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Title

An InAs/high‑k/low‑k structure: Electrontransport and interface analysis

Author(s)Ui, Toshimasa; Mori, Ryousuke; Le, Son Phuong;
Oshima, Yoshifumi; Suzuki, Toshi‑kazuCitation

AIP Advances, 7(5): 055303‑1‑055303‑8

Issue Date

2017‑05‑04

Type

Journal Article

Text version

publisher

URL

http://hdl.handle.net/10119/15733

RightsToshimasa Ui, Ryousuke Mori, Son Phuong Le,
Yoshifumi Oshima, and Toshi‑kazu Suzuki, AIP
Advances, 7(5), 2017, 055303‑1‑055303‑8. © 2017
Author(s). All article content, except whereotherwise noted, is licensed under a Creative
Commons Attribution (CC BY) license(http://creativecommons.org/licenses/by/4.0/).
[http://dx.doi.org/10.1063/1.4983176]

Description

Japan Advanced Institute of Science and Technology

AIP ADVANCES 7, 055303 (2017)

An InAs/high-k/low-k structure: Electron transport
and interface analysis
Toshimasa Ui, Ryousuke Mori, Son Phuong Le, Yoshifumi Oshima,
and Toshi-kazu Suzukia
Center for Nano Materials and Technology, Japan Advanced Institute of Scienceand Technology (JAIST), 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan(Received 23 January 2017; accepted 26 April 2017; published online 4 May 2017)

We fabricated and investigated an InAs/high-k/low-k structure in comparison
with an InAs/low-k structure, where the former and the latter are respectively obtained by bonding of InAs/Al2 O3 /AlN and InAs on low-k flexible
substrates (FS). The InAs/high-k/low-k (InAs/Al2 O3 /AlN/FS) exhibits electron
mobilities immune to interface fluctuation scattering, whereas this scattering isserious for the InAs/low-k (InAs/FS). Moreover, we find that electron sheet
concentrations in the InAs/high-k/low-k are significantly higher than those in
the InAs/low-k. From InAs/Al2 O3 interface analysis by energy-dispersive Xray spectroscopy and electron energy-loss spectroscopy, we find that the higher
electron concentrations can be attributed to natural modulation doping fromAl2 O3 to InAs. © 2017 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license(http://creativecommons.org/licenses/by/4.0/). [http://dx.doi.org/10.1063/1.4983176]

I. INTRODUCTION

InAs is an important narrow-gap compound semiconductor,1,2 applicable to mid-infrared opticaldevices,3 high-performance field-effect transistors,4–7 and also interband tunnel transistors.8,9 In
particular, heterogeneous integration of InAs devices on foreign host substrates is quite important.10–13
We previously fabricated and investigated an InAs/low-k structure, where high-quality InAs thin films
are bonded on host low-dielectric-constant (low-k) flexible substrates (FS),14–17 by using epitaxial liftoff (ELO) and van der Waals bonding (VWB) method.18,19 The InAs/low-k (InAs/FS) exhibits highelectron mobilities, where the FS with k ∼ 3, polyethylene terephthalate (PET) coated by bisaziderubber, has a merit for device applications because of a low parasitic capacitance. However, wefound a serious problem of InAs/FS interface fluctuation affecting electron mobilities15 and lowfrequency noise.17 In addition, poor heat release capability due to a low thermal conductivity of PET,κ ∼ 0.3 W/m-K, is also problematic.
Considering these problems, in this work, we fabricated and investigated an InAs/high-k/lowk structure, where a thin high-k insulator layer between InAs and the low-k FS can be beneficial
to suppress the interface fluctuation and to improve the heat release capability, almost keeping themerit of the low parasitic capacitance of the FS. We employed Al2 O3 /AlN as a high-k insulator
layer, where k ∼ 9 and κ ∼ 30 W/m-K for Al2 O3 , and k ∼ 9 and κ ∼ 300 W/m-K for AlN, to obtain
the InAs/high-k/low-k (InAs/Al2 O3 /AlN/FS). Electron transport properties of the InAs/high-k/low-k
were investigated in comparison with those of the InAs/low-k, for InAs film thicknesses from . 10 nm
to ∼ 150 nm. As a result, we find that the InAs/high-k/low-k exhibits electron mobilities immune tointerface fluctuation scattering. We also find that electron sheet concentrations in the InAs/high-k/lowk are significantly higher than those in the InAs/low-k. Interface analysis by energy-dispersive X-ray
spectroscopy (EDX) and electron energy-loss spectroscopy (EELS) for the InAs/Al2 O3 interfaceaAuthor to whom correspondence should be addressed; electronic mail: tosikazu@jaist.ac.jp

