In-line and Compact QWIP
Reference Detector at 4.6µm
Ekua Bentil1, Germano Penello1,2 and Claire Gmachl1
1Princeton
University
2Federal University of Rio de Janeiro
2010 MIRTHE Quantum Cascade Laser Workshop – November 12
Motivation

New approach to obtain a reference
signal needed in gas spectroscopy



Beam splitter
or ZnSe
To study laser fluctuation
To obtain reference intensity in direct
absorption equation: Beer-Lambert’s Law
Current methods include:

Beam splitter or ZnSe window



Lack of control over Reflection/Transmission
Unnecessary split for a reference signal
Complex algorithms to fit reference signal


Introduction of error into data
Time consumming
Reference
detector
Signal
detector
Our solution
Minimally absorbing QWIP
 In-line reflection or transmission
setup

Desired parameters




Absorb small percentage of the original beam
 Reflect or transmit > 70%
Leave majority of the beam untouched or
improved if possible
Room temperature use
Easy to implement in a sensor system
Signal
detector
Reference
QWIP detector
Reference
QWIP
detector
Signal
detector
QWIP design
E
InAlAs
InAlAs
InGaAs
z
0.40
995
1000
1005
1010
1015
1020
Thickness (A)
0.25
5
0.20
0.6
0.5
0.4
0.15
0.3
0.2
0.1
0.10
QWS
10
20
30
10
E2-E1 = 0.34452 eV
0.0
990
995
1000
1005
1010
1015
Thickness (A)
40
50
QW width (A)
Energy levels in a QW
1020
15
20
4.5
0.25
5
5.5
6
0.20
0.15
15.0
17.5
20.0
22.5
25.0
27.5
6.5
7
7.5
8
30.0
Wavelength (m)
990
Energy (eV)
E2-E1 = 0.36012 eV
0.0
Energy (eV)
Energy (eV)
0.30
0.05
4
Erwinjr
QWS
0.2
Wavelength (m)
Energy (eV)
0.35
0.30
0.4
QW width (A)
Comparison between two codes
Comparing two independent codes in order to obtain the best QW thickness
QWIP Structure
Etching
z
Contact layer
50 x QWIPS
Contact layer
InP Substrate
Metalization
z
3D view
Bonding
Possible solutions


Reflect or transmit > 70%
Leave majority of the beam untouched or
improved if possible
Reflection
Several angles of incidence
Arbitrary polarity (must have TM!)
Less disturbance on original beam
Transmission
Fixed angle of incidence (Brewster)
Fixed polarity (TM)
Easy to implement
Reflection – Fresnel Equation
θB
Brewster angle
Transmission – Fresnel Equation
θB
Brewster angle
Difficulties
High reflectance only at high incidence angles.
Size of the detector
L
L0
Projected beam on the surface L/L0
Spot size
Typical detector size around 1002 to 10002 µm2 [1]
Problem in large area detectors:

Increase dark current;

Carrier recombination
[1] Quantum Well Infrared Photodetectors – Physics and Applications, Schneider, H.; Liu, H. – Springer 2007
Solution
Reflection in a mirror polished substrate






Beam size over the sample;
Size of each detector;
Metallization on the top of the sample;
TE and TM polarities instead of only TM, as it
would be in transmission ;
Different angles instead of a fixed angle, as it
would be in transmission;
One point of concern?
InP Substrate Properties
Absorption
in a substrate
Wavelength (m)
1210 8
6
4
0.6
Transmittance
0.5
0.4
0.3
0.2
non-doped InP
0.1
n = (1.3 ~ 1.4) e17 cm
-3
n = (1.1 ~ 1.5) e18 cm
-3
0.0
1000
1500
2000
2500
3000
-1
Wavenumber (cm )
Courtesy of Xue Huang
3500
Results
Wavelength
6
5
Wavelength (m)
4
3
0.32
Erwinjr
QWS
0.30
Energy (eV)
Absorbance
0.2
0.1
0.0
-0.1
4
0.28
Wavelength (m)
0.3
7
absorption on QWIPs
4.5
0.26
5
0.24
23
1500
2000
2500
3000
3500
4000
24
25
26
27
28
29
5.5
QW width (A)
-1
Wavenumber (cm )
Absorption peak at 5,2 µm. Growth uncertainty of 1 monolayer (~3 A) explains
this behavior
Results

Processed QWIPs for reflection
Samples size (µm):
a
b
a
b
180
285
243
385
306
485
369
585
Results

Processed QWIPs for transmission
200 µm
200 µm
200 µm
3 mm
3 mm
3 mm
3 mm
Results for R-QWIPs

Fresnel equations


Fixed angle of incidence (80º and 72º)
Varying polarization (TM to TE)
Polarizer
Sample
Polarizer
ZnSe lens
MCT
MCT
Sample
ZnSe lens
Results for R-QWIPs
Incidence angle = 80o
Incidence angle = 72o
1.0
1.0
0.9
0.7
0.6
0.5
0.4
0.3
0.7
0.6
0.5
0.4
0.3
0.2
0.2
0.1
0.1
0.0
0
TM
10
20
30
40
50
Fresnel equation
QWIPs
0.8
Reflectance
Reflectance
0.8

0.9
Fresnel equations
QWIPs
60
70
80
Polarizer angle (degrees)
90
TE
0.0
0
TM
10
20
30
40
50
60
70
80
Polarizer angle (degrees)
Experimental results for shallow incidence agree well with
theoretical simulation
90
TE
Future Work




Finishing setup for actual R-QWIPs
characterization
Fabrication and characterization of T-QWIPs
Design, fabrication, characterization at
differente wavelengths for both types of
QWIPS
Implementing in an actual gas sensor