COLLEGE OF APPLIED SCIENCE(Affiliated to University of Kerala, Managed by IHRD)

Adoor, Kerala

Seminar Report

On

SPINVALVE TRANSISTOR

Submitted in partial fulfillment of the requirements for the award of the degree of BSc Electronics of University

of Kerala

Submitted by

SREENATH M

11802035

DEPARTMENT OF ELECTRONICS

COLLEGE OF APPLIED SCIENCE

ADOOR- 691523

2011-2014

College of Applied Science

(Affiliated to University of Kerala, Managed by IHRD)

Adoor, Kerala

Certificate

Certified that this is the bonafide report of seminar entitled

Spinvalve transistorDone By

SREENATH M

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

Internal Examiner External Examiner

Vinod V Rajendran Reeni Sara Thomas Santosh Babu (HOD) (Seminar coordinator) (Principal)

Department Of Electronics

College of applied scienceAdoor – 691523

2011-2014

ACKNOWLEDGEMENT

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.

SREENATH M

ABSTRACT

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.

INDEX

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

1. INTRODUCTION

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 spin-polarized 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

2. HISTORY

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[1]

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

2.1. Giant Magnetoresistance

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

measurement instruments. Other applications for magnetoresistive sensors include coin evaluation,

non-contact switching, and measurement of currents. An important issue in digital magnetic recording is

the bit density and several new technologies have pushed this density forward. Future high density

recording systems will depend increasingly on more sensitive field sensors, because of the shrinking bit

sizes and magnetic fluxes. The thin film head, the thin film media and subsequently the 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 broad measurable field range and may

further enhance bit densities.

A major advantage of using magnetoresistive sensing of magnetic fields as compared to

inductive sensing is the static measurement mode of the MR sensor: a static magnetic field can be

detected in contrast 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 of turns in the coil, but each turn adds approximately 0.5

ohm of resistance to the circuit, with a corresponding increase in thermal noise. Beyond 1 Gbit/in2, this

thermal noise of the coil becomes the main limitation preventing signal detection.

This new magnetoresistance called "Giant Magnetoresistance", was discovered in 1988 in

magnetic multilayers. It was soon called the spin valve effect because the magnetic layers act as 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 injector such as a Schottky barrier or a tunneling barrier

and a collector barrier which discriminates between scattered 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 processes to which the electrons are

subject. If the scattering processes are strong and effective, the mean 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 anti parallel (a) and parallel (b) film magnetizations.

Consider now electronic conduction in a multilayer array such as shown in Fig. 3.1 In Fig. 3.1a

the magnetic moments of successive ferromagnetic layers (Co) are anti parallel due to antiferro

magnetic coupling across the spacer layer (Cu). In (b) they are parallel due to an external magnetic field

which is strong enough to overcome the antiferromagnetic coupling. In case of Fig.3.1a , anti parallel

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'

magnetization layer as soon as it traverses the few Ångstroms of the spacer layer. In the case depicted in

Fig.3.1a, by contrast, an electron having the favored 'up' spin orientation in one magnetic layer has the

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 the array relatively

freely, and a significant difference in resistance is measured between the parallel and anti-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 transistors allows for fundamental detection of the perpendicular spin valve effect by incorporating

a spin valve into the base. The base is formed by a spin valve. A Co44Å/Cu88Å/Co8Å/Pt88Å sandwich

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), in common base.

The two magnetic layers act as polarizer and analyzer.

Figure 4.1 is the schematic cross section of the spin-valve transistor. A

Co44Å/Cu88Å/Co8Å/Pt88Å sandwich base is rf-sputtered onto the Si(100) collector substrate.A Pt

capping layer on top of the spin valve is used to make the emitter Schottky barrier larger than the

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 transistor in 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 by

collisions in the base, which affect their energy and trajectory (momentum), by optical phonon

scattering 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 relationship between the collector current

density Jc and the injected emitter current density Jinj is

………… (4.1)

Where W is the base width (=thickness) and the mean free path of the injected hot electrons

in the base. e represents the emitter efficiency, qm represents quantum mechanical transmission and

c represents the collector efficiency. Jleakage is the collector leakage current, determined by the

reverse biased collector Schottky barrier and Je is the injected emitter current. The avalanche

multiplication factor M depends on device design but if impact ionization is absent, equals one. The

leakage current of the collector may also contribute to the total collector current.

The emitter to collector current transfer ratio, or current gain is defined as:

……….(4.2)

Where the collector leakage current has been neglected. Here 0 is the common base current

gain and * is the common base current gain extrapolated to zero base

thickness. The factor represents the probability of transmission of the hot electrons through the

base. Jc is the total collector current. 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 in the base. Neglecting spin-flip scattering, we may

consider the spin up and spin down electrons to carry the current in parallel (two current models).

Following this idea, the collector current of the Co/Cu spin-valve transistor is expressed as:

……………(4.3)

() denotes the product of transmission probabilities of spin up (+) and down (-). Electrons

through each layer and interface. In first approximation we take e, c and qm similar for the two

species of electrons since these quantities reflect the properties of the semiconductors and Schottky

barriers. At saturation, all Co layers have their magnetization parallel.

