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This book provides comprehensive coverage of the new wide-bandgap semiconductor gallium oxide (Ga₂O₃). Ga₂O₃ has been attracting much attention due to its excellent materials properties. It features an extremely large bandgap of greater than 4.5 eV and availability of large-size, high-quality native substrates produced from melt-grown bulk single crystals. Ga₂O₃ is thus a rising star among ultra-wide-bandgap semiconductors and represents a key emerging research field for the worldwide semiconductor community.

Expert chapters cover physical properties, synthesis, and state-of-the-art applications, including materials properties, growth techniques of melt-grown bulk single crystals and epitaxial thin films, and many types of devices. The book is an essential resource for academic and industry readers who have an interest in, or plan to start, a new R&D project related to Ga₂O₃.

### Chapter 1. Introduction

This introductory chapter provides current and comprehensive information about gallium oxide, covering crystal structures, basic physical properties, bulk melt growth and thin-film epitaxial growth methods, and representative electrical and optical devices.

Masataka Higashiwaki

### Chapter 2. Czochralski Method

The Czochralski methodCzochralski method is one of the leading research and industrial crystal growth technologies that enables to obtain large diameter single crystals of high structural qualityStructural quality at low production costs per volume unit. A possibility of obtaining bulk β-Ga2O3 single crystalsBulk β-Ga2O3 single crystals by the Czochralski method expands the diversity of growth technologies for this compound towards large volumes and high quality suitable for epitaxial growth of layers and device fabrication. Ga2O3 is, however, thermally unstable at high temperatures and tends to decompose that has a high impact on the growth process, size, and structural quality of obtained crystals. Additionally, the growth process is also affected by electrical/optical properties of a growing β-Ga2O3 crystal. Ga2O3 thermodynamicsThermodynamics combined with new technical solutions allowed to obtain 2-inch diameter cylindrical single crystals of β-Ga2O3 of high structural qualityStructural quality with further scale-up capabilities. Czochralski-grown bulk β-Ga2O3 single crystalsBulk β-Ga2O3 single crystals can be easily doped with a diversity of elements to tune their electrical and optical properties. The bulk β-Ga2O3 single crystals can be obtained either as electrical insulators or semiconductors both with a high transparency in the UV and visible spectral regions.

Zbigniew Galazka

### Chapter 3. Vertical Bridgman Growth Method

The vertical BridgmanVertical Bridgman (VB) technique (VB) technique developed for β-Ga2O3 crystalsΒ-Ga2O3 crystal will be introduced including specific details on the VB crucible materialCrucible material determined by the measurement of the melting temperature of β-Ga2O3 and the VB growth processes of β-Ga2O3 in ambient air. The characteristic features of the crystallinity and the tentative electric characteristics of β-Ga2O3 crystalsΒ-Ga2O3 crystal grown by the VB techniqueVertical Bridgman (VB) technique will also be introduced.

Keigo Hoshikawa

### Chapter 4. Floating Zone Method, Edge-Defined Film-Fed Growth Method, and Wafer Manufacturing

This chapter describes the floating zone growth method of β-Ga2O3, the edge-defined film-fed growth of β-Ga2O3, and the manufacturing of β-Ga2O3 wafersΒ-Ga2O3 wafers . The floating zone methodFloating zone method section briefly mentions the method’s history and typical growth conditions. The section on edge-defined film-fed growth methodEdge-defined film-fed growth method discusses the history, growth sequence, and conditions. It also covers the material properties of edge-defined film-fed grown β-Ga2O3 such as twin boundariesTwin boundaries , dislocationsDislocations , nanovoidsNanovoids , residual impuritiesResidual impurities , intentional dopingIntentional doping , and dopant distributionDopant distribution . The wafer manufacturing section describes the basic wafer process and the effect of annealingAnnealing on carrier concentrationEffect of annealing on carrier concentration .

Akito Kuramata, Kimiyoshi Koshi, Shinya Watanabe, Yu Yamaoka

### Chapter 5. Plasma-Assisted Molecular Beam Epitaxy 1

Growth, Doping, and Heterostructures

Plasma-assisted molecular beam epitaxyPlasma-assisted Molecular Beam Epitaxy has been used to grow the highest quality β-Ga2O3 thin films and has shown potential to realize various efficient device structures. Growth of β-Ga2O3 is defined by the suboxide desorptionSuboxide desorption that limits growth rates at high temperatures and Ga fluxes. Growth in various orientations has been demonstrated with the (010) b-plane in particular showing promise for homoepitaxyHomoepitaxy due to high realized growth rates and materials quality. N-type dopingDoping with Sn, Ge, and Si has allowed for device structures that utilize electron conduction in this materials system. HeterostructuresHeterostructure with β-(AlxGa1-x)2O3 have been used for modulation doped field effect transistors; however, thermodynamic limitations of maximum achievable Al content have limited device performance. Expanding the growth regime through metal-oxide catalyzed epitaxy using In could help improve heterostructureHeterostructure growth for future devices.

Akhil Mauze, James Speck

### Chapter 6. Plasma-Assisted Molecular Beam Epitaxy 2

Fundamentals of Suboxide-Related Growth Kinetics, Thermodynamics, Catalysis, Polymorphs, and Faceting

This chapter sheds light on various fundamental aspects of the O plasma-assisted molecular beam epitaxy of Ga2O3. It discusses the volatile suboxide-related growth kinetics of Ga2O3 explaining the observed growth rate behavior as function of all growth parameters. The binary growth kinetics of Ga2O3 is then compared to that of its related oxides In2O3 and SnO2. During the ternary growth of (InxGa1-x)2O3, thermodynamic aspects based on different metal-oxygen bond strengths become important, which will be shown to govern the Ga- versus In-incorporation into the (InxGa1-x)2O3 thin film. More importantly, we describe how the collaborative effect of the different growth kinetics and thermodynamics of the binary oxides can lead to a strong growth rate enhancement of Ga2O3 by metal-exchange catalysis (MEXCAT) using In2O3 or SnO2 as catalyst. A brief overview of the polymorphs of Ga2O3 stabilized by different substrates is given. The surface morphology obtained by homoepitaxy will be discussed in relation to thermodynamically induced faceting. Finally, open questions will be identified that require further research.

