Conducting boron-doped single-crystal diamond films for high pressure research

Epitaxial boron-doped diamond films were grown by microwave plasma chemical vapor deposition for application as heating elements in high pressure diamond anvil cell devices. To a mixture of hydrogen, methane and oxygen, diborane concentrations of 240–1200 parts per million were added to prepare five diamond thin-film samples. Surface morphology has been observed to change depending on the amount of diborane added to the feed gas mixture. Single-crystal diamond film with a lowest room temperature resistivity of 18 mΩ cm was fabricated and temperature variation of resistivity was studied to a low temperature of 12 K. The observed minima in resistivity values with temperature for these samples have been attributed to a change in conduction mechanism from band conduction to hopping conduction. We also present a novel fabrication methodology for monocrystalline electrically conducting channels in diamond and present preliminary heating data with a boron-doped designer diamond anvil to 620 K at ambient pressure.     

Silicon rich silicon oxide films deposited by radio frequency plasma enhanced chemical vapor deposition method: Optical and structural properties

Silicon rich silicon oxide films have been deposited by plasma enhanced chemical vapour deposition using a gas mixture of silane, carbon-di-oxide and hydrogen. Silicon nanocrystals formations in the as deposited silicon rich silicon oxide films have been detected by high resolution transmission electron microscopy, scanning electron microscopy, Raman scattering and X-ray diffraction studies. Structural changes under different deposition condition have been studied by Fourier transform infrared spectroscopy. The oxygen and hydrogen bonding configurations have been obtained from Fourier transform infrared spectroscopy. Room temperature photoluminescence spectra have been observed for the as deposited films. The structural properties together with photoluminescence spectra allowed us to gain insight about the Si nanocrystal formation.     

Luminescence enhancement by hydrogenation of Si:Er,O

Erbium (Er)- and oxygen (O)-doped Cz–Si was additionally doped with hydrogen, using plasma enhanced chemical vapour deposition. Photoluminescence (PL) spectra show a large enhancement especially for samples treated with solid phase epitaxy before hydrogenation and annealing at 900°C later. Secondary ion mass spectroscopy measurements give evidence for an enhanced diffusion of O and Er at this temperature towards the surface. Etching shows that the PL does not stem from the heavily doped surface layer but from a deeper region with lower Er concentration. This conclusion is supported by the appearance of the so-called “cubic” centre with low solubility. Comparing the PL yield of the hydrogenated samples to that of samples with similar Er volume concentration but without hydrogenation still gives a large enhancement. We thus conclude that hydrogen can enhance the solubility of the cubic centre in Si:Er,O.     

Precise control over oxygen impurities in nano-crystalline silicon thin film processed with a low hydrogen dilution gas system at near room temperature

An atmosphere highly diluted with hydrogen is essential to increase the crystal fraction during formation of hydrogenated nano-crystalline (nc) or micro-crystalline (μc) silicon thin films via chemical vapor deposition (CVD). This hydrogen-rich process, however, hinders the ability for the material to find adequate use in micro-electronic devices due to contamination that results in oxygen-related problems such as donor-like doping, defect creation, or passivation. The use of neutral beam assisted chemical vapor deposition (NBaCVD), with a low hydrogen ratio (R = H₂/SiH₄) of 4, successfully deposits a highly-crystallized nc-silicon (HC nc-Si) thin film (TF) at near room temperature (<80 °C) and effectively reduces oxygen contamination by as much as 100 times when compared to conventional plasma enhanced CVD. During the formation of HC nc-Si TF via NBaCVD, energetic hydrogen atoms directly react with oxygen atoms near the surface of the nc-Si TF and remove the oxygen impurities. This is a completely different mechanism from the hydrogen-enhanced oxygen diffusion model. This technology meets the recent requirements of a high deposition rate and low temperature necessary for flexible electronics.     

