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.