1. Advanced laser and led structures, applications
Christian Weiland 4/17/17

2. outline Definition of LEDs and LASERs and what they do
Explanation of the Quantum Well structure
Different types and variants of LEDs and LASERs and their structures
How the structures are made and how they work
Specifications, advantages, disadvantages, applications

3. led P-n junction Electroluminescent
Small, long lifetime, low power consumption
LED home lighting bulbs are now cheaper than fluorescent lamps
Grown epitaxially (liquid or vapor)
Substrate lattice meant to match active layer (GaAs, GaP, InP)
Used for lighting, traffic signals, headlamps, aviation navigation lights

4. Led categories Low Current 2 mA, 2 V Standard
20 mA, V (depending on color)
Ultra-High-Output
20 mA, 2 or 4-5 V (can be seen in direct sunlight)

5. Surface emitting led (SLED)
Light emitted perpendicular to the junction plane
Emits light over a wide angle
High radiance from restricting emission to the active region
Etched well prevents heavy absorption of emitted light
Low thermal impedance in the active regions allows for high radiance emission into the optical fiber
The primary active region, where photons are emitted, has a diameter of µm
250 Mb/s

6. Edge emitting led (ELED)
Light emitted parallel to junction plane
Light confined to a narrow angle – provides better coupling
High radiance
Used in optical communications
Can be used for single mode and multimode fiber systems
Active region is a stripe that runs through the device. The polished ends are called facets.
Light is emitted through the front facet while the back facet is highly reflective.
While ELEDs have a smaller spectral range than SLEDs, they couple more power into the optical fiber and can be modulated up to 400 Mb/s – can couple 5-6 times more power

7. Superluminescent diode (sld)
Edge emitting diode that uses superluminescence
Produces high power and brightness like a laser with the low coherence of an LED
Emission band of nm
Peak wavelength and intensity depend on material composition andinjection current
Used include medical imaging and gyroscope componenets

8. Organic LEd OLED – digital displays AMOLED – better size, resolution
PHOLED - phosphorescent

9. Laser diode
Laser diodes are semiconductor diodes that are pumped with current at the p-n junction
Direct band gap semiconductor
Used for laser pointers, DVD readers, optical communications, barcodereaders, laser surgery, DNA sequencing technology
Functionality of a specific laser type depends on its qualities. For example, high coherency is good for distance measurement and narrow spectral qualities are good for telecommunications and spectroscopy
Common materials include GaAs, AlGaAs, GaP, InGaP, GaN, InGaAs, GaInAs, InP, GaInP

10. Quantum well Potential well with energy levels
Thin layer confining electrons and holes
Material embedded between two semiconductor layers of a larger band gap
Typically 5-20 nm thick
Fabricated with MBE or MOVPE
Used as waveguides in laser diodes
Multiple can be used for high optical gain

11. Double heterostructure laser
The active region, where electrons and holes coexist, is confined to the thin middle region – increased amplification
Light is reflected from the heterojunction, meaning it is confined to where amplification occurs

12. Quantum well laser
Highly efficient because electrons are concentrated in energy states that contribute to the laser
Wavelength of light determined by the width of the active region, not just the band gap
Even better efficiency with quantum wire and quantum dots

13. Quantum cascade laser Laser consisting of quantum well heterojunctions
Achieves laser emission through intersubband transition
Light as amplified at long wavelengths which is affected by the thickness of the quantum well layer
Mid-infrared produced by the laser is helpful for imaging and detection of things like explosives and toxins

14. Interband cascade laser
Mid-infrared region of the electromagnetic spectrum
Made from layers of InAs, GaSb, and AlSb
Uses interband transitions instead of intersubband transitions
Used for spectroscopic sensing

15. Separate confinement heterostructure laser
Quantum wells cannot confine light effectively
Two more layers with a lower refractive index are added
This diode is the type of almost every laser that is commercially available

16. Distributed bragg reflector laser
Single frequency laser
Has an optical cavity with a gain region between two mirrors that provide feedback
One mirror is wavelength selective with a diffraction grating and high reflectivity

17. Distributed feedback laser
Type of quantum cascade laser
Single frequency laser
Diffraction grating close to the p-n junction that filters out a single wavelength and feeds it back to the active region
Facets are AR coated because reflection is unnecessary
Used in optical communications
Threshold Current – 11 mA
Bias Current – 50 mA

18. vcsel Single frequency Optical cavity in the direction of current flow
Short active region length so that light emits from the surface of the cavity
VCSELs can be tested on the wafer which reduces fabrication costs
Wavelength typically nm – can be longer with InP active region which is good but high reflectivity causes lower output power than edge emitting lasers
Low power consumption and long lifetime because facets can’t be damaged by intensity but low power production due to high reflectivity
Used in spectroscopy, laser printing, and computer mice

19. vecsel
No p-n junction
One of the mirrors is on the outside of the structure, 1 cm from the diode
Semiconductor gain region is less than 100 nm which is much smaller than other semiconductor lasers
The waveguiding provided by the short propagation length cannot be provided by edge-emitting lasers
Industrial machines use them for their high power output

20. External-cavity diode laser
Wavelength can be tuned by rotating the diffraction grating
Usually are heterostructure diodes made of AlGaAs
Output beam is in a fixed direction but diffraction reduces the power output
Used as a data transmitter for optical communications

21. summary LEDs and LASERs are either edge-emitting or surface-emitting
Epitaxy is an important process in the fabrication of optoelectronics
Generally, LEDs emit low power light incoherently while LASERs emit high power light in a coherent beam
Light is usually emitted from the p-n junction of these devices
Quantum wells are a type of heterostructure crucial to the design of LASERs
Wavelength and power output are important factors for decided how to use an LED or LASER

22. references https://www.rp-photonics.com/semiconductor_lasers.html
STRUK_31_10.pdf
diodes/structure-fabrication.php
Unit-3-LED_Structures.pdf
good-idea

23. Key points
Edge-emitting diodes emit light perpendicular to the junction plane and produce high output power while surface-emitting diodes emit light parallel to the junction plane and have a high range of wavelength
Epitaxy is the deposition of overlayer onto a substrate that can be done either chemically or physically
Stimulated emission occurs when a photon at a certain frequency interacts with an excited electron and drops to a lower energy level
The thinnest LED in existence is 3 atoms thick
A heterojunction involves a semiconductor material being in contact with another material of a different band gap