Undoped Surface Research Articles

The Effect of Manganese and Cobalt Doping on Chromium Deposition at Pt/YSZ Cathode Interfaces

The performance of solid oxide fuel cells (SOFCs) can be degraded by chromium deposition, the amount and location of which can be affected by the compositions of the electrolyte and electrodes. Understanding compositional effects, especially at SOFC operating conditions, is important for the selection of materials that will minimize the impact of chromium deposition. In this study, the effect of YSZ doping on Cr deposition at a Pt-electrode/YSZ-electrolyte interface was investigated. Comparison between undoped and doped YSZ electrolyte surfaces after cathodic polarization for 25 hours in moist air at 800 ºC in the presence of chromia indicated that the presence of Mn or Co in the electrolyte significantly promotes Cr deposition at the electrode/electrolyte interface. The SEM micrographs of the Co-doped YSZ electrolyte surface after cathodic polarization for 10 to 45 hours clearly showed that with Co doping, deposition of Cr preferentially occurs at YSZ grain boundary.

The influence of undoped GaN surface flatness on the properties of the blue light-emitting diode wafer

We have studied the properties of blue-LED wafers grown on GaN with similar dislocation density and different surface flatness. Results indicate that the smooth surface morphology of the undoped GaN layer leads to better layer periodicity in multiple quantum well (MQW) and smooth surface morphology of LED wafers. The surface flatness of the undoped GaN layer has little effect on the forward voltage and output power of the LED. However, the reverse leakage current and the lifetime of the LED wafer grown on the flatter GaN layer improved evidently. On the other hand, wafers grown on the GaN layer with worse surface flatness show better electrostatic discharge (ESD) characteristics.

Vacancy formation and CO adsorption on gold-doped ceria surfaces

We study the effect of gold doping on oxygen vacancy formation and CO adsorption on the (1 1 0) and (1 0 0) surfaces of ceria by using density functional theory, corrected for on-site Coulomb interactions (DFT + U). The Au dopant substitutes a Ce atom in the surface layer, leading to strong structural distortions. The formation of one oxygen vacancy near a dopant atom is energetically “downhill” while the formation of a second vacancy around the same dopant requires energy. When the surface is in equilibrium with gaseous oxygen at 1 atm and room temperature there is a 0.4 probability that no oxygen atom left the neighborhood of a dopant. This means that the sites where the dopant has not lost oxygen are very active in oxidation reactions. Above 400 K almost all dopants have an oxygen vacancy next to them and an oxidation reaction in such a system takes place by creating a second vacancy. The energy required to form a second vacancy is smaller on (1 1 0) than on (1 0 0). On the (1 1 0) surface, it is much easier to form a second vacancy on the doped surface than the first vacancy on the undoped surface. The energy required to form a second oxygen vacancy on (1 0 0) is comparable to that of forming the first vacancy on the undoped surface. Thus doping makes the (1 1 0) surface a better oxidant but it has a small effect on the oxidative power of the (1 0 0) surface. On the (1 1 0) surface CO adsorption results in formation of a carbonate-like structure, similar to the undoped surface, while on the (1 0 0) surface direct formation of CO 2 is observed, in contrast to the undoped surface. The Au dopant weakens the bond of the surrounding oxygen atoms to the oxide making it a better oxidant, facilitating CO oxidation.

Influence of boron doping on roughness microcrystalline silicon

We have studied roughness of boron-doped microcrystalline Si (μc-Si) surfaces with an emphasis on the influence of heavy doping. μc-Si films were prepared using plasma-enhanced chemical vapor deposition (PECVD) with different boron concentrations in gas phase from 0% to 2%. Growth-induced roughening of μc-Si surfaces was monitored ex situ using an atomic force microscope (AFM). With an increase in the deposition time, the surface width (rms roughness), w, of undoped μc-Si surface exhibited usual behaviors; first, (a) w increased, (b) slightly dropped, (c) rose again, and then (d) gradually increased. In the case of B-doped μc-Si, w differently behaved; (a) w increased very soon, (b) slightly dropped, (a′) rose again, (b′) slightly dropped again, (c) rose, and finally (d) gradually increased. The quick increase in w indicates that boron doping promotes the nucleation, and the repeated nucleation is responsible for the behavior (a′)–(b′). Additionally, the nucleation density, that was derived using the lateral correlation length of surface heights, monotonically increased with an increase in the boron concentration. The effects of boron doping are discussed with the catalytic effects and the formation of the surface-covering layer.

