High rate deposition of a-Si:H and a-SiNx:H by VHF PECVD

Very High Frequency (VHF) plasma enhanced chemical vapour deposition (PECVD) has been applied to hydrogenated amorphous silicon (a-Si:H) and hydrogenated amorphous silicon nitride (a-SiN_x:H) films for thin film transistors (TFTs) fabrication. The effect of the excitation frequency on the deposition rate and the film quality of both films has been investigated. The films were prepared by VHF (30 MHz∼50 MHz) and HF (13.56 MHz) plasma enhanced CVD.High deposition rates were achieved in the low pressure region for both a-Si:H and a-SiN_x:H depositions by the use of VHF plasma. The maximum deposition rates were 180 nm/min for a-Si:H at 50 MHz and 340 nm/min for a-SiN_x:H at 40 MHz. For a-SiN_x:H films deposited in VHF plasma, the optical bandgap, the hydrogen content and the [Si–H]/[N–H] ratio remain almost constant regardless of an increase in deposition rate. The increase of film stress could be limited to a lower value even at a high deposition rate. The TFTs fabricated with VHF PECVD a-Si:H and a-SiN_x:H films showed applicable field effect mobility. It is concluded that VHF plasma is useful for high rate deposition of a-Si:H and a-SiN_x:H films for TFT LCD application.     

Growth behavior of CVD diamond films with enhanced electron field emission properties over a wide range of experimental parameters

In this study, diamond films were synthesized on silicon substrates by microwave plasma enhanced chemical vapor deposition (CVD) over a wide range of experimental parameters. The effects of the microwave power, CH₄/H₂ ratio and gas pressure on the morphology, growth rate, composition, and quality of diamond films were investigated by means of scanning electron microscope (SEM), X-ray diffraction (XRD), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). A rise of microwave power can lead to an increasing pyrolysis of hydrogen and methane, so that the microcrystalline diamond film could be synthesized at low CH₄/H₂ levels. Gas pressure has similar effect in changing the morphology of diamond films, and high gas pressure also results in dramatically increased grain size. However, diamond film is deteriorated at high CH₄/H₂ ratio due to the abundant graphite content including in the films. Under an extreme condition of high microwave power of 10 kW and high CH₄ concentration, a hybrid film composed of diamond/graphite was successfully formed in the absence of N₂ or Ar, which is different from other reports. This composite structure has an excellent measured sheet resistance of 10–100 Ω/Sqr. which allows it to be utilized as field electron emitter. The diamond/graphite hybrid nanostructure displays excellent electron field emission (EFE) properties with a low turn-on field of 2.17 V/μm and β = 3160, therefore it could be a promising alternative in field emission applications.     

Room temperature plasma oxidation in DCSBD: A new method for preparation of silicon dioxide films at atmospheric pressure

In this paper a new process for the preparation of thin silicon dioxide (SiO₂) film is presented: the oxidation of c-Si (1 1 1) surface in atmospheric pressure plasma at room temperature. Diffuse coplanar surface barrier discharge (DCSBD) at atmospheric pressure in air and oxygen atmosphere has been used. The oxidation rate and the thickness of oxidized layers were estimated by ellipsometry. The structure and the chemical composition of oxidized layers were investigated by infrared reflection absorption spectroscopy (IRRAS), X-ray photoelectron spectroscopy (XPS) and energy dispersive X-ray (EDX) analysis. Scanning electron microscopy (SEM) was used to observe the morphology of the layer surface. It was found that stoichiometric SiO₂ layers were obtained with oxidation rates comparable to thermal oxidation.     

Effect of nitrogen environment on NdFeB thin films grown by radio frequency plasma beam assisted pulsed laser deposition

NdFeB is a very attractive material for applications in electrical engineering and in electronics, for high-tech devices where high coercive field and high remanence are needed. In this paper we demonstrate that the deposition of nitrogen doped NdFeB thin films by pulsed laser deposition, in the presence of a nitrogen radiofrequency plasma beam, exhibit improved magnetic properties and surface morphology, when compared to vacuum deposited NdFeB layers. A Nd:YAG pulsed laser (3ω and 4ω) was focused on a NdFeB target, in vacuum, or in the presence of a nitrogen plasma beam. Substrate temperature (RT-850 °C), nitrogen gas pressure, and radiofrequency power (75–150 W), were particularly varied. The thin films were investigated by means of X-ray diffraction, atomic force microscopy, scanning electron microscopy, spectroscopic-ellipsometry, and vibrating sample magnetometry.     

