Microwave detected transient photoconductivity measurements during plasma deposition of intrinsic hydrogenated amorphous silicon

Contactless in-situ measurements of the microwave detected transient photoconductivity during growth of intrinsic a-Si:H films by plasma enhanced chemical vapour deposition are presented. It is shown, that these measurements can be performed without perturbation of the deposition process. The growth of a-Si:H films at 250° C and 120° C substrate temperature is studied and the information obtained from these measurements is discussed. In-situ characterization during growth of a multilayer structure with films deposited subsequently at 120° C, 250° C and again at 120° C is shown.     

Plasma deposition of hydrogenated amorphous silicon: Studies of the growth surface

This review focusses on the plasma-surface interactions and surface processes involved in a-Si: H thin film growth. We restrict our discussion of growth fluxes to a summary, and do not address plasma kinetics. In recent years, powerful in situ experiments have been carried out on the growing film surface, which reveal the adsorption, penetration, reaction, and elimination of precursor species, as well as the atomic-scale morphology and composition of the growth zone. Good data sets are available both for PACVD and reactive magnetron sputter deposition. These form an interesting comparison, since the former process is dominated by the hydrogen-rich radical SiH₃ at low energy, and the latter by energetic atomic Si and H. We review the key experiments and conclusions, underlining those aspects which are well established and those which remain qualitative; and we discuss the transition from amorphous to fine-grained polycrystalline film growth at high hydrogen pressures in terms of the surface mechanisms. This field is now entering a scientific stage where a detailed theory of low-temperature, plasma-assisted growth can be developed.     

Characterization of amorphous silicon films by contactless transient photoconductivity measurements

Contactless transient photoconductivity measurements of intrinsic a-Si: H films in the microwave frequency range are presented. The measurements are evaluated quantitatively and the electron drift mobility is determined. It is shown that the influence of the surface on the observed decay behaviour can be neglected. A relation between the long time decay behaviour and the position of the Fermi level is observed.     

Optical and electronic properties of hydrogenated amorphous silicon films deposited by square-wave modulated RF discharges of silane-he mixtures

The optoelectronic properties of a-Si: H films deposited under very different plasma conditions have been studied as functions of the square-wave modulation frequency of a 13.56 MHz rf discharge. Results for two extreme deposition conditions are presented. At both high and low deposition rates, the modulation of the rf discharge results in a negligible effect or a deterioration of the film properties, particularly the photoconductivity which we have found to be the most sensitive parameter. At high deposition rates, modulation of the discharge at 10 Hz results in the incorporation of particles into the film which produce enhanced absorption in the film. Therefore, for our reactor configuration, the modulation of the discharge does not produce an improvement but rather a degradation of the film properties.     

Low-temperature formation of silicon nitride films using pulsed-plasma CVD under near atmospheric pressure

Silicon nitride (SiN_X) film fabrication on polyethylene terephthalate (PET) substrates has been achieved at a low temperature (∼100 °C) by plasma enhanced chemical vapor deposition operated at near atmospheric pressures. A short-pulse based power system was employed to maintain a stable discharge of SiH₄, H₂ and N₂ in near atmospheric pressures without the use of any inert gases such as He. The deposited films were characterized by X-ray photoelectron spectroscopy. Cross sections of the films were observed by scanning electron microscope (SEM). Despite the use of N₂ in place of NH₃, a high deposition rate (290 nm/min) was obtained by this near-atmospheric-pressure plasma.     

The effects of arsenic source contamination on doped GaAs grown by MBE

The presence of parts per million (ppm) levels of impurities in the low-temperature arsenic charge used in molecular beam epitaxy (MBE) is shown to have a dramatic effect on the quality of GaAs grown. Source charge impurities play a vital role in determining the apparent variation in reported doping behaviour in MBE:GaAs.     

Temperature dependence of photodegradation in amorphous hydrogenated silicon

Measurements of steady-state photoconductivity with respect to light-induced defect generation in amorphous hydrogenated silicon (a-Si: H) show that the power index of the time evolution (long-term observation) of the photodegradation is determined by the exposure temperature and the material.     

