* Corresponding author. Fax: #81-298-47-7910. E-mail address: val@sqdp.trc-net.co.jp (V. Davydov) Journal of Luminescence 87}89 (2000) 503}505 Phonon-enhanced intraband transitions in InAs self-assembled quantum dots A.V. Baranov, V. Davydov*, H.-W. Ren, S. Sugou, Y. Masumoto ERATO Single Quantum Dot Project, JST, 5-9-9 Tokodai, Tsukuba, Ibaraki 300-2635, Japan Opto-Electronics Research Lab., NEC Corp., Tsukuba, Ibaraki 305-8501, Japan Institute of Physics, University of Tsukuba, Tsukuba, Ibaraki 305-8571, Japan Abstract Phonon-mediated intraband carrier relaxation was studied in InAs/GaAs self-assembled quantum dots (QDs) by combining resonant photoluminescence and photoluminescence excitation experiments with high spectral resolution. The phonon-related resonances were found to dominate both the luminescence and excitation spectra. Spectral widths of 1LO-phonon peaks are surprisingly small and comparable with the inverse lifetime of the LO phonons in QDs, whereas, the multiphonon resonances are much broader most likely re#ecting the width of n-phonon density of states.  2000 Elsevier Science B.V. All rights reserved. Keywords: InAs; Quantum dots; Phonon; Carrier relaxation Recent theoretical [1] and experimental [2}4] works showed that the phonon bottle-neck e!ect in semicon- ductor quantum dots (QDs) carrier relaxation may be overcome to a large extent by the nonharmonic decay of con"ned LO phonons into bulk acoustic phonons [1] or/and an e!ective inter-level relaxation involving multi- phonon scattering [2}4]. In both cases, the phonon [1] or multiphonon [2,3] resonances possess a broad spec- tral width, e.g., the large width of LO-phonon resonances observed in the resonant photoluminescence (RPL) and photoluminescence excitation (PLE) spectra ('7 meV) leads to relaxation windows of width of tens of meV around the 2LO}4LO resonances [3]. The inhomogene- ity of the stress distribution in the QDs, the weakening of k-selection rules, and the LO$LA process [3,4] were supposed to cause the large widths of the 1LO and nLO resonances. But the topic is yet to be clari"ed. We studied phonon-mediated carrier relaxation pro- cesses in InAs/GaAs self-assembled QDs at 2 K by com- bining RPL and PLE experiments with high spectral resolution (0.1 meV). The InAs QDs structures were fabricated by molecular beam epitaxy in the Stranski}Krastanow mode on Si-doped (210 cm) GaAs substrate. 1.8 monolayers of InAs were deposited at a temperature of 5003C giving rise to lens-shaped QDs with a diameter of +22 nm, a height of +10 nm, and an areal density of +1.210 cm. State-"lling experiment using an Ar laser showed that PL spectra of the sample (Fig. 1, top) exhibit several nearly equidistant peaks corresponding to optically al- lowed transitions between electron and hole levels with the same quantum numbers (enhn; n"0,1,2,3) [4,5]. The lowest transition energy (00) is 1.228 eV, energy splittings between the peaks 00, 11, 22, and 33 are equal to 53, 52, and 46 meV, respectively. An addi- tional weak transition with an energy 35 meV above the 00 one was found to contribute to the spectra. We attribute it to the 10 transition weakly allowed due to the dot shape distortion. The level separation, thus de- "ned, matches the energies of integer number of various LO phonons available in the system. The RPL and PLE spectra were excited by a wavelength-tunable narrow band cw Ti : sapphire laser. The measurements were done at low pump power ((0.2 W/cm) when RPL is only caused by recombination of the electrons and holes in 0022-2313/00/$ - see front matter  2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 2 3 1 3 ( 9 9 ) 0 0 2 2 5 - 2