© S.N. Sabki CHAPTER 9: PHOTONIC DEVICES CHAPTER 9: PHOTONIC DEVICES

© S.N. Sabki

CHAPTER 9: PHOTONIC DEVICESCHAPTER 9: PHOTONIC DEVICES

© S.N. Sabki

• Photonic devices – devices in which the basic particle of light (the photon) plays a major role

• Consider 4 groups of photonic devices:

• light emitting diodes (LEDs)

• lasers (light amplification by stimulated emission of radiation)

• photodetectors – electrically detect optical signals

• solar cells – convert optical energy into electrical energy

• Detectable range of light by human eye: from 0.4m to 0.7m

• Ultraviolet region: from 0.01m to 0.4m, infrared region: from 0.7m to 1000m

m)eV(

24.1 hvv

hc

v

c

c: speed of light

v: freq. of light

hv: energy of photon

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Figure 9.1. Chart of the electromagnetic spectrum from the ultraviolet region to the infrared region.

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)

RADIATIVE TRANSITIONS RADIATIVE TRANSITIONS

And Laser OperationAnd Laser Operation

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LASER MIRROR AND LASER LASER MIRROR AND LASER PUMPINGPUMPING

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3 processes for interaction between a photon & electron:AbsorptionSpontaneous emissionStimulated emission

2 energy levels: E1 – ground state, E2 – excited state Transition between the states with freq. v12 given by hv12=E2-E1 Atom in E1 absorbs photon – goes to E2: absorption process (fig.9.2a) atom in E2 is unstable – make a transition to E1 – giving off a photon (energy hv12): spontaneous emission (fig.9.2b) When photon (energy hv12) impinge on an atom (at E2), atom can be stimulated to E1 – stimulated emission (fig.9.2c

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Absorption An atom in a lower initial state with energy Einitial can be excited to a higher energy state Efinal (solid line) by absorbing (capturing) a photon.By the principle of conservation of energy, the energy level difference ∆E = Efinal – Einitial is provided by the energy of the photon energy hf = ∆ E. This process also called absorptionbecause it is the atom’s response to electromagnetic stimulation by the incoming photon.Stimulated absorption is responsible for the colors of dyes, which absorb at specific frequencies.

Absorption

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Spontaneous Emission

An atom or molecule in an excited state with excess storedenergy (dashed line) ∆E can release that energy spontaneously, and transition to a lower energystate (solid line) by emission of a photon with frequency f= –∆E/h (the minus sign is becausethe atom loses energy, so ∆ E is negative, but photon energies are always positive. Spontaneousemission is responsible for the light emission by fires, sunlight, LEDs, and most types of lamps.

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Stimulated Emission

An atom or molecule in an excited state with excess storedenergy (dashed line) ∆E can be stimulated to release energy by an incident photon of the exactlythe same frequency (f= ∆ E/h). The result is that the atom emits not one, but two, photons withthe same frequency, using the energy of the incident photon plus the energy stored by the excitedatom. Stimulated emission is the process that provides optical amplification in most lasers, sinceone photon in produces two photons out, an amplification factor of exactly two per event.

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Figure 9.2. The three basic transition processes between two energy levels. Black dots indicated the state of the atom. The initial state is at the left; the final state, after the transition, is at the right. (a) Absorption. (b) Spontaneous emission. (c) Stimulated emission.

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Figure 9.3. Optical absorption for (a) hv = Eg, (b) hv > Eg, and (c) hv < Eg.

• When semicond. is illuminated (if hv=Eg)– photons are absorbed to create electron-hole pairs

• If hv>Eg – an electron-hole pair is generated – the excess energy (hv-Eg) is dissipated as heat

• Processes in (a) & (b): called intrinsic transitions (or band-to-band transitions)

• If hv<Eg – a photon will be absorbed only if there are available energy state in the forbidden bandgap due to chemical impurities or defects: called extrinsic transition

• Also true for reverse situation: i.e. electron at Ec combine with hole at Ev emission of a photon (with hv=Eg)

OPTICAL ABSORPTIONOPTICAL ABSORPTION

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W0W e

Fraction of photon flux that exits from the other end of the semicond. at x=W:

mE

24.1

g

c

: photon flux

: absorption coefficient (function of hv)

c: cutoff freq.

Decreased absorption coeff. for amorphous silicon at the cutoff wavelength:


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Figure 9.5. Optical absorption coefficients for various semiconductor materials. The value in the parenthesis is the cutoff wavelength.

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LEDLED

Stands for light emitting diode.

Semiconductor device: p-n junction

forward-biased.current

emits incoherent narrow spectrum light

(due to recombination in transition region near the junction.)

Color of the emitted light depends on the chemical of the semiconducting material used.

(Near-ultraviolet, visible or infrared.)

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LEDLED

Normally constructed of (Direct Gap):

GaAs, GaAsP , GaP :

Recombinationlight

Si and Ge are not suitable because of indirect band.recombination result heat

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LEDs are p-n junctions that can emit spontaneous radiation in ultraviolet, visible or infrared regions

VISIBLE LEDsVISIBLE LEDs

• max. sensitivity of the eye: at 0.555m

• eye response – nearly ‘0’ at the visible spectrum of 0.4 & 0.7m

• Eye only sensitive to light with photon energy hv 1.8eV (or 0.7m) – semicond. of interest must have larger energy bandgap

• See fig.9.6 – the most important materials for visible LEDs: alloy GaAs1-yPy & GaxIn1-xN III-V compound system

• Notation: AxB1-xC or AC1-yDy for ternary (3 elements)

• AxB1-xCyD1-y for quarternary (4 elements)

A & B: group III

C & D: group V

x & y: mole fraction

LIGHT EMITTING DIODE (LED)LIGHT EMITTING DIODE (LED)

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Various band gaps Various band gaps different photon energies different photon energies Ultra violet :GaN 3.4 ev –infra-red: InSb 0.18evUltra violet :GaN 3.4 ev –infra-red: InSb 0.18evTernary&quarternaryTernary&quarternaryincreasing number of increasing number of available energiesavailable energies

xxPGaAs 1

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Figure 9.6. Semiconductors of interest as visible LEDs. Figure includes relative response of the human eye.

