Total Dopant Concentration Research Articles

Self-propagating room temperature synthesis of Ce0.8SmXM0.2-xO1.9(M= Gd, Nd, 0≤x≤0.2) co-doped ceria oxide-ion conductor ceramics for solid oxide fuel cells

Ce0.8SmXM0.2-xO1.9 (M = Gd, Nd, 0≤x ≤ 0.2) co-doped cerium oxide ionic conductor ceramics are prepared for the first time by a simple and time-saving self-propagating room temperature (SPRT) method. The uncalcined ceramic powders and the microstructure of the pellets sintered at 1400 °C are characterized by thermogravimetric analysis, Fourier-transformed infrared spectroscopy, scanning electron microscopy, and X-ray diffractometry. The density of the pellets is determined using the Archimedes method. The oxygen ion conductivity properties of the electrolytes are measured by electrochemical impedance spectroscopy. Impurity-free, non-porous electrolytes with high-density (>95 %) are successfully prepared. We report that the influence of dopant type and composition on the physical properties of the electrolytes prepared by the SPRT method is insignificant, with the total dopant concentration kept constant at 20 %. The dopant type and composition mainly influence the lattice parameters, grain interior conductivity of the grains, and the activation energy of the internal conductivity of the grains. Maximum oxygen ion conductivities are demonstrated with Ce0.8Sm0.1Nd0.1O1.9 and Ce0.8Sm0.15Gd0.05O1.9 samples for the Nd and Gd series, respectively. The highest ionic conductivity of 0.0677 S cm−1 and the lowest activation energy of 0.75 eV is achieved at a value of 10%Nd-10%Sm.

Investigating the physico-chemical and electrochemical properties of various aliovalent cations doped ceria powders for electrolytic applications in solid oxide fuel cells

Solid oxide fuel cells (SOFCs) are one of the most efficient energy conversion devices, yet their high operating temperatures have caused a bottleneck towards their commercialization. Due to sufficient conductivity and stability below 600 °C, doped ceria is under investigation for electrolytic applications in SOFCs. During this study, starting with Ce0.8Sm0.2O2-δ, 5% of Sm3+ was replaced by dopants of varying valencies and sizes giving Ce0.8Sm0.15X0.05O2-δ (where X = Bi3+, Mg2+, Li+), thereby aiming to keep the total dopant concentration constant. The structure-property relationship was studied using a number of techniques including XRD, Raman spectroscopy, FTIR spectroscopy and temperature dependent EIS. XRD indicated the presence of a minor phase belonging to MgO for the Mg2+ co-doped sample. Raman spectroscopy showed the presence of δ-Bi2O3 for Bi3+ doped SDC. Based on the quantitative analysis of Raman data we concluded that the materials were multi-phasic and co-dopants addition did not have a noticeable influence especially on defect concentration. In EIS data, measured from of 550–750 °C, change in conductance behavior was shown by Arrhenius plots for SCB and SCM when the test temperature was raised over 650 °C, possibly reflecting the combined conduction due to the additional phases along with SDC for these samples. Poor solubility of the non-rare earth metal ions into the ceria lattice was observed. This study suggests the need of a comprehensive analysis of doping mechanism preferably by synchrotron-based techniques to effectively incorporate low-cost non-rare earth metals into ceria lattice.

The synthesis of Eu–Tb–Ho co–doped Bi2O3–based ceramic powders for use in intermediate temperature–SOFCs

In this study, the solid–state reactions in the ambient atmosphere were used to produce Ho–Tb–Eu co–doped Bi2O3 solid ceramic powder solutions. All samples were stabilized with the cubic δ–phase, according to XRD patterns generated at room temperature. The main peak (111) on the diffraction pattern shifted to larger angles as the total dopant concentration increased, and the average crystal size decreased as well. On the temperature–based DTA curves, neither an endothermic nor an exothermic peak could be seen, indicating that no phase transition took place during the uniform heating circle. According to the conductivity measurement results, sample 5Eu5Tb15Ho had the highest conductivity of 0.288 S/cm at 750 °C and the lowest activation energy of 0.19 eV. The FE–SEM images revealed that the grain size of sample 5Eu5Tb15Ho and Ho–rich samples was not uniform across the surface, and the grain boundary line changed rapidly as the dopant concentration increased.

