Structural, magnetic and transport study of DBPLD fabricated magnetic semiconductors 1

CHAPTER 1: INTRODUCTION CHAPTER 1 INTRODUCTION 1.1 Research Background 1.1.1 Motivation to Study Diluted Magnetic Semiconductors DMSs Nowadays, almost no one can escape from computer

CHAPTER 1: INTRODUCTION

CHAPTER 1 INTRODUCTION

1.1 Research Background

1.1.1 Motivation to Study Diluted Magnetic Semiconductors (DMSs)

Nowadays, almost no one can escape from computer usage, and it plays more and more important roles in our lives Two of the most important technologies involved in computers are data processing and storage In data processing, silicon integrated circuits (ICs) are established on semiconductor materials, in which semiconductor devices generally take advantage of the charge of electrons In contrast, magnetic materials are used for data storage involving the spin of electrons

In 1996, H Ohno first published his results of preparing a GaAs-based diluted magnetic semiconductor by molecular beam epitaxy (MBE) [1] and proposed the possibilities to make use of not only the charge but also the spin degree of freedom in modern semiconductor electronics for information processing [2] This led to a new study field called spintronics With the belief in the possibility of using the spin degree

of freedom of charge carriers for the design of electronic devices with new functionalities [2, 3], spintronics has recently received considerable attention and is developing quickly The physical fundamentals of new generation devices combining standard microelectronics with spin-dependent effects arise from the interaction between spin of the carrier and magnetic ions in the material [3] Thus, it is possible to

CHAPTER 1: INTRODUCTION combine information processing and storage functionalities in one material Furthermore, semiconductor technology has experienced a continuous reduction in its working dimension Current electronic devices are getting smaller and smaller in dimension Spins of carriers become increasing important in the small devices because they can provide new functionality that can be integrated into existing semiconductor devices by combining the dissimilar properties of ferromagnetism and semiconductivity for applications ranging from nonvolatile memory to quantum computation This will give rise to revolutionary change in computer technology with increased processing speed, storage density, and even new functions

Ferromagnetic semiconductors are anticipated to be an enabling component of the next-generation spintronic devices [3] The history of magnetic semiconductors began from the late 1960’s aiming at the realization of new functionality by combining electrical transport and magnetism In spite of numerous studies, less practical applications of magnetic semiconductors has been realized Until recently, the discovery of the carrier-mediated ferromagnetism in (In,Mn)As [4] and (Ga,Mn)As [1,

2, 5] made it possible to combine complementary properties of semiconductor quantum structures and ferromagnetic systems in single devices These discoveries promoted the researches of magnetic semiconductors to fundamental materials for spintronics

DMSs are semiconductors which contain some magnetic ions as impurities in their host lattices The DMS system provides practical means to incorporate spin into semiconductor electronics, and they are expected to pave the way for the development

CHAPTER 1: INTRODUCTION

of functional semiconductor spintronics [5, 6, 7, 8] Hence，it is important for the materials to remain semiconductive properties while magnetic ions are incorporated Namely, materials with peculiar combination of ferromagnetism and semiconductive properties are required for spintronic devices In this way, novel functionalities could

be achieved if the detection of carrier spins can be controlled optically or electrically Some peculiar properties of carrier induced ferromagnetism have been reported based

on these (III,Mn)V DMSs materials mentioned above For example, a III–V

semiconductor, Mn-doped GaAs, has a Curie temperature (TC) about 110 K due to the strong p–d exchange interaction by the mobile holes [2] The magnetic element Mn has

been introduced into the nonmagnetic host-lattice of GaAs, which are widely used in semiconductor electronics, in excess of its solubility limit by low-temperature MBE [2] In this homogeneous alloy, Mn occupies Ga sites and provides magnetic moments

as well as holes, which makes (Ga,Mn)As conducting The hole-mediated ferromagnetic interaction results in ferromagnetism

Light-induced ferromagnetism was also reported by S Koshihara [9] The structure of the inducement of a ferromagnetic order by photogenerated carriers in a

novel III-V–based magnetic semiconductor heterostructure p-(In,Mn)AsyGaSb grown

by molecular beam epitaxy is shown in Fig 1-1(a) [9] It is experimentally observed that light-induced changes in Hall resistivity curves at 5 K Before the irradiation, the Hall resistivity changes nonlinearly with an external magnetic field with no hysteretic behavior, as shown by the dashed line plotted in Fig 1-1(b) After light irradiation, a hysteresis loop develops, as shown by the solid line in Fig 1-1(b), reflecting that the

CHAPTER 1: INTRODUCTION Hall resistivity directly correlates with magnetization through the skew scattering It

has been established that the ferromagnetic order in the p-(In,Mn)As is induced by the

presence of excess holes Hence, it shows an experimental evidence for a ferromagnetic order induced by photogenerated holes in this heterostructures Based

on these results, by means of photogenerated carriers, the strength of ferromagnetic spin exchange can be controlled by changing the hole concentrations in (In, Mn)As/(Ga, Al)Sb heterostructures

