Plasma Oxidation Process Research Articles

Formation of bioactive coatings on a Ti–6Al–7Nb alloy by plasma electrolytic oxidation

In an attempt to increase the bioactivity and corrosion resistance of a vanadium-free titanium alloy Ti–6Al–7Nb, the electrolytic plasma oxidation (PEO) process for surface modification was utilised. Select samples were subjected to further treatment, either thermal or alkali. The morphology, chemical composition and phase composition of the ground and treated Ti–6Al–7Nb alloy substrates were investigated using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). It was observed that during the anodic process under sparking discharge conditions, the simultaneous incorporation of calcium and phosphorus in the forming oxide layer occurs. The resulting layers were porous and exhibited the typical morphology for layers formed during the PEO process. The heat treatment of samples oxidised at 150V resulted in the in surface oxide layer forming a crystalline phases anatase and rutile. After the alkali treatment of samples oxidised at 150V, a gel-like titanate layer was formed. The bioactivity investigations in simulated body fluid (SBF) solution and with human bone marrow stromal cells (MSCs) indicated that after anodising at 150V and following alkali treatment the Ti–6Al–7Nb alloy exhibits osteoinductive properties. The electrochemical investigations showed that application of the anodising process of the Ti–6Al–7Nb alloy significantly improved its corrosion resistance in Ringer solution. The samples anodised at 80V presented the highest corrosion resistance because of the formation of the thin, compact oxide layer on the alloy surface. The approach presented here may be applied for fabricating Ti–6Al–7Nb-based implants.

Photoelectrochemical activity of as-grown, α-Fe2O3 nanowire array electrodes for water splitting

Undoped hematite nanowire arrays grown using plasma oxidation of iron foils show significant photoactivity (∼0.38 mA cm−2 at 1.5 V versus reversible hydrogen electrode in 1 M KOH). In contrast, thermally oxidized nanowire arrays grown on iron exhibit no photoactivity due to the formation of a thick (>7 μm Fe1−xO) interfacial layer. An atmospheric plasma oxidation process required only a few minutes to synthesize hematite nanowire arrays with a 1–5 μm interfacial layer of magnetite between the nanowire arrays and the iron substrate. An amorphous oxide surface layer on hematite nanowires, if present, is shown to decrease the resulting photoactivity of as-synthesized, plasma grown nanowire arrays. The photocurrent onset potential is improved after removing the amorphous surface on the nanowires using an acid etch. A two-step method involving high temperature nucleation followed by growth at low temperature is shown to produce a highly dense and uniform coverage of nanowire arrays.

Characterization of enhancement-mode AlGaN/GaN high electron mobility transistor using N2O plasma oxidation technology

In this study, an enhancement-mode (E-mode) AlGaN/GaN high electron mobility transistor (HEMT) using N2O plasma oxidation process has been performed. Before the gate metal deposition, the AlGaN barrier layer was treated by 135 W N2O plasma for 240 s. The surface native oxide of AlGaN was removed and a 4 nm high quality Ga2O3/Al2O3 compound insulator was simultaneously formed. The threshold voltage of conventional depletion-mode (D-mode) GaN HEMT was −3.6 V and this value can be shifted to +0.17 after N2O plasma oxidation treatment beneath the gate metal. This 4 nm Ga2O3/Al2O3 compound insulator oxidized by N2O can suppress the polarization charge density for E-mode operation of gate terminal and the gate leakage was also reduced simultaneously.

Improved Sensitivity of AlGaN/GaN Field Effect Transistor Biosensors by Optimized Surface Functionalization

In this paper, we optimize the AlGaN surface oxidation methods for AlGaN/GaN heterostructure field effect transistor (HFET) biological sensors. Reactive ion etching oxygen plasma, inductively coupled oxygen plasma, and piranha solution are used to oxidize AlGaN surface. After oxidation, X-ray photoelectron spectroscopy and water contact angle measurements are used to check oxidation effectiveness. Labeled streptavidin (SA) molecules are bound to the oxidized surface through linker molecules for comparison of surface modification effectiveness. Schottky diodes are fabricated to investigate the impacts of oxidation processes on electrical properties, such as Schottky barrier heights, sheet carrier concentrations, and interface trap densities. The results show that the inductively coupled plasma oxidation process has a superior behavior compared to the reactive ion etching oxygen plasma and piranha solution oxidation processes. AlGaN/GaN HFET protein sensors fabricated using the inductively coupled plasma oxidation process have exhibited improved sensitivity. An SA solution with the concentration as low as 4.73 pM were successfully detected.

