A critical review of the available experimental data obtained under conditions relevant for the deposition of high quality material shows that: 1) The dominant channel of the deposition of microcrystalline silicon (“μc-Si”) is the fragmentation of SiH4 into SiH2 and H2 followed by fast insertion of SiH2 into the surface of the growing film. The rate determining step is the ion induced dehydrogenation of the surface of the growing film. 2) The SiH2 formation followed by fast insertion into another SiH4 forming Si2H6 is the dominant channel of silane decomposition during the deposition of a-Si. Adsorption of disilane at the surface of the growing film followed by its decomposition can explain the experimental data. The published data show that the contribution of the SiH3 radical to the commonly observed growth rates of ≥ 0.1nm/s is negligible.

The plasma parameters which control the deposition of a-Si and c-Si (μc-Si and epi-Si) are identified on the basis of experimental results, and their effect on the crystalline-to-amorphous transition is explained. Ion bombardment at energies below the displacement threshold has beneficial effect on the properties of the deposit as illustrated by electrical conductivity, gas incorporation, optical absorption and light scattering data.

Real time ellipsometry results are presented which have been useful in characterizing nucleation, interface formation, and surface modification for hydrogenated amorphous silicon (a-Si:H) thin films and device structures. Earlier studies of the effect of deposition procedure on a-Si:H nucleation are reviewed, and new data on plasma-enhanced CVD p-type a-Si:H are discussed. Changes in the initial microstructural coalescence are observed when gas phase concentrations of 0.2% B2H6 are added to SiH4 to obtain the p-type films. When depositing p-type a-Si:H on undoped a-Si:H at 180°C and 250°C. ellipsometry allows detection of film formation at the rate of a few monolayers/minute by CVD (in the absence of plasma excitation) from the doping gas mixture. CVD requires both SiH4 and B2H6 in the gas. Other interesting aspects of this process are presented. Finally, in situ ellipsometry is applied with H2 plasmas and ion beams to study near-surface Si-Si bond breaking and etching. A common effect of many of these treatments is a reduction in the density of Si-Si bonds in the film bulk by 6–8%.

Measurements of the depletions of the feedstock gases in monosilane-germane and disilane-germane rf discharges indicate that the dissociation rates of germane and monosilane differ by a factor of 2 whereas the dissociation rates of germane and disilane differ by a factor of 0.8. As a result, larger germane concentrations are required in disilane-germane than in monosilane-germane discharges in order to achieve equal germanium content in the resulting films.

We have used Remote Plasma CVD with plasma excited He to grow films of hydrogenated amorphous silicon, a-Si:H, with substrate temperatures, Ts, between 38 and 400°C. These films differ from glow discharge (GD) and sputtered films, most notably in the Ts dependence of the hydrogen bonding environments (SiH, SiH2, etc.) and the photoconductivity. For RPECVD of a-Si:H, we have investigated: 1) the effect of replacing He in the plasma with Ar; 2) the silane mass spectrum and film growth rate as a function of rf power supplied to the plasma; and 3) the helium emission lines and hydrogen concentration in the films as a function of dilution of the He plasma with oxygen and nitrogen. Based on these observations we present a model for the deposition chemistry that includes SiH3 radicals as precursors to film growth.

A catalytic chemical vapor deposition (CTL-CVD) method, by which amorphous films are deposited at low temperatures only by using the catalytic or pyrolytic reactions but not by using plasma, is applied to produce amorphous silicon (a-Si) and a-Si alloys such as amorphous silicon germanium (a-SiGe), silicon carbide (a-Sic) and silicon nitride (a-SiNx). It is found that the CTL-CVD method is useful not only to produce a-Si but also to obtain high quality a-Si alloys. For instance, the photoconductivity of CTL-CVD a-SiGe of the optical band gap of 1.40 to 1.45 eV is 10−5 to 10−4 (Scm−1 ) for AM-1 of 100 mW/cm2 , keeping the photosensitivity over 104.

The transition from amorphous to crystalline silicon in films prepared in a hydrogen-diluted silane plasma has been studied. Emphasis was placed on the role played by hydrogen during film growth. Hydrogen is found to govern the film formation process by promoting the reverse process, ‘etching’. By preferentially eliminating energetically unfavorable configurations during the film growth, hydrogen controls the atomic structure, hydrogen incorporation, and grain growth of the film. It affects the total hydrogen content in the film, as well as the way hydrogen is bonded to the silicon. Excessive hydrogen dilution, however, reduces the grain size by changing the columnar grain growth to a more spherical-like grain growth, and eventually eliminates film growth. With appropriate hydrogen dilution, plasma deposition was found to yield ‘polycrystalline’ silicon films which has a large distribution of grain sizes, with the largest grains being about 2000 Å.

