Growth design for high quality AlxGa(1−x)N layer with high AlN-fraction on Si (1 1 1) substrate by MOCVD

Abstract

The constraints on increasing the AlN content in active AlGaN device layers on Si substrates for AlGaN-channel HEMTs and UV applications are explored. The essential qualifications an AlGaN semiconductor material must satisfy to be used as functional device are crack free films, a surface roughness of <1 nm, dislocation densities approaching that of GaN (∼109 cm2) and a reasonable thickness of around 1 μm; each one of the material specifications demand a growth condition that is mostly unfavourable to achieve the other one. To meet these diversified requirements thorough optimisation of growth conditions is carried out. As a result, compositions containing up to 30% AlN-fraction have been achieved. This has been accomplished by understanding the role of growth temperature and buffer design on stress, defect and surface evolution in the AlGaN epilayers. Our studies show that higher AlN-fraction containing epilayers are achievable if stress relaxation during its growth is minimised while simultaneously keeping the growth front smooth under the given growth condition. Such a growth condition can be successfully exploited if the dislocation density at the beginning of epilayer growth is lowered. A smooth heterointerface between Al0.13Ga0.87N and Al0.37Ga0.63N has been realized and formation of 2DEG at this this interface has been confirmed through C-V measurements.