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Part I

Gallium oxide

IOP Publishing

Wide Bandgap Semiconductor-Based Electronics

Fan Ren and Stephen J Pearton

Chapter 1

Low-dimensional β-Ga2O3 semiconductordevices

Suhyun Kim, Jinho Bae, Janghyuk Kim and Jihyun Kim

1.1 IntroductionCertain properties of wide bandgap semiconductors have received significantattention and have led to pivotal breakthroughs in (opto)electronics. Their lightabsorption and emission in the ultraviolet (UV) and visible regions are attractive inoptoelectronics, while their high breakdown voltage makes them suitable for high-power electronics. Furthermore, ultra-wide bandgap semiconductors, which havehigher bandgap energies than those of conventional wide bandgap materials such asSiC or GaN, can exhibit superior device performance as the relationship between thebandgap and the figures of merit is nonlinear; an increase in the bandgap causes amuch more significant increase in the figures of merit. Gallium oxide (Ga2O3) is thenewest and least mature among the major materials that have paved the way for theadvancement of ultra-wide bandgap semiconductors. Ga2O3 has five polymorphs,among which the monoclinic β phase is the most thermodynamically stable crystal.Owing to its bandgap energy of 4.6–4.9 eV, β-Ga2O3 is suitable for deep-UV solar-blind photodetectors [1, 2]. Moreover, the superior Baliga’s figure of merit forβ-Ga2O3 shows great potential for applications in high-power devices and its highchemical and thermal stabilities allow device operation under harsh conditions [3, 4].

Low-dimensional β-Ga2O3 nanostructures, including nanowires, nanobelts,nanorods, nanosheets, and nanomembranes, provide further advantages in additionto all the attractive properties of a single-crystal substrate. Therefore, these nano-structures, which can be fabricated by either top-down or bottom-up methods, havebeen widely applied in electronic and optoelectronic devices in the form of anindividual nanobelt, an array, or a bridged network of nanostructures. Usingsubstrates with higher thermal conductivity when fabricating devices based onβ-Ga2O3 nanostructures could be a solution to the low thermal conductivity, whichis a critical drawback in applying β-Ga2O3 in high-power electronics. Furthermore,

doi:10.1088/978-0-7503-2516-5ch1 1-1 ª IOP Publishing Ltd 2020

https://doi.org/10.1088/978-0-7503-2516-5ch1

the nanostructures have fewer defects and less strain when they form heterostruc-tures, while the deposition of bulk material on a substrate usually induces a latticemismatch between the material and the substrate. Through van der Waalsintegration with other two-dimensional (2D) materials, the unique properties ofβ-Ga2O3 nanostructures can be further enhanced. After a brief discussion regardingthe preparation methods and the contact properties of β-Ga2O3 nanostructures, thefabrication and characterization of various transistor structures are demonstrated inthis chapter.

1.1.1 Preparation of low-dimensional Ga2O3 nanostructures

Nanostructures can be prepared through either bottom-up or top-down methods. Thebottom-up methods for growing β-Ga2O3 nanostructures include physical evapora-tion, arc discharge, laser ablation, carbothermal reduction, microwave plasmachemical vapor deposition (CVD), and metal–organic CVD. The CVD methods aremost widely used as they allow the reproducible synthesis of high-purity β-Ga2O3nanostructures at a high deposition rate. Nanostructures with different properties andstructures can be achieved by controlling the growth parameters, such as theprecursors, catalysts, growth temperature, growth time, distance between the sourceand the substrate, and flow rates of the source gases. Gallium and oxygen arecommonly used as precursors when growing Ga2O3 nanostructures using CVDequipment. Auer et al also used these materials as the source and grew differenttypes of nanostructures including nanorods, nanoribbons, nanowires, and cones ofmonoclinic β-Ga2O3 depending on the growth temperature and the presence of Aucatalysts in the CVD process. The nanoribbons and nanorods were synthesized withand without catalysts, respectively, through the vapor–solid growth mechanism, whilethe nanowires were grown in the presence of catalysts through the vapor–liquid–solid(VLS) mechanism [5]. Kumar et al used a different catalyst, Fe, to synthesizecrystalline β-Ga2O3 nanowires through the VLS mechanism and obtained structural,morphological, and optical properties comparable to those grown using Au cata-lysts [6]. Similarly, the diameter of the β-Ga2O3 nanowires can also be effectivelycontrolled by the growth parameters; the obtained diameters are usually larger forhigher values of temperature, time, and gas flow rate in the presence of catalysts, asshown in figure 1.1 [7]. Furthermore, a larger catalyst size or smaller distance betweenthe metal source and the substrate produces nanowires with larger diameters [7].