Electron field emission from the 2D hole gas in hydrogen terminated, polycrystalline diamond

The results of field emission measurements from hydrogen and non-hydrogen terminated polycrystalline, B-doped and undoped diamond surfaces are reported. The field emission from the undoped polycrystalline diamond show different current density vs. electric field characteristics than field emission from hydrogen terminated polycrystalline B-doped diamond and from hydrogen terminated single crystal undoped diamond. These differences are attributed to differences in the conduction mechanisms, being bulk conductivity in B-doped diamond and surface conductivity in undoped hydrogen terminated single crystalline and polycrystalline diamond. In contrast to the hydrogen terminated single crystal undoped diamond case, where discrete jumps in the current density vs. electric field curves are observed [L. Gan, E. Baskin, C. Saguy and R. Kalish, Phys. Rev. Lett. 96, 196808 (2006).] the results for the undoped polycrystalline case follow the Fowler Nordheim formalism, reflecting the presence of an ensemble of 2D surface energy states due to the various angles between the crystallite surfaces and the electric field. The parameters extracted from the Fowler Nordheim representation of the data for hydrogen terminated B-doped and undoped polycrystalline diamond, yield a Fermi energy level near the surface region for of 0.34 eV below the valance band maximum for the undoped sample.

The Effect of Manganese Doping on Chromium Deposition at Pt/YSZ Cathode Interfaces

The effect of yttria-stabilized zirconia (YSZ) doping on Cr deposition at a Pt-electrode/YSZ-electrolyte interface was investigated. Comparison between undoped and Mn-doped YSZ electrolyte surfaces after cathodic polarization for 25 h in moist air at 800°C in the presence of chromia indicated that the presence of Mn in the electrolyte significantly enhances Cr deposition at the electrode/electrolyte interface. Impedance spectroscopy results showed that with a Mn-doped YSZ electrolyte the electrode polarization resistance decreased significantly with polarization time, which was attributed to the increase in the three-phase boundary length by the p-type semiconducting chromia deposits.

Nitrogen oxides and ammonia sensing characteristics of SILAR deposited ZnO thin film

Abstract Pure and Sn, Ni doped ZnO thin films were deposited on glass substrates using a novel successive ionic layer adsorption and reaction (SILAR) method at room temperature. Microstructures of the deposited films were optimized by adjusting growth parameters. The variation in resistivity of the ZnO film sensors was performed with rapid photothermal processing (RPP). The effect of rapid photothermal processing was found to have an important role in ZnO based sensor sensitivity to NO 2 , NH 3 . While the undoped ZnO film surface exhibited higher NH 3 sensitivity than that of NO 2 , an enhanced NO 2 sensitivity was noticed for the ZnO films doped with Sn and higher NH 3 sensitivity was obtained by Ni doping.

Hydrogen Evolution on Diamond Electrodes by the Volmer-Heyrovsky Mechanism: Prediction of Reversible Potentials and Activation Energies

Hydrogen evolution on (111) diamond electrode surfaces was studied by hybrid density functional theory with cluster models. The reductive hydronium ion discharge in the Volmer-Heyrovsky (V-H) mechanism to form was found to be activated when there was either a bond-inserted interstitial hydrogen defect or a substitutional boron defect in the near subsurface region. Activation energies for the Heyrovsky reaction were calculated for the doped and undoped surfaces as functions of electrode potential, ranging between 0 and , using a constrained variation theory. It is concluded that weakly adsorbed H associated with near subsurface substitutional boron defects catalyze the hydrogen evolution activity that has been observed for boron-doped diamond electrodes. Following the electron transfer, we found no barrier for forming , irrespective of whether the defect was hydrogen or boron. A barrier was found for hydrogen evolution on defect-free diamond. Calculated activation energies for the Volmer reaction step were lower than for the Heyrovsky step, which indicates the Heyrovsky step is rate limiting. An important conclusion from this work is that the boron doping of diamond not only provides conductivity, but, we propose, also gives rise to a weakly bonded hydrogen that participates in the V-H mechanism of hydrogen evolution.

