n-GaAs - Electronics Technology Network

THz wave probe for
semiconductor wafers
Terahertz Systems & Industrial Applications
25 th , January, 2008, The Royal Society, London
J. Darmo
Institute for Photonics, Vienna University of Technology, Austria

Talk outline
• Why THz waves based system
• Principles of operation
• Proof of principle
• Ellipsometry with THz waves
• Interferometry with THz wave (homogeneity mapping)

Why THz based system
Van der Pauw method
THz reflectometry
- sample cleaved from wafer
- good contacts necessary
- direct sampling of wafer

THz-TDS for semiconductor scanner
- conductivity
(doping, mobility)
- carriers’ lifetime
-thickness
(composition)
Remote measurements of carrier
concentration and mobility

Models of relevant structures
THz model of semiconductor
bulk n-type
bulk p-type
n+/n-type
p+/p-type
Drude response
2
p
Nq
2
p
2
i
2
1
m
p+/n-type
n+/p-type
p/p+/n-type
n/n+/p-type
Mobility model (n-GaAs)
n
p
n
2750
1
5450
n
0. 54
9.85e16
p 430
0. 45
1
1
p
6.00 17
e

phase shift (rad)
amplitude
attenuation (1/m)
THz model of semiconductor
1.00
0.95
0.90
0.85
0.80
0.75
0.70
0.65
0.60
0.55
0.50
0.45
0.35
0.30
0.25
0.20
0.15
0.10
0.05
0.00
rn_GaAs
5e15
5e16
5e17
5e18
rn_GaAs 1 2 3 4 5 6 7 8
5e16
5e16
frequency 5e17 (THz)
5e18
1 2 3 4 5 6 7 8
frequency (THz)
10
8
6
4
2
0
1.0
0.5
5e17
5e18
5e16
5e17
5e15
0.0
0 1 2 3 4 5 6 7 8
frequency (THz)
Summary:
Multicolour measurement:
un-ambiguous recognition (slow)
Two/three colour measurement:
distinguishable, fast
One colour measurement:
Could be confusing, needs
calibration

Talk outline
• Why THz waves based system
• Principles of operation
• Proof of principle
• Ellipsometry with THz waves
• Interferometry with THz wave (homogeneity mapping)

Targeted parameters
for semiconductorscanner (in 2005)
Semiconductor materials:
GaAs, Silicon, InP, GaN
Parameters to access:
conductivity (carrier density & mobility)
(in bulk, epitaxial layer, and 2DEG)
Range of parameters:
typically 10 16 -10 18 cm -3
(range modified to given application)
artistic rendering of scanner
Technicalities:
1. desktop format
2. >10 measurements per minute
3. user/maintenance friendly

Semiconductor scanner hardware
90cm
45cm
40cm
• compact box (90 cm x 40 cm footprint)
• 19“ instrument rack

eflection (arb. units)
reflection (arb. units)
N-GaAs: model & experiment
1.1
1.0
0.9
0.8
0.7
3.6um, 4.0um, n=3.80E14 n=5.00E14
4.0um, n=1.38E16 n=1.7E16
2.0um, n=1.00E17 n=1.50E17
2.0um, n=1.55E18 n=1.8E18
0.6
0.5
0.4
0.0 1.0 2.0 3.0 4.0 5.0
frequency [THz]

n-doping (THz data)
Comparison of THz
and standard method
1E19
1E18
N-GaAs
Thickness: 2-4 m
Substrate: undoped GaAs
1E17
1E16
1E15
min.
max.
1E14
1E14 1E15 1E16 1E17 1E18 1E19
n-doping (Hall data)
Usable range:
Doping fitting:
1e16 – 5e18 cm -3
Mobility fittting:
1e16 – 5e17 cm -3

mobility (cm 2 /Vs)
mobility (cm 2 /Vs)
GaAs & Silicon: model/experiment
8000
doping (cm -3 )
1E15 1E16 1E17 1E18 1E19
10000
GaAs
GaAs
deviation from model mobility
observed -> DC vs. HF conductivity
6000
4000
2000
1400
1200
1000
800
600
400
200
0
1E15 1E16 1E17 1E18 1E19
doping (cm -3 )
Silicon
Measurement:
N-doped GaAs:
( min : 1800 ->3000 cm 2 /Vs)
N-doped Silicon
( min : 92 ->320 cm 2 /Vs)

HEMT measurement
Heterostructure used:
n+ GaAs cap (5 nm)
Al 0.3 Ga 0.7 As (150nm)
undoped GaAs
undoped GaAs substrate

