Glow Discharge Amorphous Silicon Research Articles

Cathodic and anodic glow discharge silicon-carbon alloys (a-Si 1- xC x:H) from x = 0.5 to 1: A comparative study by photoemission (UPS) and photoluminescence (PL)

Photoluminescence (PL) and ultraviolet photoelectron spectroscopy are used to study glow discharge amorphous silicon carbon alloys (a-Si 1- x C x :H, 1 < x < 0.5) under different substrate biases. The samples were deposited simultaneously on substrates located at the cathode and the anode. The deposition conditions were those used to produce hard a-C:H (‘diamond like’). In cathodic samples, ion bombardment enhances C sp 2 bonds while in anodic samples the top of valence band is depressed. Both ion bombardment (negative bias) and electron bombardment (positive bias) decrease the PL efficiency.

Defect Properties of Cathode Deposited Glow Discharge Amorphous Silicon Germanium Alloys

ABSTRACTWe have characterized the deep defect densities and their energy distributions for a series of a-Si1-xGex:H alloys with large Ge content (0.57&lt; × &lt; 1.00) prepared by the cathode deposited glow discharge method. Our results indicate markedly superior properties for these samples. A small Urbach tail slope (about 45meV) was found for all samples in this alloy range. The defect densities were either obtained directly from the drive-level capacitance profiling or deduced from the sub-band-gap optical spectra. Both are substantially lower than the trend line determined from previous studies of a-Si1-xGex:H samples produced by conventional glow discharge and by Photo-CVD methods. However, the relation between the total defect densities and the optical spectra in the cathodic samples obeys the same defect formation model that has been used to successfully predict the defect densities in other types of a-Si1-xGex:H material.

Density of glow discharge amorphous silicon films determined by spectroscopic ellipsometry

The dielectric function of thin-film hydrogenated amorphous silicon (a-Si:H) grown on fused silica at different substrate temperatures has been investigated by spectroscopic ellipsometry. An improved interpretation of the ellipsometric data is based on a tetrahedron model that takes into account the influence of hydrogen incorporation in the amorphous network. It is shown that the film density can be derived from an accurate data interpretation, whereas the maximum value of the imaginary part of the dielectric function ε2max and the void volume fraction are not proportional to the density of a-Si:H films.

Transport properties of glow-discharge amorphous silicon grown under different deposition conditions

Abstract Hydrogenated amorphous silicon has been prepared using the glow discharge technique, with the deposition parameters being varied to give two sample groups. In the first case, substrate temperatures range from 130 to 350°C with a constant r.f. power setting. For the second group of samples, the substrate temperature is held constant at 290°C, and deposition rates of between 2 and 8 A s− 1 are used. The samples are compared on the basis of electron drift mobilities and mobility-lifetime products obtained from time-of-flight and charge collection experiments. Room-temperature mobilities are found to vary by an order of magnitude over the range of samples studied. Among the “poorer” samples, mobilities are found to be more strongly activated, indicating trapping to depths of 0.25 eV in the conduction-band tail. Transient photoconductivity data are used to derive the conduction-band-tail density of states. For samples produced under different deposition conditions, band-tail widths are found to increa...

Photoluminescence studies on silicon carbon alloys

Photoluminescence and sub-gap absorption spectroscopy are used to study glow discharge amorphous silicon carbon alloys. The samples were deposited from methane and silane, with or without hydrogen dilution of the gaseous mixture. The line-width of the emission is proportional to the Urbach energy. The emission dependence on temperature is used to estimate the localization length of trapped carriers.

Thermal quenching and relaxation in doped hydrogenated amorphous silicon deposited by plasma-enhanced chemical vapor deposition from He-diluted silane

The stability of n-type and p-type rf glow discharge amorphous silicon (a-Si:H) deposited from silane-helium mixtures has been studied. Dark dc conductivity measurements were made after quenching or slow cooling and the time dependence of isothermal relaxation at different temperatures has been investigated. The equilibration temperatures are ∼155 and ∼125 °C for n- and p-type films, respectively. These values are greater than those commonly reported on conventional rf glow discharge films (130 and 90 °C). In comparison with conventional films, at any given temperature the relaxation times are longer with activation energies of 1 and 1.39 eV for n- and p-type films. The improved stability of the films has been discussed in relation with their particular hydrogen-related structure.

