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Title

金属-半導体界面層あるいは絶縁体-半導体界面層を用いたAlGaN/GaNデバイスの閾値電圧制御

Author(s)DENG, YUCHEN

CitationIssue Date

2024-06

Type

Thesis or Dissertation

Text version

ETD

URL

http://hdl.handle.net/10119/19335

RightsDescription

Supervisor: 鈴木 寿一, 先端科学技術研究科, 博士

Japan Advanced Institute of Science and Technology

Doctoral dissertation

Threshold voltage modulationin AlGaN/GaN devices
using metal-semiconductor
or insulator-semiconductor interfacial layers

YUCHEN DENG

Supervisor: Toshi-kazu SUZUKI
Graduate School of Advanced Science and Technology
Japan Advanced Institute of Science and Technology
Materials Science

June, 2024

Abstract
We have systematically investigated eﬀects of metal-semiconductor or insulatorsemiconductor interfacial layers (ILs) in AlGaN/GaN devices, where AlO x , TiOx ,or NiOx is employed as an IL. From capacitance-voltage characteristics of metal/
IL/AlGaN/GaN devices with a metal-semiconductor IL between the gate metaland AlGaN, it is shown that the IL modulates the threshold voltage Vth , attributed
to the vacuum level step induced by the dipole of the IL. We ﬁnd negative vacuum
level steps for AlOx and TiOx ILs, and positive for NiOx , from which the IL dipole
density is estimated for each IL material. The two-dimensional electron gas carrier
concentration in the metal/IL/AlGaN/GaN devices is also modulated by the vacuum level step. Furthermore, X-Ray photoelectron spectroscopy of the IL/AlGaN
interfaces suggest a formation of NiGa in the NiOx /AlGaN interface. On the other
hand, from capacitance-voltage characteristics of metal/Al 2 O3 /IL/AlGaN/GaN
devices with an insulator-semiconductor IL between Al2 O3 and AlGaN, the ﬁxed
charge density of the Al2 O3 /IL/AlGaN interface is evaluated by the Al2 O3 thickness dependence of Vth . For AlOx and TiOx ILs, the ﬁxed charge density is higher
than that of the Al2 O3 /AlGaN interface with no IL, while lower for NiOx . The
ﬁxed charge density for an IL shows a positive correlation with the IL dipole
density, suggesting that the ﬁxed charge is related to the unbalanced IL dipole.
Furthermore, using the conductance method, we ﬁnd a low trap density of the
Al2 O3 /IL/AlGaN interface for AlOx and NiOx ILs, in comparison with that of the
Al2 O3 /AlGaN interface with no IL.
Keywords: AlGaN/GaN device, interfacial layer, threshold voltage control, vacuum level step, ﬁxed charge

Contents
Abstracti

1 Introduction1

1.1

Compound semiconductor Gallium nitride (GaN) . . . . . . . . . .

1

1.2

AlGaN/GaN heterostructures . . . . . . . . . . . . . . . . . . . . .

9

1.3

Threshold voltage control in AlGaN/GaN devices . . . . . . . . . . 13

1.4

Interfaces in AlGaN/GaN devices aﬀecting the threshold voltage . . 16

1.5

Purpose of this study . . . . . . . . . . . . . . . . . . . . . . . . . . 25

1.6

Organization of the dissertation . . . . . . . . . . . . . . . . . . . . 25

2 Eﬀects of metal-semiconductor interfacial layers (ILs) in metal/
IL/AlGaN/GaN devices

27

2.1

Metal/IL/AlGaN/GaN device fabrication . . . . . . . . . . . . . . . 28

2.2

Details in the IL formation . . . . . . . . . . . . . . . . . . . . . . . 33

2.3

Metal/IL/AlGaN/GaN devices characterization . . . . . . . . . . . 35

2.4

X-ray photoelectron spectroscopy (XPS) for the IL/AlGaN interfaces 44

3 Eﬀects of insulator-semiconductor interfacial layers (ILs) in metal/
Al2 O3 /IL/AlGaN/GaN devices

51

3.1

Metal/Al2 O3 /IL/AlGaN/GaN device fabrication . . . . . . . . . . . 52

3.2

Metal/Al2 O3 /IL/AlGaN/GaN devices characterization . . . . . . . 55

4 Conclusion and future perspectives

73

4.1

Conclusion of this work . . . . . . . . . . . . . . . . . . . . . . . . . 73

4.2

Future perspectives of this work . . . . . . . . . . . . . . . . . . . . 74

CONTENTS
A Band diagram calculation

75

A.1 metal/IL/AlGaN/GaN band diagram . . . . . . . . . . . . . . . . . 75
A.2 metal/Al2 O3 /IL/AlGaN/GaN band diagram . . . . . . . . . . . . . 78
B Conductance method

80

B.1 AC admittance aﬀected by interface traps . . . . . . . . . . . . . . 80
B.2 AC admittance in equivalent circuits for MIS devices . . . . . . . . 84
List of publications

87

Bibliography90

Acknowledgment

96Chapter 1
Introduction
1.1

Compound semiconductor Gallium nitride(GaN)

Figure 1.1: Relations between frequency and power for (a) wireless-communication
applications [1] and (b) power-switching applications [2].

Compound semiconductor is important in high-speed wireless-communication and/or high-power switching. Figure 1.1(a) shows the relation between power and frequency requirements for several wireless-communication applications [1], where
each semiconducting material has a suitable application based on its physicalproperties. For examples, Si-based transistors [3] are widely employed in mo-Chapter 1 Introduction

2

bile communications, which require a middle range frequency ∼ 1G Hz and apower ∼ 1-1000 W. Meanwhile, GaAs-based transistors [4, 5] are employed forradar communications, where working frequencies are up to ∼ 100G Hz. In future
wireless-communication, devices being capable for both high-speed and high-power
applications are important [6]. Figure 1.1(b) shows the requirements for powerswitching applications [2], where the frequency range is downscale by 10 5 orders
while the power range is upscale by the same orders compare to Fig. 1.1(a). In
these applications, the semiconductor devices are required to to handle a large
power, and a moderate switching speed is also required depending on the application. However, there is a trade-oﬀ between speed (frequency) and power forboth high-speed and high-power applications, which means that the devices canoperate at high frequency are unlikely to be capable for high-power applications,and vice versa. Gallium Nitride (GaN), a III-V compound semiconductor, shows
potentials to overcome this trade-oﬀ. GaN, a nitride compound semiconductor,
has potentials to overcome this trade-oﬀ.
In order to measure the suitability of a semiconducting materials for high-speed
and high-power applications, we employ two types of ﬁgure of merit (FoM) in thefollowing. The ﬁrst one is Johnson’s FoM, which is given byfT VBr ≤FBr vsat
,
2π

(1.1)

where fT is the cut-oﬀ frequency, FBr is the breakdown ﬁeld, and VBr is the breakdown voltage. Figure 1.2(a) shows the relation between VBr and fT , indicating that
GaN is superior than Si or GaAs in both breakdown voltage and cut-oﬀ frequency.
The other FoM is Baliga’s, which is given by3≥
ϵ0 kµFbr

4Vbr2
,
Ron

(1.2)

where ϵ0 is the vacuum permittivity, k is the dielectric constant of the semiconducting material, and Ron is the on-resistance. The relation between Ron and Vbrfor several semiconductors is depicted in Fig. 1.2(b). It is clear that GaN withrelatively low Ron at high VBr in comparison with other semiconducting materials.

Chapter 1 Introduction

3

Figure 1.2: (a) Relation between VBr and fT for several semiconductors. (b) Relation
between Ron and VBr for several semiconductors.

In high-power applications, the applied source-drain voltage is high. Moreover,due to the decrease in the gate length, the electric ﬁeld under the gate becomesstronger, leading to earlier breakdown of the semiconductor. Therefore, materials
with high breakdown ﬁeld are advantageous for power-handling applications. The
breakdown ﬁeld FBr of a semiconductor is usually related to its energy gap Eg by
the relation
FBr ∝ Egα ,(1.3)

where α ≃ 1-3, making FBr an increasing function of Eg [7]. Therefore, for highpower applications, wide band gap semiconductors are preferable. Figure 1.3 shows
the relation between the energy gap and the lattice constant for several semiconductors [8, 9], where we can ﬁnd that GaN has a large band gap Eg of 3.4 eV,corresponding to a very high breakdown ﬁeld of 3.3 MV/cm. These properties
indicate that GaN is a favorable semiconductor for high-power applications.

Chapter 1 Introduction

4

Figure 1.3: The breakdown ﬁeld Fbr as a function of the band gap Eg for several
semiconductors [8, 9].

On the other hand, there is a trend of proportional relation [10]m∗Γ ∝ Egα ,(1.4)

which leads to a relatively large eﬀective mass m∗ = 0.20 for GaN as shown in
Figure 1.4.

Figure 1.4: The eﬀective mass m∗Γ at Gamma point as a function of the band gap Eg
for several semiconductors.

The eﬀective mass m∗ aﬀects the electron transport in semiconductors. The

Chapter 1 Introduction

5

carrier velocity in a FET v is determined by the drift velocity vd at low electricﬁeld and the saturation velocity vsat at high electric ﬁeld as shown in Figure 1.5.

Figure 1.5: Carrier velocity v as a function of the electric ﬁeld.

The drift velocity at low electric ﬁeld is
vd = µE =

qτ
E,
m∗

(1.5)

where µ is the electron mobility, q > 0 is the electron charge, τ is the scattering
time, m∗ is the electron eﬀective mass, and E is the electric ﬁeld. At low electric
ﬁeld, v in a material is directly proportional to the electric ﬁeld, where the slope
is the electron mobility µ. Thus a high mobility leads to a high drift velocity.
Meanwhile, at high electric ﬁeld, carriers have high enough kinetic energy to emit
optical phonons between collisions, leading to a decrease in velocity and a consequent prohibition in further acceleration. The saturation velocity, which is thevelocity a carrier can reach before emitting a phonon, is given byr
2ℏωop
vsat =,m∗(1.6)

where ℏωop is the optical phonon energy. From Eq. (1.5) and (1.6), we ﬁnd thatboth vd and vsat are proportional to 1/m∗ , thus materials with small m∗ are
advantageous for high-speed applications.

Chapter 1 Introduction

6

Figure 1.6: Relation between drift velocity and electric ﬁeld obtained by Monte Carlosimulation [11].

Figure 1.6 shows the electron drift velocity as functions the electric ﬁeld forseveral semiconductors obtained by Monte Carlo simulation [11]. For GaN, therelatively large m∗ leads to a relatively low mobility, and a consequent low electron
velocity in low-ﬁeld region. However, in high-ﬁeld region, GaN possesses a highelectron peak velocity of 2.5 × 107 cm/s and a high electron saturation velocity≥ 1.5 × 107 cm/s as expected, owing to a large optical phonon energy ℏωop ≃ 90
meV, and a large valley-separation energy.
A large valley-separation energy ∆E can suppress the transferred-electron effect, depicted in Fig. 1.7. The energy diﬀerence between the upper valley andthe lower valley in the conduction band known as the valley-separation energy∆E. A small ∆E leads to a easier transition from the lower valley with a higher
mobility, to the upper valley with a lower mobility, resulting in a decrease in the
total mobility, hence the carrier velocity. Figure 1.8 shows the calculated energy
band structures of zinc-blende GaAs, InP, and wurtzite GaN [12, 13], where GaN
shows a large valley-separation energy of ∆E = 1.4 eV in comparison with thatof GaAs ∆E = 0.32 eV and InP ∆E = 0.52 eV, leading to a higher saturation
velocity vsat for GaN.

