J. Phys. Chem. B 2000, 104, 10449-10461
10449
FEATURE ARTICLE Optically Detected Magnetic Resonance Studies of the Surface/Interface Properties of II-VI Semiconductor Quantum Dots E. Lifshitz,*,† A. Glozman,† I. D. Litvin,† and H. Porteanu‡ Department of Chemistry and Solid State Institute, Technion, Haifa 32000, Israel, and Physik-Department E 16, Technische UniVersitaet Muenchen, 85747 Garching, Germany ReceiVed: March 6, 2000; In Final Form: July 13, 2000
The described study concentrated on the investigation of II-VI semiconductor quantum dots, prepared as colloidal species imbedded in phosphate glass, or chemically deposited on a substrate. The ultimate goal of the present research was concerned with the examination of the influence of the surface/interface quality on the optical properties of those quantum dots. This was examined by the utilization of optically detected magnetic resonance spectroscopy. This method reflected knowledge of the chemical identity of the surface/interface trapping site, and trapped electron and trapped hole recombination mechanism. It also distinguished between radiative and nonradiative processes, determined the spin-lattice relaxation, estimated the trapped electronhole exchange mechanism and the distribution of defects at the surface.
I. Introduction In recent years, there has been an increase of interest in the scientific and technological aspects of semiconductor quantum dots (QDs). These materials exhibit unique chemical and physical properties, differing substantially from those of the corresponding bulk solids.1-6 The special properties are associated with two important characteristic: (1) the quantum size effect and (2) the existence of a relatively large fraction of atoms at the surface. The synthesis, structural and electronic properties of II-VI, III-V, IV-VI and I-VII (e.g., CuCl) QDs7-11 have been studied at length. These studies showed that the aforementioned materials offer the opportunity to tune the electronic and optical properties by variating of the QD’s size (so-called quantum size effect). The theoretical aspect of the electronic properties of II-VI, III-V individual QDs have been treated in several major publications, utilizing either k‚p3,7 or atomistic pseudo-potential methods.12 The extensive efforts to produce high quality QDs was motivated by their potential use in new and emerging technologies, such as optical switches,5 efficient lasers,13 light emitting diodes,14-16 biological markers,17,18 single-electron transistors,19 photovoltaic cells,20-22 and catalysis devices.23 Currently there are three main methods for the fabrication of QDs: (1) epitaxial growth of a QD material on top of a substrate, with a different lattice constant leading to strain induced threedimensional islands, known as Stranski Krastanow (SK) QDs;24-27 (2) colloidal chemistry techniques;6-8,14 (3) inorganic precipitation from a molten glass solution.28 The SK method produces relatively large dots, with weak quantum confinement, which can be arranged in an ordered array. The in-vivo growth in a glass solution produces relatively chemically stable QDs, but it * Corresponding author. E-mail:
[email protected]. † Department of Chemistry and Solid State Institute. ‡ Technische Universitaet Muenchen.
produces large size distribution and surface imperfections. The colloidal method permits the growth of strain free QDs with controlled diameters of
