Optical Constants of Ge and GeO from Ellipsometry...GeO2 0.84 1.14 1.67 0.4429 GeO 2 1.03 1.14 30.8 0.4734 GeO 1.24 0.79 0.9723 Ge 1.36 1.36 Substrate 0.6906 Ge GeO SrTiOGeO 23 GeO

Optical Constants of Ge and GeO2

from EllipsometryT. Nathan Nunley, Nalin Fernando, Jaime Moya, Nuwanjula S. Samarasingha,

Cayla M. Nelson, Stefan Zollner

Department of Physics, New Mexico State University, Las Cruces, NM, USA

2016 DPG Frühjahrstagung

Regensburg, 9. März 2016, HL 59.10

NIR/VIS/QUV ellipsometry:

190 to 2500 nm, 77 to 800 K

Wo ist Las Cruces ?

http://ellipsometry.nmsu.edu

NSF: DMR-1505172

AFOSR: FA9550-13-1-0022

http://www.bbc.co.uk/northernireland/yourplaceandmine/images/helpabout/keyboard_250x145.jpg

http://www.bbc.co.uk/northernireland/yourplaceandmine/images/helpabout/keyboard_250x145.jpg

New Mexico State University

Flat & uniform films, at least 5 by 5 mm2,

low surface roughness, films on single-side polished substrate

Email: [email protected] http://ellipsometry.nmsu.edu

Graduate Students:

Lina Abdallah, Travis Willett-Gies, Nalin Fernando, Tarek Tawalbeh (Theory), Nuwanjula

Samarasingha, Nathan Nunley

Undergraduate Students:

Cesar Rodriguez, Khadijih Mitchell, Cayla Nelson, Jaime Moya, Jackie Cooke

2

mailto:[email protected]


BiographyRegensburg

Germany

Las Cruces, NM

Since 20103

Motorola (Mesa, Tempe)

Arizona, 1997-2005

Motorola, Freescale

Texas, 2005-2007Freescale, IBM

New York, 91-92;07-10

3

SiGe:C Metrology: How thick is my film?

4

0.00

0.05

0.10

0.15

0.20

0.25

0 1 2 3 4 5

Depth (a.u.)

Ge

ato

mic

fra

ctio

n

Si cap

(emitter)

SiGe:C base

Si substrate

theta (seconds)

-2500 -2000 -1500 -1000 -500 0 500

Lo

g In

ten

sity (

a.u

.)

2

3

simulation

data

Si1-x

Gex alloys

Energy (eV)0 1 2 3 4 5 6 7

2

0

10

20

30

40

50

Si9.06%14.93%21.10%26.86%

Si1-x

Gex alloys

Energy (eV)0 1 2 3 4 5 6 7

1

-20

-10

0

10

20

30

40

50

Six=9.06%x=14.93%21.10%26.86%

High-resolution XRD

Spectroscopic

Ellipsometry

SZ, Hildreth, Liu, Zaumseil, Weidner, Tillack, J. Appl. Phys. 88, 4102 (2000)

Si1-xGex alloys

Si1-xGex 100 thickness measurements

Need precise values of refractive index

Why Germanium ?



5

• First transistor built with Ge.

• High frequency applications (bipolar).

• Excellent infrared photodetector.

• Recent interest: PMOS channel material.

• Training students in semiconductor physics

• Why not?

SciFi CMOS cartoon:

Multi-Sample Analysis



6

• Single sample: Ellipsometry of one GeO2/Ge sample

• Unknown Ge optical constants

• Unknown GeO2 (native oxide) optical constants and thickness

• This problem is under-determined (not enough data).

• Multi-sample analysis:

• Grow thermal oxides on Ge with different thicknesses.

• All oxides identical; only thickness varies between samples.

• Fit all data simultaneously (over-determined).

• Ellipsometry measurements :

• J.A. Woollam VASE ellipsometer with Berek compensator.

• 0.5 to 6.6 eV (with halogen lamp).

• 60º-75º angle of incidence.

• Fit with parametric oscillator model.

QTH lamp

Substrate Cleaning and Thermal Oxidation



7

• Substrate cleaning:• Remove most of the native oxide.

• Leave stable (but thin) native oxide.

• No harsh chemicals (BHF, Br:Meth).

• UV ozone clean at 150ºC for 1 hour,

followed by cool-down incubation.

• Ultrasonic clean in DI water followed

by isopropanol (20 min each).

• Thermal oxidation:• 20 by 20 mm2 undoped Ge pieces.

• Single-side polished.

• Anneal in O2 (2.7 atm) at 550ºC

to avoid oxide (GeO) desorption.