2158-3226/2017/7(5)/055303/8

7, 055303-1© Author(s) 2017

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AIP Advances 7, 055303 (2017)indicates that the higher electron concentrations can be attributed to natural modulation doping fromAl2 O3 to InAs.
II. SAMPLE FABRICATIONThe InAs/high-k/low-k and InAs/low-k structures were fabricated as shown in the top of Fig. 1.
Using a heterostructure, InAs device layer (500 nm thickness)/ AlAs sacrificial layer (4 nm thickness)/ InAs buffer layer (2500 nm thickness)/ GaAs(001), we carried out ELO,14–17 separation of
the InAs device layer attached to an adhesive sheet. The InAs device layer was transferred onto
an intermediate support, a sapphire(0001) coated by resists, followed by removal of the adhesive
sheet and InAs surface cleaning using phosphoric acid. For the InAs/high-k/low-k structure, high-k
insulator deposition on the InAs was carried out; we deposited Al2 O3 (50 nm thickness) by atomic
layer deposition (ALD) using trimethylaluminum and H2 O, and AlN (30 nm thickness) by electron
cyclotron resonance (ECR) sputtering deposition using an Ar-N2 plasma and an AlN target. The
InAs/Al2 O3 /AlN was separated from the intermediate support, followed by ‘inverted’ VWB on the
low-k FS, PET coated by bisazide-rubber, to obtain the InAs/high-k/low-k (InAs/Al2 O3 /AlN/FS).
The reasons of employing Al2 O3 /AlN as a high-k insulator layer are as follows. If we employ a single layer deposition of Al2 O3 or AlN, we observe convex or concave sample warpage during VWB
as shown in Fig. 2, probably owing to a strain during the deposition, which makes the process quitedifficult. On the other hand, employing the Al2 O3 /AlN layer is advantageous to suppress sample
warpage as shown in Fig. 2, owing to strain balancing, and consequently helpful for easiness of the
VWB process. The thicknesses of Al2 O3 and AlN are optimized; we found that, for the 50-nm-thick
Al2 O3 , the 30-nm-thick AlN leads to an almost flat sample profile. In addition, the ALD depositionof Al2 O3 on InAs is suitable to avoid interface fluctuations, while the ECR sputtering deposition of
AlN causes damages of the InAs surface. Therefore, we employed the ALD deposition of Al2 O3
followed by the ECR sputtering deposition of AlN to obtain the InAs/Al2 O3 /AlN. Moreover, in orderto obtain the InAs/low-k (InAs/FS), the InAs without high-k insulator deposition was separated fromthe intermediate support, followed by the ‘inverted’ VWB on the low-k FS.
Using the InAs/high-k/low-k and the InAs/low-k, we obtained Hall-bar devices with current
flowing direction [110], by wet-etching isolation, Ohmic electrode formation, and channel thinning

FIG. 1. (Top) Schematic fabrication process of the InAs/high-k/low-k (InAs/Al2 O3 /AlN/FS) and InAs/low-k (InAs/FS) structures. (Bottom) Nomarski optical microscope images of (a) InAs/high-k/low-k and (b) InAs/low-k Hall-bar devices with current
flowing direction [110].

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FIG. 2. (Left) Schematic warpage during VWB in the case of employing a single layer deposition of Al2 O3 or AlN. (Right)
Employing the Al2 O3 /AlN layer is advantageous to suppress warpage.

by wet etching.14–17 Nomarski optical microscope images of the Hall-bar devices are shown in thebottom of Fig. 1, where the differential interference contrasts indicate same smooth surfaces for
the InAs/high-k/low-k and the InAs/low-k. The Hall-bar devices enable us to characterize electrontransport properties of the InAs/high-k/low-k and the InAs/low-k for several InAs channel thicknesses
from . 10 nm to ∼ 150 nm, where the ‘inverted’ VWB is advantageous to obtain a high crystal quality
after the thinning according to the growth-direction dislocation distribution.20
III. ELECTRON TRANSPORT PROPERTIES

