ESR Spin Density Research Articles

Magnetic Properties of Substituted Poly(thiophene)s in Their Neutral State

The magnetic behavior of undoped (neutral), substituted poly(thiophene)s is reported. In particular, the influence of the nature of the substituent (alkyl, alkoxy, thioalkyl), the substitution pattern (head-to-tail (HT) versus head-to-head−tail-to-tail (HH-TT)), and the regioregularity on the magnetic properties has been investigated. ESR spectroscopy reveals that the nature of the substituent determines the spin density, while the line width and asymmetry of the ESR signals are mainly governed by the substitution pattern and regioregularity. The spins give rise to a paramagnetic behavior. SQUID magnetometry reveals the presence of superparamagnetic order at room temperature, while ferromagnetism is observed at 5 K. The magnetic behavior observed by SQUID magnetometry does not (solely) originate from the ESR-active spin system. Its strength does therefore not depend on the ESR spin density, but seems to be governed by the supramolecular structure.

Bonding defects and optical band gaps of DLC films deposited by microwave surface-wave plasma CVD

The effects of CH 4 / C 2H 4 flow ratio and annealing temperature on the defect states and optical properties of diamond-like carbon (DLC) films deposited by novel microwave surface-wave plasma chemical vapour deposition (MW SWP CVD) are studied through UV/VIS/NIR measurements, atomic force microscopy, Raman spectroscopy and electron spin resonance analysis. The optical band gap of DLC has been tailored between a relatively narrow range, 2.65–2.5 eV by manipulating CH 4 / C 2H 4 flow ratio and a wide range, 2.5–0.95 by thermal annealing. The ESR spin density varied between 10 19 to 10 17 spins/cm 3 depending on the CH 4 / C 2H 4 flow ratio (1 : 3 to 3 : 1). The defect density increased with increasing annealing temperature. Also, there is a strong dependence of spin density on the optical band gap of the annealed-DLC films, and this dependency has been qualitatively understood from Raman spectra of the films as a result of structural changes due to sp 3/sp 2 carbon bonding network. The surfaces of the films are found to be very smooth and uniform (RMS roughness < 0.5 nm).

Formation of ultrathin Si xN y layers on silicon utilizing PECVD

We exposed crystalline Si (C-Si) substrates to a NH 3 plasma by the method of plasma enhanced chemical vapor deposition (PECVD) and the Si substrates were nitrided at a nitridation temperature, T n, between 300 and 600 °C. The nitrided layers were grown directly on n-type C-Si substrates with high resistivity for measurements of electron resonance (ESR) and Fourier transform infrared (FTIR) absorption. The intensity of the N–H IR bending band increased with increasing T n up to 450 °C, and then decreased again with a further increasing in T n. We found a strong correlation between the change in the ESR spin density, N s, and that in the N–H bending intensity, which was obtained from the spectral intensity at around 1200 cm −1. Furthermore, incorporation of N atoms in amorphous Si x N y layers may improve the structural property at T n = 450 °C for various insulating applications. In addition, we found that the atomic densities and/or composition of a-Si x N y may decrease the BHF etch rate.

Crystal structures of [Rh(η 5-C 5H 5)(C 3S 5)] and [Rh(η 5-C 5Me 5)(C 3S 5)] 2 and properties of their oxidized species

[Rh(η 5-C 5H 5)(C 3S 5)] and [Rh(η 5-C 5Me 5)(C 3S 5)] 2 [C 3S 5 2−=4,5-disulfanyl-1,3-dithiole-2-thionate(2-)] were prepared by reactions of [NMe 4] 2[C 3S 5] with [Rh(η 5-C 5H 5)Cl 2] 2 and [Rh(η 5-C 5Me 5)Cl 2] 2, respectively. Their X-ray crystal structural analyses revealed a monomeric form for the former complex and a dimeric geometry containing bridging S–Rh–S bonds for the latter in the solid state. They were reacted with bromine to afford [RhBr(L)(C 3S 5)] (L=η 5-C 5H 5 and η 5-C 5Me 5) with the Rh–Br bond and one electron-oxidation on the C 3S 5 ligand. ESR spectra and spin densities for these oxidized species are discussed.

