COLLEGE OF APPLIED SCIENCE
(Affiliated to University of Kerala, Managed by IHRD)
Submitted in partial fulfillment of the requirements
for the award of the degree of BSc Electronics of
University of Kerala
DEPARTMENT OF ELECTRONICS
COLLEGE OF APPLIED SCIENCE
College of Applied Science
(Affiliated to University of Kerala, Managed by IHRD)
Certified that this is the bonafide report of seminar entitled
Of 5th semester BSc Electronics in partial fulfillment of the requirement
for the award of the Bachelors Degree in Electronics from the University
of Kerala during the year 2011-2014
VinodV RajendranReeni Sara Thomas
Department Of Electronics
College of applied science
Adoor – 691523
First and foremost I concede the surviving presence and the flourishing refinement of
GOD ALMIGHTY for His concealed hand yet substantial supervision all through our work.
I am extremely indebted to our respected principal Sree.Santhosh for rendering us all the
facilities for the successful completion of the study phase of my seminar.
I express my sincere gratitude to the Head of Electronics Dept. Mr. Vinod Rajendran to
provide me with his wholehearted support.
I express my heartiest thanks to my seminar coordinators Mr.Manoj.G and Miss.Reeni
for the valuable guidance provided in making this seminar a successful one.
I would also like to express my thanks to all other faculty members of electronics
department at College Of Applied Science, Adoor for the valuable help provided by them. I
would like to express my sincere gratitude and thanks to all our friends for their valuable
comments and suggestions for making this work a success.
In our conventional electronic devices we use semi conducting materials for logical
operation and magnetic materials for storage, but spintronics uses magnetic materials for both
purposes. These spintronic devices are more versatile and faster than the present one. One such
device is spin valve transistor.Spin valve transistor is different from conventional transistor. In
this for conduction we use spin polarization of electrons. Only electrons with correct spin
polarization can travel successfully through the device. These transistors are used in data storage,
signal processing, automation and robotics with less power consumption and results in less heat.
This also finds its application in Quantum computing, in which we use Qubits instead of bits.
1. INTRODUCTION …………………………………………. 1
2. HISTORY …………………………………………………... 2
3. THE SPINVALVE EFFECT ……………………………….. 4
4. THE SPINVALVE TRANSISTOR ………………………… 6
5. SPINVALVE TRANSISTOR PREPARATION ………….... 10
6. ADVANTAGES ……………………………………………. 14
7. APPLICATION …………………………………………….. 15
8. RELATED WORKS ……………………………………….. 16
9. FUTURE SCOPE …………………………………………... 17
10.CONCLUSION …………………………………………….. 18
11.REFERENCE ………………………………………………. 19
Spintronics is a rapidly emerging field of science and technology that will most likely have a
significant impact on the future of all aspects of electronics as we continue to move into the 21st
century. Conventional electronics are based on the charge of the electron. To use the other fundamental
property of an electron, its spin; have given rise to a new, rapidly evolving field, known as spintronics, an
acronym for spin transport electronics that was first introduced in 1996 to designate a program of the
U.S. Defense Advanced Research Projects Agency (DARPA). Initially, the spintronics program involved
overseeing the development of advanced magnetic memory and sensors based on spin transport
electronics Studies of spin-polarized transport in bulk and low-dimensional semiconductor structures
show promise for the creation of a hybrid device that would combine magnetic storage with gain—in
effect, a spin memory transistor.
