Metal Oxide Semiconductors E-Book

Zhigang Zang

• Herausgeber: Wiley-VCH GmbH
• Kategorie: Fachliteratur
• Sprache: Englisch

Seitenzahl: 567

Veröffentlichungsjahr: 2023

Beschreibung

Metal Oxide Semiconductors

Up-to-date resource highlighting highlights emerging applications of metal oxide semiconductors in various areas and current challenges and directions in commercialization

Metal Oxide Semiconductors provides a current understanding of oxide semiconductors, covering fundamentals, synthesizing methods, and applications in diodes, thin-film transistors, gas sensors, solar cells, and more. The text presents state-of-the-art information along with fundamental prerequisites for understanding and discusses the current challenges in pursuing commercialization and future directions of this field.

Despite rapid advancements in the materials science and device physics of oxide semiconductors over the past decade, the understanding of science and technology in this field remains incomplete due to its relatively short research history; this book aims to bridge the gap between the rapidly advancing research progress in this field and the demand for relevant materials and devices by researchers, engineers, and students.

Written by three highly qualified authors, Metal Oxide Semiconductors discusses sample topics such as:

• Fabrication techniques and principles, covering vacuum-based methods, including sputtering, atomic layer deposition and evaporation, and solution-based methods
• Fundamentals, progresses, and potentials of p–n heterojunction diodes, Schottky diodes, metal-insulator-semiconductor diodes, and self-switching diodes
• Applications in thin-film transistors, detailing the current progresses and challenges towards commercialization for n-type TFTs, p-type TFTs, and circuits
• Detailed discussions on the working mechanisms and representative devices of oxide-based gas sensors, pressure sensors, and PH sensors
• Applications in optoelectronics, both in solar cells and ultraviolet photodetectors, covering their parameters, materials, and performance
• Memory applications, including resistive random-access memory, transistor-structured memory devices, transistor-structured artificial synapse, and optical memory transistors

A comprehensive monograph covering all aspects of oxide semiconductors, Metal Oxide Semiconductors is an essential resource for materials scientists, electronics engineers, semiconductor physicists, and professionals in the semiconductor and sensor industries who wish to understand all modern developments that have been made in the field.

Leseprobe

Table of Contents

Cover

Table of Contents

Title Page

Copyright

Preface

1 Metal Oxide Semiconductors: State-of-the-Art and New Challenges

1.1 Introduction

1.2

n

-Type Metal Oxide Semiconductors

1.3

p

-Type Metal Oxide Semiconductors

References

2 Fabrication Techniques and Principles

2.1 Introduction

2.2 Vacuum-Based Methods

2.3 Solution-Based Methods

References

3 Metal Oxide Semiconductors for Diodes

3.1 Introduction

3.2

P

–

N

Heterojunction Diodes

3.3 Schottky Diodes

3.4 Metal–Insulator–Semiconductor Diodes

3.5 Self-Switching Diodes

References

4 Metal Oxide Semiconductors for Transistors

4.1 Introduction

4.2 Device Structures and Mechanisms

4.3

N

-Type TFTs

4.4

P

-Type TFTs

4.5 Circuit Applications

References

5 Metal Oxide Semiconductors for Sensors

5.1 Introduction

5.2 Metal Oxide-Based Gas Sensors

5.3 Metal Oxide-Based Pressure Sensors

5.4 Metal Oxide-Based pH Sensors

References

6 Metal Oxide Semiconductors for Solar Cells

6.1 Introduction

6.2 Solar Cell Principles

6.3 Metal Oxide Solar Cells

6.4 Metal Oxide Functional Layers in Solar Cells

References

7 Metal Oxide Semiconductors for Ultraviolet Photodetectors

7.1 Introduction

7.2 Device Structures of UV Photodetectors

7.3 Important Parameters of UV Photodetectors

7.4 Materials and Performance of UV Photodetectors

7.5 Conclusion and Outlooks

References

8 Metal Oxide Semiconductors for Memory Applications

8.1 Introduction

8.2 Resistive Random-Access Memory

8.3 Transistor-Structured Memory Devices

8.4 Other Memory Devices

References

Index

End User License Agreement

List of Tables

Chapter 3

Table. 3.1 A summary of representative MIOS Schottky diodes.

Chapter 4

Table. 4.1 Representative low-temperature (<300°C),...

Table. 4.2 A summary of representative SnO TFTs deposited by sputtering.

Table. 4.3 A summary of NMOS-type inverters.

Chapter 5

Table. 5.1 Gas-sensing performance of MOS sensors toward ethanol.

Table. 5.2 Gas-sensing performance of MOS sensors toward formaldehyde.

Table. 5.3 Gas-sensing performance of MOS sensors toward acetone.

Table. 5.4 Gas-sensing performance of MOS sensors toward toluene.

Table. 5.5 Gas-sensing performance of MOS sensors toward CO

2

.

Table. 5.6 Gas-sensing performance of MOS sensors toward NH

3

.

Table. 5.7 Gas-sensing performance of MOS sensors toward NO

2

.

Table. 5.8 Gas-sensing performance of MOS sensors toward humidity.

Table. 5.9 Gas-sensing performance of MOS sensors toward CH

4

.

Table. 5.10 Gas-sensing performance of MOS sensors toward H

2

.

Table. 5.11 Gas-sensing performance of MOS sensors toward LPG.

Table. 5.12 Gas-sensing performance of MOS sensors toward warfare agent.

Chapter 6

Table. 6.1 Survey of the Cu

2

O solar cell performances, gathered from the wor...

Table. 6.2 Parameters of Cu

2

O and CuO.

Table. 6.3 PV performance of TiO

2

-based DSSCs.

Table. 6.4 Nanostructures of ZnO films, Preparation Methods, and PV paramete...

Table. 6.5 Nanostructures of Nb

2

O

5

films, preparation methods, and PV parame...

