Effect of Heating SiH4 on the Plasma Chemical Vapor Deposition of Hydrogenated Amorphous Silicon

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° 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.