Glow Discharge Samples Research Articles

Comparative study of the effect of thermal annealing on the hydrogen stability and the stress in a-C:H films deposited by electron cyclotron resonance glow discharge and direct current multipolar plasma methods

The effects of thermal annealing on the hydrogen stability and stress in two types of a-C:H films were measured. The films were prepared in a dual electron cyclotron resonance radiofrequency glow discharge or in a direct current multipolar plasma system from pure methane, at substrate bias voltages of −30 and −600 V for each type of film. Infrared absorption, elastic recoil detection analysis and internal stress measurements are used to measure some properties of the films in the as-deposited state and after annealing at increasing temperature up to 700°C. The results indicate that the two types of samples have completely different magnitudes of stresses in their as-deposited state, and that they have also different H thermal stabilities. The stresses are also affected differently by annealing. Annealing reduces the magnitude of the stresses in all cases. For annealing temperature >400°C, and for the −600 V substrate bias, the stress relaxation occurs through atomic rearrangement in the direct current plasma deposited films, while it induces a loss of adhesion in the glow discharge samples.

Persistent excited conductivity and the threshold fluence in a-Si:H under 17 MeV proton irradiation

Radiation-induced conductivity (RIC) of hydrogenated amorphous silicon (a-Si:H) has been studied under 17 MeV proton irradiation. Two kinds of a-Si:H samples were fabricated by either standard- or He-diluted glow-discharge (GD). Electric conductivity of the standard GD samples has an increase at the beginning of the proton irradiation, followed by a slower increase with further irradiation. After stopping the irradiation, the conductivity decays but remains ∼2 orders of magnitude greater than the conductivity of an unirradiated sample. This increased conductivity lasts for a long time, and is labeled persistent excited conductivity (PEC). At T=200 K, the PEC is stable over 30 h. In the He-diluted GD samples, the PEC also occurs but a threshold fluence to cause the PEC is observed. Below the threshold fluence, conductivity increases with the irradiation, but the increased conductivity decays in times less than a second after stopping the irradiation. Photoconductivity which increased in correlation with the PEC is also presented. Using the new experimental results, possible mechanisms of the PEC are discussed.

Light-induced annealing of dangling bonds in He-diluted glow discharge a-Si:H films

We report the results of a study of light-induced annealing of dangling bonds, as well as their light-induced creation in He-diluted glow discharge a-Si:H with a large amount of hydrogen at room temperature (RT) and at 75°C, by using an electron spin resonance (ESR) technique. Under illumination 0.7 W/cm2 at room temperature, dangling bonds are created by prolonged illumination, and their density tends to saturate. On the other hand, under strong illumination (1.6 W/cm2) at room temperature, dangling bonds are initially created and then photoannealed after 5 min of illumination. These results for He-diluted glow discharge samples are discussed and compared with those for standard and H2-diluted glow discharge samples.

Deconvolution of ESR Spectra and Their Light-Induced Effect in a-Si:H

It is shown that ESR spectra of dangling bonds in a-Si:H for glow discharge samples are deconvoluted into two components due to normal dangling bonds and H-related dangling bonds. The component for H-related dangling bonds has anisotropic g -values and anisotropic hyperfine coupling constants of 1 H. The distance between the dangling bond site and hydrogen site is estimated to be 2 A, using a tight binding theory. The normal dangling bonds constitute about 60% of the total spin density, while the H-related dangling bonds give about 40% for glow discharge samples. The glow discharge samples were also illuminated by a xenon lamp with 600 mW/cm 2 at room temperature. The normal dangling bonds and H-related dangling bonds are created almost equally by illumination.

An Examination of H Effusion in a-Si:H Using Infrared Spectroscopy

We present the results of H diffusion studies on glow discharge (GD) and hot wire (HW) deposited a-Si:H films studied by infrared (IR) spectroscopy. In this technique, a-Si:H films deposited on crystalline silicon are annealed isochronally for fixed times, and after each anneal the decrease in the amount of SiH infrared absorption is carefully measured. We then incorporate these data into the rate equation for loss of SiH bonds due to annealing, enabling the determination of an activation energy to break these bonds and place the H in a site that cannot be measured by IR. The activation energies for device quality GD films are similar to those found in the a-Si:H diffusion literature, suggesting that diffusion parameters can be measured by this method. On the other hand, a similar analysis of data for HW films containing similar H contents is not clear cut. GD and HW samples with lower H content, deposited at higher Ts, are also examined. We discuss these results in the context of film microstructure.