Monoclinic β-Ga2O3 can also be combined with other materials to producefunctional nanostructures for high-performance device applications. Hsieh et alsynthesized Au/Ga2O3 core–shell nanowires through the VLS mechanism andnitridized them at a relatively low temperature of 600 °C to form Au/Ga2O3/GaNnanowires (figure 1.2). The nanowires had a metal–oxide–semiconductor (MOS)structure that could be applied to vertical high-power nanoelectronics [8]. Similarly,Kumar et al investigated the influence of ammonification on the synthesis of coaxialGaN/Ga2O3 [9]. They reported that as the ammonification temperature increased,the decomposition rate of the β-Ga2O3 nanowires increased, leading to increasedGaN conversion. Therefore, coaxial GaN/Ga2O3 was formed at relatively low

Wide Bandgap Semiconductor-Based Electronics

1-2

ammonification temperatures while the complete conversion of GaN from β-Ga2O3was observed at a high temperature of 1050 °C (figure 1.2). Furthermore, Au-decorated β-Ga2O3 nanowires were also demonstrated to modify the opticalproperties of the nanostructures and enhance their photocatalytic effect [10].

Ar/O2 gases

Ga metalGa2O3 nanowire

Catalyst alloySubstrate

900°C

850°C

800°C

Increasing distance between metal source to substrate

Dec

reas

ing

tem

pera

ture

Decreasing catalystnanoparticle size

Incr

easi

ng g

row

thte

mpe

ratu

re to

850°C

Hig

her f

low

rate

ay 9

00°C

Figure 1.1. Schematic of diameter tuning of β-Ga2O3 nanowires by controlling the growth parameters.Reproduced with permission from [7]. Copyright 2017 Springer Nature.

Figure 1.2. Structural characterization of Au/Ga2O3/GaN nanowires with MOS structure. Reproduced withpermission from [8]. Copyright 2008 the American Chemical Society.

Wide Bandgap Semiconductor-Based Electronics

1-3

The top-down approach for forming nanostructures usually involves the etchingof bulk materials. For example, an inductively coupled plasma etching of a GaN-based planar LED covered with Ni nanoclusters and a SiO2 layer transformed thedevice into a nanorod LED and improved its light extraction efficiency and outputpower. Although there has been no reported research regarding the formation ofβ-Ga2O3 nanostructures through etching, another top-down approach is availablefor β-Ga2O3 owing to its crystal structure. The exceptionally high lattice constantalong one direction (a = 1.2 nm, b = 0.3 nm, and c = 5.8 nm) in monoclinic β-Ga2O3enables mechanical exfoliation using the convenient Scotch tape method [11–13].Mechanical exfoliation is commonly applied to 2D materials formed by the van derWaals interaction between each layer. The cleavage planes parallel to the (100) and(001) planes allow mechanical cleavage of β-Ga2O3 substrates into thin flakeswithout the need for etching, as shown in figure 1.3 [11]. Because the flakes areexfoliated from a single-crystal substrate, high crystallinity is maintained in thenanobelts. The exfoliated flakes show low surface roughness and are ∼20–400 nm inthickness [11, 12, 14–18]. As the exfoliation and the transfer processes are allperformed under dry conditions and the exfoliated β-Ga2O3 flakes form van derWaals heterostructures with other materials, a clean interface is observed betweenthe layers [13, 16].

1.1.2 Contact properties of β-Ga2O3 nanodevices

The study of contact properties is essential to fully characterize β-Ga2O3 nanobelt-based devices, which have a smaller contact area than the bulk-substrate-baseddevices. This is because the contact properties have a significant influence on theelectrical characteristics of the device. Therefore, optimal contact metal selectionand process optimization are essential to avoid unintentional device degradation dueto poor contact.

Generally, a Schottky contact is formed when a metal is in contact with a widebandgap semiconductor. Therefore, the contact resistance between β-Ga2O3, which

tape

Exfoliation& transfer

β-Ga2O3

β-Ga2O3

SiO2p+ SiBack gate

Photo-lithographyO

Gaa[100]

Source(Ti/Au)Drain(Ti/Au)

b[010] c[001]90°

90°103.7°

(a)

(c) (d)

(b)

Figure 1.3. Schematic of the fabrication process for field-effect transistors based on mechanically exfoliatedβ-Ga2O3 flakes. Reproduced with permission from [11]. Copyright 2016 PCCP Owner Societies.

