high-energy spectroscopic study of the iii-v diluted...

High-energy spectroscopic

study of the III-V diluted

magnetic semiconductor

Ga1−xMnxN

Master Thesis

Jong-Il Hwang

Department of Complexity Science and Engineering,

University of Tokyo

January, 2004

Contents

1 Introduction 5

2 Physical properties of Ga1−xMnxN 11

2.1 III-nitride semiconductors . . . . . . . . . . . . . . . . . . . 11

2.2 Ga1−xTxN . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

3 Principles of electron spectroscopy 27

3.1 Photoemission spectroscopy . . . . . . . . . . . . . . . . . . 27

3.2 Resonant photoemission spectroscopy . . . . . . . . . . . . . 29

3.3 X-ray absorption spectroscopy . . . . . . . . . . . . . . . . . 30

3.4 X-ray magnetic circular dichroism . . . . . . . . . . . . . . . 31

3.5 Configuration-interaction cluster model . . . . . . . . . . . . 32

4 Experimental 37

4.1 Experimental . . . . . . . . . . . . . . . . . . . . . . . . . . 37

4.2 Crystal growth and surface treatment of Ga1−xMnxN . . . . 39

5 Results and discussion 47

5.1 X-ray absorption spectroscopy and magnetic circular dichroism 47

5.2 X-ray photoemission spectroscopy . . . . . . . . . . . . . . . 53

5.3 Resonant photoemission spectroscopy . . . . . . . . . . . . . 56

6 Summary 67

Chapter 1

Introduction

The remarkable development currently being made in the fields of electron-

ics and information technologies has been made possible by exploiting the

properties of electron charge and spin. Integrated circuits used for data pro-

cessing utilize the charge of electrons in semiconductors, while data storage

media such as hard disks utilize the spin of electrons in magnetic materials.

Recently, effort to control the spin degrees of freedom in semiconductors has

been made. Various types of spin polarization have been realized in semi-

conductors, such as carrier spins, spins of doped magnetic atoms, spins in

artificialsuperlattices and nuclear spins The control of these spins can lead

to the advent of a new field - semiconductor spin electronics (semiconductor

spintronics) - involving the using the spin states inside semiconductors.

Semiconductors currently used for integrated circuits, transistors and

optical devices, such as silicon and gallium arsenide, are non-magnetic, and

the electron energy is almost independent of the spin direction. However, as

the miniaturization of such devices is developed by the progress of nanotech-

nology and crystal growth technique, exchange interaction has become more

pronounced effect, and the existence of the spin has become more tangible.

The exchange interaction can lead to spin related phenomena not only in

nanostructures but also in devices of conventional size. For instance, the

breakthrough of crystal growth technique made by development of molecu-

lar beam epitaxy (MBE), which can control the rate and direction of crystal

growth layer by layer, has enabled us to grow high quality crystals. The

emergence of MBE method has made it possible to prepare high quality

thin films of diluted magnetic semiconductors (DMS’s). The MBE method

has become to incorporate transition or rare-earth metal atoms into the

host semiconductors. The fabrication of the III-V DMS’s In1−xMnxAs and

Ga1−xMnxAs was realized by using this MBE method [1.1, 2]. These ma-

terials thus fabricated have shown ferromagnetism with p-type conduction

6 Chapter 1. Introduction

and has stimulated the extensive studies in this field. Furthermore, by mak-

ing the heterostructures between DMS’s and semiconductors using the MBE

method, newly fabricated devices are realized.

In practice, however, there are some problems in realizing new devices

using DMS’s. Up to now, the Curie temperature (TC) of the III-V DMS’s

such as Ga1−xMnxAs and In1−xMnxAs has been well below the room tem-

perature. The TC is 110 K for Ga1−xMnxAs [1.3] and 50 K for In1−xMnxAs.

Therefore, one of the issue is to develop a DMS with TC above room tem-

perature. There are many challenges to realize DMS’s with TC above room

temperature. One of the strategy to realize the ferromagnet with TC above

room temperature is to utilize GaAs that is used extensively in present-day

electronics. There are reports that the optimization of the layer thickness

and the annealing of Ga1−xMnxAs enhances the TC up to 150 K [1.4] and

that Mn δ-doped GaAs in the nominal Mn concentration of 6.3 × 1014 cm−2

have TC as high as 172 K [1.5]. Moreover, by co-doping C to make p-type

GaAs with Mn of 1 - 5 at % through ion implantation, TC rises up to 280K

[1.6]. Another challenge is the optimization of the of between the host mate-

rial and the transition or rare earth metals. A new function was proposed by

incorporating Mn into chalcopyrite semiconductors including CdGeP2 and

ZnGeP2, which shows ferromagnetism above room temperature [1.7, 8]. In

a recent theoretical study, in II-VI and III-V compound semiconductors, it

has been predicted that ferromagnetism with a very high TC occurs in sys-

tems such as p-type Ga1−xMnxN and Zn1−xMnxO [1.9]. Figure 1.1(a) shows

the schematic diagram indicating the TC of Mn-doped semiconductors cal-

culated by Dietl et al.. In the calculation, 2.5% of Mn atoms in divalent

charge state and 3.5×1020 holes par cm3. The calculation predicts that the

TC of Mn-doped GaN, InN, C and ZnO exceed above room temperature.

The stability of ferromagnetic state has been predicted in ZnO and GaN-

based DMS’s [1.10, 7]. Their calculations shows that V, Cr and Mn-doped

GaN as shown in Fig. 1.1(b). and transition-metal(except for Mn)-doped

ZnO are promising candidates for room temperature ferromagnetic DMS’s.

Motivated by such material designing, the fabrication of new ferromagnetic

DMS’s, such as ZnO:Co [1.12] and ZnO:V [1.13], were attempted.

In the III-V DMS’s, too, after the successful Mn doping into GaN [1.14],

several groups reported that Ga1−xMnxN shows an indication of ferromag-

netic behavior [1.16, 17]. GaN is a key material and essential for the progress

of electronics so that recent developments in growth technique for wurtzite

GaN has led to the fabrication of GaN-based optical and electrical devices.

However, so far, the results have been quite diverse between different re-

ports, and the occurrence of ferromagnetism is still controversial [1.21]. To

realize room-temperature ferromagnetism in DMS’s, it is necessary to elu-

7

Figure 1.1: (a) Prediction of TC for various DMS’s. 2.5 % of Mn atoms in

divalent charge state and 3.5 × 1020 holes per cm3 had been assumed [1.9].

(b) Stability of the ferromagnetic states in GaN-based DMS’s. Positive

difference means that the ferromagnetic state is more stable than the spin

glass state.

cidate the mechanism of the occurrence of ferromagnetism in the DMS’s.

Especially important is information about their electronic structure includ-

ing the interaction between the 3d electrons of the transition metal and the

band electrons of host material. As the electronic structure of Ga1−xMnxN

has not been studied experimentally so far, it is strongly desired to study

its electronic structure to see whtere ferromagnetism is possible or not.

We have investigated the electronic structure of the Ga1−xMnxN using

photoemission spectroscopy (PES) and subsequent configuration-interaction

(CI) cluster model analysis, x-ray absorption spectroscopy (XAS) and mag-

netic circular dichroism (MCD). PES and XAS are powerful tools to investi-

gate the electronic structure of solids. In the studies of DMS’s, too, PES and

XAS have played important roles to investigate their electronic structures.

CI approach is a powerful analytical tool to describe such systems in which

the Coulomb interaction term and the hybridization term is competing.

References

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287, 1019 (2000) : T. Dietl, H. Ohno, and F. Matsukura, Phys. Rev.

B. 63, 195205 (2001).

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(2001)

10 References

[1.12] K. Ueda, H. Tabata, and T. Kawai, Appl. Phys. Lett. 79, 988 (2001).

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(2001).

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Jpn. J. Appl. Phys. 40, L724 (2001).

[1.15] S. Sonoda, S. Shimizu, T. Sasaki, Y. Yamamoto and H. Hori, J. Cryst.

Growth. 237, 1358 (2002).

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K.T. McCarthy and A.F. Hebard, Appl. Phys. Lett. 79, 1312 (2001).

[1.17] K. Ando, Appl. Phys. Lett. 82, 100 (2003)

Chapter 2

Physical properties of

Ga1−xMnxN

2.1 III-nitride semiconductors

After the development of crystal growth techniques of the III-nitride com-

pounds including GaN and Ga1−xAlxN [2.1, 2], the electric and optical qual-

ities of the crystal were improved owing to the development of the crystallo-

graphic quality. Device technologies have progressed because the technique

is useful not only to binary compounds such as GaN but also to ternary and

quaternary compounds such as In1−xGaxN and InxGayAl1−x−yN.

