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Strain‑enhanced electron mobility anisotropy inIn̲xGa̲<1‑x> As/InP two‑dimensional electrongases

Author(s)Akabori, Masashi; Trinh, Thanh Quang; Kudo,Masahiro; Hardtdegen, Hilde; Schäpers, Thomas;
Suzuki, Toshi‑kazuCitation

Physica E: Low‑dimensional Systems andNanostructures, 42(4): 1130‑1133

Issue Date2010

TypeJournal Article

Text version

author

URL

http://hdl.handle.net/10119/10272

RightsNOTICE: This is the author's version of a work
accepted for publication by Elsevier. Masashi
Akabori, Thanh Quang Trinh, Masahiro Kudo, HildeHardtdegen, Thomas Schäpers, Toshi‑kazu Suzuki,
Physica E: Low‑dimensional Systems andNanostructures, 42(4), 2010, 1130‑1133,
http://dx.doi.org/10.1016/j.physe.2009.11.039

Description

Japan Advanced Institute of Science and Technology

*Manuscript

Strain-enhanced

electronmobility

anisotropyin

InxGa1-xAs/InP

two-dimensional electron gases

Masashi Akabori1, 2, *, Thanh Quang Trinh1, Masahiro Kudo1, Hilde Hardtdegen2, Thomas Schäpers2,Toshi-kazu Suzuki1, **1

Center for Nano-Materials and Technology (CNMT), Japan Advanced Institute of Science andTechnology (JAIST), 1-1, Asahidai, Nomi, Ishikawa 923-1292, Japan2

Institute of Bio- and Nanosystems (IBN-1), Jülich-Aachen Research Alliance – Fundamentals ofFuture Information Technology (JARA-FIT), Research Centre Jülich, 52425 Jülich, Germany*, **Corresponding authors: Center for Nano-Materials and Technology (CNMT), Japan Advanced

Institute of Science and Technology (JAIST), 1-1, Asahidai, Nomi, Ishikawa 923-1292, Japan
Tel: +81-761-51-1477, Fax: +81-761-51-1049, E-mail: akabori@jaist.ac.jp (M. Akabori)Tel: +81-761-51-1441, Fax: +81-761-51-1049, E-mail: tosikazu@jaist.ac.jp (T. Suzuki)(Dated: November 9, 2009)

AbstractWe

systematicallyinvestigatedelectronmobility

anisotropy

incompressively-strained,

lattice-matched, and tensilely-strained InGaAs quantum wells (QWs) grown on InP(001) by using
Hall-bar devices with various current-flowing directions. Anisotropy of electron mobility, the highest
along [1-10] direction and the lowest along [110], is systematically observed in all QWs, andwell-fitted with a sinusoidal function of the current-flowing direction angle. The mobility anisotropy isminimum in the lattice-matched case and enhanced by both compressive and tensile strains in the1QWs. We consider that random piezoelectric scattering, which is enhanced by the average normal
strain in the QW, has anisotropy and plays an important role for the observed results.

PACS: 73.50.Bk; 73.61.Ey; 73.63.Hs; 77.65.Ly

Keywords: InGaAs/InP; Two-dimensional electron gases (2DEGs); Strained quantum wells (QWs);
Electron mobility anisotropy; Random piezoelectric (PE) scattering

1. Introduction
Two-dimensional electron gases (2DEGs) in InGaAs with high indium composition play animportant role for high-speed device applications because of their very high electron mobilities andpeakvelocities.