Thin film poly-Si solar cells using PECVD and Cat-CVD with light confinement structure by RIE

Poly-Si film prepared by PECVD method showed low ESR spin density of around 1E16/cm3 and poly-Si film prepared by Cat-CVD method showed relatively low ESR spin density of around 1E17/cm3. The cell using PECVD poly-Si film as a photo-active layer showed an intrinsic cell efficiency of 7.75%. Light confinement micro-structure was introduced to the cell applying RIE method to the glass substrate surface and enhancement of Jsc was observed.

Thin film poly-Si formation by Cat-CVD method and its application for solar cells

Poly-Si thin film which has an ESR spin density of 6.9 E16/cm3 was obtained by Cat-CVD with a deposition rate of 5.4 Å/s at a relatively high deposition pressure (approx. 30 Pa) with a high hydrogen dilution (SiH4/H2=0.05). The low ESR spin density films were applied to solar cells as a photo-active layer. Although the dark V–I curves of the cell showed diode characteristics, the photo V–I curves showed very small photo-current densities, lower than 0.5 mA/cm2. A large amount of higher order silane molecules at the present high deposition pressure might be one of the causes of this small photo-current density, as well as grain boundary recombination and an incubation layer at the first stage of film deposition.

Defect Creation by Electron Beam Irradiation in Amorphous Silicon Nitride Films Compared with That by Light Soaking

ABSTRACTDefect creation by electron beam irradiation is compared with that by light soaking in a-Si1-xNx:H films. For the film with x=0.06, the ESR spin density increases by 20-keV electron beam irradiation without changes in the g-value. However, for the film with x=0.47, the ESR spin density increases with accompanying the decrease in the g-value from 2.0042 to 2.0034. The decrease in the g-value can be explained by increasing number of N atoms at the backbond site of the Si atom having the dangling bond. Light soakings does not change the g-value of the ESR signals of the films.

Trace elements in coal derived liquids: analysis by ICP-MS and Mössbauer spectroscopy

Concentrations of trace elements in coal derived liquids have been investigated by inductively coupled plasma-mass spectrometry (ICP-MS) and by Mössbauer spectroscopy. Liquefaction extracts prepared from the Argonne Premium Coals and a coal tar pitch have been examined. Microwave digestion in concentrated nitric acid has been shown as a suitable method for determining trace element concentrations in coal derived liquids by ICP-MS—for sample sizes as small as 3–20 mg. High concentrations of Fe were found for all extract samples (<265–1474 ppm). Ti, Cr, Mn, Co, Ga, Sb, Cs and Ba were measurable. Concentration distributions of trace elements found in the extracts bore little relation to the corresponding distributions in the original coals. The proportions of individual trace elements present in the original coals and found in the extracts, varied widely. Mössbauer spectroscopy of the extracts indicated that the high Fe-concentrations corresponded to the presence of organometallic-Fe compounds—and not to pyritic iron. There is evidence suggesting the presence of material derived from iron-storage proteins such as ferritin, but final proof is lacking. Our data suggest that other metallic ions detected in these coal derived liquids may be present in association with the organic material. Concentrations of paramagnetic metal species were found to be of the same order of magnitude as ESR spin-densities already found in coal liquids. Both types of paramagnetic species are suspected of causing loss of signal in solid state 13C NMR.

Optoelectronic and structural properties of amorphous silicon–carbon alloys deposited by low-power electron-cyclotron resonance plasma-enhanced chemical-vapor deposition

The optoelectronic and structural properties of hydrogenated amorphous silicon-carbon alloys (a-SiC:H) are studied over the entire compositional range of carbon content. The films are prepared using low-power electron-cyclotron resonance (ECR) plasma-enhanced chemical vapor deposition. The carbon content was varied by using different methane (or ethylene-)-to-silane gas phase ratios and by introducing the methane (or ethylene) either remotely into the plasma stream or directly through the ECR source, together with the excitation gas (hydrogen). Regardless of the deposition conditions and source gases used, the optical, structural and transport properties of the a-SiC:H alloys followed simple universal dependencies related to changes in the density of states associated with their structural disorder. The deep defect density from photothermal deflection spectroscopy, the ESR spin density, the steady state and the transient photoluminescence, the dark and photoconductivity, the temperature of the hydrogen evolution peaks and the bonding from infrared spectroscopy are correlated to the Urbach tail energy, the B factor of the Tauc plot and E04 (defined as the energy at which the absorption coefficient is equal to 104 cm−1). Silicon-rich and carbon-rich regions with very different properties, corresponding approximately to carbon fractions below and above 0.5, respectively, can be distinguished. The properties of the ECR a-SiC:H alloys are compared with those of alloys deposited by rf glow discharge.

