Shu-Han Lin

Microhardness study of amorphous hydrogenated boron carbide deposited on a cathode substrate by plasma deposition

We have grown amorphous hydrogenated boron carbide thin films on a cathode substrate by rf plasma decomposition of diborane and methane. The chemical composition, infrared absorption, optical absorption, microhardness, and adhesion of these thin films were measured. As a function of increasing diborane concentration in the feedstock, we observe increasing boron and decreasing hydrogen concentrations, increasing infrared absorption at 1300 cm−1 due to boron icosahedra, increasing optical band gaps, dramatically increased microhardness, and increased adhesion to the underlying substrates of these thin films. These results provide evidence that the presence of boron icosahedra increases microhardness, adhesion, and optical band gaps.

Electrical and electron spin resonance measurements of amorphous hydrogenated carbon nitride

Abstract The electrical conductivity and electron spin resonance spectra of amorphous hydrogenated carbon nitride and amorphous hydrogenated carbon are reported. Both measurements present evidence that the addition of nitrogen significantly decreases the density of dangling bonds, similar to the role of hydrogen in amorphous hudrogenated silicon. These results are also consistent with previously reported optical absorption and photoluminescence measurements.

13C NMR spectroscopy of amorphous hydrogenated carbon nitride

Abstract The 13 C NMR spectra of chemical vapor deposited amorphous hydrogenated carbon nitride thin films were measured and a number of sharp lines superimposed on top of a broad peak were observed. These sharp lines have been interpreted as arising from nanocrystals of nitrogen-containing aromatic rings terminated by amino groups. The concentration of these nanocrystals increases with increasing nitrogen concentration and decreases with thermal annealing. These nanocrystals are responsible for the increased structural order in these films. Similar nanocrystals are probably present in sputtered carbon nitride thin films. There is no 13 C NMR evidence of any phase of crystalline carbon nitride in either the chemical vapor deposited or sputtered films.

Luminescence study of amorphous hydrogenated carbon grown with varying self-bias voltages

As a function of increasing self-bias voltage across the plasma, the following changes are observed in the grown amorphous hydrogenated carbon thin films: the photoluminescence intensity decreases while the peak position shifts only silightly to lower energy; the optical bandgap decreases; the hydrogen concentration decreases; and the carbon dangling bond electron spin resonance signal increases. We explain all these trends in terms of increased number of electron collisions in the plasma that break more carbon-hydrogen bonds in precursor ions and molecules, leading to larger clusters of graphite-like cores coated with hydrogen in the grown films.

The role of hydrogen in the growth of amorphous hydrogenated carbon

Abstract We have investigated the role of hydrogen atoms, ions, and molecules in the growth of amorphous hydrogenated carbon by plasma chemical vapor deposition. By varying the ratio of CH 4 to H 2 in the feedstock, we varied the concentration of hydrogen atoms, ions, and molecules in the plasma. We observed that with increasing hydrogen concentrations in the plasma, the film growth rate decreases, the hydrogen concentration in the grown film decreases, and the optical bandgap decreases. We interpret these results in term of increased hydrogen etching of the carbon-hydrogen bonds that terminate the growing graphite-like crystallites. This leads to larger graphite-like crystallites, lower hydrogen concentrations in the growing film, and consequently, smaller optical bandgaps.

Doping vs alloying in amorphous hydrogenated boron carbide

Abstract We have grown amorphous hydrogenated boron carbide thin films with boron concentrations ranging from 0 to 18 atomic percent by plasma decompostion of a feedstock of diborane and methane. The chemical composition, optical bandgap, and electrical conductivity activation energy data provide convincing evidence that this material is an alloy at high boron concentrations. On the other hand, the electrical conductivity prefactors, which are proportional to the acceptor densities, demonstrate that the boron also acts like a dopant, with increasing boron concentrations leading to increasing acceptor densities. Finally, the samples with the highest boron concentrations also had the highest hydrogen concentrations — 61 atomic percent — strongly suggesting that this material is a tenuous boron-carbon matrix with plenty of methyl, methylene and borene groups hung from the matrix.

Boron carbide icosahedra and the 1280 cm−1 line in amorphous hydrogenated boron carbide

Abstract We report infrared absorption measurements that provide evidence for the presence of boron carbide icosahedra in amorphous hydrogenated boron carbide thin films. The infrared absorption spectra are dominated by an intense line at 1280 cm −1 with a FWHM of ⋍ 320 cm −1 . Similar lines have been previously reported in polycrystalline boron carbide, where boron carbide icosahedra make up the unit cell. In both systems, the linewidth narrows and the peak position shifts to higher energy with increasing carbon concentrations. From annealing studies of amorphous hydrogenated boron carbide, hydrogen plays a very small role in the 1280 cm −1 line. Finally, the integrated intensity of the 1280 cm −1 line is a sublinear function of the boron concentration, providing further evidence that the boron content of these icosahedra increases as the boron concentration of the film increases.

Search for the nitrogen dangling bond in amorphous hydrogenated carbon nitride

Abstract Motivated by the recent observation of the nitrogen dangling bond in amorphous hydrogenated silicon nitride by electron spin resonance, we report our search for the nitrogen dangling bond in amorphous hydrogenated carbon nitride using electron spin resonance. We searched in films as grown, annealed, irradiated with ultraviolet radiation, and with varying nitrogen concentrations. We are led to the conclusion that the nitrogen dangling bond state in amorphous hydrogenated carbon nitride is always below the top of the valence band, always filled with two electrons, and consequently unobservable by electron spin resonce.

Electron spin resonance in microcrystalline cubic boron nitride amorphous hydrogenated boron nitride mixed phase thin films

Electron spin resonance and transmission electron microscopy results are reported that confirm the presence of a microcrystalline as well as an amorphous phase in thin boron nitride films grown by plasma assisted chemical vapor deposition. Line broadening effects in hydrogenated and deuterated samples indicate that a broad central line can be associated with dangling bonds in the amorphous phase, whereas a four-line spectrum and a ten-line spectrum can be associated with paramagnetic boron defect centers located in the microcrystalline phase. The dangling bond concentration is ten times the concentration of the one-boron defects. The hyperfine coupling constant for the one-boron center in the cubic microcrystalline regions is only 20% of the previously reported value for a similar center in hexagonal boron nitride. The relative concentrations of the dangling bonds, the one-boron centers and the three-boron centers depend on the exact plasma conditions used to form the thin films.

Thermal and hydrogen plasma annealing of amorphous hydrogenated carbon

Abstract We have studied the changes in the optical properties of amorphous hydrogenated carbon due to variety of annealing processes. Whether the sample is annealed at 225°C for one hour in vacuum, in hydrogen gas, or in a hydrogen plasma, the photoluminescence intensity decreases, the bandgap decreases, and the intensity of the Urbach tails increases. All of these annealing processes increase the density of defect states, most likely by driving hydrogen out of the film. This is in marked contrast to the observation by one group of previous workers that the luminescence intensity increases by an order of magnitude due to a vacuum anneal.