Semiconductor Spintronics: What’s It All About?

1
Semiconductor Spintronics Whats It All About?
Bruce D. McCombe Department of Physics and
Center for Advanced Photonic and Electronic
Materials University at Buffalo The State
University of New York Buffalo, NY 14260
ONR, DARPA and CAPEM
2
The Usual Suspects (Collaborators)

• University at Buffalo
• H. Luo, X. Chen growth (ideas) and structure
• K.P. Mooney, F. Gasparini magnetism
• M. Na, C. Ruester transport/magneto-transport
• G. Kioseoglou, Y.L. Soo, S. Kim, Y.H. Kao x-ray
• M. Furis, G. Itskos, G. Kioseoglou, C. Meining,
  A. Petrou
• optics and magnetooptics
• G. Comanescu IR
• Notre Dame University
• Y. Sasaki, X. Liu, J.K. Furdyna growth
• Penn State University
• S. J. Potashnik and P. Schiffer magnetism and
  magnetotransport

3
DARPA Consortium
Organization (Consortium) UB (lead Institution),
Notre Dame U., U. of Wuerzburg, Indiana U., Naval
Research Lab., Vanderbilt U., U. of Texas,
N.C.State U., WPI, Penn State U. Focus
III-Mn-V Semiconductors and their
Heterostructures (GaMnAs, GaMnSb, InMnAs)
4
Outline

• Background What is Spintronics
• Recent Developments -- Materials and Spin
  Injection
• Ferromagnetic III-Vs -- Materials/Physics ---
  Problems
• Our Approach -- Digital AlloysGaSb/InAs with Mn
• Some Selected Results - GaMnAs, GaMnSb
• Summary and Key Issues for the Future

5
Spintronics

• Conventional electronics charge of electron
  used to achieve functionalities e.g., diodes,
  transistors, electro-optic devices (detectors and
  lasers.)

• Spintronics manipulate electron spin (or
  resulting magnetism) to achieve new/improved
  functionalities -- spin transistors, memories,
  higher speed, lower power, tunable detectors and
  lasers, bits (Q-bits) for quantum computing.

6
Conventional Electronics
Metal Oxide Semiconductor Field Effect
Transistor MOSFET
7
Spintronics
Spin Valves, Spin transistors, Switches,
Modulators, MRAM,.
Spin Transistor
Inject polarized spin from one FM contact --
modulate current by modifying spin precession
via Rashba effect (Asymmetry - spin-orbit
interact.) Depends on perpendicular electric
field on 2DEG other FM contact is analyzer
Datta and Das, APL 56, 665 (1990)
8

BUILDING BLOCKS FOR SPINTRONICS
Nonvolatile
9
New Ballistic Spin Filter Concept
Total transmission
Total polarization
Difference between polarization flux in y and y
directions
Courtesy of K. W. Kim
10
BUILDING BLOCKS FOR SPIN-PHOTONICS
11
Why Semiconductor Spintronics? He who controls
magnetism (in semiconductors) rules the world
Dick Tracy, ca. 1940

• Possible Revolutionary Advances
• Very fast, very dense memory and logic at
  extremely low power
• Spin Quantum Devices (Spin FETs, LEDs, RTDs)
• Quantum Computing at Room T
• Complete computer on a chip

12
III1-xMnxV Random Alloys (InMnAs and GaMnAs)

• Started by Munekata and Ohno (80s) IBM
  Japan
• Initially not widely received -- materials pretty
  bad (still are)
• III1-xMnxVs without precipitates (grown below
  300oC)
• Poor optical quality (no PL from LT GaAs)
• All III1-xMnxVs are heavily p-type
• No excitonic absorption from heavily doped
  samples
• Mn is dopant (acceptor) in III-Vs generally not
  desirable for alloying

13
MBE Phase Diagram Ga1-xMnxAs Random Alloys

• - Low concentration (lt 1
• insulating, not FM)
• - Higher Mn concentration
• (1 - 7 - metallic, FM)
• Clustering of a few Mn ions
• (not precipitates)
• AF exchange between Mn ions
• overcome by carrier-mediated
• exchange
• - Even Higher Mn Conc.
• (gt 8 - insulating, not FM)
• Complex situation Disorder,
• Residual Magnetism, self-
• compensation

14
MBE-grown GaMnAs Random Alloys

• Ga-Mn-As
• Ferromagnetic from about 3 - 7 Mn
• I-M-I Transitions
• Carrier mediated mechanism
• Large density of holes (comes
• with the territory - Mn on Ga
• sites is an acceptor)

Highest TC so far -- 110 K
Furdyna/Schiffer - U. of Notre Dame
15
Mechanism Carrier-Mediated Exchange Interaction
16
Issues, Approach and Goals
17
Whats Needed?

