A systematic study of material quality has been performed for a-Si:H layers deposited by plasma enhanced chemical vapour deposition at frequencies between 30–80 MHz. The effect of frequency variation was studied in combination with the variation of pressure and power density. The process conditions were optimised not only for ‘device quality’ opto-electronic properties but also for a uniformity in layer thickness better than 5 %. For every frequency an optimum pressure exists for which the properties of the deposited layer satisfy the ‘device quality’ requirements. A clear correlation is observed between the transition from the so-called α- to the γ-regime of the plasma and the dependence of the deposition rate γd with pressure pand frequency ƒ: γ d ∝ pƒ2/3.

The interface state density between a-Si:H and insulating film is very important in characteristics of a-Si:H thin film transistors. In this study, the interface state density was measured by photothermal deflection spectroscopy (PDS). Layered structures of a-SiN, 1.7:H on a-Si:H and a-SiO2.0 on a-Si:H were deposited by P-CVD method. The a-SiN1.7:H layer was grown from a gas mixture of SiH4 and NH3 and the a-SiO2.0 layer was grown from a gas mixture of SiH4 and N2O. While the interface state density of a-SiN1.7:H on a-Si:H structure was smaller than the free surface state density of a-Si:H, that of a-SiO2.0 on a-Si:H structure was larger than the free surface state density of a-Si:H. The difference in the surface state density between these specimens is discussed in terms of plasma damage of a-Si:H surface by the source gases during deposition of insulating layer. Surface of the a-Si:H was treated by plasma of NH3 or N2o gas which is dominant constituent of source gases of insulating layer. Although the surface state density of the N2o plasma-treated sample increases, that of NH3 plasma treated sample does not increase. The shape and intensity of the spectra of N2o plasma-treated sample is similar to that of the a-Sio2.0 on a-Si:H structure. These results indicate that the interface defect between a-Sio2.0and a-Si:H layer was induced by plasma damage of the a-Si:H surface.

A new approach has been developed for the analysis of real time spectroscopie ellipsometry (RTSE) data that has important applications in characterizing compositionally-graded layers in amorphous semiconductor devices. The new RTSE data analysis approach determines (i) the instantaneous deposition rate, (ii) the surface roughness layer thickness, and (iii) the dielectric function of the near-surface layer (top 20 Å) of the growing film, all without any knowledge of the deposition history. We tested this new approach by characterizing a-Si1-xCx:H (x ∼ 0.09) alloy films with continuously graded void volume fractions. The results reported here are unique in that they represent the first successful optical analysis of amorphous semiconductor structures having continuously varying properties with depth into the film.

Formation of hydrogenated amorphous silicon (a-Si:H) was carried out by rf glow discharge method in the reactive gas systems of silane. By preliminary experiments on a-Si:H for helium dilution, a relatively high deposition rate up to 2–3 μm/hr was obtained. In order to investigate the opto-electronic properties on a-Si:H films due to different dilution of helium or hydrogen, measurements on optical band gap Eop, electric conductivity and FT-IR were done. Optical emission spectra were also observed. Optical band gap value Eop on a-Si:H film for SiH4 (10%) -He (90%) was nearly constant about 1.73 eV, while that for SiH4 (10%) -H2 (90%) was increased from 1.75 eV to 1.78 eV with increase of residence time of silane molecule and it was relatively high. From the experimental results of FT-IR, ratio of bonding mode of SiH to (SiH+SiH2) for hydrogen dilution was about 90% and higher than that for helium dilution, while hydrogen concentration in the film for SiH4 (10%) -H2 (90%) was less than 10% and that for SiH4 (10%) -He (90%) was 20–30%.

