Photon Factory Activity Report 2016 #34 (2017)

BL-16A/2014G725, BL-3A/2014G726, BL-3A/2016G684

Synchrotron studies of laser MBE grown iron oxide epitaxial layers on GaN(0001)
Sergey Suturin1,*, Alexander Korovin1, Masao Tabuchi2 and Nikolai Sokolov1
1
Ioffe Institute, 26 Polytechnicheskaya, 194021 St. Petersburg, Russia
2
Synchrotron Radiation Research Center, Nagoya University, Nagoya, Japan
Hybridization of semiconducting and magnetic materials into a single heterostructure is believed to provide vast
opportunities in design of novel functional spintronic devices. The nanometer thick epitaxial films of four iron oxides(Fe3O4, αFe2O3, γFe2O3 and multiferroic metastable εFe2O3 ) exhibiting drastically different crystal structure, electric andmagnetic properties have been recently stabilized on GaN(0001) using Laser MBE growth technique. The present work is
focused on the synchrotron X-ray diffraction, X-ray absorption spectroscopy and X-ray magnetic circular dichroism
studies of the iron oxide on GaN films aimed at confirmation of their crystalline quality and adequate magnetic structure.
1 Introduction
Hybrid heterostructures combining closely spaced
semiconducting and magnetic layers are promising
candidates to be used in design of functional spintronic
devices. Unlike diluted magnetic semiconductors, thehybrid heterostructures allow separate control over themagnetic and electrical properties. Insulating properties
and reasonable optical transparency of the magnetic layer
in the visible range would be welcome in the
optoelectronic ferroic-semiconductor devices. The simple
formula iron oxides are suitable candidates for magnetic
layer material in the discussed hybrid heterostructures.
The iron oxides including Fe3O4 and a number of Fe2O3
polymorphs make up a big family of magneticallyordered materials exhibiting a rich variety of outstanding
physical properties presenting interest for technologicalapplications and fundamental studies. Ferrimagnetic
Fe3O4 (magnetite) is known for the intriguing Verwey
metal-insulatortransition.
Ferrimagnetic
γFe2O3(maghemite) is the close relative of magnetite withslightly smaller magnetization and insulating properties.
Hematite (αFe2O3) deserves interest for the Morintransition from the antiferromagnetic to a weakferromagnetic state. The metastable εFe2O3 not existing inbulk has been only few times crystallized in the form of
epitaxial layers [1, 2, 3]. This intriguing material shows
very high magnetocrystalline anisotropy with coercivity
values exceeding 2 T and exhibits multiferroic properties.
In the present work GaN was chosen as thesemiconductor widely used in numerous devices such as
bright blue LEDs, HEMT transistors for high-power,
high-frequency and high radiation resistant applications.
Placing a ferroic film next to the active zone of the GaNbased devices is supposed to add new functionality tothese devices [4,5] including spin injection into GaN.
Recently the possibility to stabilize four different ironoxide phases - Fe3O4, αFe2O3, γFe2O3 and εFe2O3 - in theform of epitaxial layers on GaN substrate by Laser MBE
has been reported [ 6 ]. The orange tint insulating
antiferromagnetic αFe2O3 phase nucleates in oxygen atintermediate temperature and is the most stable one. The
ochre tint insulating metastable εFe2O3 polymorph
nucleates in oxygen at higher temperature and exhibits
properties of a hard ferrimagnet with the magnetization

lying in-plane and coercivity exceeding 1 T. Magnetite
films are grown in nitrogen exhibiting gray metallic tint
and being conductive at RT due to half metal nature of
Fe3O4. The close to rectangular in-plane magnetization
curves measured for magnetite films show coercivity of
600 Oe. Maghemite γFe2O3 films are produced byannealing magnetite layers in oxygen. The Fe3O4 –γFe2O3 transformation is accompanied by the film color
change from gray to ochre, drop of conductivity andslight decrease of the magnetization. To grow thick
γFe2O3 layers the growth / oxidation procedure has to berepeated multiple times as the Fe3O4 oxidation affects
only the top 20 nm of the film.
In the present report we present the results ofsynchrotron studies performed on the iron oxide on GaN
layers. The X-ray diffraction (XRD), X-ray absorption
spectroscopy (XAS) and X-ray magnetic circulardichroism (XMCD) studies have been performed at PF toclarify the influence of the laser MBE technologicalparameters on the growth process, as well as on the
crystal and magnetic structure of the resulting iron oxide
films.
2 Experiment
The iron oxide films having thickness of 40-60 nm
were grown in Ioffe Institute on the prefabricated GaN(0001) / Al2O3 (0001) substrates by Laser MBE using the
KrF excimer laser to ablate Fe2O3 target. Depending on
the particular iron oxide phase the growth was performed
at 600 – 800°C in either oxygen or nitrogen atmosphere ata pressure of 0.02 – 0.2 mbar. Preliminary structural
studies have been performed in situ by high-energy
electron diffraction (RHEED) 3D reciprocal space
mapping [ 7] to confirm epitaxial stabilization of eachparticular phase. The in-plane magnetization curves havebeen measured by MOKE and VSM to confirm theexpected magnetic properties. XRD measurements havebeen carried out at BL3A beamline. The dedicatedsoftware has been developed by the authors to carry out
3D reciprocal space mapping at BL3A using the Pilatus
100K detector. The 3D mapping data visualization,
processing and modeling is carried out in a unified way
for both X-ray and electron diffraction data. XAS andPhoton Factory Activity Report 2016 #34 (2017)

