Doped Thin Films Research Articles

Elevated Curie temperature and half-metallicity in the ferromagnetic semiconductorLaxEu1−xO

Here we study the effect of La doping in EuO thin films using SQUID magnetometry, muon spin rotation ($\mu$SR), polarized neutron reflectivity (PNR), and density functional theory (DFT). The $\mu$SR data shows that the La$_{0.15}$Eu$_{0.85}$O is homogeneously magnetically ordered up to its elevated $T_{\rm C}$. It is concluded that bound magnetic polaron behavior does not explain the increase in $T_{\rm C}$ and an RKKY-like interaction is consistent with the $\mu$SR data. The estimation of the magnetic moment by DFT simulations concurs with the results obtained by PNR, showing a reduction of the magnetic moment per La$_{x}$Eu$_{1-x}$O for increasing lanthanum doping. This reduction of the magnetic moment is explained by the reduction of the number of Eu-4$f$ electrons present in all the magnetic interactions in EuO films. Finally, we show that an upwards shift of the Fermi energy with La or Gd doping gives rise to half-metallicity for doping levels as high as 3.2 %.

Ion Transport and Surface Kinetics in Oxygen Ion Conducting Nanocrystalline Oxide Thin Films

Nanocrystalline oxide thin films, e.g., doped oxides of zirconia and ceria films, exhibit extraordinary electrical and electrochemical properties that are of great interest for applications such as solid oxide fuel cells (SOFCs). In nanocrystalline doped oxide thin films fabricated by atomic layer deposition (ALD) or pulsed laser deposition (PLD), interfacial properties are often found to dominate the electrochemical properties. For example, grain boundary plays a crucial role in ionic transport and surface exchange reactions. Ionic transport across grain-boundaries(GBs) is known to be hindered due to an electrical potential barrier built up by defect segregation, while exchange reaction is enhanced near the GBs due to the higher concentration of oxide ion vacancies. Such exotic electrochemical properties of oxide thin films are closely related to the distribution of defects near GBs. Along these lines, we investigated the defect structure near GBs in doped oxide thin films as well as the surface exchange and the ionic transport inside the films associated with grain size and the grain boundary density. We report on the atomic scale study of the defect structure near a single GB of yttria-stabilized zirconia (YSZ) [1]. We show significant oxygen deficiency due to segregation of oxide-ion vacancies near the grain-boundary core with half-width of 0.6 nm, which corroborates well with electron energy loss spectroscopy (EELS) measurements. Additionally, we recently conducted near-atomic-scale EELS study on gadolinia-doped ceria (GDC) thin films [2]: Homogeneous distribution of dopants was observed in as-deposited films, while strong segregation of dopant was shown in the annealed (1100°C) film a width of 1.2 nm to 1.5 nm and an enhancement factor of 1.9. It has been reported that grain boundaries at the electrode(cathode)/electrolyte interface are of great importance in enhancing the oxygen reduction kinetics in SOFCs. Oxide-ion incorporation is greatly facilitated by these surface grain boundaries [3]. Thin-film electrolytes with high surface grain-boundary density consistently show higher exchange current density, and therefore, smaller activation loss. These experimental observations are supported by secondary ion mass spectroscopy (SIMS) and electrochemical impedance spectroscopy (EIS) studies on doped zirconia and doped ceria thin films [3,4]. Polycrystalline YSZ and YDC were annealed in an 18O2 environment so that the exchange reaction between 16O2 inside YSZ and YDC and 18O in the environment can take place. Surface mapping using NanoSIMS clearly indicates the preferential enrichment of 18O along grain boundaries. The activation resistance at the cathode interface is significantly reduced by an electrolyte surface featuring a high density of grain boundaries that facilitates high surface exchange rates. In parallel, surfaces modified by compositionally graded cation doping layers of ~1 nm facilitate enhanced oxide-ion vacancy concentration and improve the power density of the thin-film SOFC by lowering the activation loss [5]. It is known that grain boundaries consisting of grain boundary core and the adjacent space charge layer pose a blocking effect on cross-boundary transport of ions. We show that thermal annealing, which in turn facilitates defect segregation and redistribution, plays an important role on the ionic conductivity of oxide thin films [2,6]. Nanocrystalline thin films of YDC and GDC were fabricated and then post-annealed for grain growth. Interestingly, as the grain size increases, ionic transport is indeed hindered, showing higher activation energy and lower ionic conductivity than the as-deposited films, which have the smallest grains. We speculate that the uniform distribution of dopants may attenuate the space charge layer barrier height and maintain high ionic conductivity, while the annealed films have fully developed space charge layer, which impedes ionic transport. In summary, the grain size and the grain boundary effects have a huge impact on surface exchange and ionic transport. We show that fundamental understanding and surface engineering of grain structures in oxide thin films may have significant implications in improving the electrochemical performance of thin film SOFCs, and therefore, widening their applications. References J. An, J. S. Park, A. L. Koh, H. B. Lee, H. J. Jung, J. Schoonman, R. Sinclair, T. M. Gür, F. B. Prinz, Sci. Rep., 3, 2680 (2013)J. Bae et al., in-preparation J. H. Shim, J. S. Park, T. P. Holme, K. Crabb, W. Lee, Y. B. Kim, X. Tian, T. M. Gür, F. B. Prinz, Acta Mater., 60, 1-7 (2012)Y. B. Kim, J. S. Park, T. M. Gür, F. B. Prinz, J. Power Sources, 196, 10550-10555 (2011)C. C. Chao, Y. B. Kim, F. B. Prinz, Nano Lett., 9, 3626-3628 (2009)J. An, J. Bae, S. Hong, B. Koo, Y. B. Kim, T. M. Gür, F. B. Prinz, Scr. Mater., 104, 45-48 (2015)

