Innovative III-Nitride Epitaxy Approach for Low Dislocation GaN and Free-Standing GaN Substrate

Abstract

GaN and its related alloys have emerged as eminently useful materials for the fabrication of green-to-ultraviolet light-emitting diodes (LEDs), laser diodes (LD) and also high-power, high-efficiency transistor devices due to their widely tunable bandgap. The key technological challenge in the manufacturing of these high performance optoelectronic and electronic devices is the absence of GaN substrates of practical size. As a result, all epitaxial GaN films are grown on the top of a substrate with different lattice constant, such as sapphire, Si and SiC, leading to the inevitable introduction of dislocations and residual strain in the epilayers, which is known to degrade the performance and reliability of the devices. Moreover, the popularly used sapphire substrate is an insulator with low thermal conductivity and difficult to cleave beneath the GaN based LD wafers. So there is a high demand for both low dislocation GaN epilayers and freestanding substrate. In this work, we present two approaches to meet this demand. (1). The Innovative One-step Lateral Epitaxy with Serpentine Mask Structures. There are two key features to this technology, one being a unique mask structure that blocks threading dislocations using amorphous films, and the other being associated with the controlled selective nucleation directly on sapphire surface. The direction of the growth front of GaN makes two 90° turns before emerging from the serpentine channel. The growth direction at the beginning is along the substrate surface norm that subsequently turns to become parallel to the surface and finally turns towards the surface norm again when emerging from the serpentine channel. It achieves the blockage of threading dislocations from reaching the active device region. The second key feature is the selective nucleation of GaN on sapphire surfaces but not on the amorphous mask surfaces, which is the enabling factor that allows for the entire fabrication process being carried out with only one single epitaxial step, leading to significant reduction in the fabrication cost. The microstructural and optical properties of the GaN epilayer grown on serpentine masked structures will be described. Furthermore, we will summarize that this novel technique is a promising candidate for the growth of high quality III-nitride and the subsequent high-performance device fabrication including high brightness LED, laser diodes, and high-power, high-efficiency transistors. (2) Free-Standing (FS) GaN Substrate by HVPE. Recently GaN substrate has been developed to improve qualities of III-nitride devices, thermal conductivity, dislocation density, thermal and lattice mismatch, etc. due to home-epitaxial growth through various growth methods including ammonothermal, High-pressure solution growth, Na-flux and Hydride vapor phase epitaxy (HVPE). In spite of advantage of HVPE method such as high growth rate, capability of wide area wafer and already commercialized growth methods for GaN substrate with comparing other growth methods, the problems of large dislocation density, parasitic nucleation and stress controls in HVPE- GaN substrate were still remained. We will provide one simple HVPE growth method, named as the pulsed NH3-flow modulation method, to realize a crack-free GaN thick layer with high crystal quality and nearly no residual strain. A few simple self-separation methods are developed for removal of hetero-substrates to obtain FS-GaN. 2-inch FS-GaN substrates are obtained with high quality. The FWHM values are about 108 and 81 arcsec for (002) and (102) reflections. The corresponding dislocation density of the freestanding GaN is low to 3-7×106 cm-2. We will also discuss self-separation mechanisms for FS-GaN substrates grown by HVPE.