Dopant Activation Depth Profiling for Highly Doped Si:P By Scanning Spreading Resistance Microscopy (SSRM) and Differential Hall Effect Metrology (DHEM)

Heavily n-doped epitaxially grown Si layers are of great importance for source/drain (S/D) application in advanced node nMOS devices. For contact resistivity reduction, the dopant activation level is very important. Various techniques are being used to evaluate dopant activation in Si:P layers. Among these two methods are Scanning Spreading Resistance Microscopy (SSRM) and Differential Hall Effect Metrology (DHEM). SSRM uses an atomic force microscope equipped with a hard conductive probe that is scanned in contact mode on the cross-sectioned sample’s surface and measures spreading resistance. Measured resistance values as a function of depth are converted into resistivity and carrier concentration depth profiles using calibration measurements and conversion relationships. DHEM provides depth profiles of mobility, resistivity and carrier concentration through a semiconductor layer by making successive sheet resistance (Rs) and Hall voltage measurements using Hall effect/Van der Pauw techniques, as the electrically active thickness of the layer is reduced through successive oxidation steps. Controlled oxidation is achieved by electrochemical anodization. Data collected can then be processed to yield the depth profiles. In this contribution we have carried out SSRM and DHEM measurements on Si:P epi layers subjected to different processing conditions including annealing and ohmic contact fabrication. Secondary Ion Mass Spectrometry (SIMS) was used to measure the total (active + inactive) dopant profiles through the films. Effects of these processes on dopant diffusion and activation were studied and the results from DHEM and SSRM were compared.In-situ phosphorus (P) doped Si epi-layers were grown over 300mm diameter boron doped monitor wafers. While one set was kept as the reference, a second set was treated by a spike-annealing process at 1000 °C. In a third set a Ti/TiN contact fabrication process was carried out and the contact was removed before analysis. In the fourth set contact process was applied to the spike annealed wafer before removal of the Ti/TiN layers. Bulk sheet resistance measurements were made using 4-point probe (4PP) for all the samples. SSRM measurements were carried out at IMEC. Cross-shaped Van der Pauw test-patterns were formed on 8mmx8mm areas on the samples and DHEM measurements were performed at ALP. Figure below shows the dopant depth profiles obtained by SIMS and the carrier concentration profiles from DHEM and SSRM techniques for samples D02 (as deposited Si:P) and D03 (spike annealed). One can make some general observations from the data in this figure. The total dopant concentration as measured by SIMS is ~ 1.4E21/cm3. There is only a small difference in the total dopant distribution profiles (SIMS) between the as deposited sample and the spike annealed sample. However, the spike annealed sample shows much higher dopant activation as measured by DHEM. Carrier concentration is ~2.5x higher in sample D03 (~5E20/cm3) compared to sample D02 (~2E20/cm3). Activation levels measured by SSRM, however, are lower for both samples, and the peak carrier concentration value increases only slightly upon spike annealing, going from ~2E20/cm3 in sample D02 to ~2.2E20/cm3 in sample D03. DHEM clearly indicates the sharp interface between the p-type substrate and the n-type epi-layer and its depth calibration agrees well with the expected thicknesses of the epi layers. The tail of the SSRM data is much more graded. Data from other samples will be presented and discussed in the final manuscript. Figure 1

Study of Dopant Activation in Silicon Employing Differential Hall Effect Metrology (DHEM)

Differential Hall Effect Metrology (DHEM) technique was used to characterize highly n-type doped Si epi layers deposited on p-type Si wafers. Total dopant concentration, doping depth profile and post deposition annealing condition were changed for various sample sets and influence of such changes on the resistivity, mobility and carrier concentration depth profiles were studied. It was determined that samples annealed at 900 ∘C had higher activation compared to those annealed at 700 ∘C. Gradation in doping depth profiles did not result in similar gradation in resistivity values. Carrier concentration at the near-surface region was found to be lower in all samples. It is shown that electrical properties of films forming ultra-shallow junctions can be studied in detail and correlated with process parameters using DHEM data obtained at sub-nm depth resolution.

Computational Modelling of Ceria-Based Solid Oxide Fuel Cell Electrolyte Materials

