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Electrical and Electronic Properties of Interface Boundaries in Functional Structures for Microelectronics

This article explores the role of interfaces in nanoelectronic devices and investigates various novel logic and memory concepts. It discusses methods of investigation and presents experimental results on metal/semiconductor interfaces and band line-up at metal/dielectric interfaces.

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Electrical and Electronic Properties of Interface Boundaries in Functional Structures for Microelectronics

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  1. Электронные и электрические свойства границ раздела в функциональных структурах для микроэлектроники Андрей Зенкевич зав. лаб. «Новые функциональные материалы и структуры для наноэлектроники» МФТИ

  2. Outline • Introduction: the role of interfaces in nanoelectronic devices • Novel logic/memory concepts (ReRAM, MRAM, spin transistor, FeRAM, … ) and corresponding functional structures (MIM, FTJ, …) • Metal/semiconductor (dielectric) interfaces : historic excursion • Methods of investigation: I-V, C-V, ballistic electron emission microscopy (BEEM), internal photoemission (IPE), core-level photoemission spectroscopy (XPS)→ HAXPES • Experiments / results: • measurements of “effective” WF of metal in contact with dielectric • charge redistribution in MOS stacks upon ex situ and in situ biasing • band line-up at metal/dielectric interface • electronic structure vs. electron transport in M-FE-M tunnel junctions

  3. Introduction “…interface between the different materials plays an essential role in any device action. Often, it may be said that the interface is the device.” H. Kroemer, Nobel lecture (2000) • The scaling of the device lateral size → shrinking of individual layer thicknesses in the functional structure down to 1-10 nm scale • Numerous non-volatile memory concepts taking advantage of the electric resistance switching phenomena in a few nm thick layers • Once the thickness of the individual layer is in nm range, the properties are largely defined by interfaces The functionalization of the novel combination of materials in nm thick layers implies the comprehensive study of the interface properties

  4. Complementary metal-oxide-semiconductor (CMOS) scaling Moore’s Law : x2 more devices/wafer every 2 years feature size decreases by x2 every 6 years What for? speed↗ energy consumption↘ cost↘

  5. Gate Oxide Thickness 1.2 nm • SiO2 gate oxide is only 1.2 nm thick – very ‘Nano’ • only 5 atomic layers thick

  6. Why High K oxides ? • SiO2 layers <1.6 nm have high leakage current due to direct tunnelling. Not insulating • Maintain C/areafor S-D current • Replace SiO2 withthicker layer of new oxide with higher K • Equivalent oxide thickness ‘EOT’

  7. Band Offsets are a key criterion Band offsets should be > 1 V Calculated with Charge Neutrality Levels by J. Robertson, JVST B 18 1785 (2000) Si

  8. Metal gates Doped poly-Si as gate electrode has significant depletion length, td ~ 3A 1/C = 1/Cd + 1/Cox + 1/Cqm Use metal gates for small td (0.5A) Need metals of suitable work function for NMOS and PMOS, or midgap metal (TiN)

  9. Metal gates: vacuum WFs Ec (4.05) Ta (4.19) Al (4.08) Mo (4.20) Sr (4.21) V (4.30) Ti (4.33) Sn (4.42) W (4.52) Cr (4.6) Ru (4.71) Rh (4.80) Co (4.97) Pb (4.98) Re (5.1) Ni (5.1) Ev (5.17) Ir (5.27) Pt (5.34) NMOS PMOS • • Low WF metals (Φm ~ 4 eV): Al, Ta, Mo, Zr, Hf, V, Ti - high chemical activity, low thermal stability • High WF metals (Φm ~ 5 eV): Co, Pd, Ni, Re, Ir, Ru, Pt- bad adhesion, enhanced diffusion of pure metals

  10. Metal gates: motivation for research Assumption that vacuum WFmet. = WFmet.eff. on gate dielectric isincorrect! (interface dipole → Fermi level “pinning”→ “effective” WF depends on particular Me/high-k system)

  11. Spin field-effect transistor 1) The ability to inject electron spins from the source ferromagnetic contact into the semiconductor channel (spin injection). 2) The transport of the electrons through the semiconductor channel from source to drain without losing the spin information. 3) The ability to transport the electron spins from the channel into the magnetic drain contact in a spin-selective way (spin-detection).

