REAL-TIME MONITORING AND CONTROL DURING MBE GROWTH OF GAAS ALGAAS BRAGG REFLECTORS USING MULTIWAVELENGTH ELLIPSOMETRY/

A new multi-wavelength in situ ellipsometer capable of acquiring accur ate ellipsometric data at 44 wavelengths from 415 to 750 nm in less th an 1 s has been directly mounted on a molecular beam epitaxy (MBE) gro wth system. In contrast to single-wavelength ellipsometers, enough mea sured data is available to allow calculation of layer thickness, compo sition, temperature and exact angle of incidence. In situ monitoring a nd real-time analysis was used to control the growth process of GaAs/A lGaAs 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 co ntrolled very precisely, even allowing for slow ellipsometric acquisit ion rates and substrate wobble due to MBE substrate rotation. The cont rol algorithm for the two reflectors did not attempt to control the Al composition of an individual AlGaAs layer, but the measured compositi on was used to adjust the Al cell temperature for the next AlGaAs laye r. For purposes of comparison, the FastDyn fitting routine was used wi th another reflector to simultaneously control the thickness and surfa ce composition of the AlGaAs layers. An overview of the hardware and s oftware integration on the MBE system will be given. The in situ measu rements during the growth control were later compared with ex situ mea surements made with the spectroscopic ellipsometer system variable ang le spectroscopic ellipsometry (VASE). (C) 1997 Elsevier Science S.A.