Radio-frequency Hydrogen Plasma Research Articles

Improved NO2 gas-sensing performance of PPy by hydrogen plasma treatment: Experimental study and DFT verification

Improving gas selectivity and stability poses a challenge for gas sensors based on polypyrrole (PPy). Radio frequency hydrogen plasma was employed to modify the surface of PPy nanosheet. PPy treated with hydrogen plasma owns a larger specific surface area and an increased concentration of surface hole carriers due to the dispersion of hydrogen plasma, which results in excellent nitrogen dioxide (NO2) gas sensing performance for the PPy sensor. Furthermore, the increased electronic transfer ability and NO2 adsorption capacity of the PPy treated by hydrogen plasma were confirmed based on the calculations of Density Functional Theory (DFT). The stronger interaction of PPy treated with hydrogen plasma between PPy and NO2 have happened comparing to untreated PPy. PPy/H sensor shows a magnificent NO2 gas sensing performance (sensitivity and selectivity) due to the strong interaction (strong adsorption ability and electron transfer) between PPy and NO2 caused by the hydrogen plasma treatment. Organic semiconductor treatment with hydrogen plasma offers a novel method for surface modification of PPy applied to gas sensing.

Controllable Preparation of Spherical Molybdenum Nano-Powders by One-Step Reduction of APM in Radio Frequency Hydrogen Plasma

Spherical molybdenum nano-powders were in-situ ultrafast synthesized from ammonium paramolybdate (APM) raw materials in a one-step reduction method by radio frequency (RF) hydrogen plasma. Due to the extreme conditions of the RF plasma torch such as its high temperature and large temperature gradient, the injected raw APM powder was quickly gasified and then reduced into nano-sized metal molybdenum (Mo) powder. The influences of APM powder delivery rate and H2 concentration on the properties of the obtained powders were investigated. Field-emission scanning electron microscope (FESEM), transmission electron microscope (TEM), X-ray diffraction (XRD), nanolaser particle analyzer, and specific surface area method were used to characterize the morphology, phase, and particle size distribution of the powders. The results showed that the nano-sized Mo powder obtained by hydrogen plasma treatment had a quasi-spherical morphology and an average particle size of about 30 nm. The particle size could be successfully adjusted by varying H2 concentrations. In addition, spherical nano-sized MoO3 powder could be obtained when no H2 was added into the RF plasma.

Improving the photoresponse spectra of BaSi2 layers by capping with hydrogenated amorphous Si layers prepared by radio-frequency hydrogen plasma

We studied the surface passivation effect of hydrogenated amorphous silicon (a-Si:H) layers on BaSi2 films. a-Si:H was formed by an electron-beam evaporation of Si, and a supply of atomic hydrogen using radio-frequency plasma. Surface passivation effect was first investigated on a conventional n-Si(111) substrate by capping with 20 nm-thick a-Si:H layers, and next on a 0.5 μm-thick BaSi2 film on Si(111) by molecular beam epitaxy. The internal quantum efficiency distinctly increased by 4 times in a wide wavelength range for sample capped in situ with a 3 nm-thick a-Si:H layer compared to those capped with a pure a-Si layer.

Enhanced recrystallization and dopant activation of P+ ion-implanted super-thin Ge layers by RF hydrogen plasma treatment

Radio-frequency (RF) hydrogen plasma treatment, thermal annealing in a furnace, and rapid thermal annealing of high-dose P+ ion implanted p-type Ge layers have been studied by Raman scattering spectroscopy, atomic force microscopy, secondary ion mass spectrometry, electrochemical capacitance–voltage profiling, four-point probes method, and x-ray reflectometry. It was shown that low-temperature RF plasma treatment at temperature about 200 °C resulted in full recrystallization of amorphous Ge layer implanted by P+ ions and activation of implanted impurity up to 6.5 × 1019 cm−3 with a maximum concentration at the depth of about 20 nm. Rapid thermal annealing (15 s) and thermal annealing (10 min) in nitrogen ambient required considerably higher temperatures for the recrystallization and activation processes that resulted in diffusion of implanted impurity inside the Ge bulk. It was demonstrated that RF plasma treatment from the samples with front (implanted) side resulted in considerable stronger effects of recrystallization and activation as compared with the same treatment from the back (unimplanted) side. The experiment shows that nonthermal processes play an important role in enhanced recrystallization and dopant activation during the RF plasma treatment. Mechanisms of enhanced modification of the subsurface implanted Ge layer under plasma treatment are analyzed.

