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Title

Insulator‑semiconductor interface fixed charges
in AlGaN/GaN metal‑insulator‑semiconductor
devices with Al̲2O̲3 or AlTiO gate dielectrics

Author(s)Le, Son Phuong; Nguyen, Duong Dai; Suzuki,Toshi‑
kazu

Citation

Journal of Applied Physics, 123(3): 034504‑1‑
034504‑7

Issue Date

2018‑01‑19

Type

Journal Article

Text version

publisher

URL

http://hdl.handle.net/10119/15737

RightsCopyright 2018 American Institute of Physics.
This article may be downloaded for personal use
only. Any other use requires prior permission of
the author and the American Institute of Physics.
The following article appeared in Son Phuong Le,
Duong Dai Nguyen, and Toshi‑kazu Suzuki, Journalof Applied Physics, 123(3), 034504 (2018) and may
be found at http://dx.doi.org/10.1063/1.5017668

Description

Japan Advanced Institute of Science and Technology

JOURNAL OF APPLIED PHYSICS 123, 034504 (2018)

Insulator-semiconductor interface fixed charges in AlGaN/GaN
metal-insulator-semiconductor devices with Al2O3 or AlTiO
gate dielectrics
Son Phuong Le, Duong Dai Nguyen, and Toshi-kazu Suzukia)
Center for Nano Materials and Technology, Japan Advanced Institute of Science and Technology (JAIST),
1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan(Received 29 November 2017; accepted 3 January 2018; published online 19 January 2018)
We have investigated insulator-semiconductor interface fixed charges in AlGaN/GaN metalinsulator-semiconductor (MIS) devices with Al2O3 or AlTiO (an alloy of Al2O3 and TiO2) gate
dielectrics obtained by atomic layer deposition on AlGaN. Analyzing insulator-thickness dependences of threshold voltages for the MIS devices, we evaluated positive interface fixed charges,
whose density at the AlTiO/AlGaN interface is significantly lower than that at the Al2O3/AlGaN
interface. This and a higher dielectric constant of AlTiO lead to rather shallower threshold voltages for the AlTiO gate dielectric than for Al2O3. The lower interface fixed charge density also
leads to the fact that the two-dimensional electron concentration is a decreasing function of theinsulator thickness for AlTiO, whereas being an increasing function for Al2O3. Moreover, we discuss the relationship between the interface fixed charges and interface states. From the conductance method, it is shown that the interface state densities are very similar at the Al2O3/AlGaN
and AlTiO/AlGaN interfaces. Therefore, we consider that the lower AlTiO/AlGaN interface fixedcharge density is not owing to electrons trapped at deep interface states compensating the positive
fixed charges and can be attributed to a lower density of oxygen-related interface donors.
Published by AIP Publishing. https://doi.org/10.1063/1.5017668

I. INTRODUCTION

GaN-based heterojunction field-effect transistors(HFETs)1 are important devices owing to their high current
drive capability and high breakdown voltages. However,there are several disadvantages of GaN-based HFETs; selfheating effects,2–4 current collapse phenomena,5,6 and also
gate leakage currents are limiting factors for the practical useof these devices. For the suppression of gate leakage currents
and the current collapse phenomena in GaN-based devices, it
can be effective to employ metal-insulator-semiconductor(MIS) structures, which are also significant to normally off
operations,7 even though GaN-based MIS devices sometimes
exhibit unstable characteristics.8–12 As a gate dielectric of
GaN-based MIS devices, high-dielectric-constant (high-k)
insulators, such as Al2O3,13 HfO2,14,15 TaON,16 AlN,17–21
BN,22,23 and AlTiO,24 have been investigated. In GaN-based
MIS device processing, when an insulator is deposited on anegatively polarized III-N semiconductor surface, such as
Ga-face (Al)GaN, positive insulator-semiconductor interface
fixed charges tend to be generated and to cancel the negative
polarization charges.25–32 However, the existence of theinsulator-semiconductor interface fixed charges is not anecessity.29,30 Since the interface fixed charges have significant impacts on threshold voltages Vth, we expect that Vth
can be controlled by “interface charge engineering,”29 i.e.,
by controlling the interface fixed charges. In particular, if thepositive interface fixed charge density is sufficiently suppressed, a normally off operation can be expected.33,34a)Author to whom correspondence should be addressed: tosikazu@jaist.ac.jp