Table. 6.6 Efficiency record chart of inorganic PSCs with TiO

2

ETLs.

Table. 6.7 Efficiency record chart of inorganic PSCs with SnO

2

ETLs.

Table. 6.8 Efficiency record chart of inorganic PSCs with ZnO ETLs.

Chapter 8

Table. 8.1 The periodic table of the elements, summarizing the elements that...

List of Illustrations

Chapter 1

Figure 1.1 Applications of metal oxide semiconductors.

Figure 1.2 Stick and ball representation of ZnO crystal structures: (a) cubi...

Figure 1.3 Bulk structures of SnO

2

polymorphs (gray and red colors represent...

Figure 1.4 Ball and stick representations of crystal structures of

bcc

-In

2

O

3

Figure 1.5 Structure of anatase, rutile, and brookie TiO

2

.

Figure 1.6 Crystal structure Ga

2

O

3

polymorphs. (a) Rhombohedral crystal stru...

Figure 1.7 Crystal structure of binary copper oxides. (a) Cubic Cu

2

O and (b)...

Figure 1.8 (a) Crystal structure of SnO. (b) Schematic illustration of the v...

Figure 1.9 (a) Crystal structure of NiO

x

.(b) Flow diagram for NiO films ...

Chapter 2

Figure 2.1 A schematic diagram showing the sputtering process.

Figure 2.2 (a) A cross-sectional TEM image of a-IGZO TFTs.(b) Electrical...

Figure 2.3 (a) Phase stability map showing the narrow window of obtaining p-...

Figure 2.4 Schematic diagrams showing the ALD process.

Figure 2.5 SEM images of ZnO using (a) H

2

O and (b) O

3

as the oxidant. (c) XR...

Figure 2.6 Schematic diagrams showing (a) thermal evaporation and (b) e-beam...

Figure 2.7 Schematic diagrams showing the hydrothermal synthesis.

Figure 2.8 TEM images of TiO

2

colloids prepared by (a) microwave and (b) con...

Figure 2.9 A schematic diagram showing the electrospinning process and possi...

Figure 2.10 Schematic diagrams showing the growth mechanism of ZnO nanowires...

Figure 2.11 FE-SEM images in (a) low and (b) high resolutions, and (c) XRD p...

Figure 2.12 (a) Schematic diagrams showing the synthesis of TiO

2

nanosheets....

Figure 2.13 IZO film characterizations with (a) TGA of precursors, (b) a TEM...

Chapter 3

Figure 3.1 Different types of heterojunctions. (a) Type I, (b) type II and (...

Figure 3.2 (a) Typical

I–V

characteristics of

n

-ZnO/

p

-NiO heterojuncti...

Figure 3.3 (a)

I–V

characteristics under both dark and UV illumination...

Figure 3.4 A schematic energy band diagram of metal/oxide semiconductor cont...

Figure 3.5 (a)

I–V

characteristics and (b) extracted ideality factor o...

Figure 3.6 (a) Cross-sectional HRTEM images of a representative ZnO coplanar...

Figure 3.7 (a) Generic diagrams of adhesion lithography process steps for th...

Figure 3.8 (a)

J–V

characteristics of IGZO Schottky diodes with differ...

Figure 3.9 (a)

J–V

and (b)

C

−2

–V

characteristics of IGZO S...

Figure 3.10 (a)

J–V

characteristics of β−Ga

2

O

3

Schottky diodes measure...

Figure 3.11 (a)

C–V

and (b)

G/ω–V

characteristics of Al/(Zn...

Figure 3.12 (a) A schematic diagram showing the diode structure. (b) Electri...

Figure 3.13 (a) Spectral responsivity of the MgZnO-based MS and MIS photodet...

Figure 3.14 (a)

I–V

characteristics with the inset showing the device ...

Figure 3.15 (a) An SEM image of a typical SSD. (b) The depletion region form...

Chapter 4

Figure 4.1 Schematic diagrams of basic TFT structures. (a) Top-gate top-cont...

Figure 4.2 Cross-sectional views of an ideal

n

-type TFT in (a) cut-off, (b) ...

Figure 4.3 Typical (a) output and (b) transfer characteristics of

n

-type TFT...

Figure 4.4 Schematic orbital structures of the CBM states in Si and an ionic...

Figure 4.5 (a)

μ

FE

and (b)

V

ON

obtained for TFTs with different oxide c...

Figure 4.6 Transfer characteristics of IGZO TFTs gated with (a) Al

x

O

y

and (b...

Figure 4.7 (a) Transfer characteristics and (b) corresponding cross-sectiona...

Figure 4.8 Electrical performance of ITO TFTs. (a-c) Device structures and (...

Figure 4.9 Transfer characteristics of IGZO TFTs gated with (a) bare HfO

x

an...

Figure 4.10 Transfer characteristics of IGZO TFTs (a) without and (b) with O...

Figure 4.11 The excepted electronic structures and electrical properties of ...

Figure 4.12 Energy levels of different oxide semiconductors arranged in orde...

Figure 4.13 (a) Schematic diagrams of the condensation mechanism of DUV-irra...

Figure 4.14 (a) Depiction of the two different synthetic approaches. (b) Ene...

Figure 4.15 Schematics of (a) Type-I and (b) Type-II transistor architecture...

Figure 4.16 (a) Schematic diagrams showing the effects of Ga doping on hole ...

Figure 4.17 Electrical performance of SnO TFTs with (a) the output character...

Figure 4.18 (a) Phase-stability and (b) Hall mobility as a function of total...

Figure 4.19 (a) Output and (b) transfer characteristics of ambipolar SnO TFT...

Figure 4.20 Electrical performance of NiO

x

TFTs. (a) Al

2

O

3

quality. (b) Tran...

Figure 4.21 Schematic diagrams of NMOS inverters using (a) enhancement-mode ...

