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Title

Epitaxial Lift‑Off of InGaAs/InAlAs Metamorphic
High Electron Mobility Heterostructures and Theirvan der Waals Bonding on AlN Ceramic Substrates

Author(s)Jeong, Yonkil; Shindo, Masanori; Akabori,
Masashi; Suzuki, Toshi‑kazuCitation

Applied Physics Express, 1: 021201‑1‑021201‑3

Issue Date

2008‑01‑25

Type

Journal Article

Text version

author

URL

http://hdl.handle.net/10119/4840

RightsThis is the author's version of the work. It isposted here by permission of The Japan Society ofApplied Physics. Copyright (C) 2008 The Japan
Society of Applied Physics. Yonkil Jeong,
Masanori Shindo, Masashi Akabori, and Toshi‑kazu
Suzuki, Applied Physics Express, 1, 2008, 021201.

Description

Japan Advanced Institute of Science and Technology

Typeset with apex.cls <ver. 1.0.1>

Applied Physics Express

Epitaxial Lift-Off of InGaAs/InAlAs Metamorphic High Electron Mobility
Heterostructures and Their van der Waals Bonding on AlN Ceramic Substrates
Yonkil Jeong, Masanori Shindo, Masashi Akabori ∗ , and Toshi-kazu Suzuki †Center for Nano Materials and Technology,
Japan Advanced Institute of Science and Technology (JAIST),
1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan

We have carried out epitaxial lift-off (ELO) of In0.57 Ga0.43 As/In0.56 Al0.44 As metamorphic high electron
mobility heterostructures and their van der Waals bonding (VWB) on AlN ceramic substrates. Using ametamorphic heterostructure with an AlAs sacrificial layer and an InGaAs graded buffer grown on
GaAs(001), thin film Hall-bar devices on AlN ceramic substrates were successfully fabricated by ELO and
VWB. The Hall-bar devices exhibit very high electron mobilities, such as 11000 cm2 /V-s at room
temperature (RT) and 84000 cm2 /V-s at 12 K. The RT mobility is the highest ever reported for ELO
devices. This is the first report on ELO for metamorphic devices.

∗

Present address: Institute of Bio- and Nanosystems (IBN-1), Research Centre Juelich, 52425 Juelich, Ger-

many
Corresponding author. E-mail address: tosikazu@jaist.ac.jp†

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Metamorphic crystal growth of lattice-mismatched compound semiconductor systems has
attracted much attention, because it can remove the restrictions of the lattice-matching condition between device layers and substrates. In particular, high indium content InGa(Al)As
metamorphic devices, such as metamorphic high electron mobility transistors1–3) and metamorphic heterojunction bipolar transistors,4, 5) on GaAs substrates, which have practical merits in comparison with InP substrates, were extensively developed. We have freedom of choice
regarding the indium content in the metamorphic devices, which should be optimized considering the trade-off between speed and power performances.6, 7) One of the most important
issues of the metamorphic devices is to reduce unfavorable influences of crystalline defects,
because these defects, dislocations and stacking faults, often cause deterioration of device performance. Therefore, thicker metamorphic buffer layers are employed to obtain lower defectdensity in the device layers. However, there are problems associated with the thick buffers
containing high-density defects, such as increasing growth costs, high thermal resistances, andpossible leakage currents or parasitic capacitances. Because of the low thermal conductivityof the GaAs substrates compared with the InP substrates, the thermal resistance in the metamorphic devices is rather problematic.8) In order to reduce the leakage current or the parasitic
capacitance, the material choice for the buffers is restricted.
In this work, for a solution of these problems, we propose an application of epitaxial lift-off(ELO) process to the metamorphic devices. In this process, the device layer is separated fromthe substrate and the buffer, by selective etching of a sacrificial layer. Accordingly, all of the
problems associated with the buffers can be eliminated, and we can reuse the substrate and thebuffer for the next growth, reducing the costs. ELO processes are important for heterogeneous
integration (HI) in combination with bonding techniques, for example, van der Waals bonding(VWB).9–13) Thin film compound semiconductor devices, typically with thickness of ∼ 1 µm,
separated from the original substrates of >∼ 600 µm thickness by the ELO, can be integrated on
arbitrary host substrates. Recycling the original substrate, we can obtain an HI system withthe minimized mass of compound semiconductors, which are sometimes harmful materials.
Moreover, the ELO process does not need substrate thinning in contrast to the transferredsubstrate process.14, 15) Therefore, the ELO technology is favorable also from the viewpoint ofindustrial waste reduction and resource conservation. Although there have been many resultson ELO devices for GaAs lattice-matched systems,16–19) few studies have been carried out for
lattice-mismatched systems. The combination of ELO and metamorphic lattice-mismatched
growth will open up many possibilities; it is acceptable to grow a considerably thick buffer on
a conventional substrate, leading to a “virtual substrate” with a desired lattice constant andreduced defect density for the ELO device growth and the HI.
As an example of the proposal, we investigated the ELO of In0.57 Ga0.43 As/In0.56 Al0.44 As
metamorphic high electron mobility heterostructures on GaAs and their VWB on AlN ceramic

