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In‑plane isotropic magnetic and electricalproperties of MnAs/InAs/GaAs(111)B hybrid
structure

Author(s)Islam, Md. Earul; Akabori, Masashi

Citation

Physica B: Condensed Matter, 532: 95‑98

Issue Date

2017‑03‑06

Type

Journal Article

Text version

author

URL

http://hdl.handle.net/10119/15736

RightsCopyright (C)2017, Elsevier. Licensed under theCreative Commons Attribution‑NonCommercial‑
NoDerivatives 4.0 International license (CC BY‑
NC‑ND 4.0).
[http://creativecommons.org/licenses/by‑nc‑nd/4.0/] NOTICE: This is the author's version ofa work accepted for publication by Elsevier. Md.
Earul Islam and M. Akabori, Physica B: Condensed
Matter, 532, 2017, 95‑98,
http://dx.doi.org/10.1016/j.physb.2017.03.013

Description

Japan Advanced Institute of Science and Technology

In-plane isotropic magnetic and electrical properties of MnAs/InAs/GaAs(111)B
hybrid structure*Md. Earul Islam and Masashi Akabori
Center for Nano Materials and Technology (CNMT), Japan Advanced Institute of
Science and Technology (JAIST), 1-1 Asahidai, Nomi, Ishikawa 923-1292, JAPANE-mail: s1540002@jaist.ac.jp; akabori@jaist.ac.jp*Corresponding author, e-mail: s1540002@jaist.ac.jp; Tel: +81-761-51-1920

AbstractWe

characterized

in-planemagneticandelectricalpropertiesofMnAs/InAs/GaAs(111)B hybrid structure grown by molecular beam epitaxy (MBE).
We observed isotropic easy magnetization in two crystallographic in-plane
directions, [2� 110] and [01� 10] of hexagonal MnAs i.e. [1� 10] and [112� ] of cubic InAs.
We also fabricated transmission line model (TLM) devices, and observed almost
isotropic electrical properties in two crystallographic in-plane directions, [1� 10] and[112� ] ofcubic InAs. Also we tried to fabricate and characterize lateral spin-valve(LSV) devices from the hybrid structure. We could roughly estimate the spin
injection efficiency and the spin diffusion length at room temperature in [112� ]direction. We believe that the hybrid structures are helpful to design spintronic
device with good flexibility in-plane.

Keywords

Molecular beam epitaxy (MBE); MnAs, InAs, GaAs(111)B; Magnetic properties;
Transmission line model (TLM); Lateral spin-valve (LSV)

1. Introduction
In semiconductor spintronics, ferromagnetic/semiconductor hybrid structures havealready attracted attention because of their potential application for spin field effecttransistors (spin-FETs) [1]. For spin-FET application, it is necessary to haveferromagnetic source and drain for spin polarized carrier injection and detection,and semiconductor channel with spin-orbit coupling (SOC) controlled by electricfield for spin polarized carrier transport. Hexagonal NiAs-type MnAs is aferromagnetic metal with Curie temperature over room temperature [2], which canbe synthesized with III-As semiconductors in epitaxial growth system [3]. III-As
semiconductors are expected to show large SOC when the bandgap becomesnarrow. InAs is a narrow bandgap III-As semiconductor. Therefore, MnAs/InAs
hybrid structures with in-situ-formed interfaces are possible candidates for the basestructures of spin-FET application. GaAs and InP are widely-used substratematerials for III-As as well as MnAs epitaxial growths. It is known that MnAs grown
on GaAs (001) [4] sometimes shows mixed phase between hexagonal andorthorhombic structures, and also MnAs grown on GaAs(001) and InP(001)shows

anisotropic in-plane magnetization [4, 5] where c-axis of hexagonal structure is inplane-oriented to (001). On the other hand, on GaAs(111)B [6] or InP(111)B [7],only hexagonal NiAs-type MnAs with c-axis normal to the plane has been observed,

and has shown almost isotropic in-plane magnetization [8]. Moreover, consideringspin-FET application, investigation of in-plane electrical properties is also important.
However, there is no report on in-plane electrical properties using MnAs on (111)B,
even though there are some reports on GaAs(001) [9] and InP(001) [10].