Figure 4.22 Schematic diagrams and real images of the complementary circuits...

Chapter 5

Figure 5.1 Formation of electronic core-shell structures in (a)

n

-type and (...

Figure 5.2 (a) Circuitry for simultaneous resistance and work function measu...

Figure 5.3 TEM images of (a) ZnO nanoparticles (NPs) and (b) ZnO nanorods (N...

Figure 5.4 Schematic of proposed NO

2

gas-sensing mechanism of the TiO

2

senso...

Figure 5.5 Sintered block type gas sensors: (a) a digital photo. Source: Rep...

Figure 5.6 (a) An FESEM image of TiO

2

flower-like microstructures. (b) A sch...

Figure 5.7 (a) SEM and (b) TEM images of SnO

2

-Fe

2

O

3

interconnected nanotubes...

Figure 5.8 TEM images of (a) La

2

O

3

and (b) Pd- La

2

O

3

thin films. The variati...

Figure 5.9 (a) The sensitivity of obtained materials to 200-ppm ammonia as a...

Figure 5.10 Schematic illustrations of NH

3

-sensing mechanism of BP-TiO

2

sens...

Figure 5.11 The diagram of sensing mechanism for N-MXene-18 sensors.

Figure 5.12 (a) A TEM image of BP nanosheets-ZnO nanowires composites. (b) A...

Figure 5.13 The adsorption of water molecules on ZnO nanosheets.

Figure 5.14 (a) An optical microscopy image of prepared sensing device. The ...

Figure 5.15 (a) Variations of response to 1000 ppm CH

4

with temperature for ...

Figure 5.16 (a) An SEM image of different contents of CeO

2

-loaded In

2

O

3

holl...

Figure 5.17 (a) LPG response upon exposure of 2600-ppm LPG as a function ope...

Figure 5.18 Schematic diagrams showing the reaction of WO

3

/CuO NFs with oxyg...

Figure 5.19 Schematic illustrations of three common transduction mechanisms ...

Figure 5.20 (a) A schematic and (b) an optimal image showing a single Sb-dop...

Figure 5.21 (a) A schematic diagram of the measurement system used to charac...

Figure 5.22 Schematic diagrams of three pH sensors. (a) Potentiometric pH se...

Chapter 6

Figure 6.1 General principles of PVs. (a) Upon photon absorption, an electro...

Figure 6.2 (a) Schematic drawing of an all-oxide thin-film PV cell consistin...

Figure 6.3 Experimental procedures for Cu

2

O/ZnO nanopillar heterostructures....

Figure 6.4 SEM images of ZnO nanopillars and Cu

2

O/ZnO nanopillar heterostruc...

Figure 6.5 Current density vs. voltage of Cu

2

O/ZnO nanopillar solar cells un...

Figure 6.6 SEM images of several ZnO structures. (a) ZnO NWs with Cu

2

O....

Figure 6.7 Band alignment affects

V

OC

values. (a) Nonoptimized and (b) optim...

Figure 6.8 (a) A photograph of the fabricated Cu

2

O solar module. (b) A cross...

Figure 6.9 Schematic diagrams of the device structure.

Figure 6.10 A Schematic of dye solar cell.

Figure 6.11 (a) Core/shell model and the corresponding energy diagram. (b) A...

Figure 6.12 SEM images of ZnO nanostructures. (a) Bilayer urchin-like ZnO-NW...

Figure 6.13 (a) Schematic diagrams of the charge transfer process in PSCs. (...

Figure 6.14 (a–f) Interface modification by SmBr

3

. Source: Reproduced with p...

Figure 6.15 (a) The scheme of device architecture. (b) Energy band alignment...

Figure 6.16 (a) Dipole formation and (b) the energy level alignments at the ...

Figure 6.17 (a) Schematic view of different ETL with bandgap alignment and (...

Figure 6.18 (a) Schematic illustrations of

p-i-n

CsPbIBr

2

PSCs with a NiO

x

H...

Chapter 7

Figure 7.1 Categories of UV photodetectors.

Figure 7.2 Schematic structures of typical UV photodetectors. (a) Photocondu...

Figure 7.3 A schematic diagram of an operating Schottky junction in Schottky...

Figure 7.4 A schematic diagram showing an MSM detector with interdigitated f...

Figure 7.5 (a)

I

–

V

responses recorded with and without 365-nm UV illuminatio...

Figure 7.6 Variation of photodetector responsivity of UV photodetectors with...

Figure 7.7 Schematic device structures. (a) ITO/ZnO/Ag photodetectors and (b...

Figure 7.8 (a) Schematic illustrations of the fabrication process for ZnO an...

Figure 7.9 (a) Spectral photoresponse of a 400-μm diameter Sch...

Figure 7.10 (a) Scanning electron microscope and atomic force microscope ima...

Figure 7.11 (a) The morphology of bulk β-Ga

2

O

3

. (b)...

Figure 7.12 (a) The morphology of synthesized β-Ga

2

O...

Figure 7.13 (a) Schematic diagrams of Ga

2

O

3

nanowire network-based MSM photo...

Figure 7.14 Schematics showing the crystal structure of β-Ga

2

...

Figure 7.15 (a) A structure diagram of UV photodetectors based on TiO

2

/NiO

p

...

Figure 7.16 (a) The device structure of self-powered PDs. (b)

I–t

curv...

Figure 7.17 (a) A schematic diagram of Au/CIS/TiO

2

/FTO heterojunction PDs. (...

Figure 7.18 (a) The schematic illustration of performance measurement for se...

Figure 7.19 (a) A SEM image of the membrane of the SnO

2

HoMSs with an inset ...

Figure 7.20 (a) The schematic diagram, (b) the current–time curve and enlarg...

Figure 7.21 (a) A schematic diagram of a Zn

1−

x

Mg

x

O/PANI heterojunction...

