Charles M. Fortmann

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.

Control of orientation from random to (220) or (400) in polycrystalline silicon films

The control of grain orientation in polycrystalline silicon thin films on glass substrates by low-temperature techniques was investigated. Either (220) or (400) preferential grain orientation could be attained by control of source gas ratio over substrate temperatures between 250°C and 360°C. A remote type plasma chemical vapor deposition system was used with source gas mixtures of SiF 4 , H 2 and Ar. The (220) preferential films were obtained with Ar/H 2 /SiF 4 gas flow rates of 60/15/30 seem (respectively), while the (400) preferential oriented films were obtained at higher SiF 4 /H 2 ratios (SiF 4 /H 2 = 90/10 sccm). At the higher SiF 4 /H 2 ratio during the crystal nucleation stage, either randomly oriented or (400) grains formed followed by the highly preferred deposition of (400) oriented crystallites. Raman scattering and ellipsometry spectra indicated that the (400) oriented films had a very smooth surface.

Carrier transport, structure and orientation in polycrystalline silicon on glass

Abstract Polycrystalline silicon films exhibiting (220) and (400) preferential orientation in X-ray diffraction (XRD) were grown on glass substrate from gaseous mixture of SiF 4 and H 2 , respectively, using a remote type plasma enhanced chemical vapor deposition (PECVD). In particular, the grains of (400) oriented texture showed smooth surface resulting from its highly selective sticking of deposition precursor on a certain site of (100) surface. Hall mobility at room temperature of (220) and (400) oriented films rose by 12 and 7 cm 2 /Vs, respectively, with increasing the grain size and decreasing the structure fluctuation. In addition, the Hall mobility observed in these films is characterized by a thermally activated process given by the equation, μ = μ 0 exp (− Eμ /kT) where μ 0 , E μ, k and T are the extended mobility, activation energy, Boltzmann constant and temperature, respectively. The μ 0 increased with increasing the grain size up to 40 cm 2 /Vs at the grain size of 250 nm diameter, whereas E μ was kept constant at around 35 meV being independent of grain size, where a part of the free electrons is considered to be localized in the shallow traps being in ‘thermal contact’ with the conduction band.

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...

Optimization of Transparent Conductive Oxide for Improved Resistance to Reactive and/or High Temperature Optoelectronic Device Processing

Research on improved amorphous silicon-based devices has focused on materials prepared at high temperatures and/or those grown under very reactive conditions. The use of these conditions for device applications requires the development of more robust transparent conductive oxide (TCO) substrates. A thin (<10 nm) ZnO coating on a SnOx-coated glass substrate could withstand RF (13.56 MHz) and very high frequency (VHF: 144 MHz) hydrogen plasma treatments; however, the TCO was strongly reduced by a higher density, higher energy electron cyclotron resonance (ECR) hydrogen plasma or a higher temperature. Ga-doped ZnO (GZO) TCO substrates exhibited greater resistance to hydrogen plasma induced reduction. RF magnetron sputter deposited crystalline GZO thin films were deposited and optimized at temperatures higher than 150°C on glass substrates. The electron mobility and the Ga doping efficiency were improved with increasing GZO deposition temperature. The performance of a-Si:H solar cells fabricated under standard conditions (~220°C) on these GZO substrates increased with increased GZO deposition temperature. The performance of a-Si:H solar cells prepared under more reactive and/or at higher deposition temperatures on 250°C deposited GZO was also examined. Both high temperature (280°C)-deposited narrow-bandgap a-Si:H(Ar) and ECR hydrogen plasma deposited a-Si:H(Cl) based solar cells were significantly improved using high temperature deposited GZO substrates.

Comparison of Microstructure and Crystal Structure of Polycrystalline Silicon Exhibiting Varied Textures Fabricated by Microwave and Very High Frequency Plasma Enhanced Chemical Vapor Deposition and Their Transport Properties

Micro- and crystal structures of polycrystalline silicon (poly-Si) films fabricated by low temperature (≤360°C) plasma enhanced chemical vapor deposition (PECVD) were examined. Crystal orientation could be controlled by varying the source gas ratio SiF4/H2. (220) oriented films were obtained at low gas flow rate ratios while (400) preferentially oriented films were obtained at higher SiF4/H2 ratios either by a remote-type microwave PECVD or a capacitive coupled parallel electrode very high frequency (VHF) PECVD. It was found that micro- and crystal structures were a strong function of orientation; that is, the crystal lattice in the (220) oriented film was under tensile strain and the crystalline grain had strong anisotropy of grain size. In contrast, the crystal lattice in the (400) oriented film was under compressive strain and evident anisotropy in the grain size could not be found. Furthermore, it was confirmed that the deposition of SiHn and/or SiHnFm and etching by fluorinated species and their competition played an important role in the selective growth. Fluorine-related species were also effective in growing large crystalline grains. Hall mobility of electron for (400) oriented films showed a monotonic increase with carrier density and achieved a large mobility of ~10 cm2/Vs.

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).

Fabrication of Polycrystalline Silicon Films from SiF4/H2/SiH4 Gas Mixture Using Very High Frequency Plasma Enhanced Chemical Vapor Deposition with In Situ Plasma Diagnostics and Their Structural Properties.

Polycrystalline silicon (poly-Si) films were fabricated by very high frequency (VHF) plasma enhanced (PE) chemical vapor deposition (CVD) from SiF4 and H2 gas mixture with small amounts of SiH4. Reactions and growth of poly-Si in the SiF4/H2/SiH4 system were discussed together with the results obtained from in situ plasma diagnostics, and compared with those obtained by microwave PECVD (MW CVD). As a result, similar relationships among growth temperature, SiF4/H2 gas flow ratio and film structure to those obtained with MW CVD were obtained with VHF CVD. For example, growth temperature could be reduced to 100°C while keeping a high crystal fraction (>80%) when small SiF4/H2 gas flow ratios were used. In contrast, under large SiF4/H2 gas flow ratios, crystal fraction rapidly decreased with decreasing temperature. The role of fluorine-related species in the growth of poly-Si was examined in relation to film microstructure and the results obtained from plasma diagnostics. Finally, guiding principles to achieve high rate and/or low-temperature growth of poly-Si by VHF CVD using SiF4/H2 gas mixtures were discussed.

Control of Orientation for Polycrystalline Silicon Thin Films Fabricated from Fluorinated Source Gas by Microwave Plasma Enhanced Chemical Vapor Deposition

The structure of polycrystalline silicon films was controlled by selecting deposition conditions, especially gas mixing ratio of H2/SiF4, for plasma enhanced chemical vapor deposition on glass at a low temperature of 360°C. Under the H2/SiF4 ratio of 10/90 sccms, (111), (220), (311) and (400) diffraction peaks were observed by X-ray diffraction for a 0.5 µm thick film and (400) oriented crystallites grew with film thickness. The sharpness of the Raman peak around 520 cm-1 and the pseudo-dielectric function measured by an ellipsometer indicated that the crystallites have excellent regularity of the Si–Si bond compared with (220) oriented films and also have small surface roughness, which are preferable features for device applications. The selective growth of (400) oriented grains is thought to partly originate from selective etching due to fluorine related species generated under low hydrogen conditions.