Chapter 1 Introduction

7

Figure 1.7: The transferred-electron eﬀect.

Figure 1.8: The calculated energy band structures of zinc-blende GaAs, InP, andwurtzite GaN [12, 13].

In addition to the electrical properties, thermal conductivity κ is also an important property of semiconductors, especially in the high-power application. Whena FET operates with a drain current ID under a drain-to-source voltage VDS ,temperature T in the channel of the FET increases owing to the Joule heating
P = ID VDS as shown in Fig. 1.9(a), known as the self-heating. The increase in T

Chapter 1 Introduction

8

leads to a reduction in electron velocity, causing a negative drain conductance [14]gD = ∂ID /∂VDS < 0(1.7)

in the DC output characteristics of the FET shown in Fig. 1.9(b) [15], which isunfavorable in device application. In order to suppress the self-heating, high-κsemiconductor is desirable, where GaN is the case with a κ of ∼ 130 WK−1 m−1[16].

Figure 1.9: (a) Joule-heating in FETs. (b) Output characteristic of a FET considering
the self-heating[15].

We summarize the physical properties for several semiconducting materials in
Table 1.1, showing that GaN is a promising material owing to its high saturation
velocity, wide band gap and relatively high thermal conductivity.

Chapter 1 Introduction

9

Table 1.1: Comparison of properties between GaN and other semiconductors.

Mater.

1.2

Eg

µ

vsat

FBr

κ

[eV]

[cm2 /V.s]velocity [cm/s][V/cm][W/K.m]

Si

1.12

1500

1.0 ×107

3.0 ×105∼ 200-400

GaAs

1.42

8500

2.0 ×107

4.0 ×105

∼ 50

InAs

0.36

33000

∼ 4.0 × 107

4.0 ×104

∼ 30

SiC

3.33

900

2.0 ×107

3.0 ×106∼ 200-300

GaN

3.39

1100

2.7 × 107

3.3 × 106

∼ 130

AlGaN/GaN heterostructures

In addition to the properties discussed above, GaN can form a heterostructurewith AlGaN, which is a alloy of GaN and AlN, leading to further applications. In
the AlGaN/GaN heterostructure, spontaneous and piezoelectric polarization are
critical [17, 18]. The spontaneous polarization Psp comes from an intrinsic asymmetry of the bonding in equilibrium wurtzite crystal structures of GaN or AlN as
shown in Fig. 1.10(a). In a geometrically ideal wurtzite structure, the close-packed
structure leads to a lattice constant ratio c/a = 1.633 as shown in Fig. 1.10(b).
However, due to ionicity of Ga and N ions, the c-axis is distorted in the real GaN
wurtzite structure, leading to a c̃/a = 1.623 as shown in Fig. 1.10(c). This distortion leads to a net total polarization, so called the spontaneous polarization,
which would have been canceled out in the geometrically ideal case.

Chapter 1 Introduction

10

Figure 1.10: The GaN wurtzite structure and its distortion in real case.

On the other hand, the piezoelectric polarization Ppe origins from mechanical stress, i.e. tensile or compressive strain of AlGaN and GaN layers, shownin Fig. 1.12 as a consequence of the formation of AlGaN/GaN heterostructure.
Al(Ga)N has a smaller lattice constant than GaN as shown Fig. 1.11, leading to alattice constant mismatch between AlGaN and GaN. In order to compensate thismismatch, the AlGaN layer is compressed, resulting in an extra net polarization,so called the piezoelectric polarization.

Chapter 1 Introduction

11

Figure 1.11: The energy gap and corresponding wavelength as functions of the lattice
constant for several III-V compound semiconductors [8, 9].

Figure 1.12: The formation of AlGaN/GaN heterostructure, where the AlGaN layer
is compressed to compensate the lattice constant mismatch.

These two polarizations lead to a two-dimensional electron gas (2DEG) withhigh concentration at AlGaN/GaN heterointerface as shown in Fig. 1.13 [18]. Typically, an Al composition of 20-30% gives a 2DEG concentration ≃ 1013 cm−2 ,which can be utilized as the channel of a AlGaN/GaN Schottky FET. Such
FETs, namely the AlGaN/GaN high electron mobility transistors (HEMT), whose
schematic is shown in Fig. 1.14(a), are widely investigated for high-speed andhigh-power applications [19]. However, high gate leakage current despite of wideband gap [20–23] and current collapse due to surface electron trapping [24–26]
still occur in AlGaN/GaN Schottky FETs. In order to solve these problems, AlGaN/GaN metal-insulator-semiconductor (MIS) FETs, whose schematic is shown

Chapter 1 Introduction

12

in Fig. 1.14(b), have been investigated according to the advantages of gate leakage
reduction [23] and passivation to suppress the current collapse [24–26].

Figure 1.13: (a) The 2DEG at the AlGaN/GaN heterointerface. (b) Calculated sheet
charge density caused by spontaneous and piezoelectric polarizations of a GaN-face
GaN/AlGaN/GaN heterostructure [18].

Figure 1.14: (a) A schematic cross section of AlGaN/GaN Schottky FETs. (b) A
schematic cross section of AlGaN/GaN MIS FETs.

Chapter 1 Introduction

1.3

13

Threshold voltage control in AlGaN/GaN
devices

The control of threshold voltage Vth is one of the most important issues in both
Schottky and MIS FETs. Basically, Vth of a FET is the minimum gate-to-source
voltage VG that is necessary to create a conducting channel between the sourceand drain electrodes. Drain current ID and carrier density ns in the FET channel
are proportional to the diﬀerence between VG and Vth in GaN-based semiconductor
devices:
ID ∝ ns ∝ (VG − Vth ),(1.8)

shown in Figure 1.15. In this case, Vth can be deﬁned as the ID = 0 (ns )=0
intercept of the linear extrapolation for ID − VG or ns − VG relations. For diﬀerent
application, Vth with diﬀerent value, or even diﬀerent sign are demanded, making
the control of Vth an important issue.

Figure 1.15: Typical ID (ns )-VG characteristics of GaN-based FETs.

Owing to the existence of 2DEGs, carrier density ns in the channel of a AlGaN/GaN FET is > 0 at VG = 0, making the FET a normally-on devices witha ID -VG characteristic is shown in Fig. 1.16(a). However, in some applications,normally-oﬀ devices, whose ID -VG characteristic is shown in Fig. 1.16(b), are favorable from the viewpoint of fail-safe operations. Although there are several eﬀorts
to realize normally-oﬀ devices, such as partial gate recess [27], full gate recessChapter 1 Introduction

14[28], ﬂuoride plasma treatment [29], p-type (Al)GaN capping [30, 31], selective
electro-chemical oxidation [32], and ﬁn-structure formation [33], it is still diﬃcult
to establish normally-oﬀ device technology, in particular for a high positive Vth .
Thus, the positive Vth modulation under gate-region is further important.

Figure 1.16: Typical ID (ns )-VG characteristics for (a)normally-on and (b)normally-oﬀ
GaN-based devices.

On the other hand, since normally-oﬀ devices operates in forward bias region
VG > 0, a normally-oﬀ AlGaN/GaN Schottky structure will exhibits serious gateleakage IG . Meanwhile, due to the insulator insertion, a MIS structure can leadto eﬀective gate leakage reduction as shown in Fig. 1.17. Figure 1.18 shows thegate leakage reduction by MIS structure. Therefore, for normally-oﬀ devices, MIS
structure is necessary.

Chapter 1 Introduction

Figure 1.17: Schematic of gate leakage reduction in MIS devices.

Figure 1.18: Comparison of gate leakage IG between Shcottky and MIS devices.

15

Chapter 1 Introduction

1.4

16

Interfaces in AlGaN/GaN devices aﬀecting
the threshold voltage

In the AlGaN/GaN Schottky and MIS devices, the metal-semiconductor andinsulator-semiconductor interfaces are important building blocks, aﬀecting thethreshold voltage Vth of the devices.

Figure 1.19: (a) The metal-semiconductor interface. (b) The insulator-semiconductor
interface.

The metal-semiconductor interface
In the case of the Schottky devices, Vth is dominated by the metal-semiconductor
barrier height ϕS shown in Fig. 1.20(a), which can be modulated by using metals
with diﬀerent work functions [34, 35]. However, in many cases of GaN-based
Schottky contacts, the value of ϕS is not uniquely determined by the diﬀerence
between the metal work function and the (Al)GaN electron aﬃnity, and is aﬀectedby treatments of the metal-semiconductor interface [36–38]. Figure 1.20(b) shows
an example, where treatments using HCl considerably modulated ϕS in GaNbased Schottky contacts. This indicates the existence of an unintentional metalsemiconductor interfacial layer, leading to a modulation of ϕS by a vacuum level
step ∆Evac due to a dipole of the interfacial layer [39].

Chapter 1 Introduction

17

Figure 1.20: (a) The not uniquely determined metal-semiconductor barrier height. (b)Metal-semiconductor barrier height ϕB as functions of metal work functions ϕM for a
metal/GaN interface. [38].

Figure 1.21: (a) A metal-semiconductor interface with a positive ∆Evac . (b) A metalsemiconductor interface with a negative ∆Evac . (c) The Vth shifts due to ∆Evac .

Figure 1.21 shows the relation between the vacuum level step ∆Evac and theChapter 1 Introduction

18

threshold voltage Vth . A positive ∆Evac leads to a deeper Vth , while a negative one
does the opposite. Therefore, from the behavior of Vth , we can directly determine∆Evac .

The insulator-semiconductor interface
On the other hand, in the case of the MIS devices, Vth is aﬀected by the insulatorsemiconductor conduction band oﬀset φ and the ﬁxed charge density σint of theinsulator-semiconductor interface, which are shown in Fig. 1.22(a). Various insulators such as oxides Al2 O3 [40], HfO2 [41, 42], TiO2 [43], AlSiO [44, 45], AlTiO[46–52], oxynitrides TaON [53], AlON [54], and nitrides BN [55, 56], AlN [57–61]
have been employed as a gate insulator for GaN-based devices, where Vth can be
modulated by both φ and σint . Similarly to ϕS , φ is not uniquely determined by theelectron aﬃnity diﬀerence between the insulator and the semiconductor, and is affected by insulator-semiconductor interface treatments [62]. Figure 1.22(b) shows
an example, where chlorine treatments modulated φ in ZnO/AlGaN contacts.

Figure 1.22: (a) The not uniquely determined insulator-semiconductor band oﬀset.
(b) ZnO/AlGaN band oﬀset modulated by interface treatments [62].

Meanwhile, as shown in Fig. 1.23, when an insulator such as Al 2 O3 is deposited

Chapter 1 Introduction

19

on the Ga-polar (Al)GaN surface with a negative surface polarization charge, theﬁxed charge tends to be positive, neutralizing the polarization charge [63–70],
while this is not always the case [51, 71, 72].

Figure 1.23: The positive ﬁxed charge at insulator-semiconductor interface.

The band oﬀset and positive ﬁxed charge have signiﬁcant impact on the threshold voltages Vth . Assuming no charges inside the insulator, Vth is a linear functionof the insulator thickness dins , whose slope depends on the positive ﬁxed charge
density, and intercept at the dins = 0 limit depends on the band oﬀset as shownin Fig. 1.24(a). If the density of the positive ﬁxed charge is high, leading to aband diagram shown in Fig. 1.24(b), Vth becomes more negative as dins increases.
Meanwhile, if the density of the positive ﬁxed charge is low, the electric ﬁeld inside
the insulator can be reverted as shown in Fig. 1.24(c), leading to a positive slope
in Vth -dins relation. In other words, Vth will be more positive when dins increases.
On the other hand, Vth intercept at dins = 0 becomes deeper for larger band oﬀset,
and does the opposite for smaller one.

