AlGaN and InAlGaN alloys - Epitaxial growth, optical and electrical properties, and applications

The paper summarises some of recent advances made by the author's group on the growth, characterization and applications of AlGaN and InAlGaN alloys. Electrical, optical as well as optoelectronic properties have been studied. It is shown that the effect of carrier localisation in undoped AlGaN alloys enhances with increased Al contents and is related to the insulating nature of AlGaN of high Al contents. Si-doping reduces the effect of carrier localization in AlGaN alloys and a sharp increase in conductivity occurs when the Si doping concentration increases to above 10¹⁸ cm^-3. For the Mg-doped Al_xGa_1-xN alloys, p-type conduction was achieved for x up to 0.27. Due to the strong piezoelectric polarization and deep triangular potential notch in Al_xGa_1-xN (x = 0.5) heterointerface, a total of five emission lines related with the two-dimensional electron gas in heterostructure have been observed, which correspond to the recombination between the electrons from different sub-bands. The dominant optical transition at low temperatures in In_xAlyGa_1-xN quaternary alloys is due to localized exciton recombination, while the localisation effects were combined from those of InGaN and AlGaN ternary alloys with comparable In and Al compositions. Finally micro-size ultraviolet light emitters are demonstrated.