Time-resolved photoluminescence of ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}/\mathrm{G}\mathrm{a}\mathrm{N}$ multiple quantum well structures: Effect of Si doping in the barriers

The carrier recombination dynamics in a series of ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}/\mathrm{G}\mathrm{a}\mathrm{N}$ multiple quantum wells, nominally identical apart from different Si doping concentrations in the GaN barriers, was studied by time-resolved photoluminescence (PL) with excitation densities ranging from $220\hspace{0.5em}{\mathrm{n}\mathrm{J}/\mathrm{c}\mathrm{m}}^2$ to $28\hspace{0.5em}\mu {\mathrm{J}/\mathrm{c}\mathrm{m}}^2$ at 10 K and 300 K. At early time delays and with excitation densities greater than $5\hspace{0.5em}\mu {\mathrm{J}/\mathrm{c}\mathrm{m}}^2,$ at which the strain-induced piezoelectric field is screened by both photogenerated carriers and electrons from the GaN barriers, we observe a strong ${\mathrm{In}}_x{\mathrm{Ga}}_{1-x}\mathrm{N}$ PL peak initially located $\sim 60\hspace{0.5em}\mathrm{meV}$ below the absorption edge and well above an effective mobility edge. This peak decays quickly with an effective lifetime less than 70 ps and disappears into the extended states while it gradually redshifts. The amount of this PL peak redshift decreases with increasing Si doping in the GaN barriers, suggesting that the peak is due to radiative recombination of free excitons in the screened piezoelectric field.