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A Low Temperature Ohmic Contact Process for n-type Ge Substrates

K. Kakushima, R. Yoshihara, K. Tsutsui, H. Iwai

Tokyo Institute of Technology

IWJT 2013 6 May 2013

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High mobility channel candidates for n- and p-MOSFETs

I. Vurgaftman, et al., J. Appl. Phys., 89, pp. 5815 (2001).Table from S. Takagi, IEDM Short course (2011).

Channel selection

C. H. Lee, et al., IEDM p. 457 (2009).

InGaAs for n-MOSFETGe for p-MOSFET

High electron mobility with Ge channel(after Dit reduction process)

No doubt for p-MOSFET with Ge channelGe channel is also attractive for n-MOSFET

Ge n-MOSFET

3

Source/Drain junctions for Ge n-MOSFET

Large diffusion coefficient for P, As, Sb (>500oC)Incomplete activation of dopants

A. Axmann, et al., Appl. Phys., 12, pp. 173 (1977).S. Mirabella, et al., J. Appl. Phys., 113 031101 (2013).

Control of diffusion and activation of dopants are the keyfor source/drain for Ge sub.

Short period/high temperature annealing to exceed 1020 cm-3

C. O. Chui, et al., Appl. Phys. Lett., 83, 3275 (2003).

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R. Li, et al., EDL, 27, 476 (2006).

Ge MOSFET with Schottky junctions

Metal Schottky junctions have advantages for scaled MOSFETsMany reports of metal Schottky junction (PtGe2, NiGe, etc.) for Ge p-MOSFET

Advantages of metal Schottky S/D- atmoically abrupt junction- robust against short-channel effect- low parasitic resistance- low temperature process capability

J. M. Larson, TED, 53, 1048 (2006)

Lphy

Dop

antC

onc.

y position

δ δGate

σ σ

Lphy

Dop

antC

onc.

y position

δ δGate

σ σ Met

al C

onc.

y position

Gate

Lphy = Leff

Met

al C

onc.

y position

Gate

Lphy = Leff

Conventional doping Schottky junction

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Fermi level pinning at metal/Ge

Metal-induced gap states

Extrinsic defects (interface states)

Dangling bonds

Structural disorder at interfaceP. S. Y. Lim, Appl. Phys. Lett.,101, 172103 (2012).

Proposed mechanism of pinning

Metal pinnned near VB of Ge, which is favorable for p-MOSFETHowever, it gives a huge challenge for n-MOSFETA depinning process has to be developed

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Reported de-pinning process for Ge sub.

M. Kobayashi, et al., J. Appl. Phys., 105, 023702 (2009).

Insertion of thin SiN layer Formation of amourphous Ge layer

Sensitive to SiN thicknessTrade-off between parasitic resistance

Sensitive to process conditionsRestriction for thermal treatments

No intentional layer insertion to achieve a low φBn (Ohmic) contactStable process with wide process window (power, annealing, etc.)

M. Iyota, et al., Appl. Phys. Lett., 98, 192108 (2011).M. Mitsuhara, et al., JSAP Autumn meeting (2012).

De-pin process requirements for Schottky S/D

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Contents of this presentation

1. Introduction2. Stacked silicidation sputtering process3. φBn tuning with P atom incorporation4. Extraction of surface potential shift by XPS5. Conclusions

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Stacked silicidation sputtering process

n-Ge(100) sub.

Ni(5.5nm)

(a)

n-Ge(100) sub.

Si(1.9nm)/Ni(0.5nm)

8 set of Si/Ni layers

(b)

n-Ge substrate (4x1016 cm-3)

HF treatment

Diode patterning

Deposition by RF sputtering in Ar

Backside Al contact

Rapid Thermal Annealing (RTA) in N2

(a) Ni (5.5nm)

(b) Ni(0.5nm)+Si(1.9nm) x 8

(Control sample)

Control sample

NiSi2 film on Geusing stacked silicidation

10nm

Corresponds to atomic ratio of 1:2

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rms

Rou

ghne

ss (n

m)

Annealing temperature (oC)

asdepo. 200 300 400 500 600 700

0

2.0

4.0

6.0

StackedNiSi2

NiSi2 is formed over 350 oC, stable sheet resistances can be obtained up to 700 oC

050

100150200250300350400

0 100 200 300 400 500 600 700Annealing temperature (oC)

asdepo.