Morphology induced magnetic instabilities on oxide antiferromagnetic surfaces

The role of interfacial disorder and roughness has been pointed out several times in ferromagnetic-antiferromagnetic exchange coupled systems. However, effect of surface morphology on magnetic properties of bare antiferromagnetic material has been mostly ignored. Here we report our experimental observations of magnetic instabilities on high temperature air annealed NiO(100) surfaces through treatment of successive vacuum and oxygen annealing above Néel temperature. Preferential spin-orientations are found to be governed by surface morphology with enhanced roughness and oxygen diffusion process. Stable domain distribution has been observed on smoother surfaces, which are found to be mostly inactive to these annealing treatments.     

Study of the oxygen incorporation and diffusion in Sr(Ti0.65Fe0.35)O3 ceramics

For applications in high temperature fast conductometric gas sensors and oxygen membranes, several mixed conductors show promising features. In particular, acceptor-doped strontium titanate (STO) has been widely investigated for an application as a fast conductometric oxygen sensor. By a B-site substitution with 35% iron, the resulting ceramic solid solution SrTi_0.65Fe_0.35O₃ (STF35) exhibits a temperature-independent conductivity, an ideal prerequisite for a gas sensor.In the presented study, the oxygen tracer exchange behavior and the tracer diffusion of dense ceramic STF35 bulks have been investigated in the temperature range between 600 and 900 °C by means of ¹⁸O₂ tracer exchange experiments and subsequent secondary ion mass spectrometry (SIMS), resulting in the determination of k* and D* values, respectively.Furthermore, by coating the samples with a thin alkaline earth metal oxide layer (CaO), a significantly enhanced oxygen surface exchange reaction was observed. These findings are in good agreement with previous results on STO single crystals.     

Hydrogen promoted corrosion of tungsten by oxygen in an electric field: A field ion microscope study

The extensive changes in surface topography observed to occur on tungsten field ion emitters as a result of exposure to oxygen in presence of hydrogen at 78°K and at fields of 2 V/Å have been studied in detail. Field promoted diffusion of gas from the low field region of the emitter shank over the imaged area of the surface removes kink site metal atoms and subsequently deposits them on either side of the well defined diffusion paths along close packed zones; field evaporation of such atoms may also occur when the change in local surface contour causes sufficient field enhancement. Although oxygen is primarily responsible for the corrosion effects, its diffusion at such temperatures is promoted by the presence of hydrogen, and the rate of reaction is dependent, inter alia, on hydrogen pressure.     

Controllable preparation of vertically standing graphene sheets and their wettability and supercapacitive properties

Vertically standing graphene(VSG) sheets have been fabricated by using plasma enhanced chemical vapor deposition(PECVD) method.The lateral size of VSG nanosheets could be well controlled by varying the substrate temperature.The higher temperature usually gives rise to a smaller sheet size.The wettability of VSG films was tuned between hydrophobicity and hydrophilicity by means of oxygen and hydrogen plasma treatment.The supercapacitor electrode made of VSG sheets exhibited an ideal double-layer-capacitor feature and the specific capacitance reached a value up to 9.62 F·m~(-2).     

Electronic criteria for distinction between weak and strong ionosorption

Three different types of ionosorption on pellets of compressed powder of cobaltous oxide have been observed by measurements of the electrical conductance.(a) When a powder is efficiently outgassed, an instantaneous ionosorption of oxygen is characterised by an increase in conductance, while a sudden decrease of conductance appears, when hydrogen is introduced instead of oxygen.(b) A slow change of conductance starts, less than an hour after the first phase of instantaneous ionosorption, and goes on during several days. A parallel shifting of the semilog straight lines of the resistance vs absolute temperature, has generally been observed below a threshold temperature of 85°C.(c) Beyond this threshold of temperature, a simultaneous shifting and revolving of the semilog lines has been established. Their slopes increase in hydrogen and decrease in oxygen ambience. Water vapor was detected at, and beyond this threshold. This temperature, beyond which the revolving starts, is the same for hydrogen, oxygen or carbon monoxide ambience.The parallel shifting of the semilog lines and the meaning of the threshold of 85°C has been interpreted following two possible assumptions.     

