Inelastic Light Scattering from Quasi-Two Systems: Impurities in Semiconductor Quantum Well Structures and Electrons Confined at Gallium-Arsenide Surfaces.

The results of inelastic light scattering experiments on two different quasi-2-dimensional electron systems are presented here. In the first system we study the effects of spatial confinement on the energy spectrum of shallow (Si) donors. Resonant Raman experiments on GaAs(Si)-Al(,x)Ga(,1 -x)As multiple quantum well heterostructures show new features attributed to the presence of the Si impurities. The transition from 1s to 2s donor states, derived from the lowest conduction subband, has been identified. There is good agreement between the measured energy of this excitation and recent calculations. The width and lineshape of the feature give an estimate of Si diffusion during growth. Another effect attributed to the presence of the donors is a splitting of free intersubband transition peaks. The low energy peaks of the doublets behave like free intersubband transitions. The high energy peak ((TURN)2 meV above the free intersubband energy) is studied as a function of power, temperature, and GaAs well width. These results are consistent with assigning this peak to a 1s donor ground state to 1s resonant impurity state excitation. This represents the first observation of resonant impurity states in semiconductor multiple quantum well heterostructures. The second system considered is that of semi-insulating GaAs codoped with Cr and a donor (e.g., Sn or Te). GaAs is a material that has undergone extensive study with Raman spectroscopy; however, none of the previous results have shown evidence of 2-dimensional plasmas. The data presented here show an electronic feature whose position, lineshape, and power dependence are inconsistent with bulk electronic scattering, but consistent with scattering from a photoexcited 2-dimensional electron gas. The confining potential which binds the 2-dimensional electron gas is created at the surface of the GaAs by a mismatch in the Fermi levels at the surface and in the bulk. Our results show both collective and single particle-like transitions of the photoexcited electrons analogous to intersubband transitions in Si MOSFETs and heterostructure systems. By comparing these results with calculations we are able to give an estimate of the free carrier density and Fermi level mismatch.