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Optoelectrical properties of highly mismatched semiconductor materials

By Paul Harvey Jefferson


Dilute nitride alloys of III–V semiconductors, and transparent conducting group-II oxides may both be categorised as highly mismatched compounds. The small size and high values of electronegativity of nitrogen and oxygen (see figure), compared to the substituted anion, in dilute nitrides, and the cation, in transparent conducting oxides, give rise to striking properties in these materials.\ud \ud The dilute nitride alloys GaNSb, InNSb, and GaInNSb, grown by molecular beam epitaxy, have been studied. Infrared\ud absorption measurements of GaNSb are presented, showing the divergence of transitions from the valence band to E− and E+ conduction bands with increasing nitrogen incorporation. The fitting of the positions of the valence band to E+ transitions gives a value of 2.6 eV for the coupling parameter in this material. A reduction in the bandgap of InNSb from that of InSb is shown by modelling the competing effects of Moss-Burstein band filling and bandgap renormalisation. Finally, bandstructure calculations of the quaternary material GaInNSb, with dilute incorporations of nitrogen and indium, show that the material is suitable for the exploitation of the 8–14 μm atmospheric transmission window. Structural characterisation of GaInNSb shows that this material can be grown lattice matched to GaSb with nitrogen and indium incorporations of 1.8 and 8.4 per cent, respectively.\ud \ud The conducting oxide CdO, grown by metal-organic vapour-phase epitaxy, has also been studied. Analysis and simulation of infrared reflectance data, including conduction band non-parabolicity and Moss-Burstein band filling, reveal bandgap and band-edge effective mass values of 2.16 eV and 0.21 m0, respectively. In addition, high energy 4He+ ion irradiation was used to stabilise the Fermi level in CdO. Carrier statistics calculations were performed and the charge neutrality level was found to be 2.52 eV with respect to the "-point valence band maximum, corresponding to 0.36 eV above the conduction band minimum. The location of the charge neutrality level within the conduction band explains the propensity for high unintentional n-type doping, and the high conductivity observed in CdO

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