ABSTRACT The potential use of Fe doped GaN for spintronics applications requires a complete understanding of the electronic structure of Fe in all of its charge states. To address these issues, a set of 400 µm thick freestanding HVPE grown GaN:Fe crystals with different Fe-concentration levels ranging from 5×1

Axel Hoffmann

Enno Malguth

Matthew Phillips

Wolfgang Gehlhoff

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Fe-Centers in GaN as Candidates for Spintronics Applications
Enno Malguth, Axel Hoffmann1, Matthew Phillips, and Wolfgang Gehlhoff1
Microstructural Analysis Unit, University of Technology, Sydney, NSW 2007, Australia
1Technische Universität Berlin, Institut für Festkörperphysik, 10623 Berlin, Germany
ABSTRACT
The potential use of Fe doped GaN for spintronics applications requires a complete
understanding of the electronic structure of Fe in all of its charge states. To address these
issues, a set of 400 µm thick freestanding HVPE grown GaN:Fe crystals with different
Fe-concentration levels ranging from 5×1017 cm−3 to 2×1020 cm−3 was studied by means of
photoluminescence, photoluminescence excitation (PLE) and Fourier transform infrared
(FTIR) transmission experiments. The Fe3+/2+ charge transfer (CT) level was determined
to be at 2.86 ± 0.01 eV above the valence band maximum considerably lower than the
previously reported value of 3.17 ± 0.10 eV. A bound state of the form (Fe2+, hV B) with a
binding energy of 50 ± 10 meV has been established as an excited state of Fe3+. FTIR
transmission measurements revealed an internal (5E—5T2) transition of Fe
2+ around
400 eV which, until now, was believed to be degenerate with the conduction band.
Consequently, a second CT band was detected in PLE spectra.
INTRODUCTION
A possible realization of spintronic devices is to introduce transition metal (TM) ions
into a wide band gap semiconductor [1]. Theoretically, in such a diluted magnetic
semiconductor (DMS) the magnetic order of the localized magnetic centers is established
by delocalized free carriers, resulting in room temperature ferromagnetism [2, 3]. In
particular, free holes are shown to interact with the magnetic impurities via exchange
interaction [3]. Therefore, in order to predict the feasibility of carrier-mediated spin
coupling, it is crucial to know the electronic structure of the TM centers, which depends on
the present charge state and is strongly influenced by the host crystal [4]. The exact
position of the Fe3+/2+ charge transfer (CT) level within the bandgap is of particular
interest. While extensive studies have been conducted on the promising GaN:Mn material
system, e.g. [5], only little is known about the applicability of GaN:Fe for spintronics.
The spinforbidden (4T1(G)—
6A1(S)) transition of the Fe
3+ center in GaN exhibits a
structured near infra red (IR) luminescence at 1.299 eV with a 8 ms lifetime [6, 7]. By
means of photoluminescence excitation (PLE) Heitz et al. identified higher excited states,
4T2(G) and
4E(G), at 2.01 eV and 2.731 eV, respectively, as well as a shallow bound state
of the form (Fe3+,e,h) at 2.888 eV [8]. After pinpointing the Fe3+/2+ CT level at
(3.17 ± 0.10) eV, the authors drew three major conclusions [8]: (1) The binding energy of
the bound state equals ≈ 280 meV. (2) With a likely energy of approximately 390 meV the
Fe2+ (5E—5T2) transition is degenerate with the conduction band and cannot be observed.
(3) The internal reference rule fails for GaN in conjunction with both GaAs and AlN [9, 10].
Figure 1. Low-temperature (2 K) luminescence of the Fe3+ (4T1(G)—
6A1(S)) transition in
GaN for a Fe concentrations of 1×1018 cm−3. The dominating ZPL at 1.299 eV is displayed
at a different energy scale for better resolution.
In this work we present a number of optical studies on a set of high quality GaN
samples doped with iron at varying concentrations. Both charge states 3+ and 2+ are
observed. Their respective electronic structure is studied in detail as well as Fe3+/2+ CT
processes. Previous results on the Fe3+ center are determined more precisely and partly
revised. The concentration dependency of the presented results provides an indication of
the ideal Fe concentration for the potential realization of a GaN:Fe based DMS.
EXPERIMENTAL DETAILS
The samples of hexagonal crystal phase were grown along the c-axis by ATMI using
HVPE. Fe was incorporated during the growth process at concentrations ranging from
5×1017 cm−3 to 2×1020 cm−3. The substrate was removed from the ∼500 µm thick layers
