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.

Mechanisms behind green photoluminescence in ZnO phosphor powders

Atomic layer deposition(ALD), a chemical vapor deposition technique based on sequential self-terminating gas–solid reactions, has for about four decades been applied for manufacturing conformal inorganic material layers with thickness down to the nanometer range. Despite the numerous successful applications of material growth by ALD, many physicochemical processes that control ALD growth are not yet sufficiently understood. To increase understanding of ALD processes, overviews are needed not only of the existing ALD processes and their applications, but also of the knowledge of the surface chemistry of specific ALD processes. This work aims to start the overviews on specific ALD processes by reviewing the experimental information available on the surface chemistry of the trimethylaluminum/water process. This process is generally known as a rather ideal ALD process, and plenty of information is available on its surface chemistry. This in-depth summary of the surface chemistry of one representative ALD process aims also to provide a view on the current status of understanding the surface chemistry of ALD, in general. The review starts by describing the basic characteristics of ALD, discussing the history of ALD—including the question who made the first ALD experiments—and giving an overview of the two-reactant ALD processes investigated to date. Second, the basic concepts related to the surface chemistry of ALD are described from a generic viewpoint applicable to all ALD processes based on compound reactants. This description includes physicochemical requirements for self-terminating reactions,reaction kinetics, typical chemisorption mechanisms, factors causing saturation, reasons for growth of less than a monolayer per cycle, effect of the temperature and number of cycles on the growth per cycle (GPC), and the growth mode. A comparison is made of three models available for estimating the sterically allowed value of GPC in ALD. Third, the experimental information on the surface chemistry in the trimethylaluminum/water ALD process are reviewed using the concepts developed in the second part of this review. The results are reviewed critically, with an aim to combine the information obtained in different types of investigations, such as growth experiments on flat substrates and reaction chemistry investigation on high-surface-area materials. Although the surface chemistry of the trimethylaluminum/water ALD process is rather well understood, systematic investigations of the reaction kinetics and the growth mode on different substrates are still missing. The last part of the review is devoted to discussing issues which may hamper surface chemistry investigations of ALD, such as problematic historical assumptions, nonstandard terminology, and the effect of experimental conditions on the surface chemistry of ALD. I hope that this review can help the newcomer get acquainted with the exciting and challenging field of surface chemistry of ALD and can serve as a useful guide for the specialist towards the fifth decade of ALD research.

Surface chemistry of atomic layer deposition: A case study for the trimethylaluminum/water process

The photoluminescence (PL) spectra of the undoped ZnO films deposited on Si substrates by dc reactive sputtering have been studied. There are two emission peaks, centered at 3.18 eV (UV) and 2.38 eV (green). The variation of these peak intensities and that of the I–V properties of the ZnO/Si heterojunctions were investigated at different annealing temperatures and atmospheres. The defect levels in ZnO films were also calculated using the method of full-potential linear muffin-tin orbital. It is concluded that the green emission corresponds to the local level composed by oxide antisite defect OZn rather than oxygen vacancy VO, zinc vacancy VZn, interstitial zinc Zni, and interstitial oxygen Oi.

Green luminescent center in undoped zinc oxide films deposited on silicon substrates

We present a review of current research on the optical properties of ZnO nanostructures. We provide a brief introduction to different fabrication methods for various ZnO nanostructures and some general guidelines on how fabrication parameters (temperature, vapor-phase versus solution-phase deposition, etc.) affect their properties. A detailed discussion of photoluminescence, both in the UV region and in the visible spectral range, is provided. In addition, different gain (excitonic versus electron hole plasma) and feedback (random lasing versus individual nanostructures functioning as Fabry-Perot resonators) mechanisms for achieving stimulated emission are described. The factors affecting the achievement of stimulated emission are discussed, and the results of time-resolved studies of stimulated emission are summarized. Then, results of nonlinear optical studies, such as second-harmonic generation, are presented. Optical properties of doped ZnO nanostructures are also discussed, along with a concluding outlook for research into the optical properties of ZnO.

Optical properties of ZnO nanostructures.

Recent progress in processing and properties of ZnO

Point defects and luminescence centres in zinc oxide and zinc oxide doped with manganese

