Quantum Confined Electron-Hole States in ZnSe Quantum Dots
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ILawrence Livermore National Laboratory, P. 0. Box 808, Livermore, CA 94551, [email protected] 2 Department of Chemical Engineering and Materials Science, University of California, Davis, CA 95616 ABSTRACT We observed the quantum confined bandedge emission from ZnSe quantum dots and the size dependence of the energy states, spin-orbit interaction, and Stokes shift. The bandedge emission occurs in the UV-blue. The energy gap = Eg + C/dn where d is the diameter and n is 1.19 ± 0.13 and 1.21 ± 0.13 for the first and second electron-hole transitions, respectively. The separation between these transitions approaches the bulk spin-orbit splitting, while the Stokes shift decreases with particle size. Effective mass theories can not explain these results. Trap emission is observed in some samples in the green and red, resulting from Se-related traps. INTRODUCTION Recent developments in ZnSe quantum wells enabled the successful fabrication of blue and green light emitting diodes (LEDs) and diode lasers, 1-3 making ZnSe an important material system for blue emitting devices. However, ZnSe has seen intense competition recently from IIIV systems such as GaN. 4 This study of ZnSe quantum dots (QDs) was motivated by the inherent interest in quantum confined systems and by the potential for even shorter wavelength device
operation enabled by blue-shifted quantum confined energy levels of wide bandgap materials such as ZnSe. There are few reported results on ZnSe QDs.5- 7 We recently reported observation of blue light emission from ZnSe QDs in a glass matrix. 8 Here, we present a more detailed study of these QDs. We investigated the size-dependent optical and electronic properties of ZnSe QDs with absorption, photoluminescence (PL) and PL lifetime spectroscopy and compare these results to existing models of quantum confinement. EXPERIMENTAL Samples of ZnSe QDs in a potassium borosilicate glass matrix were made as described previously 5 and will be briefly described here. ZnSe powder was added to a pre-melted glass mixture and the blended mixture was re-melted at 1400'C for approximately 1.5 hours before casting the melt into small slabs. As-cast samples appear reddish-orange after overnight annealing at -350 *C. High Resolution Transmission Electron Microscopy (HRTEM) on these samples shows crystalline particles with sizes of 5.5 ± 1.7 nm. 5 Some of these as-cast samples were successively cycled through re-melting and rapid quenching. HRTEM shows particle sizes 259 Mat. Res. Soc. Symp. Proc. Vol. 571 ©2000 Materials Research Society
for quenched samples of 3.7 ± 1.1 nm. The samples were then irradiated with an Argon ion laser for various time durations. We showed that laser annealing affects the relative contributions of the bandedge and trap emission and decreases the QD size and size distribution. 9 Absorption and PL spectra were recorded with appropriate spectrometers. Optical excitation was provided by a xenon lamp or by the frequency doubled output of a femtosecond Ti:sapphire laser. A picosecond nitrogen laser and a photomu
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