• Ferromagnetic Materials (semiconductors)
• Spin Polarizers/Aligners
• Spin Injectors (and Long Spin lifetimes)
• Means of manipulating spins (B, E, light)

18
II-Mn-VI/III-V Spin Polarizer
19
Predicted Curie Temperatures
Weird Materials with low TC
Room Temp
Dietl et al., Science , (2000)
20
The 6.1 A? Materials
Why Interesting?
What Are They? (III-Vs)
21
Photo-induced Ferromagnetism
Munekata et al, PRL 78, 4617 (1997) (InMnAs)
22
Ferromagnetic Resonant Interband Tunneling Diode
(FRITD)

• Unique Band Alignment -- Novel Device
  Structures Possible
• Carriers (electrons) have high mobility and
  longer spin-coherence lifetimes
• Spin Polarizer or Spin Filter -- can be bias
  controlled

23
Digital alloys 2 Types
Atomic Layer Epitaxy
24
GaMnAs Digital Alloys
25
TEM of GaMnAs Digital Alloys (Two series of
baseline samples)

• GaAs Spacer 8 monolayers
• (? 2.3 nm)
• 8 seconds exposure ALE
• (? 0.2 monolayer of Mn)
• Clear Superlattice Structure
• No evidence of 3D Clusters

Series of identical samples parameter varied
is Mn exposure time from 1 sec to 22 sec (0.02
- 0.5 ML)
26
Magnetization MeasurementsGaMnAs Digital Alloys
MR? lt 1/3 MR,in-plane
? 0.2 ML Mn
Easy Axis in-plane HC ? 100 Oe
27
Summary of Magnetic Measurements
28
Anomalous Hall Effect
Rhall R0B RSM
Anomalous
Normal (dominates at high B)

• Magnetic Moments plus spin-orbit interaction
• Skew Scattering (RS ? ?sheet)
• Side-jump Scattering (RS ? ?2sheet)

29
MagnetoTransport Measurements Digital GaMnAs
Alloys
8 sec exposure, Higher flux -- 0.4 ML
Thermally activated resistance Samples in this
series all Ferromagnetic
30
Correlation between Positive Magnetoresistance
and FM
8 sec Mn -- 0.4 ML
31
MagnetoTransport Measurements Digital GaMnAs
Alloys
10 sec exposure, lower flux -- 0.5 ML
Dominated by Rsheet B dep.
Thermally activated resistance Samples in this
series all Ferromagnetic
32
MagnetoTransport Measurements -- Digital GaMnAs
Alloys
Anomalous Hall Effect
Lower flux, short exposure -- 0.15 ML
33
T Dependence of Sheet Resistance
0.15 ML Sample (shorter exposure)
Critical Scattering
34
Temperature Dependence of Sheet Resistance
Digital Alloys
Similar resistance, Similar magnetotransport,
Similar Curie TC , Different activation behavior
35
Summary - GaMnAs Digital Alloys

• Systematic study of digital GaAsMn alloys Mn
  layer coverages lt 0.5 monolayer -- Good
  Structural
• properties (TEM, X-ray)
• ESD and TC correlated -- maximum TC vs. Mn
  layer
• coverage local structure important --
  Clustering, AF
• spin-spin coupling, phases?
• Anomalous Hall Effect. Magnetoresistance --
  initially
• positive followed by large negative MR in FM
  regime
• Thermally activated (hopping) conductivity in
  all
• samples -- lnR ? T-1/2, T-1/4 observed
• Modified mechanism for FM -- no free holes

36
GaMnSb Digital Alloys
37
GaMnSb Digital Layers
38
Squid Magnetization Measurements
Changing Hysteresis Loops up to Room T
Magnetization at Room T
39
GaMnSb Digital Alloys-Magnetotransport
0.5 ML Mn 10 ML GaSb Spacer
40
Arrott Plot from Magnetotransport
Magnetization Persists to very High T
TC about 50 K
41
GaMnSb Digital Alloys-Magnetotransport
Magnetoresistance
Hall Resistance
0.5 ML Mn 12 ML GaSb Spacer
42
Magnetic Force Microscopy
0.5 ML Mn 12 ML GaSb Spacer
43
Possible Model

• Small (lt 20-30 nm) 2D metallic
• FM Islands of MnSb (TC gt 500 K)
• embedded in Random Matrix
• of Mn Substituting for Ga

• Large Hole density (about 10 of Mn density) due
  to random isolated Mn acceptors near the MI
  transition -- holes interact with magnetic
  islands at high T -- Two critical temperatures

44
Calculations of Curie Temperature in Digital
Alloys

• Large VBO localizes hole
• wavefunctions in vicinity
• of ferromagnetic layers
• MFT predicts Tc up to RT
• GaMnAs/GaAs SL Large
• Tc only if P gt 1020 cm-3
• GaMnAs/Al(Ga)As SL
• Strong Tc enhancement
• even for P lt 1019 cm-3!
• Digital Layers are Good

