F. Gaspari

Determination of the sp3/sp2 ratio of a-C:H by XPS and XAES

Abstract X-ray photoelectron spectroscopy and X-ray excited Auger electron spectroscopy were used to determine some of the properties of diamond like amorphous carbon films deposited using saddle field glow discharge of methane. By applying the chemical shift technique and curve fitting technique to the C1s peak in the X-ray photoelectron spectrum of the film, we were able to determine the sp 3 /sp 2 bonding ratio of the films. The ratio from this method was in agreement (±1%) with that calculated from the X-ray excited Auger electron spectrum of the same film. The method was also applied to a-C:H films doped with different impurities (boron and phosphorus) in amounts varying from 1% to 20%. We found that the sp 3 /sp 2 ratio of the film was dependent on the deposition parameters and on types of impurities and their concentrations.

Tritiated amorphous silicon films and devices

The dc saddle-field glow discharge system was used to stably bond tritium in hydrogenated amorphous silicon films. A betavoltaic battery is demonstrated using tritiated-hydrogenated amorphous silicon as the intrinsic layer in a p–i–n diode and betaconductivity was observed in a-Si:H:T films. Although the half-life of tritium is 12.5 years, the decay of tritium appeared to rapidly increase the midgap density of states which decreased the excess carrier lifetime and decreased the power from the betavoltaic battery. The betaconductivity properties of the a-Si:H:T film were also affected.

The preparation of amorphous Si:H thin films for optoelectronic applications by glow discharge dissociation of SiH4 using a direct-current saddle-field plasma chamber

Hydrogenated amorphous silicon films (a‐Si:H) have been prepared by glow discharge decomposition of silane (SiH4) in a dc saddle‐field cavity. In the saddle‐field electrode configuration, electrons can oscillate along the axis of the plasma chamber, increasing the effective path length for ionizing collisions. This allows discharge formation over a wide range of pressures, avoiding the tuning problems that are encountered with rf techniques while providing better control of the ion energy, direction of ion motion, and ion density. The high discharge current density that is obtainable at moderate pressures facilitates relatively high growth rates exceeding 5 A/s. The fraction of hydrogen incorporated can be directly controlled by coevaporating Si during the discharge deposition. a‐Si:H films exhibiting high photoconductive gains that exceed 103 under AM1 illumination have been obtained over a wide range in bonded hydrogen content, from about 5 to 25 at. %. The dependence of the optical gap on hydrogen cont...

Structural properties of a‐C:H deposited using saddle‐field glow‐discharge decomposition of methane

Diamondlike hydrogenated carbon films were deposited using saddle‐field glow discharge in pure methane. The structure of the films was studied using x‐ray photoelectron spectroscopy, x‐ray stimulated Auger electron spectroscopy, and Raman spectroscopy. It was found that for appropriate conditions of pressure and substrate bias a very high percentage of sp3 bonding could be achieved.

The optical absorption edge of diamond-like carbon: A quantum well model

This article presents an analysis of the optical absorption edge of diamond-like carbon, based on transitions between confined electronic states in quantum wells. This theory is proposed to replace the commonly-used Tauc and Urbach expressions. It uses the cluster model of sample structure: the wells correspond to islands of graphitic, sp2-bonded material embedded in a diamond-like, sp3-bonded skeleton. A percolation model is used to give the cluster size distribution. Near-edge optical absorption coefficients are determined approximately for macroscopic thin films that have low fractions of sp2 bonding. Experimental results provide preliminary confirmation of the approach.

Hydrogen effusion from tritiated amorphous silicon

Results for the effusion and outgassing of tritium from tritiated hydrogenated amorphous silicon (a‐Si:H:T) films are presented. The samples were grown by dc-saddle field glow discharge at various substrate temperatures between 150 and 300°C. The tracer property of radioactive tritium is used to detect tritium release. Tritium effusion measurements are performed in a nonvacuum ion chamber and are found to yield similar results as reported for standard high vacuum technique. The results suggest for decreasing substrate temperature the growth of material with an increasing concentration of voids. These data are corroborated by analysis of infrared absorption data in terms of microstructure parameters. For material of low substrate temperature (and high void concentration) tritium outgassing in air at room temperature was studied, and it was found that after 600h about 0.2% of the total hydrogen (hydrogen+tritium) content is released. Two rate limiting processes are identified. The first process, fast tritiu...

Luminescence in hydrogenated amorphous carbon films grown by dc saddle‐field glow‐discharge decomposition of methane

Photoluminescence in hydrogenated amorphous carbon thin films deposited using the dc saddle‐field glow‐discharge technique onto glass and single‐crystal silicon substrates was studied. Samples prepared using positive substrate bias exhibited strong broad‐band photoluminescence at room temperature. The luminescence spectrum had a major peak at 1.9 eV and two smaller peaks at 2.3 and 2.6 eV. Samples prepared using grounded or negatively biased substrates exhibited only weak photoluminescence near 2.6 eV. Infrared spectroscopy indicates that the luminescence at 1.9 and 2.3 eV is related to the presence of C–OH bonds, whereas the photoluminescence at 2.6 eV appears to be an intrinsic property of the a‐C:H films.

The preparation of hydrogenated amorphous silicon by plasma-enhanced reactive evaporation

Abstract A novel technique, plasma-enhanced reactive evaporation (PERE), has been developed for the preparation of a-Si:H. Silicon is evaporated from a molten source and deposited onto a substrate in the presence of a dc glow discharge in hydrogen or silane at pressures from about 0.01 to 0.1 Torr. The PERE method facilitates relatively high deposition rates, direct control of bonded hydrogen incorporation and uniform hydrogenation of large substrate areas. The dark electrical transport characteristics of PERE a-Si:H films are comparable to those of discharge-deposited films containing considerably more bonded hydrogen. Furthermore, the results suggest that the presence of SiHn radicals is required during film growth to obtain highly photoconductive films.

Structural, optical, and electrical properties of doped hydrogenated diamond-like amorphous carbon films deposited using the dc saddle-field glow-discharge technique

The dc saddle-field glow-discharge technique was employed to deposit undoped, phosphorus-doped and boron-doped hydrogenated diamond-like amorphous carbon films. The undoped films were grown using pure methane, while the doped films were grown using methane diluted with dopant gases (phosphine and diborane) in mole fraction ranging from 0.05 to 1×10−5. Secondary ion mass spectroscopy was used to determine the composition of these films. The results showed that various levels of doping in amorphous carbon films can be achieved predictably by using the appropriate mole fraction of dopant gases. The fractions of tetrahedral and trigonal bonds in the films were obtained using x-ray Auger electron spectroscopy. The optical energy gaps of the films were determined from optical absorption measurements. These data were correlated with the doping levels of the films. The electrical conductivities of the undoped and doped samples were determined at temperatures in the range from 225 to 380 °C. The activation energie...

Reduction of silicon dioxide by aluminum in metal–oxide–semiconductor structures

The reduction of SiO2 by Al was studied in Al/SiO2/Si structures above 350 °C. It was found that Al displaces Si in the oxide, forming an Al–O compound with an Al:O concentration ratio between 1:1 and 1.3:1. In the reacted areas, less than 1 at. % Si is left in what was originally a pure SiO2 matrix. The activation energy for the reaction in a pyrogenic oxide is approximately 2 eV. The reaction was found to be responsible for electrical failure of diodes manufactured using Al/SiO2/Si technology.