Lateral carrier transfer in Cd x Zn 1 Àx SeÕ ZnS y Se 1 Ày quantum dot layers S. Rodt,* V. Tu ¨ rck, R. Heitz, F. Guffarth, R. Engelhardt, U. W. Pohl, M. Straßburg, M. Dworzak, A. Hoffmann, and D. Bimberg Institut fu ¨r Festko ¨rperphysik, Technische Universita ¨t Berlin, Hardenbergstraße 36, 10623 Berlin, Germany Received 14 November 2002; published 26 June 2003 Lateral carrier transfer is investigated for single Cd x Zn 1-x Se/ZnS y Se 1-y quantum dots QD’s in a high- density ensemble by time-resolved spectroscopy. Following nonresonant excitation a significant probability of independent capture of electrons and holes in separate QD’s is observed. The subsequent lateral migration of carriers between adjacent QD’s leads to a slow decay component of the exciton ground-state luminescence. At low temperatures the lateral carrier transfer is restricted to phonon-assisted inter-QD tunneling, resulting in migration times of the order of several nanoseconds. The role of independent carrier capture is suppressed at high excitation densities or increased temperatures, enabling thermally activated migration. DOI: 10.1103/PhysRevB.67.235327 PACS numbers: 78.67.Hc, 72.20.Jv, 78.55.Et, 78.60.Hk I. INTRODUCTION Semiconductor quantum dots QD’s grown epitaxially in the Stranski-Krastanow mode on planar substrates are pres- ently the subject of intense studies. 1 The interest is largely driven by the simplicity of the fabrication of high-density ensembles of defect-free QD’s and their application in opto- electronic devices, like lasers and detectors. A critical aspect in such applications is the lateral interaction of QD’s, being essential for the carrier mobility within the QD layer and, thus, the steady-state carrier distribution. At sufficiently low temperatures and large inter-QD spacings the QD’s act as independent recombination centers that are statistically populated. 2 The resulting carrier dynamics and, hence, statis- tics do not allow for an equilibrium description based on average occupation numbers and a global quasi-Fermi level. Either thermally activated escape followed by recapture or tunneling, which requires dense QD ensembles with suffi- ciently low tunnel barriers, might lead to inter-QD carrier transfer. 3–6 Efficient lateral carrier migration would result in the formation of a global Fermi-level and quantum-well like statistics. As a consequence of the self-organized growth, the QD’s are neither structurally identical nor homogeneously distrib- uted, hampering the experimental investigation of the inter-QD interaction. The electronic coupling between neigh- boring QD’s has been investigated for vertically stacked QD’s, 7,8 for which the separating barrier can be controlled by modifying the spacer thickness and composition. Stacking of QD’s, which intentionally differ in size or composition, was exploited for the investigation of energy transfer pro- cesses. 9,10 Theoretical investigations predicted electronic coupling for a spacer thickness below about 6 nm, 11 and fast nonresonant energy transfer was observed for asymmetric QD pairs with tunnel barrier widths below about 5 nm. 10 Note that the energy transfer is nonresonant even for nomi- nally identical QD pairs due to the asymmetry of the built-in strain. 11 The investigation of lateral carrier transfer processes within a dense QD layer is more difficult since neighboring QD’s can neither be identified nor isolated. Here, the identi- fication of lateral energy transfer is indirect, requiring to model the average effect on the ensemble properties. For QD ensembles with densities in excess of about 10 11 cm -2 ,a decrease of the decay time on the high-energy slope of the photoluminescence PL peak was observed and taken as evidence for lateral exciton transfer from smaller to larger QD’s. 5,12–14 With increasing temperature, thermally activated escape to the wetting layer followed by recapture leads to an efficient redistribution of excitons among the QD’s, 15 estab- lishing ultimately a global quasi-Fermi level. 16 Obviously, the transfer of excitons between QD’s can occur only during the exciton lifetime and, thus, affects the initial decay of the QD luminescence in the subnanosecond region. 12–14 Exciton transfer cannot account for slower decay components extend- ing well beyond the exciton life span, which have been ob- served repeatedly for the ground-state luminescence of self- organized QD’s. 17,18 For samples with a single layer of QD’s such a slow component was tentatively attributed either to carrier feeding from tail states in the matrix 17 or to the co- existence of two different radiative exciton states, character- ized by different spectral and temporal characteristics 18,19 For stacked pairs of asymmetric QD’s such a slow compo- nent was attributed to the formation of spatially indirect excitons. 20 For colloidal CdSe QD’s Ref. 21 and alloy- disordered Cd x Zn 1 -x Se/ZnSe quantum wells 22 spin-flip from dark to bright exciton states might provide an intrinsic ex- planation for observed PL decay times, that are much longer than the radiative bright exciton lifetime. A potential model system for studies of lateral carrier mi- gration are Cd x Zn 1 -x Se layers in a ZnS y Se 1 -y matrix, which in suitable samples form a high-density ( 10 11 cm -2 ) ensemble of weakly localizing QD’s. Such properties favor inter-QD tunneling and, thus, lateral carrier migration within the QD layer. Time-resolved experiments show in addition to the well-known direct exciton decay a slow decay com- ponent with time constants of some nanoseconds for the ground-state luminescence, which is demonstrated to result from the bright exciton decay. The results of time-resolved cathodoluminescence TRCL investigations of single QD’s and time-resolved photoluminescence TRPL experiments of the ensemble indicate lateral migration of carriers in the Cd x Zn 1 -x Se/ZnS y Se 1 -y QD layer due to inter-QD tunneling. PHYSICAL REVIEW B 67, 235327 2003 0163-1829/2003/6723/2353277/$20.00 ©2003 The American Physical Society 67 235327-1