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substrates. By means of molecular beam epitaxy, we have grown a metamorphic buffer consisting of an Inx Ga1−x As step graded buffer (SGB) and an In0.56 Al0.44 As buffer. The InGaAs SGB
has x = 0.19 → 0.62 with ∆x = 0.054 and 100 nm each step thickness, and the InAlAs buffer
is 100 nm thick. After the growth of the metamorphic buffer, a very thin AlAs sacrificial layerof 3 nm thickness was grown. Finally, we have grown a modulation-doped InGaAs/InAlAs
metamorphic device layer consisting of an In0.56 Al0.44 As barrier (1.9 µm), an In0.57 Ga0.43 As
channel (30 nm), an In0.56 Al0.44 As spacer (20 nm), a Si δ-doping, an In0.56 Al0.44 As barrier(40 nm), and an In0.57 Ga0.43 As cap (10 nm). The indium contents were precisely confirmedby symmetric and asymmetric X-ray diffraction measurements. We have employed slightly
higher indium contents than the InP lattice-matched system, according to the freedom of
choice regarding the indium contents. The InGaAs SGBs are usually not preferred for metamorphic devices because of their parallel conduction or leakage currents, as well as parasitic
capacitances. However, we employed the InGaAs SGB, because there is no need to concern
its influence on device operation after the ELO. Moreover, it is advantageous for selective
etching of the AlAs sacrificial layer, in comparison with the InAlAs SGB, whose low indium
content regions are etched during the sacrificial layer etching. In the device layer, the channel and the barrier are almost lattice-matched to each other. On the other hand, there is a
4 % lattice-mismatch between the device layer and the sacrificial layer, and also between the
top of the metamorphic buffer layer and the sacrificial layer. Figure 1 shows a cross-sectionaltransmission electron microscope image of the grown structure. There is a tendency for dislocations to be confined to the bottom region, which can be attributed to the alloy hardening
gradient.20) In spite of the 4 % lattice-mismatch, the influence of the thin AlAs sacrificial layeron the device layer is not serious. It should be noted that, concerning the AlAs sacrificial layer
thickness, there is a trade-off between the device layer crystal quality and the ELO processeasiness; thicker sacrificial layers lead to easier and fast ELO processes, but lower crystal qualities. Thus, we fixed the AlAs sacrificial thickness at 3 nm, which results in a defect density
on the order of 108 cm−2 , similar to usual metamorphic devices.6, 7)
Using the metamorphic heterostructure, we have fabricated thin film Hall-bar devices
on AlN ceramic substrates by using ELO and VWB. The AlN has favorable properties fordevice applications: a high thermal conductivity suitable for efficient heat management, a highelectrical resistivity for low leakage currents, and a lower dielectric constant for lower parasitic
capacitances. Figure 2(a) shows a schematic fabrication process. After device mesa-etching,
the thin film device layer is separated by ELO etching, selective etching of the AlAs sacrificial
layer using the 12.5 % HF solution. The separated device layer is then bonded onto the AlN
ceramic substrates by a VW force, followed by the conventional photolithography and lift-off
process of Ti/AuGeNi/Au ohmic electrodes. Since the separated thin film device layers arevery fragile, we must take care in the handling and bonding processes to avoid a damage.

