Retardation in the Oxidation Rate of Nanocrystalline Silicon Quantum Dots

111

We have investigated phase separation, silicon nanocrystal (Si NC) formation and optical properties of Si oxide (SiO x , 0Ͻ x Ͻ 2) films by high-vacuum annealing and dry oxidation. The SiO x films were deposited by plasma-enhanced chemical vapor deposition at different nitrous-oxide/silane flow ratios. The physical and optical properties of the SiO x films were studied as a result of high-vacuum annealing and thermal oxidation. X-ray photoelectron spectroscopy (XPS) reveals that the as-deposited films have a random-bonding or continuous-random-network structure with different oxidation states. After annealing at temperatures above 1000°C, the intermediate Si continuum in XPS spectra (referring to the suboxide) split to Si peaks corresponding to SiO 2 and elemental Si. This change indicates the phase separation of the SiO x into more stable SiO 2 and Si clusters. Raman, high-resolution transmission electron microscopy and optical absorption confirmed the phase separation and the formation of Si NCs in the films. The size of Si NCs increases with increasing Si concentration in the films and increasing annealing temperature. Two photoluminescence (PL) bands were observed in the films after annealing. The ultraviolet (UV)-range PL with a peak fixed at 370-380 nm is independent of Si concentration and annealing temperature, which is a characteristic of defect states. Strong PL in red range shows redshifts from ϳ600 to 900 nm with increasing Si concentration and annealing temperature, which supports the quantum confinement model. After oxidation of the high-temperature annealed films, the UV PL was almost quenched while the red PL shows continuous blueshifts with increasing oxidation time. The different oxidation behaviors further relate the UV PL to the defect states and the red PL to the recombination of quantum-confined excitions.

Section: F Oxidation Effectsmentioning

confidence: 99%

Annealing and oxidation of silicon oxide films prepared by plasma-enhanced chemical vapor deposition

Chen

Tang

et al. 2004

111

“…6,7 This fabrication route is very attractive because of its ability to control the size and location of a narrow NC band and its compatibility with standard complementary metal-oxide semiconductor ͑CMOS͒ processes. In practice, high-dose ͑typically 10 16 cm −2 ͒ Si implantation in the 1 keV range into very thin ͑Ͻ10 nm͒ oxide layers followed by annealing at temperatures in the 900-1000°C range allows for the formation of two-dimensional ͑2D͒ arrays of Si NCs at tunable tunneling distances from the SiO 2 / Si interface. 6 As shown in Ref.…”

Section: Introductionmentioning

confidence: 99%

Oxidation of Si nanocrystals fabricated by ultralow-energy ion implantation in thin SiO2 layers

Coffin

Bonafos

Schamm

et al. 2006

Articles you may be interested inModeling stress retarded self-limiting oxidation of suspended silicon nanowires for the development of silicon nanowire-based nanodevices Journal of Applied Physics 110, 033524 (2011) The effect of thermal treatments in nitrogen-diluted oxygen on the structural characteristics of two-dimensional arrays of Si nanocrystals ͑NCs͒ fabricated by ultralow-energy ion implantation ͑1 keV͒ in thin silicon dioxide layers is reported. The NC characteristics ͑size, density, and coverage͒ have been measured by spatially resolved electron-energy-loss spectroscopy by using the spectrum-imaging mode of a scanning transmission electron microscope. Their evolution has been studied as a function of thermal treatment duration at a temperature ͑900°C͒ below the SiO 2 viscoelastic point. An extended spherical Deal-Grove ͓J. Appl. Phys. 36, 3770 ͑1965͔͒ model for self-limiting oxidation of embedded silicon NCs has been carried out. It proposes that the stress effects, due to oxide deformation, slow down the NC oxidation rate and lead to a self-limiting oxide growth. The model predictions show a good agreement with the experimental results. Soft oxidation appears to be a powerful way for manipulating the NC size distribution and surface density.

“…The oxidation rate is reduced to 1/3 for Si nanocrystals with an initial size of 15 nm compared with that of a Si͑100͒ substrate for 15 h oxidation at 750°C. 31 Complete oxidation of Si nanocrystals is difficult and Si nanocrystals are embedded in the oxide layer after oxidation. Oxidation can reduce the size of the nanocrystals ͑give rise to the QCE͒, convert the outer layer material into Si oxide ͑introduce the Si-SiO x interface or defects in the oxide layer͒, and create c-Si in an oxide shell ͑passivate the nanocrystal surface͒.…”

Section: Effects Of Oxidation and Annealingmentioning

confidence: 99%

Mechanisms of photoluminescence from silicon nanocrystals formed by pulsed-laser deposition in argon and oxygen ambient

Chen

et al. 2003

We have investigated the different mechanisms of photoluminescence ͑PL͒ of silicon nanocrystals due to the quantum confinement effect ͑QCE͒ and interface states. Si nanocrystals were formed by pulsed-laser deposition in inert argon and reactive oxygen gas. The collisions between the ejected species greatly influence the morphology of the Si nanocrystals and cause a transition from a film structure to a porous cauliflowerlike structure, as the ambient gas pressure increases from 1 mTorr to 1 Torr. The oxygen content of the Si nanocrystals increases with increasing O 2 ambient pressure, and nearly SiO 2 stoichiometry is obtained when the O 2 pressure is higher than 100 mTorr. Broad PL spectra are observed from Si nanocrystals. The peak position and intensity of the PL band at 1.8 -2.1 eV vary with ambient gas pressure, while intensity changes and blueshifts are observed after oxidation and annealing. The PL band at 2.55 eV shows vibronic structures with periodic spacing of 97Ϯ9 meV, while no peak shift is found before and after oxidation and annealing. Raman and transmission electron microscope measurements show consistent results in crystal size while more accurate atomic force microscope measurements reveal a smaller crystal size. X-ray diffraction reveals a polycrystal structure in the Si nanocrystals and the crystallinity improves after annealing. Combined with the PL spectra of Si nanocrystals obtained by crumbling electrochemically etched porous Si layer, the results clearly demonstrate that the PL band at 1.8 -2.1 eV is due to the QCE in the Si nanocrystal core, while the PL band at 2.55 eV is related to localized surface states at the SiO x /Si interface.