Amorphous Silicon Carbon Alloy Films Research Articles

Thermal modification of wide-bandgap hydrogenated amorphous silicon-carbon alloy films grown by plasma-enhanced chemical vapour deposition from C2H2+SiH4 mixtures

Abstract The thermal stability of hydrogenated amorphous silicon-carbon (a-Si1−xCx: H) alloy films grown by plasma-enhanced chemical vapour deposition from C2H2+SiH4 mixtures was characterized by means of infrared spectroscopy, electron spin resonance, transmittance-reflectance spectroscopy and photoluminescence (PL) spectroscopy. It is demonstrated that the network undergoes relaxation and reconstruction under the condition of low-temperature annealing. Weak C-C, Si-Si and C-Si bonds will be broken, and a new stage of hydrogen effusion and structural rearrangement will occur under the condition of high-temperature annealing. The thermal stability a-Si1−xCx:H of films increases with increase in carbon content. In carbon-poor a-Si1−xCx:H networks, the dangling bonds are the main non-radiative recombination channel, which causes a strong correlation of the PL signal with defect density. In carbon-rich a-Si1−xCx:H networks, π-bonded clusters play an important role in the luminescence process. The PL intensit...

Annealing study of the optical properties of hydrogenated amorphous silicon–carbon alloy film

Hydrogenated amorphous silicon–carbon alloy films were prepared by the glow discharge decomposition of silane and ethylene. The real and imaginary parts of the dielectric constants ϵ 1 and ϵ 2 of the film were determined at room temperature for the photon energies between 1.38 and 4.86 eV for the as-deposited ( T s=250 °C) as well as for subsequent anneals at different temperatures up to 1200 °C. The optical bandgap ( E opt) was determined from the Tauc's relation while the characteristic optical parameters such as the dispersion energy ( E d), average excitation energy ( E o), plasmon energy ( ℏ w p) and the valence electron density ( n v) were obtained from the Wemple–DiDomenico model. We observe an interesting correlation between E opt and n v which seems to indicate the coupling of the valence and conduction band through single-particle excitation. © 1997 Elsevier Science S.A. All rights reserved.

Characterization of Monolayer-Level Composition and Optical Gap Profiles in Amorphous Silicon-Carbon Alloy Bandgap-Modulated Structures

ABSTRACTOver the past few years we have applied real time spectroscopie ellipsometry (RTSE) to characterize the structural, compositional, and optical gap profiles in continuously-graded amorphous silicon-carbon alloy films (a-Si1-xCx:H). Most recently, we have extended the RTSE methods to their monolayer sensitivity and resolution limits. In this study, continuous triangular variations in the carbon content × (0.02≤x≤0.24) within ∼25 to 130 Å thick graded layers were introduced at the i/p interfaces of the n-i-p solar cell structures using continuous variations in the flow ratio z=[CH4]/{[SiH4]+[CH4]} during rf plasma-enhanced chemical vapor deposition (PECVD). A virtual interface approximation has been applied to interpret the RTSE data collected during the growth of the graded interface layers. This analysis yields C-content depth-profiles with monolayer-level resolution and a compositional uncertainty of ±0.004. Even compositional gradients in which x changes by >0.2 within a few monolayers’ thickness are readily characterized. Lastly, a continuous increase in open circuit voltage with increasing graded interface layer thickness, saturating at ΔVoc=0.1 V after 100 Å, is observed in the n-i-p solar cells with graded layers. These results demonstrate the importance of the RTSE analysis in assessing bandgap engineered device designs.

Real time spectroellipsometry characterization of optical gap profiles in compositionally-graded semiconductor structures: Applications to bandgap engineering in amorphous silicon-carbon alloy solar cells

