EMSL: Tiffany C Kaspar's Publications

Droubay T, TC Kaspar, BP Kaspar, and SA Chambers. 2009. "Cation dopant distributions in nanostructures of transition-metal doped ZnO:Monte Carlo simulations." Physical Review. B, Condensed Matter and Materials Physics 79(7):Art. No. 075324. Abstract The path from trace doping to solid solution formation involves an intermediate regime in which the doping level is a few to several atomic percent. In this regime, dopant-dopant interactions, which are driven by the spatial arrangement of dopants, are critical factors in determining the resulting properties. Conventional wisdom counts on simple probabilistic methods for predicting dopant distributions. Here, we use Monte Carlo simulations to show that widely used, straightforward statistical models, such as that of Behringer1, are accurate only in the limit of infinitesimally small surface–to-volume ratio. For epitaxial films and nanoparticles, where much of the current interest resides, dopant distributions depend strongly on the surface-to-volume ratio. We present empirical expressions that accurately predict dopant bonding configurations as a function of film or particle size, shape and dopant concentration for doped ZnO, a material of particular interest in semiconductor spintronics.

Heald SM, TC Kaspar, T Droubay, V Shutthanandan, SA Chambers, A Mokhtari, AJ Behan, HJ Blythe, JR Neal, M Fox, and G Gehring. 2009. "X-ray absorption fine structure and magnetization characterization of the metallic Co component in Co-doped ZnO thin films ." Physical Review. B, Condensed Matter 79(7):Art. No. 075202. Abstract X-ray absorption fine structure (XAFS) measurements have been used to characterize a series of Co doped ZnO films grown on sapphire substrates by pulsed laser deposition. The emphasis is on characterization of the fate of the Co dopant: metallic particles or substitutional Co2+. It is shown that analysis of both the near edge and extended fine structure can provide a measurement of the fraction of metallic Co. Any quantitative understanding of magnetism in this system needs to take account of both types of Co. Results are reported for two types of films from two different groups that show distinctly different behavior. Films grown with high concentrations of Co show varying amounts of metallic Co that could be identified as hcp or fcc Co. Another set of films were annealed in Zn vapor to induce magnetism. These also showed significant metallic Co, but of a different type similar to the CoZn intermetallic. The bulk forms of both metals are magnetic and should contribute to the magnetism. Using bulk magnetic values, there are some discrepancies with room temperature magnetic measurements. The 2 magnetic properties of the small metal particles are likely changed by their surroundings and by superparamagnetism. Low temperature magnetic measurements for one of the samples confirmed this with an estimated blocking temperature of 50K.

Kaspar TC, T Droubay, SM Heald, MH Engelhard, P Nachimuthu, and SA Chambers. 2008. "Hidden Ferromagnetic Secondary Phases in Cobalt-doped ZnO Epitaxial Thin Films." Physical Review. B, Condensed Matter 77(20):201303. doi:10.1103/PhysRevB.77.201303 Abstract The quest to discover a dilute magnetic semiconductor which is ferromagnetic at room temperature has led to extensive research on doped semiconducting oxides. However, the wide range of reported properties has raised doubts regarding the presence of intrinsic ferromagnetism in these materials. Here we explore the origin of ferromagnetism in epitaxial Co:ZnO thin films, which are paramagnetic but become weakly ferromagnetic (~0.05 μB/Co) after annealing in Zn vapor to introduce interstitial Zn. Conventional bulk materials characterization techniques indicate no phase segregation or Co reduction has occurred. However, x-ray photoelectron spectroscopy sputter depth profiling clearly indicates the presence of Co(0) in the Zn-treated films; x-ray absorption spectroscopy is utilized to identify the secondary phase as ferromagnetic CoZn (1.5 μB/Co, TC ~ 400 – 450 K). This work demonstrates that the potential for ferromagnetic secondary phases in doped oxides must be thoroughly discounted, through painstaking materials characterization, before claims of intrinsic ferromagnetism can be made.