Plasma processes are usually worked out in a small-scale environment (electrode area maximum 121 cm2, rf- and VHF- excitation frequencies). In order to meet the requirements of large area device applications they have to be upscaled. The investigations of glow discharge systems for different PECVD reactors (parallel plate- and coaxial electrodes) have shown, that the reactor design (power supply, line connection) sharply influences the large area deposition process. The voltage distribution on the driven electrode especially determines the uniformity of the deposited layer thickness. Possibilities which influence the voltage distribution on large areas will be discussed. The results of large area electrode description as an electrical line will be discussed in comparison with different reactor configurations and the optimization of the behavior of the deposition process. The experimental results of a coaxial reactor (electrode area 5000 cm2, substrate length 120 cm) show that a homogenous deposition of amorphous silicon (layer uniformity of thickness over the length better ± 7 %) by connecting the driven electrode with additional electrical devices is possible.

A remote argon/hydrogen plasma is used to deposit amorphous hydrogenated silicon. The plasma is generated in a DC thermal arc (typical operating conditions 0.5 bar, 5 kW) and expands into a low pressure chamber (20 Pa) thus creating a plasma jet with a typical flow velocity of 103 m/s. Pure silane is injected into the jet immediately after the nozzle, in a typical flow mixture of Ar:H2:SiH4=55:10:6 scc/s. The electron temperature in the jet is low (typ. 0.3 eV) : silane radicals are thought to be produced mainly by hydrogen abstraction, but also by a sequence of dissociative charge exchange and consecutive dissociative recombination. In-situ ellipsometry yields refractive indices of 3.6–4.2 at 632.8 nm and growth rates of 10–20 nm/s. FTIR analysis yields a hydrogen content of 9–25 at.% and refractive indices of 2.7–3.3 in the infrared. The SiH density decreases with increasing hydrogen content, whereas the SiH2 density increases. Above 11 at.%, the majority of hydrogen is bonded in the SiH2 configuration. The optical bandgap remains constant at approximately 1.72 eV. The photoconductivity is of the order 101–6 (Ωcm) 1–6 and the photoresponse 106.

The effect of plasma power, reactor outgassing rate and feed gas purity on the oxygen content of amorphous silicon (a-Si:H) and microcrystalline silicon (μc-Si:H) layers, prepared by the VHF (70 MHz) glow discharge technique has been investigated. The oxygen concentration in the films was measured by Secondary Ion Mass Spectrometry. The gas purity could be optionally enhanced by the application of a getter-based gas purifier mounted in the gas pipe just before the reactor. The purifier reduces the oxygen contaminants in the feed gas to levels in the ppb range. It was found that oxygen incorporation was enhanced at lower deposition rates. The deposition rate dependence of the oxygen incorporation is in excellent agreement with the proposed incorporation mechanism. Furthermore, the reactor outgassing rate and the purity of the feed gas itself were identified as the main contamination sources for the oxygen contaminants in the deposited a-Si:H films as well as in μc-Si:H films : e.g. at outgassing rates in the range of 10−6 mbar 1 s−1, at least around half of the oxygen detected in both a-Si:H and in the μc-Si:H layers originates from the feed gas if the latter is unpurified. By combining reduced outgassing rates and the use of purified feed gas, a-Si:H and μc-Si:H films with an oxygen concentration of 5×1017cm−3 and 2×1018cm−3, respectively, were deposited. These values both represent the lowest concentrations of oxygen contaminants reported to date for these kinds of materials.

We have systematically investigated the effects of addition of sub-ppm levels of boron on the stability of a-Si:H films and p-i-n devices, deposited by PE-CVD technique. The films thus produced with appropriate amounts of boron, show a significant improvement in stability, when soaked under both AM 1.5 (short-term) as well as 10×sun (long-term) illumination conditions. The opto-electronic properties of the films are quite respectable It is concluded that boron compensates the native impurities by forming donor-acceptor pairs, which reduces the “fast” defects and hence the initial degradation of the films. It is also speculated that boron may also be improving the short-term stability, by reducing the recombination of light generated electrons and holes, by converting D° into D+ states. The long-term stability appears to get affected by hydrogen dilution which seems to reduce the amount of “slow” defects. As a result of B doping of i-layer, the initial conversion efficiency of the devices decreases. It is presumed that our devices may contain an enhanced level of boron impurity, than expected, making them as worse material and to degrade less.

