Электронные и электрические свойства границ раздела в функциональных структурах

для микроэлектроники

Электронные и электрические свойства границ раздела в функциональных структурах

для микроэлектроники

Андрей Зенкевичзав. лаб. «Новые функциональные материалы и структуры

для наноэлектроники»

МФТИ

•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

OutlineOutline

IntroductionIntroduction“…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 functionalization of the novel combination of materials in nm thick layers implies the comprehensive study of the interface properties

• 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

Complementary metal-oxide-semiconductor (CMOS) scalingMoore’s Law :

x2 more devices/wafer every 2 years

feature size decreases by x2 every 6 years

What for? speed↗ energy consumption↘ cost↘

Silicon subsrate

Source Drain

Gate

Gate oxide

channel

Gate Oxide Thickness

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

1.2 nm

Why High K oxides ?

• SiO2 layers <1.6 nm have high leakage current due to direct tunnelling. Not insulating

• Maintain C/area for S-D current

• Replace SiO2 with thicker layer of new oxide with higher K

• Equivalent oxide thickness ‘EOT’

t

KC 0

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)

0 V

1.1 V

V > 1 Ve

V > 1 Vh

OxideSi

CB

VB

Si

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)

C 1

C 2

C 3

Metal gates: vacuum WFs

Pb (4.98)

Al (4.08)Mo (4.20) Sr (4.21)

V (4.30)

Ru (4.71)

Ta (4.19)

Rh (4.80)Co (4.97)

W (4.52)Sn (4.42)

Cr (4.6)

Ti (4.33)

Pt (5.34)Ir (5.27)

Ec (4.05)

Ni (5.1)Ev (5.17)

Re (5.1)

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

Assumption that vacuum WFmet. = WFmet.eff. on gate dielectricis incorrect!

(interface dipole → Fermi level “pinning”→ “effective” WF depends on particular Me/high-k system)

Spin field-effect transistor

1) The ability to inject electron spins from the source ferromagnetic contactinto the semiconductor channel (spin injection).

2) The transport of the electrons through the semiconductor channel from sourceto drain without losing the spin information.

3) The ability to transport the electron spins from the channel into the magneticdrain contact in a spin-selective way (spin-detection).

* 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).

Al2O3

Spin injection into a semiconductor (2)

Мотивации для создания памяти на альтернативных принципах

- 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 подачи определенного электрического сигнала.

Энергонезависимая память на основе эффектов резистивного переключения

Энергонезависимая память на основе эффектов резистивного переключения

3 main mechanisms of ReRAM memory:

1) Thermochemical

2) Electrochemical

Utilizes the formation of conductive bridge by

electrochemical metal deposition

and dissolution from electrodes

1) Thermochemical

3) Valence change

2) Electrochemical 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)

The main problem for commercialization: detailed understanding of switching mechanism is missing

Resistive switching memory (ReRAM) (2)Resistive switching memory (ReRAM) (2)

Memristor = memory + resistor Memristor = memory + resistor

• 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 duration of the current.”

• теоретически предложен L. Chua, IEEE Trans. Circuit Theory 18, 507 (1971)• экспериментально продемонстрирован командой HP: D. Strukov et al. Nature 453 80 (2008)

?…

What ferroelectric tunnel junctions are all about?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!

• 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 memristors*Ferroelectric tunnel memristors*

• high-density multilevel non-volatile memories • high-density multilevel non-volatile memories

Z. Wen et al. Nat. Mater. doi:10.1038/nmat3649 (2013).

• synapse-like functions for neuromorphic computing architectures

• synapse-like functions for neuromorphic computing architectures

*memrsitor = resistor with memory*memrsitor = resistor with memory

Metal-semiconductor interfaceF. 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 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!

Concept of metal induced gap states (MIGS)

Qm + Qs = Qm + Qsb + Qis = 0

Concept of charge neutrality level (CNL)

• 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

Y.-C. Yeo,T.-J. King, C. Hu, JAP 92, 7266 (2002)

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

Thermionic emission theory:

-3 -2 -1 0 1 2 3

Gate bias, V

SemiconOxideMetal

MOS stack

Ef

Ec

Ev

Vacuum level

C-V

CFB

Techniques: C-V characteristics (MOS)Techniques: C-V characteristics (MOS)

Techniques: ballistic electron emission microscopy Techniques: ballistic electron emission microscopy

