EFFECT OF MOLAR RATIO OF (CAXBAY)F-2 FILMS ON ELECTRICAL CHARACTERISTICS OF METAL FLUORIDE/INP STRUCTURES/

(CaxBay)F-2(x:y=7:3, 1:1 and 3:7)/n-type InP (100) metal-insulator-sem iconductor (MIS) structures were investigated by means of capacitance- voltage (C-V) measurements and X-ray photoelectron spectroscopy (XPS) to study the effect of the molar ratio (x:y) of the fluoride films on the quality of the interface between the fluoride and InP. The fluorid e films were deposited on the InP substrates by co-evaporating CaF2 an d BaF2 powders in a vacuum of about 1-2 x 10(-6) Torr. The best C-V ch aracteristics were obtained fur the lattice-matched (CaBa)F-2 (Ca:Ba= 1:1 in molar ratio)/InP and the next-best, for the (CaBa)F-2 (Ca:Ba=7: 3)/InP MIS structures, On the other hand, poor C-V characteristics wer e obtained for the (CaBa)F-2 (Ca:Ba=3:7)/InP and the SrF2/InP structur es, though their lattice matchings were better than that of the (CaBa) F-2 (Ca:Ba=7:3)/InP structure, It is therefore concluded that the latt ice matching: between the fluoride film and the InP substrate is proba bly the most important factor for obtaining a good-quality interface; however, the presence of a fair amount of the ''stable'' Ca-P bonds is also effective to obtain a stable interface between fluoride and InP. The XPS data indicated that fluoride films with distinct and uniform chemical compositions were formed from the top surface to the interfac e with the InP substrate even by the conventional vacuum evaporation t echnique. The peak binding energy of the XPS signal from the Ba 3d cor e level shifts to the lower energy side, while that from the Ca 2p cor e level shifts to the higher energy side with increase in the ratio of Ba to Ca, These shifts are nearly linear with the change of the mixed molar ratio. These findings can be explained by a difference in ionic radii between Ba and Ca in the fluorite structure and indicate that t he molar ratios in the fluoride films are the same as the mixed molar ratios of the source powders. Native oxides of InP such as In2O3; InPO 3 and InPO4 were found to be removed or reduced by the fluoride films during or after deposition of these films. A small amount of oxygen wa s included in the fluoride film, indicating that the fluoride film red uced the native oxide of InP and became oxidized itself.