Investigation of hBN by THz Time Domain Spectroscopy

Investigation of Hexagonal Boron Nitride by Terahertz Time-Domain Spectroscopy

Jon Leist Momentive Performance Materials, Inc.

Mira Naftaly, Richard DudleyNational Physical Laboratory

2Copyright 2010 Momentive Performance Materials Inc. All rights reserved. CONFIDENTIAL

Agenda

Why Terahertz?

Why Hexagonal Boron Nitride?

Terahertz measurements of Hexagonal Boron Nitride

Concluding Thoughts


The Terahertz Gap

10

GH

z

10

0 G

Hz

1 T

Hz

10

TH

z

10

0 T

Hz

10

00

TH

z

1 c

m

1 m

m

10

0

m

10

m

1

m

10

0 n

m

Microwave / mm-wave Sub-mm / Terahertz / Far IR mid IR NIR VIS UV

ELECTRONICS PHOTONICSTerahertz

Molecular rotation

Intramolecular vibration

Intermolecular vibration e- transitions

10

0

eV

0.8

cm

-

1 1 m

eV

8.0

cm

-

1 10

meV

80

cm

-1

10

0

meV

80

0 c

m-

1 1 e

V

80

00

cm

-1

10

eV

80

00

0 c

m-1


Spectral information is the power of the THz regime

Images from http://www.lightsources.org/images/posters/jlabposter4.jpg/

REFLECTED ABSORBED WEAKLY ABSORBED

SPECTRA

Metal

Conductors• Graphite• ITO

Water

Water-bearing tissue

Amorphous and disordered materials

• Glasses, ceramics, wood, paper, clothes

Polar solventsPolar polymers

• Polystyrene, polyvinyl, Perspex

Plastics (plastic explosives)

Dry biological materials

Non-polar oils

Non-polar polymers (PTFE, HDPE, TPX, cyclo-olefins)

III-V crystals & polycrystalline

• GaAs, GaP, BN, AlN

Polar plastics

Toxic Gases

Certain Gases• CO, HCL, HCN, N2O• Moist air

Crystalline form of some organic molecules

• Sugars, drugs, medicines, DNA, proteins


Hexagonal Boron Nitride (BN)Hexagonal Structure

Analogous to graphite/diamond:

Hexagonal crystal structure soft, lubricous

Cubic crystal structure hard, abrasive

Thermally conductive:

Single crystal, in-plane ~300 W/mK

Shape, 92% dense ~30-55 W/mK

Electrically insulating:

volume resistivity ~10-15 ohm-cm

Dielectric constant ~3.9

Loss tangent ~0.0004 at 1 MHz

Other Powder Properties:

PSD: fine 0.5-50um; coarse 100-400um

a

bc


BN Powders for a Variety of ApplicationsHeat Sink

TIM-2

Heat spreader

TIM-1

Die

Die attach

Thermal Core

Heat Sink

TIM-2

Heat spreader

TIM-1

Die

Die attach

Thermal Core

0

1

2

3

4

5

6

PPS neat 35wt% 40wt% 50wt% 60wt%

PT110 Loading

Ther

mal

Con

duct

ivity

(W/m

K)

Through Plane

In-plane (Lam)

Thermal Conductivity Increases 10X


Hot Pressed Boron Nitride

HBN HBC HBT

HBR

Binder

Porosity

Max use temperature (C)Thermal Conductivity (W/m-K)Volume Resistivity (-cm)

CTE (ppm/C)

Compressive Strength (103psi)

Boric oxide

2.10

59 / 331015

16 / 18

7%

4 / 6

None

1.95

28 / 231015

6 / 7.5

13%

0.4 / 0.8

None

1.75

22 / 19

1015

4.8 / 5.6

22%

0.1 / 0.3

Ca borate

2.00

55 / 331015

10 / 9

11%

3 / 4


Terahertz Time-Domain Spectroscopy

Ti-sapphire lasermode-locked

800 nm centre wavelength20 fs pulse length

77 MHz repetition rate

beamsplitter

probebeam

pump beam

delaystage

THz emitterbiased GaAs

balancedphotodiode

detector

Wollaston prism

ZnTe electro-optic

crystal

sample

THzbeam

parabolic mirror

parabolic mirror

parabolic mirror

parabolic mirror


Refractive indices and loss coefficients

O-ray orientation

E-ray orientation

o-ray (2)

