Feedback control of substrate temperature during the growth of near-lattice-matched InGaAs on InP using diffuse reflection spectroscopy

Diffuse reflection spectroscopy (DRS) is used to control substrate temperature to within ±2 °C of user specified setpoint during the growth of near-lattice-matched InGaAs on InP. The same growth under constant thermocouple control would result in a 50 °C rise in real substrate temperature. Feedback control is achieved using a nested proportional-integral-derivative (PID) control loop; the inner loop consists of a conventional Eurotherm-thermocouple feedback loop that controls the substrate heater power; the outer loop updates the thermocouple setpoint based on the difference between the user setpoint and the substrate (DRS) temperature using a PID control loop implemented in the control software. Frequency loop shaping, based on a dynamical model of the system obtained from an identification experiment, is used to tune the outer PID loop. In addition, the thermal disturbances that occur during effusion cell shutter operations must be rejected. In the simplest case, a single correcting step in the Eurotherm (thermocouple) setpoint is input when a shutter is toggled. Through disturbance identification and model inversion a more sophisticated disturbance rejection action from the controller can be obtained.