Elsevier

Thin Solid Films

Volume 392, Issue 1, 23 July 2001, Pages 40-49
Thin Solid Films

The effects of phosphorus on the crystallisation and photoluminescence behaviour of aerosol–gel deposited SiO2–TiO2–Er2O3–P2O5 thin films

https://doi.org/10.1016/S0040-6090(01)01017-3Get rights and content

Abstract

Optically active films in the system, SiO2–TiO2–Er2O3–P2O5 have been prepared by aerosol–gel deposition. Crystallisation and morphology features that determine the preparation of transparent films are described and discussed in relation to the experimental conditions. The photoluminescence (PL) behaviour is then studied and discussed in relation to the films composition and to the nature of the crystallographic phases present in the films. It is concluded that concentration quenching and co-operative up-conversion mechanisms quench noticeably the PL. A 20Ti–0.5Er–10P composition is compatible with the preparation of transparent films together with an efficient prevention of these parasitic mechanisms.

Introduction

Nowadays, the worldwide need for optically active devices is growing exponentially. These devices are used in numerous application fields including optical waveguide amplifiers, laser sources, optical memories, scintillators, phosphors for display, sensors… The needs are all the more important when optically active integrated devices can be fabricated, which allows a significant cost and space reduction. First of all, the fabrication of an integrated optical device requires the preparation of transparent thin films with low optical losses. Besides this, the films have to be doped with optically active elements. Rare earth elements appear to be ideal candidates because of their electronic transition that covers a wide spectral range from UV to IR.

The sol–gel process has been largely investigated over the past decades for the fabrication of rare earth-based optically active integrated devices. Due to the high technological importance of this field, a major part of the sol–gel studies has been focused on the preparation of all-optical integrated amplifiers for telecommunication applications. For that purpose, erbium-doped thin films have been the object of huge interest because the 4I13/24I15/2 energy transition of erbium ions occurs at approximately 1.55 μm, in the centre of the third telecommunication window. Most of the sol–gel studies were focused on the preparation of erbium-doped films prepared in the binary system SiO2–TiO2 (for a review, see Orignac et al. [1]). The sol–gel technique allows the preparation of a transparent glassy matrix in this system. Moreover, the refractive index of the films can be precisely adjusted through a suitable control of the TiO2 content, which is of peculiar interest for integrated waveguide fabrication.

The use of rare earth doped thin films for optically active integrated device applications requires a perfect control of the rare earth photoluminescence (PL) properties (strong PL intensity, long PL lifetime). The PL of erbium ions diluted in a silica host matrix is very sensitive to several parasitic mechanisms. It is particularly affected by concentration quenching and co-operative up-conversion mechanisms. These mechanisms are promoted as soon as the erbium concentration exceeds a threshold level, which favours erbium ions clusterisation and detrimental short range distance ion–ion interactions. For this reason, several authors proposed to co-dope their silica films with phosphorus. This element is known to favour the dilution of rare earth ions in the silica host matrix and to prevent clusterisation [2].

In a recent paper, we proposed the use of an original sol–gel deposition technique for the preparation of transparent and optically active SiO2–TiO2–Er2O3 thin films [3]. This technique, called the aerosol–gel process, is based on the sol–gel polymerisation of a liquid film deposited from an ultrasonically generated aerosol. As the aerosol–gel process is compatible with in-line technologies used for high throughput rate production, it could appear as an interesting alternative for the low cost mass production of integrated waveguides. In this paper we report on the aerosol–gel preparation of optically active transparent thin films in the system, SiO2–TiO2–Er2O3–P2O5. Crystallisation and morphology features that determine the preparation of transparent amorphous films are described and discussed in relation to the experimental conditions. The PL behaviour is then studied and discussed in relation to the film composition and to the nature of the crystallographic phases present in the films.

Section snippets

Experimental

SiO2–TiO2–Er2O3–P2O5 sol–gel thin films were prepared from tetraethoxysilane (TEOS), tetraisopropyl-orthotitanate (TIPT), erbium nitrate hydrate and phosphorus oxide. These precursors were diluted in absolute ethanol in the presence of deionised water and hydrochloric acid. A SiO2–TiO2–Er2O3 mixed solution was first prepared according to a previously published procedure [3]. Phosphorus oxide diluted in ethanol was then added under magnetic stirring. Different solution compositions were studied

Crystallisation and morphology features

The crystallisation features of the films appeared strongly influenced by the phosphorus concentration (Table 1). These features can be split in two categories. Below a phosphorus content threshold, 5% for 5Ti–5Er compositions and 10% for 20Ti–0.5Er and 20Ti–5Er compositions, the films remained amorphous after heat-treatment at 1000°C for 1 h (Table 1a). A similar trend was observed in the case of 20Ti–0.5Er films after heat-treatment at 1100°C for 1 h (Table 1b). For the same Ti/Er composition

Crystallisation and morphology features

XRD results show that the amount of phosphorus influences drastically the nature and size of crystallites constituting the films. According to the SiO2–P2O5 binary phase diagram [6], for a weak phosphorus content, a solid state SiO2 phase coexists with a liquid phase in the thermal range of 1000–1700°C. Above 1700°C, a pure liquid phase is formed. This liquid phase is likely to favour atomic diffusion and to promote the growth of large crystallites during film heat-treatment. However, we

Conclusion

Optically active transparent films in the system, SiO2–TiO2–Er2O3–P2O5 have been prepared by aerosol–gel deposition. The preparation of transparent films depended closely on the phosphorus content and heat-treatment conditions (temperature, time). This study showed that the film PL is noticeably affected by concentration quenching and co-operative up-conversion mechanisms, due to clusterisation effects related to the concentration of erbium in the films. The best PL properties were obtained for

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