Infrared, Self Assembled InAs/ GaAs Quantum Dot, Photodetectors

Michiel C. Donker

June 23, 2005

RuG

Infrared Light

SWIR = 0.7 - 3 μm 1.77 - 0.41eV

MWIR = 3 - 7 μm 0.41 - 0.18 eV

LWIR = 7 - 12 μm 0.18 - 0.10 eV

VLWIR > 12 μm < 0.10 eV

Infr

are

d

IR Detector Applications

• Medicine

8.5 μm 0.15 eV LWIR

• Astronomy

• Defense

• Environment

• Medicine

• Astronomy

• Defense

• Environment

Content

• Introduction

• Quantum Dots

• Device Fabrication

• Electronic Structure

• Characterisation and Performance

• Conclusion

HgCdTe Detectors

• Hg1-xCdxTe

• Wavelength 0.7- 25 μm

• High detectivity

• Difficult to process

0.0495 - 1.77 eV

IR

HgCdTeIR

Vbias =1.0 V

• Dark Current

• Photo CurrentHole

Electron

Quantum Confinement

InAs

Quantum Wire 1D

cross section

InAs

Quantum Dot 0D

cross section

GaAs

Bulk 3D

InAs

GaAsInAs

Quantum Well 2D

cross sectionz

E

z

GaAs GaAs

Conduction band

Valence band

InAs

50 nm

InAs: Eg =0.36 eV

GaAs: Eg= 1.52 eV

InAs

GaAs

Density of States

Bulk Quantum Well Quantum Wire Quantum Dot

g(E)= density of states

Quantum Dot: Particle in a Box

• Infinite potential barrier

• Ψ~ sin(nπx/ Lx) sin(mπy/ Ly) sin(lπz/ Lz)

• Selection rules: < Ψi I r I Ψj > = 0 111 211

x=0 x=L 1-D

Self Assembled Quantum Dots

• Host: GaAs 5.653 Å

• Quantum Dot: InAs 5.867 Å

• Stranski- Krastanov growth

GaAs

InAs

InAs GaAs

Heavely n- doped GaAs

Heavely n- doped GaAs

Electronic Structure

• Lens shaped QD’s

• Pyrimidal QD’s

GaAs conduction band

GaAs 3D valence bandWetting Layer

Wetting Layer

0 meV

56 meV63 meV

121 meV128 meV

115 meV

h 000

p 100 010

d

0 meV

31 meV

55 meV57 meV61 meV

h 100 010

h 110h 200h 020

s 000

930 meV

S.Sauvage et al. C.R. Physique 4, p1133 (2003)

IRIR

IR

Characterisation

In plane as well as normal incidence absorption

Conductionband

S P transition

ConductionBand edge

S wetting layer, conduction bandS P transition

• Intersubband: absorption e e Δn=odd non zeroΔn=even zero

S.Sauvage et al. C.R. Physique 4, p1133 (2003)

Dark and Photo Current-Thermionic emission

-Tunneling

-Thermally assisted tunneling

Energy

z direction

GaAs conduction band

InAs wetting layer

IR

-Wetting layer

-Conduction band

-Bound state

-Thermal generation e/ h pair

Vbias

Capture Probability

• phonon (lattice vibration)• e-e scattering• e-h scattering

IR

Carrier relaxation processes:

E

z

InAs

GaAs

• recombination

GaAs

S P D

phonon

phonon

The more electrons in a QD, the more charge.

ΔE= h c/ λcut off

Dark and Photo Current• Thermal generation• Tunneling• Number of electrons per QD• QD density• Number of QD layers• Capture probability

H.C. Liu, Opto Electronic Review 11, p.1-5 (2003)

77 K

Vbias (V)

x

zy ·

z

xy

S.Sauvage et al. C.R. Physique 4, p1133 (2003)

• Angle of incidence • Shape and size• Absorption cross section

• Vbias

D.Pang et el. Appl. Phys. Lett. 75, p. 2719 (1999)

20 K

Da

rk

Responsivity and Detectivity

• Responsivity (mA/ W) = =

- η = quantum efficiency = # generated electrons by photons# incident photons

Poptical power

Iphotocurrent

h v

e η g

- g = gain = =# carriers through device # generated carriers by photons τ carrier transit time

τ carrier lifetime

• Detectivity (cm Hz1/2/ W) = signal to noise ratio

- Background photon noise

- Photo electron noise; detectable carrier η, not detectable (1- η)

- Receiver circuit noise

Responsivity and Detectivity

• Responsivity (mA/ W) =Poptical power

Iphotocurrent

Res

po

nsi

vity

(A

/ W

)

Wavelength (μm)

165 eV

L. Jiang et al. Appl. Phys. Lett. 82, p.1986 (2003)

• Detectivity (cm Hz1/2/ W) = signal to noise ratio

77 K Vbias= -2.0 V QD density = 1.2 x 1010 Size 26 nm 6 nm

D = 3.6 x 1010 cm Hz1/2/ W

Conclusion

• QD: D = 3.6 x 1010 cm Hz1/2/ W

• HdCdTe: D = 2.2 x 1012 cm Hz1/2/ W

• # electrons per dot

• dot density

• dot size and shape

• spacing thicknessVbias

• transport directionz

xy

J.Philips et al. Encyclopedia of Nanoscience and Nanotechnology, 9 p. 131 (2004)

Acknowledgements

Quantum Well Camera

Paul van Loosdrecht

Questions

• Infinite potential barrier

• Ψ~ sin(nπx/ L) sin(mπy/ L) sin(lπz/ L)

• Selection rules:

• Fermi golden rule:

< Ψi I r I Ψj > = 0

x=0 x=L

Δn=odd, on the same axis

000 100 100 010100 200 000 200

XX

<1 | z | 2>= (16/9π2)/L<1 | z | 4>= (4/45π2)/L

Questions

• Interband: Photo Luminescense

• Intersubband: AbsorptionIntersubband selection rules:

e e Δn=odd strongΔn=even zero

S.Sauvage et al. C.R. Physique 4, p1133 (2003)

Interband selection rules:

h e Δn=0 strongΔn=0 odd weakΔn=0 even zero

H 000 E 000

Questions

• D= R AΔf / Inoise

• H= Hk·p + Hstrain + Vconfining potential

• En (k) = En (0) + h2k2/2m +

h2/m2 Σ’ n= band index

| <n0| k·p |j0> |2

En(0) – Ej (0)

Questions

InAs

GaAs GaAs

AlGaAs AlGaAs

Vbias

E

z

Questions

L. Jiang et al. APL 82, p.1986 (2003)

Questions

S.F. Tang et al. APL 78, p. 2428 (2001)

Questions