Real-time monitoring and control during MBE growth of GaAs/AlGaAs Bragg reflectors using multi-wavelength ellipsometry

A new multi-wavelength in situ ellipsometer capable of acquiring accurate ellipsometric data at 44 wavelengths from 415 to 750 nm in less than 1 s has been directly mounted on a molecular beam epitaxy (MBE) growth system. In contrast to single-wavelength ellipsometers, enough measured data is available to allow calculation of layer thickness, composition, temperature and exact angle of incidence. In situ monitoring and real-time analysis was used to control the growth process of GaAs/AlGaAs Bragg reflectors with a center wavelength of 1000 nm. The layer thickness is controlled very accurately even though ellipsometric data was acquired only every 3 s. The accuracy of shutter timing can be controlled very precisely, even allowing for slow ellipsometric acquisition rates and substrate wobble due to MBE substrate rotation. The control algorithm for the two reflectors did not attempt to control the Al composition of an individual AlGaAs layer, but the measured composition was used to adjust the Al cell temperature for the next AlGaAs layer. For purposes of comparison, the FastDyn fitting routine was used with another reflector to simultaneously control the thickness and surface composition of the AlGaAs layers. An overview of the hardware and software integration on the MBE system will be given. The in situ measurements during the growth control were later compared with ex situ measurements made with the spectroscopic ellipsometer system variable angle spectroscopic ellipsometry (VASE).