Dilute Magnetic Semiconductors

1. Dilute Magnetic Semiconductors Josh Schaefferkoetter February 27, 2007

2. Introduction • Spintronic devices manipulate current with charge and spin • This added degree of control will require materials that have magnetic properties in addition to the traditional electronic properties • Semiconductors doped with magnetic atoms have recently been the subject of much research

3. Semiconductor • According to band-gap theory, the conduction and valence bands overlap in metals and they are separated by a large gap in insulators • Semiconductors lie between them, the two bands are separated by a smaller gap, and electrons can be excited to the conduction band

4. Pure Semiconductors • Silicon and germanium are intrinsic semiconductors • Gallium Arsenide is a compound semiconductor • In their pure form, their conductivity is determined by thermal energy • Electronic bonds must be broken to excite valence electrons to the conduction band

5. Crystal Structure • Silicon and Germanium are Group 4 elements with electron configurations [Ne] 3s23p2 and [Ar] 3d104s24p2 • In both crystals every atom is covalently bonded to 4 others sharing an electron each • This forms a tetrahedral configuration • GaAs is an example of a 3-5 compound semiconductor

6. MBE • MBE is an important tool in material science • Most common method of fabricating thin films

7. Doping • Intrinsic semiconductors like Si or Ge are doped with other atoms • Impurities to the lattice are introduced and this changes electrical properties • If a Group 3 element is used it is p-type doping • If a Group 5 element is used it is n-type

8. Magnetism • Magnetism arises from electron spin orbit coupling and the Pauli exclusion principle • Valence electrons in ferromagnetic materials align themselves • This creates magnetic domains

9. Magnetic Doping • Doping of transition metals with magnetic properties into conventional semiconductors • Relatively easy way to add magnetic properties to familiar materials • There are certain criteria that a magnetic semiconductor must satisfy • the ferromagnetic transition temperature should safely exceed room temperature • the mobile charge carriers should respond strongly to changes in the ordered magnetic state • the material should retain fundamental semiconductor characteristics, including sensitivity to doping and light, and electric fields produced by gate charges

10. (Ga,Mn)As • Configuration • Ga [Ar] 3d10 4s2 4p1 • As [Ar] 3d10 4s2 4p3 • Mn [Ar] 3d5 4s2 • The Mn atoms replace the Ga as acceptors • This introduces a hole because of the missing p-shell electron and a local magnetic moment of 5/2

11. Dopant Concentration • Theoretically, the Curie transition temperature increases with dopant concentration • Equilibrium growth conditions only allow 0.1% Mn doping before surface segregation and phase separation occur • Low temperature MBE increases this limit to around 1%

12. Current Research • Material science • Many methods of magnetic doping • Spin transport in semiconductors

13. Ferromagnetic Origin in DMS • The current understanding of ferromagnetism in DMS based on a simple Weiss mean field theory that studies the collective distribution of magnetic moments as a single continuous field • This is an approximation of the Zener model for the local (p-d) exchange coupling between the impurity magnetic moment, S 5/2 d levels of Mn and the itinerant carrier spin polarization, s 3/2 holes of p shell in the valence band of GaAs • According to kinetic exchange-coupling, the long range ferromagnetic ordering of Mn local moments arises from the local antiferromagnetic coupling between the carrier holes in (Ga,Mn)As and the Mn magnetic moments • Introduced in the 50’s, RKKY describes interaction between two electron spins or nuclear and electron spins throught the hyperfine interaction within MF theory

14. Theoetical Methods • Mean-field theories alone often can not accurately predict certain physical parameters such as Curie temperature • The theoretical generalization neglects to account for inconsistencies in the model like physical inhomogeneities such as spatial doping fluxuations • Percolation Theory and Monte Carlo simulations have proven useful in modeling random events • Dagotto et al. have developed theoretical predictions based on two-band model

15. Substitutional Impurities • Mn dopant atoms that lie at interstitial sites rather than cation substitutional sites tend to antiferromagnetically couple to other Mn atoms, reducing the magnetization saturation • The bonding configuration also introduces a double donor, overcompensating the single donor Mn cation subs (As antisites also are double donors)

16. Annealing • Small variations in material purity and lattice consistency can have a large negative effect on the bulk electrical and magnetic properties • Mninterstitiates can be removed by annealing at temperatures near that of the growth • This process does not significantly reduce the wanted Mn atoms in the cation sites because they are bound more tightly than the defects • However this reduces the total doping concentration, so ideal concentrations depend on the functionality of equipment HALL RESISTANCE Black 110K Red 130K Green 140K