Ternary compounds IIIxIII’1−xN such as In1−xGaxN and quaternary

compounds IIIxIII’yIII”1−x−yN such as InxGayAl1−x−yN can be made using

compounds having same crystal structure. All of these III-nitride semicon-

ductors except for BN have a direct band gap and are stable in wurtzite

structure at room temperature. These binary, ternary and quaternary III-

nitride materials are promising candidates as optical devices because the

values of the band gap, 1.9 - 6.2 eV, range from the visible light to ultravi-

olet.

On the other hand, these materials are expected to be useful for the

production of new devices such as power devices and electron emitter be-

cause GaN has a large electron-velocity saturation compare to GaAs, and

AlGaN have a negative electron affinity. Since the III-nitride semiconduc-

tors are stable physically and chemically, devices prepared using III-nitride

semiconductors can work in harsh conditions compare to devices made of

conventional semiconductors.

Those characteristics of the III-nitride semiconductors are derived from

the fact that these materials include nitrogen atoms. General physical prop-

erties of the III-nitride semiconductors are described below.

12 Chapter 2. Physical properties of Ga1−xMnxN

Table 2.1: Part of the periodic table and the energies of valence electrons in

free atoms (all values are negative and are given in units of eV).

Let us show that the properties are much different between materials

belonging to row 2 and those belonging to row 3 in the periodic table. Table.

2.1 shows a part of the periodic table and the energies of valence electrons.

The number of the row in the periodic table indicates the principal quantum

number. The atoms belonging low rows in the periodic table tend to have

many electronic orbital. Therefore, the spatial distribution of electron in

these atoms tends to become extended. Most of tetrahedrally bonded semi-

conductors are formed as the number of the valence electrons becomes eight

per two atoms, as in Si, GaAs and ZnSe. Since the atomic radii belonging to

the same row in the periodic table similar, the lattice constants of compound

semiconductors consisting of atoms in the same row, such as Ge, GaAs and

ZnSe, are similar to each other.

On the other hand, in the III-V and II-VI compounds, one of the element

is charged positively while the other one is charged negatively, namely the

bonds in the compound semiconductors have the ionic nature in addition to

the covalent bond. We define the ionicity fi using the bond gap Eg according

to Phillips [2.3]. The bond gap Eg, which is the energy difference between the

bonding state and anti-bonding state while the band gap is energy difference

between the conduction-band minimum and valence-band maximum, can be

2.1. III-nitride semiconductors 13

define using the part derived from the ionicity and the covalent bond as [2.3]

E2g = E2

h + C2, (2.1)

where Eh is the energy difference between the anti- and bonding-state de-

rived from the covalent bond, namely the bond-gap energy derived from

those of atoms in the corresponding IV family. C is the energy between the

anti-bonding and bonding-state derived from the ionicity. The ionicity fi is

defined using as

fi =C2

E2g

. (2.2)

Table 2.2 shows the ionicity of semiconductors [2.3]. The ionicity fi =

0 means a complete covalent bond. In most of the III-V semiconductors,

the values of C are smaller than those of Eh while in most of the II-VI and

I-VII semiconductors, the values of C are larger than those of Eh. However,

Eh and C are same in AlN, GaN and InN.

The distance between the atoms strongly depend on the row which the

atoms belong to. As shown in Fig. 2.1, the distance decreases rapidly in row

2. One of the characteristics of the III-nitride semiconductors is very small

lattice constants compare to semiconductors consisting of atoms below row

3. The nearly atomic distances in AlN, GaN and InN are also very small as

shown in Fig. 2.1.

That the bond length is small implies a strong bonding between the

atoms, namely the cohesive energy is large. The cohesive energies of row 1,

2, 3 and 4 horizontal sequences and quasi-horizontal sequences not involving

row 1 are all shown in Fig. 2.2. Also there are large differences between

crystals belonging to row 2 and below the row 3 in cohesive energies. That

the bonding in the crystals belonging to row 2 is strong is thus indicted.

The crystals are produced easily in order of Al>GaN>InN, as shown in

Fig. 2.2. The strong bonding is the second characteristics of the III-nitride

semiconductors.

The third characteristics is that the energies of the valence electrons are

low. The energies of nitrogen s and p orbitals are low as shown in Table 2.1.

The s and p orbitals contribute to bonding substantially. The mean energy

of these electrons

Es + 3Ep

4(2.3)

are shown in Fig. 2.3 against the row in the periodic table. These energies

of the valence electron in row 2 is extremely low while the energies of atoms

below row 3 are not much different. This means that these electrons are


Table 2.2: Eh, C, and ionicity ff of various semiconductors. fi=0 means a

completely covalent bond [2.3].

strongly bound to the atom. The energies of the valence electrons are low-

ered because the atomic bond strength increases when the distances between

the atoms decreases. This tendency is conspicuous for the III-nitrides semi-

conductors. For instance, the conduction-band minimum of GaN is lower

than the valence-band maximum of GaAs.

The last characteristics worthy of special mention of the III-nitride

semiconductors is that their dielectric constants are small. The dielectric

constant is determined by the polarizability of constituent ions caused by

external electric field. Since the nitrogen atom binds the electrons strongly,

its polarizability is small. And therefore the dielectric constant is small.

Coulomb interaction in the solid is large for a small dielectric constant. This

is one of the reasons why the acceptor and/or donor level is relatively deep

in the III-nitride compounds. Free hole is difficult to be generated because

the hole is bound to acceptor through the strong Coulomb interaction.

As mentioned above, the characteristics of the III-nitride semiconduc-


Figure 2.1: Relationship between the distance and the ionicity fi of various

semiconductors. Compounds of the same row are connected by lines.

tors are considerably different from the semiconductors constituted by the

atoms below row 3. Most of these remarkable characteristics are derived

from nitrogen atoms.

Most semiconductors have the structure of diamond type, zinc-blend

type or wurtzite type. The diamond type and the zinc-blend type are the

same structure basically. Figure 2.4(a) shows a part of tetrahedral bonded

AB crystal. If the atoms are not charged, B2 atoms are located at a more

stable position in the middle of the A1 atoms. On the other hand, if the

atoms are charged, these A1 and B2 atoms attract each other, and B2 atoms

are located at just above the A1 atoms. Figure 2.4(b) and (c) shows the

wurtzite structure and zinc-blend structure, respectively. If the ionicity is

stronger, each ion is six-fold coordinated because the Coulomb attractive

energy is greater for the six-fold coordination, such as NaCl. Therefore, the

crystal structures are determined by the ionicities of the crystal as shown

in Fig. 2.5 [2.3]. All of the crystals plotted above straight solid line (fi =

0.785) in Fig. 2.5 have a rock solt structure. The zinc-blend type and the

wurtzite type can be classified not by straight line, but a curve as shown

in Fig. 2.5. Crystals near this boundary curve such as ZnSe, ZnS, AlN,

GaN and InN can be crystallized not only in the wurtzite but also in the


Figure 2.2: Gibbs free energy of atomization ∆Gs against fi [2.3].

Figure 2.3: Mean energy of sp valence electrons against the row of the

periodic table.


Figure 2.4: (a)Part of a tetrahedral bonded AB crystal. (b)Wurtzite struc-

ture. (c)Zinc blend structure.

zinc-blend type because of the ionicity is comparable to covalency in these

semiconductors.

GaN has normally wurtzite-type structure. By selecting the substrate,

the cubic GaN (zinc-blend structure) can be synthesized. In this thesis,

we consider only GaN in the wurtzite structure. The unit cell and the

reciprocal unit cell of the wurtzite structure shown in Fig. 2.6. Unlike the

zinc-blend structure, the structure of the reciplocal unit cell is hexagonal.

The lattice constants a and c of bulk (thin film) GaN are 3.189 (3.188) and

5.186 (5.183), respectively [2.4]. The band structure of wurtzite GaN has

been investigated theoretically and experimentally. The band structure has


Figure 2.5: Relationship between the covalency and the ionicity of various

semiconductors. The crystal structures are classified by the ionicity of the

crystals [2.3]

2.2. Ga1−xTxN 19

Figure 2.6: The unit cell and the reciprocal unit cell of the wurtzite structure.

(a) unit cell. (b) reciprocal unit cell.

been calculated using various methods such as the tight-binding method,

first-principle calculation and the local-density-approximation (LDA) with

self-interaction correction [2.7, 6, 8, 9]. Bulk band dispersion and surface

states of the thin-film wurtzite GaN have been investigated experimentally

by Dhesi et al. using angle-resolved photoemission spectroscopy [2.5]. Figure

2.7 shows the band mapping determined experimentally [2.5] and calculated

[2.7].

