Infra-red transmission spectroscopy of GaAs during molecular beam epitaxy

Abstract

Direct radiative heating of GaAs substrates for molecular beam epitaxy (MBE) has a very useful side-benefit: the infra-red light which heats the substrate can also serve as a light source for transmission spectroscopy. We demonstrate the use of in-situ transmission spectroscopy in two important 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°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°C. The precision of the measurement is ±2°C. We have measured GaAs substrate temperatures as low as 450°C, and the technique is easily 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 x Ga 1− x As and GaAs can be determined by measuring the Fabry-Pérot interference fringes resulting from thin layers. For a single Al x Ga 1− x As layer, the amplitude of the fringes observed as a function of time is a good measure of the index difference between the layer and the substrate. From published data about the GaAs index at high temperature, we can get the Al x Ga 1− x As index, and thus an estimate of the Al mole fraction of the layer. By counting the fringes as the layer is grown, we can determine the thickness of the layer. For a single layer of Al 0.3Ga 0.7As on GaAs, we observe fringes of magnitude 3.85%±0.62% at 1.468 μm. The optical thickness can be determined to within ±24 nm. For multi-layer structures, the variations of the transmittance with wavelength become large, so that the optical thickness of both GaAs and AlAs can be extracted from a single wavelength scan. An accurate determination of the refractive indices of these materials at high temperatures could make this technique very important for the reproducible growth of Al x Ga 1− x As-GaAs heterostructures, because a high precision calibration could be done during each growth.