Magnetic materials and magnetic devices have occupied a major place in science and
technology for most of the twentieth century and played a very important role in the emergence of the
digital computer by providing both ferrite core and plated wire memories. It was not until the early
1980s that thin-film magnetism was applied to higher-density nonvolatile random access memory .A
new path leading to the integration of magnetic devices into computer technology began to emerge
with the discovery of giant magnetoresistance (GMR at low temperatures and high magnetic
fields.Although it was known for quite some time that the current from a magnetic metal is spinpolarized and that current transport through adjacent magnetic layers depends on the spin-polarization
of those layers, neither the magnitude of the current nor the temperature at which it was observed
were of technological significance. Discoveries in this new field were quite rapid, and the path toward a
new technology started to appear quite early. The first significant GMR device was the spin valve
The magnetically sensitive transistor (also known as the spin transistor or spintronic transistor—
named for spintronics, the technology which this development spawned), originally proposed in 1990
and currently still being developed, is an improved design on the common transistor invented in the
1940s. The spin transistor comes about as a result of research on the ability of electrons (and other
fermions) to naturally exhibit one of two (and only two) states of spin: known as "spin up" and "spin
down". Unlike its namesake predecessor, which operates on an electric current, spin transistors operate
on electrons on a more fundamental level; it is essentially the application of electrons set in particular
states of spin to store information
Magnetic field sensors have found many applications: read heads in audio/video/computer
systems,magnetic random access memories (MRAMs), position/ rotation/ velocity sensors in cars/
aircrafts/satellites, electronic compass applications, measurement of currents and scientific
measurementinstruments. Other applications for magnetoresistive sensors include coin evaluation,noncontact switching, and measurement of currents. An important issue in digital magnetic recording is the
bitdensity and several new technologies have pushed this density forward. Futurehighdensity recording
systems will depend increasingly on more sensitive field sensors, because ofthe shrinking bit sizes and
magnetic fluxes. The thin film head, the thin film media and subsequentlythe introduction of
magnetoresistance heads enhanced the annual bit density increase drastically.Due to tailoring of the
magnetic materials in the base, the spin valve transistor shows a broadmeasurable field range and may
further enhance bit densities.
A major advantage of using magnetoresistive sensing of magnetic fields as compared to
inductivesensing is the static measurement mode of the MR sensor: a static magnetic field can be
detected incontrast to inductive pick-up coils for which a voltage is only generated by a temporal flux
change.For magnetic recording, the increase of density
leads to a corresponding reduction in magnetic signal.Inductive head designs have compensated for the
weakening signal by increasing the number ofturns in the coil, but each turn adds approximately 0.5
ohm of resistance to the circuit, with a correspondingincrease in thermal noise. Beyond 1 Gbit/in2, this
thermal noise of the coil becomes themain limitation preventing signal detection.
This new magnetoresistance called "Giant Magnetoresistance", was discovered in 1988
inmagnetic multilayers. It was soon called the spin valve effect because the magnetic layers actas valves
for electrons with different spin moments (spin up and spin down). The spin-valve transistor consists of
three regions: a spin-valve base, a hot electron injectorsuch as a Schottky barrier or a tunneling barrier
and a collector barrier which discriminates betweenscattered and ballistic (not scattered) electrons. The
base can be made of any magnetoresistive metal system.
3 .THE SPIN VALVE EFFECT
GMR effect can be observed in the conduction process in magnetic materials,particularly the
transition metals Fe, Co and Ni.Conduction electrons are divided in to two classes , those whose spin is
parallel to the local magnetization and those whose spin is anti parallel .The resistance to the flow of an
electronic current in a metal is determined by the scattering processesto which the electrons are
subject. If the scattering processes are strong and effective, themean free path (mfp) of an electron
between scattering processes is small and the resistance is large.Conversely, weak scattering processes
lead to a long mfp and a low resistance.
Fig 3.1: Graph of conduction in multilayer magnetic film array, showing how differential spin scattering
produces a different resistance for antiparallel (a) and parallel (b) film magnetizations.
Consider now electronic conduction in amultilayer array such as shown in Fig. 3.1 In Fig. 3.1a
the magnetic moments of successive ferromagnetic layers (Co) are antiparallel due toantiferromagnetic
coupling across the spacer layer (Cu). In (b) they are parallel due to an externalmagnetic field which is
strong enough to overcome the antiferromagnetic coupling. In case of Fig.3.1a, antiparallel moments, no
electron can traverse two magnetic layers without becoming unfavored,highly scattered species. An
electron conserves its spin orientation as it traverses a solid .Therefore if it was the
favored 'up'electron in an 'up' magnetization layer it becomes the unfavored 'down' electron in an 'up'
magnetizationlayer as soon as it traverses the few Ångstroms of the spacer layer. In the case depicted
inFig.3.1a, by contrast, an electron having the favored 'up' spin orientation in one magnetic layer hasthe
same favored orientation in all layers, and can traverse the array relatively freely. For configuration(a)
no electron traverses the array freely; for (b) half of the electron species can traverse thearray relatively
freely, and a significant difference in resistance is measured between the parallel andanti-parallel arrays.
