The application of vibrational/rotational energy to material gas (SiH4) and radicals by heating the material gas has been investigated in order to improve the properties of hydrogenated amorphous silicon (a-Si:H) for solar cells. It has been found that the optical gap (E(opt)) of a-Si:H films deposited from heated SiH4 (temperature of the gas heater: 280-380 degrees C) is narrower by about 20-30 meV than those of conventional films where SiH4 is not heated, using the same substrate temperature (T-s) and deposition rate (R(d)) This result suggests that the heated SiH4 molecules or related radicals promote reactions at or near the film-growing surface. In other words, applying vibrational, rotational, or translational energy to SiH4 gas has the same effect as raising T-s in plasma chemical vapor deposition (plasma-CVD) of a-Si:H. It is demonstrated that gas heating can improve the conversion efficiency of a-Si solar cells by reducing the damage to underlying layers during i-layer deposition.
