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Title

Electron transport properties of InAs ultrathin
films obtained by epitaxial lift‑off and van der
Waals bonding on flexible substrates

Author(s)Takita, Hayato; Hashimoto, Norihiko; Nguyen, Cong
Thanh; Kudo, Masahiro; Akabori, Masashi; Suzuki,
Toshi‑kazuCitation

Applied Physics Letters, 97(1): 12102‑1‑12102‑3

Issue Date

2010‑07‑07

Type

Journal Article

Text version

publisher

URL

http://hdl.handle.net/10119/9495

RightsCopyright 2010 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 Hayato Takita,
Norihiko Hashimoto, Cong Thanh Nguyen, Masahiro
Kudo, Masashi Akabori, and Toshi‑kazu Suzuki,
Applied Physics Letters, 97(1), 12102‑ (2010) andmay be found at
http://link.aip.org/link/APPLAB/v97/i1/p012102/s1

Description

Japan Advanced Institute of Science and Technology

APPLIED PHYSICS LETTERS 97, 012102 共2010兲

Electron transport properties of InAs ultrathin films obtained by epitaxial
lift-off and van der Waals bonding on flexible substrates
Hayato Takita, Norihiko Hashimoto, Cong Thanh Nguyen, Masahiro Kudo,
Masashi Akabori, and Toshi-kazu Suzukia兲
Center for Nano Materials and Technology, Japan Advanced Institute of Science and Technology (JAIST),
1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan

共Received 13 May 2010; accepted 9 June 2010; published online 7 July 2010兲
We investigated InAs ultrathin films on flexible substrates. InAs layers grown on GaAs共001兲 areseparated by epitaxial lift-off 共ELO兲, followed by van der Waals bonding 共VWB兲 on flexible
substrates. We employed “normal” and “inverted” VWB; in the former, top and bottom sides aremaintained during ELO and VWB, while inverted in the latter. From the InAs on flexible substrates,
we fabricated Hall-bar devices with recess etch-thinning, using which electron transport properties
depending on InAs layer thickness were characterized. For the inverted VWB, we observe very highelectron mobilities of InAs ultrathin films, such as ⬃10 000 cm2 / V s for ⬃100 nm thickness and⬃7000 cm2 / V s for ⬃20 nm. These carrier mobilities are highest not only for thin films on flexible
substrates but also for InAs thin films; higher than those of InAs films grown on GaAs共111兲A andmembranes fabricated from them. © 2010 American Institute of Physics. 关doi:10.1063/1.3459137兴
Electron devices on flexible substrates, such as polyethylene terephthalate 共PET兲 substrates, are important for lightweight, portable, and flexible electronic apparatus applications. In addition, flexible substrates with a low dielectric
constant and a high resistivity can be advantageous for highspeed circuit applications, due to a low parasitic capacitanceand a low leakage current. Thus, fabrication of high-speed
and high-performance electron devices on flexible substrates
is an interesting challenge for future electronics. However, ingeneral, there are difficulties in deposition of high quality
device active layers on flexible substrates due to strongly
limited process temperatures and noncrystalline natures offlexible substrates. Although III-V semiconductors as a device active layer have excellent electronic properties, such as
high electron mobilities and velocities, deposition of highquality III-V semiconductor thin films on flexible substrates
is problematic due to the difficulties. In fact, a lowtemperature-deposited polycrystalline InAs film of ⬃1 ␮mthickness on a flexible substrate exhibits electron mobility of⬃500 cm2 / V s,1 which is far lower than that of singlecrystalline InAs, while higher than that of polycrystalline Si.
In order to realize high-quality III-V semiconductor thinfilms on flexible substrates, van der Waals bonding 共VWB兲
of thin films, in combination with their separation from their
original substrates, is promising because it does not involvethe difficulties of the temperature limitation and the noncrystallinity of flexible substrates. By using the bonding methodwith the separation by ion-cut, a single-crystalline InP film of⬃1 ␮m thickness with electron mobility of ⬃200 cm2 / V s
was formed on a flexible substrate.2 On the other hand, epitaxial lift-off 共ELO兲 process3,4 is simple and effective for the
separation of III-V semiconductor thin films from their original substrates. While almost all studies about ELO and VWB
on host substrates had been restricted to GaAs latticematched systems with a few exceptions,5 we recently haveproposed ELO-VWB process of III-V narrow-gap semicon-

