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Title

Low‑frequency noise in AlTiO/AlGaN/GaN metal‑
insulator‑semiconductor heterojunction field‑
effect transistors

Author(s)Le, Son Phuong; Ui, Toshimasa; Nguyen, Tuan Quy;
Shih, Hong‑An; Suzuki, Toshi‑kazuCitation

Journal of Applied Physics, 119(20): 204503‑1‑
204503‑6

Issue Date

2016‑05‑27

Type

Journal Article

Text version

publisher

URL

http://hdl.handle.net/10119/15735

RightsCopyright 2016 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,
Toshimasa Ui, Tuan Quy Nguyen, Hong‑An Shih, andToshi‑kazu Suzuki, Journal of Applied Physics,
119(20), 204503 (2016) and may be found at
http://dx.doi.org/10.1063/1.4952386

Description

Japan Advanced Institute of Science and Technology

JOURNAL OF APPLIED PHYSICS 119, 204503 (2016)

Low-frequency noise in AlTiO/AlGaN/GaN metal-insulator-semiconductor
heterojunction field-effect transistors
Son Phuong Le, Toshimasa Ui, Tuan Quy Nguyen, Hong-An Shih, 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 25 March 2016; accepted 10 May 2016; published online 27 May 2016)
Using aluminum titanium oxide (AlTiO, an alloy of Al2O3 and TiO2) as a high-k gate insulator, wefabricated and investigated AlTiO/AlGaN/GaN metal-insulator-semiconductor heterojunction
field-effect transistors. From current low-frequency noise (LFN) characterization, we findLorentzian spectra near the threshold voltage, in addition to 1/f spectra for the well-above-threshold
regime. The Lorentzian spectra are attributed to electron trapping/detrapping with two specific time
constants, 25 ms and 3 ms, which are independent of the gate length and the gate voltage,corresponding to two trap level depths of 0.5–0.7 eV with a 0.06 eV difference in the AlTiO
insulator. In addition, gate leakage currents are analyzed and attributed to the Poole-Frenkel
mechanism due to traps in the AlTiO insulator, where the extracted trap level depth is consistent
with the Lorentzian LFN. Published by AIP Publishing. [http://dx.doi.org/10.1063/1.4952386]

I. INTRODUCTION

GaN-based metal-insulator-semiconductor heterojunction field-effect transistors (MIS-HFETs) with various highdielectric-constant (high-k) gate insulators, such as Al2O3,1
HfO2,2,3 ZnO,4 TaON,5 AlN,6–9 or BN,10,11 have been extensively developed, owing to the merits of gate leakage reduction and passivation effects. Although both a high k and awide energy gap (Eg ) are important for a gate insulator, there
exists a trade-off between these two properties.12 One effective method to balance k and Eg is employing aluminum titanium oxide (AlTiO, an alloy of Al2O3 and TiO2) withintermediate physical properties of Al2O3 (k  9; Eg  7 eV)
and TiO2 (k  60; Eg  3 eV). AlTiO has been used for Sibased devices13–15 and GaAs-based devices16,17 and also
should be promising for GaN-based MIS-HFETs. On theother hand, current low-frequency noise (LFN) in MIS devices might be influenced by electron traps, in particular, by
those related to gate insulators.18 In fact, in addition to 1/f
LFN spectra,19–27 Lorentzian LFN spectra are sometimes
observed,28–31 being attributed to electron traps in GaN-based
HFETs. Therefore, LFN characterization is an importantdiagnostic tool for electron traps in GaN-based MIS-HFETs.
In this work, we fabricated and investigated AlTiO/AlGaN/
GaN MIS-HFETs using an AlxTiyO (x : y ¼ 0:73 : 0:27) gateinsulator (k  14; Eg  6 eV), obtained by atomic layer
deposition (ALD). From current LFN characterization, we
find Lorentzian spectra near the threshold voltage, attributed
to electron trapping/detrapping in the AlTiO insulator, fromwhich we extracted time constants as well as trap level
depths. Moreover, gate leakage currents are analyzed andattributed to the Poole-Frenkel mechanism due to traps in
the AlTiO insulator, which are discussed in relation with theLorentzian LFN.
a)

Author to whom correspondence should be addressed. Electronic mail:tosikazu@jaist.ac.jp