Soft magnetic properties and high frequency characteristics of FeM (M = B, Hf, Zr) and pure Fe films fabricated by oblique deposition

To improve the high-frequency properties of Fe-based thin films, doped and pure Fe thin films were obliquely deposited on Si (100) substrate by RF-magnetron sputtering. Vibrating sample magnetometer (VSM) measurements show obvious in-plane uniaxial magnetic anisotropy in both doped and pure Fe thin films, and enhancement effects of doping on Soft magnetic properties were also observed. Microwave permeability measurements indicate that the resonance frequency and permeability of pure Fe films are much larger than those of doped Fe thin films. Damping factors of Fe thin films deposited at different oblique angles were further investigated.

A review on the understanding and fabrication advancement of MgB2thin and thick films by HPCVD

MgB2 thin films with superior superconducting properties are very promising for superconducting magnets, electronic devices and coated conductor electric power applications. A clear understanding of flux pinning mechanism in MgB2 films could be a big aid in improving the performance of MgB2 by the enhancement of Jc. The fabrication advancement and the understanding of flux pinning mechanism of MgB2 thin and thick films fabricated by using hybrid physical-chemical vapor deposition (HPCVD) are reviewed. The distinct kind of MgB2 films, such as single-crystal like MgB2 thin films, MgB2 epitaxial columnar thick films, and a-axis-oriented MgB2 films are included for flux pinning mechanism investigation. Various attempts made by researchers to improve further the flux pinning property and Jc performance by means of doping in MgB2 thin films by using HPCVD are also summarized.

Effects of Ho, Mn co-doping on ferroelectric fatigue of BiFeO3 thin films

Ho, Mn, and Ho, Mn co-doped BiFeO3 thin films were deposited on Pt (100)/Ti/SiO2/Si substrates using solution gelation. The ferroelectric fatigue properties were clearly improved after doping with Ho and Mn. The doped BiFeO3 thin film displayed more strongly increased ferroelectric, leakage current, and fatigue properties compared with the pure BiFeO3 thin film. Thus, the improved performance of the BiFeO3 thin films depends on the structural transformation and reduced oxygen vacancy concentration. The present work provides a feasible approach to enhance the ferroelectric and fatigue properties of BiFeO3-based thin films. Open image in new window

