Fundamentals of Optoelectronic Materials and Devices

Hsing-Yu Tuan (段興宇）

Department of Chemical Engineering, National Tsing-Hua University

光電材料與元件基礎

Textbook in this lecture

1.Optoelectronics and photonics priciples and practices, S.O. Kasap 1999

2.Solid state electronic devices, Ben G. streetman and sanjay banerjee, fifth edition

Electron energy, E

Conduction Band (CB)Empty of electrons at 0 K.

Valence Band (VB)Full of electrons at 0 K.

Ec

Ev

0

Ec+χ

(b)

Band gap = Eg

(a)

Covalent bond Si ion core (+4e)

(a) A simplified two dimensional view of a region of the Si crystalshowing covalent bonds. (b) The energy band diagram of electrons in theSi crystal at absolute zero of temperature.?1999 S.O. Kasap, Optoelectronics (Prentice Hall)

Band gap schematic of Si

Distinction between insulator, semiconductor and metal

Streetman p62

Insulator Semiconductor Metal

conduction band

valance band

> 5 eV

-very few electrons excited to valance band at room temperature

-few electrons excited to conduction band Ex, Si: 1010 / cm3 (note: Si has 5x1022 atoms/cm3)

plenty of electrons

E

CB

k–k

Direct Bandgap

(a) GaAs

E

CB

VB

Indirect Bandgap, Eg

k–k

kcb

(b) Si

E

k–k

Phonon

(c) Si with a recombination center

Eg

Ec

Ev

Ec

Ev

kvb VB

CB

ErEc

Ev

Photon

VB

(a) In GaAs the minimum of the CB is directly above the maximum of the VB. GaAs istherefore a direct bandgap semiconductor. (b) In Si, the minimum of the CB is displaced fromthe maximum of the VB and Si is an indirect bandgap semiconductor. (c) Recombination ofan electron and a hole in Si involves a recombination center .

?1999 S.O. Kasap, Optoelectronics (Prentice Hall)

Direct band gap and indirect band gap structure

Semiconductor, band gap, and wavelength

The band gap corresponds to the wavelength of light that emits from the semiconductor

顏色名 波長 (nm)

紫 380-450

藍 450-495

綠 595-570

黃 570-590

橙 590-620

紅 620-750

Eg=1240/λ eV

Carrier: electron and hole

-Some electrons were excited to the conduction band at temperature >0K -For convenience, an empty state in the valence band is referred to as a hole -Electron-hole pair (EHP): conduction band electron and the hole are created by the excitation of a valence band electron -EHPs are free charge carriers in semiconductor materials -Si, at room temperature has 1010 EHP/cm3 (Si: 5x1022 atoms/cm3)

Doped materials

Donors: P, As, Sb (Column V elements ) , n-type, provide one additional electron Acceptors: B, Ga, In, (Column III elements), p-type provide one additional hole

P+ - B-

-weakly bound -bonding strength EB~ 0.05 eV (Si bonding ~1.12 eV)

Majority carrier - electron in a n-type material hole in a p-type material Minority carrier – hole in a n-type material electron in a p-type material

n p

The nomination is not limited to Si, but for any semiconductors

Equilibrium distribution of carriers in intrinsic and doped semiconductors

g(E)

hole 1-f(E)

n Ec EF

Ei Ev

Ec Ei EF

Ev

fermi level shift

Ec Ei Ev

Intrinsic

n-type p-type p

i

Carrier action • Drift: electrons and holes move due to a electrical field E

• Diffusion electrons and holes move due to concentration gradient

E + -

• Recombination - generation Ec

Ev heat or light

Space charge in depletion region

Uniformly doped p- and n-type semiconductors before joining

Equilibrium Fermi energy

Electric field in the depletion region and the energy band diagram of a p-n junction in thermal equilibrium