Oliver Bierwagen, Patrick Vogt, Piero Mazzolini

### Chapter 7. Ozone-Enhanced Molecular Beam Epitaxy

This chapter explains homoepitaxial growth of β-Ga2O3 by using ozone-enhanced molecular beam epitaxy (MBE). First, in order to reveal the suitable surface orientation for β-Ga2O3 homoepitaxial MBE growth, we investigate the surface orientation dependence of the growth rate and surface morphology. The results show that the (010) plane is the most suitable one for growth. Next, we optimize the growth conditions of films grown on the (010) plane. In the case of unintentionally doped films, we find that smooth surface films can be obtained by optimization of the surface migration of Ga and O adatoms. On the other hand, in the case of Sn-doped films, segregation of Sn leads to roughening of the surface. A highly O-rich condition is good for obtaining a smooth surface, because it reduces segregation of Sn. By optimizing the growth conditions in this manner, one can fabricate device-quality β-Ga2O3 homoepitaxial films with precisely controllable donor concentrations over a wide range (1016–1018 cm−3) and atomically flat surfaces.

Kohei Sasaki, Shigenobu Yamakoshi, Akito Kuramata

### Chapter 8. Metalorganic Chemical Vapor Deposition 1

Homoepitaxial and Heteroepitaxial Growth of Ga2O3 and Related Alloys

This chapter is devoted to the growth of Ga2O3 and its alloys by metalorganic chemical vapor depositionMetalorganic chemical vapor deposition MOCVD (MOCVDMOCVD ) or, equivalently, metalorganic vapor phase epitaxyMetalorganic vapor phase epitaxy (MOVPE) (MOVPE). MOCVD is a standard epitaxial growth technique used for nitride, III–V, and oxide-based power devices as well as LEDs and laser diodes. It would, therefore, seem that MOCVD would be the most appropriate growth method to accelerate the development and commercialization of Ga2O3. However, molecular beam epitaxyMolecular beam epitaxy MBE (MBE) and halide vapor phase epitaxy (HVPE) were commonly used in the earlier growth studies of Ga2O3 epitaxial films. Fortunately, the broad range of knowledge available on the hardware and control systems of the MOCVD tool for the growth of device quality epitaxial films makes it easily adaptable to the growth of epitaxial Ga2O3. Device quality films grown at ~10 µm/h were demonstrated [1, 2], evidencing the ability of MOCVD to achieve the high throughput Ga2O3 epitaxial layer growth needed for high voltage power device and deep ultraviolet solar-blind photodetector commercial applications. This chapter discusses the following topics: the selection of suitable metalorganic precursorsMetalorganic precursors and oxygen sources used for the growth of Ga2O3 and (Al, Ga)2O3 alloys; the need for the careful design of the MOCVD reactors, homoepitaxial and heteroepitaxial growth on c-plane sapphire and native Ga2O3 substrates, donor and acceptor doping; and the origin and methods for mitigating or reducing unintentional impurities.

### Chapter 9. Metal Organic Chemical Vapor Deposition 2

Heteroepitaxial MOCVD Growth of -, -, and -GaO Thin Films on Sapphire Substrates

This chapter reviews the heteroepitaxial growthHeteroepitaxial growth of $$\upalpha$$- $$\upalpha$$ -Ga $$_2$$ O $$_3$$ , $$\upbeta$$- $$\upbeta$$ -Ga $$_2$$ O $$_3$$ , and $$\upvarepsilon$$-gallium oxideGallium Oxide (Ga $$_2$$ O $$_3$$ ) (Ga$$_2$$O$$_3$$) $$\upvarepsilon$$ -Ga $$_2$$ O $$_3$$ films, with a focus on those grown using the metalorganic chemical vapor deposition (MOCVD)Metalorganic Chemical Vapor Deposition (MOCVD) technique. Variations in growth conditions and substratesSubstrates result in the growth of different polymorphsPolymorph of Ga$$_2$$O$$_3$$ Gallium Oxide (Ga $$_2$$ O $$_3$$ ) or combinations of them. $$\upbeta$$-Ga$$_2$$O$$_3$$ Gallium Oxide (Ga $$_2$$ O $$_3$$ ) is $$\upbeta$$ -Ga $$_2$$ O $$_3$$ consistently reported as the dominant phase to grow at high substrateSubstrates temperatures >700 $$^{\circ }$$C. At lower substrateSubstrates temperatures, $$\upalpha$$- and $$\upvarepsilon$$- metastableMetastable phases have been observed. Other growth conditions and substratesSubstrates that have yielded $$\upalpha$$- $$\upalpha$$ -Ga $$_2$$ O $$_3$$ and $$\upvarepsilon$$-Ga$$_2$$O$$_3$$ $$\upvarepsilon$$ -Ga $$_2$$ O $$_3$$ epitaxialGallium Oxide (Ga $$_2$$ O $$_3$$ ) films are also discussed. DopingDoping of MOCVD-grown $$\upbeta$$-Ga$$_2$$O$$_3$$ Gallium Oxide (Ga $$_2$$ O $$_3$$ ) is $$\upbeta$$ -Ga $$_2$$ O $$_3$$ also briefly reviewed, where Si and Sn are the most commonly used dopantsDopants. DopingDoping concentrations between $$1\times 10^{17}$$ and $$8\times 10^{19}$$ cm$$^{-3}$$ have been achieved, with corresponding electron mobilityMobility values between $$\sim$$130 and 50 cm$$^2$$/Vs.

