My Career at University of FloridaMy Career at University of Florida

Sheng S. Li, ProfessorSheng S. Li, ProfessorDepartment of Electrical & Computer EngineeringDepartment of Electrical & Computer Engineering

Research HighlightsResearch Highlights

(1968-2006)(1968-2006)

Background

Education:Education: BS EE: National Cheng-Kung University, Taiwan, 1962BS EE: National Cheng-Kung University, Taiwan, 1962 MS EE: Rice University, Houston, Texas, 1966 MS EE: Rice University, Houston, Texas, 1966 Ph.D.EE: Rice University, Houston, Texas, 1968Ph.D.EE: Rice University, Houston, Texas, 1968 Professional: (1968-2006 at UF)Professional: (1968-2006 at UF) Assistant Professor, 1968-73, E.E.Dept. Assistant Professor, 1968-73, E.E.Dept. Associate Professor,1973-78 Associate Professor,1973-78 Professor, 1978-2006Professor, 1978-2006 Electronic Engineer, National Bureau of Standards (Electronic Engineer, National Bureau of Standards (NBSNBS), ),

DC,1975-76DC,1975-76 Visiting Professor, National Chiao-Tung University, Hsinchu Visiting Professor, National Chiao-Tung University, Hsinchu

Taiwan,1995 (7 months)/2002 (1 month) Taiwan,1995 (7 months)/2002 (1 month)

Books and MonographsBooks and Monographs

Semiconductor Physical Electronics (Plenum,1993) Semiconductor Physical Electronics (Plenum,1993) (S. Li)(S. Li)

Electrical Characterization of Silicon-on-Insulator Electrical Characterization of Silicon-on-Insulator Materials and Devices (Kluwer Academic,1995) Materials and Devices (Kluwer Academic,1995) (Li/Cristoloveanu)(Li/Cristoloveanu)

Intersubband Transitions in Quantum Wells: Physics and Intersubband Transitions in Quantum Wells: Physics and Devices (Kluwer Academic, 1998) Devices (Kluwer Academic, 1998) (LI/Su)(LI/Su)

Semiconductor Physical Electronics (2Semiconductor Physical Electronics (2ndnd edition, edition, Springer, 2006) Springer, 2006) (S. Li)(S. Li)

Research and Scholarly Achievements (I)Research and Scholarly Achievements (I)

Top 100Top 100 Research Achievement Award, Research Achievement Award,

University of Florida, 1989 University of Florida, 1989 Top 100Top 100 Research Achievement Award, Research Achievement Award,

University of Florida, 1990 University of Florida, 1990 Inaugural Professorial Excellent ProgramInaugural Professorial Excellent Program ( (PEPPEP))

award, University of Florida, 1996 award, University of Florida, 1996 University of Florida Research Foundation'sUniversity of Florida Research Foundation's (UFRF)(UFRF)

Research Professorship Award,Research Professorship Award, 2000-2003 2000-2003

Research and Scholarly Achievements (II)Research and Scholarly Achievements (II)

Chair/co-chair of Chair/co-chair of 22ndnd, 3, 3rdrd, 5, 5thth, and 6, and 6thth Int. Symposium on Long Wavelength Int. Symposium on Long Wavelength

Infrared Detectors and ArraysInfrared Detectors and Arrays (1993,95’, 96’, 98’,99’ ECS Meetings) (1993,95’, 96’, 98’,99’ ECS Meetings)

Co-chair of Co-chair of Int. Workshop on Intersubband Transitions in Quantum Wells-Int. Workshop on Intersubband Transitions in Quantum Wells-

Physics and ApplicationsPhysics and Applications. (1997). (1997)

Editor/Co-editor of the above Conference Proceedings Editor/Co-editor of the above Conference Proceedings

Published Published 153153 journal papers and journal papers and 140140 conference papers conference papers

OneOne monograph book on SOI materials and devicesmonograph book on SOI materials and devices

3 3 book chapters on QWIPs (96’,99’,03’)book chapters on QWIPs (96’,99’,03’)

