174 suggests the possibility of using these QDs with large surface density as efﬁcient active region (e.g., in a mi- crocavity laser), which could allow for plenty of new application relevant research as well as fundamental physics studies. The single GaAs QD PL spectra as a function of the temperature taken at 5.7 Wcm–2 excitation power are shown in Fig. 1(b). The graphs show a clear shift of the center of the excitonic luminescence to longer wavelengths due to the bandgap reduction with increasing temperature. The PL spectrum taken at low temperature consists of resolution-limited well sepa- rated sharp emission lines with no background. The X line is well visible up to 100 K, above this temperature X overlaps with multiexcitonic lines. Below 130 K, the emission is rather intense so integration times of 1 s were sufﬁcient to obtain reasonable signal-to-noise ratios. The temperature increase causes the PL line- width to increase, since the contribution of the phonon sidebands becomes larger. In Fig. 2, we display a typical temperature variation of the peak position energy and the homogeneous linewidths of the X PL-line of a single GaAs QD (T £ 100 K) and QD ground-state (T > 100 K) deduced from the Lorentzian proﬁle taken at 5.7 Wcm–2 excitation power. The reasons of choos- ing this power for the temperature dependence mea- surements are: (a) It was the minimum power of Fig. 2 Temperature dependence of PL energy positions (n) and homogeneous linewidth of the exciton PL line (•) and the dot ground-state (s) in a single GaAs QD. The solid line is a ﬁt of the experimental data in the range 0–100 K. The inset is an Arrhenius plot of the integrated PL intensity as a function of reciprocal temperature Nanoscale Res Lett (2006) 1:172–176 obtaining a reasonably good PL signal up to room temperature and (b) from the power dependence measurements taken at 5 K no effect was found on the linewidths of the X PL line for powers up to 7 Wcm–2. The PL peak energy shows a redshift of about 97 meV as the temperature increases from 5 K to 295 K due to the bandgap shrinkage. In contrast with the behavior of InGaAs QDs [23], the redshift of the PL peak energy of GaAs QD ground-state (X) with rising temperature follows the thermal shrinkage of the bulk GaAs and is well ﬁtted using the empirical Varshini equation (not shown here). We believe that the disagreements found in Ref. 23 have to be attributed to strain-related phe- nomena. Taking into account the spectral resolution of our setup (90 leV) we obtain for the X qanﬃﬃﬃiﬃﬃnﬃﬃtﬃﬃrﬃﬃiﬃnﬃﬃﬃsﬃﬃiﬃcﬃﬃ linewidth G of 24 ± 15 l eV at 5 K (C ¼ C2m Cr2es [24], where G is the intrinsic linewidth, Gm is the mea- sured linewidth, and Gres is the spectral resolution of our setup). This value increases only by a few leV as the temperature is raised to 40 K. Furthermore, as the sample temperature is increased, the PL spectra be- come broader due to the contribution of mainly two mechanisms: (a) Phonon coupling which leads to the appearance of a broad background that emerges at both sides of the zero-phonon line which is assigned to the phonon sidebands [25] and (b) phonon scattering, which gives rise to the broadening of the excitonic zero-phonon transition [26]. As already mentioned above, the X line is clearly visible up to 100 K. Therefore, for a more quantitative analysis of the coupling mechanism the zero-phonon line is separated from the phonon sidebands by ﬁtting only the central part of the X line by a Lorentzian. The linewidth data are ﬁtted using the following relation, which describes the temperature dependence of the excitonic peak broadening in quantum wells and bulk semicon- ductors [27] and which has also been used for QDs [28, 29]: C ¼ C0 þ cAcT þ cOpðehxLO=kBT 1Þ 1, where G0 = 84 ± 13l eV is the zero K linewidth, the second term (cAc) gives the acoustic phonon scattering, and the third term gives the scattering with optical pho- nons. hxLO represents the energy of the longitudinal optical (LO) phonon of GaAs. The phonon broadening exhibits a linear variation for T £ 40 K. The linear term in the above equation has been reported by many research groups [see e.g., Refs. 15 and 28]. In the quantum well case, the linear temperature variation accounts for the exciton absorption of acoustic pho- nons of energies much smaller than kBT [30]. The physical origin of the observed linear term in the temperature dependence of the homogeneous line- width in QDs is still under debate. At low temperature,