resulting in freestanding samples. Photoluminescence (PL) experiments were carried out
with the samples immersed in liquid helium exciting with an Ar-ion laser at 2.41 eV. The
PL was dispersed by a 1 m Spex double monochromator and detected by a liquid nitrogen
cooled germanium-detector. In photoluminescence excitation (PLE) experiments a
tungsten-halogen lamp in combination with a 275 mm double monochromator served as
tunable light source. The luminescence was dispersed and detected by a 300 mm double
monochromator and a germanium-detector, respectively. Fourier transform infrared
(FTIR) measurements were perfomed using a Bruker IFS 66v commercial Fourier
spectrometer fitted with a liquid helium cryostat. For PL and FTIR experiments the
samples were mounted in such a way that light travelled perpendicular to the c-axis,
resulting in an absorption path length equal to the samples’ widths (∼2mm), which
produced increased absorption. This method also enabled variations in the incoming light’s
polarization with respect to the c-axis.
RESULTS AND DISCUSSION
Figure 1 presents PL spectra of the internal Fe3+ (4T1(G)—
6A1(S)) transition in GaN
at 1.299 meV and its phonon sideband for different Fe concentrations [6]. Both ZPL and
Figure 2. PLE spectra of the Fe3+ (4T1(G)—
6A1(S)) luminescence at 1.299 eV of Fe-
doped GaN for various Fe concentrations (given in cm−3) at 7 K on logarithmic scale. The
luminescence was detected in an 8 meV wide window around 1.299 eV.
sideband are similar for any studied concentration (not shown here). The full width at half
maximum (FWHM) of the ZPL scales with the Fe concentration between 120 and 270 µeV,
proving the samples to be of better quality than those studied previously [7]. This is
mainly regarding the strain free incorporation of Fe on Ga site and was confirmed by
electron paramagnetic resonance (EPR) experiments.
The most pronounced features of the sideband could successfully be assigned to
phonon modes of the host crystal and local vibrational modes (LVM) [11–14]. A strong
coupling of the Fe3+ center on only E2 phonon modes as well as on A1 and E LVMs was
identified.
Figure 2 shows PLE spectra of the Fe3+ (4T1(G)—
6A1(S)) luminescence in GaN for
different Fe concentrations. Transitions into the higher excited Fe3+ states, 4T2(G) and
4E(G), are reflected by peaks at 2.01 eV and 2.73 eV respectively which appear at Fe
concentrations from 1×1018 cm−3. A smaller peak at 2.812 eV with pronounced replicas
was previously attributed by Heitz et al. to an exciton bound to the Fe3+ center
(Fe3+,e,h) [8]. However, in Ref. [8] the first phonon replica at 2.888 eV was mistaken for
the actual ZPL. The observed strong phonon coupling is typical for bound states [8] and
can be explained by a coupling on the E2 phonon mode and the A1 LVM. The replicas are
superimposed on the start of a broad UV band which represents the CT process
Fe3+(6A1(S)) + hν −→ Fe2+ + hV B (1)
in which a hole is excited into the valence band [8]. Therefore, the band’s low energy onset
equals the distance between the Fe3+/2+ CT level and the VB maximum. An exponential
fit produces an energy of (2.865 ± 0.01) eV. This is considerably lower than the value of
(3.17 ± 0.10) eV established by Heitz et al. on the basis of the questionable energy found
for the bound state [8]. As a result the binding energy of the bound state amounts to only
(50 ± 10) meV, indicating a small overlap of hole and core state wave functions. With the
CT level less close to the CB minimum, there will only be a minor hybridization of the
Fe-center’s core states with the CB. Both points imply only a weak exchange interaction.
Therefore, in line with the Fe center in other III-V materials [15–17], the bound state is
better described as an effective-mass-like complex consisting of a hole bound to a
Fe2+ center, (Fe2+,hV B).
An additional consequence of the new location of the Fe3+/2+ CT level, (0.68 ±
0.06) eV below the conduction band minimum, is that the Fe2+ (5E—5T2) transition might
be located within the band gap.
According to the PLE spectra (figure 2), the CT process (Eq. 1) initiates the most
efficient excitation process of the Fe3+ center, i.e. the capture of a free hole by an
Fe2+ center, probably by forming the discussed bound state, which subsequently relaxes
nonradiatively into the 4T1(G) state.
Fe2+ + hV B −→ (Fe2+, h) nonrad. > Fe3+(4T1) (2)
A second band resembling a CT process can be seen particularly for [Fe]=2×1020 cm−3
