Quantum Dots Infrared Photodetectors (QDIPs)

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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

InAlAs
InAs
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Outline

• Self assembled quantum dots
• Infrared photodetectors from bandgap
• engineering to artificial atoms
• QWIPs vs QDIPs

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What are quantum dots?

• A medium whose dimensions
• are of the order of the electrons
• de Broglie wavelength
• ? 3D confinement

Lx, Ly, Lz ? ldeBroglie
Density of States
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Self-Assembled Growth of Quasi-zero Dimensional
Systems

Stranski-Krastanow initial 2D growth leads to 3D
island growth
Frank-van der Merwe 2d layer by
layer
Vollmer-Weber 3D island growth
Wetting layer
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)

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MOMBE Growth of InAs/InP Quantum Dots

QD Density vs. InAs Nominal Thickness AFM image
of single dot
Tal Raz et al., PRB 2003 submitted
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MOMBE Growth of InAs/InP Quantum Rings

Tal Raz et al., APL 2003
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QDs Structures
Self organized islands are Formed 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
QD
QW
Intra-band transition
Inter-band transition
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QDs 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
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MWIR 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

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Quantum Well IR photodetector QWIPthe Bandgap
Engineering concept
-

e
Intra-band
well
barrier
well
Band to band
barrier
well

barrier
well
Man made IR detector in wide band-gap
semiconductor
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QWIP structure

Top view
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QWIPs do not work with normal incidence light
Complicated coupling technique
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Noise mechanisms in QWIPs

1. Tunneling
2. Field induced tunneling
3. Thermionics emission

Conduction band of multi-quantum wells structure

E vs. K
(c)
(b)
(a)
Recombination time 1 p sec
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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

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Device structure and image
1-10
AFM Image
110
2 electrons per dot
Device structure
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Front illumination
QDIP photoconductive spectra as function of bias
Finkman et al., PRB 2001
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Polarization dependence
Polarization dependence of 100 mV peak
Front illumination pc signal
Bound to bound tunneling
Bound to continuum
1-10
Dot shape and orientation
1 1 0
Bahir SPIE 4820 (2002)
Dual band detector with polarization selectivity
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I-V as function of temperature (dark current
full line, background radiation 300K dashed line)
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PC Spectra for various temperatures
S. Schacham et al., PRB 2003
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QDIP advantages over QWIP

• Normal incidence absorption
• Normal incidence (without grating) was
  indeed observed
• 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.

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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
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Model fit to temperature dependence
The decrease of signal with temperature is
associated with reduced polaron life time due
to increased LA phonon population with
temperature
100 meV
S. Schacham et al., PRB 2003
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Conclusion

• 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.