Amorphous Silicon

Amorphous Silicon Kazunobu Tanaka National Institute for Advanced Interdisciplinary Research, Ibaraki, Japan Eiichi Maruyama Hitachi Ltd, Ibaraki, Japan Toshikazu Shimada Hitachi Ltd, Ibaraki, Japan Hiroaki Okamoto Osaka University, Osaka, Japan Translated by Takeshi Sato, National Institute for Advanced Interdisciplinary Research, Ibaraki, Japan Amorphous silicon has substantially different properties as compared to crystalline silicon. It has therefore become recognized as a fascinating and important material in its own right, with many interesting facets that lead to a range of novel and still developing applications. Amorphous Silicon introduces the reader to this field by first discussing what is meant by the amorphous state. It details the way in which amorphous silicon is prepared, and the growth mechanism. The main structural, optical and electronic properties are then covered in detail, and there is a full chapter on the structural stability of the material, including photoinduced effects. Finally, a number of the most exciting applications of amorphous silicon are presented, including its use in solar cells, photo-sensors and liquid crystal displays. Amorphous Silicon will be of great interest to all those working in solid state physics or chemistry, materials science and electronic engineering, from postgraduate students to more experienced workers in these fields.

Kazunobu Tanaka is the author of Amorphous Silicon, published by Wiley.

Preface to the Japanese Edition

ix

(4)

Preface to the English Edition

xiii

1 Introduction

1

(26)

1.1 What is the Amorphous State?

1

(9)

1.1.1 Definition of Terms

1

(1)

1.1.2 The Thermodynamic State

2

(3)

1.1.3 Structural Disorder

5

(3)

1.1.4 Classification of Amorphous Semiconductors

8

(2)

1.2 A Short History of Amorphous Semiconductor Studies

10

(14)

1.2.1 Pioneering Work

10

(7)

1.2.2 Advent of Hydrogenated Amorphous Silicon

17

(4)

1.2.3 Academic Societies and Communities

21

(3)

1.3 Organization of the Book

24

(1)

References

25

(2)

2 Methods of Preparation and Growth Processes

27

(31)

2.1 Outline of Preparative Methods

27

(10)

2.1.1 Non-Equilibrium Processes and Amorphous Structures

27

(4)

2.1.2 Preparation of Amorphous Silicon

31

(6)

2.2 Formation of SiH(4) Plasma

37

(10)

2.2.1 Plasma Reactor Construction and Electron Energy Distribution

37

(2)

2.2.2 Electron Impact Dissociation of SiH(4)

39

(2)

2.2.3 Steady-State Conditions of the SiH(4) Plasma: Merits of Plasma Diagnosis

41

(6)

2.3 Growth Mechanisms

47

(9)

2.3.1 Lifetimes and Diffusion of Species in the SiH(4) Plasma

47

(1)

2.3.2 Adsorption/Desorption of Species and Growth Rates

48

(4)

2.3.3 Surface-Process Model

52

(1)

2.3.4 Surface Diffusion and Network Structure

53

(3)

References

56

(2)

3 Structural Properties

58

(45)

3.1 Morphology (Macroscopic Aspects)

58

(2)

3.2 Hydrogen in a-Si:H Films

60

(8)

3.2.1 Silicon-Hydrogen Bond Types

60

(3)

3.2.2 Spatial Distribution of Hydrogen

63

(4)

3.2.3 Thermal Stability of Hydrogen

67

(1)

3.3 Density

68

(3)

3.3.1 Hydrogen-Free Amorphous Silicon

68

(1)

3.3.2 Hydrogenated Amorphous Silicon

69

(2)

3.4 Structural Order

71

(8)

3.4.1 Amorphous Structure and Hierarchy of Order

71

(1)

3.4.2 Hydrogen-Free Amorphous Silicon

72

(3)

3.4.3 Hydrogenated Amorphous Silicon

75

(4)

3.5 Defects

79

(16)

3.5.1 Concepts and Classification of Defects

79

(3)