In the fabrication of a-Si photoreceptor for electrophotography (Fig.1), it is important to understand physical and chemical reactions in glow-discharge plasma [1] in order to improve film quality and to achieve high deposition rate [2] and uniformity of the film, because of its thickness of about 30 μm and large area.

High rate deposition of a-Si-layers is an important condition for an economical production of a-Si devices. We have obtained deposition rates of up to 30 μm/h by reactive DC magnetron sputtering from n-type and p-type doped monocrystalline silicon targets. The electrical properties of the films (dark conductivity, photoconductivity, activation energy) were examined as a function of the hydrogen partial pressure for the deposition rate of about 10 μm/h. The film structure is investigated by scanning electron microscope (surface and fracture) and hydrogen evolution method. The high rate deposited films show a homogeneous appearance exhibiting voids. The electrophotographic properties were examined for films on flat substrates. The charge acceptance for positive and negative charging is about 35V/pm with a low dark decay for films of the structure Al/SiOx/a-Si/SiOx. ESCA-measurements show an a-SiOx-layer of 2nm on the top. The spectral sensivitity is examined.

The deposition chemistry, optical properties, and structure of plasma-deposited fluorinated silicon nitride films are studied and compared to available results of other researchers. Fluorinated films show an increase in deposition rate and optical band gap, and a decrease in refractive index and film stress compared to unfluorinated silicon nitride films. Fluorine concentration in the films ranges from 5 to 25 atomic percent. The total hydrogen concentration of fluorinated films is lower than that of unfluorinated films. The improved thermal stability of fluorinated films as compared to unfluorinated films is explained by a significant decrease in hydrogen bonded to silicon. All films hydrolyze to some extent and films with large fluorine concentrations hydrolyze rapidly to silicon dioxide. The microstructure of fluorinated films consists of clustered silicon-fluorine and nitrogen-hydrogen regions within an amorphous silicon-nitrogen matrix.

Highly photosensitive a-SiC:H films exhibiting ημτ-product more than 10−6 cm2/V at the band gap of 1.9 eV are prepared under the high deposition rate around 6μm/h by a plasma decomposition of SiH4-C2H2 source mixture gas at higher rf power. It is also shown that the SiH4-C2H2 gas system has a feature of the carbon incorporation into the films with the content nearly equal to that in the source mixture gas and hence is suitable for the high rate deposition process. This advantage and in addition the requirement of the higher rf power for obtaining the films with good electrical performance are discussed in terms of the dissociation reaction of the source gas and the secondary gas phase reactions in the plasma.

In comparison with the Si:H films the conformational changes in the Si:H(F) networks of the films prepared by HRCVD and Spontaneous Chemical Deposition were investigated systematically in terms of the effects of the substrate temperature and the promoters. Fluorine participation in the chemical processes of the film growth favored the reduction of terminators without degradation of the Si network and promoted the propagation of closely packed Si network, leading to the crystal growth at a low temperature.

New results are reported on the effect of hydrogen dilution on the incorporation of hydrogen in undoped, unalloyed silicon films that were deposited by plasma-enhanced chemical vapor deposition in an rf-glow discharge system. A crystalline phase was detected for dilution ratios (H2: SiH 4) of 20:1 and greater. This phase change was accompanied by the appearance of higher-order silicon-hydride complexes. It was further observed that the H concentration in the films increased with dilution for ratios of 10:1 and greater. These results are compared with other recent studies of hydrogen dilution.

Extended x-ray absorption fine structure (EXAFS) measurements were made on a-Ge and a-Ge:H films which were prepared by sputtering and were annealed at temperatures between 300 and 450° C.The first nearest neighbor distance and the Debye-valler factor were investigated. These values decrease rapidly on the crystallization of the samples. The relationship between them was analyzed in respect of tvo-body potential. The potential of amorphous state is considered to be affected by unharmonic term.

