Fourier transform infrared spectroscopy (FTIR) for semiconductor material characterization Halvard Haug, 04.10.2010

Fourier transform infrared spectroscopy (FTIR) for semiconductor material characterization

Halvard Haug, 04.10.2010

Outline

• Introduction

• Theory

– Localized vibrational modes

– Infrared transmission spectroscopy

– The Michelson interferometer

• Advantages and limitations of FTIR

• The FTIR in MiNaLab

• Some examples of results

– Defects in irradiated Si

– H impurities in ZnO

– Bond concentrations in SiN x

films

• Summary


Suggested reading

• Schroder, D. K: Semiconductor material and device characterization, section 10.4 (page 585 – 592)


Molecular vibrations


May absorb IR radiation with a characteristic energy if the bond has a time-varying dipole moment

IR active vibrational modes

Not IR active

IR active


Vibrational modes in crystals

Phonons

Acoustical modes

(not IR active):

Optical modes

(IR active):


Vibrational modes in crystals

Point defects

• Destroy the translational symmetry of the lattice, altering the normal modes of vibration.

• If the frequency of the modified mode lies outside the phonon bands we get a localized vibrational mode

(LVM)

• LVMs absorb IR radiation at a characteristic frequency determined by the local atomic configuration


Infrared spectroscopy of solids

• Absorption mechanisms

– Optical phonon absorption

– Localized vibrational modes

– Free electron absorption

– Internal electronic transitions in (impurity) atoms

– (Band-to-band absorption ++)


Infrared spectroscopy

• Wave number:

Transmittance


Absorption coefficient


• Including reflection:



• Including reflection:



• Separate absorption from lattice/free electrons ( α

1

) and defects ( α

2

):



• known from defectfree reference or may be substracted

•  Calculate α

2

from T



Beers law:

The (integrated) absorption coefficient is proportional to the concentration of absorbing species:


The Michelson interferometer

Stationary mirror

Beamsplitter

Source

Movable mirror


Detector


Stationary mirror

Beamsplitter

Source

Movable mirror


Detector


Stationary mirror

Beamsplitter

Source

Movable mirror


Detector


Stationary mirror

Source


Beamsplitter

Optical path length difference

 Interference

Detector

Movable mirror

Single wavelength source

Constructive interference for

And destructive interference for

Intensity as a function of OPD:


Multiple wavelength source


Continuous source

Interferogram


Fourier transformation

Interferogram:

Spectrum:

FT


Interference from sample surfaces n d

Constructive interference for:

 Sinusoidal ”noise” over the

whole spectrum


Interference from sample surfaces d

Constructive interference for:

 Sinusoidal ”noise” over the

whole spectrum


Advantages of FTIR over dispersive spectrometers

• All resolution elements are measured all of the time

– Higher signal-to-noise-ratio (SNR) for a given measurement time and resolution

• Higher optical throughoutput

– Better SNR and sensitivity

• Short measurement time (~1-3 s) and possibility for signal averaging

– Quick measurement or high SNR

• High resolution by increasing δ max

– MiNaLab: Δ = 0.01 cm-1

• Internal wavenumber calibration system


Limitations of FTIR

• Must have a time-varying dipole moment

• Sample must be (partially) transparent to IR

 Samples must not be too conductive

(For Si: ~1 Ωcm)

• Need sufficient absorption length (For Si: ~mm)

• Need conversion factor for good quantitative analysis

• ”Single-beam technique”  Background and sample spectra measured at different times

• Detection limits typically 10 13 – 10 15 cm -3


The FTIR in MiNaLab

Name

Geometry

Spectral range

Bruker Optics - 125HR

Transmission

5 cm -1 to 50 000 cm -1

Max OPD

Resolution

50 cm

Resolves lines down to 0.001 cm -1

Sample temperature ~ 15 K or RT


The FTIR in MiNaLab


Case 1:

Investigation of oxygen-related complexes in irradiated Cz silicon

With thanks to Leonid Murin,

Joint Institute of Solid State and Semiconductor Physics,

Minsk, Belarus


0.5

T = 300 K (Bruker IFS 113v)

0.4

0.3

0.2

0.1

Background (without sample)

Raw spectrum, FZ-Si, d = 3 mm

Raw spectrum

Cz-3-J4, 24 GeV p-irr 1E16, d = 3 mm

0.0

400 600 800 1000 1200 1400

Wavenumber, cm

-1

1600 1800 2000


0.4

0.3

0.2

0.1

0.0

400

T = I / I

0.6


0.5

FZ-Si, d = 3 mm

600

CZ-Si, 24 GeV p-irr 1x10

16 cm

-2

, d = 3 mm

800 1000 1200 1400

Wavenumber, cm

-1

1600 1800 2000


3

4


A = αd = -ln(I/I

0

)