The significant advances in materials development for solid oxide fuel cells (SOFCs) and modelling techniques have challenged researchers to develop streamlined methodologies aiming at faster and more reliable fabrication of highly efficient cells. Instead of adopting a trial-and-error technique to discover the optimal composition of fuel cell component materials, alternative tools and techniques should be developed for predicting the likely best compositions in a cost-effective and timely manner. Many formulations have been studied for application as SOFC electrolytes over the years and doubly-doped ceria materials have been of considerable recent interest.Here, a simulation methodology to calculate the lattice parameter and the oxygen ion migration energy of ceria-based electrolyte formulations was developed, and the results were analysed and benchmarked against corresponding results obtained experimentally in order to verify the efficacy of the simulation methodology. To this end, a supercell consisting of 2 x 2 x 2 ceria unit cells doubly-doped with samarium and gadolinium was modelled and simulated using molecular mechanics force field methods using the CP2K computational chemistry suite. Two compositional series were modelled with the general formula Ce32-(x+y)SmxGdyO64-(x+y)/2 where x and y are the number of samarium and gadolinium dopant atoms per supercell respectively. The first compositional series, Series One, had equal amounts of dopants but different total dopant concentration with values of x = y = 1, 2, 8, or 16. Series Two had a fixed total dopant concentration (x + y = 8) but different amounts of each dopant in each structure such that x had integer values from 0 to 8 (x = 0 ... 8) and y = 8 - x. Moreover, two doping strategies were simulated for each compositional series depending on how the dopant atoms and resulting vacancies were distributed throughout the crystal. The first doping strategy places one dopant atom in two or more of the eight minicubes comprising the 2 x 2 x 2 supercell for Series One structures and, for Series Two structures, one dopant atom is placed in each minicube starting with one gadolinium atom in each minicube and then progressively replacing one gadolinium atom with one samarium atom in each of the subsequent structures of this series until there is one samarium atom in each minicube. The second doping strategy places the dopants exclusively in the four minicubes that let the dopants be as far as possible from each other. In all cases, the oxygen vacancies are chosen to be the nearest neighbours, i.e., in the nearest lattice positions, to the dopant atoms.For each structure, the lattice parameter was calculated using a cell optimisation technique utilizing the conjugate gradient method where the soft particle mesh Ewald (SPME) sums method and the DIPole Polarizable Ion Model (DIPPIM) for the long- and short-range potentials respectively. To estimate the activation energies associated with hopping of an oxygen ion from a specific site to a neighbouring site in the structures studied, the following steps were applied: The positions of the dopant atoms in the pure crystal were assigned.The at a distance within 3.0 Å was generated.The climbing image nudged elastic band (CI-NEB) method was applied to determine all the activation barriers of the doped crystal.The average energy barrier was calculated. The results obtained for the lattice parameter were comparable to those observed experimentally and exhibited the same linear trends satisfying Vegard's law, albeit the trend for the fixed dopant concentration series had a slight negative slope. Simulation-derived values of the activation energy showed the same trends as those observed experimentally. The simulations also produced transition state curves showing the saddle point and energy minima for each structure; information that could not be obtained experimentally and that can shed some light on the mechanism of ionic conduction. It was found that the position of oxygen vacancies within the crystal structure play a key role in determining the activation energy of ion hopping, and that the different patterns of doping do not affect the overall trends in lattice parameter and activation energy. They do however produce different patterns in the energy profiles contributing to the energy barriers to ion hopping within the crystal structure.This work demonstrates that computational simulations of SOFC electrolyte materials provide results comparable with those obtained experimentally and could also provide insights into the mechanisms of ionic conduction. The incongruence of the computational and experimental results was attributed to the limitations of the molecular mechanics force field methodology utilised, with the expectation that an ab initio density functional theory (DFT) calculation would yield closer conformance. Figure 1

