Photoluminescence Of Si Nanocrystals Research Articles

On the spectral difference between electroluminescence and photoluminescence of Si nanocrystals: a mechanism study of electroluminescence

Spectral shift, especially blueshift, in peak position of electroluminescence (EL) spectrum of Si nanocrystal (Si-nc) with respect to its photoluminescence (PL) counterpart has been often observed. Explanations for the spectral difference are different for different EL mechanisms adopted. To gain a relevant picture of the EL process, in this work, we analyze three EL mechanisms that are mainly applied nowadays, i.e., the model of defect light emission, that of band-filling, and that of Si-nc size selection by the carrier energy. Different Si-nc samples and working conditions are designed and their EL and PL emissions monitored according to the predictions of the three models. It is concluded that the observed EL is mainly of Si-nc-related origin. The experimental results are more consistent with the model of Si-nc size selection.

Defect engineering of Si nanocrystal interfaces

AbstractThe photoluminescence of Si nanocrystals is often assigned to exciton recombination confined within the nanocrystals. However, localized radiative or non‐radiative defects at the nanocrystal (NC) interface can drastically influence the NCs ensemble and reduce the luminescence output. In addition, size of the NCs as well as embedding host are playing a major role and have a significant influence on the optical properties. Here we summarize our work on size controlled Si NCs based on the SiOx/SiO2 superlattice approach. We will discuss how to distinguish between defects at the NC‐Si/SiO2 interface and defects in the matrix and demonstrate ways for effective NC surface passivation. The influence of different annealing ambient used for crystallization will be shown. Such perfectly defect passivated and size controlled samples will be used as a model system to discuss size dependent quantum properties such as the temperature dependent blue shift of the Si NC band gap.

A multilayered approach of Si/SiO to promote carrier transport in electroluminescence of Si nanocrystals

The electroluminescence (EL) and photoluminescence of Si nanocrystals (Si-nc) from multilayered samples of Si/SiO are investigated. Si-nc are formed within Si and SiO layers after furnace annealing. It is found that the presence of Si interlayers creates extra carrier paths for EL emission. A comparative study is further performed on a multilayered Si/SiO sample and a single-layered one with Si and SiO homogeneously mixed. Both samples have the same ratio of Si to O and the same contents of Si and O. The multilayered sample is found to have higher EL intensity, less turn-on voltage, lower resistance, and higher current efficiency than the single-layered one. The results indicate that Si interlayers in Si/SiO may act as carrier channels, which promote carrier transport and enhance the EL emission of Si-nc.

Effect of Silicon Concentration on the Photoluminescence of Si Nanocrystals Produced Using Silicon-rich Oxide Films

Effect of Silicon Concentration on the Photoluminescence of Si Nanocrystals Produced Using Silicon-rich Oxide Films

Formation and photoluminescence of Si nanocrystals in controlled multilayer structure comprising of Si-rich nitride and ultrathin silicon nitride barrier layers

The effect of ultrathin silicon nitride (Si3N4) barrier layers on the formation and photoluminescence (PL) of Si nanocrystals (NCs) in Si-rich nitride (SRN)/Si3N4 multilayer structure was investigated. The layered structures composed of alternating layers of SRN and Si3N4 were prepared using magnetron sputtering followed by a different high temperature annealing. The formation of uniformly sized Si NCs was confirmed by the transmission electron microscopy and X-ray diffraction measurements. In particular, the 1nm thick Si3N4 barrier layers was found to be sufficient in restraining the growth of Si NCs within the SRN layers upon high annealing processes. Moreover, X-ray photoelectron spectroscopy spectra shown that films subjected to post-anneal processes were not oxidized during the annealing. X-ray reflection measurements revealed that high annealing process induced low variation in the multilayer structure where the 1nm Si3N4 layers act as good diffusion barriers to inhibit inter-diffusion between SRN layers. The PL emission observed was shown to be originated from the quantum confinement of Si NCs in the SRN. Furthermore, the blue shift of PL peaks accompanied by improved PL intensity after annealing process could be attributed to the effect of improved crystallization as well as nitride passivation in the films. Such multilayer structure should be advantageous for photovoltaic applications as the ultrathin barrier layer allow better electrical conductivity while still able to confine the growth of desired Si NC size for bandgap engineering.

Alteration of band structure in Si nanocrystals

The photoluminescence of Si nanocrystals with dimensions ranging from 70 to 250 nm was investigated. Grains were prepared by mechanical ball-milling and subsequent sedimentation steps in order to segregate them in size. This was followed by heat treatment in argon atmosphere at 1000°C to reduce the density of dislocations introduced during milling. At the same temperature, an oxidation process in air was carried out in order to create a stable SiO 2/Si interface and reduce the dimensions of the grains. Crystallinity of the samples was checked by X-ray diffraction and transmission electron microscopy-related techniques. Two emission bands have been determined. One was identified with the D1 center usually assigned to dislocations. The second one is identified as being due to an excitonic recombination and shows phonon replicas characteristic for bulk silicon. This band is reported here for the first time. In the fraction of the smallest silicon grains, a gradual upshift of the excitonic line was observed with diminishing average grain diameter. This was associated with band structure perturbation due to size-confinement.

Magneto-optical effects in photoluminescence of Si nanocrystals

We report on magneto-optical studies of excitons confined in Si nanocrystallites. As a result of Zeeman splitting of the exciton ground level the whole resonantly excited photoluminescence spectrum shifts linearly to lower energy with rising magnetic field. The recombination decay time depends on magnetic field due to the change in the thermal populations of singlet-triplet sublevels of the exciton ground state. The effect is stronger for weakly confined excitons when the value of the Zeeman splitting becomes comparable with the electron-hole exchange energy. The experimental results strongly support the exciton nature of luminescing states responsible for the light emission from porous silicon.

Effect of annealing and H 2 passivation on the photoluminescence of Si nanocrystals in SiO 2

Silicon nanocrystals have been synthesized in an SiO 2 matrix using Si ion implantation. Si implantations have been performed at 35 keV with doses from 1×10 16 cm −2 to 7×10 16 cm −2, corresponding to peak excess concentrations of 3 at.% to 17 at.%. The samples have been annealed in vacuum at various temperatures to obtain Si nanoclusters of different sizes. Photoluminescence (PL) experiments show a distinct shift of the spectra towards lower energies and a decrease in intensity with increasing annealing temperature between 700 °C and 900 °C for doses higher than 5×10 16 cm −2. We also observe for these samples that the peak position appears at lower energy for higher doses. Additional annealing in H 2/N 2 results in a strong increase of the PL intensity which is attributed to the passivation of defects. Annealing the samples again in vacuum leads to a reproduction of the former PL spectra showing the reversibility of the passivation. These experiments are in qualitative agreement with the quantum confinement model and in contradiction to recent observations made on Ge nanocrystals in a SiO 2 matrix (K.S. Min et al., Appl. Phys. Lett., submitted).