A. Morales-Sánchez

Analysis of surface roughness and its relationship with photoluminescence properties of silicon-rich oxide films

It is well known that silicon-rich oxide (SRO) shows intense photoluminescence (PL). In this work, the authors studied the relationship of the surface morphology and the PL emission. PL spectra of SRO as a function of the excess silicon, temperature, and time of thermal annealing were obtained. The same samples were studied using transmission electronic microscopy and atomic force microscopy to determine their microstructure and surface morphology. A relationship between silicon agglomerates in the SRO and the surface morphology was obtained. Then, the red PL emission was related to the surface morphology. The authors found that the surface roughness is an important parameter for the high red emission of SRO.

Synthesis and characterization of PbSe nanoparticles obtained by a colloidal route using Extran as a surfactant at low temperature

Lead selenide nanoparticles (PbSe NPs) have been obtained through an easy and low cost route using colloidal synthesis in aqueous solution. The synthesis was carried out at room temperature using Extran (Na₅P₃O₁₀, NaOH and H₂O) as surfactant. Hydrochloric acid (HCl) was used to eliminate the generated by-products. The size of PbSe NPs was varied by changing the Pb:Se molar concentration. The PbSe NPs were characterized by powder x-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive x-ray analysis (EDAX), high-resolution transmission electron microscopy (HRTEM) and Raman spectroscopy. The XRD measurements showed that the PbSe NPs have the face-centered cubic phase structure. The crystal size was found to be between 14 and 20 nm as calculated from the XRD patterns and these values were corroborated with SEM and TEM. Additionally, HRTEM micrographs showed crystalline planes at (200), (220) and (111) of the PbSe NPs, in agreement with the XRD results.

DC and AC electroluminescence in silicon nanoparticles embedded in silicon-rich oxide films

Electroluminescent properties of silicon-rich oxide (SRO) films were studied using metal oxide semiconductor-(MOS)-like devices. Thin SRO films with 4 at.% of silicon excess were deposited by low pressure chemical vapour deposition followed by a thermal annealing at 1100 degrees C. Intense continuous visible and infrared luminescence has been observed when devices are reversely and forwardly bias, respectively. After an electrical stress, the continuous electroluminescence (EL) is quenched but devices show strong field-effect EL with pulsed polarization. A model based on conductive paths--across the SRO film--has been proposed to explain the EL behaviour in these devices.

The mechanism of electrical annihilation of conductive paths and charge trapping in silicon-rich oxides.

The electrical properties of silicon-rich oxide (SRO) films in metal-oxide-semiconductor-like structures were analysed by current versus voltage (I-V) and capacitance versus voltage (C-V) techniques. SRO films were thermally annealed to activate the agglomeration of the silicon excess in the form of nanoparticles (Si-nps). High current was observed at low negative and positive voltages, and then at a certain voltage (V(drop)), the current dropped to a low conduction state until a high electric field again activated a high conduction state. C-V measurements demonstrated a capacitance reduction at the same time as the current dropped, but without appreciable flat-band voltage (V(FB)) shifting. The reduction in capacitance and current was also observed after applying an electrical stress. These effects are ascribed to the annihilation of conductive paths created by Si-nps. An equivalent circuit is used to explain the capacitance and current reductions. Finally, the conduction mechanism is also analysed by making use of trap assisted tunnelling and Fowler-Nordheim tunnelling at low and high electric fields, respectively.

Strong blue and red luminescence in silicon nanoparticles based light emitting capacitors

Light emitting capacitors (LECs) were fabricated using silicon rich oxide (SRO) films as active layer. Blue and red electroluminescence (EL) was observed by changing the silicon nanoparticle (Si-np) size from 1.5 to 2.7 nm embedded in the silica matrix. EL is ascribed to the charge injection into the Si-nps embedded in the SRO films through a balanced transport network. The EL emission is observed with the naked eye and in daylight conditions on the whole area of devices. Therefore, these results prove the feasibility to obtain LECs by using simple capacitors with SRO films as the active layer.

