Luminescence characteristics of Ge nanocrystals embedded in SiO2 matrix

doi:10.1016/j.optmat.2004.08.041

Optical Materials

Volume 27, Issue 5, February 2005, Pages 948-952

https://doi.org/10.1016/j.optmat.2004.08.041 Get rights and content

Abstract

Germanium nanocrystals embedded in silicon dioxide matrix were grown using rf magnetron sputtering. The transmission electron micrograph showed the formation of nearly spherical nanocrystals evenly distributed in the oxide matrix. Annealed nanostructures exhibited strong and broad photoluminescence in the visible range at room temperature due to the quantum confinement of carriers in Ge nanocrystals. A trilayer structure consisting of the cap gate oxide, sputtered SiGe layers and thermally grown tunnel oxide was fabricated on p-Si substrates followed by rapid thermal annealing at 1000 °C in nitrogen atmosphere. The light emission from the trilayer structure manifests the quantum confinement of charge carriers in Ge nanocrystals, making it attractive for future nanocrystal memory devices.

Introduction

Recently, a considerable amount of research has been devoted to the luminescence of group-IV semiconductors of low dimensionality to study quantum-confined electronic states and to develop integrated optoelectronic devices directly on Si. Nanocrystals of indirect-gap semiconductors like Si and Ge are being widely studied as they would open new possibilities for applications in optoelectronics and microelectronics, with the main advantage of being compatible with actual device technology. In contrast to a bulk indirect semiconductor, no-phonon transitions may be possible in nanocrystals due to the confinement of the electron and hole wave functions in real space that leads to a spread of the wave function in momentum space. This gives rise to light emission from silicon-based nanostructures, which are very attractive for full-color display, especially for the integrated optoelectronics technology. Nanocrystals embedded in SiO₂ matrix offer an attractive option because it is a well characterized material known to passivate semiconductor surfaces. Most reports of visible photoluminescence (PL) from low-dimensional structures have involved Ge nanocrystals embedded in SiO₂ [1], [2], [3], [4], [5]. This PL was associated with the effect of quantum confinement in the Ge dots surrounded by the high potential barriers of SiO₂ [3], [6] or with radiative recombination between holes confined in the Ge dots and electrons localized in the structural defects in the SiO₂ at the Ge/SiO₂ interface [7].

The observed luminescence in Ge nanocrystals (NC) is interesting for their potential applications in optoelectronics. In Ge, the direct gap (0.88 eV) is close to its indirect gap (0.75 eV). So it is predicted that quantum confinement effects would appear more pronounced in Ge than in Si, and Ge nanocrystals would exhibit a direct-gap like characteristics [8], [9]. The mechanisms involved in Ge nanocrystal growth are still controversial. To obtain three-dimensionally confined systems, different groups have exploited a variety of techniques, viz., pyrolysis [3], co-sputtering [1], [10], [11], spark processing [12], reduction of oxide [13], and ion implantation [14], [15], [16], [17], [18]. Among the techniques used to produce dielectric composite films containing small semiconductor crystals, rf magnetron sputtering has several advantages, in particular, low deposition temperature and higher purity of the films compared to the melting techniques [19], [20]. In this paper, we report the formation of Ge nanocrystals embedded in SiO₂ matrix by rf magnetron sputtering. Microstructural and optical characteristics of the samples are presented. Trilayer structures have been fabricated with Ge nanocrystals embedded in SiO₂ along with tunneling and cap oxides, which can have further applications in memory devices.

Section snippets

Experimental

The Ge–SiO₂ thin films were deposited on (100) oriented p-type silicon substrates by RF co-sputtering. Prior to deposition, the Si substrates were dipped in 1% hydrofluoric acid to remove the surface native oxide followed by rinsing in de-ionized water and drying in a flux of N₂. The target used was a 3-inch n-type Si wafer masked with Ge wafer pieces of defined area. The chamber was first evacuated to a base pressure of 1 × 10⁻⁶ Torr. The target to substrate distance was kept fixed at 6 cm and the

TEM study

The size and distribution of Ge nanocrystals embedded in oxide matrix were studied using TEM micrograph. The TEM observations were carried out using a JEM 3000 F field emission microscope with an operating voltage of 300 kV. The sample studied was the one deposited at 50 W and annealed at 900 °C for 2 h. Ge nanocrystals are observed as dark patches in the plan-view micrograph as shown in Fig. 1. The distribution of Ge nanocrystals is fairly even, however, a variation of their sizes is quite

Conclusion

We have formed Ge nanocrystals embedded in SiO₂ matrix by rf magnetron sputtering. The formation of spherically shaped nanocrystals has been observed using TEM microscopy. The optical characteristics of nanocrystals have been investigated using Raman spectroscopy and photoluminescence. The appearance of strong and broad visible photoluminescence at room temperature is attributed to the quantum confinement of carriers in Ge nanocrystals having a wide distribution of crystallite sizes.

Acknowledgement

The authors are grateful to S. Maikap, Je-Hun Lee, G.S. Kar, A. Dhar, B.K. Mathur and M. Nanda Goswami for their help and many useful discussions.This work was supported in part by sponsored research projects from DST and DRDO, Government of India.

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Luminescence characteristics of Ge nanocrystals embedded in SiO2 matrix

Abstract

Introduction

Section snippets

Experimental

TEM study

Conclusion

Acknowledgement

J. Lumin.

Solid State Commun.

Sci. Technol. Adv. Mater.

Appl. Phys. Lett.

Appl. Phys. Lett.

Phys. Rev. B

Appl. Phys. Lett.

Mater. Res. Soc. Symp. Proc.

Appl. Phys. Lett.

Phys. Rev. B

Phys. Rev. B

J. Chem. Phys.

J. Phys. Chem.

Jpn. J. Appl. Phys.

Luminescence characteristics of Ge nanocrystals embedded in SiO₂ matrix