  12. Spin injection into a semiconductor (2) Al2O3 * B.-C. Min, K.I Motohashi, C. Lodder, R. Jansen “Tunable spin-tunnel contacts to silicon using low-work-function ferromagnets”, Nat. Mat. 5 817 (2006).

  13. Мотивации для создания памяти на альтернативных принципах Энергонезависимая память на основе эффектов резистивного переключения - время хранения информации (>10 лет) - быстродействие (~ 1 нс ), - энергопотребление ( < 1 пДж), - масштабирование ( <22 нм), - возможность 3-D интеграции. • основана на использовании состояния с высоким сопротивлением и состояния с низким сопротивлением для хранения информации в виде «0» и «1» с возможностью перехода между этими состояниями при подачиопределенного электрического сигнала.

  14. Resistive switching memory (ReRAM) (2) 3 main mechanisms of ReRAM memory: 1) Thermochemical 1) Thermochemical Utilizes the formation of conductive bridge by electrochemical metal deposition and dissolution from electrodes 2) Electrochemical 2) Electrochemical 3) Valence change The main problem for commercialization: detailed understanding of switching mechanism is missing Utilizes the formation of conductive bridge by formation and redistribution of oxygen vacancies in dielectric Utilizes the effect of thermal induced formation and destruction of conductive bridge (origin is under discussion)

  15. Memristor = memory + resistor • теоретически предложен L. Chua, IEEE Trans. Circuit Theory 18, 507 (1971) • экспериментально продемонстрирован командой HP: D. Strukov et al. Nature 453 80 (2008) ?… • K.F. Braun (1874): “In a large number of natural and synthetic sulfides, …, I have found that their resistance varied up to 30%, depending on direction, intensity, and durationof the current.”

  16. What ferroelectric tunnel junctions are all about? • polarization in thin film ferroelectrics (FE): spontaneous, stable, electrically switchable • polarization reversal: fast (~10 ps), dissipates low power (I~104 A/cm2) • excellent state variable for non-volatile data storage • ferroelectricity persists in ultrathin (~2 nm) epitaxial films!

  17. Ferroelectric tunnel memristors* *memrsitor = resistor with memory Z. Wen et al. Nat. Mater. doi:10.1038/nmat3649 (2013). • high-density multilevel non-volatile memories • synapse-like functions for neuromorphic computing architectures

  18. Metal-semiconductor interface F. Braun (1874): “In a large number of natural and synthetic sulfides and with very different samples, …, I have found that their resistance varied up to 30%, depending on direction, intensity, and duration of the current.” -> “rectification” “Semi-conductor” - (1911, Koenigsberger and Weiss) Galena (PbS) “cat’s whiskers” rectifiers -> detection of radio signal -> patent by Bose (1904) (receivers of electromagnetic radiation) Physical understanding: Schottky 1929 -> potential gradient across Cu/Cu2O -> differential capacitance [ C=εε0S/d(V) ] -> defects: n- and p-type semiconductors -> depletion layer -> Schottky 1938 “Semiconducting Theory of the Blocking Layer” -> Bethe 1942 “thermionic emission of electrons” -> “Bethe diode”

  19. Metal-semiconductor interface S = 0.08! • Energy band alignment of a metal-semiconductor interface according to the electron affinity rule: ΦB = φm‒ χs . • BUT does not take into account bonding at the interface!

  20. Concept of metal induced gap states (MIGS) Qm + Qs = Qm + Qsb + Qis = 0

  21. Concept of charge neutrality level (CNL) Y.-C. Yeo,T.-J. King, C. Hu,JAP92, 7266(2002) • gap states on the surface of sc or dielectric – intrinsic property • CNL → the sign changes from predominantly p- to n-type • WFeff≠WFvac →to be measured for particular metal/diel. pair

  22. Techniques: I-V characteristics (metal/semiconductor) Thermionic emission theory:

  23. Techniques: C-V characteristics (MOS) -3 -2 -1 0 1 2 3 Gate bias, V MOS stack Metal Oxide Semicon Vacuum level C-V Ec Ef CFB Ev

