Precise control of time-varying effusion cell flux in molecular beam epitaxy

We outline a method to control time-varying effusion cell flux in molecular beam epitaxy. This can be used to grow 2D nanostructures with precise continuously graded alloys, and we explore the specific case of controlling an Al cell for the growth of continuously graded Al xGa 1 − xAs parabolic quantum wells. We develop a simple model of the Al cell’s thermal dynamics, including the effects of shutter transients. This model’s parameters can be empirically derived using a beam flux monitoring ion gauge. Though simple, this model can already be used to generate near-arbitrary time-varying fluxes (within the physical limits of the cell). For growths that require higher precision than the simple model can achieve, we introduce an iterative correction scheme to eliminate any lingering errors. Using this method, we control the composition in a 3 THz Al xGa 1 − xAs parabolic quantum well array to a root mean squared error of just ± 0.18 %Al. This accuracy is achieved at the standard growth rates necessary for thick structures (0.15–0.25 nm/s). The approach is quite generally applicable to other composition profiles or other situations where time-dependent flux control is necessary, and it could be extended to other effusion cells.