Graduate School of Pure & Applied Sciences, University of Tsukuba

Kuroda Laboratory
Semiconductor Spintronics Laboratory○ Semiconductor Spintronics●Magnetic Engineering●Semiconductor Electronics○ How to Fabricate DMS→ Molecular Beam Epitaxy (MBE)
Schematic image of MBE growth chamber
Inside of the chamber is Ultra High Vacuum condition.
~10-10 Torr (ex. 1 atm = 760 Torr)
It’s a very clean condition!!

Ex. Transistor, CPU, DRAM, LED

Ex. Magnet, HDD, MO,

Utilize charge of electrons
High Speed (calculation)
Volatility

Utilize spin of electrons
Non-Volatility (storage)
Low SpeedTurbo
MolecularPump(TMP)

RotarｙPump(RP)

HeaterSubstrate

RHEED Gun
RHEED Screen→ We need much electric power to keep data.

Spintronics : Utilize both of charge and spin of electrons
Ex. Giant Magnetic Resistivity effect (GMR)

Nobel prize in 2007

Cd Evaporation source (with shutter)

Te●Future application of Semiconductor Spintronics

FeEx. Spin transistor
Gate(VG)

SourceCr
Other possible applications

Drain・High-speed high-density nonvolatile memory
・Reconfigurable logic devices/computing
Realization of no standby-power electricity
・Integrated magneto-optical devices
・Quantum information devices using spin states

Current

Semiconductor
FM(2D electron gas)
VG is used to control spin direction of carrier.
FMAt the interface between semiconductor and Drain,
Spin directions between the carrier and Drain FM are

Parallellarge current
Anti-parallel
small current
We can control current with low power.

In order to realize spintronics devices,
we should explore ferromagnetic materials
for injection and detection of a spin polarized
current into a semiconductor.

○ Diluted Magnetic Semiconductors (DMS)

Max : 7 Solids Sources& a RF plasma source

ZnMerits of MBE growth● Keeping a clean substrate surface for many hours● Precise control of growth condition
・ Substrate temperature・ Thickness of growth films (~1 Å/ s)・ Flux ratio
Composition of dopant atom (< 1 %)● Growth films in non-thermal equilibrium condition● Characterize a sample surface structure during growth
by Reflection high-energy electron diffraction (RHEED)○ Highlights of Our Recent Results
Physical Review Letters 97, 037201 (2006) , Nature Materials 6, 440 (2007)

Ferromagnetic properties of (Zn,Cr)Te with Acceptor & Donor doping

Elements for semiconductor

Intrinsic Room-temperature Ferromagnetism when Cr composition is 20 %.

●Doping of charge impurities
p-type (Acceptor) : Nitrogen (N) n-type (Donor) : Iodine (I)Transition Metals (TM)for magnetic dopant

Magnetic properties (SQUID)

Doping magnetic TM in a host semiconductor material
TM ions substitute atomic sites of a host semiconductor.

300

I-doped200

undoped(Te-rich)100

N-doped
00.00

0.05

0.10

0.15

0.20

Cr composition x

Diluted Magnetic Semiconductors (DMS)

I-doping Ferromagnetism is enhanced
N-doping Ferromagnetism is suppressed●Characters of DMSs
・ They have both properties of the host semiconductor
and doped TM ions (magnetic) .
・ They can grow epitaxially on a semiconductor surface.

Tc at various Cr composition
Curie Temperature TC [K]M-H curves

Spatial distribution properties of Cr atoms (TEM)
I-dopeUndope (Te-rich)

N-dopeFerromagnetic DMS @ Room temperature

Promising materials for semiconductor spintronics
Search for high-TC ferromagnetic semiconductors

ZnO+ Mn+ Cr+ Gd+ Co

TC ~ 900K (Sonoda) vs PM (Munekata)> RT (Asahi)> RT (Asahi, Ploog)ZnO
TC > RT (Tabata) vs PM (Kawasaki)

TiO2+ CoTC > RT (Kawasaki)

ZnTe+ CrTC ~ 300K (Saito)

Ge+ Mn+ Fe

TC ~ 120K (Park)
170K (Tanaka)

GaN・Origin of FM
Intrinsic of DMS
orExtrinsic by precipitate (ex. TM cluster) ?

・Mechanism ?
Wide variety of magnetism reported so far even for the same materials
Possible factors which may explain these contradictory results of magnetic properties
・Extrinsic precipitates of FM compounds with different phases
・Non-uniformity of magnetic impurities in the host crystal

We must fabricate DMS samples precisely.

150nm

Inhomogeneous
Attractive
Enhanced

Cr distribution
Interaction between Cr
Ferromagnetism

Homogeneous
Repulsive
Suppressed

Summary
Doping of charge impurities or deviation from stoichiometry
Tune the interaction between TM ionsControl cluster formation & magnetic properties