Control of the Oscillator Strength of the Exciton in a Single InGaN-GaN Quantum Dot

We report direct evidence for the control of the oscillator strength of the exciton state in a single quantum dot by the application of a vertical electric field. This is achieved through the study of the radiative lifetime of a single InGaN-GaN quantum dot in a $p\mathrm{\text{−}}i\mathrm{\text{−}}n$ diode structure. Our results are in good quantitative agreement with theoretical predictions from an atomistic tight-binding model. Furthermore, the increase of the overlap between the electron and hole wave functions due to the applied field is shown experimentally to increase the attractive Coulomb interaction leading to a change in the sign of the biexcitonic binding energy.

Figure 1

(a) A series of micro-PL spectra recorded under reverse bias at 4.3 K under two-photon excitation. The excitation power is 38 mW and the excitation wavelength is 830 nm. A large blueshift is observed. (b) The bias dependence of the energy shift of the experimental QD indicated in (a) (square symbols) and for the theoretical QD described in the text (triangular symbols). The continuous lines are a fit using a second order polynomial. The inset shows the dependence of the linewidth of the emission line from the experimental QD as obtained from a Lorentzian fit of the emission line. The dashed line represents the optical resolution of the setup, $570\mu \mathrm{eV}$.

Figure 2

(a) Time-resolved spectra (log scale) of the investigated QD for three values of the applied bias. The spectra were fitted with a single exponential. (b) The experimental (squares) and theoretical (circles) dependence of the radiative lifetime on the applied bias. The inset of (b) is the theoretical value of the $z$ component of the permanent dipole as a function of the bias.

Figure 3

(a) A series of micro-PL spectra as function of the bias from a different position recorded at 4.3 K. In this case, the excitation power is 34 mW and excitation wavelength is 850 nm. (b) An expanded portion of the spectra presented in (a) at 0 V and 5 V. L1 refers to the low-energy emission line and L2 refers to the high-energy line. (c) The intensity dependence of L1 and L2 on excitation power (log-log scale) at 0 V and 5 V.