Size-dependent oxygen-related electronic states in silicon nanocrystals

Abstract

Silicon nanocrystals embedded in SiO 2 were isolated with a selective etching procedure, and the isolated nanocrystals' excitonic emission energy was studied during controlled oxidation. Nanocrystals having initial diameters, d 0 , of ϳ2.9-3.4 nm showed a photoluminescence ͑PL͒ blueshift upon oxidatively induced size reduction, as expected from models of quantum confinement. Oxidation of smaller Si nanocrystals ͑d 0 ϳ 2.5-2.8 nm͒ also initially resulted in a PL blueshift, but a redshift in the PL was then observed after growth of ϳ0. 2 Detailed assessments of the relationship between Si nanocrystal size and band gap using self-consistent tight binding 3 and Quantum Monte Carlo ͑QMC͒ 4 methods predict a continuous increase in band gap with decreasing diameter down to diameters less than 1 nm. Experimental measurements have confirmed that the excitonic emission energy of Si nanocrystals increases as the nanocrystal size decreases, but the increase is smaller than that expected theoretically. 3,5,6 This discrepancy suggests the presence of excitonic recombination through a localized electronic state whose energy level lies within the band gap of the smaller Si nanocrystals.Si nanocrystals are typically embedded in silicon dioxide or surrounded by a native oxide layer, and theoretical models suggest that oxygen-related states at the nanocrystal surface can produce intragap energy levels.3,4 Wolkin and coworkers evaluated theoretically the effect of a surface silicon-oxygen double bond ͑Si= O͒ on the electronic band structure of a Si nanocyrstal.3 Their semiempirical computations predicted that the energy difference between the conduction and valence bands would increase roughly according to r −2 , but their calculations also predicted that, for nanocrystals with d Ͻ ϳ 3 nm, the Si= O bond should produce interface states that lie within the band gap. Puzder et al. used QMC calculations for small nanocrystals and interpolated to the bulk band gap with trends from density functional theory to confirm the semiempirical results. 4 More recently, reports of nonlinear optical effects in Si nanocrystal systems have invoked three-or four-level models that require, along with the Si band edges, the presence of deep-lying surface states. 7,8 To test these interface-related excitonic recombination models experimentally, we have used PL spectroscopy to measure the exciton recombination energy of Si nanocrystals in a well-controlled size range, d ϳ 2.5-3.4 nm, with and without passivation by an oxygen environment.Ensembles of Si nanocrystals were produced by implantation of 15 keV Si + ions to a fluence of 1.3ϫ 10 16 cm −2 into 15-nm-thick silicon dioxide films that were grown by thermal oxidation of lightly p-doped Si͑100͒. According to SRIM,9 a Monte Carlo simulation program, such implantations lead to a Gaussian depth distribution of silicon in the SiO 2 matrix and produce a peak excess Si concentration of 15% at a depth of 10 nm. The implanted samples were annealed at 1100°C for 5 min in Ar. Transmission electron microscopy r...

Oxidation of freestanding silicon nanocrystals probed with electron spin resonance of interfacial dangling bonds

“…29 In previous investigations of Si-NCs fabricated by other techniques, it has been found that oxidation may also result in a redshift of the NCs photoemission. 26,30,31 Theoretical calculations indicate that this could originate from the introduction of intragap energy levels by interfacial Si=O bonds, 26,27,32 and Si-O-Si surface bridges.…”

Stable, freestanding Ge nanocrystals

“…A similar HF etching procedure was recently employed to study the photoluminescence behavior of free standing Si nanocrystals. 3 Transmission electron micrographs of embedded nanocrystal samples demonstrate that nanocrystals are spherical with an average size of 5.1 nm and a distribution full width at half maximum (FWHM) of 3.9 nm [ Fig. 1(a)].…”

A Chemical Approach to 3-D Lithographic Patterning of Si and Ge Nanocrystals

“…It is likely that for the nanocrystal size range investigated here (average diameter > 3 nm), luminescence originates from size-dependent oxygen-related defect centers at the nanocrystal/matrix interface rather than from confined excitons within the nanocrystal. [12] Further studies are underway on larger nanocrystals, which can be obtained by annealing at higher temperatures, to determine the effect of oxygen dose on confinement-based luminescence.…”