Investigation of Low Temperature SiP Epitaxy on 300 mm Si Substrate

To continue the scaling down of transistors several challenges needs to be addressed, including the contact resistivity at the interface of the contact metal and the source/drain (SD) region, and the source/drain bulk resistivity. Phosphorous doped Si (Si:P) is commonly used for nFET as an alternative for (Si:C):P. In this work we report on low temperature (LT) epitaxy (below 500℃) of Si:P layers with extremely low as-grown layer resistivity. Optimal growth conditions at 400℃ led to a layer resistivity of 0.2 mOhm.cm and an active dopant concentration of ~1.15×1021 cm-3. Finally, the as-grown layer properties are compared to existing literature data.SiP layers were grown in a 300 mm ASM Intrepid® ES reduced pressure chemical vapor deposition reactor on Si(001) substrates. The Intrepid® system is equipped with a Previum® module for pre-epi cleaning at low temperatures. First, a set of Si:P layers was epitaxially grown at 470°C, with various Si precursor flows. The pressure, carrier flow and PH3 flow were kept constant. Figure 1 shows the omega-2theta scans around the symmetric (004) reflections of the Si:P/Si stacks. As expected, reducing the Si precursor flow leads to a higher substitutional P incorporation in the layer. The Si:P layer is pseudomorphic with good crystalline quality, as indicated by the thickness fringes. Figure 2 summarizes the HR-XRD angular spacing versus the SIMS content from different literature papers [1-4]. We note that for a low P doping level (<4%) the data are almost overlapping, while the spread increases for a high P content. This could be attributed different SIMS calibration for high P content. Using these calibration curves we note that the P content in the layers shown in Fig. 1 varies between ~1% to around 8.5% ± 0.5%. Figure 3 represents the resistivity versus the P content at three different temperatures (470°C, 440°C and 400°C). Definitely, the growth conditions were tuned to enable a growth with an acceptable growth rate at 400°C. The results indicate that reducing the growth temperature leads to a reduction in the layer resistivity, for the same P content. This highlights that the well-known P deactivation mechanism is correlated to the growth temperature. Figure 4 reveals AFM scans for the Si:P layers with different P content showing that smooth surface morphology can be obtained. Figure 5 compares the (micro-) Hall mobility versus the active concentration for the Si:P process developed in this frame work at LT to data obtained for DCS based process at high temperature (HT) (at 670°C). The DCS based layers were subjected to different thermal anneals [1]. The LT Si:P process follows the same trend, i.e. the Hall mobility decreases when the active dopant concentration increases. The reduction could be attributed to an increase in ionized impurity scattering. This explains the resistivity value around 0.2 mOhm.cm regardless of the P content at 400°C. Figure 6 shows the active dopant concentration as a function of the total dopant concentration. The LT process significantly outperforms the HT process in terms of ratio of active dopant over total dopant concentration. Furthermore, it shows an improvement in active dopant concentration even when compared to the layers exposed to ms laser anneal. Figure 7 summarizes the resistivity value for a Si:P layer obtained at high temperature and subsequently subjected to different anneals and the Si:P layer grown at 400°C, with a P content around 3%. Figure 8 shows the smooth surface morphology of the LT SiP layer with a resistivity of 0.2 mOhm.cm. Finally, Table 1 summarizes the key parameters for the Si:P layers found in literature and compare it to this work.To conclude, LT Si:P epitaxy paves the road towards new applications and is required for novel device architecture. However, a selective process is at utmost importance for integration schemes, which is currently under investigation.[1] E. Rosseel et al, ECS Trans. 75(8) (2016) 347.[2] K.D Weeks et al, Thin Solid Films 520 (2012) 3158.[3] M. Lee et al, ACS Appl. Electron. Mater. 1(3) (2019) 288.[4] X. Li et al, ECS Trans. 64(6) (2014) 959.[5] J.M. Hartmann et al, Semicond. Sci. Technol. 32 (2017) 104003.[6] C.N. Ni et al, VLSI Technology, Kyoto, (2015) T118-T119.[7] M. Bauer el al, ECS Trans. 33(6) (2010) 629.[8] Z. Ye et al, ECS Trans. 50(9) (2012) 1007. Figure 1

Sub-nm Near-Surface Activation Profiling for Highly Doped Si and Ge Using Differential Hall Effect Metrology (DHEM)