Physical properties of amorphous silicon-carbon alloys produced by different techniques

Results of a study of compositional, optical, electrical, and structural properties of hydrogen amorphous silicon carbide (a-SiC:H) prepared, respectively, by glow-discharge (GD) and reactive sputtering (SP) techniques at power densities varying between 1.25 · 10−2 and 1.25 · 10−1 W · cm−2 for GD samples are presented. Measurements are reported on the composition, optical and IR spectroscopy, and on the temperature dependence of electrical conductivity. All experimental observations suggest that the power density only slightly affects the physical properties of GD silicon-rich samples, whereas those of the carbon-rich SP samples depend more strongly on this deposition parameter. Finally, it is shown that the GD technique can provide films with better characteristics, whereas samples of similar composition prepared by sputtering have higher compositional disorder and are more inhomogeneous.

Comparison of the Properties of Sputtered and Glow Discharge a-Ge:H Films

ABSTRACTUndoped a-Ge:H films have been made both by sputtering and by glow discharge under various combinations of deposition parameters, and characterized structurally, electrically, and optically. A detailed comparison of subband gap spectra of sputtered and glow discharge samples has been carried out using photothermal deflection spectroscopy and the constant photocurrent method. The results show subband gap absorption in a-Ge:H to be an order of magnitude larger than in device-quality a-Si:H. For some of the films the absorption spectra deduced from transmission and PDS fit very well onto the absorption spectra inferred from CPM measurements, while for others it is clear that the match is very poor. We infer that the photoconductivity-derived spectrum does not necessarily reflect the dominant absorption process, and that this is related to the existence of a two-phase structure clearly observed by transmission and scanning electron microscopy.

Investigation of the structural disorder fluctuations in a a-Si:H films by coupled X-ray diffractometry and photodeflection spectroscopy

The full width at half maximum (FWHM) of the first diffraction peak and the width of the optical absorption edge E o (Urbach edge) have been measured on glow discharge (GD) and sputtered (SP) prepared a-Si:H samples. The coherence length L, derived from the FWHM has been plotted vs E o and also vs the optical gap E g. It is shown that L increases when E o decreases. The values of L are always found larger for GD samples than for SP films. The Si matrix relaxation by H atoms is demonstrated in GD materials through an increase of L with E g. This is not the case for SP materials.

Auger line shape analysis of hydrogenated amorphous silicon

Auger line shape analysis of the Si L 23 VV peak of hydrogenated amorphous silicon samples has been performed. Both ion-implanted and glow discharge samples were analyzed. Hydrogen concentrations were established by IR and secondary ion mass spectroscopies. The L 23 VV peak was deconvoluted into its L 23M 1M 23, L 23M 23M 23 and localized two-hole components. The relative amounts of the L 23 VV components were compared with the hydrogen concentration. The results show that Auger line shape analysis may be used as (i) a semiquantitative monitor of the hydrogenation of amorphous silicon and (ii) a probe of the valence band of hydrogenated amorphous silicon.

Thickness Dependence of the Optical and Electrical Properties of Thin A-Si:H Films

ABSTRACTOptical and electrical properties are investigated as a function of thickness for two sets of samples of a-Si:H films deposited respectively by RF magnetron sputtering and by RF glow discharge under the same conditions. The hydrogen concentration, the transmittance and reflectance spectra in the UV-visible-NIR and the resistivity were measured for both sets of samples. Relationships between index of refraction, absorption coefficient, imaginary part of the dielectric constant, optical band gap, resistivity and the thickness are found and interpreted. The thickness dependence of these parameters is related to the larger amount of inhomogeneities present in sputtered samples with respect to glow discharge samples.

Photoconductivity of lightly boron doped a-Si:H

The Fermi level effects on photoconductivity σ ph are analysed experimentally on boron doped sputtered and glow discharge samples with well-characterized boron contents, optical properties and gap defects densities. The results on σ ph and its exponent γ in SP materials indicate that the recombination kinetics are controlled by the valence band tail states profile according to Rose's model whereas in GD materials these are controlled by the position of E F relative to one single defect placed at E i − 0.05 eV with positive correlation energy.

