Nigel D. Shepherd

Workfunction tuning of zinc oxide films by argon sputtering and oxygen plasma: an experimental and computational study

Zinc oxide (ZnO) films were grown by radio frequency magnetron sputter deposition and the changes to its surface composition and workfunction induced by argon sputter cleaning and oxygen plasma treatments were characterized using x-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy and density functional theory modelling. Compared with a workfunction of 3.74?eV for the as-deposited ZnO films, a workfunction of 3.95?eV was obtained after Ar sputter cleaning and 4.21?eV after exposure to oxygen plasma. The data indicate that oxygen plasma treatment leads to a more negative ZnO surface. The dipole induced by this charge redistribution reinforces the original surface dipole, which results in an increase in the surface dipole moment and an increase in workfunction. The reverse is true for hydrocarbon contamination of ZnO surfaces. Excellent qualitative agreement between the experimental results and computational modelling was obtained. The results suggest that specific surface functionalization may be a viable method of controlling the workfunction of ZnO for use as the transparent conducting oxide in optoelectronic applications such as solar cells and organic light-emitting diodes.

Sputter deposited GaN doped erbium thin films: Photoluminescence and 1550 nm infrared electroluminescence

Erbium doped gallium nitride (GaN) thin films were deposited on Si substrates by reactive rf magnetron cosputtering of a commercial GaN target, together with a metallic erbium target. Nitrogen was employed as the reactive sputtering gas. The gallium nitride doped erbium (GaN:Er) films thus obtained, exhibited the characteristic 525, 540, 660, and 1550 nm photoluminescence emission associated with the Er+3 ion 4f–4f intraband transitions. In addition, 1550 nm IR electroluminescence (EL) emission was observed from the sputter deposited GaN:Er phosphor films. The EL device was an inverted half-stack ac thin-film EL device structure. The 1550 nm EL emission is consistent with impact excitation and subsequent 4I13/2→4I15/2 radiative relaxation of Er+3 ions. Impact excitation requires conduction electrons with sufficient energy from electrical field acceleration, to excite the transition.

Visible and near-infrared alternating-current electroluminescence from sputter-grown GaN thin films doped with Er

A demonstration of visible and near-infrared (NIR) alternating-current electroluminescence from sputter-grown GaN thin films doped with Er is reported. The alternating-current thin-film electroluminescent (ACTFEL) devices were constructed using a standard single-insulating structure, Al/GaN:Er/aluminum–titanium–oxide/indium–tin–oxide/Corning 7059 glass. Visible emissions peaked at 550 and 665 nm as well as NIR emissions centered at 1000 and 1550 nm were observed from the fabricated ACTFEL devices operating at room temperature. The visible and NIR emissions at 550, 665, 1000, and 1550 nm were attributed to the Er3+ 4f–4f intrashell transitions from the 4S3/2, 4F9/2, 4I11/2, and 4I13/2 excited-state levels to the 4I15/2 ground state, respectively. The green 550 nm emission had a larger dI/dV and a higher threshold voltage, Vth than the NIR 1550 nm emission, which could result from the need for higher electron impact energy to impact-excite the Er ion into the higher-energy excited-states for emission of gre...

Infrared emission from zinc sulfide: Rare-earth doped thin films

Infrared (IR) electroluminescent (EL) thin film phosphors were radio frequency magnetron sputter deposited by cosputtering of an undoped ZnS target together with ZnS: 1.5 mole % ErF3 or ZnS: 1.5 mole % NdF3 targets. The ZnS:ErF3 and ZnS:NdF3 thin film phosphors were annealed in a N2 ambient at temperatures ranging from 350 to 475 °C for 1 h to increase radiance. The maximum EL radiance observed was 28 μW/cm2 at 1550 nm for ZnS:ErF3, and 26 μW/cm2 at 910 nm and 15 μW/cm2 at 1060 nm for ZnS:NdF3 (at 40 V above the threshold voltage) after a 425 °C, 1 h anneal in nitrogen. For anneals above 425 °C visible emission increased, while near infrared (NIR) emission from both ZnS:ErF3 and ZnS:NdF3 was either constant or decreased. For ZnS:ErF3, the 1550 nm NIR peak decreased, but the 990 nm peak remained constant in intensity. The crystallinity of ZnS was improved by annealing, and these results are consistent with the postulate that residual defects limit the acceleration of “hot” electrons for anneals at ⩽425 °C....

