Physica E: Low-dimensional Systems and Nanostructures
Quantum confinement effects of CdTe nanocrystals sequestered in TiO2 matrix: effect of oxygen incorporation
Introduction
Quantum dots are nanometer-size semiconductor structures and represent one of the most rapidly developing areas of current semiconductor research. Size-dependent chemical reactivity [1], optical nonlinearity [2], efficient photo-electron emission [3] and melting point reduction [4] are some of the interesting properties exhibited by nanoparticles. Applications take advantage of high surface area and confinement effects, which lead to nanostructures with different properties than conventional materials, and which create opportunities for innovating principles of operation for devices and instruments.
Recently, the nanostructures of II–VI materials have attracted much attention for their great fundamental, experimental and applied interests. CdS [5], CdSe [6], and CdTe [7] are amongst the most studied cases. CdTe has a direct band gap of 1.5 eV at room temperature, and is a suitable material for electro-optical devices. Because of its large exciton Bohr diameter (15.0 nm), this semiconductor offers the possibility of studying quantum confined materials with small reduced cluster sizes (ratio of cluster size to exciton Bohr diameter) [7]. Amongst the various physical vapor deposition (PVD) techniques for the preparation of CdTe nanocrystalline composites in TiO2 matrix, RF sputtering has been the most versatile and proven one since it enables the controlled distribution of nanoparticle size and volume over a wide range [8]. Moreover, using RF magnetron sputtering, the processing at relatively low temperatures can avoid the possible reaction between the matrix and the semiconductor nanoparticles. However, nanocomposites film having large volume fraction of semiconductor particles in insulating film have not been studied in detail. It is expected that due to enhanced surface-to-volume ratio of semiconductor, nanocrystals should show different properties varied over a large range.
In the present study, RF-sputtered TiO2 thin films of thicknesses ∼500 nm deposited at two substrate temperatures, room temperature (RT) (300 K) and 373 K with high volume fraction (40–55%) of CdTe have been obtained. Generally, certain threshold values of volume fraction and crystallite size are imperative in order to observe quantum confinement effects. In the present work, systematic variation in CdTe volume fraction (and thus crystallite size) has been carried out and its effect on the shift of the optical absorption edge has been investigated. Post-deposition thermal treatment in the temperature range of 250–300°C has also been carried out in order to elucidate its role in modulating the optical absorption edge of the composite thin films. The choice of TiO2 as an insulating matrix is based on the fact that TiO2 being an optically transparent wide band gap oxide, the quantum size effects of CdTe nanocrystals embedded in TiO2 can be studied over a wide energy range. The volume fraction of CdTe (Vf) in nanocrystalline CdTe:TiO2 films were systematically varied by increasing the number of CdTe pellets on the TiO2 target.
Section snippets
Experimental
The CdTe nanocrystallites embedded in TiO2 matrix were prepared by RF magnetron sputtering from a composite target. The target consisted of TiO2 sintered disc of 2 inches diameter having a CdTe pellet of diameter 10 mm placed at its center. By varying the number of CdTe pellets on the TiO2 target, the semiconductor concentration in the deposited film can be controlled to a desired value. TiO2 and CdTe were co-sputtered with a RF power of 240–260 W in the presence of Ar gas at 0.02 mbar. The
Results and discussion
M1–M3 corresponds to nanocrystalline CdTe:TiO2 films with varying number (1–3) of CdTe pellets on the TiO2 target prepared at while M4–M6 represents the corresponding films prepared at , respectively. The different elemental compositional values obtained by EDS measurements for nanocrystalline CdTe:TiO2 films before and after thermal treatment (250–300 °C) are listed in Table 1. As shown in Table 1, by increasing the number of CdTe pellets on the TiO2 target and upon thermal
Conclusions
TiO2 thin films with high volume fraction of CdTe dispersions could be realized by using RF magnetron sputtering from a composite CdTe:TiO2 target. EDS measurements allowed determination of the composition of the samples. The films prepared at RT and 373 K were mainly amorphous in nature, however, few crystalline signatures were observed for high Vf CdTe films prepared at high Ts. Interplanar distance of CdTe:TiO2 composite thin films were found to be larger for films prepared at high Ts due to
Acknowledgments
We thank the Director, National Physical Laboratory for permission to publish this research work. Acknowledgments are also due to Dr. (Mrs.) M. Kar and Mr. K.N. Sood for optical and EDS measurements, respectively.
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