Effect of preparation conditions on the properties of glow-discharge intrinsic amorphous silicon

Abstract The influence of the preparation conditions (process pressure, substrate temperature, RF-power density and deposition time/thickness) on the optical and electrical properties of intrinsic hydrogenated amorphous silicon (a-Si:H) has been investigated with the aim of optimising such films to be used as absorbent layers of a-Si:H-based p-i-n solar cells. Highly photosensitive films have been obtained at high growth rates (6.2 A s−1) in the depletion regime using a high process pressure (1000 mTorr), a moderate substrate temperature (250°C) and a relatively high RF-power density (35.2 mW cm−2). These films have excellent properties for the application in question.

Optical study of microvoids, voids, and local inhomogeneities in amorphous silicon

Elastic light scattering has been used for a study of microstructure in amorphous hydrogenated silicon. A simple theory to get quantitative informations on the microstructure has been presented for the first time, both for Rayleigh and Mie scattering. For optimal very high frequency glow discharge amorphous silicon layers, the presence of voids with diameter between 1 and 20 nm is typical.

Absence of thermal quenching effects in undoped amorphous silicon deposited by the pecvd of he - diluted silane

The effect of thermal quenching on the optoelectronic properties of well characterized undoped rf glow discharge amorphous silicon deposited under standard conditions and from silane helium mixtures has been studied. The defect density of the “helium diluted” samples is shown to be independent of the temperature up to 300°C; this could be a sign of a better stability of these films. The data are analyzed in terms of recent equilibrium models and discussed in relation with hydrogen-related properties resulting from the particular growth conditions.

On the diffusion of boron in glow-discharge a-Si:H

Contrary to previously reported experimental results and theoretical estimates we find that there is no detectable diffusion of boron at typical deposition temperatures of glow-discharge amorphous silicon, i.e. D(285 °C) ⪅ 7 × 10 −20 cm 2 s −1 .

A spectroscopic ellipsometry study of the growth and microstructure of glow-discharge amorphous and microcrystalline silicon films

Spectroscopic ellipsometry (SE) has been used to study the surface, microstructure, and optical properties of glow-discharge deposited amorphous and microcrystalline silicon films. The SE data has been analyzed using multilayer models, effective medium approximation, and linear regression analysis. The studies have shown that with increasing rf power and silane dilution, a gradual transition from good (dense) a-Si:H to a spongy material with a large density deficit, and then to a microcrystalline material is obtained. A quantitative analysis of the microstructure of a-Si:H films with growth parameters has been carried out. Combined with infrared and x-ray diffraction measurements, the results have been explained in terms of a nucleation and growth model.

Dark discharge in hydrogenated amorphous silicon photoreceptors

We have studied the electric field, thickness, and temperature dependencies of dark discharge in corona-charged glow-discharge amorphous silicon (a-Si:H) films. Dark discharge is a basic property for image devices such as photoreceptors. The dark-discharge rate ‖dV/dt‖ is found to be proportional to the square root of the electric field E as in the form log‖dV/dt‖∝√E, and increases nearly proportionally to the thickness of the a-Si:H film, indicating a bulk emission of charges from barriers lowered by electric fields like the Poole–Frenkel emission. It was also found that the contribution of electron injection from the substrate increases significantly in the dark discharge for small film thicknesses. The temperature dependence of dark discharge revealed the barrier height to be approximately 0.9 eV. A discharge mechanism was considered on the basis of these results, and was compared to that of other conventional materials for photoreceptors such as a-Se and a-As2Se3.

Equilibrium temperature and related defects in intrinsic glow discharge amorphous silicon

We find the equilibrium temperature for intrinsic glow discharge amorphous silicon to be 195–200 °C. Defects left behind after fast cooling result in a temperature-dependent dc photoconductivity which shows small differences in the tail state recombination kinetics when compared to defects left behind in the same number after light soaking. Finally anneal kinetics of fast cool defects follow neither singly activated, mono-, nor bimolecular kinetics with a temperature dependence indicating activation energies from 1.0 to 1.4 eV. Unlike the distribution of defects left behind in similar number as a result of light soaking at room temperature, the distribution of defects resulting from fast cooling from higher temperature is shifted to higher energies and requires much longer anneal times.

Doping glow-discharge amorphous silicon by metal coevaporation

We report here on the further development of a new technique for doping plasma-deposited amorphous silicon by thermal evaporation of metal into the plasma from which the film is grown. We show that the de bias applied to the substrate has an important effect on the incorporation of the metal into the film, and on the doping efficiency. We also report on our efforts to monitor and control the evaporation by mass spectroscopy.

