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A Tale of Two Vacancies

A Tale of Two Vacancies. Peter Y. Yu Department of Physics, University of California & Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720.

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A Tale of Two Vacancies

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  1. A Tale of Two Vacancies Peter Y. Yu Department of Physics, University of California & Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720 Acknowledgments: Collaborators are Lei Liu, Wei Cheng, Zixun Ma and Samuel S. Mao. This work was supported by the Us Department Of Energy NNSA/NA-22, under Contract No. De-Ac02-05ch11231 Cardona Symposium 2010

  2. OUTLINE • INTRODUCTION • MOTIVATIONS • VACANCIES IN GROUP III-NITRIDES • FERROMAGNETISM DUE TO Ga VACANCIES • DOPING BY Gd • VACANCIES IN Cd-CHALCOGENIDES • CODOPING WITH OXYGEN • EVIDENCE OF O2 MOLECULES • CONCLUSIONS • ACKNOWLEDGMENTS Cardona Symposium 2010

  3. INTRODUCTION • Vacancies are often introduced during crystal growth at high temperature • Vacancies are important in determining the quality and electrical properties of semiconductors: • Vacancies allow impurities to diffuse more easily throughout a crystal • Vacancies involve dangling bonds and are electrically active, vacancies can cause self-compensation Cardona Symposium 2010

  4. MOTIVATIONS • Vacancies are detected mainly by two methods: High Resolution TEM and Positron Annihilation. Otherwise they are difficult to detect • Recent advances in First-Principle Density Functional Theory (DFT) make it possible to calculate the properties of vacancies • Present work attempts to study their role in • (1) room temperature Ferromagnetism in GaN:Gd • (2) incorporation of Oxygen into CdTe in the formation of O2 Cardona Symposium 2010

  5. Cardona Symposium 2010

  6. STORY 1 How Vacancies Produce Room Temperature Ferromagnetism In GaN Cardona Symposium 2010

  7. Ferromagnetism in GaN:Gd • Room temperature Ferromagnetism was reported by at least 2 groups in GaN doped with Gd : • “Magnetic, optical and electrical properties of GaN and AlN doped with rare-earth element Gd” by S. W. Choi, Y. K. Zhou, S. Emura, X. J. Lee, N. Teraguchi, A. Suzuki, and H. Asahi (2002-2006): • [Gd]:2-6% • Tc~400K • sample n-type with [e]>5x1019 cm-3. • “Gd-doped GaN: A very dilute ferromagnetic semiconductor with a Curie temperature above 300 K” byS. Dhar, L. Pérez, O. Brandt, A. Trampert, and K. H. Ploog (2005-2007) • [Gd]:1016-1019 cm-3 • Tc>300K • magnetic moment /Gd~4000mB • magnetic moment/Gd increases with defect concentration since ion-implanted sample has large moment than sample after annealing Cardona Symposium 2010

  8. Results of Ploog’s Group suggested that intrinsic defects played an important role in the Ferromagnetism.The nature and role of the intrinsic defect are, however, unclear. Our First-Principle Calculation suggests that: GaN:Gd is paramagnetic GaN containing GdGa+VN is also paramagnetic GaN containing GdGa+VGa is ferromagnetic Surprisingly GaN containing only VGA is also ferromagnetic! Ferromagnetism in GaN:Gd Cardona Symposium 2010

  9. Computation Method • Use spin-polarized DFT within the Generalized Gradient Approximation (GGA). • Electron correlation important for the d and f electrons of Gd ions are included (approximation known as GGA+U ) • Use Full-potential Linearized Augmented Plane Wave (FLAPW) as basis functions for calculating the electron eigenvalues and functions. Lei Liu, Peter Y. Yu, Zhixun Ma, and Samuel S. Mao.Ferromagnetism in GaN:Gd: A Density Functional Theory Study. Phys. Rev. Lett. 100, 127203 (2008) Cardona Symposium 2010

  10. Supercell model:GdGa7N8 Gd atoms are separated by 3 atom layers to simulate long range interaction between Gd atoms. Cardona Symposium 2010

