FWD: Univ. PhD Dissertation Defense/Xin Jiang

Xin Jiang xjiang at stanford.edu
Mon Dec 1 08:56:31 PST 2003



Speaker:		Xin Jiang
		(Research Advisor: Professor J. Harris)

Title:		Tunnel Spin Injectors For Semiconductor Spintronics

Date:		4 December, 2003

Time:		9:00 a.m.

Place:		CIS-X Auditorium



	Spin-based electronics, often referred to as "spintronics",
is a research field of intense current interest, which aims to
develop novel sensor, memory and logic devices by manipulating the
spin states of electrons or holes in semiconducting materials.  This
talk will focus on electrical spin injection into semiconductors,
which is a prerequisite for spintronics and, in particular, on
tunnel based spin injectors operable above room temperature.  Two
types of spin injectors are discussed: a magnetic tunnel transistor
(MTT) injector and a magnetic tunnel injector formed from CoFe/MgO.

The MTT is a three terminal device comprised of a metal emitter and
a metal base, separated by an insulating tunnel barrier, and a
semiconductor collector: in this talk the base contains
ferromagnetic materials and the emitter is formed from either
non-magnetic Cu or ferromagnetic CoFe.  The MTT is a hot electron
device operating at electron energies of ~1-3 eV above the Fermi
level.  Due to spin-dependent electron scattering in the base
region, the collector current of the MTT is highly spin-polarized,
as demonstrated by a magneto-current of more than 3400%.  The basic
concept and transport properties of the MTT will be discussed and
will be compared with the properties of a simpler two terminal
tunnel injector device comprised of a ferromagnetic CoFe emitter and
a novel MgO tunnel barrier.

	A quantum well light emitting diode structure is used to
detect the spin polarization of the injected electron current within
the semiconductor.  The electron spin polarization is determined
optically by measuring the circular polarization of the
electroluminescence (EL) from the quantum well when the
spin-polarized electrons recombine with unpolarized holes.  Using an
MTT injector a spin polarization of ~10% is found for an injection
electron energy of ~2 eV at 1.4 K.  This moderate spin polarization
is most likely limited by significant electron spin relaxation at
high energy.  Much higher spin injection efficiency is found using
the CoFe/MgO tunnel injector with spin polarization values of ~50%
at 100K.  The temperature and bias dependence of the EL polarization
provides insight into spin relaxation mechanisms within the


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