(a) (b)

Fig 1-1(a) Structure of the sample Direction of light irradiation is shown by an arrow;

(b) Hall resistivity ρHallobserved at 5 K before (dashed line) and after (solid line) light irradiation, showing that theρHallcorrelates with magnetization directly through the skew scattering [9]

H Ohno et al [10] show electric-field control of ferromagnetism in a thin-film semiconducting alloy, using an insulating-gate field-effect transistor structure, as shown in the insets of in Fig 1-2 Under the condition of different gate biases which correspond to different directions of depletion of holes, hysteresis loop in Hall

CHAPTER 1: INTRODUCTION resistivity was observed to transformed from ferromagnetic to paramagnetic as a response with the depletion of the carriers, as shown in Fig 1-2 Besides these reports, injection of polarized spins into the semiconductor [6, 7] have also been reported These features are a consequence of the controllable carrier density for the ferromagnetic semiconductors

Fig 1-2 Hall resistance versus field curves under three different gate biases The insets

show the structure of the sample, magnetic semiconductor (In, Mn)As field-effect transistors[10]

However all these peculiar features can only be exhibited at low temperature in a

cryostat, as the highest TC of DMSs obtained to date is about 170 K in Ga 1-xMnxAs

[11] From the application point of view, a ferromagnetic TC beyond room temperature

is strongly required The development of practical semiconductor spintronics devices

will thus require the development of new DMSs with TC above room temperature

CHAPTER 1: INTRODUCTION

In this study field, much attention has been paid to wide band gap semiconductors,

because theoretical calculations predicted that the Tc of DMSs based on Ⅲ- (GaN) Ⅴand Ⅱ- (ZnO) Ⅵ compounds could be raised to above 300K [12] Ab initio band calculations by Sata [13] also predicted the stability of ferromagnetism (FM) in p-type

Zn1-xMnxO, and antiferromagnetism (AF) in n-type Zn1-xMnxO Similar calculations

predict a FM phase for both carrier-undoped and n-type ZnO substituted with Fe, Co,

or Ni [14]

Spurred by these predictions, a large number of research groups are now working

in this field Besides the GaN and ZnO based compounds, many kinds of materials, including Mn doped Ⅱ- and Ⅵ Ⅲ- compounds semiconductors have been Ⅴextensively studied, such as Ga1-xMnxN [15], Ga1-xMnxAs [1], (In,Mn)As [16],

Cd1-xMnxTe [17], Zn1-xMnxO(M=TM) [18] and Zn1-xMnxO [19] Ⅱ-Ⅵ compounds semiconductor includes a variety of compounds consisting of various combinations of

Ⅱ-group cations (such as Zn and Cd, etc.) and group- anions (such as O and Te, etc.) Ⅵ[18-26]

During this period, several reports of high Tc ferromagnetic DMSs appeared,

including room temperature ferromagnetism in Co-doped TiO2 [27], ZnO based compounds [18] , GaN based compounds [28] and SnO2 [29] From the physical point

of view, oxide semiconductors can be host compounds for magnetic semiconductors since the capability of high electron doping and the rather heavy effective electron

mass for the oxide semiconductors could supply the possibility to realize high TC [22] Room temperature ferromagnetic semiconductors, anatase and rutile phase Co-doped

CHAPTER 1: INTRODUCTION TiO2 were discovered with a combinatorial approach [27, 30] Many studies have

emerged on the discovery of high TC ferromagnetic oxide semiconductors so far, including ZnO based DMSs [18, 31, 32] These reports excited the hope for the development of practical semiconductor spintronics technologies

Among the above mentioned oxides, ZnO based diluted magnetic semiconductors, considered to be potential room temperature dilute magnetic semiconductors attracted much more interesting The first ZnO based DMS was reported by T Fukumura, Tokyo Institute of Technology of Japan, in which Mn-doped ZnO was fabricated as a new class of II–VI magnetic semiconductor [22] Some of the properties were similar

to typical magnetic semiconductors [19, 22, 33]: the absorption due to d–d transition of

the Mn ion, and the large magnetoresistance at low temperature Moreover, the spin glass magnetic behavior of ZnO doped with the other transition metals (TM) synthesized with a combinatorial approach were observed [34, 35] After that, there have been many reports on the fabrication of transition-metal-doped ZnO Both bulk and thin film specimens have been synthesized So far, Ti-, V-, Cr-, Mn-, Fe- , Co-, Ni-doped [22, 34-41], as well as (Mn, Sn)-doped [42], (Fe, Co)-doped [43] and (Fe, Cu)-doped [44] ZnO have been reported

Currently, a lot of experimental and theoretical researches focused on dilute magnetic semiconductors are based on ZnO doped with transition metal ions Among

3d transition metals, Co-doped and Mn-doped ZnO have been commonly considered to

be ferromagnetic

In view of dopant, Co is a good candidate dopant due to its high spin state and high