Yao Yao, Robert F. Davis, Lisa M. Porter

### Chapter 10. Halide Vapor Phase Epitaxy 1

Homoepitaxial Growth of β-Ga2O3 on β-Ga2O3 Substrates

Homoepitaxial growth of β-Ga2O3 on β-Ga2O3 substrates by halide vapor-phase epitaxyHalide vapor phase epitaxy (HVPE) using GaCl and O2 was investigated by both thermodynamic analysisThermodynamic analysis and growth experiments. The thermodynamic analysis clarified that growth of Ga2O3 is expected at high temperatures around 1000 °C using an inert carrier gas. The experimental results revealed that homoepitaxial growth of unintentionally dopedUnintentionally doped (UID) layers with a low effective donor concentrationEffective donor concentration (Nd − Na) of less than 1013 cm−3 is possible at 1000 °C on β-Ga2O3 (001) substrates with a high growth rate of up to 28 μm/h. Furthermore, HVPE growth of intentionally Si-doped β-Ga2O3 layers was investigated by supplying SiCl4, which revealed that n-type carrier density almost equal to the Si-doping concentration can be controlled in the range of 1015–1018 cm−3. The carrier mobilityMobility decreased with increasing Si impurity concentration and was about 150 cm2/V·s at room temperature for a layer with a carrier density of 3.2 × 1015 cm−3. Thus, the intentionally Si-doped homoepitaxial layers grown on β-Ga2O3 substrates can be applicable for the production of β-Ga2O3-based power devices.

Yoshinao Kumagai, Keita Konishi, Ken Goto, Hisashi Murakami, Bo Monemar

### Chapter 11. Halide Vapor Phase Epitaxy 2

Heteroepitaxial Growth of α- and ε-Ga2O3

Halide vapor phase epitaxy of metastableMetastable α- and ε-Ga2O3(-Ga2O3 is reviewed. The both polymorphs were grown using GaCl and O2 as precursors. Phase-pure corundumCorundum α-Ga2O3(-Ga2O3 was heteroepitaxially grown on (0001) sapphire at temperatures of approximately 550 °C or lower. The n-type electrical conductivity was controlled by Ge dopingDoping using GeCl4GeCl4 as the dopant source, and very low resistivity of 8.6 mΩ cm was achieved. Epitaxial lateral overgrowth was shown to be effective in improving the crystal quality, and the dislocationDislocation density was reduced from 1010 cm−2 to less than 5 × 106 cm−2 in the laterally grown wing region. Morphology of α-Ga2O3 islands was controlled such that inclined facets well develop, and dislocation density above mask openings remarkably decreased due to dislocationDislocation bending caused by the inclined facets. Orthorhombic ε-Ga2O3(-Ga2O3 was grown on (0001) GaN and (0001) AlN using virtually the same growth recipe as that used for α-Ga2O3(-Ga2O3 . Fundamental material properties of ε-Ga2O3, such as the optical bandgapBandgap energy, thermal stabilityThermal stability , and thermal expansionThermal expansion coefficient, were investigated using the epitaxial film.

Yuichi Oshima

### Chapter 12. Mist Chemical Vapor Deposition 1

Heteroepitaxial Growth of α-Ga2O3 Thin Films on Sapphire Substrates

This chapter summarizes fundamental issues of corundum-structured gallium oxide (α-Ga2O3)(-Ga2O3 , which is obtained by heteroepitaxy on sapphire substratesSapphire substrates and is featured by its large bandgap (~5.3 eV), bandgap engineeringBandgap engineering (3.7 to ~9 eV), and existing corundum-structured p-type oxide such as α-Ir2O3(-Ir2O3 , as well as low epitaxy cost on inexpensive sapphire substratesSapphire substrates .

Shizuo Fujita

### Chapter 13. Mist Chemical Vapor Deposition 2

Heteroepitaxial Growth of ε-Ga2O3 on Various Substrates

ε-Ga2O3Ε-Ga2O3 is one of the five polymorphs of Ga2O3, which has attracted considerable attention because it exhibits unique polarization and ferroelectric properties. In this chapter, we describe the growth of ε-Ga2O3 thin films via mist chemical vaporMist chemical vapor deposition (CVD) depositionChemical vapor deposition (CVD) (CVD). In the epitaxial growth of ε-Ga2O3Ε-Ga2O3 thin films, we demonstrated that various substratesVarious substrates allow ε-Ga2O3 growth via mist CVD because of the atomic arrangementAtomic arrangement between the surface of the substrate and the ε-Ga2O3 growth surface. Further, a method for distinguishing the hexagonalHexagonal and orthorhombicOrthorhombic structures of ε-Ga2O3 using X-ray diffractionX-ray diffraction (XRD) (XRD) φ-scansΦ-scans was explained in detail. In our experiments, all ε-Ga2O3Ε-Ga2O3 thin films grown via mist CVDChemical vapor deposition (CVD) exhibited orthorhombicOrthorhombic crystal structure, which permitsRotational domains rotational domainsDomains. Furthermore, the growth mechanism of these rotational domains was explained using the atomic arrangementAtomic arrangement of ε-Ga2O3 and the crystal structures of the substrates. Finally, bandgap engineeringBandgap engineering from 4.5 to 5.9 eV was demonstrated via mist CVD with the incorporation of InIncorporation of In and AlIncorporation of Al.

Hiroyuki Nishinaka

### Chapter 14. Pulsed Laser Deposition 1

Homoepitaxial Growth of β-Ga2O3 on β-Ga2O3 Substrates

This chapter examines homoepitaxial β-Ga2O3 thin films fabricated by pulsed laser depositionPulsed laser deposition. The recent availability of high-quality native substrates affords investigations using a wide range of growth conditions to achieve single crystalline films by pulsed laser depositionPulsed laser deposition. Deposition parameter optimization and structural, electrical and chemical film characterization are presented for undoped and impurity-doped films. Films grown on commercially available (010) Fe compensation-doped substrates fabricated by the edge-defined film-fed growth technique and a developing Czochralski method are examined. Hall effectHall effect results from PLD Si-doped β-Ga2O3 films are compared with data produced from other vapor phase epitaxial growth techniques. The influence of substrate orientationOrientation on film properties is also studied with (010) and (001) crystals. Implementation of a PLD n+ regrowthRegrowth layer in a transistor device is demonstrated to achieve low ohmic contact resistance. Results from β-(AlxGa1−x)2O3 film studies show the potential utility of heterostructuresHeterostructure for field-effect transistor 2DEG formation.