Wide Bandgap Semiconductor-Based Electronics

1-4

is an n-type wide bandgap semiconductor, and the contact metal depends on theSchottky barrier height (SBH), ΦB. The SBH for n-type semiconductors satisfies thefollowing equation:

Φ = Φ −q q m E ,B EA

where q is the electron charge, Φm is the work function of the contact metal, and EEAis the electron affinity of the n-type semiconductor. In the case of devices consistingof ohmic contacts such as metal–oxide–semiconductor field-effect transistors(MOSFETs), high contact resistance can be responsible for the degradation of theelectrical properties. On the other hand, the Schottky contact properties determinethe performance of devices such as metal–semiconductor field-effect transistors(MESFETs) or Schottky barrier diodes (SBDs). Hence, to fabricate and characterizeβ-Ga2O3 nanobelt devices that suit the purpose of a device, the contact propertiesbetween various metals and β-Ga2O3 nanobelts were investigated.

1.1.3 The ohmic contacts of β-Ga2O3 nanobelt devices

The first approach to form ohmic contacts is to select a metal electrode with thesmallest SBH, considering the electron affinity of β-Ga2O3 and the work function ofthe contact metal. This led to the study of various metal electrodes that can achievethe lowest contact resistance and excellent ohmic contact properties in β-Ga2O3nanobelt-based devices. Yao et al analyzed the electrical properties of nine differentcontact metals, namely Ti, In, Ag, Sn, W, Mo, Sc, Zn, and Zr, and evaluated thecontact properties of each metal electrode [19]. The study showed that Au-cappedTi metal electrodes have the best ohmic properties among the aforementioned ninemetal electrodes. Considering the existence of the Schottky barrier, which iscalculated by the work function of Ti and the electron affinity of the unintentionallydoped β-Ga2O3 (4.33 eV and ∼4.00, respectively), an underlying mechanism forforming the ohmic contact is observed, which will be described later.

Ohmic contact in the bulk-substrate β-Ga2O3 devices was achieved through pre-treatments such as ion implantation, reactive ion etching, and plasma bombardmentin the defined region before the metal electrode deposition [20]. This pre-treatmentprocess is not commonly used for nanobelt-based devices because it causesunpredictable damage to the β-Ga2O3 and the underlying substrate, moreover itsohmic properties are not reproducible. Instead, post-treatments such as annealingafter metal electrode deposition are mainly used for ohmic contact confirmation.The annealing process is expected to reduce the damage before and after thefabrication process.

Bae et al investigated the annealing conditions of β-Ga2O3 nanobelt-based devicesand their influence on the contact properties of the contact metal (figure 1.4) [13].The results confirmed that the electrical characteristics of the Ti/Au contacts weresignificantly improved, compared to the as-deposited device, when the annealingprocess was performed at 500 °C. Research has also shown that out-diffusion ofoxygen from β-Ga2O3 to Ti, leading to the formation of oxygen vacancies as donors,significantly improves the electrical properties of devices. In addition, TixOy, which

Wide Bandgap Semiconductor-Based Electronics

1-5

has a lower work function due to the reaction between the out-diffused oxygen andTi, has a significant influence on the formation of the ohmic contact.

This result is consistent with that reported by Zhen et al, who performedAr-ambient annealing on a β-Ga2O3 nanobelt FET and confirmed that the contactresistance decreased from ∼430 to ∼0.387 Ω mm−1 before and after annealing,respectively [15]. Thus, it was confirmed that most Ti/Au contacts can form anohmic contact through annealing at ∼450 °C–500 °C. However, Bae et al confirmedthat oxygen diffusion degrades the metal electrode at temperatures at or above700 °C [13]. Therefore, multilayer metal deposition such as Ti/Al/Au and Ti/Al/Ni/Auis being investigated to achieve stable operation at high temperatures.

1.1.4 β-Ga2O3 nanobelt Schottky contacts

The Schottky contact is essential for the fabrication of structures such as MESFETsand SBDs, which are commonly used in power devices. Since β-Ga2O3 has beenattracting attention as a high-voltage device due to its wide bandgap and ultrahighbreakdown field, studies of the Schottky contact for bulk substrates have beenconducted. Farzana et al proposed guidelines for selecting a Schottky contact metalby calculating the SBH for four metals—Pd, Ni, Pt, and Au—using threeindependent methods (I–V, C–V, and internal photoemission) (figure 1.5) [21].The suitable contact metal for a Schottky barrier device was determined byanalyzing the current transport mechanism of different contact materials.However, there have been no specific studies regarding the Schottky contact forβ-Ga2O3 nanobelt devices, although Ni or Pt is used as a Schottky contact fornanobelt devices based on the existing research on bulk substrates. Bae et alproposed the use of Au-capped Ni contacts in β-Ga2O3 nanobelt devices as thetop-gate electrode for MESFET devices and measured their electrical properties[22, 23]. Swinnich et al fabricated a flexible substrate-based SBD. They deposited aPt Schottky contact on the β-Ga2O3 nanobelt to fabricate an SBD that showed anexcellent breakdown voltage even when bent [24]. However, Kim et al demonstrated

