W. Futako

Role of the hydrogen plasma treatment in layer-by-layer deposition of microcrystalline silicon

We have investigated the role of hydrogen in hydrogenated microcrystalline silicon (μc-Si:H) formation using hydrogen plasma treatments, in particular examining the possibility of subsurface reaction due to permeating hydrogen atoms, which leads to the crystallization of hydrogenated amorphous silicon (a-Si:H). It is demonstrated that the hydrogen plasma treatment of a-Si:H film on the anode using a cathode covered by a-Si:H film, which is inevitably coated during the deposition period, gives rise to the deposition of μc-Si:H over the a-Si:H layer, i.e., chemical transport takes place. It is also found that the pure hydrogen plasma treatment using a clean cathode induces only etching of the a-Si:H layer. These results imply that the present hydrogen plasma condition does not cause crystallization of a-Si:H but only etching, and that careful experimentation is required to determine the real subsurface reaction due to atomic hydrogen.

Wide band gap amorphous silicon thin films prepared by chemical annealing

High quality wide gap hydrogenated amorphous silicon films were prepared using a hydrogen chemical annealing technique involving the deposition of thin amorphous silicon films followed by a hydrogen radical (and/or ion) treatment. Thick films were prepared by repeating this process many times. The substrate temperature and the hydrogen treatment time can be used to select optical band gaps ranging from 1.8 to 2.1 eV. Low dangling bond defect densities in the as-deposited films ranging from 3 to 8×1015 cm−3 were measured over the entire optical band gap range. The light induced dangling bond densities were less than those found in standard high quality amorphous silicon. The optical band gap is strongly correlated to the medium range structure characterized by the dihydride density. The electronic transport and stability are correlated with the Si–Si bonding environments and the associated short range order including bond angle and bond length distributions.

FABRICATION OF NARROW-BAND-GAP HYDROGENATED AMORPHOUS SILICON BY CHEMICAL ANNEALING

Hydrogenated amorphous silicon films with optical band gaps narrower than 1.7 eV have been prepared by a chemical annealing process involving the sequential deposition of 10–30 A of amorphous silicon followed by a plasma treatment, hydrogen and hydrogen–argon plasma treatments were investigated. Thick homogeneous films were built up by repeating the sequence many times. The formation of microcrystalline structure could be completely suppressed by the proper choice of the substrate temperature and hydrogen–argon mixture. Argon radical impingement results in hydrogen abstraction from the growth surface resulting in films with hydrogen contents as low as 3 at. %. These low hydrogen content films had a correspondingly low optical band gap of ∼1.6 eV. Raman spectra analysis indicates that the silicon–silicon bonding environment is independent of the optical band gap. However, the optical band gap is very sensitive to the content and type of hydrogenated structure present in the material. Analysis of the electr...

Band gap tuning of a-Si:H from 1.55 eV to 2.10 eV by intentionally promoting structural relaxation

Abstract Substrate temperature and sequential treatments are used to prepare a-Si:H films with band gaps ranging from 1.55 to ∼2.1 eV. Low band gap materials were prepared at higher substrate temperature using a sequential process involving the deposition of thin (≲5 nm) a-Si:H layers followed by an Argon radical (and/or ion) treatment. Larger band gap materials were prepared at lower substrate temperatures using a hydrogen chemical annealing process. The series was used to determine the relationship among the deposition conditions, the opto-electronic characteristics, and the atomic bonding structures in a-Si:H. The band gap is correlated to the total di-hydride content. The local silicon–silicon bonding environments, the hydrogen, and mono-hydride content and the mono- to di-hydride ratio are not well correlated to the band gap. Electronic transport is correlated with the local silicon–silicon bonding environment, but not the di-hydride content.