When a p-type and a n-type material meet together: PN-Junction

the same as before

[ ])(1)()()( 1122 EfENEfEN −•

[ ])(1)()()( 2211 EfENEfEN −•

Electrons transfer rate from 1 to 2 is proportional to

Electrons transfer rate from 2 to 1 is proportional to

At equilibrium, not net transfer electrons, so these must be equal

[ ])(1)()()( 2211 EfENEfEN −•

[ ])(1)()()( 1122 EfENEfEN −•=

[ ] [ ] 1/)(1/)(

21

11221222211211

21 11

)()(,

−−−− +=+

=

−=−

kTEEkTEE FF ee

EfEfso

fNfNNfNfNfNNfN

EF1=EF2 dEF

dx =0

Invariance of the Fermi level at equilibrium

In the beginning of diffusion

Empty sites The number of filled states

Einstein Relation

• Under equilibrium conditions the Fermi level inside a materials (or inside a group of materials in intimate contact) is invariant as a function of position; that is dEF/dx = dEF/dy=dEF/dz=0 under equilibrium conditions

P-N junction:

Space charge distribution

Rectangular approximation of the space charge distribution

Space charge

-The n-side near the junction Becomes depleted of majority carriers And therefore has exposed positive donor ions -Similar situation happens to p-side -The region on both sides of the junction M consequently becomes depleted of free carriers in comparison with the bulk p and n regions, called – space charge layer, known as depletion region

A p-n junction with abrupt doping changes at the metallurgical junction

Energy band diagram of an abrupt junction at thermal equilibrium

There will be a bulit-in voltage created in the junction

PN junction:summary

nno

xx = 0

pno

ppo

npo

log(n), log(p)

-eNa

eNd

M

x

E (x)

B-

h+

p n

M

As+

e–

Wp Wn

Neutral n-regionNeutral p-region

Space charge region Vo

V(x)

x

PE(x)

Electron PE(x)

Metallurgical Junction

(a)

(b)

(c)

(e)

(f)

x

–Wp

Wn(d)

0

eVo

x (g)

–eVo

Hole PE(x)

–Eo

Eo

M

ρnet

M

Wn–Wp

ni

Properties of the pn junction.

?1999 S.O. Kasap, Optoelectronics (Prentice Hall)

E=-dV/dx

charge density

potential

Story: The carrier concentraion difference between the n and p regions causes the carriers to diffuse. The diffusion, however, Leads to a charge imbalance. the charge imbalance in turn produces an electric field, which counteracts the diffusion so that in thermal equilibrium the net flow of carriers Is zero. The charged region near the metallurgical junction where the mobile carriers have been reduced is called the depletion region

Vo: built-in-potential

no carrier in the depletion region

Energy band diagram in an applied field

V

n-Type Semiconductor

Ec

EF − eV

A

B

V(x), PE(x)

x

PE(x) = ?eV

Energy band diagram of an n-type semiconductor connected to avoltage supply of V volts. The whole energy diagram tilts becausethe electron now has an electrostatic potential energy as well

EElectron Energy

Ec − eV

Ev− eV

V(x)

EF

Ev

?1999 S.O. Kasap, Optoelectronics (Prentice Hall)

n-type semiconductor connected to a voltage

-The applied voltage makes the energy band bend and cause the potential energy difference between two sides of n-type semiconductor -EF(A)-EF(B)=eV, meaning the electrostatic PE barrier between A and B

E=-dV/dx

Depletion layer width and energy band diagrams of a p-n junction under various

biasing conditions

Thermal-equilbrium condition

Forward-bias condition

Reverse-bias condition ( )[ ] 2/12 Dbisn qNVVxW −=≈ ε

The width of depletion layer can be externally adjusted by the applied voltage

Ec

Ev

Ec

EFp

M

EFn

eVo

p nEo

Evnp

(a)

VI

np

Eo–E

e(Vo–V)

eV

EcEFn

Ev

Ev

Ec

EFp

(b)

(c)

Vr

np

e(Vo+Vr)

EcEFn

Ev

Ev

Ec

EFp

Eo+E (d)

I = Very SmallVr

np

Thermalgeneration Ec

EFnEv

Ec

EFp

Ev

e(Vo+Vr)

Eo+E

Energy band diagrams for a pn junction under (a) open circuit, (b) forwardbias and (c) reverse bias conditions. (d) Thermal generation of electron holepairs in the depletion region results in a small reverse current.