Treatment of toluene by using adsorption and nonthermal plasma oxidation process

A nonthermal plasma (NTP) reactor packed with γ-Al 2O 3 pellets was applied to the treatment of gaseous toluene. The NTP reactor employed was a dielectric barrier discharge (DBD) reactor. The process consisted of two sequential modes, i.e, adsorption of toluene followed by NTP oxidation of the adsorbed toluene. In the adsorption step, a dilute toluene gas stream was concentrated in the adsorbent bed for a given period, and in the next step, the adsorbed toluene was oxidized to CO and CO 2 by creating NTP in the reactor. Experiments were conducted with a toluene molar flow rate of 102 μmol min −1. In the early stage of the NTP oxidation, the concentrations of CO and CO 2 formed from the oxidation of the adsorbed toluene abruptly increased, and then gradually decreased. The time required to oxidize the adsorbed toluene was shorter when the discharge power was higher. By this method, the gaseous toluene was effectively treated with reasonable electrical energy consumption. The FTIR spectrum showed that the only byproduct of toluene oxidation other than CO and CO 2 was ozone. The energy consumed for the treatment of toluene was found to be 41.2 J μmol −1.

Oxidative strategies in lignin chemistry: A new environmental friendly approach for the functionalisation of lignin and lignocellulosic fibers

Novel processing methods and product concepts are required to extend the role of lignin for future biomass and biofuel applications in emerging platforms such as the biorefinery. The possible strategies of lignin valorisation are focused into two main directions, namely the selective functionalisation of the lignin polymer or in its oxidative depolymerization to get polyfunctional monomeric compounds. Here we report a panel of biocatalysis, organometallic catalysis, biomimetic catalysis and plasma oxidation processes developed by our research group for the activation of the environmental friendly oxidants oxygen and hydrogen peroxide in the oxidative functionalisation of lignin and lignin model compounds.

Plasma treatment studies of MIS devices

Abstract Silicon nitride films have emerged as the possible future dielectrics for ultra large scale integration (ULSI). Because the interface state density of silicon nitride/silicon interface in metal insulator semiconductor (MIS) configuration is more than an order of magnitude larger than that of silicon dioxide/silicon interface, plasma treatment studies on silicon nitride films have been undertaken for the possible improvement. Accordingly, silicon nitride films of various composition have been prepared by plasma enhanced chemical vapor deposition (PECVD) system using silane(SiH4) and ammonia(NH3) with nitrogen(N2) as the diluent and MIS devices have been fabricated with as well as without plasma treated silicon nitride as the insulator. A considerable improvement in the silicon nitride/silicon interface is observed on ammonia plasma treatment while nitrous oxide(N2O) plasma treatment studies have resulted in the establishment of a novel plasma oxidation process.

The effect of laser crystallization on the carrier transport and electroluminescence from nanocrystalline Si/SiO2 multilayers

AbstractPeriodic amorphous Si (a‐Si)/SiO2 multilayers, fabricated in a conventional plasma enhanced CVD system by alternatively changing the deposition of ultrathin a‐Si film and in‐situ plasma oxidation process, formed (nc‐Si)/SiO2 multilayers after the subsequent KrF laser‐crystallization. Stable electroluminescence (EL) can be achieved at room temperature from laser crystallized multilayers by evaporating ITO and Al as electrodes. The influence of laser energy density on the cur‐ rent‐voltage characteristics and EL property was investigated. It is found that the EL efficiency of laser crystallized sample is changed with laser energy density, and the EL intensity is comparable with that of furnace annealed sample. Our results indicate that the laser crystallization may be an appropriate approach to obtain light‐emitting nanocrystalline Si for future applications. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

TiO2 films in the rutile and anatase phases produced by inductively coupled RF plasmas

Abstract Films of titanium dioxide (TiO 2 ) in the rutile and anatase phases have been obtained in a cylindrical pyrex-like glass vessel inductively coupled to a 13.56 MHz radio frequency generator. Rutile films were developed from commercially pure (CpTi) titanium samples used as targets in an argon/oxygen plasma. Each CpTi target was biased at − 3000 V by means of a direct current supply. The plasma ion acceleration at such a potential leads to a kinetic energy transference to the target whose temperature rises to ∼ 670 °C, favourable to the rutile formation. The anatase films were obtained from sputtering the titanium targets over glass and silicon electrically floated substrates. The latter were placed 3 cm away from the target. The substrates attained a temperature resulting from the amount of heat transferred by the plasma particles, in turn, a function of the plasma parameters. Thus, under the prevailing experimental conditions, substrate temperatures lay in the ∼ 280 to 325 °C range. Atomic titanium is assumed to be sputtered out of the target and oxidised, evolving into the anatase phase on the surface of the glass or silicon. The obtained film phases did not require annealing after the plasma oxidation process. The characterisation of the film samples was conducted by means of X-ray diffraction (XRD), Raman spectroscopy and scanning electron microscopy (SEM).