XMCD studies have been carried out at BL16 beamline atroom temperature.
3 Results and Discussion
Fig. 2 shows intensity profiles taken along the specularrods for ~40 nm films of the four different iron oxide
phases grown on GaN. The profiles have been preciselyaligned using the Al2O3 and GaN substrate reflections asa reference. Reflection indexing was facilitated knowing
the epitaxial relations extracted from our preliminary
RHEED studies:(008) reflections on the specular XRD profile of αFe2O3
film in Fig. 2. One can assume that the transition layer
between GaN and αFe2O3 is a rich in stacking faults
mixture of αFe2O3 and εFe2O3 phases. This layer is likely
responsible for the non-zero MOKE signal in theotherwise supposed to be antiferromagnetic αFe2O3 layers.

αFe2O3 (001) || GaN(001); αFe2O3 [110] || GaN[1-10]
εFe2O (001) || GaN(001); εFe2O3 [100] || GaN[1-10]
γFe2O3 (111) || GaN(001); γFe2O3 [11-2] || GaN[1-10]
Fe3O4 (111) || GaN(001); Fe3O4 [11-2] || GaN[1-10]
The given epitaxial relations help in clarifying why thefour iron oxides having so different lattice structure andlattice parameters grow epitaxially on GaN. The ironoxide lattice basically consists of closely packed frame ofoxygen atoms with iron atoms sandwiched in between theoxygen planes in octahedral and tetrahedral slightly
distorted sites (Fig. 1). GaN has a similar lattice structurewith closely packed frame of nitrogen atoms and Ga
atoms in between. Our diffraction studies confirm that thedifferent iron oxides nucleate on GaN so that the oxygen
planes are aligned parallel to the surface and the O-O-O
triangles within the oxygen planes are aligned withrespect to the N-N-N triangles in the GaN(0001) planes(Fig. 1). In this way the 7% lateral lattice matching
between the film and the substrate is achieved.

Fig. 1: Lattice matching between GaN(0001), αFe2O3(0001), εFe2O3 (001), Fe3O4 (111) and γFe2O3 (111).
The different arrangement of the Oh and Td iron sites aswell as the different stacking order of the oxygen planes(AB-AB in αFe2O3, ABC-ABC in Fe3O4 / γFe2O3,
ABCA-ABCA in εFe2O3) account for slightly different
distances between the oxygen planes and slightly shifted
specular reflections for different oxides. In all oxides
except αFe2O3 the monolayer consists of two oxygen
planes leading to twice higher reflection frequency.
Interestingly our diffraction data shows the presence ofa few nanometers thick non-hematite layer presumably
located at the interface of αFe2O3 / GaN film. This isevidenced by the presence of the two bumps
approximately at the position of εFe2O3 (004) and εFe2O3

Fig. 2: XRD specular intensity profiles measured in
epitaxial iron oxide layers on GaN(0001).
A noteworthy feature of the shown profiles is thehighly asymmetrical shape of the GaN peak slopes. Thisis an evidence of interference between crystal truncation
rods of the flat GaN (0001) surface and the transition
layer. This kind of interference often results in noticeable
shift of the film peak maximum and can be used toestimate the distance at the interface. Interestingly there
exists a small but noticeable difference between γFe2O3
and Fe3O4 intensity profiles. Magnetite and maghemite
are known to crystallize in the same cubic lattice withlattice parameters rather close to each other. The only
difference (besides iron oxydation state) is the presenceof octahedral vacancies in the γFe2O3 lattice. According
to the presented XRD profiles, the Fe3O4 to γFe2O3transformation is accompanied by slight decrease of the(111) interlayer distance (see the 555 reflection) due to Fe
diffusion from the layer to the surface where the reaction
with oxygen takes place.
The lattice parameters extracted from the specularprofiles are: a=8.37 Å (tabular value a=8.33 Å) for
γFe2O3; a=8.53 Å (tabular value a=8.40 Å) for Fe3O4,
c=13.708 Å (tabular value c=13.747 Å) for αFe2O3, andc=9.481 Å (tabular value c=9.471 Å) for εFe2O3. The
tendency is that the iron oxide lattice is slightly expanded
perpendicular to the surface.
The off-specular reflections of the iron oxide films on
GaN have been studied by XRD 3D reciprocal space
mapping. A number of equivalent epitaxial relations
related to GaN (0001) surface symmetry have been foundin addition to the epitaxial relations discussed above.
Placed over hexagonal GaN (0001) surface the iron oxide
lattice may be rotated around surface normal by 180 deg
for αFe2O3, γFe2O3, Fe3O4 and by ±120 deg for εFe2O3.