SCL

?1999 S.O. Kasap, Optoelectronics (Prentice Hall)

Forward and reverse bias effect for a pn junction

Fermi level equilibrium

Forward bias reduces the eVo to e(Vo-V), so the electrons at Ec in the n-side can overcome the the potential barrier and diffuse to the p-side A reverse bias, V=-Vr, Vr adds to the built-in potential Vo, so the P.E barrier becomes e(Vo+Vr), so there is hardly any reverse current.

Fabrication of Light Emitting Diode using a pn junction structure

Light Emitting Diodes: Principle

hυ - Eg

Eg (b)

V

(a)

p n+

Eg

eVo

EF

p n+

Electron in CBHole in VB

Ec

Ev

Ec

Ev

EF

eVo

Electron energy

Distance into device

(a) The energy band diagram of a p-n+ (heavily n-type doped) junction without any bias.Built-in potential Vo prevents electrons from diffusing from n+ to p side. (b) The appliedbias reduces Vo and thereby allows electrons to diffuse, be injected, into the p-side.Recombination around the junction and within the diffusion length of the electrons in thep-side leads to photon emission.

?1999 S.O. Kasap, Optoelectronics (Prentice Hall)-made by a junction consists of p-side with heavily-n-doped-side (n+)

Depletion region extends mainly into p-side built in voltage

-the recombination of injected electrons in the depletion region as well as in the neutral p-side and results in spontaneous emission of photons -the recombination zone is called the active region -light emission from EHP recombination as a result of minority carrier injection is called injection electroluminescence

-P-layer has to be narrow (a few microns) to allow the emitted photons escape without being reabsorbed

LED device structure

Light output

Insulator (oxidep

n+ Epitaxial layer

A schematic illustration of typical planar surface emitting LED devices. (a) p-layergrown epitaxially on an n+ substrate. (b) First n+ is epitaxially grown and then p regionis formed by dopant diffusion into the epitaxial layer.

Light output

pEpitaxial layers

(a) (b)

n+Substrate Substrate

n+

n+

Metal electrode

?1999 S.O. Kasap, Optoelectronics (Prentice Hall)

Narrow

Light output

p

Electrodes

LightPlastic dome

Electrodes

Domedsemiconductor

pn Junction

(a) (b) (c)

n+n+

(a) Some light suffers total internal reflection and cannot escape. (b) Internal reflectionscan be reduced and hence more light can be collected by shaping the semiconductor into adome so that the angles of incidence at the semiconductor-air surface are smaller than thecritical angle. (b) An economic method of allowing more light to escape from the LED isto encapsulate it in a transparent plastic dome.

Substrate

?1999 S.O. Kasap, Optoelectronics (Prentice Hall)

LED device structure

p

GaAs is around 160

LED device illustration

LED semiconductor materials

ηexternal = Pout (optical) x 100% IV

1993 年日亞(Nichia)發展高效率藍光LED!

LED materials – mainly III-V based direct band gap materials

- II-VI compounds are hard to be doped, so not usually be used

0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6λ

1.7Infrared

GaAs1-yPy

InP

In1-xGaxAs1-yPyAlxGa1-xAs

x = 0.43

Indirectbandgap

Free space wavelength coverage by different LED materials from the visible spectrum to theinfrared including wavelengths used in optical communications. Hatched region and dashedlines are indirect Eg materials.

