Investigation of organic / ferromagnet interface and magnetoresistive characteristics of small molecule organic semiconductors

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LEABHARLANN CHOLAISTE NA TRIONOIDE, BAILE ATHA CLIATH TRINITY COLLEGE LIBRARY DUBLIN OUscoil Atha Cliath The University o f Dublin. Terms and Conditions of Use of Digitised Theses from Trinity College Library Dublin. Copyright statement. All material supplied by Trinity College Library is protected by copyright (under the Copyright and Related Rights Act, 2000 as amended) and other relevant Intellectual Property Rights. By accessing and using a Digitised Thesis from Trinity College Library you acknowledge that all Intellectual Property Rights in any Works supplied are the sole and exclusive property of the copyright and/or other I PR holder. Specific copyright holders may not be explicitly identified. Use of materials from other sources within a thesis should not be construed as a claim over them.. 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Electronic or print copies may not be offered, whether for sale or otherwise to anyone. This copy has been supplied on the understanding that it is copyright material and that no quotation from the thesis may be published without proper acknowledgement.. Investigation o f Organic/Ferromagnet Interface and M agnetoresistive Characteristics o f Small. M olecule Organic Sem iconductors. By. Huseyin Tokuc. A thesis subm itted for the degree o f D octor o f Philosophy. in the U niversity o f Dublin. School o f Physics,. Trinity C ollege Dublin,. A ugust 2013. Declaration. I h e r e b y d e c la re th a t th is th e s is h a s n o t b e e n s u b m itte d as an e x e rc ise fo r a. d e g re e in a n y o th e r u n iv e rsity .. T h is th e s is is e n tire ly m y o w n w o rk , e x c e p t fo r th e a d v ic e an d a s s ista n c e . m e n tio n e d in th e a c k n o w le d g e m e n ts .. I a g re e to a llo w th e lib ra ry o f T rin ity C o lle g e D u b lin c o p y o r len d a se c tio n o r. th is e n tire th e sis o n re q u est.. H u se y in T o k u c. 29/08/2013 J TRINITY COLLEGE. LIBRARY DUBLIN ^. II. Summary. Spin injection, transport and dynam ics in organic sem iconductors are o f grow ing. interest as the field o f organic spin electronics begins to take shape during the last. decade. T his new field o f research aim s to m anipulate the electron spin degree o f . freedom in organic based electronic devices, such as organic-light em itting diodes,. organic field effect transistors, and organic radio-frequency identification tags.. O rganic m aterials offer the prom ise to future lightw eight, low -pow er, and. inexpensive electronics on flexible substrates. H ow ever, to realize such device. applications the fundam ental processes that govern electron spin dependent. injection into and transport w ith organic sem iconductors m ust be understood. In. general, organic sem iconductors have long spin relaxation tim es (~ 10'^ s), w eak. hyperfm e interactions, and little spin orbit scattering because the m olecules are. m ainly com posed o f the elem ents carbon and hydrogen. H ow ever, the m obility in. organics is too low, so it is unclear w hether the spin-polarized electrons can travel. distances over 100 nm , w hich is required for several device applications.. In this report, the focus is on the organic based m agnetic tunnel ju n ctio n s (M TJ). and quality o f organic/ferrom agnetic interfaces w hich directly govern the spin. injection into organic sem iconductor. The basic idea is to use organic film s as a. barrier betw een M gO or AlOx and top ferrom agnetic electrode in vertical spin valve. stacks.. The interface quality o f organic/ferrom agnet and organic/insulator/ferrom agnet. m ultilayers w ere studied using a sim ple m ethod called as ferro m a g n etic film. I I I. th ickn ess m a g n e tiza tio n (FFT M ), w here the organic layers are Alq^, CuPc and. Znq2 . The results clearly sh ow ed that a m agnetic dead layer forms at. organic/ferrom agnet interface; ~ 0 .9 nm for A lq 3 , ~ Inm for Znq2 , and ~ 1.3 nm for. C uPc. Then, introducing an insulator layer at the organic/ferrom agnet interface. reduces the dead layer th ickness by m ore than 50 %. A nother remarkable. observation is that the bottom ferrom agnetic layer w as o x id ized through the organic. layer w h en the ferrom agnetic/organic bilayer w as ex p o sed to air.. S pin polarized transport properties o f Alq^, CuPc, ZnPc and Z nqz w ere studied. u sin g M gO /organ ic and A lO x/organ ic based hybrid organic spin va lv es in a vertical. structure. The stacks w ere fabricated u sin g a com bination o f magnetron sputtering. tool and an organic evaporation chamber. U V lithography and sh adow m asking. p ro cesses w ere used to pattern the stacks into m icro n -size jun ction s. The key. elem en t in our lithographically patterned, exch an ge-b iased tunnel ju n ction s w ith an. A lq j spacer layer, w h ich enables them to exh ibit u sefu l M R at room -tem perature, is. the C o F e B /M g O spin injector.. M agn etoresistive characteristics o f the C oF eB /M gO /organ ic based hybrid d ev ices. w ere studied and the results su ggested that the d e v ic e s sh ow ed better perform ance. w hen the M gO layer w as clean ed u sin g A r-ion etch in g before d ep osition o f organic. layer. In the ca se o f air e x p o sed M gO , the d e v ic e s sh ow ed - 1 2 % M R w hich is. independent o f the A lq j th ick n ess (t= 2 -8 nm). H ow ever, the d ev ices sh ow ed . m axim u m ~ 50 % M R in the p resence o f A lqs w h en the M gO layer w as cleaned. b efore d ep o sitin g the Alq? layer.. M agnetotransport properties o f Znq2 and C uPc based hybrid d ev ices w ere studied. u sin g an exch an ged bias and bottom -pin ned MTJ stack, w here the barrier w as air-. I V. e x p o se d M g O /Z n q 2 (t= 0 -2 n m ) o r a ir-e x p o s e d M g O /C u P c (0 -2 nm ). In th e ca se o f . Znq?, th e M R w a s in d e p e n d e n t o f th e o rg a n ic th ic k n e ss an d ~ 15 % o f M R w as. o b se rv e d at 3 0 0 K fo r / = 1 an d 2 nm re sp e c tiv e ly . H o w e v e r, th e M R e x h ib it a. stro n g th ic k n e s s d e p e n d e n c e w h e n th e o rg a n ic w a s C u P c , w h e re th e M R at room . te m p e ra tu re w a s 13 % a n d 6 % fo r / = 1 n m an d t = 2 nm , re sp e c tiv e ly .. M a g n e to tra n s p o rt p ro p e rtie s o f th e A lO x /o rg an ic/A lO x b a se d h y b rid sp in v a lv e s. p re p a re d b y u sin g in -situ s h a d o w m a s k in g w e re s tu d ie d , w h e re th e o rg a n ic lay er. w a s A1q3, C u P c, Z n P c an d Z n q 2 . T h e fiill sta c k w a s C o F e (1 0 n m )/A 1 0 x (1.5. n m )/o rg a n ic (t)/A 10x (1.5 n m )/C o F e (18 nm ). T h e re s u h s a b s o lu te ly s h o w e d n o M R . at 3 0 0 K w h e n t = 1 n m fo r Z nq?, Z n P c an d C u P c an d t= 2 nm fo r Alq.v T h e ro o m . te m p e ra tu re M R w a s 2 % fo r Alq^ w h e n t = 1 nm . A t low te m p e ra tu re ra n g e (~ 20. K ) th e m a x im u m o rg a n ic th ic k n e ss w h ic h s h o w e d a m e a s u ra b le M R signal w a s 3. nm fo r A iqs an d 1 n m fo r th e o th e r o rg a n ic b a rrie rs. T h e d e v ic e s s h o w e d stro n g . te m p e ra tu re d e p e n d e n t I-V . F u rth e rm o re , m u lti-s te p tu n n e lin g is c o n sid e re d to. g o v ern th e tra n s p o rt m e c h a n is m a n d b e h a v e d as a so u rc e o f sp in p re c e s s io n w h e n . th e e le c tro n s o c c u p y th e in te rm e d ia te sites in th e o rg a n ic layer.. V. Acknowledgments. I w ould like to express m y deepest gratitude to m y supervisor Prof. M ichael C oey. w ho gave m e the o p portunity and the support to w ork in his lab and com plete a PhD. in T rin ity C o llege D ublin. As a supervisor, his fruitful suggestions, invaluable. com m ents, positive attitude and friendly behaviour during m y PhD m otivated me. for m ore focusing on m y research.. I w ould like to thank P r o f G regory J. Szulczew ski, A ssist. P r o f H useyin Kurt, Dr.. K aan O guz and Dr. C iaran F ow ley for their interest and skilful guidance during my. PhD . Special thanks to A ssist. P r o f H useyin Kurt w ho helped m e to design and set. up a new deposition system and Dr. K aan O guz w ho shared his house w ith m e in. m y first years in D ublin and becam e a good friend in m y life.. I w ant to thank our postdocs Dr. M unusam y V enkatesan, A ssist. P r o f Plam en S.. Stam enov, Dr. K arsten Rode, Dr. L orena M. M onzon and Dr. R em y L assalle-B alier. for their invaluable com m ents and helps. Spacial thanks to Assist. P r o f Plam en S.. Stam enov and Dr. M unusam y V enkatesan for spending too m uch tim e to m easure. m y sam ples in SQ U ID or PPM S as w ell as scientific discussions.. I feel fortune have group m em bers w ho m ade the tim e in m y PhD enjoyable and. shared the any sort o f problem . Dr. Sim one A lborghetti, Dr. D am aris Fernandes,. D avide B etto, Jong C. Lau, Kiril B orisov, A m ir Sajad E sm aeily and H ongjun Xu. I. am esp ecially thankful to Dr. Inam M. M irza for being a nice housem ate and. invaluable discussions about the science and life.. VI. CONTENTS. S u m m a r y .............................................................................................................................................................. I l l. A c k n o w l e d g m e n t s .................................................................................................................................... V I. CH APTER 1 ............................................................................................................ 1. 