Y.W. Lam

Characterization of heavily boron-doped diamond films

A series of boron-doped polycrystalline diamond films grown by hot-filament-assisted chemical vapor deposition was studied with transmission electron microscopy (TEM), secondary-electron microscopy, Raman spectroscopy, Hall-effect and current-voltage measurements. The grain size of the diamond films decreased as the resistivity of the films decreased. A drastic change of the Raman spectra with the film resistivity was observed. The zone-center optical phonon line at 1332 cm−1 downshifted and decreased as the film resistivity decreased, and even disappeared in the sample with the lowest resistivity. On the other hand, broad peaks around 465 cm−1 and 1220 cm−1 appeared and their intensity increased with decreasing resistivity. We attributed these phenomena to the breakdown of the k=0 selection rule of the Raman scattering owing to the heavy boron incorporation in the film. A TEM study showed that all the films were of the cubic diamond structure. No boron-carbon compound was found in these films. This finding agreed well with the Raman result. Increased mobility and non-valence-band conduction were observed. As to the defect study, twinning was found to be the dominant feature in these films, and an increase of dislocation density was also observed with decreasing resistivity of the films.

The influence of boron doping on the structure and characteristics of diamond thin films

Abstract Diamond thin films doped with various boron concentrations were grown by hot-filament-assisted chemical vapor deposition. The charge carrier density in the films ranges from 3.4 × 10 17 to 1.8 × 10 21 cm −3 . TEM studies show that all films are of cubic diamond structure with no impurity phase. As the boron concentration increases, the average grain size of the film decreases from 10 to 1 μm. A significant decrease in plane defect density and a slight increase in dislocation density were also observed as boron content increases. Penetration twin, caused by secondary nucleation, was found to be a common feature in films with high boron contents. It is considered that the small grain size in heavily boron doped samples is mainly caused by the formation of these twins. Significant differences were observed from the Raman spectra of the films with different boron content. As the boron concentration increases, the zone center optical phonon line at 1332 cm −1 downshifts and weakens while broad peaks around 465 and 1220 cm −1 appear and their intensities increases. This is considered to be the result of the breakdown of the k = 0 selection rule due to the high boron content.

Deposition and properties of a-C:H films on polymethyl methacrylate by electron cyclotron resonance microwave plasma chemical vapor deposition method

Abstract a-C:H films were deposited on polymethyl methacrylate (PMMA) using electron cyclotron resonance (ECR) microwave plasma (MP) decomposition of CH 4 diluted in Ar gas. The effect of substrate on the deposition process, optical properties and structure of the a-C:H films was studied. A transparent polymer-like carbon film formed on PMMA, while a semi-transparent diamond-like carbon (DLC) formed on silicon under the same growth conditions. The a-C:H films grown under certain conditions were found to substantially improve the wear resistance of PMMA substrates.

Physical properties of a-C:H films prepared by electron cyclotron resonance microwave plasma chemical vapor deposition

Abstract a-C:H films have been deposited on silicon and glass using electron cyclotron resonance microwave plasma decomposition of CH4 diluted with Ar gas at 0–160 V rf negative bias and 65–250 W microwave power. The deposition rates, absorption coefficients, optical bandgaps, refractive indices and internal stresses of the a-C:H films grown under varying preparation parameters have been measured. The microstructure of the carbon films has been evaluated by Raman spectroscopy. It has been found that the properties and structure of the carbon films are strongly dependent upon the rf substrate bias voltage. At lower rf biases (0 and 40 V), the films are transparent polymer-like carbon films with lower refractive indices, lower internal stresses, and higher optical bandgaps; at higher rf biases (80, 120 and 160 V), they are semi-transparent diamond-like carbon films with higher refractive indices, higher internal stresses and lower optical bandgaps. Microwave power has little influence on the properties and structure of the a-C:H films.

