Glow discharge deposition of hydrogenated amorphous silicon films involves; (A) the electron collisions which produce the reactive species, (B) the gas reactions these species undergo while diffusing or drifting to the surfaces, and (C) the surface reactions involved in film growth and gas processing. I will first describe our knowledge of the electron and gas reactions in these discharges, then of the surface reactions, and finally I will offer some conjectures regarding the influence of these different surface reactions and bombardments upon film properties.

Films of a-Si:H were deposited by dual ion beam sputtering using a new configuration in which both the argon and hydrogen beam sources are directed at the silicon target. This geometry also permits independent control of the hydrogen and argon energy and particle flux. Infrared absorption mealurents show that even for high hydrogen concentrations, the 2000 cm-1 Si-H stretching band is dominant. This result is in contrast with the more conventional configuration in which the H soyrce is directed at the substrate, resulting in films with dominant 2100 cm-1 mode. This suggests that the precursors resulting in H-incorporation are different for the two configurations. In fact, IR reflectance and SIMS analysis of the silicon sputtering target reveal hydrogen is incorporated, peaking at about 30 Å below the target surface. A strong increase in the photo and dark dc conductivity occurs as the hydrogen ion enery is reduced below 30 eV, suggesting the importance of preventing high energy back-scattered H ion bombardment of thS film. At a H ion energy of 8eV, the values are 2x10-5 (AM1) and 2x10-9 (ohm-cm-1), respectively. Spectroscopic ellipsometry measurements of films reveal a Si-Si bond packing greater than that of low Hcontent a-Si prepared by LPCVD even up to H contents as high as 24%. Above 25% a microstructural transition is observed, verified by SEM, resulting in an increase in the density of voids, (which appears to be responsible for a sudden drop in the hydrogen-induced compressive stress) and accompanied by a shift in the dominant stretching mode energy.

CO2 laser irradiation onto growing a-Si:H surface caused a decrease of the optical bandgap down to 1.63 eV and a significant increase of the photosensivity. Primary effect of the laser irradiation was a conventional substrate heating, while the optical bandgap narrowing could not be explained by heating effect alone.

Hydrogenated amorphous silicon (a-Si:H) thin films have been deposited from silane using a novel photo-enhanced decomposition technique. The system comprises a hydrogen discharge lamp contained within the reaction vessel; this unified approach allows high energy photon excitation of the silane molecules without absorption by window materials or the need for mercury sensitisation. The film growth rates (exceeding 4 Å/s) and material properties obtained are comparable to those of films produced by plasma-enhanced CVD techniques. The reduction of energetic charged particles in the film growth region should enable the fabrication of cleaner semiconductor/insulator interfaces in thin-film transistors.

Chemical vapor deposition techniques, in particular plasma enhanced CVD, have been used to produce high quality a-Si:H materials. Continuing research is directed toward increased device performance, improved stability, and translation of scale to commercial production. A part of this effort is the evaluation of alternate CVD techniques which in addition to providing technical options for high efficiency and long term stability are likely to lead to improved understanding of the relationships between deposition processes and material properties. A relatively new technique for depositing a-Si:H is photo-CVD which utilizes ultraviolet light to initiate the decomposition of silane or disilane. The best results from both materials properties and device efficiency points of view have been achieved using mercury sensitized photo-CVD. Recently, a 10.5% efficient a-Si:H p-i-n photovoltaic cell, fabricated by photo-CVD, was reported [1]. A limitation in photo-CVD has been preventing deposition on the UV transparent window. In this paper we describe a new photo-CVD reactor with a moveable UV-transparent Teflon film and secondary gas flows to eliminate window fouling. The deposition and opto-electronic characterization of intrinsic a-Si:H and a-SiGe:H and p-type a-SiC:H are described. Finally, preliminary results of p-i-n solar cells are presented.

Hydrogenated amorphous silicon may be deposited at relatively low temperatures, where the density of defects may be expected to be low, by the chemical vapor deposition (CVD) of higher silanes. This method is an attractive alternative to plasma deposition techniques. We describe here the preparation of a-Si:H and related alloys incorporating carbon, germanium, and fluorine. a-Si:H films were deposited on heated substrates in the range 365°C-445°C by CVD of Si2H6 and Si3H8. The optical gap (Eg) ranged from 1.4 to 1.7 eV and the properties of films deposited from either Si2 H6 or Si3 H8 were quite similar. Wide band gap (Eg=2 eV) alloys of a-SiC:H doped with boron were prepared by CVD of disilane, methyl silane, and diborane. We also prepared variable band gap a-SiC:H alloys by substituting F2C= CFH for methylsilane, and these films were found to have approximately 1–2% fluorine incorporated. The dark conductivity of the boron doped a-SiC:H alloys dep~sited from either carbon source ranged from ix10-7 to 6x10-7 (ohm-cm)-1. We also prepared low band aap alloys of Si and Ge by CVD of trisilane and germane. The band gap of a film containing 20% Ge was 1.5 eV; however, the photoconductivity of the film was relatively low.