0021-8979/2018/123(3)/034504/7/$30.00

However, despite many reports on the interface fixed charges,
their sufficient control is a remaining issue. Moreover, their origin is not fully elucidated even though they are attributed topositively ionized oxygen donors in some cases. Therefore, further investigations on insulator-semiconductor interface fixedcharges for GaN-based MIS devices are very necessary andimportant towards Vth control and normally off operations.
In this work, we investigated insulator-semiconductor
interface fixed charges in AlGaN/GaN MIS devices with Al2O3
or AlTiO (an alloy of Al2O3 and TiO235–37) gate dielectrics,
which are deposited on an AlGaN/GaN heterostructure byatomic layer deposition (ALD). AlTiO has, depending on its
composition, intermediate physical properties between Al2O3(k  9 and Eg  7 eV) and TiO2 (k  60 and Eg  3 eV),37 being
useful to balance the trade-off between k and energy gap Eg.
Previously, we fabricated AlTiO/AlGaN/GaN MIS devices
with excellent characteristics, indicating that AlTiO can be
an important candidate for a gate dielectric of GaN-based
MIS devices.24 The present work involves a comparative
study on insulator-semiconductor interface fixed charges in
Al2O3/AlGaN/GaN and AlTiO/AlGaN/GaN MIS devices.
By analyzing linear insulator-thickness dependences of Vth,we evaluated insulator-semiconductor interface fixedcharges. As a result, we find that the fixed charge density at
the AlTiO/AlGaN interface is significantly lower than thatat the Al2O3/AlGaN interface. In addition, we also discuss
the relationship between the interface fixed charges andinterface states. It is suggested that the lower AlTiO/AlGaN
interface fixed charge density is not owing to electrons
trapped at deep interface states.

123, 034504-1

Published by AIP Publishing.

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J. Appl. Phys. 123, 034504 (2018)

II. DEVICE FABRICATION

Using an Al0.27Ga0.73N(30 nm)/GaN(3000 nm) heterostructure grown by metal-organic vapor phase epitaxy on sapphire(0001), we fabricated AlGaN/GaN MIS devices with
Al2O3 or AlTiO gate dielectrics. The device fabrication wasstarted with Ti/Al/Ti/Au Ohmic electrode formation. Aftersurface treatments using organic solvents, oxygen plasma ashing, and an ammonium-based solution, insulator films of
Al2O3 or AlTiO as gate dielectrics with several thicknesses
dins ¼ 6–29 nm were deposited on the AlGaN surface by
ALD. The Al2O3 films (k  9 and Eg  7 eV) were obtainedby using trimethylaluminum (TMA) and H2O as precursors,
and the AlxTiyO films (x:y ¼ 0.73:0.27, k  13–14, andEg  6 eV) were by using TMA, tetrakis-dimethylamino titanium (TDMAT), and H2O. After post-deposition annealing in
H2-mixed Ar at 350  C, Ni/Au gate electrode formation completed the device fabrication. As a result, we obtained Al2O3/
AlGaN/GaN and AlTiO/AlGaN/GaN MIS devices, whosecross sections are schematically shown in Fig. 1(a), with
70 lm  70 lm gate electrodes surrounded by the Ohmicelectrodes as shown by top-view optical images in Fig. 1(b).
III. INSULATOR-SEMICONDUCTOR INTERFACE FIXEDCHARGESIn order to investigate insulator-semiconductor interface
fixed charges, we examined threshold voltages Vth of the
Al2O3/AlGaN/GaN and AlTiO/AlGaN/GaN MIS devices,by measuring capacitance-voltage (C–V) characteristics
between the gate and the grounded Ohmic electrodes. Since
GaN-based MIS devices sometimes exhibit unstable Vthdepending on the sweeping range of the gate voltage VG,8–12
we checked Vth stability; starting from VG0  0, C–V characteristics were measured under VG ¼ VG0 ! –15 V with asweep rate of 0.36 V/s. Figure 2 shows an example of the

FIG. 2. Checking Vth stability of the Al2O3/AlGaN/GaN and AlTiO/AlGaN/
GaN MIS devices with dins ¼ 19 nm: C–V characteristics measured at 1 MHz
under VG ¼ VG0 ! –15 V with a sweep rate of 0.36 V/s, where VG0 ¼ 0, 1, 2,
3, 4, and 5 V.