Series Preface. Preface. 1 Principles of Luminescence (Adrian H. Kitai). 1.1 Introduction. 1.2 Radiation Theory. 1.3 Simple Harmonic Radiator. 1.4 Quantum Description. 1.5 Selection Rules. 1.6 Einstein Coefficients. 1.7 Harmonic Perturbation. 1.8 Blackbody Radiation. 1.9 Dipole-Dipole Energy Transfer. 1.10 Energy Levels in Atoms. 1.11 Crystal Field Splitting. Acknowledgement. References. 2 Phosphor Quantum Dots (Debasis Bera, Lei Qian and Paul H. Holloway). 2.1 Introduction. 2.2 Nanostructured Materials. 2.3 Quantum Dots. 2.4 Relaxation Processes of Excitons. 2.5 Blinking Effect. 2.6 Surface Passivation. 2.7 Synthesis Processes. 2.8 Optical Properties and Applications. 2.9 Perspective. Acknowledgement. References. 3 Color Conversion Phosphors for LEDS (Jack Silver and Robert Withnall). 3.1 Introduction. 3.2 Disadvantages of using LEDs without Color Conversion Phosphors. 3.3 Phosphors for Converting the Color of Light Emitted by LEDs. 3.4 Survey of the Synthesis and Properties of some Currently Available Color Conversion Phosphors. 3.5 Multi-Phosphor pcLEDs. 3.6 Quantum Dots. 3.7 Conclusions. Acknowledgements. References. 4 Development of White OLED Technology for Application in Full-Color Displays and Solid-State Lighting (T. K. Hatwar and Jeff Spindler). 4.1 Introduction. 4.2 Generation of White Light. 4.3 White OLEDs for Display Applications. 4.4 White OLED Tandem Architecture. 4.5 White OLEDs Based on Triplets. 4.6 White OLEDs Based on Conjugated Polymers. 4.7 White OLEDs for Solid-State Lighting. 4.8 Advanced Manufacturing of Large-Area Coatings. 4.9 Future Outlook. Acknowledgements. References. 5 Polymer Light-Emitting Electrochemical Cells (Jun Gao). 5.1 Introduction. 5.2 LEC Operating Mechanism and Device Characteristics. 5.3 LEC Materials. 5.4 Frozen-Junction LECs. 5.5 Planar LECs. 5.6 Conclusions and Outlook. References. 6 LED Materials and Devices (Tsunemasa Taguchi). 6.1 Introduction. 6.2 LED Structures and Effi ciencies. 6.3 Typical LEDs and Features. 6.4 Generation of White Light. 6.5 Devices and Applications. 6.6 Future Prospects. 6.7 Conclusions. References. 7 Thin Film Electroluminescence (Adrian H. Kitai). 7.1 Introduction. 7.2 Background of EL. 7.2.4 Sphere-supported thin film EL. 7.3 Theory of Operation. 7.4 Electroluminescent Phosphors. 7.5 Device Structures. 7.6 EL Phosphor Thin Film Growth. 7.7 Full-Color Electroluminescence. 7.8 Conclusions. References. 8 AC Powder Electroluminescence (Feng Chen and Yingwei Xiang). 8.1 Background. 8.2 Structure and Materials of AC Powder EL Devices. 6 LED Materials and Devices (Tsunemasa Taguchi). 6.1 Introduction. 6.2 LED Structures and Effi ciencies. 6.3 Typical LEDs and Features. 6.4 Generation of White Light. 6.5 Devices and Applications. 6.6 Future Prospects. 6.7 Conclusions. References. 7 Thin Film Electroluminescence (Adrian H. Kitai). 7.1 Introduction. 7.2 Background of EL. 7.3 Theory of Operation. 7.4 Electroluminescent Phosphors. 7.5 Device Structures. 7.6 EL Phosphor Thin Film Growth. 7.7 Full-Color Electroluminescence. 7.8 Conclusions. References. 8 AC Powder Electroluminescence (Feng Chen and Yingwei Xiang). 8.1 Background. 8.2 Structure and Materials of AC Powder EL Devices. 8.3 The Mechanism of Light Emission for AC ZnS-Powder-EL Device. 8.4 EL Characteristics of AC Powder EL Materials. 8.5 Preparation of Powder EL materials. 8.6 Limitations of AC Powder EL Devices. 8.7 Applications of ACPEL. References. Index.

Luminescent materials and applications

/pdf/solid-state-luminescence-136rzt9zp6.pdf

Solid State Luminescence

Positron lifetime spectroscopy and cathodoluminescence were employed to study luminescence centers in ZnO. The samples were high-purity polycrystalline ceramics sintered at temperatures ranging from 800 to 1,400 C for 2 to 40 h. Scanning electron microscopy shows that as annealing temperatures and/or times increase, the average grain size increases and can reach 30 {mu}m for samples sintered at 1,200 C. At the same time, the positron bulk lifetime approaches theoretically estimated single-crystal values, while the integrated luminescence intensity increases significantly. A further increase of the sintering temperature beyond 1,200 C results in a decrease in the luminescence intensity, in good agreement with the only weak luminescence observed in single-crystalline material. The positron lifetime spectra clearly show the existence of one dominant vacancy-type defect, most likely a complex involving V{sub Zn}, or the divacancy, V{sub Zn}V{sub O}, independent of sample thermal history. The concentration of this center steadily decreases with increasing sintering temperature. It is concluded that the yellow luminescence centers are related to charged zinc vacancies trapped in the grain boundary regions. The authors propose that the observed broadness of the spectra likely originates from the modification of the electronic configuration of the luminescence centers due to their complex environment.more » A direct connection between the positron and the luminescence results could not be established; instead, they appear to reflect two relatively independent aspects of the samples. It could be shown, however, that positron annihilation measurements can be used effectively to monitor the evolution of the microstructure of the samples, in good agreement with scanning electron micrographs.« less

The influence of processing conditions on point defects and luminescence centers in ZnO

Mn‐activated Zn2SiO4 and Ce‐activated Y2SiO5 multilayer thin film electroluminescent (EL) devices were prepared by rf magnetron sputtering. The EL response of the devices under different voltages, frequencies, and pulse widths, as well as the transferred charge and decay characteristics were studied. The EL device using Zn2SiO4:Mn as the phosphor layer is shown to achieve a brightness of over 200 cd/m2 at 400 Hz and a field of 3×106 V/cm (twice the threshold field) and a maximum efficiency of 0.78 lm/W, with a decay time of 0.6 ms. The main characteristics of the oxide phosphors include (a) strong trailing edge excitation in nonsymmetrical EL devices, (b) a narrow transferred charge loop, and (c) a time response sensitive to device structures. Compared with the nonsymmetrical structure, the more symmetrical double‐insulated structure is shown to increase the efficiency of the EL device by generating strong EL excitation from both positive and negative voltage pulses.

Adrian H. Kitai

Papers

Point defects and luminescence centres in zinc oxide and zinc oxide doped with manganese

Luminescent materials and applications

Solid State Luminescence

The influence of processing conditions on point defects and luminescence centers in ZnO

Electroluminescence of the oxide thin film phosphors Zn2SiO4 and Y2SiO5