Application of the Nuclear Reaction Analysis Online Technique to Study the Diffusion of Deuterium in Metals

Nuclear reaction analysis online technique has been applied to study the diffusion of deuterium in metals. Investigations ensuring the application of the new method have been performed. These investigations include the development of a device for diffusion annealing of samples in the chamber of an accelerator and an algorithm for taking into account the effect of radiation defects on the diffusion coefficients. Test measurements of the diffusion coefficients of deuterium in nickel in the temperature range from 130 to–60°С have been performed. For negative temperatures, experimental data on the diffusion of a hydrogen isotope in a metal have been obtained for the first time by a direct method and it has been shown that the online nuclear reaction analysis provides reliable data.     

The water-forming reaction on palladium

The water-forming reaction on Pd has been studied on a PdSiO₂Si (Pd-MOS) structure in the temperature range 323–473 K. The reaction is found to be of the Langmuir-Hinshelwood type with the formation of OH beeing rate limiting. Since the Pd-MOS structure works as a sensitive hydrogen detector unique information on the behaviour of hydrogen during this catalytic reaction has been obtained. The reaction can be described in a model where the hydrogen atoms on the Pd surface have a large temperature activated lateral mobility and with no evidence of beeing in hot precursor states. At T = 473 K this means that for oxygen coverages ? 0.01 monolayers all hydrogen adsorbed will also react with oxygen. For smaller oxygen coverages unreacted hydrogen will not initially desorb towards the vacuum but towards the internal Pd surface of the Pd-MOS structure. Futhermore, hydrogen adsorption is blocked by adsorbed oxygen. The sticking coefficient for hydrogen on the bare Pd surface is, however, close to one and only weakly temperature dependent. An effect giving rise to a hysteresis in the work function versus oxygen coverage curve during oxygen adsorption - desorption is also discussed.     

Measurement of diffusion process of iron atoms under high pressure of hydrogen by time-domain analysis of nuclear resonant scattering of X-rays

We applied the time-domain analysis of nuclear resonant scattering (NRS) of X-rays for the study of the hydrogen-induced enhancement of atomic diffusion. The time-domain analysis of NRS is a powerful technique for studying diffusion processes on an atomic scale. The NRS measurement combined with high-pressure technique enables the direct measurement of self-diffusion processes under high hydrogen pressures. In this preliminary experiment, self-diffusion in 4 μm thick ⁵⁷Fe foils at 0.8 GPa was investigated. The samples of the ⁵⁷Fe were encapsulated with MgO or NaCl. Faster decays caused by diffusion of Fe atoms were observed in the time spectra of NRS at high temperatures. This enhancement of diffusion is believed to be the hydrogen-induced effect. In the present experiment, hydrogen should have been supplied to the samples by reaction with water originally adsorbed on NaCl/MgO powder particles. It was concluded that the diffusion of ⁵⁷Fe atoms under high pressure can be studied by nuclear resonant scattering of X-rays using a compact cubic-anvil press. The NRS method can also be extended to the study of atomic diffusion in the subsurface region by doping ⁵⁷Fe layer(s) at known depths.     

Diffusion of hydrogen in the β-phase of pd-H studied by small energy transfer neutron scattering

The diffusion of hydrogen in β-PdH_x has been studied by quasielastic neutron scattering. It is shown that the diffusion occurs through jumps between adjacent octahedral interstitial sites. The observed integrated quasielastic intensities cannot be described by a simple Debye-Waller factor. The phase transition from the β-phase to the α-phase has also been studied. No dramatic changes in the scattering patterns were observed. It is concluded that the diffusion mechanism is remarkably similar for the low concentration α-phase and the high concentration β-phase.     

Parameter space map of an electron cyclotron resonance plasma in acompact chamber