22 NBS Special Publications (77’,79’) NBS Special Publications (77’,79’)

Short courses: One in China two in Taiwan (90’,93’,02’)Short courses: One in China two in Taiwan (90’,93’,02’)

Supervised Supervised 3535 Ph.D. and Ph.D. and 4545 M.S. students M.S. students

Highlights of ResearchHighlights of Research

Studies of transport properties in semiconductor Studies of transport properties in semiconductor materials materials (DARPA,NBS, NSF, AFOSR)(DARPA,NBS, NSF, AFOSR)

DLTS characterization of radiation induced defects in DLTS characterization of radiation induced defects in GaAs solar cells and semiconductor materials GaAs solar cells and semiconductor materials (NASA)(NASA)

Defect characterization in SOI materials and devicesDefect characterization in SOI materials and devices(Rome AFB, Harris Semiconductors) (Rome AFB, Harris Semiconductors)

QQuantumuantum WWellell IInfrarednfrared PPhotodetectorhotodetectorss ( (QWIPsQWIPs) ) for for LWIR staring LWIR staring FFocal ocal PPlane lane AArray rray (FPA)(FPA) applications. applications. (1989-2004) (1989-2004) (DARPA, ONR, AFRL, ARL, ARO, BMDO, ADT)(DARPA, ONR, AFRL, ARL, ARO, BMDO, ADT)

CuInSeCuInSe22 ( (CISCIS) thin film solar cells. (1991-2006) ) thin film solar cells. (1991-2006) (NREL)(NREL)

Quantum Well Infrared Photodetectors (Quantum Well Infrared Photodetectors (QWIPsQWIPs) ) for Long Wavelength Infrared Imaging Arraysfor Long Wavelength Infrared Imaging Arrays

Fundamentals and Fundamentals and

Practical ApplicationsPractical Applications

Applications of IR Detector ArraysApplications of IR Detector Arrays

Industrial

•Electronics

Medical

•Astronomy•Infrared target detection

Space Military

•Automotive Industry

•Weather Forecasting

(MWIR,LWIR)&VLWIR) (LWIR)(MWIR)&(LWIR)

Applications of VLWIR Applications of VLWIR (> 14 micron) Detectors(> 14 micron) Detectors

•Deep Space Astronomy

•Early detection of long range missiles

•Atmospheric pollution monitoring

QWIP Research InitiativeQWIP Research Initiative

In 1990 DARPA issued a RFP called for the In 1990 DARPA issued a RFP called for the development of GaAs/AlGaAs QWIP FPA for development of GaAs/AlGaAs QWIP FPA for LWIR imaging arrays applications LWIR imaging arrays applications

DARPA funded DARPA funded fourfour research projects: research projects: UF (Li), AT&T Bell Lab, Rockwell, and Martin- UF (Li), AT&T Bell Lab, Rockwell, and Martin- Marietta) for 3 years to develop new QWIP Marietta) for 3 years to develop new QWIP devices and QWIP FPAs devices and QWIP FPAs

Intersubband Transition Quantum Well Infrared Photodetector (QWIP)

MBE grown QWIP structure

(3-6 nm)

(GaAs)

Energy Band Diagrams and IntersubbandTransitions in n- and p-type QWIPs

Ec

GaAs

AlGaAs

Energy Band Diagrams and IntersubbandTransitions in Multi-color QWIPs

Theoretical Considerations

• 1-D Time Independent Schrödinger Equation

)()()(

2 2

2

*

2

zEzzVzm

, z (growth direction of QW)

- Boundary Conditions : Continuities of z and *' /mz

• Transfer Matrix Method (TMM) - Calculations of energy levels and transmission coefficients

2

11

2

1

2221

1211

1

1

1

0 where,

TA

AT

BB

A

TT

TT

B

A

NX

NN

N

A1B1

AN

Intersubband Transition

• Absorption Coefficient:

fluxenergyincident

timeandvolumecellunitpertransitionofnumberNet )(

BZ if

iffiEE

ffMd

cn

e22

22

22

2/

2/1

2

24

k

• 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

if

izf

EE

p

mf

2

*

||2

f (oscillator strength)

bb

GaAs Substrate

Calculated Peak Detection Wavelengths for an Calculated Peak Detection Wavelengths for an n-type GaAs/Aln-type GaAs/AlxxGaGa1-x1-xAs QWIP with AlAs Mole Fraction As QWIP with AlAs Mole Fraction xx