0021-8979/2016/119(20)/204503/6/$30.00

II. DEVICE FABRICATION AND BASIC
CHARACTERISTICS

Using an Al0.27Ga0.73N(30 nm)/GaN(3000 nm) heterostructure obtained by metal-organic vapor phase epitaxy on
sapphire(0001), we fabricated AlTiO/AlGaN/GaN MIS devices as follows. On the AlGaN/GaN heterostructure, Ti/Al/Ti/
Au Ohmic electrodes were formed, and device isolation wasachieved by Bþ ion implantation. After AlGaN surface treatments, a 29-nm-thick AlxTiyO (x : y ¼ 0:73 : 0:27) film as agate insulator was deposited by ALD using trimethyl aluminum (TMA), tetrakisdimethylamino titanium (TDMAT), andH2O.17 Considering the trade-off between k and Eg as wellas the break-down field Fb , we employed AlxTiyO withx : y ¼ 0:73 : 0:27, which has k  14 (the static dielectric
constant obtained by capacitance measurements of metal-insulator-metal structures at 1 MHz), Eg  6 eV, and Fb  6.5
MV/cm.17 Hall-effect measurements show an electron
mobility of 1450 cm2/V-s, a sheet electron concentration of
9:1  1012 cm2 , and a sheet resistance of 470 X=ⵧ of thetwo-dimensional electron gas (2DEG) of the AlTiO/AlGaN/
GaN. Finally, the formation of Ni/Au gate electrodes on the
AlTiO followed by an annealing at 350  C for 30 min in
H2-mixed (10%) Ar ambience completed the device fabrication. As a result, we obtained AlTiO/AlGaN/GaN MISHFETs, MIS capacitors, and ungated two-terminal (2T) devices as shown in Fig. 1. The MIS-HFETs have gate lengthsof LG ¼ 0:26; 0:56; and 1:1 lm, a source-drain spacing of
5 lm, a source-gate spacing of 2 lm, and a channel widthof W ¼ 50 lm. The MIS capacitors have a gate area of
100 lm  100 lm, distanced 25 lm apart from the Ohmicelectrode. The ungated 2T devices have channel lengths(electrode spacings) of L ¼ 2–16 lm and a channel width of
W ¼ 100 lm. It should be noted that the fabrication andcharacteristics (including LFN characteristics) of the baseline AlGaN/GaN Schottky-HFETs have been reported inRef. 27.

119, 204503-1

Published by AIP Publishing.

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J. Appl. Phys. 119, 204503 (2016)

FIG. 1. Schematics of the AlTiO/
AlGaN/GaN (a) MIS-HFETs, (b) MIS
capacitors, and (c) ungated 2T devices.

Figure 2 shows examples of output and transfer characteristics of the AlTiO/AlGaN/GaN MIS-HFETs withLG ¼ 0.26 lm, where VD is the drain-source voltage, VG isthe gate-source voltage, ID is the drain current, IG is the gate

FIG. 2. (a) Output characteristics, and transfer characteristics at (b)VD ¼ 10 V and (c) VD ¼ 0.1 V of the AlTiO/AlGaN/GaN MIS-HFET withLG ¼ 0.26 lm. The drain current ID, the gate current IG, and the transconductance gm are normalized by the channel width W.

current, and gm is the transconductance. (Considering LFNcharacterization, we employ the definition of currents in theunit of [A] without normalization by the channel width;
the vertical axis of Fig. 2 shows ID/W and IG/W in the unitof [mA/mm].) As shown in the output characteristics ofFig. 2(a), we obtain drain currents as high as 700 mA/mm.
The transfer characteristics at VD ¼ 10 V in the saturation
regime (Fig. 2(b)) and at VD ¼ 0.1 V in the linear regime(Fig. 2(c)) both exhibit significantly small IG of 108 A/mm
range or less, about 7 and 4 orders of magnitude smaller for
forward and reverse biases, respectively, than those of
Schottky-HFETs, owing to good insulating properties of the
AlTiO. The small gate leakage currents lead to small drain
off-currents shown in Figs. 2(b) and 2(c). In addition, we
observe a bump of IG for high VD as shown in Fig. 2(b),attributed to a self-heating effect; the bump disappears forsmall VD as in Fig. 2(c). The obtained device characteristics
suggest that AlTiO can be an important candidate as a gateinsulator for GaN-based MIS-HFETs. We also characterized
the AlTiO/AlGaN/GaN MIS capacitors. Figure 3 shows
capacitance-voltage (C-VG) characteristics at 1 MHz under
VG ¼ 15 V ! þ6 V and the 2DEG concentration ns under
the gate calculated by the integration of C as a function of
VG. The C-VG characteristics exhibit a quite small hysteresis
less than 30 mV under VG ¼ 15 V ! þ6 V and VG¼ þ6 V ! 15 V with a sweep rate of 0.36 V/s. From thecapacitance plateau, we can estimate the dielectric constantof the AlTiO insulator, k  14, which is consistent with thecapacitance measurements of the metal-insulator-metal
structures. The inset of Fig. 3 shows current density-voltage(J-VG) characteristics of the MIS capacitors, being consistent
with the gate leakage of the MIS-HFETs.