In0.49AlxGa0.51-xP

?1999 S.O. Kasap, Optoelectronics (Prentice Hall)

Evolution of light source

Tuning band gap by alloying or doping

y=0.45, λ=630 nm Red light

-Nitrogen doped indirect bandgap GaAs1-yPy allows can emit green, yellow, orange LEDs -Al doped SiC, GaN are can emit blue emission, however, Al doped SiC is indirect band gap and GaN is very expansive

Alloy doping

LED characteristics

λ∆ Ehcvc // ==λ

Ehcvc // ==λ

E

Ec

Ev

Carrier concentrationper unit energy

Electrons in CB

Holes in VBhυ

1

0

Eg

hυ1

hυ2

hυ3

CB

VBλ

Relative intensity

1

0λ

1λ

2λ

3

∆λ∆hυ

Relative intensity

(a) (b) (c) (d)

Eg + kBT

(2.5-3)kBT

1/2kBT

Eg1 2 3

2kBT

(a) Energy band diagram with possible recombination paths. (b) Energy distribution ofelectrons in the CB and holes in the VB. The highest electron concentration is (1/2) kBT aboveEc . (c) The relative light intensity as a function of photon energy based on (b). (d) Relativeintensity as a function of wavelength in the output spectrum based on (b) and (c).

?1999 S.O. Kasap, Optoelectronics (Prentice Hall)

-The energy of an emitted photon from an LED is not equal to the Eg -Electron (hole) concentration’s peak position is 1/2kBT above Ec or Ev and direct recombination is proportional to the concentration -The linewidth is defined as width between half-intensity equal to △hv , normally is around 2.5-3 kBT

λ∆

E=hv

V

2

1

(c)

0 20 40I (mA)0

(a)

600 650 7000

0.5

1.0

λ

Relativeintensity

24 nm

∆λ

655nm

(b)

0 20 40I (mA)0

Relative light intensity

(a) A typical output spectrum (relative intensity vs wavelength) from a red GaAsP LED.(b) Typical output light power vs. forward current. (c) Typical I-V characteristics of ared LED. The turn-on voltage is around 1.5V.?1999 S.O. Kasap, Optoelectronics (Prentice Hall)

Output spectrum of a red GaAsP LED

- =24 nm is around 2.7kBT -Turn-on voltage increase with the energy bandgap Eg, vlue LED is 3.5-4.5 V yellow LED is around 2 V, GaAs infraed LED is around 1 V

Ehcvc // ==λ

Turn-on voltage:1.5V

λ∆

Peak emission

White light LED

• White light for lighting – long service life, electricity effective, low driving voltage, safe • White light LED: First example: Blue LED + YAG (yttrium aluminum garnet, 釔鋁石榴石) yellow phosphor

(currently most popular, low cost & high efficiency) (at 1996) - RGB LEDs (Red:green:blue = 3:6:1) - UV LED (GaN) + RGB phosphors

Various combination of white LED

Evolution of light source • LED has 80% lower energy vs. incandescent • LED has 39% lower energy vs. CFL

Courtesy of Osram

compact fluorescent light

Efficiency now is the best.

Light output V.S. Efficiency

Cost problem of LED

Coutesy of LLF

(five years ago)

LED照明甜蜜點提前到2012-02-22 01:38 工商時報

• 市場原預期，眾所期盼的LED照明「甜蜜點」必須

等到下半年才看到；但根據LEDinside最新調查，由於韓國地區競爭激烈，取代40瓦的LED燈泡報價，今年1月已跌破10美元，甚至北美、英國最低價也逼進10美元；LED照明甜蜜點提前來臨，預期可帶動龐大的需求商機。

• 照明被視為LED綠能產業的最大商機，過去因為LED價格過高，雖然擁有省電、壽明長等優點，但仍無法與白熾、省電燈泡競爭；業界普遍將取代40瓦照明的LED燈泡跌破10美元、取代60瓦的LED燈泡跌至10美元至12美元時，即達到所謂市場需求「甜蜜點」，一旦落在此價位之下，需求曲線將大幅翻揚。

Is the technology revolution similar?

High tech product combined with LED

水立方