Photothermal deflection spectroscopy (PDS) is a sensitive method used to measure weak optical absorption of sub-bandgap radiation. A method utilizing Fourier transformation is presented which allows removal of optical interference effects and noise which is typically present in PDS data taken on a-Si:H, yet leaves the underlying PDS spectra undistorted. The method greatly facilitates comparison of PDS spectra taken on different samples, and simplifies further analysis. Examples using a-Si samples produced under different deposition conditions are presented to demonstrate the utility of the method.

Hydrogen's role in the doping efficiency and long term stability of hydrogenated amorphous silicon is not well understood. The introduction of dopants has been observed to cause an increase in the deposition rate and in the formation of dangling bonds. We have performed NMR spin echo double resonance measurements on heavily doped amorphous silicon samples prepared via plasma deposition in order to ascertain the dopanthydrogen structural units. The resulting dopant concentrations were 2.0 and 0.5 at. % boron and/or phosphorus in compensated and singly doped material respectively. Our results indicate no direct dopant-hydrogen bonding in the compensated or n-type samples. However, in the p-type material the data indicate that approximately 40% of the boron atoms are directly bonded to a hydrogen.

A comparison of 1H nuclear magnetic resonance (NMR) measurements in freshly prepared a-Si:H films on substrates with those removed from the substrates shows that a large fraction of the trapped H2 molecules are released when the films are removed from the substrates. After the films on substrates have been aged for several months, an equal amount of H2 has diffused out of the film. This gradual aging appears-to stabilize after several months. Once aged, H2 can be diffused into the films by immersion in H2 gas. The diffusion time is around 2 weeks at 300 K which is much slower than in vitreous quartz or 7059 glass at the same temperature. Diffusion, which is enhanced during the aging process, could proceed via the remnant columnar structure in the film. This speculation is supported by the inability to detect, using pake-doublet experiments, significant changes in the H2 content and by the recent observation of oxygen diffusion along remnant columnar structure in device quality films.

We measured the diffusion coefficient of H in a-Si:H between 36°C and 690°C via H2-evolution and the pressure rise in a closed ampoule. Our measurement temperatures reached ∼110°C above the highest T reported to date. The diffusion coefficient for unalloyed a-Si:H, DH=1.66×10−3 exp [−1.45 eV/kT] cm2 s−1, agrees with the earlier, lower-T, measurements. Between 250°C and 690°C the H-diffusion mechanism in a-Si:H appears to remain the same, with DH ranging from 10−17 cm2s−1 to 10−10cm 2s−1.

The effects of deposition parameters on the H microstructure of plasma deposited amorphous silicon (a-Si:H) and amorphous silicon carbide (a-SiC:H) are measured via multiple quantum nuclear magnetic resonance (MQ NMR). These studies indicate clusters of 5 to 7H atoms exist in a-Si:H films prepared at temperatures ranging from 113 to 324°C. In the range from 270 to 324°C, only these small clusters exist, but lower temperature films also contain larger clusters. Annealing studies indicate H rearranges in a-Si:H prior to evolution. Deposition temperature and annealing temperature have similar effects on H concentration in a-Si:H, but deposition temperature control the density and microstructure of the film. The addition of dopant atoms also affects the H microstructure, with phosphorous causing larger H clusters to form, and boron reducing clustering in a-Si:H films. This perturbation of the film's microstructure suggests that the effects of dopant addition are more complex in amorphous than in crystalline semiconductors. The concentration of carbon atoms also effects H microstructure of a-SiC:H in a complex way. We surmise that H microstructure, rather than H content, determines amorphous semiconductors properties.

Using a semiempirical potential energy function for Si, thin amorphous films of Si have been simulated on crystalline Si by a partial melting and subsequent quenching process. After relaxation at 450°K, the dominant structural feature was a dense free surface skin with a void layer underneath for both c-Si(100) and c-Si(111) substrates and for film thicknesses up to 16Å. Tetrahedral coordination was maintained throughout the amorphous region and structural differences were noted in the α-Si/c-Si interface region for the two orientations considered.

