C. H. Seager

Mechanisms behind green photoluminescence in ZnO phosphor powders

We explore the interrelationships between the green 510 nm emission, the free‐carrier concentration, and the paramagnetic oxygen‐vacancy density in commercial ZnO phosphors by combining photoluminescence, optical‐absorption, and electron‐paramagnetic‐resonance spectroscopies. We find that the green emission intensity is strongly influenced by free‐carrier depletion at the particle surface, particularly for small particles and/or low doping. Our data suggest that the singly ionized oxygen vacancy is responsible for the green emission in ZnO; this emission results from the recombination of a photogenerated hole with the singly ionized charge state of this defect.

CORRELATION BETWEEN PHOTOLUMINESCENCE AND OXYGEN VACANCIES IN ZNO PHOSPHORS

By combining electron paramagnetic resonance (EPR), optical absorption, and photoluminescence (PL) spectroscopy, a strong correlation is observed between the green 510 nm emission, the free‐carrier concentration, and the density of singly ionized oxygen vacancies in commercial ZnO phosphor powders. From these results, we demonstrate that free‐carrier depletion at the particle surface, and its effect on the ionization state of the oxygen vacancy, can strongly impact the green emission intensity. The relevance of these observations with respect to low‐voltage field emission displays is discussed.

The dc voltage dependence of semiconductor grain‐boundary resistance

A model is developed to describe the potential barriers which often occur at grain boundaries in polycrystalline semiconductors. The resistance of such materials is determined by thermionic emission over these barriers. The dc grain‐boundary current density as a function of applied voltage is calculated using several forms for the density of defect states within the boundary region. In all cases, the currents are Ohmic at low voltages; they can attain a quasisaturated level at intermediate voltages, and they display a sharp bias dependence at high voltages. The details of the intermediate and high‐voltage characteristics are found to depend strongly on the grain‐doping density and on the density and energy distribution of defect states at the grain boundary. Contrary to previous assertions, we find that the large current‐voltage nonlinearities found in real materials are most likely associated with defect‐state densities that decrease above the zero‐bias Fermi level. The results of the model are compared ...

Zero-bias resistance of grain boundaries in neutron-transmutation-doped polycrystalline silicon

We have characterized the electrical transport properties of neutron‐transmutation‐doped polycrystalline silicon. Zero‐bias measurements of resistance have been made as a function of temperature on both bulk specimens and individual grain boundaries in this material. Below a doping level of ∼2×1015 phosphorus/cm3, the bulk resistance has a nearly Arrhenius behavior with an activation energy of ∼0.55 eV; above this donor concentration the resistivity is markedly curved on an Arrhenius plot with values of slope which decrease with decreasing temperature. Potential probe measurements show that a large spread in grain‐boundary impedances exist in these higher‐doped specimens. We compare our data to theoretical expressions for current flow across grain‐boundary potential barriers and good agreement is observed; these comparisons indicate that the largest grain‐boundary state densities observed in our samples consist of ∼6×1011 available single‐electron‐states/cm2 located within ∼0.2 eV from the center of the f...

Electrical properties and conduction mechanisms of Ru‐based thick‐film (cermet) resistors

This paper presents an experimental study of the electrical conduction mechanisms in thick‐film (cermet) resistor. The resistors were made from one custom and three commercially formulated inks with sheet resistivities ranging from 102 to 106 Ω/⧠ in decade increments. Their microstructure and composition have been examined using optical and scanning electron microscopy, electron microprobe analysis, x‐ray diffraction, and various chemical analyses. This portion of our study shows that the resistors are heterogeneous mixtures of metallic metal oxide particles (∼4×10−5 cm in diameter) and a lead silicate glass. The metal oxide particles are ruthenium containing pyrochlores, and are joined to form a continuous three‐dimensional network of chain segments. The principal experimental work reported here is an extensive study of the electrical transport properties of the resistors. The temperature dependence of conductance has been measured from 1.2 to 400 K, and two features common to all resistors are found. Th...

Passivation of grain boundaries in polycrystalline silicon

Preferential diffusion of various gases down the grain boundaries in polycrystalline silicon is shown to promote significant changes in the density of defect states in these regions. A plasma of monatomic hydrogen provides a significant reduction in both the state density and the accompanying grain‐boundary potential barrier while plasmas of oxygen, nitrogen, and sulfur hexafluoride are shown to increase this density of states. Boundaries passivated with hydrogen have as much as a factor of 1000 larger transconductance after treatment. Hydrogenated barriers are stable over long periods at 375 °C and essentially indefinitely at 23 °C. The results have important implications for the development of low‐cost thin‐film silicon photovoltaic devices.

Role of carbon in GaN

GaN samples, containing various concentrations of carbon and doped intentionally with silicon, have been grown heteroepitaxially on sapphire using metal–organic chemical-vapor deposition. These samples have been characterized by a variety of electrical and optical techniques, and the resulting experimental data are compared to density-functional-theory calculations of the formation energies and electronic states of substitutional and interstitial carbon in hexagonal GaN. We find that in samples where the silicon concentration exceeds that of carbon, carbon sits in the N substitutional site, acting as an acceptor and partially compensating the material. However, when carbon densities exceed those for Si, GaN becomes semi-insulating due to carbon occupation of both N and Ga substitutional lattice sites, and a new luminescence peak appears at ∼3 eV. Calculated formation energies of carbon in both sites are strong functions of both the Fermi level and growth stoichiometry. The former dependence gives rise to ...

Green photoluminescence efficiency and free-carrier density in ZnO phosphor powders prepared by spray pyrolysis

Abstract Electron paramagnetic resonance, optical absorption, and photoluminescence spectroscopy have been combined to characterize ZnO powders that were prepared by spray pyrolysis. We generally observe a good correlation between the 510 nm green emission intensity and the density of paramagnetic isolated oxygen vacancies. In addition, both quantities increase with free-carrier concentration n e , as long as n e 18 cm −3 . At higher free-carrier concentrations, both quantities decrease. A model is proposed involving the isolated oxygen vacancy as the luminescence center.

Grain boundary recombination: Theory and experiment in silicon

Calculations have been made of the barrier heights and recombination velocities at semiconductor grain boundaries subject to uniform illumination with above‐band‐gap light. A detailed balance approach has been coupled with a solution of the current continuity equation to yield a full solution of the problem subject to the approximation that majority carrier currents can be properly described by thermionic emission expressions. Silicon bicrystal data are presented and shown to be in good agreement with the predictions of the theory.

Grain boundary states and varistor behavior in silicon bicrystals

The energy density of states in a Si grain boundary has been quantitatively determined for the first time. Since the deconvolution scheme used in this determination is a technique previously unapplied to real materials, the applicability of the model and the validity of the results were experimentally verified by comparing conductance and capacitance data. Additionally, a high‐voltage varistor characteristic (highly non‐Ohmic current) was observed. This shows for the first time that a simple grain boundary without intergranular additives is capable of a strong varistor behavior (nonlinearity coefficient α≳20).