Sol–gel matrix for YAG:Ce phosphors in pc-LEDs

In conventional silicone resins two organic methyl groups are attached to silicon in the repeat unit -[OSiMe₂]-. This accounts for matrix disintegration and yellowing [8] when used as matrix for YAG:Ce phosphors in white pc-LEDs. When sol–gel synthesis based on MTEOS, TEOS, and silica particles employed, this ratio can be significantly reduced. We chose an initial formulation, where one silicon atom statistically comes to only 0.61 methyl groups.

Sol–gel syntheses, however, differ in principle from the production of silicone resins. Ethanol is set free from the hydrolysis and condensation reaction. This volatile component must escape from the film during its manufacture. Therefore, shrinkage occurs perpendicular to the substrate plane. At the same time, a three-dimensional network is successively created due to the multiple reaction possibilities of the precursors. Consequently, tensile stresses parallel to the substrate plane occur within the film. When these stresses exceed the cohesive strength within the material, cracking of the film takes place [20‐22].

The above considerations are especially important for mostly inorganic and thus brittle film materials. One strategy to increase the critical film thickness, above which cracking occurs, is to reduce brittleness by introducing non-condensing groups that preserve some flexibility. In our case the use of a fraction of MTEOS (Me–Si(OEt)₃) serves this purpose. Additionally, pre-densified SiO₂ particles (“LEVASIL”), that do not undergo shrinkage during film preparation, are employed. Such SiO₂-based sol–gel matrix films were prepared by spin-coating. After drying at 80 °C, the samples were annealed at 300 °C. The resulting film thickness is displayed in Fig. 1a. The film thickness develops as a function of the spinning rate as one would expect from this setup: For a spinning rate of 300 RPM a film thickness of up to 6.8 µm can be obtained, for spinning rates above 3000 RPM a constant film thickness of ~2.5 µm is established. The cross-sectional SEM view of a sample prepared with 5000 RPM appears homogeneous; no cracking becomes apparent. Because silicon is the main component of both the layer and the glass substrate, the image suffers from weak brightness contrast between the two areas, but the layer can be easily distinguished from the glass substrate by its different texture.

To do justice to the truth one has to admit that cross-sectional SEM imaging of small sample spots easily suggests the absence of film defects. In fact, only large area imaging of film surfaces allows a reliable assessment of the extent of crack formation, the quantification of which is difficult [22]. Therefore, we have investigated larger sections (50 × 50 mm²) of film surfaces with Laser Scanning Microscopy (LSM, data not shown). Here we distinguish between defect-free films, samples that show individual cracks, and surfaces where interconnected cracks separate distinct layer fields. These results are collected in Table 1 along with the respective final film thicknesses.

Thermal curing of sol–gel films is a dynamic process where removal of volatile components and network formation occur simultaneously. The investigated samples were treated by placing them in a pre-heated oven, whereby the layers experience high heating rates. Under these conditions, it can be assumed that a regime “drying before gelation” as classified by Cairncross et al. [23] is established.

The table describes the quality of the layers as a function of the thickness after film deposition (spin speed) and treatment temperature. As a general trend, thinner as-deposited films (high spinning rates) are less prone to defect formation. This is directly understandable because they undergo less densification and thus suffer from less tensile stresses. It should be noted that the determination of the coating thickness by profilometry is probably subject to a measurement inaccuracy of 0.1–0.2 µm.

The dependence on the temperature is not so straightforward to describe: Samples prepared at 1500 RPM, for example, do not crack up to a treatment temperature of 200 °C. Between 200 and 400 °C, a maximum of the defect formation is passed through, whereas on the other hand, crack-free samples are obtained again at higher temperatures. It can be assumed that at lower temperatures the material is not yet fully cross-linked and the remaining flexibility prevents the formation of cracks by structural relaxation. In the medium temperature range, a rigid network has already formed at the film surface, while the removal of volatile components and reaction products, especially from deeper layer regions, is not yet complete. Cracks will initiate at the brittle surface. When the as-deposited samples are rapidly exposed to temperatures above 450 °C, the whole film is densified, and consolidation and densification are balanced.

YAG:Ce particles were mechanically stirred into matrix sols in mass ratios from 20% up to 80%. The viscosities of the sols are in the range of 0.01–0.02 Pa s, therefore they slow down the sedimentation of the YAG particles (d₅₀ 7–17 µm) only slightly. The character of the mixture is therefore more like a slurry than a stable dispersion, and rapid processing is appropriate. One could try to improve this property by adding viscous additives such as PEG. However, such a procedure would contradict the goal of a low organic content of the matrix. Later thermal burn-out of such components could also cause porosity in the composite. It seems therefore advantageous to deal with the described disadvantages of the YAG dispersion.

