CHAPTER 1: INTRODUCTION CHAPTER 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 National University of Singapore 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 National University of Singapore 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 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 National University of Singapore 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 ρ Hall observed at K before (dashed line) and after (solid line) light irradiation, showing that the ρ Hall correlates 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 National University of Singapore 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 Ga1-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. National University of Singapore 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- Ⅴ(GaN) and Ⅱ-Ⅵ (ZnO) 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-Ⅵ and Ⅲ- Ⅴ compounds semiconductors have been extensively 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-Ⅵ anions (such as O and Te, etc.) [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 National University of Singapore 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 National University of Singapore 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 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. Zn1-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 National University of Singapore 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. National University of Singapore 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-xCoxO: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 Zn1-xCoxO National University of Singapore 10 CHAPTER 3: DBPLD SET-UP AND EXPERIMENTAL PROCEDURES characterize the contribution of carrier response of the film to the magnetic moment and Hall resistance. The electrical resistance of the films was measured for the temperature range 80 – 300 K using a home-made 4-probe method. Specimens were placed on a cold finger of the liquid-helium cryostat. The cryostat chamber was evacuated during measurements. The samples were analyzed by XRD (Philips, X’PERTMRD) via a Cu Kα source to identify the different crystal planes, possible phases and crystallographic orientation formed in the films. Detailed lattice structure and possible precipitates were confirmed by a JEM 2010F HRTEM. The crystallographic orientations of the Zn1-xCoxO films were also determined using the XDD beam line at SSLS. The diffractometer is a Huber 4-circle system, with a high-precision of 0.0001° in step size for omega. The X-ray beam was conditioned to select the Cu Kα1 radiation (8.048 keV in photon energy) using a Si (111) channel-cut monochromator. An DI NanaScope AFM was used to study the surface morphologies of the films. XPS was performed using a Physical Electronics Quantum 2000 Scanning Microprobe system with a monochromatized Al Kα source to characterize the compositions of the films. The elemental composition was determined by calculating the relative peak areas at specific binding energies. Chemical states of elements were also determined from the XPS spectra. A UV- 3101 spectrometer (Shimadzu) was employed to characterize the optical transmittance or absorbance spectra. PL was excited by a He–Cd laser operating at 325 nm and captured by a charge coupled device (CCD) camera through a monochromator. Valence-band PES measurements were performed using the Surface, Interface and Nanostructure Light Science (SINS) beamline at the SSLS in NUS. The data were obtained at room temperature and at a pressure less than 4×10-10 Torr. The Fermi level National University of Singapore 50 CHAPTER 3: DBPLD SET-UP AND EXPERIMENTAL PROCEDURES was determined from the cutoff point of the valence-band spectra. All the spectra were normalized to the incident photon flux. The valence-band spectra were obtained by using the light source with hν ≈ 135.8 eV. To study the partial DOS in details, 3d, p and s partial density of states of ZnO near EF, theoretical valence band was calculated by FEFF801. It was obtained from the local density approximation (LDA) band structure calculation. The parameters used for simulations are shown in Table 3-1. Table 3-1 Main structural parameters of ZnO used for the calculation partial DOS by FEFF801 Space = P 63 m c a = 3.249 Å b = 3.249 Å c = 5.205 Å α = 90.0° β = 90.0° γ = 120° National University of Singapore 51 CHAPTER 4: DEPENDENCE OF Zn1-xCoxO