APPLIED PHYSICS LETTERS
VOLUME 83, NUMBER 22
1 DECEMBER 2003
Luminescent spin-valve transistor
Ian Appelbaum,a) K. J. Russell, D. J. Monsma, V. Narayanamurti, and C. M. Marcus
Gordon McKay Laboratory, Harvard University, Cambridge, Massachusetts 02138
M. P. Hanson and A. C. Gossard
Materials Department, University of California, Santa Barbara, California 93106
共Received 29 July 2003; accepted 7 October 2003兲
A magneto-optical sensor, the luminescent spin-valve transistor, is demonstrated, showing direct
control of a light source using a magnetic field. By manipulating the relative magnetizations of
thin-film ferromagnets in the transistor’s base, the luminescence intensity is modulated by
approximately 200%. © 2003 American Institute of Physics. 关DOI: 10.1063/1.1630838兴
Since its discovery in 1988,1–3 magnetic-field sensors
based on the giant magnetoresistance 共GMR兲 of magnetic
multilayers have been the subject of much research that has
led to important technologies.4 – 6 One such sensor, the spinvalve transistor 共SVT兲, modulates a hot-electron current
passing perpendicularly through the multilayers.7–9 The relative magnetizations of its constituent magnetic layers control
the collector current, since attenuation in these films is exponentially dependent on the electron mean free path 共mfp兲 of
minority spin 共anti-aligned with magnetization兲 and majority
spin 共aligned with magnetization兲 electrons. The SVT, unlike
devices which make use of the GMR of in-plane currents in
magnetic multilayers, is capable of extraordinarily large
共100–1000%兲 changes in 共collector兲 current at constant
bias.10,11
A magnetic sensor with the sensitivity of the SVT but
with an optical signal output would have a distinct advantage
over similar existing devices because this additional measurement channel could enable the construction of, for instance, high spatial resolution magnetic-field-sensing arrays
and displays or remote-sensing devices. In this letter, we
report on the design and operation of a light-emitting,
magnetic-field-sensing device based on the SVT.
The recently developed light-emitting metal-base
transistor12 can be modified into a luminescent SVT by substitution of a magnetic multilayer for the otherwise normalmetal base. A schematic band diagram of the unbiased device
is shown in Fig. 1共a兲. When the emitter is biased with voltage V e 关as in Fig. 1共b兲兴, electrons tunnel through an insulating barrier and travel ballistically through the magnetic
multilayer base. The spin-dependent mfp in the first ferromagnetic layer spin-polarizes the hot-electron current passing through it, and the second ferromagnet acts as a spin
analyzer. Changing the relative magnetizations of the ‘‘polarizer’’ and ‘‘analyzer’’ modulates the hot-electron current.
If the emitter energy, eV e , exceeds the Schottky barrier
共SB兲 height, these electrons have enough energy to couple
with states in the collector.13 Because the collector consists
of a p – i – n junction for light emission, a collector bias V c
must be applied to draw the injected electrons into the luminescence region for recombination in an undoped quantum
well 共QW兲. This ballistic electron emission luminescence oca兲
Electronic mail: appeli@deas.harvard.edu
curs when the collector bias energy eV c exceeds the difference between the SB and the QW bandgap energy (E g ). 12,14
These conditions are shown schematically in Fig. 1共b兲.
The light-emitting collector heterostructure portion of
the device was grown via molecular-beam epitaxy with the
following structure: heavily doped p-type GaAs substrate,
300 nm p-type GaAs buffer layer doped to 5⫻1018 cm⫺3 ,
300 nm p-type Al0.30Ga0.70As doped to 5⫻1018 cm⫺3 , 10
nm GaAs undoped QW, 100 nm n-type Al0.30Ga0.70As doped
to 2⫻1017 cm⫺3 , and a 20 nm n-type GaAs cap layer doped
to 2⫻1017 cm⫺3 . All n-type doping is with Si, all epitaxial
p-type doping is with Be, and substrate doping is with Zn.
These collector heterostructure parameters were chosen
FIG. 1. Schematic band diagram of the luminescent SVT with V e ⫽0 and
V c ⫽0 共a兲, and eV e ⬎SB and eV c ⬎E g ⫺SB 共b兲.
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© 2003 American Institute of Physics
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Appl. Phys. Lett., Vol. 83, No. 22, 1 December 2003
FIG. 2. Optical profile of the 900⫻900 ␮ m2 luminescent SVT. The two
triangular-shaped structures form tunnel junction and ohmic contact to the
underlying 500⫻500 ␮ m2 magnetic multilayer base. The ‘‘⬎’’-shaped region is an insulating layer for wire bonding. The small diamond shape is an
alignment mark.
to provide an n-type Schottky interface at the surface and a
hole-rich recombination region in the otherwise undoped
GaAs QW. The n-type surface doping is required so that the
injected electrons have long lifetimes as majority carriers
before recombining radiatively in the QW. Additionally, the
doping results in a built-in field which pulls electrons into
the semiconductor. This doping must be relatively low to
ensure Schottky contact, but not so low that the surface
depletion region extends into the p – i – n junction, which
would cause leakage upon reverse biasing. The p-type doping level beneath the undoped QW must be significantly
higher than that of the n-type region to compensate the electron population in the QW.
