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

New multi-wavelength in-situ ellipsometer acquiring accurate ellipsometric data at 44 wavelength form 415 to 750 nm in less than 1s is directly mounted on a MBE growth system. Compared to single wavelength ellipsometers enough measured data are available to have access to layer thickness, composition, temperature and exact angle of incidence. In- situ monitoring and real time analysis was used to control the process of GaAs/AlGaAs Bragg reflectors with center wavelength of 1000 nm. The layer thickness is controlled very accurately even though ellipsometric data was acquired only every 3 seconds. The accuracy of shutter timing can be made very precisely even for slow ellipsometric acquisition rates and substrate wobble due to MBE substrate rotation. The control algorithm for 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. In comparison for another reflector, the FastDyn fitting routine were used to simultaneously control the thickness and surface composition of the AlGaAs layers. An overview about 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 spectroscopic ellipsometer system VASE.