Transcript Introduction to Spintronics

DILUTE MAGNETIC SEMICONDUCTORS
Josh Schaefferkoetter
February 27, 2007
INTRODUCTION
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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
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PURE SEMICONDUCTORS
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Silicon and germanium are intrinsic semiconductors
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Gallium Arsenide is a compound semiconductor
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In their pure form, their conductivity is determined by thermal energy
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Electronic bonds must be broken to excite valence electrons to the
conduction band
CRYSTAL STRUCTURE
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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
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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
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MAGNETISM
Magnetism arises from
electron spin orbit coupling
and the Pauli exclusion
principle
 Valence electrons in
ferromagnetic materials
align themselves
 This creates magnetic
domains
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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
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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
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(GA,MN)AS
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Configuration
Ga
 As
 Mn
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[Ar] 3d10 4s2 4p1
[Ar] 3d10 4s2 4p3
[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
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DOPANT CONCENTRATION
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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
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Material science
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Many methods of
magnetic doping
Spin transport in
semiconductors
FERROMAGNETIC ORIGIN IN DMS
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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
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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
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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
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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 110K
Red
130K
Green 140K
TRANSITION TEMPERATURES
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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).
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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).
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F. Matsukura, E. Abe, and H. Ohno, “Magnetotransport Properties of (Ga, Mn)Sb,” J. Appl. Phys. 87, 6442 (2000).
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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.
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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
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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
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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).
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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).
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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).
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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).
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
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SPIN TRANSISTOR
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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 metaloxide 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
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CURRENT RESEARCH
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Weitering et al. have made numerous advances
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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.