Optical and Magneto-Optical Studies of Doped III -v Quantum Well Structures.

The main theme of this thesis is the optical study of strained semiconductor structures. This includes using photo-luminescence (PL) and photo-luminescence excitation (PLE), both with and without a magnetic field. The principal structures employed are a series of asymmetric modulation doped rm Al_{x}Ga_ {1-x}As-rm In_{y }Ga_{1-y}As-GaAs quantum wells (AMDQWs) in which a high density of electrons occupies up to two subbands (n = 1,2) in the strained In _{rm y}{Ga}_ {rm 1-y}As quantum well. Several interesting phenomena due principally to the high-electron density, are discussed (supported by self-consistent calculations). The first experimental evidence is reported for the indirect fundamental bandgap (in wave-vector space), developed when a magnetic field is applied parallel to the plane of the layers. The PL undergoes a large approximately quadratic shift. This is a consequence of the allowed transitions in an increasingly indirect gap band structure. Of particular interest is the Fermi Energy Edge Singularity (FEES) observed in AMDQWs with significant occupation of the second subband (n = 2). The FEES is a many body effect observed in PL and PLE as an excitonic enhancement near the Fermi energy (E_{rm F}). From the characteristic temperature dependent broadening and decrease of PLE peak height in a Schottky gated AMDQW, a minimum electron density in n = 2 of 0.4 x 10 ^{11} cm^{ -2} is established for the clear observation of FEES behaviour. In samples where E_ {rm F} is close to the subband separation E_2-E_1 magneto -oscillations in the PL intensity of E_2 , are observed; E_{21} is attributed to hybridisation of n = 1 electrons near E_{rm F}, with n = 2 states. It is shown that the E_{21 } oscillations can be accounted for in terms of oscillations in the occupation of n = 1 Landau level states near E_2. Other phenomena discussed include Resonant Polaron Coupling between occupied LLs and LO phonons. PL results are presented for a Double Barrier Resonant Tunnelling Structure (DBRTS) in which a strained (InGa)As layer forms a pre-well in the emitter accumulation region. This allows observation of PL from the emitter region as well as from the GaAs QW, sandwiched between the two barriers. Thus providing direct information on the charge distribution of the DBRTS under operation, in good agreement with magneto-transport measurements.