Structural and optical properties of thin lead oxide films produced by reactive direct current magnetron sputtering

Abstract

The structural and optical properties of thin lead oxide films were studied. Thin films were prepared by reactive dc magnetron sputtering of lead (Pb) targets in an Ar/O2 mixture. The structure has been determined by x-ray diffraction measurements, which show that crystalline lead oxide films of different composition (PbO, PbO1.44, Pb2O3, and PbO2) have been formed upon increasing oxygen flow (partial pressure). This result is confirmed by Raman spectroscopy. The effect of postdeposition annealing on the structural properties of PbO films reveals that the film structure is governed by both energetics and kinetics. X-ray reflectivity measurements were used to determine the thickness, density, and roughness of the films. The calculated film density values are almost equal to the bulk density of the material, showing that compact, nearly void free films are formed. The optical properties of the films have been studied from the reflectance and transmittance spectra recorded by optical spectroscopy measurements from 10 000 (1.24 eV) to 50 000 (6.21 eV) cm−1. From these data we have determined optical properties such as the dielectric function, the optical band gap Eg and the refractive index n as well as the film thickness.

Role of substrate temperature on the growth mechanism and physical properties of spray deposited lead oxide thin films

“…E g values of 1.9 and 2.5 eV were found for the tetragonal and orthorhombic structure of PbO, respectively [28]. For Pb 3 O 4 an experimental value of 2.1 to 2.2 eV is reported, while for PbO 2 a value of 4.2 has been measured for α-PbO 2 phase and a value of 1.5 eV has been reported for β -PbO 2 [29,30]. The band gap values obtained in this work are consistent with the values reported in the literature [28].…”

“…Lead oxide thin films have been laid on by both physical and chemical deposition methods. Physical methods reported in the literature include pulsed laser-assisted deposition [17], dc and rf magnetron sputtering [18], while chemical methods include spray pyrolysis [13,14] and electro deposition [19]. Amongst the chemical methods used to fabricate lead oxide thin film, spray pyrolysis method is simple, inexpensive, and capable of producing films on large scale area for industrial applications.…”

Origin of n- and p-type conductivity in undoped α-PbO: role of defects

“…For crystalline materials, interstitial defects induce a large lattice distortion into their immediate neighbourhood that results in their high formation energy [15]. In this regard, the crystal lattice of polycrystalline α-PbO is unique, because the distance between weakly interacting layers is large enough to accommodate a foreign atom (c 0 = 5.07Å [14]). Therefore, along with the vacancy defects, formation of the interstitials is expected to be inherent to the α-PbO lattice.…”

“…Because the size of the band gap is crucial in order to correctly define the appearance of the defect states inside the band gap, the band structure properties have been evaluated for the optimized lattice parameters. However, for calculations of the defect formation energy, an adjustment of the interlayer distance by using the experimental data of the lattice constant (c 0 = 5.07Å [14]) has been performed. It was found that only the formation energy of the Pb interstitial is considerably affected by the mismatch in the lattice parameters, caused by the poor incorporation of Pb atom of large atomic radius between the layers (the formation energies of the vacancy defects have been pointed out not to be affected by GGA limitation in our previous work [5]).…”

“…An alignment of the top of the valence band (E V ) for supercell with charged defect to that in the bulk is performed with help of the correction term ∆V [15]. Since for accurate calculation of the formation energy of interstitial defects the mismatch in the interlayer distance (c 0 =5.51Å) occurring for the lattice parameters optimized with GGA in comparison to the experimental data (c 0 =5.07Å [14]) is essential, the experimental value has been used for those calculations. In this case, an application of the so-called band gap error ∆E G [21] is required to compensate for a reduction of the band gap size [5].…”