The sum of the transmission probability factors for the two spin channels can then be written as:

……………… (4.4)

At the coercive field, this quantity becomes:

………………(4.5)

Where WCo expresses the sum of all Co layer widths (total Co thickness) which is valid for equally

thick layers, ½W is half of the total Co thickness; WCu is the total Cu thickness, the majority (minority)

MFPs in the Co layers and Cu the MFP in the Cu layer. The factor 2 in eqn appears because the two

parallel channels are equal for antiparallel magnetizations. The values of the collector current in the

parallel (P) and antiparallel (AP) magnetic configurations are then obtained. The typical properties in the

spin valve transistor are thus:

1. Perpendicular GMR can be measured down to tri-layers

2. Exponential amplification of the magneto resistance occurs because the transfer is

exponentially dependent on the electron mean free path in the base

3. Electron energy can be varied so electron spectroscopy can be performed by changing emitter

Schottky 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

dependent scattering centers can be located accurately and, in contrast to common - CPP MR,

the relative change in collector current CC(%) is not decreased by spin independent scattering

processes such 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

valve transistor 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

independent boundary scattering reduces the CIP magneto resistance largely in thin sandwiches. Also,

fundamental parameters of the effect, such as the relative contributions of interface and bulk spin

dependent scatterings are difficult to obtain using the CIP geometry. Measuring with the Current

Perpendicular to the Planes (CPP) solve most of these problems, mainly because the electrons cross all

magnetic layers, but a practical difficulty is encountered: the perpendicular resistance of the ultra thin

multi layers 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 at least two magnetic layers with antiparallel orientation. Because many electrons travel almost

parallel to the layers in the CIP-GMR, and do not cross many layers, the adjacent layers must have the

antiparallel 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 resistant state

as the AF-coupled state (“self averaging”). In CIP-GMR the electric field is independent of position in the

film, but the current density depends on the perpendicular direction to the film. The characteristic

length scale is the longest mean free path. For CPP transport, the electric field depends on the

perpendicular position in the film, but the current density is independent of position in the film. The spin

diffusion length is the new length scale.

5. VACUUM BONDING: SPIN VALVE

TRANSISTOR PREPARATION

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

inserted into loadlock F and transported using beam G to the main chamber

A after approximately 1 hour pumping. Multilayers can be deposited using a computer controlled

rotating table and deposition shutters.

Fig. 5.2 High vacuum DC/RF magnetron sputters system.

The properties of the system are: background pressure typically 10-9 mbar, three magnetron

sputter guns, variable substrate-target distance, heated substrate table, RF and DC power supplies.

Twelve different samples can be sputtered in one run using the specially designed substrate rotator, of

which 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.

Deposition occurs via 5. Substrate selection is via magnetically coupled beam 3. In this way optimized

GMR multi layers 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 of transistors to micron dimensions. For this reason, the emitter has to be thinned down to

about 1 to 5 micron. A major requirement is that the emitter substrate needs a highly doped region for

ohmic contact 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 to

come 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 the

most 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 performed

chemo-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 this case an etch stop on n++ is not possible.

Moreover, it is difficult to grow defect free device n-layers wafers on a buried n++ layer, sufficiently high

doped for ohmic contact formation. This problem also plays a role in etch stops using highly B doped p++

Si and KOH, TMAH or EPW. Another disadvantage 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

without misfit dislocations. Electrochemical etch stops using P/N junctions require KOH etching at

elevated temperatures with the additional buried n++ layer problem.

5.4 Completed spin valve transistor structures

Following emitter thinning, the base region is defined using photolithography: photo resist

prebaked at 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 large sputter induced leakage currents after ion beam etching, a short TMAH

silicon etch is necessary to remove the damaged silicon surface next to the

base. For the H₂O₂/HF base etch this is not required since it does not introduce defects and grows a

surface passivating SiO₂ automatically. Since the H₂O₂/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 ohmic contact to a printed circuit board,

aluminum wires are ultrasonically bonded to the base and emitters and is ready for electrical

characterization.

5.5 Processing other semiconductors

Experiments with Germanium collectors have also been performed. The difference in

preparation before metal deposition with Si is that Ge can be etched using HF/H₂O₂ (1/10). This etchant

does not 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

provided excellent bonds. Photo resist 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 good Schottky

barriers: an AlAs layer has been grown in situ over the epitaxial GaAs layer, providing protection of the

GaAs surface. (it is even possible to use a buried AlAs layer as an etch stop with citric acid/H₂O₂

etchants.

This top layer is removed using a very selective HF 2% (1 min) dip as a final cleaning step prior to

bonding. We found nearly ideal Schottky barriers using this method. Another technique for preparing

ideal Schottky barriers on GaAs involves substrate heating (5500°C) before deposition.

6.ADVANTAGES

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

7. APPLICATIONS

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 off-

current. 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 spin-

electronic integrated circuit architectures for nonvolatile memory and reconfigurable logic employing

spin-MOSFETsarealsoproposed.

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

9. FUTURE SCOPE

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.

10. CONCLUSION

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. Exponential

amplification of the magneto resistance occurs because the transfer is exponentially dependent on the

electron mean free path in the base. Electron energy can be varied so electron spectroscopy can be

performed by changing emitter Schottky barrier height (or tunnel bias). Measurements can be done at

cryogenic and room temperature. Since the scattering processes appear as products in the transfer

equation., the spin dependent scattering centers can be located accurately and, in contrast to common

CPP-MR, the relative change in collector current CC(%) is not decreased by spin independent scattering

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

common metals.

11. REFERENCES

[1] Dr S.S. Verma ,” Spintronics for the Ultimate in Performance”

Electronics for You, VOL. 34 NO. 8. August 2002, Pages 110-113 Websites:

[2] "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).

[3] "Electronic measurement and control of spin transport in silicon",

Nature, Vol 447,(2007)