FIRST-PRINCIPLES EXPLORATION FOR HALF METALLIC MATERIALS RONGQIN WU (B.Sc.,Fujian Normal University) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF PHYSICS NATIONAL UNIVERSITY OF SINGAPORE 2006 Acknowledgements I would like to thank my supervisor, Professor Feng Yuanping for his guidance, advice and kindness throughout all my research work. He is always encouraging me whenever I have a new idea and never complains on the pains he takes in revising my manuscripts. My thanks also goes to Singaporeans. My scholarship, which has been supporting my life and research activities all these years, came from their hard working. I am deeply indebted to my mother and father for bringing me into this wonderful world and supporting me on each of my decision conditionlessly . Finally, I wish to thank the following: Peng Guowen(for allowing me to use his wonderful Tex template for this thesis); Liu Lei, He Jun and Sun Yiyang(for all the good and bad times we had together); Yang Ming and Shen Lei (for giving me delicious Chinese foods). Rongqin Wu December 2006 i Table of Contents Acknowledgements i Summary vi List of Tables ix List of Figures x Introduction 1.1 Semiconductor Spintronics Devices . . . . . . . . . . . . . . . . . . 1.2 Dilute magnetic semiconductors and half metals . . . . . . . . . . . 1.3 Problems with dilute magnetic semiconductors and half metals . . . 1.3.1 Clustering of magnetic cations in dilute magnetic semiconductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.3.2 Lattice mismatch between known half metals and wide gap semiconductors . . . . . . . . . . . . . . . . . . . . . . . . . ii 1.3.3 Low efficiency of spin injection from half metals to semiconductors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4 Objectives of this study . . . . . . . . . . . . . . . . . . . . . . . . 1.5 Outlines of this thesis . . . . . . . . . . . . . . . . . . . . . . . . . . Density functional theory for materials design 14 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 2.2 Adiabatic approximation . . . . . . . . . . . . . . . . . . . . . . . . 15 2.3 Hartree-Fock approximation . . . . . . . . . . . . . . . . . . . . . . 19 2.4 Density Functional Theory . . . . . . . . . . . . . . . . . . . . . . . 22 2.5 Local density approximation . . . . . . . . . . . . . . . . . . . . . . 26 2.6 Bloch’s theorem and plane wave basis sets . . . . . . . . . . . . . . 29 2.7 Pseudopotential method . . . . . . . . . . . . . . . . . . . . . . . . 33 2.8 VASP code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 Cu-doped GaN and Mg-doped AlN dilute magnetic semiconductors 38 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38 3.2 Calculation details . . . . . . . . . . . . . . . . . . . . . . . . . . . 41 3.3 Electronic and magnetic properties of Cu-doped GaN . . . . . . . . 43 3.4 Electronic and magnetic properties of Mg-doped AlN . . . . . . . . 46 iii 3.5 3.6 Origin of spin polarization and ferromagnetism in Cu-dope GaN and Mg-doped AlN . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55 Magnetism in BN Nanotubes Induced by Carbon Doping 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61 4.2 Computational details . . . . . . . . . . . . . . . . . . . . . . . . . 63 4.3 Results and discussions . . . . . . . . . . . . . . . . . . . . . . . . 64 4.4 Magnetism in C-doped BN nanotubes . . . . . . . . . . . . . . . . . 71 4.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 Possible half metals: NiO in wurtzite and zinc-blend structure 75 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 5.2 Computational details . . . . . . . . . . . . . . . . . . . . . . . . . 78 5.3 Results and discussions . . . . . . . . . . . . . . . . . . . . . . . . . 80 5.3.1 Structural and electronic properties of NiO in wurtzite structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3.2 61 80 Structural and electronic properties of NiO in zinc-blend structure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87 5.4 Realization of NiO in wurtizte and zinc-blend structure . . . . . . . 92 5.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 93 Ab initio study on the interface of CrSb/GaSb heterojunction 99 iv 6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Computational Details . . . . . . . . . . . . . . . . . . . . . . . . . 