Doctor-blading proved to be more suitable for the production of composite layers than the spin-coating process used for pure matrix films. Figure 2 shows surface images of coatings prepared from dispersions with different YAG:Ce contents. In all images bulges, presumably from underlying YAG:Ce particles, can be seen. The average distance between the protrusions decreases when the particle content is increased. The surface of the composite film containing 20 mass% YAG:Ce shows several cracks, which sometimes merge into each other. The extent of these defects is considerably smaller, when 50 mass% are employed. The surfaces of composite films that were prepared using dispersions that contain 70 mass% YAG:Ce are completely crack-free.

The production of composite films obeys the same principles that were already mentioned for the pure matrix films: The installation of the rigid YAG:Ce phosphor particles reduces the shrinkage of the film; tensile stresses and associated crack formation are minimized. As the space between YAG:Ce grains decreases due to the higher load, the sol–gel matrix has to bridge smaller distances. In this respect the YAG:Ce particles in the composite films perform the same function as the SiO₂ particles from “Levasil” in the pure matrix layers.

The cross-sectional image in Fig. 3 reveals that the composite coatings with ~125 µm can be prepared significantly thicker than the pure matrix films. Due to the roughness of the fracture surface, the image gives the impression of a high layer porosity. Therefore, a cross section of the sample was prepared by FIB technique and examined at higher magnification. This cross section shows a firm integration of the YAG:Ce grains into the sol–gel matrix. Individual gaps in between the surface of the particles and the matrix may be induced by the heat input of the Ga-beam during sample preparation [24]. However, the rough surface of the mechanically generated fracture surface is obviously caused by the breaking out of particle agglomerates.

In addition to the mechanical integration into the matrix, the question of its protective function for the fluorescent particles and for the underlying LED chip against the ambient atmosphere arises. This may become especially important for next-generation LEDs which use more sensitive converters such as quantum dots or nitrides [16, 17].

Oxygen and humidity permeation measurements would therefore be obviously appropriate, but the materials are too brittle to prepare specimen suitable for such measurement setups. Ellipsometric porosimetry [25] could be used to characterize the water vapor condensation into possible pores of the matrix films. Unfortunately, these films are too thick for such a measurement and their refractive index contrast to the glass substrates is too low.

ALD [26] is a versatile tool to deposit dense and conformal barrier films of e.g. alumina [27], hafnia [28], or hafnia-silica laminates [28, 29]. Apart from the additional application of protective layers on our composites, investigations on the penetration of the gaseous ALD precursors used can allow conclusions about the tightness of the matrix materials.

At the edges of doctor-bladed samples some surface cracking may occur as the local film thickness exceeds 200 µm. We purposely chose such a specimen and applied a tenfold ALD procedure of alternate alumina and hafnia deposition of 5 nm each. For our processing temperature of 150 °C it is known that these compositions are formed in their amorphous phase [26]. The SEM overview of the cross section in Fig. 4 shows that both the outer surface and the crack surfaces open to the outside are coated (1). But there are also voids between the YAG:Ce particles and the matrix (2) where both materials remain uncovered by ALD. In these regions, the sol–gel material obviously provides effective protection against the access of gaseous media. Also within closed pores (3) no material deposition takes place.

An enlargement of a section shows the lamellar structure of the ALD coating. Because of the comparable atomic numbers of their main components, the first alumina coating on the YAG:Ce and sol–gel matrix material is hardly visible, while the second hafnia coating appears much brighter.

The upper part of Fig. 5 shows a V-shaped crack that tapers downwards. Its inner side is evenly coated with the ALD material; the laminates on both walls of the crack meet at its narrowest point at the bottom. This constriction of the crack coming from above reaches down to a horizontal cavity; the original access to it is now closed by the ALD deposition. The inner surface of the lower cavity is only partially covered with less than 10 of the ALD laminate layers, preferably near the former gap. Figure 5 shows how ALD has sealed a crack within the YAG:Ce matrix composite, blocking access to the surrounding atmosphere. This is a particularly impressive example of how sol–gel matrix and ALD complement each other in terms of their blocking effect.

For functional tests composites of YAG:Ce phosphors (70 mass%) in a sol–gel matrix were drop-casted on LED-chips (1 × 1 mm²) as shown in the left part of Fig. 6. After curing at 150 °C the film remains adherent and results in a functional device (right part).

The standard pc-LEDs under investigation using silicones as matrix material for their YAG:Ce phosphors are operated with 0.75 A. At higher currents the LED temperatures exceeds 150 °C causing degradation and yellowing of the PDMS matrix. In the left part of Fig. 7 the emission of such a device is shown. After 1500 h operating time a slight decrease in emission due to aging phenomena is observed

In contrast to the silicone-based devices LEDs using the sol–gel matrix material can be run at 1.9 A which leads to a strongly enhanced brightness. Despite the more stringent conditions, even after 1500 h of operation, only a moderate reduction in emission is evident (right part of Fig. 7). This observation is presumably due to the reducing of the methyl/Si ratio from 2 (PDMS) to 0.61 and even 0.16 for the sol–gel matrix. Our sol–gel material clearly outperforms the previous state-of-the-art silicones.