Surface photoabsorption monitoring of III-V and GaN MOVPE surfaces

Surface photoabsorption method is based on the measurement of the reflectance of p-polarized light at near Brewster angle. When the surface changes due to the adsorption and the desorption of constituent atoms and/or the surface is modified chemically, the reflectance can respond sensitively to both changes in anisotropic and isotropic dielectric constants of the surface. For the two-fold symmetric surfaces such as (0 0 1) oriented III-V surfaces, the anisotropic part can be extracted by taking a difference between the reflectances measured in the two orthogonal directions of [1 1 0] and [(1) over bar 1 0] and the spectrum thus obtained corresponds to the optical transition of anisotropic bond on the surface, i.e., the electronic transitions in the visible region and the vibrational transitions in the infrared region. The vibrational transition of the anisotropic GaN bonding can be observed by the nitridation of GaAs(0 0 1) surface. For the surfaces with symmetries other than two-fold, the anisotropic and the isotropic parts are indivisible but the reflectivity changes sensitively with the surface stoichiometry. Three surface phases of As-rich (2 x 2), root 19 x root 19 and Ga-rich (1 x 1) can be distinguished for the three-fold GaAs(1 1 1)B surface. For the three-fold wurtzite GaN(0 0 0 1) surface, Ga-rich and N-rich stoichiometry can be monitored in MOVPE. (C) 1998 Elsevier Science B.V. All rights reserved.