16924
J. Phys. Chem. C 2010, 114, 16924–16928
Chemistry of Silicon Nanocrystal Surfaces Exposed to Ammonia† Navneethakrishnan Salivati,‡ Nimrod Shuall,§ Joseph M. McCrate,‡ and John G. Ekerdt*,‡ Department of Chemical Engineering, UniVersity of Texas at Austin, Austin, Texas 78712, and D.C. Sirica Ltd., Nesher, 36680 Israel ReceiVed: December 21, 2009; ReVised Manuscript ReceiVed: February 25, 2010
Silicon nanocrystals (NCs) 8-10 nm in diameter are grown on SiO2 surfaces in an ultrahigh-vacuum chamber using hot wire chemical vapor deposition. These NCs are subjected to varying exposures of deuterated ammonia (ND3). The surface chemistry of Si NCs is studied using X-ray photoelectron (XP) spectroscopy and temperature-programmed desorption (TPD). The dissociative adsorption of ND3 on Si NCs results in the formation of ND2 species prior to TPD, and Si3N (nitride) formation is observed after TPD. D2 desorption is observed only from the monodeuteride species at 780 K. In separate experiments, a hot tungsten filament is used to predissociate ND3 before adsorption on the NC surface. XP spectra reveal that ND2 species form initially, and as the dose is increased, ND species dominate. After TPD, a SixNy species is observed. D2 desorption is observed from the mono-, di-, and trideuteride species when ND3 is predissociated. Irrespective of the technique used for dosing ammonia, TPD spectra do not contain any ND3 fragments, indicating that the ND2 species are not thermally stable on the NC surface. The photoluminescence (PL) emitted from Si NCs (diameter ∼ 4.1 nm) is reported for an excitation wavelength of 405 nm. PL is observed only when the hot filament is used to predissociate ND3, and this can be attributed to the presence of di- and trideuteride species on the nanocrystal surface, which results in better passivation. 1. Introduction A variety of systems containing nanometer-sized silicon (Si) features, such as porous Si,1 silicon/silicon dioxide superlattices,2 Si nanoparticles,3 and Si nanowires,4 are known to emit light despite the indirect band gap of bulk Si. These materials are promising candidates for light emitting diode (LED) applications, and LEDs based on nanocrystalline Si with turn-on voltages as low as 1.4 V have been developed.5,6 In such materials, the surface passivation has an important role because the ratio of surface atoms to the total number of atoms is quite large. Nonradiative recombination of excitons can take place at the defect states due to unpassivated dangling bonds at the interface.7 Surface reconstructions reduce the number of dangling bonds but result in severe distortion of the surface bonds, creating defect states within the bandgap.8–10 An understanding of the surface chemistry is therefore imperative for the development of nanocrystal (NC)-based light emitting devices. We have previously examined the surface chemistry of deuterium-terminated Si nanocrystals11 and explained the influence of surface passivation on the optoelectronic properties.12 Hydrogen/deuterium treatment can relieve the strained surface bonds formed due to reconstructions and eliminate the dangling bonds. However, hydrogen passivated Si surfaces tend to oxidize, and there is a need to explore additional passivating agents. Herein, we explore the surface chemistry of deuterated ammonia (ND3) on Si NCs. The dissociative adsorption of ammonia on Si(100)8,13,15–18 and Si(111)8,14–17 surfaces has been studied extensively using X-ray photoelectron spectroscopy (XPS) and temperatureprogrammed desorption (TPD). The dissociative adsorption of †
Part of the “D. Wayne Goodman Festschrift”. * Corresponding author. Phone: 512.471.4689. Fax: 512.471.7060. E-mail:
[email protected]. ‡ University of Texas at Austin. § D.C. Sirica Ltd.
ammonia on Si(100) at 300 K leads to the formation of NH2(a) and H(a) species. A recent study by Kim et al. utilizing highresolution core-level photoemission spectroscopy indicated the presence of some NH species on Si(100) at 500 K.18 Formation of NH(a) is observed on Si(111) in addition to NH2(a) at temperatures above 200 K.14 Dufour et al. have also reported the presence of N(a) on Si(111) at 300 K.15 As the temperature is increased, N-H bond dissociation takes place; formation of NH(a) species is favored between 300-600 K. The decomposition of ammonia on porous Si surfaces at 300 K predominantly results in the formation of NH2 and H with a little NH (