Nongeminate radiative recombination in sputtered and glow dischargea-Si:H

We report measurements of the excitation intensity dependence of the photoluminescence efficiency of the 1.4-eV band in both sputtered and glow discharge prepared $a$-Si:H. We observe that sputtered samples show very weak nongeminate radiative recombination in comparison with glow discharge samples of comparable low-temperature luminescence efficiency and midgap state density. A structural model is tentatively proposed to account for these differences.

Determination of the fluorine concentration in silicon films grown from the disproportionation of SiF 2

Amorphous silicon layers containing fluorine were prepared both by the pyrolytic decomposition of SiF 2 gas and by d.c. glow discharge decomposition in low pressure SiF 2-SiF 4 gas mixtures. The fluorine content was determined using a nuclear technique based on the reaction 19F(p,α) 16O ∗ and calibrated with fluorine-implanted silicon layers. The glow discharge samples contained fluorine in concentrations ranging from about 1% to above 15%. Pyrolytically deposited samples showed only about 1%F. X-ray photoelectron spectroscopy (XPS) measurements were consistent with the values obtained by the nuclear method, although they always gave a lower calculated fluorine concentration. The differences are probably a consequence of the loss of fluorine from the outermost layers sampled by the XPS technique. IR absorption measurements confirmed the presence of fluorine.

Comparative study of hydrogen evolution from amorphous hydrogenated silicon films

The results of hydrogen evolution from amorphous hydrogenated silicon (a-Si:H) films prepared by glow discharge decomposition of SiH 4, sputtering in an ArH 2 plasma and vacuum evaporation with the addition of hydrogen are compared. Two evolution processes can be distinguished: a low temperature evolution due to the release of molecular hydrogen through voids or cracks and an evolution at high temperatures due to effusion from compact a-Si:H material, which is dominated by diffusion of atomic hydrogen. The low temperature evolution for sputtered and evaporated films is related to molecular hydrogen that is adsorbed dissociatively on internal surfaces during film growth and for glow discharge samples to the presence of polysilane-like intergrain material. Differences in the position of the high temperature evolution peaks are attributed to a higher degree of disorder in evaporated and sputtered films compared with that in glow discharge samples.

Ac conductivity and optical properties of amorphous Si(H) sputtered at 77 K

The ac conductivity of amorphous Si(H) films deposited at 77 and 300 K by dc sputtering in an argon-hydrogen mixture was measured from 300 to 4.2 K. The fact that ${\ensuremath{\sigma}}_{\mathrm{ac}}\ensuremath{\propto}T$ over a restricted temperature range, as was previously observed on glow discharge samples, was taken as evidence for the quantum-mechanical tunneling theory. The discovery that at low temperatures (30 K) ${\ensuremath{\sigma}}_{\mathrm{ac}}$ becomes essentially temperature independent, as do the chalcogenide glasses, can be explained by Elliott's correlated barrier hopping theory. While the optical properties of Si(H) films sputtered at 300 K are quite similar to those reported for rf-sputtered films at 500 K, those for films deposited at 77 K show interesting differences. The optical properties of films deposited at 77 K were measured at 123 K without annealing, and remained unchanged at room temperature. The vibrational mode at 850 ${\mathrm{cm}}^{\ensuremath{-}1}$ for films deposited at 77 K increases with respect to the 900-${\mathrm{cm}}^{\ensuremath{-}1}$ mode as a function of increasing hydrogen content and eventually becomes strongest. This would imply, if we judge from the existing literature, a high concentration of ${(\mathrm{Si}{\mathrm{H}}_{2})}_{n}$. This strong concentration of Si---H bonds decreases the Si oscillator strength homogeneously as is shown by the decrease and shift to higher energy of the peak in the imaginary part of the dielectric constant. The values of the refractive index are appreciably smaller in films deposited at 77 K than in films deposited at 300 K. On the other hand, the structure of both films as observed in transmission electron microscopy is quite similar and also close to the structure previously observed in pure $a\ensuremath{-}\mathrm{S}\mathrm{i}$.