indroduction to spintronics

7/29/2019 Indroduction to Spintronics

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Introduction to

Spintronics and Spin LEDs

Arnab Bose(124070018)

Dept. of Electrical Engineering, IIT-B


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Overview of this presentation


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Why Spintronics?

Fundamental Physical Limitation of Modern Technology

Alternatives like Single Electron Transistor, Organic

Semiconductor, Spintronic Devices

Spin is the Extra dimension other than Chage which can be

controlled

Aiming more packing density, lesser power consumption andmore operation speed and so on.

Basics of Spintronics is Spin injection, Spin transfer and

Spin detection


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Spin Injection:

1) Optical Injection

2) DMS Aligner

3) Half Metals 4) Ferro magnets

1)From Circular Polarised light ( Optical

Injection) Conservation of Angular Momentum

Angular momentum of Photons & for electrons


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2) Injection from Ferromagnetic Sources:

Unpaired electrons in d orbital

Spin Bands are asymmetrical

Degree of Spin polarization

is defined by : S,X= +

Fig-1

Its advantage is high Curie Temperature, large saturation of magnetization ,low

coercivities and well developed fabrication technology.

Disadvantage: conductivity mismatch problem.

Solution : Rashbas tunnel barrier & Schottky reversed biased tunnelling.


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3) Injection from DMS

(Diluted Magnetic Semiconductor)

Advantage: Lesser conductivity mismatch problem

Ex: II-Mn-VI kind of paramagnetic materials like CdMnTe, ZnMnSe, .

Under presence of applied H field they have conduction band

(Zeeman) splitting with interaction ofsp-d orbitals with localised Mn+

ions.

When unpolarised current is passed from non-magnetic materials tothis DMS, injected electrons are quickly scattered in lower spin sub

bands and as a result they become spin polarised along the direction

of the magnetic field.

And then after they can be drifted or diffused to the semiconductor


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3) Injection from Half-Metals

Due to the ferromagnetic decoupling in half-metallic ferromagnets,

one of the spin subbands generally either the majority spin or

spin-up sub-band exhibits a metallic density of states ideally

100% injection efficiency.

It is the density of state orientation that determines the orientation

of spin not the mobility of the different spin oriented electrons.

Heusler half metal Shows large magnetoresistance effect in tunnel junction at

room temperature.

Few like Fe3Si, Co2MnGa, Co2MnSi have very high Curie

temperature and excellent lattice match with GaAs


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Spin transport

Drift or diffusion. Spin life time Relaxation process

EY(ElliottYafet) recombination ( It dominates in small-gap and large

spinorbit coupling semiconductors and it is the primary reason for

spin relaxation in semiconductor)

DP (DyakonovPerel) recombination (it dominates in middle-gap

materials and at high temperatures for systems with sufficiently low

hole densities),Materials lacking Inversion Symmetry,Ex:GaAs

BAP(BirAronovPikus) recombination (It dominates in p+-doped

semiconductors at lower temperatures.

Hyperfine Interaction Mechanism in which magnetic interaction between

electrons & nuclei resulting spin decoherence of confined electrons in QW.


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Spin Detection

1) Optical Detection

2) Spin Hall Effect

3) Magnetoresistance.


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Spin Optical Detection

Rate of transition: Rif= 2 |Mif|2 ()

Where Mif= < f |HI | i > , HI = E

Condition of transition:m= 1 or 0, & l= 1

Conduction Band Heavy Hole Light Hole

| 1/2 , > | 3/2 , 3/2 > | 3/2, >

Transition Matrix element Mif = < f |H| i> mj Emission | Mif|2

CB HH < 3/2, +3/2 || 1/2,+ > -1 + |< px| | px >|2

CB HH < 3/2, -3/2 || 1/2, - > +1 - |< px| | px >|2

CB LH < 3/2, +3/2 || 1/2, - > -1 + 1/6|< px| | px >|2

CB LH < 3/2, -3/2 || 1/2,+ > +1 - 1/6|< px| | px >|2


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Optical Selection

Optical selection rules are application

at point.

inj=nnn+n

CB HH,CB HH are 3 times more probablethan CB LH,CB LH transition.

CP =I(

+

) I(

)

I +

+ I(

)=

(3 +)(3+)

3 + +(3+)=- inj

2

For QW & QD due to strain degeneracy in HH &LH bands are removed & we neglect transition

in to LH from CB

CP=I(

+

) I(

)

I +

+ I(

)

33

3+3= - inj

Fig-3


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Device Band Structure

Active well QW or QD Confinement in Spatial Co-ordinate MoreSpin life time for QD than QW.

Spin relaxation time() is more for Homovalent QW than HeterovalentQW. QD less sensitive to Temperature than QW, in which Spinpolarization decrease with same range of temperature.

Forwider QW T-2 ,in narrow QWs(


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Device Geometry

Fig-2: spin-LED under the

(a) Faraday, (b) quasi-Voigt and (c) oblique Hanle effect geometries


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Faraday Geometry:

1) It is most commonly used geometry

and selection rules are well understood.

2) H is parallel to direction of propagation

of light and surface normal

or growth direction.

3) Its disadvantage is that due to its

shape anisotropy very large

magnetic field is required.


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Quasi-Voigt geometry

It is less commonly used and it is used to

characterise edge emitting LEDs.

H and direction propagating photons are parallel

and they are perpendicular to the growth direction.

Faradays selection rules are no longer valid here.

Photon reabsorption through the aligner is lesser as photonsdoes not pass through it.

Competitively lesser H required here and so forsmall band

gap semiconductormaterial where electronics properties are

affected with large magnetic field this geometry is suitable.


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Ambiguity regarding Spin detection

Ideally we should have placed Spin Aligner directly

adjacent to then QW or WDs. But to reduce the

diffusion of magnetic impurities inside active regionwe need to place spacer of few Angstrom range.

When large H is applied degenerate bands get spitted

(Zeeman Effect) and hence carrier density may attainnet spin polarization which does not have any relation

with the injected spin from Aligner.


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Conclusion

Spin LEDs, LASERs can be used in secured optical

communication, cryptography, quantum computing and manymore.

Still it is challenging to build efficient Spin LEDs for room

temperature operation.


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References:

1)Fig 1 from Phd dissertation on FERROMAGNET/SEMICONDUCTOR BASED

SPINTRONIC DEVICES by Prof. Dipankar Saha

2) Fig-2 & Fig-3 from TOPICAL REVIEW Spin-polarized light-emitting diodes and

lasers by M Holub and P Bhattacharya, J. Phys. D: Appl. Phys. 40 (2007) R179

R203

3) Spintronics: Fundamentals and applications by Igor Zutic, Jaroslav Fabian, S.

Das Sarma from arXiv:cond-mat/0405528v1 [cond-mat.other] 21 May 2004

4) acta physica slovaca vol. 57 No. 4 & 5, 565 907

5) Spintronics: A Spin-Based Electronics Vision for the Future, S. AScience 294,

1488 (2001). Wolfet al., DOI: 10.1126/science.1065389

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