Electronic states of Mn4+ ions in p-type GaN
B. Han and B. W. Wesselsa)
Department of Materials Science and Engineering and Materials Research Center,
Northwestern University, Evanston, Illinois 60208, USA
M. P. Ulmer
Department of Physics and Astronomy, Northwestern University, Evanston, Illinois 60208
The electronic states of manganese in p-type GaN are investigated using
photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies. A series of
sharp PL lines at 1.0 eV is observed in codoped GaN and attributed to the intra d-shell
transition 4T2(F)-4T1(F) of Mn4+ ions. PLE spectrum of the Mn4+ [4T2(F)-4T1(F)] luminescence
reveals intra-center excitation processes via the excited states of Mn4+ ions. PLE peaks
observed at 1.79 and 2.33 eV are attributed to the intra d-shell 4T1(P)-4T1(F) and 4A2(F)-4T1(F)
transitions of Mn4+, respectively. In addition to the intra-shell excitation processes, a broad
PLE band involving charge-transfer transition of the Mn4+/3+ deep level is observed, which is
well described by the Lucovsky model. As determined from the onset of this PLE band, the
position of the Mn4+/3+ deep level is 1.11 eV above the valence band maximum, which is
consistent with prior theory using ab initio calculations. Our work indicates 4+ is the
predominant oxidation state of Mn ions in p-type GaN:Mn when the Fermi energy is lower
than 1.11 eV above the valence band maximum.
a)
Electronic Mail: [email protected]
1
Transition metals in III-V semiconductors have been previously extensively studied
because of their scientific importance and technological applications.1,2 Recently there has
been renewed interest due to the discovery of ferromagnetism in semiconductors alloyed with
magnetic elements.3,4 One system that has received considerable attention is manganese in
GaN.5-7 This system has been reported to be a ferromagnetic semiconductor with a high Curie
temperature due to a large p-d hybridization and small spin-orbit coupling.4-6,8 The
ferromagnetic properties of GaN:Mn depend strongly on the electronic configuration of the
Mn ions,9,10 which is determined by the oxidation state of Mn and the local crystal field.1,2
The 2+ (d5) and 3+ (d4) oxidation states are typically observed for substitutional manganese in
III-V semiconductors. Both the d4 and d5 configurations have been observed by electron
paramagnetic resonance and optical studies.11-14 The Mn3+/2+ acceptor level was determined to
be 1.4-1.8 eV above the valence band maximum.11,12
Recent ab initio calculations indicated that besides Mn2+ and Mn3+, the oxidation state
of Mn4+ could also be stabilized in wurtzite GaN (with a tetrahedral local crystal field) in that
the Mn4+/3+ transition level is 1.1 eV above valence band maximum.15 However, the position
of the Mn4+/3+ level in GaN has not been experimentally determined; the electronic states of
Mn4+ haven’t been established as well. It should be noted that the Mn4+ oxidation state has not
been previously observed in a tetrahedral crystal field in that the Mn4+/3+ transition level is
predicted to be resonant with the valence band in most semiconductors.1,2
In
this
work
the
optical
properties
of
GaN:Mn-Mg
are
investigated
by
photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies to determine
the Mn4+/3+ energy level position, as well as the excited state energy levels of the Mn4+ ions. A
series of sharp PL lines at 1.0 eV is observed in p-type GaN:Mn heavily codoped with the
2
acceptor Mg. Their peak energies remain invariant with increasing temperature up to 200 K,7
which is characteristic of intra d-shell transitions.1,2 From the transient decay properties of the
1.0 eV PL, they are attributed to the spin allowed intra shell 4T2(F)-4T1(F) transitions of Mn4+
ions due to their short lifetimes of 75±10 µsec.7 In this work by examining the excitation
behavior of the 4T2(F)-4T1(F) PL transition using PLE spectroscopy, the position of the
Mn4+/3+ deep level is determined and compared to ab initio calculations.15 The energy levels
of the 4T2(F), 4T1(P), and 4A2(F) excited states are also measured.
GaN:Mn epilayers codoped with Mg were grown by atmospheric pressure metal-organic
vapor phase epitaxy as previously described.11,16,17 For PLE measurements a 250 W quartzhalogen lamp was used as the excitation source, spectrally dispersed by a 0.75m SPEX
grating monochromator. The luminescence was detected by a cooled Ge detector. A Zeiss
MM12 double prism monochromator is employed as a band-pass filter to define the detection
window. The PLE spectra were normalized to account for the spectral responsivity of the
lamp and the SPEX monochromator. For PL spectroscopy the excitation source was either the
325 nm line of a He-Cd laser, or the 973.8 nm line of a semiconductor laser. The sample
temperature was varied between 20 and 300 K using a closed-cycle helium cryostat.