PROPERTIES ON PROCESSING PARAMETERS CHAPTER DEPENDENCE OF Zn1-xCoxO PROPERTIES ON PROCESSING PARAMETERS 4.1 Introduction Today, fabrication of room temperature DMSs remains a difficult challenge. DBPLD is a novel approach for doping impurities in thin films [1, 2]. It is also an appropriate way to synthesize new materials [3]. It is a non-equilibrium growth process that allows us to obtain doping levels that exceeds the limit at thermal equilibrium. This method is thus helpful in suppressing the possible magnetic dopant precipitates in DMSs. DBPLD also allows in situ control of dopant concentration in the films [2]. It is an appropriate way to synthesize new materials with a large range of dopant concentration, as it can fabricate the films with different stoichiometries under identical conditions. Using this method, we synthesized Co-doped ZnO (Zn1-xCoxO) thin films on c-plane sapphire substrates with different related Co concentrations. The properties of Zn1-xCoxO thin films grown on c-plane sapphire substrates were focused to depend on substrate position, growth temperature and vacuum pressure, especially growth temperature. There is also an optimum experimental conditions to synthesize the Zn1-xCoxO thin films without apparent precipitates. Based on our experimental results, Co has been doped into ZnO successfully, and they show typical semiconductor behaviors, whose transport properties can be explained by a hopping National University of Singapore 52 CHAPTER 4: DEPENDENCE OF Zn1-xCoxO PROPERTIES ON PROCESSING PARAMETERS conduction mechanism. 4.2 Co Concentration Dependence on Substrate Position Using XPS spectra, the Co concentration of the film was obtained by calculating the relative areas at specific binding energies of Zn 2p3/2 and Co 2p3/2. Figure 4-1 shows a typical relationship of Co concentration and substrate position. Co concentration increases as the substrate position approaches the Co target. Variation in Co concentration at different substrate positions allows us to control the quantity of dopants in the host lattice. It is the main way to adjust Co concentration via the DBPLD method under our experimental conditions. Co concentration x 0.25 0.20 0.15 0.10 Excimer laser 10 Hz, ~3J/cm -5 5x10 Torr o 650 C 0.05 0.00 Substrate Position Fig. 4-1 Co concentration dependence on substrate position. National University of Singapore 53 CHAPTER 4: DEPENDENCE OF Zn1-xCoxO PROPERTIES ON PROCESSING PARAMETERS 4.3 Structure Dependence of Specimen Prepared on different Growth Temperatures Figure 4-2 shows the Co concentration as a function of growth temperature for a particular substrate position (position 2). The Co concentration increases with increasing growth temperature. It increases slowly below 650℃, and rapidly above 650℃. We attribute the Co concentration dependence on temperature to the different activation energies of cobalt and ZnO [4]. It may be understood by the “diffusion” and “evaporation” growth stages by the deposition theories [5]. Although a higher growth temperature favors increased Co doping in the ZnO host lattice, it also leads to crystal structures with large grain size [5]. It was found experimentally that when the growth temperature reaches around 650 ℃, both the Co concentration and crystal structures obtained are satisfactory. Co Concentration x 0.07 0.06 Excimer laser 10 Hz, ~3J/cm -5 5x10 Torr Substrate Position 0.05 0.04 0.03 0.02 0.01 400 500 600 700 800 900 o Growth temperature ( C) Fig 4-2 Co concentration dependence on growth temperature. National University of Singapore 54 CHAPTER 4: DEPENDENCE OF Zn1-xCoxO PROPERTIES ON PROCESSING PARAMETERS Figure 4-3 shows a typical XRD pattern of a Co-doped ZnO thin film grown on a c-plane sapphire substrate, showing that the films contain wurtzite structure with c-axis aligned with that of the c-plane sapphire substrate. In our studies, clear XRD patterns with only (0002) and (0004) peaks of Zn1-xCoxO thin films [denoted by (0002) f and (0004) f , respectively] were obtained in a relatively large range of experimental conditions. It is well known that Bragg’s law for XRD are based on the ideal conditions during diffraction. These conditions are based on that crystals are perfect. However these conditions never actually exist. To determine the quality of crystal structures, a well-defined technique, with the term of Full-width at half maximum, or FWHM, can be used by XRD peak