Transcript Dilute Magnetic Semiconductors

Dilute Magnetic Semiconductors
(DMS)
NAN ZHENG
COURSE: SOLID STATE II
INSTRUCTOR: ELBIO DAGOTTO
SEMESTER: SPRING 2008
DEPARTMENT OF PHYSICS AND ASTRONOMY
THE UNIVERSITY OF TENNESSEE
KNOXVILLE
Outline
 Introduction: spintronics and DMS
 DMS materials
 (Ga,Mn)As
 (Ga,Mn)N
 Transitional metal doped oxide
 Magnetic mechanism studied by the Mean Field
Approach
 Summary
Introduction: Spintronics and DMS
 Spintronics: Spin-based electronics
 Idea: a combination of microelectronics and magnetic storage
technique.
Mass Storage

Searching for Materials??
Integrated Circuit
Introduction: Spintronics and DMS
 Diluted Magnetic Semiconductor (DMS):
Traditional semiconductors doped with transition
metals

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
Why “Dilute”?
Small doping concentration (a few %)
Why “Magnetic”?
Display ferromagnetisation
Why “Semiconductor”?
While preserving the semiconducting properties
Introduction: Spintronics and DMS
 Criteria of ideal materials for
spintronics:
 Room temperature
ferromagnetisation
 Fit into current electronic
technique
Theoretical
predictions
by Dietl, Ohno et al.
Various DMS displays room temperature ferromagnetism!
DMS materials I: (Ga,Mn)As
 First DMS material, discovered in 1996 by Ohno et al
using molecular beam epitaxy (MBE), a
breakthrough in experiment.
 Curie temperature Tc  110 K at optimal doping

Max TC ~ 110K
x ~ .05
[Ohno et al., APL 69, 363 (1996)]
DMS materials I: (Ga,Mn)As
 Metal to Insulator Transition (MIT)
Resistance measurements on
samples with different Mn
concentrations:
Metal
 R  as T 
Insulator  R  as T 
MIT happens at TC for
intermediate
Mn
concentrations
(0.035~0.053)
[Ohno, JMMM 200, 110 (1999)]
DMS materials I: (Ga,Mn)As
 Annealing Effect (observed in other DMSs as well)
Resistance

decreases
with
annealing time, up to 2 hrs, and then
increases again
WHY??
Two regimes at annealing time
Below 2h, T , FM , metallicity ,
lattice constant 
Origin related to defects, details unknown
DMS materials II: (Ga,Mn)N
 First room temperature
DMS discovered in 2001 Highest TC
in Dietl’s
using metal organic
chemical vapor deposition prediction
(MOCVD) method.

 High curie temperature


Experiment: up to Tc  800K
Theory: up to Tc  940K


DMS materials III:
Transition metal doped oxide
 Room temperature
ferromagnetism discovered
in Mn doped ZnO through
reactive magnetron cosputtering and fast
annealing in 2001.
 Material:


Mn doped ZnO
Co doped TiO
 Reported
TC up to 400K
Hysteresis curve at Room temperature
for Mn doped ZnO(Sn)
Magnetic Mechanism and Physical Properties
 Carrier-mediated mechanism:
Interaction between
hole spin and Mn
local moment is
AFM, giving rise to
an effective FM
coupling between
Mn spins
Doping magnetic atoms (eg. Mn: S=5/2)
Itinerant carriers (holes or electrons), s=1/2
[Dietl et al., PRB 55, R3347(1997)]
Magnetic Mechanism and Physical Properties
 Two basic approaches to understand magnetism in
DMS

Mean Field Theory based on Zener model
Scenario 1

Scenario 2
Clusters formed by magnetic atoms are responsible for
ferromagnetism
Magnetic Mechanism and Physical Properties
 MF approach further
explained:
 (A) High carrier density:
Carrier (electrons or holes,
depending on doping)
mediated interaction leads to
ferromagnetism.
 (B) Low carrier density:
Percolation network is
formed, carriers hop from
site to site freely, aligning Mn
moments within the cluster
network.
Pearton et al, Mat. Sci. Eng. R 40 (2003)
Magnetic Mechanism and Physical Properties
 How good is Mean Field Theory?
 Its reliability is case dependent.
Various MFT calculation for (Ga,Mn)As
Various MFT calculation for (Ga,Mn)N
Summary and Outlook
 Room temperature DMS already realized, while
explanation on the origin of ferromagnetism still
under refinement.
 Further development on mean field approach in
DMS:
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Monte Carlo simulations on local moment (eg. Mn)
distribution
Incorporation of defect structures (implied by annealing effect)
Correlation effects in the hole sub-system