We present new results of kinetic studies of the deposition of high quality a-Si:H which strongly support the reaction mechanism suggested in our earlier papers: 1. SiH4 → SiH2; 2. SiH2 + SiS4 → Si2H6 (SiH2 + Si2H6 → Si3H6); 3. Si2H6 → 2a-Si:H (Si3H8 → 3a-Si:H). The “SiH3 mechanism”, as promoted by several workers, is in contradiction with these experimental facts.

The di- and trisilane, which have a much higher reactive sticking coefficient than monosilane, play the role of reactive intermediates which facilitate the heterogeneous decomposition of silicon carrying species at the surface of the growing film. The values of the reactive sticking coefficient of Si2H6 and Si3H8 depend on the surface coverage by chemisorbed hydrogen; they increase with decreasing surface coverage. Under the conditions of the growth of high quality a-Si:H films the reactive sticking coefficient of disilane amounts to 10−4 to 10−2 which is in a good agreement with recent data of other authors.

The rate determining step of the growth of high quality a-Si:H films is the desorption of hydrogen from the surface of the growing film. This can be strongly enhanced by ion bombardment at impact energy of <100 eV. In this way, homogeneous, good quality films were deposited at rates up to 1800 Angströms/min, and there is a well justified hope that this rate can be further increased.

A novel method for preparing photoconductive Si thin films termed “Spontaneous Chemical Deposition”, is proposed, in which silane is decomposed spontaneously by gas phase reactions with fluorine at reduced pressure. With the external parameters in the gas phase reaction such as a gas flow ratio of SiH4 to F2 and the reaction pressure and temperature, the Si-network structure of the films can be controlled intentionally, resulting in a reduction of the hydrogen content, CH and a variety of the films from “amorphous” to “microcrystalline”.

Both microcrystalline and epitaxially grown Si thin films were prepared at low substrate temperature from different precursors under control of atomic hydrogen. With the precursors, SiHn (n≤3), generated by plasma-induced decomposition of Sil4, microcrystalline Si was formed through the island-like clusters with the aid of atomic hydrogen, while two-dimensional growth was caused with the precursors, SiHnFm (n+m≤3) generated from SiF4.

The spatial and energetic characteristics of rf and dc parallel-plate deposition discharges are discussed, along with their implications to plasma chemistry. We discuss the results and interpretation of our recent measurements of silane, disilane, germane, and mixed-gas stochiometry. These yield the initial dissociation branching between even and odd dangling-bond radicals, as well as the relative roles of higher silanes and silyl germanes (produced by the discharge) in the chemistry. Our deposition model and supporting data are discussed, as are various causes of film-quality variations which this suggests.

We present a detailed study of the variables (reactor geometry, gas pressure, rf power and silane dilution) which control the flux and energy of the ions impinging on the substrate in capacitively-coupled rf glow discharge systems. The ion flux and ion energy, measured with an electrostatic energy analyzer, were found to be largely dependent on the reactor geometry, which was quantified by the ratio of the area of the grounded surfaces Sg to the area of the rf electrode Sc. Special attention was paid to the effects of the dilution of silane in hydrogen, helium and argon. Helium and hydrogen dilution were found the most effective means to increase ion bombardment while argon dilution has little effect on the ion energy distribution. Furthermore, it is likely that ion bombardment at moderate energy (Eion < 70 eV) does not induce any serious degradation of the electronic properties of a-Si:H films.

Results are presented on the properties of a-Si:H thin films deposited with a remote hydrogen plasma. An essential feature of the reactor design is the use of an alumina (rather than quartz) tube to contain the hydrogen plasma for low oxygen contamination of the films. High doping efficiency is demonstrated for both P-and B-doped amorphous films, and the effects of high silane dilution and deposition temperature are illustrated with P-doped amorphous and microcrystalline silicon films.

A series of glow discharge a-Ge:H films has been produced at substrate temperatures, Ts, between 100 and 350°C. The films were structurally characterized using Differential Scanning Calorimetry (DSC), Gas Evolution (GE), Transmission Electron Microscopy (TEM) and Scanning Electron Microscopy (SEM). The DSC and GE results are substrate dependent. Two exothermic peaks are seen in DSC spectra for films deposited on aluminum foil while only one peak, identified by Raman measurements as the crystallization peak, is seen for samples deposited on beryllium, NaCl, carbon coated mica, crystalline silicon and 7059 glass. The crystallization peak temperature of films deposited on 7059 glass decreases with increasing Ts until 250°C where it then remains constant up to Ts=350°C. GE results show a relative increase of high temperature evolution with an increase in Ts. A well defined island/tissue structure seen in TEM micrographs of low Ts films disappears at higher Ts values. SEM measurements show columnar structure present only in films produced at Ts≤250°C. All the structural measurements point to a more stable material of higher density for Ts>250°C.

