Effects of Surface Orientation and Molar Ratio of (SrxCay)F2 Films on Electrical Characteristics of Metal/Fluoride/GaAs Structures

The (Sr x Ca y )F 2 ( x : y =3:7, 1:1 and 7:3)/GaAs (100), (111)A and (111)B metal-insulator-semiconductor (MIS) structures were investigated by the capacitance-voltage (C-V) measurements and the X-ray photoelectron spectroscopy (XPS) to study the effects of the surface orientation of GaAs and the mol ratio ( x : y ) of the fluoride films on the quality of the interface between these fluoride films and GaAs. The fluoride films were deposited on the GaAs substrates by co-evaporating SrF 2 and CaF 2 powders in a vacuum of 1–2×10 -6 Torr. The C-V measurements indicated that the best result was obtained for the (SrCa)F 2 (Sr:Ca=1:1 in molar ratio)/GaAs (100) surface and the next, for the (Sr 0.3 Ca 0.7 )F 2 /GaAs (111)B surface. Very poor C-V characteristics were obtained for the GaAs (111)A surface for all the molar ratios of Sr and Ca studied here. It is concluded that the lattice matching between the fluoride film and GaAs plays an important role in the formation of a good-quality interface for the GaAs (100)-oriented surface, while the presence of a large amount of stable Ca–As bonds is more effective for GaAs (111)-oriented surfaces. The XPS data indicated that the native oxide of GaAs, which is mainly composed of As 2 O 3 and Ga 2 O 3 , was removed upon evaporation of the fluoride films. Sr is considered to reduce the native oxide and was oxidized itself. Sharp peaks of the XPS data were observed and the binding energies of these peaks varied nearly linearly with the molar ratio of Sr and Ca. These indicate that stoichiometric (Sr x Ca y )F 2 films were formed even by the conventional vacuum-evaporation technique. Elemental Sr and/or Ca was observed to exist at the interface with GaAs.