From room-temperature measurements of the Hall-bar devices, as shown in Fig. 3, we obtained
electron mobilities µ and electron sheet concentrations ns as functions of the InAs channel thickness d,
where the error bars of d come from thickness measurements.15 The InAs/low-k for d . 15 nm exhibitsµ rapidly decreasing with decrease in d, µ ∝ dγ (γ ' 5-6) attributed to serious interface fluctuation
scattering or thickness fluctuation scattering;15,21–26 when there is a bonding interface fluctuation,
the confinement potential fluctuates with a thickness fluctuation in the Cartesian coordinate, leading
to µ ∝ dγ behavior. On the other hand, for the InAs/high-k/low-k, we do not observe µ ∝ dγ behavior,

FIG. 3. Electron mobilities µ and electron sheet concentrations ns as functions of the InAs channel thickness d for the
InAs/high-k/low-k and InAs/low-k structures. The blue solid line for µ shows µ ∝ d γ (γ ' 5.2). The black solid curve for ns
is obtained by Poisson-Schrödinger calculation.

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AIP Advances 7, 055303 (2017)indicating that the InAs/high-k/low-k exhibits µ immune to interface fluctuation scattering evenfor d . 10 nm. This can be attributed to the fact that the interface fluctuation of the InAs/Al2 O3
obtained by ALD is smaller than that of the InAs/FS obtained by VWB. The InAs/low-k for d & 15
nm exhibits µ dominated by Coulomb scattering, where µ slowly decreases with decrease in d.15
The InAs/high-k/low-k exhibits similar µ attributed to Coulomb scattering, but slightly lower thanthat of the InAs/low-k. This suggests more Coulomb scattering centers near the interface for the
InAs/high-k/low-k, as discussed later. We also find that ns in the InAs/high-k/low-k is significantly
higher (1012 cm−2 order) with a smaller dispersion than in the InAs/low-k (1011 cm−2 order). The
observed higher ns with the smaller dispersion suggests that electrons are supplied from Al2 O3 to
InAs through the InAs/Al2 O3 interface.
IV. INTERFACE ANALYSISIn order to examine the possibility of the electron supply from Al2 O3 to InAs, we carried out
InAs/Al2 O3 interface analysis by using a scanning transmission electron microscope (STEM) withan acceleration voltage of 120 kV. Figure 4(a) shows a high angle annular dark field (HAADF) STEM
image near the InAs/Al2 O3 interface, where the STEM sample thickness is around 50 nm or less, andthe origin of the position x defined later corresponds to the interface. EDX maps for In-Lα, As-L,
Al-Kα, and O-Kα near the InAs/Al2 O3 interface were obtained in the STEM as shown in Fig. 4(b).
EDX intensities (integrated along y direction parallel to the interface and normalized) as functions

FIG. 4. (a) A HAADF STEM image near the InAs/Al2 O3 interface. (b) EDX maps obtained in STEM for In-Lα, As-L,
Al-Kα, and O-Kα near the InAs/Al2 O3 interface. (c) EDX intensities as functions of the position x.

