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Электронные и электрические свойства границ раздела в функциональных структурах для микроэлектроники. Андрей Зенкевич зав. лаб. «Новые функциональные материалы и структуры для наноэлектроники» МФТИ. Outline. Introduction: the role of interfaces in nanoelectronic devices

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Электронные и электрические свойства границ раздела в функциональных структурах

для микроэлектроники

Андрей Зенкевич

зав. лаб. «Новые функциональные материалы и структуры

для наноэлектроники»

МФТИ


Outline свойства границ раздела в функциональных структурах

  • 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


Introduction свойства границ раздела в функциональных структурах

“…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


Complementary metal-oxide-semiconductor (CMOS) scaling свойства границ раздела в функциональных структурах

Moore’s Law :

x2 more devices/wafer every 2 years

feature size decreases by x2 every 6 years

What for? speed↗ energy consumption↘ cost↘


Gate Oxide Thickness свойства границ раздела в функциональных структурах

1.2 nm

  • SiO2 gate oxide is only 1.2 nm thick – very ‘Nano’

  • only 5 atomic layers thick


Why High K oxides ? свойства границ раздела в функциональных структурах

  • 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’


Band offsets are a key criterion
Band Offsets are a key criterion свойства границ раздела в функциональных структурах

Band offsets should be > 1 V

Calculated with Charge Neutrality Levels by J. Robertson, JVST B 18 1785 (2000)

Si


Metal gates
Metal gates свойства границ раздела в функциональных структурах

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)


Metal gates vacuum wfs
Metal gates: vacuum WFs свойства границ раздела в функциональных структурах

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


Metal gates: motivation for research свойства границ раздела в функциональных структурах

Assumption that vacuum WFmet. = WFmet.eff. on gate dielectric

isincorrect!

(interface dipole → Fermi level “pinning”→

“effective” WF depends on particular Me/high-k system)


Spin field-effect transistor свойства границ раздела в функциональных структурах

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).


Spin injection into a semiconductor (2) свойства границ раздела в функциональных структурах

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).


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.


Resistive switching memory ( альтернативных принципахReRAM) (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)


Memristor альтернативных принципах = 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.”


What ferroelectric tunnel junctions are all about? альтернативных принципах

  • 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!


Ferroelectric tunnel альтернативных принципахmemristors*

*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


Metal semiconductor interface
Metal-semiconductor interface альтернативных принципах

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”


Metal semiconductor interface1
Metal-semiconductor interface альтернативных принципах

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!


Concept of metal induced gap states migs
Concept of metal induced gap states (MIGS) альтернативных принципах

Qm + Qs = Qm + Qsb + Qis = 0


Concept of charge neutrality level cnl
Concept of charge neutrality level (CNL) альтернативных принципах

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


Techniques альтернативных принципах: I-V characteristics (metal/semiconductor)

Thermionic emission theory:


Techniques альтернативных принципах: 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


Techniques альтернативных принципах: ballistic electron emission microscopy


Techniques альтернативных принципах: internal photoemission


Techniques альтернативных принципах: x-ray photoelectron spectroscopy (XPS)


Techniques альтернативных принципах: x-ray photoelectron spectroscopy (XPS)

Hf4f

BE=hν – KE–WF

Vacuum

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


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


XPS results: Au/HfO interface by XPS2 (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


XPS results: Au/HfO interface by XPS2 (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


Preview of xps results monitoring wf eff changes
Preview of XPS results monitoring interface by XPSWFeff 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


Electronic properties of Fe/ interface by XPSGd/Al2O3/Si MOS stack

Injection of spin-polarized electrons into Si

Al2O3

Gd

Fe

Ferromagnetic metal


Sample preparation, HAXPES analysis interface by XPS

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


HAXPES on interface by XPSFe/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


Effect of interface by XPSGd 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


Ferroelectric tunnel junctions: interface by XPS

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


Materials of choice: interface by XPSMgO(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


MgO interface by XPS/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


MgO interface by XPS/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


BaTiO interface by XPS3

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


Hard X-ray photoelectron spectroscopy (HAXPES) @ DESY interface by XPS

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”


Determination of band line-up at interface by XPSPt/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


Determination of band line-up at interface by XPSPt/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


Determination of band line-up at interface by XPSPt/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


Determination of band line-up at Cr/BaTiO interface by XPS3 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


-15 interface by XPS

-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).


Band line-up in interface by XPSPt/BaTiO3/Cr junction

(BaTiO3 polarized toward Pt)

M.Minnekaev et al. MEE 109 227 (2013).


Transport measurements on interface by XPSPt/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





Выводы interface by XPS

  • масштабирование латеральных размеров устройств наноэлектроники → уменьшение толщин функциональных структур

  • новые концепции логических и запоминающих устройств подразумевают использование новых комбинаций материалов

  • границы раздела играют решающую роль при функционировании гетероструктур → необходимо детальное исследование

  • всестороннее моделирование границ раздела металл/диэлектрик не дает полного объяснения их электронных и электрических свойств → необходимы экспериментальные исследования

  • несколько приведенных примеров демонстрируют экспериментальные подходы к исследованию электронных свойств границ раздела в многослойных структурах

  • взаимное расположение электронных зон коррелирует с электрическими (транспортными) свойствами функциональных структур для созданий устройств энергонезависимой памяти


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