Diamond films grown by hot filament chemical vapor deposition from a solid carbon source

Continuous diamond films have been grown on mirror-polished silicon, polycrystalline copper, SiC and high oriented pyrolytic graphite substrates using hot filament assisted chemical vapor deposition. Instead of using conventional gaseous carbon source, a graphite plate was used as the carbon source with hydrogen as the only feeding gas. No substrate pretreatment such as diamond powder scratching or negative biasing was needed. When carbon-13 was used as the carbon source, only diamond-12 was detected on carbon-13 indicating that deposition rather than direct conversion of carbon to diamond occurred. High nucleation density and conversion efficiency were obtained for the present growing process. Diamond films grown by this method were found to be particularly pure in diamond content.

Electron affinity and surface re-ordering of homoepitaxial diamond (100)

The change in electron affinity and the re-ordering of the diamond surface upon annealing of a homoepitaxially grown diamond (100) thin film were investigated by ultraviolet photoemission spectroscopy, low energy electron diffraction (LEED), and Auger electron spectroscopy. When the sample was heated to ~1250° C, the electron affinity of the sample changed from negative to positive and the LEED pattern changed from (2×1/1×2) to (1×1). When the sample was further annealed at ~900° C, the electron affinity and the LEED pattern reverted to negative and (2×1/1×2) respectively. The sample was then heated to ~1100° C, followed by annealing at ~900° C again, and the same phenomenon was observed. These observations were considered to be associated with the disordering/re-ordering behavior of the diamond surface upon annealing.

Electron affinity and effect of annealing on heavily boron-doped diamond films

A series of heavily boron-doped polycrystalline diamond films was studied. It was found that the electron affinity of the diamond surfaces increased with increasing doping concentration and the Fermi level was pinned at similar to 0.35 eV above the valence band maximum (VBM). After annealing at similar to 500 degrees C, the electron affinity of all samples decreased and the Fermi level increased to 0.87 eV. These changes were concurrent with a decrease in the concentration of both boron and oxygen, and an increase in that of BxOy relative to elemental boron on the diamond surfaces. A model was proposed to explain the dependence of electron affinity on boron concentration, and to explain other changes observed upon annealing.

Probing of microvoids in high-rate deposited a-Si : H thin films by variable energy positron annihilation spectroscopy

In this paper, positron annihilation measurements have been carried out on a-Si : H thin films deposited by plasma-enhanced chemical vapor deposition (PECVD) at high and low rates by means of the variable energy positron beam Doppler-broadening technique. The depth profiles of microvoids in the films grown under different conditions have been determined. We found a smaller void fraction in the surface region of all films compared to the bulk, and a smaller void fraction in low rate than in high growth rate films. By plotting S and W parameters in the (S, W) plane, we have shown that the vacancies in all of the high-rate and low-rate deposited intrinsic samples, and in differently doped low-rate samples are of the same nature, although there appears to be a higher density of defects in the boron than phosphorus doped films. The depth profiles of the microvoid-like defects in the a-Si : H films are extracted by use of the VEPFIT program.

Study Of Gate-to-source Parasitic Capacitance In A-si Tft

The gate-to-source parasitic capacitance in a-Si TFT consists of both geometric and apparent ones, in whlich the former is caused by the geometric overlapping between gate and source electrodes, and the latter is originated from the field overlapping due to fringe of electric field by gate and source electrodes in a-Si TFT. This presentation has given a detailed analysis on both geometric and apparent parasitic capacitances so that a contour map used to determine the storage capacitance in a-Si TFT for active matrix LCD has been drawn up.

Interfacial Effects On Characteristics Of A-si Tft

The concept of effective density of interface states at a-SiNx:H/a-Si:H interface has been used to analyse the performance of a-Si TFT theoretically, such as field effect, flat band voltage, subthreshold characteristics, as well as experimental research on a-Si TFT. The transfer characteristics of various a-Si TFT and the sub-threshold characteristic parameter S, with different fabrication conditions , have verified the conclusions drawn up from the theoretical analysis in this paper.