The effect of argon dilution on deposition of microcrystalline silicon by microwave plasma enhanced chemical vapor deposition

Microwave plasma-enhanced chemical vapor deposition (PECVD) is a very promising method for industrial scale fabrication of microcrystalline silicon solar cells since the technique is well applicable for large areas, and high deposition rates can be obtained. We have investigated the effect of Ar dilution on the growth process and the material properties of microcrystalline silicon. The major benefit of Ar addition in the MWPECVD process, using H₂ and SiH₄ as reactant gases, is an improved stabilization of the plasma, in particular at low pressure and MW power. We show, however, that material properties of the microcrystalline silicon layers deteriorate if we partly substitute H₂ by Ar during the deposition. The density of the layers - as expressed by the refractive index - decreases, and the defect density (measured by Fourier transform photocurrent spectroscopy) increases with increasing Ar flow. Investigation of the plasma by optical emission study shows that Ar atoms play a very active role in the dissociation processes of H₂ and SiH₄. Substitution of H₂ by Ar decreases the SiH^? emission and increases the Si^? emission. On the other hand, the H_α/H_β ratio increases upon substitution of H₂ by Ar. The latter effect shows that Ar addition does not lead to higher electron temperatures and we conclude that the changes of SiH^? and Si^? emissions are due to dissociation of SiH₄ by Ar^? (quenching reactions). The precise role of Ar in MWPECVD of microcrystalline silicon needs further investigation, but we conclude that the usage of this gas should be minimized in order to maximize the quality of the silicon layers.     

Effect of HCl addition on the crystalline fraction in silicon thin films prepared by hot-wire chemical vapor deposition

In an effort to increase the crystalline fraction of silicon films directly deposited on a glass substrate by hot-wire chemical vapor deposition, the effect of HCl addition was studied. The silicon film was deposited on a glass substrate at 320 °C under a reactor pressure of 1333 Pa at the wire temperature of 1600 °C with 10%SiH₄–90%He at a fixed flow rate 100 standard cubic centimeter per minute (sccm) and HCl varied at 0, 10, 16 and 28 sccm. With increasing HCl, the crystalline fraction of silicon was increased as revealed by Raman spectra but the growth rate was decreased.     

Dynamic behavior of concrete at high strain rates and pressures: I. experimental characterization

Understanding the behavior of concrete and mortar at very high strain rates is of critical importance in a range of applications. Under highly dynamic conditions, the strain-rate dependence of material response and high levels of hydrostatic pressure cause the material behavior to be significantly different from what is observed under quasistatic conditions. The behavior of concrete and mortar at strain rates of the order of 10⁴ s⁻¹ and pressures up to 1.5 GPa are studied experimentally. The mortar analyzed has the same composition and processing conditions as the matrix phase in the concrete, allowing the effect of concrete microstructure to be delineated. The focus is on the effects of loading rate, hydrostatic pressure and microstructural heterogeneity on the load-carrying capacities of the materials. This experimental investigation uses split Hopkinson pressure bar (SHPB) and plate impact to achieve a range of loading rate and hydrostatic pressure. The SHPB experiments involve strain rates between 250 and 1700 s⁻¹ without lateral confinement and the plate impact experiments subject the materials to deformation at strain rates of the order of 10⁴ s⁻¹ with confining pressures of 1–1.5 GPa. Experiments indicate that the load-carrying capacities of the concrete and mortar increase significantly with strain rate and hydrostatic pressure. The compressive flow stress of mortar at a strain rate of 1700 s⁻¹ is approximately four times its quasistatic strength. Under the conditions of plate impact involving impact velocities of approximately 330 ms⁻¹, the average flow stress is 1.7 GPa for the concrete and 1.3 GPa for the mortar. In contrast, the corresponding unconfined quasistatic compressive strengths are only 30 and 46 MPa, respectively. Due to the composite microstructure of concrete, deformation and stresses are nonuniform in the specimens. The effects of material inhomogeneity on the measurements during the impact experiments are analyzed using a four-beam VISAR laser interferometer system.     

