Accuracy of AlGaAs growth rates and composition determination using RHEED oscillations

Reflection high-energy electron diffraction (RHEED) oscillations are widely used in molecular beam epitaxy (MBE) as a technique to calibrate material growth rates. The growth rates are used to predict the composition of the following growth run. For many applications, the predicted composition uncertainties of a few percent are adequate, but some applications, like vertical cavity lasers (VCSELs) and distributed Bragg reflectors, demand greater accuracy. To improve the accuracy in determining the composition of MBE-grown films, it is obvious we need to understand the uncertainties and limitations associated with RHEED as an MBE tool. In this study, we investigate several aspects of RHEED intensity oscillations of the specular spot during growth of AlAs, GaAs, and AlGaAs on GaAs substrates, including the effects of beam positioning, substrate size, different growth rates, and incident beam along the [0(-1)1] (corresponding to the 4x reconstruction) and [011] (corresponding to the 2x reconstruction) direction. Additionally, we examine "beat" phenomena in the RHEED oscillations (due to nonuniformity in growth rate across the sample) and beam flux transients and their implications on composition. For the two largest factors, electron beam positioning and flux transients, the overall uncertainty can be reduced with careful experimental technique. The lower end of the range corresponds to technique that minimizes the error, while the upper number corresponds to allowing these factors to be essentially uncontrolled. We also present a procedure that uses the measured variance in the growth rates to calculate the composition with the smallest mean square error.