Effect of rapid thermal annealing on Si rich SiO2 films prepared using atom beam sputtering technique

doi:10.1016/j.surfcoat.2009.02.059

Surface and Coatings Technology

Volume 203, Issues 17–18, 15 June 2009, Pages 2506-2509

https://doi.org/10.1016/j.surfcoat.2009.02.059 Get rights and content

Abstract

We report here the synthesis of silicon-nanoclusters embedded in SiO₂ by atom beam co-sputtering technique. A sputtering target consisted of 40% and 60% area of Si pieces glued on a fused silica plate. A co-sputtered film containing Si embedded in SiO₂ having different compositional fractions of Si was deposited on fused silica and c-Si substrates. Annealing was done in N₂ atmosphere for 1 min at temperatures ranging from 700–900 °C to precipitate silicon nanoclusters. Samples were characterized using Raman and FTIR spectroscopy. In IR absorption spectra, the blue shift of the Si–O–Si asymmetric stretching band, from 1017 cm^− 1 for as-deposited samples, to 1070 cm^− 1 for samples annealed at 900 °C, indicates that phase separation of Si and SiO₂ has been completed and films consist of Si particles embedded in an SiO₂ matrix. Raman spectrum showed the emergence of 513 cm^− 1 due to nanocrystalline silicon after RTA at 900 °C.

Introduction

The important advancements in the study of silicon nanocrystals embedded in amorphous silica thin films (a-SiO₂) have been motivated by the potential application of Si-nc in active light sources as well as in memory devices. Silicon rich silicon oxide (SRO) films consisting of silicon nanocrystals may be used as passivating material for silicon electronic devices [1], [2] or as an integral part of non-volatile memory devices [3]. SRO films have also been used with thermal SiO₂ to increase capacitance and yield of storage capacitors in memory cells [4]. Torre et al. [5] reported the production of stable and efficient LEDs based on Si-nc embedded in SiO₂.

Silicon rich silicon oxide films (i. e. SiO_x films where 0 < x < 2) prepared by variety of methods such as ion implantation of Si⁺ into SiO₂ [6], chemical vapor deposition [7], RF co-sputtering [8], or reactive RF magnetron sputtering [9], which often involves conventional furnace annealing at elevated temperatures [10] for the formation of silicon nanocrystals. The long annealing time associated with conventional furnace annealing may not be compatible with the future development of integrated circuit processes, as it may lead to Ostwald ripening processes, where larger precipitates grow at the expense of smaller ones. Rapid thermal annealing (RTA) was introduced to provide high-temperature annealing with a short duration. This technique has proved to be very effective in increasing the growth or recrystallization rate of Si and Ge compared with conventional furnace annealing [11]. RTA helps to increase the density of nanocrystals in hydrogenated nanocrystalline silicon (nc-Si:H) films deposited by PECVD. Raman spectroscopy analysis showed that RTA at 1000 °C is the optimal processing condition for the recrystallization of the film. Rapid thermal annealing (RTA) can induce high-density, size-limited recrystallization in nc-Si:H films. Recently, Choi et al. [12], [13] presented Raman results of Ge nanocrystals embedded in amorphous SiO₂ (a-SiO₂) films synthesized by the RTA process.

In the RF magnetron co-sputtering process, the presence of a magnetic field results in higher sputtering from a narrow circular region, resulting in possible non-uniformity in the samples, if the number of samples is large. Such nonuniformity does not occur in the wide-source atom beam co-sputtering due to the large size of the atom beam (2 in. diameter in the present case). Avasthi et al. [14] presented a novel route for atom beam co-sputtering for synthesizing metal polymer nanocomposites for optical applications. Singha et al. [15] synthesized germanium nanocrystals in SiO₂ matrix by atom beam sputtering technique. Kabiraj et al. [16] and Mishra et al. [17] also used atom beam sputtering technique for the controlled growth of Cu, Ag and Au nanoparticles in silica matrix.

Kabiraj et al. [16] showed that atom beam sputtering is a suitable technique for obtaining metal nanoclusters embedded in suitable matrix because of low rate of deposition, uniformity and homogeneity of the deposited films and controlled growth of nanoparticles. In the present work, we attempt to follow the method of rapid thermal annealing on atom beam co-sputtered Si rich SiO_x films to form Si-nc in SiO₂. In this paper we have studied optical absorption, FTIR and Raman spectra of silicon nanocrystals embedded in SiO₂ synthesized using atom beam co-sputtering and rapid thermal annealing technique.

Section snippets

Experiments

A fast neutral atom source installed at Inter University Accelerator Centre, New Delhi is used for co-sputtering of Si and SiO₂. The saddle field source provides medium area beam of essentially atomic neutrals and requires DC power which is suitable for sputtering from conducting, dielectric and insulating targets. The atom source is mounted at an angle of 45° facing towards the sputtering target as shown in Fig. 1. The substrate holder is mounted on a motor and the distance between sputtering

Results and discussion

The IR absorbance spectrum in the range 700–1300 cm^− 1 of as deposited and annealed Si-rich silicon oxide films is shown in Fig. 2, Fig. 3 for 40% and 60% Si compositional fraction respectively. For 40% Si compositional fraction, absorption bands at 820 and 1017 cm^− 1 are observed. For 40% Si compositional fraction of co-sputtered films, the main absorption band at 1017 cm^− 1 for the as deposited sample shifts to 1064 cm^− 1 for the sample annealed at 900 °C, as shown in Fig. 2. For 60% Si

Conclusions

From Raman spectra and analysis based on a phonon confinement model, it may be concluded that Si nanocrystals with size 3.2 nm embedded in amorphous SiO₂ matrix are formed after rapid thermal annealing at 900 °C. Amorphous Si clusters are formed at annealing temperatures less than 900 °C. From FTIR results it may be concluded that RTA causes phase separation in the films.

Acknowledgements

The financial support from Inter University Accelerator Centre, New Delhi is kindly acknowledged. The authors would like to thank Mr S. R. Abhilash, Mr. Chetan Gurada and Mr. Radhekrishna Dubey for their kind help.

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Effect of rapid thermal annealing on Si rich SiO2 films prepared using atom beam sputtering technique

Abstract

Introduction

Section snippets

Experiments

Results and discussion

Conclusions

Acknowledgements

Physica E

J. Cryst. Growth

Thin Solid Films

Thin Solid Films

Appl. Surf. Sci.

NIMB

Solid State Commun,

J. Non-Cryst. Solids

Solid State Commun.

Jpn. J. Appl. Phys.

Jpn. J. Appl. Phys.

IEEE Electron Device Lett.

IEEE Trans. Electron Device Lett.

Appl. Phys. Lett.

Effect of rapid thermal annealing on Si rich SiO₂ films prepared using atom beam sputtering technique