Chemical bath deposition of Fe-doped ZnS thin films: Investigations of their ferromagnetic and half-metallic properties

The nanocrystalline thin films of Fe-doped ZnS were deposited onto glass substrates by a chemical bath deposition (CBD) method. X-ray diffraction patterns reveal the formation of cubic zinc blende phase of ZnS. The incorporation of iron in ZnS lattice changed the lattice parameters of the ZnS structure. The purity of doped films was confirmed by the absence of secondary phases or iron clusters. SEM images show the growth of almost spherical clusters with slight change in size between un-doped and Fe-doped ZnS samples. The M–H curves indicate that doped ZnS thin films exhibit room temperature ferromagnetism. The magnetization properties as a function of applied field and doping concentration were studied in detail. The density functional calculations were performed using Full Potential Linearized Augmented Plane Wave (FP-LAPW) approach as is employed in Elk-code. The strong p–d hybridization between sulfur and iron resulted in ferromagnetic stable state and half-metallicity. The half-metallic iron doped ZnS thin films have potential applications in future spintronic devices.

Modeling Water Reduction on Doped Ceria Thin Films

In this work, the non-linear electrochemical impedance (NLEIS) response of doped ceria thin films under reducing conditions was modeled using a stirred tank model. Two different scenarios were considered: a uniform film scenario and a surface-enriched scenario where the electron concentration in the surface was increased. Modeling results show that NLEIS is not only capable to distinguish between the different rate-determining cases, but also is sensitive to changes in the thermodynamics of the surface.

Effect of Nd doping on the resistive switching properties of BiFeO3 thin films

Bismuth ferrite (BiFeO3) and Nd-doped BiFeO3 thin films have been grown on Pt/Ti/SiO2/Si substrates with pulsed laser deposition using the same growth conditions. Different resistive switching behaviours have been observed on the pure and doped BiFeO3 thin films. The leakage current is deceased after Nd doping, which benefits the fabrication of high-density resistive switching memory with low power consumption. The conduction mechanisms are discussed to understand the effect of Nd doping on the resistive switching properties of BiFeO3 thin films.

Enhancement of rotatable anisotropy in ferrite doped FeNi thin film with oblique sputtering

Rotatable anisotropy of stripe domain (SD) was investigated in a ferrite doped FeNi thin film with different oblique angles. Rotation of SD under an in-plane magnetic field was observed by magnetic force microscopy, suggesting the existence of rotatable anisotropy. A rotatable anisotropy field Hrot was derived from the fitting curves of the in-plane resonance field versus the angle between the orientation of easy axis and applied field. As the oblique angle increases, an increase of Hrot from 305 Oe to 468 Oe was observed and the perpendicular anisotropy increased as well, indicating a correlation between rotatable anisotropy and perpendicular anisotropy.

Facile synthesis of ZnO nanostructures by spray pyrolysis technique and its application as highly selective H2S sensor

ZnO nanorods were facilely grown on glass substrates using spray pyrolysis technique and they were successfully used to detect hydrogen sulfide at room temperature. Desired molar concentration of zinc chloride precursor was optimized to produce high quality ZnO nanorods. The plausible growth mechanism of ZnO nanorods was schematically depicted and analyzed. Further, the influence of transition metal doping (Co, Ni and Cu) on the structural, morphological, optical and hydrogen sulfide sensing properties was investigated. Sensing response of the undoped and doped thin films were also investigated in the presence of eight different vapors. The sensing response indicated the highly selective nature of ZnO nanorods towards hydrogen sulfide. Among the doped elements, Ni-doped ZnO film showed an excellent sensing response of 321 towards 100ppm of hydrogen sulfide with the response and recovery times of 12 and 90s, respectively.