A First-Principles Analysis of Hydrogen Interaction in Ti-Doped NaAlH4 Surfaces: Structure and Energetics

First principles density functional theory studies have been carried out to investigate the hydrogen interactions in Ti-doped NaAlH4 (001) and (100) surfaces. In both surfaces, Ti was found to energetically favor the interstitial sites formed by three neighboring AlH4- units and interact directly with them. The resulting local structure corresponds to a formula of TiAl3Hx with x = 12 before hydrogen desorption starts. The hydrogen desorption energies from many positions of TiAl3Hx are reduced considerably as compared with that from the corresponding clean, undoped NaAlH4 surfaces. The almost invariant local environment surrounding Ti during dehydrogenation makes the TiAl3Hx complex a precursor state for the formation of experimentally observed TiAl3. The importance of the complex has been explored by analyzing the structures and energetics accompanying hydrogen desorption from the complex and from the neighboring AlH4- units. The TiAl3Hx has extended effects beyond the locally reducing hydrogen desorption energy. It facilitates low-energy hydrogen desorption by either transferring hydrogen to the TiAl3Hx complex or reducing hydrogen desorption energy in the neighboring AlH4- by linking these AlH4- units with the complex structure. The possible mechanisms for forming octahedral AlH6(3-) were also identified in the vicinity of TiAl3Hx. Desorbing hydrogen atoms between Ti and Al atoms causes a symmetrical expansion of Ti-Al bonds and leads to the formation of octahedral AlH6(3-).

Structural characterizations and electronic properties of Ti-doped SnO2(110) surface: A first-principles study

The Ti-doped SnO2(110) surface has been investigated by using first-principles method with a slab model. The geometrical optimizations and band-structure calculations have been performed for four possible doping models. Our results indicate that the substitution of Ti for sixfold-coordinated Sn atom at the top layer is most energetically favorable. Compared to the undoped surface, those Sn and O atoms located above Ti atom tend to move toward the bulk side. Besides the surface relaxations, the doping of Ti has significant influences on the electronic structures of SnO2(110) surface, including the value and position of minimum band gap, the components of valence and conduction bands, the distributions of the charge densities, and the work function of the surface. Furthermore, the effects introduced by the substitution of Ti atom observed in the experiments can be well explained when the sixfold-coordinated Sn atom at the first layer is replaced by Ti atom.

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.

Evidence for valence-charge fluctuations in the3×3−Pb∕Si(111)system

We have investigated the 1/3 monolayer $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}\text{\ensuremath{-}}\mathrm{Pb}∕\mathrm{Si}(111)$ system with core level photoemission at temperatures of 300 and $120\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. At both temperatures the $\mathrm{Pb}\phantom{\rule{0.3em}{0ex}}5{d}_{5∕2}$ and $5{d}_{3∕2}$ core levels are split into two states with an intensity ratio of 2:1, as in the related $\mathrm{Sn}∕\mathrm{Ge}(111)$ and $\mathrm{Sn}∕\mathrm{Si}(111)$ systems. On doping the surface with $\mathrm{Cs}$ (an electron donor) or oxygen (an electron acceptor) the ratio of the intensities of the two states change strongly, suggesting the states have filled or empty dangling bonds. The results are interpreted in terms of charge fluctuations with one third of $\mathrm{Pb}$ atoms acting as charge donors and two thirds as acceptors in the undoped phase. Four chemically shifted $\mathrm{Si}\phantom{\rule{0.3em}{0ex}}2p$ core level peaks are found whose energies are almost constant for the doped and undoped surfaces, and whose relative intensities change significantly on doping. These states of $\mathrm{Si}$ are assigned to two distinct kinds of atoms bonded to the two states of $\mathrm{Pb}$, and to deeper $\mathrm{Si}$ atoms. The shifts are explained in the framework of the charge fluctuation model [M. G\othelid, M. Bj\orkvist, T. M. Grehk, G. Le Lay, and U. O. Karlsson, Phys. Rev. B 52, R14 352 (1995)], and the integer pseudo charge model [G. Ballabio, G. Profeta, S. de Gironcoli, S. Scandolo, G. E. Santoro, and E. Tosatti, Phys. Rev. Lett. 89, 126803 (2002)]. Within the family of group IV metals adsorbed on group IV (111) semiconductor surfaces, $\mathrm{Pb}∕\mathrm{Si}(111)$ is similar to $\mathrm{Pb}∕\mathrm{Ge}(111)$ and $\mathrm{Sn}∕\mathrm{Ge}(111)$ with $\mathrm{Sn}∕\mathrm{Si}(111)$ being the exceptional case.

