Optical evidence of polaron interaction in InAsÕGaAs quantum dots M. Bissiri, G. Baldassarri Ho ¨ ger von Ho ¨ gersthal, A. S. Bhatti,* M. Capizzi, and A. Frova Istituto Nazionale di Fisica della Materia, Dipartimento di Fisica, Universita ` di Roma ‘‘La Sapienza,’’ Piazzale Aldo Moro 2, I-00185 Roma, Italy P. Frigeri and S. Franchi CNR-MASPEC Institute, Parco Area delle Scienze, I-43100 Parma, Italy Received 16 November 1999; revised manuscript received 24 February 2000 Photoluminescence PL and resonant PL RPL have been performed at low temperatures in a number of InAs/GaAs quantum dots QD’s whose emission energies range from 1.4 to 1.08 eV. A simple, standard electron-phonon interaction model reproduces PL and RPL spectra well. The value of the electron-phonon interaction S is large for small QD’s and evolves to small values for large, well-formed QD’s. This trend is consistent with recent experimental results in InAs QD’s and provides an experimental basis to recent theo- retical speculations. I. INTRODUCTION Quantum dots QD’s are good candidates for optoelec- tronic devices, such as optical switches or semiconductor lasers. Carriers are strongly confined and the linear and non- linear optical emission of QD’s should be greatly enhanced relative to those of bulk materials. 1 Although the density of states is atomiclike, the photoluminescence PL bands in QD’s are quite broad because of the finite spread in the dot- size distribution, a major handicap for the development of many devices. 2,3 The electron-phonon interaction in QD’s has been the subject of thorough theoretical and experimental investiga- tions because it gives rise to multiphonon processes in the carrier relaxation and radiative decay. Moreover, it acts as an additional source of line-shape broadening. Multiphonon structures in the PL and Raman spectra of nanosize quantum dots permit us to estimate the effective strength of the electron-phonon interaction, namely, the Huang-Rhys factor S, from the intensity ratio between terms neighboring in the optical spectra. In II-VI QD’s, S varies over almost one order of magnitude, with values much higher than those predicted on the basis of simple adiabatic models assuming a local charge neutrality because of carrier confinement. 1 In CdSe, e.g., S ranges from 0.035 see Ref. 4 to 1.3 see Ref. 5 compared with a predicted value of S 10 -2 . Quite high values of S have been measured also for PbS 0.7 and CuBr 1.0 QD’s. 6,7 The electron-phonon interaction in the weakly ionic III-V compounds is expected to be one order of magnitude smaller than that in the II-VI compounds. 8 Therefore, in the former compounds it should be masked easily by inhomogeneous broadening due to fluctuations in the QD shape and/or size. Only two estimates of S in InAs QD’s can be found in the literature, at least to the best of our knowledge. 9,10 In well- formed QD’s emitting at 1.05 eV, S ranges from 0.012 to 0.02. 9 These values are 5 times larger than in bulk InAs 0.0033. In InAs QD’s emitting at 1.311 eV, S is instead 0.5; see Ref. 10. Finally, a much higher value of S 1.25 has been estimated in a standard Huang-Rhys approach from the PL spectra of a single InP/Ga x In 1 -x P QD. 11 In summary, in all compounds S varies from sample to sample by more than one order of magnitude, with values one or two orders of magnitude higher than those predicted by simple models for both bulk II-VI and III-V compounds. In order to reconcile experimental results and theoretical pre- dictions, several refinements have been introduced into the standard adiabatic approach. 8–10,12 Alternatively, a break- down of the adiabatic approximation, due to both external and internal effects in the exciton-phonon system, has been proposed. 13 Finally, magnetospectroscopy in self-assembled InAs QD’s has suggested that electrons and longitudinal- optical LO phonons are always in a strong coupling regime and form an everlasting mixed electron-phonon mode, a novel type of electron-phonon behavior in heterostructure physics. 14 In the present work, we perform a systematic investiga- tion of the electron-phonon interaction in a series of InAs QD’s differing in size and/or shape. The emission energies of the samples investigated here cover a wide range of energies, from 1.4 to 1.08 eV. S values have been derived directly from the ratio of phonon replica in PL or resonant PL RPL spectra and/or by a fit to a simple adiabatic model. We find that S decreases for decreasing QD emission energy. This result is consistent with those previously reported in the op- posite limits of small QD’s emitting at high energy or, vice versa, of large QD’s emitting at low energy, Ref. 10 and Ref. 9, respectively. We suggest that the effective electron- phonon interaction depends on QD morphology, namely, size, shape, strain distribution, and inhomogeneities, in agreement with recent theoretical predictions. 10 II. EXPERIMENTAL DETAILS Low temperature PL and RPL measurements have been performed on InAs/GaAs heterostructures grown by molecu- lar beam epitaxy MBE or atomic layer MBE ALMBE in a Varian GenII chamber, under various growth conditions. This has allowed us to investigate samples whose emission energy ranges from few tens of meV below the two- PHYSICAL REVIEW B 15 AUGUST 2000-I VOLUME 62, NUMBER 7 PRB 62 0163-1829/2000/627/46425/$15.00 4642 ©2000 The American Physical Society