Ion current density measurements were made in an electron cyclotron resonance (ECR) plasma reactor for both argon and oxygen discharges. Spatial changes in the ion current density were also recorded across the reactor diameter for changes in pressure and power. These measurements revealed a minimum in the ion current density on the reactor axis. This observation has been explained as a consequence of the shape of the ECR region, which, in turn, is dependent on the mode of coupling. Current density measurements were made as a function of reactor pressure and microwave power for two different axial locations in the system. A Langmuir probe was also used at these two locations to measure the electron temperature as a function of these process conditions. It was observed that the ion current density and/or plasma density measured downstream from the ECR zone, increased significantly in the low-pressure/high-microwave power region. Results from this region of the operating parameter space have not previously been reported. Further existing models do not predict this observed increase in plasma density or ion current density. It has been proposed that a rarefication of the gas in the ECR region, as a result of gas heating, has acted to increase the outward diffusion of electrons from the ECR zone and, thus, has increased the ambipolar diffusion of ions to the downstream location. This proposal has been partially validated by experimental results in which the ion energy was measured as a function of reactor pressure and gas flow rate. The shape of the oxygen parameter space map differs significantly from that for Ar. The principal reasons for these changes are a number of different inelastic electron scattering mechanisms which effect the transport electrons out of the ECR zone and through ambipolar diffusion also the transport of ions. The second factor is the production of negative ionic species which varies with reactor pressure and, thus, T_e     

Low temperature diffusion of oxygen in titanium and titanium oxide films

Work in our laboratory and elsewhere indicates that the thermal ignition characteristics of Ti-based pyrotechnics are controlled by diffusion of oxygen from a surface coating into the bulk of the metal. Diffusion of oxygen in Ti at temperatures greater than 700 K has previously been modeled using Fick's law. No work has been reported at lower temperature, and the results from previous high temperature oxidation studies are both inconsistent and insufficient to define diffusion behavior for the temperature range 400 to 800 K. We have conducted Ti oxidation experiments in oxygen ambients of 3 to 700 Torr, at temperatures from 400 to 800 K, for periods from 1 to 100 h. Oxygen concentration profiles were determined by Auger analysis combined with sputter depth profiling. Calibration of the Auger atomic concentrations were confirmed by Rutherford backscattering spectroscopic measurements. Results show growth of four distinct oxide regions which are consistent with the titanium-oxygen phase diagram. Growth of the oxide regions is independent of the oxygen partial pressure and appears to be diffusion controlled. Fick's law with a constant diffusion coefficient was assumed, and an Arrhenius expression for the oxygen diffusion coefficient in each region was evaluated from the experimental data. Activation energies for oxygen diffusion range from about 9 to 43 kcal/mol, and indicate that the thermal ignition mechanism is controlled by growth of a TiO₂ region adjacent to the gas-solid interface.     

Transformation of hydrogen silsesquioxane properties with RIE plasma treatment for advanced multiple-gate MOSFETs

Keeping in line with Moore's law requires increasing efforts in the development of alternative electronic devices. Multiple-gate transistors are very promising in order to suppress short-channel effects and to increase the current drive. Nevertheless, the fabrication of such devices represents a strong challenge for silicon process technology. One of the key steps of the process consists in shrouding the silicon fins in an isolating matrix, using a flowable oxide, namely hydrogen silsesquioxane (HSQ). The general objective of this work is to show that the properties of HSQ can be modified by appropriate thermal or plasma treatments to modulate its characteristics in terms of etching selectivity and surface topography. SEM characterization has shown that HSQ exhibits excellent planarization and gap fill capabilities, while AFM analysis, on 100 nm thick HSQ films deposited by spin on, reveals a roughness as low as 3 nm. Various oxygen plasma treatments have been applied to densify the HSQ films. Fourier transform infra-red spectroscopy (FT-IR) has shown very interesting qualitative and quantitative informations. Chemical and physical transformations from a Si-O-Si cage-like structure into an Si-O-Si network one have been observed. It is shown that exposure to oxygen plasma at high power (290 W) for a long time (20 min) or thermal curing at high temperature improves the resistance to wet etching using 1% hydrofluoric acid (HF). This densification technique holds the remarkable property to transform HSQ into an SiO₂-like structure.     