2.2 Ga1−xTxN

DMS’s based on GaN have been extensively studied because of the intrinsic

high potential of GaN and the theoretical prediction that ferromagnetism

with a very high TC occurs in systems such as p-type Ga1−xMnxN [2.9, 7]. In

this section, the physical properties of DMS’s based on GaN are summarized.

Dietl et al. have reported the theoretical study which predicts that fer-

romagnetism occurs in Ga1−xMnxN. for the Mn concentration of 2.5 at %

per unit cell and the hole concentration of 1020 cm−3 [2.9]. It has also been

predicted the TC exceeds the room temperature. On the other hand, based

on a self-consistent electronic structure calculation using the local spin den-

sity approximation (LSDA), Sato et al. have studied V-, Cr-, Mn-, Fe-, Co-

and Ni-doped GaN-based DMS’s in the ferromagnetic state. The d orbitals

of the transition metal splits in to t2g and eg states by the crystal field. The


Figure 2.7: Band mapping of the wurtzite GaN. The solid line and dots

indicate band dispersion decided by the calculation [2.7] and experiment

[2.5].

3dt2g orbitals, which have the symmetry as functions of xy, yz and zx, hy-

bridize well with p orbitals of valence band, so that they form bonding states

tb and their anti-bonding counterpart ta as schematically shown in Fig. 2.8

[2.7]. Thus it has been predicted that the anti-bonding ta states and non-

bonding e states appear in the band gap of the host semiconductors and are

partially occupied. They have also reported that the V-, Cr- and Mn-doped

GaN is a promising candidate to realize room temperature ferromagnetic

DMS’s because the ferromagnetic state is stable in these materials. There

is many attempts to produce GaN-based DMS’s to realize room tempera-

ture ferromagnetic DMS’s. So far, it has been reported that GaN doped

with Cr [2.11], Mn [2.15, 16, 17], Fe [2.12, 13, 14] Gd [2.18], Tb [2.19] were

synthesized. Ferromagnetism in the GaN-based DMS’s has been reported

for Cr-doped [2.11], Mn-doped [2.16, 17] and Gd-doped [2.18]. Below, we

shall overview GaN doped with transition-metal ions. Sonoda et al. have

reported that Ga1−xMnxN prepared by NH3-MBE with Mn concentration

of 9 at % has TC exceeding the room temperature [2.16]. Surprisingly, the

estimated value of the TC is 940K as shown in Fig. 2.9. In that report,

the coercivity Hc and residual magnetization Mr of Ga0.91Mn0.09N at 300K

have been estimated to be 85 Oe and 0.77 emu/g, respectively. However,

Ando have reported using MCD that Ga1−xMnxN prepared by NH3-MBE

2.2. Ga1−xTxN 21

Figure 2.8: Schematic electronic structure of the transition metal atom sub-

stituting the Ga site in GaN [2.7].

Figure 2.9: Temperature dependence of the magnetization at 0.1 T of

Ga0.91Mn0.09N prepared by NH3-MBE [2.16]


Figure 2.10: Temperature dependence of sheet resistance (a) and Hall resis-

tance (b) of Ga0.93Mn0.07N prepared by RF-plasma-MBE [2.17].

[2.22]is a paramagnetic DMS and the ferromagnetism of the sample arises

from an unidentified material that is not detected by the x-ray diffraction

[2.21]. Overberg et al. have prepared Ga1−xMnxN with Mn concentration

of 7 at % [2.17] and reported its magnetic and magnetotransport properties

as shown in Fig. 2.10. Based on the anomalous Hall effect, negative mag-

netoresistance and magnetic hysteresis at 10 K, they have concluded that

Ga1−xMnxN have TC between 10 and 25 K because the anomalous Hall term

vanishes at 25 K as shown in Fig. 2.10.

On the other hand, Kondo et al. have reported that Ga 1-xMn xN

grown by RF-plasma-assisted-MBE shows primarily paramagnetic behav-

ior [2.13]. They have systematically investigated the properties of epilayers

that the electron concentration decreases and the resistivity increases with

increasing Mn concentration. They have estimated the effective spin number

of the paramagnetic component as S ≈ 2.5. This implies that Mn atoms be-

comes Mn2+ ions due to compensation. Probably the compensation is caused

by defects which provide the electrons, consistent with the previous report

[2.20]. Also, they have reported that epilayers with high Mn concentrations

(∼ 1020 cm−3) have a positive paramagnetic Curie temperature as shown

in Fig. 2.11. The positive paramagnetic Curie temperature suggests the

presence of ferromagnetic spin exchange between Mn ions. From these mea-

surements, they have proposed that the conduction type of Ga1−xMnxN in

the high Mn concentration region is p-type with low hole concentration while

in general, the paramagnetic Curie temperature become negative in high Mn

concentration region because of the presence of the anti-ferromagnetic ex-

change interaction between the Mn ions.

Thus, so far, the magnetic property of Ga1−xMnxN have been quite

2.2. Ga1−xTxN 23

Figure 2.11: Curie-Weiss plot for Ga0.98Mn0.02N [2.15].

diverse between the difference reports, and the occurrence of the ferromag-

netism is still controversial. As the electronic structure of Ga1−xMnxN has

not been studied experimentally so far, it is desirable to study its electronic

structure to see whether there is a possibilities of the ferromagnetism in this

system.

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K.T. McCarthy and A.F. Hebard, Appl. Phys. Lett. 79, 1312 (2001).

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H. Asahi, Solid. Stat. Comm. 216, 625 (1999).

[2.19] K. Hara, N. Ohtake and K. Ishii, Phys. Stat. Sol. 216, 625 (1999).

[2.20] Y.L. Soo, G. Kioseoglou, S. Kim, S. Huang, Y.H. Kao, S. Kuwabara,

S. Owa, T. Kondo and H. Munekata, Appl. Phys. Lett. 79, 3926

(2001).

[2.21] K. Ando, Appl. Phys. Lett. 82, 100 (2003).

[2.22] S. Sonoda, H. Hori, Y. Yamamoto, T. sasaki, M. Sato, S. Shimizu, K.

Suga and K. Kindo, IEEE Trans. Magn. 38, 2859 (2002).

Chapter 3

Principles of electron

spectroscopy

3.1 Photoemission spectroscopy

Photoemission spectroscopy (PES) is a powerful tool to directly investigate

the electronic structure of materials. Photoemission is a phenomenon that

a material irradiated by light emits electrons to outside. By absorbing the

light, the an electron is excited from occupied state to the vacuum (unoc-

cupied) state. PES measures the distribution of the kinetic energy of these

electrons. Knowing the kinetic energy Ek of the emitted electrons, one can

deduce how strong the electron was bound to the material. Owing to the

energy conservation law,

EVkin = hν − EB − Φ, (3.1)

where hν is the energy of the incident photon, EB is the binding energy of

the electron relative to the Fermi level EF and Φ is the work function which

is the energy required for an electron at EF to escape from solid through the

surface and to reach the vacuum level Evac, that is, Φ = Evac - EF . Here,

let us ignore the correlation effects between the electrons and assume that

according to Fermi distribution function these electrons occupy energy level

up to EF , namely we assume the one-electron approximation. In actual PES

measurements, since both the sample and the electron energy analyzer are

grounded, the measured kinetic energy Ekin of the photoelectron is referred

to EF . We obtain, then,

Ekin = hν −EB. (3.2)

The energy -EB is approximately regarded as the energy of the electron

inside the sample material before the photoemission. Therefore, the energy

distribution inside the material can be directly mapped by the distribution of

28 Chapter 3. Principles of electron spectroscopy

the kinetic energies of photoelectrons emitted with monochromatic incident

photons.

Figure 3.1: Schematic diagram of photoemission spectroscopy. The density

of states N(E) can be obtained by measuring the photoemission spectra

I(E).

Figure 3.1 schematically shows how the electronic density of states

(DOS) is measured by the electron distribution curve (EDC), that is the

photoemission spectrum (I(E)). In this thesis, photoemission spectra will

be displayed with the binding energy EB for the horizontal axis and the

density of states of photoelectrons for vertical axis.