Nanocrystalline silicon (nc-Si) has been fabricated by a very-high-frequency plasma cell attached to an ultra-high-vacuum chamber using SiH4 gas. Nanocrystalline Si is formed in the gas phase of the plasma cell and is extracted out of plasma cell through the orifice to the ultra-high-vacuum chamber. The shape of nc-Si is spherical or octahedral with the diameter of 3–30nm. Giant Si particles about 100nm in diameter are also formed at the lower cell pressure condition. A 1000keV transmission electron microscopy measurement has revealed that the core region of giant Si particle with the diameter about 30nm was crystalline and the shell region is amorphous. We have demonstrated that the spread of particle size can be decreased using pulsed gas supply of H2 into SiH4 plasma.

The conductivity and the structural properties of thin films deposited by Hot-Wire Chemical Vapor Deposition (HW-CVD) from silane and hydrogen at a substrate temperature of 220 °C are shown to be strongly dependent on the filament temperature, Tfil, and process pressure, p. Amorphous silicon films are obtained at low pressures, p < 3 × 10−2Torr, for Tfil ∼ 1900 °C and FH2 = FSiH4. At this TfilJU, high deposition rates are observed, both with and without hydrogen dilution, and no silicon was deposited on the filaments. At Tfil ∼ 1500 °C, a transition from a-Si:H for p > 0.3 Torr to microcrystalline silicon (μc-Si:H) for p < 0.1 Torr occurs. In this temperature regime, silicon growth on the filaments is observed. /ic-Si:H growth both without hydrogen dilution and also in very thin films (∼ 0.05 μm) is achieved. Raman and X-Ray spectra give typical grain sizes of 10 – 20 nm, with a crystalline fraction higher than 50%. For both, Tju ∼ 1500 °C, p > 0.3 Torr and Tfil ∼ 1900 °C and p ∼ 2.7 × 10−2Torr, an increase of the crystalline fraction from 0 to ∼ 30% is observed when the hydrogen dilution, FH2/FSiH4, increases from 1 to > 4.

Polysilicon (poly-Si) thin films have been obtained using hot-wire chemical vapor deposition (HWCVD) from silane-hydrogen mixtures. The films were prepared at low substrate temperatures (down to 200°C) and at very high deposition rates (up to 40 Å/s). They showed good crystalline properties and no amorphous phases were detected. The films can also be efficiently doped by adding diborane or phosphine to gas phase. In this paper, an overview of the properties of the poly-Si films, intrinsic and p and n-doped, deposited at our laboratory by HWCVD is presented and discussed. The properties of the material and the features of the deposition technique which are interesting for their application in photovoltaics are emphasized.

In this study, we have deposited polycrystalline silicon (poly-Si) thin films by hot-wire Chemical Vapor Deposition (CVD) using hydrogen and disilane as the reactive gases. We selectively activate hydrogen and let disilane bypass the hot tungsten filament assembly and enter the reactor downstream from hydrogen. This may provide a better process chemistry, and by this approach, we have deposited poly-Si films at a substrate temperature as low as 310°C and at a growth rate as high as 100 Å/min. The substrate temperature is more than 2000C lower and the growth rate is more than twice higher compared to those of LPCVD poly-Si films. The effect of hydrogen flow rate, disilane flow rate and substrate temperature on the deposition rate and structural properties of the polysilicon films are investigated. The deposited films are characterized by transmission electron microscopy, scanning electron microscopy, Raman spectroscopy and X-ray diffraction.

We have studied the growth of undoped and n+ μc-Si:H (:CI) films by Remote Plasma CVD using SiH4/SiH2Cl2/H2/He mixtures. It was found that the μc-Si film can be fabricated by increasing flow rate of SiH2Cl2 and/or H2. The deposited undoped μc-Si film exhibited a maximum crystalline volume fraction of 85 %, obtained from Raman spectroscopy. The n-type μc-Si film, deposited with SiH4/SiH2Cl2/H2/PH3/He mixtures, shows a room temperature conductivity of 2 S/cm, conductivity activation energy of 29.8 meV and optical band gap of-2.0 eV. The optical band gap of n-type μc-S1 deposited using SiH2Cl2 is much higher compared to conventional μc-Si film.