ZnO thin film sensors for detecting dimethyl- and trimethyl-amine vapors

Undoped and Al-doped ZnO thin films were deposited on quartz substrates using an indigenously developed modified chemical vapor deposition technique. Microstructures of the deposited films were optimized by adjusting various growth parameters. The baseline resistance of the ZnO film sensors was stabilized by annealing the as-deposited films periodically in an oxidizing and reducing ambient at maximum operating temperature. The sensor parameters were studied by exposing the optimized ZnO thin films to low concentrations of dimethylamine (DMA) and trimethylamine (TMA), the chemical species released from harvested fish and other seafood with ageing. The effect of operating temperature was found to play a vital role in determining the sensitivity and resolution of the sensor. While the undoped ZnO film surface, sensitized with palladium, exhibited higher DMA sensitivity than that of TMA, an enhanced TMA sensitivity was noticed with the ZnO films doped with aluminum.

Improved interatomic potentials for silicon-fluorine and silicon-chlorine.

Improved sets of empirical interatomic potentials for silicon-fluorine and silicon-chlorine are presented. The Tersoff-Brenner potential form has been reparameterized using the density-functional theory (DFT) cluster calculations of Walch. Halogenated silicon cluster energetics computed with DFT are, on average, within several tenths of an eV of the energies of the corresponding clusters with the reparameterized empirical potential for both Si-F and Si-Cl. Using the reparameterized Tersoff-Brenner potentials, molecular-dynamics simulations of F and Cl atom exposure to undoped silicon surfaces are in excellent agreement with published data on etch probability, halogen coverage at steady state, and etch product distributions.

Influence of surface Pd doping on gas sensing characteristics of SnO 2 thin films deposited by spray pirolysis

In this article the analysis of steady state and transient gas sensing characteristics of undoped and Pd surface doped SnO 2 films, deposited by spray pyrolysis, is described. The influence of parameters such as air humidity (2–50% RH), operation temperature (25–500 °C) and Pd surface concentration (0–1% ML Pd) on gas response to CO and H 2 (0.1–0.5%), response time, shape of sensitivity S( T) curves and activation energy of τ(1/ kT) dependencies are discussed. A mechanism based on a chemisorption model is proposed to explain how Pd influences the gas sensing characteristics of SnO 2 films.

Investigation of Current-Voltage Characteristics of Oxide Region Induced by Atomic Force Microscope on Hydrogen-Terminated Diamond Surface

The current-voltage characteristic of the atomic force microscope (AFM) field-assisted local oxidized region on an undoped hydrogen-terminated (H-terminated) diamond surface is investigated. The barrier height on the top of the valence band between the undoped H-terminated diamond surface and the AFM oxidized surface is estimated by Fowler-Nordheim (F-N) tunneling current analysis. By fitting the parameter of the slope in the F-N plot, the barrier height is estimated to be 61 meV in the electrically isolated conductive island structure. On the other hand, the barrier height is also estimated to be 72 meV in the lateral tunneling diode.