broadening analysis. In general, the smaller FWHM, the higher the film crystallity. A plot of FWHM values of the (0002) f peak of the films as a function of growth temperature in Fig. 4-4 is given, where f denotes the film. The value of FWHM of the (0002) f peak was the same order of magnitude in a relatively large range of growth temperatures. The smallest value of FWHM occurred near the growth temperature of 650 ℃, showing the best crystal structure of the film. An increase in the FWHM was observed when the growth temperature was lower than 500 ℃, particularly when it was lower than 400 ℃, indicating a deteriorating crystal structure. From this experimental result, we concluded that the optimum condition for the growth of Zn1-xCoxO thin film is at the growth temperature of 650 ℃, laser fluence of 3J/cm3, laser repetition rate of 10 Hz, vacuum pressure of 5×10-5 Torr [6,7]. The following results are based on the specimens obtained under these conditions. National University of Singapore 55 20 30 Excimer laser 10 Hz, ~3J/cm -5 5x10 Torr Co 0.015 40 Co-doped ZnO (0004) C-Al2O3 (0006) Co-doped ZnO (0002) Intensity (a.u.) CHAPTER 4: DEPENDENCE OF Zn1-xCoxO PROPERTIES ON PROCESSING PARAMETERS 50 60 70 80 2θ (Degree) Fig. 4-3 A typical XRD pattern of a Co-doped ZnO thin film grown on a c-plane sapphire substrate. 3.0 Excimer laser 10 Hz, ~3J/cm -5 5x10 Torr Co ~ 0.015 FWHM (degree) 2.5 2.0 1.5 1.0 0.5 0.0 300 400 500 600 700 800 900 o Growth temperature ( C) Fig. 4-4 FWHM of rocking curves for the (0002) f peak of Zn1-xCoxO thin films dependence on growth temperature. National University of Singapore 56 CHAPTER 4: DEPENDENCE OF Zn1-xCoxO PROPERTIES ON PROCESSING PARAMETERS 4.4 Dependence of Semiconductor Transport Properties of the Specimen Prepared at Different Growth Temperatures and Vacuum Pressures Figure 4-5 shows the dependence of Hall resistivity ( ρ H ) and carrier density (N) of Zn1-xCoxO films with a Co concentration of x = 0.05 on growth temperature. When the growth temperature was greater than 650 ℃, the Hall resistivity decreased, while carrier density increased. This temperature effect could be understood by the thermodynamics of nucleation theory [5]. The film grown at higher temperature tends to be uniform crystal structures with larger grain size and small number of defects such as boundaries. Higher carrier density arises from fewer electron traps in the films. Therefore, lower resistivity is a consequence of a higher carrier density. This effect of growth temperature also coincides with the optimum growth temperature discussed above. National University of Singapore 57 CHAPTER 4: DEPENDENCE OF Zn1-xCoxO PROPERTIES ON PROCESSING PARAMETERS 0.6 2.00E+019 1.50E+019 0.5 0.4 1.00E+019 0.3 5.00E+018 0.2 0.1 0.00E+000 0.0 400 0.7 2.50E+019 Excimer laser 10 Hz, ~3J/cm -5 5x10 Torr Co ~0.05 Carrier density (1/cm ) Hall resistivity (Ω cm) 0.8 Density Resistivity 0.9 500 600 700 800 900 o Growth temperaturet ( C) Fig. 4-5 Dependence of Hall resistivity and carrier density of Zn1-xCoxO thin films with x around 0.05 on growth temperature. In the light of Figure 4-6 we can see the relationship of the Hall mobility ( µ H ) and growth temperature. It is shown that the Hall mobility increases with growth temperature. Films grown at higher temperatures have less crystal defects, hence less scattering arising at crystal defects. In this case, the decrease in resistivity dominates this process, and causes the Hall mobility to increase according to the formula µH ∝ n ρH (4-1) where ρ H is the Hall resistivity. National University of Singapore 58 60 Hall Mobility (cm /VS) CHAPTER 4: DEPENDENCE OF Zn1-xCoxO PROPERTIES ON PROCESSING PARAMETERS Excimer laser 10 Hz, ~3J/cm -5 5x10 Torr Co ~ 0.05 40 20 400 500 600 700 800 900 o Growth temperature ( C) Fig. 4-6 Relationship between Hall mobility and growth temperature for Zn1-xCoxO thin films with Co concentration x around 0.05. In the remaining sections of the chapter, we shall discuss the relationship between transport properties dependence on carrier density at different growth temperatures and vacuum pressures. Figure 4-7 and 4-8 describe an overview of the ρ H , µ H and Hall coefficient ( R H ) as a function of the carrier density (N). The data scattered in the figures are probably due to non-uniformity of the films fabricated by PLD methods. The hopping conductance in a doped semiconductor can be expressed by a