e-ray

o-ray (2)

e-ray


Refractive indices and loss coefficients

o-ray (2)

e-ray

o-ray (2)

e-ray

o-ray (2)

e-ray

o-ray (2)

e-ray

o-ray (2)

e-ray

o-ray (2)

e-ray

o-ray (2)

e-ray

o-ray (2)

e-ray


Relationship between porosity and refractive index O-ray orientation

E-ray orientation

o-ray (2)

e-ray

no

ne


Relationship between porosity and loss coefficient O-ray orientation

E-ray orientation

o-ray (2)

e-ray


Size of scattering elements

o-ray (2)

e-ray

o-ray


Summary

THz time-domain spectroscopy revealed significant differences among four grades of hot pressed boron nitride (BN), in terms of their refractive indices and transmission loss.

The analysis confirmed that both porosity and binder tend to be segregated at c-plane edges of the BN platelets. An indication of the sizes of particles and pores was also obtained.

THz time-domain spectroscopy was demonstrated as a technique capable of providing insight into the structure of optically opaque ceramic materials.

Both HBN and HBC grades have potential use as optical materials at THz frequencies

Future work includes investigation of pyrolytic BN, BN coatings on Si, and BN incorporated into nonpolar polymers


Terahertz Facilities at NPL

http://www.npl.co.uk/electromagnetics/terahertz/products-and-services/thz-time-domain-spectroscopy

The National Physical Laboratory (United Kingdom) utilizes a powerful collection of equipment to perform Terahertz measurements on a routine basis. Please contact them if you are interested in studying your materials in this frequency range.


Questions?


Appendix


Known absorption bands in BN*

k

* Geick et. al., Physical Review 146 (2), 543-547 (1966)

1 T

Hz

10

TH

z

10

0 T

Hz

Far IR / Terahertz mid IR NIR

10

0 c

m-1

10

00

cm

-1

10

00

0

cm-1

10

cm

-1

Molecular rotation

Intramolecular vibration

Intermolecular vibration

500

1000

1500

2000

2500

Lo

ss c

oef

fici

ent

(cm

-1)


Alternative solutions for Terahertz optics

n (cm-1) at 1 THz

Elastic Modulus

(MPa)

CTE (ppm/C)

LDPE

HDPE

PTFE

PP

TPX

HRFZ-Silicon

Germanium

GaAs

Crystalline sapphire

Crystalline quartz

Fused SilicaHBN

HBR

HBC

HBT

Tlimit (C)

1.51

1.53

1.43

1.50

1.46

0.2

0.3

0.6

0.6

0.4

200-400

0.3

0.6

0.6

118-350

98

130

260

110

150

150-200

0.3

0.6

0.6

0.4

TC (W/m-K)

0.3

0.3

0.6

0.6

0.4

3.42

4.00

3.60

0.1

0.6

0.5

1.9 x 104

>104

8.6 x 104

850

560

450

2.55

6.1

5.8

159

60

46

3.41/3.07

2.15/2.11

1.95

1.0/1.0

0.1

2.0

3.6 x 105

9.7/7.7 x 104

7.3 x 104

1400

1100

1100

5

7.6/14

0.55

36

9.5/6.1

1.46

2.08/2.20

1.95/2.13

2.05/2.20

2.01/2.20

0.4

2.3

0.4

2.0

6.2 x 104

4.8 x 104

2.0 x 104

2.0 x 104

550*

850*

850*

850*

4/6

3/4

0.4/0.8

0.1/0.3

59/33

55/33

28/23

22/19

Resistivity (ohm-

cm)>1015

1017

1018

1017

>1016

2.4 x 105

52

104

1014

1018

1010

>1015

>1015

>1015

>1015

R

4.1%

4.4%

3.1%

4.0%

3.5%

30%

36%

32%

30% / 26%

13%

10%

12% / 14%

10% / 13%

12% / 14%

11% / 14%


Example of time-domain data & resultant spectra

)(

)(ln

2)(

1

2

21

E

E

dd

)(2

)()(1)(

21

12

dd

cn


Alignment of Terahertz beam


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Investigation of hBN by THz Time Domain Spectroscopy

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