Chapter 1 Introduction

20

Figure 1.24: (a) Vth as functions of dins . Theoretical band diagram of AlGaN/GaN
MIS devices where (b) ﬁxed charge density is high, band oﬀset is large and (c) ﬁxed
charge density is low, band oﬀset is small.

Figure 1.25 shows examples where a high-density positive ﬁxed charge of1013 /cm2 order has been observed, and the threshold voltage shifts negatively
with increase in the insulator thickness. Even in fully recessed-gate normally-oﬀ
devices, a high-density positive ﬁxed charge of 1013 /cm2 order has been observed,
leading to a relatively low threshold voltage as shown in Fig. 1.26. On the other
hand, when the positive interface charge is suppressed, positive threshold voltage
shifts can be achieved as shown in Fig. 1.27. Changes in intercept due to theband oﬀset changes between diﬀerent insulator-semiconductor interface can alsoChapter 1 Introduction

21

be observed.

Figure 1.25:

Negative threshold voltage shifts in (a) Al2 O3 /AlN/GaN [63], (b)Al2 O3 /AlGaN/GaN, Al2 O3 /GaN/AlGaN/GaN [68], and (c) Al2 O3 /AlGaN/GaN, AlTiO/AlGaN/GaN devices [48].

Figure 1.26: (a) Threshold voltages of fully-gate-recessed Al2 O3 /GaN MIS-FETs. (b)Theoretical band diagram of the MIS-FETs, showing a theoretical Vth of 10 V [73].

Chapter 1 Introduction

22

Figure 1.27: Positive threshold voltage shifts in (a) AlTiO/AlGaN/GaN [51], (b)Al2 O3 /AlInN/GaN [71], and (c) CuO/AlGaN/GaN, NiOx /AlGaN/GaN devices [72].

The behaviors of φ and σint also can be attributed to the existence of an unintentional insulator-semiconductor interfacial layer with a dipole, where the bandoﬀset is modulated by ∆Evac due to the dipole, and the ﬁxed charge density of themetal/interfacial layer/AlGaN interface can be interpreted as a dipole unbalance.
In order to control the interface of the AlGaN/GaN devices, an insertion of an
intentional metal-AlGaN or insulator-AlGaN interfacial layer should be eﬀective
as shown in Fig. 1.28.

Chapter 1 Introduction

23

Figure 1.28: (a) The metal-semiconductor and (b) the insulator-semiconductor interface with an intentional interfacial layer.

Previously, intentional metal-(Al)GaN interfacial layers have been studied [74,75]. An example is shown in Fig. 1.29, where an insertion of insulator interfacial
layer modulate the barrier height eﬀectively.

Chapter 1 Introduction

24

Figure 1.29: Barrier height modulation due to the insertion of a GaOx interfacial
layer.

On the other hand, MIS device performance improvements by an intentional
insulator-(Al)GaN interfacial layer have been reported [76–78], where an example
is shown in Fig. 1.30. However, the eﬀects of a insulator-(Al)GaN interfacial layeron the ﬁxed charge and band oﬀset have not been elucidated, and systematiccomparison between the eﬀects of a metal-(Al)GaN interfacial layer and that of ainsulator-(Al)GaN interfacial layer have been lacking.

Figure 1.30: Performance improvements in MIS FETs due to the insertion of a GaOx
interfacial layer [78].

Chapter 1 Introduction

1.5

25

Purpose of this study

Eﬀects of metal-semiconductor or insulator-semiconductor interfacial layers (ILs)in AlGaN/GaN devices were systematically investigated, where AlO x , TiOx , or
NiOx obtained by metal layer oxidation are employed as an IL. From this study,
useful insights into the controls of threshold voltage for GaN-based devices are
expected.

Figure 1.31: Schematic of AlGaN/GaN Schottky and MIS devices utilizing intentional
interfacial layers (ILs).

1.6

Organization of the dissertation

This dissertation includes four chapters. The content of each chapter is summarized as follows.
Chapter 1 is the introduction to this research, which introduces the generalbackground on compound semiconductors, the GaN-based semiconductors andrelated devices, the interfaces in AlGan/GaN devices, and the purpose of thisstudy.
Chapter 2 is the investigation on the eﬀects of metal-semiconductor interfacial
layers (ILs) in metal/IL/AlGaN/GaN devices. We fabricated metal/IL/AlGaN/
GaN devices and obtained their capacitance-voltage (C-V ) characteristics, fromwhich we found a modulation in the threshold voltage Vth due to the vacuum level
step ∆Evac induced by the dipole of the metal-AlGaN IL. From the vacuum levelsteps, the IL dipole density is estimated for each IL material. Moreover, using Hallmeasurements for the metal/IL/AlGaN/GaN devices, it is shown that we foundthat the 2DEG carrier concentration is also modulated by ∆Evac .

Chapter 1 Introduction

26

Chapter 3 is the investigation on the eﬀects of insulator-semiconductor interfacial layers (ILs) in metal/Al2 O3 /IL/AlGaN/GaN devices. We fabricated metal/
Al2 O3 /IL/AlGaN/GaN devices and obtained their C-V characteristics. From theAl2 O3 thickness dependence of Vth , we found the ﬁxed charge density of the Al2 O3 /IL/AlGaN interface showing a positive correlation with the IL dipole density.
Furthermore, we characterized the interface trap density in the metal/Al 2 O3 /IL/
AlGaN/GaN devices using the conductance method.
Chapter 4 concludes this work and discusses the future perspectives of thework.

Chapter 2Eﬀects of metal-semiconductor
interfacial layers (ILs) inmetal/IL/AlGaN/GaN devices
Eﬀects of metal-semiconductor interfacial layers (ILs) in metal/IL/AlGaN/GaN
devices were investigated, where AlOx , TiOx , or NiOx obtained by metal layer
oxidation are employed as an IL. We fabricated the metal/IL/AlGaN/GaN devices and obtained their capacitance-voltage (C-V ) characteristics, from which we
found a modulation in the threshold voltage Vth due to the vacuum level step
∆Evac induced by the dipole of the metal-AlGaN IL. From the vacuum level steps,
the IL dipole density is estimated for each IL material. Using Hall measurements
for the metal/IL/AlGaN/GaN devices, it is shown that we found that the 2DEG
carrier concentration is also modulated by ∆Evac . By X-ray photoelectron spectroscopy (XPS), we investigated the chemical shifts in the IL/AlGaN interface,where formation of NiGa is suggested in NiOx /AlGaN interface.

Chapter 2 Metal-semiconductor interfacial layers (ILs)

2.1

28

Metal/IL/AlGaN/GaN device fabrication

Using an Al0.24 Ga0.76 N (20 nm)/GaN (3 µm) heterostructure grown by metalorganic vapor phase epitaxy on a sapphire (0001) substrate, we fabricated themetal/IL/AlGaN/GaN devices. The heterostructure schematic and basic electricalproperties are shown in Fig. 2.1.

Figure 2.1: The AlGaN/GaN heterostructure schematic and basic electrical properties.

The fabrication main ﬂow is shown in Fig. 2.2, consisting of Ohmic electrodeformation, IL metal deposition, IL oxidation and gate electrode formation. After
Ti-based Ohmic electrode formation, thin Al, Ti or Ni metal layers with several thicknesses in the nm-range were deposited on AlGaN. Annealing at 350 ◦ C
in air was carried out next, to obtain thin oxide ILs of AlO x , TiOx , or NiOx .
The fabrication was completed by formation of Ni gate metal covered by Au.
Metal/AlGaN/GaN devices with no IL were also fabricated for comparison.

Figure 2.2: Metal/IL/AlGaN/GaN device fabrication main ﬂow.

Chapter 2 Metal-semiconductor interfacial layers (ILs)29

Ohmic electrode formation
The process ﬂow of the Ohmic electrode formation is shown in Fig. 2.3 withdetailed conditions. This process includes 4 parts, surface treatment, patterning,
Metal deposition and annealing.
Surface treatment was carried out by using organic solutions such as acetone,
methanol, and deionized water (DIW), followed by O2 plasma ashing to fully
remove organic impurities on the sample surface. After that, the samples were put
in a semico-clean solution, a dilute solution of tetra-methyl ammonium hydroxide(TMAH) and N(CH3 )4 OH to remove oxide layer at the surface, then washed in
DIW.
Patterning by lithographic was carried out next, consisting of resist coating,
exposure, and developing. Before resist coating, the samples were baked to remove
remaining water. Lift-oﬀ resist LOL2000 and positive resist TSMR-8900 were
coated using spincoater, followed by baking for resist hardening. Exposure forthe coated samples was carried out by mask-less aligner (MLA) using laser lightof wavelength 405 nm. Then the development was done by TMAH, followed bywashing in DIW, and O2 plasma ashing to remove remaining resist. Patterning
was ﬁnished by a semico-clean solution treatment to remove oxide layer, which
was formed after long time exposure in the air.
Metal deposition was then carried out by resistance heating evaporator (RHE).
After vacuuming until the pressure reached the low of 10−4 Pa, Ti/Al/Ti/Au metals were deposited with the thickness of 5/100/200/50 nm. After the deposition,lift-oﬀ process was carried out by a resist remover, 1-methyl-2-pyrrolidinone-based
solution (commercial name: 1165), follow by necessary wet surface treatment.
Finally, annealing in N2 atmosphere at 575 ◦ C formed the Ohmic contact
between metal and semiconductor.

Chapter 2 Metal-semiconductor interfacial layers (ILs)30

Figure 2.3: Process ﬂow of the Ohmic electrode formation.

IL metal deposition
The process ﬂow of the IL metal deposition is shown in Fig. 2.4 with detailed
conditions. This process includes 3 parts, surface treatment, patterning, andmetal deposition. Metal deposition was carried out by RHE, where Al. Ti or
Ni thin metal layers were deposited with several thicknesses of ∼ nm.

Chapter 2 Metal-semiconductor interfacial layers (ILs)31

Figure 2.4: Process ﬂow of the IL metal deposition.

IL oxidation
The process ﬂow of the IL oxidation is shown in Fig. 2.5 with detailed conditions.
This process includes 2 parts, surface treatment and annealing. The annealing
was carried out in air atmosphere at 350 ◦ C, forming the thin metal oxide AlOx ,TiOx , or NiOx ILs. The determination of conditions for oxidation annealing will
be explained in the next section.

Figure 2.5: Process ﬂow of the IL oxidation.

Chapter 2 Metal-semiconductor interfacial layers (ILs)32

Gate electrode formation
The process ﬂow of the gate electrode formation is shown in Fig. 2.6 with detailed
conditions. This process includes 3 parts, surface treatment, patterning, and metal
deposition. Since TMAH is capable to dissolve oxide ILs, semico-clean treatment
was omitted. For the resist coating, addition layer is necessary to protect oxide ILs
from development process due to the same reason. Thus, we employed an electron
beam resist GL2000, which cannot be dissolved by TMAH but removed by O 2
plasma ashing. Metal deposition was carried out by RHE, where Ni/Au metal
was deposited with the thicknesses of 5/100 nm. The gate electrode formationﬁnished the fabrication of metal/IL/AlGaN/GaN devices.

Figure 2.6: Process ﬂow of the gate electrode formation.