100A

tom

ic p

erce

ntag

e (%

)

50

00.1 0.2 0.3

Distance (μm)(a) (b)

100

Ato

mic

per

cent

age

(%)

50

00.1 0.2 0.3

Distance (μm)

TiAuGaO

TiAuGaO

Figure 1.4. Atomic percentage profiles of contact metal and β-Ga2O3 obtained using energy dispersive x‐rayspectrometry (a) before and (b) after annealing at a temperature of 500 °C. Reproduced with permission from[13]. Copyright 2017 the Electrochemical Society.

Wide Bandgap Semiconductor-Based Electronics

1-6

Pt/Au electrode degradation during high-temperature operations. Therefore, furtherstudies are required to confirm stable operation under harsh conditions [25].

The greatest advantage of the β-Ga2O3 nanobelts is that they can be integratedwith other 2D materials to fabricate devices that can utilize the advantages of bothmaterials. Therefore, there have been attempts toward the integration and bandgapengineering of β-Ga2O3 nanobelts with mechanically exfoliated 2D materials. Forexample, Yan et al fabricated a graphene barristor device utilizing the Schottkycontact between exfoliated graphene and a β-Ga2O3 nanobelt to achieve a highbreakdown field of 5.2 MV cm−1 [26]. Kim et al used graphene as a transparent top-gate electrode for β-Ga2O3 nanobelts, minimizing the dark current and shadoweffect under the electrode (figure 1.6) [27]. Further studies are underway to formmultifunctional Schottky contacts that cannot be applied to metal electrodes, suchas transparent electrodes or junction FETs, through strain-free van der Waalsbonding between the β-Ga2O3 nanobelt and 2D materials.

100

10–2

Cur

rent

den

sity

(A/c

m2 )

10–4

10–6

10–8–3 –2 –1 0 1

Voltage (V)

PdNiPtAu n~1.09

n~1.03

n~1.04n~1.05

Figure 1.5. Schottky diode properties of β-Ga2O3 nanobelt FETs with various Schottky contact metals.Reproduced with permission from [21]. Copyright 2017 AIP Publishing.

Grapheneβ-G

a 2O3

Figure 1.6. Schematic of β-Ga2O3/graphene heterostructure photodetector devices. Graphene is used as thetransparent top-gate electrode of the photodetector. Reproduced with permission from [27]. Copyright 2019the American Chemical Society.

Wide Bandgap Semiconductor-Based Electronics

1-7

1.2 β-Ga2O3-based nanoelectronic devicesTo explore β-Ga2O3-based nanoelectronic devices, there have been several studies onthe fabrication and characterization of various types of FETs and SBDs usingβ-Ga2O3 nanobelts. The β-Ga2O3 nanobelt, which can be easily exfoliated from thebulk crystal and inherits the quality of the bulk crystal, is more suitable fornanoelectronic devices because of its superior quality compared to the otherβ-Ga2O3 nanostructures [28]. Generally, the β-Ga2O3 nanobelts used for fabricatingdevices have thicknesses in the range of tens to hundreds of nanometers and areexfoliated along the (100) or (001) directions [29]. Moreover, β-Ga2O3 exhibits lowthermal conductivity of 11–27 W m−1 K−1, which is notably lower compared tocompeting materials such as SiC (360–490Wm−1 K−1) and GaN (150–200Wm−1 K−1)[4, 30]. Therefore, research on improving the thermal conductivity of β-Ga2O3-baseddevices by integrating with other substrates or materials is essential for the developmentof β-Ga2O3-based power electronic devices. As β-Ga2O3 nanobelts are easily transferredto various substrates and materials, they can be applied in research on the electrical andthermal properties of β-Ga2O3-based nanoelectronic devices on various substrates inpreference to conventional bulk crystals. In addition, β-Ga2O3 nanobelts can becombined with other semiconductor materials to fabricate various types of new devices.

1.2.1 Single β-Ga2O3 nanobelt-based field-effect transistors

Hwang et al successfully demonstrated the possibility of fabricating MOSFETsusing β-Ga2O3 nanobelts for the first time [12]. Since then, there have been variousstudies regarding the fabrication of β-Ga2O3-based nanoelectronic devices usingnanobelts.

