Modulation Spectroscopy Study of Strained-Layer Semiconductor Microstructures.

Strained-layer semiconductor microstructures are of great importance for both fundamental interest and technological applications. A study of such systems can reveal a great amount of information of the physics of reduced dimensionality, strain effects, as well as knowledge for material and structural characterization. In this thesis, electroreflectance, photoreflectance and piezoreflectance modulation spectroscopy methods were employed to investigate a number of strained microstructures. We have for the first time positively identified the optical features in modulation spectroscopy associated with the two-dimensional electron gas properties in the pseudomorphic modulation-doped quantum well structures. We have proposed a proper modulation spectroscopy lineshape function for the intersubband transitions in a modulation -doped quantum well with two-dimensional electron gas. This lineshape allows us to extract the Fermi level position relative to the electron subband in addition to the intersubband transition energies and broadening, which in turn yields the sheet electron density present in the well channel. We have measured the Ge composition dependence of the E_1 and "E_ {rm o}^'" (will be defined later in chapter 5) transition energies at 300K in a series of strained Si_{rm 1-x}Ge_{rm x} /Si layers (0 < x < 0.23) using photoreflectance. In our strained samples the energy of the E_1 peak is essentially the same as in unstrained material, in agreement with deformation potential theory. The "E_{rm o}^'" feature, which exhibits only a very weak variation with Ge content, may also contain a strain-shifted E_1 + Delta_1 component. We have investigated a number of short-period strained Si-Ge microstructures fabricated on (001) Ge using piezoreflectance, photoreflectance and Schottky barrier electroreflectance at 77K. In addition to bulk Ge these samples consisted of Ge quantum wells sandwiched between ordered short-period strained Si-Ge superlattice regions. No evidence for pseudodirect transitions in the Si-Ge superlattices was found by any of the above measurements although quantum transitions in the Ge quantum wells were observed in piezoreflectance. The photoreflectance and Schottky barrier electroreflectance spectra measured to date derive from the bulk-like Ge regions of the sample, in contrast to previously published data on similar samples.