J. I. Wong

Strong violet and green-yellow electroluminescence from silicon nitride thin films multiply implanted with Si ions

Strong visible electroluminescence (EL) has been observed from a 30 nm silicon nitride thin film multiply implanted with Si ions and annealed at 1100 °C. The EL intensity shows a linear relationship with the current transport in the thin film at lower voltages, but a departure from the linear relationship with a quenching in the EL intensity is observed at higher voltages. The EL spectra show two primary EL bands including the predominant violet band at ∼3.0 eV (415 nm) and the strong green-yellow band at ∼2.2 eV (560 nm). Two weak bands including the ultraviolet band at ∼3.8 eV and the near infrared band at ∼1.45 eV emerge at high voltages. The evolution of each EL band with the voltage has been examined. The phenomena observed are explained, and the EL mechanisms are discussed.

Influence of nanocrystal size on optical properties of Si nanocrystals embedded in SiO2 synthesized by Si ion implantation

Si nanocrystals (nc-Si) with different sizes embedded in SiO2 matrix have been synthesized with various recipes of Si ion implantation. The influence of nanocrystal size on optical properties, including dielectric functions and optical constants, of the nc-Si has been investigated with spectroscopic ellipsometry. The optical properties of the nc-Si are found to be well described by the four-term Forouhi-Bloomer model. A strong dependence of the dielectric functions and optical constants on the nc-Si size is observed. For the imaginary part of the dielectric functions, the magnitude of the main peaks at the transition energies E1 and E2 exhibits a large reduction and a significant redshift in E2 depending on the nc-Si size. A band gap expansion is observed when the nc-Si size is reduced. The band gap expansion with the reduction of nc-Si size is in good agreement with the prediction of first-principles calculations based on quantum confinement.

Charging effect on current conduction in aluminum nitride thin films containing Al nanocrystals

The presence of Al nanocrystals (nc-Al) in AlN thin films is found to enhance the current conduction of the thin film system greatly due to the formation of tunneling paths of nc-Al arrays, and the nc-Al∕AlN system shows a quasi-two-dimensional transport following a power law. However, charge trapping in nc-Al reduces the current conduction because of the increase in the tunneling resistance and/or the breaking of some tunneling paths due to Coulomb blockade effect. The current conduction also evolves with a trend towards one-dimensional transport due to the breaking of some transverse tunneling paths as a result of the charge trapping.

Influence of charge trapping on electroluminescence from Si-nanocrystal light emitting structure

We report a study on the influence of charge trapping on electroluminescence (EL) from Si nanocrystal (nc-Si) distributed throughout a 30nm SiO2 thin film synthesized by Si+ implantation into an oxide film thermally grown on a p-type Si substrate. The electron and hole trapping in the nc-Si located near the indium tin oxide gate and the Si substrate, respectively, cause a reduction in the EL intensity. The reduced EL intensity can be recovered after the trapped charges are released. A partial recovery can be easily achieved by the application of a positive gate voltage or thermal annealing at hot temperatures (e.g., 120°C) for a short duration. The present study highlights the impact of charging in the nc-Si on the light emission efficiency and its stability of nc-Si light-emitting devices.

Electrically tunable white-color electroluminescence from Si-implanted silicon nitride thin film

Visible electroluminescence (EL) with two composite bands, i.e., a violet band and a green-yellow band has been observed from Si-implanted silicon nitride thin films. By varying the intensity ratio of the two composite EL bands in terms of the injection current, strong white-color EL can be achieved at certain injection currents (e.g., ~265 mA/cm(2)). The observed transition in EL color from violet to white under different injection conditions is studied based on the understanding that the violet band is originated from silicon nitride matrix while the green-yellow band is related to the implanted Si. The Si-implanted silicon nitride thin film offers the possibility of electrically tunable white-light Si-based light emitters.

Effect of annealing on charge transfer in Ge nanocrystal based nonvolatile memory structure

Ge nanocrystals (nc-Ge) embedded in the gate oxide of the nonvolatile memory structure were synthesized by Ge ion implantation followed by thermal annealing at 800 °C for various durations. Large changes in the structural and chemical properties of the Ge+-implanted oxide have been observed, and they have been found to possess a significant impact on the charge transfer in the oxide layer. The distribution and concentration of the nc-Ge and dissolved Ge atoms which serve as both the charge storage and transfer sites in the oxide are affected by the annealing. Two charge transfer mechanisms, i.e., the lateral charge diffusion along the Ge-distributed layer and the charge leakage from the charge storage sites to the Si substrate via the charge transfer sites, have been identified based on the charge retention behaviors. Both mechanisms are enhanced by the annealing as a result of the change in the distribution and concentration of the charge transfer sites.

Charging effect of Al2O3 thin films containing Al nanocrystals

In this work, Al2O3 thin film containing Al nanocrystals (nc-Al) is deposited on Si substrate by radio frequency sputtering to form a metal-insulator-semiconductor structure. Both electron and hole trapping in nc-Al are observed. The charge storage ability of the nc-Al/Al2O3 thin films provides the possibility of memory applications. Charging in the nc-Al also leads to a change in the dc resistance of the thin films, namely, the electron trapping in the nc-Al leads to an increase in the resistance, whereas the resistance is reduced if there is hole trapping in the nc-Al.

Annealing effect on the optical properties of implanted silicon in a silicon nitride matrix

Optical properties of implanted Si in a silicon nitride (Si3N4) thin film have been determined with spectroscopic ellipsometry based on the Tauc–Lorentz (TL) model and the Bruggeman effective medium approximation. It is shown that the suppressed dielectric functions of the implanted Si are dominated by the energy transitions related to the critical point E2. The effect of thermal annealing on the dielectric functions of the implanted Si has been investigated. The analysis of the dielectric functions based on the evolution of the TL parameters can provide an insight into the structural changes in the implanted Si embedded in the Si3N4 matrix caused by the annealing.

Evolution of electroluminescence from multiple Si-implanted silicon nitride films with thermal annealing

Influence of thermal annealing on electroluminescence (EL) from multiple-Si-implanted silicon nitride films has been investigated. A reduced injection current and an enhanced EL intensity have been obtained simultaneously by increasing the annealing temperature, which results in a higher EL quantum efficiency. In addition, four emission bands are identified, and their peak energies, intensities, and full widths at half maxima are found to change with annealing temperature. A model is proposed to illustrate the carrier transport, the mechanisms of the four emission bands, and the evolution of the EL bands with annealing as well. The two major bands and the minor ultraviolet band are explained in terms of the recombination of the injected electrons in either the silicon dangling-bond (≡Si0) states or the nitride conduction band with the injected holes in either the band tail states above the nitride valence band or the valence band itself, while the minor near infrared band is attributed to the Si nanocryst...

The influence of the implantation dose and energy on the electroluminescence of Si+-implanted amorphous SiO2 thin films

Visible and infrared (IR) electroluminescence (EL) has been observed from a metal–oxide–semiconductor-like (MOS-like) structure with Si nanocrystals (nc-Si) embedded in the gate oxide fabricated with low-energy ion implantation. The EL spectra are found to consist of four Gaussian-shaped luminescence bands with their peak wavelengths at ~460, ~600, ~740, and ~1260 nm, respectively, among which the ~600 nm band is the dominant one. Different nanocrystal distributions are achieved by varying the implanted Si ion dose and implantation energy. The nanocrystal distribution is found to play an important role in the EL. The influence of the applied voltage, the implantation dose, and implantation energy on the luminescence bands has been investigated.