Kevin D. Leedy

### Chapter 15. Pulsed Laser Deposition 2

Heteroepitaxial Growth of GaO and Related Alloys

We review heteroepitaxial growth of Ga$$_2$$O$$_3$$ and related alloys by pulsed laser deposition (PLD). First, we briefly summarize the history of PLD and discuss its evolution and development since its breakthrough in the 1980s with the focus on combinatorial material synthesis. Then, the impact of strain on the lattice constant of rhombohedral, pseudomorphic (Al, Ga)$$_2$$O$$_3$$ thin films is introduced and the determination of thin film composition from X-ray diffraction measurements is outlined. For monoclinic Ga$$_2$$O$$_3$$ layers the influence of key growth parameters on growth rate and surface morphology is discussed. Electrical transport properties of monoclinic thin films doped by silicon or tin are presented and compared to that of homoepitaxial layers. For ternary thin films growth parameters strongly influence the chemical composition in addition to growth rate and morphology. High oxygen pressures and/or low growth temperatures are necessary for a stoichiometric transfer of the target composition to the epilayer which is explained by the desorption of gallium suboxides occurring otherwise. Further, we resume solubility limits and the dependence of structural, optical and vibrational properties on the alloy composition of monoclinic (In, Ga)$$_2$$O$$_3$$ and (Al, Ga)$$_2$$O$$_3$$ thin films.

Holger von Wenckstern, Daniel Splith, Marius Grundmann

### Chapter 16. Low Pressure Chemical Vapor Deposition

This chapter reviews the growth and material characterization of β-Ga2O3 grown via the low pressure chemical vapor deposition (LPCVD)Low pressure chemical vapor deposition (LPCVD) method. The growth of β-Ga2O3 thin filmsβ-Ga2O3 thin film , with Si as a demonstrated effective and controllable n-type dopant, on off-axis c-sapphire and native Ga2O3 substratesGa2O3 substrate are discussed. LPCVDLow pressure chemical vapor deposition (LPCVD) growth of β-Ga2O3Ga2O3 substrate rod structuresGa2O3 rod structure on 3C-SiC substrates3C-SiC substrate is also discussed. From the crystal structural characterization and electron transport measurements, LPCVDLow pressure chemical vapor deposition (LPCVD) -grown β-Ga2O3 materials exhibit high quality with great promises for high power electronicPower electronics and short-wavelength optoelectronic device applications.

Hongping Zhao

### Chapter 17. First-Principles Calculations 1

Electronic and Structural Properties of GaO and Alloys with InO and AlO

In this chapter, we show how hybrid density functional theory can be used to elucidate the basic properties of Ga$$_2$$O$$_3$$, such as crystal structureCrystal structur e, Brillouin zoneBrillouin zone, and band structureBand structure. We also demonstrate how it can predict the properties of Ga$$_2$$O$$_3$$ alloysAlloys and heterojunctions with In$$_2$$O$$_3$$ In $$_2$$ O $$_3$$ or Al$$_2$$O$$_3$$ Al $$_2$$ O $$_3$$ . The results can guide experimental exploration of materials and design of devices.

Hartwin Peelaers, Chris G. Van de Walle

### Chapter 18. First-Principles Calculations 2

Doping and Defects in GaO

Gallium oxide has rapidly developed as a prime candidate for next-generation power electronics owing to the availability of high-quality material (e.g., large single-crystal substrates and epitaxial films) in conjunction with its favorable optical and electronic properties. While a number of advances have been made in improving the crystal quality, improving control over the free carrier and defect concentrations in this wide-band gap semiconductor are critical to realize the full potential of Ga$$_2$$O$$_3$$-based devices. Identifying the most suitable dopantsDopants and the origins of the prominent deep-level defects, as well as possible passivationPassivation techniques, is therefore an essential next step in further developments. In this chapter, we use hybrid functionalHybrid functionals calculations to investigate the behavior of a number of native defectsNative defects, candidate dopantsDopants, and extrinsic impuritiesImpurities in $$\upbeta$$-Ga$$_2$$O$$_3$$ and to elucidate how theory has helped in understanding the defect-related properties of this material.

Joel B. Varley

### Chapter 19. Structural Properties 1

Characterization of Defects in β-Ga2O3 Substrates by Transmission Electron Microscopy and Related Techniques

This chapter describes structural evaluation of β-Ga2O3β-Ga2O3 crystals grown by edge-defined film-fed growth process using etch pitting, focused ion beamFocused Ion Beam (FIB) scanning ion microscopyScanning Ion Microscopy (SIM) , transmission electron microscopyTransmission Electron Microscopy (TEM) , and related techniques. Three types of defectsDefect have been found in the crystals. First, arrays of edge dislocationsEdge dislocation were observed corresponding to etch pitEtch pit arrays on a $$(\bar{2}01)$$-oriented wafer after etchingEtching with hot H3PO4. In some of the dislocationsDislocation , the line-shaped appearance is slightly deformed due to an inhomogeneous strain field. Next, platelike nanovoidsNanovoid which correspond to etch pitsEtch pit on the (010) plane were observed. Although the crystallographic configurations of the defectsDefect are different from that of nanometer-sized crystalline grooves which have been previously reported, they are both classified as similar type of nanovoidsNanovoid . Finally, twin lamellaeTwin lamella as well as regular large twinsTwin were observed in the crystal. The twin lamellae correspond to shallow V-grooves revealed after the chemical etchingEtching .

Osamu Ueda

### Chapter 20. Structural Properties 2

Crystallographic Defects in -GaO and X-Ray Topography Analysis

Crystallographic defectsCrystallographic defect, such as dislocationsDislocation and stacking faultsStacking fault, in $$\upbeta$$-Ga$$_2$$O$$_3$$ are analyzed. A slip planeSlip plane model is proposed, which is derived from the close-packed oxygen subcell. Possible Burgers vectorsBurgers vector of dislocationsDislocation are the translation vectors on the closed packed planes, $$(\overline{2}01)$$, $$(\overline{1}0\overline{1})$$, (310), and $$(3\overline{1}0)$$. DislocationsDislocation are observed using synchrotronSynchrotron radiation X-ray topographyX-ray topography and are identified based on the proposed slip planeSlip plane model. Stacking faultsStacking fault on the $$(\overline{2}01)$$ plane are revealed in the X-ray topographs and are enclosed by a single partial dislocationPartial dislocation loop slipping on the plane.