a-Si:H films rf sputtered in 10 mT He and 0.5 mT H2, in contrast to those prepared in Ar/H2 or Xe/H2, exhibit port depositional oxidation when sputtered at power levels of 0.55 – 3.3 W/cm2 (100 – 600W). SEM measurements show that the morphology of the films evolves from a cracked, microcrystalline, glassy one at higher rf power levels to a porous one at low power levels. This trend is also correlated with a sharply increasing density of Si-H2 dihydride bonds at decreasing rf levels. Auger depth profiles of the oxygen concentration in the films agree with a diffusion controlled process of a source solute of O2 molecules into a semi-infinite solid solvent. Yet IR absorption measurements indicate that the oxygen is confined to the internal surfaces of the microvoids. The diffusion constant is a very sensitive functi n of the sputtering conditions. At low rf power levels, it is as low as 10−16 – 10−1 cm2 /sec at room temperature, but increases to ∼10−13 at 250 – 350°C.These results are discussed in terms of a porous network of microvoids, through which the O2 “squeezes” into the film.

When silane, with more than 0.1% of diborane in the gas mixture, are co-pyrolyzed at temperatures below 540°C, an amorphous silicon-boron alloy is formed. The composition of the solid film was determined by SINS and Auger spectroscopy. The amorphous nature of the films was established by X-ray diffraction, laser Raman spectroscopy, and TEM electron diffraction. Electrical and optical properties of this material are reported. Oxidation and chemical etching of this material were also studied.

Hydrogenated amorphous silicon (a-Si:H) is well known by its applications for solar energy conversion devices in a large scale. Hydrogen has the capability of terminating the dangling bonds in the amorphous silicon structure and improves the optical properties for effective utilization of the solar spectrum. With this, the quantification of the hydrogen content in a-Si:H is an important task in its characterization.

Auger electron spectroscopy induced by Ar+ ion bombardment (IAES) has been shown to be sensitive to the presence of hydrogen in a-Si:H [1,2]. The Si-L 2VV peak has a distinctively new shape as compared to electron- xcited. Auger were only some slight differences can be observed [3]. contribution to the IAES to that particular peak coming from de-excitations inside and outside the solid can be identified [4].

In this work, IEAS of the Si-L23VV peak were obtained for a-Si:H prepared by glow discharge, grystalline silicon, and crystalline silicon implanted with silicon ions. The relative differences in the spectra are characterized. In particular the changes in the relative amplitudes of the so-called “bulk-like” peak to the “atomic-like” peak as a fuction of hydrogen content are obtained.

The dependence of the optical bandgap on the hydrogen concentration is measured for amorphous silicon films prepared under different glow-discharge conditions. A deviation from the usually accepted linear dependence is found for hydrogen concentrations above 12 at.‰ We find that in this concentration region an increase of hydrogen incorporated as SiH2 is responsible for this behaviour.

Experimental and computational results for three classes of experiments are reported. NMR measurements of phosphorus in amorphous hydrogenated silicon indicate that bonding rearrangements in bulk films are possible. Monte Carlo simulations of film growth demonstrate that surface diffusion is the dominant physical phenomenon responsible for the bulk hydrogen content of a-Si:H films. Finally, the gas phase chemistry of silane, as elucidated by our new “diesel” reactor, is considerably more complicated than many authors assume in models for thin films grown from silane.

By simulating the growth morphology of plasma-deposited amorphous silicon films on patterned substrates using a Monte-Carlo technique, we find the effective sticking coefficient for the dominating mass transporting species to be quite different depending on the film growth mechanism. The physical-vapor-deposition-like growth regime producing defective materials is associated with species like SiH2 and SiH which have a large effective sticking coefficient, while the chemical-vapor-deposition-like process producing “device quality” materials is linked to species like SiH3 which has a much smaller effective sticking coefficient. In a triode reactor the physical-vapor-deposition-like process is suppressed since species with small effective sticking coefficients are selected. However, the growth rate decreases rapidly as the substrates are moved away from the plasma. There seems to be an incompatibility between good material quality and fast growth rate in the plasma deposition of amorphous silicon in either diode or triode reactors.

Systematic studies have been made on preparation of Si thin films from SiF4 under control over the flow of atomic hydrogens. The gas phase reactions taking place in the mixture of fragments (SiFn) resulting from plasma-induced dissociation and atomic hydrogens were widely investigated by a mass spectroscopy. Chemically active species,i.e., SiF2H and SiH2F were found as those related to the growth of films. The growth in the vicinity of substrates involves either endothermic or radical-enhanced reaction for the propagation of the three dimensional Si networks, accompaning release of terminators such as H and F. Accordingly, Si thin films with structures from amorphous to crystalline were obtained by controlling the flow of atomic hydrogen. A marked improvement in the hole-transport was established in the Si films containing hydrogen less than 5–6 at % due to the reduction in the tail states near the valence band.