CHAPTER 1: INTRODUCTION solubility limit in ZnO Though many experimental results have been obtained on Co-doped ZnO materials, there exist discrepancies pointed out by different groups for the magnetic properties The materials were deemed to be ferromagnetic [37, 45] and nonferromagnetic [46-48]

There are many similar reports on ferromagnetic behaviors for Co doped ZnO For example, Ueda group [18] reported this ferromagnetic behavior, but with low reproducibility It was reported that Zn1-x(Co0.5Fe0.5)xO could enhance the ferromagnetism [47] W Prellier et al synthesized high-quality Co-doped ZnO thin films using the pulsed laser deposition (PLD) technique on (0 0 0 1)-Al2O3 substrates [37] The Zn1-xCoxO films exhibit ferromagnetism with a Curie temperature close to

room temperature for x = 0.08 and at 150K for x = 0.05 Zn 1-xCoxO films with the atomic fraction in the range 0.035–0.115 prepared by sputtering show ferromagnetic behavior with Curie temperatures higher than 350 K [49] Hyeon-Jun Lee et al [45] characterized Zn1-xCoxO powder and thin films fabricated by the sol–gel process and

found that for x less than 0.25, no secondary phase was observed The Co-doped ZnO

thin film showed ferromagnetism above 350 K [45] Dana A [50] demonstrated the reversible 300K ferromagnetic ordering in a DMS, achieved in Co2+:ZnO

However there were also different magnetic behaviors for Co-dope ZnO Sometimes, even for the compounds with similar compositions, the magnetic behaviors were reported to be different For example, Jae Hyun Kim et al [36] characterized Zn1-xCoxO (x = 0.25) films grown on sapphire (0001) substrates by pulsed laser deposition The homogeneous Zn1-xCoxO (x = 0.25) film show spin-glass

CHAPTER 1: INTRODUCTION behavior at low temperature and high temperature Curie–Weiss behavior with a large negative value of the Curie–Weiss temperature, indicating strong antiferromagnetic exchange coupling between Co ions in Zn1-xCoxO It is thought that DMS properties will not be produced for the homogeneous bulk samples of Zn0.9Co0.1O [47] Cobalt-doped ZnO (Zn1-xCoxO) thin films prepared by reactive magnetron cosputtering were reported by Zhigang Yin, et al [51] In their opinion, ferromagnetism can be realized in Zn1-xCoxO without carrier incorporation

As for this situation, the reports for the studies on the Co-doped ZnO materials are considered to be controversy Hence the origin of magnetism has been studying since

then In this study field, studies on the sp-d exchange interactions are important Many results were reported to show the sp-d exchange interactions in the Co-ZnO system

For example, Co–ZnO inhomogeneous magnetic semiconductor were synthesized

on the subnanometer scale [38] Based on their experimental results, room temperature ferromagnetism and large negative magnetoresistance was found at room temperature,

as shown in Fig 1-3 It is thought that the large negative magnetoresistance may be related to spin-dependent hopping and the magnetic-field-induced change in the localization length

CHAPTER 1: INTRODUCTION

Fig 1-3 Dependence of sheet resistance R and the MR ratio on the magnetic field

applied measured at 4.8 K (a) and 293 K (b), respectively [38]

Similar results was reported by [35], in which combinatorial laser molecular-beam epitaxy method was employed to fabricate epitaxial ZnO thin films doped with Co-doped samples After examined the magnetoresistance in laser-deposited

Zn1-xCox O:Al (x = 0.02–0.25) thin films, the observed MR features of the samples with different Co contents were explained in terms of the weak localization, s–d

exchange coupling between the conducting electrons and localized spins of magnetic

Co ions, and spin–disorder scattering [52] K Andoa et al [53] reported the observation of huge magneto-optical effects in Zn1-xCoxO films, showing a strong

mixing of the sp bands of the host ZnO with Co2+ d orbitals, indicating that Zn CoxO

CHAPTER 1: INTRODUCTION

is a diluted magnetic semiconductor with wide optical band

In contrast, the ferromagnetic behaviors of Co-doped ZnO were also questioned For instance, S C Wi et al [47] studied electronic structures of homogeneous bulk samples of Zn0.9Co0.1O using photoemission spectroscopy and x-ray absorption spectroscopy, and considered that the DMS properties observed in Zn1-xCoxO thin films are likely to be extrinsic Zn1-xCoxO thin films were paramagnetic for x < 0.12

and room-temperature ferromagnetic for x > 0.12 However the ferromagnetism

originates from the nanometer-sized Co clusters

The controversial results between research groups suggest that the ferromagnetism strongly depends on the technical methods for the fabrication of the samples In our view, the possible reasons may be due to the sensitivity of properties to the microstructures or second phases involved Hence it is preferred to prepare the materials under the identification conditions