Density of Si–H bonds responsible for structural flexibility in a-Si:H

Modulation of infrared absorption of the Si–H stretching mode following a modulated excitation can be observed at around 2000 cm−1 as a photoinduced bleaching band in a photomodulation spectrum of a-Si:H as reported previously by Vardeny et al. Here we have studied how this modulated vibrational spectrum is affected by photodegradation assuming that the spectral change comes from breaking of weak Si–Si bonds next to a Si–H bond. By evaluating quantitatively the spectral change induced by photodegradation as well as the simultaneous change of ESR spin density, density of Si–H or weak Si–Si bonds, which are precursor sites for structural flexibility in a-Si:H, can be evaluated to be ∼1.4–2.4×1018 cm−3. It is interesting that the evaluated value is only about 0.024–0.04% of the total density of Si–H bonds contained in the specimen used here.

Control of Microstructure and Optoelectronic Properties of Si:H Films by Argon Dilution in Plasma-Enhanced Chemical Vapor Deposition from Silane

Structural and optoelectronic properties of thin films of silicon-hydrogen binary alloy (Si:H) deposited from silane and argon mixture in a rf glow discharge plasma have been studied for different argon dilutions and rf powers. It has been observed that with low rf power density ( 30-70 mW/cm3) increase of argon dilution up to 95% reduces the microstructure in the films, as determined from IR absorption spectra. Simultaneously, increase in refractive index and decrease in ESR spin density have been observed. Above 95% argon dilution or with higher rf powers, transmission electron microscopy (TEM) studies reveal a dominance of the columnar growth mechanism, and the optoelectronic properties of the films deteriorate. At 99% argon dilution, microcrystallites appear to form within columnar regions. Addition of a small amount of hydrogen to the silane-argon plasma improves the network significantly, which is manifested by the changes in the dark conductivities in the different rf power regimes.

Decay Behavior of Light-Induced ESR in Hydrogenated Amorphous Silicon-Nitrogen Alloys

Light-induced ESR signals created by short-time illumination in Si-rich hydrogenated amorphous silicon-nitrogen films do not completely disappear after cessation of illumination. The remaining fraction of light-induced ESR spin density increases with increasing illumination time and illumination intensity, suggesting that a structural change proceeds with illumination.

Relation Between Defect Density and Conductivity Changes with Light-Soaking and Annealing in a-Si:H

ABSTRACTIt is found in a-Si:H and N-doped a-Si:H that the ESR spin density Ns increases and saturates with light-soaking slower than the dark- and photoconductivities (αd and αp) do. In the recovery process by annealing, the change in Nsalso occurs slower than αd and αp. From the measurement of the activation energy for αd the change in αd is found to originate mainly from the movement of the Fermi level Ef. In order to elucidate the origin of different behaviors between Ns and αd or αp, the light-induced ESR and the constant photocurrent method measurements have been carried out.

Thermal equilibrium behavior of defect density and conductivity in a-Si 1−xGe x:H alloys

The thermally induced increase in the ESR spin density and the thermally induced decrease in the do conductivity were observed after fast cooling from elevated temperature. These phenomena are thought to be determined by a thermal equilibration process.

Achievement of More than 10% Efficiency for a Single-Junction 100cm2 a-Si Solar Cell and Development of a New-Type Module Structure

ABSTRACTA total area conversion efficiency of 10.2% has been achieved for a 1Ocm×1Ocm integrated-type single-junction amorphous silicon (a-Si) solar cell submodule. It is the highest conversion efficiency ever reported for an a-Si solar cell with an area of 100cm2, including multi-junction cells. The effective area conversion efficiency is as high as 11.3%. The high efficiency is obtained by improving the quality of the i-layer and the p/i buffer layer, as well as by utilizing a highly textured, high-quality transparent electrode. The quality of the i-layer plays a dominant role in the performance of a-Si solar cells, especially in high efficiency cells. Techniques that control the properties of the high-quality a-Si films for the i-layer are described. Electric conductivity, ESR spin density and the Raman spectra of high-quality a-Si:H films are investigated as well as their thickness-dependence and substrate-dependence.A Through-Hole Contact (THC) integrated-type submodule has been developed as a new-type a-Si solar cell module structure. Numerical simulations on the output power of the structure show that the output power can be significantly improved by the THC structure.

