INFRARED TRANSMISSION SPECTROSCOPY OF GAAS DURING MOLECULAR-BEAM EPITAXY

Direct radiative heating of GaAs substrates for molecular beam epitaxy (MBE) has a side-benefit: the infra-red light which heats the substrate can also serve a light source for transmission spectroscopy. We demonstrate the use of in-situ transmission spectroscopy in two applications, temperature measurement and growth rate measurement. The substrate temperature can be accurately determined by measuring the position of the band-gap absorption edge. The band gap of GaAs shifts about 50 mv per 100 degree C in the usual temperature range of MBE growth, and has been well characterized previously. We can measure the position of the absorption edge to better than 5 mv, so a temperature can be determined with an accuracy of better than 10 degree C. The precision of the measurement is plus or minus 2 degree C. We have measured GaAs substrate temperatures as low as 450 degree C, and the technique is extendable to much lower temperatures. We have used this technique to calibrate the thermocouple used to control our substrate heater during normal MBE growth. The growth rates of Al//xGa//1// minus //xAs and GaAs can be determined by measuring the Fabry-Perot interference fringes resulting from thin layers.