Chapter 8

Figure 8.1 Catalogues of standard semiconductor memories.

Figure 8.2 (a) A schematic diagram of RRAMs. (b) Representative

I–V

ch...

Figure 8.3 High-resolution TEM images of (a) Ti

4

O

7

and (b) Ti

5

O

9

filaments....

Figure 8.4 A schematic showing the deposition of amorphous IGZO and

I–V

...

Figure 8.5 Schematic diagrams showing the resistive switching mechanisms of ...

Figure 8.6 (a) Five configurations used to measure

I

reset

−

I

comp

characterist...

Figure 8.7 Schematic diagrams of flash memory devices with (a) NOR and (b) N...

Figure 8.8 Schematic diagram of a (a) normal thin-film transistor, (b) float...

Figure 8.9 Transfer characteristics of IGZO transistors (a) without and (b) ...

Figure 8.10 (a) A schematic of the device structure. Transfer characteristic...

Figure 8.11 Schematic diagrams showing the different operating mechanisms of...

Figure 8.12 (a) A schematic showing the device structure of the ferroelectri...

Figure 8.13 (a) The multilevel states of the ferroelectric TFTs. (b) Schemat...

Figure 8.14 Schematic diagrams of (a) TSAS and (b) OMTs.

Figure 8.15 Schematic diagrams of (a) and (c) ion migration, and (b) and (d)...

Figure 8.16 (a) Device structure and cross-sectional TEM image of IGZO memor...

Figure 8.17 (a) Cross-sectional TEM images and a schematic diagram of the de...

Guide

Cover

Table of Contents

Title Page

Copyright Page

Preface

Begin Reading

Index

End User License Agreement

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Metal Oxide Semiconductors

Synthesis, Properties, and Devices

Zhigang ZangWensi CaiYong Zhou

The Authors

Prof. Zhigang ZangChongqing University174 Shazheng Street1206 Main Building, Campus AChongqingChina400044

Dr. Wensi CaiChongqing University174 Shazheng Street1410 Main Building, Campus AChongqingChina400044

Prof. Yong ZhouChongqing University174 Shazheng Street1412 Main Building, Campus AChongqingChina400044

Cover Image: © Yuichiro Chino/Getty Images

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Print ISBN: 978-3-527-35225-8ePDF ISBN: 978-3-527-84254-4ePub ISBN: 978-3-527-84256-8oBook ISBN: 978-3-527-84255-1

Preface

Electronics are undergoing a disruptive evolution, transitioning from heavy, rigid, and bulky devices to lightweight, soft, and flexible ones. This shift has given rise to emerging applications such as epidermal devices, artificial skins, and transparent displays, which have the potential to revolutionize our daily lives. However, traditional semiconductors like silicon (Si) face challenges in meeting the requirements of flexibility, lightweightness, transparency, and biocompatibility for these new applications.

In recent years, metal oxide semiconductors have emerged as a novel class of semiconductors and have garnered significant attention. Compared to traditional Si-based technologies, oxide semiconductors offer unique advantages such as high transparency and flexibility, making them highly promising for next-generation wearable and transparent electronics. Currently, thin-film transistors (TFTs) with an InGaZnO channel layer have already been commercialized for driving high-definition and energy-saving LCDs in smartphones, tablets, PC monitors, and OLED TVs. Additionally, oxide semiconductors are being studied for applications in diodes, gas sensors, solar cells, photodetectors, and memory devices, challenging conventional semiconductor technologies.

Despite rapid advancements in the materials science and device physics of oxide semiconductors over the past decade, the understanding of science and technology in this field remains incomplete due to its relatively short research history and the differences in chemical bonding between oxides and covalent-type semiconductors. Nevertheless, oxide semiconductors hold significant promise, particularly in the backplane of advanced displays and gas sensors, with applications in solar cells, photodetectors, and memory devices on the horizon. Regrettably, no comprehensive monograph has been published to date that covers all aspects of oxide semiconductors. This book aims to bridge the gap between the rapidly advancing research progress in this field and the demand for relevant materials and devices by researchers, engineers, and students.

The monograph provides a current understanding of oxide semiconductors and their applications in electronics and optoelectronics. It covers fundamentals, synthesizing methods, and applications in diodes, TFTs, gas sensors, solar cells, UV detectors, and memories. We present state-of-the-art information along with fundamental prerequisites for understanding, discuss the current challenges in pursuing commercialization, and provide our insights to guide the future directions of this field.

We express our gratitude to our colleagues at Chongqing University for their assistance in organizing the seminar and editing this book. Special thanks go to Dr. Huaxin Wang for editing Chapter 6 and Dr. Shuangyi Zhao for editing Chapter 7. We also extend our appreciation to the editor of Wiley for inviting us to write this book, and we are delighted to have the opportunity to publish this book on the novel class of metal oxide semiconductors.

Zhigang ZangWensi CaiYong Zhou

1Metal Oxide Semiconductors: State-of-the-Art and New Challenges

1.1 Introduction

Metal oxide semiconductors (MOS) are abundant materials found in the Earth's crust and are commonly used in traditional ceramics. However, they differ significantly from conventional inorganic counterparts like silicon and III–V compounds in various aspects. These distinctions encompass materials design concepts, electronic structure, charge transport mechanisms, defect states, thin-film processing, and optoelectronic properties. As a result, oxide semiconductors enable the realization of both established and innovative functionalities [1].

In comparison to inorganic semiconductors, oxide semiconductors possess unique characteristics. These include exceptional carrier mobilities even in the amorphous state, resilience against mechanical stress, compatibility with organic dielectric and photoactive materials, and high optical transparency. These properties make oxide semiconductors particularly appealing for various applications.

