My Career at University of Florida
Sheng S. Li, Professor
Department of Electrical & Computer Engineering
Research Highlights
(1968-2006)
Background
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Education:
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BS EE: National Cheng-Kung University, Taiwan, 1962
MS EE: Rice University, Houston, Texas, 1966
Ph.D.EE: Rice University, Houston, Texas, 1968
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Professional: (1968-2006 at UF)
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Assistant Professor, 1968-73, E.E.Dept.
Associate Professor,1973-78
Professor, 1978-2006
Electronic Engineer, National Bureau of Standards (NBS),
DC,1975-76
Visiting Professor, National Chiao-Tung University, Hsinchu
Taiwan,1995 (7 months)/2002 (1 month)
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Books and Monographs
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Semiconductor Physical Electronics (Plenum,1993) (S. Li)
Electrical Characterization of Silicon-on-Insulator
Materials and Devices (Kluwer Academic,1995)
(Li/Cristoloveanu)

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Intersubband Transitions in Quantum Wells: Physics and
Devices (Kluwer Academic, 1998) (LI/Su)
Semiconductor Physical Electronics (2nd edition,
Springer, 2006) (S. Li)
Research and Scholarly Achievements (I)
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Top 100 Research Achievement Award,
University of Florida, 1989
Top 100 Research Achievement Award,
University of Florida, 1990
Inaugural Professorial Excellent Program (PEP)
award, University of Florida, 1996
University of Florida Research Foundation's (UFRF)
Research Professorship Award, 2000-2003
Research and Scholarly Achievements (II)

Chair/co-chair of 2nd, 3rd, 5th, and 6th Int. Symposium on Long Wavelength
Infrared Detectors and Arrays (1993,95’, 96’, 98’,99’ ECS Meetings)
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Co-chair of Int. Workshop on Intersubband Transitions in Quantum WellsPhysics and Applications. (1997)
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Editor/Co-editor of the above Conference Proceedings
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Published 153 journal papers and 140 conference papers
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One monograph book on SOI materials and devices
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3 book chapters on QWIPs (96’,99’,03’)
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2 NBS Special Publications (77’,79’)
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Short courses: One in China two in Taiwan (90’,93’,02’)
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Supervised 35 Ph.D. and 45 M.S. students
Highlights of Research
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Studies of transport properties in semiconductor
materials (DARPA,NBS, NSF, AFOSR)
DLTS characterization of radiation induced defects in
GaAs solar cells and semiconductor materials (NASA)
Defect characterization in SOI materials and devices
(Rome AFB, Harris Semiconductors)
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Quantum Well Infrared Photodetectors (QWIPs) for
LWIR staring Focal Plane Array (FPA) applications.
(1989-2004) (DARPA, ONR, AFRL, ARL, ARO, BMDO, ADT)
CuInSe2 (CIS) thin film solar cells. (1991-2006) (NREL)
Quantum Well Infrared Photodetectors (QWIPs)
for Long Wavelength Infrared Imaging Arrays
Fundamentals and
Practical Applications
Applications of IR Detector Arrays
•Automotive
Industry
•Electronics
Industrial
(MWIR)&(LWIR)
•Weather
Forecasting
•Infrared target
detection
•Astronomy
Space
(MWIR,LWIR)&VLWIR)
Medical
(LWIR)
Military
Applications of VLWIR
(> 14 micron) Detectors
•Deep Space
Astronomy
•Early detection of
long range missiles
•Atmospheric
pollution monitoring
QWIP Research Initiative

In 1990 DARPA issued a RFP called for the
development of GaAs/AlGaAs QWIP FPA for
LWIR imaging arrays applications

DARPA funded four research projects:
UF (Li), AT&T Bell Lab, Rockwell, and MartinMarietta) for 3 years to develop new QWIP
devices and QWIP FPAs
Intersubband Transition Quantum
Well Infrared Photodetector (QWIP)
(3-6 nm)
MBE grown QWIP
structure
(GaAs)
Energy Band Diagrams and Intersubband
Transitions in n- and p-type QWIPs
Ec
AlGaAs
GaAs
Energy Band Diagrams and Intersubband
Transitions in Multi-color QWIPs
Theoretical Considerations
• 1-D Time Independent Schrödinger Equation
 2 2



V
(
z
)

 ( z )  E ( z )
*
2
2
m

z


, z (growth direction of QW)
- Boundary Conditions : Continuities of  z  and  z  / m
'
*
• Transfer Matrix Method (TMM)
- Calculations of energy levels and
transmission coefficients
 A1  T11 T12   AN 
, where BN  0


