Amorphous Semiconductors E-Book

Kazuo Morigaki

• Herausgeber: John Wiley & Sons
• Kategorie: Fachliteratur
• Serie: Wiley Series in Materials for Electronic & Optoelectronic Applications
• Sprache: Englisch

Seitenzahl: 452

Veröffentlichungsjahr: 2016

Beschreibung

Amorphous semiconductors are subtances in the amorphous solid state that have the properties of a semiconductor and which are either covalent or tetrahedrally bonded amorphous semiconductors or chelcogenide glasses.

• Developed from both a theoretical and experimental viewpoint
• Deals with, amongst others, preparation techniques, structural, optical and electronic properties, and light induced phenomena
• Explores different types of amorphous semiconductors including amorphous silicon, amorphous semiconducting oxides and chalcogenide glasses
• Applications include solar cells, thin film transistors, sensors, optical memory devices and flat screen devices including televisions

Leseprobe

Wiley Series in Materials for Electronic and Optoelectronic Applications

www.wiley.com/go/meoa

Series Editors

Professor Arthur Willoughby, University of Southampton, Southampton, UK

Dr Peter Capper, Formerly of SELEX Galileo Infrared Ltd, Southampton, UK

Professor Safa Kasap, University of Saskatchewan, Saskatoon, Canada

Published Titles

Bulk Crystal Growth of Electronic, Optical and Optoelectronic Materials, Edited by P. Capper

Properties of Group‐IV, III–V and II–VI Semiconductors, S. Adachi

Charge Transport in Disordered Solids with Applications in Electronics, Edited by S. Baranovski

Optical Properties of Condensed Matter and Applications, Edited by J. Singh

Thin Film Solar Cells: Fabrication, Characterization and Applications, Edited by J. Poortmans and V. Arkhipov

Dielectric Films for Advanced Microelectronics, Edited by M. R. Baklanov, M. Green and K. Maex

Liquid Phase Epitaxy of Electronic, Optical and Optoelectronic Materials, Edited by P. Capper and M. Mauk

Molecular Electronics: From Principles to Practice, M. Petty

CVD Diamond for Electronic Devices and Sensors, Edited by R. S. Sussmann

Properties of Semiconductor Alloys: Group‐IV, III–V and II–VI Semiconductors, S. Adachi

Mercury Cadmium Telluride, Edited by P. Capper and J. Garland

Zinc Oxide Materials for Electronic and Optoelectronic Device Applications, Edited by C. Litton, D. C. Reynolds and T. C. Collins

Lead‐Free Solders: Materials Reliability for Electronics, Edited by K. N. Subramanian

Silicon Photonics: Fundamentals and Devices, M. Jamal Deen and P. K. Basu

Nanostructured and Subwavelength Waveguides: Fundamentals and Applications, M. Skorobogatiy

Photovoltaic Materials: From Crystalline Silicon to Third‐Generation Approaches, G. Conibeer and A. Willoughby

Glancing Angle Deposition of Thin Films: Engineering the Nanoscale, Matthew M. Hawkeye, Michael T. Taschuk and Michael J. Brett

Spintronics for Next Generation Innovative Devices, Edited by Katsuaki Sato and Eiji Saitoh

Physical Properties of High‐Temperature Superconductors, Rainer Wesche

Inorganic Glasses for Photonics ‐ Fundamentals, Engineering, and Applications, Animesh Jha

Forthcoming

Materials for Solid State Lighting and Displays, Adrian Kitai

Amorphous Semiconductors

Structural, Optical, and Electronic Properties

Kazuo Morigaki,

Emeritus, University of Tokyo, Japan

Sándor Kugler,

Department of Theoretical Physics,Budapest University of Technology and Economics,Hungary

Koichi Shimakawa

Centre of Innovative Photovoltaic Systems,Gifu University, Japan

This edition first published 2017© 2017 John Wiley & Sons Ltd

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Library of Congress Cataloging‐in‐Publication Data