Chapter 2 Metal-semiconductor interfacial layers (ILs)

2.2

33

Details in the IL formation

Determination of oxidation conditions
Figure 2.7 shows the schematic of Hall devices for bear AlGaN/GaN structures,and the 2DEG carrier concentrations ns0 obtained form them at VG = 0 as afunction of the anneal temperature. A serious decrease in ns0 is observed for anneal
temperature > 350 ◦ C, suggesting an oxidation on the AlGaN surface which should
be avoided. Therefore, anneal temperature for IL oxidation is determined to be
350 ◦ C.

Figure 2.7: Schematic of Hall devices for AlGaN/GaN and the 2DEG carrier concentrations ns0 at VG = 0 obtained form them as a function of anneal temperature.

Figure 2.7 shows the schematic of Hall devices for IL/AlGaN/GaN structures,and the 2DEG carrier concentrations ns0 obtained form them at VG = 0 as functions of anneal time, in comparison with a AlGaN/GaN structure with no IL.
Decreases in ns0 are observed right after the IL metal deposition (anneal time =0) due to possible surface barrier height changes. More importantly, after a 5
minutes anneal, we ﬁnd further ns0 decreases in IL/AlGaN/GaN, indicating the
progress of interface reactions between the ILs and AlGaN. ns0 remain almost
constants for further annealing, indicating a complete reaction in IL/AlGaN interfaces, and a complete oxidation of ILs. Therefore, we determined a over 5
minutes anneal time for the IL oxidation.

Chapter 2 Metal-semiconductor interfacial layers (ILs)34

Figure 2.8: Schematic of Hall devices for IL/AlGaN/GaN and the 2DEG carrier concentrations ns0 at VG = 0 obtained form them as a function of the anneal time. Anneal
temperature is ﬁxed at 350 ◦ C.

Surface morphology of the ILs
Figure 2.9 shows examples of atomic force microscope (AFM) images for the surfaces of the AlOx , TiOx , and NiOx ILs in comparison with the AlGaN surface,indicating that the surface morphology is unchanged after the IL formation. On
the other hand, due to the ∼ nm surface roughness, it is diﬃcult to accurately
evaluate the thicknesses for ILs of same order. Therefore, the IL thicknesses are
conﬁrmed by the capacitance of the ILs shown later.

Figure 2.9: Atomic force microscope (AFM) images of the surfaces of AlGaN, AlOx ,TiOx , and NiOx .

Chapter 2 Metal-semiconductor interfacial layers (ILs)

2.3

35

Metal/IL/AlGaN/GaN devices characterization

Current-frequency (J-V ) characterizations
Figure 3.5 shows the top view of a fabricated capacitor and the conﬁgurationof J-V (DC) characterization for the capacitors, using semiconductor parameter
analyzer.

Figure 2.10: Top view of a fabricated capacitor and the conﬁguration of J-V (DC)characterization.

Figure 2.11 shows examples of current-voltage (J-V ) characteristics of the capacitors, where one device is picked up for each IL, under application of the gate
voltage VG with respect to the grounded Ohmic electrode. The IL leads to lower
leakage, which can be attributed to both the vacuum level steps ∆Evac (equivalently the eﬀective barrier height ϕS ) and the IL barrier eﬀects. Thus, it is diﬃcult
to accurately evaluate ∆Evac (or ϕS ) from the J-V characteristics.

Chapter 2 Metal-semiconductor interfacial layers (ILs)36

Figure 2.11: J-V characteristics of metal/AlGaN/GaN and metal/IL/AlGaN/GaN
devices.

On the other hand, ∆Evac can be evaluated from the capacitance-voltage (C-V )characteristics.

Capacitance-voltage (C-V ) characterizations
Figure 2.12 shows the band diagram of a metal/IL/AlGaN/GaN device, where ϕS0
is the barrier height with no vacuum level step, ϕS is the eﬀective barrier height,∆Ec is the AlGaN-GaN conduction band oﬀset, σGaN /q ≃ 2.1 × 1013 cm−2 andσAlGaN /q ≃ 3.2 × 1013 cm−2 give the polarization charge densities [79–83], andσD is the IL dipole density. From this band diagram, we obtain the vacuum level
step∆Evac =q(σD − σGaN )
CIL

(2.1)

Chapter 2 Metal-semiconductor interfacial layers (ILs)37

and the threshold voltage
Vth =ϕS0 ∆Evac σAlGaN − σGaN ∆EC
−
−
−
q
q
CAlGaNq(2.2)

using the IL capacitance CIL and the AlGaN capacitance CAlGaN 1 . Since thethreshold voltage with no IL is given by
Vth0 =

ϕS0 σAlGaN − σGaN ∆EC−−q
CAlGaNq(2.3)

assuming ∆Evac = 0, we obtain the threshold voltage shift
Vth − Vth0 = −

∆Evac
,
q

(2.4)

which can be used to evaluate ∆Evac .

Figure 2.12: The band diagram of the metal/IL/AlGaN/GaN devices.

Figure 3.7 shows the conﬁguration of the C-V (AC) characterization for thecapacitors, using LCR meter.
1

see Appendix A for detailed derivation

Chapter 2 Metal-semiconductor interfacial layers (ILs)38

Figure 2.13: The conﬁguration of C-V (AC) characterization at f = 1 MHz.

Figure 2.14 shows examples of C-V characteristics at frequency f = 1 MHz,
where one device is picked up for each IL. The 2DEG concentration ns obtainedby integrating the C-V characteristics is also shown in Fig. 2.14, and can beﬁtted by qns ≃ Ctot (VG − Vth ) using the total capacitance Ctot given by 1/Ctot =1/CAlGaN +1/CIL . From the ﬁtting, Vth , Vth0 , CAlGaN , and CIL are obtained. For the
AlOx , TiOx , and NiOx ILs with several thicknesses, we ﬁnd 1/CIL ≲ 0.4 cm2 /µF,
≲ 0.1 cm2 /µF, and ≲ 0.4 cm2 /µF, corresponding to the IL thicknesses of ≲ 3 nm,≲ 3 nm, and ≲ 4 nm, respectively, assuming typical dielectric constants ∼ 7 of
AlOx , ∼ 30 of TiOx , and ∼ 10 of NiOx .

Chapter 2 Metal-semiconductor interfacial layers (ILs)39

Figure 2.14: C-V and ns -V characteristics of metal/AlGaN/GaN and metal/IL/
AlGaN/GaN devices.

The obtained Vth − Vth0 and ∆Evac of all the devices are shown in Fig. 2.15
as functions of 1/CIL , where Vth − Vth0 and −∆Evac are shown in the left andright vertical axes, respectively. It is shown that Vth − Vth0 < 0 for the AlOx and
TiOx ILs, indicating that the vacuum level step ∆Evac is positive, while for theChapter 2 Metal-semiconductor interfacial layers (ILs)40

NiOx ILs, Vth − Vth0 > 0 indicating that the vacuum level step ∆Evac is negative.
Moreover, we ﬁnd an almost proportional relation between ∆Evac and 1/CIL foreach IL, indicating that the IL dipole density
σD =∆Evac CIL
+ σGaNq(2.5)

is almost constant for each IL material. Figure 2.16 shows the averaged IL dipole
densities in comparison with σAlGaN , σD /q ≃ 2.5 × 1013 cm−2 of the AlOx ILs,
σD /q ≃ 3.5 × 1013 cm−2 of the TiOx ILs, and σD /q ≃ 0.4 × 1013 cm−2 of the NiOx
ILs, where the error bars stand for the three-sigma standard deviations. This
indicates that the AlOx /AlGaN and TiOx /AlGaN interfaces are nearly neutral,
while the NiOx /AlGaN interface is quite negatively charged.

Figure 2.15: The threshold voltage shifts Vth − Vth0 and vacuum level steps ∆Evac of
all the devices as functions of 1/CIL .

Chapter 2 Metal-semiconductor interfacial layers (ILs)41

Figure 2.16: The interfacial layer (IL) dipole density σD in comparison with the AlGaN
polarization charge density σAlGaN .

Based on these obtained results, Figure 2.17 shows the calculated 1D PoissonSchrödinger band diagrams of the metal/AlGaN/GaN and metal/IL/AlGaN/GaN
devices at VG = 0 V, where we assume a 2 nm IL thickness. The AlOx and TiOx ILs
slightly lower the eﬀective barrier height owing to the almost neutral AlO x /AlGaN,
TiOx /AlGaN interfaces and the consequent positive ∆Evac , while NiOx strongly
increase it owing to the quite negatively charged NiOx /AlGaN interface and theconsequent negative ∆Evac . Note that the change in eﬀective barrier heights do
not equal to the extracted ∆Evac due to the existence of 2DEG.

Chapter 2 Metal-semiconductor interfacial layers (ILs)42

Figure 2.17: Calculated 1D Poisson-Schrödinger band diagrams of the metal/AlGaN/
GaN and metal/IL/AlGaN/GaN devices.

Moreover, we fabricated metal/IL/AlGaN/GaN and metal/AlGaN/GaN Hallbar devices shown in Fig. 2.18. From Hall measurements under a magnetic ﬁeld
B = 0.32 T, the 2DEG carrier concentration ns0 at VG = 0 was obtained. The
relation between qns0 /Ctot and ∆Evac is also shown in Fig. 2.18, being consistent
with qns0 /Ctot = −Vth = ∆Evac /q + Vth0 shown by the dashed line, indicatingthat ns0 is also modulated by the vacuum level step ∆Evac due to the dipole. In
particular, the NiOx ILs strongly reduce the 2DEG concentration.

Figure 2.18: Schematic of Hall devices for metal/IL/AlGaN/GaN and the relation
between ns0 and ∆Evac . The dashed line shows qns0 /Ctot = −Vth = ∆Evac /q + Vth0 .

Chapter 2 Metal-semiconductor interfacial layers (ILs)43

The fact that ∆Evac is either positive or negative can be attributed to thereaction at the IL/AlGaN interface. The tie-line conﬁgurations for metal-(Al)GaN systems have been studied [84, 85], showing that TiN is stable in a Ti-(Al)Ga-N
system, while NiGa is stable in a Ni-(Al)Ga-N system as shown in the Fig. 2.19.
Considering that AlN is also stable in a Al-(Al)Ga-N system, due to the formationof TiN or AlN, nitrogen vacancy VN donors can be generated at the TiOx /AlGaNor AlOx /AlGaN interface, and the ionized donors act as positive IL dipole charges
leading to the positive ∆Evac . On the other hand, due to the formation of NiGa,
gallium vacancy VGa acceptors can be generated at the NiOx /AlGaN interface,
and the ionized acceptors act as negative IL dipole charges leading to the negative∆Evac .

Figure 2.19: Tie-line conﬁgurations for metal-Ga-N systems[85].

Chapter 2 Metal-semiconductor interfacial layers (ILs)

2.4

44

X-ray photoelectron spectroscopy (XPS) for
the IL/AlGaN interfaces

In order to examine the IL/AlGaN interface reaction, we employed X-ray photoelectron spectroscopy (XPS) to investigate the chemical shifts in the interfaces.
Figure 2.20 shows the conﬁguration of the XPS measurement using a photon energy ≃ 1.5k eV and a take-oﬀ angle of 35°, leading to a inelastic mean free pathof electron ≃ 2 nm [86]. Bear AlGaN, metal/AlGaN (before oxidation), and
IL/AlGaN (after oxidation) are investigated, where the thicknesses of metals andILs were chosen to be < 2 nm in order to study the interface.

