Carrier Dynamics in III--Nitrides Studied by Time-Resolved Photoluminescence

Abstract

Recent results on time-resolved photoluminescence (PL) studies for Al-rich AlGaN alloys, InAlGaN quaternary alloys, InGaN/InAlGaN and GaN/AlN quantum wells (QWs), GaN quantum dots (QDs), waveguides, and microsize emitters are reviewed. New developments in time-resolved PL experimental methods are briefly described. It is shown that Si doping reduces the effect of carrier localization in Al$_{x}$Ga$_{1-x}$N alloys with high Al contents 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 $1 \times 10^{18}\mathrm{cm}^{-3}$. Time-resolved PL studies suggest that the carrier localization effects in In$_{x}$Al$_{y}$Ga$_{1-x}$N quaternary alloys are the summation of those in AlGaN and InGaN alloys with comparable In and Al compositions. As a result, the quantum efficiency of In$_{x}$Al$_{y}$Ga$_{1-x-y}$N quaternary alloys is enhanced over AlGaN alloys with a comparable Al content. Similar results have been obtained for InAlGaN-based QWs. Studies on AlGaN/GaN heterostructures with high Al content show that the PL emission lines associated with a two-dimensional electron gas (2DEG) persist at temperatures up to $220\mathrm K$, in sharp contrast to the AlGaAs/GaAs system in which the 2DEG emission lines are observable only at $T < 20\mathrm K$. For AlN/GaN QWs, a conduction-band offset parameter of roughly 70% is obtained for the AlN/GaN heterostructures and the radiative lifetimes of the interband transitions of electrons and holes in the $n=1$ and $n=2$ subbands are measured to be $0.12\mathrm{ns}$ and $0.44\mathrm{ns}$, respectively, whereas the relaxation of the electrons and holes from the $n=2$ to $n=1$ subbands is only about $33\mathrm{ps}$. Time-resolved studies on GaN quantum dots reveal a large redshift in the PL spectra as well as large decay lifetime due to the built-in electric field in GaN/AlN QDs, implying that GaN QDs with size larger than a certain level will not be good candidates for optoelectronic-device applications. Time-resolved PL is also utilized to probe the light-propagation property in GaN/AlGaN QW submicrometer waveguides and the speed of generated polaritons is determined to be approximately $1.3 \times 10^{7}\mathrm{m}/\mathrm{s}$. Time-resolved electroluminescence spectroscopy has also been developed and employed to study the size dependence of the switching characteristics of the microsize LEDs and reveals that the switching speed increases systematically as the size of micro-LEDs is reduced.