102 6.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 103 6.4 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 Concluding remarks 99 113 7.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 7.2 Future works . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115 v Summary First principles calculations based on Density Functional Theory (DFT) have been performed to explore new half metallic materials for spintronics applications. Based on these calculations, 1) Cu-doped GaN, Mg-doped AlN and C-doped BN nanotubes are predicted to be half metallic dilute magnetic semiconductors (DMS) without magnetic cations. They are potential DMS completely different from conventional ones; 2) zinc-blend NiO and wurtzite NiO are proposed to be potential half metals with lattice constants matched to wide gap semiconductors such as SiC, AlN, GaN and ZnO and thus they are potential electrodes for these wide gap semiconductors. While there have been numerous reports on room temperature ferromagnetism of conventional DMS, identification of the origin of the ferromagnetism remains a challenge as the magnetic dopants always cluster together in the host semiconductors. The magnetic attraction might originate from the magnetism of the magnetic dopants. A possible way to alleviate this problem is to use non-magnetic dopants to fabricate DMS. Calculations on Mg-doped AlN, Cu-doped GaN and C-doped BN showed spin polarization in theses systems even though there are no magnetic vi elements. In both Mg-doped AlN and Cu-doped GaN, their band structures are half metallic and ferromagnetism can be expected provided that sufficient dopants can be incorporated. NiO has an anti-ferromagnetic ground state and crystalizes in rock-salt structure. Being anti-ferromagnetic, its applications are quite limited. On the other hand, wide gap semiconductors such as SiC, AlN, GaN and ZnO crystalize in zinc-blend or wurtzite structure and there have been neither experimental nor theoretical reports of suitable half metallic spin electrodes for them. Calculations showed that if NiO can crystalize in zinc-blend or wurtzite structure, it might change its antiferromagnetism to ferromagnetism and could have a half metallic band structure. In addition, the lattice constants of NiO in these two structures are quite close to those of wide gap semiconductors in correspondent structures. Thus it has the potential to act as spin electrode for wide gap semiconductors. Existing experiments on electrical injection from half metals to semiconductors are far from satisfactory with poor efficiency, which might be associated with the properties of the half metal-semiconductor interface. Systematic first principles studies have been carried out on the properties of transition metal pnictides and group III-V semiconductors interfaces with particular concentration on the energy band discontinuity for the heterostructrues. The results suggest that high efficiency of electrical spin injection can be expected from CrSb to GaSb. The high efficiency can be attribute to the band alignment at CrSb/GaSb interface. vii Publications [1] R. Q. Wu, L. Liu, G. W. Peng and Y. P. Feng, ”Magnetism in BN nanotubes induced by carbon doping”, Appl. Phys. Lett. 86, 122510 (2005). [2] R. Q. Wu, G. W. Peng, L. Liu and Y. P. Feng, ”Wurtzite NiO: a potential half metal for wide gap semiconductors”, Appl. Phys. Lett. 89, 082504 (2006) [3] R. Q. Wu, G. W. Peng, L. Liu and Y. P. Feng, ”Cu-doped GaN: A New Dilute Magnetic Semiconductor from First-principles Study”, Appl. Phys. Lett. 89, 062505 (2006) [4] R. Q. Wu, L. Liu, G. W. Peng and Y. P. Feng, ”Ferromagnetism in Mg-doped AlN from first principles study”, Appl. Phys. Lett. 89, 142501 (2006) [5] R. Q. Wu, L. Liu, G. W. Peng and Y. P. Feng,” Zinc-blend NiO: a potential half metal for wide gap semiconductor”, Phys. Rev. B (under review). [6] R. Q. Wu, G. W. Peng, L. Liu and Y. P. Feng, ”Possible graphitic-boronnitride-based metal-free molecular magnets from first principles study”, J. Phys.: Cond. Matter. 