Hard amorphous (diamond-like) carbons

It is shown that substitutional doping of an amorphous semiconductor is possible and can provide control of the electronic properties over a wide range. a-Si and Ge specimens have been prepared by the decomposition of silane (or germane) in a radio-frequency (r.f.) glow discharge. Doping is achieved by adding carefully measured amounts of phosphine or diborane, between 5 × 10−6 and 10−2 parts per volume, to obtain n- or p-type specimens. The room temperature conductivity of doped a-Si specimens can be controlled reproducibly over about 10 orders of magnitude, which corresponds to a movement of the Fermi level of 1·2 eV. Ion probe analysis on phosphorus doped specimens indicates that about half the phosphine molecules in the gaseous mixture introduce a phosphorus atom into the Si random network; it is estimated that 30–40% of these will act as substitutional donors. The results also show that the number of incorporated phosphorus atoms saturates at about 3 × 1019 cm−3, roughly equal to the number ...

Electronic properties of substitutionally doped amorphous Si and Ge

Thermal sensors based on the seebeck effect

Diamond-like carbon (DLC) films deposited by cathodic vacuum arc evaporation (CVAE) have attracted worldwide interest from research groups and industry since the beginning of the 1990s. Hydrogen-free amorphous carbon (a-C) coatings were first deposited by CVAE about two decades after the first description of hydrogenated a-C coatings (a-C:H) deposited by glow-discharge techniques. This paper highlights the development and broad potential of hard a-C coatings deposited by direct (DCVAE) and filtered (FCVAE) cathodic arc evaporation, including pulsed arc. DLC films offer a wide range of exceptional physical (optical, electrical), chemical (interaction with media), mechanical (hardness, elastic modulus), biomedical and tribological properties. Monolithic tetrahedrally-bonded hydrogen-free coatings (ta-C) provide the highest hardness, while various softer a-C coatings are also useful in some applications. Many film properties such as electrical conductivity and surface energy can be modified by alloying with elements such as H, N, Si, B, F, P and metals. Recent research and industrial solutions for generating DLC coatings by CVAE of carbon-based cathodes are described, and hybrid methods using metal cathodes and gas-phase sources are discussed. Coatings containing additional elements and having complex architectures are also discussed, and selected properties for various coating types are presented. The number of industrial applications of ta-C and a-C coatings continues to increase, mainly for tribological coatings to reduce wear and friction. Various applications of coatings deposited by CVAE are described, including data hard disks, engine parts, razor blades, valve seals, decorative coatings, cutting and forming tools, biomedical products and others.

60 years of DLC coatings: Historical highlights and technical review of cathodic arc processes to synthesize various DLC types, and their evolution for industrial applications

The thermoelectric properties of N‐type and P‐type Si‐Ge alloys have been reviewed and detailed calculations for the efficiency of a thermoelectric generator made from a 70% Si‐30% Ge alloy have been made over the temperature range from 300 to 1300 K. A model employing one valence band and two conduction bands has been used. A generator of standard material, optimally doped, and infinitely segmented will have an efficiency of 12.1% operating over this range. If the lattice thermal conductivity can be reduced to its minimum value without upsetting the electrical properties, then the efficiency can be raised to an ultimate maximum of 23.3%. A more modest increase in efficiency to 14.7% could be obtained by a 2.4 volume percent of finely dispersed second‐phase precipitates which would act as phonon scatterers. The utility/futility of GaP additions and grain‐boundary scattering as methods to increase the efficiency is discussed.

The maximum possible conversion efficiency of silicon‐germanium thermoelectric generators

Amorphous carbon films have been prepared in a r.f. glow discharge from various hydrocarbon gases. Electrical conductivity and optical absorption measurements show a strong dependence on the preparation gas and on the substrate temperature T d. Particularly for specimens prepared from ethylene, varying T d from 230 to 300°C increased the conductivity by ten orders of magnitude and decreased the optical gap from 2·0 to 0·8 eV. The hydrogen content of the specimens, measured by the thermal evolution technique, lies between 30% and 45% and shows no systematic dependence on T d or the preparation gas. Results are also presented for films which have been prepared from gas mixtures containing small predetermined amounts of phosphine, diborane or nitrogen. The addition of each dopant gas increases the specimen conductivity by between one and three orders of magnitude, depending on T d. It is concluded that for both undoped and doped specimens, conduction is predominantly by hopping in a region of fairly...

Properties of hydrogenated amorphous carbon films and the effects of doping

Hall mobility and conductivity measurements have been made as a function of temperature on a series of n- and p-type a—Si specimens prepared by the glow discharge technique and doped with phosphorus or boron atoms. The Hall effect exhibits an interesting double reversal in sign : n-type samples show a Hall potential in the direction normally expected for holes and the opposite applies to p-type specimens. In lightly doped n-type samples, the observed Hall mobility μmH has a magnitude of about 0.1 cm2V−1see−1, essentially independent of temperature; this is in agreement with theoretical predictions for transport in the extended states. On increasing the donor density, μmH begins to show a temperature dependence which varies systematically with the doping level. The conductivity and Hall data for n-type specimens have been analysed in terms of a model with two parallel conduction paths, one in the extended electron states and the other through the localized impurity levels where transport takes pla...

Hall effect and impurity conduction in substitutionally doped amorphous silicon

Thermoelectric power and conductivity measurements have been made as a function of temperature on a series of a-Si specimens doped substitutionally with phosphorous. The samples were prepared by the glow discharge decomposition of silane containing predetermined concentrations of phosphine. The thermoelectric power of all the samples investigated was negative as expected. For the lightly doped specimens the thermoelectric and conductivity results support the model developed from previous work. Electron transport is in the extended states, but below room temperature there is an increasing contribution from tail state hopping. At higher doping concentrations the data can be interpreted in terms of transport through the extended states and the localized donor states, as suggested by the analysis of recent Hall effect results. The value of the intercept S 0 on the S axis at 1/T = 0 depends strongly on the position of the Fermi energy ∊F with respect to the mobility edge at ∊c. The movement of ∊F with...

Thermoelectric power in phosphorous doped amorphous silicon

Hydrogen content, electrical properties and stability of glow discharge amorphous silicon

Photothermal deflection spectroscopy has been used to measure optical absorption for two series of amorphous silicon-based alloys prepared by the decomposition of silane/ammonia or silane/nitrous oxide mixtures to produce a-SiNx or a-SiOx respectively. Measurements have been made in the energy range 1 to 5 e V. Each sample shows a well-defined exponential edge and a low-absorption region. The inverse slope of the exponential edge increases monotonically with nitrogen or oxygen content. The low-absorption region is analysed and discussed in terms of the defect-state density. For the silicon nitride samples the results can be interpreted in terms of silicon dangling-bond defect states. On the incorporation of some nitrogen the density of defect states increases, but the defect remains at a certain energy above the valence band edge. As stoichiometry is approached the number of such states decreases. For the silicon oxide samples, oxygen incorporation results in a decrease in the density of silicon ...

D. I. Jones

Papers

Properties of hydrogenated amorphous carbon films and the effects of doping

Hall effect and impurity conduction in substitutionally doped amorphous silicon

Thermoelectric power in phosphorous doped amorphous silicon

Hydrogen content, electrical properties and stability of glow discharge amorphous silicon

Optical absorption and defects in amorphous SiNx and SiOx