 B   T

 1   21 T22   BN 
TX 
AN
A1
2

1
T11
2
A1
B1
AN
Intersubband Transition
• Absorption Coefficient:
Net number of transition per unit cell volume and time
incident energy flux
 ( )  
4 2e 2

cn
2dk
1
/2
2


M
f

f
BZ 2 2 fi f i  E f  Ei   2   / 22
2   f | pz |  i 
  f  *
m
E f  Ei
2
f (oscillator strength)
• Light Coupling: Normal incidence absorption is forbidden in an n- QWIP.
- 45o incidence for single detector and 1-D array detector
- Grating coupler: 1-D (lamellar), 2-D (cross), random grating coupler.
- C (corrugated)-QWIP
- E (Enhancement) -QWIP
b
GaAs Substrate
Calculated Peak Detection Wavelengths for an
n-type GaAs/AlxGa1-xAs QWIP with AlAs Mole Fraction x
GaAs/AlxGa1-xAs
Schematic diagram of the conduction band of a bound-to quasibound (BQB) transition QWIP under bias condition
–
continuum
en ergy
–
A GaAs
bound
state
cross
section
TEM
–
conduction
band
o
• 3
• 2
GaAs
1
“dark current”
mechanisms
position
–
photocurrent
Absorption of IR photons
can photo-excite electrons
from the ground state of
the quantum well into the
continuum states,
producing a photocurrent.
Three dark current
mechanisms are also
shown: ground state
tunneling (1); thermally
assisted tunneling (2); and
thermionic emission (3).
The inset shows a crosssection transmission
electron micrograph (TEM)
of a QWIP sample.
QWIP Performance (1)
• Dark currents (Id):
(1) Thermionic emission
(2) Thermally assisted tunneling
(3) Direct or trap-assisted tunneling
(1)
(2)
hn
• Dark current calculation:
(3)
(Thermionic emission):
Aemw*
Id  2
 L p

F
 F 

1  
 vs 
2
 f ( E )T ( E , F )dE
E0
Dark Current (A)
10
10
10
10
10
10
10
0
-2
-4
77K
-6
-8
-10
-12
0
1
2
3
4
Bias Voltage (V)
5
QWIP Performance (2)
• Spectral Responsivity (Ri)
Ri 
e

g 
g ,
hc
1.24
• Photoconductive Gain (g)
1  pc   L
g
T
Npc
• Quantum Efficiency ( )
   (1  RC )(1  e  ml )
 I ph 

