Closed-Loop Thickness Control of Resonant-Tunneling Diode Mbe Growth Using Spectroscopic Ellipsometry

The room temperature electrical characteristics of AlAs/InGaAs/InAs resonant-tunneling diode (RTD) devices are sensitive to sub-monolayer thickness changes, and present a challenging case for closed-loop control. We prepared stacked RTD structures by MBE in which the strained AlAs barrier thicknesses were controlled based on an in situ spectroscopic ellipsometry (SE) system. The short-term and long-term reproducibility of RTDs grown under SE-based control was compared with similar samples grown by “dead-reckoning” (timing-based shutter openings coupled with pre-growth flux calibrations). SE-based calibration can compensate for long-term flux drift, but closed-loop thickness control did not provide a significant improvement over dead-reckoning in terms of short-term RTD reproducibility.