2, IQE Inc., Greensboro, North Carolina, United States
The wide band gap gallium nitride is an excellent material for the production of high power, high speed transistors as well as semiconductor lasers and light emitting diodes (LEDs). While GaN bulk substrates are still too expensive for practical application in solid state lightning, much attention is drawn towards GaN on Si technology. One possibility to integrate GaN on Si is the heteroepitaxial growth using erbium oxide as a buffer. Er2O3 reduces the lattice mismatch between Si and GaN. However due to thermal expansion coefficient mismatch the cracks are observed after the growth of GaN thick layers. If the substrate is patterned into separate areas, the stress can be relieved by elastic relaxation of the film at the pattern edges. Therefore cracks can be prevented to occur in the formed films. This study is aimed to develop growth technology of GaN on Si substrates using Er2O3 as buffer layer in the application field of photonics.
Two types of substrates were studied in this work for GaN growth. The first one was designed to fabricate a wave guide in the visible region. Therefore 300 nm-thick Er2O3 (111) layers were prepared on Si(111) substrate by molecular beam epitaxy (MBE). Afterward, the substrates were covered with photoresist, exposed to UV light and finally wet etched to produce the slab like patterns. The chemical etching kinetics was analysed in detail by determining the etching rates and activation energy. The optimization of the wet etch process was necessary in order to get well defined 100x100um structures. For the second set of samples a plane structure with several periods of Er2O3/Si layers were grown on Si(111) by MBE. The latter structure can act as a Bragg reflector with the reflection maximum at a wavelength of 480 nm. Prior GaN growth, the structures were analysed by spectrometer and optical microscopy. The finite difference time domain (FDTD) modelling was performed to explain the light propagation in the designed structures both in the patterned sample and the plane one. The close-coupled showerhead metalorganic chemical vapour deposition reactor (MOCVD) was used to grow GaN on a plane substrate with Bragg reflector structure. The morphological and structural properties of GaN were investigated by X-ray diffraction (XRD) and atomic force microscopy (AFM).