Enhancement of the photocatalytic efficiency of WO3 nanoparticles via hydrogen plasma treatment

Surface defect engineering is able to effectively enhance the photocatalytic performance of WO3 nanoparticles. In this paper, radio frequency hydrogen plasma was employed to create surface defects on WO3 nanoparticles. X-ray photoelectron spectroscopy (XPS) and electron spin resonance (ESR) analysis confirmed that hydrogen plasma modification increases the density of oxygen vacancies on the surface of WO3. The broadening of characteristic WO3 peaks in Raman spectra indicates the increase of oxygen vacancies by increasing voltage in hydrogen plasma treatment. The sample treated with hydrogen plasma at 20 volts shows enhancement in photocurrent density by an order of magnitude, attributable to the band-gap narrowing and subsequent increase of quantum yield in the visible range. Consistent results were also obtained from photocatalytic O2 evolution from water oxidation.

Application of low-temperature low-pressure hydrogen plasma: treatment of artificially prepared corrosion layers

Abstract The aim of this work is the application of low-temperature low-pressure hydrogen plasma on artificially prepared corrosion layers, so called plasma chemical reduction. It is necessary to use samples with artificially prepared corrosion layers because it is impossible to use the real artifacts for fundamental research. The bronze was chosen as a sample material. Formation of corrosion layers on the bronze samples was carried out in concentrated hydrochloric acid vapors with the addition of sand. The radio-frequency hydrogen plasma was generated in the flowing regime at a pressure of 160 Pa. Different values of supplied power were chosen as well as different discharge modes: continuous or pulsed mode with varied duty cycles. By the combination of supplied power and mode factors, we selected two values of effective power. The process of plasma chemical reduction was monitored by optical emission spectroscopy (OES) and simultaneously, the sample temperature was measured. Rotational temperatures were calculated from OH radicals spectra. Changes in the structure and elemental composition were determined using scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX).

Temperature of hydrogen radio frequency plasma under dechlorination process of polychlorinated biphenyls

It has been reported that RF (radio frequency) hydrogen plasmas promote the dechlorination process of PCBs (polychlorinated biphenyls) under irradiation of MW (microwave). A relative emission intensity spectroscope system was used for single-shot imaging of two-dimensional temperature distributions of RF hydrogen plasmas generated in chemical solutions with several mixing ratios of isopropyl alcohol (IPA) and insulation oil under MW irradiation. Our experimental results showed that the plasma generation frequencies for the oil-contaminating solutions were higher than that for the pure IPA solution. In addition, the plasma temperature in the compound liquids including both oil and IPA was higher than that in the pure IPA and oil solutions. A combination of the plasma temperature measurements and plasma composition analysis indicated that the hydrogen radicals generated in a chemical solution containing the equal volumes of IPA and oil were almost the same amounts of H and H+, while those produced in the other solutions were mainly H.

Excellent field emission characteristics from few-layer graphene–carbon nanotube hybrids synthesized using radio frequency hydrogen plasma sputtering deposition

The growth of few-layer graphene (FLG) on carbon nanotubes (CNTs) was realized by using radio frequency hydrogen plasma sputtering deposition. A defect nucleation mechanism and a two dimensional growth model of the FLG were proposed, and field emission characteristics of these FLG–CNT hybrids were studied. They show excellent field emission properties, with a low turn-on electric field (0.98V/μm) and threshold field (1.51V/μm), large field enhancement factor (∼3980) and good stability behavior, which are much better than those of the as-grown CNT arrays. The sharp edges and the low work function of the hybrids are believed to be responsible for the improved field emission properties.