In this paper, we report on in-plane magnetic and electrical properties of a
MnAs/InAs/GaAs(111)B hybrid structure grown by molecular beam epitaxy (MBE)for spin-FET application. Also we report on a trial of spin-valve measurements
using the hybrid structure.

2. Sample preparation
We used semi-insulating GaAs(111)B substrates for MBE growth. The detail of
MBE growth is discussed in elsewhere [11]. The MnAs/InAs was grown directly on
the substrates without buffer layer. The nominal thicknesses of MnAs and InAs
were ~200 nm and ~1.2 µm, respectively, as shown in Fig. 1. We confirmed goodstacking of MnAs(0001)/InAs(111)B on GaAs(111)B and parallel alignment ofMnAs [2� 110], InAs[1� 10] , and GaAs [1� 10] by X-ray diffraction study [11]. After thegrowth, we removed indium metals on backside of the samples by polishing.

[0110][2110]

[112]
[110]

FIG. 1. Layer stacking of MnAs/InAs/GaAs.

To measure magnetic properties we cut the samples to 5 mm X 5 mm square chips,
and then loaded the chips into a superconducting quantum interference device(SQUID) magnetometer.
To measure electrical properties we performed transmission line model (TLM)
device fabrication using photolithography. First, we patterned MnAs electrodes byphotolithography, and then carried out wet chemical etching of MnAs using
H 3 PO 4 :H 2 O 2 :H 2 O = 3:1:200. The etching rates for MnAs and InAs were ~100
nm/min and ~20 nm/min, respectively. The electrode width and length were ~180µm and ~90 µm, respectively. Next, we patterned InAs channels also by
photolithography, and then carried out wet chemical etching of InAs by
H 3 PO 4 :H 2 O 2 :H 2 O = 3:1:50 with the etching rate of ~100 nm/min. The channel
width was ~200 µm close to the electrode width, and the length (L) was varied
between 20 µm and 70 µm. Figure 2(a) shows an optical microscope photograph ofa fabricated TLM device, and significant side-etching behaviors in both MnAs andInAs can be seen. Therefore, the wet chemical etching is not suitable for precise
MnAs electrode formation.
We also tried to fabricate lateral spin-valve (LSV) devices. With considering theside etching of MnAs as mentioned above, we used the combination of electronbeam lithography (EBL) and Ar+ ion beam etching. Two electrode lengths of ~1 µm
and ~3 µm were prepared to obtain different coercive magnetic fields for antiparallel magnetization status. The electrode spacing (L) was varied between 1 µm
and 3 µm. InAs channels were defined by EBL and wet chemical etching similar toTLM device fabrication with expanded resist patterns with considering the sideetching of InAs. The channel width was designed ~20 µm. Figures 2(b) and (c)show an optical microscope photograph and a schematic of LSV device,respectively. Even though MnAs electrodes almost can be kept, however, the side

etching of InAs channel still can be seen due to underestimation of InAs sideetching.

(a)

MnA

200 µmGaAs(b)

200 µm

20 µm

GaA

InAs
MnAs

IVNL

MnAs MnAs
Ti/Au

Ti/Au(c)InABGaAs
FIG. 2. Optical microscope photographs of (a) TLM device and (b) LSV

We could confirm the etching process is reproducible in TLM and LSV devices
fabrication with significant side etching. Therefore, we need dry etching process toimprove both MnAs electrode and InAs channel formations.

3. Magnetic properties
Figure 3 shows magnetization curves at 300 K as a function of applied magnetic
field (H) parallel to two in-plane directions [2� 110] and [01� 10] of hexagonal MnAs.

We confirmed ferromagnetic behaviors at 300 K, and found easy and almost
isotropic magnetization along in-plane directions which is similar to MnAs onGaAs(111)B [8]. The saturation magnetization (M S ) and the coercive field (H C ) are~300 emu/cm3 and ~320 Oe, respectively. Here, we have neglected diamagnetic
influence of GaAs and InAs into the magnetization because of higher magnetic
moment (~100 times) of ferromagnetic MnAs than diamagnetic GaAs and InAs ofthe sample.

3Magnetization [emu/cm ]-H ǁ MnAs [2110]
H ǁ MnAs[0110]

Magnetic fieldFIG. 3. Magnetization curves at 300 K of [2� 110] and [01� 10] directions of hexagonal MnAs.