Figure 2.20: Conﬁguration of the XPS measurement and schematics of samples.

Figure 2.21 shows the wide scan spectra for the bear AlGaN and the IL/AlGaN
samples, from which we can identify the peak positions of each element.

Chapter 2 Metal-semiconductor interfacial layers (ILs)45

Figure 2.21: Wide scan spectra for the bear AlGaN and the IL/AlGaN.

Afterward, high resolution scan for each highlighted peak was carried out.
Before each scan, calibrations to remove the extrinsic chemical shift (equivalently
the binding energy peak position shift) due to the potential diﬀerence between
samples and detector are done by using C-C binding energy from C1s spectra.
Figure 2.22 shows the C1s spectra, illustrating diﬀerent bonding states of carbon(C-C, C-OH, or C=O). The dots are measurement data and the red lines are theﬁtted curves given by the sum of individual Gaussian peaks for each bonding state.
We adjusted the peak positions of C-C for every spectra to 284.8 eV, ﬁnishing thecalibration of peak positions.

Chapter 2 Metal-semiconductor interfacial layers (ILs)46

Figure 2.22: C1s spectra with Gaussian peaks ﬁtting.

Figure 2.23 shows the O1s spectra for each samples. For AlOx , negligible
change was observed in the peak positions, while for TiOx and NiOx , the peaks
shift to high energy side. The XPS binding energy of an element is a measureof the electronic environment (equivalently the bonding state) of that element.
When an additional element is introduced, the electronegativity of the additional
element determines the shift in binding energy peak position. Table 2.1 showsthe electronegativity for the elements involved in the IL/AlGaN interfaces. Whenbonds with high electronegativity element such as O increase, the electron density
around the base element decreases, leading to an increase in binding energy. On
the contrary, when bonds with low electronegativity element such as a metal (Al,Ti or Ni) increase, electron density around the base element increases, leading
to a decrease in binding energy. Therefore, the unchanged peak positions for
AlOx /AlGaN suggest that the Al metal was almost oxidized completely before theTable 2.1: Electronegativity for the elements involved in the IL/AlGaN interface[87].

Elements

O

N

Ni

Ga

Al

Ti

Electronegativity

3.44

3.04

1.91

1.81

1.61

1.54

Chapter 2 Metal-semiconductor interfacial layers (ILs)47

anneal oxidation, while the high energy shifts for TiO x /AlGaN and NiOx /AlGaN
indicate further oxidation due to the metal deposition and the anneal oxidation.
However, since oxygen atoms can exist in both the oxide ILs and the IL/AlGaN
interfaces, the O1s spectra is not a strong indicator to the interface reaction.

Figure 2.23: O1s spectra for each IL material.

We further focused on N1s and Ga3d spectra, where nitrogen and gallium
atoms are considered to mainly exist in the interface. Figure 2.24 shows the N1sspectra for each IL material. For AlOx /AlGaN and TiOx /AlGaN, we ﬁnd negligible peak position change, suggesting that the bonding state of nitrogen merelyChapter 2 Metal-semiconductor interfacial layers (ILs)

48changed during the interface reaction. While for NiOx /AlGaN, peak positions
shift to the high energy side, suggesting an increase in N-O bonding.

Figure 2.24: N1s spectra for each IL material.

Figure 2.25 shows the Ga3d spectra for each IL material. In the case of AlO x /AlGaN, negligible peak position change is observed. While for TiO x /AlGaN, highenergy shifts are observed, suggesting an increase in Ga-O bonding. Therefore, it
is possible that not only VN donors, but also oxygen donors exist in the TiOx /AlGaN interface. Furthermore, low energy shifts are observed for NiO x /AlGaN,
suggesting an increase in Ga-Ni bonding. This result supports the formation

Chapter 2 Metal-semiconductor interfacial layers (ILs)49

of NiGa and the consequently existence of VGa acceptors in the NiOx /AlGaN
interface.

Figure 2.25: Ga3d spectra for each IL material with Gaussian peaks ﬁtting.

Figure 2.26 shows the Al2p, Ti3p, and Ni3p spectra for the AlO x /AlGaN,
TiOx /AlGaN, and NiOx /AlGaN respectively. Sightly high energy shifts in AlO x /AlGaN and TiOx /AlGaN suggest increases in Al-O (or N), Ti-O (or N) bonding.
Meanwhile, low energy shifts are observed for NiOx /AlGaN, being consistent withthe behavior of Ga3d spectra in supporting the formation of NiGa.

Chapter 2 Metal-semiconductor interfacial layers (ILs)50

Figure 2.26: Al2p, Ti3p, and Ni3p spectra for each IL material respectively.

According to the XPS results, models of AlOx (TiOx )/AlGaN, and NiOx /AlGaN
interface are depicted in Figure 2.27, where the oxygen donors and V Ga acceptors
are generate at the AlOx (TiOx )/AlGaN and NiOx /AlGaN interface receptively.

Figure 2.27: Models of AlOx (TiOx )/AlGaN, and NiOx /AlGaN interface.

Chapter 3
Eﬀects of insulator-semiconductor
interfacial layers (ILs) inmetal/Al2O3/IL/AlGaN/GaN
devicesAccording to the results in previous chapter, we carried out further investigation oneﬀects of insulator-semiconductor ILs in metal/Al2 O3 /IL/AlGaN/GaN devices using the AlOx , TiOx , or NiOx IL. We fabricated the metal/Al2 O3 /IL/AlGaN/GaN
devices and obtained their C-V characteristics. From the Al2 O3 thickness dependence of Vth , we ﬁnd the ﬁxed charge density of the Al2 O3 /IL/AlGaN interface,showing a positive correlation with the IL dipole density. In addition, by frequency(f )-dependent C-V characteristics, we characterized the interface trap density inthe metal/Al2 O3 /IL/AlGaN/GaN devices using the conductance method. Temperature dependence of the f -dependent C-V characteristics is further investigated, where the time constant and the active energy of the interface traps are
estimated.

Chapter 3 Insulator-semiconductor interfacial layers (ILs)

3.1

52

Metal/Al2O3/IL/AlGaN/GaN device fabrication

Using the same AlGaN/GaN heterostructure, we fabricated metal/Al 2 O3 /IL/
AlGaN/GaN capacitor devices with the AlOx , TiOx , or NiOx ILs, where Al2 O3 are
employed as the gate insulators. The fabrication main ﬂow is shown in Fig. 3.1,
consisting of Ohmic electrode formation, IL metal deposition, IL oxidation, insulator deposition, and gate electrode formation. After the IL oxidation, several
thickness of Al2 O3 gate insulators were formed by atomic layer deposition (ALD)
using TMA (trimethyl aluminum) and water. The fabrication was completed byformation of Ni gate metal covered by Au. Metal/Al2 O3 /AlGaN/GaN capacitor
devices with no IL were also fabricated for comparison.

Figure 3.1: Metal/Al2 O3 /IL/AlGaN/GaN device fabrication main ﬂow.

Ohmic electrode formation
The process is the same to the one of previous chapter.

Chapter 3 Insulator-semiconductor interfacial layers (ILs)53

IL metal deposition
The process is the same to the one of previous chapter.

IL oxidation
The process is the same to the one of previous chapter.

Insulator deposition
The process ﬂow of the insulator deposition is shown in Fig. 3.2 with detailed conditions. The surface treatment of the no IL devices includes the TMAH treatment
in order to remove surface oxidation, while this is unnecessary for devices with
ILs.

Figure 3.2: Process ﬂow of the insulator deposition.

The Al2 O3 insulators are obtained by atomic layer deposition (ALD), using
trimethyl aluminum Al(CH3 )3 (TMA), and water H2 O as precursors. Figure 3.3
shows the chemical structure of the TMA, which react with water as
2Al(CH3 )3 + 3H2 O → Al2 O3 + 6CH4 ,forming the Al2 O3 insulator layer of one mono-layer. By controlling the the total
numbers of the reaction, it is possible to accurately control the thickness of Al 2 O3
as shown in Fig. 3.4.

Chapter 3 Insulator-semiconductor interfacial layers (ILs)

Figure 3.3: Chemical structure of TMA.

Figure 3.4: ALD deposition sequences.

Gate electrode formation
The process is the same to the one of previous chapter.

54

Chapter 3 Insulator-semiconductor interfacial layers (ILs)

3.2

55

Metal/Al2O3/IL/AlGaN/GaN devices characterization

Current-voltage (J-V ) characterizations
Figure 3.5 shows the top view of a fabricated capacitor and the conﬁgurationof J-V (DC) characterization for the capacitors using semiconductor parameter
analyzer.

Figure 3.5: Top view of a fabricated capacitor and the conﬁguration of J-V (DC) or
C-V (AC) characterization.

Figure 3.6 shows examples of current-voltage (J-V ) characteristics of the capacitors, where one device is picked up for each IL and each Al 2 O3 thickness, underapplication of the gate voltage VG with respect to the grounded Ohmic electrode.
In comparison to the metal/IL/AlGaN/GaN devices, leakage is reduced dramatically due to the Al2 O3 gate insulator, leading to a negligible contribution fromILs.

Chapter 3 Insulator-semiconductor interfacial layers (ILs)56

Figure 3.6: J-V characteristics of metal/Al2 O3 /AlGaN/GaN and metal/Al2 O3 /IL/
AlGaN/GaN devices.

Capacitance-voltage (C-V ) characterizations
Figure 3.7 shows the conﬁguration C-V (AC) characterization for the capacitors
using LCR meter.

Chapter 3 Insulator-semiconductor interfacial layers (ILs)57

Figure 3.7: The conﬁguration of C-V (AC) characterization at f = 1M Hz.

Figure 3.8 shows examples of C-V characteristics at f = 1 MHz. The 2DEG
concentration ns obtained by integrating the C-V characteristics is also shown in
Fig. 3.8, and ﬁtted by qns ≃ Ctot (VG − Vth ) using the total capacitance Ctot givenby 1/Ctot = 1/CAl2 O3 + 1/CAlGaN + 1/CIL .

Figure 3.8: C-V characteristics of metal/Al2 O3 /AlGaN/GaN and metal/Al2 O3 /IL/
AlGaN/GaN devices.

Chapter 3 Insulator-semiconductor interfacial layers (ILs)58

From the ﬁtting, we obtain Vth and Ctot . Examples of 1/Ctot are shown in
Fig. 3.9 as functions of the Al2 O3 thickness dAl2 O3 , which give CAl2 O3 = kins ϵ0 /dAl2 O3from the slope, and CAlGaN and CIL from the intercept at dAl2 O3 = 0. On the other
hand, Fig. 3.10 shows examples of Vth as functions of the dAl2 O3 , where the AlOx
and TiOx ILs lead to more negative Vth shifts, while the NiOx ILs lead to lessnegative ones.

Figure 3.9: The inverse of total capacitances 1/Ctot .

Figure 3.10: The threshold voltages Vth depending on the insulator thicknesses dAl2 O3 .