Photon Factory Activity Report 2016 #34 (2017)reference datairon oxides / GaN

L3

Kim 2009
Tseng 2009
Kuepper 2004

FeO
Fe II

L2

Fe3O4

Absorption

Fe II

TEY

Interestingly the εFe2O3 off-specular reflections were
found to be elongated along [100] in-plane direction. An
example is shown in Fig. 3 representing a reciprocal
space cut carried out parallel to the surface throughεFe2O3 (113) and εFe2O3 (112) off-specular reflections.
When reciprocal lattice nodes of two/three orientational
domains coincide, reflections have four- or six-ray stars.
The observed reflection elongation is probably due todisorder caused by coexistence of the three orientational
domains. Presumably anti-phase boundaries are formed
upon island coalescence with different orientation orphase shift. The latter is favored by the fact that εFe2O3
surface cell is 6 times larger than that of the GaN (Fig.
3d).

Fe2O3
Fe2O3Fe3O4
Fe2O3
Fe2O3thin Fe2O3

Fe2O3

Fe2O3704 708 712 716 720 724 728

704 708 712 716 720 724 728

Energy, eV

Energy (eV)Oh

Fe2O3 thickFe2O3 thinFe2O3

Fe2O3 thick

L3

Fe2O3
Fe2O3

total electron yieldFe2O3Fe3O4

L2

Fe2O3 thin

TEY

Oh

Fe II

Fe3O4

Fe II

Td
705 706 707 708 709 710 711

716 718 720 722 724 726

Energy, eVbciron oxides / GaN

Tdd

XMCD and XAS results
X-ray absorption spectroscopy (XAS) is known
to be a highly appropriate tool to investigate core levelatomic structure responsible for oxidation state andcrystallographic environment of individual atoms. Withcircular polarized light it is possible to probe the atom
selective magnetization. The L23 spectra of transition
metals are usually dominated by the dipole-allowed 2p to
3d transitions. They consist of L3 and L2 main peaks
resulting from spin-orbit coupling. Each of the main
peaks is further split by crystal field (eg, t2g). This splitting
provides a highly sensitive tool to probe the coordination
environment and the corresponding magnetization.
The Fe L23 XAS spectra measured in the ironoxide films grown on GaN are shown in Fig. 4a. They canbe compared to the reference spectra adopted from [8, 9,
10] and shown in Fig. 4b. In all the studied samples theL3 peak exhibits two major components: a larger peak at
709.5 eV and a smaller satellite at 708 eV. This spectral
shape is characteristic of iron oxides as opposed to themetallic iron [11].

Fe2O3

XMCD

Fig. 3: Off-specular XRD maps. Reciprocal space cuts
parallel to the surface showing star-like shapes of εFe2O3(113) (a) and εFe2O3 (112) (b) reflections. Reciprocal
space cut perpendicular to the surface showing the Laueoscillations on the streak passing through εFe2O3 (112)reflection (c). Schematic explanation of stacking fault
formation principle (d).