In recent decades, MOS have garnered significant attention in various research fields, including optoelectronics, thin-film transistors (TFTs), photocatalysts, gas sensors, solar cells, and memristors [1–14], as shown in Figure 1.1. MOS can be categorized into two types based on their conductivity: n-type, where electrons are the majority carriers, and p-type, where holes are the majority carriers. These semiconducting properties arise from factors such as doped aliovalent cations or oxygen nonstoichiometry [15, 16]. Among the n-type MOS, ZnO, SnO2, TiO2, In2O3, and Ga2O3 are widely studied concerning synthesis, characterization, and applications [17–21]. As for the p-type MOS, research efforts primarily focus on CuxO (CuO and Cu2O), SnO, and NiOx [22–24].

1.2 n-Type Metal Oxide Semiconductors

1.2.1 ZnO

Zinc oxide (ZnO), as an n-type semiconductor, has sparked significant research interest due to its distinctive physical and chemical properties [25–27]. In the field of materials science, ZnO is classified as a II–VI compound semiconductor, possessing a covalence level between ionic and covalent semiconductors. Its attributes, such as a direct wide bandgap (Eg ∼ 3.3 eV at 300 K), large free exciton binding energy (60 meV), and high thermal and mechanical stability at room temperature, make ZnO a promising candidate for various applications in electronic devices, optoelectronics, gas sensors, and laser technology [28, 29]. Additionally, ZnO can be utilized as an energy collector due to its piezo- and pyroelectric properties, as well as a photocatalyst for hydrogen production [30–32].

Figure 1.1 Applications of metal oxide semiconductors.

Source: Reprinted with permission from Refs. [9–14]. Copyright 2015 American Chemical Society, 2016 American Chemical Society, 2020 American Chemical Society, 2020 The Royal Society of Chemistry.

The crystal structure of ZnO can be classified into three types: wurtzite (B4), zinc blende (B3), and rocksalt (B1), as shown in Figure 1.2. Thereinto, wurtzite is the most stable thermodynamic phase under ambient conditions. Wurtzite ZnO possesses a hexagonal structure (space group C6mc) with lattice parameters a = 0.3296 nm and c = 0.52065 nm. The structure of ZnO can be simply described as a number of alternating planes composed of tetrahedrally coordinated O2− and Zn2+ ions, stacked alternately along the c-axis. The tetrahedral coordination in ZnO results in noncentral symmetric structures and consequently piezoelectricity and pyroelectricity [33].

Figure 1.2 Stick and ball representation of ZnO crystal structures: (a) cubic rocksalt (B1), (b) cubic zinc blende (B3), and (c) hexagonal wurzite (B4). The gray and red spheres denote Zn and O atoms, respectively.

Figure 1.3 Bulk structures of SnO2 polymorphs (gray and red colors represent Sn and O atoms, respectively). (a) Rutile (P42/mnm) and CaCl2-type (Pnnm), (b) R-PbO2-type (Pbcn), (c) pyrite-type (Pa), (d) ZrO2-type (Pbca), (e) fluorite-type (Fmhm), and (f) cotunnite-type (Pnam).

Source: Reproduced from Gracia et al. [40]/with permission of American Chemical Society.

1.2.2 SnO2

SnO2 is also a wide-bandgap (∼3.6 eV) n-type semiconductor, belongs to the group-IV compounds, and exhibits remarkable transparency and conductivity simultaneously [20, 34, 35]. The unique chemical, electronic, and optical properties of SnO2 have led to extensive research on its applications in various devices, including solar cells [36], catalytic materials [37], and gas sensors [38]. Additionally, SnO2 serves as a distinctive transparent metal oxide and finds wide-ranging applications as transparent conducting oxide electrodes in optoelectronic devices. This is due to its excellent chemical and thermal stabilities in atmospheric environments, as well as its high optical transmission properties [39].

SnO2 possesses several polymorphs including rutile-type (P42/mnm), CaCl2-type (Pnnm), a-PbO2-type (Pbcn), pyrite-type (Pa), ZrO2-type orthorhombic phase I (Pbca), fluorite-type (Fmm), and cotunnite-type orthorhombic phase II (Pnam) with ninefold coordination, as shown in Figure 1.3. All these structures are sequentially obtained when the most commonly available and stable rutile phase is subjected to a high mechanical pressure [40].

1.2.3 In2O3

Another widely studied n-type semiconductor is indium oxide (In2O3), which has a bandgap ranging from 3.5 to 3.7 eV. It finds numerous applications in electronic and optoelectronic fields such as solar cells, gas sensors based on TFTs, and Schottky contacts and diodes [21, 41]. In2O3 can exist in two well-established crystal structures: body-centered cubic (bcc) and rhombohedral (rh), as depicted in Figure 1.4[42]. The phase of In2O3 thermodynamically stable under ambient conditions adopts the body-centered cubic bixbyite structure, with the space group Ia (#206) and a lattice constant of 10.118 Å. The rhombohedral structure is stabilized under high-pressure conditions. The rhombohedral cell belongs to the Rc space group with lattice constants a = 5.478 Å and c = 14.51 Å. There are six formula units per hexagonal cell, and the volume per formula of 62.85 Å3 for the rh phase is much smaller than the value of 64.72 Å3 for the ambient bcc phase.

Figure 1.4 Ball and stick representations of crystal structures of bcc-In2O3 (a, b) and rh-In2O3 (c, d). In atoms are pale pink, and O atoms are dark red. The viewing directions and the in-plane orientation are indicated in the figure.

Source: Reproduced from Zhang et al. [42]/with permission of American Chemical Society.