ductors obtained by lattice-mismatched growth with nanoscale thin sacrificial layers,6 which can be applied for formation of high-quality thin films on flexible substrates. In thiswork, we investigated InAs on flexible substrates obtainedby the ELO-VWB process. As a result, we realized InAs
ultrathin films down to ⬃20 nm thickness with very highelectron mobilities on flexible substrates.
By means of molecular beam epitaxy, we grew a heterostructure for ELO-VWB shown in Fig. 1共a兲, InAs layer
共500 nm兲/AlAs sacrificial layer 共4 nm兲/InAs buffer layer
共2500 nm兲/GaAs共001兲. For reference, a structure shownin Fig. 1共b兲, InAs layer 共500 nm兲/GaAs共001兲, was also
grown. The heterostructure and the reference structure, respectively, exhibit room-temperature electron mobilities of7600 cm2 / V s and 7100 cm2 / V s, and sheet concentrationsof 4.8⫻ 1012 cm−2 and 3.2⫻ 1012 cm−2. In these structures,
there are dislocation density distributions along the growth
direction.7,8 From transmission electron microscopy, we
estimate dislocation density in the 500 nm InAs layer ofthe heterostructure, ⬃7 ⫻ 108 cm−2 near the surface and⬃2 ⫻ 109 cm−2 at the InAs/AlAs interface. Meanwhile, theInAs layer of the reference structure possesses estimateddislocation density ⬃2 ⫻ 109 cm−2 near the surface andⲏ1010 cm−2 at the InAs/GaAs interface. These indicate thatthe crystal quality of the InAs layer in the heterostructure isbetter than that in the reference structure.
Figure 2 shows the process of ELO and VWB on a flexible substrate. The flexible substrate is a PET substratea兲

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

0003-6951/2010/97共1兲/012102/3/$30.00

FIG. 1. 共a兲 Heterostructure for ELO-VWB. 共b兲 Reference structure.

97, 012102-1© 2010 American Institute of Physics

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012102-2

Appl. Phys. Lett. 97, 012102 共2010兲

Takita et al.

FIG. 3. 共Color兲 Optical microscope images of 共a兲 an isolated InAs Hall-bar
device mesa on a flexible substrate 共FS兲 and 共b兲 a Hall-bar device on a FS
with the resist patterning of the active region for recess etch-thinning.

18000

normal VWB on FS
inverted VWB on FS
Ref. on GaAs

16000

Electron mobility [cm2/V-s]

14000
12000
10000
8000
6000
4000
2000
0

Sheet electron concentration [cm ]共340 ␮m兲 coated by bisazide-rubber OMR85 共⬃500 nm兲.
Using ⬃2 ⫻ 2 mm2 size samples of the heterostructure, we
carried out ELO, separation of the 500 nm InAs layer attached to an adhesive sheet 共35 ␮m兲, acrylic-coated polymer
sheet Adwill D-201, by HF selective wet-etching of the sacrificial layer. After the ELO, we realized two types of VWB
shown in Fig. 2; one is “normal” VWB, in which top andbottom sides are maintained during ELO and VWB, and theother is “inverted” VWB, resulting in the top-bottom inversion. In the normal VWB process, the separated InAs layer isbonded on a flexible substrate, followed by adhesive sheet
removal. On the other hand, in the inverted VWB, the separated InAs layer is first bonded on an intermediate support, aresist-coated sapphire substrate. After removal of the adhesive sheet, the InAs layer is transferred onto a flexible substrate by VWB, and then the intermediate support is removed
by resist dissolution. If ELO process could be carried out
after VWB of the heterostructure on a flexible substrate in
the beginning, this leads to inverted VWB on the flexible
substrate. However, this was impossible because a suppleness of the adhesive sheet is necessary for progression of theselective wet-etching of the nanoscale thin sacrificial layer,
whose thinness is to keep the crystal quality of the InAs
layer. In order to realize firm VWB via deionized water, we
employed oxide removal from the InAs surface and hydrophilic treatments using oxygen plasma of the flexible substrate surface. As a result, we obtained normal and inverted
VWB InAs layers on flexible substrates, both of which areavailable for subsequent device fabrication process.
Owing to the dislocation density distribution along the
growth direction in the InAs layer, the normal VWB InAs
layer has lower dislocation density in the top side, while theinverted VWB InAs layer has lower dislocation density inthe bottom. Therefore, we expect a different crystal quality
between normal and inverted VWB after etch-thinning. Thiscan be studied by characterization of electron transport properties depending on the layer thickness using InAs Hall-bar
devices with recess etch-thinning. We fabricated the Hall-bar
devices from the normal VWB InAs on flexible substrate, theinverted VWB InAs on flexible substrate, and also the reference InAs on GaAs共001兲, as follows. First, device mesas
were isolated by wet etching. Figure 3共a兲 shows an optical
microscope image of a device mesa on a flexible substrate.
There was no penetration of wet etchant into the InAs/
flexible substrate interface due to the firm bonding, as confirmed by observation from the backside of the flexible