Comparison of electrical and electro-optical characteristics of light-emitting capacitors based on silicon-rich Si-oxide fabricated by plasma-enhanced chemical vapor deposition and ion implantation

This work presents electrical and electro-optical studies performed on light-emitting capacitors with silicon-rich silicon oxide fabricated by plasma-enhanced chemical vapor deposition and by the implantation of Si ions in thermally grown SiO2. The influence of the fabrication technique and silicon content on electrical, electro-optical, and emission spectra characteristics has been studied. Results on the electrical behavior show a significant dependence on both the fabrication technique and Si content that translates in variations on electroluminescence with fabrication technique and silicon excess.

Emission mechanisms of Si nanocrystals and defects in SiO 2 materials

Motivated by the necessity to have all silicon optoelectronic circuits, researchers around the world are working with light emitting silicon materials. Such materials are silicon dielectric compounds with silicon content altered, such as silicon oxide or nitride, enriched in different ways with Silicon. Silicon Rich Oxide or silicon dioxide enriched with silicon, and silicon rich nitride are without a doubt the most promising materials to reach this goal. Even though they are subjected to countless studies, the light emission phenomenon has not been completely clarified. So, a review of different proposals presented to understand the light emission phenomenon including emissions related to nanocrystals and to point defects in SiO2 is presented.

Photoconduction in silicon rich oxide films obtained by low pressure chemical vapor deposition

Photoconduction properties of silicon rich oxide (SRO) thin films were studied under different illumination conditions. In the past, Al/SRO/Si structures showed a high photocurrent in spite of the fact that an opaque Al layer was on the active area. In order to elucidate this observation, new Al/SRO/Si structures were tested, but this time they were also measured horizontally. SRO thin films were deposited on silicon wafers by low pressure chemical vapor deposition technique using SiH4 (silane) and N2O (nitrous oxide) as reactive gases at 700°C. 1%–12% silicon excess was used. Structures with a single SRO layer and with a double layer were fabricated in order to have a barrier to isolate the silicon from the active SRO layer. The results show that all structures have a higher current when light shines on them than when they are in the dark. It is proposed that the photocurrent is produced in the SRO bulk, and an explanation for these observations is given.

Analysis of the luminescent centers in silicon rich silicon nitride light-emitting capacitors

An analysis of the luminescent center and its effect on the optical, electrical and electro-optical properties of silicon rich silicon nitride (SRN) films deposited by low pressure chemical vapor deposition is reported. As-deposited SRN films emit a broad photoluminescence (PL) spectrum in the visible range where the maximum peak shifts from ?490 to ?590 nm as the silicon excess increases. After thermal annealing, a PL blue-shift is observed and it is ascribed to a compositional-dependent change in the concentration of defect states within the films. A correlation between the PL peak energy with the optical band-gap indicates that the luminescence is related to the band tail carrier recombination in the SRN films. Light emitting capacitors (LECs) based on fluor-doped tin oxide SnO2:F (FTO)/SRN active layer/n-Si substrate emit a broad electroluminescent spectra where the maximum emission peak blue-shifts when the polarity is changed from reverse to forward bias. In the reverse bias, the electroluminescence (EL) is related to the states of valence band tail and Si dangling bonds (K0 centers), while in the forward bias the EL is originated from electronic transitions from the conduction band minimum to K0 centers. A model based on the trap assisted tunneling carrier transport is correlated with the proposed EL radiative recombination process in the FTO/SRN/n-Si structures. A discussion of the differences between the PL and EL spectra is reported. The results open new alternatives toward the development of Si-based light emitters where two different EL spectra can be obtained changing the polarity.

DC Electroluminescence Efficiency of Silicon Rich Silicon Oxide Light Emitting Capacitors

We analyze the influence of the fabrication technique and the silicon excess on the power efficiency and evolution with time of the electroluminescence of silicon rich silicon oxide in metal-oxide-semiconductor like light emitting capacitors under direct current. The silicon rich silicon oxide layers have been fabricated using two different techniques, namely plasma enhanced chemical vapor deposition and silicon ion implantation. Six different silicon excesses have been studied, ranging from 6 at. % to 15 at. %. It is shown that both the power efficiency and external quantum efficiency increase with the silicon excess due to a decrease in the electroluminescence current threshold. The maximum value of the power efficiency has been found to be (2.6±0.3)×10-5 in the ion implanted sample with 15 at. % silicon excess. Significant differences in the evolution of the electroluminescence with time are found depending on the fabrication technique.