The use of silicon substrates for GaN based device structures is attractive for many reasons, particularly in terms of cost and scale, but there are also inherent drawbacks. In addition to the large lattice mismatch between silicon and GaN, which causes a high dislocation density, there is a particularly large thermal expansion coefficient mismatch between the two materials. This introduces tensile stress, on cooling from the growth temperature of around 1000C, and can result in wafer bowing and cracking of the GaN.
Using guidance provided by XRD, AFM and TEM analysis, MOCVD growth schemes incorporating the use of AlN nucleation layers and interlayers, AlGaN graded layers and AlN/GaN superlattices have been used to reduce tensile strain in the GaN, producing uncracked surfaces and virtually flat wafers. In addition, SiN interlayers have been integrated into the structures to reduce dislocation density. Using these we have grown GaN films on Si which have an AFM roughness down to 0.11nm, with pit density of 4x108 cm-2, and have XRD rocking curve FWHM of 500 and 550 arcsec for the (002) and (101) reflections respectively. A functional InGaN/GaN LED structure has also been realised on such a template.