GaAs/AlxGa1-xAs

Schematic diagram of the conduction band of a bound-to quasi-Schematic diagram of the conduction band of a bound-to quasi-

bound (BQB) transition QWIP under bias conditionbound (BQB) transition QWIP under bias condition

GaAs

AGaAs

continuum–

–

–

–

“dark current” mechanisms

•

photocurrent

bound state

conduction band

ener

gy

position

cross section TEM

1

2•

3

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 cross-section transmission electron micrograph (TEM) of a QWIP sample.

o

QWIP Performance (1)

(3)

(2)(1)

Photocurrent

• Dark currents (Id): (1) Thermionic emission (2) Thermally assisted tunneling (3) Direct or trap-assisted tunneling

• Dark current calculation: (Thermionic emission):

0

),()(

122

*

E

s

p

wd dEFETEf

vF

F

L

AemI

0 1 2 3 4 5

100

10-10

10-6

10-12

10-8

10-4

10-2

77K

Bias Voltage (V)

Dar

k C

urr

ent

(A)

h

QWIP Performance (2)

• Spectral Responsivity (Ri)

,24.1

gghc

eRi

)1)(1( lmC eR

%124

PI ph

• Quantum Efficiency ( )

• Photoconductive Gain (g)

c

c

Np

pg

1

T

L

• Detectivity (D* )

* 1/ 2 1/ 2 /dD A f 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

p (m) = 1.24/E(eV)

E1

E1

Ec

E1=0.4 eV, p1=3.1 m

E2

E2=0.124 eV, p2=10 m

20 periods MWIR QWIPs 20 periods

LWIR QWIPs

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 and (b) transmission coefficient of a high- strain (HS) InGaAs/AlGaAs/InGaAs LWIR triple-coupled (TC-) QWIP. 18

(a) Conduction band diagram

of a TC-QWIP

(b) Transmission coefficient of an InGaAs/AlGaAs/InGaAs TC-QWIP

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 Layer diagram of four-band QWIP device structure and the deep groove 2-D periodic grating structure. Each pixel represent groove 2-D periodic grating structure. Each pixel represent a 640x128 pixel area of the four-band focal plane arraya 640x128 pixel area of the four-band focal plane array

Four-color QWIP Normalized Responsivity

0

0.2

0.4

0.6

0.8

1

3 5 7 9 11 13 15

Wavelength (micron)

Res

po

nsi

vity

(ar

b.u

)

QWIP Technology for IR FPAQWIP Technology for IR FPA

1. 1. Advantages:Advantages: Highly uniform large format (640x480) GaAs/AlGaAs Highly uniform large format (640x480) GaAs/AlGaAs

QWIP Focal Plane Array (FPA) can be fabricated for QWIP Focal Plane Array (FPA) can be fabricated for LWIR imaging array applications.LWIR imaging array applications.

High yield and reproducibility using GaAs QWIP Tech.High yield and reproducibility using GaAs QWIP Tech. Extremely low NEExtremely low NEmmhas been has been

achievedachieved

in GaAs E-QWIP.in GaAs E-QWIP.

2. 2. Drawback:Drawback: High dark current limits the operating temperature for High dark current limits the operating temperature for

QWIP to around 80K for 9QWIP to around 80K for 9m detection peak. m detection peak.

An InAn In0.60.6GaGa0.40.4As/GaAs Quantum Dot As/GaAs Quantum Dot

Infrared Photodetector (QDIP)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.

In0.6Ga0.4As QDs

Vb

Semi-insulating (100) GaAs substrate

0.5m GaAs, n=21018cm-3

1m GaAs, n=21018cm-

3

10600ÅGaAs (i)

0.5m GaAs Buffer

Cross- Sectional TEM for an InCross- Sectional TEM for an In0.60.6GaGa0.40.4As/GaAs As/GaAs

QDIP with High Operating Temperature (250 K)QDIP with High Operating Temperature (250 K)

•Cross- sectional transmission electron microscopy (TEM) of the QDIP structure.