Kaspar TC, T Droubay, SM Heald, P Nachimuthu, CM Wang, V Shutthanandan, CA Johnson, DR Gamelin, and SA Chambers. 2008. "Lack of ferromagnetism in n-type cobalt-doped ZnO epitaxial thin films." New Journal of Physics 10:Art. No. 055010. doi:10.1088/1367-2630/10/5/055010 Abstract Epitaxial thin films of cobalt-doped ZnO (Co:ZnO) were deposited by pulsed laser deposition (PLD) on both c-plane and r-plane sapphire (Al2O3). The films exhibited high structural quality with narrow x-ray diffraction (XRD) rocking curve peak widths. X-ray absorption spectroscopy (XANES and EXAFS) confirmed well-ordered Co substitution for Zn in ZnO without the formation of secondary phases. A wide range of n-type conductivities (10-4 – 105 -cm) was achieved by controlling the deposition conditions, post-annealing in vacuum, and/or addition of Al during deposition. Despite the high structural quality of the Co:ZnO thin films, no significant room temperature ferromagnetism was observed under any processing or treatment conditions. The lack of ferromagnetism indicates that itinerant conduction band electrons alone are not sufficient to induce ferromagnetism in Co:ZnO, even when the carrier concentration is a significant fraction of the magnetic dopant concentration. The implications of this observation are discussed.

Li Y, TC Kaspar, T Droubay, AG Joly, P Nachimuthu, Z Zhu, V Shutthanandan, and SA Chambers. 2008. "A Study of H and D doped ZnO epitaxial films grown by pulsed laser deposition." Journal of Applied Physics 104(5):Article no. 053711. doi:10.1063/1.2975219 Abstract We examine the crystal structure, electrical and optical properties of ZnO epitaxial films grown by pulsed laser deposition in a H2 or D2 ambient. We compared with pure ZnO films grown in O2 and vacuum. N-type electrical conductivity is enhanced by two to three orders of magnitude as a result of growing in H2 or D2. Temperature dependent Hall effect measurements reveal small (a few meV) carrier activation energies, along with carrier concentrations of 2-7 x 1018 cm-3, and mobilities of 20-40 cm2/Vs in ZnO films doped with H or D in the 1018 cm-3 range. We have modeled the low-temperature electrical properties of H- and D-doped ZnO films using variable range hopping and surface layer conductivity models, but our data do not fit well with these models. Rather, it appears that growth in H2 or D2 promotes the formation of an exceedingly shallow or conduction-band degenerate donor state, possibly associated with H or D substitution at O sites in the lattice.

Li Y, TC Kaspar, T Droubay, Z Zhu, V Shutthanandan, P Nachimuthu, and SA Chambers. 2008. "Electronic properties of H and D doped ZnO epitaxial films." Applied Physics Letters 92(15):Art. No. 152105. Abstract ZnO epitaxial films grown by pulsed laser deposition in an ambient of H2 or D2 exhibit qualitatively different electronic properties compared to films grown in vacuum or O2, or bulk single crystals annealed in H2. These include temperature-independent resistivities of ~0.1 -cm, carrier (electron) concentrations in the 1018 cm-3 range, mobilities of 20-40 cm2/V-sec, and negligible (a few meV) activation energies for conduction. These transport properties are consistent with H (D) forming an ultra-shallow donor or conduction band states not achievable by post-growth annealing in H2.

Ney A, K Ollefs, S Ye, T Kammermeier, V Ney, TC Kaspar, SA Chambers, F Wilhelm, and A Rogalev. 2008. "Absence of Intrinsic Ferromagnetic Interactions of Isolated and Paired Co Dopant Atoms in Zn1−xCoxO with High Structural Perfection." Physical Review Letters 100(15):Art. No. 157201. doi:10.1103/PhysRevLett.100.157201 Abstract We report element specific structural and magnetic investigations on Zn1−xCoxO epitaxial films using synchrotron radiation. Co dopants exclusively occupy Zn sites as revealed by x-ray linear dichroism leading to a high degree of structural perfection. Comparative magnetic field dependent measurements by x-ray magnetic circular dichroism and conventional magnetometry consistently show purely paramagnetic behavior for isolated Co dopant atoms with a magnetic moment of 4.8μB. However, the total magnetization is reduced by ~30%, indicating that Co-O-Co pairs are antiferromagnetically coupled. We find no sign of intrinsic ferromagnetism in Co:ZnO films.