0

)

2

1

0

400 600


16 cm

-2

, d = 3 mm

FZ-Si, d = 3 mm

800 1000 1200 1400

Wavenumber, cm

-1

1600 1800 2000


- A ref

/I

0

))

1.2


1.0

0.8

0.6

0.4

0.2

0.0

400 600


16 cm

-2

, d = 3 mm

800 1000 1200 1400

Wavenumber, cm

-1

1600 1800 2000


α = A/d

4.0

3.5


3.0

2.5

2.0

1.5

1.0

0.5

0.0

400 600 800

N

O

= 1.06x10

18

cm

-3


16 cm

-2

, d = 3 mm

1000 1200 1400

Wavenumber, cm

-1

1600 1800 2000


α = A/d

0.40

0.38

0.36

0.34

0.32

0.30

0.28

0.26

0.24

0.22

0.20

800


N

VO

= 8.5x10

15

cm

-3

850

I

2

O


16 cm

-2

, d = 3 mm

1000 900 950

Wavenumber, cm

-1


1050

α – α

0.45

VO

0.40

I

2

O

0.35

0.30

IO

2

, I

2

O

2


16 cm

-2

, d = 3 mm

O

2i

IO

2

, I

2

O

2

O

2i

0.25

T = 300 K

0.20

800 850

O i

1100 900 950 1000

Wavenumber, cm

-1

1050


1150

0.8

0.6

0.4

0.2

0.0

-0.2

800

V

2

O

2

VO

850

C i

O i

VO

-

α, low temperature

1 - Cz1-I4 ([C

S

= 5x10

16

cm

2 - Cz3-J4 ([C

S

< 1x10

15

cm

-3

) RT irr 1x10

16

cm

-2

p

+

26 GeV

-3

) RT irr 1x10

16

cm

-2

p

+

26 GeV

3 - Cz3-J3 ([C

S

< 1x10

15

cm

-3

) DAC: RT irr 1x10

16

cm

-2

p

+

26 GeV - hot irr

IC i

O i

IC i

1

18

O i

I

2

O

2

IO

2

I

2

O

2

IO

2

I

2

O

2

3

O

2i

O

2i

VO

2

900 950

Wavenumber, cm

-1

1000 1050

T = 20 K

1100


Case 2:

Hydrogen-related defects in hydrothermally grown ZnO


Hydrogen in ZnO

• H is an important impurity in ZnO, present during crystal growth

• Interstitial (free) H acts as a donor, independent of EF

• May also passivate acceptors

 Contributing to inherent n-type conductivity

• H forms strong O-H bonds in the ZnO structure

 Localized vibrational modes

 Frequency in the range ~3200-3600 cm-1 (close to that of

the free O-H bond), characteristic of the atomic environment

 FTIR a useful tool to study OH-related complexes


Isotopic shifts

• O-H vibrational mode modeled as an harmonic oscillator

• Predicted frequency shift of when D (m = 2 amu) is substituted for H (m = 1 amu)

• Similar shifts observed in experiments  used for identification of

H-related absorption lines


As-grown spectra

H-? OH-Li

Zn

Ni

Zn


Spectrum after treatment in H

2

/D

2

gas

1.35  O-H

H-Y

H-Y

H-X

H-X


H-X, thermal stability

E a

= 2.8 eV t = 30 min


H-X, molecular model

• Defect complex with an impurity atom and two O-

H bonds

• Mg, Fe, Si ? c-akse


Case 3:

Determination of bond concentrations and

Si/N-ratio in a-SiN x

:H dielectric thin films

Figures from

Wright et. al., ”Plasma-enhanced chemical vapour-deposited silicon nitride films; The effect of annealing on optical properties and etch rates”, Solar

Energy Materials & Solar Cells (2008)



Halvard Haug, 04.10.2010 n(633 nm)

Summary

• Defects in a crystal material may give rise to localized vibrational modes (LVMs) that absorb IR radiation at a characteristic frequency

• FTIR is a relatively simple, non-destructive tool suitable for studying such LVMs and other material properties

• Both qualitative and quantitative (with conversion factors) investigations are possible

• Samples must be thick enough for sufficient absorption and transparent in the IR-range (not too conductive)


Fourier transform infrared spectroscopy (FTIR) for semiconductor material characterization Halvard Haug, 04.10.2010

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