Role of oxygen vacancies in visible emission and transport properties of indium oxide nanowires

We report on the effect of oxygen vacancies on the defect-related emission and the electronic properties of In2O3 nanowires. The nanowires were synthesized by vapor phase transport and had diameters ranging from 80–100 nm and lengths over 10–20 μm, with a growth direction of [0 0 1]. The as-grown nanowires connected in an FET type of configuration show n-type conductivity, which is ascribed to the presence of intrinsic defects like oxygen vacancies in the nanowire. The resistivity, transconductance, field effect mobility and carrier concentration of the In2O3 nanowires were determined to be 1.82 × 10−2 Ω cm, 11.2 nS, 119 cm2 V−1 s−1 and 4.89 × 1017 cm−3, respectively. The presence of oxygen vacancies was also confirmed by photoluminescence measurements, which show a strong UV emission peak at 3.18 eV and defect peaks in the visible region at 2.85 eV, 2.66 eV and 2.5 eV. We present a technique of post-growth annealing in O2 environment and passivation with (NH4)2S to reduce the defect-induced emission.

Defect structure and chemical bonding of p-type ZnO:Sb thin films prepared by pulsed laser deposition

X-ray photoelectron spectroscopy and x-ray diffraction were used to understand the local environment of Sb in pulsed laser deposited ZnO:Sb films, which switched to p-type electrical conductivity from as-deposited n-type behavior after being annealed at 600 °C under an O2 flow. Tensile in-plane stress was observed in the as-deposited n-type films, and its magnitude increased after the annealing induced p-type switch. This stress can be explained by larger Sb sitting on smaller Zn sites in both the as-deposited and annealed films. Annealing resulted in oxygen incorporation and a more electronegative environment for the cations, as indicated by shifts to higher binding energy of both the Zn2p and Sb3d XPS peaks. The XPS results also show the formation of Sb2O5 in the p-type samples, and indicate a reorganization of the local coordination of Sb. Based on these results and density functional theory computations of the formation and ionization energies of defect complexes, we propose that acceptor complexes are responsible for the p-type conductivity.

The influence of high dielectric constant aluminum oxide sputter deposition on the structure and properties of multilayer epitaxial graphene

High dielectric constant aluminum oxide (Al(2)O(3)) is frequently used as the gate oxide in high electron mobility transistors and the impact of its deposition by radio frequency (RF) magnetron sputtering on the structural and electrical properties of multilayer epitaxial graphene (MLG) grown by graphitization of silicon carbide (SiC) is reported. Micro-Raman spectroscopy and temperature dependent Hall mobility measurements reveal that the processing induced changes to the structural and electrical properties of the MLG can be minimal when the oxide deposition conditions are optimal. High-resolution transmission electron microscopy (HRTEM) analysis confirms that the Al(2)O(3)/MLG interface is relatively sharp and that our thickness approximation of the MLG using angle resolved x-ray photoelectron spectroscopy (ARXPS) is accurate. An interface trap density of 5.1 × 10(10) eV(-1) cm(-2) was determined using capacitance-voltage techniques. The totality of our results indicates that ARXPS can be used as a nondestructive tool to measure the thickness of MLG, and that RF sputtered Al(2)O(3) can be used as a high dielectric (high-k) constant gate oxide in multilayer graphene based transistor applications.