In real systems, ignoring the correlation effect described above is inad-

equate to interpret the photoemission spectra For instance, photoemission

3.2. Resonant photoemission spectroscopy 29

spectra are affected by the relaxation of the entire electron system in the

photoemission final state, in addition to the one-electron energy. In response

the hole produced by the photoemission process, the surrounding electrons

tend to screen the hole to lower the total energy of the system. There-

fore, considering the entire electron system, the binding energy EB is given

by the energy difference between the N -electron initial state ENi and the

(N − 1)-electron final state EN−1f , as

EB − Φ = EN−1f − EN

i . (3.3)

That is, EB is the energy that is required to produce the hole with photoe-

mission process, including the relaxation energy of the total electron sys-

tem. Thus, the consideration of correlation effect makes the interpretation

of photoemission spectra complicated. However, this helps to obtain the

information about the electron correlation in the materials.

3.2 Resonant photoemission spectroscopy

Resonant photoemission spectroscopy (RPES) is a powerful technique to

extract the impurity atom derived photoemission spectrum in the valence

band. The capability of synchrotron radiation that one can continuously

vary the photon energy is exemplified in RPES measurements.

Figure 3.2 shows a schematic diagram of RPES. The direct photoemis-

sion process of a valence d electron is denoted as

p6dN + hν → p6dN−1 + e−, (3.4)

where e− denotes the photoelectron. Here, we assume that the p core level

is completely filled by six electrons. On the other hand, the absorption

from the p core level to the valence d state occurs with the tunable photon

energy. After the subsequent super-Coster-Kro nig decay, the final electronic

configuration p6dN−1 is reached through

p6dN + hν → p5dN+1 → p6dN−1 + e−. (3.5)

The energy level in the final states created by these two process have the

same energy and the same electronic configuration. Therefore, these pro-

cesses can interfere, resulting in a resonantly enhancement of the photoe-

mission intensity from the d orbitals, and hence in the so-called Fano profile

[3.1]. Since the enhancement occurs only for d orbitals, one can obtain the

information about the d partial DOS. Using this method, one can selec-

tively extract a orbital component from the valence-band spectrum because


Figure 3.2: Schematic diagram of resonant photoemission process.

the energy of the absorption where the enhancement occurs is different be-

tween the elements. This method is suitable for transition metal impurity

systems because weak signals in normal photoemission measurements can

be enhanced by RPES.

3.3 X-ray absorption spectroscopy

The measurements of photo-absorption by excitation of a core-level electron

into unoccupied states as a function of photon energy is called x-ray absorp-

tion spectroscopy (XAS). The probability of the excitation is proportional

to the product between the DOS of the unoccupied states the transition

3.4. X-ray magnetic circular dichroism 31

probabilities. XAS, which measure the photo-absorption intensity including

the excitation probability, therefore, takes mainly the information about the

density of unoccupied states, because the DOS of the core is relatively sharp

compare to the unoccupied states in energy.

The photo-absorption intensity is given by

Iµ(hν) =∑f

〈f | Tµ | I〉2δ(Ei −Ef − hν). (3.6)

Here, T is the dipole transition operator and µ is the index of light polar-

ization. The 2p core-level XAS spectra of transition metal compounds well

reflect the 3d electronic states in the 3d transition metal compounds includ-

ing the symmetry and the crystal field splitting of the 3d orbitals. XAS is

also selective in a elemental because the energy of excitation is adjusted to

the energy proper to element.

There are two measurement modes for XAS, namely the transmission

mode and the yield mode. In the transmission mode, the intensity of the x-

ray is measured before and after the samples and the ratio of the transmitted

photons is recorded. Alternatively, one can obtain the absorption cross

section by measuring decay products of the core hole which is created in the

absorption process. This is the yield mode measurement and is standard for

soft x-rays. In this thesis, the total electron yield method is adopted.

3.4 X-ray magnetic circular dichroism

Using circularly polarized light in XAS, the absorption intensity depends

on the helicity of the incident light. This method is called x-ray magnetic

circular dichroism (XMCD). XMCD is defined as the difference between the

absorption intensities for right- and left-handed circularly polarized light

when the polarized light is parallel and antiparallel to the magnetization

direction of the magnetic materials in a magnetic field.

One of the advantages of XMCD measurement is also that it is an ele-

ment specific measurement method like RPES and XAS. Another advantage

is that XMCD measurements reflect the spin and orbital polarization of lo-

cal electronic states. The value of the spin and orbital moments can be

separately estimated by using magneto-optical sum rules [3.2, 3]. For the

2p-3d MCD analysis, the value of the spin mspin and orbital morb magnetic

quantum numbers are given by

mspin = −6∫L3

(I+ − I−)dω − 4∫L3+L2

(I+ − I−)dω∫L3+L2

(I+ + I−)dω, (3.7)

morb = −4∫L3+L2

(I+ − I−)dω

3∫L3+L2

(I+ + I−)dω(10 − n3d)

(1 +

7〈TZ〉2〈SZ〉

)−1, (3.8)


where I± is the absorption intensity for the positive (negative) helicity, 〈TZ〉is the expectation value of the magnetic dipole operator and 〈SZ〉 is equal

to half mspin. Thus, XMCD measurement is a powerful tool to investigate

the magnetic moments in materials [3.4, 5].

3.5 Configuration-interaction cluster model

In order to consider the correlation interaction between electrons, we con-

sider the hybridization between the Slater determinants instead of a sin-

gle Slater determinant, The hybridizing between the Slater determinants

leads to so-called configuration-interaction (CI). In this thesis, core-level

and valence-band spectra will be analyzed using CI calculation on a MnN4

cluster. The CI cluster-model analysis has been a useful framework for un-

derstanding the electronic structure of DMS [3.6, 7, 8].

In the CI picture, we consider a tetrahedral MnN4 cluster with the

central Mn atom. Here, the distortion from the Td symmetry tetrahedron in

the wurtzite structure is ignored because the magnitude of the distortion is

small. The wave function of the ground state ψg, which we call N -electron

state, is spanned by linear combinations of charge transfer state as

ψg = a0|dn〉 + a1|dn+1L〉 + a2|dn+2L2〉 + · · · . (3.9)

The final state wave functions of Mn 2p core-level photoemission ψc and

Mn 3d valence-band ψv are also spanned by linear combinations of charge

transfer state as,

ψc = b0 | cdn〉 + b1 | cdn+1L〉 + b2 | cdn+2L2〉 + · · · , (3.10)

ψv = c0 | dn〉 + c1 | dn+1L〉 + c2 | dn+2L2〉 + · · · , (3.11)

where c and L denotes holes in the valence band and ligand p orbitals,

respectively, and n = 5 for the ground state of the Mn+2. The anion-to-3d

orbital charge-transfer energy is defined by ∆ ≡ E(dn+1) − E(dn), and the

3d-3d Coulomb interaction energy is defined by U ≡ E(dn+1) + E(dn−1) −2E(dn), where E(dlk) is the center of gravity of the dlLk multiplet. The

multiplet splitting is expressed using Racah parameters B and C, which

are fixed at the values of the free Mn2+ ion (B = 0.119 eV, C = 0.412

eV) [3.9]. The average Coulomb interaction Q between the Mn 3d electron

and the Mn 2p core hole is fixed at U/Q = 0.8. In the tetrahedral cluster

model, one-electron transfer integrals between the 3d and ligand p orbitals

are given by Slater-Koster parameters (pdσ) and (pdπ). We have utilized

the relationship (pdσ)/(pdσ) = -2.16 according to Harrison [3.10]. One can

3.5. Configuration-interaction cluster model 33

also define the charge-transfer energy ∆eff and 3d-3d Coulomb interaction

energy Ueff with the respect to the lowest term each multiplet, are given by

∆eff = ∆ + (70B − 35C)/9 + 7C, (3.12)

Ueff = U + (14B − 7C)/9 + 14B + 14C. (3.13)

Figure 3.3 schematically shows the energy diagram of a cluster in the

neutral (N -electron system) and positively ionized ((N -1)-electron system)

and negatively ionized ((N+1)-electron system) state. Photoemission pro-

cess corresponds to the process from the N -electron system to the (N -1)

electron system and inverse photoemission process corresponds to the pro-

cess from the N -electron system to the (N -1) electron system. In practice,

the ∆, U and (pdσ) are parameterized, which are called electronic structure

parameter, and are so chosen as reproduce the photoemission spectrum.

Figure 3.3: Schematic energy-level diagram from a dn transition metal im-

purity in a host semiconductor.

Using the fitted parameters ∆, U and (pdσ), one can estimate the ex-

change constant Nβ between the 3d electron and the electron at the top of

the valence band of host semiconductor [3.6] by the second-order perturba-

tion with respect to charge transfer as

Nβ = −16

S

(1

−δeff + Ueff

+1

δeff

)(1

3(pdσ) − 2

√3

9(pdπ)

)2

. (3.14)

References

[3.1] U. Fano, Phys. Rev. 124, 1866 (1961).

