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Chapter

11

SEMICONDUCTOR

OPTOELECTRONICS

Semiconductor based optoelectronic devices form an important component of modern information

age. The following �gures provide an overview of important optoelectronic processes and devices.

© Prof. Jasprit Singh www.eecs.umich.edu/~singh

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Immunity to electromagnetic interference

AAAAAAAAAAAAAAAAAANon-interference of two or

more crossed beams

High parallelism

High speed–high bandwidth

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Beam steering for reconfigurable interconnects

Special-function devices

AAAAAAAAAAAAAAAAAAWave nature of light for

special devices

Nonlinear materials

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Photonics-electronics coupling

Can be transmitted without distortion due to electrical storms, etc.

Unlike electrical signals, optical signals can cross each other without distortion

Two-dimensional information can be sent and received

Potential bandwidths for optical communication systems exceed 1013 bits per second

Free space connections allow versatile architecture for information processing

Interference or diffraction of light can be used for special applications

New logic devices can be created

The best of electronics and photonics can be exploited by optoelectronic devices

ADVANTAGES OF OPTICAL DEVICES

Challenges: How does one harness the tremendous potential?


InSb PbS Ge Si GaAs CdSe GaP CdS SiC GaN ZnS

GaAs1-yPyHgCdTe

Infrared Red Green Violet Ultraviolet

Orange Yellow Blue

λ (µm)

Human eye repsonse

Eg (eV)

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6

6.0 3.0 2.0 1.5 1.0 0.9 0.8 0.7 0.6 0.5 0.45 0.4 0.35

CdTe

OPTOELECTRONICS: MOTIVATIONS FROM SYSTEM DEMANDS

DISPLAY APPLICATIONS: Light emitters covering red, green, blueOPTICAL MEMORIES: Short wavelength light emittersCOMMUNICATIONS: Light emitters/detectors operating at low absorption/dispersion

points of optical fibers (1.55 µm, 1.3 µm)

SEMICONDUCTOR BANDGAPS (WAVELENGTHS) AND HUMAN EYE RESPONSE


+

CONDUCTION

BAND

Photon

VALENCE

BAND

ABSORPTION PHONON-ASSISTED

ABSORPTION

Photon

Phonon

DIRECT BANDGAP INDIRECT BANDGAP

ωh > Eg

LIGHT ABSORPTION IN SEMICONDUCTORS

• Energy conservation hω > Eg• Momentum conservation

ki = kf for strong processes• Indirect gap materials have weak absorption

102

105

103

104

101.0 1.5 2.0 2.5 3.0

Ge

GaAsCdTe

Si

GaP

PHOTON ENERGY (eV)

AB

SOR

PTIO

N C

OE

FFIC

IEN

T (c

m–1

)

Electron-hole pair generation rate

Pop: Optical intensity (Watts/cm2)

Direct gap materials:

hω, Eg in units of eV

α(hω) ~5.6 x 1041/2hω–Eg

hω( (

GL =αPophω


hω

_

+

_

+hω

hω

VR

p+n+i

Signal

W

_

+

EV

Carriers are collected from the depletion region

x=0 x=W

hω

Ec

eVR

P-I-N PHOTODETECTORS

The detector is reverse biased to collect any electron-hole pairs created by light absorption.

Photocurrent:

IL = eAJph(0)[1– exp (–αW)]Jph = Optical photon particle currentW = Depletion regionA = Device area


SEMICONDUCTOR LIGHT EMITTERS FOR COMMUNICATION APPLICATIONS

Light emitters for long haul communication systems must emit light at λ = 1.55 µm or λ = 1.3 µm.

• GaAs lasers (λ ~0.88 µm) are used for local area networks where distances to be covered are only a few kilometers.

• InGaAsP lasers are used for long haul communication. Optical pulses travel ~40-50 km and are then separated by repeater lasers. Optical fiber amplifiers are also used to boost the signal.

1.55 µm: Lowest loss point in optical fibers

1.3 µm: Lowest dispersion point in optical fibers

0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

100

50

20

10

5

2

1

0.5

0.2

0.1

0.05

–OH absorption peaks

Infrared absorption tail from lattice transitions

Rayleigh scattering

FIB

ER

AT

TE

NU

AT

ION

(dB

km

–1)

WAVELENGTHS, MICRONS

1.55µm loss ~ 0.2 dB/km1.3µm loss ~ 0.5 dB/km


LIGHT-EMITTING DIODE: GENERAL PRINICPLES

The LED is a forward biased p-n diode. Electrons (holes) are injected into p-side (n-side) region where they recombine with holes (electrons) to emit photons.

