Formation of Nb2O5 matrix and Vis-NIR absorption in Nb-Ge-O thin film
© Abe.; licensee Springer. 2012
Received: 9 April 2012
Accepted: 14 June 2012
Published: 25 June 2012
This paper investigates the crystal structure and optical absorption of Ge-doped Nb-oxide (Nb-Ge-O) thin films prepared by RF sputtering. A wide-gap material, Nb2O5, is selectively produced as a matrix to disperse Ge nanocrystals through compositional optimization with Ge chip numbers and oxygen ratio in argon. The optical-absorption spectra are obviously shifted to visible (vis) and near-infrared (NIR) regions, suggesting that a composite thin film with Ge nanocrystals dispersed in Nb2O5 matrix exhibits quantum-size effects. Accordingly, the two valuable characteristics of the Nb2O5 matrix and the vis-NIR absorption are found to be retained simultaneously in Nb-Ge-O thin films.
Quantum-dot solar cells have attracted much attention because of their potential to increase conversion efficiency . Specifically, the optical-absorption edge of a semiconductor nanocrystal is often shifted due to the quantum-size effect. The optical band gap can then be tuned to the effective energy region for absorbing the maximum intensity of the solar radiation spectrum. Furthermore, quantum dots produce multiple electron–hole pairs per photon through impact ionization, whereas bulk semiconductor produces one electron–hole pair per photon.
A wide-gap semiconductor sensitized by semiconductor nanocrystals is a candidate material for such use. Wide-gap materials such as TiO2 and ZnO can only absorb the ultraviolet (UV) part of the solar radiation spectrum. Hence, the semiconductor nanocrystal supports the absorption of visible (vis) and near-infrared (NIR) light. Up to now, various nanocrystalline materials (InP , CdSe , CdS [4, 5], PbS , and Ge [7, 8]) have been investigated as sensitizers for TiO2. Alternatively, the wide-gap semiconductor ZnO was also investigated, since the band gap and the energetic position of the valence band maximum and conduction band minimum of ZnO are very close to that of TiO2. Most of these composite materials were synthesized through chemical techniques, although physical deposition, such as sputtering, is also useful. In addition, package synthesis of composite thin film is favorable for low-cost production of solar cells. Package synthesis requires a specific material design for each deposition technique, for example radio frequency (RF) sputtering  and hot-wall deposition . The present study proposes a new composite thin film with Ge nanocrystals dispersed in Nb2O5 matrix by RF sputtering. According to the material design, based on differences in the heat of formation , Ge nanocrystals are thermodynamically stable in an Nb2O5 matrix, since Nb is oxidized more than Ge because the heat of formation of GeO2 exceeds that of Nb2O5. In addition, nanocrystalline Ge dispersed in the Nb2O5 matrix may exhibit quantum-size effects due to the wide band gap of 3.4 eV in Nb2O5. However, it is difficult to forecast how Nb oxides (typically NbO, NbO2, and Nb2O5) will be formed during the preparation process. Among these compounds, only Nb2O5 satisfies the present objective. In the current study, the composition of Ge-doped Nb-oxide (Nb-Ge-O) thin film is varied widely to produce single-phase Nb2O5 as the matrix, while retaining vis-NIR absorption due to the presence of Ge nanocrystals.
An Nb-Ge-O thin film was prepared by RF sputtering from a composite target. Specifically, 5 × 5 mm2 Ge-chips were set on a 4-in.-diameter ceramic Nb2O5 target. The chamber was first evacuated to a vacuum of 1.5 × 10−7 Torr, and the thin film was deposited on a Corning #7059 glass substrate cooled by water. The substrate was cleaned with an acetone ultrasonic bath for 60 min to remove surface contaminations, dried using nitrogen air gun, and finally sputter-etched at an applied power of 200 W for 1 min. The distance between the target and the substrate was kept constant at 73 mm. The total gas pressure of argon or argon and additional oxygen was fixed at 2.0 × 10−3 Torr. RF power and deposition time were kept constant at 200 W and 90 min, and no RF bias was applied to the substrate. The Nb-Ge-O thin films thus deposited were successively post-annealed at 923 K for 60 min in a vacuum to crystallize Ge nanocrystals and the Nb-O matrix. The Nb-Ge-O thin film was structurally characterized using X-ray diffraction (XRD, Rigaku RAD-X, Rigaku Corporation, Tokyo, Japan) with Cu Kα radiation. The optical-absorption spectrum of the film was measured using UV–vis-NIR spectroscopy (Shimadzu UV5300, Shimadzu Corporation, Nakagyo-ku, Kyoto, Japan), and the composition of the film was analyzed using energy-dispersion spectroscopy (EDAX Phoenix, NJ, USA), operating at 10 kV with standard samples of KNbO3 to calibrate the analyzed results for elements Nb and O, and with Bi4Ge3O12 for element Ge. Nanoscale elemental mapping was performed using scanning transmission electron microscopy (STEM, Hitachi HD-2700, Hitachi, Ltd., Tokyo Japan) in EDX mode (EDAX model: Genesis) operating at 200 kV with an energy resolution of 150 eV. Ion milling was performed during sample preparation.
Results and discussion
These results indicate that the Nb-Ge-O thin films can selectively produce the Nb2O5 matrix and the vis-NIR absorption simultaneously, despite the package synthesis by RF sputtering. One-step synthesis of a composite package with Ge nanocrystals dispersed in Nb2O5 matrix therefore has the potential to yield low-cost production of next-generation solar cells.
A new composite thin film with Ge nanocrystals dispersed in Nb2O5 matrix has been proposed as a candidate material for quantum-dot solar cells. It should be pointed out that single-phase Nb2O5 appears in a restricted composition range from 1.0 to 1.8 at.% Ge as a result of compositional optimization based on the Ge chip number and oxygen ratio in argon. Furthermore, the optical absorption edge shifts toward the lower-photon-energy region as the Ge content increases. In particular, onset absorption can be confirmed at 1.0 eV with 1.5 at.% Ge, favorably covering the desirable energy region for high conversion efficiency. Elemental mapping indicates that the isolated Ge nanocrystals are dispersed in the Nb2O5 matrix. Thus, two valuable characteristics, the selective production of Nb2O5 and vis-NIR absorption, are simultaneously retained in Nb-Ge-O thin films.
SA is a group leader of Research Institute for Electromagnetic Materials.
The present work was supported by a Grant-in-Aid for Scientific Research from the Japan Society for the Promotion of Science (No. 18360338). The author gratefully acknowledges the valuable comments and continuous encouragement of President T. Masumoto [Research Institute for Electromagnetic Materials (DENJIKEN), Sendai, Japan]. The author is also grateful to Mr. N. Hoshi and Y. Sato (DENJIKEN) for assisting in the experiments.
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