1.2.4 TiO2

As a member of transition metal oxides, TiO2 is a well-known n-type semiconductor [43–46]. It exhibits excellent electronic and optical properties, making it highly suitable for various applications in the fields of gas sensors [47], solar cells [48], and photocatalysis [49]. In its natural form, TiO2 exists in three different phase structures, namely anatase (tetragonal), brookite (orthorhombic), and rutile (tetragonal), as illustrated in Figure 1.5. These phases have energy bandgaps of 3.2 eV (anatase), 3.02 eV (brookite), and 2.96 eV (rutile) [50, 51]. Among these phases, anatase and rutile are widely applied due to their superior stability, while rutile TiO2, with a tetragonal structure containing six atoms per unit cell, exhibits a slight distortion in the TiO6 octahedron [52]. Anatase TiO2 also possesses a tetragonal structure, but with a slightly larger distortion of the TiO6 octahedron. In general, rutile TiO2 is more thermodynamically stable than anatase under typical temperature and pressure conditions.

Figure 1.5 Structure of anatase, rutile, and brookie TiO2.

Source: Reproduced from Macwan et al. [51]/with permission of American Chemical Society.

1.2.5 Ga2O3

Ga2O3 is an emerging ultra-wide bandgap semiconductor with a bandgap of 4.8 eV. This unique property enables Ga2O3 to simultaneously achieve high breakdown voltage and low on-resistance, making it highly promising for both direct current (DC) and radiofrequency (RF) applications [53, 54]. As a result, Ga2O3 offers exciting prospects in various fields such as electronics (high-power devices, field-effect transistors) [55], optoelectronics (solar cells, solar-blind ultraviolet photodetectors) [56–58], and sensors (gas sensors, radiation detectors) [59]. Ga2O3 exists in five polymorphs, including corundum (α), monoclinic (β), defective spinel (γ), orthorhombic (δ), and hexagonal (ε), as shown in Figure 1.6[53]. Among these, β-Ga2O3 is the most stable phase under ambient conditions, while the other phases can transform into β-Ga2O3 through heating treatment [60].

1.3 p-Type Metal Oxide Semiconductors

1.3.1 Copper Oxides (CuO/Cu2O)

Tenorite (CuO) and cuprite (Cu2O) are two stable phases in copper oxides at an ambient environment. As p-type semiconductors, CuO and Cu2O separately possess a band gap of 1.9–2.1 and 2.1–2.6 eV [61]. Both binary copper oxides are attractive for versatile applications such as solar cells, TFTs, gas sensors, and photoelectric detectors [62–67]. When annealed at a high temperature, Cu2O could transform to CuO by the oxidation reaction [63]. In addition to these two stable phases, paramelaconite (Cu4O3) is another metastable mixed-valence intermediate compound between Cu2O and CuO and could decompose into Cu2O and CuO beyond the stability limit under vacuum environment at 670–800 K. Figure 1.7 illustrates these crystal structures of Cu2O and CuO [61].

Figure 1.6 Crystal structure Ga2O3 polymorphs. (a) Rhombohedral crystal structure of corundum-like α-Ga2O3. (b) Monoclinic β-Ga2O3 crystal structure. (c) Cubic defective spinel lattice structure of γ-phase Ga2O3. (d) Orthorhombic ε-Ga2O3 structure.

Source: Reproduced from Zhang et al. [53]/with permission of AIP Publishing.

Figure 1.7 Crystal structure of binary copper oxides. (a) Cubic Cu2O and (b) Monoclinic CuO.

Source: Reproduced from Gupta et al. [61]/with permission of Elsevier.

1.3.2 SnO

Recently, tin monoxide (SnO) has garnered significant attention due to its inherent p-type conductivity, structural stability, and electric properties in ambient environment [68]. The bandgap of SnO ranges from 2.2 to 3.0 eV at 300 K [69]. Figure 1.8 illustrates the crystal structure of SnO and the hybridization of valence-band maximum (VBM) [62]. It is worth noting that the energy level of Sn 5s is in close proximity to that of O 2p near the VBM, as well as Sn 5p near the conduction band. This arrangement effectively reduces hole localization and results in high hole mobility. The presence of Sn vacancies and oxygen interstitials contributes to the p-type conductivity observed in SnO [70]. The native p-type behavior of SnO makes it suitable for various applications in the fields of TFTs [71], gas sensors [72], and optoelectronics [69].

Figure 1.8 (a) Crystal structure of SnO. (b) Schematic illustration of the valence-band maximum (VBM) hybridization in SnO.

Source: Reproduced from Wang et al. [62]/with permission of Elsevier.

1.3.3 NiOx

NiOx is a widely studied p-type semiconductor that exhibits a green color in ambient environments and possesses a wide bandgap ranging from 3.6 to 4.0 eV [73]. Due to its exceptional thermodynamic stability and unique optical/chemical properties [74, 75], NiOx has attracted significant research interest in various fields, including solar cells [76], supercapacitors [77], catalysis [78], electrodes for lithium-ion batteries [74], and gas sensors [79].

Figure 1.9a illustrates the crystal structure of NiO. Nalage et al. proposed a sol–gel synthesis method for NiO thin films [80]. In this typical procedure, nickel acetate serves as the source of Ni, and the flow diagram for the production of NiO films is depicted in Figure 1.9b. The NiO films prepared through this process and subsequently sintered at 700 °C exhibited an electron carrier concentration and mobility of 3.75 × 1019 cm−3 and 4.2 × 10−5 cm2 V−1s−1, respectively, along with a bandgap of 3.47 eV.

In various application scenarios like TFTs, photocatalysts, gas sensors, solar cells, and memristors, the morphological and structural characteristics of metal oxides play a crucial role in determining their performance. Factors such as crystal phases, terminal atom species, crystal size, shape, defects, vacancies, and crystal planes significantly influence the ultimate performance of these materials. Therefore, depending on the desired performance requirements, it is possible to design and improve the synthesis parameters while adhering to a fixed preparation strategy. This approach allows us to obtain the desired metal oxide with specific properties, albeit with some compromises between performance merits such as sensitivity and stability.

Figure 1.9 (a) Crystal structure of NiOx.