Hirotaka Yamaguchi

### Chapter 21. Structural Properties 3

Vacancy Defects Studied with Positron Annihilation Spectroscopy

PositronPositron annihilation spectroscopy has been applied to study vacancy defects in Ga2O3. Both positron lifetimePositron lifetime and Doppler broadeningDoppler broadening experiments have been performed, both in bulk single crystals and in thin films. The results show that Ga vacancyGa vacancy defects are efficiently formed in thin film growth and account for the electrical compensationElectrical compensation in semi-insulating and highly resistive Ga2O3. Their concentrations are very low in n-type material. In (InxGa1−x)2O3 alloys, the nature and behavior of the cation vacancy defects change from Ga2O3-like to In2O3-like along with the crystalline phase. Further work is important to elucidate the details of the vacancy formation mechanisms and the origins of the exceptionally strongly anisotropic positronPositron annihilation signals.

Filip Tuomisto

### Chapter 22. Electrical Properties 1

Donors and Acceptors

Studies of SiSi , GeGe donorsDonor and FeFe , MgMg as acceptorsAcceptor in β-Ga2O3 through temperature-dependent van der Pauw and Hall effect measurementsHall effect measurement of samples grown by a variety of methods are presented in this chapter. SiSi and Ge are identified as shallow donorsShallow donor with donor energy of 30 meV instead of DX states. FeFe is a deep acceptor with its energy level 860 meV below the conduction band edge. MgMg -doped samples present an activation energyActivation energy of 1.1 eV, but the type could not be resolved. Unintentional donorsDonor and acceptorsAcceptor are also discussed including intrinsic defects (i.e., vacancies) and extrinsic impurities. Last, an unintentional donor with energy of 110 meV is presented and the impacts of its incomplete ionizationIonization to power devices are discussed.

### Chapter 23. Electrical Properties 2

Electron Transport Studies in β-Ga2O3

This chapter discusses the electron transportElectron transport properties and their implications on power devices under very high electric fields (>1 MV/cm). A combination of first-principles calculation of microscopic interactions, Monte Carlo simulation of electron transportElectron transport , and TCAD simulationTCAD simulation on power transistors manifests a bottom-up view of the hot electron dynamics in β-Ga2O3Β-Ga2O3 devices. While the focus of the chapter is on impact ionizationImpact ionization and resulting avalanche breakdownAvalanche breakdown , a brief overview on low-field and moderately high-field transport are also presented. The role of crystal orientation on breakdown is discussed in detail. Compact model parameters for breakdown phenomena are provided. Estimates of breakdown field are shown through simple analytical calculations and also through a TCAD simulationTCAD simulation .

Krishnendu Ghosh, Avinash Kumar, Uttam Singisetti

### Chapter 24. Electrical Properties 3

Traps in β-Ga2O3: From Materials to Transistors

TrapsTraps in ultra-wide bandgap semiconductors (UWBG)UWBG are problematic for devices due to the very wide range of performance degradation phenomena they can cause, from high leakage currents, dynamic resistance and voltage dispersion in transistorsTransistors , to high dark currents, low quantum efficiencies and low responsivities in various optoelectronic devices. For β-Ga2O3β-Ga2O3 , the early stage of development for this promising UWBGUWBG semiconductor and associated lack of knowledge of many basic materials properties add more challenges as many of the trapTraps states are unknown and their physical sources are poorly understood at present. This chapter summarizes the state of knowledge concerning deep levelDeep levels defectsDefects in β-Ga2O3β-Ga2O3 materials and early stage transistorsTransistors . Deep levelDeep levels transient and optical spectroscopiesDLTS (DLTS/DLOS)DLOS are the primary characterization methods being focused on here. DLTS/DLOS measurements made on β-Ga2O3 materials prepared by several growth methods and irradiated by high energy particles are discussed. Defect spectroscopyDefect spectroscopy measurements made directly on β-Ga2O3 transistorsTransistors are also described. Several traps that are in common across the range of materials and devices are revealed, several unique traps are identified, and by comparing with theory and other physical characterization results, the potential physical sources for several trapsTraps are considered. Finally, this chapter attempts to correlate defect levels found in β-Ga2O3β-Ga2O3 transistors with the fundamental materials studies, leading toward possible identification of specific defectsDefects as primary sources for transistorTransistors instabilities such as threshold voltage shifts.

Aaron R. Arehart, Steven A. Ringel

### Chapter 25. Electrical Properties 4

Band Offsets and Interface State Density Characterization of Dielectric/Ga2O3 Interfaces

This chapter reviews recent literature on band offsetsBand offsets of dielectricsDielectrics and semiconductors to gallium oxideGallium oxide (Ga2O3) and its ternary alloy, aluminum gallium oxideAluminum gallium oxide . Band diagramBand diagram principles are reviewed, and an X-Ray photoelectron spectroscopyX-Ray photoelectron spectroscopy method for accurate band offset measurement is discussed. Interface state densityInterface state density measurements of Ga2O3 MOS capacitorsMOS capacitor are reviewed, and Terman methodTerman method results for the HfO2/β-Ga2O3 and ZrO2/β-Ga2O3 are presented. Fowler-Nordheim tunnelingFowler-Nordheim tunneling model was used to fit the leakage currentLeakage current for the HfO2/β-Ga2O3 MOS structure. A need for new dielectricsDielectrics with both wide bandgapWide bandgap and high dielectric constant is noted, and possible new directions for research are presented.

Marko J. Tadjer, Virginia D. Wheeler, David I. Shahin

### Chapter 26. Electrical Properties 5

Defects in β-Ga2O3 Crystals and Their Influence on Schottky Barrier Diode Characteristics

β-Gallium oxide is promising for use in semiconductorSemiconductor power devicesPower device . First, the type and character of crystal defectsDefect , such as dislocationsDislocation and voids, are described. Next, I describe the fabrication and measurement of Schottky barrier diodes (SBD) on the entire surface and investigate the relation between the leakage currentLeakage current and defectsDefect , as revealed mainly by the etch-pit method. The dislocationsDislocation that appeared on the (010) surface became SBD leakage paths, whereas the dislocations on the ($$\bar{2}01$$) and (001) surfaces apparently had no relation with the SBD leakage current. Voids that extend in [010] direction and appeared on all surface orientationsOrientation did not affect the SBD leakage currentLeakage current .

