Hydrogenated amorphous silicon-carbon alloys are prepared using electron-cyclotron resonance (ECR) plasma-enhanced chemical-vapor deposition. Hydrogen is used as the excitation gas in the resonance chamber while silane and methane (or ethylene) are introduced in the main chamber. A minimum of 95% hydrogen dilution is used. The microwave power is kept constant at 150 W. The effect of the type of carbon source gas, silane to carbon source gas ratio, deposition pressure, substrate temperature and hydrogen dilution on the deposition rate, bandgap and Urbach energy are studied. The photoconductivity and the Urbach energy of the ECR-deposited films are compared to those prepared with glow discharge with the same bandgap.

We present the results of studies on the light-induced stability of undoped layered hydrogenated amorphous silicon films grown with alternating substrate temperature between optimal and non optimal temperatures for device-quality films. Compared to the single layer films grown at optimal substrate temperature, the layered films show improved stability in the lightinduced state. Under intense light illumination of 3 W/cm2, the steady-state defect density of the layered film reached a saturation of 2×1016 cm−3, while the single layer film saturates at about 6×1016 cm−3. It is found that in the completely degraded state the photoconductivity in the layered film is also improved by a factor of two compared to the single layer film.

Hydrogen or helium dilution of silane has been used to improve charge transport characteristics or deposition rate for thick amorphous silicon layers. In both cases, mechanical instability, such as peeling-off, due to high strain energy stored in the a-Si:H film is one of the major concerns. Growing the a-Si:H film at temperatures below 150°C brought the residual stress in the film to a level low enough to prevent delamination. The defect density increased at the same time, but it could be recovered by annealing at 160°C for 100 hours without affecting the stress state. In the hydrogen-diluted material, the deposition condition for optimum mobility and defect density values corresponded to the onset of the microcrystalline formation, and this relationship was confirmed by Transmission Electron Microscopy (TEM). Existence of a small amount of an ordered phase in the helium-diluted material, which was hardly observed by the TEM, was revealed by IR absorption spectra and supported by the change in ratio of photo-todark conductivity compared to that of undiluted material.

Plasma enhanced chemical vapor deposition (PECVD) of a-Si:H with silane or silane and hydrogen at temperatures lower than 200°C commonly results in films with significant dihydride bonding and a high defect density. In this paper, we demonstrate the formation of monohydride dominant a-Si:H films using rf parallel plate PECVD at 35°C at deposition rates greater than 100 Å/min. In the as-deposited state, these films have low dark conductivity (∼10−9 S/cm) and low photoconductivity. Annealing the films at 150°C caused the monohydride dominant films to show photo to dark conductivity ratio near 105. Our results also indicate that an increase in monohydride fraction is not linked with a decrease in deposition rate.

Hydrogenated silicon suboxide and subnitride films were deposited by a remote plasma enhanced chemical-vapor deposition (RPECVD) process. Rapid thermal annealing (RTA) of these alloys eliminated all of the bonded hydrogen and formed a two phase system. The microstructure of the annealed films showed silicon crystallites (c-Si) surrounded by amorphous layers. The amorphous layers were identified as SiO2 and Si3N4 in the annealed suboxides and subnitrides, respectively. The as-deposited films showed band edge photoluminescence (PL); however, no PL was observed from the annealed films. This indicates that there was no significant suboxide or subnitride bonding in the amorphous layers at the metallurgically-sharp c-Si-SiO2 and c-Si-Si3N4 interfaces, respectively.

Direct nanoscale observation on the nucleation and growth of hydrogenated amorphous and microcrystalline silicon on graphite substrates was made using scanning tunneling microscopy, atomic force microscopy, and Raman scattering spectroscopy. Nucleation of hydrogenated silicon clusters is initiated through the nucleation sites created by reactive hydrogen species coming from the source gas plasma. The difference in spatial distribution of nucleated clusters at the initial stage of deposition between a-Si:H and μc-Si:H is ascribed to the difference in the number density of nucleation sites which results in difference in the diffusion length of a SiH3 radical at the initial stage of deposition on the graphite substrate. The RMS roughness of μc-Si:H films is larger than that of a-Si:H when the film thickness is larger than 10 Å, which is opposite to the behavior at the initial nucleation stage on the graphite substrate.