Hydrogenated amorphous silicon (a-Si:H) films were prepared by mercury photosensitized decomposition of silane using a low-pressure mercury lamp. The deposition rate showed an activation type for substrate temperature (the activation energy: 0.13 eV), because the deposition rate would be determined by the rate of hydrogen elimination from the hydrogen saturated surface. Moreover, the relationship was found between the Si-H2 bond density in a- Si:H films and the gas phase reactions.

Chemical reactions were systematically investigated with regard to the propagation of Si-network in the vicinity of the growing surface by using various precursors, SiHn, SiHnFm, and SiHnCim (n+m≤3) generated by plasma-induced decomposition of SiH4, SiF4, SiH2Cl2 and SiHCl3. Atomic hydrogen was an effective agent to promote the propagation reaction due to its strong chemical affinity for silicon and its ability to diffuse through the Si-network. A preliminary analysis was made of the kinetics of the propagation reactions.

Amorphous silicon (a-Si:H) and amorphous carbon (a-C:H) films have been deposited by electron cyclotron resonance (ECR) microwave plasma enhanced CVD. A high deposition rate of ˜ 25 Å/sec and a light-to-dark conductivity ratio of 1 × 105 for a- Si:H films have been achieved by the ECR process using a pure silane plasma. ECR microwave plasmas have been analyzed by in situ optical emission spectroscopy (OES) and have shown a strong H * emission at 434 nm indicating higher chemical reactivity than RF plasmas. The linear correlation between the film deposition rate and the SiH* emission intensity of ECR silane plasma suggests that SiH* species are related to the neutral radicals which are responsible for the a-Si:H film deposition. Hard and soft a-C:H films have been deposited by ECR with and without RF bias power, respectively. The RF bias to the substrate is found to play a critical role in determining the film structure and the carbon bonding configuration of ECR deposited a-C:H films. Raman spectra of these films indicate that ECR deposition conditions can be optimized to produce diamond films.

A series of films of amorphous hydrogenated germanium has been produced using the r.f. glow discharge technique by varying the H2/GeH4 ratio in the gas plasma from 0 to 50 while keeping all other deposition parameters constant. Electronic, optical, and structural characterization has been performed. No significant changes in optical and electronic properties were observed for this range of dilution, in contrast to repeated reports on the hydrogenated silicon-germanium alloy system. However, small systematic changes in structure are observed using TEM techniques. We conclude that the observed differences are unimportant in determining the optical and electronic properties of this material.

A systematic study of the effect of the trace impurity of oxygen and water vapor in the processing environment on the infra-red (IR), optical and electrical properties, and defect density of sputtered a-Si:H films is reported. The results are that small contents of water introduced into the argon plasma extract H-atoms from the growth surface, thereby reducing total hydrogen content and polyhydride formation in films deposited at low substrate temperatures, <150°C. This leads to a decrease of the defect density, and hence, to improvements in the photoconductivity (Δσ) and the associated quantum efficiency-mobility-lifetime(ημτ) product.

Structural, optical and electrical properties of a-Si:H films prepared with the VHF Glow Discharge system were studied as function of deposition temperature, particularly in the range below 100°C. Usually, this material has poor optoelectronic properties. The a-Si:H material discussed in this work shows a surprisingly low hydrogen content, a low dihydride to monohydride ratio, good optical and electrical properties.

Species formed during the decomposition of silane-methane-hydrogen mixtures ([SiH4]x:[CH4]y:[H2]z), by spatial plasma separation technique using the TCDDC (Two Consecutive Decomposition and Deposition Chambre) systeml, are evaluated by mass spectrometric analysis and related with the structural and electro-optical properties for either amorphous (a-) or microcrystalline (μc-) thin films. Results obtained show that in the plasma region, the main reaction is the direct fragmentation of SiH4 by electron impact whilst near the growing surface, the main detected species are excited. The kind of species and their intensity depend strongly on the power density (dp), mixture gas ratio (g = CH4 /SiH4+CH4), static electromagnetic (ξ, B) fields and r.f. frequency (f), used. Since CH4 has a threshold decomposition higher than that one of SiH4, the species presented at the plasma region are, mainly, methyl, dimethyl and CH2-CH2 graphitic-like chains depending, mostly, on dp and on g. By diluting the mixture in H2, we observe the existence of active H2 species that, for high dp, may lead to a transition from the amorphous to microcrystalline phase, as well as a carbon incorporation in the amorphous tissue as graphitic-like bonds. This allow us to infer the merit of the TCDDC system in producing a-/μc-thin films that can be applied to photovoltaic and other applications.