4. Electrical properties
Figures 4(a) and (b) show current (I)-voltage (V) characteristics of TLM devices
with the channels along [1� 10] and[112� ] of cubic InAs, respectively. Each curve

seems almost linear. Therefore, we can conclude MnAs/InAs interface is Ohmiccontact. Figure 5 shows their TLM plots. We found almost isotropic behaviors intwo in-plane directions. The extracted electrical parameters, sheet resistance (R S )of InAs channel, contact resistance (R C ) between MnAs/InAs and specific contactresistance (r C ) are summarized in Table 1. From the extracted data, we see almost
no in-plane dependency of the basic electrical properties having little higher
specific contact resistance of ~4 X 10-4 Ω-cm2 through MnAs/InAs interface thantypical Ohmic contact of III-V semiconductors [12]. We note that Hall effect of thehybrid structure indicates p-type conduction with sheet resistance of ~6.3 Ω/sq
mainly from MnAs layer. In addition, from TLM measurements, we extracted InAs
sheet resistance of ~950 Ω/sq, which is significantly higher than that of MnAs.
Therefore, the MnAs/InAs interface can act as p+/n+ junction, which includes atunnel barrier. The tunnel barrier may increase specific contact resistance.

Current [A/cm]

L~25.5 µm
L~35.5 µm
L~45 µm
L~ 53.5 µm
L~ 63.5 µm(a)-I ǁ InAs [110]Current [A/cm]

Voltage [V]
L~23.5 µm
L~33.5 µm
L~42 µm
L~ 51.5 µm
L~ 60.5 µm(b)-I ǁ InAs [112]Voltage [V]FIG. 4. Current-voltage characteristics of TLM devices with the channels along (a) [1� 10]
and (b) [112� ] of cubic InAs.

Resistance [Ω-cm]

I ǁ InAs[110]I ǁ InAs[112]Channel length [µm]FIG. 5. TLM plots along [1� 10] and [112� ] of cubic InAs.

Table 1 Electrical properties from TLM plots.
In-plane

RSRC

L t = RC / RSr C = (R C )2 / R S

direction[Ω/sq.][Ω-cm][µm][Ω-cm2][1� 10]

950

0.61

6.42

4.3 X 10-4

945

0.60

6.34

4.2 X 10-4

[112� ]

5. Spin-valve measurements
Figure 6 shows non-local SV signal curves in MnAs electrode spacing of 1 µm
along [112� ] direction at room temperature. The signal does not seem so clear,
however, SV like dips with hysteresis can be found around +25 and -15 mT. The

asymmetric dips to zero magnetic field might be caused by hysteresis of magnet
system. The dips in principal correspond to anti-parallel status, and we confirmed
anti-parallel could be realized around several 10’s mT by magnetic force
microscopy using a patterned MnAs sample with similar configuration. We also
checked electrode spacing dependence as shown in Fig. 7, and roughly obtainedthe spin injection efficiency (P i ) of ~8.5% and the spin diffusion length (L S ) of ~0.7
µm. The values seem hopeful compared with values in ex-situ-formed
CoFe/InGaAs [13], however, we note that there are some problems in device
fabrication like etchant penetration and in data reliability like asymmetric behaviorof spin-valve signal curves to zero magnetic field.

FIG. 6. Non-local SV signal curves in MnAs electrode spacing of 1 µm along[112� ]direction at room temperature.

FIG. 7. Electrode spacing dependence of SV signal amplitude.

6. Conclusion
We confirmed isotropic in-plane magnetization and isotropic basic electricalproperties of MnAs/InAs/GaAs(111)B hybrid structure grown by MBE. M S and H C
at 300 K are ~300 emu/cm3 and ~320 Oe, respectively. r C of MnAs/InAs interface
at room temperature is ~4 X 10-4 Ωcm2. Also we tried to fabricate and characterize
LSV devices. We roughly obtained the spin injection efficiency of ~8.5% and thespin diffusion length of ~0.7 µm at room temperature. We believe that the hybrid
structures are helpful to design spintronic device with good flexibility in-plane.

Acknowledgement
This work was supported by The Murata Science Foundation 2016 in part.

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