  11. GaN:Gd is Paramagnetic f electrons Band Structure of GaN:Gd: f-electrons in Gd ions are magnetized but the coupling between the moments is paramagnetic Cardona Symposium 2010

  12. GdGa6N8 Supercell containing Gd and VGa Cardona Symposium 2010

  13. GaN containing GdGa+VGa is ferromagnetic! Ferromagnetism in GaN:Gd Top valence bands are 100% Polarized! Cardona Symposium 2010

  14. What is the source of the strong coupling between the Gd ions? Answer: coupling with the spin of holes introduced by Ga vacancies Ferromagnetism in GaN:Gd Spin-resolved DOS of Ga Vacancy in GaN Pratibha Dev, Yu Xue, and Peihong Zhang. Defect-induced intrinsic magnetism in wide-gap III-nitrides. Phys. Rev. Lett. 100, 117204 (2008). Cardona Symposium 2010

  15. How come the Gd magnetic moment is so large? Ferromagnetism in GaN:Gd • Local strain of Gd ion induces vacancies in the vicinity • The magnetic moment of Gd is enhanced by the 3 spins of each Ga vacancy nearby GdGa6N8 10 0.7 Cardona Symposium 2010

  16. Summary: Gd introduces Ga vacancies by producing tensile local strain Ga vacancies produce holes When the hole wave functions are localized enough (as in case of nitrides) they become spin polarized according to Hund’s Rule Coupling between the Gd and hole spins produce a strong ferromagnetic state. When the hole or Ga vacancy concentration is much higher than the Gd concentration the magnetic moment of Gd appears to be enhanced Ferromagnetism in GaN:Gd Cardona Symposium 2010

  17. Cardona Symposium 2010

  18. STORY 2 How Cd Vacancies allow O2 Molecules to be incorporated into CdTe Cardona Symposium 2010

  19. CdTe is an important semiconductor for thin film solar cells Oxygen is a common impurity in the manufacture of solar cells. O replacing Te (OTe) is an isovalent impurity. Since electronegativity of O>>electronegativity of Te, O will attract an electron to form O- which is a shallow acceptor. Mass of O<<Mass of Te so vibration of OTe is highly localized around O. These are called local vibration mode (LVM). LVM are very sharp and, therefore, sensitive probes of light impurities BACKGROUND Cardona Symposium 2010

  20. Infrared Absorption Spectra of CdTe:O (G. Chen, I. Miotkowski, S. Rodriguez, and A. K. Ramdas, Phys. Rev. B 75, 125204 (2007).) • Sample Growth strategies: • CdO to provide oxygen and ExcessCd to suppress VCd • A single sharp line is observed: • 0 = 349.8 cm-1 • FWHM = 0.24 cm-1 • Selection rule: • 1  5 Cardona Symposium 2010

  21. High Frequency Mode when VCd Is Present In CdTe:O without excess Cd Chen et al. observe 2 high frequency modes at : 1 = 1096.78 cm-1; 2 = 1108.35 cm-1. At high T n1 and n2 merged into one mode with frequency:1104cm-1. They attributed these 2 modes to vibration of a complex: OTe-VCd Cardona Symposium 2010 21

  22. 1 2 Ec E||c Te Model Proposed by Chen et al. Te Te 3 N = 1 VCd 1 c n1 O Cd Cd n2 1 N = 0 Cd Cardona Symposium 2010 22

  23. Dynamic Switching of VCd-O complex 0* As T increases Oxygen switches between sites: 1, 2 , 3 and 4 Cardona Symposium 2010 23

  24. Activation Energy for Switching 1/T W=42 meV Cardona Symposium 2010 24

  25. L. Zhang, J. T-Thienprasert, M.-H. Du, D. J. Singh, and S. Limpijumnong, Phys. Rev. Lett. 102, 209601 (2009) calculated, from first principle, the frequency of the OTe-VCd complex and obtained <500 cm-1. Similar high frequency modes (>1000 cm-1)have also been observed by Chen et al. in CdSe (G. Chen, J. S. Bhosale, I. Miotkowski, and A. K. Ramdas, Phys. Rev. Lett. 101, 195502 (2008)) so this complex is rather common in Cd chalcogenides. What is the identity of this complex of O in CdTe and CdSe? How to explain the dynamic switching? MOTIVATION FOR PRESENT WORK Cardona Symposium 2010 25

