No abstract

We report a systematic investigation of the effect of ion bombardment during the growth of amorphous silicon-germanium alloy films from silane and germane rf-glow discharge. Independent control of the plasma and the ion flux and energy is obtained by using a triode configuration. The ion contribution to the total deposition rate can reach 20% on negatively biased substrates. Although the Si and Ge composition of the film does not depend on the ion flux and energy, the optical, structural and electronic properties are drastically modified at low deposition temperatures when the maximum ion energy increases up to 50 eV, and remain constant above 50 eV. For a Ge atomic concentration of 37% and a temperature of 135°C, the optical gap decreases from 1.67 to 1.45 eV. This is correlated with a modification of hydrogen bonding configurations. Silicon dihydride sites disappear and preferential attachment of hydrogen to silicon is reduced in favour of germanium. Moreover the photoconductivity increases which shows that ion bombardment is a key parameter to optimize the quality of low band gap amorphous silicon-germanium alloys.

We have developed a back-channel-oxidized thin-film transistor (TFT) structure which does not require the conventional etching of the n+-a-Si:H layer from the channel region. Key processes in the fabrication of this structure are the deposition of a very thin (less than 10 nm) n+-a-Si:H layer with low resistivity (∼50 Ω cm), and an oxygen plasma treatment to change the n+-a-Si:H layer above the channel region into dielectric oxide. With a thin (∼50 nm) a-Si:H layer, the back-channel-oxidized TFT structure makes it possible to obtain much better “ON” characteristics than are obtained with conventional channel-etched TFTs. To gain insight into the underlying physical mechanism we investigated the back-channel electrical characteristics of both types of TFTs as a function of temperature, and found that back-channel-oxidized TFTs had much better back-channel characteristics than channel-etched TFTs, which is due to a lower density of back-channel interface states.

No abstract

No abstract