Defect annealing processes for polycrystalline silicon thin-film solar cells

A variety of defect healing methods was analyzed for optimization of polycrystalline silicon (poly-Si) thin-film solar cells on glass. The films were fabricated by solid phase crystallization of amorphous silicon deposited either by plasma enhanced chemical vapor deposition (PECVD) or by electron-beam evaporation (EBE). Three different rapid thermal processing (RTP) set-ups were compared: A conventional rapid thermal annealing oven, a dual wavelength laser annealing system and a movable two sided halogen lamp oven. The two latter processes utilize focused energy input for reducing the thermal load introduced into the glass substrates and thus lead to less deformation and impurity diffusion. Analysis of the structural and electrical properties of the poly-Si thin films was performed by Suns-V_OC measurements and Raman spectroscopy. 1 cm² cells were prepared for a selection of samples and characterized by I–V-measurements. The poly-Si material quality could be extremely enhanced, resulting in increase of the open circuit voltages from about 100 mV (EBE) and 170 mV (PECVD) in the untreated case up to 480 mV after processing.     

On-line characterisation of radiofrequency magnetron sputter deposition of SiOx using elastic recoil detection

A radiofrequency Ar/O₂ magnetron discharge is used to deposit silicon suboxide (SiO_x, 0 ≤ x ≤ 2) films. In this paper we demonstrate how we can apply the high-energy ion beam technique elastic recoil detection (ERD) on-line, i.e. in situ during deposition, to continuously monitor the growing layer thickness and depth resolved composition. From the ERD data the deposition rates of distinctly silicon and oxygen and the composition of the growing film are determined. Using this method several growth conditions can be investigated and compared in a fast and reliable manner on a single substrate. In this work we report the variation of the growth rate and of the composition of the growing layer as a result of the variation of the rf power and show the consequence of keeping the O₂ flow constant in comparison to keeping the O₂ partial pressure constant.     

Titanium oxide thin films synthesis by pulsed – DC magnetron sputtering

Titanium oxide thin films (1–4 μm) were deposited on the porous Hastelloy-X substrates using the pulsed – DC magnetron sputtering technique and characterized by X–ray diffraction (XRD) and scanning electron microscopy (SEM) methods. Firstly, the films were deposited at different distances between the magnetron and the substrate, as magnetron current and pressure in the deposition chamber were constant. The distance between the magnetron and the substrate was changed from 3 cm to 7 cm, and the deposition rate varied between 10.1 nm/min to 6.0 nm/min. Secondly, pressure influence for the deposition rate was investigated. The deposition rate decreased nearly 15% with the decrease of oxygen pressure from 1.3 to 6.0 Pa. Finally, the influence of the bias (applied to the substrate for the increase of deposition rate) on thin films phase and microstructure was investigated.The experimental results showed that formation of pure titanium oxide thin films was observed in all experimental cases. Only crystallite sizes and orientation were changed. The results showed that there is a possibility to change porosity and uniformity of the growing film by changing oxygen partial pressure during deposition or bias application to the substrate. The existence of columnar boundaries and nanocrystalline structure in the films was observed.     

The effect of complexing agents on the oriented growth of electrodeposited microcrystalline cuprous oxide film

Three conventional complexing agents, including lactic acid, citric acid and EDTA, are applied in the electrodeposition of microcrystalline cuprous oxide (Cu₂O) film on indium tin oxide glass substrate. Both scanning electron microscopy and X-ray diffraction have been performed to characterize the morphology and texture of microcrystalline Cu₂O film. It is found that the stability constant of copper-based complex compound can obviously influence the deposition overpotential of Cu₂O, and the overpotential can significantly alter the growth priority of different planes, which results in oriented growth of Cu₂O grains. The quantitative relationships between the stability constant and the deposition overpotential of different complexing agents, as well as the relationship between the overpotential and the formation energy of microcrystalline cuprous oxide's (1 1 0), (1 1 1) and (2 0 0) planes are calculated, respectively.     

Thin-film silicon solar cells applying optically decoupled back reflectors

Thin-film silicon solar cells often apply a metal back reflector (BR) separated from the silicon layers by a thin rear dielectric of thickness around 80 nm or a white paint combined with a thick rear dielectric of several micrometers. In this work, we investigate the optical performance of microcrystalline silicon (μc-Si:H) solar cells applying BRs of various topographies. In contrast to a standard 80 nm-ZnO/Ag BR design, for which the BR nearly strictly follows the texture of the underlying μc-Si:H layers, placing the Ag BR far from the μc-Si:H layers allows for a variation of the BR topography. Irrespective of the investigated BR topographies and also for a conventional white paint BR, long distances (of several micrometers) between the BR and the μc-Si:H layers are found to be detrimental for the light trapping. Optical simulations based on both rigorous and scalar scattering theory have been performed to understand the impact of the diverse BR designs on the optical cell performance.     