4. SPIN VALVE TRANSISTOR
4.1 Spin valve transistor principle
The perpendicular electron transport and exponential mean free path dependence in metal
base transistorsallows for fundamental detection of the perpendicular spin valve effect by incorporating
aspin valve into the base. The base is formed by a spin valve. A Co44Å/Cu88Å/Co8Å/Pt88Åsandwich
base is sputtered onto a Si (100) collector substrate. The emitter is negatively biased(forward) using a
DC current source, the collector substrate is in reverse (positive voltage bias), incommon base.
The two magnetic layers act as polarizer and analyzer.
Co44Å/Cu88Å/Co8Å/Pt88Å sandwichbase is rf-sputtered onto the Si(100) collector substrate.APt
capping layer on top of the spin valve is used to make the emitter Schottky barrier larger thanthe
collector barrier, in order to decrease quantum mechanical reflections at the collector barrier.This can
also be seen in the schematic energy band diagram of the bonded Co/Cu spin-valve transistorin Fig4.2.
Fig. 4.2 Schematic energy band diagram of the spin-valve transistor under forward bias.
4.2 Current Transfer
The emitter bias accelerates the electrons over the emitter barrier, after which they constitute
the hot, quasi-ballistic electrons in the base. The probability of passing the collector barrier is limited
bycollisions in the base, which affect their energy and trajectory (momentum), by optical
phononscattering in the semiconductors and by quantum mechanical reflections at the base-collector
interface.For a metal base transistor with a single metal base film the relationshipbetween the collector
current density Jc and the injected emitter current density Jinj is
Where W is the base width (=thickness) and
the mean free path of the injected hot electrons
in thebase. e represents the emitter efficiency, qm represents quantum mechanical transmission and
crepresents the collector efficiency. Jleakageis the collector leakage current, determined by the
reversebiasedcollector Schottky barrier and Je is the injected emitter current. The avalanche
multiplicationfactor M depends on device design but if impact ionization is absent, equals one. The
leakage currentof thecollector may also contribute to the total collector current.
The emitter to collector currenttransfer ratio, or current gain is defined as:
Where the collector leakage current has been neglected. Here 0 is the common base current
gainand * is the common base current gain extrapolated to zero base
thickness. The factor
representsthe probability of transmission of the hot electrons through the
base. Jcis the total collectorcurrent. In the spin-valve transistor
under consideration, the collector current of the Co/Cu spinvalve transistor depends exponentially on
the spin dependent hot electron mean free paths inthe base. Neglecting spin-flip scattering, we may
consider the spin upand spin down electrons to carrythe current in parallel (two current models).
Following this idea, the collector current of the Co/Cu spin-valve transistor isexpressed as:
( ) denotes the product of transmission probabilities of spin up (+) and down (-). Electrons
through each layer and interface. In first approximation we take
qmsimilar forthe two
species of electrons since these quantities reflect the properties of the semiconductors andSchottky
barriers. At saturation, all Co layers have their magnetization parallel.
