[show abstract][hide abstract] ABSTRACT: We report micro-photoluminescence studies of single GaN/AlN quantum dots grown along the (0001) crystal axis by molecular beam epitaxy on Si(111) substrates. The emission lines exhibit a linear polarization along the growth plane, but with varying magnitudes of the polarization degree and with principal polarization axes that do not necessarily correspond to crystallographic directions. Moreover, we could not observe any splitting of polarized emission lines, at least within the spectral resolution of our setup (1 meV). We propose a model based on the joint effects of electron-hole exchange interaction and in-plane anisotropy of strain and/or quantum dot shape, in order to explain the quantitative differences between our observations and those previously reported on, e.g. CdTe- or InAs-based quantum dots.
Physical Review B 06/2008; 77(23):235315. · 3.77 Impact Factor
[show abstract][hide abstract] ABSTRACT: We present single dot spectroscopy of hexagonal GaN/AlN self-assembled quantum dots (QDs) grown by MBE along the (0001) axis. The GaN quantum dots are grown on an AlN epilayer on Si (111) substrate, with dot densities between 108 and 1011 cm-2. We study the micro-photoluminescence spectra of a few quantum dots. In the energy range corresponding to the smaller dots we observe several groups of peaks, each group corresponding to the emission of a unique quantum dot. These groups are identified through their time-correlated spectral diffusion. The measured linewidth of the transition is 2 meV (resolution limited).
[show abstract][hide abstract] ABSTRACT: We study wurtzite GaN/AlN quantum dots (QDs) by time-resolved photoluminescence. The properties of nitride based nano-objects are significantly affected by strong built-in electric fields existing in this crystalline phase. These fields induce a spatial separation of electrons and holes in the quantum dot, a Stark shift of the transition energy, and they slow their radiative recombination down to tens of microseconds of decay time for the largest quantum dots. Such long decay times are responsible for the screening of internal fields by using moderate excitation intensities, thus modifying drastically both the energy and the rate of photon emission. Consequently it is difficult to perform spectroscopic studies of these QDs in the regime where a single electron-hole (e-h) pair is present at the most in each dot. Using a pulsed laser excitation, we study the time-resolved dynamics of carrier recombinations, waiting for complete recombination of all electron-hole pairs in the dots. We clearly exhibit the conditions where no screening of the internal electric field occurs. We especially obtain the transition energy and the mono-exponential decay, i.e the radiative recombination time, of the last e-h pair present in the dot, for a wide range of QD sizes. These results are analyzed within an envelope function model for the electron and hole confinement. The effective electric field, averaged over the strain distribution in the dots, is determined to be of 9.0 ± 0.5 MV/cm.
[show abstract][hide abstract] ABSTRACT: We report microphotoluminescence studies of single GaN∕AlN quantum dots grown by molecular beam epitaxy on Si(111) substrates. Small groups of emission lines characterize each single dot, with linewidths mostly limited by our experimental setup to 1 or 2 meV. By using time-dependent microphotoluminescence, we observe both the continuous and discontinuous spectral diffusion of these lines, assigned to the trapping of charges at defects in the vicinity of the dots. We show that this trapping takes place on a large variety of time scales, which depend on the photogeneration. It induces energy shifts that also cover some range, yielding both some unresolved broadening and discrete positions of the emission line. We propose that this results from different local configurations, mainly in terms of the distance between the defects and the dots.
Physical Review B 11/2006; 74(19):195319. · 3.77 Impact Factor
[show abstract][hide abstract] ABSTRACT: Wurtzite GaN/AlN quantum dots �QDs� are studied by time-resolved photoluminescence. Careful measurements allow us to reach the regime of a single electron-hole pair per dot, evidenced by a monoexponential decay of the luminescence and a stop of the time-dependent shift of the emission energy. The transition energy and the radiative lifetime of the QD ground state are measured in the absence of any electric field screening effect, for a wide range of QD heights. These results are analyzed within an envelope function model for the electron and hole confinement. The effective electric field, averaged over the strain distribution in the dots, is determined to be of 9.0±0.5 MV/cm.
Physical Review B 03/2006; 73(11):113304. · 3.77 Impact Factor