Studies on destruction of silicon tetrachloride using microwave plasma jet

A new method for destroying silicon tetrachloride has been proposed, which is based on a microwave plasma jet that operates at atmospheric pressure using hydrogen as work gas. The influence of input power (P) and silicon tetrachloride concentration (φ) on the percent destruction and removal of SiCl₄ was investigated. And the reclaimed solid byproducts were characterized by SEM, EDX and XRD. Species in the plasma, which were identified by atomic emission spectroscopy were found to include no halogen. Results indicate that the destruction efficiency of silicon tetrachloride can reach 96% when P = 800 W and φ = 1.0%, and the main solid byproduct was Si. The silicon deposited on the molybdenum substrate of the plasma reactor was yellow and typical nano-sized particles with grain size of 54 nm.     

Structural and biocompatible characterization of TiC/a:C nanocomposite thin films

In this work, sputtered TiC/amorphous C thin films have been developed in order to be applied as potential barrier coating for interfering of Ti ions from pure Ti or Ti alloy implants. Our experiments were based on magnetron sputtering method, because the vacuum deposition provides great flexibility for manipulating material chemistry and structure, leading to films and coatings with special properties. The films have been deposited on silicon (001) substrates with 300 nm thick oxidized silicon sublayer at 200 °C deposition temperature as model substrate. Transmission electron microscopy has been used for structural investigations. Thin films consisted of ~ 20 nm TiC columnar crystals embedded by 5 nm thin amorphous carbon matrix. MG63 osteoblast cells have been applied for in vitro study of TiC nanocomposites. The cell culture tests give strong evidence of thin films biocompatibility.     

Effect of the cooling rate on the phase transformation of Astaloy CrL powders modified with SiC addition

This paper discusses issues related to modifying the structure of material depending on the amount of silicon carbide additive and on the applied cooling rate. The study was conducted for Astaloy CrL modified powder with 1 wt.% or 3 wt.% SiC addition and 0.6% C introduced through mechanical alloying. It was concluded that the MA process as well as the addition of silicon have a significant influence on lowering the temperature of iron alloyed with 1.5 wt.% Cr and 0.2 wt.% Mo transformation. Moreover, changing the cooling rate has a very significant influence on formation and control of the sintered material’s microstructure. It was further observed that silicon carbide decomposes and silicon diffuses into the Astaloy CrL base powder.     

A ZnO/PET assembly study: Optimization and investigation of the interface region

Zinc oxide thin films are deposited on polyethylene terephthalate (PET) by r.f. magnetron sputtering process from a ceramic target in oxygen–argon plasmas. Structural studies show that the thin films are highly oriented along the (0 0 2) direction of the würtzite phase when the oxygen partial pressure is lower than 0.2 Pa. The crystallinity is accentuated when the oxygen partial pressure of the sputtering gas is increased from 0 to 0.02 Pa. The composition of the films determined by Rutherford backscattering spectrometry (RBS) varies in a wide range and it is necessary to add a few amount of oxygen in the plasma composition to establish the stoichiometry. The oxygen partial pressure is found to influence also the microstructure and consequently the density of the coatings.Various cold plasmas are used to treat the polymer surface before the deposition of zinc oxide films. Wettability measurements show an increase in the polar component of the PET surface free energy whatever the nature of the plasma used for the treatment. This increase is more obvious with the carbon dioxide plasma. XPS examinations of the CO₂ plasma treated PET surface in optimized conditions show a functionalisation of the polymer surface. The carbon dioxide plasma treatments of PET surface are found to enhance the peeling energy. The adhesion level depends also on the sputtering parameters, mainly the oxygen partial pressure and the r.f. power which influence the coating properties. The zinc oxide/PET interface is studied by XPS at the different stages of deposition and at various take-off angles. AFM observations show a regular growth of zinc oxide layers with smooth topographies on PET films. The different findings obtained from C1s, O1s, Zn2p3/2, Zn3p peaks and Auger Zn L₃M_4.5M_4.5 peak are corroborated and discussed. New chemical bonds between the polymer and the further coming zinc oxide thin layer are evidenced.     