  26. First-Principledensity-functional theorybased on the GGA-PBE potential(J.P. Perdew, K. Burke, and M. Ernzerhof, Phys. Rev. Lett. 77, 3865 (1996)) Two commercial softwares: VASP (Vienna ab initio simulation package ) and MedeA (by Material Design ) Computational Method Cardona Symposium 2010 26

  27. Tests Of Softwares Cardona Symposium 2010 27

  28. Ball-and-Stick model of the cell: Cd31Te32O2 containing a VCd (blue ball) and a O2 molecule (red balls). The golden and green balls represent Te and Cd atoms, respectively. New Model of Oxygen-VCd Complex in CdTe O-O is oriented along the [111] axis and is displaced from the Cd site. Symmetry of complex is C3v Cardona Symposium 2010 28

  29. Defect Formation Energies in CdTe *S. H. Wei, S. B. Zhang, and A. Zunger J. Appl. Phys. 87, 1304 (2000). Cardona Symposium 2010 29

  30. Normal Modes of O2 Molecule in CdTe Vacancy O-O stretch:1112.5 cm-1 Rocking of the O2 molecule:192.1 cm-1 A1 Modes Libration Mode at 315.7 cm-1 Rocking of the O2 molecule:176.6 cm-1 E Modes Cardona Symposium 2010 30

  31. O-O Stretching Mode of O2 in gas form is not infrared-active since it has even parity O2 in VCd of CdTe has no inversion symmetry and therefore can be infrared-active. The calculated charge difference between the two O atoms in CdTe is ~0.05e and the bond length is ~0.13 nm (0.1208 nm in gaseous O2) giving an electric dipole moment of ~3 Debye. IR activity of O-O Stretching Mode in CdTe Cardona Symposium 2010 31

  32. Existence of Two IR modes in VCd-O2 n1 peak (singlet) n2 peak (doublet)>n1 The energy to rotate the O2 molecule by 90o~ energy of the libration mode=39 meV Cardona Symposium 2010

  33. Summary: Oxygen replacing Te has a LVM at ~350 cm-1. In the presence of VCd Oxygen prefers to form molecule inside the VCd The O2 molecule is oriented along the [111] direction but displaced from the center of the vacancy. The two O atoms occupy in-equivalent sites so the O-O stretching mode is IR-active Charge transfer to the neighboring Te atoms weakens the O-O bond and lowers the O-O stretching mode frequency. The calculated O-O stretching mode frequency is in good agreement with experiment The existence of two modes at low T and their convergence at high T are also explained by theory LVM of Oxygen in CdTe Cardona Symposium 2010

  34. Cardona Symposium 2010

  35. Using first-principle density-functional theory we have studied the electronic and vibrational properties of vacancies in CdTe and GaN. In GaN vacancies can be induced by replacing the cations with large rare-earth ions like Gd. The Ga vacancies produce holes which are spin polarized. They strongly coupled to each other and to the Gd spins. These results explains recently reported observation of ferromagnetism in GaN:Gd above room temperature and the enhancement of the magnetic moment per Gd by intrinsic defects. In CdTe the cation vacancies are large thus allowing small molecules like oxygen to be located inside them and forming a new kind of molecular complex. The vibrational modes of these molecular-vacancy complexes in CdTe explain the sharp high frequency local vibration modes reported in CdTe. CONCLUSIONS Cardona Symposium 2010

  36. CONGRATULATIONS MANUEL! Cardona Symposium 2010

  37. ANNOUNCEMENT: FOURTH EDITION OF YU & CARDONA IS NOW AVAILABLE! Cardona Symposium 2010

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