Contact resistivity at the metal-semiconductor interface is a strong function of the carrier concentration in the near surface. For next generation 2D/3D MOSFET transistors, industry has been investigating very highly doped materials (>1x1021cm-3) approaching alloy concentrations in Silicon and various Silicon-Germanium materials. All the while, contact development in Ge NMOS has favored co-doping approaches. Regardless of the material system, it is very important to accurately measure near-surface dopant activation. In this paper, we use Differential Hall Effect Metrology (DHEM) implemented on the ALPro™ tool to characterize the activation profiles in both materials. The method employs electrochemical means of reducing the electrically active thickness of a semiconductor film and determining the sheet resistance and mobility of the thinned down layer using Van der Pauw/Hall effect type measurements. By repeating this process and using differential equations, depth profiles of mobility and carrier concentration are obtained [1] at sub-nm resolutions.It is difficult to achieve high n-type dopant activation in Ge. Point defects in Ge are known to behave as acceptor sites and reduce the overall free electron density, particularly near the surface. Minimization of these defects is essential to achieving high performance Ge NMOS. Accurate analysis of dopant activation within 5 nm of the surface is critical to understanding effectiveness of various process steps on improving contact resistance. We have previously published a study on the effect of Ge co-doping with Sb and P on near-surface dopant activation. In Ge with only P-doping it was found that only ~5x1018cm-3 or 5% of the total surface dopants (~1x1020cm-3) were electrically active. With Sb as a co-dopant, the active electron concentration increased to ~2x1019cm-3 with a similar P chemical concentration. Prior work has shown similar increases in activation with co-doping but did not capture the significant decrease in electron concentration by 4x within 10 nm of the surface. Here, we further study the effects of the implantation and annealing processes on near-surface dopant activation in n-type Ge. Samples were prepared by growing 3 µm thick epi-Ge layers on Si substrates. Sample 1 was capped with 20 nm SiO2 and Sample 2 was capped with 20 nm Al2O3 to minimize implant damage and control implant depth. Both samples were implanted first with Sb (dose: 6x1014cm-2, energy: 65 keV) followed by P (dose: 6x1014cm-2, energy: 90 keV). Post-implant anneal was done at 500°C for 10 s in N2 to activate the dopants. ALProTM and SIMS measurements were done to characterize active dopant concentration and total chemical dopant concentration, respectively. The Al2O3 capped sample (Sample 2) showed significantly lower carrier concentration than the SiO2 capped sample (Sample 1) throughout the depth of the profile (Figure 1). Carrier concentration at the surface was lower by nearly an order of magnitude, decreasing from ~2x1019cm-3 to ~2x1018cm-3. Additionally, the peak concentration decreased from 1x1020cm-3 to 5x1019cm-3 at a depth of ~30 nm in both cases. The variations in the carrier profile for the Al2O3 capped sample also indicate greater process variation. SIMS profiles show that there is significant diffusion of Al into the Ge matrix after implant and anneal. Al chemical concentration is 1x1020cm-3 at the Ge surface and steadily decays up to a depth of more than 300 nm. This indicates that there is significant damage caused during the implantation process and the activation anneal is not enough to remove these defects. Al atoms then diffuse from the Al2O3 capping layer into the highly defective Ge substrate. Al is a p-type dopant in Ge and counter-dopes the Sb and P implants to reduce overall electron concentration in Sample 2. It is important to note the high resolution DHEM technique was able to resolve this important phenomenon right near the surface of the material.Additionally, we present early data on very highly phosphorus-doped (~1x1021cm-3) epitaxial silicon thin films grown on Si substrates. In very highly doped Silicon materials, the effect of near-alloy levels of dopant in the matrix on dopant deactivation is not well-known. Samples were obtained with different levels of dopant concentration above ~1x1021cm-3and different anneal temperatures. It was found that near-surface activation in these un-capped samples led to a lowering of activation in the <20nm region from the surface. Extensive post-DHEM metrology (using TEM and SIMS analysis) will also be presented demonstrating control and precision of DHEM data.

(Invited) Differential Hall Effect Metrology (DHEM) Sub-Nm Profiling and Its Application to Dopant Activation in n-Type Ge

With silicon CMOS technology approaching its limits due to continued device scaling, many potential new channel materials have been proposed. Germanium shows considerable promise due to having higher electron and hole mobilities compared to silicon. In addition, the electron and hole mobilities are similar, making it a viable option for CMOS technology. High performance Ge PMOS has been demonstrated, but NMOS has lagged behind due to the difficulty in making low resistance contacts to n-type Ge. Contact resistance is a strong function of the carrier concentration at the semiconductor surface, and it is extremely difficult to achieve high electron activation in Ge. Point defects in Ge are known to behave as acceptor sites and reduce the overall electron activation, particularly near the surface. Minimization of these defects is essential to achieving high performance Ge NMOS. Techniques such as fluorine passivation and co-doping with Sb and P have been shown to reduce the defect density and improve dopant activation, but the impact of these methods in the near-surface regime has largely been uncharacterized. Accurate analysis of dopant activation within 5 nm of the surface is critical to understanding the effectiveness of such processes on improving contact resistance.Differential Hall Effect Metrology (DHEM) is a powerful method that provides dopant activation depth profiles at sub-nm resolution. The method employs electrochemical means of reducing the electrically active thickness of a semiconductor film and determining the sheet resistance and mobility of the thinned down layer using Van der Pauw/Hall effect type measurements. Repeating this process and using differential equations, depth profiles of mobility and carrier concentration are obtained [1].We have previously used the ALProTM tool employing the DHEM technique to study the effect of Ge co-doping with Sb and P on near-surface dopant activation [2]. In Ge with only P-doping, the active dopant concentration was ~5x1018cm-3 at the surface despite the P chemical concentration being ~1x1020cm-3, indicating that only 5% of the implanted dopants were electrically active. With co-doping of Sb and P, the active electron concentration increased to ~2x1019cm-3 with a similar P chemical concentration. Prior work has shown similar increases in activation with co-doping but did not capture the significant decrease in electron concentration by 4x within 10 nm of the surface. Here, we further study the effects of the implantation and annealing process on near-surface dopant activation in n-type Ge. Samples were prepared by growing 3 µm thick epi-Ge layers on Si substrates. Sample 1 was capped with 20 nm SiO2 and Sample 2 was capped with 20 nm Al2O3 to minimize implant damage and control implant depth. Both samples were implanted first with Sb (dose: 6x1014cm-2, energy: 65 keV) followed by P (dose: 6x1014cm-2, energy: 90 keV). Post-implant anneal was done at 500°C for 10 s in N2 to activate the dopants. ALProTM and SIMS measurements were done to characterize active dopant concentration and total chemical dopant concentration, respectively.The Al2O3 capped sample (Sample 2) showed significantly lower dopant activation than the SiO2 capped sample (Sample 1) throughout the depth of the profile (Figure 1). Active electron concentration at the surface was lower by nearly an order of magnitude, decreasing from ~2x1019cm-3 to ~2x1018cm-3. Additionally, the peak concentration decreased from 1x1020cm-3 to 5x1019cm-3 at a depth of ~30 nm in both cases. The variations in the carrier profile for the Al2O3 capped sample also indicate greater process variation. SIMS profiles show that there is significant diffusion of Al into the Ge matrix after implant and anneal. Al chemical concentration is 1x1020cm-3 at the Ge surface and steadily decays up to a depth of more than 300 nm. This indicates that there is significant damage caused during the implantation process and the activation anneal is not enough to remove these defects. Al atoms then diffuse from the Al2O3 capping layer into the highly defective Ge substrate. Al is a p-type dopant in Ge and effectively counter dopes the Sb and P implants to reduce overall electron concentration in the Al2O3 capped sample. It is important to note the high resolution DHEM technique was able to resolve this important phenomenon right near the surface of the material.

