Elsevier

Applied Surface Science

Volume 168, Issues 1–4, 15 December 2000, Pages 328-331
Applied Surface Science

Effects of deposition and post-fabrication conditions on photoluminescent properties of nanostructured Si/SiOx films prepared by laser ablation

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Abstract

Pulsed laser ablation (PLA) from a Si target in an inert He ambient has been used to produce Si/SiOx nanostructured thin films on Si substrates. After the film exposition to atmospheric air, they exhibited photoluminescence (PL) signals with peak energy between 1.58 and 2.15 eV. It was found that both natural and thermal oxidation of the films can cause dramatic changes in PL properties giving rise to the appearance or considerable enhancement of fixed PL peaks around 1.6–1.65 and 2.2–2.25 eV. Quite different time-dependent PL degradation behavior under continuous laser irradiation gives an evidence for different mechanisms responsible for the PL peaks. Possible origins of PL are discussed.

Introduction

Because silicon is an indirect bandgap material with a small bandgap, it cannot emit visible light. This property significantly complicates the spreading of Si-based technologies to optoelectronics. However, recent observations of visible PL in porous silicon [1] and in other Si-based nanocrystalline structures (see, e.g. [2]) gave a promise for the development of some applications.

Pulsed laser ablation (PLA) is one of the best candidates for the production of the Si-based nanocrystallites [3], [4], [5], [6]. Due to the peculiarities of laser plasma formation and evolution, Si nanoscale clusters can be effectively produced by laser ablating a Si target into an inert gas ambient (see, e.g. [7]). Si/SiOx films fabricated by PLA were found to contain Si nanoscale particles and exhibit visible PL [3], [4], [5], [6], [8]. Nevertheless, many aspects of the phenomenon remain unclear, and the physical origin of the observed photoluminescence is still controversial.

In this paper, we study PL properties of Si-based films produced by PLA and the effect of post-deposition oxidation. In the PL studies, the main attention has been given to the visible and near-infrared spectral ranges (S-band), which are the most important for optoelectronics applications.

Section snippets

Experimental setup

The ablation of material from a rotating Si target was produced by a pulsed KrF laser (λ=248 nm, pulse length 15 ns FWHM, repetition rate 30 Hz). The radiation was focused on a focal spot 2×1 mm2 on the target at the incident angle of 45°. The fluence was 7–10 J/cm2 giving a radiation intensity of about 5×108 W/cm2. The laser-induced plasma plume expanded perpendicularly to the target surface. The substrates, identical to the target, were placed on a rotating substrate holder at 2 cm from the target.

Experimental and discussion

In our experiments, PL signals were observed only after the exposition of the films to oxygen, i.e. after the replacement of He in vacuum chamber by atmospheric air. PL spectra could differ for the films prepared under different deposition conditions. For example, a change of the He pressure from 3 Torr to 150 mTorr in different deposition experiments caused a progressive blue shift of the PL peak position from 1.58 to 2.15 eV. However, a wider displacement of the PL peak position by this method

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