The sum of thetransmission probability factors for the two spin channels can then be written as:
At the coercive field, this quantity becomes:
Where WCoexpresses the sum of all Co layer widths (total Co thickness) which is valid for
equallythick layers, ½W is half of the total Co thickness; WCuis the total Cu thickness,
majority(minority) MFPs in the Co layers and
Cu the MFP in the Cu layer. The factor 2 in
eqnappearsbecause the two parallel channels are equal for antiparallel magnetizations. The values of
the collectorcurrent in the parallel (P) and antiparallel (AP) magnetic configurations are then
obtained.The typical properties in the spin valve transistor arethus:
1. Perpendicular GMR can be measured down to tri-layers
2. Exponential amplification of the magnetoresistance occurs because the transfer isexponentially
dependent on the electron mean free path in the base
3. Electron energy can be varied so electron spectroscopy can be performed by changing
emitterSchottky barrier height (or tunnel bias)
4. Measurements can be done at cryogenic and room temperature
5. Since the scattering processes appear as products in the transfer equation., the spin
dependentscattering centers can be located accurately and, in contrast to common -CPPMR, the
relativechange in collector current CC(%) is not decreased by spin independent scattering
processessuch as in the Cu layers or in the semiconductors
6. As a consequence of the direct MFP dependence of the transmission across the base, the spin
valvetransistor allows quantification of spin dependent electron MFPs
7. The output is a high impedance current source.
4.3 Resistance measurement
Resistance of the multilayer can be measured with Current In Plane (CIP) or Current
Perpendicular to the Plane (CPP) configurations. CIP is the easiest experimental approach of electrical
transport in magnetic multi layers. But the drawback of CIP configuration is that the spin valve effect is
diminished by shunting because many electron travel within one layer because of channeling.
Uncoupled multi layers or sandwitches with thick spacer layer suffer from this problem.Spin
independentboundary scattering reduces the CIP magnetoresistance largely in thin sandwiches.Also,
fundamentalparameters of the effect, such as the relative contributions of interface and bulk spin
dependentscatterings are difficult to obtain using the CIP geometry. Measuring with the
CurrentPerpendicular to the Planes (CPP) solve most of these problems, mainly because the electrons
crossall magnetic layers, but a practical difficulty is encountered: the perpendicular resistance of the
ultrathin multilayers is too small to be measured by ordinary techniques.
Fig. 4.3 a. CIP-GMR: shunting and channeling of electrons in the magnetic and nonmagnetic
layers versus b. CPP-GMR: perpendicular electrons cross all magnetic layers, no shunting at antiparallel
alignment.As shown in Fig. 4.3, a high resistant state (in zero field) can only be obtained if electrons
cross atleast two magnetic layers with antiparallel orientation. Because many electrons travel almost
parallelto the layers in the CIP-GMR, and do not cross many layers, the adjacent layers must have
theantiparallel orientation, i.e. they need an antiferromagnetic coupling. In the case of CPP-GMR the
electrons cross all layers, and a random orientation of the layers produces the same high resistantstate
as the AF-coupled state (“self averaging”). In CIP-GMR the electric field is independent of position in the
film, but the current density dependson the perpendicular direction to the film. The characteristic length
scale is the longest mean freepath. For CPP transport, the electricfield depends on the perpendicular
position in the film, but thecurrent density is independent of position in the film. The spin diffusion
length is the new lengthscale.
5. VACUUM BONDING: SPIN VALVE
5.1. Schematic process flow
In vaccum bonding initially cleaning process is done.For this 1 micron tetra ethyl ortho silicate
(TEOS) SiO2 is used as protecting layer and the Si fragments are etched away using HF/HNO3 at room
temperature isotropic etch.Preparation scheme is shown in figure 5.1.
Fig. 5.1 Schematic process flow for the preparation of vacuum bonded spin valve transistors
5.2 Deposition of the base layers
For deposition of the base layers a DC-RF magnetron sputtering machine is used. The robot is
insertedinto loadlock F and transported using beam G to the main chamber
A after approximately 1hour pumping. Multilayers can be deposited using a computer controlled
rotating table and depositionshutters.
Fig. 5.2 High vacuum DC/RF magnetron sputters system.
The properties of the system are: background pressure typically 10-9 mbar, three magnetron
sputterguns, variable substrate-target distance, heated substrate table, RF and DC power supplies.
Twelvedifferent samples can be sputtered in one run using the specially designed substrate rotator,
ofwhich a schematic picture is shown in Fig. 5.2.
Fig. 5.3 Substrate rotator for multiple in-situ sample preparation.