1.1 I n t r o d u c t i o n ......................................................................................................................................... 1. 1.2 S p i n ELECTRONICS............................................................................................................. 2. 1.2.1 Electron tunnelin g ........................................................................................................................ 2. 1.2.2 Tunneling criteria...........................................................................................................................5. 1.2.3 Anisotropic magnetoresistance and giant m agnetoresistance............................................... 6. 1.2.4 Tunneling m agnetoresistance..................................................................................................... 8. 1.2.5 Physical origin o f tunneling magnetoresistive e ffe c t................................................................ 8. 1.2.6 Spin electronics: oven /iew ......................................................................................................... 11. 1.3 O r g a n i c s e m i c o n d u c t o r s ..........................................................................................................12. 1.3.1 Introduction................................................................................................................................. 12. 1.3.2 n-conjugation.............................................................................................................................. 14. 1.3.3 Charge conduction in n-conjugated system s........................................................................... 15. 1.3.4 Carrier injection and transport in organics.............................................................................. 17. 1.3.5 Materials fo r organic electronics...............................................................................................20. 1.4 O r g a n i c s p i n e l e c t r o n i c s ........................................................................................................ 25. 1.4.1 Life o f spin in organics................................................................................................................26. 1.4.2 Organic spin valves: overview ...................................................................................................28. 1.5 S u m m a r y ...................................................................................................................................................35. 1.6 B i b l i o g r a p h y ........................................................................................................................................37. CHAPTER 2 ......................................................................................................... 45. V I I. 2 .1 I n t r o d u c t i o n 4 5. 2 .2 T h i n f i l m d e p o s i t i o n ..................................................................................................................... 4 6. 2.2.1 S p uttering process arid its b a sic s .....................................................................................................46. 2 .2.2 M agnetron s p u tterin g .........................................................................................................................48. 2 .2.3 Thermal e v a p o r a tio n .......................................................................................................................... 49. 2 .3 T h i n f i l m d e p o s i t i o n s y s t e m s ................................................................................................. 5 0. 2.3.1 The Shannrock m a g n etro n sp u tterin g s y s t e m ............................................................................... 50. 2.3.2 Ultra high vacuum organic & m e ta l therm a l evaporation s y s te m ............................................52. 2.3.3 O ther organic evaporation s y s te m s .................................................................................................65. 2 .4 D e v i c e f a b r i c a t i o n t e c h n i q u e s .......................................................................................... 6 9. 2.4.1 Photolithography te c h n iq u e ..............................................................................................................69. 2.4.2 In-situ shadov^ m asking te c h n iq u e .................................................................................................. 76. 2 .5 C h a r a c t e r i z a t i o n TECHNIQUES............................................................................................. 7 7. 2.5.1 The Resistance-Tem perature (R-T) r i g .............................................................................................77. 2.5.2 SQUID m a g n e to m e te r ......................................................................................................................... 78. 2.5.3 X-ray diffraction and .X-ray reflectivity.............................................................................................79. 2.5.4 A tom ic Force M icroscopy................................................................................................................... 82. 2 .6 B i b l i o g r a p h y ........................................................................................................................................ 8 4. CHAPTER 3 ...................................................................................................... 85. 3 .1 I n t r o d u c t i o n ........................................................................................................................................ 8 5. 3 .2 O v e r v i e w ..................................................................................................................................................8 6. 3 .3 E l e c t r o n i c s t r u c t u r e o f m e t a l / o r g a n i c i n t e r f a c e .........................................8 7. 3.3.1 M etal-organic sem iconductor c o n ta c t.............................................................................................87. 3.3.2 M etal-insulator-organic sem ico n d u cto r c o n ta c t.......................................................................... 88. 3 . 4 C a r r i e r i n j e c t i o n m e c h a n i s m i n t o a n o r g a n i c s e m i c o n d u c t o r. f r o m a m e t a l ....................................................................................................................................................8 9. VIII. 3.4.1 Thermionic em ission m ec h a n ism ..................................................................................................... 90. 3.4.2 Field em ission m e c h a n ism ................................................................................................................. 91. 3.4.3 Back-flow tu n n elin g ............................................................................................................................ 92. 3 . 5 S p e c t r o s c o p i c a n d p h y s i c a l c h a r a c t e r i z a t i o n o f m e t a l / o r g a n i c . IN T E R F A C E ............................................................................................................................................................ 9 3. 3 . 6 M a g n e t i c a n d p h y s i c a l p r o p e r t i e s o f o r g a n i c - f e r r o m a g n e t a n d . o r g a n i c - i n s u l a t o r - f e r r o m a g n e t i n t e r f a c e s .................................................................... 9 8. 3.6.1 Ferromagnetic film thickness m agn etisa tio n m e th o d (FFTM)...................................................99. 3.6.2 SiOi/Aiqj-Co interface......................................................................................................................100. 3.6.3 Si02/Alqs-LIF-Co in terfa ce............................................................................................................... 104. 3.6.4 The e ffe c t o f an AIO^ layer a t th e Alqi/CoFe in te r fa c e ............................................................. I l l. 3.6.5 TEM, EDX and EELS analysis o f CoFe-AIO^-AlqyAIO^-CoFe str u ctu re..................................... 114. 3.6.6 Si02/Znq2-Co in terfa ce.................................................................................................................... 118. 3.6 .7 SiOi/CuPc-CoFe interface.................................................................................................................121. 3 . 7 C o n c l u s i o n ...........................................................................................................................................1 2 7. 3 . 8 BIBLIOGR.4PHY...................................................................................................................................... 1 2 9. CHAPTER 4 ................................................................................................133. 4 .1 I n t r o d u c t i o n ..................................................................................................................................... 1 3 3. 4 .2 S p in VALVE t e s t s t a c k s ............................................................................................................... 1 3 5. 4.2.1 Calibration ofAiO„ barrier prepared by using sh a d o w m asking te ch n iq u e......................... 135. 4.2.2 Calibration o f th e MgO cleaning p r o c e s s .................................................................................... 142. 4.2.3 M agnetoresistive characteristics o f MgO/LIF ba sed M TJs...................................................... 144. 