measurement results for dins ¼ 19 nm, at 1 MHz frequency with
VG0 ¼ 0, 1, 2, 3, 4, and 5 V. Although we find rather stable Vth,weak Vth shifts take place after positive bias applications, probably owing to charging effects of trapped electrons. Thus, todetermine Vth, we employ VG0 ¼ 0 V to avoid the charging
effects. Figure 3(a) shows C–V characteristics of the MIS
devices with dins ¼ 6–29 nm, measured at 1 MHz under VG¼ 0 ! 15 V with a sweep rate of 0.36 V/s. As shown in Fig.
3(b), the sheet concentration of the two-dimensional electrongas (2DEG) under the gate, ns, can be obtained by integrating
C as a function of VG, from which we can determine Vth.
Figure 4 shows the band diagram of AlGaN/GaN MIS
devices, considering the interface fixed charges. From this,
we obtain
Drins  qns
DrAlGaN  qns
dins þ
dAlGaNkins e0kAlGaN e0¼ VG þ w=q þ EF =q(1)

using the elementary charge q > 0, the vacuum permittivity
e0, the insulator-semiconductor interface fixed charge density
rins, the polarization charge densities rGaN and rAlGaN, thedielectric constants kins and kAlGaN, the thicknesses dins anddAlGaN, the 2DEG Fermi energy EF, and w ¼ /  u  DEC
defined in Fig. 4, where Drins ¼ rins  rGaN and DrAlGaN¼ rAlGaN  rGaN. For VG ¼ Vth (ns ¼ 0 and EF ¼ 0), we findVth ¼ Drins
DrAlGaNdins 
dAlGaN þ w=q
kins e0kAlGaN e0(2)giving a linear dins-dependence of Vth with a slope ofDrins/(kinse0). The 2DEG concentration ns under the gate isapproximately given by
qns ’ C0 ðVG  Vth Þ(3)

as experimentally confirmed in Fig. 3(b), where
FIG. 1. (a) Schematic cross sections and (b) top-view optical images of thefabricated Al2O3/AlGaN/GaN and AlTiO/AlGaN/GaN MIS devices.

1
dins
dAlGaN¼
þ:
C0 kins e0 kAlGaN e0(4)

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J. Appl. Phys. 123, 034504 (2018)

FIG. 3. (a) C–V characteristics of the Al2O3/AlGaN/GaN and AlTiO/AlGaN/GaN MIS devices with dins ¼ 6–29 nm, measured at 1 MHz under
VG ¼ 0 ! –15 V with a sweep rate of 0.36 V/s. (b) The 2DEG sheet concentration ns obtained by integrating C as functions of the gate voltage VG, fromwhich we can determine Vth.

For VG ¼ 0, ns ¼ ns0 is given by
qns0 ’ C0 Vth¼

FIG. 4. The band diagram of AlGaN/GaN MIS devices, considering theinterface fixed charges.

Drins dins =ðkins e0 Þ þ DrAlGaN dAlGaN =ðkAlGaN e0 Þ  w=q
dins =ðkins e0 Þ þ dAlGaN =ðkAlGaN e0 Þ(5)which is a nonlinear function of dins.
According to Eq. (4), 1/C0 is a linear function of dins.
Experimentally, C0 is estimated by C at VG ¼ 0 V as plottedin Fig. 5(a), where we can confirm the linear relation. Fromthe slopes, we obtain dielectric constants kins ¼ 9.4 and 13.4
for Al2O3 and AlTiO, respectively, being consistent withseparated experimental results using metal-insulator-metal
structures (not shown). From the intercept, we obtainkAlGaN ¼ 9.5 (using dAlGaN ¼ 30 nm). Figure 5(b) shows theexperimentally determined Vth as functions of dins. We findlinear dependences obeying Eq. (2), indicating that the interface fixed charges dominate Vth and also rather shallower Vthfor AlTiO than for Al2O3. By fitting using Eq. (2), we obtain
Drins/q ¼ 1.5  1013 cm2 and 7.3  1012 cm2 at the Al2O3/
AlGaN and AlTiO/AlGaN interfaces, respectively. The latter
gives a significantly lower rins than the former, which may
be attributed to a lower-density of oxygen donors25,31,32 at

