InSb-added TiO2 nanocomposite films by RF sputtering
© Abe; licensee Springer. 2013
Received: 19 April 2013
Accepted: 30 May 2013
Published: 7 June 2013
This study investigates the preparation of InSb-added TiO2 nanocomposite films by RF sputtering. The optical absorption spectra are obviously shifted to visible and near-infrared regions. High-resolution transmission electron microscopy indicates that sphere-shaped InSb nanocrystals with a size of about 15 nm are dispersed in a matrix. The X-ray diffraction result reveals that the matrix forms a phase mixture of TiO2 and In2O3, which is also produced by decomposing the InSb during postannealing at 723 K. Therefore, the absorption shift is clearly due to quantum size effects of the InSb nanocrystals embedded in the wide-gap oxides TiO2 and In2O3.
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 quantum size effects. The optical band gap can then be tuned to an 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. 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. 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 those of TiO2. Most of these composite materials were synthesized through chemical techniques, although physical deposition, such as sputtering, is also useful. In addition, one-step synthesis of a 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 [10, 11] and hot-wall deposition . The present study proposes a new composite thin film with InSb-added TiO2 produced by RF sputtering. InSb nanocrystals may exhibit relatively high absorption efficiency due to a direct band structure with 0.17eV  and an exciton Bohr radius of 65.5 nm . According to the material design, based on differences in the heat of formation [10, 11], InSb nanocrystals are thermodynamically stable in an TiO2, since Ti is oxidized more than InSb because the free energy of oxidation in InSbO4, which is a typical oxide of InSb, exceeds that of the TiO2[15, 16]. In addition, nanocrystalline InSb dispersed in the oxide matrix may exhibit quantum size effects, due to the wide band-gap of 3.2 eV in TiO2 with anatase structure . However, it is difficult to forecast how the composite will be formed in the one-step synthesis, since the compound semiconductor, InSb, may have decomposed during the preparation process. In the current study, the composition of InSb-added TiO2 nanocomposite film is varied widely to find a composite with vis-NIR absorption due to the presence of InSb nanocrystals embedded in the wide-gap oxide matrix.
An InSb-added TiO2 nanocomposite film was prepared by RF sputtering from a composite target. Specifically, 5 × 5 mm2 InSb chips, which were cleaved from a 2-in diameter InSb (100) wafer, were set on a 4-in diameter ceramic TiO2 target. The chamber was first evacuated to a vacuum of 1.5 × 10−7 Torr. InSb-added TiO2 nanocomposite films were deposited on a Corning #7059 glass substrate (Norcross, GA, USA) cooled by water. The distance between the target and the substrate was kept constant at 73 mm. The total gas pressure of argon or argon with diluted oxygen was fixed at 2.0 × 10−3 Torr. RF power and deposition time were kept constant at 200 W and 60 min, and no RF bias was applied to the substrate. The InSb-added TiO2 nanocomposite films thus deposited were successively annealed at temperatures from 623 to 923 K in 50 K steps for 60 min in a vacuum to crystallize both InSb and TiO2. The film was structurally characterized using X-ray diffraction (XRD, Rigaku RAD-X, Rigaku Corporation, Tokyo, Japan). The optical-absorption spectrum of the film was measured using UV–vis-NIR spectroscopy (Shimadzu UV3150, 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 MnTiO3 to calibrate the analyzed results for elements Ti and O and with InSb for elements In and Sb. The nanoscale structure was observed using high-resolution transmission electron microscopy (HRTEM, Hitachi H-9000NAR, Hitachi, Ltd., Tokyo, Japan) operating at 300 kV. Ion milling was performed during sample preparation.
Results and discussion
InSb-added Al-oxide thin film, which is a similar composite containing InSb nanocrystals, produces a mean grain size of 8 nm during postannealing at 723 K, with similar concentrations of 9.5 at.% In and 13.5 at.% Sb . The present result provides InSb nanocrystals of nearly twice this size. In addition, no inclusion of In2O3 is seen in the InSb-added Al-oxide thin films, while this does appear in the present study (Figures 2 and 3). These different results are probably due to the difference in the free energy of reaction between the two oxides, TiO2 and Al2O3. Specifically, Al2O3 with its smaller free energy of reaction is thermodynamically more stable than TiO2. InSb-added Al-oxide thin films also exhibit a narrower size distribution in the InSb nanocrystals compared with that of the SiO2 matrix , whose free energy of reaction is close to that of the TiO2. The thermodynamic stability of the matrix may affect the aggregation of the InSb nanocrystals during postannealing, although the size distribution of the InSb nanocrystals dispersed in the multiphase matrix, TiO2 and In2O3, is not estimated here, due to a difficulty of finding InSb nanocrystals in the HRTEM image containing three kinds of crystals, InSb, TiO2, and In2O3.
The present results indicate that InSb-added TiO2 nanocomposite films provide a composite with InSb nanocrystals embedded in a multioxide matrix composing TiO2 and In2O3 and exhibiting vis-NIR absorption due to quantum size effects of the InSb nanocrystals. One-step synthesis of a composite thin film therefore has potential for low-cost production of next-generation solar cells.
InSb-added TiO2 nanocomposite films have been proposed as candidate materials for quantum dot solar cells. It should be pointed out that composite thin films with InSb nanocrystals dispersed in a multiphase composing TiO2 and In2O3 appear in a restricted composition range from 12 to 18 at.% (In + Sb), because of compositional variation. The optical absorption edge shifts toward the vis-NIR range, favorably absorbing a desirable energy region for high conversion efficiency. A HRTEM image indicates that the composite thin film contains spherical InSb nanocrystals with a size of approximately 15 nm. This size is sufficiently small to exhibit quantum size effects. InSb-added TiO2 nanocomposite films also produce In2O3, due to decomposition of the added InSb during postannealing. The electrical properties are not studied at all in the present study. However, the photocurrent of the composite may be enhanced by including In2O3, since the carrier mobility of the phase mixture of TiO2 and In2O3 is higher than that of the pure TiO2. Therefore, a multioxide matrix of TiO2 and In2O3 with InSb nanocrystals should be useful for next-generation solar cells.
SA is a group leader of the 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. 24360295). The author gratefully acknowledges the valuable comments of President T. Masumoto (Research Institute for Electromagnetic Materials (DENJIKEN), Sendai, Japan). The author is also grateful to Mr. N. Hoshi (DENJIKEN) for assisting in the experiments.
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