Source: Reproduced from Goel et al. [75]/with permission of Elsevier. Copyright 2020, Elsevier.

(b) Flow diagram for NiO films synthesis by sol–gel method.

Source: Reproduced from Nalage et al. [80]/with permission of Elsevier.

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2Fabrication Techniques and Principles

2.1 Introduction

Owing to their vast application potential in the over-increasingly developed Internet of Things (IoT) and artificial intelligence (AI), oxide semiconductors, encompassing both n-type and p-type, have gained increasing attention in recent years. To attain devices with exceptional performance, the implementation of high-quality oxide layers is generally required.

The quality of prepared oxide semiconductors heavily relies on the employed deposition methods. Currently, both vacuum- and solution-based methods are utilized for depositing oxide semiconductors. Vacuum-based methods, such as sputtering, atomic layer deposition (ALD), and evaporation, have been reported to exhibit excellent uniformity over large areas and precise thickness control, making them ideal for applications in devices like thin-film transistors (TFTs) and diodes. On the other hand, solution-based methods have demonstrated significant potential in depositing oxide semiconductors with reduced dimensions, thus being highly promising for sensing applications. Additionally, there is a growing interest in solution-processed oxide thin films due to their compatibility with existing printing techniques, which aligns with the increasing demand for printable electronics. This chapter primarily focuses on discussing the common fabrication techniques and principles of oxide semiconductors.

2.2 Vacuum-Based Methods

The deposition techniques of oxide semiconductors can be mainly separated into two categories: vacuum-based methods and solution-based methods. Vacuum-based techniques, when compared to their solution-based counterparts, provide distinct advantages such as precise thickness control and the ability to achieve highly conformal films at low deposition temperatures. Sputtering, ALD, and evaporation are among the most commonly utilized vacuum-based methods for depositing oxide semiconductors. Here, in this subchapter, oxide semiconductors are reviewed based on their respective deposition techniques.

2.2.1 Sputtering

As a type of physical vapor deposition, sputteringcan be separated as direct current (DC), radio frequency (RF), reactive and magnetron sputtering. Sputtering refers to the physical removal of atoms from the surface of the target material through collision with energetic particles (i.e. an ionized gas known as plasma) on the surface of materials. The atoms that are removed from the target surface due to this energy bombardment are then transferred to the adjacent surface (i.e. substrate), forming a film with the desired thickness. A typical sputtering growth process is shown in Figure 2.1.

Compared with other vacuum-based methods, sputtering offers several advantages, including (i) suitability for various materials, including metals, insulators, alloys, and composites, (ii) high film quality and good step coverage, (iii) good uniformity over large areas, and (iv) potential low-temperature or even room-temperature deposition. These advantages have positioned sputtering as one of the most used vacuum techniques for depositing high-quality oxide semiconductor films, particularly in thin-film transistors.

Initially, research on sputtered oxide semiconductors primarily focused on binary compounds, i.e. ZnO and SnO2. For example, Baud and coworkers conducted a study on the structural and optical properties of ZnO films sputtered using a ZnO target, showing that the composition of Zn and O exhibited a narrow dependence on the sputtering parameters [1]. The O/Zn atomic ratio ranged from 0.95 to 1.06, with an increase observed in correlation with the total pressure and oxygen partial pressure, while a decrease was observed with an increase of sputtering power.

Notably, to achieve high mobility, crystallization of ZnO is necessary, which poses challenges in achieving good uniformity and reproducibility over a large area. Using amorphous oxides with good uniformity is an alternative. An adorable achievement in sputtering of oxide semiconductors was reported by Hosono and coworkers in depositing amorphous InGaZnO (IGZO) TFTs [2]. As shown in the cross-sectional TEM images (Figure 2.2a), the deposited a-IGZO layer was uniform and dense, showing excellent step coverage without any observable grain boundaries. By varying the O2 partial pressure during sputtering (i.e. from 3.1% to 3.7% for O2/(O2 + Ar)), the conductivity of the a-IGZO films was controlled from 10−3 to 10−6 S cm−1 (Figure 2.2b). The prepared IGZO TFTs exhibited a remarkable current on/off ratio of ∼108, a mobility of 12 cm2 V−1s−1 and a subthreshold swing of 0.2 V dec−1, showcasing the potential of sputtering for depositing high-performance TFTs on flexible substrates.

Figure 2.1 A schematic diagram showing the sputtering process.

Figure 2.2 (a) A cross-sectional TEM image of a-IGZO TFTs.

Source: Reproduced with permission from Yabuta et al. [2]. © 2006, AIP Publishing.

(b) Electrical conductivity as a function of O2 partial pressure.

Source: Reproduced from [2]/with permission of AIP Publishing.

The requirement of complementary circuits necessitates the preparation of both n-type and p-type oxide semiconductors with the same methods. Currently, the sputtering of n-type oxide semiconductors, especially IGZO, has been successfully commercialized. However, the deposition of p-type oxide semiconductors with comparable performance to n-type ones remains limited. Such an issue has been partially addressed with the demonstration of sputtered p-type SnO, as reported by Fortunato et al. [3]. The sputtering window for achieving high-performance p-type SnO films is relatively narrow, with parameters including post-annealing conditions (temperature and atmosphere), oxygen partial pressure, and sputtering power affecting the overall film quality. For example, Alshareef and coworkers reported a comprehensive study on the deposition of high-performance SnO film using DC reactive magnetron sputtering [4]. Oxygen partial pressure and the whole deposition pressure were carefully controlled by them, with the former varying from 3 to 25% and the latter from 1.2 to 2.2 mTorr. XRD patterns indicated that the phase formation strongly depended on both parameters, resulting in polycrystalline SnO with or without metallic phases only within a narrow window between oxygen partial pressure of 7% and 15%, and pressures of 1.5 and 2 mTorr. The deposition phase map in Figure 2.3a suggested that for each process pressure between 1.5 and 2.0 mTorr, the deposited film composition exhibited a specific trend. A higher strain value was found in mixed-phase (mp-) SnO, indicating the presence of more lattice defects, which might cause additional carrier scatterings. Compared with pure-phase SnO, mp-SnO exhibited improved Hall mobility, reaching a peak hole mobility of 18.7 cm2 V−1s−1 for films consisting of c. 3 at% β-Sn second phase (Figure 2.3b). DFT simulations suggested that defects (i.e. Sn interstitial and oxygen vacancy) generated under Sn-rich growth conditions gave a more metallic character to the valence band of the SnO, contributing to the higher mobility. The high hole mobility might attribute to the balance between the modulation of valence band at low concentrations of the β-Sn second phase (<3 at%) and the increased charge-carrier scattering at higher concentrations of the β-Sn second phase. Using mp-SnO with the highest mobility, TFTs were fabricated on both rigid glass and flexible polyimide substrates, showing a linear field-effect mobility of 6.75 and 5.87 cm2 V−1s−1, respectively.