substrate. Next, nonalloy Ohmic Ti/Au electrodes are formedby conventional metal evaporation and lift-off. Furthermore,
we carried out resist patterning of the device active
regions for recess etch-thinning. As shown in Fig. 3共b兲, we
obtained Hall-bar devices on flexible substrates with the resist patterning, using which we repeated recess etch-thinningby H3PO4 – H2O2 – H2O wet-etchant and measurements. The
InAs layer thickness was determined by scanning confocal
laser microscope measurements, with cross-checking by astep profiler. Electron transport properties were characterized
by Hall measurements at room temperature.
Figure 4 shows electron mobilities and sheet concentrations as functions of InAs thickness obtained by measurements of many devices. Both the normal and inverted VWB
InAs give higher electron mobilities than that for the reference, indicating that our ELO-VWB method is effective torealize high-quality InAs ultrathin films on flexible substrates. Moreover, in comparison with the normal VWB, the-2

FIG. 2. 共Color兲 The process of ELO and VWB on a flexible substrate 共FS兲.
Top and bottom sides are maintained in the normal VWB, while inverted in
the inverted VWB.

0

100

200
300
400
InAs thickness [nm]

500

14

10

normal VWB on FS
inverted VWB on FS
Ref. on GaAs

13

10

12

10

11

10

0

100

200
300
400
InAs thickness [nm]500

FIG. 4. 共Color兲 Room-temperature electron mobilities 共above兲 and sheet
concentrations 共below兲 for normal VWB on flexible substrate 共FS兲, inverted
VWB on FS, and reference on GaAs共001兲, as functions of InAs thickness.

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012102-3

Appl. Phys. Lett. 97, 012102 共2010兲

Takita et al.

inverted VWB InAs after thinning exhibits high electron mobilities as expected, such as ⬃10 000 cm2 / V s for⬃100 nm thickness and ⬃7000 cm2 / V s for ⬃20 nm.
While the normal VWB and the reference give electron mobilities monotonically decreasing with decrease in the thickness, being consistent with the dislocation density distributions, the inverted VWB InAs does not. For the inverted
VWB InAs, with decrease in the thickness, the electron mobility once slightly increases for the thickness down to⬃100 nm, which is consistent with the dislocation density
distribution but followed by a decrease for the thickness
ⱗ100 nm. Due to the Fermi energy pinning of InAs,9,10
there are conduction electrons accumulated at the surfaces
and the interfaces 共InAs/flexible substrate or InAs/GaAs兲 inspite of the undoped InAs growth. The observed sheet electron concentrations are dominated by the electrons at thesurfaces and the interfaces, and thus do not exhibit pronounced thickness dependence. The reference exhibits higher
sheet electron concentrations than the normal and inverted
VWB, owing to the interface electrons caused by highdensity dislocations at the InAs/GaAs interface. The decreasein the electron mobilities for the inverted VWB InAs of the
thickness ⱗ100 nm can be attributed to scattering of thesurface electrons by the interface random potential, and also
scattering of the interface electrons by the surface random
potential, which should affect the electron mobilities also forthe normal VWB and the reference.
For the inverted VWB InAs, we obtain highest carrier
mobilities not only for thin films on flexible substrates but
also for InAs thin films. Previous studies show that InAs
grown on GaAs共001兲 exhibits electron mobilities similar to
those of our reference, such as ⬃5000– 8000 cm2 / V s for⬃500 nm thickness, ⬃2000 cm2 / V s for ⬃100 nm,11,12 indicating difficulties of realization of high-quality InAs ultrathin films due to the high-density dislocations at the InAs/
GaAs共001兲 interface. On the other hand, growth of InAs on
GaAs共111兲A provides lower defect densities at the InAs/
GaAs interface, and thus better thin films, whose electron
mobilities are ⬃3000– 4000 cm2 / V s for 50–100 nm thickness. Moreover, free-standing InAs membranes fabricatedfrom the InAs films on GaAs共111兲A exhibit high electron
mobilities such as ⬃7500 cm2 / V s for 50–100 nm
thickness.13 Our inverted VWB InAs ultrathin films exhibit
even higher electron mobilities. This can be applied to InAs

field effect transistors 共FETs兲,14 in particular thin-film FETs
on flexible substrates, a kind of thin body III-V
semiconductor-on-insulator FETs.15 In addition, we can expect an extension to applications to InAs tunnel FETs.16–18
In summary, we investigated InAs ultrathin films on
flexible substrates obtained by ELO-VWB. We obtained very
high electron mobilities of InAs ultrathin films on flexible
substrates. This can open up InAs thin-film device applications on flexible substrates.
This work was partially supported by a research grantfrom the Foundation for Technology Promotion of Electronic
Circuit Board.
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