  24. Techniques: ballistic electron emission microscopy

  25. Techniques: internal photoemission hν

  26. Techniques: x-ray photoelectron spectroscopy (XPS)

  27. Techniques: x-ray photoelectron spectroscopy (XPS) Hf4f BE=hν – KE–WF Vacuum hν EF KE Δ interface dipole Ec Ec WFmet e- ΔBE=BEd0 – BEd1 Core level e- Ev Ev Metal Dielectric BEd0 BEmet 22.5 20 17.5 15 BEd1 Binding energy, eV • XPS: core level binding energy (BE) of dielectric is measured wrt. metal EF • interface electric dipole → shift of dielectric BE wrt. metal gate EF

  28. Probing of electronic properties at metal/dielectric interface by XPS continuous metal layer Thermal treatments in different environments: UHV, O2, FGA… XPS probe depth metal layer < 5 nm Potential distribution φ(x) x dielectric • continuous metal layer is too thick for XPS analysis metal NCs metal NCs • 2-5 nm metal layer is usually NOT continuous on dielectric (NCs) • modeling: φ=φ(x) induced by NCs is equivalent to continuous layer within ~0.05 V • opportunity: open dielectric surface makes the structure sensitive to environment 5-10 nm 5-10 nm 5-10 nm

  29. XPS results: Au/HfO2 (1) O1s Au4f Hf4f -0.65eV -0.55eV Annealing inO2 T=5000C 0.9eV 0.74eV UHV annealing T=5000C -0.25eV -0.29eV Exposed to air As grown (UHV preann. 3000C) 21 19 17 15 14.00 534 533 532 531 530 529 528 90 88 86 84 82 Binding energy, eV

  30. XPS results: Au/HfO2 (2) O1s HfO2 Au V++ V0 ∆BEHf4f = e- -0.5±0.1 0.6±0.1 -0.2±0.1 ∆BEO1s = EF -0.2±0.1 0.7±0.1 -0.5±0.1 BEHf4f BEHf4f Hf4f

  31. Preview of XPS results monitoring WFeff changes HfO2 Au V++ e- Hf4f • Model: • high concentration of VO in high-k, increases during annealing in reducing atm. • electrons from VO form electric dipole at Me/high-k interface driving WFeff toward Si mid-gap , particularly, for p-FET metal gates • the value of electric dipole corresponds to core level line shift wrt. metal EF EF BEHf4f BEHf4f

  32. Electronic properties of Fe/Gd/Al2O3/Si MOS stack Injection of spin-polarized electrons into Si Al2O3 Gd Fe Ferromagnetic metal

  33. Sample preparation, HAXPES analysis Gd 30 Å Fe Points of RBS & HAXPES analysis RBS @ E=1.5 MeV 11 Å • HAXPES @ DESY: BW2 (DORIS III) and P09 (PETRA III) stations • E = 4.5-6 keV • normal emission 6 Å 2 Å 1.4 1.5 1.6 1.7 1.8 1.9 Energy, MeV HAXPES 2011 Fe: ~ 75 Å Gd: 2 - 30 Å Al2O3: 100 Å Si

  34. HAXPES onFe/Gd/Al2O3/Si based MOS stack Fe3s Al3s Si2p Fe3soxide 2 Å Gd Si2poxide 30 Å Gd Binding energy, eV • the band alignment change at the Fe/Al2O3 interface is cleary visible in the Al 2s peak shift wrt. Fe 3s depending on the Gd thickness

  35. Effect of Gd marker onWeff of Fe in contact with Al2O3 • Gd IL thickness changing 0 - 3 nm in Fe/Gd/Al2O3/Si → • decreaseof Fe WFeff = 4.5 – 3.7 eV

  36. Ferroelectric tunnel junctions: electronic vs. electric (transport) properties A. Gruverman et al. Nano Lett. 9, 3539 (2009). Purpose of the work: • prepare functional ferroelectric tunnel junction • directly correlate its electronic structure with electron transport properties

  37. Materials of choice: MgO(001)/Pt/BaTiO3/Cr WFPt = 5.6 eV, dPt ≈ 0.1 nm WFCr = 4.5 eV, dCr  0.5 nm aBTO = 4.033 ÅaPt = 3.976 Å (aBTO – aPt)/aPt≈ 1.8% • Pt underlayer → compressed in-plane stress in BTO → tetragonal distortion normal to the surface → expected enhanced ferroelectricity • metal electrodes → better charge screening at the interfaces • Pt vs. Cr electrodes: different WFs and screening lengths