The thermal decomposition of disilane in a helium flow has been used for the deposition of hydrogenated amorphous sil icon (a-Si:H) films on various substrates at rates higher than 1 μm/min, and a-SI:H films with thickness up to 20 μm have been deposited in this manner. Their optical properties, photoconductivity, minority carrier diffusion length, gap state density, etc., have been measured. The electronic properties of these films are superior to those of chemical vapor deposited a-Si:H films heretofore reported, indicating that the thermal decomposition of disilane in a hel ium atmosphere is a promising technique for the deposition of a-SI:H films.

Hydrogenated amorphous silicon films were produced from silane/hydrogen and silane/helium gas mixtures by RF glow discharge. We examined the optical and electrical properties of films produced with these gas mixtures, at various RF power levels and silane fractions. Film quality was analyzed by measuring the dark and photoconductivity, optical band gap, and activation energy. Optical emission spectroscopy was also used as a diagnostic tool for studying the plasma during glow discharge depositions. Experimental results indicate that amorphous silicon films made from silane/helium mixtures exhibit improved optoelectronic properties, higher deposition rates, and higher emission intensity ratios (ISiH/IH) as compared to films produced from silane/hydrogen mixtures. In preparing films from silane/helium mixtures, the onset of dust/powder formation occurs at considerably higher RF powers as compared to silane/hydrogen, thus making this approach an attractive commercial option for depositing films at high rates.

Preliminary results of a comparative study of some optical and ESR properties of aSi:H films prepared by rf sputtering on a cold substrate in 10 mtorr of either He, Ar, or Xe and 0.5 mtorr H2 are presented. In all cases the concentration of Si-H and Si-H2 bonds, the optical gap and the dangling bond spin density all generally increase as the rf power is decreased from 3.3 to 0.27 W/cm2. However, whereas the optical energy gap of He/H2 sputtered films ranges from 1.26 eV to 2.13 eV, the gap of Ar/H2 and Xe/H2 films sputtered under these conditions only changes from 1.54 to 1.94 and 1.41 to 1.71 eV, respectively. The dangling bond spin densities are lowest (~1017 cm-3) in the Ar/H2 sputtered films at high rf power and highest (~5x1018 cm-3) in Xe/H2 sputtered films at low power.

Amorphous and microcrystalline germanium films prepared by glowdischarge and molecular beam deposition were hydrogenated after deposition, using a 3cm Kaufman ion source. The hydrogen profiles were determined using the N15p nuclear resonance reaction. We found that the surface region hydrogen concentration depended on ion beam modification of the material, and the bulk concentration was determined by the hydrogenation conditions and deposition conditions. The light and dark conductivities wee measured before and after hydrogenation. Several orders of magnitude change in the ratios of the conductivities were measured under optimum hydrogenation conditions. The aclivation energy for electrical conductivity was measured and found to be dependent on film structure thickness and hydrogenation conditions.

Solid Phase Epitaxy (SPE) of amorphous silicon thin films can be employed to build novel device structures for VLSI applications. One way of achieving SPE is to use a room temperature silicon implant to amorphize a polysilicon layer followed by a thermal treatment to promote epitaxial growth. Both vertical SPE, in which the epitaxial film is grown directly on silicon substrate, and lateral SPE, in which the epitaxial growth is extended over a thin layer of oxide using the vertical SPE region as a seed, have been realized using this approach. This paper presents results obtained by cross-sectional TEM analysis of the epitaxial films, with particular emphasis on the effects of implant schedule and annealing conditions on the epitaxial regrowth.

There have been many studies of the vibrational and optical properties of a- Si and a-Ge that have correlated systematic variations of these properties with films grown and/or annealing temperatures, and have attributed these variations to changes the local atomic structure and intrinsic network disorder that are correlated with the thermal history of a particular sample. The most frequently proposed atomic structure parameter associated with this intrinsic disorder has been the width of the bond angle distribution. We attempted to isolate the effects of bond angle disorder on the vibrational and electronic properties of a-Si using a Bethe Lattice structure that avoids some of the uncertainties introduced by the uncontrolled variation in other atom structure parameters. Using this approach, we show that increases in the width of the bond angle distribution can account for trends in the calculated vibrational and optical properties wth thermal hisrory that are in good qualitative agreement with trends reported in the experiments.

Because Multiple Quantum NMR coherences occur only between spins which are coupled together by the dipole interaction, this technique has been used to study the clustering of hydrogen in amorphous silicon. The clustered hydrogen was found to be associated with the broad line of the single quantum NMR spectra. For device quality films, the average cluster size is approximately six protons. The concentration of these five to seven atom defects increases with increasing hydrogen content until, at very high hydrogen content, the clusters are replaced by a continuous network of silicon-hydrogen bonds.