Ni (5.5nm)

StackedNiSi2

(10nm) agglomeration

wide processwindow

Shee

t res

ista

nce

(Ω/s

q.)

Sheet resistance by 4-point probe method Surface roughness (rms) by AFM

Smooth surface can be obtained up to 600 oC annealing

A thermally stable metal with wide process window (350oC~600oC) can be obtained with stacked silicidation sputtering process

on n-Ge

on n-Ge

Thermal stability of NiSi2 on Ge sub.

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Reaction of NiSi2 and Ge substrate

56kJ/mol2SiNiGeGeNiSi2 −+=+

22 OGeNiSi ++1760kJ/mol2SiONiGe 2 ++=

Reaction do not proceed

Under presence of oxygen atomsNiSi2 can be decomposed by Ge

-Formation of SiO2 is confirmed-Further stability can be expected by adopting oxygen atom/molecule blocking capped layer

Main composition is NiSi2 at 500oC annealing with thin NiGe (<1nm)

∆Hf data from Y.Q.Lu, J. Alloy. Comp., 491, p64 (2010)

851852853854855856857

Binding energy (eV)

Inte

nsity

(a.u

.)

NiGeNiSiNiSi2

500 oC800 oC

Ni 2p3/2hν=7940 eVTOA=80o

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Diode characteristics of NiSi2/n-Ge sub.

NiSi2 showed stable φBn with ideality factor within 1.2 up to 500 oC

asdepo. 400 500 600

Annealing temperature (oC)

Stacked NiSi21.0

2.0

3.0

Idea

lity

fact

or

Stacked NiSi2

Ni(5.5nm)0.50

0.55

0.60

0.45

φ Bn

(eV)

Ni(5.5nm)

NiSi2 by stacked silicidation process is useful to preserve ideal metal/Ge interface

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φBn tuning with P atom incorporation

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φBn tuning by P atom incorporationwith stacked silicidation process

n-Ge(100) sub.

Si(1.9nm)/Ni(0.5nm)

Ni3P(0.68nm)Si(1.9nm)

7 set of Si/Ni layers

Incorporation P atoms at metal/Ge interface

The first Ni layer was replaced with Ni3P layer

Corresponds to P atoms of 4.6x1015 /cm2

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Ge diode characteristics with P incorporation

Anode voltage (V)

Cur

rent

den

sity

(A/c

m2 )

0 0.5 1.0-0.5-1.0

30

2010

0

-10

-20-30

600oC500oC

400oCasdepo.

n-Ge (100)(Nd=4x1016cm-3)

Anode voltage (V)

Cur

rent

den

sity

(A/c

m2 )

0 0.5 1.0-0.5-1.0

102

101

100

10-1

10-2

10-2

600oC500oC

400oC

asdepo.

An Ohmic contact to n-type Ge substrate can be achievedat annealing temperature over 400 oC

Stable Ohmic characteristics can be obtain up to 600 oC

Increase in reverse current after annealingAfter annealing, Ohmic contact can be obtainedFurther increase in current with temperature

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Cross-sectional images of NiSi2:P/Ge

Surface agglomeration with interface reaction and SiO2

A thin bright contrast at interface

A 1-nm-thick layer with a bright contrast with P atoms might be the key to realize Ohmic contact for n-type Ge sub.

Flat surface and interface

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Extraction of surface potential shift by XPS

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Potential profile extraction of Ge sub.

[ ]dzzEEIeEJz

)(sin)( 00sin ψθθλ +−⋅⋅= ∫

∞−

I(E-E0) : a typical spectrum with a peak energy of E0Ψ(z) : potential profile in Si substrateθ: take off angleλ: Inelastic mean free pass (IMFP)

Energy

Spectrum is the convolution of potential

Peak shift

Energy

Ge 2p

VB

CBφBn

2

21)( ⎟

⎟⎠

⎞⎜⎜⎝

⎛⋅−≈ zqNz

sGe

bs ψε

ψψ

Nb: doping concentrationψs: surface potential 3/2

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Surface potential extraction by XPS

Spectrum shifts due to band bending in Ge substratesA shift of 0.3eV is the average binding energy of each photoelectron

Surface bends down with P incorporation

without incorporation

P incorporation

1220 1219 1217

Binding energy (eV)

1216

Inte

nsity

(a.u

.) Ge 2p3/2 spectrahν=7940 eVTOA=80o

500 oC 1min

0.3 eV

GeNiSi2 10nm

hν=7940eV

IMFP(λ)~10nm

TOA=80o

BE

ψs

Z

distance from surface Ge(3λ~36nm)0

ψs,P with P incorporation

NiSi2 only

Ge 2p3/2

(3λ~30nm)

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NiGeGe sub.Ge 2p3/2hν=7940 eVTOA=80o

500 oC 1min

NiGeGe sub.

