Closed-loop control of resonant tunneling diode barrier thickness using in situ spectroscopic ellipsometry

Abstract

We examine the use of spectroscopic ellipsometry (SE) for fully automated, in situ real-time control of the barrier thickness for resonant tunneling diodes (RTDs) grown by molecular-beam epitaxy (MBE). The RTDs in this study utilize AlAs barriers, an InGaAs well with an InAs subwell, and lattice-matched InGaAs spacer layers, grown on an InP(100) substrate. Pseudodielectric functions for the strained AlAs barriers were generated from SE data acquired during MBE growth, using simultaneous photoemission oscillation measurements for in situ thickness calibration. For closed-loop control, the measured dielectric functions were utilized in conjunction with a virtual substrate model to derive the instantaneous layer thickness from real-time SE data. Repeatability was tested by growing several series of AlAs barriers and complete RTDs under fully automated control, with shutter actuation based on the real-time thickness determined from SE. The results show that by using SE for real-time control, barrier thickness repeatability on the order of ±0.1 ML can be achieved.