Silicon-based light emitting diodes on silicon and glass substrates using a low temperature multilayered nanocrystalline structure

Si nanocrystals have been prepared by hydrogenation and subsequent annealing of as-deposited amorphous Si layers on glass and Si substrates. The hydrogenation process has been performed at 350°C under radio frequency hydrogen plasma. The nanocrystallites were processed by sequential reactive ion etching to allow light emission. Photoluminescence (PL) measurements demonstrate that the nanocrystallites emit light in the range of 500–570nm. The evolution of nanocrystals has been studied using scanning electron microscopy, while atomic force microscopy and transmission electron microscopy have been utilized to examine the structure of the Si nanocrystals. Multilayer luminescent Si nanocrystals have been fabricated using alternating layers of Si nanocrystals and Si oxy-nitride. Bilayer structures have higher efficiency than a single layer structure, while multilayers with three layers of luminescent nanocrystals and above did not show a higher PL intensity. Transparent light emitting diodes have been realized based on multilayer luminescent Si nanocrystals that displayed bright emission which was visible to the naked eye in a bright room.

Optical property improvement of InAs/GaAs quantum dots grown by hydrogen-plasma-assisted molecular beam epitaxy

An order-of-magnitude increase of photoluminescence (PL) efficiency at room temperature has been observed in the GaAs/InAs quantum dots (QDs)-in-a-well structure grown with in situ irradiation of atomic hydrogen supplied by a radio-frequency hydrogen-plasma source. The enhancement in PL intensity rapidly increases with the hydrogen flow rate and is stable with a variation of excitation power in the radio-frequency plasma source. Extensive thermal annealing of grown samples up to 634 °C did not show any significant degradation of photoluminescence intensity compared with the reference sample. The reduction of nonradiative recombination centers in the as-grown sample causes the greatly enhanced luminescence property. In addition to PL enhancement the authors observed that the H-assisted growth of InAs QDs has suppressed bimodal distribution of QD shape. In contrast to the hydrogen-plasma-assisted growth, irradiation by hydrogen in molecular form has a detrimental effect on the optical properties of similar structures. The high thermal stability of improved optical properties suggests that the formation of the defects which are responsible for nonradiative recombination channels is suppressed during H-assisted epitaxy although in situ defect passivation by atomic hydrogen cannot be completely ruled out.

Hydrogen plasma treatment for improving bulk passivation quality of c-Si solar cells

We have investigated the effects of radio frequency hydrogen plasma (H-plasma) treatment on bulk passivation qualities of cast poly-crystalline silicon (poly c-Si) wafers. When the hydrogen treatment was performed directly to poly c-Si wafers, the plasma damage was introduced and it was difficult to improve the minority carrier lifetime by optimizing the plasma condition. Hydrogenated amorphous silicon oxide (a-SiO x :H) films deposited by RF-PECVD using a mixture of SiH 4, H 2 and CO 2 were employed as the protection and the passivation layers. We found that the lifetime of poly c-Si substrates passivated by a RF-PECVD a-SiO x :H films with additional H-plasma treatment showed an improvement of the overall effective lifetime. Annealing up to 400 °C also enhances the further diffusion of hydrogen, and the effective lifetime of the samples with H-plasma treatment increased 20–30% compared to the samples without treatment. This result clearly indicated that the H-plasma treatment improves the bulk passivation quality of poly c-Si wafers.

Thickness-Dependent Reversible Hydrogenation of Graphene Layers

In this work, graphene layers on SiO(2)/Si substrate have been chemically decorated by radio frequency hydrogen plasma. Hydrogen coverage investigation by Raman spectroscopy and micro-X-ray photoelectron spectroscopy characterization demonstrates that the hydrogenation of single layer graphene on SiO(2)/Si substrate is much less feasible than that of bilayer and multilayer graphene. Both the hydrogenation and dehydrogenation process of the graphene layers are controlled by the corresponding energy barriers, which show significant dependence on the number of layers. The extent of decorated carbon atoms in graphene layers can be manipulated reversibly up to the saturation coverage, which facilitates engineering of chemically decorated graphene with various functional groups via plasma techniques.

