Reflectance-difference spectroscopy: a new probe of crystal growth by MBE and OMCVD

Reflectance-difference spectroscopy (RDS) is a normal-incidence optical technique that uses symmetry to enhance the typically low sensitivity of optical probes to surface phenomena. In RDS, the difference between reflectances parallel and perpendicular to the two principal optic axes in the plane of the surface is determined experimentally by modulation techniques. Contributions from the nearly isotropic bulk and randomly oriented surface species largely cancel in subtraction, preferentially leaving those from the lower symmetry surface. Being optical, RDS can be used in the reactive, relatively high-pressure sample ambients of organometallic chemical vapor deposition (OMCVD) reactors as well as the ultrahigh-vacuum sample environment of molecular beam epitaxy (MBE). MBE results for the (001) AlGaAs system show that reflectance-difference signals are sensitive to either surface chemistry or surface structure according to photon energy.