Microstructure and Electronic Band Structure of Pulsed Laser Deposited Iron Fluoride Thin Film for Battery Electrodes

Battery electrodes in thin-film form are free of the binders used with traditional powder electrodes and present an ideal platform to obtain basic insight to the evolution of the electrode-electrolyte interface passivation layer, the formation of secondary phases, and the structural underpinnings of reversibility. This is particularly relevant to the not yet fully understood conversion electrode materials, which possess enormous potential for providing transformative capacity improvements in next-generation lithium-ion batteries. However, this necessitates an understanding of the electronic charge transport properties and band structure of the thin films. This work presents an investigation of the electron transport properties of iron fluoride (FeF2) thin-film electrodes for Li-ion batteries. FeF2 thin films were prepared by pulsed-laser deposition, and their phase purity was characterized by electron microscopy and diffraction. The grown materials are polycrystalline FeF2 with a P42/mnm crystallographic symmetry. Room-temperature Hall measurements reveal that as-deposited FeF2 is n-type: the Hall coefficients were negative, electron mobility was 0.33 cm2/(V s) and resistivity was 0.255 Ω cm. The electronic band diagram of FeF2 was obtained using a combination of ultraviolet photoelectron spectroscopy, photoluminescence, photoluminescence excitation and optical absorption, which revealed that FeF2 is a direct bandgap, n-type semiconductor whose band structure is characterized by a 3.4 eV bandgap, a workfunction of ∼4.51 eV, and an effective Fermi level that resides approximately 0.22 eV below the conduction band edge. We propose that the shallow donor levels at 0.22 eV are responsible for the measured n-type conductivity. The band diagram was used to understand electron transport in FeF2 thin film and FeF2-C composite electrodes.

Effect of surface adsorption and non-stoichiometry on the workfunction of ZnO surfaces: A first principles study

ZnO has been actively studied for potential usage as a transparent conducting oxide (TCO) for a variety of applications including organic light emitting diodes and solar cells. In these applications, fine-tuning the workfunction of ZnO is critical for controlling interfacial barriers and improving the charge injection (or outcoupling) efficiencies. We have performed plane wave periodic density functional theory calculations to investigate the effect of different surface absorbents and surface defects (including surface non-stoichiometry) on the workfunction of ZnO. The aim was to understand the underlying mechanism of workfunction changes, in order to engineer specific workfunction modifications. Accurate calculations of workfunctions of polar surfaces were achieved by introducing balancing pseudo charges on one side of the surface to remove the dipolar effect. It was found that increasing the surface coverage of hydrocarbons (-CH3) decreased the workfunction, while adsorption of highly electronegative-F and -CF3 groups and increases in surface O/Zn ratio increased the workfunction of ZnO. The increase of workfunction was found to be directly correlated to the enhancement variation of surface dipole moment due to adsorptions or other surface modifications. Introducing surface absorbents that increase surface dipole moment can be an effective way to increase workfunction in ZnO TCOs.

The influence of MoOx gap states on hole injection from aluminum doped zinc oxide with nanoscale MoOx surface layer anodes for organic light emitting diodes

The effective workfunction of Al doped ZnO films (AZO) increased from 4.1 eV to 5.55 eV after surface modification with nanoscale molybdenum sub-oxides (MoOx). Hole only devices with anodes consisting of 3 nm of MoOx on AZO exhibited a lower turn-on voltage (1.5 vs 1.8 V), and larger charge injection (190 vs 118 mA/cm2) at the reference voltage, compared to indium tin oxide (ITO). AZO devices with 10 nm of MoOx exhibited the highest workfunction but performed poorly compared to devices with 3 nm of MoOx, or standard ITO. Ultraviolet photoelectron, X-ray photoelectron, and optical spectroscopies indicate that the 3 nm MoOx films are more reduced and farther away from MoO3 stoichiometry than their 10 nm equivalents. The vacancies associated with non-stoichiometry result in donor-like gap states which we assign to partially occupied Mo 4d levels. We propose that Fowler-Nordheim tunneling from these levels is responsible for the reduction in threshold voltage measured in devices with 3 nm of MoOx. A schematic...