Determination of the degree of dissociation in an inductively coupled hydrogen plasma using optical emission spectroscopy and laser diagnostics

The knowledge of absolute concentrations of radicals in molecular discharges, such as the density of atomic hydrogen in H2 plasmas, is crucial for understanding processes at the surface and is, therefore, of great interest in a variety of plasma applications. Reliable spectroscopic methods are indispensable for the determination of the degree of dissociation in processing plasmas. In this paper, we describe a novel optical emission spectroscopy method called two-gas actinometry (TGA). It is based on using a second actinometer gas to take into account dissociative excitation, which is of particular importance at comparatively low degrees of dissociation. TGA is employed for measuring the characteristic dependence of the degree of dissociation, in a wide range of plasma parameters, in an inductively coupled radio-frequency hydrogen plasma. Absolute densities with high accuracy are obtained using two-photon absorption laser-induced fluorescence spectroscopy.

Investigation of coupling between chemistry and discharge dynamics in radio frequency hydrogen plasmas in the Torr regime

We present the results of a study of a capacitively coupled hydrogen discharge by means of a one-dimensional numerical fluid model and experiments. The model includes a detailed description of the gas-phase chemistry taking into account the production of H− ions by dissociative attachment of H2 vibrational levels. The population of these levels is described by a Boltzmann vibrational distribution function characterized by a vibrational temperature TV. The effect of the dissociative-attachment reaction on the discharge dynamics was investigated by varying the vibrational temperature, which was used as a model input parameter. Increasing the vibrational temperature from 1000 to 6000 K affects both the chemistry and the dynamics of the electrical discharge. Because of dissociative attachment, the H− ion density increases by seven orders of magnitude and the H− ion density to electron density ratio varies from 10−7 to 6, while the positive ion density increases slightly. As a consequence, the atomic hydrogen density increases by a factor of three, and the sheath voltage drops from 95 to 75 V. Therefore, clear evidence of a strong coupling between chemistry and electrical dynamics through the production of H− ions is demonstrated. Moreover, satisfactory agreement between computed and measured values of atomic hydrogen and H− ion densities gives further support to the requirement of a detailed description of the hydrogen vibrational kinetics for capacitively coupled radio frequency discharge models in the Torr regime.

Comparison of excessive balmer α line broadening of inductively and capacitively coupled RF, microwave, and glow-discharge hydrogen plasmas with certain catalysts

From the width of the 656.3-nm Balmer /spl alpha/ line emitted from inductively and capacitively coupled radio frequency (RF), microwave, and glow-discharge plasmas, it was found that inductively coupled RF helium-hydrogen and argon-hydrogen plasmas showed extraordinary broadening corresponding to an average hydrogen atom energy of 250-310 and 180-230 eV, respectively, compared to 30-40 and 50-60 eV, respectively, for the corresponding capacitively coupled plasmas. Microwave helium-hydrogen and argon-hydrogen plasmas showed significant broadening corresponding to an average hydrogen atom energy of 180-210 and 110-130 eV, respectively. The corresponding results from the glow-discharge plasmas were 33-38 and 30-35 eV, respectively, compared to /spl ap/ 4 eV for plasmas of pure hydrogen, neon-hydrogen, and xenon-hydrogen maintained in any of the sources. Similarly, the average electron temperatures T/sub e/ for helium-hydrogen and argon-hydrogen inductively coupled RF and microwave plasmas were high (43 200 /spl plusmn/ 5% K, 18 600 /spl plusmn/ 5% K, 30 500 /spl plusmn/ 5% K, and 13 700 /spl plusmn/ 5% K, respectively); compared to 9300 /spl plusmn/ 5% K, 7300 /spl plusmn/ 5% K, 8000 /spl plusmn/ 5% K, and 6700 /spl plusmn/ 5% K for the corresponding plasmas of xenon-hydrogen and hydrogen alone, respectively. Stark broadening or acceleration of charged species due to high electric fields cannot explain the inductively coupled RF and microwave results since the electron density was low and no high field was present. Rather, a resonant energy transfer mechanism is proposed.

