Gallium nitride (GaN) is a wide bandgap material that is already extensively used in industrial production of optoelectronic devices (light emitters) that operate in the blue and ultraviolet wavelength range. GaN is interesting not only because it has a wide bandgap (3.5 eV), it is also chemically and physically inert and is environmental friendly for device production, as opposed to the lower bandgap III-V materials based on GaAs or InP.
Despite recent efforts there is still no bulk GaN substrates with sufficient crystal quality and size and sufficiently low price that they can be used commercially in devices. The problem in growing GaN is that because there are no native substrates the growth is done on foreign substrates (heteroepitaxy). When GaN is grown heteroepitaxially there will be a built in strain in the material because of the lattice mismatch and thermal expansion difference between the materials. This produces a number of different defects that degrades the crystal quality. Native GaN substrates are therefore strongly in demand, and many growth techniques are presently pursued worldwide to develop such substrates.
Hydride Vapor Phase Epitaxy (HVPE) is the growth technique most commonly used for growing bulk GaN material. Certainly it is the most developed method, and it can produce a high growth rate(> 100 μ/hr). HVPE has comparatively simple growth chemistry and is relatively economic compared to other growth techniques.
In the work involved in this PhD thesis the focus has been to optimize the growth of thick 2" diameter GaN layers on sapphire with a vertical HVPE system.
Paper I deal with how to grow thick GaN layers(> 300 μm) on sapphire, and reviews some of the problems that are involved in the growth process. The emphasis is on the growth related defects in such thick GaN layers.
In Paper II we have shown that it is possible to grow GaN with a very high growth rate and with good crystal quality.
Paper III shows that it is possible to produce a free-standing GaN layer that is strain free. This is done by separating the GaN layer from the sapphire substrate using a laser lift-off (LLO) process.
In paper IV the optical and structural properties of thick freestanding GaN layers are reported.
In paper V we have shown that it is possible to grow a GaN layer on sapphire over a 2" area that is virtually strain free over most part of the grown area. This is due to induced cracking in the starting sapphire substrate during the cool down phase.
In paper VI we have grown the GaN material on a starting template consisting of a two-step epitaxial lateral overgrowth (ELO) GaN layer grown by MOVPE on a sapphire substrate. The self separated GaN layer was then evaluated and the result showed a virtually strain free material.
In paper VII positron annihilation studies of a thick GaN layer is performed. The studies were done to identify native point defects (Ga vacancies) in the as-grown non-intentionally doped n-type GaN.
Paper VIII deals with μ-Raman scattering profiling studies on a thick free-standing GaN layer. The studies provided the vertical strain distribution and the evolution of the crystalline quality with increasing layer thickness.
Linköping: Linköpings universitet , 2005. , 45 p.
2005-06-15, Hörsal Planck, Fysikhuset, Campus Valla, Linköping, 10:15 (Swedish)
Seifert, Werner, Prof.