Intervalley splitting and intersubband transitions in n-type Si/SiGe quantum wells: pseudopotential vs. effective mass calculation

摘要

Intervalley mixing between conduction-band states in low-dimensional Si/SiGeheterostructures induces splitting between nominally degenerate energy levels.The symmetric double-valley effective mass approximation and the empiricalpseudopotential method are used to find the electronic states in differenttypes of quantum wells. A reasonably good agreement between the two methods isfound, with the former being much faster computationally. Aside from being anoscillatory function of well width, the splitting is found to be almostindependent of in-plane wave vector, and an increasing function of themagnitude of interface gradient. While the model is defined for symmetricenvelope potentials, it is shown to remain reasonably accurate for slightlyasymmetric structures such as a double quantum well, making it acceptable forsimulation of multilayer intersubband optical devices. Intersubband opticaltransitions are investigated under both approximations and it is shown that inmost cases valley splitting causes linewidth broadening, although under extremeconditions, transition line doublets may result.