DIFFERENT JAPANESE GROUPS (1-5) HAVE RECENTLY PRODUCED GOOD QUALITY HYDROGENATED AMORPHOUS SILICON FILMS BY PHOTODISSOCIATING SILANE (SIH4) OR DISILANE (SI2H6) USING DIFFERENT TYPES OF ULTRAVIOLET LIGHT SOURCES. IN ORDER TO INVESTIGATE THE NEW POSSIBILITIES OF THIS TECHNIQUE, WE HAVE DEVELOPED SIMILAR EXPERIMENTS, THIS YEAR, IN STRASBOURG, BY USING EITHER COHERENT (LASER) OR INCOHERENT (LAMP) UV LIGHT SOURCES.
The photolytic deposition of hydrogenated amorphous silicon has been studied. The active gas, silane or disilane, is dissociated by 2 different ultraviolet light sources:
a low pressure mercury lamp, providing photons of 253 nm wavelength to dissociate silane molecules in the presence of mercury vapour by a mercury photosensitized reaction;
a high power pulsed excimer laser, operating at 193 nm to dissociate directly molecules of reactant gas, either by a 1-photon excitation of disilane or a 2-photon excitation of silane.
Work has been conducted to optimize the different parameters governing the deposition when silane is dissociated with the 2 different light sources. A comparison has been made of the gas phase processes resulting from silane and disilane photodissociation with the pulsed excimer laser. Finally, some important structural, optical and electrical properties of photochemical vapour deposition hydrogenated amorphous silicon films, prepared in various experimental conditions, have been reported.
THIN FILM DEPOSITION AT LOW TEMPERATURE OF HYDROGENATED AMORPHOUS SILICON BY DISSOCIATING SILANE AND DISILANE WITH UV LIGHT IS NOW WELL RECOGNIZED AS AN INTERESTING TECHNIQUE. WE HAVE COMPARED, THE PHOTODISSOCIATION PROCESSES OF SIH4 AND SI2H6 UNDER PULSED EXCIMER LASER AT 193 NM.
THE EXPERIMENTAL RESULTS HAVE SHOWN THAT THE DISSOCIATION OF SI2H6 RESULTS FROM BOTH ONE AND TWO-PHOTON ABSORPTION WHEREAS SIH4 ONLY ABSORBS TWO PHOTONS. THEY GAVE US ACCESS TO THE CORRESPONDING ABSORPTION CROSS SECTIONS AT 193 NM.
THE ESTABLISHMENT OF A STATIONARY STATE IN THE GAS PHASE HAS ALSO BE DEMONSTRATED IN THE TWO SITUATIONS, WHICH PROVE THE PRESENCE, IN THE CINETIC CHEMICAL MODEL, OF REVERSE REACTIONS WHICH REGENERATE THE INITIAL REACTANT GAS.
THE DEPOSITION YIELD OF THE TWO GASES HAS BEEN DEDUCED FROM EXPERIMENTAL DATA AS A FUNCTION OF VARIOUS PARAMETERS SUCH AS THE INITIAL PRESSURE, LASER ENERGY DENSITY AND IRRADIATION TIME. THE INCREASE OF THE DEPOSITED YIELD AT LOW PRESSURE HAS BEEN DEMONSTRATED, IN GOOD AGREEMENT WITH THE EXPERIMENTAL RESULTS ALREADY PUBLISHED IN THE LITERATURE.
SILANE AND DISILANE, DESPITE OF THEIR DIFFERENT WAY OF EXCITATION (ONE OR/AND TWO-PHOTON PROCESSES) PRESENT THE SAME BEHAVIOUR WHEN REGARDING THE GAS PHASE COMPOSITION UNDER PULSED UV LIGHT (193 NM).