18, 569 (2006). [7] R. Q. Wu, L. Liu, G. W. Peng and Y. P. Feng, ”First principles study on the interface of CrSb/GaSb heterojunction”, J. Appl. Phys. 99, 093703 (2006) [8] R. Q. Wu, G. W. Peng, L. Liu and Y. P. Feng, ”Properties of VAs/GaAs from first principles study”, J. Phys.: Conf. Series, 29, 150 (2006) [9] L. Liu, R. Q. Wu, Z. H. Ni, Z. X. Shen and Y. P. Feng, ”Phase transition mechanism in KIO3 single crystals”, J. Phys.: Conf. Series, 28, 105 (2006) viii List of Tables 3.1 ΔE(EF M − EAF M ) (in unit of meV)at a concentration of 3.70% of magnetic 3d MTM and Cu in GaN. . . . . . . . . . . . . . . . . . . 5.1 List of known w-half metals and wide gap semiconductors and their lattice constants. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 45 77 Calculated lattice constants a and c, internal parameter u, majority band gap Eg and spin-flip gap Egsp of w-NiO for various values of U-J. 82 5.3 Lattice constant a, majority band gap Eg and spin-flip gap Esp g with different values of U-J . . . . . . . . . . . . . . . . . . . . . . . . . 89 ix Chapter 6. Ab initio study on the interface of CrSb/GaSb heterojunction to ternary Heusler compounds half metals, the atomic environment discontinuity is reduced at the binary transition metal pnictides/group III-V semiconductors, which holds the possibility of excluding the occurrence of massive interface states in the minority spin and keeping the half-metallicity. To date CrSb is the second ZB transition metal pnictide which has been successfully grown on GaAs, AlGaSb and GaSb substrates and showed ferromagnetism with high Curie temperature (> 400K).13 Among all transition metal pnictides and group III-V semiconductors, the lattice mismatch between CrSb and GaSb (their structure is illustrated in Fig. 6.1 )is the smallest and thus it is possible to grow CrSb on GaSb with fine crystallinity at the interface. However, an interface with fine crystallinity does not guarantee a high polarization of the spin current. The electronic properties of the interface should be studied in order to understand the efficiency of spin injection. The following sections give a detailed study on the electronic structure at the interface layers and the band alignment at the interface. 6.2 Computational Details All calculations are based on the spin polarized density functional theory (DFT) implemented in the VASP plane wave code.14, 15 Generalized gradient approximation (GGA) is used for the exchange-correlation functional. The Vanderbilt ultrasoft pseudopotentials16 are used to represent the electron-ion interactions. For Gallium, 3d electrons are treated as valence state. Two six-layer slabs, one for ZB-CrSb (001) and the other for GaSb (001) were stacked along c direction to 102 Chapter 6. Ab initio study on the interface of CrSb/GaSb heterojunction Figure 6.1: Ball and stick model of the unit cell (left) and supercell (right) of zinc-blend GaSb. form the interface. The in-plane lattice parameter a was set to that of the GaSb and the c was optimized. The atomic coordinates in GaSb layers were fixed while those in CrSb were fully relaxed. A kinetic energy cutoff of 500 eV is used. A 6×6×1 K-mesh according to the Monkhorst-Pack scheme17 is adopted to sample the rrreducible Brillouin zone (IBZ). All these parameters are carefully checked to ensure an energy convergence of meV. 6.3 Results and Discussion The calculated equilibrium lattice constant a of ZB GaSb is 6.20 ˚ A. For ZB CrSb a is 6.16 ˚ A, which agrees well with previous similar calculations.18, 19 The lattice 103 Chapter 6. Ab initio study on the interface of CrSb/GaSb heterojunction constant mismatch is about 0.5%. We show in Fig. 6.2 the spin-resolved band structure for a CrSb tetragonal supercell with in-plane lattice constant a slightly expanded to that of GaSb and c optimized (to model the strain imposed on CrSb from GaSb substrate). All concerned energies are also shown in Fig. 6.2. All these values are measured with respect to the averaged electrostatic potential. The majority and the minority spin band structures of CrSb show that the halfmetallicity is conserved upon strain effect. The minority spin has a band gap of 1.68 eV. The actual value is larger than this due to the underestimation of the CBM of DFT in LDA scheme. The spin-flipping gap is estimated to be 0.97 eV. This value is still underestimated but still high enough to exclude the majority spin to be flipped by perturbations like thermionic emission. 