Effect of the Helium Background Gas Pressure on the Structural and Optoelectronic Properties of Pulsed-Laser-Deposited PbS Thin Films.

This work focuses on the dependence of the features of PbS films deposited by pulsed laser deposition (PLD) subsequent to the variation of the background pressure of helium (PHe). The morphology of the PLD-PbS films changes from a densely packed and almost featureless structure to a columnar and porous one as the He pressure increases. The average crystallite size related to the (111) preferred orientation increases up to 20 nm for PHe ≥ 300 mTorr. The (111) lattice parameter continuously decreases with increasing PHe values and stabilizes at PHe ≥ 300 mTorr. A downshift transition of the Raman peak of the main phonon (1LO) occurs from PHe = 300 mTorr. This transition would result from electron-LO-phonon interaction and from a lattice contraction. The optical bandgap of the films increases from 1.4 to 1.85 eV as PHe increases from 50 to 500 mTorr. The electrical resistivity of PLD-PbS is increased with PHe and reached its maximum value of 20 Ω·cm at PHe = 300 mTorr (400 times higher than 50 mTorr), which is probably due to the increasing porosity of the films. PHe = 300 mTorr is pointed out as a transitional pressure for the structural and optoelectronic properties of PLD-PbS films.