Functional Materials. Optoelectronic devices

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Phebe Whitehead
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1 Functional Materials Lecture 2: Optoelectronic materials and devices (inorganic). Photonic materials Optoelectronic devices Light-emitting diode (LED) displays Photodiode and Solar cell Photoconductive materials Photodiodes: p/n-junction in reverse bias or p-i-n junction photodetectors Solar cell electricity source Diode lasers 1

Photo- vs Electroluminescence Photoluminescence:

Emission energy lower than absorption energy.

LEDs Light-emitting diodes A semiconductor diode

electrically biased in the forward direction of

2 Photo- vs Electroluminescence Photoluminescence: Emission of light caused by absorption of light. Fluorescence. Electroluminescence: Emission of light caused by application of an electric potential. Solid inorganic semiconductors show photoluminescence at low temperatures. Molecules can fluoresce also at room temperature. Emission energy lower than absorption energy.(longer wavelength). 1. LEDs Light-emitting diodes A semiconductor diode that emits narrow-spectrum light when electrically biased in the forward direction of the p-n junction. This effect is electroluminescence. The colour of the emitted light depends on the bandgap of the semiconducting material used (tuning of composition). 2

LED materials Direct bandgap materials Mainly III-V semiconductors,

n-algaas/gaas/p-algaas, n-inp/p-ingaasp/p-inp heterojunctions

Blue: InGaN based on p-gan (Shuji Nakamura of Nichia Corporation,

3 LED materials Direct bandgap materials Mainly III-V semiconductors, later also other semiconductors Infrared: data transmission n-algaas/gaas/p-algaas, n-inp/p-ingaasp/p-inp heterojunctions Visible: displays Red: GaAsP (Nick Holonyak (1962) ) Green: AlGaP Blue: InGaN based on p-gan (Shuji Nakamura of Nichia Corporation, 1990 s) Charge injection edge-emitting surface-emitting E direct vs indirect bandgap 3

White LEDs White: tv, lighting Colour mixing (RGB): stability, dynamic control control Phosphors: blue InGaN/GaN LED structure with Y 3 Al 5 O 12 :Ce ("YAG ) phosphor coating on top, to mix yellow

4 White LEDs White: tv, lighting Colour mixing (RGB): stability, dynamic control control Phosphors: blue InGaN/GaN LED structure with Y 3 Al 5 O 12 :Ce ("YAG ) phosphor coating on top, to mix yellow light with the blue by down-conversion (1996). Near-UV LED with europium-based red and blue emitting phosphors plus green emitting ZnS:Cu, Al. Homoepitaxially grown ZnSe on a ZnSe substrate emits simultaneously blue light from its active region and yellow light from the substrate (company Sumitomo). Blue, green, and red LEDs; These can be combined to produce any color, including white light. Properties of LEDs Advantages: More light per watt than incandescent light bulbs best luminous efficacy for LED: lm/w. a conventional watt incandescent light bulb: ~15 lm/w standard fluorescent lights: up to 100 lm/w. Low power consumptions (on-set voltage only 2-4 V), small No colour filters needed Suitable for dimming and frequent on/off switching Wide viewing angle (compared to LCD) important for displays Disadvantages More expensive Requires correct current supply Temperature sensitive Spectrum limited 4

5 Why no silicon for LEDs? Very low photoluminescence (IR) Radiative recombination slow in silicon (indirect bandgap) Disordered form of silicon Isoelectric doping Sponge structure Porous silicon: network of fine Si wires. Confinement visible PL and EL, colour tuning. All-silicon optoelectronics? To read: Light from silicon by Coffa, S. ieee.org/print/1886 Lasers as light sources coherent monochromatic light stimulated emission: emission induced by exposing species to same wavelength light as will be emitted. cavity: light reflects back and forth and stays in the emitting material. population inversion: large number of excited species. Avalanche process: radiative decay > burst of light Usually optical pulse to create the population inversion. Usually bulky 5

technology Edge-emitting Charge injection E - - - + + + Ex.

6 Diode lasers Based on semiconductors (III-V) Population inversion created by injection of electrons and holes. Structure is a pn-junction. Progress Band gap engineering color tuning Quantum-well structures, heterojunctions Blue (II-VI, GaN) Epitaxial growth, thickness control (confinement) Integration with Si technology Edge-emitting Charge injection E Ex. n-algaas/gaas/p-algaas Quantum well lower bandgap material Laser development Surface-emitting microlasers Photolithography manufacture arrays of lasers in a plane Quantum Cascade laser Periodic series of thin layers of varying material composition (superlattice) Intersubband transitions Colour determined by physical dimensions instead of bandgap. 6

$Photonic materials Electrons photons Glass optical fiber with cladding (low refractive index) Long distance$ Decrease in intensity due to scattering and absorption high quality silica Infrared signals silica: λ up to 2.

Decrease in intensity due to scattering and absorption high quality silica Infrared signals silica: λ up to 2.

7 Photonic materials Electrons photons Glass optical fiber with cladding (low refractive index) Long distance voice, video, data transfer x faster than copper wire and higher capacity. Decrease in intensity due to scattering and absorption high quality silica Infrared signals silica: λ up to 2.5 µm ZrF 4 : λ up to 8 µm Crystalline ZrF 4 fibers for short distance communication Fiber optics n core > n cladding Minimal absorption 1.3 µm (IR). 7

8 Photonic materials Problems and development of the technology Smearing out of signals (dispersion) due to photons with steeper paths in the fibers (slower). Stepped-index fibers graded-index fibers Optical amplifiers (repeaters): doping with luminescent La, Er, which fluorescence in infrared + modulation with the input signal amplification of optical signal Polymer fibers (PMMA): cheaper but optically lower quality Several signals simultaneously without mixing up (multiplexing). Future challenges Integration with electronics, conversion both ways. So far: speed limiting factor. All-optical networks. Photonic transistor? 8

Luminous Equivalent of Radiation

Intensity vs λ Luminous Equivalent of Radiation When the spectral power (p(λ) for GaP-ZnO diode has a peak at 0.69µm) is combined with the eye-sensitivity curve a peak response at 0.65µm is obtained with