104 Chapter 6. Ab initio study on the interface of CrSb/GaSb heterojunction 4.0 4.0 CBM=3.73 VBM=2.15 Ef=2.76 2.0 Energy (eV) Energy (eV) 2.0 0.0 -2.0 0.0 -2.0 -4.0 -4.0 -6.0 -6.0 Z A M G Z (a) Majority spin R X G Z A M G Z R X G (b) Minority spin Figure 6.2: Band structures of the majority spin (a) and the minority spin (b) along with concerned energies,Ef of the majority spin, CBM and VBM of the minority spin. Unit: eV We now turn to the electronic properties of the CrSb layers near the interface. By energy comparison it is found that the interface configuration in the form of ./Ga/Sb/Ga/Sb/Cr/Sb/Cr/Sb . is energetically favored.Due to the well matched lattice constants between CrSb and GaSb, no noticeable atomic relaxation is observed. Fig. 6.3 plots the spin DOS of Cr atoms in the three layers nearest to the interface. A cutoff radius of 1.50 ˚ A and a denser k-mesh are used for the DOS calculation. As shown by the DOS in Fig. 6.3 (a) and (b), the bulk property is recovered at the second Cr layer to the interface. For the first Cr layer to the interface, the DOS is slightly different. The VBM of the minority spin is shifted about 0.2 eV towards the Fermi level of the majority spin but no shift is observed for CBM of the minority spin. This shift-up indicates the occurrence of interface 105 Chapter 6. Ab initio study on the interface of CrSb/GaSb heterojunction states but there is no states at Fermi level. Therefore one can see that the halfmetallicity is perfectly reserved at the interface layers. The absence of massive interface states is not a surprise in view of the coherence of the interface and the small difference in electronegativity between Ga and Cr (1.81 for Ga and 1.66 for Cr in Pauling scale). 106 Chapter 6. Ab initio study on the interface of CrSb/GaSb heterojunction 4.0 2.0 (a) 0.0 DOS (states/eV) -2.0 -4.0 4.0 2.0 (b) 0.0 -2.0 -4.0 4.0 2.0 (c) 0.0 -2.0 -4.0 -4 -3 -2 -1 Energy (eV) Figure 6.3: The spin density of states projected on the third (a), the second (b) and the first (c) layer Cr atom to the interface. The Fermi level Ef is set to zero. Positive and negative values represent the majority and the minority spin states, respectively. The vertical dotted lines serve as an indicator. 107 Chapter 6. Ab initio study on the interface of CrSb/GaSb heterojunction Band alignment at the interface is another determinant for spin injection for a HMF to a Sc. To obtain the energy alignment at the interface the well-established “bulk plus line up” procedure where the macroscopic averaged electrostatic potentials are applied for references as proposed by van de Walle et al,20 is applied. The macroscopic averaged electrostatic potential (dash line) along the c direction is plotted in Fig. 6.4 (a) along with in-plane-averaged electrostatic potential (solid line). The difference between the two macroscopic averaged electrostatic potential ΔV (VGaSb -VCrSb ) is estimated to be 0.46 eV. Based on this lineup, one can obtain the energy alignment as shown in Fig. 6.4 (b). Here the experimental value 0.70 eV of GaSb band gap is used to determine the CBM. For the majority spin, the energy difference between the Fermi level and the CBM of GaSb is conventionally defined as Schottky barrier height (SBH). In this case the SBH is 0.89 eV with Fermi level lying below the CBM. This suggests that a Schottky barrier will be formed for n-GaSb and a reverse bias should be applied for majority spin tunneling into GaSb. Most of all, one finds that the CBM of CrSb lies slightly above that of GaSb by 0.08 eV. The actual value can be larger as mentioned above. This character suggests the capability of efficient spin injection. With the CBM of the minority spin lying above that of n-GaSb, the majority spin can be directly injected to n-GaSb with less probability of being flipped to the conduction bands of the minority spin under the applied reverse bias. Thus highly polarized spin current can be injected to n-GaSb. 108 