Closed-loop control of MBE growth for AlAs/GaAs distributed Bragg reflectors using in-situ pyrometric interferometry monitoring
Reproducible control of layer thickness and composition is crucial to the performance and ease of manufacturing of many optoelectronic devices including vertical cavity surface emitting lasers (VCSELs). To achieve this objective, it is highly desirable to develop a continuous in situ monitoring and control scheme during the epitaxial growth. Such a system will not only improve the run-to-run growth reproducibility, but also relax the stringent precision requirement for growth rates calibration. Pyrometric interferometry (PI) has previously been used for in situ monitoring and control of GaAs/AlAs/lnGaAs VCSELs growth during molecular beam epitaxy (MBE) with considerable success. Compared with reflectometry and ellipsometry, PI can readily be installed in most existing MBE systems with minimum retrofitting, and the detected signal is relatively insensitive to optical alignment and substrate wobbling during rotation. The schematic of the model-reference control system is shown. To control layer switching, both the simulated and the reference PI data are modeled as amplitude and phase-modulated (AM and PM) signals. A least-square fitting algorithm is employed to track (demodulate) the "phase (/spl theta/)" (measured in radian) of the signal, which is used as criteria for layer switching. The results show that a highly reproducible PI signal can be obtained without a precise knowledge of the growth rate. We conclude by discussing (i) possible approaches for incorporating real-time adaptive control techniques, (ii) the extension of the current control scheme to multiple wavelength monitoring, and (iii) the implication of these studies on the prospect of realizing an intelligent MBE growth system.