Strongest Hole Confinement
Courtesy of Jerry Meyer, NRL
45
Summary/Future Directions
46
Magnetization MeasurementsGaMnAs Digital Alloys
Easy Axis in- plane HC ? 100 Oe
47
Temperature Dependence of Sheet Resistance
Crossover
Crossover
48
Magnetotransport Measurements Digital GaMnSb
Alloys
Neg. Magnetores. much smaller than GaMnAs
TG 273C, 50 repetitions 9 ML GaSb spacer ?
0.2 ML Mn p-type
49
Infrared Absorption Measurements
GaAsMn diffused (Linnarsson et al. PRB 55,
6938 (1997).
Impurity Band Transitions ?
Sample 01223C
MBE GaAsMn epilayer low density (TS 590 C)
semiconducting p ?
50
Photoluminescence Measurements Random GaAsMn
Mn Acceptor
LO
Data taken at NHMFL
51
Photoluminescence Measurements Random GaAsMn
Band Edge Region
Mn Acceptor Region
52
Photoluminescence Measurements Digital GaMnAs
Alloys
Mn Acceptor Region
Band Edge Region
53
REFLECTIVITY of GaMnAs DIGITAL ALLOYS
Curie temperature around 40 K Highly resistive
Sheet hole density 2-3 x 1010 cm-2 at room T
Curie temperature around 30 K Highly resistive
cant estimate sheet hole density
Not Ferromagnetic insulating
Optical, magnetic and transport properties are
Correlated
54
Magnetization Measurements Dependence on Bonding
Digital Alloys
55
MAGNETISM in Ga-Mn-As(Random Alloys)

• Low concentration (lt 1 - insulating not
  Ferromagnetic)
• - Magnetic properties determined by spins of
  individual Mn2 (S 5/2 ) - Paramagnetic
• Higher Mn concentration (between 1 and 8 -
  metallic Ferromagnetic)
• - Clustering of a few Mn ions (not precipitates
  which are larger)
• - Antiferromagnetic exchange between Mn ions
  overcome by carrier-mediated exchange

• Even Higher Mn Concentration ( gt 8 -
  insulating not
• Ferromagnetic)
• - Complex situation Disorder Residual
  Magnetism self- compensation

56
GaMnAs Phase Diagram
(H. Ohno , J. of Magnetism and Magnetic Materials
200,(1999))
57
Spin Injection
Spin injection into semiconductors
Abstract Spin injection (spin polarized
current) results from the passage of a current
through a contact between a ferromagnet and a
semiconductor. Depending on the type of contacts,
either the majority or the minority carriers may
be polarized. An analysis is made of the
influence of a magnetic field on such spin
injection and conditions for its observation are
discussed. Soviet Physics - Semiconductors 10,
698 (1976). A. G. Aronov and G. E. Pikus B. P.
Konstantinov Institute of Nuclear Physics and A.
F. Ioffe Physicotechnical Institute Academy of
Sciences of the USSR, Leningrad
58
GaAs/MnGa Superlattices
Substrates (100) GaAs Materials/Structures
8-16 ML GaAs/4 ML MnGa Growth temperature
275oC Deposition rate monitored with RHEED
oscillations Growth Mode MBE for GaAs/Atomic
Layer Epitaxy for MnGa
4 ML of MnGa (2 periods of Mn and Ga depositions)
59
Interface Formation in InAs/GaSb
GaSb
GaSb
InSb
GaAs
InAs
InAs
InSb interface bonds
GaAs Interface Bonds

• Two types (and combinations) of Interfaces InSb
  and GaAs
• Can Control Type During Growth
• Interface type affects Electrical and Optical
  Properties

60
Interface Effects on Band coupling
The k p coupling across the interface depends
on Overlap Integral of the electron and hole
subband envelope functions
Interface layer -- lower barrier
Interface layer -- additional barrier
Very simple(minded) Picture
61
Mn INCORPORATION

• Intrinsic Problem
• - Presence of precipitates for high Mn
  concentrations

• Past lessons
• - ?-doping is a highly effective method for
  increased doping concentration
• - Digital alloys result in high quality materials

• Our Approach III-V/Mn digital alloys
• - Increase Mn concentration and improve
  structural quality
• - 2-D spin systems and interlayer coupling
• - Optical and transport properties in ordered
  alloy (in 1D) systems

62
TEM of GaMnSb Digital Alloy
High resolution
Low resolution
63
R vs. T GaMnAs Digital Alloy(16 ML GaAs/lt1 ML
Mn)
Samples are all insulating (resistance vs.
Temperature)
Ferromagnetic up to About 40 K

• ? expT0/T1/2
• for variable-range
• hopping

Behavior observed over wide temperature range
64
MATERIALS ISSUES

• Mn is a p-type dopant for III-Vs (good and bad)
• Low growth temperature required (275oC)

Low Curie Temperature
65
Magnetization Measurements Digital GaMnAs
Alloys
Correlated Behavior
ESD Effective Spin density from saturation
Magnetization (S 2.5, g 2)
66
X-RAY REFLECTIVITY
Periods of digital alloys from Bragg peaks agree
well with thickness measured in situ
Weakly ferromagnetic sample -- 12- monolayer GaAs
spacers -- shows best crystal quality
67
Magnetization (Squid)
Easy Axis in-plane
68
III1-x Mnx Vs for Spintronics

• Materials Issues
• Precipitates for high Mn concentration
• Poor Structural/Optical Properties

• Past Lessons
• ?-doping highly effective for increasing
  concentration
• Digital alloys result in high quality materials