  38. MgO/Pt/BaTiO3: structural properties (XRR/XRD/HRTEM) 020Pt 020Pt BTO(5 nm)/Pt(12 nm)/MgO 020BTO 020MgO 220BTO 220MgO 220Pt 220Pt M. Minnekaev et al. MEE 109 227 (2013). AZ et al. TSF 520 4586 (2012). Pt MgO BTO • heteroepitaxial BTO/Pt growth on MgO(100) • strained BTO on Pt → tetragonal distortion normal to the surface → enhanced ferroelectricity? 5 nm

  39. MgO/Pt/BaTiO3: ferroelectric properties PFM images for a bare ~10 nm thick BTO film on Pt underlayer: a) amplitude, b) phase. Amplitude (c) and phase (d) hysteresis loops taken for 3 nm thick BTO layer through the top Cr/Au electrode. AZ et al. APL 102 062907 (2013). • Heteroepitaxial BaTiO3 (2-10 nm) films as grown on Ptunderlayer: • ferroelectric • single domain state • polarized downward

  40. BaTiO3 Pt Possible origin of BTO downward polarization in contact with Pt Band line-up at the Pt/BTO interface Vacuum P Vacuum - + - + - + WF Ec - - - φ - Eg e- CNL EF Ev Pt4f7/2 Ti2p3/2

  41. Hard X-ray photoelectron spectroscopy (HAXPES) @ DESY  HAXPES instrument • hν up to 12 KeV  probe depth up to 20 nm  investigation of electronic & chemical properties of realistic functional structures • P09 station @ DESY: in situ biasing  • studying of “devices under operation”

  42. Determination of band line-up at Pt/BaTiO3 interface by HAXPES Pt4f7/2 Pt(20 nm) EF MgO(100) 466 76 74 72 70 68 464 462 460 458 456 12 10 8 6 4 2 0 Binding energy, eV

  43. Determination of band line-up at Pt/BaTiO3 interface by HAXPES Pt4f7/2 Pt(20 nm) EF MgO(100) Ti2p3/2 VBM 466 76 74 72 70 68 464 462 460 458 456 BaTiO3(20 nm) 12 10 8 6 4 2 0 STO:Nd(100) Binding energy, eV

  44. Determination of band line-up at Pt/BaTiO3 interface by HAXPES Pt4f7/2 Pt(20 nm) EF MgO(100) Ti2p3/2 BaTiO3(~5 nm) VBM Pt(10 nm) MgO(100) Ti2p3/2 Pt4f7/2 466 76 74 72 70 68 464 462 460 458 456 BaTiO3(20 nm) 12 10 8 6 4 2 0 STO:Nd(100) Binding energy, eV

  45. Determination of band line-up at Cr/BaTiO3 interface by HAXPES Cr Cr2p3/2 EF MgO(100) Cr(15 nm) BaTiO3(10 nm) Ti2p3/2 Pt MgO(100) VBM Cr2p3/2 466 572 580 578 576 464 462 460 458 574 456 BaTiO3(20 nm) 12 10 8 6 4 2 0 STO:Nd(100) Binding energy, eV Ti2p3/2

  46. -15 -10 -5 0 Band gap of ultrathinBaTiO3 films (REELS) Reflection electron energy loss spectroscopy (REELS) REELS E0= 0.5 – 5 keV Eg Kinetic energy, eV M.Minnekaev et al. MEE 109 227 (2013).

  47. Band line-up in Pt/BaTiO3/Cr junction (BaTiO3 polarized toward Pt) M.Minnekaev et al. MEE 109 227 (2013).

  48. Transport measurements on Pt/BaTiO3/Cr tunnel junctions . AZ et al. APL 102 062907 (2013). Model: W.F. Brinkman, JAP 41 1915 (1970); A. Gruverman et al. NL 9(10) 3539 (2009) • TER effect (~10) upon FE polarization reversal (Uc = ±2.5 V, t=1 s) • the model of e- tunneling across a trapezoidal potential barrier fits I-V curves with experimentally determined barrier profile • the derived Δϕ upon BTO polarization reversal: ΔϕPt= +0.42 eV, ΔϕCr= ‒0.03 eV

  49. Useful elements present for microelectronic devices (1970’s)

  50. Useful elements for microelectronic devices (1990’s)

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