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

We examine the use of spectroscopic ellipsometry (SE) for fully automated, in situ real-time control of the barrier thickness for resonant tunneling d iodes (RTDs) grown by molecular-beam epitaxy (MBE). The RTDs in this study utilize AlAs barriers, an InGaAs well with an InAs subwell, and lattice-mat ched InGaAs spacer layers, grown on an InP(100) substrate. Pseudodielectric functions for the strained AlAs barriers were generated from SE data acqui red during MBE growth, using simultaneous photoemission oscillation measure ments for in situ thickness calibration. For closed-loop control, the measu red dielectric functions were utilized in conjunction with a virtual substr ate model to derive the instantaneous layer thickness from real-time SE dat a. Repeatability was tested by growing several series of AlAs barriers and complete RTDs under fully automated control, with shutter actuation based o n the real-time thickness determined from SE. The results show that by usin g SE for real-time control, barrier thickness repeatability on the order of +/-0.1 ML can be achieved. (C) 2000 American Vacuum Society. [S0734-211X(0 0)02503-8].