4 .3 ALQ3 b a s e d h y b r i d o r g a n i c s p i n v a l v e s ..................................................................... 1 4 6. 4.3.1 M agnetoresistive characteristics o f air exp o sed MgO/Aiq^ b a sed hybrid d ev ic es 147. 4.3.2 M agnetoresistive characteristics o f argon ion cleaned M gO/Alq^ b a sed hybrid. devices 152. IX. 4 . 3 .3 M a g n e t o r e s i s t i v e c h a r a c te r is tic s o f A IO JA Iqi/AIO^ b a s e d h y b r id d e v ic e s p r e p a r e d b y . u s in g in -s itu s h a d o w m a sl< in g ........................................................................................................................................161. 4 .4 CUPC BASED HYBRID ORGANIC DEV ICES....................................................................169. 4 .4 .1 M a g n e t o r e s i s t i v e c h a r a c te r is tic s o f M g O /C u P c b a s e d h y b r id d e v i c e s p r e p a r e d u sin g. U V l i t h o g r a p h y ......................................................................................................................................................................1 6 9. 4 . 4 .2 M a g n e t o r e s i s t i v e c h a r a c te r is tic s o f A iO jC u P c /A iO ^ b a s e d h y b r id d e v ic e s p r e p a r e d b y. in s itu s h a d o w m a sl< in g .....................................................................................................................................................1 7 4. 4 .5 M a g n e t o r e s i s t i v e c h a r a c t e r i s t i c s o f A lO x / Z n P c / A l O x b a s e d. HYBRID d e v i c e s PREPARED BY IN SITU SHADOW MASKING.............................................177. 4 .6 ZNQ2 b a s e d h y b r i d o r g a n i c DEVICES....................................................................... 180. 4 .6 .1 M a g n e t o r e s i s t i v e c h a r a c te r is tic s o f M g O /Z n q 2 b a s e d h y b r id d e v i c e s p r e p a r e d u sin g. UV l i t h o g r a p h y ......................................................................................................................................................................1 8 0. 4 . 6 .2 M a g n e t o r e s i s t i v e c h a r a c te r is tic s o f A l O j Z n q /A I O ^ b a s e d h y b r id d e v ic e s p r e p a r e d b y . u s in g in -situ s h a d o w m a sl< in g ........................................................................................................................................1 8 3. 4 .7 C o n c l u s i o n ...........................................................................................................................186. 4 .8 B i b l i o g r a p h y ....................................................................................................................... 188. C H A P T E R S ............................................................................................... 191. 5.1 C o n c l u s i o n ...........................................................................................................................191. 5.2 F u t u r e w o r k ........................................................................................................................ 192. X. LIST OF FIGURES. Figure 1.1. W ave function decays ex p onentially w ithin the energy b a rrie r......................... 3. Figure 1.2. B and diagram o f m etal/insulator/m etal stru c tu re ...................................................5. Figure 1.3. A G M R spin valve structure. The dashed arrow show s the free layer. w hile the continuous arrow show s the pinned layer. The current flow s. perpendicular to the p la n e ...................................................................................................6. Figure 1.4. T he figure o f conduction in a G M R structure (a) in parallel alignm ent. and (b) in antiparallel alig n m en t....................................................................................... 7. Figure 1.5. Schem atic representation o f DOS; (a) for a norm al m etal and (b) for a. ferrom agnetic m etal (arrow s show m ajority and m inority spin s ta te s )...................... 9. Figure 1. 6 . Schem atic diagram o f spin dependent tunneling via the Julliere model.. The top (a) and bottom (b) panels show the parallel and antiparallel. alignm ent o f the ferrom agnetic electrodes, resp ectiv ely ........................................... 10. Figure 1.7. Band structure o f p-type Si (a) and rubrene (b) [22]...........................................14. Figure 1.8. sp" hybridization o f a carbon atom and ti / o bonds betw een. neighbouring carbon ato m s.............................................................................................. 15. Figure 1.9. M olecular structure o f A lqj (a) and two geom etrical isomers: m eridional. (b) and facial (c)................................................................................................................. 22. Figure 1.10. M olecular structure and atom ic orientations o f m etal phthalocyanine 23. Figure 1.11. B is(8-hydroxyquinoline) zinc(II), dihydrate (a) and anhydrous bis(8-. hydroxyquinoline) zinc(II) (b )........................................................................................ 25. Figure 1.12. Spin diffusion length, 4, versus spin relaxation tim e, r̂ , for different. classes o f m aterials [22]................................................................................................... 28. XI. F igure 1.13. Schem atic view o f an hybrid state at a ferrom agnet/organic m olecule. interface, show ing the density DO S o f a ferrom agnetic m etal and an organic. m olecule w hen they are isolated (a) or in the contact (b,c) [9 0 ]............................. 35. Figure 2.1. Sputtering process, show ing the ejected atom s from the target surface as. a result o f striking energetic ions to the target surface.............................................. 46. Figure 2.2. DC sputtering system show ing the sputtered target atom s and form ation. o f a thin film on the substrate surface............................................................................48. Figure 2.3. The m agnetron sputtering system show ing the plasm a confined near the. target area w here the m agnetic field is s tr o n g ............................................................ 49. Figure 2.4. The Sham rock system , show ing cham ber B, C and D as w ell as a. schem atic draw ing o f the w hole s y s te m .......................................................................51. Figure 2.5. O verall view o f the organic U H V ch a m b e r.......................................................... 53. Figure 2.6. The bottom plate o f the organic U H V cham ber, show ing the triple. m olecular organic evaporation assem bly, m etal heating sources and a. schem atic draw ing o f one o f the evaporation pockets in the organic. evaporation asse m b ly ........................................................................................................ 54. Figure 2.7. Therm al evaporation sources, show ing alum ina coated boat (top left. panel), baffled tantalum box heater (bottom left panel), tantalum . m icroelectronics box heater (bottom right panel) and m etal plated tungsten. rods (top right p an el)..........................................................................................................56. F igure 2.8. Show ing the tools for A1 deposition. Firstly, single A1 pellet is put inside. the BN crucible and the crucible is then surrounded by m olybdenum thin. sh e et....................................................................................................................................... 56. XI I. Figure 2.9. The bottom side o f the organic U H V cham ber show ing the electrical and. cooling c o n n e c tio n s........................................................................................................... 57. Figure 2.10. The thickness/rate m onitor (Q C M )..................................................................... 58. Figure 2.11. The picture o f the load-lock show ing its a s se m b ly ......................................... 60. Figure 2.12. Sam ple m ounting and transferring. A 25 m m Si02 w afer is attached on. the copper substrate holder (a), the substrate h o ld er is m ounted on the. transfer arm in the load-lock (b) and the rotational assem bly to hold the. substrate in the m ain cham ber (c )................................................................................... 60. Figure 2.13. Picture o f the shadow m ask and linear m otion m e c h a n is m .......................... 62. Figure 2.14. Picture o f the ion gun and its pow er su p p ly ...................................................... 64. Figure 2.15. Picture o f C am ellia..................................................................................................66. Figure 2.16. Picture o f the organic evaporator......................................................................... 67. Figure 2.17. The bell-jar evaporation system and a schem atic d ra w in g ........................... 68. Figure 2.18. U V exposure, show ing the U V light p assing only from som e specific. areas through the m etal m ask; the features on the m ask are transferred to the. substrate after develo p in g ................................................................................................. 70. Figure 2.19. T he picture o f the OAI m ask a li g n e r ..................................................................71. Figure 2.20. Picture o f the M illa tro n .......................................................................................... 