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Le, Nguyen, and Suzuki

J. Appl. Phys. 123, 034504 (2018)of a large dins, ns0 in Eq. (5) approaches to Drins/q, indicatingthat a normally off operation can be expected for sufficiently
suppressed interface fixed charges, satisfying Drins < 0, i.e.,rins < rGaN. However, in the both cases, we observe Drins> 0, i.e., rins > rGaN.
Even though Drins is obtained experimentally, in order toevaluate rins, it is necessary to assume rGaN. Hereafter, we
assume rGaN/q ¼ 2.1  1013 cm2 obtained by a theoretical
calculation.38 This leads to rins/q ¼ 3.6  1013 cm2 and2.8  1013 cm2 at the Al2O3/AlGaN and AlTiO/AlGaN interfaces, respectively. In addition, these values should be compared with rAlGaN/q. Although DrAlGaN/q ¼ 1.5  1013 cm2for Al0.27Ga0.73N/GaN is obtained theoretically,39 several
experiments show lower DrAlGaN, about 85% of the theoretical one.40–42 Thus, we assume DrAlGaN/q ¼ 1.3  1013 cm2,i.e., rAlGaN/q ¼ 3.4  1013 cm2. Based on the assumptions,
we summarize rins compared with rAlGaN in Fig. 6, where thedotted line corresponds to neutral insulator-semiconductor
interfaces, i.e., rins þ (rAlGaN) ¼ 0. We obtain that theAl2O3/AlGaN interface is nearly neutral,25 while the AlTiO/
AlGaN interface is rather negatively charged owing to thelower rins. By fitting Vth as functions of dins with Eq. (2), we
also obtain w ¼ 2.0 and 1.3 eV for the Al2O3/AlGaN/GaN and
AlTiO/AlGaN/GaN MIS devices, respectively. From these,
we obtain band diagrams of the AlGaN/GaN MIS devices by
Poisson-Schr€odinger calculation43 as shown in Fig. 7, where
we can confirm that the AlTiO/AlGaN interface is negatively
charged. The directions of the electric fields in Al2O3 andAlTiO at VG ¼ 0 V are opposite, leading to the fact that ns0 is
a decreasing function of dins for AlTiO, whereas being an
increasing function for Al2O3.
IV. RELATION WITH INSULATOR-SEMICONDUCTORINTERFACE STATESIt should be noted that electrons trapped at deep interface states with very long time constants can act as (quasi)
negative interface fixed charges.30 Therefore, the interface
fixed charge measurements might be influenced by electrons
at deep interface states compensating the positive fixed

FIG. 5. (a) 1/C0, (b) Vth, and (c) ns0 at VG ¼ 0 of the Al2O3/AlGaN/GaN and
AlTiO/AlGaN/GaN MIS devices, as functions dins with fitting curves.

the AlTiO/AlGaN interface. This lower rins and the higher
kins of AlTiO lead to rather shallower Vth. Figure 5(c) showsthe experimentally obtained ns0 as functions of dins, whose
non-linear dependences are fitted by Eq. (5). We find that ns0
is a decreasing function of dins for AlTiO, whereas being an
increasing function for Al2O3. From Eq. (5), we obtain@ns0
C0ðDrins =q  ns0 Þ¼@dins kins e0(6)which implies that Drins/q > ns0 leads to increasing ns0 withdins, while Drins/q < ns0 leads to decreasing ns0. Thus, for
Al2O3 and AlTiO, ns0 is an increasing and a decreasing function of dins, respectively. It should be noted that, in the limit

FIG. 6. A comparison between the insulator-semiconductor interface fixedcharge density rins and AlGaN polarization charge density rAlGaN.

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J. Appl. Phys. 123, 034504 (2018)

charges. In particular, there is a possibility that the lower
AlTiO/AlGaN interface fixed charge density is owing toelectrons trapped at deep interface states. In order to consider
this possibility, we examined the interface states at Al2O3/
AlGaN and AlTiO/AlGaN by frequency dependent C–V
measurements. Figure 8 shows examples of the measurement
results, C–V characteristics at 100 Hz–1 MHz for the Al2O3/
AlGaN/GaN and AlTiO/AlGaN/GaN MIS devices withdins ¼ 14–19 nm. In any cases, no frequency dispersion isobserved for negative bias voltages, showing that the Vthdetermination is not affected by the measurement frequency.
On the other hand, for positive bias voltages, frequency dispersions are observed, suggesting insulator-semiconductor
interface states.
The conductance method44 was applied to the frequency
dependent C–V characteristics to evaluate the interface state
density.30,45–51 Assuming the equivalent circuit shown in theinsets of Fig. 9, which consists of an interface state capacitance Ci, an interface state conductance Gi, and an AlGaN
capacitance CAlGaN in parallel, with an insulator capacitance
Cins connected in series, we obtained the frequency dependence of Gi for the Al2O3/AlGaN/GaN and AlTiO/AlGaN/
GaN MIS devices. Figure 9 shows examples of obtained
Gi/x as functions of frequency f, where x ¼ 2pf, exhibiting
single-peaked behavior. As shown by the curves in Fig. 9,
the single-peaked behavior is well fitted by52
Gi q2 Di lnð1 þ x2 s2 Þ
¼
;
x
2xs
FIG. 7. Band diagrams of the Al2O3/AlGaN/GaN and AlTiO/AlGaN/
odinger
GaN MIS devices at VG ¼ 0 V, obtained by 1D Poisson-Schr€calculation.