In situ ellipsometric studies have been performed during the nucleation and growth of hydrogenated amorphous silicon (a-Si:H) films prepared by the hot wire chemical vapour deposition (HWCVD) method in order to understand the growth mechanism of these films. For a comparison with films deposited by plasma enhanced chemical vapour deposition (PECVD), the hot wire deposition was carried out under similar conditions and reactor geometry as for the PECVD process. It is observed from the kinetic ellipsometry measurements that low filament temperature (TFil) and low gas pressure favour the growth of more dense films, but at lower deposition rates. Moreover, for a given set of conditions an increase in substrate temperature (Ts) leads to a higher final value of the film density with a different growth behaviour in the initial stage. Thus, the filament temperature in the hot wire method seems to have a similar effect on the film density as the rf power has in the PECVD process, which has been observed earlier. Film density and surface roughness obtained from spectroscopic ellipsometry using a tetrahedron model which takes into account the effect of hydrogen on the dielectric function, is used to get information about the film microstructure. A correlation between this microstructure, the growth behaviour and the electronic properties as the defect density or the ambipolar diffusion length in the films is also reported.

Hydrogenated amorphous silicon (a-Si:H) with a gaps narrower than 1.7 eV were made by repeating the deposition of a thin layer (1–3 nm thick) and the treatment of growing surface with a mixture of H and Ar*. Crystallization induced by permeation of hydrogen into the subsurface at high substrate temperature (>200C) was efficiently prevented by treating with a mixture of H and Ar*. The activation of growing surface may arise from releasing a part of hydrogen on surface by treating with Ar*. High quality a-Si:H films containing hydrogen of 3 atom % with a gap of 1.6 eV were made by chemical annealing with a mixture of H and Ar*.

A novel plasma based chemical vapour deposition (PECVD) technique employing electron cyclotron wave resonance (ECWR) for plasma excitation was applied to the deposition of hydrogenated nanocrystalline silicon (nc-Si:H) films. nc-Si:H films were produced at deposition rates up to 8Å/sec (TS = 200°C) with a pure SiH4 plasma in contrast to the conventional glow discharge technique where the high hydrogen dilution usually needed leads to considerable lower deposition rates. The amorphous-to-nanocrystalline phase transition was investigated in dependence of substrate temperature, the hf-power and magnetic field mandatory for ECWR, and SiH4-flow into the plasma. With the knowledge of the plasma parameters derived from single probe measurements, and the intensities of excited plasma species detected by means of optical emission spectroscopy we can qualitatively describe the silane-plasma dissociation behaviour. The nanocrystalline phase is found to be always deposited when the dissociation degree of the SiH4 plasma is almost saturated.

Hydrogenated amorphous silicon (a-Si:H) samples containing 17O (3 x 1019 cm-3, nuclear spin, I, = 5/2) and 13C (3 - 4 x 1017cm-3, I = 1/2) isotope impurities were used in the detailed pulsed electron spin resonance (ESR) and pulsed electron-nuclear double resonance (ENDOR) measurements to obtain information on the role of O and C impurities in lightinduced metastable dangling bonds in a-Si:H. No hyperfine structure related to 17O and 13C atoms was observed in echo detected ESR, electron spin echo envelope modulation (ESEEM) of pulsed ESR as well as pulsed ENDOR (Davies and Mims sequences), which suggests that O and C impurities are not directly involved in the formation of light-induced dangling bonds.

We 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.

The deposition of a-Si:H films on crystalline silicon substrates was monitored in situ by transient photoconductivity measurements in the microwave frequency range. At the start of the deposition a drastic increase of the interface recombination velocity was observed, followed by a rapid decrease. The implications of these results for the structure of the interface are discussed. Changes of the interface after deposition were detected without a change of the temperature, even at 250'C: The long relaxation time of the structure of the interface will be discussed. Ex situ results on the samples produced will be compared to the in situ results in view of the passivation properties of a-Si:H films on c-Si substrates.

High quality a-Si:H films were deposited by pulsed VUV (157nm) laser CVD, allowing digital control of the deposition process. Nucleation and growth on native oxide-covered Si (100) and on H-terminated Si (111) surfaces were studied in situ by FTIR transmission spectroscopy with sub-monolayer sensitivity. The film thickness was monitored simultaneously using a quartz crystal microbalance (QCM) with comparable resolution. The in situ spectra reveal that the nature of the substrate surface significantly influences the hydrogen bonding configuration in the interface region. In both cases the assumed cluster growth during the initial stage is characterized by a band around 2100cm−1, which is assigned to SiHX (x = 1 – 3) surface modes. This band broadens until the clusters coalesce and band saturation occurs. At this time a second band starts to grow at 2000cm−1, which is attributed to bulk SiH bonds. Difference spectra calculated for layers at different depths with definite thickness and the deconvolution of the broad feature observed during the very first stage of film growth indicate the existence of a hydrogen-rich layer at the substrate surface whose composition and thickness depend on the properties of the substrate surface.

