Linköping Studies in Science and Technology. Dissertatins
No. 541

Electronic structure and optical properties of InGaAs and InAsP semiconductor quantum wires

Johan Hammersberg

Akademisk avhandling

som för avläggande av teknologie doktorsexamen vid Tekniska Högskolan I Linköping kommer att offentligt försvaras i hörsal Planck, Fysikhuset, Linköpings Universitet, torsdagen den 11 juni 1998, kl 10.15. Fakultetsopponent är Prof. Jörn M. Hvam, Mikroelektronikcentret, DTH Lyngby, Danmark.

Abstract

In recent years intensive studies of semiconductor nanostructures or so-called semiconductor quantum structures have been performed in many research groups around the world. The research interest in quantum structures arises both from their potential to expose new physical phenomena in condensed matter physics, allied with fundamental quantum mechanical effects, and from their potential application in electronic and optoelectronic devices.

The key feature of quantum structures is their nanometer size ~10 nm, which should be compared with the lattice constant in semiconductor crystals ~0.5 nm. The nanometer size modifies markedly the electronic structure in the semiconductor crystal due to quantum mechanical effects. The electronic structure is thus strongly related to both the size and the material properties of the semiconductor of which the quantum structure is fabricated. Basic research and understanding of the electronic structure and the optical properties of these structures are necessary to facilitate the development of new electronic and optoelectronic devices.

Here I present an investigation of the electronic structure and optical properties of InGaAs/InP and InAsP/InP semiconductor quantum wires. A quantum wire has nanometer wide cross-section (~10 x 10 nm) and a macroscopic length. Both the InGaAs and the InAsP quantum wires are completely buried within InP. The InGaAs quantum wires are lattice-matched to the surrounding InP barrier, that is equal lattice constants. The InAsP quantum wires, on the contrary, are not since they have a larger lattice constant than InP. Combined effects of quantum confinement, strain and modulation doping are investigated here. Strained hetero-epitaxy is an important technological method to modify the electronic structure in semiconductors. Doping is a necessity to introduce and control the excess carriers in electronic and optoelectronic semiconductor devices. Combining quantum wire confinement with strain and modulation doping in a single structure, one obtains a complex system here characterized by investigations of quantum wires of successive widths. Unpolarized and polarized photoluminescence spectroscopy combined with extremely high magnetic fields (up to 28 T) have been used to experimentally characterize their electronic structure and optical properties. The photoluminescence spectra give direct information on the optical transition energies in the quantum wires, and the extremely high magnetic fields make it possible to affect their electronic structure with an external and adjustable parameter. These experiments resolve effects due to quantum confinement and strain that result in energy shifts in the electronic structure and shifts of the luminescence peaks, for example. Polarized photoluminescence measurements also give direct information on the symmetry of the electronic wave functions and effects due to the broken symmetry in quantum wires with a non-uniform strain distribution. The measurements, furthermore, give information on the carrier density and the number of populated electron energy levels (subbands) in modulation doped quantum wire structures. These experiments have been combined with theoretical modeling using k·p, finite element and self-consistent calculations to obtain a better understanding of the electronic structure and optical properties of InGaAs and InAsP semiconductor quantum wires.

Department of Physics and Measurement Technology
Linköping University, S-581 83 Linköping, Sweden
Linköping 1998

ISBN 91-7219-236-4 ISSN 0345-7524