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DILUTE MAGNETIC SEMICONDUCTORSJosh SchaefferkoetterFebruary 27, 2007
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
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
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
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
MBE MBE is an
important tool in material science
Most common method of fabricating thin films
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
MAGNETISM Magnetism arises from
electron spin orbit coupling and the Pauli exclusion principle
Valence electrons in ferromagnetic materials align themselves
This creates magnetic domains
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
(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
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%
CURRENT RESEARCH
Material science Many methods of
magnetic doping
Spin transport in semiconductors
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
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
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)
ANNEALING Small variations in material purity and lattice consistency can
have a large negative effect on the bulk electrical and magnetic properties
Mn interstitiates 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 110KRed 130KGreen 140K
TRANSITION TEMPERATURES
F. Matsukura, H. Ohno, A. Shen, and Y. Sugawara, “Transport Properties and Origin of Ferromagnetism in (Ga,Mn)As,” Phys. Rev. B 57, 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,” Science 295, 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,” Science 291, 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. Forum 798, B10.57.1 (2004).
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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
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
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.