Surface Patterning of Ceramic Phosphor Plate for Light Extraction

Light-Emitting Diodes (LEDs) are expected to replace traditional lighting sources in the near future due to their energy-efficiency, optical design flexibility and good reliability over traditional lighting sources. III-V nitride blue LEDs with powdered phosphors have been used commercially to get white emission. However, due to scattering losses, thermal issues as well as the surface reactivity with common encapsulants, LEDs fabricated with powdered phosphors have limitations in achieving high luminous efficacy, high chromatic stability and good color-rendering properties. Solid, non-scattering phosphors could avoid many of these limitations, but issues of light extraction and coupling of excitation radiation to the phosphor require development to insure efficient operation. Photonic crystal structures fabricated into or on non-scattering phosphors can be used to address these challenges. In this thesis, a lift-off process with bilayer resist system is developed to create nanopatterns. A photonic crystal structure is fabricated by low cost molecular transfer lithography (MxL) with bi-layer resist system on non-scattering phosphor plate used for white emission to increase the extraction efficiency. In Chapter 1, some basic background concepts which appear frequently in this thesis are introduced. These concepts include the Stokes shift and backscattering phenomenon for powder phosphors as well as non-scattering phosphors. In Chapter 2, a non-scattering single crystal phosphor with a patterned surface is proposed to replace the powdered phosphors used for color converted LEDs. A non-scattering phosphor YAG:Ce ceramic phosphor plate (CPP) patterned with TiO2 photonic crystal structure is selected for convenience to demonstrate the concept. The physical origin of light extraction of the proposed structure is discussed. The simulation principles and results are discussed in this chapter to find the optimized photonic crystal structure for light extraction. In Chapter 3, a lift-off procedure developed in this work is demonstrated; involving molecular transfer lithography (MxL) process based on water-soluble nanostructured PVA templates. Nickel hard masks are fabricated using this process and a novel bi-layer resist system suitable for simple, high yield lift-off process. Using this process, TiO2 photonic crystal structures are fabricated on YAG: Ce CPP substrates. In Chapter 4, the optical performance of the fabricated samples is evaluated and discussed. The forward emission of the CPP is measured by placing the CPP on top of a simple blue LED source. The extraction efficiency of the light from the patterned CPP is increased by over 4.5 times compared to the non-patterned CPP. The photonic crystal structure also demonstrates collimation of the emitted light from the CPP in forward direction. A 32.6 % of improvement in forward conversion efficiency for a 300 nm photonic crystal (PhC) patterned TiO2-YAG:Ce CPP has been achieved relative to non-patterned YAG:Ce CPP. In conclusion, this thesis demonstrates that the use of non-scattering ceramic phosphors has advantages for fabricating efficient phosphor converted LED structures. The concept and fabrication process is also applicable to other ceramic phosphor plate designs for improved light extraction. A lift-off process using bi-layer resists and using nanopatterned PVA templates is developed to fabricate nanostructures used in this study. The process could be extended to large scale, low lost nanostructure fabrication for a very wide variety of applications due to its simplicity and scalability.