Chapter 6. Ab initio study on the interface of CrSb/GaSb heterojunction 6.0 Energy (eV) 4.0 VGaSb=0.19(eV) (a) VCrSb=-0.27(eV) 2.0 0.0 -2.0 -4.0 CrSb layers GaSb layers CrSb layers GaSb layers -6.0 6.0 (b) Energy (eV) 5.0 4.0 3.0 2.0 Minority CBM=3.46(eV) Majority Ef=2.49(eV) Minority VBM=1.88(eV) 1.0 0.0 CBM=3.38(eV) VCrSb=-0.27(eV) VGaSb=0.19(eV) -1.0 Figure 6.4: The in-plane (solid line) and the macroscopic (dash line) averaged electrostatic potentials (a) and the energy alignment (b) based on (a). CBM and VBM in the left are for minority spin while Ef are for majority spin of CrSb. Unit: eV. 109 Chapter 6. Ab initio study on the interface of CrSb/GaSb heterojunction 6.4 Conclusion By first principle spin density functional theory calculation on the CrSb/GaSb heterostructure, the electronic properties and energy alignment at the interface have been obtained. By DOS calculation, the study showed that the CrSb can maintain its half-metallicity at the interface. The band alignment suggests the existence of a Schottky barrier and the probability for majority spins to be directly injected into the SC with less possibility of being flipped to conduction bands of the minority spin. These characters suggest that the CrSb/GaSb heterojunction is a potential robust spin current injector. 110 Bibliography [1] G. Schmidt, D. Ferrand, L. W. Molenkamp, A. T. Filip and B. J. van Wees, Phys. Rev. B 62, R4790 (2000) [2] C. J. Hill, X. Cartoix´a, R. A. Beach, D. L. Smith and T. C. McGill, condmat/0010058 (4 October 2000) [3] D. L. Smith and R. N. Silver, Phys. Rev. B 64, 045323 (2001) [4] E. I. Rashba, Phys. Rev. B62, 16267 (2000) [5] H. J. Zhu, M. Ramsteiner, H. Kostial, M. Wassermeier, H.-P. Sch¨onherr and K. H. Ploog, Phys. Rev. Lett. 87, 016601 (2001) [6] A. T. Hanbicki, B. T. Jonker, G. Itskos, G. Kioseogou and A. Petrou, Appl. Phys. Lett., 80, 1240 (2002) [7] A. T. Hanbicki, O. M. J van’t Erve, R. Magno, G. Kioseoglou, C. H. Li, G. Itskos, R. Mallory, M. Yasar and A. Petrou, Appl. Phys. Lett, 82, 4092 (2003) [8] R. A. de Groot, M. F. M¨ uller, P. G. van Engen and K. H. J. Buschow, Phys. Rev. Lett. 50, 2024 (1983) 111 BIBLIOGRAPHY [9] R. J. Soulen Jr., J. M. Byers, M. S. Osofsky, B. Nadgorny, T. Ambrose, S. F. Cheng, P. R. Broussard, C. T. Tanaka, J. Nowak, J. S. Moodera, A. Barry and J. M. D. Coey, Science, 282, 85 (1998) [10] X. Y. Dong, C. Adelmann, J. Q. Xie and C. J. Palmstrom, Appl. Phys. Lett. 86, 102107 (2005) [11] S. Picozzi, A. Continenza and A. J. Freeman, IEEE Trans. Magn. 38, 2895 (2002); S. Picozzi, A. Continenza and A. J. Freeman, J. Appl. Phys. 94, 4723 (2003); S. Picozzi, A. Continenza and A. J. Freeman, J. Phys. Chem. Solids 64, 1697 (2003) [12] H. Akinaga, T. Manago and M. Shirai, Jpn. Appl. Phys. 39, L1118 (2000) [13] J. H. Zhao, F. Matsukura, K. Taramura, E. Abe, D. Chiba and H. Ohno, Appl. Phys. Lett, 79, 2776 (2001) [14] G. Kresse and H. Hafner, Phys. Rev. B 47, 558 (1993), 48,13115 (1994) [15] G. Kresse and J. Furthmuller, Comput. Mater. Sci. 6, 15 (1996) [16] D. Vanderbilt, Phys. Rev. B 41, 7892 (1990) [17] H. J. Monkhorst and J. D. Pack, Phys. Rev. B 13, 5188 (1976) [18] B. G. Liu, Phys. Rev. B 67, 172411 (2003) [19] M. Shirai, J. Appl. Phys. 93, 6844 (2003) [20] C. G. V. de Valle and R. M. Martin, Phys. Rev. B 35, 8154 (1987) 112 Chapter Concluding remarks 7.1 Conclusions This study aimed to explore new half metallic materials by theoretical calculations. New dilute magnetic semiconductors (DMS) free of magnetic cations have been designed. This kind of new DMS might have the potential to avoid or alleviate the clustering problem in conventional DMS. Two new half metals have been proposed. Their lattice constants are suitable for epitaxial growth on wide gap semiconductors. In detail, the main findings in this thesis include: Calculations on the electronic structures of Cu-doped GaN and Mg-doped AlN showed that both Cu and Mg can induce spin polarization in the host semiconductors and have finite magnetic moment for each dopant even though they not show any ferromagnetism in their natural phases and nitrides. However, the mechanism of the spin polarization is different. In Cu-doped GaN, the spin-polarization 113 