Preparation and characterization of spark-anodized Al-alloys: Physical, chemical and tribological properties

Aerospace applications and energy saving strategies in general boosted the interest and the research in the field of light weight materials, typically on alloys based on aluminium. Aluminium itself does not provide sufficient mechanical strength for structural parts, but there exists a lot of recently developed alloys containing silicon, copper, magnesium, zinc or manganese in various combinations and compositions exhibiting excellent mechanical properties. These alloys require surface treatments or coatings to withstand corrosive ambient conditions. Among those treatments known as chromate replacements, plasma oxidation processes were used for different applications, especially if the surfaces have to face mechanical load or severe environmental conditions. In this work, specimens of different aluminium alloys have been plasma oxidized by micro-arc treatment in silicate and phosphate solutions. The ceramic coatings were characterized with respect to phase composition, micro-hardness and corrosion stability. In addition, the tribological performance of the coatings was investigated using a ball-on-disc tribometer with reciprocating motion against sintered alumina ball. The typical worn surfaces of the Al substrate and the ceramic coatings were observed by a scanning electron microscope. Applying same wear conditions, the wear rates in different depth of the coatings are nearly similar. However, in a defined depth of the coatings, wear rate gradually decreases with wear duration. During friction process, a-stable transfer layer consisting of oxides was formed on the tribo-contact area of the coatings. The friction coefficient in a steady friction state is in the range of 0.8.

Different mechanism to explain the 1∕f noise in n- and p-SOI-MOS transistors fabricated on (110) and (100) silicon-oriented wafers

The investigation of the low frequency noise in p- and n-channel semiconductor-on-insulator–metal oxide semiconductor transistors fabricated on (100) and (110) silicon-oriented wafers using high advanced processes has been carried out. While for both orientations the 1∕f noise in the n-channel transistors can be explained through the ΔN formalism, the p-channel ones seem to follow the Hooge model. In addition, the new orientation presents a noise level higher than the conventional one. Furthermore, the authors showed that when there is a gap of one decade between the Si(100) and Si(110) p-channel transistors, this one has been reduced to a factor of 2 in the case of the n-channel ones. Finally, by combining the highly advanced microwave-excited high-density plasma oxidation process with the alkali-free five-step cleaning process, it has been possible to fabricate on both surfaces a very high quality oxide with a very low density of traps.

Rod- and wire-like morphologies of tin oxide developed with plasma oxidation after electrodeposition

A new manufacturing method has been developed to prepare films composed of Sn-rich SnO 2 wires and rods using electrodeposition and subsequent plasma oxidation of pure Sn. The morphology of Sn-rich tin oxide grains varied significantly depending on the deposition current density. After a DC plasma oxidation process, the Sn, SnO, and SnO 2 phases were obtained as spherical grains when the previous electrodeposition was carried out at 6 A/dm 2 current density. The wire morphology was obtained only when the electrodeposition current was below 3 A/dm 2. The film produced at 1.5 A/dm 2 and then plasma oxidized showed wire morphology with single crystals of SnO 2 that formed in the (110) direction.

Smart modification of magnetron sputtered TiN surfaces for stimulated differentiation

The surface electrical properties of TiN films are revisited in this work aiming at a controlled surface modification of their in vitro behaviour. A plasma oxidation process is used to tune the originally high electrical conductance of TiN to that of a wide band gap semiconductor. A capacitively coupled plasma reactor with Ar:O mixtures was used to oxidize the surfaces of thin (250 nm) magnetron sputtered TiN films deposited onto Si substrates. Both optical (UV–Vis reflectance spectroscopy) and structural (X-ray diffraction diagrams) properties were traced to follow a graded modification. Pluripotent human mesenchymal stem cells (hMSCs) were exposed to the resulting TiN surfaces in both proliferation and osteoblastic differentiation biomolecular environments. HMSCs response was evaluated by fluorescence microscopy (double staining with 594 and 488 nm emissions). Cells adhered to TiN and modified TiNO show that, completely oxidized surfaces are better adapted for proliferation purposes while unmodified surfaces are ideal for differentiation as denoted by the development of characteristic cytosolic prolongations.