FIG. 3. C-VG characteristics of the AlTiO/AlGaN/GaN MIS capacitor at
1 MHz and the 2DEG concentration ns under the gate calculated by the integration of C as a function of VG. The inset: J-VG characteristics.

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J. Appl. Phys. 119, 204503 (2016)

III. LOW-FREQUENCY NOISE CHARACTERISTICS

For LFN characterization of the devices, we employed ameasurement system consisting of a shielded probe station,a low-noise pre-amplifier (LNA, Stanford SR570), and a
dynamic signal analyzer (DSA, Agilent 35670A).27,32 We
first measured LFN in the AlTiO/AlGaN/GaN ungated 2T
devices. Figure 4 shows examples of measurement results ofcurrent noise power spectrum density (PSD), SI, as a functionof the frequency f for the Ohmic regime of the ungated 2T
devices with L ¼ 2 and 16 lm. We observe pure 1/f LFN
spectra satisfying SI =I 2 ’ K=f with a constant factor K,
where the DC current I is varied by changing the 2T noiseless bias voltage V. In the same way as the previous work,27
we can identify the contributions from the Ohmic contacts
and the ungated 2DEG channel by analyzing KW dependingon the channel length L shown in the inset of Fig. 4. As aresult, we obtain the factor Kc for one Ohmic contact and the
Hooge parameter aug for the ungated 2DEG, Kc W ’
1:5  1012 cm and aug ’ 4:0  104 , which are similar toother AlGaN/GaN devices in the previous work.
We next measured LFN in the AlTiO/AlGaN/GaN MISHFETs. Figure 5(a) shows examples of measurement resultsof drain current noise PSD, SID , for the linear regime of the
AlTiO/AlGaN/GaN MIS-HFETs with LG ¼ 0:26 lm at fixedgate voltages VG. We confirm the relation SID / ID 2 ; where
the drain current ID is varied by changing the noiseless drain
voltage VD. In the well-above-threshold regime, VG ⲏ  4 V,
pure 1/f LFN spectra satisfying SID ’ KHFET ID 2 =f with a constant factor KHFET were obtained. From the pure 1/f LFN
spectra, we obtain Hooge parameters a in the intrinsic gatedregion as in the previous work.27 Fig. 5(b) shows a for
AlTiO/AlGaN/GaN MIS-HFETs as a function of the 2DEG
concentration ns under the gate given in Fig. 3, with theresults for AlN/AlGaN/GaN MIS-HFETs, where the AlN
gate insulator was deposited by RF magnetron sputtering,8,9
and AlGaN/GaN Schottky-HFETs, as well as aug for theungated 2DEG. We observe a universal behavior, which canbe attributed to fluctuations in the intrinsic gate voltage
through the extrinsic source resistance.27 On the other hand,near the threshold voltage, VG  7–5 V, we find non-1/f

FIG. 5. (a) LFN spectra normalized by the drain current square, SID =ID 2 , for
the AlTiO/AlGaN/GaN MIS-HFETs with LG ¼ 0.26 lm. (b) Hooge parameter a as a function of ns with the results for AlN/AlGaN/GaN MIS-HFETs
and AlGaN/GaN Schottky-HFETs, as well as aug for the ungated 2DEG.

LFN spectra with Lorentzian behaviors. No Lorentzian LFN
spectra are observed for AlN/AlGaN/GaN MIS-HFETs and
AlGaN/GaN Schottky-HFETs, which show pure 1/f LFN
spectra for all gate biases.27 Therefore, the Lorentzian LFN
spectra are attributed to the AlTiO gate insulator.
Hereafter, we discuss the Lorentzian LFN spectra.
Figure 6(a) shows the observed spectra depending on thegate voltage VG, where we can confirm that all the spectra
exhibit Lorentzian behavior. The Lorentzian spectra can bewell-fitted by a superposition of two Lorentzians
SID ð f Þ
A1
A2
¼
þ
;
2
ID 2
1 þ ð 2pf s1 Þ
1 þ ð 2pf s2 Þ2

FIG. 4. LFN spectra normalized by the current square, SI/I2, as functions offrequency f for the AlTiO/AlGaN/GaN ungated 2T devices. The inset: KW
depending on the channel length L.