Hydrogenated indium oxide window layers for high-efficiency Cu(In,Ga)Se2 solar cells

High mobility hydrogenated indium oxide is investigated as a transparent contact for thin film Cu(In,Ga)Se2 (CIGS) solar cells. Hydrogen doping of In2O3 thin films is achieved by injection of H2O water vapor or H2 gas during the sputter process. As-deposited amorphous In2O3:H films exhibit a high electron mobility of ∼50 cm2/Vs at room temperature. A bulk hydrogen concentration of ∼4 at. % was measured for both optimized H2O and H2-processed films, although the H2O-derived film exhibits a doping gradient as detected by elastic recoil detection analysis. Amorphous IOH films are implemented as front contacts in CIGS based solar cells, and their performance is compared with the reference ZnO:Al electrodes. The most significant feature of IOH containing devices is an enhanced open circuit voltage (VOC) of ∼20 mV regardless of the doping approach, whereas the short circuit current and fill factor remain the same for the H2O case or slightly decrease for H2. The overall power conversion efficiency is improved from 15.7% to 16.2% by substituting ZnO:Al with IOH (H2O) as front contacts. Finally, stability tests of non-encapsulated solar cells in dry air at 80 °C and constant illumination for 500 h demonstrate a higher stability for IOH-containing devices.

(Invited) High Performance Nano-Carbon/Silicon Solar Cells Via Strategic Doping Processes

Hybrid approaches that combine silicon (Si) and nano-carbon (nano-C) materials-based thin films (i.e., single walled carbon nanotube (SWNT) films or graphene) have recently led to novel and promising photovoltaic devices. The main advantage is that the intrinsically high photovoltaic efficiency of Si can be realized in a cost-effective manner owing to the low-temperature solution processibility inherent to the fabrication of nano-C/Si junctions. Indeed, efficiencies up to 15% have recently been reported, which surpasses the best efficiency of any polymer/Si hybrid solar cells developed with a similar motivation towards reduced fabrication cost. This superiority is mainly due to the excellent electrical properties of the nano-C materials in comparison to conventional conjugated polymers. In this talk, we demonstrate that easy to handle metallocenes and related complexes can be used to both p-type and n-type dope SWNT thin films, using a simple spin coating process [1]. We report n-SWNT/p-Si photovoltaic devices that are >450 times more efficient than the best solar cells of this type currently reported and show that the performance of both our n-SWNT/p-Si and p-SWNT/n-Si devices is related to the doping level of the SWNT. Furthermore, we establish that the electronic structure of the metallocene or related molecule can be correlated to the doping level of the SWNT, which may provide the foundation for controlled doping of SWNT thin films in the future [1]. In a separate approach we will show how HF treatment of SWNTs followed by current stimulation is a very effective method for oxygen removal with broad applications for carbon electronics [2]. Finally, we will describe how direct solution casting of silver nanowires (AgNWs) onto the SWNT/Si junctions leads to a significant improvement of photovoltaic properties owing to enhanced carrier transport in the bilayer AgNW/SWNT composites [3]. This improvement becomes particularly more pronounced with increasing junction area, which results in a significantly high efficiency of ~11 % in SWNT/Si solar cells with the largest junction area (49 mm2) developed so far [3]. [1] X. Li, L.M. Guard, J. Jiang, K. Sakimoto, J.-S. Huang, J. Wu, J. Li, L. Yu, R. Pokhrel, G.W. Brudvig, S. Ismail-Beigi, N. Hazari and A.D. Taylor (2014) Controlled doping of carbon nanotubes with metallocenes for application in hybrid carbon nanotube/Si solar cells, Nano Letters, 14 (6), 3388–3394. DOI: 10.1021/nl500894h [2] X. Li, J-S. Huang, S. Nejeti, L. McMillon, S. Huang, C.O. Osuji, N. Hazari, A.D. Taylor (2014), The Role of HF in Oxygen Removal from Carbon Nanotubes: Implications for High Performance Carbon Electronics, Nano Letters, 14 (11), 6179-6184. DOI: 10.1021/nl502401c [3] X. Li, Y. Jung, J-S. Huang, T. Goh , and A.D. Taylor (2014), Device Area Scale-Up and Improvement of SWNT/Si Solar Cells Using Silver Nanowires, Advanced Energy Materials, 4 (12), 1400186. DOI: 10.1002/aenm.201400186

