Light Emitting Diodes

1. Light Emitting Diodes EE 698A Kameshwar Yadavalli

2. Outline • Basics of Light Emitting Diodes (Electrical) • Basics of Light Emitting Diodes (Optical) • High internal efficiency designs • High extraction efficiency structures • Visible Spectrum LED’s • White-Light LED’s • The promise of solid state lighting

3. From Light-Emitting Diodes, Fred Schubert. PN junction diode in forward bias, the electron-hole recombination leads to photon emission I = Is(eeV/kT-1) Threshold voltage Vth = Eg/e I = IseeV/ηkT where η is the ideality factor LED-Electrical Properties-PN junctions Double Heterostructure is used to confine the carriers, improving the radiative recombination rate

4. LED-Electrical Properties-Hetero junctions Grading of the heterojunction is done to reduce the resistance seen by carriers From Light-Emitting Diodes, Fred Schubert.

5. LED-Electrical Properties-Hetero junctions From Light-Emitting Diodes, Fred Schubert.

6. LED-Electrical Properties-Carrier loss • The confinement barriers are typically several hundred meV (>>kT) • Due to Fermi-Dirac distribution of carriers in the active region, some carriers will have energy higher than that of the barriers • In AlGaAs/GaAs and AlGaN/GaN the barriers are high • In AlGaInP/GaInP the barriers are lower resulting in higher leakage currents From Light-Emitting Diodes, Fred Schubert.

7. LED-Electrical Properties-Blocking layers • Electron Blocking Layers are required to prevent electron escape at high injection current densities From Light-Emitting Diodes, Fred Schubert.

8. LED-Optical Properties-Efficiency • ηint = # of photons emitted from active region per second # of electrons injected in to LED per second = Pint / (hν) I / e • ηextr = # of photons emitted into free space per second # of photons emitted from active region per second = P / (hν) Pint / (hν) From Light-Emitting Diodes, Fred Schubert.

9. LED-Optical Properties-Emission Spectrum • The linewidth of an LED emitting in the visible range is relatively narrow compared with the entire visible range (perceived as monochromatic by the eye) • Optical fibers are dispersive, limiting the bit rate X distance product achievable with LED’s • Modulation speeds achieved with LED’s are 1Gbit/s, as the spontaneous lifetime of carriers in LED’s is 1-100 ns From Light-Emitting Diodes, Fred Schubert.

10. LED-Optical Properties-Light Escape Cone • Total internal reflection at the semiconductor air interface reduces the external quantum efficiency. • The angle of total internal reflection defines the light escape cone. sinθc = nair/ns • Area of the escape cone = 2πr2(1-cosθc) • Pescape / Psource = (1-cosθc)/2 = θc2/4 = (nair2/ns2)/4 From Light-Emitting Diodes, Fred Schubert.

11. Light intensity in air (Lambertian emission pattern) is given by Iair = (Psource/4πr2) X (nair2/ns2) cosΦ Index contrast between the light emitting material and the surrounding region leads to non-isotropic emission pattern LED-Optical Properties-Emission Spectrum From Light-Emitting Diodes, Fred Schubert.

12. LED-Optical Properties-Epoxy encapsulants • Light extraction efficiency can be increased by using dome shaped encapsulants with a large refractive index. • Efficiency of a typical LED increases by a factor of 2-3 upon encapsulation with an epoxy of n = 1.5. • The dome shape of the epoxy implies that light is incident at an angle of 90o at the epoxy-air interface. Hence no total internal reflection. From Light-Emitting Diodes, Fred Schubert.

13. Temperature dependence of emission intensity • Emission intensity decreases with increasing temperature. • Causes include non-radiative recombination via deep levels, surface recombination, and carrier loss over heterostucture barriers. From Light-Emitting Diodes, Fred Schubert.

14. High internal efficiency LED designs • Radiative recombination probability needs to be increased and non-radiative recombination probability needs to be decreased. • High carrier concentration in the active region, achieved through double heterostructure (DH) design, improves radiative recombination. R=Bnp • DH design is used in all high efficiency designs today. From Light-Emitting Diodes, Fred Schubert.

15. High internal efficiency designs • Doping of the active regions and that of the cladding regions strongly affects internal efficiency. • Active region should not be heavily doped, as it causes carrier spill-over in to the confinement regions decreasing the radiative efficiency • Doping levels of 1016-low 1017 are used, or none at all. • P-type doping of the active region is normally done due to the larger electron diffusion length. • Carrier lifetime depends on the concentration of majority carriers. • In low excitation regime , the radiative carrier lifetime decreases with increasing free carrier concentration. • Hence efficiency increases with doping. • At high concentration, dopants induce defects acting as recombination centers. From Light-Emitting Diodes, Fred Schubert.