Figure 1 shows the 20K PL spectra of codoped GaN pumped by (a) below gap and (b)
above gap laser excitation. A series of sharp peaks at 1.0 eV dominates both spectra with the
full width at half maximum of 3-8 meV. Although positions of the major sharp PL peaks are
independent of excitation wavelength (indicated by the dotted lines in Fig. 1), a strong
dependence of their relative intensities on the excitation wavelength is observed, indicating
these PL transitions are related to different Mn4+ centers formed in codoped GaN.7 A series of
lower intensity PL peaks are also observed on the low energy side of the PL spectra, which
3
are attributed to phonon replicas of the major peaks. In Fig. 1, the major peaks and their
corresponding phonon replicas are linked by the arrows. The phonon energy is determined to
be 64±1 meV.
To determine the excitation mechanisms for the 1.0 eV PL peaks, as well as the
electronic states of the Mn4+ ions in codoped GaN, the PLE spectra of the Mn4+ [4T2(F)-4T1(F)]
luminescence were measured at low temperature. When measuring the PLE spectra, the Mn4+
luminescence was detected in a 70-meV window around the PL peaks at 1.03 eV. Figure 2(a)
shows the 20 K PLE spectrum of the 1.03 eV peak. A sharp increase of the PLE response with
decreasing photon energy is observed at the low energy side (hν < 1.15 eV), which results
when the energy of the dispersed excitation photons is tuned close to the detection window
around 1.03 eV. The shape of the PLE spectrum indicates the presence of a broad excitation
band with the threshold energy ~1.2 eV. Due to its large spectral bandwidth, this band is
attributed to a PLE transition between a deep impurity level and the conduction or valence
band (i.e. the photoionization of a deep level). Assuming the conduction/valence band to have
a parabolic shape, the spectral dependence of the photoionization cross section can be
determined using a Lucovsky model whereby σ ν ~ (hν − Ei ) 3 / 2 /(hν ) 3 , where hν is the
photon energy and Ei is the ionization energy of the deep level.18 By fitting the PLE band
using the Lucovsky model [shown as the dashed line in Fig. 2(a)], the ionization energy of the
deep level is determined to be 1.11 eV. It should be noted that the observed ionization energy
is in good agreement with ab initio calculations for Mn in GaN, where the Mn4+/3+ transition
level is calculated to be 1.1 eV above the VBM.15 Consequently, the PLE band with the Ei of
1.11 eV is attributed to the photoionization of holes from the Mn3+ ions to the valence band.
The PLE processes of this Mn4+/3+ broad band are summarized in Eq. (1):
4
Mn 4+ [ 4T1 (F)] + hν → Mn 3+ + h + → Mn 4+ [ 4T2 (F)] → Mn 4+ [ 4T1 (F)] + hν 0
(1)
Holes are directly excited from the Mn4+[4T1(F)] ground state to the valence band when the
photon energy is larger than 1.11 eV, forming Mn3+ ions. The recombination of holes with the
Mn3+ centers subsequently excites the 3d shell electrons to Mn4+[4T2(F)] states, followed by
the 4T2(F)-4T1(F) PL transitions of Mn4+ ions at 1.0 eV (Fig. 1). A similar excitation process
involving charge transfers has been described for intra d-shell transition of Fe3+ ions in GaN.19
Three other bands at 1.79, 2.33 and 3.3 eV are distinctly observed upon subtracting the
broad band from the PLE spectrum, as shown in Fig. 2(b). The intensities of the two peaks at
1.79 and 2.33 eV decrease with increasing photon energy after reaching the maximum, which
is characteristic of an intra center transition. Consequently they are attributed to the intra dshell excitations of Mn4+ ions. According to the Tanabe-Sugano diagram of a d3 ion (Mn4+) in
the tetrahedral crystal field (GaN), the 1.79 and 2.33 eV PLE peaks are attributed to spin
allowed intra d-shell transitions from the ground 4T1(F) state to the 4T1(P) and 4A2(F) excited
states, respectively.2 The intra d-shell excitation processes can be explained as follows: when
the pump energy is tuned resonantly with one of its excited states, the Mn4+ ion is excited
from the ground state to the corresponding excited state. The excited Mn4+ ion subsequently
relaxes non-radiatively to the first excited 4T2(F) state before the 4T2(F)-4T1(F) PL transition at
1.0 eV.