3.5.2 Electron Spin Resonance Spectroscopy Studies: Dangling Bonds Versus Floating Bonds

82

(3)

3.5.3 Defect Density and Hydrogen in Films

85

(4)

3.5.4 Thermal Equilibrium Process in Defect Formation

89

(6)

3.6 Impurities

95

(1)

3.7 Doping

96

(4)

3.7.1 Doping Characteristics and Efficiencies: Phosphorus and Boron

96

(2)

3.7.2 Hydrogen in Doped a-Si:H

98

(1)

3.7.3 Electron Spin Resonance Studies

99

(1)

References

100

(3)

4 Optical and Electrical Properties

103

(84)

4.1 Disorder and Electronic Structure

103

(15)

4.1.1 Electronic Structure of Amorphous Semiconductors

103

(2)

4.1.2 Conduction Band and Valence Band

105

(2)

4.1.3 Electronic State Near the Band Edge

107

(4)

4.1.4 Localized Levels Based on Structural Defects

111

(7)

4.2 Structure of the Optical Absorption Edge

118

(14)

4.2.1 Features of the Optical Absorption Spectrum

118

(2)

4.2.2 Inter-Band Transition Region

120

(4)

4.2.3 Band-Tail Transition

124

(4)

4.2.4 Optical Transitions Caused by Structural Defects

128

(4)

4.3 Electrical Properties

132

(15)

4.3.1 Mechanism of Electrical Conduction

132

(4)

4.3.2 DC Conductivity and Thermoelectric Power

136

(4)

4.3.3 Hall Effect

140

(2)

4.3.4 Impurity Doping Effect

142

(5)

4.4 Transport and Recombination of Excess Carrier

147

(21)

4.4.1 Carrier Capture and Non-Radiative Recombination

147

(3)

4.4.2 Recombination and Photoconductivity Under Steady Illumination Conditions

150

(5)

4.4.3 Carrier Transport Characteristics Under Transient Illumination Conditions

155

(4)

4.4.4 Mobility and Lifetime of Carriers

159

(7)

4.4.5 Photoluminescence

166

(2)

4.5 Silicon Alloy Materials and Multilayer Film Properties

168

(11)

4.5.1 Basic Properties of Silicon Alloy Materials

168

(2)

4.5.2 Properties of Multilayer Films

170

(9)

References

179

(8)

5 Structural Stability and Photo-Induced Effects

187

(31)

5.1 Nature of the Non-Equilibrium Solid

187

(2)

5.2 Photo-Induced Effects in Amorphous Semiconductors

189

(5)

5.2.1 Photodarkening and Photostructural Changes

189

(1)

5.2.2 Photo-Induced Metastable ESR Centers

189

(3)

5.2.3 Structural Origins of Photo-Induced Metastability

192

(2)

5.3 The Staebler-Wronski Effect

194

(14)

5.3.1 Description of the Phenomenon

194

(3)

5.3.2 Aspects of Kinetics

197

(3)

5.3.3 Structural Origins and Mechanisms of Defect Formation: Microscopic Models

200

(5)

5.3.4 Phosphorus-Doped a-Si:H

205

(3)

5.4 Thermal Stability

208

(7)

5.4.1 Diffusion and Desorption of Hydrogen and Crystallization of Amorphous Films

208

(4)

5.4.2 Crystallization Processes in Multilayer Films

212

(3)

References

215

(3)

6 Applications

218

(36)

6.1 Features and Applications of Amorphous Semiconductors

218

(6)

6.2 Examples of Applications of Amorphous Semiconductors

224

(29)

6.2.1 Solar Cells

224

(7)

6.2.2 Solid-State Photosensors

231

(5)

6.2.3 Field-Effect (Thin-Film) Transistors for Liquid Crystal Displays

236

(6)

6.2.4 Photoreceptor Drums for Electrophotography and Laser-Beam Printers

242

(3)

6.2.5 Amorphous Selenium Vidicon-Type Image Pick-Up Tubes

245

(8)

References

253

(1)

Index

254

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Summary

Author Biography

Table of Contents

Supplemental Materials