Spring 1 is wound up using manipulator 2. Samples 6 are mounted on rotating table 4.
Depositionoccurs via 5. Substrate selection is via magnetically coupled beam 3. In this way optimized
GMRmultilayers and sandwiches can be found quickly.
5.3 Emitter wafer thinning
After bonding, the emitter substrate has to be thinned down to dimensions which allow
definition oftransistors to micron dimensions. For this reason, the emitter has to be thinned down to
about 1 to 5micron. A major requirement is that the emitter substrate needs a highly doped region for
ohmiccontact formation (the emitter barrier contact is reverse biased, in contrast to the collector
contact).In so called BESOI (Bond and Etch back Silicon On Insulator) several techniques are known
tocome to a small device layer:
1. Grinding and polishing
2. Etch stop layers
Grinding and polishing is a possibility for the required device layer thickness, and would be
themost obvious way for standard wafer size. Thickness variations of about 0.5 microns are
achievable.In grinding one has to be careful with subsurface damage and the final etching has to be
performedchemo-mechanically. However for small samples chemo mechanical polishing is not used.
Using etch stop layers thickness variation of about 5 microns is obtained. Etch stops using HF anodic
etching usually provide fast etching of p type and n++ type, so in thiscase an etch stop on n++ is not
Moreover, it is difficult to grow defect free device n-layerswafers on a buried n++ layer, sufficiently high
doped for ohmic contact formation. This problemalso plays a role in etch stops using highly B doped p++
Si and KOH, TMAH or EPW. Anotherdisadvantage of this technique is that it is difficult to grow defect
free layers on top of this layer .Addition of larger Ge to the B atoms provides stress free etch stop layers
withoutmisfit dislocations. Electrochemical etch stops using P/N junctions require KOH etching at
elevatedtemperatures with the additional buried n++ layer problem.
5.4 Completed spin valve transistor structures
Following emitter thinning, the base region is defined using photolithography: photoresist
prebakedat 900°C was used to protect the base either during wet etching (10:H₂O₂/1:HF) during 20
seconds(for Co/Cu) or using ion beam etching during
30 minutes. To reduce the largesputter induced leakage currents after ion beam etching, a short TMAH
silicon etch is necessary toremove the damaged silicon surface next to the
base. For the H₂O₂/HF base etch this is not requiredsince it does not introduce defects and grows a
surface passivatingSiO₂automatically. Since theH₂O₂/HF tends to attack the photoresist, care has to be
taken not to etch longer than 1 minute. After the base etching procedure,the substrate is glued using
conducting room temperature curingepoxy with its backside ohmiccontact to a printed circuit board,
aluminum wires are ultrasonically bonded to the base and emittersand is ready for electrical
5.5 Processing other semiconductors
Experiments with Germanium collectors have also been performed. The difference in
preparationbefore metal deposition with Si is that Ge can be etched using HF/H₂O₂(1/10). This etchant
doesnot attack photo resist and consequently, the surface can be protected with a single(hard baked)
photo resist layer.First experiments with epitaxially grown n-GaAs films on n+ GaAs substrates have also
providedexcellent bonds. Photoresist was employed to offer protection during a H₂SO₄/H₂O₂/H₂0
fragment etch. There are new ways to obtain both a very clean GaAs surface and very goodSchottky
barriers: an AlAs layer has been grown in situ over the epitaxial GaAs layer, providingprotection of the
GaAs surface. (it is even possible to use a buried AlAs layer as an etch stop withcitric acid/H₂O₂etchants.
This top layer is removed using a veryselective HF 2% (1 min) dip as a final cleaning step prior to
bonding. We found nearly ideal Schottkybarriers using this method. Another technique for preparing
ideal Schottky barriers on GaAsinvolves substrate heating (5500°C) before deposition.
