gad bahir – technion nanotechnology workshop 22.05.03 quantum dots infrared photodetectors (qdips)...

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


OutlineOutline

Self assembled quantum dots

Infrared photodetectors from bandgap

engineering to “artificial atoms”

QWIPs vs QDIPs


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


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


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


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


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


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


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


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


QWIP structureQWIP structure

Top view


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


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


”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


Device structure and imageDevice structure and image

AFM Image

Device structure

[1-10]

[110]

2 electrons per dot


Front illuminationFront illuminationQDIP photoconductive spectra as function of bias

Finkman et al., PRB 2001


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]


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

]


PC Spectra for various temperatures

PC Spectra for various temperatures

S. Schacham et al., PRB 2003


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.


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


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


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.

gad bahir – technion nanotechnology workshop 22.05.03 quantum dots infrared photodetectors (qdips)...

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