The2DEGsintheInGaAs

areusuallyobtained

bymeans

ofpseudomorphic/lattice-matched (PM/LM) growth on InP(001) or metamorphic (MM) growth onGaAs(001). The 2DEGs in the MM InGaAs sometimes exhibit large anisotropy in electron transport
such as higher mobilities along [1-10] direction and lower ones along [110] direction. While theanisotropy in the case of MM InGaAs on GaAs(001) had been attributed to anisotropic lattice
relaxation, interface roughness or residual strain due to anisotropic cross-hatch morphology (CHM)[1], it was recently claimed that they cannot be the dominant origin of the anisotropy and indium
composition modulation due to the anisotropic CHM is the most important origin [2]. However, insome cases there is no large mobility anisotropy (< 10 %) even in the MM InGaAs with theanisotropic CHM [3]. In addition, large mobility anisotropy (> 15 %) has been observed also in a PM
InGaAs quantum well (QW) on InP(001) with neither lattice relaxation nor CHM [4]. These suggest adifferent origin of the mobility anisotropy with no relation to lattice relaxation or CHM.
In this work, we paid attention to strain in InGaAs QWs. We carried out PM/LM growth ofInGaAs/InP heterostructures on InP(001) by metal-organic vapor phase epitaxy (MOVPE) in order toobtain high electron mobility 2DEGs in compressively-strained, lattice-matched, and tensilely-strained2InGaAs QWs. We systematically investigated the electron mobility anisotropy in the InGaAs QWs byusing Hall-bar devices with various current-flowing directions.

2. Sample preparation
We grew 4 types of InGaAs/InP modulation-doped heterostructures for 2DEGs, 30 nm-InP / 10nm-n-InP / 20 nm-InP / 10 nm-InxGa1-xAs / 300 nm-InP / InP(001) substrate, by MOVPE with N2
carrier gas, trimethylindium (TMIn), trimethylgallium (TMGa), arsine (AsH3), phosphine (PH3), anddisilane (Si2H6) at 670°C [5]. The differences in each growth are the partial pressures of TMIn andTMGa and the growth time for InxGa1-xAs.
After MOVPE growth, we carried out X-ray diffraction (XRD) and atomic force microscope (AFM)
measurements of grown structures. Figure 1 shows (004) XRD measurement results with simulation
curves. We confirmed the PM/LM growth of the heterostructures. By the simulation curves fitted tosub peaks from InGaAs, we also confirmed indium composition of x = 0.75, 0.62, 0.53, and 0.44 in
InxGa1-xAs; InGaAs QWs of x = 0.75 and 0.62 are compressively strained, that of x = 0.53 islattice-matched in average, and that of x = 0.44 is tensilely strained. Figure 2 shows AFM images ofthe surfaces of grown structures. Very smooth morphologies with step-flows are observed. The
roughness of each surface is less than 1 monolayer height in root-mean-square, and the directions of
the step-flows are not related to crystal orientation. Thus, the step-flowing direction originates fromsubstrate manufacturing, and almost smooth InGaAs/InP interfaces are expected.
For the investigation of the electron mobility anisotropy, we fabricated 6-terminal Hall-bar devices(50-µm-width and 200-µm-length) with 8 current-flowing directions by conventional photolithography
techniques. Figure 3a shows an optical micrograph of the Hall-bar device. The current-flowing
directions are defined by the angles from [1-10] direction as schematically shown in Fig. 3b.

3. Hall measurement results and discussion
We carried out Hall measurements of the Hall-bar devices at various temperatures with a He-flow

3cryostat. At low temperatures less than 50K, evident anisotropy of electron mobilities are observed ineach QW, while no anisotropy is observed in the higher temperature region. Figure 4 shows theelectron mobilities at 10 K as a function of the angle from [1-10] direction or current-flowing
directions. The data points and the error bars are determined by using the average and variations ofsheet electron concentrations in 8 Hall-bar devices, respectively. The average values of the sheet
electron concentrations are also shown in Fig.4, which slightly depend on indium composition. For
fitting curves, we assumed the following Matthiesen-type equation;