Kaspar TC, T Droubay, V Shutthanandan, SM Heald, CM Wang, DE McCready, S Thevuthasan, JD Bryan, DR Gamelin, AJ Kellock, MF Toney, X Hong, C Ahn, and SA Chambers. 2006. "Ferromagnetism and structure of epitaxial Cr-doped anatase TiO2 thin films." Physical Review. B, Condensed Matter and Materials Physics 73(15):155327 (12 p.). doi:10.1103/PhysRevB.73.155327 Abstract The materials and magnetic properties of Cr-doped anatase TiO2 thin films deposited on LaAlO3(001) and SrTiO3(001) substrates by oxygen-plasma-assisted molecular beam epitaxy have been studied in detail to elucidate the origin of ferromagnetic ordering. Cr substitution for Ti in the anatase lattice, with no evidence of Cr interstitials, segregation, or secondary phases, was independently confirmed by transmission electron microscopy (TEM) with energy dispersive x-ray (EDX) spectroscopy, extended x-ray absorption fine structure (EXAFS), and Rutherford backscattering spectrometry (RBS) in the channeling geometry. Epitaxial films deposited at ~0.1 Å/s were found to have a highly defected crystalline structure, as quantified by high resolution x-ray diffraction (XRD). These films were also ferromagnetic at room temperature with a moment of ~0.5 B/Cr, Curie temperatures in the range of 400 – 700°C, and exhibited shape and in-plane magnetocrystalline anisotropy. However, no free carrier spin polarization was observed by Hall effect measurements, raising questions about the mechanism of magnetism. Films deposited slowly (~0.015 Å/s) possessed a nearly perfect crystalline structure as characterized by XRD. Contrary to expectations, these films exhibited negligible ferromagnetism at all Cr concentrations. Annealing in vacuum to generate additional oxygen defects and free carrier electrons did not significantly increase the ferromagnetic ordering in either fast- or slow-grown films. These results contradict both oxygen-vacancy-derived free-carrier-mediated exchange and F-center-mediated bound magnetic polaron exchange mechanisms, and instead indicate the primary role of extended structural defects in mediating the ferromagnetic ordering in doped anatase films.

Kaspar TC, T Droubay, DE McCready, P Nachimuthu, SM Heald, CM Wang, AS Lea, V Shutthanandan, SA Chambers, and MF Toney. 2006. "Magnetic properties of epitaxial Co-doped anatase TiO2 thin films with excellent structural quality." Journal of Vacuum Science and Technology B--Microelectronics and Nanometer Structures 24(4):2012-2017. doi:10.1116/1.2216723 Abstract The heteroepitaxy of Co-doped anatase TiO2 on LaAlO3(001) has been refined with the goal of determining the relationship between structural quality and magnetic ordering. By significantly reducing the deposition rate and substrate temperature, well-ordered Co:TiO2 films with unprecedented crystalline quality were obtained by oxygen-plasma-assisted molecular beam epitaxy, as characterized by x-ray diffraction. These films exhibit uniform Co doping, with no evidence of Co segregation or secondary phases throughout the film depth or on the surface. Despite the improvement in crystalline quality and Co distribution, the films exhibit negligible ferromagnetism, with saturation moments of only ~0.1 B/Co. This loss of ferromagnetism is in stark contrast to faster-grown Co:TiO2 films, where a higher growth rate and substrate temperature typically result in lower crystalline quality, a highly non-uniform Co distribution, and average saturation moments of ~1.2 B/Co. The presence of ferromagnetism in faster-grown Co:TiO2 does not appear to arise from intrinsic point defects present in the bulk material, such as charge-compensating oxygen vacancies, but is instead attributed to the presence of extended structural defects.

Shutthanandan V, S Thevuthasan, T Droubay, TC Kaspar, A Punnoose, J Hays, and SA Chambers. 2006. "Quantification of Dopant Concentrations in Diluted Magnetic Semiconductors using Ion Beam Techniques." Nuclear Instruments and Methods in Physics Research. Section B, Beam Interactions with Materials and Atoms 249(1-2):402-405. doi:10.1016/j.nimb.2006.04.038 Abstract It has recently been demonstrated that magnetically doped TiO2 and SnO2 show ferromagnetism at room-temperature and Curie temperatures above room temperature. However, accurate knowledge of dopant concentrations is necessary to quantify magnetic moments in these materials. Rutherford Backscattering spectrometry (RBS) is one of the powerful techniques to quantify magnetic transition metal dopant concentrations in these materials. However, in some cases, the interference of RBS signals for different dopants and substrate elements in these materials makes analysis difficult. In this work, we demonstrate that particle induced x-ray emission (PIXE) can be successfully used to quantify the magnetic transition element dopants in several room temperature ferromagnetic materials synthesized using three different synthesis methods: oxygen plasma assisted molecular beam epitaxy, ion implantation and wet chemical methods.