72. Figure 2.21. A picture o f the first U V photom ask (a) and zoom in (b). (c) show s the. spinner and the hot plate for so ft-b ak in g .......................................................................75. Figure 2.22. D evice patterning p ro c e s s ...................................................................................... 75. Figure 2.23. Junctions produced by shadow m asking technique and an SEM image. (the scale bar is 100 |am)................................................................................................... 77. Figure 2.24. The picture o f the R -T r i g ..................................................................................... 78. Figure 2.25. SQ U ID m ag n eto m eter................................................................. 79. XI I I. Figure 2.26. X-ray diffraction................................................................................................... 80. Figure 2 2 1 . a) FWHM for a real and b) ideal XRD peak [7].............................................. 80. Figure 2.28. Phillips X-Pert Pro system ................................................................................. 82. Figure 2.29. Nanoscope 3a M ultimode Atomic Force M icroscope..................................... 83. Figure 3.1. Formation o f a metal-sem iconductor interface and energy level. alignments in separate (a) and contact (b) positions................................................ 88. Figure 3.2. Energy levels alignment o f A l/A lq3 interface (a) and Al/LiF/Alq^. interface........................................................................................................................... 89. Figure 3.3. Illustrating different injection mechanism for the metal-semiconductor. interface where JTE, JEF and Jback represent the thermionic emission, field. emission and back scattering, respectively................................................................ 90. Figure 3.4. Comparison o f the XPS spectra o f the Alq. ̂ core levels before and after. deposition o f Co layers with different thicknesses [20].......................................... 93. Figure 3.5. Showing the XPS spectra o f Co (2p3/2) core levels o f the Co layers o f. various thicknesses deposited on Alq? [20]...............................................................94. Figure 3.6. TEM images showing the metal/organic interface (a) taken from ref. [20]. and (b) taken from r e f [2]............................................................................................ 96. Figure 3.7. Showing the interface feature o f the A lq 3 /Co/Au structure (a) TEM. image, (b) EELS image [10]........................................................................................ 97. Figure 3.8. a) Schematic illustration o f different magnetic interfaces: (i) sharp,. atomically flat interface, (ii) diffuse interface with a reached interlayer where. the metal is no longer all ferromagnetic, (iii) a rough but sharp interface, and. (iv) very rough but sharp interface, b) The magnetic moment is plotted as a. function o f the nominal thickness o f ferromagnetic metal, t . ...............................100. X I V. F ig u re 3 .9 . T h e sta c k s u se d for m a g n e tic d ead layer in v e s tig a tio n s h o w in g u n ca p p ed. (a) and ca p p e d (b ) C o la y e r ...................................................................................................... 101. F ig u re 3 .1 0 . R o o m tem p eratu re m a g n e tiz a tio n c u r v e s for c o b a lt layers o f d ifferen t. n o m in a l th ic k n e s s e s o n S iO : (a) and F F T M p lo t o f th e data, s h o w in g th e. e ffe c t o f in tro d u cin g a 3 .5 nm A1 c a p p in g la y er ( b ) .......................................................101. F igu re 3 .1 1 . F F T M p lo ts o f th e A lq V C o in terfa ce s h o w in g th e e ffe c t o f th e A1. c a p p in g la y er (a) and th e b ila y er w ith Alq^ o n top ( b ) .............................................. 103. F igu re 3 .1 2 . A F M im a g e o f S i0 2 /A lq 3 (2 0 n m ) (a) and S i0 2 /A lq 3 (2 0 n m ) LiF (2 n m ). ( b ) ....................................................................................................................................................... 105. F igu re 3 .1 3 . T E M im a g e o f S iO :/ A lq 3 (4 0 n m )/L iF (2 n m )/C o (5 n m )/A l(7 ) m u ltila y er. in norm al dark field m o d e (a) and h igh r e so lu tio n m o d e ( b ).......................................106. F igu re 3 .1 4 . T h e X R R r e fle c tiv ity o f the S i0 2 /A lq 3 (4 0 )/L iF (2 )/C o (5 )/A l(7 ) stack.. T h e b lack lin e is the ex p erim en ta l data and th e red lin e is the f i t . .......................... 107. F igure 3 .1 5 . M a g n e tisa tio n curv'es o f C o layer w ith v a r y in g LiF th ic k n e ss and. co rresp o n d in g m u h i-la y e r ......................................................................................................... 108. F igu re 3 .1 6 . F F T M p lo ts o f the A lq^/C o in terface s h o w in g the e ffe c t o f the LiF. b u ffer layer a) at 3 0 0 K and b) at 4 K .................................................................................. 109. F igu re 3 .1 7 . M a g n e tisa tio n s w itc h in g cu r v e s o f C o layers at 4 K s h o w in g th e e ffe c t. o f the L iF b u ffer layer a) w ith L iF and b) w ith o u t L i F . .............................................109. F igu re 3 .1 8 . R ela ted m u lti-la y e r sta ck s for F igu re 3 .1 7 , s h o w in g the C o layer. d e p o site d eith er on LiF or A lq j ...............................................................................................110. F igu re 3 .1 9 . M a g n e tisa tio n c u rv es o f the C o F e layer s h o w in g th e e ffe c t o f A lO x at. the A lq j/C o F e in terfa ce..............................................................................................................113. F igu re 3 .2 0 . S h o w in g the T E M , E D X and E E L S data o f S i0 2 /C o F e (8 )-A 1 0 x (1 .5 )-. A lq 3( 10 )-A 1 0 x ( 1.5 )C o F e ( 18) sta c k ......................................... ................ ...............115. XV. F ig u re 3. 2 1 . T E M , E D X and E E L S data, s h o w in g the e le m e n ta l c o m p o s itio n in the. m id d le o f the Alq^ la y e r ............................................................................................................ 1 1 6. F ig u re 3 .2 2 . E le c tr o n in te n sity p r o file s tak en from d ifferen t r e g io n s o f th e top and. b o tto m in te r fa c e s o f S i0 2 /C o F e ( 8 ) - A 1 0 x ( 1 .5 ) - A lq 3 ( 1 0 ) - A 1 0 x ( 1 .5 ) C o F e ( 1 8 ) .. 1 1 7 . F ig u re 3 .2 3 . T h e sta ck s s h o w in g that th e C o la y er is eith er d e p o site d on SiO z or. Z n q ..................................................................................................................................................... 1 1 9. F ig u re 3 .2 4 . R o o m tem p eratu re m a g n e tisa tio n cu r v e s o f C o layers w ith d ifferen t. un d er la y er and to p la y er (a ) and F F T M p lo t o f th e data, s h o w in g the related. m a g n e tic d ead la y e r s ( b ) ............................................................................................................1 1 9. F ig u re 3 .2 5 . A F M (a ) and X R R (b ) m e a su r e m e n ts o f a 10 nm th ic k Z n q 2 film. s h o w in g sm o o th s u r fa c e ............................................................................................................ 120. F ig u re 3 .2 6 . T E M and e le c tr o n in te n sity p r o file s h o w in g the sharp in terfa ce b e tw e e n. Z n q 2 and C o .................................................................................................................................... 121. F ig u re 3 .2 7 . R o o m tem p eratu re m a g n e tisa tio n s w itc h in g c u r s e s o f S i0 2 /C u P c ( 5 ) -. C o F e (t)-A 1 0 x (1 .5 ) (a ) and S i0 2 /C u P c ( 5 ) - A 1 0 x ( 1 .5 ) - C o F e (t ) - A 1 0 x ( 1 .5 ) (b ) .... 123. F ig u re 3 .2 8 . S h o w in g th e ro o m tem p eratu re m a g n e tisa tio n s w itc h in g c u r v e s o f C o F e . la y er w ith th e c o m b in a tio n o f d ifferen t b o tto m and top la y e r s (a ) and FFT M . p lo t s h o w in g th e e ffe c t o f A lO x o n th e fo rm a tio n o f th e m a g n e tic d ead layer. ( b ) ........................................................................................................................................................ 1 2 4. F ig u re 3 .2 9 . A F M im a g e o f S i0 2 /C u P c ( 2 n m ) s in g le la y er and R M S ro u g h n e ss o f the. d iffe r e n t C u P c s in g le la y ers w ith th ic k n e s s ran gin g from 2 to 8 n m ..................... 1 2 6. F ig u re 3 .3 0 . T E M im a g e o f an u n p a tte m e d s ta c k ..........................................................................1 2 6. F ig u re 4. 1. T M R r e sp o n s e o f th e A lO x b a sed M TJ d e v ic e s , c o m p a r in g th e fo rm a tio n. o f A lO x in d iffe r e n t p rep aration c o n d itio n s .......................................................................1 3 7. X V I. F ig u re 4.2. T M R re s p o n se o f th e sta c k D , sh o w in g th e e ffe c t o f th e p o s itio n s a n d. ty p e s o f fe rro m a g n e tic e le c tro d e s in th e s ta c k .............................................................. 138. F ig u re 4.3. T h e I-V and c o n d u c ta n c e g ra p h s o f the J-4 in set 1 (a ,b ) an d J-5 in set 2. (c ,d ).............................................................................................................................................. 