Rueben J. Mendelsberg

Dynamically Modulating the Surface Plasmon Resonance of Doped Semiconductor Nanocrystals

Localized surface plasmon absorption features arise at high doping levels in semiconductor nanocrystals, appearing in the near-infrared range. Here we show that the surface plasmons of tin-doped indium oxide nanocrystal films can be dynamically and reversibly tuned by postsynthetic electrochemical modulation of the electron concentration. Without ion intercalation and the associated material degradation, we induce a > 1200 nm shift in the plasmon wavelength and a factor of nearly three change in the carrier density.

Extracting reliable electronic properties from transmission spectra of indium tin oxide thin films and nanocrystal films by careful application of the Drude theory

Analysis of the transmittance and reflectance of transparent conducting oxide thin films and nanocrystal films can be accurately modeled using the Drude free electron theory to extract electrical transport properties if enough care is taken. However, several fits starting from different initial guesses are needed before confidence in the extracted Drude parameters can be obtained. Film thickness, optical carrier concentration, and optical carrier mobility can be reliably derived when using either a fully empirical or semiempirical model for the ionized impurity scattering. The results are in good agreement with those based on more arduous spectroscopic ellipsometry measurements. Furthermore, fitting the reflectance along with the transmittance reduces the uncertainty, but does not significantly affect the values of the extracted parameters.

Plasma potential mapping of high power impulse magnetron sputtering discharges

Pulsed emissive probe techniques have been used to determine the plasma potential distribution of high power impulse magnetron sputtering (HiPIMS) discharges. An unbalanced magnetron with a niobium target in argon was investigated for a pulse length of 100 μs at a pulse repetition rate of 100 Hz, giving a peak current of 170 A. The probe data were recorded with a time resolution of 20 ns and a spatial resolution of 1 mm. It is shown that the local plasma potential varies greatly in space and time. The lowest potential was found over the target’s racetrack, gradually reaching anode potential (ground) several centimeters away from the target. The magnetic presheath exhibits a funnel-shaped plasma potential resulting in an electric field which accelerates ions toward the racetrack. In certain regions and times, the potential exhibits weak local maxima which allow for ion acceleration to the substrate. Knowledge of the local E and static B fields lets us derive the electrons’ E×B drift velocity, which is abou...

Achieving high mobility ZnO:Al at very high growth rates by dc filtered cathodic arc deposition

Achieving a high growth rate is paramount for making large-area transparent conducting oxide coatings at a low cost. Unfortunately, the quality of thin films grown by most techniques degrades as the growth rate increases. Filtered dc cathodic arc is a lesser known technique which produces a stream of highly ionized plasma, in stark contrast to the neutral atoms produced by standard sputter sources. Ions bring a large amount of potential energy to the growing surface which is in the form of heat, not momentum. By minimizing the distance from cathode to substrate, the high ion flux gives a very high effective growth temperature near the film surface without causing damage from bombardment. The high surface temperature is a direct consequence of the high growth rate and allows for high-quality crystal growth. Using this technique, 500–1300 nm thick and highly transparent ZnO : Al films were grown on glass at rates exceeding 250 nm min −1 while maintaining resistivity below 5 × 10 −4 � cm with electron mobility as high as 60 cm 2 V −1 s −1 . (Some figures in this article are in colour only in the electronic version)

Determining the nonparabolicity factor of the CdO conduction band using indium doping and the Drude theory

Due to their high intrinsic electron mobility, CdO-based materials are gaining interest as transparent conductive oxides. By creating model dielectric functions based on the Drude theory, accurate fits to the measured transmittance and reflectance of CdO and CdO : In thin films were achieved without using a frequency dependent Drude damping parameter. Difference in the model between undoped and In-doped CdO showed that the Burstein–Moss shift is not the only mechanism which improves the transparency in In-doped samples. Comparing the Drude analysis with Hall measurements revealed a nonlinear relationship between the free-electron effective mass and the carrier concentration, an effect which is caused by the nonparabolicity of the CdO conduction band. Analysis of 50 CdO : In thin films grown by pulsed filtered cathodic arc showed the nonparabolicity factor was C = (0.5 ± 0.2) eV−1 and the band-edge effective mass was (0.16 ± 0.05)me. Knowledge of the effective mass allows for optical measurements of carrier mobility, which was less than or equal to the measured Hall mobility in these films due to the large electron mean free path compared with the grain size.

