Formation Of Si Nanocrystallites Research Articles

Comparative Investigation of Structural and Optical Properties of Si-Rich Oxide Films Fabricated by Magnetron Sputtering

RF magnetron sputtering of two separate silicon and oxide (SiO2 or Al2O3) targets in pure argon plasma was used for deposition of Six(SiO2)1-x and Six(Al2O3)1-x films with x=0.15-0.7 on long fused quarts substrate. The effect of post-fabrication treatments on structural and light emitting properties of the films with different x values was investigated by means of Raman scattering, electron paramagnetic resonance and X-ray diffraction as well as by photoluminescence (PL) methods. The formation of amorphous Si clusters upon deposition process was found for the both types of films. The annealing treatment at 1150°C during 30 min results in formation of Si nanocrystallites (Si-ncs). The latter were found to be larger in Six(Al2O3)1-x films than that in Six(SiO2)1-x counterparts with the same x values and are under tensile stresses. The investigation of photoluminescence properties of annealed films of both types revealed the appearance of visible-near infrared light emission. The Six(SiO2)1-x films demonstrated one broad PL band which peak position shifts gradually to from 1.4 eV to 1.8 eV with the x decrease. Contrary to this, for the Six(Al2O3)1-x films two overlapped PL bands were observed in the 1.4-2.4 eV spectral range with peak positions at ~2.1 eV and ~1.7 eV accompanied by near-infrared tail. Comparative analysis of PL spectra of both types samples showed that the main contribution to PL spectra of Six(SiO2)1-x films is given by exciton recombination in the Si-ncs whereas PL emission of Six(Al2O3)1-x films is caused mainly by carrier recombination either via defects in matrix or via electron states at the Si-ncs/matrix interface.

Si-rich Al2O3 films grown by RF magnetron sputtering: structural and photoluminescence properties versus annealing treatment

Silicon-rich Al2O3 films (Six(Al2O3)1−x) were co-sputtered from two separate silicon and alumina targets onto a long silicon oxide substrate. The effects of different annealing treatments on the structure and light emission of the films versus x were investigated by means of spectroscopic ellipsometry, X-ray diffraction, micro-Raman scattering, and micro-photoluminescence (PL) methods. The formation of amorphous Si clusters upon the deposition process was found for the films with x ≥ 0.38. The annealing treatment of the films at 1,050°C to 1,150°C results in formation of Si nanocrystallites (Si-ncs). It was observed that their size depends on the type of this treatment. The conventional annealing at 1,150°C for 30 min of the samples with x = 0.5 to 0.68 leads to the formation of Si-ncs with the mean size of about 14 nm, whereas rapid thermal annealing of similar samples at 1,050°C for 1 min showed the presence of Si-ncs with sizes of about 5 nm. Two main broad PL bands were observed in the 500- to 900-nm spectral range with peak positions at 575 to 600 nm and 700 to 750 nm accompanied by near-infrared tail. The low-temperature measurement revealed that the intensity of the main PL band did not change with cooling contrary to the behavior expected for quantum confined Si-ncs. Based on the analysis of PL spectrum, it is supposed that the near-infrared PL component originates from the exciton recombination in the Si-ncs. However, the most intense emission in the visible spectral range is due to either defects in matrix or electron states at the Si-nc/matrix interface.

Formation of Si Nanocrystallites in Al-Added Amorphous Si Films by Electron Beam Irradiation

Si nanocrystallites were formed in Al-added amorphous Si films (Al/a-Si, a-AlxSi1-x) by the irradiation of a focused electron beam. In-situ heating of the a-Al0.025Si0.975 films was performed at temperatures up to 400 °C. The size, shape, and concentration of the Si crystallites varied significantly with the film temperature and electron beam irradiation time. Si nanocrystallites with a mean size of ∼10 nm were formed when the films were kept at 200 °C, and irradiated using an electron beam with a current density of 15.7 pA/cm2. The total crystallite volume fraction in the films increased from ∼9.2 to ∼94.8% with increasing temperature from 100 to 400 °C. The estimated activation energy for the crystallization in the a-Al0.025Si0.975 film under the electron beam irradiation was 0.78±0.05 eV.

Photoluminescence of Silicon Nanocrystals Embedded in Silicon Oxide

The bonding structure and the optical properties of silicon-rich silicon oxide films which were prepared using plasma-enhanced chemical vapor deposition (PECVD) with thermal oxidation were studied in detail. The composition and bonding structures were explored using X-ray photoelectron spectroscopy (XPS). The luminescent properties were studied using Raman spectroscopy and photoluminescence (PL) measurements. Results show that high-temperature (approximately 1000 degrees C) annealing of as-deposited Si-rich silicon oxide gives rise to phase separation and formation of crystalline Si phases in the oxide films. Upon high temperature annealing, the PL (550-900 nm) intensity was significantly reduced in stoichiometric oxide or in low Si-content (prepared with [N2O]/[SiH4] gas flow ratio > 2.5) films whereas an opposite trend was found in higher Si-rich samples. Particularly, for the red band (650 nm) and the infrared band (750 nm), the PL intensity was significantly enhanced by annealing slightly Si-rich samples ([N2O] / [SiH4] = 2.5) at 1100 degrees C. This observation was explained by the formation of Si nanocrystallites via phase separation reaction and the removal of defect-related absorption centers during annealing.

Photoluminescence from high-pressure-annealed silicon dioxide

Effect of treatment at high temperature (HT) (up to 1200°C) under high hydrostatic pressure (HP) (up to 1.5GPa) on the photoluminescence (PL) of silicon dioxide films was investigated. The authors found that the PL intensity of the oxide films grown in pure oxygen can be significantly enhanced by the HT-HP treatment. Four PL peaks, at wavelengths of about 570, 620, 720, and 950nm, were found. The 570 and 950nm peaks are attributed to the defect centers and amorphous Si, respectively. The luminescence at 620 and 720nm is attributed to the quantum confinement effect involving the silicon nanocrystallites embedded in the oxide film. The PL intensities are strongly governed by the preparation conditions of the as-grown oxide layer as well as by the annealing conditions. Raman study indicates that both amorphous and crystalline Si phases coexist in the HT-HP processed samples. The formation of Si nanocrystallites is attributed to the phase separation effect involving Si suboxide.

High-resolution TEM study of the Er distribution in Er-doped SiO2 films prepared by laser ablation

High-resolution TEM was used to study the Er distribution and the formation of Si nanocrystallites in Er-doped SiO2 films fabricated by laser ablation. Formation of Er nanoparticles up to 40nm in diameter were observed and characterised.

Positron/positronium annihilation in nanocrystalline silicon thin films

Positron and positronium annihilation investigations were applied to nanocrystalline silicon (nc-Si) thin films, for the first time. The nc-Si thin films with average grain diameters of 3–5 nm show intense blue luminescence at room temperature. The nanometer-sized Si crystallites formed in amorphous Si (a-Si) matrix give rise to this luminescence. Very highS-parameters up to 0.62 were observed in the as-grown a-Si thin film suggesting positronium formation in the a-Si layer. The average lifetime of the positrons in the a-Si was determined to be about 520 ps. TheS-parameters dropped significantly to 0.53 by crystallization of the thin film at 800 °C for 10 seconds, which was almost the same to the value observed in bulk Si (100) substrate. Further crystallization from 60 seconds to 1 hour showed smaller change in theS-parameters than that from the a-Si to 10 seconds. The large change in theS-parameters due to the annealing might be caused by the formation of Si nanocrystallites in a-Si matrix suggesting that positron is a sensitive probe for structural investigations of the nc-Si materials.