Effect of r.f. hydrogen plasma annealing on the properties of Si/SiO 2 interface: a spectroscopic ellipsometry study

The effect of radio frequency hydrogen plasma annealing on the interface properties of Si/SiO 2 structures is studied by spectroscopic ellipsometry in the spectral range 280–430 nm. By thermal oxidation of (1 1 1)Si at 850 °C a 1 nm thick interface with a composition of Si 0.6(SiO 2) 0.4 is formed, which becomes thinner (0.5 nm) and its composition changes to Si 0.8(SiO 2) 0.2 after hydrogen plasma annealing. For the as-grown oxide, a shift in the characteristic peaks in the 〈ε 2〉 spectrum towards lower energies and a split of the E 1 peak at 3.4 eV are observed and are attributed to the high interfacial stress, the level of which is estimated to be 1.1×10 10 dyn/cm 2. These phenomena disappear after annealing of the Si/SiO 2 structure.

Surface analysis of LiNbO3 single crystals modified by radiofrequency hydrogen plasma

Surface analysis of LiNbO3 single crystals modified by radiofrequency hydrogen plasma

Surface analysis of LiNbO3 single crystals modified by radiofrequency hydrogen plasma

Lithium niobate (LiNbO 3 ) is a widely used ferroelectric material with high electro-optic, non-linear optic, birefringent and photorefractive activities. Modification of the surface of LiNbO 3 single crystals is essential for its application in optoelectronics. We accomplished the surface modification of single-domain LiNbO 3 wafers (z-cut) in the hydrogen plasma of radiofrequency (r.f.) discharge (13.56 MHz) under pressures of 0.5-3 mbar. Modification of the wafer surface is demonstrated mainly by oxygen and lithium depletion. The overall depth of the modified surface region is ∼0.5 μm. There is a thin near-surface layer (<10 nm) where the niobate structure is destroyed and only niobium oxide remains there. Towards the bulk the niobate structure is preserved, although with many point defects. From a depth of 1 μm the genuine niobate composition remains. A double inversion of the spontaneous polarization in the layer was found after reoxidation of the plasma-processed sample. The polarity inversion is adjoined to the internal fields induced by point defect gradients in the plasma-modified surface. The following methods are used: neutron depth profiling for lithium depth distribution; RBS for heavier element depth profiles; x-ray diffraction for evaluation of crystallinity changes; and XPS for composition and chemical bonding of elements in the actual surface.

Li depth profile anomaly after radio-frequency hydrogen plasma processing of single crystal LiNbO3

Lithium niobate is processed in the hydrogen plasma of the radio-frequency glow discharge. This treatment causes a strong chemical reduction and outlet of lithium from the sample surface. The Li depth profiles are measured by Neutron Depth Profiling. The diffusion coefficient and the flux of escaping Li atoms are determined by fitting the experimental and calculated curves. An anomalous Li depth profile is measured if the plasma treatment is followed by reoxidation.

Structure observed in measured electron energy distribution functions in capacitively coupled radio frequency hydrogen plasmas

Electron energy probability functions measured with a passively compensated Langmuir probe in asymmetric capacitively coupled hydrogen and deuterium plasmas exhibit structure. The otherwise relatively continuous distribution appears to have an abrupt peak in electron density near 5 eV. This structure occurs at a higher energy in deuterium than hydrogen and there is a correlation between floating potential and the voltage at which the structure is observed in the second derivative of the I(V) characteristic. While the cause of the structure has yet to be clarified, spectroscopic observations and computer-based hydrogen models indicate that the high energy tail of the distribution is strongly modulated during the radio frequency cycle. The effect of this modulation on plasma properties and probe measurements has yet to be explored.