[3.2] B.T. Thole, P. Carra, R. Sette and G. ven der Laan, Phys. Rev. Lett.

68, 1943 (1992).

[3.3] P. Carra, B.T. Thole, M. Altarelli and X. Wang, Phys. Rev. Lett. 70,

694 (1993).

[3.4] C.T. Chen, Y.U. Idzerda, H.J. Lim, N.V. Smith, G.H. Ho, E. Pelle-

grion and F. Sette, Phys. Rev. Lett. 75, 152 (1995).

[3.5] J. Stohr, J. Elect. Spectrosc. Relat. Phenom. 75, 253 (1995).

[3.6] T. Mizokawa and A. Fujimori, Phys. Rev. B. 48, 14150 (1993).

[3.7] J. Okabayashi, A. Kimura, T. Mizokawa, A. Fujimori T. Hayashi and

M. Tanaka, Phys. Rev. B. 59, 2486 (1999).

[3.8] T. Mizokawa, T. Nambu, A. Fujimori, T. Fukumura and M. Kawasaki

Phys. Rev. B. 65, 085209 (2002).

[3.9] S. Sugano, Y. Tanabe and H. Kamimura, Multiplets of Transition

Metal Ions in Crystals (Academic, New York, 1970).

[3.10] W.A. Harrison, Electronic structure and the Properties of Solids

(Dover, New York 1989).

Chapter 4

Experimental

4.1 Experimental

The photoemission experiments were performed at BL-18A of the Photon

Factory, High Energy Accelerator Research Organization. The measure-

ments were made in an ultra high vacuum below 1× 10−10 Torr at room

temperature. Photoelectrons were collected using a VG CLAM hemispher-

ical analyzer in the angle-integrated mode. The total energy resolution

including the monochromator, the electron analyzer and the temperature

broadening was estimated to be 200meV from the Fermi edge of a metal.

Core-level photoemission spectra were taken using a MgKα source (hν =

1253.6 eV). The resolution was estimated to be ∼ 0.8 eV from the Au 4f

core-level spectrum. Satellite emission of the Kα1,2, Kα3,4 and Kα5,6 has

been numerically subtracted. The photoemission spectra were referenced to

the Fermi edge of a metal in electrical contact with the sample. Also, the

Au 4f7/2 core-level binding energy set to 84.0 eV.

In the electron energy analyzer, the ejected electrons are retarded by an

amount VR before entering the analyzer. The analyzer is a band pass filter

only transmitting electrons with energy equal to the pass energy Epass before

reaching the detector as shown in Fig. 4.1. Then we obtain the relation,

EVkin + Φ = Ekin = eVR + Epass + ΦA = hν + EB, (4.1)

where EVkin and Φ are kinetic energy of photoelectron and a work function of

the sample, respectively. Ekin is kinetic energy of photoelectron measured

from Fermi level EF . VR and Φ are the retarding voltage and the work func-

tion of the electron energy analyzer. The voltages of the outer hemisphere,

the inner hemisphere and the entrance slit are shown in Fig. 4.2. In the

hemispherical electron energy analyzer, two concentric hemispheres of mean

radius R0 are mounted. The potential V is applied between them so that

38 Chapter 4. Experimental

Figure 4.1: Relation ship between the kinetic energy of photoelectron Ekin,

the pass energy EPass and the retarding voltage VR.

the outer hemisphere is negative and the inner hemisphere is positive with

respect to V (R0) = VR which is the equipotential surface in the middle of

hemispheres. The entrance and exit slits are both centered on the sphere of

radius R0. The relationship between Epass and V is given by

eV = Epass

(R2

R1

− R1

R2

). (4.2)

The voltage on the inner and outer hemispheres, Vin and Vout, are given by

eVin = eVR + 2Epass

(R0

R1

− 1

), (4.3)

eVout = eVR + 2Epass

(R0

R2− 1

). (4.4)

Then one can calculate VR and Epass from the voltages of the outer hemi-

sphere (Vout), the inner (Vin) and the entrance slit (VR) using these expres-

sions because R0, R1 and R2 are constants which depend on the size of the

analyzer.

4.2. Crystal growth and surface treatment of Ga1−xMnxN 39

Figure 4.2: Sketch of the hemispherical energy analyzer.

4.2 Crystal growth and surface treatment of

Ga1−xMnxN

Ga1−xMnxN (0001) thin films with x = 0.0, 0.02 and x = 0.042 used in this

study were grown by molecular beam epitaxiy with an RF-plasma nitrogen

source and elemental sources of Ga and Mn on a sapphire (0001) substrate

[4.1]. Figure 4.3 shows the layer structure of the sample. After nitridation

of the substrate, a 3-nm-thick AlN buffer layer was grown on it followed by

the growth of a 100-nm-thick GaN buffer layer. On top of the GaN layer,


Sn-doped 100-nm-thick n-GaN layer was grown to secure the conduction

of the sample. Finally, a 110-nm-thick Ga1−xMnxN epitaxial layer was de-

posited on top of it at substrate temperature 550◦C. The crystal polarity

of the Ga1−xMnxN is determined to be Ga-terminated hexagonal surface by

reflection high-energy electron diffraction (RHEED) [4.1]. All the samples

thus prepared were paramagnetic from room temperature down to 4K.

Figure 4.3: Sample layer structure of the Ga1−xMnxN.

For surface cleaning, we made N+2 ion sputtering followed by anneal-

ing up to 500◦C. Sputtering of the GaN (0001) surface by Ar+ and N+2

ion beams have been investigated by Lai et al. using synchrotron-radiation

photoemission spectroscopy [4.2]. For Ar+ ion sputtering, the N atoms are

preferentially removed and a Ga-enriched GaN surface is produced. The ex-

cess Ga atoms on the Ar+ ion sputtered surface aggregate to form metallic

Ga clusters at temperatures above 350◦C as shown in Fig. 4.4(a). On the

other hand, a well-ordered GaN (0001) 1×1 surface can be obtained by N+2

sputtering instead of Ar+. In addition to acting as a sputtering ion, the

N+2 ion serves as a reactant which compensates for the preferential loss of

N atoms caused by the physical bombardment, resulting in the reduction of

deficiency of nitrogen atoms as shown in Fig. 4.4(b). The difference between

the effects of the N+2 and Ar+ ion sputtering also appears in the valence-band

photoemission spectra as shown in Fig. 4.5. For Ar+ sputtering, a shoulder

band appears just below the Fermi level upon annealing the sample above

350◦C. This new band is attributed to the 4s and 4p states of metallic Ga.

The effectiveness of the N+2 ion sputtering for nitride compounds such as

CrN is also reported [4.3]. In our measurements of Ga1−xMnxN, the diff-


Figure 4.4: Ga 3d and N 1s photoemission spectra of the GaN (0001). (a)

Sputtered by 1 kV Ar+ for 10 min at room temperature and then annealed

to 623, 723 and 823 K. (b) Sputtered by 1 kV N+2 for 10 min at room

temperature and then annealed to the indicated temperatures [4.2].

ences between N+2 and Ar+ ion sputtering is confirmed in the photoemission

spectra and low-energy electron diffraction (LEED). Figure 4.6 shows the

valence-band photoemission spectra of Ga0.98Mn0.02N after 2.0 kV N+2 or 1.5

kV Ar+ ion sputtering followed by annealing up to 500◦C. As stated above,

a shoulder band just below the Fermi level and a Fermi edge appear for

Ar+ ion sputtering while they do not appear for N+2 ion sputtering. The

spectrum for Ar+ ion sputtering is broad as a whole compare to that for N+2

ion sputtering. Figure 4.7 shows the LEED pattern of Ga0.98Mn0.02N after

the N+2 or the Ar+ ion sputtering followed by annealing up to 500◦C. The

LEED pattern of the Ar+ sputtered surface is broad and weak than that of

N+2 ion sputtered surface.

In our measurements of Ga1−xMnxN, the cleanliness of the surface was

checked by LEED and core-level XPS measurements. Figure 4.8 shows the

LEED pattern of Ga1−xMnxN (0001). The O 1s and C 1s core-level peaks

were diminished below the detectable limit by repeated N+2 ion sputtering


Figure 4.5: Valence-band photoemission spectra of the GaN (0001) surfaces

sputtered by (a) 1 kV Ar+ and (b) 1 kV N+2 for 10 min at room temperature.

The sputtered GaN samples are subsequently annealed to 623, 723, 823 K

[4.2].

and annealing, and a clear low-energy electron diffraction (LEED) pattern

was obtained, reflecting an ordered clean surface.