Names: Morigaki, Kazuo, author. | Kugler, Sándor, 1950– author. | Shimakawa, Koichi, author.Title: Amorphous semiconductors : structural, optical, and electronic properties / Kazuo Morigaki, Sándor Kugler and Koichi Shimakawa.Description: Chichester, West Sussex, United Kingdom : John Wiley & Sons, Ltd, [2017] | Includes bibliographical references and index.Identifiers: LCCN 2016040352| ISBN 9781118757925 (cloth) | ISBN 9781118758205 (epub) | ISBN 9781118757949 (Adobe PDF)Subjects: LCSH: Amorphous semiconductors.Classification: LCC QC611.8.A5 M 2017 | DDC 537.6/226–dc23LC record available at https://lccn.loc.gov/2016040352

A catalogue record for this book is available from the British Library.

Series Preface

Wiley Series in Materials for Electronic and Optoelectronic Applications

This book series is devoted to the rapidly developing class of materials used for electronic and optoelectronic applications. It is designed to provide much‐needed information on the fundamental scientific principles of these materials, together with how these are employed in technological applications. The books are aimed at (postgraduate) students, researchers and technologists, engaged in research, development and the study of materials in electronics and photonics, and industrial scientists developing new materials, devices and circuits for the electronic, optoelectronic and communications industries.

The development of new electronic and optoelectronic materials depends not only on materials engineering at a practical level, but also on a clear understanding of the properties of materials, and the fundamental science behind these properties. It is the properties of a material that eventually determine its usefulness in an application. The series therefore also includes such titles as electrical conduction in solids, optical properties, thermal properties, and so on, all with applications and examples of materials in electronics and optoelectronics.

The characterization of materials is also covered within the series in as much as it is impossible to develop new materials without the proper characterization of their structure and properties. Structure–property relationships have always been fundamentally and intrinsically important to materials science and engineering.

Materials science is well known for being one of the most interdisciplinary sciences. It is the interdisciplinary aspect of materials science that has led to many exciting discoveries, new materials and new applications. It is not unusual to find scientists with a chemical engineering background working on materials projects with applications in electronics. In selecting titles for the series, we have tried to maintain the interdisciplinary aspect of the field, and hence its excitement to researchers in this field.

Arthur WilloughbyPeter CapperSafa Kasap

Preface

This book deals with amorphous semiconductors, which are typical disordered systems. Many textbooks and monographs concerning amorphous semiconductors have already been published. In this book, we treat their structural, optical, and electronic properties; in particular, recent developments in these areas are included and are emphasized as features of the book, because since amorphous semiconductors exhibit a disordered nature, there exist difficult problems to be resolved in the interpretation of experimental results. So, we attempt to present recent experimental data and interpretations in order to help in obtaining a deeper understanding of these properties.

The book is composed of eight chapters. The basic ideas for understanding the properties of amorphous semiconductors mentioned above are presented in these chapters, particularly preparation techniques in Chapter 2 and theoretical fundamentals in Chapter 4. In Chapter 3, structural properties are treated, with particular emphasis on neutron diffraction measurements and structural modeling of computer simulations. In Chapter 5, hydrogenated amorphous silicon is treated and emphasis is put on recent developments in the nature of shallow and deep states and of light‐induced defects, particularly as investigated by magnetic resonance measurements. In Chapter 6, amorphous chalcogenides are treated, with particular emphasis on their basic glass science, their unique properties, and light‐induced effects in them. In Chapter 7, other amorphous materials are treated, particularly amorphous carbon and amorphous semiconducting oxides. In Chapter 8, applications of amorphous semiconductors are mentioned, particularly those of hydrogenated amorphous silicon and amorphous chalcogenides. The eight chapters were written by the authors as follows: Kazuo Morigaki (KM) wrote Chapters 1 and 5 (except for Section 5.4.2, by (KS)), Sándor Kugler (SK) wrote Chapters 3 and 4, and Koichi Shimakawa (KS) wrote Chapters 2, 6, 7 (with contributions from SK), and 8. We hope that the book will be useful for students in physics, chemistry, and materials science as well as for researchers and engineers interested in amorphous semiconductors.

