Sr. Johnson et al., Feedback control of substrate temperature during the growth of near-lattice-matched InGaAs on InP using diffuse reflection spectroscopy, J CRYST GR, 202, 1999, pp. 40-44
Diffuse reflection spectroscopy (DRS) is used to control substrate temperat
ure to within +/-2 degrees C of user specified setpoint during the growth o
f near-lattice-matched InGaAs on InP. The same growth under constant thermo
couple control would result in a 50 degrees C rise in real substrate temper
ature. Feedback control is achieved using a nested proportional-integral-de
rivative (PID) control loop: the inner loop consists of a conventional Euro
therm-thermocouple feedback loop that controls the substrate heater power,
the outer loop updates the thermocouple setpoint based on the difference be
tween the user setpoint and the substrate (DRS) temperature using a PID con
trol loop implemented in the control software. Frequency loop shaping, base
d on a dynamical model of the system obtained from an identification experi
ment, is used to tune the outer PID loop. In addition, the thermal disturba
nces that occur during effusion cell shutter operations must be rejected. I
n the simplest case, a single correcting step in the Eurotherm (thermocoupl
e) setpoint is input when a shutter is toggled. Through disturbance identif
ication and model inversion a more sophisticated disturbance rejection acti
on from the controller can be obtained. (C) 1999 Elsevier Science B.V. All
rights reserved.