Inte

nsity

(ar

b.un

it)

12 10 8 6 4 2 0

Binding Energy (eV)

Ga0.98Mn0.02NAIPES (hν = 45 eV)

2 kV N2+ sputtering

1.5 kV Ar+ sputtering

Figure 4.6: Difference between N+2 and Ar+ ion sputtering in the valence-

band photoemission spectra of Ga0.98Mn0.02N (0001).

Figure 4.7: Difference of between the N+2 and the Ar+ ion sputtering in the

LEED pattern of Ga0.98Mn0.02N (0001) surface.


Figure 4.8: LEED pattern of the Ga1−xMnxN (0001) surface.

References

[4.1] T. Kondo and H. Munakata, Oyobuturi 71, 1274 (2002). (in Japanese)

[4.2] Y.H. Lai, C.T. Yeh, J.M. Hwang, H.L. Hwang, C.T. Chen and W.H.

Hung, J. Phys. Chem. B. 105 10029 (2001).

[4.3] I. Bertoti, M. Mohai, P. H. Mayrhofer and C. Mitterer, Surf. Interface.

Anal. 34, 740 (2002)

Chapter 5

Results and discussion

5.1 X-ray absorption spectroscopy and mag-

netic circular dichroism

We have made an x-ray absorption spectroscopy (XAS) measurement to

obtain the information about the valence state of the Mn ion in Ga1−xMnxN.

In contrast to photoemission spectroscopy, absorption experiment provides

us with a direct insight into the 3d valence electronic structure of Mn since

the dipole selection rule ensures that the Mn 2p core electron is excited to

these states.

Figure 5.1 shows the Mn absorption spectrum of Ga0.958Mn0.042N. The

two groups of peaks are associated with Mn 2p3/2 and 2p1/2 spin-orbit dou-

blet. If the final state is delocalized, the line shape of the absorption edges is

reflects the density of unoccupied states above the Fermi level. However, the

rich structures of the observed spectrum are typical for localized 3d states

such as those of the Ga1−xMnxAs [5.1]. Comparing the line shape observed

here with calculated absorption spectrum, one can obtain the information

about the valence state of a Mn in the Ga1−xMnxN. Calculations for the d3,

d4, d5 … configuration in a tetrahedral crystal field have been done by van

der Laan [5.2] for various magnitude of the crystal field.

Figures 5.2 and 5.3 show the experimental spectrum compared with the

calculations for the d4 and d5 ground states in a tetrahedral crystal field,

respectively. In Fig. 5.2, the calculations for all values of the crystal-field

splitting 10Dq do not correspond to the experimental spectrum, particularly

for the 2p1/2 peak. In contrast, d5 ground state as shown in Fig. 5.3, the

calculation with the values of 10Dq = 0.0 - 0.5 eV well correspond to the

experimental spectrum. This leas us to conclude the experimental spectrum

to be that of the d5 ground state in the tetrahedral crystal field, namely,

the valence state of Mn in the Ga1−xMnxN is close to 2+ and S ∼ 5/2 as

48 Chapter 5. Results and discussion

1.2

1.0

0.8

0.6

0.4

0.2

0norm

. tot

al y

ield

(ar

b.un

it)

660655650645640Photon Energy (eV)

experimental back ground

Ga0.958Mn0.042N XAS (T = 80K)

Mn 2p3/2

Mn 2p1/2

Figure 5.1: Absorption spectrum of the Mn 2p in Ga0.958Mn0.042N at 80 K.

The two peaks are associated with Mn 2p3/2 and 2p1/2.

in Ga1−xMnxAs [5.1]. This also indicates that the Mn ion acts an acceptor

in the host GaN and produces a hole, consistent with the previous report

[5.3, 4].

We have also measured magnetic circular dichroism in x-ray absorption

spectroscopy. This is called x-ray magnetic circular dichroism (XMCD).

Its utmost strength is the element-specific, quantitative determination of

the spin and orbital magnetic moments. Figure 5.4 shows the Mn 2p core

absorption spectra excited with circular polarized light of either helicity at

80K in an applied field of 2 T. The spectra taken with different light helicity

show differences, showing circular dichroism, especially at 2p3/2 absorption

peak. The circular dichroism is shown in the lower panel of Fig. 5.4. We

obtain the value of 3.8% at the 2p3/2 peak.

According to the XMCD sum rules [5.5, 6] as indicated by Eq. 3.7 and

3.8 in chapter 3, one can estimate the spin and orbital magnetic moment of

the Mn ion in Ga1−xMnxN. The sum rules are

mspin = −6∫L3

(I+ − I−)dω − 4∫L3+L2

(I+ − I−)dω∫L3+L2

(I+ + I−)dω, (5.1)

5.1. X-ray absorption spectroscopy and magnetic circular dichroism 49

Inte

nsity

(ar

b.un

it)

660655650645640635

Photon Energy (eV)

Ga0.958Mn0.042N XAS comparison (d

4)

Mn 2p1/2

Mn 2p3/2

10Dq = 1.5

1.0

0.5

0.0

experimental

Figure 5.2: Comparison between the experimental spectrum and calcula-

tions which assume tetrahedral coordination and the d4 ground state for

Ga1−xMnxN. [5.2]. The experimental spectrum do not corresponds to the

calculated spectrum for all the values of 10Dq adopted here.


Inte

nsity

(ar

b.un

it)

660655650645640635Photon Energy (eV)

10Dq = 1.5

1.0

0.5

0.0

experimental

Mn 2p3/2

Mn 2p1/2

Ga0.958Mn0.042N XAS comparison (d 5)

Figure 5.3: Comparison between the experimental spectrum and calcula-

tions which assume tetrahedral coordination and the d5 ground state for

the Ga1−xMnxN. [5.2]. The experimental spectrum well corresponds to the

calculated spectra for the values of 10Dq = 0.0 - 0.5 eV.

5.1. X-ray absorption spectroscopy and magnetic circular dichroism 51

Inte

nsity

(ar

b.un

it)

660655650645640635

Photon Energy (eV)

µ+

µ−

(µ+ − µ−) x 10

Ga0.958Mn0.042N XMCD (T = 80K, B = 2T)

XMCD

Figure 5.4: Absorption spectra of Mn 2p in Ga0.958Mn0.042N excited by po-

larized light, measured at 80 K in an applied field of 2 T. The lower panel

shows the XMCD.

morb = −4∫L3+L2

(I+ − I−)dω

3∫L3+L2

(I+ + I−)dω(10 − n3d)

(1 +

7〈TZ〉2〈SZ〉

)−1, (5.2)

where∫L3

and∫L3+L2

are the values of the integral in the 2p3/2 region and

the 2p3/2 plus 2p1/2 region, respectively. The energy range of the 2p3/2 and

the 2p3/2 plus p1/2 are chosen as 635 - 648 and 635 - 660 eV, respectively.

The XAS and XMCD spectra and their integral are shown in Fig. 5.5.

The obtained values of the spin and orbital moments aremorb ∼ 0.0 µB/atom

and mspin ∼ 0.14 µB/atom, respectively. The value of morb ∼ 0.0 µB/atom

is consistent with the fact that the high spin d5 system should be isotropic

because all the d orbital are occupied by an electron with up spin. From

the vales of the magnetic moment thus obtained, magnetization per unit

volume is estimated to be 2.7 emu/cm−3. This value is compared with the

value obtained by a magnetization measurements using a Superconducting

Quantum Interference Device (SQUID) magnetometer as shown in Fig. 5.6.

The value of the magnetization obtained by SQUID is 2.5 emu/cm−3 sim-

ilar to the magnetization obtained by XMCD. From these measurements

and subsequent estimation of the magnetization, it is confirmed that Mn in

Ga1−xMnxN is divalent and are the paramagnetic.


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660655650645640635Photon Energy (eV)

µ+

µ−

(µ+ − µ−) x10 integral

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660655650645640635Photon Energy (eV)

(µ+ + µ−) integral

Ga0.958Mn0.042N XAS (T = 80K)

Ga0.958Mn0.042N XMCD (T = 80K, B = 2T)

Figure 5.5: Integral of Mn 2p XAS and XMCD spectra for Ga0.958Mn0.042N.

5.2. X-ray photoemission spectroscopy 53

Figure 5.6: Magnetization curve of Ga0.958Mn0.042N measured by SQUID.

The value at 80 K in an applied field 2 T is about 2.5 emu/cm−3.

5.2 X-ray photoemission spectroscopy

So far the electronic structure of Ga1−xMnxN has not been studied exper-

imentally. To see whether there is a possibility of ferromagnetism in this

system, it is desirable to study its electronic structure.