1220 12151219 1218 1217 1216Binding energy (eV)

1220 12151219 1218 1217 1216Binding energy (eV)

interface

bulk

interface

bulk

Inte

nsity

(au)

Inte

nsity

(au)

deconvolutiondeconvolution

ψs = -0.43 eVψs,P = 0.14 eV

A potential shift of 0.57 eV can be extracted by deconvolution

BE

ψs

Z

distance from surface Ge(3λ~36nm)0

ψs,P with P incorporation

NiSi2 only

Ge 2p3/2

(3λ~30nm)

Deconvolution of Ge 2p3/2 spectra

w/o P with P

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Specific contact resistance with P incorporation

-1.0 -0.5 0.0 0.5 1.0

2.01.51.00.50.0

-0.5-1.0-1.5-2.0

Cur

rent

(mA)

Voltage (V)

60µm

30µm

60µm

30µm

as-sputtered

700oC

600oC

300oC

200 300 400 500 700

10-1

10-2

10-3

Con

tact

resi

stan

ce (Ω

cm2 )

Annealing temperature (oC)600

n-Ge (4x1016cm-3)

Less dependency on annealing temperatureStill, this process has strong advantage for meal Schottky S/D application

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0 1.0 2.0 3.0 4.0 5.01015

1017

1019

1021

1023

Diffusion depth (nm)

P co

ncen

tratio

n (c

m-3

)

NiSi2 Ge sub.

500 oC 1min

Nd = 4.0x1016 cm-3

⎟⎠

⎞⎜⎝

⎛=DtxNtxN s 2

erfc),(

Ns: solid solubility limitD : diffusion constant

D = 2.3x10-17

Ns = 3.0x1019 cm-3

Number of P atoms= 4.6x1015 atoms/cm2

P

x

Discussion: P atom diffusion into Ge?

Contribution of φBn tuning→Low temperature JV measurement in progress

Model from XPS measurementsP atom diffusion <1nm for 300oCCannot explain the contact resistance

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ConclusionsStacked Ni silcidation process for Ge sub.

- initial flatness of Ge substrate can be maintained- position of the interface is defined- stable surface and interface up to 600oC annealing

Effective Schottky barrier height (φBn) reduction - effective φBn reduction to Ohmic with P incorporation- stable properties up to 600oC annealing

Surface potential shift with P incorporation is confirmed- Shift of 0.57eV, near CB of Ge, has been extracted by XPS

Constant specific contact resistance against annealing - Contribution of φBn shift in addition to P atom diffusion

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Si(1.9nm)/Ni(0.5nm)

Ni3P(0.7nm)

B(0.3nm)

Si sub.

Si(1.9nm)/Ni(0.5nm)

Ni3P(0.7nm)

B(0.3nm)

Si sub.

1023

1022

1021

1020

1019

NiSi2 Si sub.

0 10 20Depth (nm)

B co

ncen

tratio

n (c

m-3)

before

B

after 500oC annealing

1022

1021

1020

1019

1018

NiSi2 Si sub.

0 10 20Depth (nm)

P co

ncen

tratio

n (c

m-3)

before

after 500oC annealing

P

B or P incorporation with stacked silicidation process

SIMS B and P profile

Feasibility study: NiSi2 on Si substrate with P and B atoms

Most of the B and P atoms remain at NiSi2/Si sub. Interface(some of them diffuse out toward surface)

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Depinned report with epi-NiGe2 formation

φBn tuning 0.60 → 0.37eV

Laser irradiation epitaxial NiGe2 growthNiGe NiGe2

NiGe2 is not a stable form for bulk nickel germanide

NiGe2 might be formed under high stress or non-equilibrium thermal treatmentRole of P atoms and thin bright layer are the keys for our Ohmic contact

P. S. Y. Lim, et al., Appl. Phys. Lett., 101, 172103 (2012).