Probing of bulk and 2D carriers
original
THz
in-plane field component
– sheet conductance
normal field component
– optical transitions in QW
2DEG
d

carrier sheet density (THz data)
Summary of the HEMTs measurement
1E12
InGaAs/GaAs
InGaAs/InP A
Design of HEMTs:
• GaAs/AlGaAs (d-doping)
• GaAs/InGaAs system
• InP/InGaAs
(single and double doping)
1E11
AlGaAs/GaAs
InGaAs/InP B
Conclusions:
• structure model
improvement needed
• calibration works
1E11
1E12
carrier sheet density (Hall data)

Talk outline
• Why THz waves based system
• Principles of operation
• Proof of principle
• Ellipsometry with THz waves
• Interferometry with THz wave (homogeneity mapping)

Ellipsometry & THz waves
p
p
s
s
polarizer
/4 shifter /4 shifter
analyzer
THz ellipsometry:
• wide frequency range available (inherent spectroscopic feature)
• optical components to manipulate polarization of THz needed (!)
• direct evaluation of y and D from FT spectra of E p and E s (standard approach)

Ellipsometry & THz waves
• Electro-optic detector
‣ intrinsic polarization sensitivity
DS
~ cos sin 2
2sin
cos 2
P.C.M. Planken et al., J.Opt.Soc.Am. B313
• THz emitter
‣ electrically controlled (modulated) polarization
~
Phase: linear – circular – elliptical ‘polarization‘
~
Amplitude: ellipticity

Ellipsometry & THz waves
ellipsometric angle D
200
195
190
185
180
175
170
AlGaAs/GaAs
2.90
3.00
3.10
3.20
3.30
ellipsometric angle D
200
195
190
185
180
175
170
AlGaAs/GaAs
=
0.000
0.005
0.010
0.020
165
160
lambda
thickness
0.20 0.25 0.30 0.35 0.40
ellipsometric angle y
Heteroepitaxy case:
• high sensitivity to composition and doping
• high sensitivity to 2DEG presence
(anisotropy)
Homoepitaxy case:
• higher sensitivity for highly doped (strongly
absorbing) layers
ellipsometric angle D
165
160
185
180
175
0.20 0.25 0.30 0.35 0.40
plain GaAs
ellipsometric angle y
GaAs/GaAs
=
0.05
0.01
0.02
170
0.30 0.35 0.40
ellipsometric angle y

carrier density (10 18 /cm 3 )
Ellipsometry demonstration
3.0
2.5
2.0
1.5
standard THz-TDS
THz ellipsometry
- tests on epitaxial layers (n-GaAs)
- tests on heterostructure samples
(2DEG) pHEMT (InGaAs/InP)
AlGaAs/GaAs (MODFET)
1.0
0.5
0.5 1.0 1.5 2.0 2.5 3.0 3.5
Hall carrier density (10 18 /cm 3 )

Talk outline
• Why THz waves based system
• Principles of operation
• Proof of principle
• Ellipsometry with THz waves
• Interferometry with THz wave (homogeneity mapping)

eflectivity change (%)
reflectivity change in phase (rad)
THz model of semiconductor
0.6 1% modulation of doping
0.5
0.4
0.3
0.2
0.1
0.0
10 19
10 17 10 18 10 16
0 2 4 6 8
0.10
0.08
0.06
0.04
0.02
0.00
-0.02
-0.04
10% modulation of doping
10 18
10 17
10 16 10 19
0 2 4 6 8
frequency (THz)
frequency (THz)
Accessibility range
n-GaAs 1e16 – 5e19
p-GaAs 5e16 – 2e19
n-Si 2e16 – 1e19
p-Si 5e16 – 2e19
(layer’s thickness and structure dependent)

THz interferometer
Schematic drawing of setup

Parameters homogeneity map
4x10 11
3x10 11
AlGaAs/GaAs based 2DEG
map of 2DEG density
scanned area: 16x16 mm
step size: 0.5 mm
2x10 11
4x10 11
3x10 11
2x10 11
AlGaAs/GaAs based 2DEG
Map of 2DEG density after treatment
(sample subjected to surface oxidation)
Scanner area: 16x16 mm
Step size: 0.25 mm
1x10 11

Lehighton Electronics, Inc. (PA)
LEI 1600/1605 – contactless alternative to Hall measurement
( 10 GHz microwave technology)
LEI 1500/1510 – resistivity mapping (Eddy current measurement)
RS300/LEI 88 – sheet resistivity
LEI 1600 LEI 1500 RS 300
LEI 1605 LEI 1510 LEI88

Summary
• semiconductor scanner demonstrator is built and tested on
epitaxial layers
• potential of ellipsometry for THz wave probe evaluated
• spatial mapping of elecrical parameters with THz wave
probe is demontrated

Those who contributed
scanner team
D. Dietze, T. Prikoszowitch,
P. Kelly, and K. Unterrainer
Institute for Photonics, Vienna University of Technology, Austria
fs-oscillator with PhC fiber delivery team
T. Le and A. Stingl
Femtolasers Produktions GmbH, Austria