Bonding and electronic structures of amorphous SiNx:H

The bonding properties and annealing effects for glow-discharge amorphous silicon nitride (GD a-SiNx:H; x=0–0.33) have been investigated by means of x-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), and optical measurements. After annealing, a fraction of the N atoms were effused from the film. Shifts of the XPS spectra (Si2p and N1s lines) toward higher binding energy with x are observed. The opposite and large shifts are also observed in the AES spectra (SiLVV and NKLL lines). The peak binding energy EB(Si2p) of the Si2p is connected with a weighted average coordination number nav at a Si site in the SiNn (n=0,...,4) configurations, and the chemical shift per Si–N bond in the Si2p core level is 0.7 eV. For EB(Si2p) and the peak kinetic energy EK(SiLVV), a linear reduction in (EB+EK) with x is found. This result implies that the approach of the N atoms to the Si–Si bonds serves to shift the Si3p native bonding states downward. This model is consistent with the changes of the optical gap and slope B in the Tauc equation as a function of x. A reduction in EB(N1s)+EK(NKLL) with x is also found, indicating that the shifts of the N1s and NKLL are chemical shifts.

I n s i t u investigation of the growth of rf glow-discharge deposited amorphous germanium and silicon films

An in situ investigation of the growth of rf glow-discharge amorphous germanium (a-Ge:H) and silicon (a-Si:H) films using fast real-time spectroscopic phase-modulated elipsometry is presented. The influence of the conditions of preparation is studied in both cases. The same behavior is obtained in a-Ge:H and a-Si:H films deposited in similar conditions. In particular, the initial stage of the growth can be described by a nucleation process in both cases, whatever the conditions of preparation. The incomplete coalescence of the nuclei leads to the formation of a surface roughness on a ∼40-Å scale which is observed during film growth. In comparing real-time ellipsometry measurements performed at different wavelengths, a correlation between the internuclei distance and the thickness of the surface roughness is observed. An enhancement of the surface mobility of the reactive species due to an increase of substrate temperature and/or ion-bombardment energy results in an increase in the density of the nucleation centers. The influence of ion bombardment and gas pressure is discussed by comparing the growth of a-Si:H film deposited in a rf discharge to the results of previous studies using a low-pressure multipole plasma.

Frequency-dependent transport in glow-discharge amorphous silicon

Abstract The frequency-dependent conductivity of glow-discharge a-Si:H has been measured in the temperature range 90–350 K. The bulk and interfacial relaxations are separated by measurement. The interfacial relaxation may originate from a Schottky barrier and is interpreted in terms of a trap-released model. The origin of the bulk a.c. loss is discussed in terms of hydrogen-related two-level systems (TLS) and electron transfer between defect states. The bulk a.c. loss can best be explained by the model for tunnelling of atoms.

Surface photovoltage measurement of light instability of amorphous silicon films

The effect of prolonged light exposure on photovoltaic quality of glow discharge amorphous silicon films has been studied by the surface photovoltage technique. Two principal results have been observed. (i) The minority-carrier diffusion length degrades but stabilizes at 0.25–0.3 μm. (ii) The space-charge width collapses at open circuit voltage under as little as 0.0001 sun bias illumination. The combination of these two effects suggests that it is gross changes in the built-in electric fields that most seriously degrade the fill factor in amorphous silicon solar cells, rather than changes in μτ.

Intrinsic stress and hydrogen bonding in glow-discharge amorphous silicon films

We have found a strong correlation between intrinsic stress and hydrogen bonding in glow-discharge amorphous silicon (a-Si:H) films. The stress of high-deposition-rate (HDR) films continuously changes from tensile to compressive with increasing rf power. In contrast, low-deposition-rate (LDR) films indicated compressive stress in all the rf power range which was varied. However, adding a small amount of Ar to SiH4 in the LDR film deposition changes its stress from compressive to tensile as in the HDR films. IR absorption spectroscopies showed that films with compressive stress always indicated a smaller ratio of absorptions at 2070 and 2000 cm−1 α(2070)/α(2000) than the value Rc(=0.9–1.2), whereas those with tensile stress showed a larger ratio. Consequently, zero or very weak stress is reached at the ratio approximately equal to Rc, regardless of varying deposition parameters. It is also shown that annealing alters the stress of all the HDR films to strong tensile, and that α(2070)/α(2000) decreases, differing from the correlation where the rf power and Ar mixing ratio are varied.

The effects of linear strain on the electronic properties of glow discharge amorphous silicon

The conductivity of n- and p-type a-Si specimens has been investigated as a function of tensile and compressive strains. All changes were reversible. For a given type of strain a unidirectional movement of ϵ f takes place in both n- and p-type specimens. The dependence of the elasto-conductivity on ϵ c− ϵ f can be interpreted mainly in terms of changes in the conductivity pre-factor σ 0.