Gallium nitride on low temperature cofired ceramic templates for Schottky junctions

Abstract

In this work aluminum, silicon and zinc oxide were used as intermediate layers for thin film growth on cofired glass ceramic substrates. The motivation behind this work is a direct deposition of nitride thin films on the surface of the ceramic substrate, eliminating the die and attach techniques. Ceramics have unique applications due too the nature of their mechanical processing, and their physical resilience and chemical inertness. The low melting of the glass ceramics from a device processing perspective and their rough, inhomogeneous surface presents a challenge for device fabrication. Oxide materials can be applied by a variety of techniques compatible with large device areas and arbitrary shapes to apply a surface texture to improve thin film properties for device fabrication. Ideally these techniques could be applied to any substrate that meets the thermal budget of the thin film process. Solution coating was found to be a good candidate for applying coatings since it can deposit many different oxide materials over large areas, for relatively low cost, and surface tension of the liquid phase helps to planarize the surface. Several (>7) microns of coating materials were found to be needed to reduce the appearance of the ceramic surface features. Deposition of GaN on the surface of the oxide coatings was performed using a Flow Modulation Epitaxy (FME) style deposition in conjunction with a unique hollow cathode plasma source. These features are designed to lower the overall temperature requirements for GaN growth by providing additional Ga migration time during growth and by using nitrogen plasma as an alternative to thermal decomposition of ammonia. Ni/Au Schottky junctions fabricated on sapphire using ceramic compatible temperatures and FME show leaky characteristics with high ideality factors, indicating tunneling is a significant contributor to carrier transport through the junction. The same Ni/Au GaN devices fabricated on ZnO coated ceramics was found to produce ohmic junctions. The density of surface states is a likely candidate for this behaviour.