We have applied a real time spectroellipsometry data analysis procedure developed previously [S. Kim and R. W. Collins, Appl. Phys. Lett. 67, 3010 (1995)] to characterize depth profiles in the optical gap for compositionally-graded semiconductor alloy thin films prepared by plasma enhanced chemical vapor deposition. The analysis procedure employs a two-layer (four-medium) optical model consisting of the ambient, a thin surface roughness layer and outer-layer (5–15 Å) whose properties are to be determined, and a pseudo-substrate that contains the past history of the graded-layer deposition. The ellipsometric spectra (2.3–4.0 eV) are analyzed to provide, not only the depth-profile of the optical gap and alloy composition for the graded layer, but also the instantaneous deposition rate and the surface roughness layer thickness versus time or accumulated layer thickness. To apply the previous analysis approach, it was necessary to (i) parameterize the dielectric function of the alloys as a continuous function of composition over the desired alloy range and (ii) express the optical gap as an accurate function of alloy composition. As an example, we have applied the extended analysis to obtain the depth-profile of the optical gap and alloy composition with <15 Å resolution for a hydrogenated amorphous silicon-carbon alloy (a-Si1−xCx:H) film prepared by continuously varying the gas flow ratio z=[CH4]/{[CH4]+[SiH4]}. In order to demonstrate the technological importance of such structures, the graded layer has been incorporated at the p/i interface of widegap a-Si1−xCx:H p-i-n solar cells, and improvements in open-circuit voltage have been observed.

Advances in the Characterization of Compositionally-graded Layers in Amorphous Semiconductor Solar Cells by Real Time Spectroellipsometry

AbstractWe have developed a real time spectroellipsometry data analysis procedure that allows us to characterize compositionally- graded amorphous semiconductor alloy thin films prepared by plasma-enhanced chemical vapor deposition (PECVD). As an example, we have applied the analysis to obtain the depth-profile of the optical gap and alloy composition with ≤15 Å resolution for a hydrogenated amorphous silicon-carbon alloy (a-Si1−xCx:H) film prepared by continuously varying the gas flow ratio z=[CH4]/{[CH4]+[SiH4]} in the PECVD process. The graded layer has been incorporated at the p/i interface of widegap a-Si1−xCx:H (x∼0.05) p-i-n solar cells, and consistent improvements in open-circuit voltage have been demonstrated. The importance of the graded-layer characterization is the ability to relate improvements in device performance directly to the physical properties of the interface layer, rather to the deposition parameters with which they were prepared.

Optical characterization of continuous compositional gradients in thin films by real time spectroscopic ellipsometry

An analysis procedure for real time spectroscopic ellipsometry data is described that allows one to characterize compositional gradients in thin films prepared by continuously varying the deposition parameters. This approach provides the time evolution of (i) the instantaneous deposition rate, (ii) the surface roughness layer thickness, and (iii) the near-surface dielectric function and composition of the film with a depth resolution as high as 7 Å. We apply the analysis to obtain the depth profile of the relative void volume fraction for an amorphous silicon-carbon alloy film prepared by plasma-enhanced chemical vapor deposition.

Growth of clean amorphous silicon–carbon alloy films by hot-filament assisted chemical vapor deposition technique

Amorphous silicon–carbon (a-SiC) alloy films were deposited by using the hot-filament chemical vapor deposition technique. The gas mixture containing different concentrations of silane in methane with additional dilution in hydrogen formed the source gas. Films were characterized for infrared and optical absorption and Raman scattering. The films deposited under ‘‘starving’’ hot-filament conditions exhibited properties characteristic of stoichiometric carbon-free a-SiC material.

Correlation of structure and optical properties of an annealed hydrogenated amorphous silicon-carbon alloy film

A hydrogenated amorphous silicon-carbon (a-Si 0.76C 0.24:H) film has been prepared via the glow discharge decomposition of SiH 4 and C 2H 4. The optical constants of this alloy film have been determined as a function of annealing temperature T a for photon energies between 1.5 and 4.75 eV. The refractive index n and its imaginary part k show small but significant variation with annealing temperature. The optical energy gap E opt also exhibits interesting variation with annealing temperature, decreasing with increasing annealing temperature up to T a = 650°C and then increasing above this temperature. Further, E opt is found to be correlated with the inverse of the full width at half-maximum of the SiC and the SiO IR stretch absorption mode, which seems to indicate that the changes in E opt are structural in origin and that phonon order correlates well with electronic order.