Silanes and their organo-derivatives are of interest as precursors and dopants in chemical vapor deposition (CVD) to prepare (modified) thin films with unique surface properties. Applications of these films are widespread: transitors, laser diodes, optoelectronic devices, etc. Film growth, however, is capital intensive and mechanistically complex, while film characterization is time-consuming and often requires sophisticated instrumentation. A tool is needed to prescreen precursors' CVD behavior, preferrably a technique requiring a minimum of time and sample to provide information directly related to aspects of the film growth process.

We investigate deuterium incorporation into deuterated-hydrogenated amorphous silicon grown by the saddle-field glow-discharge of deuterium and silane. The presence of HD and SiH3D in the discharge suggests strong gas phase mixing. The dominant process of deuterium incorporation into the growing film appears to be the direct reaction between deuterium and the growth surface.

Amorphous hydrogenated silicon (a-Si:H) films were deposited from disilane at substrate temperatures between 180 and 390 °C using a F2-laser (157 nm) in a parallel configuration. Material properties such as hydrogen content, SiH and SiH2 group concentration, photo-and dark conductivity, band-gap energy and the Urbach parameter were determined as a function of the deposition temperature. The material with the best optical and electronical properties was found for a substrate temperature of 260 °C. Using argon as the buffer gas instead of helium results in films of poor quality.

The Effects of fluorine atoms on laser-induced crystallization have been studied. The defect states of hydrogenated amorphous silicon (a-Si:H) film, which are related to weak hydrogen bonds, were reduced by involving fluorine atoms, before laser annealing. In laser annealing, the crystallinity was also enhanced to 63 %, while that of a-Si:H was 45 % at the energy density of 300 mJ/cm2. It may be considered that the fluorine atoms reduce the hydrogen atoms cause to the degradation of film in laser annealing, and increase the heat capacity which results in promotion of crystallinity in melt-solid phase transition.

A new concept of Reactive CVD is proposed for the low-temperature growth of thin films by CVD and discussed in terms of general features of such film growth. One demonstrated example of the reactive CVD, called Spontaneous Chemical Deposition, features gas phase reactions of silane with fluorine is outlined in terms of the general characteristics of the film growth and properties.

In addition, a second example of reactive CVD, called Reactive Thermal CVD and involving thermal CVD of poly-SiGe from germanium fluoride and disilane, is discussed and low-temperature growth using this method is reviewed.

Hydrogenated amorphous silicon containing chlorine (a-Si:H (Cl)) films have been grown by plasma enhanced chemical vapor deposition from a mixture of silane and dichlorosilane with a dichlorosilane concentration up to 60%. We report on the film properties in the as-deposited state and the behavior of the films under both high intensity pulsed laser illumination and long-term AMI illumination. With increasing dichlorosilane concentration the films show an increased resilience to the creation of light induced defects, as determined from the constant photocurrent method. After 900 hours under AMI illumination, the defect density shows a minimum (< 1016 cnr−3) for a 10 % mixing ratio.

Polycrystalline silicon thin films have been deposited at fast growth rates (50 Å/s) by hotwire chemical vapour deposition (HW-CVD) from SiH4/H2 gas mixtures at low substrate temperature (400–500°C). The surface morphology of these films consists of 0.5 – 2.0μm dendritic grains as seen by electron microscopy. The films have a columnar morphology with grains starting from the substrate either on glass or c-Si. Even the 150 nm thick initial layer is polycrystalline. The preferential crystalline orientation of the poly-Si film is apparently not governed by the radiative source but strongly depends on the type and orientation of the substrate. A strong hydrogen dilution (>90%) of silane is essential to obtain poly-Si films with optimal crystalline structure.