Figure 5.7 shows the N 1s core-level photoemission spectra of x = 0,

0.02 and 0.042. Cleary, the peak position of N 1s is shifted towards lower

binding energies with increasing Mn concentration. The relative values of

the shift from the N 1s core level in GaN is 0.5 eV for x = 0.02 and 0.9 eV

for x = 0.042. The energy shift is also observed in the valence-band spectra,

as shown in Fig. 5.8. The amount of the shift in the valence-band spectra

is the same as the shift of the N 1s peak. This indicates that the Fermi

level is shifted downward with Mn doping, that is, the doped Mn atoms

supply holes into the n-type semiconductor. In the valence-band spectrum,

one can see that in going from x = 0.0 to x = 0.042, a new feature is created

above the valence band maximum (VBM) as denoted by shaded area in Fig.

5.8. This Mn-induced change is considered due to the appearance of Mn 3d

character because in XPS measurement the relative cross section of Mn 3d

to N 2p is large.


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404 402 400 398 396 394 392

Binding Energy (eV)

Ga1-xMnxN N 1s

x = 0.0

x = 0.02

x = 0.042

hν = 1253.6 eV

Figure 5.7: N 1s core-level photoemission spectra. The peak position of the

N 1s is shifted toward lower binding energy with increasing Mn concentra-

tion.

5.2. X-ray photoemission spectroscopy 55

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10 5 0

Binding Energy (eV)

x = 0.0

x = 0.02

x = 0.042

Ga1-xMnxN hν = 1253.6 eV

Figure 5.8: Valence-band photoemission spectra. The shift is also observed

with increasing Mn concentration as in the N 1s core level. The amount of

the shift in the valence-band spectra is same as the shift of the N 1s peak.

The shaded area indicates the change above the valence-band maximum

caused by increasing the Mn concentration.


Figure 5.9 shows the Mn 2p core-level spectra of the x = 0.042 sample.

The broad peak at 667 eV is due to Mn L2,3M2,3M4,5 Auger emission. The

spectrum shows a spin-orbit doublet, each component of which shows a

satellite structure on the higher binding energy side separated by ∼ 5 eV.

The presence of the satellite structure indicates strong Coulomb interaction

among the 3d electrons and strong hybridization between the 3d electrons

and the valence orbitals. We have analyzed the spectrum using the CI cluster

model to obtain the electronic structure parameters. We have assumed a

tetrahedral MnN4 cluster with the central Mn atom. Here the distortion from

the tetrahedron in the wurtzite structure is ignored because the magnitude

of the distortion is small. The calculated spectrum has been broadened with

a Gaussian and a Lorentzian. We have assumed that the valence of the Mn

is 2+ as indicated by the XAS study. We have also ignored additional holes

which enter into the top of the valence band of GaN because the carrier

concentration is negligibly small, based on the fact that the samples are

highly resistive, presumably due to charge compensation [5.4]. The satellite

structure is well reproduced with parameter values ∆ = 4.0 eV ± 1.0, U

= 6.0 ± 1.0 eV and (pdσ) = 1.5 ± 0.1 eV. In the bottom panel of Fig.

5.9, the calculated spectrum has been decomposed into the 2pd5 and 2pd6L

components of the final state configurations. Excited core-hole states, which

corresponds to the satellite, consist of 2pd5 configuration.

5.3 Resonant photoemission spectroscopy

We have also investigated the valence band using RPES. RPES is a power-

ful technique to extract the Mn 3d derived photoemission spectrum in the

valence band. For RPES, the Mn 3p-to-3d absorption occurs at the photon

energy above 50 eV. Interference between the normal photoemission and

3p-to-3d transition followed by a 3p-3d-3d super-Coster-Kronig decay gen-

erates a resonance enhancement of the Mn 3d-drived photoemission. From

such measurements, one can obtain the resonantly enhanced Mn 3d partial

density of state (PDOS).

Figure 5.10 shows the absorption spectra of Ga1−xMnxN. One can see

that in going from x = 0.0 to 0.042, a peak at 50 eV appears and grows

in intensity. This peak represents the Mn 3p-to-3d absorption. From the

absorption spectrum, on-resonance and off-resonance photon energies are

found to be 50 and 48.5 eV, respectively.

Figure 5.11 shows the valence-band spectra of the x = 0.042 sample

taken at various photon energies in the Mn 3p-to-3d core excitation region.

The intensities have been normalized to the photon flux. All binding en-


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664 660 656 652 648 644 640 636

Binding Energy (eV)

experimental calculation

Ga0.958Mn0.042Nhν = 1253.6 eV

Mn LMV Auger

Mn 2p1/2

2pd 5

2pd 6L

Mn 2p3/2

Figure 5.9: Mn 2p core-level (dots) XPS spectrum of Ga0.958Mn0.042N and

its CI cluster-model calculations (solid line). In the bottom panel, the calcu-

lated spectrum has been decomposed into the 2pd5 and 2pd6L components

of the final-state configurations.


Figure 5.10: Absorption spectra for x = 0.0, 0.02 and 0.04 of Ga1−xMnxN

recorded by the total electron yield method. The peak at 50 eV is due to

the Mn 3p-3d absorption.

ergies are referenced by the Fermi level (EF ). In going from hν = 47 to

50 eV, one can see that the peak at the binding energy of 5 eV grows in

intensity. By subtracting the off-resonance spectrum from the on-resonance

one, we obtained the Mn 3d-derived spectrum as shown in the bottom panel

of Fig. 5.11. Here, the photon energy dependence of the cross section of

the N 2p atomic orbital has been considered. The difference spectrum that

corresponds to the Mn 3d PDOS reveals a peak at EB = 5 eV and a shoulder

at EB = 2 eV. A satellite also appears at EB = 9 - 13 eV, at a higher finding

energy than that of Ga1−xMnxAs [5.8]. The Mn 3d PDOS thus obtained

is compared with other Mn-doped DMS’s as shown in Fig. 5.12. The line

shape of the Mn 3d PDOS in the GaxMnxN is close to those of III-V DMS’s

such as In1−xMnxAs and Ga1−xMnxAs rather than that of Zn1−xMnxO. If

one compares the Mn 3d PDOS with that deduced by the first-principles


Table 5.1: Electronic structure parameters for substitutional Mn impurities

in semiconductors and estimated p-d exchange constant Nβ for Mn2+. ∆,

U and (pdσ) given in units of eV.

Material ∆ U (pdσ) Nβ Reference

Ga1−xMnxN 4.0 6.0 -1.5 -1.6

Ga1−xMnxAs 1.5 3.5 -1.0 -1.0 [5.10]

In1−xMnxAs 1.5 3.5 -0.8 -0.7 [5.10]

Zn1−xMnxO 6.5 5.2 -1.6 -2.7 [5.11]

Zn1−xMnxS 3.0 4.0 -1.3 -1.3 [5.11]

Zn1−xMnxSe 2.0 4.0 -1.1 -1.0 [5.11]

Zn1−xMnxTe 1.5 4.0 -1.0 -0.9 [5.11]

Cd1−xMnxS 3.0 4.0 -1.0 [5.12]

Cd1−xMnxSe 2.5 4.0 -1.0 [5.12]

Cd1−xMnxTe 2.0 4.0 -1.0 [5.12]

LDA calculation [5.7], the main feature EB = 1 - 8 eV can be reproduced

qualitatively whereas the satellite cannot be explained by the same calcula-

tion.

The CI cluster-model calculation can well explain not only the main

structure but also the satellite, as shown in Fig. 5.13. As shown in the

bottom panels of 5.13, the main peak largely consists of d5L final state

and the satellite consist d5 final states. The CI calculation reproduces the

difference spectra with identical parameters as in the case of Mn 2p core-

level within the error bars, ∆ = 4.0 ± 1.0, U = 6.0 ± 1.0, (pdσ) = -1.5 ±0.1 eV.

Using the electronic structure parameters ∆, U and (pdσ) thus obtained,

one can estimate the p-d exchange constant Nβ for the Mn2+ ion in the GaN

host by the second order of perturbation with respect the hybridization term.

As indicated by Eq. 5.3 in chapter 3, Nβ is given by

Nβ = −16

S

(1

−δeff + Ueff+

1

δeff

)(1

3(pdσ) − 2

√3

9(pdπ)

)2

. (5.3)

The value ofNβ thus estimated is -1.6 eV, much larger than that of Ga1−xMnxAs

[5.8]. This result is consistent with the theoretical study by Dietl et al. [5.9].

The electronic structure parameters, ∆, U and (pdσ), p-d exchange

constant Nβ are listed in Table 5.1 with those of other Mn-doped DMS’s.

