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"Amorphous Chalcogenide Semiconductors and Glasses" describes developments in the science and technology of this class of materials. This book offers an up-to-date treatment of chalcogenide glasses and amorphous semiconductors from basic principles to applications while providing the reader with the necessary theoretical background to understanding the material properties technology of this class of materials. This book offers an up-to-date treatment of chalcogenide glasses and amorphous semiconductors from basic principles to applications while providing the reader with the necessary theoretical background to understanding the material properties. Chalcogenides form a special class of materials, which have one or more of the elements from the chalcogen group, Group VI in the Periodic Table (S, Se. or Te) as a constituent; the chalcogen is mixed with other elements to form various "new" compounds and alloys. Chalcogenides are noncrystalline solids because their structure is "amorphous" or "glassy". Such structures have totally different properties than crystalline solids. Chalcogenide glasses have a number of very interesting and useful properties, which have been already exploited in the commercialization of new devices.



Chapter 1. Introduction

We begin with the terminology and definition of several words, which may be somewhat confusing. Comparison of crystal and amorphous materials is made from physical standpoints. Among many non-crystalline solids, we shed light on oxide and chalcogenide glasses with brief histories. The readers will see how glass has made an impact on the present society. We also see the importance of unified understanding of glasses containing VIb elements in the periodic table. There are many kinds of chalcogenide glasses, which will be discussed in terms of atomic elements.
Keiji Tanaka, Koichi Shimakawa

Chapter 2. Structure

Atomic and microscopic structures of chalcogenide glasses are discussed from theoretical and experimental points of view. Starting with discussion on an ideal glass structure, we will see continuous studies performed for grasping atomic structures in disordered materials. Experimental methods and deduced results for the short-range and medium-range structures (orders) in glasses are introduced. Structural defects, which are likely to produce localized states in the bandgap, are discussed. In addition to these atomic structures, we shed light upon inhomogeneity and nano-structures in chalcogenide glasses.
Keiji Tanaka, Koichi Shimakawa

Chapter 3. Structural Properties

This chapter describes physical properties governed by normal atomic bonds. One of the biggest and long-standing problems is glass transition, at which specific heat, thermal expansion, and viscosity exhibit marked changes. Thermal crystallization is also studied extensively, specifically in relation to phase-change memories. We also take brief views of structural properties at low and room temperatures. Importance of the atomic coordination number, which affects structural properties, is also discussed. There exist magic coordination numbers at 2.4 (Phillips) and 2.67 (Tanaka); the origin of these numbers is discussed. We also refer to ion transport.
Keiji Tanaka, Koichi Shimakawa

Chapter 4. Electronic Properties

Electronic density of states in the extended and localized states govern optical and electrical properties. We see, in this chapter, that studies on electronic properties have yielded a lot of valuable ideas, such as Tauc gap, mobility edge, and charged defects. In addition, concepts originally proposed for crystals such as polaron and Urbach edge bear special importance in chalcogenide glasses. We also consider optical nonlinearity, which is prominent in the chalcogenide glass. Electrical conduction mechanisms, under dc and ac electric fields, are also discussed. It is suggested that the Meyer–Neldel law is important to obtain full understanding of the transport mechanisms. The final section refers to composition dependence of the bandgap energy.
Keiji Tanaka, Koichi Shimakawa

Chapter 5. Photo-Electronic Properties

Photo-excited electrons relax to ground states through several ways. One of the processes can be probed through photoluminescence, in which the most puzzling feature in amorphous chalcogenides may be the so-called half-gap rule of the peak energy. The origin will be discussed. Another photo-electronic property is the photoconduction. Most amorphous chalcogenides are good photoconductors, for which steady-state and transient characteristics are briefly discussed. Finally, we refer to a carrier avalanche effect in a-Se films, which has been applied to highly sensitive vidicons.
Keiji Tanaka, Koichi Shimakawa

Chapter 6. Light-Induced Phenomena

Light-induced structural changes are the most exciting phenomena in amorphous chalcogenides. We will overview thermal and photon effects and their mechanisms. These include bulk effects such as irreversible, reversible, and transitory changes. There exist also photo-chemical reactions, the most known being photodoping. In addition, these changes are either scalar (isotropic) or vector (anisotropic) upon excitation of linearly polarized light. It is also shown that computer simulations aid to understand the mechanisms. Light-induced phenomena in oxide, a-Si:H, and polymers are briefly discussed for comparison.
Keiji Tanaka, Koichi Shimakawa

Chapter 7. Applications

A variety of applications, present and potential, of non-crystalline insulators and semiconductors including amorphous chalcogenides are described in a “tree growth manner.” History and trend of optical devices, fibers, and waveguides are described. Great success has been attained in phase change memories (DVDs), x-ray medical image sensors, highly sensitive vidicons, and xerography. We refer also to other applications such as holographic memories, nonlinear devices, solar cells, and ionic devices.
Keiji Tanaka, Koichi Shimakawa

Chapter 8. Future Prospects

In this chapter, we summarize notable problems that are unresolved and also try to predict future prospects of amorphous chalcogenides.
Keiji Tanaka, Koichi Shimakawa


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