Modification in structure of La and Nd co‐doped epitaxial BiFeO3 thin films probed by micro Raman spectroscopy

The influence of La and Nd co‐substitution on structure, electric and magnetic properties of epitaxial thin films of BiFeO3 (BFO) was examined. We demonstrate structural phase transition in co‐doped La and Nd BFO thin films using Raman spectroscopy. Based on group theoretical analysis of the number and symmetry of Raman lines, we provide strong experimental evidence that the structure has been changed from rhombohedral to monoclinic due to co‐doping in BFO. The change in structure was also reflected in morphology of these films. Room temperature magnetic hysteresis curves showed that doped films exhibit enhanced ferromagnetic properties with remnant magnetization of ~10 emu/cm3 and coercive field of 1.2 kOe. The enhanced magnetic properties highlight the potential applications of doped BLNFO thin film for smart devices. Copyright © 2015 John Wiley & Sons, Ltd.

Crossover between weak anti-localization and weak localization by Co doping and annealing in gapless PbPdO2 and spin gapless Co-doped PbPdO2

The magnetotransport properties of Pb(Pd,Co)O2 and PbPdO2 thin films were investigated. In magnetoconductance curves, we observed a crossover between weak anti-localization (WAL) and weak localization (WL) depending on the annealing and Co doping in PbPdO2 thin films. For the Pb(Pd,Co)O2 case showing WAL signals, the ex-situ annealing weakens the Pd-O hybridization by stabilizing Co3+ states and generating Pd1+ states, instead of Pd2+, so that the spin-orbit coupling (SOC) strength is significantly reduced. It causes the dominant magnetotransport mechanism change from WAL to WL. This annealing effect is compared with the PbPdO2 case, which possesses WL signals. The annealing process stabilizes the oxygen states and enhances the Pd-O hybridization, and consequently the SOC strength is enhanced. Our experimental results are well explained by the Hikami-Larkin-Nagaoka theory in terms of two important physical parameters; SOC strength-related α and inelastic scattering length lϕ.

Electrochemical growth and characterization of iron doped cadmium sulfide thin films

Cadmium Sulfide and Ferrous doped Cadmium Sulfide thin films have been prepared on different substrates using an electrodeposition technique. Linear sweep voltammetric analysis has been carried out to determine deposition potential of the prepared films. X-ray diffraction analysis showed that the prepared films possess polycrystalline nature with hexagonal structure. Surface morphology and film composition have been analyzed using Scanning electron microscopy and Energy dispersive analysis by X-rays. Optical absorption analysis showed that the prepared films are found to exhibit Band gap value in the range between 2.3, 2.8eV for Cadmium Sulfide and Ferrous doped Cadmium Sulfide.

Improved resistive switching properties by nitrogen doping in tungsten oxide thin films

Abstract In this study, nitrogen-doped WO x thin films are investigated for the improvement of resistive switching (RS) properties. Compared to WO x thin films, nitrogen-doped WO x thin films exhibit a higher on/off current ratio (a separation of ~ 2 orders of magnitude), better endurance (> 100 cycles), narrower current dispersion, and longer retention characteristics (> 10 4 s). Electrical measurements, X-ray diffraction, and X-ray photoelectron spectroscopy demonstrate that nitrogen in WO x :N thin films forms WN nanoclusters and W x (O, N) phases, which are beneficial to improve the RS properties in WO x thin films; WN nanoclusters can locally enhance the electric field to form stable conductive filament while W x (O, N) phases can suppress random migrations of oxygen ions (O 2 − ), leading to stable RS characteristics. Our findings suggest that nitrogen doping method can lead further optimization of the RS characteristics in WO x thin films.

