Luminescent properties of Y2SiO5:Ce thin films
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The luminescent properties of yttrium silicate doped with cerium (Y2SiO5:Ce) phosphor thin films were investigated. A detailed investigation (cathodoluminescence (CL), photoluminescence (PL) and Gaussian peak fits) was first done on the luminescent mechanism of Y2SiO5:Ce phosphor powders in order to understand and find a plausible mechanism that could assist in future research to be done. Luminescence in Y2SiO5:Ce occurs due to characteristic transitions in the Ce3+ ion itself. Splitting of the 4f energy level into the 2F5/2 and 2F7/2 energy levels is due to the 4f1 electron in Ce3+ having the ability to exhibit a +1/2 and -1/2 spin. This creates the expectation of a luminescent spectrum with two main peaks in the blue region (between 400 and 500 nm). Y2SiO5:Ce has two different monoclinic crystal structures. A low temperature (synthesized at temperatures less than 1190 °C) X1 - phase (much weaker luminescent intensity, with space group P21/c) and a high temperature (synthesized at temperatures above 1190 °C with a melting temperature at 1980 °C) X2 - phase (space group B2/c). In each of these two phases there are two possible Y3+ sites in the Y2SiO5 matrix. The most plausible explanation for the broad band luminescent spectra obtained from excitation and emission results in this research study is that the two different sites of the Ce3+ ion (Ce can substitute Y) (A1 and A2) in the host matrix are responsible for two sets of visible peaks. The difference in orientation of the neighbour ions in the crystal structure will be responsible for the broadening of the band emission. Three sets of Y2SiO5:Ce thin films were grown with pulsed laser deposition (PLD) by using a 248 nm KrF and a XeCl (λ = 308 nm) excimer laser. The thin films were grown on Si (100) substrates with different process parameters in order to investigate the surface morphology and luminescent properties. Process parameters that were changed during the growth process using a KrF laser were the O2 ambient pressure (vacuum, 10 mTorr and 1 Torr), the fluence (3 ± 0.3 and 1.6 ± 0.1 J.cm-2), the substrate temperature (400 and 600 ºC) and the gas species (N2, O2 and Ar at 455 mTorr). The laser pulse frequency and the amount of pulses were kept constant at 8 Hz at 4000 pulses. The increase in the pressure to 1 Torr O2 shows a definite increase in particle size and roughness. The increased fluence led to bigger particle and grain sizes. The surface structure of the thin film ablated at 400 ºC substrate temperature is less compact (lesser agglomeration of particles than the 600 °C). The increase in substrate temperature definitely resulted in a rougher surface layer. Ablation done in N2 gas resulted in small particles of mostly 20 nm in diameter. Ablation in O2 gas produced bigger particles of 20, 30 and 40 nm as well as an agglomeration of these particles into bigger size clusters of about 80 to a 100 nm. Ablation in Ar gas showed particle sizes of mostly 30 nm. The particles are more spherically defined and evenly distributed on the surface in comparison with the agglomerated particles grown in O2 gas. Thin film morphology and other characteristic properties strongly depend on the gas pressure during PLD. An increase to 1 Torr O2 gas thus resulted in bigger particle sizes and the higher fluence also led to bigger particles with a decrease in particle density. The higher substrate temperature resulted in a rougher surface layer and ablation in Ar gas at 455 mTorr compared to N2 and O2 gas resulted in bigger and less agglomerated particles being formed. CL scanning images were obtained to investigate the effect of a tin oxide (SnO2) coated layer on the light output. The CL scan results of the uncoated and tin oxide coated thin films showed a definite increase in luminescent intensity with the uncoated thin film which indicates the photon absorption effect of the extra tin oxide coated layer. The tin oxide acts as a coated layer to prevent electron stimulated reactions with the phosphor surface and thus inhibits degradation. CL measurements that were done showed that the increased O2 ambient (1 Torr) resulted in a higher CL intensity compared to the thin films ablated in vacuum. This is in agreement with the PL results where the nano – particles’ shape ensure better light output due to fewer photons being totally reflected internally. The ablation in high fluence also showed a higher CL and PL intensity with the vacuum and 1 Torr thin films compared to the low fluence. The higher substrate temperature (600 ºC) results in better intensities due to the rougher surface formed. The thin film ablated in 455 mTorr Ar gas showed a higher CL intensity than the other two thin films. This is due to the spherically shaped and the less agglomerated particles on the surface of the substrate. A 144 hr CL degradation study was done on the thin film ablated in Ar gas (coulomb dose of 1.4 x 104 C.cm-2) at an O2 pressure of 1 x 10-6 Torr, 2 keV electron energy and 10 μA electron beam current. There was a definite decrease in the CL intensity measured at 440 nm while a second broad band peak emerged at 650 nm, which increased with an increase in the degradation time; leading to a broad spectrum ranging from 400 to 850 nm. The blue colour again changed (the same as with the powders) to a whitish colour. The degradation results were again ascribed to the formation of SiO2 with a defect level at 1.9 eV (650 nm). The XPS analysis showed that a SiO2 layer formed on the surface under electron bombardment. The thin films are therefore also degrading but are more chemically stable than the phosphor powders. The light output intensity however; is lower.