New Infrared Material, J. N. Schulman and T. C. McGill, Applied Physics Letters, May 15, 1979.

4. Electronic Properties of the AlAs-GaAs (001) Interface and Superlattice, J. N. Schulman and T. C. McGill, Physical Review B, to be published.

5. Ideal CdTe/HgTe Superlattices, J. N. Schulman and T. C. McGill, submitted to the Journal of Vacuum Science and Technology.

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