High indium metamorphic HEMT on a GaAs substrate

Metamorphic growth of device structures on GaAs substrates has advanced rapidly in recent years. High quality electronic and optical devices have been demonstrated. Also long-term reliability has been achieved with low noise MHEMT devices. Most of the development emphasis has been with structures conventionally grown on InP substrates. This work is motivated by the lower cost, larger diameter, and greater robustness of GaAs substrates compared to InP substrates. However an important characteristic of metamorphic growth is the degree of freedom in choosing the In/sub x/(GaAl)/sub 1-x/As composition and consequently the lattice constant between GaAs and InAs. Consequently new device structures can be achieved which are not possible by pseudomorphic growth on either GaAs or InP substrates. In this effort, solid source MBE was used to grow metamorphic HEMT structures with high indium content. For the conventional MHEMT, the indium concentration is graded to In/sub 0.52/Al/sub 0.48/As to expand the lattice constant to that of InP. Here the indium content was graded to In/sub 0.64/Al/sub 0.36/As to achieve a larger lattice constant than InP. The resulting surface roughness was examined by AFM. For a 25 /spl mu/m x 25 /spl mu/m area, the RMS roughness was 12/spl Aring/ which is very similar to the roughness present in the conventional MHEMT with less indium content.