Controlled Passivation and Luminescence Blue Shifts of Isolated Silicon Nanocrystals
- PDF / 365,306 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 99 Downloads / 533 Views
I6.2.1
Controlled Passivation and Luminescence Blue Shifts of Isolated Silicon Nanocrystals Julie S. Biteen1, Anna L. Tchebotareva2, Albert Polman1,2, Nathan S. Lewis1, and Harry A. Atwater1 1 California Institute of Technology, 1200 E. California Blvd., Pasadena, CA 91125 2 FOM Institute for Atomic and Molecular Physics, Kruislaan 407, 1098 SJ Amsterdam, The Netherlands
ABSTRACT We have performed a comparative study of oxide- and nonoxide-passivated silicon nanocrystals to probe the role of the silicon/oxygen interface in low coverage, non-interacting silicon nanocrystal systems. Ensembles of Si nanocrystals characterized by a narrow distribution and diameters of 2–5 nm were synthesized by ion implantation into SiO2 films followed by a hightemperature anneal in Ar. The nanocrystals were removed from the SiO2 film matrix and deposited on Si substrates using a chemical etch in HF, leaving a hydrogen-terminated surface. A natural oxide layer grows on these surfaces in air. We characterized the morphology of the samples with atomic force microscopy (AFM) and the spectroscopic properties with photoluminescence (PL) and X-Ray photoelectron spectroscopy. We found that the PL energy of Si nanocrystals can be shifted by particle size reduction and hydrogen or oxygen termination. Further, PL peak energy shifts upon etching and oxidation were consistent with the model of Wolkin et al. that proposes that for very small radii, a silicon-oxygen double bond will produce deep interface states which red shift the luminescence. INTRODUCTION Silicon nanocrystals with diameters below 5 nm show room temperature photoluminescence (PL)[1]. This emission is generally attributed to quantum size effects[2], but the origin of PL is only partially understood. It is widely accepted that excitonic emission is responsible for a portion of the luminescence, with quantum confinement leading to visible PL as the structure’s dimensions become smaller than the Bohr radius of an exciton[2]. The quantum confinement effect scales with confinement radius, r, as r -2[3]. However, the results of experiments in several groups have shown that, although the PL energy of silicon is indeed blue shifted as its dimensions decrease, the shift is smaller than expected[4,5]. Recent theoretical models suggest that localized states at the surface of these materials can account for the relatively wide range of experimental data found in the literature[6,7]. Wolkin and coworkers proposed an interface state related to the presence of Si=O double bonds on the nanocrystal surface. Their results show that while the energy difference between conduction and valence bands does indeed increase as r -2, the Si=O double bond leads to interface states that lie within the expected band gap and cause the emitted photons to have reduced energies. The inclusion of this surface group leads to calculated energy levels that are consistent with experimental PL data[6]. More recently, reports of optical gain and lasing in Si nanocrystal systems[8,9] have invoked three- or four-level mode
Data Loading...