Progressive annealing of a-Si:D and a-Si:DH samples between 250 and 600°C is found to eliminate the broad central deuteron magnetic resonance (DMR) spectral component associated with weakly bonded D. The well-resolved doublet arising from tightly bound D diminishes in intensity. The narrow central line associated with microvoid-contained molecular D2 and HD increases and then decreases for the warmest anneals. DMR relaxation times, line shapes, and hole-burning spectra reflect changes in sample morphology upon annealing.

It has recently become feasible that theory Will be able to predict the structure of solids “ab-initio”, using only the atomic number of the constituent atoms as input. This is based on recent advances in density-functional theory and pseudopotential theory. A simple physical introduction of the concepts underlying these theories is presented. Special emphasis is given to examining the structure and effective correlation energies of defects in amorphous Si.

This paper describes a calculation of twofold-coordinated (or divalent) intrinsic bonding defects in a-SiSn:H alloy films. The motivation for this study comes from experimental studies of the electronic and optical properties of a- Si, Sn:H alloys which indicate dramatic changes in the electronic and photoelectronic properties for small concentrations of Sn (approximately 1–2 at. %). We have used a cluster Bethe lattice structural model and an empirical tight-binding Hamiltonian to investigate the electronic properties of tetrahedrally bonded Sn atoms and neutral Sn defect centers (T2o and T3o) and in an a-Si host. We find that: (C) fourfoldcoordinated Sn atoms simply promote a reduction in the optical bandgap, with the energy gap disappearing for Sn concentrations of about 20 to 30 at. %; (2) neutral dangling bonds (T2o) or threefold-coordinated Sn atoms generate a localized state in the gap that is iower in energy than the corresponding neutral Si atom dangling bond; and (3) divalent (T2o) or twofold-coordinated Sn atoms give rise to two states in the gap, an occupied state that is lower in energy that either the Sn or Si dangling bond, and an empty state that is just below the conduction band edge. We show that the electronic and optical properties of the a-SiSn:H alloys can be understood in terms of a model in which there are relatively high densities of unhydrogenated Sn divalent sites and/or Sn dangling bonds.

The conventional view of the electronic structure of hydrogenated amorphous silicon is: (1) the material is characterized by a mobility gap of about 1.8 eV, with exponential band tails due to disorder and deep defect states arising from silicon dangling bonds (T3 centers); (2) substitutional doping occurs because of the formation of chargedimpurity/dangling-bond pairs, e.g. P4+ – T3-, at the substrate temperature; (3) the effective correlation of the T3 center is about 0.4 eV; (4) T3o centers are the predominant recombination center; (5) the three intrinsic ESR signals are due to electrons on T3o centers, electrons in the conduction band tail, and holes in the valence band tail. It is the purpose of this paper to demonstrate that this model is in sharp disagreement with an array of basic experimental data, and much of the evidence presented in its favor is based on self-inconsistent logic. We conclude that it is very likely that large concentrations of charged intrinsic defect pairs are present in all hydrogenated amorphous silicon films.

The puzzles regarding the magnitude of the free electron mobility in hydrogenated amorphous silicon are examined. It is suggested that highlevel double injection produces a metastable increase in the carrier mobility by neutralizing positively and negatively charged defect states thereby eliminating long-range potential fluctuations. Since these defect states cannot be neutralized under low-level or single injection, they both contribute to the modulation of the conduction band and increase the freecarrier scattering. If the latter is the predominant scattering mechanism, the neutralization of charged defects directly leads to a mobility increase under double-injection conditions. We discuss the various implications of this model, and present recent experimental results in agreement with these ideas.

The role of the neutral dangling bond defect upon photocarrier processes in undoped amorphous hydrogenated silicon (a-Si:H) is discussed. The evidence that the dangling bond is a simple recombination center is reviewed, and it is shown that this model does not account for photocurrent response time measurements. Experimental data pertinent to the role of electrical contacts upon response time measurements are presented, and it is concluded that contact effects do not account for response-time measurements. The possibility that the dangling bond is primarily an electron trap is discussed.

IR re-excitation of non-equilibrium carriers in undoped a-Si:H has been used to probe the profile of the distribution of deep traps. In a dualbeam experiment, after turning off a pump light, the trapped carriers are re-excited by an IR probe light which causes the photoconductivity(PC) to pass through a maximum, σmax, before settling down towards its steady state value, σs. σmax depends on the time interval td between turning off the pump light and turning on the probe light in a manner Δσ=σmax-σs∞td-α(T) Based on the multiple trapping (MT) model, the distribution of deep traps has been deduced from the temperature dependences of α(T).