Makoto Kasu

### Chapter 27. Optical Properties

Fundamental Absorption Edge and Emission Properties of β-Ga2O3

This chapter provides fundamental optical properties of β-Ga2O3. Valence band orderingValence band ordering was investigated by polarizedPolarized transmittanceTransmittance and reflectanceReflectance measurements. AnisotropicAnisotropic optical properties were also investigated by spectroscopic ellipsometrySpectroscopic ellipsometry measurements. The optical anisotropy in a biaxialBiaxial crystal as well as the gradual increase in the absorption coefficientAbsorption coefficient were recognized as origins of the scattering in optical bandgapBandgap energies in a range 4.5–5.0 eV. Temperature-dependent exciton resonance energies were studied by using polarizedPolarized reflectanceReflectance measurement. The large changes in the exciton resonance energies with temperature were found to be originated from the exciton—longitudinal optical phononOptical phonon interaction. Correlation between blue luminescenceLuminescence (BL) intensity and formation energy of oxygen vacancy (VO) was found by measuring temperature-dependent cathodoluminescenceCathodoluminescence spectra. Suppression of the BL band in the heavily nitrogen-doped epitaxial films was shown as an evidence for the decrease in the VO concentration by N-doping and resultant high resistivity in the N-doped epitaxial films.

Takeyoshi Onuma

### Chapter 28. Phonon Properties

Phonon and Free Charge Carrier Properties in Monoclinic-Symmetry -GaO

We present and discuss the complete set of infrared-active phononPhonons modes in monoclinic-symmetry crystalMonoclinic crystal symmetry modification gallium oxide (gallia, $$\upbeta$$-Ga$$_2$$O$$_3$$). The phononPhonons mode set is obtained from a comprehensive analysis of generalized spectroscopic ellipsometryGeneralized spectroscopic ellipsometry data in the farinfrared and infrared spectral regions investigating various n-type electrically conductive single crystalSingle crystal samples with different free electron volume density parameters cut under different orientations. The analysis of the ellipsometry data is performed using an eigendielectric displacement vector summation (EDVS) approach. In this approach, the effect of the free charge carriersFree charge carriers onto the lattice modes of intrinsic $$\upbeta$$-Ga$$_2$$O$$_3$$ are removed by calculation. Density functional theoryDensity functional theory calculations are performed in the general gradient approximation and all phononPhonons modes at the Brillouin-center and their displacement direction dependencies are obtained. TransverseTransverse optical phonons and longitudinal opticalLongitudinal optical phonons phononPhonons mode parameters polarized within the monoclinic plane as well as perpendicular to the monoclinic plane agree excellently between experiment and theory. We also present and discuss the directional limiting frequency parameters within the monoclinic plane, the shape and anisotropyAnisotropy of the reststrahlen band, and the order of the phononPhonons modes in semiconductors with polar phononPhonons modes and monoclinic crystal structure. We further present and discuss the effect of coupling of longitudinal opticalLongitudinal optical phonons phononsPhonons with free charge carriersFree charge carriers, leading to longitudinal-phonon-plasmon coupled modes. We reveal that the coupled modes, which affect electric and thermal transport, change amplitude, frequency, and direction within the monoclinic plane as a function of free electron concentration. Finally, we show optical Hall effectOptical Hall effect measurements, and provide experimentally determined effective electron mass parameters in $$\upbeta$$-Ga$$_2$$O$$_3$$ for moderately-doped n-type samples.

Mathias Schubert, Alyssa Mock, Rafał Korlacki, Sean Knight, Bo Monemar, Ken Goto, Yoshinao Kumagai, Akito Kuramata, Zbigniew Galazka, Günther Wagner, Marko J. Tadjer, Virginia D. Wheeler, Masataka Higashiwaki, Vanya Darakchieva

### Chapter 29. Thermal Properties

In this chapter, an overview of the current research progress on the thermal properties of beta-gallium oxide (β-Ga2O3) is provided. Thermal properties of β-Ga2O3 are of great significance to the device reliability and performance in its potential applications. Previous research through both computational and experimental studies on β-Ga2O3 using various methods is reviewed. The most notable findings are the relatively low and highly anisotropicAnisotropic thermal conductivityThermal conductivity . At room temperature, the [010] direction has the highest thermal conductivityThermal conductivity of around 25 W/mK, while that in the [100] direction is measured to be the lowest, which is around 13 W/mK. We also make comparison between β-Ga2O3 and GaNGaN , another widely used semiconductor for power electronicsPower electronics . The relatively low thermal conductivity of β-Ga2O3 compared to GaNGaN may present a major challenge for its potential applications. Another important thermal property, heat capacityHeat capacity , of β-Ga2O3 at room temperature is measured to be 18.7 J/mol K. On the other hand, the effective thermal conductivityThermal conductivity in β-Ga2O3 thin filmThin film is shown to be larger than other gate oxides, providing a possibility of using it as gate dielectrics in GaNGaN device contacts. The thermal properties discussed in this chapter might be useful for thermal management and design of β-Ga2O3 devices.

Zeyu Liu, Tengfei Luo

### Chapter 30. Scintillation Properties

ScintillatorsScintillator are key materials for radiation detectorsRadiation detector for a wide range of applications such as medicine, security, well-logging, environmental monitoring and high energy physics. In recent years, Ga2O3 has attracted much attention as a scintillator. In this chapter, we introduce some backgrounds of scintillatorsScintillator and scintillation detectorsScintillation detector , which are followed by recent R&D of Ga2O3 for scintillation detectors. In this chapter, in order to improve the scintillation properties, we have examined some dopants for Ga2O3. The dopants are rare earthRare earth , ns2Ns2 and alkali earthAlkali earth ions. In addition to the investigations of dopants, we have also investigated the transparent ceramicTransparent ceramic form. These results are introduced in the present chapter.