VISIBLE LEDs (Cont.)VISIBLE LEDs (Cont.)

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Figure 9.7. (a) Compositional dependence for the direct- and indirect-energy bandgap for GaAs1-yPy. (b) The alloy compositions shown correspond to red (y = 0.4), orange (0.65), yellow (0.85), and green light (1,0).

Example: 0<Y<1 0<Y<0.45 Direct usually 0.4 used for LEDs 0.45<Y<1Indirect

YY PGaAs 1

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Figure 9.8. Quantum efficiency versus alloy composition with and without isoelectronic impurity nitrogen.

Quantum efficiency

• Efficiency without N2 drops sharply in the composition range 0.4<y<0.5 because the Eg changes direct to indirect at y=0.45

• Efficiency with N2 is higher for y>0.5 but decreases with increasing y – caused by the increasing separation between direct & indirect Eg

Indirect Indirect can emit light if we can emit light if we add nitogenadd nitogen..

YY PGaAs 1

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• Fig. 9.9(a) – Direct-Eg LED emits red light, fabricated on GaAs substrate

• Fig. 9.9(b) – Indirect-Eg LED emits orange, yellow or green light, fabricated on GaP substrates

• For high-brightness blue LEDs (0.455-0.492m) – II-VI compounds (ZnSe), III-V (GaN), IV-IV (SiC)

• II-IV based devices – have short lifetimes (not good)

• SiC (indirect Eg) low brightness

• Direct Eg: GaN (Eg=3.44eV), III-V semicond. (AlGaInN)

• High quality GaN has been grown on sapphire (insulator) substrate


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Figure 9.9. Basic structure of a flat-diode LED and the effects of (a) an opaque substrate (GaAs1-yPy) and (b) a transparent substrate (GaP) on photons emitted at the p-n junction.


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Figure 9.10. III-V nitride LED grown on sapphire substrate.

• Blue light originates from the radiative recombination in the GaxIn1-xN region (sandwiched between p-type AlxGa1-xN & n-type GaN – larger Eg)


© S.N. SabkiFigure 9.11. Diagrams of two LED lamps.

• Visible LED can be used for full-color displays, full-color indicators & lamps with high efficiency & high reliability

• LED lamps contains an LED chip & plastic lens (as optical filter & to enhance contrast)


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Figure 9.12. LED display formats for numeric and alphanumeric: (a) 7-segment (numeric); (b) 5 x 7 array (alphanumeric).8

Basic formats for LED displays

• Fig. 9.12(a) – (7 segments) displays no. from 0 to 9

• Fig.9.12 (b) – (5x7 matrix array) dispalys alphanumerics (A-Z & 0-9)

• LEDs are 3 times as efficient as incandescent lamps & can last 10 times longer


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• Application: multicolor, large-area flat panel display (attributes low power consumption & excellent emissive quality with a wide viewing angle

• Fig. 9.13(a) – 2 representative organic semicond. (tris aluminium [AlQ3] & aromatic diamine)

• Fig. 9.13(b) – transparent substrate – transparent anode (ITO) – diamine (hole transport) – AlQ3 (electron transport) – cathode

• Fig. 9.13(c) – electrons are injected from cathode toward heterojunction interface (AlQ3/diamine) – holes are injected from anode toward the interface

ORGANIC LEDsORGANIC LEDs

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Figure 9.13. (a) Organic semiconductors. (b) OLED cross sectional view. (c) Band diagram of an OLED.

ORGANIC LEDs (Cont.)ORGANIC LEDs (Cont.)

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Application 1: opto-isolators (input or control signal is decoupled from the output

• Fig.9.14 – opto-isolator having infrared LED (light source) & photodiode (detector)

• Input signal applied to LED will generate light detected by the photodiode light is converted back to an electrical signal as I that flows thru load resistor

• Application 2: communication system (transmission of an optical signal thru optical fiber)

• Optical fiber – a waveguide at optical frequencies

INFRARED LEDINFRARED LED

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Figure 9.14. An opto-isolator in which an input signal is decoupled from the output signal.

INFRARED LED (Cont.)INFRARED LED (Cont.)

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•The surface-emitting infrared InGaAsP LED – for optical fiber communication

• Light emitted from the central surface area & coupled into the optical fiber

• Heterojunctions –

• increase efficiency (from the confinement of the carriers by the InP higher Eg)

• also serve as an optical window to the emitted radiation (higher Eg – confining layers do not absorb radiation from the lower Eg – emitting region)


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Figure 9.17. Small-area mesa-etched GaInAsP/InP surface-emitting LED structure.


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Ultimate limit on how fast the LED can vary the light output depends on the carrier lifetimesIf I is modulated at angular freq. , the light output P():

21

)0()(

PP

2

1

2

f

21

P(0): light output at =0

: carrier lifetime

The modulation bandwidth f (freq. at which the light output is reduced to

at =0


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© S.N. Sabki CHAPTER 9: PHOTONIC DEVICES CHAPTER 9: PHOTONIC DEVICES