PRECISELY CONTROLLED COMPOSITIONAL GRADIENTS IN MBE GROWN AIGAAS/GAAS STRUCTURES

An increasing number of interesting new heterojunction structures rely on compositional gradients rather than abrupt heterojunctions. The use of rapid shuttering with a variable duty cycle is currently the most commonly used way of achieving the desired grading in molecular beam epitaxial (MBE) growth since thermal source ramping to achieve such grading suffers from problems of thermal lag and non-steady-state temperature distributions in the evaporation sources. We get around these obstacles by using reflective high energy electron diffraction (RHEED) oscillations to measure actual compositions during trial gradings and then use these direct data to fine tune the thermal ramps to achieve the desired compositional profile. The result is a graded layer with a precise reproducible profile grown without the complications of multiple interfaces and possible non-linear effective average properties inherent in the rapid shuttering technique. These layers can later be inserted during actual growth runs within more complex structures.