Excimer laser photolysis of molybdenum hexacarbonyl with buffer gas

We have studied the photolysis of molybdenum hexacarbonyl by focused excimer laser radiation. It was found that electronically excited Mo atoms detected in the focus region of a KrF laser are due to a direct two photon absorption transition. The upper limit of the energy for complete dissociation of Mo(CO)₆ has been derived from these results. Two photon dissociation in the gas phase should be the dominant process at metal film deposition on substrates positioned perpendicularly and near to the focus. Adding buffer gases to the organometallic vapor particle formation was observed in the whole irradiated gas volume. The analysis of scattered He-Ne laser light yields information about the density and size of these particles. Some conclusions are drawn with respect to structured metal film deposition with high spatial selectivity.Dedicated to Prof. Dr. Herbert Welling on the occasion of his 60th birthday     

Visible photoluminescence from hydrogenated amorphous carbon films prepared by pulsed laser ablation of polymethyl methacrylate (PMMA)

Visible photoluminescence (PL) has been observed from the hydrogenated amorphous carbon (a-C:H) films prepared by ArF pulsed laser ablation of polymethyl methacrylate (PMMA) in the presence of hydrogen gas (H₂). With increasing hydrogen concentration the PL intensity increases and the intensity maximum blue-shifted. The PL intensity also increases after hours of light illumination, exhibiting a light soaking enhancement. Increasing the deposition temperature decreases the PL and increases the ratio of sp³/sp² bonds.Z.F. Li is on leave from Department of Physics, Nanjing University, Nanjing 210093, P.R. China.     

Doping properties of microcrystalline silicon prepared by mercury sensitized photochemical vapor deposition

Conductivity of photo-CVD microcrystalline silicon (c-Si:H) in wide range of dopant gas concentration (10^–53/SiH₄, B₂H₆/SiH₄<10^–2) is investigated. As compared with a-Si:H, the conductivity of the film is improved more than two orders of magnitude by microcrystallization for a wide range of dopant concentration at a deposition temperature of as low as 150°C. This indicates the suitability of photo-CVD for low temperature processing. A conductivity minimum is found at a doping ratio of about B₂H₆/SiH₄=1×10^–5.     

Relationship between optical and structural properties of hydrogenated amorphous silicon

A series of experimental studies has been made on the relationship between optical and structural properties of hydrogenated amorphous silicon (a-Si:H) prepared under various conditions. It has been clarified by analysing the results that the shape of the energy spectrum near the band edge and the distribution of the valence-band tail states depend primarily on the structural disorder of the Si network in a-Si:H. On the other hand, the total content and the bonding mode of bonded hydrogen have little effects on these electronic properties of a-Si:H. It has also been found that the distribution of the valenceband tail states might be related to other unidentified factor(s) besides the structural disorder. The present results have been compared with those of the previous experimental and theoretical studies.     

Low-temperature fabrication of silicon nitride films by ArF excimer laser irradiation

The low-temperature fabrication of silicon nitride films by ArF excimer laser irradiation has been studied. Two fabrication methods are presented. One is photoenhanced direct nitridation of a silicon surface with NH₃ for very thin gate insulators, and the other is photo-enhanced deposition of silicon nitride films with Si₂H₆ and NH₃ gases for stable passivation films. The ArF excimer laser irradiation dissociates the NH3 gas producing NH and NH₂ radicals which proved effective in instigating the nitridation reaction. The quality of both films has been much improved and the growth temperature has been lowered by using laser irradiation. These photo-enhanced processes seem to be promising ULSI techniques because they do not depend on high temperatures and are free from possible reactor contamination.     

LIF diagnostics of C2 radical behaviour in a laser CVD environment

A laser-induced fluorescence (LIF) system was developed to diagnose the reaction and transport of radicals in the ArF-laser-assisted CVD environment. The C₂(a ³ II _u) radicals were produced by the multi-photon dissociation of C₂H₂. The transport of the radical was directly measured by LIF. The disappearance rate of the radical in C₂H₂ was also determined. By using the values determined, the in-flux of the C₂ radicals onto a substrate was investigated, based on a diffusion model.     

Electron spectroscopy measurements on hydrogen implanted graphite and comparison to amorphous hydrogenated carbon films (a-C:H)

Highly oriented pyrolytic graphite (HOPG) as well as polycrystalline graphite (pc-graphite) were bombarded with 3.5 keV H⁺ ions by means of a Penning ion source. The implanted graphite was characterized by in situ electron spectroscopy techniques such as UPS, XPS and EELS. Our UPS valence band measurements of the hydrogen saturated graphite reveal it to be an insulating phase, and XPS measurements show a shift of the C1s core level to higher binding energy with respect to pristine graphite. This behavior is explained by a Fermi energy shift upon hydrogen bombardment of graphite.In addition, a close resemblance in the electronic structure of hydrogen bombarded graphite and amorphous hydrogenated carbon films (a-C:H) is shown which suggests the modification of pristine graphite to an amorphous network [1] of mostly tetrahedrally bonded carbon atoms by hydrogen implantation.     

Photo-chemical vapor deposition of hydrogenated amorphous silicon films

A photo-chemical vapor deposition, using ultraviolet light excitation and a mercury photo-sensitization, was investigated for depositing hydrogenated amorphous silicon films from SiH₄. The photoelectric and structural properties were examined to characterize the deposited films. Those properties were depended strongly on substrate temperature, and the films which were deposited at a substrate temperature more than 200°C contained dominant SiH configurations. A relatively large single crystalline grain size of about 0.5 m was observed in a 1.0 m thick film, which was obtained at a substrate temperature as low as 200°C. Phosphorus impurity doping into the films and Pt-Schottky diode fabrication were also attempted.     