The mechanical properties of ionoplast interlayer material at high strain rates

Ionoplast material has been recently introduced and extensively used as interlayer material for laminated glass to improve its post-glass breakage behavior. Due to its sound mechanical performance, the applications of laminated glass with ionoplast interlayer have been widely extended to the protection of glass structures against extreme loads such as shock and impact. The properties of this material at high strain rates are therefore needed for properly analysis and design of such structures. In this study, the mechanical properties of ionoplast material are studied experimentally through direct tensile tests over a wide strain rate range. The low-speed tests are performed using a conventional hydraulic machine at strain rates from 0.0056 s⁻¹ to 0.556 s⁻¹. The high strain-rate tests are carried out with a high-speed servo-hydraulic testing machine at strain rates from approximately 10 s⁻¹ to 2000 s⁻¹. It is found that the ionoplast material virtually exhibits elasto-plastic material properties in the strain rate range tested in this study. The testing results show that the material behavior is very strain-rate dependent. The yield strength increases with strain rate, but the material becomes more brittle with the increase in strain rate, with the ultimate strains over 400% under quasi-static loading, and decreasing to less than 200% at strain rate around 2000 s⁻¹. The testing results indicate that simply applying the static material properties in predicting the structure responses of laminated glass with ionoplast interlayer subjected to blast and impact loads will substantially overestimate the ductility of the material and lead to inaccurate predictions of structure response. The testing results obtained in the current study together with available testing data in the literature are summarized and used to formulate the dynamic stress–strain curves of ionoplast material at various strain rates, which can be used in analysis and design of structures with ionoplast material subjected to blast and impact loads.     

Tuning of charge carrier density of ZnO nanoparticle films by oxygen plasma treatment

In this work, the charge carrier density of ZnO nanoparticle films was modified after deposition and annealing by an oxygen plasma treatment. The respective films were utilized as active layers in thin film transistors. For a discussion of the plasma–surface interaction on the molecular level, the electrical behavior of the layers was investigated which in general is highly sensitive to low level variations in defect or doping densities. A treatment with remote oxygen plasma at 400 W for 10 s led to a shift of the turn-on voltage from ?12 to 4 V and a reduction of the off-current by more than two orders of magnitude. A model for the influence of oxygen species adsorbed to ZnO nanoparticle surfaces on electrical characteristics of ZnO nanoparticle thin film transistors is introduced.     

Characterization of laser-induced damage in silicon solar cells during selective ablation processes

Selective laser ablation of silicon nitride layers on crystalline silicon wafers was investigated for solar cell fabrication. Laser processing was performed with a nanosecond UV laser at various energy densities ranging from 0.2 to 1.5 J cm^?2. Optical microscopy was used as a simple mean to assess the ablation threshold that was correlated to the temperature at the interface between the silicon nitride coating and the silicon substrate. Minority carrier lifetime measurements were performed using a microwave photo-conductance decay technique. Band to band photoluminescence spectroscopy proved to be a sensitive technique to qualify the laser-induced damage to the silicon substrate. The crystalline structure of silicon seemed to be maintained after silicon nitride ablation as shown by UV reflectivity measurements. Laser parameters corresponding to fluences of around 0.4 J cm^?2 were found to achieve selective ablation of SiN_x without causing detrimental damage to the surrounding material.     

Effect of cooling rate and Mg content on the Al–Si eutectic for Al–Si–Cu–Mg alloys

This study investigates and clarifies the qualitative and quantitative effects of Mg content and cooling rate (ranging from 0.5 to 4 °C/s), on the modification of the silicon eutectic structure and on the undercooling of the silicon eutectic growth temperature (ΔT_Si-eut) in the series of Al–Si–Cu–Mg alloys. The critical Mg content to produce a notable improvement in the silicon eutectic by 1.5 modification levels (regardless of the cooling rate) is 0.6 wt.% Mg. A similar increase in the modification level was also observed when the cooling rate was increased to a maximum of 4 °C/s, regardless of the Mg content. Measurements of the area and roundness of the silicon particles showed a good correlation with the modification level. The undercooling (ΔT_Si-eut) increased by up to ~ 23 °C at a relatively high Mg content and cooling rate and up to ~ 14 °C when the Mg content was increased from 0.4 to 0.6 wt.%.