1.1.2 Fabrication Method

There exist various techniques for fabricating magnetic oxide semiconductors [54] Due to relatively high thermal equilibrium solubility of transition metal in ZnO, fabrication of the bulk specimen is sometimes possible The thermal nonequilibrium processes such as thin film growth by PLD process are usually performed High temperature growth sometimes leads to high crystalline quality of thin film; such a condition may be close to the thermal equilibrium state, where the solubility of the dopant is limited On the other hand, low temperature processing sometimes leads to

CHAPTER 1: INTRODUCTION the appearance of metastable ferromagnetic secondary phases [55-57] Many deposition techniques were employed, such as MBE [1, 15, 34], PLD [18, 20-23, 39,58], sputtering [49], chemical vapour deposition [59], and sol-gel method [60], ion implantation [42], molecular beam epitaxy, and solid-state reaction These experiments have been plagued by highly process-dependent properties which often result in low reproducibility PLD has also been used to deposit nanoclusters of various sizes [61,62]

Among the fabrication methods mentioned above, laser ablation is being widely employed on various applications of material processing, such as thin-film deposition, cluster formation, chemical reactions, and surface modifications (synthesis of nanoclusters) [63] In particular, PLD has emerged as one of the versatile techniques for the deposition of thin films of a variety of materials such as metals, semiconductors, and ceramic for various applications [63] Laser-ablated plume is the source of thin films in PLD, which primarily consists of electrons, ions, neutrals, and ionized species

As the plume expands in an ambient medium, the ablated species from the target undergo collisions with the atoms and molecules of the ambient medium, resulting in scattering and slowing down of the plume The peripheral region of the expanding plume consists predominantly of particles of high kinetic energy; the collisional processes are enhanced by high reactivity of the charged species that may assist in the formation of oxides The hydrodynamic expansion of the plume, the composition, and size distribution of clusters depend not only on initial conditions, but also on the laser intensity, pulse width, and ambient gas pressure The growth and deposition of

CHAPTER 1: INTRODUCTION the films are determined by the thermodynamic parameters of the target material and initial conditions such as temperature and substrate materials

Dual beam pulsed laser deposition (DBPLD) is a novel approach to doping impurities in thin films [64] It is also a suitable way to synthesize new materials [65]

In this method, two plumes are generated by laser ablation The components including atoms, molecules, electrons, ions and clusters from targets interact within their overlapping plasma area Hence, a new kind of material can be synthesized on the substrate [65]

1.2 Objective of the Research

As mentioned above, many results of DMSs have been reported However, in particular, there is a great deal of controversy in the magnetic properties of the Co-doped ZnO reported by different groups In the case of TM-doped ZnO thin films, some research groups claim non-ferromagnetic behaviors [22], whereas others hold out the ferromagnetism of similar materials [18] One reason may be due to the poor reproducibility of processing methods The controversial results probably originate from different qualities of thin films fabricated by the different methods, and the properties are micro-structural sensitive Hence, synthesis of materials under identical experimental conditions is helpful Another reason is that the ferromagnetism may even originate from metal clustering [66] Hence, the main issues in this field include determining the distribution of impurities and local magnetization, and the understanding of the magnetic ordering and underlying mechanisms [67]

CHAPTER 1: INTRODUCTION The fabrication of room temperature DMSs is still a great challenge Thus, Co-doped ZnO is one of the candidates to study

Taking account of the above considerations, we propose to fabricate Co-doped ZnO (Zn1-xCoxO) films by the DBPLD method The reasons for choosing this topic are as follows

In the field of DMSs study, DMSs based on ZnO compound semiconductors are the most hopeful candidate for room temperature ferromagnetic semiconductors Co is a good candidate dopant due to its high spin state and high solubility limit in ZnO [34]

On the other hand, DBPLD is a novel approach to dope impurities in thin films [68] It

is also an appropriate way to synthesize new materials [65] As it can synthesize the films in a large range of dopant concentrations uner identical experimental conditions Currently, the films in a large DBPLD provides a relatively convenient way to fabricate DMSs By means of this method, we plan to fabricate room temperature DMSs We will study the stoichiometry and structure dependence properties on processing parameters, and try to reach an available stoichiometry range We will investigate the underlying magnetic mechanisms This research is under the project entitled “Magnetic Semiconductor for Spintronics Materials” with the project code of DSI/03-200001, which was financially supported by A*STAR (Agency for Science, Technology and Research) of Singapore The objectives of this project are summarized

as follows:

(1) DBPLD deposition of Co-doped ZnO thin films

Probably synthesis of single-phase oxide DMS is a must to explore the

CHAPTER 1: INTRODUCTION

magnetic properties arising due to the strong sp–d exchange interaction [69]

DBPLD is a novel approach to dope impurities in thin films There are few successful attempts on synthesizing new materials using a method at present [65]

It is the first time that DBPLD is used to fabricate DMSs Now that ZnO based DMSs has not been conclusively synthesized, hence we will make use of DBPLD

to investigate systematically the influence of the controllable processing parameters on film properties This is our first objective

