Elsevier

Journal of Crystal Growth

Volume 261, Issue 4, 1 February 2004, Pages 549-556
Journal of Crystal Growth

Structure and photoluminescence of Ge nanoparticles with different sizes embedded in SiO2 glasses fabricated by a sol–gel method

https://doi.org/10.1016/j.jcrysgro.2003.08.081Get rights and content

Abstract

Ge nanocrystals embedded in bulk SiO2 glasses were prepared by combining the sol–gel process and the heat treatment in H2 gas. The size of Ge nanoparticles decreases with a reduction in content of Ge in the starting materials. Silica gel glasses doped with Ge nanoparticles showed a strong room-temperature photoluminescence (PL) with peaks at 568, 607, 672, 722 and 775 nm. The peak position of PL spectrum scarcely depends on the Ge/Si ratio. However, the PL intensity increases sharply as the Ge/Si ratio is reduced. The PL arises from the Ge clusters of diameter of <1–2 nm with a molecular character instead of nanocrystalline Ge with the diamond structure.

Introduction

Indirect bandgap semiconductors such as bulk Si and Ge have very poor room-temperature luminescent efficiency. However, the discovery of efficient visible luminescence in porous Si [1], and other nanometer structures [2], [3] opens a possibility for indirect-gap semiconductors as materials for photoelectron applications. In semiconductor nanocrystals, quantum confinement effect plays an essential role in optical absorption and luminescence process. Bulk Ge has smaller electron and hole effective masses and a larger dielectric constant than bulk Si. Then, it is considered that quantum confinement effects would appear more pronounced in Ge nanocrystals than in Si nanocrystals [4], [5].

Nanocrystals embedded in a SiO2 matrix offer an attractive option because SiO2 is a well-characterized material known to passivate semiconductor surfaces. For Ge, most visible photoluminescence (PL) was reported from low-dimensional structures involving Ge nanocrystals embedded in SiO2 [4], [5], [6], [7], [8], [9], [10], [11], [12]. Maeda and co-workers [5] observed PL at room temperature in Ge microcrystals embedded in SiO2 matrix prepared by radio frequency magnetron co-sputtering for the first time. Since then, a variety of methods have been used to synthesize Ge nanocrystals in SiO2 films, such as co-sputtering of Ge and SiO2 [4], [5], [6], Ge ion implantation [7], [8], atmospheric pressure chemical vapor deposition (APCVD) [9], deposition of Ge on SiO2 and subsequent high temperature oxidation, [10] and H2 reduction of SixGe1−xO2 [11], [12], etc. However, these methods require highly sophisticated equipment. In addition, it is sometimes not easy to find the conditions for reproducing samples that emit visible light.

The sol–gel method has been providing a more practical approach for preparing semiconductor and metal microcrystallites dispersed in a SiO2 glass. There are many reports of preparing CdS, CuCl, CdTe PbS, Au and Ag microcrystallites embedded in SiO2 glass using the sol–gel method [13], [14], [15], [16], [17], [18]. Nogami and Abe [19] first prepared Ge nanocrystals embedded in silica containing 7 wt% Ge by the sol–gel method using Si (OC2H5)4(TEOS) and GeCl4 as starting materials, and observed visible PL from the Ge nanocrystals. However, preparation of Ge microcrystallites of controlled size by the sol–gel process was not reported. Very recently, we have succeeded in producing Ge microcrystallites embedded in SiO2 gel glasses by a sol–gel process using 3-trichlorogermanium propanoic acid (Cl3–Ge–C2H4–CO2H) as Ge source [20]. In our previous paper [20], we reported the results of optical transmission measurements, which demonstrate the quantum size effects. This work is an extension of the previous work and we report here structure and PL of the Ge nanoparticles with different sizes embedded in SiO2 gel glasses. Silica gel glasses doped with Ge nanoparticles showed a strong room temperature PL. The PL arises from the Ge clusters of diameter of <1–2 nm with a molecular character instead of nanocrystalline Ge with the diamond structure.

Section snippets

Preparation of Ge nanoparticles with different sizes embedded in SiO2 gel glasses

Ge/SiO2 gel glasses were prepared by the sol–gel method using Cl3–Ge–C2H4–CO2H and TEOS as starting materials, as described in our previous paper [20]. A block of the SiO2–GeO2 glass with different Ge/Si ratios was prepared by the sol–gel method. Then it was reduced for 3 h at 600°C under a flowing (1.0 l/h) H2 gas to from Ge nanoparticles with different sizes in this SiO2 glass matrix.

Characterization and PL

The Ge nanocrystals were characterized and analyzed by X-ray diffraction (XRD) (Rigaku D/max-IIIC with Cu Kα

Preparation of Ge nanoparticles with various sizes

The gel glasses heated at 600°C in an air gas atmosphere are transparent and colorless. The germanium ions are incorporated as an oxide state in a silicon–oxygen network, forming homogeneous GeO2–SiO2 glasses. When these transparent glasses were heated in H2 gas, their color turned to brown. The extent of coloration increases with an increase in the Ge content in the starting materials. According to Nogami and Abe [19], in the reaction of GeO2–SiO2 glass with H2 gas, Si4+ is much more stable

Conclusion

We have prepared Ge nanoparticles with different sizes in silica glasses by sol–gel method using Cl3–Ge–C2H4–COOH as Ge source. The size of Ge nanoparticles decreases with a reduction in the content of Ge in the starting materials. Silica gel glasses doped with Ge nanoparticles showed a strong room temperature PL with peaks at 568, 607, 672, 722 and 775 nm. The PL may arise from the Ge clusters of diameter of <1–2 nm with a molecular character instead of cubic Ge nanocrystals.

Acknowledgements

The authors would like to thank Professor Daming Huang and Professor Xun Wang for their valuable assistance.

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