Chapter 3 Insulator-semiconductor interfacial layers (ILs)59

Figure 3.11 shows the band diagram of a metal/Al2 O3 /IL/AlGaN/GaN device,from which the threshold voltage Vth is given by
ϕ σint − σGaN φ σAlGaN − σGaN ∆EC
−
− −
−
q
CAl2 O3q
CAlGaNq
σGaN − σint
σAlGaN − σGaN ϕ − φ − ∆EC=
dAl2 O3 −+,kAl2 O3 ϵ0
CAlGaN
q

Vth =

(3.1)

where CAl2 O3 = kAl2 O3 ε0 /dAl2 O3 is the the Al2 O3 capacitance given by the Al2 O3dielectric constant kAl2 O3 and the thickness dAl2 O3 , ϕ is the metal-Al2 O3 barrierheight, φ is the eﬀective Al2 O3 -AlGaN conduction band oﬀset, σint = σD − σeD isthe ﬁxed charge density of the Al2 O3 /IL/AlGaN interface due to the unbalanced
IL dipole1 . It should be noted that Vth is a linear function of dAl2 O3 , where theslope is (σGaN − σint )/(kAl2 O3 ε0 ), and the intercept is −(σAlGaN − σGaN )/CAlGaN +(ϕ − φ − ∆EC )/q. From the former, we can evaluate σint due to the unbalanced
IL dipole. Also the latter is determined by ϕ − φ, where φ is aﬀected by the thevacuum level step ∆Evac induced by the dipole of the insulator-AlGaN IL.

Figure 3.11: The band diagram of the metal/Al2 O3 /IL/AlGaN/GaN devices.
1

see Appendix A for detailed derivation

Chapter 3 Insulator-semiconductor interfacial layers (ILs)60

Using Eq. (A.8), we evaluate σint /q shown in Fig. 3.12 as functions of 1/CIL witherror bars standing for the three-sigma asymptotic standard errors of the linear
ﬁtting. Although the error bars are slightly large, σint /q is almost independentof 1/CIL for each IL, and the averaged σint /q is ≃ 3.2 × 1013 cm−2 for no IL,≃ 3.6 × 1013 cm−2 for the AlOx ILs, ≃ 3.9 × 1013 cm−2 for the TiOx ILs, and≃ 2.7 × 1013 cm−2 for the NiOx ILs.

Figure 3.12: The relation between the ﬁxed charge densities σint of all the devices and
1/CIL .

Using the IL dipole density σD obtained for each IL material in the previous
section, the relation between σint and σD is demonstrated in Fig. 3.13, showing a
positive correlation. This suggests that the ﬁxed charge is related to the unbalanced IL dipole.

Chapter 3 Insulator-semiconductor interfacial layers (ILs)61

Figure 3.13: The ﬁxed charge density σint correlated with the IL dipole density σD .

On the other hand, assuming a metal/Al2 O3 barrier height ϕ = 4.6 eV [51],the band oﬀset φ is evaluated to be φ ≃ 2.1 ± 1 eV for no IL, φ ≃ 2.5 eV for the
NiOx ILs, and φ ∼ 0-5 eV for the AlOx or TiOx ILs as shown in Fig. 3.14 where
the uncertainty comes from the asymptotic standard errors of the linear ﬁtting.
Owing to this large uncertainty, unfortunately it is diﬃcult to discuss φ aﬀectedof the vacuum level step ∆Evac .

Figure 3.14: The relation between the band oﬀset φ of all the devices and 1/CIL .

Chapter 3 Insulator-semiconductor interfacial layers (ILs)62

Figure 3.15 shows the calculated 1D Poisson-Schrödinger band diagrams ofthe metal/Al2 O3 /AlGaN/GaN and metal/Al2 O3 /IL/AlGaN/GaN devices at VG =0 V, where the Al2 O3 thickness is 22 nm. The Al2 O3 /AlOx /AlGaN and Al2 O3 /TiOx /AlGaN interfaces are almost neutral, while the Al2 O3 /NiOx /AlGaN interface is negatively charged due to a suppression of the positive ﬁxed charge.

Figure 3.15: The calculated 1D Poisson-Schrödinger band diagrams of the metal/
Al2 O3 /AlGaN/GaN and metal/Al2 O3 /IL/AlGaN/GaN devices.

Capacitance-voltage-frequency (C-V -f ) characterizations
Due to the insertion of insulator-semiconductor ILs, the number of interface increased as shown in Fig. 3.16. This may lead to an increase of interface traps in
the metal/IL/AlGaN interface.

Chapter 3 Insulator-semiconductor interfacial layers (ILs)

63

Figure 3.16: The interface increase due to the insertion of an insulator-semiconductorIL.

Thus, we characterized the trap densities of the Al2 O3 /IL/AlGaN interfaces
by f -dependent C-V measurements for f = 100-1M Hz, using the conﬁguration
shown in Fig. 3.17.

Figure 3.17: The conﬁguration of C-V -f (AC) characterization at f = 100-1M Hz.

Examples of f -dependent C-V characteristics of devices with dAl2 O3 = 22 nm
are shown in Fig. 3.18. Negligible frequency dispersion is observed at negative bias
region for all devices, showing that the Vth determination is not aﬀected by themeasurement frequency. On the other hand, frequency dispersions are observedfor positive biases except for TiOx ILs. By using the conductance method [88],which is widely employed for interface characterization [48, 68, 89], the interface
trap density was evaluated using the equivalent circuits shown in the inset ofChapter 3 Insulator-semiconductor interfacial layers (ILs)64

Fig. 3.182 .

Figure 3.18: C-V -f characteristics of metal/Al2 O3 /AlGaN/GaN and metal/Al2 O3 /IL/AlGaN/GaN devices.

In those equivalent circuits, Git is the interface trap conductance, and Cit is theinterface trap capacitance. When we carry out C-V measurements, the equivalentcircuit will be transform into the measured circuit shown in Fig. 3.19(right), where
Cm and Gm are the measured capacitance and conductance, respectively.

Figure 3.19: The equivalent circuit of the MIS capacitor and the measured circuit.
2

see Appendix B

Chapter 3 Insulator-semiconductor interfacial layers (ILs)

65

Therefore, using 1/C0 =(1/CAl2 O3 )+(1/CIL ), we have the total impedance givenby11+(equivalent circuit)jωC0 jωCAlGaN + jωCit + Git
1=(measured circuit).
jωCm + Gm

(3.2)

1
1
1
=
−
jω(CAlGaN + Cit ) + Git
jωCm + Gm jωC0
jω(C0 − Cm ) − Gm.
=(jωCm + Gm )jωC0

(3.3)

Z=

Thus,

As a result,(jωCm + Gm )jωC0
jω(C0 − Cm ) − Gm(−ω 2 C0 Cm + jωC0 Gm )[jω(C0 − Cm ) + Gm ]=−
ω 2 (C0 − Cm )2 + Gm 2
ω 2 (C0 − Cm )C0 Gm + ω 2 C0 Cm Gm=+ j × ...
ω 2 (C0 − Cm )2 + Gm 2jω(CAlGaN + Cit ) + Git =(3.4)

Equating two sides for Eq. 3.4 and extracting the real part, we obtainGit
ωC0 2 Gm
= 2,ω
ω (C0 − Cm )2 + Gm 2

(3.5)

from which we can calculate the interface trap conductance from the measurement
data.
Figure 3.20 shows examples of Git /ω (ω = 2πf ) as functions of frequency f ,exhibiting single-peaked behaviors except for the TiO x ILs.

Chapter 3 Insulator-semiconductor interfacial layers (ILs)66

Figure 3.20: Git /ω as functions of frequency with ﬁtting curves for metal/Al2 O3 /AlGaN/GaN and metal/Al2 O3 /IL/AlGaN/GaN devices.

These single-peaked behavior is described byGit
q 2 Dit ln(1 + ω 2 τ 2 )
=
,
ω
2ωτ

(3.6)

where the electron trapping time constant τ is given by the peak frequency fp =1/(πτ ) and the interface trap density Dit by the peak value Git /ω ≃ 0.4q 2 Dint asshown in Figure 3.213 .

3

see Appendix B

Chapter 3 Insulator-semiconductor interfacial layers (ILs)

67

Figure 3.21: The interface trap density Dit and the electron trapping time constant τ
extractions form Git /ω as a function of measured frequency f.

The absence of single-peaked behavior for the TiOx ILs suggests that the interface trap have a very long trapping time constant. The peak values of Git /ω
for all the devices are shown Fig. 3.22 as functions of frequency, from which we
extract the Dit and τ shown in Fig. 3.23. For an interface trap at the energy E,
the time constant τ at temperature T is given byτ = e(EC −E)/kB T /(vth σe NC ) = τ0 e(EC −E)/kB T ,(3.7)

using the conduction band bottom energy EC , the electron thermal velocity vth ,the capture cross section of the trap σe , and the conduction band eﬀective densityof states NC , where τ0 = 1/(vth σe NC ). Even though the τ0 is ambiguous due tothe uncertain σe , by assuming a wide range of τ0 ∼ 0.1-103 ps, Dit as functions of
EC − E are evaluated and shown in Fig. 3.24, where the error bars correspond to
the widely assumed τ0 .

Chapter 3 Insulator-semiconductor interfacial layers (ILs)

Figure 3.22: Peak values of Git /ω as functions of measured frequency f.

Figure 3.23: Dit as functions of τ .

68

Chapter 3 Insulator-semiconductor interfacial layers (ILs)69

Figure 3.24: The interface trap density Dit as functions of the energy EC − E.

The shallow interface trap densities are Dit ≃ 2.5 cm−2 eV−1 for no IL, Dit ≃1.6 cm−2 eV−1 for the AlOx ILs, and Dit ≃ 0.6 cm−2 eV−1 for the NiOx ILs. Thismeans that Dit for no IL is higher than those for the AlOx and NiOx ILs, despiteof the apparently weak frequency dispersion in the C-V characteristics for no IL
shown in Fig. 13. We consider that this behavior can be explained as follows. In
Fig. 14 for no IL, in addition to the observed peak, we ﬁnd the tail of another next
peak of Git /ω at ≲ 1 MHz for forward biases, with a peak frequency rather higher(≫ 1 MHz) than the measurement frequency range, and a peak value comparable
to the observed peak at ∼ kHz. When there is such another next peak, Cit in theequivalent circuit becomes quite large in the measurement frequency range, andthus we will obtain the measured capacitance Cm ≃ CAl2 O3 , which hardly shows
frequency dispersion.
While the equivalent circuit is usually described by Eq. 3.6 andq 2 Dit atan(ωτ )
Cit =
,
ωτ

(3.8)

where Dit is the trap density and τ is the trapping time constant, for the equivalentcircuit with such another kind of traps, we should use
q 2 Dit ln(1 + ω 2 τ 2 ) q 2 D̃it ln(1 + ω 2 τ̃ 2 )
Git
=
+
ω
2ωτ
2ωτ̃

(3.9)

Chapter 3 Insulator-semiconductor interfacial layers (ILs)

70

and
Cit =

q 2 Dit atan(ωτ ) q 2 D̃it atan(ωτ̃ )
+
ωτ
ωτ̃

(3.10)

with the density D̃it and the time constant τ̃ of the another kind of traps. By assuming CAl2 O3 = 300 nF/cm2 , CAlGaN = 400 nF/cm2 , Dit = 2.5 × 1013 cm−2 eV−1 ,τ = 10−4 s, D̃it = 3.0 × 1013 cm−2 eV−1 , and τ̃ = 10−8 s, which are realistic forthe experiments, we carried out a model calculation to illustrate this modiﬁcation. Git /ω, Cit , and Cm were calculated as shown in Figs. 3.25, 3.26, and 3.27
respectively for the cases without (blue curves) and with (red curves) the next
peak. In the measurement frequency range, we observe similar Git /ω for the cases
without and with the next peak, except that the tail of the next peak can befound at ≲ 1 MHz as in the measured results. Moreover, in the measurement
frequency range, while the variation of Cit is 0 - µF/cm2 for the case without thenext peak, Cit remain at µF/cm2 order for the case with the next peak. As theresults, frequency dispersion can be observed in the case without the next peak. In
the case with the next peak, however, due to the high Cit ∼µF/cm2 , the measured
capacitance becomes Cm ≃ CAl2 O3 , which hardly shows frequency dispersion. This
explains the observed weak frequency dispersion for no IL, and implies that it isdiﬃcult to discuss the trap density only by apparent C-V characteristics.

