dc.description.abstract | The objective of this research is to grow high quality GaN/AlGaN high electron mobility transistors (HEMTs) on silicon substrates by metal-organic chemical vapor deposition (MOCVD). A number of graded AlxGa1-xN/AlyGa1-yN superlattice buffer layers with different thickness and aluminum composition are designed to investigate to how these superlattice buffers improve the material quality using transmission electron microscopy and x-ray diffraction.
The biggest challenge for epitaxial growth of GaN on silicon substrates is to overcome the problems of crystal defects, such as dislocations, cracks and wafer bow, caused by the mismatch of lattice constant and the thermal expansion coefficient between Si and GaN. A common solution is to grow a step-graded AlGaN buffer layer, such as Al0.8Ga0.2N/Al0.5Ga0.5N/Al0.2Ga0.8N, before growing the active layer to avoid wafer deformation caused by excessive stress.
In this study, a graded Al0.8Ga0.2N/Al0.2Ga0.8N superlattice is used to replace the Al0.5Ga0.5N layer in the conventional step-graded AlGaN buffer layer. The quality of GaN and wafer bow can be improved by adjusting the thickness and period number of the superlattice. By increasing the period number of the superlattice, the residual compressive stress becomes smaller and the wafer bow decreases. With this superlattice structure, the wafer bow of a GaN HEMT structure with a total thickness of 5 μm, grown on 6-inch 1 mm-thick low-resistivity silicon substrate, is only 44 μm. Besides, a higher period number of superlattice leads to a lower dislocation density and higher carrier mobility. Low-temperature Hall measurements on the GaN/AlGaN HEMT structure show that electron mobility reaches 32,000 at 10K. The full-width at half maximum of the GaN <002> and <102> x-ray rocking curve is 472 arcsecond and 510 arcsecond, respectively.
The aforementioned Al0.8Ga0.2N/Al0.2Ga0.8N graded superlattice is also applied to the epitaxy on 6-inch 675 μm-thick low-resistivity silicon substrates. By inserting another 15 pairs of Al0.2Ga0.8N/GaN superlattice between the Al0.2Ga0.8N step graded buffer layer and the GaN buffer layer, a wafer bow as low as 2.6 μm has been achieved on a 4 μm-thick HEMT wafer.
This study shows that the superlattices proposed in this research can reduce not only the dislocation density of GaN, but also the bow of the epitaxial wafer to less than 30 μm, which is the specification of production line. | en_US |