Photoluminescent materials emit light under illumination via the phenomenon of photoluminescence. Scientifically, photoluminescence (PL) is the radiative emission of light from a material following excitation to a higher energy state. Using Plasma Quest Limited’s thin film deposition technology, coatings of ZnS:Mn exhibiting strong photoluminescent characteristics have been deposited onto silicon, glass and flexible plastic substrates at low temperature. The thin film photoluminescent coatings have also been incorporated as the active light-emitting layer within thin film Electroluminescent devices deposited entirely using the HiTUS sputter deposition equipment.
A reactive sputtering process is employed depositing ZnS:Mn thin films from a Zn target doped with Mn (Zn:Mn) in a partial pressure of argon and hydrogen sulphide (H2S) gases. This material has been deposited using varying concentrations of Mn within the Zn target and a value of 0.6 wt.% Mn has been shown to yield coatings with the highest luminescent intensity.
By increasing the flow rate of H2S for fixed target powers the films are seen to become highly transparent exhibiting an average visible (400 to 800 nm) transmission of approximately 90 % when referenced to an uncoated glass slide. The deposition rate is also seen to increase for constant target power with the introduction of more H2S gas into the process. This peaks at approximately 50 nm/min for a target power density of just 2.55 Wcm-2, with any further increase in partial pressure of H2S having a detrimental effect on the photoluminescent intensity. Deposition is carried out with no substrate heating or post annealing.
The luminance of the coatings on glass has been measured by an independent source. Excitation was via a N2 gas laser (337 nm) and results show that the intensity of the luminescent emission exhibits the broad spectrum characteristic of Mn doped ZnS, with a peak intensity between 585 and 600 nm, dependent upon process conditions. An image taken of an 800 nm thick ZnS:Mn coating deposited onto a glass microscope slide and illuminated with a standard UV light source can be seen below. The intensity of light emission from HiTUS deposited ZnS:Mn thin films deposited at room temperature has been shown to be significantly higher than similar thin films deposited at elevated substrate temperatures and post deposition annealed using other sputtering technologies. The low temperature nature of the HiTUS ZnS:Mn process also makes it compatible with flexible displays, conformable lighting and indicator applications.