Oxygen ion conductivity in ceria-based electrolytes co-doped with samarium and gadolinium

In a systematic study, two compositional series of ceria-based oxides, both co-doped with Sm and Gd, were synthesised using a low temperature method and evaluated as oxygen ion-conducting electrolytes for Intermediate Temperature Solid Oxide Fuel Cells (IT-SOFCs). Series one, Ce1-2xSmxGdxO2-x, had equal concentrations of Sm and Gd but varying total dopant concentration. Series two, Ce0.825SmxGd0.175-xO1.9125, had a fixed total dopant concentration but the Sm:Gd concentration ratio was varied. The materials were characterised using scanning and transmission electron microscopy, inductively coupled plasma mass spectrometry and X-ray diffraction. Impedance spectra were recorded on dense pellets of these materials. From these, total, bulk and grain boundary conductivities and capacitances along with activation energies, pre-exponential constants and enthalpies of ion migration and defect association were obtained. These gave a detailed insight into the fundamental conduction processes in the materials. Ce0.825Sm0.0875Gd0.0875O1.9125 had the highest total ionic conductivity at temperatures of 550 °C and above and also demonstrated an enhanced conductivity with respect to its singly-doped parent compounds, Ce0.825Sm0.175O1.9125 and Ce0.825Gd0.175O1.9125, at 400 °C and above. This compares favourably with previously-reported values and has promising implications for the development of IT-SOFCs.

Study on the doping effect of spin coated Al and In doped and (Al/In) co-doped ZnO thin films for near-infrared plasmonic applications

Al and In doped and (Al/In) co-doped ZnO films were fabricated by spin coating for near infrared plasmonic applications. The total dopant concentration was fixed at 3 at.%, in which In and Al were varied sequentially. The as-coated films were annealed in air at 400 °C followed by second annealing at 300 °C in reduced atmosphere. Films selected with respect to carrier concentration (n) were further investigated for plasmonic applications. Films with a particular ratio of Al and In only exhibited n~1020/cm3, necessary for plasmonic behaviour in near infrared. Structural studies were carried out by X-Ray diffractometer and band gap variations by calculating Urbach energy. Plasma frequency calculated from reflectance–transmittance spectra was theoretically verified using Drude-Lorentz equation. Al and In doped films and Al:In = 3:7 film exhibited metallic behaviour near to the wavelength 2000 nm. The films possessed minimum loss compared to those prepared by vacuum techniques. The properties obtained confirmed suitability of the films for plasmonic applications in the near infrared.

