Hydrogen Plasma Processing Research Articles

Inductively coupled hydrogen plasma processing of AZO thin films for heterojunction solar cell applications

Al-doped ZnO (AZO) thin films deposited by means of RF magnetron sputtering were processed in a low frequency inductively coupled plasma of H2, aiming at heterojunction (HJ) solar cell applications. A variety of characterization results show that the hydrogen plasma processing exerts a significant influence on the microstructures, electrical and optical properties of the AZO films. The incorporation of hydrogen under the optimum treatment simultaneously promoted the transmittance and conductivity due to the hydrogen associated passivation effect on the native defects and the formation of shallow donors in the films, respectively. A p-type c-Si based HJ solar cell with a front AZO contact was also treated in as-generated non-equilibrium hydrogen plasma and the photovoltaic performance of the solar cell was prominently improved. The underlying mechanism was discussed in terms of the beneficial impacts of high-density hydrogen plasma on the properties of AZO itself and the hetero-interfaces involved in the HJ structure (interface defect and energy band configuration).

Influence of hydrogen patterning gas on electric and magnetic properties of perpendicular magnetic tunnel junctions

To identify the degradation mechanism in magnetic tunnel junctions (MTJs) using hydrogen, the properties of the MTJs were measured by applying an additional hydrogen etch process and a hydrogen plasma process to the patterned MTJs. In these studies, an additional 50 s hydrogen etch process caused the magnetoresistance (MR) to decrease from 103% to 14.7% and the resistance (R) to increase from 6.5 kΩ to 39 kΩ. Moreover, an additional 500 s hydrogen plasma process decreased the MR from 103% to 74% and increased R from 6.5 kΩ to 13.9 kΩ. These results show that MTJs can be damaged by the hydrogen plasma process as well as by the hydrogen etch process, as the atomic bonds in MgO may break and react with the exposed hydrogen gas. Compounds such as MgO hydrate very easily. We also calculated the damaged layer width (DLW) of the patterned MTJs after the hydrogen etching and plasma processes, to evaluate the downscaling limitations of spin-transfer-torque magnetic random-access memory (STT-MRAM) devices. With these calculations, the maximum DLWs at each side of the MTJ, generated by the etching and plasma processes, were 23.8 nm and 12.8 nm, respectively. This result validates that the hydrogen-based MTJ patterning processes cannot be used exclusively in STT-MRAMs beyond 20 nm.

High current H2+ and H3+ beam generation by pulsed 2.45 GHz electron cyclotron resonance ion source

The permanent magnet 2.45 GHz electron cyclotron resonance ion source at Peking University can produce more than 100 mA hydrogen ion beam working at pulsed mode. For the increasing requirements of cluster ions (H2(+) and H3(+)) in linac and cyclotron, experimental study was carried out to further understand the hydrogen plasma processes in the ion source for the generation of cluster ions. The constituents of extracted beam have been analyzed varying with the pulsed duration from 0.3 ms to 2.0 ms (repetition frequency 100 Hz) at different operation pressure. The fraction of cluster ions dramatically increased when the pulsed duration was lower than 0.6 ms, and more than 20 mA pure H3(+) ions with fraction 43.2% and 40 mA H2(+) ions with fraction 47.7% were obtained when the operation parameters were adequate. The dependence of extracted ion fraction on microwave power was also measured at different pressure as the energy absorbed by plasma will greatly influence electron temperature and electron density then the plasma processes in the ion source. More details will be presented in this paper.

Low-Temperature Polycrystalline Silicon Thin Film Transistor Nonvolatile Memory Using Ni Nanocrystals as Charge-Trapping Centers Fabricated by Hydrogen Plasma Process

Processes for fabricating a Ni nanocrystal (NC)-assisted low-temperature polycrystalline silicon thin film transistor (LTPS-TFT) nonvolatile memory device of noble stack below 600 °C were successfully developed. The NCs were fabricated in H-plasma atmosphere by heating a nanosized Ni film to realize an appropriate nanoparticle distribution. Results show that NCs with a number density of ∼5×1011 cm-2 and a particle diameter of 4 to 12 nm can successfully be fabricated as charge-trapping centers for enhancing the device performance. The results also indicate that the data retentions at the initial time and after 104 s for a SiO2/Ni-NCs/Si3N4/SiO2 gate under the present stack of devices are about 2.2 and ∼1.1 V, respectively.

Scanning Kelvin-probe study of the hydrogen-terminated diamond surface in ultrahigh vacuum

Atomic-force and Kelvin-probe microscopies were employed in ultrahigh vacuum to image the surface topography and contact potential of the hydrogen-terminated and unterminated surfaces of diamond. A variation of about 25 meV in the contact potential was measured on a length scale of 20 nm and ascribed to differently orientated surface domains resulting from hydrogen-plasma processing of the sample. Shifts in the work function arising from sample heating in vacuum and the adsorption of C60 were measured. The Fermi level was found to be 0.7 and 1.1 eV below the valence band maximum for C60 coverages of 1 and 4 monolayer, respectively.