Takayuki Yanagida, Go Okada, Noriaki Kawaguchi

### Chapter 31. Field-Effect Transistors 1

Introduction of Early Transistor Developments for Power Switching and RF Applications

In this chapter, we describe the early development of gallium oxide transistorsTransistor that led to the most common gallium oxide device: n-type, depletion-mode, metal insulator semiconductor field effect transistorTransistor . We relate the enticing material properties of gallium oxide described in earlier chapters to the requirements in design and fabrication of marketable devices for low-loss powerPower switching and radio frequencyRadio Frequency (RF) applications. A route for device optimization is shown by maximizing electric field in the drift regionDrift region while reducing parasitic resistance. We analyze the developments that will be required to overcome device-related technical barriers such as achieving low contact resistance, reducing effects of self-heating, and increasing device gain. The chapter concludes with recent research and goals for gallium oxide transistorsTransistor moving forward with subsequent chapters detailing these topics.

Neil Moser, Andrew Green, Kelson Chabak, Eric Heller, Gregg Jessen

### Chapter 32. Field-Effect Transistors 2

Ga2O3 Field-Effect Transistors for Power Switching and Radiation-Hard Electronics

Ga2O3Ga2O3 has exploded onto the semiconductor landscape for next-generation power electronics because its enticing material properties, most notably a large critical field strength stemming from its ultrawide bandgap, promise miniaturized circuits and systems with high conversion efficiency. Intense pursuit of Ga2O3Ga2O3 power devicesPower device galvanized by the demonstration of a high-voltage Ga2O3 field-effect transistor (FET) in 2012 has brought about tremendous advancements in this new technology, whose strong radiation tolerance and high thermal stability also befit harsh-environment applications that impose stringent reliability requirements. This chapter reviews the designs and properties of various types of depletion-Depletion mode and enhancement-modeEnhancement mode Ga2O3 FETs—which have predominantly been lateral devices—for power switchingPower switching and radiation-hardRadiation-hard electronics. The development of vertical Ga2O3Ga2O3 transistors based on a low-cost, highly manufacturable ion implantationIon implantation doping process will also be presented.

Man Hoi Wong, Masataka Higashiwaki

### Chapter 33. Field-Effect Transistors 3

β-(AlxGa1−x)2O3/Ga2O3 Modulation-Doped Field-Effect Transistors

The availability of monoclinic aluminum gallium oxide (β-(AlxGa1−x)2O3) alloysAlloy and their staggered band alignment with β-Ga2O3 makes it possible to realize two-dimensional electron gas (2DEG)Two-dimensional Electron Gas (2DEG) through modulation doping. This brings unique advantages in improving the electrical transportElectrical transport properties in β-Ga2O3, and at the same time, it provides a great platform for the evaluation of a range of the fundamental materials and electrical properties of β-Ga2O3. In this chapter, we describe the early efforts in the design and epitaxy of the heterostructuresHeterostructure and discuss the confirmation of a 2DEGTwo-dimensional Electron Gas (2DEG) through temperature-dependent Hall measurements as well as the first observation of the quantum oscillationsQuantum oscillation in the material system. We will further discuss the realization of modulation-doped field-effect transistorsField effect transistors (MODFETs)Modulation-Doped Field Effect Transistors (MODFETs) , which have contributed to the evaluation of the saturation velocitySaturation velocity and demonstration of high breakdown field in the material system. We will also discuss the methods to improve the 2DEGTwo-dimensional Electron Gas (2DEG) density and present the first double heterostructureHeterostructure MODFETModulation-Doped Field Effect Transistors (MODFETs) (DH-MODFET)Double Heterostructure MODFET (DH-MODFET) . The early efforts on β-(AlxGa1−x)2O3/Ga2O3 heterostructuresHeterostructure confirmed that the modulation-doped structure could be a promising architecture for electronic device applications.

Yuewei Zhang, Sriram Krishnamoorthy, Siddharth Rajan

### Chapter 34. Field-Effect Transistors 4

Nano-Membrane β-Ga2O3 Field-Effect Transistors

β-Ga2O3Β-Ga2O3 -based field-effect transistor (FET)Field effect transistor is regarded as a promising candidate for the next-generation power electronics due to its ultrawide bandgap of 4.5–4.8 eV, estimated critical field of 8 MV/cm and decent intrinsic electron mobility limit of 250 cm2/Vs, yielding a high BFOM of more than 3000, which is several times higher than GaN and SiC. Meanwhile, β-Ga2O3Β-Ga2O3 crystal also possesses a unique property that it has a large lattice constant of 12.23 Å along [100] direction, which allows a facile cleavage into thin belts or nano-membranesNano-membrane . Therefore, by transferring β-Ga2O3Β-Ga2O3 nano-membrane from its bulk substrate to a foreign substrate, we can fabricate β-Ga2O3 on insulator FETs and then explore material and device potentials before β-Ga2O3 epitaxy technology becomes mature and the cost of the epi-wafers reduces significantly. In this chapter, we will focus on the nano-membraneNano-membrane -based FETs and their electrical interfaces, demonstrating record high drain current density of the devices, minimize the self-heating effectSelf-heating effect by the integration of nano-membrane on high thermal conductivityThermal conductivity substrates and expand research direction toward a low-power and wide bandgap logic application.

Hong Zhou, Jinhyun Noh, Hagyoul Bae, Mengwei Si, Peide D. Ye

### Chapter 35. Field-Effect Transistors 5

Vertical Ga2O3 Fin-Channel Field-Effect Transistors and Trench Schottky Barrier Diodes

Recently, significant progresses have been made on the demonstration and development of vertical gallium oxide power devices. The goal of this chapter is to give a brief review on vertical gallium oxide FinFETs and Schottky barrier diodes based on fin-channel structures. In both devices, vertical fin-shaped channels are the key to achieving high breakdown voltages and low on-resistances simultaneously. The chapter starts with a short introduction of vertical transistor concepts and development in various wide band gap semiconductors, then moves on to topics including vertical gallium oxide FinFET structures, device operation, high voltage design and baseline fabrication process flow. Fundamental electrical performance will be discussed, followed by our analysis on threshold voltage control, drain-induced barrier lowering effects and breakdown mechanisms. Finally, we will briefly mention our key results on vertical trench Schottky barrier diodes.