Techniques: internal photoemissionTechniques: internal photoemissionhν

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

Metal

• XPS: core level binding energy (BE) of dielectric is measured wrt. metal EF

Binding energy, eV

WFmet

BEmet

Dielectric

EF

Ec

Ev

BEd0ΔBE = BEd0 – BEd1

Δ interface dipole

Ec

Ev

BEd1

BE=hν – KE–WF

• interface electric dipole → shift of dielectric BE wrt. metal gate EF

e-

e-

KE

hν

Hf4f

22.5 20 17.5 15

Core level

Vacuum

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

Probing of electronic properties at metal/dielectric interface by XPS

• continuous metal layer is too thick for XPS analysis

dielectric

XPS probe depth continuous metal layer

metal layer < 5 nm

Thermal treatments in different environments:UHV, O2, FGA…

• 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

Potential distribution φ(x)

metalNCs

metalNCs

x

5-10 nm 5-10 nm 5-10 nm

XPS results: Au/HfO2 (1)

As grown(UHV preann. 3000C)

Exposed to air

UHV annealing T=5000C

Annealing in O2 T=5000C

Binding energy, eV

90 88 86 84 82

Au4fO1s Hf4f

534 533 532 531 530 529 528

-0.65 eV

0.9 eV

-0.25 eV

21 19 17 15 14.00

-0.55 eV

0.74 eV

-0.29 eV

XPS results: Au/HfO2 (2)

V++

HfO2Au

e-

Hf4fB

EH

f4f

BE

Hf4

f

EF

∆BEHf4f =

∆BEO1s =

-0.2±0.1

-0.2±0.1

0.6±0.1

0.7±0.1

-0.5±0.1

-0.5±0.1

O1s

V0

Preview of XPS results monitoring WFeff changes

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

V++

HfO2Au

e-

Hf4fB

EH

f4f

BE

Hf4

f

EF

Electronic properties of Fe/Gd/AlElectronic properties of Fe/Gd/Al22OO33/Si MOS stack /Si MOS stack Injection of spin-polarized electrons into Si

Gd

Al2O3

Fe

Ferromagnetic metal

21.04.23HAXPES 2011

Sample preparation, HAXPES analysisSample preparation, HAXPES analysis

RBS @ E=1.5 MeV

1.4 1.5 1.6 1.7 1.8 1.9Energy, MeV

Fe

Gd

2 Å

6 Å

11 Å

30 Å

Fe: ~ 75 Å

Al2O3: 100 Å

Si

Gd: 2 - 30 Å

Points of RBS & HAXPES analysis

• HAXPES @ DESY: BW2 (DORIS III) and P09 (PETRA III) stations

E = 4.5-6 keV

normal emission

HAXPES on Fe/Gd/Al2O3/Si based MOS stack

Binding energy, eV

Al3s

Fe3s

Fe3soxide

Si2poxide

Si2p

30 Å Gd

2 Å Gd

• 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 Gd marker on Weff of Fe in contact with Al2O3

• Gd IL thickness changing 0 - 3 nm in Fe/Gd/Al2O3/Si → decrease of Fe WFeff = 4.5 – 3.7 eV

• prepare functional ferroelectric tunnel junction

• directly correlate its electronic structure with electron transport properties

• prepare functional ferroelectric tunnel junction

• directly correlate its electronic structure with electron transport properties

A. Gruverman et al. Nano Lett. 9, 3539 (2009).

Ferroelectric tunnel junctions: electronic vs. electric (transport) properties

Ferroelectric tunnel junctions: electronic vs. electric (transport) properties

Purpose of the work: Purpose of the work:

Materials of choice: MgO(001)/Pt/BaTiO3/CrMaterials of choice: MgO(001)/Pt/BaTiO3/Cr

aBTO = 4.033 Å aPt = 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

WFPt = 5.6 eV, dPt ≈ 0.1 nm

WFCr = 4.5 eV, dCr 0.5 nm

MgO/Pt/BaTiO3: structural properties (XRR/XRD/HRTEM)MgO/Pt/BaTiO3: structural properties (XRR/XRD/HRTEM)

•heteroepitaxial BTO/Pt growth on MgO(100)•strained BTO on Pt → tetragonal distortion normal to the surface →

enhanced ferroelectricity?

MgO Pt

5 nm

020Pt

020MgO

220MgO

220Pt

020Pt

020BTO

220Pt

220BTO

BTO

AZ et al. TSF 520 4586 (2012).

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

BTO(5 nm)/Pt(12 nm)/MgO

Heteroepitaxial BaTiO3 (2-10 nm) films as grown on Pt underlayer:

•ferroelectric•single domain state •polarized downward

MgO/Pt/BaTiO3: ferroelectric propertiesMgO/Pt/BaTiO3: ferroelectric properties

AZ et al. APL 102 062907 (2013).

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.