Its advantages are ; traditional transistors use on and off currents to create bits the binary zero and
one of computer information, quantum spin valve transistor will use up and down spin states to
generate the same binary data. Currently logic is usually carried out using conventional electrons, while
spin is used for memory. Spintronics will combine both. In most semiconducting transistors the relative
proportion of up and down carrier types are equal. If ferromagnetic material is used as the carrier
source then the ratio can be deliberately skewed in one direction. Amplification and/or switching
properties of the device can be controlled by the external magnetic field applied to the device. One of
the problems of charge current electrons is that we pack more devices together, chip heats up. Spin
current releases heat but it is rather less
Spin valve transistors have huge potential for incorporation in stable, high sensitivity magnetic
field sensors for automotive, robotic, mechanical engineering and data storage applications. It finds its
application towards quantum computer, a new trend in computing here we use qubits instead of bits.
Qubit exploit spin up and spin down states as super positions of zero and one. They have the advantage
over conventional semi conductor chips that do not require power to maintain their memory state. This
may also be used as magnetically controlled parametric amplifiers and mixers, as magnetic signal
processors for control of brush less dc motors as magnetic logic elements.
8. RELATED WORKS
Scientists have recently proposed new class of spin transistors, referred to as spin-filter
transistor (SFT) and spin metal-oxide-semiconductor field-effect transistor (spin MOSFET), and their
integrated circuit applications. The fundamental device structures and theoretically predicted device
performance are theoretically calculated predicted. The spin MOSFETs potentially exhibit significant
magnetotransport effect such as large magneto-current and also satisfy important requirements for
integrated circuit applications such as high transconductance, low power-delay product, and low offcurrent. Since the spin MOSFETs can perform signal processing and logic operations and can store
digital data using both of the charge transport and the spin degree of freedom, they are expected to be
building blocks for a memory cell and logic gates on spin-electronic integrated circuits. Novel spinelectronic integrated circuit architectures for nonvolatile memory and reconfigurable logic employing
Now researcher Christian Schoenenberger and colleagues at the University of Basel, Switzerland,
describe a carbon nanotube transistor operating on a same principle, opening a promising avenue
toward the introduction of spin-based devices into computer chips. A device consisting of a single
carbon nanotube connected to two magnetic electrodes that control the orientation of the electrons’
spins have been developed
There are major efforts on going at ibm, Motorola in developing RAM based on spin valves, such
devices called MRAMs have demonstrated faster speeds, high density, low power consumptions and no
volatility. They are promising replacement for semi conducting rams currently used. Also researches
are going on to replace Pt with suitable combinations of metal (low cost alloys) in order to make it
affordable at minimum cost.
Now it is clear that spin valve transistor is more versatile and more robust but it needs further
fabrication methods to improve magnetic sensitivity of collector current. The greatest hurdle for
spintronic engineers may be controlling all that spin. To do it on a single transistor is already feasible,
while to do it on a whole circuit will require some clever ideas.
In the spin-valve transistor perpendicular GMR can be measured down to tri-layers.
Exponentialamplification of the magnetoresistance occurs because the transfer is exponentially
dependent onthe electron mean free path in the base. Electron energy can be varied so electron
spectroscopy canbe performed by changing emitter Schottky barrier height (or tunnel bias).
Measurements can bedone at cryogenic and room temperature. Since the scattering processes appear
as products in thetransfer equation., the spin dependent scattering centers can be located accurately
and, in contrast tocommon CPP-MR, the relative change in collector current CC(%) is not decreased by
spin independentscattering processes such as in the Cu layers or in the semiconductors.
However the key question will be whether any potential benefit of such technology will be
worth the production cost. Spin valve transistors and other spin devices will become affordable by using
*1+ Dr S.S. Verma ,” Spintronics for the Ultimate in Performance”
Electronics for You, VOL. 34 NO. 8. August 2002, Pages 110-113 Websites:
"Perpendicular hot electron spin-valve effect in a new magnetic field sensor:
The spin-valve transistor " D. J. Monsma, J. C. Lodder, Th. J. A. Popma,
B.Dieny, Phys. Rev. Lett. 74, 5260, (1995).
 "Electronic measurement and control of spin transport in silicon",
Nature, Vol 447,(2007)