Stavrinou, Paul Nicholas; (1995) A study of InP-based strained layer heterostructures. Doctoral thesis (Ph.D.), University College London (United Kingdom).

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Abstract

The small layer dimensions required in low dimensional structures, such as multiple quantum wells (MQW), allows constituent materials to be used which are lattice mismatched in bulk form. The primary aim of this thesis is the study of InP based electroabsorption devices which comprise lattice mismatched induced strained layer MQW material. Experimental and theoretical investigations are undertaken into the band-edge electronic structure and associated optical properties of strained InASxP1-x/ InP quantum wells. The devices are designed for application around 1.06 ?m and, for the first time, demonstrate growth of this QW system by solid source molecular beam epitaxy. Modelling is also used to study the properties of three layer InP/ InASxP1-x / InzGa1-zAs QW's which are proposed for their internal staircase-like potential. The associated effective built-in field allows a blue shift of the fundamental absorption edge and offers improved device performance over the conventional red shifting MQW devices. Theoretical descriptions of MQW subband states are based on the effective Hamiltonian model (k.p) which includes coupling between conduction and valence bands as well as the effects of biaxial strain. The approach is extensively discussed and alternative approximations for describing band-edge structure are assessed. To account for the presence of an electric field, a tunnelling resonance method using a k.p description is used to find the quasi-bound states of the structure. Excitonic properties are computed using variational techniques thereby giving the ability to simulate optical absorption spectra using phenomenological broadening of both the exciton line and the continuum. Excellent agreement is found with experiment. Optical absorption spectroscopy and bias dependent photocurrent spectra from p- i(MQW)-n structures is presented along with supporting cross-section TEM and X-ray diffraction measurements. In the context of these results, limits to pseudomorphically strained growth and routes to strain relaxation are discussed. The latter is shown to be an important factor in determining the absorption characteristics of devices.

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