Fabrication of diamond single-hole transistors using AFM anodization process

By the field-assisted local anodization technique using an atomic force microscope (AFM), a single-hole transistor has been fabricated on an undoped hydrogen-terminated diamond surface where p-type conduction occurs on the subsurface region. A dual side-gated FET structure has been applied to modulate the island potential in the single-hole transistor. The island size is 230 nm×230 nm, and the width of the barrier is approximately 100 nm. Measurements of the current–gate voltage characteristic at a temperature of 4.6 K show significant non-linearities including a current oscillation suggestive of single-hole transistor behavior. The oscillation that is significantly affected by the application of the side gate potential is explained by the shrinkage of the conductive island with the expansion of the depletion region.

Characterization of proteins and fibroblasts on thin inorganic films.

The ability of biomaterial surfaces to regulate cell behavior requires control over surface chemistry and material microstructure. One of the goals in the development of silicon-based biomedical devices such as biosensors or drug delivery systems is improved biocompatibility which may be achieved through the deposition or adsorption of thin films. In this study, films of single crystal silicon, stoichiometric and low stress silicon nitride, doped and undoped polysilicon, as well as Arg-Gly-Asp (RGD) peptide adsorbed surfaces characterized in terms of protein adsorption or cellular adhesion for a period of four days. Protein adsorption studies using fibrinogen and albumin, two proteins implicated in cellular adhesion and surface activity, reveal that low stress silicon nitride surfaces have a 223%+/-2.50% greater protein adsorption compared to undoped polysilicon surfaces, followed by silicon nitride, unmodified silicon, and doped polysilicon surfaces, respectively. The thickness of the adsorbed albumin and fibrinogen layer on various thin films was measured by ellipsometry and compared to contact angle measurements. The greatest cellular adhesion was observed on undoped polysilicon, followed by unmodified (control) silicon, low stress silicon nitride, silicon nitride, and doped polysilicon surfaces. Cellular binding supports the differential protein adsorption found on modified and unmodified silicon surfaces. Understanding the biological response to thin films will allow us to design more appropriate interfaces for implantable diagnostic and therapeutic silicon-based microdevices.

Influence of Li impurities on the adsorption of D 2O on MgO (1 0 0)

The aim of the present work is to elucidate the role played by lithium impurities in the D 2O adsorption on MgO(1 0 0) surfaces at room temperature and on H and D diffusion processes by using electron stimulated desorption (ESD). The growth of the D 2O layer has been studied by recording the intensity of O +, OD + and D + desorbed ions. Whereas for undoped MgO surfaces the O + intensity decreases with coverage, in Li-doped MgO surfaces the intensity increases. These findings together with the observed threshold energy of D + desorbed ions at 55±5 eV indicate that adsorbed D 2O undergoes preferentially total dissociation on the MgO : Li system. This threshold energy corresponds to the Mg 2p excitation, which means that the D species are bonded to the Mg atoms.

Optimization of post-CMP cleaning process for elimination of CMP slurry-induced metallic contaminations

We have invetigated the slurry-induced metallic contaminations of undoped and doped silicate oxide surface during the post-chemical mechanical polishing (CMP) cleaning process. The metallic contaminations by CMP slurry were evaluated in the four different oxide films, such as plasma enhanced tetra-ethyl-ortho-silicate glass (PE-TEOS), O3-boro-phospho-silicate glass (O3-BPSG), PE-BPSG, and phospho-silicate glass (PSG). Prior to entering the post-CMP cleaner, all films were polished with KOH-based slurry. The total X-ray fluorescence (TXRF) measurements showed that all oxide surfaces are heavily contaminated by potassium and calcium during polishing, which is due to the CMP slurry. The polished O3 BPSG films presented higher potassium and calcium contaminations compared to PE-TEOS film because of a mobile ion gettering ability of phosphorus. For PSG oxides, the slurry-induced mobile ion contamination increased with an increase of phosphorus content. In addition, the polishing removal rate of PSG oxides had a linear relationship with phosphorus content.