17. Transition Temperatures • F. Matsukura, H. Ohno, A. Shen, and Y. Sugawara, “Transport Properties and Origin of Ferromagnetism in (Ga,Mn)As,” Phys. Rev. B57, R2037 (1998). • A. M. Nazmul, T. Amemiya, Y. Shuto, S. Sugahara, and M. Tanaka, “High Temperature Ferromagnetism in GaAs-Based Heterostructures with Mn Delta Doping”; see http://arxiv.org/cond-mat/0503444 (2005). • F. Matsukura, E. Abe, and H. Ohno, “Magnetotransport Properties of (Ga, Mn)Sb,” J. Appl. Phys.87, 6442 (2000). • X. Chen, M. Na, M. Cheon, S. Wang, H. Luo, B. D. McCombe, X. Liu, Y. Sasaki, T. Wojtowicz, J. K. Furdyna, S. J. Potashnik, and P. Schiffer, “Above-Room-Temperature Ferromagnetism in GaSb/Mn Digital Alloys,” Appl. Phys. Lett.81, 511 (2002). • Y. D. Park, A. T. Hanbicki, S. C. Erwin, C. S. Hellberg, J. M. Sullivan, J. E. Mattson, T. F. Ambrose, A. Wilson, G. Spanos, and B. T. Jonker, “A Group-IV Ferromagnetic Semiconductor: MnxGe1−x,” Science295, 651 (2002). • Y. Matsumoto, M. Murakami, T. Shono, T. Hasegawa, T. Fukumura, M. Kawasaki, P. Ahmet, T. Chikyow, S. Koshihara, and H. Koinuma, “Room-Temperature Ferromagnetism in Transport Transition Metal-Doped Titanium Dioxide,” Science291, 854 (2001). • M. L. Reed, N. A. El-Masry, H. H. Stadelmaier, M. E. Ritums, N. J. Reed, C. A. Parker, J. C. Roberts, and S. M. Bedair, “Room Temperature Ferromagnetic Properties of (Ga, Mn)N,” Appl. Phys. Lett.79, 3473 (2001). • S. Cho, S. Choi, G.-B. Cha, S. Hong, Y. Kim, Y.-J. Zhao, A. J. Freeman, J. B. Ketterson, B. Kim, Y. Kim, and B.-C. Choi, “Room-Temperature Ferromagnetism in (Zn1−xMnx)GeP2 Semiconductors,” Phys. Rev. Lett.88, 257203 (2002). • S. B. Ogale, R. J. Choudhary, J. P. Buban, S. E. Lofland, S. R. Shinde, S. N. Kale, V. N. Kulkarni, J. Higgins, C. Lanci, J. R. Simpson, N. D. Browning, S. Das Sarma, H. D. Drew, R. L. Greene, and T. Venkatesan, “High Temperature Ferromagnetism with a Giant Magnetic Moment in Transparent Co-Doped SnO2−δ,” Phys. Rev. Lett.91, 077205 (2003). • Y. G. Zhao, S. R. Shinde, S. B. Ogale, J. Higgins, R. Choudhary, V. N. Kulkarni, R. L. Greene, T. Venkatesan, S. E. Lofland, C. Lanci, J. P. Buban, N. D. Browning, S. Das Sarma, and A. J. Millis, “Co-Doped La0.5Sr0.5TiO3−δ: Diluted Magnetic Oxide System with High Curie Temperature,” Appl. Phys. Lett.83, 2199–2201 (2003). • H. Saito, V. Zayets, S. Yamagata, and K. Ando, “Room-Temperature Ferromagnetism in a II–VI Diluted Magnetic Semiconductor Zn1−xCrxTe,” Phys. Rev. Lett.90, 207202 (2003). • P. Sharma, A. Gupta, K. V. Rao, F. J. Owens, R. Sharma, R. Ahuja, J. M. Osorio Guillen, B. Johansson, and G. A. Gehring, “Ferromagnetism Above Room Temperature in Bulk and Transparent Thin Films of Mn-Doped ZnO,” Nature Mater.2, 673 (2003). • J. Philip, N. Theodoropoulou, G. Berera, J. S. Moodera, and B. Satpati, “High-Temperature Ferromagnetism in Manganese-Doped Indium–Tin Oxide Films,” Appl. Phys. Lett.85, 777 (2004). • H. X. Liu, S. Y. Wu, R. K. Singh, L. Gu, D. J. Smith, N. R. Dilley, L. Montes, M. B. Simmonds, and N. Newman, “Observation of Ferromagnetism at over 900 K in Cr-doped GaN and AlN,” Appl. Phys. Lett.85, 4076 (2004). • S. Y. Wu, H. X. Liu, L. Gu, R. K. Singh, M. van Schilfgaarde, D. J. Smith, N. R. Dilley, L. Montes, M. B. Simmonds, and N. Newman, “Synthesis and Characterization of High Quality Ferromagnetic Cr-Doped GaN and AlN Thin Films with Curie Temperatures Above 900 K” (2003 Fall Materials Research Society Symposium Proceedings), Mater. Sci. Forum798, B10.57.1 (2004). 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53

18. Spin Transistor • Spin transistors would allow control of the spin current in the same manner that conventional transistors can switch charge currents • This will remove the distinction between working memory and storage, combining functionality of many devices into one

19. Datta Das Spin Transistor • The Datta Das Spin Transistor was first spin device proposed for metal-oxide geometry, 1989 • Emitter and collector are ferromagnetic with parallel magnetizations • The gate provides magnetic field • Current is modulated by the degree of precession in electron spin

20. Current Research • Weitering et al. have made numerous advances • Ferromagnetic transition temperature in excess of 100 K in (Ga,Mn)As diluted magnetic semiconductors (DMS's). • Spin injection from ferromagnetic to non-magnetic semiconductors and long spin-coherence times in semiconductors. • Ferromagnetism in Mn doped group IV semiconductors. • Room temperature ferromagnetism in (Ga,Mn)N, (Ga,Mn)P, and digital-doped (Ga,Mn)Sb. • Large magnetoresistance in ferromagnetic semiconductor tunnel junctions.