From the PLE spectrum of the Mn4+ [4T2(F)-4T1(F)] luminescence, a comprehensive
electronic diagram of the Mn4+ ions in hexagonal GaN can be constructed and is shown in Fig.
3. Four crystal-field states of Mn4+ ions are identified, whose energy positions give a wealth
of information for detailed crystal-field calculations that are beyond the scope of this paper.
5
It is of interest to note that the energy separation between the 4T2(F) and 4T1(F) states of
Mn4+ in GaN is 1.0 eV, approximately twice as large as the energy separation of V2+ (also 3d3
configuration) in the tetrahedral crystal field of II-VI compounds;20,21 i.e. the crystal field
splitting of the 4F multiplet of Mn4+ in GaN is twice of the splitting for V2+ in II-VI
compounds.1,2 The observed stronger crystal field is presumably a combined result of i) the
small lattice constant of GaN, ii) the 4+ charge state of manganese, and iii) the high electron
affinity of nitrogen.19 A large crystal-field splitting has been observed for other transition
metals in GaN.19,22
In addition to the three PLE peaks discussed above, another band with the threshold
energy of 3.3 eV is also observed. The intensity of this peak increases monotonically with
increasing photon energy, which is characteristic of an excitation process between a discrete
level and the conduction or valence band. From the threshold energy of 3.3 eV, this band is
attributed to photoionization of electrons from negatively charged MgGa- acceptors to the
conduction band in that the MgGa0/- (or Mg3+/2+) level is 0.2 eV above the valence band
maximum23 and the low temperature bandgap energy of GaN is ~3.5 eV. This attribution is
consistent with the presence of high concentration (~ 1019 cm-3) Mg2+ ions in our films.23,24
The PLE processes of 3.3 eV band can be described as follows: electrons are excited from
negatively charged MgGa- (i.e. Mg2+) ions to the conduction band for a photon energy greater
than 3.3 eV. The free electron is subsequently trapped at the Mn4+ related center. The electron
occupied trap then captures a hole and a bound exiton is formed at the Mn4+ center.
Recombination of this bound exiton excites the 3d shell electrons of Mn4+ to the 4T2(F) state
through an Auger type process.25 The observed 1.0 eV peak is a result of the 4T2(F)-4T1(F)
6
radiative transition of Mn4+. Alternatively the 3.3 eV PLE band can be explained by a Dexter
energy transfer process.26,27,28
Finally it should be noted that since the Mn4+ ions have a d3 configuration the magnetic
properties of the p-type codoped GaN:Mn would be expected to be different than material
which has predominantly Mn2+ (d5) or Mn3+ (d4) ions.9-12
In summary, the optical properties of Mn ions in the 4+ oxidation state in tetrahedral
crystal field (of p-type GaN:Mn-Mg) were measured. The electronic states of Mn4+ ions in
codoped GaN:Mn-Mg are determined by PLE spectroscopy of the Mn4+ [4T2(F)-4T1(F)]
luminescence. The Mn4+/3+ level is 1.11 eV above the VBM, which is in excellent agreement
with the ab initio calculations. Two intra d-shell transitions at 1.79 and 2.33 eV are observed
in the PLE spectrum and attributed to the photon excitations of the Mn4+ ions from the ground
state 4T1(F) to different excited states 4T1(P) and 4A2(F), respectively. The Mn4+ [4T2(F)-4T1(F)]
luminescence can also be excited by photon ionization of nearby Mg2+ ions through an Auger
process.
This work is supported by NASA Grant No. NAG5-1147, NSF SPINS program under
Grant No. ECS-0224210, and ONR Grant No. N00014-01-0012.
7
References:
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6
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Kim, Appl. Phys. Lett. 82, 4755 (2003).
7
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(2004).
8
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and Y. E. Ihm, J. Appl. Phys. 93, 6793 (2003).
9
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1988), Vol. 25.
10
P. Kacman, Semicond. Sci. Technol. 16, R25 (2001).
11
R. Y. Korotkov, J. M. Gregie, and B. W. Wessels, Appl. Phys. Lett. 80, 1731 (2002).
12
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5159 (2002).
13
W. Gebicki, J. Strzeszewski, G. Kamler, T. Szyszko, and S. Podsiadlo, Appl. Phys. Lett.
76, 3870 (2000).