model of random Miller-Abrahames network [8] as follows: ρ3 = ρ0e α N / 3a (4-2) where ρ3 is the resistivity corresponding to the hopping mechanism, a is the state National University of Singapore 59 CHAPTER 4: DEPENDENCE OF Zn1-xCoxO PROPERTIES ON PROCESSING PARAMETERS radius, N is the concentration vertices of the Miller-Abrahams network and α an constant. In Fig. 4-7, the ρ H appears to vary exponentially with the carrier density N. This can be explained by the hopping conduction mechanism in the presence of the impurities in the crystal [8]. Figure 4-8(a) and 4-8(b) describe the relationship of the Hall mobility and Hall coefficient with the carrier density of the samples obtained at different growth temperatures, respectively. It shows that a higher mobility is obtained at a higher growth temperature. This is due to less scattering at crystal defects in the films grown at relative higher temperatures. The RH follows a linear dependence on the carrier density N in the log scale, as shown in Fig. 4-8(b). The negative slope indicates a n-type semiconductor. 1.0 0.8 0.7 0.6 Excimer Laser 10 Hz, 3J/cm -5 5x10 Torr o 850 C o 750 C o 650 C 0.5 0.4 Linear fit -ln(ρ) Hall resistivity (Ω cm) 0.9 1.0x10 1.5x10 2.0x10 2.5x10 3.0x10 1/3 N 0.3 0.2 0.1 0.0 -0.1 18 5.0x10 19 1.0x10 19 19 1.5x10 2.0x10 Carrier density N (1/cm ) Fig. 4-7 Hall resistivity ρ H versus carrier density of series of Zn1-xCoxO thin films deposited at different growth temperatures. The insets shows the − ln ρ H and − N follow a linear dependence. National University of Singapore 60 CHAPTER 4: DEPENDENCE OF Zn1-xCoxO PROPERTIES ON PROCESSING PARAMETERS 80 60 Hall Mobility (cm /Vs) 70 50 (a) Excimer Laser 10 Hz, 3J/cm -5 5x10 Torr o 850 C o 750 C o 650 C 40 30 20 10 18 19 10 10 Carrier density (1/cm ) 10 Hall coefficient (cm /Vs) (b) Excimer Laser 10 Hz, 3J/cm -5 5x10 Torr o 850 C o 750 C o 650 C Linear Fit 0.1 18 10 19 10 Carrier density (1/cm ) Fig. 4-8(a) Hall mobility µ H versus carrier density of series of Zn1-xCoxO thin films deposited at different growth temperatures; (b) Hall coefficient R H versus carrier density of series of Zn1-xCoxO thin films deposited at different growth temperatures. National University of Singapore 61 CHAPTER 4: DEPENDENCE OF Zn1-xCoxO PROPERTIES ON PROCESSING PARAMETERS 0.5 0.4 0.3 Excimer Laser 10 Hz, 3J/cm o 650 C -3 5x10 Torr -4 5x10 Torr -5 5x10 Torr Linear Fit -ln(ρ) Hall resistivity (Ω cm) 0.6 1x10 0.2 2x10 1/3 N 3x10 4x10 0.1 0.0 19 1x10 19 2x10 19 3x10 19 4x10 Carrier density (1/cm ) Fig. 4-9 Hall resistivity ρ H versus carrier density of series of Zn1-xCoxO thin films deposited at different vacuum pressures. Figure 4-9 shows the Hall resistivity dependence on carrier density of the specimen prepared at different vacuum pressures. We observed that ρ H has a similar dependence on carrier density, following the Eq. (4-2). In general, at this range of ambient vacuum pressure, the Co doped ZnO films show a semiconductive transport behaviour. As we know, the growth of high quality of host ZnO thin film tends to be obtained at a lower ambient vacuum pressure due to higher energy of deposition flux, though there are some controversy reports [8,9]. In our experiments, clear XRD patterns with only (0002) and (0004) peaks of Zn1-xCoxO thin films were obtained in a relatively large range of ambient vacuum pressure, indicating that the ZnO based films with a majority of c-axis growth under the condition of a relative large range of National University of Singapore 62 CHAPTER 4: DEPENDENCE OF Zn1-xCoxO PROPERTIES ON PROCESSING PARAMETERS ambient pressure. The FWHM values of the (0002) f peak of the thin films with the Co concentration around 0.02 from XRD patterns are almost the same, around 0.24° , indicating that the crystalline quality is similar. Under our experimental conditions, the ambient vacuum pressure of 5×10-5 Torr can be available to fabricate the Co doped ZnO thin film. Figure 4-10(a) shows the Hall mobility µ H versus carrier density N. There is no apparent relationship between the Hall mobility and carrier density. Hence, we would like to discuss the mobility from the mechanism of scattering only, and we are interested in the order of magnitudes of the mobilities and the major factors affecting these numbers. It is known that the primary factor influencing mobility is the average time between scattering processes. There are three important mechanisms. They