The thickness of the nanobelt was in the range of 20–100 nm and TEMobservations showed that it is mainly exfoliated along the (100) direction. Theenergy-dispersive x-ray spectroscopy and the absorption spectra of the (100)β-Ga2O3 nanobelt confirmed that the properties of the bulk crystal are maintainedin the nanobelt. The fabricated β-Ga2O3 nanobelt MOSFET was fabricated bydepositing a Ti/Au metal electrode followed by transferring a 100 nm thick β-Ga2O3nanobelt onto an SiO2/Si substrate, as shown in figure 1.7.

The β-Ga2O3 nanobelt MOSFET exhibits μFE and SS values of ∼70 cm2 V−1 s−1

and ∼200 mV dec−1, respectively, as shown in figure 1.8. These values were relativelylow compared to those of bulk β-Ga2O3 devices, indicating that the metal contactand interfaces have not been optimized. The device exhibited n-type semiconductorbehavior originating from atomic defects or impurities in the β-Ga2O3. Thefabricated β-Ga2O3 nanobelt MOSFET exhibited a high gate modulation of ∼107

even under a high drain voltage of 20 V.Kim et al fabricated back-gated MOSFETs using ∼200 nm thick β-Ga2O3

nanobelts that were mechanically exfoliated from unintentionally n-type doped(∼3 × 1017 cm−3) commercial β-Ga2O3 substrates and studied their stability andelectrical characteristics under various operating temperatures in the range of 25 °C–250 °C [11]. They reported that the electrical conductance of the β-Ga2O3 MOSFET

Wide Bandgap Semiconductor-Based Electronics

1-8

increased with temperature and the activation energy was approximately 0.25 eV,implying that the increased conductivity of the β-Ga2O3 nanobelts was caused byactivation of deep donor level oxygen vacancies at higher temperatures. Furthermore,an electrical breakdown was not observed in these measurements for a VDS of upto +40 V and a VGS of −60 V between 25 °C and 250 °C.

Tadjer et al reported the fabrication of enhancement-mode (E-mode) FETs basedon β-Ga2O3 nanobelts using a high-kHfO2 gate dielectric [31]. The threshold voltagewas +2.9 V at a VDS of 100 mV, which, according to the authors, originated from alarger conduction band offset between HfO2 and β-Ga2O3. A maximum IDS ofabout 11.1 mA mm−1 and an on-resistance (RON) of about 818 Ω mm weremeasured at VGS = 18 V, and at VG < 0 a large off-state current leakage wasobserved, which originated from the traps at the HfO2–β-Ga2O3 or β-Ga2O3–SiO2interfaces. Electron spin resonance (ESR) measurements were performed at room

Thermally grown SiO2(285 nm)

Metal (Ti/Au)

Ga2O3

285 nm SiO2

Si wafer

Nanomembrane Ga2O3 transfer

Cubic Ga2O3 cleaved and mechanically exfoliated

3 μm

Optical image of final devices Source/drain/back gate

Figure 1.7. Schematic process flow for the fabrication of β-Ga2O3 nanobelt field-effect transistors. Reproducedwith permission from [12]. Copyright 2014 AIP Publishing.

103 120

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LG = 3 μm

IG

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VDS = 20 VVDS = 10 V

T = 300 K101

10–1

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–18

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A)

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hres

hold

swin

g (m

V/d

ec.)

I D (μ

A)

I D (μ

A)

0.3

0.2

0.1

0.0 0.0

T = 300 K

W = 1 μmLG = 3 μmT = 300 K

Figure 1.8. (a) Drain current (ID) versus back-gate-to-source voltage (VBG), showing an on–off current ratio of∼107 and n-type semiconductor behavior. (b) Field-effect mobility (μFE) and (c) subthreshold swing (SS) versusVBG. Reproduced with permission from [12]. Copyright 2014 AIP Publishing.

Wide Bandgap Semiconductor-Based Electronics

1-9

temperature to investigate the origin of the electrons in the β-Ga2O3 nanobelts andthe results indicated that the conductivity originated from the presence of oxygenvacancies in the β-Ga2O3 nanobelts: the density of oxygen vacancies was measuredto be 2.3 × 1017 (±50%) cm−3.