(figure 2) starting at (3.245 ± 0.02) eV. Its origin can be tentatively identified as the CT
resulting in the excited Fe2+ state, 4T2(G). This assumption is supported by the
observation of the (5E—5T2) transition of Fe
2+ presented later on in this work. The two
Fe2+ states will be shown to be separated by 386 meV which perfectly agrees with the
energetic difference of (380 ± 20) meV between the onsets of the alleged CT bands
observed here.
The intensity of the Fe3+ related characteristics of the PLE spectra in figure 2 suggest
that at Fe concentrations of 1×1018 cm−3 and 5×1017 cm−3 there are fewer and no ions in
the 3+ charge state, respectively. At these concentrations the Fe3+ luminescence is
primarily excited via an external generation of free holes recombining with Fe2+ centers
(equation 2). The origin of free holes is probably a two step process involving the deep
defect centers responsible for the yellow luminescence [18]. This assumption is also
supported by the fact that a wide excitation band starting at 2.1 eV appears in both PLE
spectra of the yellow luminescence [18] and of the Fe3+ emission (figure 2). Considering
that the Fe3+ luminescence in figure 1 was excited at 2.41 eV, here the hole transfer to
Fe3+ centers represents the dominating excitation process at any studied Fe concentration.
FTIR transmission spectra of the Fe-doped GaN samples are displayed in figure 3. A
richly structured absorption band around 395 meV can be observed for any concentration.
It can be unambiguously assigned to the (5E—5T2) transition of Fe
2+ based on three
arguments: (i) The observed absorption structure is found in the same energy region and is
of similar shape as the Fe2+ transition in GaAs, GaP and InP, i.e. between 340 and
450 meV [19]. (ii) The shape and amplitude of the absorption feature change significantly
with the Fe concentration. (iii) Its position equals the distance between the two CT bands
yielded by PLE measurements. Consequently, the spectra in figure 3 confirm the presence
of iron in the 2+ charge state at any Fe concentration studied, as was concluded on the
Figure 3. FTIR transmission spectra of the (5E—5T2) transition of Fe
2+ in GaN for different
Fe concentrations. The noisy signal for [Fe]= 2×1020 cm−3 is the result of the very high
overall absorption of this sample.
basis of PLE experiments. Fe2+ is a d6 system with a 5D ground state. In compound
semiconductor crystals of zinc blende structure, the ground state splits under the impact of
the crystal field of Td symmetry into the ground state,
5E, and the excited state, 5T2.
Spin-orbit-interaction causes these states to split into 5 and 6 sublevels, respectively. The
resulting electronic structure has been established successfully for GaAs, GaP and InP [19].
However, in hexagonal crystals of wurtzit structure the crystal field is of c3V symmetry
causing an additional splitting which has not been resolved so far and which is reflected by
the complex absorption structure appearing in figure 3. According to Boltzmann’s law, the
elevated temperatures encountered during the experiments caused most of the ground
states to be populated. This produced a multitude of absorption lines which are difficult to
assign.
It can be seen from figure 3 that an increase in Fe concentration produces a broad
absorption band at the position of the Fe2+ absorption around 400 meV, superimposing
the finely structured absorption spectrum. The mechanisms behind this are most likely
inter-center interaction and localized strain, which occur at high Fe densities and cause the
single (5E—5T2) lines to broaden.
CONCLUSION
A consistent picture of iron in GaN has been established in this work. Both charge
states 3+ and 2+ were detected for different Fe concentrations. The respective electronic
structure was established. Significant findings relevant to potential spintronics application
include: the position of the CT level (2.865 eV above the VB), the existence of the bound
state, (Fe2+, h), constituting a transient shallow acceptor state, and the confirmation that
the Fe2+ (5E—5T2) transition is located within the band gap. These results will hopefully
stimulate further research activities towards carrier-mediated ferromagnetism in GaN:Fe.
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Fe-Centers in GaN as Candidates for Spintronics Applications

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Fe-Centers in GaN as Candidates for Spintronics Applications

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