(1)

where s1 and s2 are time constants, and A1 and A2 areLorentzian prefactors. Figure 6(b) shows an example of thefitting. As a result of the fitting for LG ¼ 0.26, 0.56, and
1.1 lm, we obtain time constants s1, s2, shown in Fig. 7(a),and Lorentzian prefactors A1, A2, as functions on the gate
voltage VG. Two specific time constants, s1  25 ms ands2  3 ms, are independent of VG and LG. The bias independent time constants suggest that the Lorentzian spectra are
attributed to two electron trap levels inside the AlTiO insulator. In general, such time constant s at temperature T is given
by s ¼ 1=ðvth re Nc ÞeEa =kB T ¼ s0 eEa =kB T , where kB is theReuse of AIP Publishing content is subject to the terms at: https://publishing.aip.org/authors/rights-and-permissions. Download to IP: 118.83.33.166 On: Fri, 27 May 2016
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FIG. 6. (a) Lorentzian LFN spectra depending on the gate voltage VG. (b)An example of fitting of a LFN spectrum at VG ¼ 6.0 V by a superpositionof two Lorentzians.

Boltzmann constant, vth is the electron thermal velocity, re is
the electron capture cross-section, Nc is the effective densityof states in the conduction band, and Ea is the trap level
depth. Although we do not know the electron capture crosssection, we can estimate the trap level depths in the AlTiO
insulator. As shown in Fig. 7(b), even if we assume a wide
range of re ¼ 1016–1014 cm2 and an electron effective
mass between m* ¼ 0.3m0 for Al2O333 and m* ¼ 30m0 for
TiO234,35 (m0: the bare electron mass), the trap level depths
are estimated to be Ea1 ¼ ð1=bÞlnðs1 =s0 Þ ’ 0:50:7 eVand Ea2 ¼ Ea1  DEa with DEa ’ 0:06 eV. On the other
hand, the Lorentzian prefactors depend on VG. Normalized
Lorentzian prefactors, products of the prefactor and the gatearea, A1 LG W and A2 LG W, are shown in Fig. 8, as functionsof (a) the gate voltages VG and (b) the 2DEG concentration
ns under the gate. We find that A1 LG W and A2 LG W are independent of the gate area, i.e., the Lorentzian prefactors areinversely proportional to the gate area as a natural consequence of the law of large numbers. We observe decreases in
the normalized Lorentzian prefactors with an increase in VG
as well as ns, implying that the effects of electron trapping/detrapping are more dominant for lower 2DEG concentrations ns. This suggests that Lorentzian LFN near the threshold voltage can be a good indicator of the quality of gateinsulators. On the other hand, for the well-above-threshold

J. Appl. Phys. 119, 204503 (2016)

FIG. 7. (a) Time constants s1 and s2 as functions on the gate voltage VG. (b)The relation between the time constant and the electron trap level depth Ea.

regime, i.e., for large ns, Lorentzian LFN is buried in 1/f
spectra.
In order to understand the meaning of the Lorentzianprefactors, we consider a general Lorentzian current PSD
normalized by the current square
SI
A
2psaL
1
¼
¼
;
I2 1 þ ð 2psf Þ2
N 1 þ ð 2psf Þ2(2)with a prefactor A (in the unit of [1/Hz]) and a specific time
constant s (or a specific frequency f0 ¼ 1/2ps), where we
define a dimensionless Hooge-like parameter aL for Lorentzian
LFN by A ¼ aL =Nf0 ¼ 2psaL =N using the total carrier number N. Since the current fluctuation dI is given by
ðdIÞ2
¼
I2

ð1

SI
p aL
;
df ¼
2
I
2N
0

and Burgess theorem36 gives"#ð dIÞ2 1 ðdlÞ2 ð dN Þ2
;
¼
þ
I2
N
N l2

(3)

(4)

where dl and dN are the fluctuations of the mobility l andthe carrier number N, respectively, we obtain

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J. Appl. Phys. 119, 204503 (2016)

FIG. 9. The Hooge-like parameters aL1 and aL2 as functions of ns.

gate insulator; for forward biases, it is difficult to obtainaccurate F owing to effects of the interface states.38 The
electric field can be evaluated by
F¼

FIG. 8. Normalized Lorentzian prefactors, products of the prefactor and thegate area, A1 LG W and A2 LG W, as functions of (a) VG and (b) ns.

