Modulation spectroscopy of group IV, III-V, and II-VI semiconductors and their heterostructures
Abstract
Modulation spectroscopy techniques are applied to the investigation of the electronic band structure of isotopically enriched germanium as well as to the study of the quantum confinement effects of the electronic structure of semiconductor heterostructures. The modulation techniques based on piezo-modulation, electro-modulation, photo-modulation, and wavelength-modulation are used in reflection or transmission. A modulated reflectivity study of strained ZnSe/Zn$\sb{1-x}$Cd$\sb{x}$Se single quantum wells show the effects of quantum confinement in the blue spectral region. The quantum well energy levels were identified by a theoretical model based on the $\vec k{\cdot}\vec p$ transfer-matrix formalism which takes into account the strain in the layers. Transitions involving the above-barrier quasi-bound continuum states observed in GaAs/Al$\sb{1-x}$Ga$\sb{x}$As asymmetric quantum wells are compared and contrasted to bound to bound state transitions in symmetric single quantum wells. The variation of the density of states above the barrier energies is observed in the piezo-modulated reflectivity spectra. The evolution of discrete multiple quantum well energy levels into superlattice minibands is studied with piezo-modulated reflectivity. As series of GaAs/Al$\sb{1-x}$Ga$\sb{x}$As multiple quantum well samples were grown with varying barrier thicknesses separating the quantum wells. A systematic study shows the broadening of quantum well energy levels in to superlattice minibands as the barrier thickness decreases. It is shown through modulated reflectivity that quantized energy levels may form in the surface layer of GaAs/Al$\sb{1-x}$Ga$\sb{x}$As heterostructures. These are energy levels confined between the surface and a quantum barrier underneath the surface layer. Such structures also clearly reveal the existence of resonances in the density of states above a single quantum barrier. Isotope effects on the electronic structure of germanium are studied with modulated reflectivity and transmission. Four of the naturally occurring germanium isotopes have been separated and crystallized into single crystal samples. The variation of the direct gaps with isotope mass are studied by modulated reflectivity. The variation of the indirect gaps and the associated L-point Brillouin zone boundary phonons are deduced from modulated transmission studies.
Degree
Ph.D.
Advisors
Ramdas, Purdue University.
Subject Area
Condensation
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