Ahn et al demonstrated back- and top-gated MOSFETs using Al2O and SiO2 asthe dielectrics of the top and back gates, respectively [32]. The ID–VDS outputcharacteristics of the β-Ga2O3 nanobelt FETs operating at 25 °C with either or bothfront and back gates were analyzed. Channel modulation was observed to improvewhen both gates were used rather than when using either of the gates. The IDS waseffectively modulated by VGS with good saturation and sharp pinch-off character-istics. Moreover, there was no electrical breakdown up to biases of VDS = +100 Vand VGS = −100 V, showing stable performance. A saturation mobility (μ) of∼1.35 cm2 V−1 s−1 was achieved, which was determined using the following equation:

μ = × ×LW

m C2

( ),2 G

where ID,SAT is the drain saturation current, m is the slope of a regression fit to thestraight-line portion of the (ID,SAT)

1/2–VG transfer curve, W is the gate width, L isthe gate length, and CG is the gate capacitance. The authors noted that the mobilityin depletion-mode (D-mode) structures was often overestimated as the β-Ga2O3nanobelt FET is a bulk conduction channel device.

Zhou et al demonstrated the fabrication of E-mode and D-mode β-Ga2O3nanobelt-based MOSFETs by optimizing the thickness of a β-Ga2O3 nanobelt[33]. They used Sn-doped β-Ga2O3 nanobelts with a doping concentration of 2.7 ×1018 cm−3 as the channel layer. Ar plasma treatment was implemented to decreasethe contact resistance. As a result, a relatively high maximum drain current density(ID,MAX) of 600 mA mm

−1 was achieved by a D-mode MOSFET using Sn-doped94 nm thick β-Ga2O3 nanobelt. The authors reported that the threshold voltage (Vth)shifted from the negative to positive direction as the thickness was graduallyreduced. The dependence of Vth on the thickness of the β-Ga2O3 nanobelt wasattributed to the surface depletion effect caused by the dangling bond on theβ-Ga2O3 nanobelt surface. Meanwhile, the E-mode β-Ga2O3 nanobelt FETsdemonstrated a high breakdown voltage of 185 V and an average electrical field(Eav) of 2 MV cm

−1, demonstrating the great potential of β-Ga2O3 nanobelt-basedFETs in future power devices.

β-Ga2O3 nanobelt-based high-power devices are attracting attention due to theiradvantages, which include minimizing power loss due to their downsizing andeconomizing power system through efficiency maximization. Among the availablefabrication techniques, the field plate (FP) technique is widely used owing to its easeof fabrication and efficiency. The ability to disperse concentrated electric fields atspecific locations through a gate FP, source-connected FP, or multiple FPs hasalready been confirmed in devices such as GaN and AlGaN, which are commonlyused for the fabrication of conventional power device materials. A study forincreasing the breakdown voltage of β-Ga2O3 devices by applying the FP technique

Wide Bandgap Semiconductor-Based Electronics

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was also conducted on a nanobelt-based device. Bae et al conducted a study tomaximize the breakdown voltage of a quasi-2D power device by introducing asource-connected FP to a β-Ga2O3 nanobelt MESFET (figure 1.9) [23]. Themaximum value of the electric field on the β-Ga2O3 surface was decreased from∼11 to ∼6 MV cm−1 by introducing the source FP, which is smaller than thebreakdown field of β-Ga2O3. The three-terminal off-state breakdown voltage of thefabricated devices was improved to 314 V, which is twice that of the conventionaldevices, confirming the possibility of a high-power nanodevice.

A high ID,MAX of 1.5/1.0 A mm−1 for D/E-mode β-Ga2O3 nanobelt FETs was

achieved by increasing the doping concentration of the β-Ga2O3 nanobelts from3.0 × 1018 to 8.0 × 1018 cm−3, respectively [34]. Further, a lower contact resistance of0.75 Ωmm and a higher electric field velocity of 7.3 × 106 cm s−1 were achieved fromthe high doping channel compared with those of the low doping channel. In addition,the self-heating effect was studied using thermo-reflectance measurements [35]. Theresults showed that even at a low bias power regime (P = VDS × ID = 1.2Wmm

−1), orin unbiased devices, the temperature of the device was 35 °C higher than roomtemperature. Moreover, the device temperature increased more significantly in higherpower bias regimes, leading to reduced electron mobility, reliability, and breakdownvoltage.

Furthermore, the authors reported thermodynamic investigations of a β-Ga2O3nanobelt FET on a sapphire substrate, which has half the ΔT compared to the SiO2/Si substrate [36]. The thermal resistances were measured to be 4.6 × 10−2 and 1.47 ×10−1 mm2 KW−1 for the sapphire and SiO2/Si substrates, respectively. As a result ofthe reduced self-heating, an ID,MAX of 535 mA mm

−1 was achieved on the sapphiresubstrate, which is 2.5× higher than that on the SiO2/Si substrate. These results showthat incorporating β-Ga2O3 channels in substrates with high thermal conductivitycan help solve the low thermal conductivity problems of β-Ga2O3 in powerelectronics applications.