The value of each parameter for Ga1−xMnxN is large compare with those


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12 8 4 0Binding Energy (eV)

Ga0.958Mn0.042NAIPES

51

50

48.5

47

difference 50 - 48.5 eV

hν=53 eV

Figure 5.11: A series of photoemission spectra of Ga0.958Mn0.042N for various

photon energies around the Mn 3p-3d core excitation threshold. the differ-

ence between the on-resonant (hν = 50 eV) and the off-resonant (hν = 48.5

eV) spectra is shown in the bottom panel.


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12 8 4 0Binding Enegy relative to VBM (eV)

In0.84Mn0.16As

Ga0.931Mn0.069As

Ga0.958Mn0.042N

Zn0.93Mn0.07O

Mn 3d PDOS

Figure 5.12: Mn 3d PDOS in various DMS’s. The line shape of the Mn

3d PDOS in the GaxMnxN is close to those of the III-V DMS’s such as

In1−xMnxAs and Ga1−xMnxAs [5.10] rather than that of Zn1−xMnxO [5.11].


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14 12 10 8 6 4 2 0Binding Energy (eV)

experimental calculation

Ga0.958Mn0.042N Mn 3d PDOS

d 4

d 6L

2

d 5L

Figure 5.13: Mn 3d partial density of state (PDOS) of Ga1−xMnxN for the x

= 0.042 sample obtained by RPES (dots) and its CI cluster-model analysis

(solid line). In the bottom panel, the calculated spectrum is decomposed in

to d4, d5L and d6L2 components of final-state configurations.


of other III-V and II-VI DMS’s, where ∆, U and (pdσ) are 1.5, 4.0, -1.0

eV for Ga1−xMnxAs [5.10], 1.0, 3.5, -0.8 eV for In1−xMnxAs [5.10], 2.0, 4.0,

-1.1 eV for Zn1−xMnxSe [5.11] and 3.0, 4.0, -1.2 eV for Cd1−xMnxSe [5.12].

The present parameters are close to those for Zn1−xMnxO [5.11] for which

∆, U and (pdσ) are 6.5, 5.2 and 1.6 eV, respectively. The charge-transfer

energy ∆ increases with increasing electronegativity of the anion. The one-

electron transfer integral (pdσ) increases with decreasing distance between

the transition-metal atom and the anion atom. The large value of U for

Ga1−xMnxN may be attributed to the low polarizability of the N atom.

While the differences between ∆ and U are similar to those for Ga1−xMnxN,

Ga1−xMnxAs and In1−xMnxAs, contribution of (pdσ) becomes substantial in

Ga1−xMnxN and gives the large Nβ. One of the reasons why ferromagnetism

does not occur in the Ga1−xMnxN samples used in this study in spite of the

large p-d exchange constant Nβ may be attributed to the lack of hole carrier

which mediate ferromagnetic coupling between Mn ions because of charge

compensation in Ga1−xMnxN [5.4].

References

[5.1] H. Ohldag, V. Solinus, F.U. Hillebrecht, J.B. Goedkoop, M. Finazzi,

F. Matsukura and H. Ohno, Appl. Phys. Lett. 76, 2928 (2000).

[5.2] G. van der Laan and I.W. Kirkman, J. Phys. :Condens. Matter. 40,

4189 (1992).

[5.3] Y.L. Soo, G. Kioseoglou, S. Kim, S. Huang, Y.H. Kao, S. Kuwabara,

S. Owa, T. Kondo and H. Munekata, Appl. Phys. Lett. 79, 3926

(2001).

[5.4] T. Kondo, S. Kuwabara, H. Owa and H. Munekata, J. Cryst. Growth.

237, 1353 (2002).

[5.5] B.T. Thole, P. Carra, R. Sette and G. ven der Laan, Phys. Rev. Lett.

68, 1943 (1992).

[5.6] P. Carra, B.T. Thole, M. Altarelli and X. Wang, Phys. Rev. Lett. 70,

694 (1993).


(2001)

[5.8] J. Okabayashi, A. Kimura, T. Mizokawa, A. Fujimori, T. Hayashi and

M. Tanaka, Phys. Rev. B. 59, 2486 (1999).

[5.9] T. Dietl, H. Ohno, F. Matsukura, J. Cibert, and D. Ferrand, Science

287, 1019 (2000); T. Dietl, H. Ohno, and F. Matsukura, Phys. Rev.

B. 63, 195205 (2001).

[5.10] J. Okabayashi, T. Mizokawa D.D. Sarma and A. Fujimori, Phys. Rev.

B. 65, 161203 (2002).

[5.11] T. Mizokawa, T. Nambu, A. Fujimori, T. Fukumura and M. Kawasaki,

Phys. Rev. B. 65, 085209 (2002).

[5.12] T. Mizokawa and A. Fujimori, Phys. Rev. B. 48, 14150 (1993).

Chapter 6

Summary

The electronic structure of the III-V diluted magnetic semiconductor

Ga1−xMnxN was studied by high-energy spectroscopy. The x-ray absorption

spectroscopy study has revealed that Mn in Ga1−xMnxN is divalent and acts

as an acceptor and produces a hole in the host GaN. From x-ray magnetic

circular dichroism, it is shown that these Mn2+ ions are responsible for the

paramagnetism of Ga1−xMnxN. The photoemission spectroscopy and sub-

sequent configuration-interaction cluster-model analysis have revealed that

the electronic structure of Ga1−xMnxN can be treated as a many-electron

system. The p-d exchange constant Nβ is found to be larger than that of

Ga1−xMnxAs. Althogh it is still unknown whether the p-d exchange cou-

pling between the magnetic ions is substantial or not, Ga1−xMnxN would

be a promising candidate for a room temperature ferromagnetic DMS as

predicted theoretically if a high hole concentration were realized.

Acknowledgement

It is my great pleasure to express my special gratitude to the following people

for their help concerning my master thesis.

First of all, I would like to express my deepest gratitude to Prof. Atsushi

Fujimori for having introduced me into the field of photoemission spectros-

copy. His gentle and clear advice, together with his deep insight into physics,

always encouraged me in a delightful way.

I would like to thank Prof. Takashi Mizokawa for his kind guidance, and

valuable discussion. In particular, I owe a great deal in the cluster-model

calculation to him.

The experiments at Photon Factory were supported by a number of

people. I am particularly indebted to the members of Kinoshita group, Dr.

T. Okuda, Ms. A. Harasawa, Dr. T. Wakita, and Prof. T. Kinoshita, for

their valuable technical support during the beamtimes. The experiments at

Spring-8 were supported by Dr. J. Okamoto and Dr. K. Mamiya for their

helpful technical support during the beamtime.

Let me also thank Prof. H. Munekata, Dr. T. Kondo and Mr. H. Owa.

They provided us with such interesting and excellent samples of Ga1−xMnxN

with the valuable advice and discussions. The perfect quality of the samples

based on their skilled crystal growth technique provided us with useful and

trustworthy data.

The life during the master course is commemorated with the joyful

members of the Fujimori group : Dr. K. Okazaki, Mr. K. Tanaka, Mr. H.

Yagi, Mr. H. Wadachi, Mr. K. Ebata, Mr. M. Kobayashi, Mr. M. Takizawa,

Mr. M. Hashimoto, and Ms. Y. Shimazaki. Mr. Y. Ishida helped me at

Photon Factory and his energetic attitude toward the experiment encouraged

me during the beamtime. M. H. Yagi helps how to operate photoemission

instruments and maintenance of the photoemission spectroscopy equipment

from my early stage of the master course.

I would also like to thank members of the Mizokawa group : Dr. J.-Y.

Son, Dr. J. Quilty, Mr. D. Asakura, Mr. M. Ikeda, Mr. N. Ueda, Ms. M.

Kurokawa, Mr. S. Hirata, Mr. T.-T. Tran, Mr. A. Shibata, Mr. K. Takubo,

Mr. Y. Fujii. Mr. S. Hirata and Mr. K. Takubo supported me during the

70 Chapter 6. Summary

experiment at Mizokawa laboratory.

I cannot miss the contribution of alumni of : Dr. J. Okabayashi, Dr. T.

Yoshida. Especially I like to thank Dr. J. Okabayashi for a lot of advice.

I like to thank the University of Tokyo which has opened a way for me

and my junior to study after having graduated from Korea University and

given me a chance to study in the splendid environment.

Finally, I would like to express my special thanks to my brother and

my parents with my respect.

January 2004,

Jong-Il Hwang

high-energy spectroscopic study of the iii-v diluted...

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