140. F ig u re 4.4. T M R an d I-V re sp o n se s o f the stack o f C o F e (1 0 n m )/A 1 0 x (D )/C o F e. ( 18nm ) as a fu n c tio n o f t e m p e r a t u r e ................................................................................141. F ig u re 4.5. S c h e m a tic re p re se n ta tio n o f sam p le p re p a ra tio n u sin g A r-io n c le a n in g. p r o c e s s ........................................................................................................................................ 143. F ig u re 4.6. R A an d T M R re sp o n se o f th e sam p le s w ith d iffe re n t c le a n in g tim e for. M gO s u rfa c e ..............................................................................................................................144. F ig u re 4.7. I-V c h a ra c te ristic s o f M g O (2.5 n m )/L iF (t) b ase d h y b rid d e v ic e s w h e n. t= 0 n m (a) and t= \ n m ( b ) . .................................................................................................. 146. F ig u re 4.8. T M R c h a ra c te ristic s o f M g O (2.5 n m )/L iF (t) b a se d h y b rid d e v ic e s w h en. t=Q n m (a) an d ?=1 n m ( b ) .................................................................................................... 146. F ig u re 4.9. E v o lu tio n o f air e x p o se d M gO /A lq^ b ase d s ta c k ..................................................148. F ig u re 4.10. T M R c h a ra c te ristic s o f a ir e x p o se d M g O (2 n m )/A lq 3 (/) b a se d h y b rid . d ev ice s w ith d iffe re n t th ic k n e sse s o f Alq^ at 10 m V . A lso , the curv'e fo r /= 2. n m at 500 m V is in c lu d e d .....................................................................................................149. F ig u re 4.11. R e sista n c e -a re a p ro d u c t p lo tte d as a fiin ctio n o f A lq j b a rrie r th ic k n e ss. in M g O /A lq 3 M T Js. T h e red c u rv e sh o w s th e fittin g ..................................................150. F ig u re 4.12. B ias d e p e n d e n t T M R c h a ra c te ristic s o f M gO/Alq.^ b ase d h y b rid. d e v ic e s ......................................................................................................................................... 152. F ig u re 4.13. E v o lu tio n o f th e c le a n e d M g O (2.5 nm VA lq^ (t) b ase d s ta c k s ......................153. F ig u re 4.14. R o o m te m p e ra tu re T M R c h a ra c te n s tic s o f arg o n ion c le a n e d M gO (2.5. n m )/A lq 3 (t) b a se d h y b rid d e v ic e s ....................... 154. X V I I. Figure 4.15. The left panel shows our experimental data while the right panels show. the data taken from R e f [21], In both data, an exponential increase o f RA. with Alqs thickness was seen and the RA changed slightly with temperature.. The stack used in R e f [21] was Co (8 nm)/A10x (0.6 nmVAlq^ (1.6. nm)/NiFe (10 nm )........................................................................................................ 155. Figure 4.16. M agnetoresistance curves as a function o f applied bias measured from. contaminated or uncontaminated MgO based tunnel junctions, where the. data taken from our study (a) and literature (b) [ 11] (note that the scales on. the graphs are different). The black solid lines in both figures represent the. TM R o f devices which have contaminated interfaces........................................... 157. Figure 4.17. Bias dependent TMR curves o f the MgO (2.5 nm)/Alq3 { r = 0 , 1, 2 and 3. nm) based hybrid devices............................................................................................158. Figure 4.18. Comparison o f the normalized bias dependent TM R curves, where the. data is taken from our study (a) and R e f [22] (b). Note that the polarity o f. the applied bias in both figures is reversed and the bias scales are different.... 159. Figure 4.19. Two-step tunneling and spatial distribution o f Gaussian DOS o f the. intermediate sites under an applied bias with different electrical polarity. [22] 161. Figure 4.20. MR (a) and I-V (b) curves o f the device as a function o f temperature. when t= l nm. The inset shows the variation o f junction resistance and MR as. a ftjnction o f tem perature............................................................................................ 162. Figure 4.21. MR and I-V characteristics o f the device when I = 2 nm. The MR data. was measured at 15 K and the inset figure shows the variation o f device. resistance as a ftinction o f tem perature.....................................................................163. X V I I I. F ig u re 4.22. M R , I-V a n d d l/d V c h a ra c te ris tic s o f th e d e v ic e w h e n t=3 n m . T h e. in set sh o w s th e v a ria tio n o f d e v ic e re s is ta n c e w ith te m p e r a tu re ............................ 164. F ig u re 4 .2 3 . T h ic k n e ss d e p e n d e n t re s is ta n c e o f o rg a n ic d e v ic e s, w h e re th e d a ta in. (a) re p re s e n t o u r stu d y w h ile th e d ata in (b) is ta k e n fro m R e f [2 3 ]. T h e . d a sh e d line in (a ) is an e y e s -to -g u id e p lo t. T h e sta c k in (b) is T a (2 )/C o F e B . (2)/A 10x (~ 1.5)/Alq3 (t)/C o (2 0 ), w h e re th e n u m b e rs in p a re n th e s e s are the. la y e r th ic k n e s s e s in n a n o m e tre . T h e ju n c tio n siz e in (a) an d (b) is 2 5 0 x 2 5 0. |jm ^ a n d 3 0 0 x 3 0 0 |i m ', re s p e c tiv e ly ................................................................................. 166. F ig u re 4.24. S c h e m a tic b a n d d ia g ra m fe rro m a g n e t/A lq j/fe r ro m a g n e t s tn ic tu re (a ). an d in v e rse c u rre n t d e n s ity as a fu n c tio n o f Alq,? th ic k n e ss (b) [ 2 3 ] .....................167. F ig u re 4.25. E v o lu tio n o f M g O /C u P c b ase d M T J s ta c k s .........................................................170. F ig u re 4.26. T M R c h a ra c te ris tic s o f th e first se t o f sa m p le at 3 00 K (a) an d at 50 K. (b). T h e in set in (b) sh o w s the v a ria tio n o f T M R w ith te m p e ra tu re fo r all. s a m p le s ........................................................................................................................................171. F ig u re 4.27. T e m p e ra tu re d e p e n d e n t re sista n c e b e h a v io r o f the first set o f sa m p le (a). and R A p ro d u c t v ersu s sq u a re ro o t o f a re a p lo t ( b ) .....................................................172. F ig u re 4.28. M R c h a ra c te ristic s o f th e sec o n d set o f sam p le s at 3 0 0 K (a) an d a t 20. K ( b ) ............................................................................................................................................. 173. F ig u re 4.29. T M R c h a ra c te ristic at 18 K (a ) an d th e te m p e ra tu re d e p e n d e n t I-V. c h a ra c te ristic s (b ) o f C u P c b ase d d e v ic e w h e n t= \ nm . T h e d l/d V cu rv e w as. c a lc u la te d fro m the I-V d a ta m e a su re d at 18 K (c). T h e in set in fig u re (b). sh o w s th e v a ria tio n o f M R and re sista n c e as a fu n c tio n o f te m p e r a tu re .............. 175. F ig u re 4.30. M R (a), I-V (b) an d d l/d V (c) c h a ra c te ris tic s o f 2 n m th ic k C u P c b a se d. d e v ic e , w h e re th e M R c u rv e w a s m e a s u re d a t 17 K ................................................... 176. X I X. F igure 4.31. M R (a), I-V (b) and d l/d V (c) characteristics o f 1 nm thick ZnPc based. device, w here the M R curve w as m easured at 15 K. The inset in figure (c). show s the M R and norm alized resistance o f the device as a function o f. tem p eratu re........................................................................................................................ 178. F igure 4.32. M R (a), I-V (b) and d l/d V (c) characteristics o f 3 nm thick ZnPc based. device, w here the M R curve w as m easured at 15 K. The inset in (c) shows. the norm alized resistance o f the device as a function o f tem p eratu re................... 179. Figure 4.33. R esistance o f a set o f ju n c tio n s w ith increasing ZnPc th ic k n e ss.................. 180. Figure 4.34. E volution o f M gO /Z nq 2 based M TJ stack s...................................................... 181. Figure 4.35. T M R characteristics o f M gO (2 nm )/Z nq 2 (t) based M TJs (a) and. variation o f RA as a function o f tem perature (b )......................................................182. F igure 4.36. Bias dependent TM R curves o f M gO (2 nm )/ Znq2 (t) based ju n ctio n s.. 182. Figure 4.37. M R (a) and I-V (b) characteristics o f Znq: based device w hen t= l nm,. w here the M R curve in (a) w as m easured at 20 K ................................................... 184. Figure 4.38. M R curves o f Z n q 2 based device w hen t=2 nm at 18 K for 50 mV (a). and 100 mV (b )................................................................................................................. 184. F igure 4.39. M R (a) and I-V (b) characteristics o f Z nq 2 based device w hen t=10 nm,. w here R-H curve was m easured at 18 K applying 500 mV. Also, a zoom in. view o f the I-V curve is show n in (c ).......................................................................... 185. XX. Chapter 1. INTRODUCTION. 