Figure 3.25: The calculated Git /ω as functions of the frequency.

Chapter 3 Insulator-semiconductor interfacial layers (ILs)71

Figure 3.26: The calculated Cit as functions of the frequency.

Figure 3.27: The calculated Cm as functions of the frequency.

On the other hand, we do not observe the second step of the C-V characteristics for the NiOx IL shown in Fig. 3.18, indicating that electron trapping at theinterface traps does not take place. This is owing to weak eﬀects of Git and Dit in
the measurement frequency range, because of the peak frequency of Git /ω in theChapter 3 Insulator-semiconductor interfacial layers (ILs)72

100 Hz range and the small peak value due to the low Dit . Thus, it is possible toconclude that the AlOx and NiOx ILs suppress the interface trap density.

Chapter 4
Conclusion and future
perspectives
4.1

Conclusion of this work

In this work, we systematically investigated eﬀects of metal-semiconductor or
insulator-semiconductor interfacial layers (ILs) in AlGaN/GaN devices, where
AlOx , TiOx , or NiOx is employed as an IL. From capacitance-voltage characteristics of metal/IL/AlGaN/GaN devices, it is shown that the metal-AlGaN IL
leads to modulation of the threshold voltage Vth , attributed to the vacuum level
step induced by the IL dipole. We ﬁnd negative vacuum level steps for AlO x
and TiOx , and positive for NiOx , from which the IL dipole density is estimated,indicating that the AlOx /AlGaN and TiOx /AlGaN interfaces are nearly neutral,
while the NiOx /AlGaN interface is negatively charged. From Hall measurements,
we ﬁnd that the 2DEG carrier concentration in the metal/IL/AlGaN/GaN devices is also modulated by the vacuum level step. In addition the these results,
we also carried out X-ray photoelectron spectroscopy (XPS) to examine the reaction at the IL/AlGaN interface. we ﬁnd an increase in Ni-Ga bonding at theNiOx/AlGaN interface, suggesting the existence of ionized VGa acceptors, whichlead to the negatively charged NiOx/AlGaN interface. On the other hand, from
capacitance-voltage characteristics of metal/Al2 O3 /IL/AlGaN/GaN devices, theChapter 4 Conclusion and future perspectives

74

ﬁxed charge density of the Al2 O3 /IL/AlGaN interface is evaluated. For AlOx and
TiOx , the ﬁxed charge density is increased in comparison with no IL, while decreased for NiOx . The ﬁxed charge density shows a positive correlation with the
IL dipole density, suggesting that the ﬁxed charge is related to the unbalanced IL
dipole. Furthermore, using the conductance method, we show that it is possible
to obtain small trap densities of metal/Al2 O3 /IL/AlGaN interfaces, comparing to
the interface with no IL.

4.2

Future perspectives of this work

Although we ﬁnd a correlation between the IL dipole density and ﬁxed charge density, the relation between the dipole density and the insulator- semiconductor bandoﬀset remains unclear due to the large uncertainty in the intercept of the Al 2 O3thickness dependence of Vth . Experiments using very small insulator thickness arenecessary to improve the accuracy in the intercept extraction. Moreover, utilizing
the Vth modulation by ILs, we should be able to carry out a normally-oﬀ operation
for AlGaN/GaN MIS-FETs, where ILs such as TiOx can be employed for accessregion to reduce series resistance, while ILs such as NiOx can be employed for gateregion to shift Vth positively in order to achieve normally-oﬀ.

Appendix A
Band diagram calculation
A.1

metal/IL/AlGaN/GaN band diagram

Figure A.1: Theoretical band diagram of the metal/IL/AlGaN/GaN devices under VG= Vth .

Chapter A Band diagram calculation

76

Figure A.1 shows the theoretical band diagram of the metal/IL/AlGaN/GaN devices at gate voltage VG = Vth , where ϕS0 is the barrier height with no vacuum
level step, ϕS is the eﬀective barrier height, ∆Ec is the AlGaN-GaN conduction band oﬀset, ψAlGaN is the total potential change through the AlGaN layer,
σGaN /q ≃ 2.1 × 1013 cm−2 and σAlGaN /q ≃ 3.2 × 1013 cm−2 give the polarization
charge densities [79–83], and σD is the IL dipole density.
Based on the 1-dimension Poisson equation, the potential ψ inside the IL or
AlGaN is given by
∂ 2ψ
ρ
=− .
2
∂x
ε

(A.1)

Taking the integral of Eq. A.1, we have
∂ψ
= F,
∂x

(A.2)

where F is the electric ﬁeld strength inside the IL or AlGaN. We assume that the
IL dipole and the polarization charge are at and/or near the interface, thereby wecan apply Gauss law. Figure A.2 shows a Gauss box covering both AlGaN and
GaN, determining the electric ﬁeld inside the AlGaN FAlGaN , and the electric ﬁeld
inside the GaN FGaN ,Figure A.2: Applying Gauss box in band diagram for AlGaN/GaN.

with a total charge inside the box ρ = σAlGaN − σGaN . Based on the Gauss

Chapter A Band diagram calculation

77

equation, we haveρσAlGaN − σGaN=εε
σAlGaN − σGaN=,
kAlGaN ε0

FAlGaN − FGaN =(A.3)

where kAlGaN is the dielectric constant of AlGaN. For GaN part, since VG = Vth ,the 2DEG is completely depleted, lead to a ﬂat band in GaN where FGaN = 0
Therefore, we have the total potential change through the AlGaN layer
σAlGaN − σGaN
dAlGaNkAlGaN ε0
σAlGaN − σGaN.
=
CAlGaNψAlGaN = FAlGaN dAlGaN =(A.4)

The same analysis can be applied for the IL/AlGaN/GaN as shown in Fig. A.3.
Assuming no bulk charge inside the ILs. the total charge inside the box is ρ =σD − σAlGaN + σAlGaN − σGaN = σD − σGaN , where the polarization in the AlGaN
cancels out. Therefore, we obtain the total potential change through the ILs,
namely the vacuum level step
∆Evac /q =σD − σGaN.
CIL

Figure A.3: Applying Gauss box in band diagram for IL/AlGaN/GaN.

(A.5)

Chapter A Band diagram calculation

78

Using these results and Figure A.1, we obtain∆EC
ϕS0 ∆Evac−− ψAlGaN −qqq
ϕS0 σD − σGaN σAlGaN − σGaN ∆EC
=
−
−
−
q
CIL
CAlGaN
q

Vth =

(A.6)

for the metal/IL/AlGaN/GaN devices.

A.2

metal/Al2O3/IL/AlGaN/GaN band diagram

Figure A.4: Theoretical band diagram of the metal/Al2 O3 /IL/AlGaN/GaN devices
under VG = Vth .

Figure A.4 shows the theoretical band diagram of the metal/Al 2 O3 /IL/AlGaN/GaN
devices at gate voltage VG = Vth , where where CAl2 O3 = kAl2 O3 ε0 /dAl2 O3 is the
Al2 O3 capacitance given by the Al2 O3 dielectric constant kAl2 O3 and the thickness dAl2 O3 , ϕ is the metal-Al2 O3 barrier height, φ is the eﬀective Al2 O3 -AlGaNChapter A Band diagram calculation79

conduction band oﬀset, ψAlGaN is the total potential change through the AlGaN layer, ψAl2 O3 is the total potential change through the Al2 O3 layer, σint =σD − σeD is the ﬁxed charge density of the Al2 O3 /IL/AlGaN interface due to theunbalanced IL dipole. A similar analysis in the previous section can be allied
toAl2 O3 /IL/AlGaN/GaN assuming no bulk charge inside the Al2 O3 as shown in
Fig. A.5. The total charge inside the box is now ρ = σD − σeD − σGaN = σint − σGaN ,where the polarization in the AlGaN cancels out while the IL dipole does not due
to the unbalance. Therefore, we obtain the total potential change through the
Al2 O3
ψAl2 O3 = FAl2 O3 dAl2 O3 =σint − σGaN
dAl2 O3 .
kAl2 O3 ε0(A.7)

Figure A.5: Applying Gauss box in band diagram for Al2 O3 /IL/AlGaN/GaN.

Using these results and Figure A.4, we obtainφ∆EC
ϕ− ψAl2 O3 − − ψAlGaN −qqq
σGaN − σint
σAlGaN − σGaN ϕ − φ − ∆EC=
dAl2 O3 −+,kAl2 O3 ϵ0
CAlGaNq

Vth =for the metal/Al2 O3 /IL/AlGaN/GaN devices.

(A.8)

Appendix B
Conductance method
B.1

AC admittance aﬀected by interface traps

At an insulator-semiconductor interface, an interface trap with a electron occupation probability P at energy E, as shown in Fig. B.1, is given by a time-dependent
rate equation∂P= γ(1 − P )n − αP,
∂t

(B.1)

where n is the semiconductor electron density at the interface, and γ and α areproportional constants. In Eq. B.1, the ﬁrst term is related to the electron trapping, while the second term is related to the detrapping. The constant γ is givenby
γ = vth σ(B.2)

where vth is the electron thermal velocity and σ is the electron capture crosssection.

Chapter B Conductance method

81

Figure B.1: A trapped electron at energy E at the insulator-semiconductor interface.

If we separate P , n and surface potential V into two contributions, form direct
current (DC) parts and alternative current (AC) parts receptively, we obtain
P = P0 + Pe exp(jωt)
n = n0 + n
e exp(jωt)(B.3)

V = V0 + Ve exp(jωt).
The P0 component at energy E is given by Fermi-Dirac distribution
1.
1 + exp[(E − Ef )/kB T ]

(B.4)

dP0
exp[(E − Ef )/kB T ]/kB T=−
dE(1 + exp[(E − Ef )/kB T ])2
P0 (1 − P0 )
=−
.
kB T

(B.5)

P0 (E) =Thus, we have

Substitute the DC components into Eq. B.1, we obtain
α = γn0 (1 − P0 )/P0 =1 − P0,τ P0with τ =

1
.
γn0

(B.6)

In the range of small AC signal modulation, α is approximated to Eq. B.6. From
Eq. B.1, we can obtainjω Pe exp(jωt) = γ(1 − P )n −(1 − P0 )P
.
τ P0

(B.7)

Chapter B Conductance method82

The right hand side of Eq. B.7 equals to(1 − P0 )Pγ(1 − P )nτ P0 − (1 − P0 )P
=
τ P0
τ P0
e
[1 − P0 − P exp(jωt)][n0 + n
e exp(jωt)]P0 /n0 − (1 − P0 )(P0 + Pe exp(jωt))
=
τ P0
2
n
eP0 /n0 − P0 n
e/n0 − Pe − PeneP0 exp(jωt)/n0=exp(jωt)τ P0
P0 (1 − P0 )en/n0 − Pe[enP0 exp(jωt)/n0 + 1]=
exp(jωt).
τ P0γ(1 − P )n −(B.8)
In case of small AC signal modulation, we have n
e ≪ n0 , thus neP0 exp(jωt/n0 ) ≪ 1.
With this approximation, Eq. B.7 turns intojω Pe exp(jωt) ≃P0 (1 − P0 )en/n0 − Peexp(jωt).
τ P0