(7)

where Di is the interface state density and s is the trapping
time constant, giving the peak frequency fp ¼ 1/(ps) and thepeak value of Gi/x ’ 0.4q2Di. The observed peaks are summarized in Fig. 10(a), where we find very similar behavior

FIG. 8. C–V characteristics of the
Al2O3/AlGaN/GaN and AlTiO/AlGaN/
GaN MIS devices with dins ¼ 14–19 nm,
measured at 100 Hz–1 MHz.

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J. Appl. Phys. 123, 034504 (2018)

FIG. 9. Gi/x as functions of frequency
with fitting curves for the Al2O3/
AlGaN/GaN and AlTiO/AlGaN/GaN
MIS devices. Insets: The small-signal
equivalent circuit.

FIG. 10. (a) The peak value of Gi/x as functions of peak frequency fp and(b) the interface state density Di as functions of the energy (EC  E), for the
Al2O3/AlGaN/GaN and AlTiO/AlGaN/GaN MIS devices.

for Al2O3/AlGaN/GaN and AlTiO/AlGaN/GaN and also fordifferent dins, suggesting that the behavior is dominated byinterface states with very similar densities at Al2O3/AlGaN
and AlTiO/AlGaN. From the peaks, we can obtain the relationship between Di and s. Moreover, s for an interface stateat the energy E is given by s ¼ s0 exp ½ðEC  EÞ=kB T using
the Boltzmann constant kB, temperature T, and the conduction band bottom energy EC, where s0 is a time constantdetermined by the capture cross section of the trap. Thus,
using s0, we can estimate the relationship between Di and(EC  E). Even though s0 is ambiguous, assuming a wide
range of s0 ¼ 1–100 ps, we show Di as functions of (EC  E)
in Fig. 10(b), where the error bars correspond to the wide
range of s0 values. This indicates a very similar shallow
interface state density Di 2  1013 cm2 eV1 of Al2O3/
AlGaN and AlTiO/AlGaN and suggests that deep interface
state densities are also similar, even though the interface
fixed charge density rins is rather lower at AlTiO/AlGaN.
Thus, we should conclude that there is no correlation
between the interface fixed charges and the interface statesin our case, as reported in Ref. 32. This suggests that thelower rins at AlTiO/AlGaN is not owing to electrons trapped
at deep interface states, compensating the positive fixedcharges. Since interface states generally have a U-shaped
density of states, from the shallow interface state density
above, we can expect a deep interface state density ofⱗ1013 cm2 eV1 or less. On the other hand, the difference
between rins/q at Al2O3/AlGaN and that at AlTiO/AlGaN is0.8  1013 cm2. Thus, it is not plausible that the difference is due to trapped electrons at the deep interface states.
Although the material origin of the lower rins at AlTiO/
AlGaN is not clear, it is possible to tentatively assume alower density of oxygen-related interface donors, wherestrong Ti-O bonding may suppress donor formation.

034504-7

Le, Nguyen, and Suzuki

V. CONCLUSION

We have investigated insulator-semiconductor interface
fixed charges in Al2O3/AlGaN/GaN and AlTiO/AlGaN/GaN
MIS devices. The AlTiO/AlGaN interface gives significantly
lower-density interface fixed charges and rather shallower
threshold voltages. The lower interface fixed charge density
also leads to the fact that the 2DEG concentration is adecreasing function of the AlTiO thickness, whereas being
an increasing function of the Al2O3 thickness. Moreover, we
discuss the relationship between the interface fixed charges
and interface states. Since the interface state densities arevery similar at Al2O3/AlGaN and AlTiO/AlGaN, it is suggested that the lower interface fixed charge density at AlTiO/
AlGaN is not owing to electrons trapped at deep interface
states, compensating the positive fixed charges. Thus, alower density of oxygen-related donors at the AlTiO/AlGaN
interface can be assumed, where strong Ti-O bonding may
suppress donor formation. We consider that the results canprovide a clue towards Vth control and normally off operations of GaN-based MIS devices.
ACKNOWLEDGMENTS

This work was supported by JSPS KAKENHI Grant
Nos. 26249046 and 15K13348.
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