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 Ga 1− 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 μ 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 μ m to 0.1 μ m all of the above effects are diminished, and the absolute threading dislocation densities increase.