"#
2
2 ðdlÞ2 ð dN Þ
;
aL ¼
þ
N
p l2

(5)

showing that the Hooge-like parameter is related to fluctuations of the mobility and carrier number. According to thedefinition, we evaluate Hooge-like parameters for the two
Lorentzian components of the LFN in the AlTiO/AlGaN/
GaN MIS-HFETs, aLi ¼ Ai N=ð2psi Þ (i ¼ 1, 2). Figure 9shows aL1 and aL2 as functions of ns for LG ¼ 0.26, 0.56, and
1.1 lm, where we find aL1 ’ aL2 . This suggests the same origin of the two trap levels in the AlTiO gate insulator. We
tentatively assume an on-site Coulomb effect.37 Although
two electrons (with opposite spins) can occupy one traplevel, the on-site Coulomb repulsion shifts the effective level
for the second electron shallower; this gives DEa ’ 0:06 eV.

Dr  qns
;
ke0

(6)

similar to Refs. 39 and 40, where q is the electron charge, e0
is the vacuum permittivity, and Dr ¼ r – r0 is the difference
between the interface fixed charge r at AlTiO/AlGaN
and the polarization charge r0 of GaN. We find Dr=q
¼ 7:6  1012 cm2 from measurements of the MIS capacitors with several insulator thicknesses,41,42 giving the relation between
pﬃﬃﬃJ and F shown in Fig. 11(a); Poole-Frenkel
plots (J/F– F) exhibit linear dependence in the high fieldregime, indicating the Poole-Frenkel p
conduction
mechanism
ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
ﬃ
given by J ¼ BF expðb/Þ expðb q3 F=ðpje0 ÞÞ, whereb ¼ 1=kB T, B is a constant, j is related to the dielectric constant, and / is the trap level depth in the AlTiO. Although
we obtain j  15 from the Poole-Frenkel currents, its meaning is not clear. There exists a controversy on the meaning of
the value;43 we consider that j can be either the static one44
or four times of the dynamic (optical) one.45 On the other
hand, we can unambiguously obtain a trap level depth in the

IV. TRAP LEVEL DEPTH IN COMPARISON WITH AN
ESTIMATION BY POOLE-FRENKEL CURRENTS

In order to confirm the AlTiO trap level depth obtainedby the LFN, we investigated gate leakage currents of the
MIS capacitors. Figure 10 shows the gate leakage current
density J as a function of VR ¼ –VG at T ¼ 280–380 K, dominated by the Poole-Frenkel conduction mechanism as shown
below. It should be noted that we focus on reverse biases inorder to accurately evaluate the electric field F in the AlTiO

FIG. 10. Gate leakage current density J as a function of VR ¼ –VG at
T ¼ 280–380 K for the MIS capacitor.

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J. Appl. Phys. 119, 204503 (2016)

FIG. p11.
ﬃﬃﬃ (a) Poole-Frenkel plots(J=F- F). (b) B expðb/Þ as a function of 1/T.

AlTiO insulator / ’ 0:6 eV by plotting B expðb/Þ as afunction of 1/T shown in Fig. 11(b); / is consistent with thetrap level depth obtained from the Lorentzian LFN, indicating that the traps in the AlTiO insulator dominate both theLorentzian LFN and the Poole-Frenkel leakage currents.
V. CONCLUSION

We fabricated and investigated AlTiO/AlGaN/GaN
MIS-HFETs. From LFN characterization, in addition to 1/f
spectra for the well-above-threshold regime, we findLorentzian spectra near the threshold voltage, with two specific time constants 25 ms and 3 ms, corresponding totrap level depths of 0.5–0.7 eV. In addition, gate leakage
currents are analyzed and attributed to the Poole-Frenkel
mechanism due to traps in the AlTiO insulator, where theextracted trap level depth is consistent with the Lorentzian
LFN, indicating that the traps in the AlTiO insulator dominate both the Lorentzian LFN and the Poole-Frenkel leakage
currents. The results exemplify the importance of LFN characterization for GaN-based MIS-HFETs as a diagnostic tool.
ACKNOWLEDGMENTS

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