 P 
 124%


• Detectivity (D* )
D*  Ad 1/ 2 f 1/ 2 / NEP
Detectivity vs. Cutoff Wavelength for N-Type QWIPs
25
The Energy Band Diagram of a Two-Stack
Two-Color MW/LW IR BC-QWIP
20 periods
MWIR QWIPs
20 periods
LWIR QWIPs
Ec
E1
E2
E1
E2=0.124 eV, p2=10 m
E1=0.4 eV, p1=3.1 m
p (m) = 1.24/E(eV)
Dark I-V and Spectral Response Curves for an
InGaAs/AlGaAs MWIR BC-QWIP
14
Dark I-V and Spectral Response Curves for a
GaAs/AlGaAs LWIR BC-QWIP
15
The Energy Band Diagram of an InGaAs/
AlGaAs /InGaAs Triple-coupled (TC-) QWIP
(a)
Conduction band diagram
of a TC-QWIP
(b) Transmission coefficient of
an InGaAs/AlGaAs/InGaAs TCQWIP
(a) Conduction band diagram and (b) transmission coefficient of a high- strain
(HS) InGaAs/AlGaAs/InGaAs LWIR triple-coupled (TC-) QWIP.
18
Dark I-V and Spectral Responsivity Curves for
the InGaAs/AlGaAs/InGaAs TC- QWIPs
5 periods
10 periods
19
Layer diagram of four-band QWIP device structure and the deep
groove 2-D periodic grating structure. Each pixel represent
a 640x128 pixel area of the four-band focal plane array
Four-color QWIP
Normalized Responsivity
Responsivity (arb.u)
1
0.8
0.6
0.4
0.2
0
3
5
7
9
11
Wavelength (micron)
13
15
QWIP Technology for IR FPA
1. Advantages:
 Highly uniform large format (640x480) GaAs/AlGaAs
QWIP Focal Plane Array (FPA) can be fabricated for
LWIR imaging array applications.
 High yield and reproducibility using GaAs QWIP Tech.
 Extremely low NET 1020 mK/K has been achieved
in GaAs E-QWIP.
2. Drawback:
 High dark current limits the operating temperature for
QWIP to around 80K for 9m detection peak.
An In0.6Ga0.4As/GaAs Quantum Dot
Infrared Photodetector (QDIP)
• A thicker spacer (600Å) of GaAs was
used instead of a larger band gap
material to block the dark currents.
• Using a thicker spacer layer one could
reduce the dark currents without
blocking the photocurrent in the
QDIP.
Vb
0.5m GaAs,
n=21018cm-3
600ÅGaAs (i)
10
In0.6Ga0.4As
QDs
1m GaAs, n=21018cm3
0.5m GaAs Buffer
Semi-insulating (100) GaAs substrate
Cross- Sectional TEM for an In0.6Ga0.4As/GaAs
QDIP with High Operating Temperature (250 K)
•Cross- sectional transmission electron microscopy (TEM) of the QDIP structure.
•QD density is 1.21010 cm-2
•Average size of the QDs is 26 nm in diameter and 6nm in height
• QDs grown by self-assemble mode using MBE techniques.
Spectral Responsivity for an
InGaAs/GaAs QDIP
120.0
160K
30
100.0
RESPONSIVITY (mA/W)
40
-0.7v 200K
-0.6v
•Spectral responsivity vs.
wavelength measured
at 100 K to 260 K.
20
10
80.0
180K
0.2v
0
6.5 7.5 8.5 9.5 10.511.5
60.0
40.0
200K
260K
220K
240K
20.0
0.0
6.5
7.5
8.5
9.5
10.5
WAVELENGTH (m)
11.5
•The bias voltages were
chosen to achieve the
maximum photocurrent
to dark current ratio.
QWIP Focal Plane Arrays (FPAs) using In- bump
Bonding to Silicon CMOS MUX
QWIP Focal Plane Array Using In-bump
bonding on Si CMOS MUX
IR radiation
QWIPs
Si CMOS MUX
readout
QWIP Array
In- bump
Twelve 640x512 QWIP focal plane arrays on a
3 inch GaAs wafer.
QWIP Phoenix Camera using
JPL QWIP FPA
One frame of video image taken with the 9 µm
cutoff 640x512 pixel QWIP Phoenix camera.
Picture of the 640x512 hand-held long wavelength
QWIP camera (QWIP Phoenix™).
Photo of a 640x486 LWIR QWIP Camera
Developed by JPL and Amber
(
(
a
)
Palm Size QWIP Camera by JPL
Image of Fire Taken with Dual Band
QWIP Camera
m
m
Photos Taken by a 640x486 LWIR QWIP
Camera (a) in a Parking Lot (b) Blades of a
Fast Turning Wheel.
(
(
a
)
Image of Stars taken with QWIP Camera
A cross-section of a single pixel of an EQWIP on the
left, and a cutout section of a fully hybridized
EQWIP FPA.
Enhanced QWIP
(Single Pixel)
n+ GaAs
contact
MQW Cavities
Pixel
Unit Cell
n+ GaAs
Spacer
Incident
Radiation
MQW
Grating
n+ GaAs
contact
Reflector
Metal
Epoxy
Silicon ROIC
Contact
Reflector
Indium Bump
The NET for 40 m pitch EQWIP arrays at 60
– 80 K with f/2 FOV and 300 K background.
C a lc u la te d N E D T (m K )
100
90
f/2 , 3 0 0 K b a c k g ro u n d
80
70
60
8 0 K , 1 .5 V
50
7 7 K , 1 .5 V
40
30
7 0 K , 1 .5 V
20
6 0 K , 1 .0 V
10
0
0
1
2
3
4
5
 in t (m s e c )
The NET for 40 m pitch EQWIP arrays at 60 – 80 K with f/2 FOV and
300 K background. The curves are extrapolated from measured data.
Images Taken by an EQWIP FPA
Camera
(a) Image from 256x256 , 40 m pitch EQWIP FPA
taken at 72 K, -1.5 V bias and f/2 optics.
(b) Image from 256x256 , 40 m pitch EQWIP FPA taken at
80.5 K, -2 V bias and f/2 optics.
An uncorrected image taken with a 640x480
format 25 m pitch EQWIP FPA at 60 K. The
peak wavelength is 8.6 m.
Recent Feedback from former students
1. Let me take this opportunity to thank you again for the positive
impact that you have had on my life. I know that I was not the easiest
student to deal with, but you were fair and flexible and helped me to
complete my education in a positive way. I am sure that there are
many students like me who you helped and who have made
significant contributions to our industry. It is amazing to think of the
change in the semiconductor industry over the last thirty years or so,
and what total impact your students have made. Thanks again for
letting me be a part of this industry- From Dr. Wade Krull (85’), Vice
President, SemEquip, Inc. N Billerica, MA (email 4/10/06).
2. Congratulation to your retirement with great honor. You are such a
great scholar and professor with warm heart. Even though I have
been working with many engineers from the prestigious schools such
as Cornell, MIT, Illinois, etc, the knowledge I learned from you is
definitely world class and is my backbone and strength.- From Dr. K.C
Hwang (90’), manager, Raytheon RF Component, Andover MA
(3/9/06 email).