Excimer-laser-assisted RF glow-discharge deposition of amorphous and microcrystalline silicon thin films

We combine the deposition of Hydrogenated amorphous Silicon (a-Si:H) by rf glow discharge with XeCl-excimer laser irradiation of the growing surface in order to obtain different kinds of silicon films in the same deposition system. In-situ UV-visible ellipsometry allows us to measure the optical properties of the films as the laser fluence is increased from 0 up to 180 mJ/cm² in separate depositions. For fixed glow-discharge conditions and a substrate temperature of 250° C we observe dramatic changes in the film structure as the laser fluence is increased. With respect to a reference a-Si:H film (no laser irradiation) we observe at low laser fluences (15–60 mJ/cm²) that the film remains amorphous but demonstrates enchanced surface roughness and bulk porosity. At intermediate fluences (80–165 m/Jcm²), we obtain an amorphous film with an enhanced density with respect to the reference film. Finally, at high fluences (165–180 mJ/cm²), we obtain microcrystalline films. The in-situ ellipsometry measurements are complemented by ex-situ measurements of the dark conductivity, X-ray diffraction, and Elastic Recoil Detection Analysis (ERDA). Simulation of the temperature profiles for different film thicknesses and for three laser fluences indicates that crystallization occurs if the surface temperature reaches the melting point of a-Si:H ( 1420 K). The effects of laser treatment on the film properties are discussed by taking into account the photonic and thermal effects of laser irradiation.Presented at LASERION 93, Munich, June 21–23, 1993     

Validity of quasi-static capacitance-voltage measurements applied to hydrogenated amorphous silicon diodes

We have reexamined the validity of quasi-static capacitance-voltage (C-V) measurements when applied to hydrogenated amorphous silicon (a-Si: H) diodes. Displacement currents with the application of a linear ramp voltage to an a-Si:H Schottky diode exhibit a slow response with time constants ranging 0.1–1 s which cannot be measured completely by the conventional measurements. The measured capacitance and the effective density of gap states obtained from the measurement depend on the timing of current observation even when the small value of the order of 0.01 V/s is chosen for the ramp rate. We propose a possible means to realize the true quasi-staticC-V measurement of a-Si:H diodes.     

CO2 laser CVD of a-Si:H: in situ gas analysis and model calculations

4 and disilane Si₂H₆ induced by continuous wave CO₂ laser irradiation has been investigated under the conditions of chemical vapor deposition (CVD) of amorphous hydrogenated silicon a-Si:H. At the very position of depositing the thin film the stationary chemical composition of the processing gas is probed in situ by an effusive molecular beam which passes through a differential pumping stage into a quadrupole mass spectrometer (QMS). With SiH₄ as educt and SF₆ as a sensitizer, SiH₄ and Si₂H₆ are found in the processing gas while Si₃H₈ or higher silanes are lacking. Si₂H₆ and SF₆ lead to SiH₄, Si₂H₆, and Si₃H₈, but higher silanes are missing. The experimentally determined composition of the processing gas is semi-quantitatively reproduced by model calculations based on the assumption of stationary local equilibrium conditions and applying thermodynamic and spectroscopic data (molecular statistics). The mass balance of the processing gas entering and leaving the CVD chamber states an atomic ratio Si:H of 1:2 for the gas phase species forming the solid deposit on the reactor walls. This finding together with theoretical considerations indicates the intermediate Si₂H₄ to be the dominating gas phase species forming the a-Si:H thin films. Received: 17 July 1998/Accepted: 20 July 1998     

Lateral growth rates in laser CVD of microstructures

Lateral growth rates of Ni spots deposited on absorbing substrates by decomposition of Ni(CO)₄ with visible Kr⁺ laser light have been measured. The experimental data are consistent with the calculated temperature distributions. The mechanism of decomposition is thermal with an apparent chemical activation energy of 22±3 kcal/mole for the temperature range 350 KT500 K.     

CO2 laser CVD of copper lines with twofold periodic structures

A continuous wave CO₂ laser beam was vertically directed onto glass substrates to deposit copper via CO₂ laser CVD by decomposition of copper hexafluoroacetylacetonate. With fixed substrates deposits of volcano like shape were obtained after short irradiation times. Upon further processing the volcano structures became completely filled. With moving substrates the laser CVD process led to stripes of deposits. Their transverse and longitudinal profiles were dependent on the processing conditions: Uniform stripes were obtained as well as periodically structured traces even with twofold patterns.