Nanoscale Res Lett (2006) 1:172–176 Fig. 3 Autocorrelation measurements under cw laser excitation obtained from the exciton photon of a single GaAs QD at (a) 5 K and (b) 77 K. The insets display the corresponding PL spectra (a) (b) 175 we found an acoustic-phonon broadening cAc = 1.0 ± 0.1 leV K–1 in a single QD, which suggests that the linewidth does not depend strongly on the temperature in this range. This value is slightly larger compared to the values obtained from InGaAs QDs reported by Bayer et al. [15] and Borri et al. [31] but smaller than the value obtained by Urbaszek et al. [24]. At higher temperature, a slight contribution of the phonon side- bands to the linewidth data deduced from the Lorentzian ﬁt of the central peak of the X PL-line cannot be excluded. We estimate an upper limit for optical-phonon broadening (cOp) in a single QD to be 30 ± 2 meV from the ﬁt of the above equation which suggests that a strong electron-LO-phonon interaction occurs. The integrated PL intensity of the ground state transition of the single QD is shown in the inset of Fig. 2. The integrated intensity remains almost con- stant up to 100 K and shows an exponential quenching at higher temperatures. An activation energy EA = 68 ± 3 meV is derived from the data ﬁt of the integrated intensity using [32]: I = I0/[1 + Cexp( – EA/ kBT)], where I is the integrated PL intensity, I0 is the PL intensity at 5 K, C is the transition rate (the ratio of the thermal escape rate to radiation recombination rate), and EA is the activation energy. The measured activation energy is nearly half of the total barrier height [33] of 140 meV (i.e., the sum of the barrier heights for electrons and holes). A possible explana- tion is that the carriers behave as correlated electron- hole pairs [34]. Autocorrelation measurements have been per- formed to demonstrate single photon generation as a function of temperature under cw excitation of the single GaAs QD. Figure 3(a), (b) show the measured normalized correlation function g(2)(s) of the (X) QD emission at 5 K and 77 K, respectively. The corre- sponding PL spectra are shown in the insets. Both traces exhibit a clear dip in the correlation counts for the time delay s = 0 ns, indicating a strong photon antibunching. The g(2)(s) is ﬁtted by a function of the form: g(2)(s) = 1 – aexp( – |s|/tm), where a accounts for the background present in the measurements and tm is the antibunching time constant. The values of 1 – a = g(2)(0) obtained from the ﬁt are 0.06 for the trace (a) which shows a nearly perfect single photon emitter and 0.45 for trace (b). In both cases, g(2)(0) < 0.5 is a signature of a single quantum emitter. The measured g(2)(0) does not reach its theoretical value of zero because of the presence of a weak uncorrelated background at low temperature. At high temperatures, a stronger background contributes to the PL spectra due to the acoustic phonon sidebands [25] and the presence of the transitions involving holes in the excited states [20], which leads to the reduction of the photon antibunching from the X line. In the pres- ence of a background the value of the g(2)(0) is increased by a factor of 1 – q2, where q = S/(S + B) is the ratio of signal S to background B counts [35]. From the PL spectra (insets of Fig. 3) q was determined to be 0.97(0.75) for T = 5 K (77 K). The resulting values for g(B2)(0) are 0.06 (0.44), which are in good agreement with the g(2)(0) values. In summary, we have studied the temperature dependence of the luminescence of single unstrained self-assembled GaAs quantum dot (QD) structures. Single QD spectroscopy showed that the QDs are characterized by intense room temperature PL emis- sion. Surprisingly, it was found that at room temperature the linewidth of the single QD is comparable to the ensemble inhomogeneous broadening. The single quantum emission nature was demonstrated at elevated temperatures by photon correlation measurements. The QD PL emission energies, homogeneous linewidths and the thermally activated behavior were discussed. Acknowledgements The authors would like to thank J. Kuhl for helpful discussions and K. v. Klitzing for his interest and support.