Ultrafast Time-Resolved Photoluminescence of Zn-Se-Te Multilayers with Type-II ZnTe/ZnSe Quantum Dots Maurice C-K. Cheung a , Ian R. Sellers b , Igor L. Kuskovsky c , Alexander N. Cartwright a , Bruce D. McCombe b a Department of Electrical Engineering, University at Buffalo, The State University of New York, Buffalo, New York b Department of Physics, University at Buffalo, The State University of New York, Buffalo, New York c Department of Physics, Queens College at CUNY, Flushing, New York ABSTRACT Time-resolved photoluminescence (TRPL) across the full spectrum of a 240 layer ZnTe/ZnSe super-lattice structure has been performed using a femtosecond Ti-Sapphire laser and a streak camera system for detection. A significant change of the PL lifetime is observed across the emission spectrum decreasing smoothly from ~ 100 ns at 2.35 eV, to less than a few nanoseconds at 2.8 eV. The increase of the PL lifetime in the low energy region of the emission spectrum is strong evidence to support the formation of type-II quantum dots (QDs) from excitons bound to clusters of Te-atoms. In such QDs the confined holes and the free electrons are spatially separated, thus increasing the radiative lifetime. This result is consistent with recent photoluminescence (PL) measurements in which evidence of the combined contribution of excitons bound isoelectronically to Te-atoms, and type-II QDs was observed. Keywords: time-resolved photoluminescence, photoluminescence, isoelectronic center, type-II quantum dot, quantum dot, ZnSeTe 1. INTRODUCTION Recently ZnSe 1-x Te x dilute alloys (x ≤ 4 %) have been under extensive investigation due to their interesting optical properties. 1-4 For example, autocorrelation experiments have shown that isoelectronic centers (ICs) 2, 3 formed by two or more excitons bound to Te-atoms can emit single photons. 5 This may be interesting for quantum cryptography applications. Also, ZnTe/ZnSe multiple quantum well (MQW) and superlattice structures have been grown in hope of achieving tunable optical properties via quantum confinement effects (e.g. Reference 6). However, the photoluminescence (PL) of these structures is heavily dominated by IC emission, and explicit experimental evidence of the PL emission from quantum confined excitons was not observed until recently. 7 The structure in Reference 7 that shows evidence of quantum confinement in an epitaxially grown Zn-Se-Te multilayer system with a 240 period ZnTe/ZnSe superlattice structure. During the layer-by-layer growth, Te-atoms are introduced in sub-monolayer quantities, by migration enhanced epitaxy. 4 Despite the intention to build a superlattice, transmission electron microscopy showed no evidence of quantum well formation. 8 Nevertheless, the PL of this structure exhibits strong evidence of the formation of ZnTe/ZnSe quantum dots (QD) with a type-II band alignment. Type-II QDs have attracted considerable attention since they have been predicted to exhibit the Aharanov-Bohm effect for neutral particles. 9 Unfortunately, the PL of the QDs in the Zn-Se-Te systems is convoluted with the luminescence of excitons bound to ICs (Te n≥2 ) within the so called “green band”. 7 Moreover, PL studies indicate that there is a smooth transition in the electronic properties from ICs to type-II QDs, 7 so the clear distinction between IC and QDs is of high interest. In this letter, we present spectrally- and time-resolved photoluminescence (TRPL) results of this ZnSe:Te multilayer system. Furthermore, a full spectrum analysis of the PL decay clearly confirms the evolution of the Te ICs into type-II ZnTe/ZnSe quantum dots (QDs). Ultrafast Phenomena in Semiconductors and Nanostructure Materials XII edited by Jin-Joo Song, Kong-Thon Tsen, Markus Betz, Abdulhakem Y. Elezzabi Proc. of SPIE Vol. 6892, 68921A, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.764112 Proc. of SPIE Vol. 6892 68921A-1 2008 SPIE Digital Library -- Subscriber Archive Copy