MOCVD growth of GaN on Si through novel substrate modification techniques.

摘要

GaN is a semiconductor material with great potential for use in high power electronics and optoelectronics due to the high electron mobility, high breakdown voltage, high thermal stability, and large direct bandgap of GaN. Si is a desirable substrate material for GaN heteroepitaxy due to the low cost of production, large wafer sizes available, and current widespread use in the electronics industry. The growth of GaN/Si devices suffers from the lattice and CTE mismatches between GaN and Si and therefore multiple methods of strain reduction have been employed to counter these effects. In this work we presented two novel methods of substrate modification to promote the growth of device quality GaN on Si.;Initial work focused on the implantation of AlN/Si(111) substrates with N+ ions below the AlN/Si(111) interface. A reduction in the initial compressive stress in GaN films as well as the degree of tensile stress generation during growth was observed on implanted samples. Optical microscopy of the GaN surfaces showed reduced channeling crack density on implanted substrates. Transmission electron microscopy (TEM) studies showed a disordered layer in the Si substrate at the implantation depth which consisted of a mixture of polycrystalline and amorphous Si. Evidence was provided to suggest that the disordered layer at the implantation depth was acting as a compliant layer which decoupled the GaN film from the bulk Si substrate and partially accommodated the tensile stress formed during growth and cooling. A reduction in threading dislocation (TD) density on ion implanted substrates was also observed.;Additional studies showed that by increasing the lateral size of AlN islands, the tensile growth stress and TD density in GaN films on ion implanted substrates could be further reduced. XRD studies showed an expansion of the AlN lattice on implanted substrates with larger lateral island sizes. The final tensile growth stress of films on implanted substrates was further reduced by utilizing thinner buffer layers and increasing the implantation depth of N+ ions.;Final studies were presented on a method of etching Si(001) substrates in order to fabricate trenches with Si{110} sidewalls. It was shown in these studies that GaN could be preferentially grown on Si{110} sidewalls such that GaN(0002)//Si{110}. The result was non-polar GaN "fins" which vertically overgrew Si(001) ridges. Further studies showed that high V/III, low temperature, and low pressure was required to promote the lateral growth of the GaN(0002) which was necessary to obtain a fully coalesced film.