Adsorption of fetal bovine serum on H/O-terminated diamond studied by atomic force microscopy

We investigate adsorption of fetal bovine serum (FBS), a crucial component for cell growth, on intrinsic CVD mono-crystalline diamond with hydrogen and oxygen surface terminations. The surface terminations are prepared by oxygen and hydrogen plasma process. The FBS adsorption is done by immersing diamond into McCoy's 5A medium supplemented with 15% FBS for 10 s–24 h. After rinsing the samples are characterized in the McCoy's medium and for comparison also in air by atomic force microscopy (AFM). By optimized measurements of AFM topography in oscillating regime and by using advanced AFM regimes such as phase imaging, nanoshaving, and force spectroscopy we characterize FBS thickness, adhesion, conformation and selectivity on the H/O-diamond surfaces. We find that FBS is adsorbed in about the same monolayer thickness (2–4 nm) on both H/O-diamond surfaces, yet the protein conformation is different. We present a schematic model.

Dielectric and Electrode Thin Films for Stack-Cell Structured DRAM Capacitors with sub 50-nm Design Rules

TiO2 thin films with high dielectric constants (83 - 100) were grown on sputtered and atomic-layer-deposited (ALD) Ru electrode at a growth temperature of 250oC using the atomic- layer-deposition method. The as-deposited films were crystallized with rutile structure. Further improvements in the leakage current characteristics are achieved by proper doping of Al and Hf, thereby an equivalent oxide thickness (tox) ~ 0.6 nm with small enough leakage current density (1 x 10-7 A/cm2 at 0.8V) was achieved. These good properties are also realized from 3D structured hole-type capacitors. The rutile structured TiO2 films were also obtained from Ru electrode which is deposited by an ALD process using a newly developed noble precursor. In addition, it was confirmed that the Ru metal film step coverage on the capacitor hole structure with an aspect ratio of 17 is > 90 %. The thermal stability of the Ru electrode on severe 3D holes was improved with the adoption of hydrogen plasma process.

Electronic properties of plasma hydrogenated diamond surfaces: A microscopic study

The quality of hydrogen termination of undoped diamond surfaces is investigated on a microscopic level by applying atomic force microscopy, Kelvin force microscopy, and scanning electron microscopy. The results show that high temperature hydrogenation in a microwave plasma can lead to microscopic inhomogeneities at the surface, both in morphology and electron emission properties. These inhomogeneities are due to a very thin (1 nm) layer of non-diamond material on the surface, which appears to be inherently generated by the hydrogen plasma process. By scanning in contact AFM an underlying diamond surface can be exposed, which retains properties typical for hydrogenated diamond and exhibits significantly improved electron emission properties.

Hydrogenated Nanoporous Diamond Films

Nanoporous diamond films up to 20 µ m thick with a pore size of 1-1.5 nm and porosities from 0.4 to 0.6 are produced by selective etching in air. The effect of hydrogen plasma processing on the IR absorption and electron paramagnetic resonance spectra of the films is investigated. The results indicate that the concen- tration of bonded hydrogen in the hydrogenated nanoporous diamond films attains 20 at %. The kinetics of hydrogen desorption are studied as a function of temperature.

Enhancement of field emission from hydrogen plasma processed carbon nanotubes

An effective approach to improve the field emission site density and large area uniformity of carbon nanotube (CNT) films has been presented. I– V characteristics, Raman spectra, as well as surface morphology and microstructure of the CNT films before and after the hydrogen plasma process (HPP) were investigated. Results showed that after the HPP nano-nodes appeared along the tubes and the field electron emission site density increased dramatically from 10 3 to 10 6 cm −2. A model for the formation of the nodular CNTs was proposed. We attributed this improvement to the changing of the electronic and/or geometrical structure of the local emission region in CNTs.

Remote hydrogen plasma processing of ZnO single crystal surfaces

We have studied the effects of remote hydrogen plasma treatment on the defect characteristics in single crystal ZnO. Temperature-dependent (9–300 K) and excitation intensity-dependent photoluminescence spectra reveal that H-plasma exposure of ZnO effectively suppresses the free-exciton transition and redistributes intensities in the bound-exciton line set and two-electron satellites with their phonon replicas. The resultant spectra after hydrogenation exhibit a relative increase in intensity of the I4 (3.363 eV) peak, thought to be related to a neutral donor bound exciton, and a peak feature at 3.366 eV with a distinctly small thermal activation energy. Hydrogenation also produces a violet 100 meV wide peak centered at ∼3.15 eV. Remote plasma hydrogenation yields similar changes in room-temperature depth-dependent cathodoluminescence spectra: the emission intensity increases with hydrogenation mostly in the violet and near-ultraviolet range. Subsequent annealing at 450 °C within the same plasma environment completely restores both the photoluminescence and cathodoluminescence spectra in the subband gap range. The appearance of another bound-exciton feature at 3.366 eV and a relative intensity increase of the donor-bound exciton at line I4 with H-plasma exposure, and the reversibility with annealing of the spectral changes, indicate a direct link between hydrogen indiffusion and appearance of a shallow donor.

