Origin of high oscillator strength in green-emitting InGaN/GaN nanocolumns

Optical characterization has been performed on an InGaN/GaN nanocolumn structure grown by nitrogen plasma assisted molecular beam epitaxy not only in macroscopic configuration but also in a microscopic one that can be assessed to a single nanocolumn. The photoluminescence (PL) decay monitored at 500 nm is fitted with a double exponential curve, which has lifetimes of 0.67 and 4.33 ns at 13 K. These values are two orders of magnitude smaller than those taken at the same wavelength in conventional InGaN/GaN quantum wells (QWs) grown toward the C orientation. PL detection of each single nanocolumn was achieved using a mechanical lift-off technique. The results indicate that the very broad, macroscopically observed PL spectrum is due to the sum of the sharp PL spectrum from each nanocolumn, the peak energy of which fluctuates. Moreover, unlike conventional QWs, the blueshift of a single nanocolumn is negligibly small under higher photoexcitation. These findings suggest that carrier localization as well as the piezoelectric polarization field is suppressed in InGaN/GaN nanocolumns.