1.1 Introduction. M agnetism features in m any research areas not only as a branch o f science but also due. to its im portant applications in areas such as m agnetic sensors and m agnetic m em ories.. In the 2 T ' century, technological developm ent is providing denser, faster and sm aller. electronic devices as w ell as low pow er consum ption in order to im prove device. functionalities. T echniques such as U V lithography and electron beam lithography. allow fabrication o f m icron or nano-scale m agnetic devices. T hese tiny devices enable. to study o f basic m agnetic phenom ena such as m agnetoresistive [1,2] and spin torque. effects [3]. M agnetic effects add to the functionality o f electronic devices. H ence, the. use o f the spin p roperty o f the electrons in devices gives rise to the nam e spin. electronics.. 1.2 Spin electronics. Spin is the intrinsic angular m om entum o f electrons and it is a purely quantum . m echanical phenom ena. E lectrons have tw o different spin states, spin up and spin down. corresponding to the angular m om entum states ms = -1/2 and ms = +1/2. It is possible to. build devices in w hich the spin property o f electrons is controlled. Spin electronic. devices m ay have sm all operation size w hich can offer high inform ation storage density. and fast operation speed because direction o f spins can sw itch on tim es o f the order o f a. nanosecond. M oreover, spin electronic devices such as m agnetic random access. m em ories (M R A M s) are non-volatile, so that w hen the pow er goes off, the spins can. keep their polarization directions, w hereas in the conventional devices such as in. dynam ic random access m em ories (D R A M s) capacitors lose their charges. In DRAM . chips all the m em ory elem ents are refreshed by reading and re-w riting the contents.. T his situation requires a constant pow er supply, w hich is the reason w hy D R A M s lose. their m em ory w hen the pow er turns o f f H ow ever, M R A M does not need to be. refreshed and no continuous pow er is required to keep the inform ation. All these. consideration m ean that spin electronics is an attractive research area for the. inform ation storage industry.. 1.2.1 E lectron tu n n elin g. T he transport m echanism o f electrons w ithin an insulating layer is the tunneling. phenom enon w hich is a purely quantum m echanical effect. E lectrons have both particle. 2. and w ave like properties. T unneling is a w ave like effect o f electrons. W hen electrons. com e across an energy barrier, their w ave fiinctions do not end abruptly but can pass. through the barrier even though the electron energy is less than the energy barrier. height. Then, sam e electrons have a probability o f a p p e a n n g on the oth er side o f the. barrier. This effect has no classical counterpart. Figure 1.1 show s the tun n elin g effect.. F ig u re 1 1 . W a v e fu n c tio n d ecay s e x p o n e n tia lly w ith in th e e n e rg y b a rrie r. In a m etal/insulator/m etal contact, electrons tunnel from one electrode to the other and. there exists a tunnel current if a potential difference is created betw een the electrodes.. The typical band diagram o f m etal/insulator/m etal ju n ctio n un d er a bias v o h ag e is. show n in Figure 1.2. The tunneling current from the left electrode to th e right electrode. depends on the density o f states o f the first electrode, p (E), and that o f the second. electrode, p (E + eV ), the square o f m atrix elem ent |M|", w hich represents the probability. o f transm ission through the barrier, occupation probabilities that states in the left. electrode, f (E), and probability [l-f(E + e V )] that the states in the right electrode are. em pty. The resulting tu nneling current is given by the follow ing expression;. 3. + 00. h ^ r = I p , ( E ) . P r ( E + e V H M \ ^ . f ( E ) . [ l - f ( E + e V ) ] d E (1) — 00. and the total tunneling current is described as. h o ta i — h->r ( 2 ). For this type o f ju n ctio n s, Sim m ons generalized a tunnel current density and expressed. it as follow ing form ula [4];. w here J(V ) is the tunnel current density as a function bias voltage V. A and /o are the. constant term s o f A u y j l r r i e / h and e / 2 n h , respectively, w here rrig is the effective m ass. o f the electron, (p is the average barrier height, d is the barrier thickness. This term . show s a linear response at sm all voltages. H ow ever, it has a n o n-linear characteristic. for larger voltages, w hich is evidence that current is due to tunneling electrons in. M /I/M structures. It can also be seen that tu nneling current show s exponential. dependence on the barrier thickness and average barrier height. T his expression can be. used only for sym m etric structures, the left and right electrodes are identical. For. asym m etric structures, B rin k m a n ’s calculation is used [5].. (3). 4. ev. F ig u re 1.2. B a n d d ia g ra m o f m e ta l/in s u la to r/m e ta l stn ic tu re. 1.2.2 Tunneling criteria. A s it is discussed in section 1.2.1, tun n elin g is a purely quantum m echanical effect that. occurs on the atom ic scale. This event can be clearly observed in m any tunnel ju n c tio n . devices in laboratory conditions. Therefore, it is w orthw hile to outline the criteria for. the expected tunneling behavior. First o f all, exponential dependence o f the tunnel. current on the barrier thickness, barrier height and applied bias will result in a n o n ­. linear current/voltage curve in the tu nneling lim it. A lso the differential conductance. w ill be quadratically dependent on the applied bias. N ext, the resistance-area pro d u ct. should rem ain constant for the sam e barrier thickness, indicating o f a uniform barrier. w ithout any hot-spots. Furtherm ore, the resistance o f a device, in the tunneling regim e,. should not change excessively w ith the tem perature. The criterion accepted for the. tunneling m odel is slight increase w ith decreasing tem perature (-1 0 -2 0 %). I f the. resistance decreases w ith the decreasing tem perature, it is m ore likely due to the. m etallic shorts in the barrier [6]. It is im portant to satisfy these criteria w hen the. observed T M R in m olecular ju n ctio n s is being interpreted.. 5. 1.2.3 A nisotropic m agnetoresistance and giant m agnetoresistance. The effect o f spin on th e electrical resistance w as firstly observed by T hom son [7], He. dem onstrated that the electrical resistance o f a ferrom agnetic c onductor depended on. w h eth er the current flow ing through the c onductor was perpendicular or parallel to the. m agnetization o f th e sam ple. W hen the current flow s parallel to the m agnetization. direction o f the sam ple, a stronger scattering process occurs and h igher resistance is. observed, p||. H ow ever, w hen the current and m agnetization direction are perpendicular. to each other, low er scattering and low er resistivity are observed, px. This effect has. been called as anisotropic m agnetoresistance effect (A M R ). T he effect is o f order 1% in. ferrom agnetic m etals. A lthough the A M R effect had previously show n the relationship. b etw een m agnetization and electrical resistance, the discovery o f G iant. M agnetoresistance (G M R ) effect in 1988 is considered as the birth o f spin electronics.. It w as independently discovered for Fe/C r m ultilayers [1] and for Fe/C r/Fe trilayers [2].. A typical G M R device consists o f tw o ferrom agnetic layers (FM ) separated by a. nonm agnetic (N M ) spacer layer w hich behaves as a ‘spin v a lv e ’. The schem atic o f spin. valve structure is show n in F igure 1.3.. FM IMM FM. Figure 1.3. A GMR spin valve structure. T he dashed arrow sh ow s the free layer w h ile the continuous. arrow show s the pinned layer. The current flo w s perpendicular to the plane.. 6. The operation principle o f these devices depends on the relative alignment o f. magnetizations o f the two FM layers one o f which is pinned while the other one is free. to rotate under external magnetic field. If the magnetization directions o f the magnetic. layers are antiparallel, the system shows high resistance. W hen they are aligned. parallel, the resistance reaches its minimum value. Figure 1.4 summarizes the operating. principle o f a simple GMR structure. In Figure 4a, only the spin up electrons can pass. without any scattering through the whole structure and spin down electrons scatter. within the both layers then conduction is supplied by spin up electrons. However, in. Figure 4b, while only the spin down electrons scatter within the first layer, the spin up. electrons do not scatter and for the second FM layer vice versa then system has a big. resistance. The significant change o f the resistance has been called as giant. magnetoresistance and the value o f the GMR is defined by the following expression;. G M R = { - ^ -----^ xlOO (4) V / ? t i /. where and R || are the resistances. respectively.. F M N M F M. M (a). o f the antiparallel and parallel alignment.. F ig u re 1.4. T h e figure o f co n d u c tio n in a G M R stru c tu re (a) in p a ra lle l a lig n m e n t an d (b ) in a n tip arallel. alig n m en t. 7. 1.2.4 Tunneling m agnetoresistance. The development o f the spin electronics has been helped by the discovery o f tunneling. magnetoresistance (TMR) effect. A simple TMR device consists o f two FM layers. which are separated by a NM insulating spacer layer. In the TMR devices, the. insulating layer should be very thin so that electrons can tunnel easily without any. scattering. The thickness o f the insulating layers generally is about 1-2 nm. The. operating principle o f the TMR devices like that o f the GM R devices is based on the. relative alignment o f the FM electrodes. However, the physical origin o f TMR depends. on the quantum mechanical tunneling phenomena o f the wave function o f the electrons.. 