Figure 2.3 (a) Phase stability map showing the narrow window of obtaining p-type SnO. (b) Room temperature Hall mobility of the pure SnO and mp-SnO films.

Source: Reproduced from Caraveo-Frescas et al. [4]/with permission of American Chemical Society.

2.2.2 Atomic Layer Deposition (ALD)

Atomic layer deposition (ALD) is a chemical gas phase method capable of depositing thin films over a large area with good uniformity. This method typical involves four essential steps in each cycle, (i) precursor exposure to the substrate, (ii) evacuation or purging of the precursors as well as byproducts from the chamber, (iii) reactant species exposure, typically oxidants or other reagents, and (iv) evacuation or purging of the reactants and byproduct molecules from the chamber. A typical growth process of ALD is shown in Figure 2.4. ALD offers advantages such as excellent conformality, precise thickness control, and relatively low growth temperature, all of which are beneficial for nanofabrication.

Figure 2.4 Schematic diagrams showing the ALD process.

The initial reports of using ALD in depositing oxides mainly focused on oxide insulators, i.e. hafnium oxide, aluminum oxide, titanium oxide and their mixtures. This emphasis was due to the highly conformal nature of ALD methods, enabling the deposition of ultra-thin films with high density. These materials have potential applications not only as dielectric layers in TFTs to achieve low operating voltage but also as insulator layers in MIM diodes and potentially in RRAMs. More recently, ALD has also been reported in depositing high-performance oxide semiconductors, demonstrating performance comparable to those deposited by sputtering. For example, Tsai et al. demonstrated ALD-deposited flexible ZnO TFTs with a mobility of over 20 cm2 V−1s−1[5]. IGZO TFTs with a mobility of ∼70 cm2 V−1s−1 and a current on/off ratio exceeding 108 were reported by Park and coworkers [6].

As a binary oxide with a bandgap of 3.37 eV, ZnO has shown great potential in solar cells and TFTs. The polycrystalline structure of ZnO itself contributes to the high reliability of film quality during deposition. Synthesizing ZnO with ALD typically involves the use of diethyl zinc (DEZ) as the precursor, while the reaction temperature might vary, although keeping at a relatively low value. Deposition of ZnO at a high temperature could result in the generation of a great number of defects, i.e. oxygen vacancies, thereby increasing carrier mobility [7]. However, the deposition with a low temperature could effectively passivate the film defects, showing a reduced mobility but an increase in current on/off ratio [8].

Oxygen source is another important factor that affect the performance of ALD-ZnO films. Liu's work suggested that compared with H2O, strong oxidants such as O3 yield smaller average grain sizes when used with DEZ (Figure 2.5a, b) [9]. The XRD patterns in Figure 2.5c also demonstrated the additional changes in the preferred grain orientation of ZnO film, indicating the tunability of growth direction through oxidant selection.

Figure 2.5 SEM images of ZnO using (a) H2O and (b) O3 as the oxidant. (c) XRD patterns of the ZnO films.

Source: Reproduced with permission from Chen et al. [9]. © 2020, ELSEVIER.

Another highly used binary oxide is In2O3, which is also capable of depositing using ALD with high mobility and good transparency [10]. Similar to ZnO, the growth temperature of In2O3 also plays a crucial role, since temperatures higher than 200 °C might result in In2O3 films with metal-like behavior owing to low electrical resistivity and high carrier concentration, leading to crystallinity and molecular orbital ordering. Achieving a semiconducting nature is thus doable by reducing the growth temperature. Additionally, methods such as post-annealing in an oxygen-rich environment, N2O plasma treatment, and doping strategies have been previously reported to reduce the overall carrier concentration [11–13].

Deposition of high-quality SnO2 is also doable using ALD, with a wide range of tin precursors including TDMASn, dimethylamino-2-methyl-2-propoxy-tin(II) [Sn(dmamp)2], tetrakis(dimethylamino)propyl tin(IV) [Sn(DMP)4] being reported previously [14–16]. However, the commercialization of ALD-SnO2 is still limited, possibly due to the lack of commercially available Sn precursors. In 2019, Kim and coworkers used commercially available tetraethyltin (TET) as the precursor and H2O2 as the reactant to deposit SnO2 films [17]. No significant changes in properties were found within a wide range of growth temperature from 250 to 400 °C, showing a high density of ∼6.2 g cm−3 and an optical bandgap of 3.7–3.9 eV, comparable to that of bulk SnO2. Negligible impurities remained in the films grown over the entire temperature range. Although the microstructure and crystallinity of SnO2 films varied with the growth temperature and number of cycles, the films exhibited negligible changes of electrical properties with the growth temperature and had low resistivity values around 1020 cm−3, suitable for a wide range of applications.

Apart from the great success made in ALD-deposited n-type oxide semiconductors, p