NOVELTY - A semiconductor thin film-forming apparatus (500) has reaction chamber (502), pedestal (506) for supporting substrate(s) comprising elements of group IB and group IIIA, group VIA element supply unit (520) for providing vaporized group VIA elements (526), and plasma unit (530,550) for generating high density plasma region (534). During reaction in reaction chamber, vaporized group VIA elements (526) flow through region (534) and transform into ionized element (526), and ionized group VIA elements (526) diffuse into substrate to form IB-IIIA-VIA2 compound semiconductor thin film.    USE - Apparatus is used for forming semiconductor thin film for silicon solar cell.    ADVANTAGE - The efficiency of the selenium reaction or sulfur reaction performed in the apparatus is improved, with reduced usage amount of selenium or sulfur element. The semiconductor thin film is economically formed using the apparatus, with high productivity and reduced impact on environment, and without using hydrogen selenide and hydrogen sulfide.    DETAILED DESCRIPTION - A semiconductor thin film-forming apparatus has a reaction chamber, a pressure control unit (504) connected with the reaction chamber to control a pressure in the reaction chamber, a pedestal disposed in the reaction chamber for supporting substrate(s) comprising elements of group IB and group IIIA, a group VIA element supply unit (520) connecting with the reaction chamber for providing vaporized group VIA elements (526) into the reaction chamber, and a plasma unit disposed in the reaction chamber for generating a high density plasma region in the plasma unit. During reaction in the reaction chamber, vaporized group VIA elements (526) flow through high density plasma region and transform into ionized group VIA elements (526), and ionized group VIA elements (526) diffuse into substrate(s) to form IB-IIIA-VIA2 compound semiconductor thin film.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of the semiconductor thin film-forming apparatus.    Semiconductor thin film-forming apparatus (500)    Reaction chamber (502)    Pressure control unit (504)    Pedestal (506)    Temperature control units (508,514,522,542)    Substrate (510)    Group VIA element supply units (520,540)    Group VIA element container (524,544)    Group VIA elements (526,546)    Carrier gas supply units (528,548)    Plasma units (530,550)    High energy density plasma region (534)