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

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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

• 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

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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

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MBE

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

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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
• 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

• 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

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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

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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 50s, RKKY describes
  interaction between two electron spins or nuclear
  and electron spins throught the hyperfine
  interaction within MF theory

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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

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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)

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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 110K Red 130K Green
140K
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Transition Temperatures

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  Transport Properties and Origin of
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  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,
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  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-d, 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-d Diluted
  Magnetic Oxide System with High Curie
  Temperature, Appl. Phys. Lett. 83, 21992201
  (2003).
• H. Saito, V. Zayets, S. Yamagata, and K. Ando,
  Room-Temperature Ferromagnetism in a IIVI
  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 IndiumTin
  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.
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• S. Y. Wu, H. X. Liu, L. Gu, R. K. Singh, M.
  van Schilfgaarde, D. J. Smith, N. R. Dilley, L.
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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

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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.