Growth and characterization of PbTe films by magnetron sputtering
Introduction
PbTe semiconductor is of interest due to its potential application as a thermoelectric material, infrared detectors and lasers in the 3–30 μm range [1]. For its use in any of the above applications epitaxial PbTe on Si wafers is desirable. In particular the monolithic integration of narrow gap AIVBVI semiconductors and Si devices is highly attractive for infrared (IR) optoelectronic applications [2]. Epitaxial growth of PbTe semiconductor on Si wafers is required [3]. The growth of epitaxial PbTe is hindered by the large mismatch of ∼19% between the lattice parameters of PbTe and the Si substrate and the considerable difference in their thermal expansion coefficients [4]. Usually some intermediate buffer layer solves the mismatch problem. Fluoride buffer layers, such as CaF2 and BaF2, are generally used to reduce thermal and lattice strains [3], [5]. Fluoride buffers have been successfully used for the growth of AIVBVI (1 1 1) oriented films on Si (1 1 1) substrates [6]. Epitaxial growth of PbTe films directly on the Si wafer is desirable for the simplification of the fabrication process. Epitaxial growth of PbTe directly on Si to fabricate a PbTe/Si heterostructure on p-type Si (1 0 0) by hot-wall epitaxy (HWE) without an intermediate buffer layer, was studied by Yang et al. [7]. The growth of epitaxial PbTe also on Si (1 1 1) by the HWE technique had been undertaken by Vaya et al. [8]. Furthermore, it might be of interest to investigate the effect of an oxide such as SiO2 also on the PbTe properties. Such an oxide might also act as a buffer and influence the tendency of epitaxial film formation [9].
In the present work the growth of PbTe thin film deposited on Si (1 1 1) substrate by RF magnetron sputtering is addressed. In order to optimize the conditions of the process, sputtering of PbTe on the Si wafers was performed by varying the power while maintaining the time constant or by keeping a constant power and altering the time of deposition. X-ray diffraction (XRD), Auger spectroscopy and high-resolution electron microscopy (HRTEM) were used to characterize the films. Optical transmission spectra were also measured.
Section snippets
Experimental
In our attempt to eliminate the use of buffer layers and simplify the PbTe fabrication process, thin films were grown on Si (1 1 1) substrates by RF magnetron sputtering. No extra cleaning procedures of the Si wafers were used except for the removal of organic impurities (by alcohol and acetone). It was of interest to see the effect of oxide and since native oxide exists usually on wafers this served as the first step to receive preliminary information. Simultaneously with the silicon, glass
Results and discussion
Fig. 1 illustrates XRD profiles of the PbTe film sputter deposited on silicon and glass substrates, which were in-situ heated during the deposition. Fig. 2 compares XRD profiles resulting from deposition on unheated substrates. As can be seen a considerable difference exists between the films deposited on the heated and unheated substrates. Also, a difference in the results was observed when the substrates are different. Sputtering at 40 W in the time range of 30–60 min and in situ substrate
Conclusion
Single crystal-like PbTe films can be obtained on in situ heated silicon (1 1 1) substrate by magnetron sputtering. It is likely that the reason for this might be the smaller mismatch between the interplanar spacing of Si (1 1 1) and PbTe (2 0 0) relative to that between the lattice parameters. HRTEM shows the presence of a thin amorphous, probably oxide layer, ∼2–4 nm thick at the interface between the substrate and the PbTe layer. Electron diffraction indicates an almost epitaxial relation between
Acknowledgements
The authors would like to thank Mrs. A Pelleg for proofreading the manuscript. The help of V. Ezersky and D. Mogilyanski is highly appreciated. The author would also like to thank Prof. M. Talianker for his help.
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