  A high sensitivity atom flux sensor based on atomic absorption spectroscopy
has been designed and implemented to control electron beam evaporators and
effusion cells in a molecular beam epitaxy system. Using a high-resolution
spectrometer and a two-dimensional charge coupled device detector in a
double-beam configuration, we employ either a non-resonant line or a resonant
line with low cross section from the same hollow cathode lamp as the reference
for nearly perfect background correction and baseline drift removal. This setup
also significantly shortens the warm-up time needed compared to other sensor
technologies and drastically reduces the noise coming from the surrounding
environment. In addition, the high-resolution spectrometer allows the most
sensitive resonant line to be isolated and used to provide excellent
signal-to-noise ratio.