KS acknowledges Professors Keiji Tanaka (Hokkaido University, Japan), Safa Kasap (University of Saskatchewan, Canada), Jai Singh (Charles Darwin University, Australia), Takeshi Aoki (Tokyo Polytechnic University, Japan), Tomas Wagner, Miloslav Frumar (University of Pardubice, Czech Republic), and Noboru Yamada (Kyoto University, Japan), and Dr. Alexander Kolobov (Advanced Institute of Science and Technology, Tsukuba, Japan) for many fruitful discussions. KS also wishes to thank the Grant Project ReAdMat funded by the ESF for financial support. SK expresses his thanks to Krisztian Kohary (University of Exeter, UK) for helpful discussions. SK must also thank Tokyo Polytechnic University for providing him with computer facilities for his large‐scale computer simulations.

KM acknowledges Professor Harumi Hikita (Meikai University, Urayasu, Japan) for preparing some of the figures in Chapter 5.

Kazuo MorigakiSándor KuglerKoichi Shimakawa

1Introduction

1.1 General Aspects of Amorphous Semiconductors

Amorphous solids are typical disordered systems. Two classes of disorder can be defined, namely, compositional disorder as seen in crystalline binary alloys and topological disorder as seen in liquids. Amorphous solids have topological disorder. However, short‐range order, that is, chemical bonding of constituent atoms, exists in amorphous covalent semiconductors. Spatial fluctuations in the bond lengths, bond angles, and dihedral angles (Figure 1.1) give rise to tail states in the band gap region, that is, below the edge of the conduction band and above the edge of the valence band. The edges of the conduction and valence bands are called mobility edges and the band gap is called a mobility gap. These edges are the boundaries between delocalized and localized states. This is illustrated in Figure 1.2. Such boundaries are caused by disorder; this is called Anderson localization. In an amorphous network, translational order does not exist, so the Bloch theory of crystalline solids is not applicable, but the tight‐binding model, the Hartree–Fock approximation or the density functional method can be applied for understanding the electronic properties of amorphous semiconductors, as shown in Chapter 4.

Figure 1.1 Short‐range order structure and dihedral angle φ in amorphous silicon.

Figure 1.2 Schematic illustration of density of states in an amorphous semiconductor. See text for details.

Although the above considerations are based on a continuous random network, actual samples have a structure deviating from an ideal random network, that is, the coordination of the constituent atoms deviates from the normal coordination following the 8 − N rule [1], where N denotes the relevant column number in the periodic table. Here, we consider elements only in columns IV–VI of the periodic table. An additional rule can be given as follows: Z (the valency) = N if N < 4. For instance, the normal coordination of amorphous silicon is fourfold, but threefold‐coordinated silicon atoms are also present. These are called structural defects. For amorphous selenium, since the normal coordination is twofold, the structural defects are onefold‐ and threefold‐coordinated selenium atoms. The band tails and structural defects affect the optical and electronic properties of amorphous semiconductors. Thus it is very important to elucidate the electronic structures of these states in order to understand these properties. An important experimental means of doing this is magnetic resonance. For instance, the electronic structures of these localized states can be elucidated from electron spin resonance (ESR); that is, their symmetry can be determined from a g‐value measurement. However, the principal axes of symmetry are randomly oriented in an amorphous network, and that it makes more difficult to identify defects in amorphous semiconductors than in crystals. This identification is normally performed by comparison between observed ESR spectra and computer‐simulated spectra. In addition, electron–nuclear double resonance (ENDOR) provides us with a powerful means for identification of defects, as shown in Chapter 5, in which pulsed electron magnetic resonance measurements in particular are presented in detail.

