K. Vanheusden

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.

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.

Defect‐dipole alignment and tetragonal strain in ferroelectrics

We show the alignment of defect dipoles along the direction of the spontaneous polarization in polycrystalline Pb(Zr,Ti)O3 and BaTiO3 ferroelectric ceramics using electron paramagnetic resonance (EPR). The alignment is demonstrated via orientation dependent paramagnetic centers in the polycrystalline materials and computer modeling of the EPR line shapes. It is shown that defect dipoles can become aligned by oxygen vacancy motion in the octahedron about a negatively charged center for the oxygen vacancy‐related dipole complexes or by defect displacement and domain realignment in the lattice for isolated defect centers. We find that the alignment is not observed in nonferroelectric materials such as SrTiO3, and is destroyed in ferroelectric materials by heating above the Curie temperature. These observations suggest an interplay between distortion in the unit cell and the ability to align defect dipoles, as is the case more generally for ferroelectric dipole alignment. We also directly observe aligned intr...

Alignment of defect dipoles in polycrystalline ferroelectrics

Using electron paramagnetic resonance (EPR), we show the alignment of defect dipoles along the direction of the spontaneous polarization in polycrystalline BaTiO3 ceramics by subjecting the capacitors to a dc bias at elevated temperatures. The alignment is demonstrated to occur via orientation dependent EPR signals in the polycrystalline perovskite lattice. The alignment of the defect dipoles is found to strongly enhance the light sensitivity of the defects. This observation may have important implications for photorefractive applications.

IMPACT OF PB DOPING ON THE OPTICAL AND ELECTRONIC PROPERTIES OF ZNO POWDERS

Electron paramagnetic resonance (EPR), optical absorption, and photoluminescence (PL) spectroscopy have been combined to characterize Pb‐doped ZnO ceramic powders. We observe a decrease in the 2.26 eV emission peak and a concomitant smearing of the band edges, narrowing the effective gap of the grains to ≊2 eV with increasing lead content. Both phenomena are at least in part attributed to the formation of a separate PbO‐like phase, likely residing at the grain boundaries. The free‐carrier concentration in the grains was also observed to decrease with increasing Pb content. Our EPR results suggest that this may be due to electron transfer from oxygen vacancy donors to substitutional Pb centers, acting as electron traps.

Mechanism for anneal‐induced interfacial charging in SiO2 thin films on Si

H‐induced positive charging is observed at both the top Si/SiO2 and bottom SiO2/Si interfaces in Si/SiO2/Si structures. The response of the H‐induced positive charge to thermal annealing and electron injection is very different from that of simple oxygen vacancy hole traps in SiO2. To explain this H‐induced positive charging, we propose a scheme in which H reacts to form positively charged over‐coordinated oxygen centers in close proximity to both top and bottom interfaces. The annealing‐induced entity may also provide a natural explanation for the origin of the fixed oxide charge that forms during oxidation of Si.

Local Ce environments and their effects on optical properties of SrS phosphors

In this study, we use electron paramagnetic resonance (EPR), optical absorption, and photoluminescence (PL) spectroscopies to determine the various Ce environments in SrS phosphor materials and how these affect absorption and emission properties. As the Ce concentration is increased from 450 to 7500 ppm, the total EPR‐active Ce3+ and optical absorption signals increase linearly with Ce concentration; by contrast, the PL intensity saturates at fairly low Ce concentrations (1000 ppm Ce). We suggest that the nonlinear behavior of the PL arises from the presence of nonradiative deexcitation pathways such as defects associated with Ce sites, or Ce–Ce pairs.

Chemical kinetics of mobile-proton generation and annihilation in SiO2 thin films

The chemical kinetics of mobile-proton reactions in the SiO2 film of Si/SiO2/Si structures were analyzed as a function of forming-gas anneal parameters in the 300–600 °C temperature range. Our data show that the initial buildup of mobile protons is limited by the rate of lateral hydrogen diffusion into the SiO2 films. The final density of mobile protons is determined by the cooling rate which terminates the annealing process and, in the case of subsequent anneals, by the temperature of the final anneal. To explain the observations, we propose a dynamical equilibrium model which assumes a reversible interfacial reaction with a temperature-dependent balance.

H+ and D+ associated charge buildup during annealing of Si/SiO2/Si structures

Abstract Si/SiO2/Si structures produced by high temperature processing have been annealed in atmospheres containing forming gas (5% H2, 95% N2 or 5% D2, 95% N2) at temperatures in the range 200–900°C. For temperatures ≥ 500°C spontaneous positive electrical charging in the SiO2 layer is observed. Electron and hole injection experiments confirm that the charging is not due to holes but must be attributed to protons or deuterons. Both static and mobile species are formed dependent upon the confined (Si/SiO2/Si) or unconfined (SiO2/Si) nature of the structure of the sample. Application of an electric field ∼ 105 V cm−1 results in motion of the mobile ions with an activation energy estimated to be 0.81 ± 0.02 eV both for deuterons and protons. A √mass factor is evidenced resulting in the conclusion that the protons are more mobile than the deuterons. The physical origin of the charging phenomenon is attributed to the formation of overcoordinated oxygen atoms with the H or D attaching themselves to oxygens in strained SiOSi bonds near the Si/SiO2 interface, the positive chargeds state is stabilised by loss of an electron to the Si conduction band. It is assumed that strongly bound overcoordinated oxygens near the interface constitute the fixed oxide charge species while more weakly bound forms migrating from SiOSi unit to SiOSi unit are the origin of the mobile species.