Layer-transfer process for silicon-on-insulator with improved manufacturability

We observe hydrogen platelet buildup in single-crystalline silicon caused by hydrogen-plasma processing. The platelets are aligned along a layer of lattice defects formed in silicon before the plasma processing. The buried-defect layer is formed by either silicon-into-silicon or argon-into-silicon implantation. We discuss the platelet nucleation, growth, and merge phenomena and discuss applicability of the plasma hydrogenation to silicon-on-insulator (SOI) wafer fabrication by layer transfer.

Field emission characteristics of nodular carbon nanotubes

A configuration of carbon nanotubes (CNTs) whose surfaces were fully covered with nanoscale nodes was developed in our laboratory. We named this material nodular CNTs or NCNTs. A hydrogen plasma process was used to turn common CNTs into NCNTs. Combined with this process, a technique has been developed to make planar NCNTs field emitters. Field emission experiments showed that planar NCNTs emitters had superior field emission characteristics. Specifically, the emission site density was increased by the order of 106/cm2. A suggestion was given to interpret the formation of NCNTs and a simple model was also proposed to explain the improvement of field emission properties. This technique was found to be an easy way to change the geometric character of CNTs in large quantities and will be useful for creating planar field emitters used in field emitter light sources or display devices.

Effects of impurity concentration, hydrogen plasma process and crystallization temperature on poly-crystalline films obtained from PECVD a-Si:H layers

In this work we report the effects of impurity concentration and crystallization temperature on the crystalline orientation and final layer resistivity of poly-crystalline films obtained from a-Si:H layers deposited by PECVD at 225 °C followed by a hydrogen plasma process in the same PECVD equipment in which they are deposited. Films were characterized electrically, by X-ray diffractometry and by transmission electron microscopy. Crystallized films were used to fabricate poly-Si TFTs.

Nodular Carbon Nanotubes and Their Field EmissionCharacteristics

A new configuration of carbon nanotubes (CNTs) has been discovered in ourlaboratory and their surfaces are fully covered with nano-sized node-likestructures. The node structures have almost the same size of 40-50 nm. Werefer to this material as the nodular carbon nanotube (NCNT). Fieldemission scanning electron microscopy shows that after a hydrogen plasmaprocess in our homemade plasma equipment, the common carbon nanotubes werechanged into NCNTs. The experimental result demonstrates that this materialhas good field emission characteristics, low threshold field, stable andsuitable emitting current, especially for the emission site density up tothe order of 106cm-2.

On the Applicability of Plasma Assisted Chemical Micropatterning to Different Polymeric Biomaterials

A plasma process sequence has been developed to prepare chemical micropatterns on polymeric biomaterial surfaces. These patterns induce a guided localized cell layover at microscopic dimension. Two subsequent plasma steps are applied. In the first functionalization step a microwave ammonia plasma introduces amino groups to obtain areas for very good cell adhesion; the second passivation step combines pattern generation and creation of cell repelling areas. This downstream microwave hydrogen plasma process removes functional groups and changes the linkages of polymer chains at the outermost surfaces. Similar results have been obtained on different polymers including polystyrene (PS), polyhydroxyethylmethacrylate (PHEMA), polyetheretherketone (PEEK), polyethyleneterephthalate (PET) and polyethylenenaphthalate (PEN). Such a rather universal chemical structuring process could widen the availability of biomaterials with specific surface preparations.

Nitrogen–Hydrogen Plasma Processing of Thin Vanadium Films

The chemical and microstructural processes induced in vanadium films by nitrogen–hydrogen plasma processing were characterized by x-ray diffraction, electron microscopy, and resistivity measurements. Conditions were identified under which vanadium nitride layers can be produced by plasma processing.

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.

Low temperature electrical surface passivation of MBE-grown pin diodes by hydrogen and oxygen plasma processes

The electrical surface passivation of mesa-shaped pin diodes was investigated by three different processes: (1) wet thermal oxidation at 700°C, (2) plasma enhanced oxidation at 360°C, (3) plasma enhanced hydrogenation at room temperature. With the plasma-assisted low-temperature processes (2) and (3) low values of leakage currents were achieved compared with the reference process (1), whereas after removal of the oxide passivation layer by HF and subsequent water rinsing the leakage current was significantly higher. According to our model, current transport is supported by mobile charges at the Si surface which are mainly present after process (2) and after removal of the thermal oxide passivation layer by HF. The plasma hydrogen termination of the Si surface lasts for several days since buried hydrogen serves as a reservoir against the decay of Si–H x surface sites by desorption.

The effect of a hydrogen plasma on the diamond (110) surface

Hydrogen plasma processing has become a valuable method for generating smooth C(100) and C(111) surfaces. Herein, the generality of the method is examined by applying it to the C(110) surface. Using scanning force microscopy (SFM) and low energy electron diffraction (LEED), it is found that plasma conditions appropriate for smoothing C(100) and C(111) result in (111) faceted pits on the C(110) surface. LEED indicates that a freshly polished C(110) surface also consists of (111) facets despite its optically smooth appearance.