1.2.5 Physical origin o f tunneling m agnetoresistive effect. The physical origin o f the operation o f TMR structures is based on spin dependent. scattering process o f the tunnel electrons at the insulator/ferromagnet interface. It is. well known that ferromagnetic materials such as Fe, Co and Ni have a different density. o f states (DOS) for the spin up (m ajority electrons) and spin down electrons (minority. electrons) at the Fermi level because o f the exchange interaction between the electrons.. These electrons at the Fermi level are spin polarized. In contrast, for normal metals the. num ber o f the spin up and down electrons is the same at the Fermi level (Figure 1.5).. E E. F e rm i le v e l ______. (a) (b). D e n s i t y o f s t a t e s. Figure 1.5. Schem atic representation o f D O S: (a) for a nonnal metal and (b) for a ferrom agnetic metal. (arrows sh ow majority and m inority spin states). T he different D O Ss lead to different transport behavior o f the m ajority and m inority. spin electrons in a FM m aterial. C onsidering a spin polarized tunneling in a F M /I/FM . structure in a parallel m agnetization alignm ent and large DOS for the m ajority electrons. (up-spin). The up-spin and dow n-spin electrons create tw o conductance channels in a. different flow rate due to the different am ount o f spin polarized electrons and they can. find them selves in a region o f the sam e spin state in the oth er FM layer. In the. antiparallel case, m agnetization direction o f the second FM layer is reversed and the. spin dependent DO S changes, resulting in large am ount o f m ajority electrons scatter. due to the lack o f available spin states at the Ferm i level. In the Julliere m odel, it is. assum ed that the conductance is proportional to the density o f states o f the. ferrom agnetic electrodes and a form ula show ing the relation betw een tunneling. 2P,P: ( 5 ). 9. m agnetoresistance and spin polarization o f the ferromagnetic electrodes can be written. as [8]:. with P i, 2 denotes the spin polarizations o f the ferromagnetic electrodes and it is given. by. p _ f 1̂.2 ~ ^1.2\ . . . W2+^i,2/. where D | 2 and £>1 , 2 are the densities o f states o f the electrodes at the Fermi energy (Ep). for the majority-spin and minority-spin bands, respectively. Figure 1.6 illustrates the. spin dependent tunneling in a FM /I/FM structure in the case o f parallel and antiparallel. alignment o f the ferromagnetic electrodes.. ( a). D e n s ity o f s ta te. Figure 1.6. Schem atic diagram o f spin dependent tunneling via the .lulliere model. The top (a) and bottom. (b) panels sh ow the parallel and antiparallel alignm ent o f the ferrom agnetic electrodes, resp ectively. 10. 1.2.6 Spin electronics: overview. A lth o u g h th e first e x p e r im e n t reported b y Ju lliere at lo w tem p era tu re s h o w e d 14%. T M R ratio for C o /o x id iz e d G e /F e sta ck but h is resu lts w e r e n o t rep ro d u ced [8],. S ig n ific a n t c h a n g e s in th e m a g n e to r e sista n c e at ro o m tem p eratu re w e r e o b s e r v e d for a. C o F e /A b O .V C o stack w ith T M R ratio o f \ l% [9] and F e /A b O ^ /F e ju n c tio n s w ith a. T M R ratio o f 20% [1 0 ], S in c e that tim e s c ie n tis ts h a v e b e e n tr y in g to im p r o v e the. T M R ratios o f M T Js b y u s in g d iffe r e n t ferro m a g n ets (F e , C o , N i a llo y s ) , d ifferen t. barriers and d ifferen t p h y s ic a l treatm en ts. O n e o f the m o st w id e ly u s e d a p p r o a c h e s is. su b se q u e n t a n n e a lin g o f the m u ltila y e r s. A n n e a lin g o f m u ltila y e r s u n d er th e su ita b le. c o n d itio n s can g r e a tly e n h a n c e th e T M R ratio [1 1 ], F urtherm ore, p o ly c r y s ta llin e C o F e . [1 2 ] and a m o rp h o u s C o F e B [1 3 ] e le c tr o d e s s h o w a T M R ratio o f 50% and 70%>,. r e s p e c tiv e ly w ith an A lO x barrier. A n o th e r m eth o d in order to o b ta in a h ig h T M R ratio. is to u se o f e le c tr o d e s w h ic h h a v e h ig h er sp in p o la r iz a tio n at th e F erm i le v e l. S o m e . m a teria ls su ch as CrO^, F e 3 0 4 , Lao vSrojM nO ^, Lao vCao iM n O j, S r 2F e M o 0 6 ty p e s and. H e u sle r a llo y s lik e N iM n S b or C o^ M n Si [1 4 ] s h o w s h a lf m e ta llic b e h a v io r w ith . e le c tr o d e s o f o n ly o n e sp in su b -b a n d at the Ferm i le v e l. B y u s in g th is ty p e o f . e le c tr o d e s , it h as b e e n p o s s ib le to o b tain v e r y large T M R ratios at lo w tem p eratu res.. H o w e v e r , th e T M R ratio d e c r e a se s and v a n is h e s at near the r o o m tem p era tu re for m a n y . o f th em [1 5 ]. T h e last m eth o d to a c h ie v e h ig h T M R ratios is to u s e a c r y sta llin e M g O . barrier instead o f an a m o rp h o u s A I2 O 3 sp a cer layer. It is w e ll k n o w n that cr y sta llin e . structures h a v e p e r fe c tly ordered a to m s in sp a c e and the num ber o f structural d e fe c ts is. le s s than for a m o rp h o u s m a terials. T o u s e a c r y s ta llin e M g O barrier r esu lts in co h eren t. tu n n e lin g w ith lo w sc a tterin g in th e barrier. T h u s, it is p o s s ib le to o b ta in la rg e T M R . ratios co m p a red to th e M T Js w ith an a m o rp h o u s A I2O 3 barrier. T h is w a s p r e d ic te d b y. 1 1. B utler et al. [16], In 2004, giant tunneling m agnetoresistance w as observed in M TJs. u sing crystalline M gO barrier [17,18], Y uasa et al. observed 180 % T M R at room . tem perature and 250 % at low tem perature in a Fe/M gO /F e M T Js grow n by m olecular. beam epitaxy [17], Independently, Parkin et al. also reported 220 % T M R ratio at room . tem perature using the stack o f C oF e/M gO /C oF e [18], Furtherm ore, D jayapraw ira et al.. o btained 300 % at low tem perature and 230 % at room tem perature with. C oF eB /M gO /C oF eB m u ltilayer grow n by m agnetron sputtering, w here C oFeB layers. are am orphous but M gO layer is (001) textured [19], Later, T M R ratios was achieved. the v a lu e o f 500 % for C oF eB /M gO /C oF eB stack at room tem perature [20], Hence,. single crystalline M gO has been w idely used as a barrier layer in m any m agnetic tunnel. ju n ctio n s. A t 300 K, the highest TM R , 604% , has been observed in. C oF eB /M gO /C oF eB structure by perform ing som e critical annealing treatm ents and. suppressing the Ta diffusion into the active region in the TM R stack [21],. 1.3 Organic semiconductors. 1.3.1 Introduction. In organic sem iconductors the m ost attractive point for spin electronic applications is. the w eakness o f the spin scattering m echanism . In general, organic sem iconductors. consist o f m ostly light elem ents (C, H, N or O ) and the strength o f the spin-orbit. interaction is proportional to Z^, w here Z is the atom ic num ber. Long spinlife tim e due. to the low spin-orbit coupling and w eak hyperfm e interactions should allow the spin. polarized carriers to travel over large distances in the organic sem iconductors despite. 12. their sm all m obility [22], O ne o f the fundam ental differences betw een organic and. inorganic sem iconductor is the charge transport m echanism . In inorganic. sem iconductors charge carriers are delocalized and m oves in broad bands. E lectrons. and holes m ove w ith high m obility. H ow ever, in organic sem iconductors the overlap o f . the orbitals o f adjacent m olecules is sm all, leading to narrow band w idth w ith little. dispersion. T herefore, electrons m ove w ith low m obility. F or exam ple, the hole. m obility o f rubrene, one o f the best organic sem iconductor, is about 10 cm ^ V 's'* at. room tem perature w hile the m obility o f p-type Si is about 450 c m 'V ’s ''. C harge carrier. transport in organics can be described by either band or hopping transport depending on. the tem perature and the degree o f order in the m aterial. H ighly pure m olecular crystals. ten d to conform to band transport at low tem peratures. H opping transport dom inates in. am orphous organic sem iconductors and tends to occur betw een localized m olecular. states. Figure 1.7 com pares the band structure o f inorganic p-type Si and organic. rubrene. T hese fundam ental differences betw een organic and inorganic m aterials arise. from the nature o f the bonding properties. W hereas the organic m aterials are van de. W aals bonded solids leading to a w eaker interm olecular bonding, inorganic. sem iconductors are covalently bonded. This difference show s up in the m echanical and. therm odynam ic properties o f those m aterials such as reduced hardness or low er m elting. point for the organics, and even m ore im portantly, in the different optical or charge. transport properties. 13. X -2. -10 -12. r XL. 3. 2 LUMO. 1 > 0 >. HOMOU j. 2. 3, X r Y Z r. (a) (b). Figure 1.7. Band structure o f p-type Si (a) and rubrene (b) [22]. 1.3.2 ;r-conjugation. I n an organic semiconductor, carbon atoms are connected by single or double bonds. which are formed from a combination o f s and p atomic orbitals. I n the ground state,. the electronic configuration o f the carbon atom is I s '2 s '2 p '. When two carbon atoms. com e closer, covalent bonds form between them and 2s and 2p orbitals mix to form. new hybrid sp, sp~ and sp^ orbitals which are suitable for the qualitative description o f. atomic bonding properties. I n the case o f sp and sp"" hybridization, all the electrons are. strongly localized resulting in extremely poor charge conduction. I n the case o f sp ' one. s orbital combines with two p orbitals and />-) orbitals and this combination creates. three hybrid sp^ orbitals. However, p^ orbital which is perpendicular to the plane o f sp '. orbitals remains unhybridized. I n a carbon chain the overlapping s p ' orbitals create o-. bonds while the p^ overlapping orbitals create 7t-bonds. I n a carbon chain the. electrons delocalize across all the contiguous 7i-bonds in the molecule, resulting in. conducting properties o f the molecules.. 