Threading Dislocations in InGaAs/GaAs (001) Buffer Layers for Metamorphic High Electron Mobility Transistors

In order to evaluate various buffer layers for metamorphic devices, threading dislocation densities have been calculated for uniform composition In (x) Ga1-x As device layers deposited on GaAs (001) substrates with an intermediate graded buffer layer using the L (MD) model, where L (MD) is the average length of misfit dislocations. On this basis, we compare the relative effectiveness of buffer layers with linear, exponential, and S-graded compositional profiles. In the case of a 2 mu m thick buffer layer linear grading results in higher threading dislocation densities in the device layer compared to either exponential or S-grading. When exponential grading is used, lower threading dislocation densities are obtained with a smaller length constant. In the S-graded case, lower threading dislocation densities result when a smaller standard deviation parameter is used. As the buffer layer thickness is decreased from 2 mu m to 0.1 mu m all of the above effects are diminished, and the absolute threading dislocation densities increase.