(2) Study on the structures of Zn1-xCoxO thin films

Motivated by a desire to illuminate the physical mechanisms underlying of magnetic behaviours in DMS, we will study the structure of the thin films, which has a very high potential for elucidating the physics behind the magnetic properties in Zn1-xCoxO thin films

(3) Characterization of the magnetic and electrical properties of Zn1-xCoxO thin films Magnetism and electricity are the most important properties for a DMS In this argument, we shall be concerned about the features of the magnetic and semiconductor properties for the films Are the films ferromagnetic or

nonferromagnetic and what is the Tc? Are they semiconducting, metallic or

insulating, and what are their transport properties? As the properties seem to be micro-structural sensitive, the third objective of this research is to characterize the magnetic and electrical properties of the Zn1-xCoxO thin films with different

Co concentrations prepared under identical experimental conditions In the meantime, some results of samples which were obtained under different

CHAPTER 1: INTRODUCTION experimental conditions and different Co concentrations were also supplied (4) Investigation on the magnetic mechanisms

It is necessary to give our claim for the magnetic behaviours of Zn1-xCoxO thin films Since there is no a consensus on the origin of magnetism, it is important to answer the question on the origin of ferromagnetism after precluding second phases Thus the final objective of this research is to investigate the underlying magnetic mechanisms

1.3 Outline of the Thesis

This thesis is organized as follows:

Chapter 1 and 2 present some auxiliary results and preliminary facts, including an

introduction to the research area, literature review and the scope of our research work The motivation and the related research topics are also highlighted in the chapters In addition, the objective of our research is given

Chapter 3 describes the experimental procedures used in this research, including

the DBPLD experimental system set-up, samples preparations, characterisation methods and equipments

The DBPLD method is discussed in Chapter 4 The effect of substrate position,

growth temperature and vacuum pressure on the properties of Zn1-xCoxO thin films are discussed, and a suggestion of an optimum experimental condition for synthesizing the

Zn1-xCoxO thin films without apparent precipitates is provided The experimental results showed that our attempt to dope Co into ZnO was successful

CHAPTER 1: INTRODUCTION

Chapter 5 presents the results on structural characterizations, including crystal

structures, chemical state and electronic structures of the Zn1-xCoxO thin films For our study on the crystal structures, we employed high-resolution transmission electron microscopy (HRTEM) to verify the existence of Co clusters in the lattice on the nanometer scale Investigations on chemical states and electronic structures, including valence band photoemission spectroscopy (PES), were also used to provide indirect and direct information on the electronic structures From our experimental results, DBPLD grown Zn1-xCoxO films show good crystallinity, particularly for the films with

relative low Co concentrations (x < 0.1) With increasing Co concentration x, apart

from Co atoms substituting the Zn-site in ZnO, some Co atoms locate at the center of the octahedral site rather than at tetrahedral sites

Chapter 6 focuses on the most important properties for a DMS; magnetic and

electrical properties The magnetic and transport behaviours of the Zn1-xCoxO thin films in a large range of Co concentrations were characterized by the vibrating sample magnetometer (VSM), the alternating gradient magnetometer (AGM), the superconducting quantum interference device (SQUID), the Hall effect measurements

and a home-made 4-point R-T (resistance dependence on temperature) system From

our experimental results, we conclude that Zn1-xCoxO with x < 0.1 is a candidate material with both magnetic and semiconductor properties at room temperature However the improvement of magnetism was limited The transport behaviors can be explained by a hopping mechanism The electrical states of Co ions are suggested to be localized

CHAPTER 1: INTRODUCTION

Chapter 7 investigates the possible mechanisms of magnetism Two self-designed

experiments were used to study the magnetic anisotropy and the correlation between

magnetism and carrier density (sp-d interaction) First, we study the role of crystalline

anisotropy by preparing the Zn1-xCoxO thin films with c-axis perpendicular and parallel

to the substrate surfaces Second, using a hand-held UV (ultra-violet) light source in a Hall effect measurement system and an AGM system, we compared the magnetic

behaviours (M-H loops) of the films at different carrier densities to obtain evidence to

show that the magnetism and carrier density are directly correlated The results of an angle dependent abnormal Hall effect experiment are also given In view of our experimental results, we conclude that there is no magneto-crystalline anisotropy in

Zn1-xCoxO thin films, and there is no correlation between the origin of domain and carriers in the Zn1-xCoxO thin films Finally, possible mechanisms for the magnetism are proposed

CHAPTER 2: LITERATURE REVIEW

CHAPTER 2 LITERATURE REVIEW

2.1 Literature Review on Applications of DMSs

DMSs have attracted great interest as potential candidates for materials in various applications of fabrication of new devices, which could increase processing speed and storage density, as they are believed to add a degree of freedom of spin in semiconductors Ferromagnetic semiconductors have emerged as important materials for spintronic applications [1] Some of them are given as follows

In recent years spin transport has attracted considerable attention as it offers a possibility for a type of transistor [2], quantum computation and quantum logic [3,4]