Dependence of anomalous phosphorus diffusion in silicon on depth position of defects created by ion implantation

Transient enhanced diffusion of phosphorus in silicon has been investigated for implants below and above the threshold for a complete amorphization. Rapid thermal processes (electron beam) and conventional furnaces have been used for the annealing. In the case of implants below amorphization, a strong enhanced diffusion, proportional to the amount of damage produced, has been observed. The extent of the phenomenon is practically independent of the damage depth position. In contrast to this, the formation of extended defects at the original amorphous-crystalline interface makes the diffusivity strongly dependent on depth in the case of post-amorphized samples. No enhanced diffusion effect is observed if the dopant is confined in the amorphous layer, while a remarkable increase in the diffusivity is detected for the dopant located in the crystalline region beyond the amorphous-crystalline interface.Damage distribution after implantation and its evolution during annealing have been determined by double crystal x-ray diffraction and correlated to anomalous P diffusivity. A qualitative distribution of the interstitial excess in solution in the silicon lattice during annealing is proposed for the two different cases. These point defects, released by the dissolution of the interstitial clusters produced by the implanted ions, have been identified as responsible for the observed enhanced P diffusion.     

Electrical properties and proton conduction of gadolinium doped ceria

The electrical properties and proton conduction of Gd_0.1Ce_0.9O_1.95 (10GCO) were investigated via impedance spectroscopy in different atmospheres and various gas concentration cells. In oxygen atmosphere, GCO is nearly a pure oxygen ionic conductor, while in hydrogen GCO behaves as a mixed conductor of oxygen ions, electrons and protons. Depending on the temperature, the total conductivity is usually enhanced by one to two orders of magnitude in hydrogen than in air/oxygen due to mixed conduction. By examining ionic transport properties of oxygen ions and protons using gas concentration cells we have discovered that the ionic transport properties depend largely on the gas atmospheres and change from one type to the other. Proton conduction generally exists in GCOs, and becomes significant in hydrogen atmospheres, which normally results in a contribution between 5 to 10 % of the total conductivity for 10 GCO. A maximum value of 17 % of the contribution by protons has been observed. The reduction of Ce⁴⁺ to Ce³⁺ of the sample in reduced atmospheres causes the formation of additional oxygen vacancies and electrons, associated also with the creation of protons. All these charge carriers are responsible for the electrical and transport properties of the investigated GCO materials. Paper presented at the 5th Euroconference on Solid State Ionics, Benalmádena, Spain, Sept. 13–20, 1998.     

Production of nanocrystalline silicon layers using the plasma enhanced chemical vapor deposition from the gas phase of silicon tetrafluoride

The production of silicon layers using plasma enhanced chemical vapor deposition in the mixture of silicon tetrafluoride and hydrogen is reported. The samples have been analyzed by X-ray diffraction, Raman spectroscopy, and secondary ion mass spectrometry. The phase composition of the layers is nanocrystalline silicon with the crystalline-domain sizes from 3 to 9 nm in dependence of the conditions of the process. The samples are characterized by intense photoluminescence at room temperature.     

The positive muon and the positron as probes of defects

Conclusions A brief but hopefully general comparison has been made between muons and positrons as probes for the study of defects in metals. Since muon experiments are not only more demanding in manpower, cost and availability than positron experiments, they should be carefully designed in light of the knowledge that the muon is extremely sensitive to both intrinsic and extrinsic defects. Initial experiments should provide estimates of the muon diffusion coefficients as a function of sample temperature. High temperature hydrogen diffusion measurements do provide quidelines, although so far most of the observations of muon trapping have been made at low sample temperatures where hydrogen diffusion data do not exist. Given that the diffusion constant is known as a function of temperature, high-purity Fe²⁶ after low temperature electron irradiation is therefore a good candidate to study with muons. Since the defect type and concentration can be controlled in electron irradiated samples, such investigations could confirm the stated values of the diffusion constants in Fe thereby providing a new method for evaluating diffusion coefficients below Stage III in different metals.All the samples should be initially characterized by other less costly techniques to obtain, where possible, the concentration and type of extrinsic and intrinsic defects. Both transverse and longitudinal measurements should be made to unravel the question of different diffusion mechanisms versus defect-, impurity- or self-trapping.Provided that the full capabilities of the muon are utilized and coupled with complementary techniques, i.e., positron annihilation, the muon will constitute a useful new probe in deepening our understanding or defects in metals.Work supported by the US Dept. of Energy under contract DE-AC02-76CH00016.