Probing Individual Localization Centers in an $\mathrm{InGaN}/\mathrm{GaN}$ Quantum Well

Photoluminescence (PL) spectroscopy with subwavelength lateral resolution has been employed to probe individual localization centers in a thin $\mathrm{InGaN}/\mathrm{GaN}$ quantum well. Spectrally narrow emission lines with a linewidth as small as 0.8 meV can be resolved, originating from the recombination of an electron-hole pair occupying a single localized state. Surprisingly, the individual emission lines show a pronounced blueshift when raising the temperature, while virtually no energy shift occurs for increasing excitation density. These findings are in remarkable contrast to the behavior usually found in macro-PL measurements and give a fundamental new insight into the recombination process in semiconductor nanostructures in the presence of localization and strong internal electric fields. We find clear indications for a biexciton state with a negative binding energy of about $-5\pm 0.7\mathrm{meV}$.

Figure 1

PL spectra from apertures of different sizes at $T=4\mathrm{K}$.

Figure 2

PL spectra measured using an aperture of 350 nm for different temperatures. The dashed line indicates the energy shift of an emission peak originating from a single localized state.

Figure 3

Energy shift versus temperature (a) of macro-PL spectra (aperture size $\varphi =25\mu \mathrm{m}$) and (b) of a single line (marked by the dashed line in Fig. 2). The dashed line corresponds to the expected Varshni shift of the band gap [17]. (c) Linewidth of two single lines versus temperature. The expected line broadening due to phonon scattering (after Xu et al. [18]) is marked by a dash-dotted line. The solid lines are guides to the eye.

Figure 4

Energy shift of a single PL line versus excitation density compared to the energy shift of the spectral weight of an ensemble ($\varphi =1\mu \mathrm{m}$). The temperature is 4 K. Inset: Power-dependent evolution of PL spectra obtained with an aperture size of 250 nm at $T=4\mathrm{K}$.