III-Nitride Ultraviolet Photonic Materials ― Epitaxial Growth, Optical and Electrical Properties, and Applications

摘要

This paper summarizes some of the recent advances made by our group on the growth, characterization and applications of AlGaN alloys with high Al contents. Recently, our group has achieved highly conductive n-type Al_xGa_(1-x)N for x as high as 0.7 (a resistivity value as low as 0.15 ohm-cm has been achieved). Prior to this, only insulating Al_xGa_(1-x)N (x > 0.5) can be obtained. Our success is largely attributed to our unique capability for monitoring the optical qualities of these layers - the development of the world's first (and presently only) deep UV picosecond time-resolved optical spectroscopy system for probing the optical properties of Ill-nitrides [photoluminescence (PL), electro-luminescence (EL), etc.] with a time-resolution of a few ps and wavelength down to deep UV (down to 195 nm). Our time-resolved PL results have shown that we must fill in the localization states (caused by alloy fluctuation) by doping before conduction could occur. The density of states of localization states is about 10~(18)/cm~3 in this system. It was also shown that Al_xGa_(1-x)N alloys could be made n-type for x up to 1 (pure A1N). Time-resolved photoluminescence (PL) studies carried out on these materials have revealed that Si-doping reduces the effect of carrier localization in Al_xGa_(1-x)N alloys and a sharp drop in carrier localization energy as well as a sharp increase in conductivity occurs when the Si doping concentration increases to above 1 x 10~(18) cm~(-3). For the Mg-doped Al_xGa_(1-x) N alloys, p-type conduction was achieved for x up to 0.27. The Mg acceptor activation energy as a function of Al content has been deduced. Mg-δ-doping in GaN and AlGaN epilayers has been investigated. We have demonstrated that δ-doping significantly suppresses the dislocation density, enhances the p-type conduction, and reduces the non-radiative recombination centers in GaN and AlGaN. A1N epilayers with high optical qualities have also been grown on sapphire substrates. Very efficient band-edge PL emission lines have been observed for the first time with above bandgap deep UV laser excitation. We have shown that the thermal quenching of the PL emission intensity is much less severe in A1N than in GaN and the optical quality of A1N can be as good as GaN. From the low temperature (10 K) emission spectra, as well as the temperature dependence of the recombination lifetime and the PL emission intensity, the binding energies of the bound excitons and free excitons in A1N were deduced to be around 16 meV and 80 meV, respectively. From this, the energy bandgap of A1N epilayers grown on sapphire was found to be around 6.11 eV at 10 K. The observed large free exciton binding energy implies that excitons in A1N are extremely robust entities. This together with other well-known physical properties of A1N may considerably expand future prospects for the application of III-nitride materials.