For the first time ion energy distributions (IED) of different ions from silane-hydrogen (SiH4-H2 ) RF plasmas are presented, i.e. the distributions of SiH3+, SiH2+ and SiH2+. The energy distributions of SiH3+ and SiH3+ ions show peaks, which are caused by a charge exchange process in the sheath. A method is presented by which the net charge density in the sheath is determined from the plasma potential and the energy positions of the charge exchange peaks. Knowing the net charge density in the sheath and the plasma potential, the sheath thickness can be determined and an estimation of the absolute ion fluxes can be made. The flux of ions can, at maximum, account for 10% of the observed deposition rate.

The intermediate layers (IL) exist at the boundary between liquid (plasma, gas) and solid phases during production processes of a-Si:H. The IL properties differ from the properties of the boundary regions anomaly. The substance in IL is in a strong nonequilibrium state of the bifurcation or the cascade of bifurcations. The processes in the IL determine the properties and the structure of the material being produced.

Proton and deuteron NMR in hydrogenated amorphous silicon yield quantitative measures of species-specific structural configurations and their dynamics. Populations of silicon-bonded and molecular hydrogens correlate with photovoltaic quality, doping, illumination/dark anneal sequences, and with infrared and other characterizations. High quality films contain substantial populations of nanovoid-trapped molecular hydrogen.

In this work the microstructures of 2–3 hydrogen at.% hot-wire CVD a-Si:H films were characterized by 1H nuclear magnetic resonance (NMR). Significant differences were found between the hydrogen distribution in these samples and that in conventional plasma-enhanced CVD samples. Among other things, the broad resonance line in the hot-wire a-Si:H is 50 kHz wide, which is much broader than that observed 25–35 kHz in PECVD a-Si:H films. Moreover, a 0.5 kHz resonance absorption hole width due to intrinsic dipolar interactions is obtained using the hole-burning technique. Surprisingly, approximately 90 percent of the hydrogen atoms give rise to the 50 kHz line and only a very small percentage of the hydrogen atoms give rise to the much narrower resonance line.

Six samples of pc-Si:H were prepared by PECVD at 200°C with plasma excitation frequencies ranging from 13 MHz to 95 MHz. Four samples were prepared with gas mixtures of 3% SiH 4 in H2. For these samples the plasma excitation frequencies ranged from 13 MHz to 95 MHz. Two samples at a plasma excitation frequency of 95 MHz were prepared at 5% and 8% SiH4 in H2. All samples were deposited on aluminum foil and etched off using dilute HCI to form powdered samples. These samples were studied by nuclear magnetic resonance (NMR), electron spin resonance (ESR) and infrared (IR) absorption measurements. The broad and narrow components of the free induction decay (FID) in the NMR measurements were compared to the respective components that occur in a-Si:H. The broad components in the various samples of μc-Si:H are similar in width to the broad component in a-Si:H, but the narrow component is narrower by a factor of two as compared to the narrow components in μ-Si:H. The narrow component in pic-Si:H samples exhibits a beat pattern similar to a previously observed Pake doublet. ESR measurements performed between 20 K and 300 K show that the spin densities, which can be attributed to silicon dangling bond states, increase as a function of plasma excitation frequency. The hydrogen content of each μc-Si:H sample was estimated from the NMR measurements, and these estimates are in good arrangement with the hydrogen content estimated from IR measurements.

Incorporation and kinetics of hydrogen during plasma post-hydrogenation and thermal treatment are discussed for a-Si:H and a-Ge:H films. For material of low hydrogen content, the hydrogen surface concentration reached by plasma treatment equals the hydrogen concentration obtained by deposition at the same temperature and under similar plasma conditions. Enhancements of the hydrogen diffusion coefficient and of hydrogen solubility observed for plasma treatment at temperatures ≤400°C and ≤300°C for a-Si:H and a-Ge:H, respectively, are attributed to a plasma induced rise of the surface hydrogen chemical potential.

Increases in the frequency of the silicon monohydride (Si-H) bond-stretching vibration in hydrogenated silicon nitrides, over and above what can be attributed to chemical induction effects, are shown to be generated by a repulsive interaction between the H-atom of the Si-H group, and a near-neighbor SiN-H group. This interaction increases the effective bond-stretching force constant of the H-atom of the Si-H group. This effect has been verified by ab-initio quantum chemistry calculations that have been used to calculate the energy of the H-atom of the Si-H group as a function of the relative separation between near-neighbor Si-H and SiN-H groups. Decreases in the bond-stretching frequencies of SiN-H groups in heavily hydrogenated nitrides are attributed to H-bonding effects associated with near-neighbor pairs of SiN-H groups.