VOLUME 74, NUMBER 20 PHYSICAL REVIEW LETTERS 15 MA+ 1995 Ultranarrow Luminescence Lines from Single Quantum Dots M. Grundmann, * J. Christen, t N. N. Ledentsov, ~ J. Bohrer, and D. Bimberg Institut fiir Festkorperphysik, Technische Universitat Berlin, Hardenbergstrasse 36, D 106-23 Berlin, Germany S. S. Ruvimov, ~ P. Werner, U. Richter, ~ U. Gosele, and J. Heydenreich Max Plan-ck Instit-ut fur Mikrostrukturphysik, Weinberg 2, D 0612-0 Halle, Germany V. M. Ustinov, A. Yu. Egorov, A. E. Zhukov, P. S. Kop'ev, and Zh. I. Alferov A. F Ioffe . Physical Techn-ical Institute, Politekhnicheskaya 26, 194021 St Pete. rsburg, Russia (Received 23 May 1994) We report ultranarrow (( 0. 15 meV) cathodoluminescence lines originating from single InAs quantum dots in a GaAs matrix for temperatures up to 50 K, directly proving their 6-function-like density of electronic states. The quantum dots have been prepared by molecular beam epitaxy utilizing a strain-induced self-organizing mechanism. A narrow dot size distribution of width 12 ~ 1 nm is imaged by plan-view transmission electron microscopy. Cathodoluminescence images directly visualize individual dot positions and recombination from a single dot. A dense dot array ( — 10" dots/cm') gives rise to a distinct absorption peak which almost coincides with the luminescence maximum. PACS numbers: 73. 20.Dx, 36. 40. — c, 78.55. — m, 78.60.Hk Semiconductor quantum dots (QD's) are expected to exhibit exciting new electronic and optical properties such as an atomiclike 6-function density of states (DOS) not found for three-, two-, and one-dimensional structures, and strongly increased exciton binding energies and oscillator strength [1 — 3]. Thus great effort is currently underway to fabricate nm-scale QD's. Because of full quantization in all three dimensions the eigenspectrum of a single dot always consists of a discrete set of eigenenergies depending only on the number of atoms making up a dot. Variations of strain or shape lead to additional continuous variation of the eigenenergies from dot to dot. The finite carrier lifetime will introduce Lorentzian broadening 5(E) of finite width I . Thus, taking into account only ground-state transitions (the quantization effects in our dots are so high that only one electron level exists), the recombination spectrum Rz of N quantum dots is a series of sharp lines, N R~(hv) = g S(hv — E„). (1) n=l This is especially valid for high temperatures where kgT » I and has to be experimentally verified in order to prove zero-dimensional DOS. No such spectroscopic confirmation for the existence of 6-function DOS in these structures is available in the literature so far. Size fluctuations between single dots will lead to a statistical distribution 1 (E) of the eigenenergies, characterized by a spectral width W. In the limit of a large number of dots, as, e. g. , sampled by photoluminescence, the recombination spectrum of the dot ensemble R is given by the convolution of the single QD spectrum with the distribution function. Since I » W, the spectrum basically rejects the statistical fluctuations between dots, R (hv) = S(h v — E)P(E) dE — P(h v) . (2) The few spectra of nm-scale dot systems reported up to now represent the dot ensemble average R, are usually several 10 meV broad (e.g. , [4,5]), and cannot reflect the zero-dimensional character of the electronic properties. In this Letter we present for the first time direct evidence for a b-function density of states of single nm-scale QD's, observed by cathodoluminescence (CL) imaging up to 50 K. Several approaches for in situ fabrication of QD's were reported hitherto: formation of microcrystallites in a glass matrix [6], growth on stepped surfaces [7], formation of corrugated superlattices on self-organized microscopically ordered faceted surfaces [8], and strain-induced self- organized growth of QD's [4,5, 9 — 13]. We employ the latter approach and fabricate structures consisting of a plane of InAs QD's inserted into a GaAs quantum well, confined by short period GaAs-A103Gao7As super- lattices. The samples are grown by elemental source molecular beam epitaxy (MBE) on GaAs(001) substrates. Growth rates are 0.8 p, m/h for GaAs and 0.3 p, m/h for InAs. Arsenic pressure was (2 — 3) X 10 6 torr. After oxide desorption, a 0. 5- p, m-thick GaAs buffer is grown at 600'C, then 200 A. of the Alo3Gao7As is deposited, followed by a2 nm/2 nm GaAs-Alo 3Gao 7As superlat- tice (5 periods), and a 7 nm GaAs layer. Then the substrate temperature is lowered to 450'C and the de- sired amount of InAs is deposited. Afterward 5 nm of GaAs is grown at 450 'C, then the substrate temper- ature is increased to 600 C, and a 2-nm-thick GaAs layer is grown. This layer is followed by a 2 nm/2 nm GaAs-A1„3Gao7As superlattice (5 periods) and 20 nm of A103Ga07As; a 5 nm GaAs layer is grown on the top for surface protection. ReAection high-energy electron diffraction (RHEED) patterns are monitored during the 0031-9007/95/74(20)/4043(4) $06. 00 1995 The American Physical Society