(B.9)

We can rewrite Eq. B.9 as
P0 (1 − P0 ) ne
P0 (1 − P0 ) q Ve
Pe ≃,=1 + jωτ P0 n0
1 + jωτ P0 kB T

(B.10)

at temperature T where kB is Boltzmann constant. The total small signal AC
admittance Yi is calculated by taking sum over all interface traps, i.e.
Yi = Git + jCit ωZjωq=
Dit (E)PedEe
V Z
Dit (E)P0 (1 − P0 )
jωq 2
dE
=
kB T
1 + jωτ P0
Z
Dit (E)
dP0
2
= jωq
−
dE,
1 + jωτ P0
dE(B.11)

with interface trap density Dit .
AC admittance for the low temperature limit
For low temperature, we have the approximation−

dP0≃ δ(E − Ef ).
dE(B.12)

Chapter B Conductance method

83

Substitute into Eq. B.11, we haveZ

Dit (E)Git + jCit ω = jωq
1 + jωτ P0
2
jωq Dit (Ef )≃1 + jωτ /2
2



dP0
−
dE


dE
(B.13)

Extract Git and Cit , we obtainGitωτ q 2 Dit=ω
2(1 + ω 2 τ 2 /4)
q 2 DitCit =.
1 + ω 2 τ 2 /4

(B.14)

AC admittance for a discrete interface trap
For a discrete interface trap, the interface trap density Dit becomes the delta
function of the energy, i.e.
Dit (E) = D0 δ(E − E0 ).
Substitute into Eq. B.11, we haveZ
Dit (E)dP0
2
Git + jCit ω = jωq−
dE
1 + jωτ P0dE
exp[(E0 − Ef )/kB T ]jωq 2 D0=
1 + jωτ P0 (E0 ) kB T (1 + exp[(E0 − Ef )/kB T ])2jωq 2 D0=
. (for Ef = E0 )4kB T (1 + jωτ /2)(B.15)(B.16)

Extract Git and Cit , we obtainGitωτ q 2 D0=
ω
8kB T (1 + ω 2 τ 2 /4)
q 2 D0.
Cit =4kB T (1 + ω 2 τ 2 /4)(B.17)

Chapter B Conductance method

84

AC admittance for general continuous interface traps
For general continuous interface traps, the interface trap density Dit becomes aconstant, not dependent of the energy. Equation B.11 now isZ
Dit (E)dP0
2
Git + jCit ω = jωq−
dE
1 + jωτ P0
dE


Z
1
dP0
2
≃ jωq Dit−
dE
1 + jωτ P0
dE
Z 1
dP0
2
= jωq Dit
0 1 + jωτ P0
2
q Dit ln (1 + jωτ )=τ(B.18)

Extract Git and Cit , we obtainGit
q 2 Dit ln(1 + ω 2 τ 2 )=ω
2ωτ

(B.19)

q 2 Dit atan(ωτ )
Cit =
ωτ

(B.20)

which we employed in Chapter 3. The time constant τ is1
Ec − Ef
exp [(Ec − Ef )/kB T ]τ (Ef ) =∝ exp=.
γn0 (Ef )vth σNc
kB T

B.2(B.21)

AC admittance in equivalent circuits for MIS
devices

Without considering the interface traps, Fig. B.2 shows the equivalent circuit of ametal/Al2 O3 /IL/AlGaN/GaN device, where we have


qns
1
1
1
VG =
= qns
+
+
Ctot
CAl2 O3 CIL CAlGaN
by assuming a linear relation between qns and VG .

(B.22)

Chapter B Conductance method

85

Figure B.2: The equivalent circuit of a metal/Al2 O3 /IL/AlGaN/GaN device without
considering the interface traps.

On the other hand, by assuming an additional trapped electron density nit at
the IL/AlGaN interface due to the insulator-semiconductor interface traps, theequivalent circuit of a metal/Al2 O3 /IL/AlGaN/GaN device is reconstructed asshown in Fig. B.3. VG is now given by
q(ns + nit )VG == q(ns + nit )
etot
C



1

1
+
CAl2 O3 CIL


+

qns
CAlGaN.

(B.23)

Figure B.3: The equivalent circuit of a metal/Al2 O3 /IL/AlGaN/GaN device considering the interface traps.

Chapter B Conductance method
Thus, the total capacitance is now given by


1
1
1
qns
=
+
+
e
CAl2 O3 CIL
q(ns + nit )CAlGaN
Ctot


1
1
1
=
+
+
C
CIL
CAlGaN + (nit /ns )CAlGaN Al2 O3

1
1
1
=
+
+
.
CAl2 O3 CIL
CAlGaN + Cit

86(B.24)

This result indicates that there is an additional capacitance contributed by theinterface traps Cit = (nit /ns )CAlGaN , being parallel to the AlGaN capacitance.
If we consider the contribution from interface traps as an AC admittance, theequivalent circuit of a metal/Al2 O3 /IL/AlGaN/GaN device can be reconstructed
as shown in Fig. B.4, which we employed in Chapter 3.

Figure B.4: The reconstructed equivalent circuit of a metal/Al2 O3 /IL/AlGaN/GaN
device considering the interface traps.

List of publications
Journal[1] Duong Dai Nguyen, Takehiro Isoda, Yuchen Deng, and Toshi-kazu Suzuki,“Normally-oﬀ operations in partially-gate-recessed AlTiO/AlGaN/GaN ﬁeldeﬀect transistors based on interface charge engineering”, Journal of Applied
Physics 130 (1), 014503, 2021.
[2] Kazuya Uryu, Yuchen Deng, Son Phuong Le and Toshi-kazu Suzuki, “Electron mobility enhancement in n-GaN under Ohmic-metal”, AIP Advances 13,
075002, 2023.
[3] Duong Dai Nguyen, Yuchen Deng, and Toshi-kazu Suzuki, “Low-frequency
noise in AlTiO/AlGaN/GaN metal-insulator-semiconductor ﬁeld-eﬀect transistors with non-gate-recessed or partially-gate-recessed structures”, Semiconductor Science and Technology 38, 095010, 2023.
[4] Yuchen Deng, Jieensi Gelan, Kazuya Uryu and Toshi-kazu Suzuki, “AlGaN/GaN
devices with metal-semiconductor or insulator-semiconductor interfacial layers: Vacuum level step due to dipole and interface ﬁxed charge ”, Journal ofApplied Physics 135, 084504, 2024.

Conference[1] Hirotomo Demura, Yuchen Deng, Duong Dai Nguyen, and Toshi-kazu Suzuki,“Threshold voltages of AlGaN/GaN metal-insulator-semiconductor devices
with AlN or Al2O3 gate insulators”, 46th Int. Symp. on Compound Semiconductors, TuB1-6, oral presentation (2019).

List of publications

88[2] Duong Dai Nguyen, Yuchen Deng, and Toshi-kazu Suzuki, “Impacts of nitrogen plasma surface cleaning on threshold voltages of AlTiO/AlGaN/GaN
MIS devices”, 51st Int. Conf. on Solid State Devices and Materials, PS-4-17,
poster presentation (2019).
[3] Takehiro Isoda, Jumpei Shimura, Duong Dai Nguyen, Yuchen Deng, andToshi-kazu Suzuki, “Threshold voltage control in AlGaN/GaN MIS devices
with AlTiO gate insulators and gate-recess structures”, The papers of technical meeting on electron devices, IEE Japan, EDD-21-032, oral presentation(2021).
[4] Duong Dai Nguyen, Takehiro Isoda, Yuchen Deng, and Toshi-kazu Suzuki,“Interface-charge-engineered normally-oﬀ AlTiO/AlGaN/GaN ﬁeld-eﬀect transistors”, 82nd JSAP Autumn Meeting, 13a-N305-4, oral presentation (2021).
[5] Hideyuki Matsuyama, Yuchen Deng, and Toshi-kazu Suzuki, “Eﬀects of insulator/semiconductor interfacial layer insertion in AlGaN/GaN MIS devices
with AlN gate insulators ”, The papers of technical meeting on electron devices, IEE Japan, EDD-22-021, oral presentation (2022).
[6] Yuchen Deng, Hibiki Anaba, Hideyuki Matsuyama, Kazuya Uryu and Toshikazu Suzuki, “Eﬀects of AlN/AlGaN interfacial control layers in AlN/AlGaN/
GaN devices”, 2022 JAIST international Symposium Quantum Devices andMetrologies, poster presentation (2022).
[7] Kazuya Uryu, Yuchen Deng, Son Phuong Le and Toshi-kazu Suzuki, “Electron mobility enhancement in n-GaN under Ohmic-metal”, 70th JSAP Spring
Meeting, 17p-A301-14, oral presentation (2023).
[8] Kazuya Uryu, Yuchen Deng, Son Phuong Le and Toshi-kazu Suzuki, “Electron mobility enhancement in n-GaN under Ohmic-metal: characterization bymulti-probe-Hall measurement ”, The papers of technical meeting on electron
devices, IEE Japan, EDD-23-024, oral presentation (2023).
[9] Kazuya Uryu, Yuchen Deng, and Toshi-kazu Suzuki, “Characterization of recess Ohmic contact to AlGaN/GaN heterostructure using single or hybrid
electrode structures”, 84th JSAP Autumn Meeting, 22p-B201-12, oral presen-List of publications

89

tation (2023).
[10] Kazuya Uryu, Yuchen Deng, and Toshi-kazu Suzuki, “Recess Ohmic contact
to AlGaN/GaN heterostructure using single or hybrid electrode structures”,
55th Int. Conf. on Solid State Devices and Materials, N-3-04, oral presentation (2023).
[11] Yuchen Deng, Jieensi Gelan, Kazuya Uryu and Toshi-kazu Suzuki, “Threshold Voltage Modulation of AlGaN/GaN Devices via Vacuum Level Step Control Using Thin Oxide Interlayers”, 2023 International Workshop on Dielectric Thin Films for Future Electron Devices – Science and Technology –, S6-2,
oral presentation (2023).

Award[1] Student Poster Presentation Award forYuchen Deng, Hibiki Anaba, Hideyuki Matsuyama, Kazuya Uryu and Toshikazu Suzuki, “Eﬀects of AlN/AlGaN interfacial control layers in AlN/AlGaN/
GaN devices”, 2022 JAIST international Symposium Quantum Devices andMetrologies, poster presentation (2022).
[2] IWDTF Young Researcher Award forYuchen Deng, Jieensi Gelan, Kazuya Uryu and Toshi-kazu Suzuki, “Threshold
Voltage Modulation of AlGaN/GaN Devices via Vacuum Level Step Control
Using Thin Oxide Interlayers”, 2023 International Workshop on DielectricThin Films for Future Electron Devices – Science and Technology –, S6-2,
oral presentation (2023).

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Acknowledgment
First of all, I would like to express my sincere gratitude to Prof. Toshi-kazu
Suzuki for guiding me through this work. I also want to express my thanks to
Prof. Masashi Akabori for his support during my course, and to Prof. Toshu An
for his instructions in both minor research and doctoral dissertation. Moreover,sincerely thanks to Prof. Eisuke Tokumitsu and Prof. Taketomo Sato for giving
me very helpful instructions during my work on doctoral dissertation. Finally,
Many thanks to my former and current lab members, Hirotomo Demura, Duong
Dai Nguyen, Jumpei Shimura, Hideyuki Matsuyama, Hibiki Anaba, Jeensi Gelan
and Kazuya Uryu for their kindly supports during experiments.