Improved quality a-SiC:H films prepared by photo chemical vapour decomposition of silane and acetylene

Abstract Hydrogenated amorphous and microcrystalline silicon carbon alloy films have been grown by photo-CVD using C2H2 as a source gas of carbon. The hydrogenated amorphous silicon carbon (a-SiC:H) film with a band gap of ~2.0 eV prepared at a very low hydrogen (LD) concentration exhibits better photo-electronic properties compared to that at high hydrogen dilution (HD) having a similar optical gap. Notwithstanding a high deposition rate, the high photosensitivity (∼ 106), the low density of the defect states (∼ 6 × 1016cm−3) and the Urbach energy parameter (72 meV) for the a-SiC:H film prepared at low hydrogen dilution and pressure are impressive. On the other hand, low pressure along with high hydrogen dilution have been found to be conducive to microcrystalline silicon carbon alloy (μc-Si:H) formation. Interestingly, crystallites are of silicon while carbon remains in the amorphous and grain boundary regions.

Structural defects in glow discharge a-Si:C:H investigated by positron annihilation spectroscopy

Glow discharge deposited hydrogenated amorphous silicon carbon alloy films with varying carbon content are studied for structural defects by positron lifetime spectroscopy. Measurements of optical properties like refractive index (n) and band gap (Eg) which indirectly reflect the varying carbon content in the films have also been made. The increased defect density in these films with increasing carbon content is very clearly indicated by the positron lifetime data. The nature of these defects and their relative concentrations are determined from the positron lifetime and intensity values. Interestingly a continuous transformation from one type of defect to the other with increasing carbon is indicated by these results.

Deposition of a-Si xC 1-x:H alloy films by tetrode RF sputtering

Hydrogenated amorphous silicon-carbon alloy (a-Si x C 1- x :H) film was deposited by a tetrode RF sputtering technique applying the plasma decomposition of propane C 3H 8 and H 2/Ar mixed gas. The carbon content 1 - x or the composition of C and Si in a-Si x C 1- x :H films was controlled by changing the flow rate ratio, R p, of C 3H 8 to mixed gas. The dependence of the properties of the deposited films on R p were studied using measurements of optical absorption in the wavelength range of 300–650 nm, infrared (IR) absorption spectroscopy, electron spin resonance (ESR) and X-ray photoelectron spectroscopy (XPS). The results showed that the composition of C and Si in a-Si x C 1- x :H films and the value of the optical energy band gap are controlled by R p adjustment during the film deposition.

Hydrogen-assisted surface reactions during the growth of sputtered a-SiC:H films

Hydrogenated amorphous silicon-carbon alloy films have been prepared by simultaneous RF reactive magnetron sputtering of silicon and graphite in an H2 + Ar gas mixture. Infrared and Raman measurements have been performed for samples prepared under different conditions. It is found that the intensity of the band related to silicon-carbon vibrations at approximately 760 cm-1 in the in spectra decreases with increasing quantity of hydrogen in the gas mixture. A model is used which describes the deposition as a three-stage process: sputtering of the target, transport of the sputtered material in the plasma and film growth processes on the substrate. Analysis of the experimental data allows one to conclude that for a carbon content in the films below 20% carbon is mainly incorporated as carbon-related clusters (CH3) and for a carbon content above 20% hydrogen-assisted desorption of CH3 and/or CH4 groups takes place on the substrate, which significantly lowers the carbon content in the films.

Electrical characteristics of metal/a-Si xC 1- x:H contacts

The current-voltage ( J- V) characteristics of metal contacts on the surface of hydrogenated amorphous silicon-carbon alloy (a-Si x C 1- x :H) films were studied through a comparison with the optical properties of the films. The a-Si x C 1- x :H films were deposited by a tetrode RF sputtering technique applying the plasma decomposition of C 3H 3 (100%) and H 2/Ar(H 2 20%) mixed gas. The partial pressure ratio of C 3H 8 to H 2/Ar, R p, in the film deposition chamber was chosen as a variable factor for the film deposition. The J- V characteristics of metal/a-Si x C 1- x :H contacts were measured using a metal/a-Si x C 1- x :H/n-c- Si/Al diode. Al was used as contact metal on a-Si x C 1- x :H surfaces. The measurement results of the J- V characteristics of the diodes showed that the characteristics are concerned with the optical energy gap E GO and the Fermi level E F which depend on R p.

Structure of hydrogenated amorphous silicon-carbon alloys as investigated by extended x-ray-absorption fine structure.