Ma et al reported abnormal positive threshold voltage (Vth) shifts under negativebias stress conditions while operating β-Ga2O3 nanobelt MOSFETs [37]. This isattributed to the surface depletion effects originating from the surface state of the

404

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)

0

0 50 100 150 200 250 300 350 50 100 150 200 250 300 350VDS (V) VDS (V)

VDS = 145 V VDS = 314 V

S G

SiO2

β-Ga2O3

D S G

SiO2

β-Ga2O3

D

FP

(a) (b)

Figure 1.9. Off-state three-terminal hard breakdown results of the fabricated β-Ga2O3 nanoFET (a) withoutand (b) with the source-connected field-modulating plate. The insets show the schematic of each device.Reproduced with permission from [23]. Copyright 2019 The Royal Society of Chemistry.

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molecules absorbed on the β-Ga2O3 nanobelt surface. The surface depletion effectswere moderated by passivating with an ALD–Al2O3 layer. These results reveal theimportance of proper passivation on the β-Ga2O3 surface.

1.2.2 β-Ga2O3 nanobelt-based heterostructured transistors

The 2D β-Ga2O3 nanobelts have been attracting increased interest as potentialnanoscale building blocks for future high-power (opto)electronic devices. Thecombination of 2D materials and β-Ga2O3 nanobelts is expected to produce a greatsynergy. The concept of 2D heterostructures was first demonstrated in FETs byintegrating mechanically exfoliated β-Ga2O3 nanobelts and h-BN flakes [16].

As h-BN has an extraordinarily flat and clean surface, the h-BN dielectricprovides a minimal density of charged impurities on its interface with theβ-Ga2O3 nanobelts. Deformations or faults in each layer and in the interfacebetween the β-Ga2O3 nanobelts and h-BN flakes were not observed from thecross-sectional TEM images, indicating the formation of a van der Waals hetero-structure. The fabricated β-Ga2O3/h-BN heterostructured transistors demonstratedlow gate leakage as well as a high IDS on–off ratio of ∼107. Furthermore, the authordemonstrated that the top-gate threshold voltage can be linearly controlled by aback-gate voltage using a dual gate operation.

Bae et al introduced a gate FP structure by integrating a 2D material on aβ-Ga2O3 nanobelt MESFET (figure 1.10) [22]. After the precise transfer of h-BN onthe β-Ga2O3 channel, half of the gate electrode was defined on h-BN to introduce agate FP structure. The h-BN gate FP, like the source FP, mitigated the electric fieldconcentrated at the hot gate edge, preventing premature breakdown of the device,securing the reliability of the device, and thereby improving the breakdown voltagesignificantly. The three-terminal off-state breakdown voltage of the fabricated h-BNgate FP β-Ga2O3 nanobelt MESFET was 344 V, which exhibited stability underhigh-voltage operation conditions.

Figure 1.10. (a) SEM image of the fabricated β-Ga2O3 FET with a h-BN gate FP device. (b) Three-terminaloff-state breakdown voltage of the fabricated device. Reproduced with permission from [22]. Copyright 2018AIP Publishing.

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Furthermore, a heterostructure n-channel depletion-mode β-Ga2O3 junction field-effect transistor (JFET) was fabricated by integrating a β-Ga2O3 nanobelt with aWSe2 flake as shown in figure 1.11 [38]. The p-type WSe2 flakes were used as thep-gate for the JFET instead of Ga2O3, which has not yet been implemented. Thefabricated WSe2–Ga2O3 heterojunction p−n diode displayed proper rectifyingbehavior with a high rectifying ratio of ∼105. The fabricated JFET exhibitedexcellent transistor characteristics with a high IDS on–off ratio of ∼108, a lowsubthreshold swing of 133 mV dec−1, and a three-terminal breakdown voltage of+144 V. This synergetic integration of 2D materials and β-Ga2O3 nanobeltsintroduces β-Ga2O3 as a nanoscale building block for future high-power devicesand opens the possibility of more diverse forms of β-Ga2O3 nanobelt-basedheterostructures.

