Deposition of Intrinsic and Doped Semiconductor Thin Films for a-Si:H TFT

http://link.springer.com/content/pdf/10.1007/978-3-642-93406-3_20.pdf

Amorphous Silicon Carbide Thin Films Produced in the Glow Discharge Deposition System

The electronic and optical properties of device quality hydrogenated amorphous silicon (a-Si:H) films grown by electron cyclotron resonance (ECR) plasma deposition were studied together with in-situ plasma characteristics. Hydrogen and helium plasmas, excited by 50-250 watts of 2.45 GHz microwave power under ECR conditions, were used to decompose silane at 6 to 20 mtorr pressures during the deposition of a-Si:H films at a 297 C substrate temperature. Both the electron temperature and density, and ion flux are measured near the deposition surface using plane and cylindrical Langmuir probes. An attempt is made to correlate these plasma properties with the light and dark photoconductivity, optical gap, refractive index, and subband gap photoconductivity.

An Investigation of the Influence of Plasma Characteristics on the Electronic and Optical Properties of Device Quality a-Si:H Grown by Electron Cyclotron Resonance Plasma Deposition

An investigation of the structural properties of hydrogenated amorphous silicon (a-Si:H) thin films prepared by plasma enhanced chemical vapour deposition of silane (SiH4) was done using a combination of atomic force microscopy (AFM), photoluminescence, infrared and UV spectroscopy. Films were prepared with rf power ranging from 100-250 W. For every rf power employed, substrate temperature were varied from room temperature to 300Â°C. The deposition rate was found to be slightly increasing with an increase of rf power while decreasing as the substrate temperature increases. The AFM images can be classified into three groups: most smooth (rms: 1.2nm), intermediate rms (2.4-3.6 nm) and highest roughness (rms: 4.9 nm). The transition to rougher films at higher substrate temperature is attributed to a change in the deposition process. The IR vibrational spectra obtained from FTIR spectroscopy display modes which can be characterized as predominantly hydrogen motions. On the basis of these identifications, it is found that samples produced on high-temperature have SiH, SiH2 and (SiH2)n groups with very little SiH3. In contrast, the ir spectra of samples produced on room-temperature are dominated by vibrational modes of SiH3 and (SiH2)n. At low rf power, the spectrum is dominated by a strong absorption bands at 2000 cm-1 associated with SiH stretching bond and also 630 cm-1 associated with SiH bending. At high rf power, an additional absorption band at around 2090 cm-1 which corresponds to (SiH2)n stretching mode and SiH2 stretching mode becomes more pronounced. The optical energy gap obtained from UV spectroscopy decreases with increasing of rf power and substrate temperature. This decrement is due to the drop of hydrogen content. At low substrate temperature, photoluminescence spectrum of a-Si consists of a relatively broad band with its main peak around 1.4 eV. The spectrum shifts to lower energies (around 1.37 eV) and its intensity decreases with increasing temperature. It is suggested that this is due to an activated non-radiative recombination (relaxation) process where exciton are captured by deep traps and this become more probable as temperature increases.

Structural properties of hydrogenated amorphous silicon (A-SI:H) thin film grown via radio frequency plasma enhanced chemical vapor deposition (RF PECVD)

Hydrogenated amorphous silicon films were produced from silane/hydrogen and silane/helium gas mixtures by RF glow discharge. We examined the optical and electrical properties of films produced with these gas mixtures, at various RF power levels and silane fractions. Film quality was analyzed by measuring the dark and photoconductivity, optical band gap, and activation energy. Optical emission spectroscopy was also used as a diagnostic tool for studying the plasma during glow discharge depositions. Experimental results indicate that amorphous silicon films made from silane/helium mixtures exhibit improved optoelectronic properties, higher deposition rates, and higher emission intensity ratios (ISiH/IH) as compared to films produced from silane/hydrogen mixtures. In preparing films from silane/helium mixtures, the onset of dust/powder formation occurs at considerably higher RF powers as compared to silane/hydrogen, thus making this approach an attractive commercial option for depositing films at high rates.

D. J. Olsen

Papers

Cowparison Studies of Hydrogenated Amorphous Silicon Films Prepared From Silane-Hydrogen and Silmne-Helium Mixtures