• 1-10 hours; 35 to 130 nm thick.

• Some spots, but OK.

• GeO2 unstable and water-soluble,

measure soon.

Hg lamp

Novascan PSD

ULVAC MILA-5000 RTA

GeO2 on Ge

Deal-Grove Oxidation Model



8B.E. Deal and A.S. Grove, J. Appl. Phys. 36, 3770 (1965)

tBAdd 2

X-ray reflectance of typical sample (550ºC, 1 h, 33 nm)



9

(Å)

GeO2 Ge

LayerElectron Density

(eÅ-3)

Bulk Electron

Density (eÅ-3)

Thickness

(nm)

Roughness

(nm)

GeO2 0.84 1.14 1.67 0.4429

GeO2 1.03 1.14 30.8 0.4734

GeO 1.24 0.79 0.9723

Ge 1.36 1.36 Substrate 0.6906

Ge

GeO

SrTiO3GeO2

GeO2 density r: 90% of bulk

Lower r near surface

Higher r near substrate

Thickness: 33 nm

Roughness: 0.5 nm (2%, neglect)

GeO2

Ge

Ge wafer with native oxide (2.3 nm)



10

• Measurement right after cleaning, excellent fit to data.

• Herzinger-Johs parametric oscillator model for Ge.

Jellison-Sales method for transparent glasses:

D at 75º below band gap determines oxide thickness (2.3 nm)

Photon Energy (eV)

0 1 2 3 4 5 6 7

<

1>

<

2 >

-20

-10

0

10

20

30

0

5

10

15

20

25

Ge wafer with 34 nm thermal oxide (one hour)



11

• Cleaned, then oxidized for one

hour at 550ºC, 33 nm by XRR.

• 5% non-uniformity.

• Poor fit in deep UV (above 6 eV)

Photon Energy (eV)

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0

in d

egre

es

D in

degre

es

0

20

40

60

80

100

-100

0

100

200

300

Photon Energy (eV)

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0

%D

epola

rization

-6

-3

0

3

6

9

12

Ge wafer with 89 nm thermal oxide (5 hours)



12

• Cleaned, then oxidized for

five hours at 550ºC.



Photon Energy (eV)

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0

in d

egre

es

D in

degre

es

0

20

40

60

80

100

-100

0

100

200

300

Photon Energy (eV)

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0

%D

epola

rization

-5

0

5

10

15

20

25

30

Ge wafer with 136 nm thermal oxide (10 hours)



13

• Cleaned, then oxidized for

ten hours at 550ºC.


• 4 nm spectral bandwidth.


Photon Energy (eV)

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0

in d

egre

es

D in

degre

es

0

20

40

60

80

100

-100

0

100

200

300

Photon Energy (eV)

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0

%D

epola

rization

-10

0

10

20

30

40

50

Preliminary optical constants for Ge and GeO2



14

• Determined using multi-sample analysis (2, 34, 52, 89, 136 nm)

Similar to Jellison/UNL data

Higher amplitude than Aspnes

Broader spectral range

(0.5 to 6.6 eV)

Bigger differences in UV.

NMSU NMSU

UNC-CH

Much broader spectral range.

Absorption begins at 6.5 eV.

Tauc-Lorentz oscillator fit.

Remaining issues



15

• Depolarization of reflected light

• Thickness non-uniformity

(20 mm sample size)

• Insufficient monochromator resolution

(4 nm)

• Does the density vary between samples?

• Need to improve XRR fits for some

samples.

• Is there a density gradient in the oxide?

• Do we need to consider surface roughness

(no AFM yet)?

• Is there an interfacial layer at the GeO2/Ge

interface?

• Is a simple Tauc-Lorentz oscillator sufficient

for GeO2?

• Need FTIR-SE to study phonons.


Summary

• Developed UV-ozone clean for thermal oxidation of Ge.

• Performed thermal oxidation of Ge at 550ºC for 1 to 10 hours.

• Multi-sample ellipsometry fit of ellipsometric angles.

• Dielectric function of Ge and GeO2 from 0.5 to 6.5 eV.

Optical constants for Ge and GeO2

Ge GeO2

16

http://ellipsometry.nmsu.eduNSF: DMR-1505172

AFOSR: FA9550-13-1-0022

Optical Constants of Ge and GeO from Ellipsometry...GeO2 0.84 1.14 1.67 0.4429 GeO 2 1.03 1.14 30.8 0.4734 GeO 1.24 0.79 0.9723 Ge 1.36 1.36 Substrate 0.6906 Ge GeO SrTiOGeO 23 GeO

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