are scattering by crystal defects, dopant atoms and phonons. Scattering by phonons and dopant atoms together contribute mainly to the mobilities. The values of mobility of the specimen prepared at different vacuum pressure are scattered indicating that the effect of vacuum pressure on the mobility is less than that of other effect, such as growth temperature [Fig. 4-8(a)]. In Figure 4-10(b), the dependence of the Hall coefficient on Hall carrier density of the specimen prepared under different vacuum pressures is similar to that obtained under different growth temperatures shown in Fig. 4-7(c). It confirms that the films show typical semiconductor behaviours. Under our experimental conditions, though experimental results are scattered, however the trends are similar. National University of Singapore 63 CHAPTER 4: DEPENDENCE OF Zn1-xCoxO PROPERTIES ON PROCESSING PARAMETERS 40 Excimer Laser 10 Hz, 3J/cm o 650 C 30 Hall Mobility (cm /Vs) (a) 20 10 -3 5x10 Torr -4 5x10 Torr -5 5x10 Torr 18 10 19 10 Hall resistivity (Ω cm) Carrier density (1/cm ) 0.1 (b) Excimer Laser 10 Hz, 3J/cm o 650 C -3 0.01 5x10 Torr -4 5x10 Torr -5 5x10 Torr Linear Fit 18 10 19 10 Carrier density (1/cm ) Fig. 4-10(a) Hall mobility versus carrier density of series of Zn1-xCoxO thin films deposited at different vacuum pressures; (b) Hall coefficient versus carrier density of series of Zn1-xCoxO thin films deposited at different vacuum pressures in the log scale. National University of Singapore 64 CHAPTER 4: DEPENDENCE OF Zn1-xCoxO PROPERTIES ON PROCESSING PARAMETERS 4.5 Summary Summarize this chapter, we have the following conclusions: z The DBPLD method was utilized to synthesize Zn1-xCoxO thin films for the purpose of DMSs. z Co concentration could be adjusted flexibly via changing the substrate position using the DBPLD method. Co concentration increases as the substrate position approaches the Co target. z The growth temperature played an important role not only in doping concentration, but also in the structures and properties of the films. Hall mobility increased with increasing growth temperature. The effect of vacuum pressure on the mobility is less than that of growth temperature. z The optimum growth condition was found to be around growth temperature 650 ℃, laser fluence 3J/cm3, laser repetition rate 10 Hz, vacuum pressure 5x10-5 Torr. z Based on our experimental results, the Zn1-xCoxO thin films show typical doped semiconductor behaviors, whose transport properties can be explained by a hopping conduction mechanism [8]. National University of Singapore 65 [...]... distribution of impurities and local magnetization, and the understanding of the magnetic ordering and underlying mechanisms [67] National University of Singapore 13 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... 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... 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 ] National University of Singapore 25 CHAPTER 2: LITERATURE REVIEW 2.2.3 RKKY Interaction and Spin Glass For a diluted system with a picture of magnetic. .. parameter of CoO is a = 4.26 Å, and the shortest Co-O distance is 2 .13 Å [47] The Co-Co distance in CoO is 3. 01 Å [48] The electronic configuration and state of Co2+ is 3d7 and 34F9/2 ,1, respectively [49] The ground state of Co2+ in an intermediate and strong cubic crystal field is 4T1g(dε5dγ2) and 2Eg(dε6dγ), respectively [50] Fig 2-7 Crystal structure of CoO [ 31] 2.5 Crystalline Anisotropy A magnetic. .. 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?... 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... 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... 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. .. National University of Singapore 17 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... 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 . such as Ga 1- x Mn x N [15 ], Ga 1- x Mn x As [1] , (In,Mn)As [16 ], Cd 1- x Mn x Te [17 ], Zn 1- x Mn x O (M=TM) [18 ] and Zn 1- x Mn x O [19 ]. Ⅱ-Ⅵ compounds semiconductor includes a variety of compounds. CHAPTER 1: INTRODUCTION CHAPTER 1 INTRODUCTION 1. 1 Research Background 1. 1 .1 Motivation to Study Diluted Magnetic Semiconductors (DMSs) Nowadays, almost. distribution of impurities and local magnetization, and the understanding of the magnetic ordering and underlying mechanisms [67]. National University of Singapore 13 CHAPTER 1: INTRODUCTION