Effect of InP Doping on the Phase Transition of Thin GeSbTe Films

We report the crystallization and phase-transition behavior of GeSbTe thin films doped with indium phosphorus (InP). Pure GeSbTe thin films and InP-doped GeSbTe thin films were prepared by use of an rf magnetron sputtering method. After thermal annealing, electrical and optical changes in the thin films were observed. Sheet resistance and reflectance measurements revealed that InP doping suppresses crystallization of GeSbTe. X-ray diffraction analysis confirmed that addition of In and P atoms inhibits the phase transition from face-centered cubic to hexagonal closed-packed. Nucleation of the doped GeSbTe thin films was delayed at an annealing temperature of 100°C; after thermal annealing, neither segregation nor formation of a secondary phase occurred. These results indicate that InP doping improves the amorphous stability of GeSbTe thin films. It is believed this enhanced amorphous stability is a result of the formation of multiple, strong crosslinks by the In and P atoms.

Mechanical stress field assisted charge de-trapping in carbon doped oxides

Leakage current and dielectric breakdown effects are conventionally studied under electrical fields alone, with little regard for mechanical stresses. In this letter, we demonstrate that mechanical stress can influence the reliability of dielectrics even at lower field strengths. We applied tensile stress (up to 8MPa) to a 33% porous, 504nm thick carbon doped oxide thin film and measured the leakage current at constant electrical fields (up to 2.5MV/cm). The observed increase in leakage current at relatively low electric fields suggests that mechanical stress assists in trap/defect mediated conduction by reducing the energy barrier potential to de-trap charges in the dielectric.

Effects of Y doping on multiferroic properties of sol–gel deposited BiFeO3 thin films

Polycrystalline BiFeO3 (BFO), Bi0.99Y0.01FeO3 (BYF1), Bi0.97Y0.03FeO3 (BYF3), Bi0.95Y0.05FeO3 (BYF5) and Bi0.90Y0.10FeO3 (BYF10) thin films have been fabricated on Pt(111)/Ti/SiO2/Si(100) substrates by sol–gel method. The effects of Y doping on the structural, optical, morphological, ferroelectric and magnetic properties of BFO thin film were analyzed by X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscope (SEM), ferroelectric and magnetic measurement system, respectively. XRD, Rietveld refinement and Raman analysis demonstrated that the phase of all films was a single perovskite-type rhombohedral structure. A structural transition from the main R3c to the single phase R-3 m was obtained with the BYF10 thin film. SEM showed that the Y doping was beneficial to the crystal growth of BFO films. The ferroelectric test indicated that the Y doping in BYF1 and BYF3 thin films was effective to reduce the leakage current density of the BFO films. The leakage current density of BYF1 thin film was reduced by two orders of magnitude compared to that of the BFO film. The BYF10 thin film had a superior remanent polarization compared to that of other BFO film due to the distorted deformation of FeO6 octahedra. The magnetic test indicated that a substantially enhanced magnetization was observed in the BYF3 thin film.

Effect of hydroxyl groups on hydrophilic and photocatalytic activities of rare earth doped titanium dioxide thin films

Rare earth (Y, La and Nd) doped TiO2 thin films were prepared on glass slides by sol-gel method. The photocatalytic decomposition of methylene blue in aqueous solution was used as a probe reaction to evaluate their photocatalytic activities. The effects of hydroxyl groups on hydrophilic and photocatalytic activities were investigated by means of techniques such as X-ray diffraction (XRD), atomic force microscopy (AFM), Fourier transform infrared (FTIR), optical contact angle, UV-Visible spectroscopy and VIS spectroscopy. The results showed that an appropriate doping of rare earth could cause the TiO2 lattice distortion, inhibited phase transition from anatase to rutile, accelerated surface hydroxylation and produced more hydroxyl groups, which resulted in a denser surface and smaller grains (40–60 nm), and a significant improvement in the hydrophilicity and photoreactivity of TiO2 thin films. The optimal content of rare earth was between 0.1 wt.% and 0.3 wt.%. Moreover, the modification mechanism of rare earth doping was also discussed.