Fe2O3

Fe2O3

Fe2O3XMCD

a

E (eV)
reference data

Fe3O4

Fe3O4

Oh
704

706

708

Oh
710

Energy, eV

712

714

704

706

708

710

712

714

Energy (eV)Fig. 4: XAS and XMCD spectra obtained from the ironoxide epitaxial films on GaN. Reference data adopted
from [8,9,10] is shown for comparison.
In thick 40 nm αFe2O3 film the splitting is mostpronounced originating from the pure octahedral
coordination of FeIII ions in hematite [8,11,12]. In 40 nm
γFe2O3 and εFe2O3 films the L3 splitting is lesspronounced in agreement with earlier studies of thesematerials: γFe2O3 [8, 12] and εFe2O3 [9]. While theseparation between the 708 eV and 709.5 eV components
is the same as in αFe2O3, the intensity drop between thepeaks becomes less pronounced due to the presence of a
FeIII Td line right in the middle between the two Oh peaks.
Interestingly the same shape of L3 edge as in γFe2O3 andεFe2O3 is observed during the onset of αFe2O3 growth(measured in a 5 nm film) indicating the presence of Td
sites. Noteworthy the non-hematite nature of the few nm
thick transition layer formed between the GaN substrate
and the pure αFe2O3 phase is also confirmed by ourdiffraction measurements. It is noteworthy that in the only
one existing paper describing the XAS of εFe2O3 [9] thePhoton Factory Activity Report 2016 #34 (2017)

spectrum is more like in αFe2O3 with high contrast L3
splitting while our data show resemblance to γFe2O3. The
latter seems more natural as unlike in αFe2O3 in εFe2O3
the Td iron sublattice is known to exist in addition to thethree distorted Oh sublattices. In contrast to the pure
trivalent oxides, the Fe3O4 films exhibit an extra shoulder
at 706.5 eV and a considerably higher satellite at 709.5
eV. These features are known to be characteristic of Oh
FeII sites in magnetite [11, 13, 12, 14, 15]. The shape of
the L2 peaks in the studied iron oxide films bears somesimilarity to the L3 peak exhibiting the FeII low energy
shoulder in Fe3O4 and a better resolved satellite in αFe2O3.
XMCD measurements have been carried out toinvestigate magnetic nature of the individual Fe
sublattices within the iron oxide films on GaN allowing
comparison to the known properties of the corresponding
bulk materials. Not surprisingly no dichroic signal has
been detected in the expected to be aniferromagnetic (atleast far from the GaN interface) αFe2O3 films. As for theother three ferrimagnetic iron oxides, their corresponding
XMCD spectra are shown in Fig. 4e. The general trend isthat in the iron oxides the L3 XMCD spectra aredominated by the two negative peaks corresponding to
the Fe magnetic moments in Oh sites aligned parallel tothe field and a positive peak in between the Oh peaks
corresponding to the Fe magnetic moments in Td sitealigned antiparallel to the field. In the pure FeIII oxides(γFe2O3, εFe2O3) the higher energy Oh peak is dominant(see reference spectra in Fig. 4f adopted from [8, 9]). In
the mixed II / III Fe3O4 the low energy peak is higher andwider due to the presence of Oh FeII ions [14, 16]. The net
magnetization in ferromagnetic iron oxides results inantiparallel alignment of the octahedral and tetrahedral Fe
sites. In contrast to the XMCD spectra of metallic iron forwhich the L3 peaks measured at opposite light helicities
show different heights, in iron oxides these peaks areslightly shifted in energy. The ratio between theoccupation of the tetrahedral and octahedral sites can bein principle determined by the ligand field multiplet
analysis of the XMCD shape.
Conclusion
We have applied XRD, XAS and XMCD
synchrotron methods to study Fe3O4, αFe2O3, γFe2O3 andεFe2O3 iron oxide films epitaxially deposited onto
GaN(0001) surface by means of Laser MBE. The
structural studies have confirmed that the thick films aremostly single phase and have lattice orientation
characterized by alignment of closely packed oxygen
planes parallel to the GaN(0001) substrate surface. The
presence of a non-hematite transition layer have been
detected in αFe2O3 films in the vicinity of the interface.
Interestingly in εFe2O3 layers the reflections were foundto be considerably elongated along the [100] direction.
This is supposed to be due to columnar structure of
εFe2O3 films whereas the individual columns are
separated by the antiphase boundaries. The XAS spectra
measured in the studied iron oxides were found to be
characteristic of the particular phases, giving additional
proof of the phase purity. Interestingly the XAS spectrum

obtained in this work for the exotic εFe2O3 phase wassimilar to that of γFe2O3 is contrast to the known result
from [9] where the εFe2O3 nanoparticles showed αFe2O3like absorption. The XMCD spectra of the ferromagnetic
iron oxide films on GaN allowed visualization of thesublattice specific magnetization behavior being in good
agreement with the known results for bulk αFe2O3,
γFe2O3 and nanoparticulate εFe2O3.
Acknowledgement
The authors wish to acknowledge V. V. Lundin forproviding GaN / Al2O3 wafers, the beamline staff at PF
and for kind assistance in conducting experiments.
Beamtime at PF was granted for projects 2014G726,
2014G725, 2016G684. Support for this work has been
received from Government of the Russian Federation(Program P220, project No.14.B25.31.0025) and from
Nagoya University.
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* suturin@mail.ioffe.ru