14. u n h y b n d i z e d o rb ita l. s p ' h y b n d o rb ita l. Figure 1.8. sp" hybridization o f a carbon atom and n / o bonds between neighbouring carbon atoms.. 1.3.3 Charge conduction in ;r-conjugated system s. In organic m aterials, van der W aals forces are responsible for the interm olecular. binding and those forces are m uch w eaker than the covalent and ionic bonds o f . inorganic crystals. T herefore, organic m aterials are less rigid than the inorganics. Thus,. electrical injection o f charge carriers into the organic m aterials can create a distortion o f . the surrounding lattice due to the C oulom b interaction. C onduction electrons attract the. positiv ely charged ions and repel the negatively charged ions in the surrounding lattice. on their path. The lattice distortion accom panies the charge carriers through the organic. m aterial, resulting in form ation o f a q u asi-particle called p o la w n . Polarons can be. either positively or negatively charged and th ey carry the spin o f the accom panying. electrons. O n the oth er hand, the m ovem ent o f the conduction electrons is hindered by. the resulting lattice distortion w hich behaves as a potential w ell, and decreases the. m obility.. 15. Hopping and band type transport are possible transport mechanisms in organic. materials. The band type conduction is expected for highly ordered organic materials at. low temperature when the mean free path o f the charge carriers exceeds the. intermolecular distance [23], Valance and conduction bands form by overlapping the. HOMO and LUM O levels o f the molecules, respectively. For disordered systems such. as organic thin films, charge transport between the localized molecular states is via the. hopping m echanism which strongly depends on the temperature, electric field, trap. states in the material and carrier concentration [24-28], Vapor deposited amorphous. organic materials have a disordered structure, resulting in disordered HOMO and. LUMO levels [29], Therefore, the band conduction mechanism does not apply in these. materials due to the disordered energy levels. The charges are localized on the. molecular sites due to the disorder so the conduction mechanism becomes via phonon-. assisted tunneling or hopping from one localized site to another site. The hopping. probability from one site to another was formulated by Mott in his variable range. hopping model [30];. where is the hopping probability o f the electrons from the / site to the / site, R is the. distance between the two sites, £/ and £ , are the energy levels o f an electron at the two. sites and a corresponds to the exponential decay o f the wave function in a potential. barrier. From the equation one can deduce that the transport o f the charge carriers is. affected by the disorder o f the position and energy o f the hopping sites. In disordered. systems, conduction is dominated by hopping process and at low temperatures. conductance is satisfactorily explained by M ott’s variable range hopping model. In. ( 7 ). 16. general, conductance, proportional to the pro b ab ility o f such a hop, as a function o f . tem perature is given by [30]. G oc. w here T is the tem perature, To is the characteristic tem perature and d is the dim ension.. T he m odel predicts that variable range hopping length, R, increases w ith decreasing. tem perature as w here d is the dim ension.. 1.3.4 Carrier injection and transport in organics. In the organic based electronic devices such as O L ED s, tw o different m echanism s. govern the device operation, nam ely carrier injection lim itation at the interfaces and. space-charge lim itation o f the current in the organic. The injection lim iting process. occurs w hen the interface barrier is so high as to control injection o f the carriers from . m etal to the organic layer. The three m echanism s w hich govern the injection lim itation. process are F ow ler-N ordheim tunneling, R ichardson-S chottky therm ionic em ission and. backflow o f the injected carriers. These m echanism s are briefly explained in C hapter 3.. In contrast, space-charge lim itation occurs w hen the charge carriers are injected from a. good m etallic contact w hich acts as an inexhaustible carrier reservoir. U sing som e basic. equations such as the Poisson equation, the continuity equation tog eth er w ith the. boundary conditions, drift-diffiision equation, and related equations for free and trapped. charge carrier densities the relation betw een th e current density and external voltage. can be found by m aking analytical solutions in som e special cases and the result is. show n in the follow ing M ott-G u m ey equation [31];. 17. 9 JsCLC — g (9 ). w here e is the dielectric co nstant o f the m aterial, £o is the perm eability o f free space, ju is. the charge carrier m obility w hich is independent o f electric field and d is the w idth o f . the organic spacer betw een tw o electrodes. This relation is obtained in the case o f . perfect insulator w ithout intrinsic carriers and traps as w ell as neglecting the difftision.. In the case o f traps w h ich have discrete en erg y levels, the current is generally low er and. the quadratic field d ependence is retained and the equation (9) is m odified by a factor 9. w hich is the ratio o f free carriers to the total num ber o f carriers;. w here n and «i are the n u m b er o f free and trapped charges, respectively. If the traps are. energetically distributed in the spacer, th ey w ill be filled w ith electric field and the. current w ill increase faster than quadratic until all traps are filled. Thus, taking account. these assum ptions a relation betw een the trap-charge lim ited current density (TCLC ). and voltage in the presence o f the exponential trap distribution can be expressed as. follow ing;. where Nc is the density o f states in the conduction band, N, is the effective density o f . traps, / = E / k s T and E, is the trap energy [32], O n the oth er hand, electron m obility. obeys the P oole-Frenkel m odel in the organic sem iconductors. In general, charge. carriers w hich m ove in an insulator are trapped in the localized states, and random . therm al fluctuations m ay assist them by giving enough energy to escape from the trap. states and m ove in the conduction band. B asically, the P oole-Frenkel m odel explains. the effect o f electric field w hich m oves th e charge carriers to the conduction band in the. n. ( n + 7it) ( 10 ). ( 1 1 ). 18. a b s e n c e o f larg e th e rm a l flu c tu a tio n s. So, th e field d e p e n d e n t m o b ility o b e y s th e. fo llo w in g P o o le -F re n k e l (P F ) re la tio n ;. pi{F) = ( 12). w h e re /? is a c o n s ta n t , juo is th e z e ro -fie ld m o b ility an d F is th e e le c tric field. I f th e. fie ld d e p e n d e n t m o b ility is ta k e n in to a c c o u n t, th e tra p -fre e S C L C is g iv e n b y the. M u rg a tro y d a p p ro x im a tio n [33];. A n o th e r im p o rta n t p a r a m e te r w h ic h m u st be in v o lv e d to u n d e rs ta n d th e J-V . c h a ra c te ris tic s o f th e d e v ic e s is th e te m p e r a tu re - d e p e n d e n t m o b ility . In th e S C L C . re g im e w ith an e x p o n e n tia l tra p d is trib u tio n , th e field an d te m p e ra tu re d e p e n d e n t. m o b ility c a n be e x p re s s e d by th e m o d ifie d P F e q u a tio n ;. w'ith. 1 1 (15). T e f f T To. w h e re A E is th e a c tiv a tio n e n e rg y fo r h o p p in g at z e ro -e le c tric field . To is a n e m p iric a l. p a r a m e te r an d jupf is th e m o b ility at T = T q an d. Ppf — (1 6 ). T h is m o d e l s h o w s re s u lts c o m p a ra b le w ith th e lite ra tu re d ata, a p a rt fro m th e p re fa c to r. ^PF. F o r e x a m p le in R e f [32] th e c a lc u la te d p a ra m e te rs fro m e x p e rim e n ta l d a ta fo r. 19. electron-only devices are that |ipF= 1.2xlO“* cm W .s, To = 430 K, AE = 0.43 eV and Ppf. = 4.1xl0""'^ J(cmA/^)'^^, w hereas those p aram eters for hole only devices in Ref. [34,35]. are 3.5xlO '^cm ‘/V .s, To = 600 K, AE = 0.48 eV and Ppf = 4 ,6 x l 0 '- ' JCcm/V)"^. 1.3.5 M aterials for organic electronics. O rganic sem iconductors can be classified into two m ajor groups such as low m olecular. w eight m aterials and polym ers. The low m olecular w eight m aterials consist o f small. m olecule or oligom ers, m ade o f a single m onom er or a few repetitions. On the other. hand, polym ers are m ade o f an unlim ited sequential repeat o f the m onom er units. Both. types o f m aterials are Ti-conjugated system form ed by / / orbitals o f 5/?’-hyridized. carbon atom s in the m olecules. H ow ever, an im portant difference betw een those two. classes o f organic m aterials is seen w hen it com es to process them to form thin films.. W hereas the thin film form o f the sm all m olecule organic m aterials can be obtained. from the gas phase by sublim ation or evaporation, polym er based thin film s are. processed from a solution by spin coating [36]. In this thesis, small m olecule organic. sem iconductors are used in the experim ents. Here, the general properties o f som e. fam ous sm all m olecule organic sem iconductors used in this thesis are briefly. m entioned.. 20. 1.3.5.1 T ris (8-h yd roxiq u in olin e) A lum in um. Tris (8-hydroxiquinoline) A lum inum (A lq j) is a coordination com plex w herein A1 is. bounded to three 8-hydroxyquinoline ligands, giving the chem ical form ula. A1(C9H6N0)3 and it is a 7t-conjugated sm all m olecule organic sem iconductor. Alq^ is. m ost com m only used in O L E D s as an
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