Electrically driven luminescence of nanocrystalline Si/SiO 2 multilayers on various substrates

Amorphous-Si:H/SiO 2 multilayers were fabricated on various substrates by alternatively changing the ultra-thin amorphous Si films deposition and in situ plasma oxidation processes. Uniform silicon nanodots with controllable size were formed by combining the rapid thermal annealing and furnace annealing techniques as revealed by cross-sectional transmission electron microscopy. Light-emitting diodes based on Al/(nc-Si/SiO 2) multilayers/Si substrate/Al structures were fabricated and electrically driven luminescence was demonstrated at room temperature. It was found that the substrate had a significant impact on the carrier injection efficiency and the luminescence intensity. The turn-on voltages of the device on p +-Si and p-Si substrate were 2.2 and 8 V, respectively. Meanwhile, electroluminescence intensity of the device on p +-Si substrate was five times higher than that on p-Si substrate under the same power supply, which means the heavily doped substrate is helpful for enhancing the carrier injection efficiency.

Resistive switching of aluminum oxide for flexible memory

The unipolar resistive switching of the Al∕AlxOy∕Al structure is investigated for nonvolatile memory. Following the production of aluminum oxide film (AlxOy) by plasma oxidation, a high ratio of on-state and off-state currents (⩾104) is achieved, and characteristics of switching endurance are reported. Due to the good ductility of aluminum, the performance of resistive switching on a flexible substrate is not degraded by severe substrate bending. The low process temperature of the plasma oxidation process is advantageous for the fabrication of flexible electronic devices and modern interconnection processes.

Formation of Ultrathin SiON Films on Si Substrates Having Different Orientations

We investigated the formation of ∼1–3 nm ultrathin SiON films by the plasma nitridation of oxidized Si substrates having different orientations, namely, (100), (110), and (111). Oxidation was performed by dry, wet, and plasma oxidation processes with rapid thermal oxidation (RTO), in situ steam generation (ISSG), and a slot plane antenna (SPA), respectively. Nitridation was performed by a plasma nitridation process with SPA. The thickness of base SiO2 films prepared by a plasma oxide process was independent of substrate orientation unlike in the dry and wet oxidation processes. Furthermore, no significant discrepancy in nitrogen areal density was observed among the SiON films grown by the plasma nitridation of differently oriented oxidized Si substrates. Our results suggest that SiON films with arbitrary nitrogen densities and thicknesses can be prepared by oxidation and plasma nitridation irrespective of the Si substrate orientation.

Growth of thin Si oxide in a cyclic oxygen plasma environment below 200 °C

The growth of Si oxide by means of a cyclic radio-frequency (rf) plasma oxidation process has been explored in a low temperature range of 100–200 °C. The growth mechanism exhibits Cabrera–Mott (CM) oxidation, that is, the transport of mobile ionic species is assisted by an electric field. The low activation energy of 0.3 eV is attributed to the small size of O − and the assistance of the electric field. The oxide becomes off-stoichiometric as one approaches to the exterior surface of the oxide layer.

Surface Modification of CrN Coating by Plasma Oxidation Process

In this study, the CrN coating was first deposited on tool steels by cathodic arc evaporation technique and then this coating was subjected to oxygen glow discharge to activate the oxidation process in order to form oxide layer on the surface of the CrN coating. The varied properties of CrN coatings with or without oxidation treatment were studied in terms of hardness, adhesion, microstructure and surface morphology by using XRD, AFM, Rockwell indentation, Nano-indentation tests. The result turned out that the effect of the plasma oxidation process on the hardness of CrN coating was improved due to the oxide layer on the surface.

Polarity-Free Resistive Switching Characteristics of CuxO Films for Non-volatile Memory Applications

Resistive switching characteristics of CuxO films grown by plasma oxidation process at room temperature are investigated. Both bipolar and unipolar stable resistive switching behaviours are observed and confirmed by repeated current–voltage measurements. It is found that the RESET current is dependent on SET compliance current. The mechanism behind this new phenomenon can be understood in terms of conductive filaments formation/rupture with the contribution of Joule heating.

High-density plasma silicon oxide thin films grown at room-temperature

The fabrication at room-temperature of thin (<8nm) silicon oxide films has been achieved, in a high-density helicon plasma source reactor using Ar/O2 mixture, exhibiting relatively low concentration of fixed oxide and interface charges, after annealing. The developed plasma oxidation process was employed in order to fabricate SOI-MOSFETs at low thermal budget with competitive operating characteristics.