Zongyang Hu, Wenshen Li, Huili Grace Xing

### Chapter 36. Diodes 1

Vertical Geometry Ga2O3 Rectifiers

There is increasing interest in SchottkySchottky rectifiersRectifier made on wide bandgapWide bandgap semiconductors because of their fast switching speed, which is important for improving the efficiencyEfficiency of inductive motor controllers and power supplies, as well as their low forward voltage drop and high-temperature operability. The advantage of simple SchottkySchottky rectifiersRectifier over p-n diodesDiode is the shorter switching times due to the absence of minority carriers. This, however, leads to higher on-state resistance (RON) values than in p-i-n rectifiersRectifier . The availability of a wider bandgap than SiSi improves the rectifierRectifier performance, with lower on-state resistance at a given reverse voltage. Both GaNGaN and SiCSiC power SchottkySchottky diodesDiode have demonstrated shorter turn-on delays than comparable SiSi devices. Recently, vertical geometry rectifiersRectifier fabricated on thick epitaxial layers of β-Ga2O3 on conducting substrates grown by edge-defined film-fed growth (EFG) have shown promising performance in terms of high reverse breakdown voltageBreakdown voltage (VB > 1 kV) and low RON, leading to good power figure-of-meritsFigure-of-merit $$(V_{\text{B}}^{2} /R_{\text{ON}} )$$ Electrical breakdownBreakdown caused by impact ionization process will preferentially occur at the contact periphery if the maximum electric field in these areas is not reduced by proper edge terminationEdge termination design. This chapter summarizes recent advances in the design and implementation of Ga2O3Gallium oxide vertical geometry rectifiersRectifier .

Jiancheng Yang, Minghan Xian, Randy Elhassani, Fan Ren, S. J. Pearton, Marko J. Tadjer, Akito Kuramata

### Chapter 37. Diodes 2

All-Oxide pn-Heterojunction DiodesHeterojunction diode with β-Ga2O3

Due to the unipolarity and small predicted hole mobilities of $$\upbeta \text {-}\text {Ga}_{2}\text {O}_{3}$$, bipolar homojunction diodesDiode are not of relevance for this material. A valid alternative is the realization of heterojunction diodesHeterojunction diode, especially with other, p-type oxideP-type oxide semiconductors, as they also exhibit a high chemical stability as well as typically large bandgaps. Possible candidates for such p-type oxideP-type oxide semiconductors are e. g., nickel oxide, cuprous oxide, or zinc cobalt oxide. Here, the properties of $$\upbeta \text {-}\text {Ga}_{2}\text {O}_{3}$$/p-type oxideP-type oxide semiconductor heterojunction diodesHeterojunction diode are discussed for the different p-type semiconductorsP-type semiconductor and different device layoutsDevice layout. Characteristic properties like ideality factorIdeality factor, rectificationRectification ratio ratioRectification ratio, built-in potentialBuilt-in potential, and breakdownBreakdown voltage are compared for the different devices.

Daniel Splith, Peter Schlupp, Holger von Wenckstern, Marius Grundmann

### Chapter 38. Photodetectors

Overview of Ga2O3-Based Photodetectors

This chapter provides an overview of Ga2O3-basedGa2O3 photodetectorsPhotodetector , which have been the subject of many published papers. I classify and summarize these detectors in several tables, roughly according to the device types: film-based photodetectorsPhotodetector (photoconductorsPhotoconductor , metal-semiconductor-metal photodetectors, and alloy-film-based photodetectorsPhotodetector ), vertical SchottkySchottky photodiodes that use bulkBulk single crystals, heterojunctionHeterojunction photodetectorsPhotodetector , and nanostructure-basedNanostructure photodetectors. In each class of detectors, some interesting devices are spotlighted.

Takayoshi Oshima

### Chapter 39. Image Sensors

Ga2O3/Se Photodiodes for Image Sensor Applications

The advent of next-generation broadcasting systems such as 8K Super Hi-Vision8 K Super Hi-Vision (SHV) has increased the demand for high-performance cameras. However, the low sensitivity of 8K8 K cameras is challenging. To address this issue, we have developed the stacked complementary metal-oxide semiconductor (CMOS) image sensor overlaid with a gallium oxide (Ga2O3Gallium oxide (Ga2O3) )/crystalline seleniumCrystalline Selenium (c-Se) (c-Se) photodiode. Using Ga2O3Gallium oxide (Ga2O3) decreased the dark current resulting from the injection of holes from the electrode. In addition, Ga2O3Gallium oxide (Ga2O3) doped with tin (Sn), which has higher carrier concentrationCarrier concentration , effectively reduced the operating voltage because the depletion layerDepletion layer spread into c-SeCrystalline Selenium (c-Se) more easily as compared to that in non-doped Ga2O3. Furthermore, the crystallization of Ga2O3Gallium oxide (Ga2O3) improved the crystal orientation of the SeSelenium formed on β-Ga2O3Β-Ga2O3 , thereby decreasing the dark current.

### Chapter 40. Gallium Oxide Materials and Devices

A Personal Recent History

Gallium oxideGallium oxide has recently been found to be of high interest as the widest bandgapBandgap semiconductorSemiconductor for which single crystalsSingle crystal bulkBulk substratesSubstrates are available and whose electronic conductivityConductivity can be controlled by n-typeN-type dopingN-type doping. Because wide-bandgapWide bandgap semiconductorsSemiconductor lead to high breakdownBreakdown voltagesBreakdown voltage in small length scales, and the resistive losses over small length scales are low, the material has several attractive attributes for high-voltage electronic diodesDiodes and switches. In this chapter, I give a personal account of the materials science and physics of this exciting new semiconductorSemiconductor material and its initial use in device demonstrations. The intention of the personal account is to share the twisted and connected paths that lead one to a specific research direction—this is certainly true for how I ended up being interested in gallium oxideGallium oxide as an interesting semiconductorSemiconductor material.

Debdeep Jena