Fabrication of Device-Quality Wide-Gap a-Si:H Films at Very Low Substrate Temperatures

ABSTRACTWide-gap a-Si:H films with device quality (Tauc’s optical gap &gt; 1.9eV, σph under AMI.5, 100mW/cm2 illumination ≥ 10−5, Ω−1cm−1, a σph/σ a≥106) have been fabricated. These films are deposited at low substrtate temperatures (TS≤80°C ) either by diluting SiH4 with H2 or optimizing the plasma parameters in a capacitively–coupled RF plasma–CVD reactor. Reduction in the SiH2 bond density and the ESR spin density are also observed. In this study, good film quality is always accompanied by a small deposition rate. Furthermore, σph is nearly the same if the deposition rate and Ts is the same, regardless of other deposition parameters. This suggests that the surface reactions or structural relaxations at the film-growing surface can produce high–quality a–Si:H films even at low TsS, if the deposition rate is low. Results in thermal annealing, light exposure, and solar cell performance confirm that these films have device quality and wide bandgap.

Material parameters in thick hydrogenated amorphous silicon radiation detectors

Transient photoconductivity measurements of basic material parameters: carrier mobility, mobility-lifetime product and the ionized dangling bond density of thick hydrogenated amorphous silicon detectors are presented. We found that only a fraction (∼ 30 – 35%) of the total defect density as measured by ESR is ionized when the detector is biased into deep depletion. The measurements on annealed samples done to relate the ionized dangling bond density and the ESR spin density also showed that this fraction is about 0.3. The time dependence of defect relaxation was found to be a stretched exponential.

Approach from a-Ge Films for Development of High-Quality a-SiGe Films

a-Ge:H films fabricated by means of a separated ultrahigh-vacuum reaction chamber, called the super chamber, were systematically studied. In the conventional glow-discharge method, there is a very narrow substrate-temperature region for fabricating high-density a-Ge:H films; that is, a minimum deposition rate and a maximum refractive index were obtained at about 250°C. From the point of view of optoelectrical properties, it was clear that not only rigidity of the film network but also total hydrogen content are important. In order to satisfy the two above-mentioned factors simultaneously, a low substrate-temperature high hydrogen-dilution method was effective, and film properties of a-Ge:H were largely improved; δd∼4.0×10-5 Ω-1 cm-1, δph∼1.5×10-4 Ω-1 cm-1, B value ∼803 (eV·cm)-1/2, and the ESR spin density ∼1.5×1017 cm-3.

Bonding properties of amorphous SiN x:H films with low density of Si-H bonds

Amorphous SiN x:H films were prepared by rf glow discharge of SiH 4dN 2H 2 mixtures at 300 °C, and investigated by means of SiH vibrational and optical absorption, ESR, and electrical and BHF etching measurements. The N content x increases with an increase in both N 2/SiH 4 ratios. The SiH absorptioon disappears at x above 0.9. A strong correlation among the ESR spin density, BHF etch-rate and breakdown strength, and oxygen contamination imported after the fabrication is found.

Preparation and Properties of a-SiGe:H Films Fabricated with a Super Chamber (Separated Ultra-High Vacuum Reaction Chamber)

High-quality a-SiGe:H films with low impurity concentrations were studied using a separated ultra-high vacuum reaction chamber system called the super chamber. The ESR spin density and the tail characteristic energy of the a-SiGe films (Eopt 1.5 eV) were 6.5×1015 cm-3 and 46 meV, respectively. These values were much lower than those for films fabricated in a conventional chamber as well as those for a-Si films. Structural properties, such as the refractive indices and thermal effusion of hydrogen, were also measured. The results suggest that impurity reduction contributed not only to an improvement in the optoelectrical properties, but also the formation of a rigid a-SiGe network.