K. Schade

Oxygen-rich phase segregation in PECVD a-SiOx:H semi-insulators☆

Abstract Photosensitive, low H content (CH = 4–6 at.%) a-SiOx:H, x « 1 , semi-insulating films were deposited by means of the admixture of small amounts of oxygen during plasma enhanced chemical vapour deposition (PECVD) to the monosilane process gas. Oxygen incorporation into the films was favoured by a rotating two-pole magnetic field (0–27 mT) and by increasing the power-source frequency from 1.2 to 13.56 MHz. Film characterization was performed measuring UV/VIS/IR transmission, photo- and dark conductivities and determining the gap state distribution by the constant photocurrent method (CPM). IR-analysis included also the previously neglected formation of dihydride configurations. Three SiOSi stretching vibrations at 980, 1030 and 1080 cm−1 assigned to different oxydation states were found. Thus, the reported shift of the SiO stretching vibration frequency with increasing oxygen concentration is caused by a changing distribution of bonding configurations with a different oxidation state. Evidence for the formation of a clustered O-rich phase was found in films with an oxygen content as low as 0.4 at.%. The deposition rate dependence of the defect state density was consistent with a model of surface reactions including a two-step dangling bond termination via an intermediate SiOSi state whose first step of SiOSi formation is rate determining and whose second step is the recombination of two SiOSi configurations releasing O2 and leaving two SiSi bonds. The distribution of oxygen induced defect states was calculated from measured CPM data.

Boron doped a-SiCx:H films from B(C2H5)3SiH4

Abstract Amorphous hydrogenated silicon films with a hydrogen content of about 1022 cm-3 were deposited in a capacitively coupled radial flow reactor from silane-triethylboron mixtures. Film composition and optical properties were investigated.

Environmentally nonpolluting boron doping of a-Si1−xCx:H with a liquid boron source

The deposition of boron doped a-Si 1−x C x :H films using a boron source which is much less toxic and more stable than B 2 H 6 , namely the liquid B(C 2 H 5 ) 3 , was investigated for a wide range of deposition conditions. These investigations revealed similarities and differences compared to films deposited from silane, hydrocarbon and diborane considering boron doping, carbon and hydrogen incorporation, optical and electrical properties.

Optical emission spectroscopy of silane/argon magnetron discharges

Optical emission spectroscopy was used as an in-situ diagnostic technique during the deposition of a-Si:H films. Emission spectra of rf magnetron discharges of a 90:10 argon-silane mixture were monitored and correlated to the parameters of the deposition process. All emission intensities are mean species residence time dependent. The admixture of hydrogen to the reaction gas lowers the mean electron energy and thus the silane dissociation degree.

Improved quality of high deposition rate a-Si:H films prepared at usual substrate temperature

Abstract In order to optimize the deposition conditions for the growth of a-Si:H deposited at high rates, a numerical model of the plasma process was developed which takes into account the generation of powder at frequencies between 3.6–27.12 MHz. In accordance with the results of the simulation, high deposition rates, 8 nm/s, were obtained by using the magnetically enhanced radio frequency glow discharge decomposition of SiH 4 in a coaxial-type diode system. The effect of the frequency-change above the ion sheath transition frequency limit was studied in combination with the variation of substrate temperature and deposition rate. By increasing the frequency from 3.6 to 27.12 MHz at substrate temperature of 250°C, the material quality of the a-Si:H films was improved (photo- to dark-conductivity ratio 10 5 , defect densities 10 16 cm −3 , and an optical gap of 1.9 eV) and applications in photoelectric devices at deposition rates up to 4.5 nm/s were realized.

Power dissipation in rf glow discharges

Abstract It is shown that caloritmetric power measurements at both electrodes of commercial PECVD reactors using a water cooling system are an additional tool for reactor characterization.

A new helix plasma source for VHF PECVD of a-Si:H

Abstract The helix type plasma source is a resonator consisting of a cylindrical conductor, which is closed on one side by a bottom plate and an inner conductor of spiral form (helix) measuring λ 4 . This plasma source works at 30 to 200 MHz without a matching network. Undoped a-Si:H films were deposited using a He up-stream excitation of SiH4, injected near the substrate in a 20–80 Pa pressure regime. For all runs high gas flow rates of 500–1000 sccm He, with 93 sccm SiH4, were used to minimize contamination within the resonator; the wafer platen temperature was 350°C and the substrate was grounded. The helix plasma source is capable of obtaining deposition rates of 1.0 to 2.8 nm/s; the samples prepared had a high Tauc gap, Eg = 1.82–1.95 eV.