R.K. Sharma, L. Horák, K. Ridzoňov, J. Holovský
Czech Technical University in Prague,
Keywords: molybdenum oxide, thin film, pulsed laser deposition, high-transparency, photothermal deflection spectroscopy (PDS)
Summary:Molybdenum oxide is an intensively studied material, thanks to its high bandgap, high work function and potentially photochromic, and plasmonic properties. Furthermore, its layered structure allowed hydrogen intercalation and high environmental sensitivity. In this contribution, we employed Pulsed Laser Deposition (PLD) from stoichiometric MoO3 and metal Mo target at temperature range of 25 °C – 500 °C and oxygen pressure variation of 0.1 mbar – 0.4 mbar to deposit high transparency MoO3 layers. The combination of Photothermal Deflection Spectroscopy (PDS) and Spectral Ellipsometry is applied to accurately track all the optical properties. The X-ray diffraction and Scanning Electron Microscopy (SEM) are used to monitor crystallinity and surface morphology. Higher temperature leads to crystallization first in monoclinic beta-MoO3 phase and then in orthorhombic alpha-MoO3 in accordance with other reports. Higher pressure is promoting crystallinity, but also increasing surface roughness. The lowest overall absorptance was achieved for predominantly amorphous layers grown from stoichiometric target at room temperature (25 °C) and highest pressure (0.4 mbar), mainly because of high porosity. For polycrystalline layers, the lowest absorptance is obtained when α-MoO3 phase is dominating (500 °C and 0.4 mbar). Similar behaviour can also be seen for stoichiometric target, however with considerable surface roughness. For the Mo metallic target, both amorphous and α-MoO3 polycrystalline (low absorbing variants) are reproduced with 30% higher fluence, because for the same deposition conditions, its showing higher sub-gap absorptance. We have also noticed that for Mo metallic target, most of the grains (50 % or more) are parallel to the orientation (010) and surface is more smoother in comparison to stoichiometric MoO3.