Optimizing rhombohedral Bi2O3 conductivity for low temperature SOFC electrolytes

Rhombohedral Bi2O3 has not been extensively evaluated as an electrolyte material for solid oxide fuel cells (SOFCs) due to the greater potential of the more highly conductive cubic phase. Due to the higher structural stability of rhombohedral Bi2O3, improving the conductivity of the material was undertaken to improve the performance of the electrolyte. Since the rhombohedral phase is generally only assessed in Bi2O3 with high total dopant concentrations, the phase stability window was mapped out in the lower dopant regime. Reported herein is a phase diagram of the rhombohedral Bi2O3 at a total dopant concentration below 10%. Within the phase stability window, the rhombohedral phase was also optimized in terms of the dopant cation and the total cation substitution rate. It was found that increasing the size of the cation dopant and decreasing the total dopant concentration required to stabilize the rhombohedral phase, ultimately maximized the conductivity. Overall, the La and Y double-doped Bi2O3 exhibited a conductivity of 3 × 10−2 S/cm at 500 °C, which is over three times higher than the conductivity of the more traditional SOFC electrolyte GDC.

Investigation on structural, optical, thermal and gamma photon shielding properties of zinc and barium doped fluorotellurite glasses

Fluorotellurite glasses based on zinc and barium fluorides with total dopant concentration limited to 40 mol% in tellurite glass matrix were obtained by melt quenching technique. Explicit effects of both the zinc and barium fluorides on the tellurite glass network were observed from their response to various studies. The tendency of fluorine atoms to break the network was observed from the formation of more non-bridging sites instead of bridging sites from Raman measurements. Formation of these non-bridging fluorine sites instead of non-bridging oxygen led to an increase in the optical band gap of these samples from 3.36 eV to 3.52 eV. The reduction in the glass transition temperature from 316 °C to 299 °C observed with zinc fluoride doping is due to the breakage in the rigid network structure, while the increase from 316 °C to 329 °C with barium fluoride doping is correlated to the effect of size of barium atoms in the network. Radiation shielding properties calculated in terms of parameters like mean free path, mass attenuation coefficient etc., showed that the presence of more heavily doped elements like barium in the glass matrix led them to exhibit better shielding properties which can be utilized as replacement materials for concrete and lead based glasses as efficient shielding materials.

Impacts of doping concentration on the saturable characteristics of Tm-Ho codoped fiber saturable absorber

Impacts of Tm ion concentration and Ho ion concentration on the saturable behaviors of Tm-Ho codoped fiber saturable absorbers and the output characteristics of the passively Q-switched laser systems are investigated and analyzed both at the initial lasing state and the stable passive Q-switching state. Simulations show that, varying concentrations of Tm and Ho ions have different impacts on the temporal evolution processes but similar effects on the macroscopic characteristics of the laser system. The root for the impacts of dopant concentrations is the population of the 3H6 energy level and the cavity loss it induces. For Tm ions, the rise of the Tm concentration improves the population of the 3H6 energy level directly, while, for Ho ions, higher Ho concentration leads to larger recovery rate of the 3H6 energy level, thus increasing the population of the 3H6 energy level indirectly. As for limited total dopant concentration, the Tm:Ho concentration ratio can be optimized for different applications.

Quantification of Sheet Resistance in Boron‐Diffused Silicon Using Micro‐Photoluminescence Spectroscopy at Room Temperature

A micro‐photoluminescence‐based technique is presented, to quantify and map sheet resistances of boron‐diffused layers in silicon solar cell precursors with micron‐scale spatial resolution at room temperature. The technique utilizes bandgap narrowing effects in the heavily‐doped layers, yielding a broader photoluminescence spectrum at the long‐wavelength side compared to the spectrum emitted from lightly doped silicon. By choosing an appropriate spectral range as a metric to assess the doping density, the impacts of photon reabsorption on the analysis can be avoided; thus, an accurate characterization of the sheet resistance can be made. This metric is demonstrated to be better representative of the sheet resistance than the surface doping density or the total dopant concentration of the diffused layer. The technique is applied to quantify sheet resistances of 12‐μm‐wide diffused fingers in interdigitated back‐contact solar cell precursors and large diffused areas. The results are confirmed by both 4‐point probe and time‐of‐flight secondary‐ion mass spectrometry measurements. Finally, the practical limitations associated with extending the proposed technique into an imaging mode are presented and explained.