Chapter 7. Concluding remarks results from the strong hybridization between Cu-3d state and N-2p state of the nearest N atoms of Cu. In AlN, results showed that an Al vacancy can introduce strong spin polarization to the four N atoms nearest to the vacancy site while a substitutional Mg atom at the vacancy site saturates only part of the magnetic moment. In each case, the dopant-related net magnetic moments are found to couple ferromagnetically and the band structures are half metallic. Formation energy calculations showed that the formation energy of an substitutional Cu in GaN is 2.0 eV at optimum conditions and thus non-equilibrium method are required to growth Cu-GaN dilute magnetic semiconductors. For a substitutional Mg in AlN, the formation energy was found to be 0.34 eV. With this formation energy, 7% of Mg can be incorporated into AlN at 1500 K, which should generate an observable ferromagnetism at room temperature. Calculations on C-doped BN nanotubes showed that C becomes spin polarized with a magnetic moment of 1.0 μB on either B or N lattice site. Although this system favors an anti-ferromagnetic ground state, it gives a hint that DMS might also be fabricate by anion substitution at some particular host semiconductors. NiO has an anti-ferromagnetic rock-salt ground state with insulating nature. Calculations on the band structure of NiO in zinc-blend structure showed that it is likely half metallic and ferromagnetic and the lattice constant a being around 4.4 ˚ A. This lattice constant is close to those of wide gap semiconductors. The shift from anti-ferromagnetism to ferromagnetism may be attributed to the structural transition as well as the change of atomic coordinates. The half metallicity and the ferromagnetism survive the structural stress and zinc-blend NiO can be expected 114 Chapter 7. Concluding remarks to act as spin electrodes for wide gap semiconductors. Similarly, wurtizte NiO also has a half metallic band structure and ferromagnetic ground state. However, due to the inclusion of on-site Coulomb parameter U-J, the feasibility of growing NiO in these two structures cannot be evaluated by total energy calculation in this study. The study on the electronic properties of CrSb/GaSb interface shows that high efficient spin injection can be hoped from CrSb half metal to GaSb semiconductor. Nearly 100% polarization of the spin current can be obtained by electrical injection. 7.2 Future works This study presented only the preliminary electronic properties of the proposed new half metallic materials. Further theoretical studies are still needed for experimental guidance. More detailed theoretical studies on the magnetic properties of Cu-doped GaN and Mg-doped AlN DMSs are required. The Curie temperatures of these two DMS should be studied. Clustering tendency of the dopants in these two DMSs should also be compared with that of 3d DMS. Calculations on NiO in zinc-blend and wurtzite structure should be done by more complicated parameter-free Self-Interaction Corrected Local Spin Density Approximation (SIC-LSDA).1, In this method, the total energies of NiO in zinc-blend and wurtzite structures can be compared to that of in rock-salt structure. This is of great importance as it demonstrate the feasibility of experimental growth. 115 Chapter 7. Concluding remarks In addition, studies of the possibility of new DMS based on anion substitution should be carried out. As a first step, our calculations showed that substitutional C in AlN and N in ZnO favor a spin polarized ground state. This might an indication of DMS based on anion substitution. 116 Bibliography [1] J. P. Perdew and A. Zunger, Phys. Rev. B 23, 5048 (1981). [2] A. Svane, Phys. Rev. B 51, 7924 (1995). 117 [...]