The atmospheric pressure thermal decomposition of disilane to produce a Si:H films was studied between 450 and 520° C in a laminar flow reactor. The structural properties of the films exhibited a strong dependence not only on substrate temperature but also on reactor flow position. Films deposited at higher substrate temperatures had less dihydride bonding in the films while the monohydride bonding remained relatively constant. Despite a decrease in the total hydrogen content of the films as the deposition temperature increased from 450 to 500° C, the AM1 photoconductivity increased over an order of magnitude and the hole diffusion length of the films doubled. In addition to the changes in structural and electrical properties as a function of substrate temperature, variations in these properties were also observed as a function of flow position. At early flow positions the amount of dihydride bonding in the films was small and the AM1 photoconductivity and hole diffusion lengths of the films increased. The flow dependence of the film properties is explained by changes in the film precursors due to the evolution of the gas phase chemistry.

Hydrogen NMR data on three microcrystalline silicon samples grown by plasma decomposition using different H2/SiH4 gas ratios are examined. A decrease of the total atomic percent hydrogen content from 12 at.% to 6 at.% is seen as the gas ratio is reduced from 98/1 to 20/1. 20 to 40% of this total hydrogen content is determined to be non-bonded molecular hydrogen. In addition, evidence for SiH2 units is seen in the spectrum.

Triple-quadrupole mass spectrometry (TQMS) showed disilane and aminosilanes, SiH4-n(NH2)n, to be the principal products of an ammonia-silane discharge. Disilane can be completely eliminated from the plasma by operating at high power and high NH3/SiH4 ratio, which results in almost complete conversion of silane to aminosilanes by reaction with activated ammonia. Films so grown had no detectable Si-H bonding and greatly reduced ESR spin density in the dark. The triaminosilane radical appears to be a key deposition precursor, progressively decomposing and condensing towards Si3N4 with increasing substrate temperature.

The role of deposition condition on content and bond configurations of a–Si:H/μc–Si:H and a–Si:C:H doped films were investigated through IR spectra and correlated with transport properties. The microstructure, morphology, chemical composition and electro-optical properties were inferred from normal X–ray power diffraction, scanning electron microscopy (SEM), Rutherford Back Scattering (RBS), dark conductivity, visible and IR measurements.

The films under investigation were prepared in a Two Consecutive Decomposition and Deposition Chamber (TCDDC[1]) system under various deposition conditions such as: power density, dp; H2 partial pressure; substrate temperature, Ts; electromagnetic static fields (ξG and IB) with or without U.V. light assisting the process.Transition of a–Si:H to μc-Si:H (doped) films is accomplished by structure variation on bond configurations, hydrogen contents, CH, and transport properties. When oxygen is present during the deposition, films deposited with U.V. light assisting the process have O2 incorporated as Si=O while at high dp levels (without U.V. light) O2 appears in the matrix as Si–O bonds. Such behaviour is explained by changes on bond configurations and on the way how hydrogen is attached in the network.

A novel method of the preparation of a-Si:H films by employing VHF(144MHz band) plasma is described. Optical emission spectroscopy is employed as a diagnostic tool in the plasma. Due to high generation efficiency of radicals, a-Si:H films are deposited at high deposition rates at low power density without sacrificing electrical properties. Criteria for the deposition of microcrystalline Si films are also discussed.

The Urbach energy E0 of undoped, Boron-, Nitrogen- and Phosphorus-doped a-Si:H deposited at 320°C from hydrogen-diluted silane mixtures has been measured by PDS. Its variations with impurity concentrations were related to those of the optical gap, of the defect density and of the width of the Raman TO peak. Taking into account the change in Hydrogen bonding in these materials, the physicochemical nature of the Valence Band Tail (VBT) states is discussed. A formal correlation between the dispersion of the Si-Si bond angle distribution and the width of the VBT is proposed within the Keating formalism.

The recombination rate dependence of the high temperature saturated dangling bond density is explored. Solar cell RBA (reverse bias anneal) and FBA (forward bias anneal) are studied as a function of temperature and bias voltage. The defects that are responsible for the SWE (Staebler-Wronski effect)[1] are shown also to be related to the RBA and FBA effects. At annealing temperatures greater than 150°C, forward bias increases the density of dangling bonds in the i-layer, while reverse bias decreases the density. The temperature dependence of FBA and RBA are investigated. The density of dangling bonds was found not to be a function of cooling rate when bias is maintained during cooling. The dangling bond density is proportional to the square root of the recombination rate, rather than the linear relationship predicted by simple kinetic models. The results are more consistent with a equilibrium description of the SWE.