On Space Charge and Spatial Distribution of Defects in Yttria-Stabilized Zirconia

Space charge in 8YSZ is examined theoretically. The Poisson-Boltzmann equation is solved by free energy minimization. The Helmholtz free energy of a crystal containing various defects is minimized subject to two constraints: The total dopant concentration is fixed and the maximum amount of surface charge is limited by site density. Large dopant concentration leads to defect concentrations which deviate from Boltzmann distribution. When free energy is minimized assuming all defects attain their equilibrium distributions, the corresponding electrochemical potentials are equilibrated through the crystal. These calculations correspond to long annealing times. Calculations are also conducted for a case in which only oxygen vacancies are allowed to equilibrate at lower temperature after a high temperature anneal. At lower temperatures, cations are frozen corresponding to high temperature anneal. In this case, only the electrochemical potential of oxygen vacancies equilibrates. In all cases Y segregates to grain boundaries/free surfaces lowering the corresponding oxygen vacancy concentration, consistent with reported effect of space charge on grain boundary conductivity. The results show a significant concentration of oxygen vacancies form defect complexes with Y dopant. Thus, free oxygen vacancy concentration is considerably lower than the usually assumed electroneutrality condition based on total dopant concentration.

Characterization of electrical properties in axial Si-Ge nanowire heterojunctions using off-axis electron holography and atom-probe tomography

Nanowires (NWs) consisting of P-doped Si/B-doped Ge axial heterojunctions were grown via vapor-liquid-solid synthesis using a combination of Au and AuGa catalyst particles. Off-axis electron holography (EH) was used to measure the electrostatic potential profile across the junction resulting from electrically active dopants, and atom-probe tomography (APT) was used to map total dopant concentration profiles. A comparison of the electrostatic potential profile measured from EH with simulations that were based on the APT results indicates that Ga atoms unintentionally introduced during AuGa catalyst growth were mostly electronically inactive. This finding was also corroborated by in situ electron-holography biasing experiments. Electronic band structure simulations guided by the experimental results helped to provide a much better explanation of the NW electrical behavior. Overall, this work demonstrates that the combination of EH, APT, in situ biasing, and simulations allows a more complete understanding of NW electrical properties to be developed.

Dysprosium and Gadolinium Double Doped Bismuth Oxide Electrolytes for Low Temperature Solid Oxide Fuel Cells

Herein, we developed a novel double dopant bismuth oxide electrolyte system with dysprosium (Dy) and gadolinium (Gd). The effect of the co-dopants on phase stability and electrical properties was investigated. Phase transformation from cubic to rhombohedral was observed as Gd dopant concentration increased and consequently resulted in conductivity degradation. The stabilization of high temperature cubic phase was achieved with a total dopant concentration as low as ∼12 mol% with 8 mol% Dy and 4 mol% Gd double dopant composition (8D4GSB) and this composition showed one of the highest total conductivity reported at this low temperature regime. In addition, the long-term stability of DGSB electrolytes was investigated.

Deposition of borophosphosilicate glass films using the TEOS–dimethylphosphite–trimethylborate system

Modernization of horizontal low pressure deposition system has been performed. The liquid source delivery system using the bubblers has been developed. The PSG and BPSG film deposition processes and film properties using TEOS-Dimethylphosphite-TEB system have been studied. It is shown that the use of dimethylphosphite allows varying the phosphorus concentration in the wide range. It is found that the optimal range of the total boron and phosphorus concentration ensuring the acceptable topology planarity and resistance to defect formation during storage is 8.7±0.3 wt% when the phosphorus concentration is 3.0—3.8 wt%. It is found that at use of the TEOS-DMP-TEB system the depletion of the phosphorus concentration along reaction zone does not occur, and the total dopant concentration is practically constant. At the same time the deposition rate of BPSG films is 9.0—10.0 nm/min and the good film thickness uniformity are ensured. The as-deposited films have “mirror-like surface” that is proof of minimal surface roughness. The BPSG films with optimal composition are characterized by the reduced reaction capability against atmospheric moisture.

Terbium and Tungsten Co-doped Bismuth Oxide Electrolytes for Low Temperature Solid Oxide Fuel Cells

We developed a novel double dopant bismuth oxide system with Tb and W. When Tb was doped as a single dopant, a Tb dopant concentration more than 20 mol% was required to stabilize bismuth oxides with a high conductivity cubic structure. High temperature XRD analysis of 25 mol% Tb-doped bismuth oxide (25TSB) confirmed that the cubic structure of 25TSB was retained from room temperature to 700oC with increase in the lattice parameter. On the other hand, we achieved the stabilization of high temperature cubic phase with a total dopant concentration as low as ~12 mol% with 8 mol% Tb and 4 mol% W double dopants (8T4WSB). Moreover, the measured ionic conductivity of 10T5WSB was much higher than 25TSB, thus demonstrating the feasibility of the double dopant strategy to develop stabilized bismuth oxide systems with higher oxygen ion conductivity for the application of SOFC electrolytes at reduced temperature. In addition, we investigated the long-term stability of TSB and TWSB electrolytes.