So you have established your binary GaAs and AlAs growth rates using Little Known MBE facts: RHEED oscillations (1) and now your thoughts are moving to ternaries. The AlxGa1-xAs ternary is fully miscible. [Al] > ~85% are indirect gap materials. If you are using AlGaAs as a carrier confining cladding layer you may want [Al] from 30-40%. So how do we calculate our ternary growth rate?
Well conveniently algebra of epitaxy holds. First find your GaAs growth rate of 0.7ML/s and your AlAs growth rate of 0.3ML/s, separately. Then when you open the two cells’ shutters together you will get Al0.3Ga0.7As growing at 1ML/s. Just remember to suitably increase your As flux to ensure good RHEED oscillations for each measurement and good stoichiometric crystal growth.
On the other hand, you can approach the problem from an entirely different angle. In the growth of InGaAs (for example) you can first accurately determine your GaAs growth rate and then (at a suitably low temperature to ensure unitary In sticking coefficient: <540 °C but good adatom mobility: >500°C) you can add a little In and grow InxGa1-xAs. The resulting increase in growth rate will allow you to determine the InAs growth rate (GR) since:
GRInGaAs = GRGaAs +GRInAs
This conveniently means we you can accurately determine your growth rate and composition for any and all AlxGa1-xAs or InxGa1-xAs ternary alloys on a single sample within a matter of minutes. How very efficient of you.