MBE growth of continuously-graded parabolic quantum well arrays in AlGaAs

Parabolic quantum wells have unique properties that make them attractive for certain applications. However, growing them with molecular beam epitaxy is much more difficult than growing standard rectangular wells, as parabolic wells require a smooth, precise variation of the composition during growth. Typically, such composition variations have been approximated using the digital alloy technique, but there are limits on the quality of wells that can be produced this way. In our approach, we instead create a smooth parabolic potential in AlxGa1-xAs by varying the Al cell flux as a function of time. To compensate for thermal lag in the effusion cell, we develop a simple linear dynamical model, which can be inverted to find the temperature input required for a desired composition profile. With this approach, the composition in a 3 THz AlxGa1-xAs parabolic quantum well can be controlled to a root-mean-squared error of 0.4% Al. This approach can be easily generalized to other structures, and, importantly, can be used at typical growth rates (1.5 - 2.5 angstrom/s), which allows for the growth of parabolic well arrays.