In this book, we consider two types of material as examples of amorphous semiconductors, namely, amorphous silicon and other column IV elemental semiconductors, and amorphous chalcogenides, including amorphous metal chalcogenides. Their preparation, structure, and optical and electronic properties are presented in Chapters 2, 3, 5, 6, and 7. In Chapter 3, definitions of crystalline and noncrystalline structures are given, and the structures of amorphous silicon, hydrogenated amorphous silicon (a‐Si:H), and amorphous selenium are treated theoretically and experimentally in more detail.

1.2 Chalcogenide Glasses

Amorphous chalcogenides are also known as chalcogenide glasses, because they exhibit a glass transition. The details of the glass transition and the structural, optical, and electronic properties of these material are dealt with in Chapter 6.

1.3 Applications of Amorphous Semiconductors

Amorphous semiconductors are widely today used as device materials. Devices using a‐Si:H include, for example, solar cells and thin‐film transistors. Devices using amorphous chalcogenides include, for example, phase‐change memories, direct x‐ray image sensors for medical use, high‐gain avalanche rushing amorphous semiconductor vidicons, and optical fibers and waveguides. These are treated in Chapter 8.

There are several comprehensive books about amorphous semiconductors [1–5], as well as books about hydrogenated amorphous silicon [6] and amorphous chalcogenides [7].

References

1

Mott, N.F. and Davis, E.A. (1979)

Electronic Processes in Non‐Crystalline Materials

, 2nd edn, Clarendon Press, Oxford.

2

Elliott, S.R. (1990)

Physics of Amorphous Materials

, Longman Scientific and Technical, Harlow.

3

Morigaki, K. (1999)

Physics of Amorphous Semiconductors

, World Scientific, Singapore and Imperial College Press, London.

4

Singh, J. and Shimakawa, K. (2003)

Advances in Amorphous Semiconductors

, Taylor & Francis, London and New York.

5

Kugler, S. and Shimakawa, K. (2015)

Amorphous Semiconductors

, Cambridge University Press, Cambridge.

6

Street, R.A. (1991)

Hydrogenated Amorphous Silicon

, Cambridge University Press, Cambridge.

7

Tanaka, K. and Shimakawa, K. (2011)

Amorphous Chalcogenide Semiconductors and Related Materials

, Springer, New York.

2Preparation Techniques

There are many preparation techniques, depending on what kind of material is needed. Quenching from the liquid state, also termed melt‐quenching (MQ), is a popular technique for so‐called glasses. To meet the requirements of thin‐film forms, a variety of techniques such as evaporation, sputtering, and chemical vapor deposition (CVD) are adopted. Ion bombardment and the use of powerful light on crystalline solids can also produce amorphous materials. Here we briefly summarize the principal methods for preparing hydrogenated amorphous silicon (a‐Si:H) films and amorphous chalcogenides.

2.1 Growth of a‐Si:H Films

a‐Si:H films, as well as a‐Ge:H, a‐C:H, and related compound films, are prepared only by condensation from the gas phase. The most popular preparation techniques for these films belong to the class of CVD techniques. Two CVD techniques are known: one is glow‐discharge deposition, which is now also called plasma‐enhanced chemical vapor deposition (PECVD), and the other is hot‐wire chemical vapor deposition (HWCVD), as described in the following.

2.1.1 PECVD Technique

This was previously called glow‐discharge deposition, when a‐Si:H films were deposited by decomposition of SiH4 gas with the help of a glow discharge [1]. It is now termed plasma‐enhanced chemical vapor deposition [2]. The application of an RF field (usually at 13.6 MHz; RF PECVD) generates a plasma in a reaction chamber. By introducing PH3 or B2H6 gas into the SiH4, n‐ or p‐type Si:H films can be prepared [1]. RF PECVD provides high‐quality uniform films, but the deposition rate can be slow as 0.3 nm/s. The development of this technique, with control of the gas pressure (0.05–2 Torr), RF power (10–100 mW/cm2), substrate temperature (150–350 °C), and so on, has led to a huge range of commercial applications, such as large‐area photovoltaics and thin‐film transistors.