Electronic energy levels and energy relaxation mechanisms in self-organized InAs/GaAs quantum dots M. J. Steer,* D. J. Mowbray, ² W. R. Tribe, ‡ M. S. Skolnick, and M. D. Sturge § Department of Physics, University of Sheffield, Sheffield S3 7RH, United Kingdom M. Hopkinson, A. G. Cullis, and C. R. Whitehouse Engineering and Physical Sciences Research Council Central Facility for III-V Semiconductors, Department of Electronic and Electrical Engineering, University of Sheffield, Sheffield, S1 3JD, United Kingdom R. Murray Department of Material Science and Engineering, University of Liverpool, Liverpool, L69 3BX, United Kingdom Received 28 May 1996 We report a spectroscopic investigation of the electronic energy levels and carrier-relaxation mechanisms in self-organized InAs/GaAs quantum dots. Power-dependent photoluminescence PL and photoluminescence excitation PLE are used to study the energy-level structure. Two excited states, 74 and 120 meV above the luminescent ground state, are identified. As expected for a zero-dimensional system, it is not possible to observe PL from the ground state of the dots when exciting between the energies of the ground and first excited state due to the discrete, atomiclike nature of the electronic states. Selectively excited PL and PLE reveal two mechanisms for the relaxation of carriers from the excited states to the ground state: a nonresonant mechanism dominant in the upper state, and a resonant mechanism, involving the emission of one or more LO phonons of well-defined energy, which is dominant in the lower excited state. The resonant mechanism is shown to be a consequence of the distribution of energy-level spacings in the inhomogeneous ensemble of dots; preferentially selecting dots with an energy-level spacing close to an integer multiple of the LO phonon energy. S0163-18299602548-9 I. INTRODUCTION Quantum dots provide the ultimate quantum system with three-dimensional carrier confinement resulting in atomic- like, discrete electronic energy states. In addition to allowing the study of physics in a zero-dimensional semiconductor system, these discrete energy levels are expected to result in a number of advantageous properties for electronic and electro-optic device applications. Quantum dot lasers are pre- dicted to exhibit both low threshold current densities, 1 and low- or zero-temperature dependence of the threshold current, 1 while the use of quantum dots may offer possibili- ties for low-power nonlinear devices. However, for device applications to be a realistic prospect the quantum dots must satisfy a number of requirements. These include large carrier confinement and energy-level separations kT , large areal densities, high optical quality, and uniform size and shape. Of the many techniques proposed and investigated for the fabrication of quantum dots, perhaps the most promising is that of self-organized growth. 2–5 Dots prepared by this tech- nique appear to be capable of satisfying all of the above requirements, although further improvements in size and shape uniformity are desirable. Self-organized growth may occur when a thin layer of one semiconductor is grown epi- taxially on a second semiconductor of a different lattice con- stant. For intermediate values of lattice mismatch the initial two-dimensional growth transforms, above a certain critical thickness, to nonuniform three-dimensional growth, resulting in a spatial modulation of the epitaxial layer thickness. This is known as the Stranski-Krastanov growth mechanism. The small areas of three-dimensional growth, which sit on a thin two-dimensional layer the so-called wetting layer, form the quantum dots. Although initially observed in the InAs-on- GaAs system, self-organized dots have now been observed in a wide range of material systems. 4,6,7 For the InAs-on-GaAs system, for which there is a 7% lattice mismatch, the result- ant InAs dots have a typical base size 10–25 nm and height 2–10 nm, 2,8 the actual size being dependent to some extent upon the growth conditions. These dimensions are small enough that strong quantum effects are observed. In this paper we present a study of the electronic energy levels and carrier relaxation mechanisms in self-organized InAs/GaAs quantum dots. The latter topic is of particular importance in zero-dimensional systems since it has been predicted that their discrete, atomiclike energy levels may inhibit the efficient carrier relaxation by single phonon emis- sion, which occurs in systems with continuous energy levels. 9 Unless other efficient relaxation mechanisms are possible, i.e., multiphonon, 10 Auger, 11 or long-range reso- nance energy transfer, 12 carrier relaxation rates will be very slow, with serious implications for device performance. II. EXPERIMENTAL DETAILS The samples were grown by solid source molecular-beam epitaxy using conditions very similar to those of Moison et al. 2 The structure consisted of a thin layer of InAs depos- ited on a GaAs buffer layer, which in turn was grown on an undoped GaAs substrate. At the growth temperature used T g =500–520 °C the transformation from two- to three- PHYSICAL REVIEW B 15 DECEMBER 1996-II VOLUME 54, NUMBER 24 54 0163-1829/96/5424/177387/$10.00 17 738 © 1996 The American Physical Society