a-Si and μc-Si were grown from SiF4 with H2 dilution in a DC glow discharge. The crystallinity of films deposited over a range of substrate temperatures and SiF4/H2 flow ratios was studied by Raman spectroscopy and the boundary between microcrystalline and amorphous Si was determined. We find that μc-Si can be grown from SiF4 with less H2 dilution than from SiH4. In the SiF4/H2 system, the etching by of F atoms appears responsible for μc-growth; H atoms play an important role in balancing growth and etching reactions.

A model for the role of H atoms as an etchant specie during deposition of microcrystalline (μc) films of Si:H and SiC:H is explored. Growth rates and etch rates of films made by Hg-sensitized photo-CVD have been measured as a function of reactor pressure (between 5 and 0.5 torr) and H2 dilution (up to 30:1). Gas phase reactions and diffusion to the substrate of depositing and etching species, have been modelled. It is found that high H radical flux (not necessarily high H2 dilution) promotes μc film growth. There are two surface etching reactions by H radicals: (i) selective etching of uncoordinated Si surface atoms (amorphous phase) from the film, leaving behind the more etch-resistant μc phase; (ii) in SiC:H alloy systems, a selective etching of C species, causing a decrease in C incorporation into the growing microcrystalline film. Films with increasing carbon content do not contain a μc-SiC phase because hydrogen is not eliminated from the carbon containing film precursors, thus inhibiting the development of crystalline Si-C network.

We present room temperature measurements on the electron drift velocity in hydrogenated amorphous silicon (a-Si:H) for large electric fields (1.3 × 105 to 3 × 105 cm/s). The experimental time resolution was ˜ 1ps and velocities as high as 8 × 105 cm/s were observed. The electron transport was non-dispersive, and the velocity was found to be a superlinear function of the applied field.

Measurements of the dependence of the D-center electron paramagnetic resonance absorption signal upon the incident microwave power are reported in undoped hydrogenated amorphous silicon (a-Si:H) for specimens prepared at differing deposition temperatures and also as the state of a given specimen was varied by illumination and subsequent annealing. These measurements are sensitive to electron spin relaxation processes of the D-center. Substantial variation in spin-relaxation behavior was found, corresponding to approximately one order of magnitude in the spin relaxation rate; no significant variations in absorption lineshape were observed. A model for these spin relaxation observations invoking two differing local microstructures near the D-center is proposed. The model indicates that illumination increases the density of defects in one microstructure but can irreversibly diminish the density in a second.

The longstanding controversy over the anomalously large subgap optical absorption energies in n-type (1.1 eV) and p-type (1.3 eV) hydrogenated amorphous silicon (a-Si:H) is described and resolved. Adler suggested that these large values are incompatible with a positive effective correlation energy of the dangling bond defect and a 1.7 eV bandgap. Kocka proposed that dopant-defect pairing deepens each dangling bond transition energy by about 0.5 eV in doped a-Si:H. I assume no deepening due to pairing, a positive correlation energy of 0.2 eV consistent with the observation of dark electron spin resonance in undoped a-Si:H, and dangling-bond relaxation energies of 0.2 to 0.3 eV which are indicated by previous theoretical and experimental work. The postulate of vertical optical transitions then reduces the anomaly from about 0.9 eV to 0.4 eV. This residual anomaly may be explained by electronic-level deepening in doped a-Si:H caused by disorder-induced potential fluctuations of 0.2 eV half-width.

Photoelectron yield spectroscopy is used to study the occupied density of states (DOS) in undoped and doped a-Si, Ge:H alloys. We find a shift in the top of the valence band to lower energy as the Ge content is increased. The width of the defect band becomes abruptly narrower when Ge is initially introduced. This change is accompanied by a significant shift in the relative position of the Fermi level towards midgap. The defect peak tracks the valence band throughout the entire composition range. The intrinsic valence band tail in the alloys is found to be an exponential with a characteristic slope of 50 to 60 meV independent of composition. Boron and phosphorous doping affect the DOS of the alloys in a manner similar to that measured in a-Si:H.