Emitted photon energy ~EgEmitted spectral linewidth ~kBTUpper limit to LED switching time ~1 ns electron-hole recombination time

+

_

Electron injection

Photons willemerge fromthe device

_ _

+Hole injection

p nPhotonsBuried region

TOP LAYER

EFnEFp

+

+ +

_ _

_ _ _ _ _ _ _ _ _ __ _ _ _ _ _ _ _ _ _ _

+ + + + ++ + + + + + + +


LED: ELECTRON-HOLE RECOMBINATION TIME

• Electrons and holes recombine in LEDs via a process called spontaneous recombination.• Energy-momentum conservation rules apply.Recombination rate αfe(k) • fh(k)

fe(k): probability of finding an electron with momentum hkfh(k): probability of finding a hole with momentum hk

CONDUCTION BAND

VALENCE BAND

Eg

Ec

Ev

k

h2k2

2m*

hω

e

h

h2k2

2m*

• Electrons-holes recombination time is a function of carrier density.At high carrier densities the e-h recombination time approaches a nanosecond in most direct gap semiconductors.

10–5

10–6

10–7

10–8

10–9

10–10

1014 1015 1016 1017 1018

Nd (for holes injected into an n-type semiconductor)

n = p (for excess electron hole pairs injected into a region)

Rad

iativ

e L

ifet

ime

(τ r

)(s)

Typical carrier densities for laser operation Low

Injection Regime

τ o

Carrier occupation is degenerate fe = fh = 1

Semiconductor GaAs Temperature is 300 K

1019cm–3

~


_ _

Eg hω

CONDUCTION BAND

VALENCE BAND

hω

No Photons Spontaneous Emission

+ +

__

+

Photons

CONDUCTION BAND

VALENCE BAND

Coherent Emission

Stimulated Emission

+

hωhω

hω

STIMULATED EMISSION AND SPONTANEOUS EMISSION

SPONTANEOUS EMISSION: Responsible for light emission in LEDs• Electron-hole recombination in the absence of photons• Outcoming photons are incoherent, i.e., have random phases• Electron-hole recombination lifetime is limited by ~1 ns

STIMULATED EMISSION: Responsible for light emission in laser diodes• Electron-hole recombination in the presence of other photons• Photons produced are coherent, i.e., have the same phase• Electron-hole recombination lifetime is

1τstim

nphτspon

=

nph: photon number


THE LASER STRUCTURE: FORWARD BIASED P-N DIODE AND OPTICA CAVITY

Output light

Roughened surfaces

Optically flat and polished parallel faces

L

n-type

p-type

I

Cladding region

Active region

Cladding region

z y

x

Optical cavity, produced by cleaving the crystal causes photons to be reflected back into the cavity. The photon build-up starts the stimulated emission responsible for lasing

Polished face

Polished face

Optical modes in the cavity

Active region

p- region n- region

DIE

LE

CT

RIC

CO

NST

AN

T

DISTANCE PERPENDICULAR TO THE CAVITY (z)

Confined optical wave

The light wave is confined in the cavity by the waveguide of the laser structure


Stimulated emission

Spontaneous emission

Below threshold Above threshold

Jth J

INJECTED CURRENT DENSITY

LIG

HT O

UT

PUT

LASER OPERATION: GAIN AND LIGHT OUTPUT

• Gain = emission coefficient – absorption coefficient• As more and more electrons and holes are injected into the active region of the laser the gain increases.• When the gain overcomes the laser in the cavity, photon build-up occurs and lasing starts.

Light output in the lasing mode is very small below threshold. It increases rapidly once the laser is in the above threshold state.

1.42 1.46 1.50

160

120

80

40

0

-40

-80

1.5 x 10 18 cm-3

2.5 x 10 18 cm-3

Photon Energy (eV)

Gai

n (c

m-1

)


hω

hω

hω

____

e-h in bands

++++++

n = nthJ > Jth

Cavity resonant modes

cavity loss

hω

GA

IN

0

GA

IN

0

GA

IN

0

Gain spectrum Light emission

PHO

TO

N IN

TE

NSI

TY

PHO

TO

N IN

TE

NSI

TY

kBT

PHO

TO

N IN

TE

NSI

TY

Dominant mode

++++++

_____

_

_____

_

n < nthJ < Jth

++++

hω

hω

SPECTRAL OUTPUT OF A SEMICONDUCTOR LASER

Below threshold the laser acts like a light emitting diode. There is no coherence in the light output.

At threshold, photon number starts to increase in the cavity. The photon output in the lasing mode starts to increase.

Above threshold most of the current injected results in photon emission in the lasing mode. Photon spectral output is very sharp and light coming out is coherent.

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