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Gad Bahir – Technion Nanotechnology Workshop 22.05.03
Quantum Dots Infrared Photodetectors (QDIPs)
Quantum Dots Infrared Photodetectors (QDIPs)
Gad Bahir
Collaboration: E. Finkman, (Technion) D. Ritter (Technion) S. Schacham (Ariel) P. Petroff (USCB USA) F. Julien (CNRS France)M. Gendry (Lyon France)
Graduate students T. Raz
M. Girzel N. Shual
CROSS SECTION T
CONDUCTION BAND DIAGRAM
GaA 70 Å
AxGA1-x S
00 Å
InAs
InAlAs
Gad Bahir – Technion Nanotechnology Workshop 22.05.03
OutlineOutline
Self assembled quantum dots
Infrared photodetectors from bandgap
engineering to “artificial atoms”
QWIPs vs QDIPs
Gad Bahir – Technion Nanotechnology Workshop 22.05.03
What are quantum dots? What are quantum dots?
A medium whose dimensions are of the order of the electron’s de Broglie wavelength 3D confinement
Lx
Ly
Lz
Lx, Ly, Lz deBroglie
Bulk
Energy
D(E
)
Quantum W ell
Energy
D(E
)
E 1 E 2
Quantum Dot
Energy
D(E
)
E 1 E 2
Quantum W ire
Energy
D(E
)
E 1 E 2
Density of States
Gad Bahir – Technion Nanotechnology Workshop 22.05.03
Self-Assembled Growth of Quasi-zero Dimensional Systems
Self-Assembled Growth of Quasi-zero Dimensional Systems
Frank-van der Merwe: 2d layer by layer
Stranski-Krastanow: initial 2D growth leads to 3D island growth
Vollmer-Weber: 3D island growth
Increasing Strain
AFM images of Surface InAs QDs
GaAs/InAs (UCSB-Technion)
InP/InAlAs/InAs (France-Technion)
SiGe/Si (France-Technion)
InP/InGaP/InAs (Technion)
Wetting layer
Gad Bahir – Technion Nanotechnology Workshop 22.05.03
MOMBE Growth of InAs/InP Quantum Dots
MOMBE Growth of InAs/InP Quantum Dots
1.76 ML 1.83 ML 1.97 ML 2.17 ML 2.38 ML
QD Density vs. InAs Nominal Thickness
AFM image of single dot
Tal Raz et al., PRB 2003 submitted
Gad Bahir – Technion Nanotechnology Workshop 22.05.03
MOMBE Growth of InAs/InP Quantum RingsMOMBE Growth of InAs/InP Quantum Rings
0
3
6
9
12
15
Hei
gh
t [n
m]
RING
DOT [1 1 0]
0 100 200 300 400
0
3
6
9
12
15
Hei
gh
t [n
m]
[1 1 0]
Distance [nm]
Tal Raz et al., APL 2003
Gad Bahir – Technion Nanotechnology Workshop 22.05.03
QDs StructuresQDs Structures
Intra-band transition
Inter-bandtransition
Self organized islands areFormed after a few Monolayers of layer by Layer growth.
Typical Dimensions:
15-25 nm lateral size
5-8 nm vertical heights
Barrier
Wetting layer
Substrate
QDQW
Gad Bahir – Technion Nanotechnology Workshop 22.05.03
QDs propertiesQDs properties
The presence of a discrete energy spectrum distinguishes quantum
dots from all other solid state systems and caused them to be called
“artificial atoms” The atom like properties make QDs a good venue for studying the
physics of confined carriers and also could lead to novel device
applications in the field of quantum computing, optics and
optoelectronics. These “artificial atoms” can, in turn, be positioned and assembled into
complexes that serve as a new material.