1
1 sin 2 , ( )  I
A(1)where  is the angle of the current-flowing direction from [1-10], and I and A are isotropic andanisotropic mobilities, respectively. Each curve is fitted well to the experimental data. Systematic
anisotropy is observed in each QW, and the behavior is symmetric with respect to [1-10] direction.
From XRD measurements, the anisotropy cannot originate from anisotropy of lattice relaxation. In
addition, since step-flowing directions are not related to crystal orientation from AFM measurements,
the anisotropy also cannot originate from anisotropy of interface roughness scattering.
Figure 5 shows the anisotropy parameter I/A as a function of indium compositions of InGaAs orlattice-mismatches of InGaAs with respect to InP. I and A are the fitting parameters in Fig.4. The
result strongly suggests that the anisotropy of electron mobility can be enhanced by thelattice-mismatch between InGaAs and InP, or the strain in InGaAs QWs. Concerning thestrain-relating modulation of the electron mobility, recently, an importance of random piezoelectric(PE) scattering in strained QWs has been pointed out [6]. In the case of zinc-blende structure, PEpolarization is induced by shear strains. In ideal strained QWs, there is no shear strain resulting in noPE polarization. In contrast, in real strained QWs with imperfection, there are non-vanishing random
shear strains, which are proportional to average normal strains. Therefore, the PE polarization takes
place randomly in the real strained QWs with imperfection and scatters conductive electrons in QWs.
The electron mobility anisotropy enhanced by the strain in InGaAs QWs can be attributed to the4anisotropy of the random PE scattering associated with the strain. Since a (001) surface of azinc-blende structure has anisotropic nature between [110] and [1-10] directions, surface
reconstruction and/or migration of III-group adatoms become anisotropic during III-V epitaxial growth.
The anisotropy during growth can introduce anisotropic correlation length of randomness, which has
been discussed in the case of 2DEGs in GaAs with extremely high electron mobilities [7]. In the caseof 2DEGs in strained InGaAs QWs, the anisotropy of random PE scattering also can be caused byanisotropic correlation length originating from the anisotropy during growth. In addition, since As-Pexchange at InGaAs/InP interfaces can result in weak normal strains causing non-vanishing random
shear strains with imperfection, the electron mobility anisotropy even in the LM case can originate
from the anisotropy of random PE scattering. In the case of the tensilely-strained QW (x = 0.44), theelectron mobility anisotropy seems larger than that in the compressively-strained QWs (x = 0.62 and0.75). Since the random PE scattering probability is increased with not only the strain but also the PE
constant e14 and the effective electron mass me*, large e14 and/or large me* can be related to the originsof the larger anisotropy in the case of low indium composition x. It should be noted that a difference ofthe growth anisotropy also can affect the anisotropy of random PE scattering.

4. Summary
In summary, we investigated the electron mobility anisotropy in PM/LM grown InxGa1-xAs/InP
QWs (x = 0.75, 0.62, 0.53, and 0.44) by using Hall-bar devices with 8 current-flowing directions.
Although no lattice relaxation and no CHM are confirmed by XRD and AFM measurements, weobserved the systematic anisotropy of electron mobilities in each QW by Hall measurements at lowtemperatures. The minimum anisotropy is observed in the LM case of x = 0.53. These results suggestthat the electron mobility anisotropy can be enhanced by the strain in InGaAs QWs and attributed tothe anisotropy of the random PE scattering associated with the strain. This is a possible origin of theelectron mobility anisotropy which has been often observed in InGaAs QWs.

5

Acknowledgement
The authors thank Mr. K. Wirtz of Research Centre Jülich for his support in MOVPE.

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Figure captions
FIG. 1. X-ray diffractions of grown structures. Dots and curves indicate experimental data andsimulation data, respectively.
FIG. 2. Atomic force microscope images of the surfaces of grown structures.
FIG. 3. (a) Optical micrograph of a Hall-bar device. (b) Current-flowing directions of Hall-bar devices.
FIG. 4. Electron mobilities at 10K as a function of current-flowing direction angles. Data points andthe error bars are determined by using the average and variations of sheet electron concentrations in 8Hall-bar devices, respectively. Solid curves are fitting results with assuming sinusoidal functions. Thevalues of the sheet electron concentrations are also shown.
FIG. 5. Anisotropy parameter as a function of indium compositions of InGaAs or lattice-mismatches ofInGaAs with respect to InP.

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