Spin valve sransistor


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Spin valve sransistor

  1. 1. 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
  2. 2. 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 transistor Done 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 VinodV RajendranReeni Sara Thomas (HOD) (Seminar coordinator) Santosh Babu (Principal) Department Of Electronics College of applied science Adoor – 691523 2011-2014
  3. 3. 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
  4. 4. 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.
  5. 5. 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
  6. 6. 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 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
  7. 7. 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 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
  8. 8. 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.
  9. 9. 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
  10. 10. 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.
  11. 11. 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. Figure 4.1 is the schematic cross section of the spin-valve transistor. A 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.
  12. 12. 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 ………… (4.1) 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.
  13. 13. The emitter to collector currenttransfer ratio, or current gain is defined as: ……….(4.2) 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: ……………(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 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:
  14. 14. ……………… (4.4) At the coercive field, this quantity becomes: ………………(4.5) 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 the 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
  15. 15. 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
  16. 16. 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.
  17. 17. 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
  18. 18. 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.
  19. 19. 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 possible. 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++
  20. 20. 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 characterization. 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.
  21. 21. 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.
  22. 22. 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
  23. 23. 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.
  24. 24. 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 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
  25. 25. 9.FUTURESCOPE 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.
  26. 26. 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. 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 common metals.
  27. 27. 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)