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...
TRANSCRIPT
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
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
New Mexico State University
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 ?
2016 DPG Frühjahrstagung
Regensburg, 9. März 2016, HL 59.10
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
2016 DPG Frühjahrstagung
Regensburg, 9. März 2016, HL 59.10
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
2016 DPG Frühjahrstagung
Regensburg, 9. März 2016, HL 59.10
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
2016 DPG Frühjahrstagung
Regensburg, 9. März 2016, HL 59.10
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)
2016 DPG Frühjahrstagung
Regensburg, 9. März 2016, HL 59.10
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)
2016 DPG Frühjahrstagung
Regensburg, 9. März 2016, HL 59.10
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)
2016 DPG Frühjahrstagung
Regensburg, 9. März 2016, HL 59.10
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• 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)
2016 DPG Frühjahrstagung
Regensburg, 9. März 2016, HL 59.10
12
• Cleaned, then oxidized for
five hours at 550ºC.
• 7% 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
-5
0
5
10
15
20
25
30
Ge wafer with 136 nm thermal oxide (10 hours)
2016 DPG Frühjahrstagung
Regensburg, 9. März 2016, HL 59.10
13
• Cleaned, then oxidized for
ten hours at 550ºC.
• 1% non-uniformity.
• 4 nm spectral bandwidth.
• 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
-10
0
10
20
30
40
50
Preliminary optical constants for Ge and GeO2
2016 DPG Frühjahrstagung
Regensburg, 9. März 2016, HL 59.10
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
2016 DPG Frühjahrstagung
Regensburg, 9. März 2016, HL 59.10
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.
New Mexico State University
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