Nanoscale Res Lett (2006) 1:172–176 Moreover, only few studies of high temperature pho- ton emission statistics from SK-grown QDs [18–20] have been performed. In this work we study the tem- perature dependence of the emission of a single, hier- archically self-assembled GaAs/AlGaAs QD. At room temperature we observe bright PL emission with FWHM of 8.5 meV. Remarkably, the width of a single-QD emission is comparable to the ensemble inhomogeneous broadening of 12.4 meV. By com- bining a micro-photoluminescence (l-PL) and a Han- bury-Brown and Twiss (HBT) correlation setup, we demonstrate a single photon emitter from the excitonic transition up to 77 K. Also, the QD-PL emission energies, homogeneous linewidths and the thermally activated behavior are discussed. The investigated samples were grown on GaAs (001) substrate by a solid-source molecular beam epi- taxy system equipped with an AsBr3 etching unit. The GaAs QDs are obtained by overgrowing a GaAs sur- face containing self-assembled nanoholes [12] with 7 nm Al0.45Ga0.55As, 2 nm GaAs, 100 nm Al0.35- Ga0.65As, 20 nm Al0.45Ga0.55As and 20 nm GaAs [7]. The 2-nm-thick GaAs layer ﬁlls up the nanoholes in the underlying AlGaAs barrier leading to the forma- tion of inverted GaAs QDs below a thin GaAs quan- tum well (QW). For single-QD and ensemble investigations, the QD density is chosen as 1 · 108 cm–2 and 4 · 109 cm–2, respectively. To perform single QD PL spectroscopy, 1 lm2 mesa structures were fabricated by optical lithography and wet etching. The sample was mounted in a cold-ﬁnger helium ﬂow cryostat which can be moved by computer- controlled xy-linear translation stages for exact posi- tioning with a spatial resolution of 50 nm. For the excitation of our sample structure, the laser light was focused by a microscope objective (with numerical aperture = 0.6) to a spot diameter of 2 lm. The same microscope objective was used to collect the QD emission. The collected luminescence was then spec- trally ﬁltered by a 0.5 m focal length monochromator equipped with a liquid nitrogen cooled charge coupled device (CCD) for PL measurements. The samples were excited by a continuous wave (cw) laser emitting at 532 nm (see Ref. [21] for further details). For photon statistics measurements, the PL light is sent to a modiﬁed HBT correlation setup [22]. The HBT con- sisted of a 50/50 non-polarizing beam splitter and two single-photon counting avalanche photodiodes (SAP- Ds) each providing a time resolution of ~700 ps. The SAPDs output signals were used to trigger the start and stop channels of a time-to-amplitude converter the output of which was stored in a PC-based multichannel analyzer. In this way, a histogram n(s) of photon 173 correlation events as a function of the time delay s = tstop – tstart was recorded. Figure 1(a) compares the room temperature PL emission from an unstrained single GaAs QD located at 1.545 eV (solid line) and a PL spectrum from ensemble GaAs QDs peaked at 1.528 eV (dashed line) from a different sample with high QD-density taken at excitation power of 480 Wcm–2 with integration times of 1 s. The PL lines located at 1.429 eV, 1.581 eV, and 1.708 eV are assigned to bulk GaAs, the ﬁrst excited state of the single GaAs QD, and the quantum well, respectively. The PL peak energy of the ensemble shows a redshift compared to the single QD because the high-density sample is characterized by slightly larger QDs. The ensemble spectrum was shif- ted to higher energy by 17 meV for comparison. The ground state PL linewidth of the single GaAs QD at room temperature is 8.5 meV, which is comparable to the inhomogeneous linewidth of the ensemble GaAs QDs ( 12.4 meV). This is a surprising result, since it is commonly assumed that the FWHM of a QD ensemble is mainly determined by the size/composition ﬂuctua- tions of the QDs. While this remains true at low tem- perature, Fig. 1(a) shows that the room-temperature emission-linewidth of our QDs is dominated by the homogeneous broadening of the single QD emission. Such a result, which is attributed to the good size homogeneity of our QDs ( ± 6% in height [8]), (a) (b) Fig. 1 (a) PL spectra measured at room temperature from a single GaAs QD (solid line) and from an ensemble of GaAs QDs (dashed line). For the excitation of the ensemble sample, the laser light was focused using a 5 · microscope objective to a spot diameter of 20 lm and about 104 QDs were being excited. The ensemble spectrum was shifted to higher energy by 17 meV for comparison. (b) PL spectra of a single GaAs QD as a function of the temperature