Dopant-induced band filling and bandgap renormalization in CdO:In films

Published in J. Phys D: Appl. Phys. 46 (2013) 195102. http://dx.doi.org/10.1088/0022-3727/46/19/195102 Dopant-induced band filling and bandgap renormalization in CdO:In films Yuankun Zhu 1 , Rueben J. Mendelsberg 2,3 , Jiaqi Zhu 1 , Jiecai Han 1 and Andre Anders 2 Center for Composite Materials and Structures, Harbin Institute of Technology, Harbin 150080, China Plasma Applications Group, Lawrence Berkeley National Laboratory, Berkeley, California, 94720 Materials Science Division, Argonne National Laboratory, Argonne, Illinois, 60439 Email: zhujq@hit.edu.cn (Jiaqi Zhu) Acknowledgment Research was supported by the LDRD Program of Lawrence Berkeley National Laboratory, by the Assistant Secretary for Energy Efficiency and Renewable Energy, Office of Building Technology, of the U.S. Department of Energy under U.S. Department of Energy Contract No. DE-AC02-05CH11231. Additional support was provided by the National Natural Science Foundation of China (Grant No.51072039 and 51222205), and the Ph.D. Programs Foundation of the Ministry of Education of China (20112302110036). DISCLAIMER This document was prepared as an account of work sponsored by the United States Government. While this document is believed to contain correct information, neither the United States Government nor any agency thereof, nor The Regents of the University of California, nor any of their employees, makes any warranty, express or implied, or assumes any legal responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by its trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof, or The Regents of the University of California. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof or The Regents of the University of California.

Observation of multiple charge states and high ion energies in high-power impulse magnetron sputtering (HiPIMS) and burst HiPIMS using a LaB6 target

The charge-state-resolved ion energies of high-power impulse magnetron sputtering (HiPIMS) discharges were measured, using a LaB6 target, as a function of charging voltage, pulse length, pulse frequency and ?on/off? time ratio within applied HiPIMS bursts. The highest charge states can reach ?+2? and ?+3? for boron and lanthanum ions, respectively. At high discharge powers, the B/La ion ratio can exceed the respective atom ratio in the target producing B-rich plasma with up to 98% boron ions. In the case of two-segmented bursts with high ?on/off? time ratios, La3+ is the dominating lanthanum ion species and the ion energy distribution of B+ shows a pronounced high-energy tail extending up to 750?eV. The measured plasma compositions, ion charge states and ion energies are discussed within the established framework of HiPIMS discharges and the recent postulation that potential humps are associated with drifting ionization zones. The recorded high B/La ion ratios are a result of complex effects related to particle fluxes in the HiPIMS plasma of compound targets, as explained with the help of an expanded schematic representation of self-sputtering and gas atom recycling. The high energies of the B+ ions are based on a combination of the self-sputtering of boron, backscattering of incident boron ions on lanthanum atoms in the target and acceleration by the potential hump. Further evidence for potential humps is provided by the observed charge-state dependence of ion energies and features between the thermal peak and high-energy tail of the ion energy distribution functions.

A synchronized emissive probe for time-resolved plasma potential measurements of pulsed discharges

A pulsed emissive probe technique is presented for measuring the plasma potential of pulsed plasma discharges. The technique provides time-resolved data and features minimal disturbance of the plasma achieved by alternating probe heating with the generation of plasma. Time resolution of about 20 ns is demonstrated for high power impulse magnetron sputtering (HIPIMS) plasma of niobium in argon. Spatial resolution of about 1 mm is achieved by using a miniature tungsten filament mounted on a precision translational stage. Repeated measurements for the same discharge conditions show that the standard deviation of the measurements is about 1-2 V, corresponding to 4%-8% of the maximum plasma potential relative to ground. The principle is demonstrated for measurements at a distance of 30 mm from the target, for different radial positions, at an argon pressure of 0.3 Pa, a cathode voltage of -420 V, and a discharge current of about 60 A in the steady-state phase of the HIPIMS pulse.

High Rate Deposition of High Quality ZnO:Al by Filtered Cathodic Arc

High quality ZnO:Al (AZO) thin films were prepared on glass substrates by direct current filtered cathodic arc deposition. Substrate temperature was varied from room temperature to 425oC, and samples were grown with and without the assistance of low power oxygen plasma (75W). For each growth condition, at least 3 samples were grown to give a statistical look at the effect of the growth environment on the film properties and to explore the reproducibility of the technique. Growth rate was in the 100-400 nm/min range but was apparently random and could not be easily traced to the growth conditions explored. For optimized growth conditions, 300-600 nm AZO films had resistivities of 3-6 x 10-4 ?Omega cm, carrier concentrations in the range of 2-4 x 1020 cm3, Hall mobility as high as 55 cm2/Vs, and optical transmittance greater than 90percent. These films are also highly oriented with the c-axis perpendicular to the substrate and a surface roughness of 2-4 nm.

Localized surface plasmon resonance of degenerate semiconducting nanocrystal ensembles with various shape distributions

Nanocrystals of degenerate semiconductors come in a variety of shapes, which have profound influence on the localized surface plasmons which are supported. Ensembles of such nanocrystals are never perfect and will always show a distribution of shapes. By embedding the Drude model into Mie scattering theory, the effect of the shape inhomogeneity on the absorbance spectrum of a nanocrystal ensemble can be approximated for a few common cases such as nanorods and nanodiscs as well as general ellipsoids. Using various distributions of aspect ratios, broadening and shifting of the various plasmonic absorption peaks is observed for nanorod and nanodisc ensembles. Similar behavior is also observed for ensembles of nearly spherical nanocrystals, which emphasizes the importance of accounting for nanocrystal shape inhomogeneity when investigating broadening mechanisms of nanocrystal plasmonic absorbance.