The wafer was processed using standard shadow-mask
and photolithographic techniques to fabricate SVTs. After
cleaning the surface with dilute NH4 OH, a magnetic
multilayer consisting of 50 Å Ni84Fe16 , 50 Å Cu, 50 Å Co,
100 Å Al was then deposited via thermal evaporation at high
vacuum to form a 500⫻500 ␮ m2 base Schottky contact.
Next, 1000 Å of Al2 O3 was deposited via electron-beam
evaporation over a portion of the multilayer base to form an
insulating region. To grow the tunnel oxide, the sample was
exposed for 300 s to UV O3 . Two 600 Å Al emitters were
deposited via thermal evaporation, partially on the Al2 O3
insulator for wire bonding. These conductors form two tunnel junctions on the base layer. The sample was then processed into device mesas approximately 2 ␮m high and of
area 900⫻900 ␮ m2 by patterning with photolithography and
etching with NH4 OH/H2 O2 /H2 O 1:1:5 for 45 s. An image of
the completed device taken with an interferometric optical
profiler is shown in Fig. 2.
Electrical contact to the collector substrate was made by
cold-pressing In into the back surface. Wire bonds to the two
Al stripes provide emitter and base contacts after ramping a
current through both tunnel junctions until one shorted. All
measurements were done in an optical cryostat at 77 K. The
luminescence was collected with a 0.2 numerical aperture
lens, and the spectra were recorded with a Thermo-Oriel
MS257 spectrograph with a cooled CCD camera, a diffraction grating with 150 lines/mm, and a blaze wavelength of
800 nm.
Appelbaum et al.
FIG. 3. Spectra of emitted luminescence under constant emitter and collector voltages, with magnetic field such that the magnetizations of the ferromagnetic layers are parallel (⫺200 G) and antiparallel 共30 G兲.
The luminescent SVT is operated at constant emitter bias
and collector bias. An emitter bias energy far greater than the
SB (⬇0.8 eV) is needed to inject a substantial hot-electron
current, but this voltage must be kept lower than the breakdown voltage for the tunnel junction; we use V e ⫽⫺1.7 V,
which gives an emitter–base current of 2.6 mA. A collector
bias energy exceeding the difference between the QW bandgap energy (⬇1.5 eV) and the SB (⬇0.8 eV) is needed to
partially null the built-in field at the p – i – n junction; we use
V c ⫽0.9 V.
At an applied external magnetic field (⫺200 G) in excess of the coercive fields of Ni84Fe16 and Co, the magnetizations of the ferromagnetic layers are aligned parallel. The
hot electrons traveling through the base have a long mfp in
both ferromagnetic layers, and a relatively large fraction of
electrons can couple with semiconductor conduction band
states and radiatively recombine in the QW, emitting light.
Figure 3 shows the optical spectrum of this luminescence.
After reducing the magnetic field to zero and then increasing
in the opposite direction to a value 共30 G兲 between the two
coercive fields, the magnetizations are aligned antiparallel
and inelastic scattering in the base attenuates the hot-electron
current, leading to a reduced optical signal, also shown in
Fig. 3.
Cycling the external field and measuring the collector
current yields a hysteresis curve typical of SVTs, as shown in
Fig. 4共a兲. A collector current change of ⬇133% is evident
from this data. Simultaneous collection of emitted luminescence spectra allows the construction of a similar hysteresis
curve shown in Fig. 4共b兲, where the peak heights of the collected luminescence spectra are plotted on the same magnetic
field axis as Fig. 4共a兲. The two hysteresis curves have a similar shape, but Fig. 4共b兲 shows a luminescence change of
⬇200%, significantly more than the collector current change
shown in Fig. 4共a兲. This suggests that, whereas the luminescence is due only to hot electrons injected into the conduction band, the collector current is at least partially due to an
induced leakage current which dilutes the change in collector
current.
These data show that the light emitted from a luminescent spin-valve transistor can be used as a complimentary
signal channel yielding information about the magnetic field.
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Appelbaum et al.
Appl. Phys. Lett., Vol. 83, No. 22, 1 December 2003
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optically-read magnetic random access memory, and read
heads for magnetic hard drives interfaced with optical networks.
The authors acknowledge support from the NSF under
#ECS-9906047, the ONR under #N00014-02-1-0994 and the
Harvard NSEC and MRSEC. This material is based upon
work supported by the Defense Advanced Research Programs Agency 共DARPA兲 under Award No. MDA972-01-10024 and The California Institute of Technology.
1
FIG. 4. Simultaneously collected hysteresis curves for collector current 共a兲
and luminescence spectrum peak 共b兲 at constant emitter and collector bias.
If the low luminescence signal can be increased by enhancing the observed luminescence efficiency 共for instance, by
changing the QW material or using a Bragg reflector deep
within the collector兲 or by increasing the collector current
共for instance, by implementing high-voltage avalanche multiplication in the collector Schottky depletion region兲, we
expect this type of device to be useful for remote-sensing
magnetic fields, magnetic-field visualization displays,
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