8
14
M. Zajac, R. Doradzinski, J. Gosk, J. Szczytko, M. Lefeld-Sosnowska, M. Kaminska, A.
Twardowski, M. Palczewska, E. Grzanka, and W. Gebicki, Appl. Phys. Lett. 78, 1276
(2001).
15
U. Gerstmann, A. T. Blumenau, and H. Overhof, Phys. Rev. B 6307 (2001).
16
R. Y. Korotkov, J. M. Gregie, B. Han, and B. W. Wessels, Physica B 308, 18 (2001).
17
R. Y. Korotkov, J. M. Gregie, and B. W. Wessels, Physica B 308, 30 (2001).
18
G. Lucovsky, Solid State Commun. 3, 299 (1965).
19
R. Heitz, P. Maxim, L. Eckey, P. Thurian, A. Hoffmann, I. Broser, K. Pressel, and B. K.
Meyer, Phys. Rev. B 55, 4382 (1997).
20
Lemanhhoang and J. M. Baranowski, Phys. Status Solidi B 84, 361 (1977).
21
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22
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23
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24
It'
s important to note that in Mn-Mg codoped GaN, the Fermi energy is presumably
located between the Mn3+/4+ and Mg2+/3+ transition levels (indicated as the dashed line in
Fig. 3). Most of the Mg ions are thereby negatively charged (in the 2+ states).
25
X. Z. Wang and B. W. Wessels, Mater. Sci. Forum 196-201, 663 (1995).
26
D. L. Dexter, J. Chem. Phys. 21, 836 (1953).
27
S. Kim, S. J. Rhee, X. Li, J. J. Coleman, and S. G. Bishop, Appl. Phys. Lett. 76, 2403
(2000).
9
28
In the Dexter model, electrons are excited from negatively charged MgGa- (i.e. Mg2+) ions
to the conduction band when the photon energy is greater than 3.3 eV, resulting in the
formation of Mg3+ ions. Mg2+ bound exitons are subsequently formed when the electrons
excited to the conduction band are trapped by the Mg3+ ions. The energy released from
recombination of the bound exitons is then transferred by a Dexter non-radiative
mechanism to nearby Mn4+ ions [D. L. Dexter, J. Chem. Phys. 21, 836 (1953)], inducing
an excitation of Mn4+ ions from the ground 4T1(F) state to one of the excited states. The
excited Mn4+ ions subsequently relax non-radiatively to the first excited state 4T2(F),
followed by the 1eV PL emission. Similar excitation process has been observed for Er3+ in
GaN and Mg codoped GaN [S. Kim, S. J. Rhee, X. Li, J. J. Coleman, and S. G. Bishop,
Appl. Phys. Lett. 76, 2403 (2000)].
10
Figure captions:
FIG. 1. Comparison of the intra d-shell transition of Mn4+ ions in the PL spectra excited by (a)
below gap (973.8 nm semiconductor) and (b) above gap (325 nm He-Cd) laser lines. The
major PL peaks and their phonon replicas are linked by arrows.
FIG. 2. (a) The calibrated PLE spectrum of codoped GaN:Mn-Mg at 20 K; (b) the 20 K PLE
spectrum after subtracting the Lucovsky fit.
FIG. 3. Energy levels of Mn4+ ions in wurtzite GaN. The energies are given for a crystal
temperature of 20 K.
11
PL Intensity ( arb units)
Figure 1
100
10
(a) 973.8 nm IR
excitation
(b) 325 nm UV
excitation
1
0.92
0.96
T = 20K
1.00
1.04
1.08
Energy (eV)
12
Figure 2
PL Intensity (arb. units)
(a)
Lamp Background
Calibrated PLE Response
10
Lucovsky Fit
T=20K
E =1.11 eV
th
1
1.0
1.5
2.0
2.5
3.0
3.5
Energy (eV)
4.0
2
4
1
th
4
1
8
+2/+3
T (F)- A (F)
E =3.3 eV
1
4
12
Mg
T (F)- T (P)
16
4
PL Intensity (arb. units)
(b)
4
0
1.0
T = 20 K
1.5
2.0
2.5
3.0
Energy (eV)
3.5
4.0
13
Figure 3
Conduction Band
2.3 eV
4
A1(F)
4
T1(P)
4
T2(F)
1.8 eV
3.3 eV
1.0 eV
Mn4+/3+
4
T1(F)
EF
1.11 eV (Lucovsky fit)
Mg3+/2+
Valence Band
14