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AIP Advances 7, 055303 (2017)of the position x are shown√ in Fig. 4(c). with fitting curves. The fitting curves√ are given by the error
function, [1 − erf[(x − x0 )/ 2σ]]/2 (for In-Lα and As-L) or [1 + erf[(x − x0 )/ 2σ]]/2 (for Al-Kα andO-Kα), with x 0 = 0 for O-Kα as the definition of the origin corresponding to the interface position,and x0 ' 0 for In-Lα, As-L, and Al-Kα.
Moreover, in the STEM, we carried out EELS near the InAs/Al2 O3 interface. Figure 5 shows
EELS spectra around the O-K edge for x = 1.5-+3.5 nm. While clear O-K edge peaks at 541 eV
are observed for positive x, clear satellite peaks at 532 eV are observed only for x & 1.5 nm. Figure 6
shows (a) O-K edge peak and (b) satellite
peak intensities as functions of the position x, which√
can be also fitted by [1 + erf[(x − x0 )/ 2σ]]/2 using the error function. For comparison, the EDX
O-Kα intensity is also shown. We find the onset position x0 ' 0 for the O-K edge peak, which isconsistent with the EDX O-Kα intensity, while x0 ' 1.3 nm for the satellite peak. In Al2 O3 , there
are inevitable oxygen-vacancies, which act as donors.27,28 It has been reported that, non-ionized
oxygen-vacancy donors (occupied by electrons) in Al2 O3 can cause the satellite peak, while ionized
oxygen-vacancy donors (unoccupied by electrons) do not give the satellite peak.29 Therefore, we
conclude that oxygen-vacancy donors are ionized near the InAs/Al2 O3 interface (x . 1.3 nm), while
not ionized for the Al2 O3 -inside (x & 1.3 nm). This indicates that natural modulation doping takes
place at the InAs/Al2 O3 interface; oxygen-vacancy donors near the interface supply electrons fromAl2 O3 to InAs leading to the higher ns , and become ionized donors. The ionized donors act as
Coulomb scattering centers near the interface, giving the slightly lower µ. The situation is similar to
the natural modulation doping taking place at InAs/AlSb interfaces,30,31 where deep donors due toantisite defects in AlSb supply electrons from AlSb to InAs, leading to a high electron concentrationin InAs.

FIG. 5. EELS spectra for x = 1.5-+3.5 nm, showing O-K edge peaks at 541 eV and satellite peaks at 532 eV.

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FIG. 6. (a) O-K edge peak and (b) satellite peak intensities as functions of the position x, with the EDX O-Kα intensity.

V. POISSON-SCHRÖDINGER CALCULATION

In order to confirm the natural modulation doping picture quantitatively, we carried out PoissonSchrödinger calculation.32 Figure 7 shows examples of the calculated energy band profile and electron
distribution at 300 K, where we plot the conduction band bottom energy E c , the valence band top
energy E v , the oxygen-vacancy donor level E D , the Fermi energy E F , and the electron concentration ρ

FIG. 7. Examples of the calculated energy band profile and electron distribution at 300 K for (a) InAs/Al2 O3 and (b) InAs/FS,
showing the conduction band bottom energy E c , the valence band top energy E v , the oxygen-vacancy donor level E D , theFermi energy E F , and the electron concentration ρ (in the unit of cm 3 ).

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AIP Advances 7, 055303 (2017)(in the unit of cm 3 ), for (a) InAs/Al2 O3 and (b) InAs/FS modeled as free-standing InAs. The energy
gaps of Al2 O3 and InAs are set to be 6.8 eV and 0.37 eV, respectively, with the conduction band
offset of 3.0 eV.33,34 We employ Fermi level pinning at the InAs surface E F E c = 56 meV, giving
1011 cm−2 order electron sheet concentrations for the InAs/FS. As shown in Fig. 7(a), we observe anelectron accumulation in InAs near the InAs/Al2 O3 interface, and a depletion region in Al2 O3 . We
obtain a depletion length in Al2 O3 xdep ' 1.3 nm in good agreement with the EELS result, assuming
1.6 × 1019 cm−3 oxygen-vacancy donors in Al2 O3 with a level of E c E D = 2.6 eV, located ∼ 4 eV
above the valence band top.35 Moreover, from the calculation, we obtain ns as a function of d forthe InAs/Al2 O3 , as shown in the black solid curve of Fig. 3. We find good agreement between thecalculated and experimental results, which supports the natural modulation doping picture.
VI. SUMMARY

In summary, we investigated the InAs/high-k/low-k (InAs/Al2 O3 /AlN/FS) in comparison with
the InAs/low-k (InAs/FS). While interface fluctuation scattering is serious for the InAs/low-k, wefind that the InAs/high-k/low-k exhibits electron mobilities immune to this scattering, attributed
to small InAs/Al2 O3 interface fluctuation obtained by ALD. We also find higher electron sheet
concentrations in the InAs/high-k/low-k than in the InAs/low-k. From EDX and EELS, we conclude
that natural modulation doping takes place at the InAs/Al2 O3 interface leading to the higher electron
concentrations, as supported by Poisson-Schrödinger calculation.
ACKNOWLEDGMENTS

This work was supported by JSPS KAKENHI Grant Number 26249046, 15K13348. We would
like to thank K. Higashimine for helpful discussions.
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