Single dot exciton spectra
Gammon Science 1996
Gad Bahir – Technion Nanotechnology Workshop 22.05.03
MWIR and LWIR ApplicationsMWIR and LWIR Applications
•Thermal imaging, night vision, reconnaissance
• Chemical spectroscopy
• Optical remote sensing
• Atmospheric applications
• Medical diagnostics
• Vegetation recognition
• Fire fighting, Crime Prevention, Forensics
• Space-based Remote Sensing, Astronomy
Gad Bahir – Technion Nanotechnology Workshop 22.05.03
Quantum Well IR photodetector – QWIPthe Bandgap Engineering concept
Quantum Well IR photodetector – QWIPthe Bandgap Engineering concept
e-
barrier
barrier
well
well
well
barrierwell
Band to band
Intra-band
Man made IR detector in wide band-gap semiconductor
Gad Bahir – Technion Nanotechnology Workshop 22.05.03
QWIP structureQWIP structure
Top view
Gad Bahir – Technion Nanotechnology Workshop 22.05.03
QWIPs do not work with normal incidence lightQWIPs do not work with normal incidence light
LINEAR GRATING
LWIR
GaS/AIGaAs MULTI-QUANTUM WELL
SAWTOOTH GRATING
Au/Ge
N+ GaAs CONTACT LAYER
Complicated coupling technique
Gad Bahir – Technion Nanotechnology Workshop 22.05.03
Noise mechanisms in QWIPsNoise mechanisms in QWIPs
E vs. K||
(c)
(a)
(b)
Conduction band of multi-quantum wells structure (a) Tunneling(b) Field induced tunneling(c) Thermionics emission
fG
VG
N
S
th
opt
4
Recombination time ~1 p sec
Gad Bahir – Technion Nanotechnology Workshop 22.05.03
”From bandgap engineering to “artificial atomsQWIP vs. QDIP
From bandgap engineering to “artificial atoms” QWIP vs. QDIP
QWIP Limitations:
Polarization Selection Rule – QE immediately limited to 50% Short lifetime of photoexcited electrons – carriers relax back to the ground
state before they can escape from the quantum well (~10 ps)
QDIP (expected) Advantages:
3D Confinement - intrinsically sensitive to normal incidence photoexcitation Much longer relaxation (~100 ps) / capture times (“phonon bottleneck”) -
leads to increased gain and thus, higher responsivity and detectivity
Gad Bahir – Technion Nanotechnology Workshop 22.05.03
Device structure and imageDevice structure and image
AFM Image
Device structure
[1-10]
[110]
2 electrons per dot
Gad Bahir – Technion Nanotechnology Workshop 22.05.03
Front illuminationFront illuminationQDIP photoconductive spectra as function of bias
Finkman et al., PRB 2001
Gad Bahir – Technion Nanotechnology Workshop 22.05.03
Polarization dependencePolarization dependence
-2
0
2
4
6
8
0 0.1 0.2 0.3 0.4 0.5
V=0VV= - 0.25VV= - 0.5VV= - 0.75VV= - 1VV= - 1.25VV= - 1.5V
file: a:\1.tblfile: c:\michael\isprav\1b.tbl
# M 875
EPH
eV]
Sp
ectr
al r
esp
onse
(a.
u)
Measurements of signal dependence on bias voltage (reverse)Front illumination, T=15K, pin F, sens=10-7
0
0.25
0.50
0.75
1.00
0 30 60 90
file: a:\int2.tblfile: c:\michael\isprav\int2a.tbl
# M 875
file: d:\michael\michael\int2a.tbl [ 1 -1 0 ] Polarization angle () [ 1 1 0 ]
Res
pon
sivi
ty (
a.u)
Measurements of integrated signal dependence on polarizationFront illumination, T=15K, V= - 1.25V, pin F, sens=10-8
[1-10]
[110]
Polarization dependence of 100 mV peak
Dot shape and orientation
[1-10]
Front illumination pc signal
Dual band detector with polarization selectivity
Bound to continuum
Bound to bound + tunneling
Bahir SPIE 4820 (2002)
[1 1 0]
Gad Bahir – Technion Nanotechnology Workshop 22.05.03
I-V as function of temperature (dark current full line, background radiation 300K
dashed line)
I-V as function of temperature (dark current full line, background radiation 300K
dashed line)
10-14
10-11
10-8
10-5
10-2
-2 -1 0 1 2
T=15K (OPEN DIODE)
T=15KT=20KT=30KT=40KT=50KT=60KT=70K
# M 875
V [Volt]
I [A
]
Gad Bahir – Technion Nanotechnology Workshop 22.05.03
PC Spectra for various temperatures
PC Spectra for various temperatures
S. Schacham et al., PRB 2003
Gad Bahir – Technion Nanotechnology Workshop 22.05.03
QDIP advantages over QWIPQDIP advantages over QWIP
1. Normal incidence absorption
Normal incidence (without grating) was indeed observed
2. Phonon bottleneck
Absence of phonon bottleneck in most experimental results. There is no advantage to QDIP over QWIP ?
The QD does not “work” as an artificial atom and we have to consider strong interaction between carriers and lattice vibrations.
Gad Bahir – Technion Nanotechnology Workshop 22.05.03
Compatition between tunneling and decay
Compatition between tunneling and decay
Following bound to bound excitation, electrons can either tunnel out and become free carriers or decay back.
As the temperature is raised, the decay rate of the 100 meV signal increases due to increased LA phonon concentration while tunneling is independent of temp.
Polaron formalism for coupling strength between electron and phonon.Bound to continuum 250 meV peak
Bound to bound + tunneling
Gad Bahir – Technion Nanotechnology Workshop 22.05.03
Model fit to temperature dependence
Model fit to temperature dependence
S. Schacham et al., PRB 2003
The decrease of signal with temperature is associated with reduced polaron life time due to increased LA phonon population with temperature
100 meV
Gad Bahir – Technion Nanotechnology Workshop 22.05.03
ConclusionConclusion
Unlike bulk material or quantum wells, the relaxation in QDs is not due to emission
of one LO phonon, but is a results of multiphonon process.
There is no need for the two electron states to differ exactly by one LO phonon
energy, i.e. no phonon bottleneck.
The atom-quantum dot analogy should not be carried too far: unlike electron in an
isolated atom, carriers in semiconductor quantum dot, which contain a few
thousands of atoms in a nearly defect free 3D crystal lattice interact strongly with
lattice vibrations and in a unique way which should be studied.