1.2.3 β-Ga2O3 nanobelt-based Schottky barrier diode

An SBD is a widely used structure in high-power or high-frequency devices becauseof low voltage loss in forward connections and easy driving in high-frequencyenvironments. Using β-Ga2O3, which has been attracting attention as a potentialnear-future high-power material, various SBD devices have been fabricated. Forexample, Yang et al fabricated a Schottky rectifier using a Si-doped β-Ga2O3epitaxial layer and analyzed its electrical properties and breakdown voltage [39]. Theresults showed an excellent on–off ratio of 3 × 107 that was not affected bytemperature. Moreover, the device exhibited a breakdown voltage of more than

Fabrication sequence

WSe2

D(Ti/Au)

S(Ti/Au)(a) (b) (c)

(d) (e)

101

10–1

10–3

10–5

10–7

10–9–50 –40 –30 –20

VGS (back-gate) (V)

I Ds (

mA

/mm

)

–10 0

β-Ga2O3

β-Ga2O3(~320 nm thick)

WSe2(~55 nm thick)

SiO2 p++-Si Back-gate (Ti/Au)

Pt/Au

Pt/Au

G

D S

VDs = +20 VFigure 1(a)Figure 1(b)

Figure 1.11. (a)−(c) Optical images of the fabrication sequence of the WSe2/Ga2O3 heterojunction JFET. Thescale bars represent 10 μm. (d) IDS–VGS transfer characteristics of the device at VDS = +20 V (a) before and (b)after the multilayer WSe2 was transferred. (e) Schematic illustration of the WSe2/Ga2O3 heterojunction JFETon the SiO2/Si substrate. Reproduced with permission from [38]. Copyright 2018 the American ChemicalSociety.

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1600 V, which demonstrates the rapid progress in this material system, exhibitingimpressive power switching applications.

There have also been studies on the use of SBDs in β-Ga2O3 nanobelt devices. Forinstance, Swinnich et al fabricated a high-power flexible SBD by transferringβ-Ga2O3 nanobelts onto a polyimide substrate using the Scotch tape method anddepositing ohmic and Schottky electrodes [24]. A breakdown voltage of −119 V anda critical breakdown field of 1.2 MV cm−1 were measured, which demonstratedsuperior stability in high-power operations even after the bending test. This studyhas opened the possibility of various applications of β-Ga2O3 by implementingβ-Ga2O3 nanobelt SBDs fabricated on versatile flexible substrates.

Yan et al fabricated a β-Ga2O3/graphene vertical barristor heterostructure byintegrating a β-Ga2O3 nanobelt with graphene (figure 1.12) [26]. Taking advantageof the easy integrability of β-Ga2O3 nanobelts with other 2D materials, Yan’s groupfabricated a heterostructure device by overlapping a β-Ga2O3 nanobelt on agraphene flake. The fabricated barristor device could switch the current throughthe gate bias and exhibited an excellent on–off ratio of over 104. The device achieveda remarkable breakdown field of 5.2 MV cm−1 in a direction perpendicular to the(100) plane, which is much better than any other previously reported lateral FETdevice. This study not only utilized a β-Ga2O3 nanobelt-based device to fabricateheterojunctions, but also achieved stability in high-voltage driving.

1.3 ConclusionDifferent growth and functionalization techniques have been used to control theproperties of β-Ga2O3 nanostructures. For device applications, the ohmic andSchottky contacts to the nanostructures were investigated by introducing a 2Dmaterial with metallic properties or different metals that have been commonly usedfor wide bandgap materials. Although the research on β-Ga2O3 nanoelectronics isstill in its early stages, there have been several studies on the fabrication andcharacterization of devices based on β-Ga2O3 nanostructures. Most β-Ga2O3-basednanoelectronics devices have been fabricated using β-Ga2O3 nanobelts, which inheritthe characteristics of single-crystal β-Ga2O3. Using the advantages of β-Ga2O3nanobelts, such as their large bandgap, high breakdown field, and thermal and

Vd

(a) (b)

(100) planea[100]

c[001]

b[010]Vs

Vbg

Vds

E-fie

ld

10 μm

Figure 1.12. (a) Schematic of a β-Ga2O3/graphene vertical barristor heterostructure. (b) Optical microscopeimage of a fabricated device. Reproduced with permission from [26]. Copyright 2018 AIP Publishing.

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chemical stabilities, various types of high-performance electronics includingMOSFETs, MESFETs, SBDs, and JFETs have been demonstrated. β-Ga2O3nanobelts are considered potential candidates for establishing nanoscale deviceplatforms for heat management and future novel nanoelectronic applications.

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Chapter 1 Low-dimensional β-Ga2O3 semiconductor devices1.1 Introduction1.1.1 Preparation of low-dimensional Ga2O3 nanostructures1.1.2 Contact properties of β-Ga2O3 nanodevices1.1.3 The ohmic contacts of β-Ga2O3 nanobelt devices1.1.4 β-Ga2O3 nanobelt Schottky contacts

1.2 β-Ga2O3-based nanoelectronic devices1.2.1 Single β-Ga2O3 nanobelt-based field-effect transistors1.2.2 β-Ga2O3 nanobelt-based heterostructured transistors1.2.3 β-Ga2O3 nanobelt-based Schottky barrier diode

1.3 Conclusion References