A significant part of the work is concentrated on spin-polarized electronic transport through the DMS [3,4] Compared with the most common use of ferromagnetic transition metals, such as Fe, Co, Ni and their alloys as the magnetic electrodes for experiments on spin-polarized tunneling of ferromagnet/insulator/ferromagnet tunnel junctions, Ref [5] give an example of spin-polarized transport in diluted GaMnAs/AlAs/GaMnAs ferromagnetic semiconductor tunnel junctions Tunneling magnetoresistance (TMR) and related phenomena were extensively studied in magnetic tunnel junctions due to the applications in digital storage and magnetic sensor technology

They are potential in providing functionality and enhanced performance in

CHAPTER 2: LITERATURE REVIEW semiconducting devices, including spin-based field-effect transistors (FETs), spin polarized lasers, light-emitting diodes (LEDs), nonvolatile magnetic semiconductor memory, and perhaps quantum computing [1,6] For nonvolatile memory storage applications, ferromagnetism is used to store data for extended periods of time [7] By manipulating spins, rather than charge, it is anticipated that more energy-efficient memory storage will be developed [8] In quantum computing devices, the spin states would be used to construct “qubits,” theoretically enabling the manipulation of huge amounts of data [9]

DMSs offer the possibility of optical spintronic devices, such as spin light-emitting diodes [10-14], spin-polarized solar cells [12] and magneto-optical switches[13] Some

of them have been demonstrated by II–VI and III–V DMSs as the pivotal spin-injection components, but to date, these devices can only operate at cryogenic temperatures because of either the absence of ferromagnetism or the low Curie

temperature Tc for the ferromagnetic phase transition [3,10,14,15]

2.2 Literature Review on Predictions for DMSs

Generally, there are two kinds of theories to predict the origin of ferromagnetism in DMSs One is based on the first-principles approach [16-18], the another one is the Hamiltonian model method by T Dietl [19,20] Despite basic differences, both methods gave similar predictions for the ferromagnetism for some kinds of new materials Let’s give a general review of these issues as follows

CHAPTER 2: LITERATURE REVIEW

2.2.1 Model Based on the First-Principles Approach

In this model, the possibility of ferromagnetic DMSs at room temperature was

predicted by the first principle KKR (Korringa-Kohn-Rostoker) Green’s function

method based on the local density approximation [17,21-23]

In a DMS system, the transition metal impurities not only have local moments

different from the host atoms, but they also strongly affect the moments of the

neighboring host atoms They show a very different behavior from sp electrons since

they have a local moment which can align either parallel or anti-parallel to the host

moments As the local density of states (DOS) of impurities strongly reflects such a

specific behavior of the local magnetic moments, the local moments of ions can be

calculated from the local DOS of the impurities K.Sato [21] gives an example of a

study on the stability of the ferromagnetic states of DMS (Mn-doped ZnO) based on

local density approximation According to this theory, Tc can be determined by a

competition between the ferromagnetic and antiferromagnetic interactions

Figure 2-1 shows the total DOS and local density of d states at the Mn site in the

25% hole doped (Zn,Mn)O both for the ferromagnetic state and the anti-ferromagnetic

state [21] In the ferromagnetic case of hole doping, some of O atoms were substituted

with N atoms, and the hybridization between 2p states of N and 3d up spin states of

Mn is wider and larger than the band width, which lead to depressing of the

hybridization between the down spin states of Mn Hence the high spin state of Mn2+ is

realized by the double exchange mechanism In the anti-ferromagnetic case, the total

DOS is symmetric in the spin direction, and the Mn-d band is narrower by the super

CHAPTER 2: LITERATURE REVIEW

exchange mechanism In other words, the feature of the Mn-d states stabilizes the ferromagnetic state, and the Mn-d state is wider in the case of ferromagnetic state than

that of the anti-ferromagnetic state It is universally thought that the carrier induced ferromagnetism in DMS is explained by the double exchange mechanism In Fig 2-2

we give that total magnetic moments per transition metal atom in ZnO It is found that the ferromagnetic state was the ground state for Co in ZnO under the condition of proper doping

In these calculations, the stability of the ferromagnetic state in ZnO-, ZnS-, ZnSe-, ZnTe-, GaAs- and GnN-based DMS is investigated systematically, and it is suggested

that V- or Cr-doped ZnO, ZrS, ZnSe and ZnTe are candidates for high-Tc

ferromagnetic DMSs V-, Cr or Mn-doped GaAs and GaN are also candidates for

high-Tc ferromagnets It is also shown that Fe-, Co- or Ni-doped ZnO is ferromagnetic

In particular, their magnetic states are controlled by changing the carrier density

CHAPTER 2: LITERATURE REVIEW

Fig 2-1 Total DOS (solid line) and local density of Mn-d states (dotted line) in the

hole doped (Zn, Mn)O Hole doping is up to 25% [21]

CHAPTER 2: LITERATURE REVIEW

Fig 2-2 Chemical trend of the magnetic states for 3d transition metal atom doped ZnO

Total magnetic moments per transition metal atom are also shown [21]