... good expitaixal growth, the lattice mismatch between a half metallic ferromagnet and a substrate semiconductor should not exceed 5% However, while some of the known half metallic ferromagnets can serve as spin electrodes for group II-VI and III-V semiconductors in zinc-blend structures with moderate band gaps, none of them can act as spin electrode for group II-VI and III-V semiconductors with wide band... structures (SiC, AlN, GaN and ZnO) For wide gap semiconductor in zinc-blend structure, the lattice constants are around 4.5 ˚ while the lattice constants of known half metallic ferromagnets in the same A 5 Chapter 1 Introduction structure range from 4.8-6.0 ˚.30–41 Similarly, the lattice constants a of known A half metallic ferromagnets in wurtzite structure are too large for these wide gap semiconductors... Chapter 2 Density functional theory for materials design 2.1 Introduction In principle, electronic properties of materials can be understood by solving the quantum mechanics Schr¨dinger equation First principles calculations based on o density functional theory (DFT) use fundamental physical lows and constants only, without empirical parameters Up to date, first -principles calculations have been proven... explanation and much interest has been given to DMS Half metals are another class of spin electrodes in addition to DMS due to their 100% polarization of carriers at Fermi level Recently, a lot of density functional theory calculations have been carrier out to search for possible half metals and some have been verified by experiments For instance, CrSb is a half metal that has been experimentally grown on... and VBM in the left are for minority spin while Ef are for majority spin of CrSb Unit: eV 109 xiii Chapter 1 Introduction 1.1 Semiconductor Spintronics Devices The mass, charge and spin of electrons in the solid state lay the foundation of the information technology we use today Integrated circuits and high-frequency devices made of semiconductors, used for information processing and... electrons in semiconductors Massive storage of information-indispensable for information technology-is carried out by magnetic recording (hard disks, magnetic tapes, magneto-optical disks) using spin of electrons in ferromagnetic materials It is the quite natural to ask if both the charge and the spin freedom of electrons can be used to further enhance the performance of semiconductor devices One may then... polarization near 100% Thus Schmidt et al proposed that two materials: dilute magnetic semiconductors (DMS) and half metals, can be efficient spin electrodes and much research has been carried out to study these two type of materials The following section will give a brief introduction of these materials 1.2 Dilute magnetic semiconductors and half metals DMS, obtained by doping conventional compound semiconductors... one of the most powerful tools for carrying out theoretical studies of electronic and structural properties of materials and advanced functional materials engineering Prediction of the electronic and structural properties of a material requires calculations of the quantum-mechanical total energy of the system and subsequent 14 Chapter 2 Density functional theory for materials design minimization of... Hartree-Fock method has found broad applications in atomic theory, but has only limited suitability for the majority applications to condensed matter For condensed matter theory the area 21 Chapter 2 Density functional theory for materials design of special interest concerns low density valence electrons for which correlations of electrons with antiparallel spins, neglected in Hartree-Fock method, yield... dopants for GaN and AlN respectively The spin-polarized electronic properties of Cu-doped GaN and Mg-doped AlN are studied by density functional theory calculations The magnetic coupling properties of these two systems is also studied In addition, C-doped BN nanotubes is also studied for interests in spin polarization in metal-free system To explore new half metals in zinc-blend and wurtzite structures for . FIRST-PRINCIPLES EXPLORATION FOR HALF METALLIC MATERIALS RONGQIN WU (B.Sc.,Fujian Normal University) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT. been performed to explore new half metallic materials for spintronics applications. Based on these calculations, 1) Cu-doped GaN, Mg-doped AlN and C-doped BN nan- otubes are predicted to be half metallic. mismatch between a half metallic ferromagnet and a substrate semiconductor should not exceed 5%. However, while some of the known half metallic ferromagnets can serve as spin electrodes for group II-VI