THE ROLE OF SUBSTRATE QUALITY ON MISFIT DISLOCATION FORMATION IN PSEUDOMORPHIC HIGH-ELECTRON-MOBILITY TRANSISTOR STRUCTURES

We examined the role of substrate quality on the epitaxial microstruct ure and performance of pseudomorphic InGaAs/AlGaAs/GaAs high electron mobility transistors (HEMTs). High resolution x-ray diffraction. high resolution x-ray topography, and transmission electron microscopy indi cate that, for a given channel layer thickness, the misfit dislocation density is always lower for HEMT structures grown on substrates havin g lower threading dislocation densities. Furthermore, the onset of mis fit dislocation formation occurs at higher channel layer thicknesses f or HEMTs grown on substrates having a lower threading dislocation dens ity when compared to those grown on substrates with a higher threading dislocation density. However, the ratio of the density of misfit disl ocations to threading dislocations is greater than one on the low disl ocation density substrates, which demonstrates that other misfit dislo cation nucleation sources (i.e., surface particulates) are significant when there is an insufficient density of threading dislocations. The Hall conductivity measurements show that the performance of HEMT struc tures improves with higher substrate quality, showing that the range o f epitaxial layer metastability increases with improved substrate equa lity. (C) 1997 American Institute of Physics.