Year

1992

Degree Name

Doctor of Philosophy

Department

Department of Physics

Abstract

In this thesis semiconductor structures with strained (InGa)As layers are studied. The principal structures employed are a series of asymmetric modulation doped AlxGa1-xAs-InyGa1-yAs-GaAs quantum wells (AMDQWs) in which a high density of electrons occupies up to two subbands (n = 1,2) in the strained InyGa1-yAs quantum well. A detailed study of these structures by photo-luminescence (PL) and photo-luminescence excitation (PLE), both with and without a magnetic field forms a major part of this thesis. Along with characterisation of the AMDQWs and determination of bandstructure parameters,several interesting phenomena, arising in these samples due to the high electron density, strongly asymmetric band-structure and strain are discussed (supported by self-consistent calculations). Reported here is the first experimental evidence for the indirect fundamental bandgap (in wave-vector space) which is developed when a magnetic is applied parallel to the plane of the layers of the AMDQWs . The observed P L undergoes a large approximately quadratic shift with field, an order of magnitude larger than the expected diamagnetic shift. This observation is a consequence of the allowed transitions in an increasingly indirect gap bandstructure. There has been considerable interest recently in the Fermi Energy Edge Singularity (FEES), particularly in AMDQWs containing two occupied subbands. The FEES is a many body effect observed in PL and PLE as an excitonic enhancement near the Fermi energy; its characteristic signature is a strong temperature dependent broadening and decrease of peak height. From the temperature dependence of PL and PLE measurements of a Schottky gated AMDQW with a significant n = 2 population at zero bias, a minimum density of ns,2⋍0.4 x 1011 cm- 2 is established for the clear observation of FEES behaviour. In samples where the Fermi energy, EF is close to the subband separation E2 - E1 a strong excitonic peak near the energy of the transition of the second subband E21 is attributed to hybridization of electrons in the n = 1 subband near the Fermi energy, with n = 2 states. Of particular interest is the interpretation of pronounced magneto-oscillations in the intensity of such P L transitions. In the work presented here it is shown that these oscillations can be largely accounted in terms of oscillations in the occupation of n = 1 states near EF, driven by Landau level (LL) quantization. PLE measurements show that variations in oscillator strength of E21 with field are negligible. Other phenomena associated with many body effects are discussed, these include Resonant Polaron Coupling between occupied LLs and LO phonons (recently reported) and magnetic field dependent oscillations of LL transition energies about the expected linear dependence.

PL results are presented for a Double Barrier Resonant Tunnelling Structure which incorporates a strained (InGa)As layer. In one bias direction under operation the (InGa)As layer forms a pre-well in the emitter accumulation region. This allows for the first time observation of PL from the emitter region. PL measurements from the prewell and the GaAs Q W (sandwiched between the two barriers) provide direct information on the charge distribution of the DBRTs under operation which are in good agreement with the magneto-transport measurements made.

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