•QD density is 1.21010 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 Spectral Responsivity for an InGaAs/GaAs QDIPInGaAs/GaAs QDIP

0.0

20.0

40.0

60.0

80.0

100.0

120.0

6.5 7.5 8.5 9.5 10.5 11.5

0

10

20

30

40

6.5 7.5 8.5 9.5 10.511.5

WAVELENGTH (m)

RE

SP

ON

SIV

ITY

(m

A/W

)

180K

160K

200K

220K

240K

260K

-0.7v-0.6v

0.2v

200K•Spectral responsivity vs. wavelength measured at 100 K to 260 K.

•The bias voltages were chosen to achieve the maximum photocurrent to dark current ratio.

QWIP Focal Plane Arrays (FPAs) using In- bump QWIP Focal Plane Arrays (FPAs) using In- bump Bonding to Silicon CMOS MUXBonding to Silicon CMOS MUX

QWIP Focal Plane Array Using In-bump QWIP Focal Plane Array Using In-bump bonding on Si CMOS MUXbonding on Si CMOS MUX

IR radiation

QWIPs Si CMOS MUX

readout

In- bump

QWIP Array

Twelve 640x512 QWIP focal plane arrays on a 3 inch GaAs wafer.

QWIP Phoenix Camera using QWIP Phoenix Camera using JPL QWIP FPAJPL QWIP FPA

Picture of the 640x512 hand-held long wavelength QWIP camera (QWIP Phoenix™).One frame of video image taken with the 9 µm

cutoff 640x512 pixel QWIP Phoenix camera.

Photo of a 640x486 LWIR QWIP CameraDeveloped by JPL and Amber

((a)

Palm Size QWIP Camera by JPLPalm Size QWIP Camera by JPL

Image of Fire Taken with Dual Band Image of Fire Taken with Dual Band QWIP CameraQWIP 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 CameraImage 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)

Reflector Metal

IncidentRadiation

n+ GaAs contact

MQW Grating

n+ GaAs contact

Enhanced QWIP(Single Pixel)

Reflector Metal

IncidentRadiation

n+ GaAs contact

MQW Grating

n+ GaAs contact

PixelUnit Cell

MQW Cavitiesn+ GaAsSpacer

ContactReflector

Indium Bump

Silicon ROICEpoxy

PixelUnit Cell

MQW Cavitiesn+ GaAsSpacer

ContactReflector

Indium Bump

Silicon ROICEpoxy

Enhanced QWIP(Single Pixel)

Reflector Metal

IncidentRadiation

n+ GaAs contact

MQW Grating

n+ GaAs contact

Enhanced QWIP(Single Pixel)

Reflector Metal

IncidentRadiation

n+ GaAs contact

MQW Grating

n+ GaAs contact

PixelUnit Cell

MQW Cavitiesn+ GaAsSpacer

ContactReflector

Indium Bump

Silicon ROICEpoxy

PixelUnit Cell

MQW Cavitiesn+ GaAsSpacer

ContactReflector

Indium Bump

Silicon ROICEpoxy

The NET for 40 m pitch EQWIP arrays at 60 – 80 K with f/2 FOV and 300 K background.

100

90

80

70

60

50

40

30

20

10

0

Ca

lcu

late

d N

ED

T (

mK

)

0 1 2 3 4 5

in t (m sec)

f/2 , 300 K b ackg ro u n d

60K , 1 .0 V60K , 1 .0 V

70K , 1 .5 V70K , 1 .5 V

77K , 1 .5 V77K , 1 .5 V

80K , 1 .5 V80K , 1 .5 V

100

90

80

70

60

50

40

30

20

10

0

Ca

lcu

late

d N

ED

T (

mK

)

0 1 2 3 4 5

in t (m sec)

f/2 , 300 K b ackg ro u n d

60K , 1 .0 V60K , 1 .0 V

70K , 1 .5 V70K , 1 .5 V

77K , 1 .5 V77K , 1 .5 V

80K , 1 .5 V80K , 1 .5 V

The NET 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 studentsRecent 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).