2.2.2 T Dietl’s Model

In this model, the tendency toward ferromagnetism have been explained within a

mean-field picture in which uniform itinerant-carrier spin polarization mediates a

long-range ferromagnetic interaction between the magnetic ions with spin S [19,24]

Curie temperature can be obtained between the ferromagnetic and anti-ferromagnetic

CHAPTER 2: LITERATURE REVIEW interactions It is here assumed that there exist two spin subsystems, i.e., carrier spins

and localized spins at magnetic ions, interacting through the sp-d interaction Having a

nonzero magnetization increases the free energy of the localized spin system, but reduces the energy of the carrier systems via spin-splitting of the bands (no energy

gain if no carriers are present) The free energy penalty reduces as temperature (T) is reduced, and balances with the energy gain of the carrier system at T = Tc The magnitude of Tc as a function of ion and hole concentrations can be explained by this model In terms of this, T.Dietl predicted that Tc of GaN and ZnO can be raised above

300 K if they were p-typed doped with 5% of Mn and 3.5x1020 holes per cm3 We can refer to Fig 2-3

Fig 2-3 Computed values of Tc for various p-type semiconductors containing 5% of

Mn and 3.5x1020 holes per cm3 [19]

CHAPTER 2: LITERATURE REVIEW

2.2.3 RKKY Interaction and Spin Glass

For a diluted system with a picture of magnetic impurities in the semiconductor carrier sea, research on RKKY (Rudermann, Kittel, Kasuya and Yoshida) and spin glass will be reviewed as follows

As we know, the motion of different electrons in condensed systems is not independent; it is highly correlated RKKY interaction in magnetic semiconductors can

be understood as the interaction between a local magnetic impurity and the surrounding electron gas [25] This is caused by the superposition of the charge density oscillations of the spin-up and spin-down electrons, giving rise to a spin density oscillation That is, an impurity placed in a gas of free electrons causes an oscillatory perturbation (Friedel oscillations) of the electron density around the ion The physical origin for this perturbation is the scattering of the free electrons at the impurity

potential In DMSs, 3d-impurities possess a magnetic moment This oscillatory

polarization can be understood from the charge oscillations Following density functional theory, spin-up and spin-down electrons feel a different potential of the form )V±(r)≅υc(r)±υx(r)m(r , which means that spin-up and spin-down electrons are scattered differently Due to different phase shift, the oscillation of the spin-up electron density is shifted relative to the oscillation of the spin-down density The superposition of these two charge densities yields an oscillatory magnetization which decays according to the dimensionality of impurity considered RKKY-interaction explains that atoms at a given distance from the impurity feel either a positive or a negative polarization and consequently have magnetic moment of respective

CHAPTER 2: LITERATURE REVIEW orientation

Spin glass is defined as [26] a random, mixed interacting, magnetic system characterized by a random, yet co-operative, freezing of spins at a well-defined

temperature, freezing temperature (Tf ), below which a highly irreversible, metastable

frozen state occurs without the usual long-range spatial magnetic order Spin glass is the class of materials exhibiting the frozen state transition The basic ingredients of the frozen state are randomness, mixed interactions and frustration There are two prerequisites for the spin glass The first is the randomness in either position of the spins; the second is properties relative to co-operative nature, such as the possibilities

of the random anisotropy and peculiar metastabilities (time and aging dependences) occurring especially in reaction to a magnetic field, some of which are listed as follows [26]

(1) Magnetization for T << Tf

It is S-shape of hysteresis loop for T << Tf This is featured as the lack of saturation

and there is a tendency towards saturation but full saturation is never attained The decrease of magnetization with increase of concentration in a constant field can be explained by not only the long range RKKY interactions, but also the peculiar anisotropy which plays a major role in preventing the external field from fully rotating the frozen moments to its direction The various clusters, which are originally randomly oriented, can point along the field direction after the external magnetic field overcomes an array of local anisotropy axes This requires the field energy to rotate the cluster moment away from its anisotropy-pinned “frozen” orientation

CHAPTER 2: LITERATURE REVIEW

(2) Resistivity for T << Tf

The temperature variation of the resistivity in a spin glass system shows a p

T

(p>1) dependence The dependence of resistivity is highly damped, non-coherent and

localized spin fluctuations

(3) Spin glasses in a field

a FC (field-cooled) magnetization has a clear kink in the plateau at Tf;

b In the plot of inverse of the FC susceptibility as a function of temperature, Tf

can be determined at the onset of the plateau The onset temperature not only shifts downward with increasing magnetic field, but becomes smeared

c Under a general experimental criterion, the boundary of the spin-glass phase

seems not to be static

2.3 Physics of DMSs

DMSs have attracted considerable attention in recent years Their structures and magnetic, electrical and optical properties are very interesting, and they have great potential for practical applications It is also of great scientific importance to understand the physics of DMSs, which are helpful to understand the origin of the properties

There are two types of magnetic interactions [27]:

(1) strong (sp-band-edge)- magnetic ions exchange interaction (sp-d) in the form

of Kondo-like interaction, –J sp-d

S σ, where J sp-d is the exchange constant, S

the magnetic ion moment and σ the spin operator associated with the

CHAPTER 2: LITERATURE REVIEW valence-band-edge hole or the conduction-band-edge electron The interaction between the magnetic spins and the spins of the band edge carrier causes the dramatic enhancement of band edge Zeeman splittings and related properties

in an external magnetic field The coupling of magnetic moment of

transition-metal ion and the spin of the charge carriers is called the sp-d

exchange interaction This kind of exchange interaction is the physical origin

of the variety of interesting physical phenomena of DMSs, such as magneto-optical and magneto-electrical effects;

(2) weaker magnetic ion-magnetic ion exchange interaction (d-d) with the form of Heisenberg interaction -2J dd

S i S j, where J dd is the antiferromagnetic coupling

between spins separated by Rij Among the magnetic ion spins at different

sites, which determines the overall magnetic behavior including the transition from paramagnetic to a spin glass phase at low temperature It is known to be antiferromagnetic and of short-range

There are two different mechanisms of the interactions between the sp band electrons and the magnetic d electrons [28] The first is the normal exchange mechanism originating from the 1/r coulomb type interaction potential The potential exchange

tends to align the spins of the band electrons with the spins of TM ions The potential exchange interaction does not depend on the magnetic ions and host lattices The

second is the most important and is the kinetic mixing of the sp band and d- electrons

due to the hybridization of their wavefunctions As an illustration, consider Ⅱ- Ⅵsemiconductor (Cd1-xMnxTe) (see [29]) The two components of the sp-d exchange are

CHAPTER 2: LITERATURE REVIEW

as follows First, the s-d exchange constant N0α (N0α=∆E c /Sz, i.e

, where ∆E

)()

(↓ − ↑

=

∆E c E c E c c expresses conduction-band-edge spin splitting, N0 the concentration of cation sites, α the s-d exchange integral) is dominated by the potential exchange contribution and the modification of the sp-orbitals by the potential exchange is weak and local Though the s-orbital of the conduction band does not mix with the d-orbital, it is still influenced by the magnetic ion It is ferromagnetic if N0α >

0 Second, the kinetic exchange interaction is much stronger than the potential

exchange interaction The p-d exchange constant N0β (N0β =∆E v/Sz , i.e

, where )

()

(↓ − ↑

=

∆E v E v E v ∆ stands for valence-band-edge spin splitting and E v β is

the p-d exchange integral) is dominated by the kinetic exchange contribution It is

ferromagnetic if N0β >0 In this system, the p-d exchange constant per unit volume

−

=

d v v eff d

pd

E E U S

V N

εε

CHAPTER 2: LITERATURE REVIEW

Fig 2-4 A schematic diagram of the p-d repulsion effects for a ferromagnetic system

with anion p states by the cation d states (a) shows the atomic unpolarized level (b)

shows the exchange split atomic levels (c) shows the crystal field split level (d) shows the final interacting states Shaded areas denote the host crystal bands [29]

The fundamental sp band gap remains direct at k =0 for all concentration The hybridization has a small effect on the energy band structure only and it is the

dominant source of both sp-d and Mn-Mn exchange interactions The p-d repulsion effects of the anion p states by the cation d states for a ferromagnetic system (Ⅱ- Ⅵsemiconductor (Cd1-xMnxTe) is shown in Fig 2-4 Both of the occupied and

unoccupied d-states are further split by the Td crystal field into doubly degenerated eg states and triply degenerated t2g states The eg-orbital does not mix with the anion

p-orbital owing to the symmetry The t 2g mixes with the p-orbitals

The giant Zeeman splitting of the semiconductor band structures, which includes

CHAPTER 2: LITERATURE REVIEW

magneto-optic effects, is a direct result of the sp-d exchange coupling Zeeman

splitting refers to the splitting of the semiconductor band structure by an external

magnetic field In DMSs, the splitting can be huge due to the sp-d exchange interactions The effective magnetic field on the sp-band electrons is amplified by the magnetic moment of the transition metal ion through the sp-d exchange interaction The sp-d exchange interaction induces magneto-optical such Farady effect and Kerr

effect, magnetic-field-induced metal-insulator transition, bound magnetic polaron, and magneto-electrical feature like giant magneto-resistance

2.4 Related Materials

2.4.1 ZnO

ZnO has a wurtzite structure, in which each ion is connected to four counterpart ions

by the sp3 tetrahedral bond The wurtzite structure can be considered as a close-packed

hexagonal structure with a basis of two atoms: O2- at (000), Zn2+ at (

1,3

2

An atom in the wurtzite structure has four atoms of another type as its nearest neighbors and twelve atoms of the same type as second nearest neighbors ZnO belongs to the P63mc (186)

space group The lattice parameter of ZnO is a = 3.249 Å, and c = 5.2052 Å [30]