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One of the successful procedures is as follows. The sacrificial layer etching in the HF solution
is carried out using the resist on the device layers for the device mesa-etching. The resist
improves the yield of the ELO process, supporting the device layers and suppressing theirdeformation and damage. For example, we employed the resist of ∼ 0.2 µm-thick LOL-2000
consisting of polydimethylglutarimide and ∼ 2 µm-thick OMR-85 consisting of rubber andbisazide on it; the purpose of the former is the enhancement of adherence between the resist
and the device layers. During the sacrificial layer etching, there was a weak [110] directionroll-up tendency of the device layers, which direction is different from [100] for pseudomorphic
strained systems.21) This can be attributed to a weak strain caused by small lattice-mismatch
between the channel and the barrier. After the sacrificial layer etching, the device layer withthe substrate is moved from the HF solution into deionized water, in which the device layer
with the resist is transferred onto the AlN substrate. After drying, the resist is removed
from the device layer on the AlN by O2 plasma ashing. By dropping a small amount ofwater near the interface between the device layer and the AlN, and subsequent drying, we
can obtain a sufficiently firm VWB for the following electrode formation process. The AlN
ceramic substrate has the root mean square surface roughness >15∼ nm, which is rather roughand disadvantageous for the VWB. In order to obtain firmer bonding, hydrophilic surface
pre-treatments using O2 plasma were effective. Figure 2(b) shows a picture of a separated
device layer in deionized water during the process (left), and an optical microscope image ofa successfully fabricated Hall-bar device bonded on the AlN ceramic substrate, whose current
flowing direction is [100] (right). The outer size of the device is about 900 × 1400 µm2 . The
channel width is 50 µm, and the maximum mesa width of the device including contact regions
is 400 µm. For this device size, the needed sacrificial layer etching time was 20-30 min. On
the other hand, for the ELO of larger samples, it is effective to use a supporting wax on thesurface. Larger area samples up to 3 × 3 mm2 were successfully epitaxial-lifted off with the
wax on the surface, whose needed sacrificial layer etching time was several hours, and bonded
on an AlN substrate, followed by a wax removal. The larger samples can be used for various
characterizations, such as X-ray diffraction and photoluminescence measurements. Detailed
characterization results will be discussed elsewhere.
Hall measurements were carried out for the ELO Hall-bar device. Figure 3 shows temperature dependence of electron mobility and sheet electron concentration, with results for the
on-wafer counterpart before ELO. We obtain very high electron mobilities for the ELO device,
such as 11000 cm2 /V-s at room temperature (RT) and 84000 cm2 /V-s at 12 K. The measurements were stable and reproducible. The obtained RT mobility is the highest ever reported
for ELO devices,16–18) owing to the high indium content and the successful ELO and VWB
processes. Moreover, the mobility is higher than that obtained by the transferred-substrateusing the substrate etching-out technique for an InP lattice-matched system.15) We confirmed

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that, with high reproducibility, these high electron mobilities are obtained for many devices
and all the current flowing crystalline directions. The mobility behavior of the ELO Hall-bar
device is almost the same as that of the on-wafer counterpart. Since the ohmic contacts of the
on-wafer counterpart are non-alloyed and shallow, influences of the parallel conduction of themetamorphic buffer are not so prominent. However, in the high temperature region, we finddifferences in the sheet electron concentration; the on-wafer counterpart exhibits higher values
than the ELO device, due to the parallel conduction of the metamorphic buffer, which also
causes a parasitic capacitance. The ELO device is completely free from these problems. Thisis a proof of that the restriction in the metamorphic buffer material choice can be removed.
In summary, based on the proposal of the application of ELO to metamorphic latticemismatched growth, we have successfully carried out ELO of In0.57 Ga0.43 As/In0.56 Al0.44 As
metamorphic heterostructures and their VWB on AlN ceramic substrates. The obtained devices show very high electron mobilities, owing to the high indium content and the successful
ELO and VWB processes. We consider that this technology will be useful for many futureapplications including the HI on arbitrary host substrates, such as silicon, ceramic, glass, andplastic.
Acknowledgment
This work was supported by KAKENHI 18560300. A part of this work was conducted in KyotoAdvanced Nanotechnology Network, supported by “Nanotechnology Network” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

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Figure captions
Figure 1:
A cross-sectional transmission electron microscope image of the InGaAs/InAlAs metamorphic
heterostructure.
Figure 2:(a) A schematic of the device fabrication process. (b) A separated device layer in deionizedwater during the process (left) and a successfully fabricated Hall-bar device on an AlN ceramic
substrate (right).
Figure 3:
Temperature dependence of electron mobility and sheet electron concentration of the ELO
and the on-wafer Hall-bar devices.

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Fig. 1. A cross-sectional transmission electron microscope image of the InGaAs/InAlAs metamorphic
heterostructure.

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Fig. 2. (a) A schematic of the device fabrication process. (b) A separated device layer in deionizedwater during the process (left) and a successfully fabricated Hall-bar device on an AlN ceramic substrate(right).

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Fig. 3. Temperature dependence of electron mobility and sheet electron concentration of the ELO
and the on-wafer Hall-bar devices.

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