The structure of 1.5-2.0 eV band gap amorphous silicon films prepared by chemical annealing

Abstract Structural properties of band gap tuned (1.5–2.0 eV) hydrogenated amorphous silicon were investigated. The short range order associated with the local silicon–silicon bonding was measured by X-ray, Raman spectroscopy, optical absorption spectrum and weight density analysis. The atomic size and/or the distance between nearest neighbor silicon atoms did not appear to change as a function of the hydrogen content or the band gap. The short range order, the silicon bond length and bond angle distributions, were invariant as the band gap varies from 1.5–2.0 eV. Other structural properties were also investigated. The hydrogen thermal desorption spectrum and the infra-red absorption spectrum were also measured. While, the hydrogen evolves from largest amount of hydrogen and/or high Si–H2 content materials at lower temperatures, this feature is not correlated with the band gap. The band gap was strongly correlated to the Si–H2 density determined by the infra-red absorption analysis (correlation coefficient >0.92).

High-Rate Growth of Stable a-Si:H

Correlation between the gas phase species in silane plasma measured by mass spectrometry and the properties of hydrogenated amorphous silicon (a-Si:H) films deposited by plasma enhanced chemical vapor deposition (PECVD) has been investigated. The authors have specially been interested in the higher-order silane related species in the plasma, whose contribution to the film growth is considered to be the cause of light-induced degradation in the film quality, especially at high growth rate. In this study, they varied excitation frequency, gas pressure and power density to vary the growth rates of a-Si:H films ranging from 2 {angstrom}/s to 20 {angstrom}/s. Molecular density ratio of trisilane, representative of higher silane related radicals, to monosilane has shown a clear correspondence to the fill factor after light soaking of Schottky cells fabricated on the resulting films.

Extremely Narrow Band Gap,∼1·50Ev, Amorphous Silicon

Hydrogenated amorphous silicon (a-Si:H) films were prepared by a layer-by-layer (LBL) argon treatment technique. Thin amorphous silicon layers are first deposited and then treated by Ar. Thick films are built up by repeatedly the process many times. By reducing the deposition rate during deposition time (T, sec), a-Si:H with the gaps narrower than 1·55eV were prepared at substrate temperature lower than 300°C. These narrow-gap films contained less than 2 at.% hydrogen and had rigid Si network. Also, these narrow gap films exhibited good light soaking stability.

Photoconductivity gain over 10 at a large electric field in wide gap a-Si:H

Abstract Wide band gap amorphous silicon films and p–i–n diodes were prepared by a chemical annealing method. Films had dark conductivities consistent with the large band gaps, low impurity levels, intrinsic conduction, and corresponding low thermal generation rates. Primary photo current measurements of these films was consistent with an electron μτ of 10 −8 cm 2 /V. To provide blocking at large electric fields, p and n-layers were optimized . Since it is known that at electric fields greater than 10 6 V/cm other amorphous materials such as a-Se exhibit avalanche multiplication, the wide band gap amorphous silicon p–i–n diodes were used to probe the prospect of avalanche multiplication in the amorphous silicon system.

Gas Phase Diagnosis of Disilane/Hydrogen RF Glow Discharge Plasma and Its Application to High Rate Growth of High Quality Amorphous Silicon

Gas phase diagnosis of disilane/hydrogen plasma was carried out using mass spectrometry. At high growth rate (20 A/s) conditions using pure disilane as a source gas, the partial pressure of disilane molecules measured by mass spectrometry was more than one order of magnitude higher than in the case when mono-silane was used as a source gas. The stability of amorphous silicon films prepared from disilane was improved by the hydrogen dilution technique, although the disilane partial pressure in this condition was much higher than in the case when mono-silane was used as a source gas for device quality films. The relation between the gas phase species and the stability of the resulting films is studied. It was found that increase in disilane related signal intensity do not decrease film stability directly.

Modulation of Growing Surface with Atomic Hydrogen and Excited Argon to Fabricate Narrow Gap a-Si:H

Hydrogenated amorphous silicon (a-Si:H) with a gaps narrower than 1.7 eV were made by repeating the deposition of a thin layer (1--3 nm thick) and the treatment of growing surface with a mixture of H and Ar*. Crystallization induced by permeation of hydrogen into the sub-surface at high substrate temperature (>200 C) was efficiently prevented by treating with a mixture of H and Ar*. The activation of growing surface may arise from releasing a part of hydrogen on surface by treating with Ar*. High quality a-Si:H films containing hydrogen of 3 atom % with a gap of 1.6 eV were made by chemical annealing with a mixture of H and Ar*.