In situ process diagnostics using coherent anti-Stokes Raman spectroscopy (CARS) have been performed under state-of-the-art a-Si:H film deposition conditions in a reactor designed for a-Si:H solar cell fabrication. The silane plasma of device-quality a-Si:H film depositions was monitored by measuring the silane ν1 band CARS spectrum to determine the depletion induced by the RF glow discharge. The silane depletion is linearly dependent on the RF power in the region of 4 to 12 W with a slope of O.5%/mWcm−2. The depletion is also dependent on the SiH4 flow rate starting with a 50% depletion at a low flow rate of 5.6 sccm and asymptotically approaching an 8% depletion at a high flow rate of 80 sccm. The a-Si:H film deposition rate is systematically measured as a function of the flow rate and the RF power. Linear correlations between the silane depletion and the film deposition rate are observed. The results are discussed in terms of primary electron impact dissociations of silane and the residence time of SiH4 molecule in the glow discharge region.

The relation between the thermal equilibration of the defect structure in n-type doped hydrogenated amorphous silicon (a-Si:H) and the deposition conditions has been investigated. When the deposition rate is increased by raising the rf power the equilibration time constants become longer, though the thermal equilibrium processes are qualitatively similar to those in samples grown under optimal conditions. Models relating the slower equilibration rate to the deposition-induced microstructure are explored.

The preparation of hydrogenated amorphous silicon by the radio-frequency (RF) glow discharge technique is shown to be strongly dependent on the plasma excitation frequency. We have investigated the influence of this parameter on the deposition rate, on the hydrogen content and on the opto-electronic properties of the amorphous silicon films, over the range 25 to 150 MHz. A large variation of the deposition rate is observed, while most of the material properties remain practically unchanged. The results should be of considerable interest for mass production of low-cost amorphous silicon thin film devices.

We have investigated the properties of several hydrogenated amorphous silicon (a-Si:H) films prepared by the plasma-enhanced chemical vapor decomposition (PECVD) of silane. This reactor is singular because it operates at pressures as low as 10 mTorr and uses infrared lamps to heat the growth environment. Structural characterization showed that all the samples were amorphous. Most films had hydrogen concentrations about 5%, optical gaps near 1.6 eV, and neutral dangling-bond concentrations of the order of 5 × 1016 Cm−3. Electrical activation energies were typically in the range 0.5 – 0.6 eV and the photoconductivity under ∼100 mW/cm2 white light was 10−4 (Ω-cm)−1. The dark conductivity was near 10−8(Ω-cm)−1. No significant change in dark conductivity or photoconductivity occurred after several hours of exposure to light.

A new method of producing high quality hydrogenated amorphous silicon (a-Si:H) and its compound films is presented. An SiH4 and H2 gas mixture is decomposed without using any plasmas or photochemical excitation, but using only thermal and catalytic reactions between deposition gas and a heated tungsten catalyzer. Photoconductivity of a-Si:H films produced by, this method reaches 10−3 (Ωcm)−1 and photosensitivity exceeds 105 for illumination of AM-1 light of 100 mW/cm2, highly efficient boron- or phosphorus-doping into the films is achieved, and also the optical band gap of the films is easily controlled without apparent degradation of the properties by adding GeH4 gas to the deposition gas.

Deposition of SiNx films by ArF laser induced chemical vapor deposition has been investigated. The films exhibit excellent electrical properties; the high breakdown voltage and the low fixed charge are the same as in films deposited by LPCVD, but the BHF etching rate of them is larger by a factor about 4 than that prepared by the plasma CVD. The diffusion length of the radicals contributing to the deposition was estimated from the distribution of the deposition rate as a function of the deposition parameters. The optical emission from the radicals produced by ArF laser irradiation was also studied. Using these results, we discuss the mechanism of the deposition.

We calculate the average surface potential barrier for incorporation of H, Si, SiHb (n=1–4) into films of a-Si:H as well as crystalline Si(111) surfaces. In the first case a local amorphous configuration for the surface is employed through a representative cluster(Si29 H1 0 ) forming 5, 6, 7 Si atom rings. For the crystalline surface, several layers of Si atoms are considered. Pairwise superposition of combined Morse and Thomas-Fermi-Moliére interatomic potentials is assumed for the total interaction between the incoming species and the surface.