We have studied the local structure of hydrogenated amorphous silicon-carbon alloy films, a-${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{C}}_{\mathit{x}}$:H, by measuring the extended x-ray-absorption fine structure at the Si K edge. We find that first-coordination-shell average bond lengths are 2.35 A\r{} for Si-Si and 1.86 A\r{} for Si-C and are constant with concentration to within \ifmmode\pm\else\textpm\fi{}0.015 A\r{}. By comparing the composition of the first coordination shell around Si with the average concentration, we show that the alloy tends to be chemically ordered, in that heteroatomic bonds are preferred.

Improvement of photoconductivity of a-SiC:H films by introducing nitrogen during magnetron sputtering process

Nitrogen has been introduced in the preparation of hydrogenated amorphous silicon carbon alloy films by magnetron sputtering of silicon in a methane-argon gas mixture. The partial pressure of nitrogen was changed, and its effects on the structural, optical, electrical, and optoelectronic properties of the deposited films were investigated. The SiN bond becomes predominant with increasing partial pressure of nitrogen pN above 10%, at which the optical band gap is rapidly increased. The concentration of silicon and carbon in the films remained almost unchanged by changing pN up to 20%. It was found that the photoconductivity is also almost unchanged up to 20%, while the optical band gap is increased. This indicates that the photoconductivity is improved over a wider optical band gap, as compared with previous studies when the optical band gap was changed by changing deposition conditions but without introducing nitrogen.

The Structure of Hydrogenated Amorphous Silicon-Carbon Alloys as Investigated by Exafs

ABSTRACTWe have studied the local structure of hydrogenated amorphous silicon-carbon alloy films, a-Si1−xCx:H, by measuring the extended x-ray absorption fine-structure (EXAFS) at the Si K edge.We find that first coordination shell average bond lengths are 2.35 Å for Si-Si and 1.86 Å for Si-C and are constant with concentration to within × 0.015 Å.By comparing the composition of the first coordination shell around Si with the average concentration we show that the alloy tends to be chemically ordered, in that heteroatomic bonds are preferred.

Doping effects of aluminium on the properties of hydrogenated amorphous silicon–carbon alloy films prepared by magnetron sputtering

Abstract Aluminium has been introduced into hydrogenated amorphous silicon-carbon alloy films prepared by reactive magnetron sputtering, using composite targets of silicon and aluminium in a methane-argon gas mixture. The doping effects of aluminium on the structural, optical, electrical and optoelectronic properties of the films were investigated. The ratio of silicon to carbon depended on the fractional aluminium content z The concentration of hydrogen bonded to silicon or carbon tended to decrease in the large-z range. These results are interpreted in terms of a gradual change in deposition process. From a thermoelectric power experiment, the type of conduction was found to change from n to p in the range of z< 1 × 10−2. Around z=1 × 10−2, the activation energy of the dark conductivity changed without changing the optical bandgap. These results indicate that doping is effective in this range of z In the large-z range, the films exhibit alloying behaviour as shown by a decrease in the optical bandgap, an increase in the dark conductivity and a quenching of photoconductivity effect.

Thermal Equilibration in Boron-Doped Hydrogenated Amorphous Silicon-Carbon Alloy Films

Cet equilibre est etabli par le mouvement des liaisons H. Ces films, a teneur en H variable, sont prepares en changeant la temperature de depot. On etudie la correlation entre la temperature d'equilibre et la teneur en H

The effect of carbon on the density of states of hydrogenated amorphous silicon-carbon alloy films prepared from silane and propane

The compositional dependence of the density of states near the Fermi level of hydrogenated amorphous silicon-carbon alloys from silane-propane mixtures was examined. Optical and electrical measurements reveal two distinct regions of behaviour: a low propane concentration regime and a high propane concentration regime. We propose that at low propane concentration there is only a small carbon contribution to the formation of the silicon-based lattice and at high propane concentration a new structure is formed.

Phosphorus Doping and Photoinduced Changes in Hydrogenated Amorphous Silicon-Carbon Alloy Films

Hydrogenated amorphous silicon carbon alloy films have been prepared by the RF glow discharge technique. The effect of phosphorus doping on the optoelectronic properties has been studied at different silane/methane flow compositions. The reduction of doping efficiency in wider-band-gap material has been attributed to increasing disorder with carbon incorporation. The spectral photoresponse data have been analysed to show that the effect is consistent with current models for doping in tetrahedral amorphous semiconductors. The light-induced changes in the properties have been explained in terms of the relative effects of phosphorus and/or carbon incorporation.