Ev

Ti2p3/2

Ec

BaTiO3Vacuum

EF

WF

Vacuum

Pt4f7/2

Pt

e-

--

--

++

---

+

PCNL

g Ti2p BTO Pt4f F Pt Ti2p Pt4f Pt/BTOE (E VBM) (E E ) (E E )

φ Eg

Possible origin of BTO downward polarization in contact with PtPossible origin of BTO downward polarization in contact with PtBand line-up at the Pt/BTO interfaceBand line-up at the Pt/BTO interface

HAXPES instrument

Hard X-ray photoelectron spectroscopy (HAXPES) @ DESY

Hard X-ray photoelectron spectroscopy (HAXPES) @ DESY

• 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 Pt/BaTiO3 interface by HAXPES Determination of band line-up at Pt/BaTiO3 interface by HAXPES

04812 26107074 687276458462 456460464466

Binding energy, eV

EF

Pt4f7/2

MgO(100)Pt(20 nm) Pt4f F Pt(E E )

Pg Ti2p BTO Ti2p Pt4ft4f F P Pt/ Tt B OE (E VBM) (E) E )(E E

Determination of band line-up at Pt/BaTiO3 interface by HAXPES Determination of band line-up at Pt/BaTiO3 interface by HAXPES

04812 26107074 687276458462 456460464466

Binding energy, eV

EF

Pt4f7/2

MgO(100)Pt(20 nm) Pt4f F Pt(E E )

g Pt4f F Pt Ti2p Pt4f Pt/BTi2p BTO TO(E VE (E E ) (E E )BM)

Ti2p3/2

VBM

STO:Nd(100)

BaTiO3(20 nm) Ti2p BTO(E VBM)

Determination of band line-up at Pt/BaTiO3 interface by HAXPES Determination of band line-up at Pt/BaTiO3 interface by HAXPES

04812 26107074 687276458462 456460464466

Binding energy, eV

EF

Pt4f7/2

MgO(100)Pt(20 nm) Pt4f F Pt(E E )

g Ti2p BTO Pt4f F Pt Ti2p Pt4f Pt/BTOE (E VBM) (E E ) (E E )

Ti2p3/2

VBM

STO:Nd(100)

BaTiO3(20 nm) Ti2p BTO(E VBM)

Pt4f7/2Ti2p3/2

MgO(100)Pt(10 nm)

BaTiO3(~5 nm)Pt4f Ti2p Pt/BTO(E E )

EFCr2p3/2

Ti2p3/2VBM

Ti2p3/2

04812 2610458462 456460464466

Binding energy, eV

MgO(100)Pt

BaTiO3(10 nm)Cr(15 nm)

STO:Nd(100)

BaTiO3(20 nm)

MgO(100)Cr

Cr2p Ti2p Cr/BTO(E E )

Ti2p BTO(E VBM)

Cr2p F Cr(E E )

g Ti2p BTO Cr2p Ti2p Cr/BTO Cr2p F CrE (E VBM) (E E ) (E E )

574578 572576580

Cr2p3/2

Determination of band line-up at Cr/BaTiO3 interface by HAXPES Determination of band line-up at Cr/BaTiO3 interface by HAXPES

Band gap of ultrathin BaTiO3 films (REELS)Band gap of ultrathin BaTiO3 films (REELS)

Ti2p BTO Cr2p Ti2p Cr/BTO Cr2p Fg Cr(E VBM) (E E )E (E E ) M.Minnekaev et al. MEE 109 227 (2013).

-15 -10 -5 0

REELS E0= 0.5 – 5 keV

Kinetic energy, eV

Eg

Reflection electron energy loss spectroscopy (REELS)

Reflection electron energy loss spectroscopy (REELS)

Band line-up in Pt/BaTiO3/Cr junction (BaTiO3 polarized toward Pt)

Band line-up in Pt/BaTiO3/Cr junction (BaTiO3 polarized toward Pt)

g Ti2p BTO Cr2p Ti2p Cr/BTO Cr2p F CrE (E VBM) (E E ) (E E ) M.Minnekaev et al. MEE 109 227 (2013).

Transport measurements on Pt/BaTiO3/Cr tunnel junctionsTransport measurements on Pt/BaTiO3/Cr tunnel junctions

.

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

AZ et al. APL 102 062907 (2013).

1A 2A 3B 4B 5B 6B 7B 8B 1B 2B 3A 4A 5A 6A 7A 8A

H He

Li Be B C N O F Ne

Na Mg Al Si P S Cl Ar

K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr

Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe

Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn

Fr Ra Ce

Useful elements present for microelectronic devices (1970’s)

1A 2A 3B 4B 5B 6B 7B 8B 1B 2B 3A 4A 5A 6A 7A 8A

H He

Li Be B C N O F Ne

Na Mg Al Si P S Cl Ar

K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr

Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe

Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn

Fr Ra Ce

Useful elements for microelectronic devices (1990’s)

1A 2A 3B 4B 5B 6B 7B 8B 1B 2B 3A 4A 5A 6A 7A 8A

H He

Li Be B C N O F Ne

Na Mg Al Si P S Cl Ar

K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr

Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I Xe

Cs Ba La Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn

Fr Ra Ce

Useful elements for microelectronic devices (2000’s)

Выводы Выводы• 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