16. P-N junction displacement • Displacement of the P-N junction causes significant change in the internal quantum efficiency in DH LED structures. • Dopants can redistribute due to diffusion, segregation or drift. From Light-Emitting Diodes, Fred Schubert.

17. Doping of the confinement regions • Resistivity of the confinement regions should be low so that heating is minimal. • High p-type conc. in the cladding region keeps electrons in the active region and prevents them from diffusing in to the confinement region. • Electron leakage out of the active region is more severe than hole leakage. From Light-Emitting Diodes, Fred Schubert.

18. Non radiative recombination • The concentration of defects which cause deep levels in the active region should be minimum. • Also surface recombination should be minimized, by keeping all surfaces several diffusion lengths away from the active region. • Mesa etched LEDs and lasers where the mesa etch exposes the active region to air, have low internal efficiency due to recombination at the surface. • Surface recombination also reduces lifetime of LEDs. From Light-Emitting Diodes, Fred Schubert.

19. Lattice matching • Carriers recombine non-radiatively at misfit dislocations. • Density of misfit dislocation lines per unit length is proportional to lattice mismatch. • Hence the efficiency of LED’s is expected to drop as the mismatch increases. From Light-Emitting Diodes, Fred Schubert.

20. High extraction efficiency structures • Shaping of the LED die is critical to improve their efficiency. • LEDs of various shapes; hemispherical dome, inverted cone, truncated cones etc have been demonstrated to have better extraction efficiency over conventional designs. • However cost increases with complexity. From Light-Emitting Diodes, Fred Schubert.

21. The TIP LED employs advanced LED die shaping to minimize internal loss mechanisms. The shape is chosen to minimize trapping of light. TIP LED is a high power LED, and the luminous efficiency exceeds 100 lm/W. TIP devices are sawn using beveled dicing blade to obtain chip sidewall angles of 35o to vertical. High extraction efficiency structures Krames et. al, Appl. Phys. Lett., Vol. 75, No. 16, 18 October 1999

22. Visible spectrum LEDs The plot charts the gains made in luminous efficiency till date. From Light-Emitting Diodes, Fred Schubert.

23. Visible spectrum LEDs • The emission spectrum of the blue, green and red LEDs indicate that the green LED has a wider spectrum. • Alloy broadening leads to spectral broadening that is greater than 1.8 kT linewidth. From Light-Emitting Diodes, Fred Schubert.

24. White-light LEDs • White light can be generated in several different ways. • One way is to mix to complementary colors at a certain power ratio. • Another way is by the emission of three colors at certain wavelengths and power ratio. • Most white light emitters use an LED emitting at short wavelength and a wavelength converter. • The converter material absorbs some or all the light emitted by the LED and re-emits at a longer wavelength. • Two parameters that are important in the generation of white light are luminous efficiency and color rendering index. • It is shown that white light sources employing two monochromatic complementary colors result in highest possible luminous efficiency. From Light-Emitting Diodes, Fred Schubert.

25. White-light LEDs • Wavelength converter materials include phosphors, semiconductors and dyes. • The parameters of interest are absorption wavelength, emission wavelength and quantum efficiency. • The overall energy efficiency is given by η = ηext(λ1/ λ2) • Even if the external quantum efficiency is 1, there is always an energy loss associated with conversion. • Common wavelength converters are phosphors, which consist of an inorganic host material doped with an optically active element. • A common host is Y3Al5O12. • The optically active dopant is a rare earth element, oxide or another compound. • Common rare earth elements used are Ce, Nd, Er and Th. From Light-Emitting Diodes, Fred Schubert.

26. White-light LEDs • Phosphors are stable materials and can have quantum efficiencies of close to 100%. • Dyes also can have quantum efficiencies of close to 100%. • Dyes can be encapsulated in epoxy or in optically transparent polymers. • However, organic dyes have finite lifetime. They become optically inactive after 104-106 optical transitions. From Light-Emitting Diodes, Fred Schubert.

27. White LEDs based on phosphor converters A blue GaInN/GaN LED and a phosphor wavelength converter suspended in a epoxy resin make a white Light LED. The thickness of the phosphor containing epoxy and the concentration of the phosphor determine the relative strengths of the two emission bands From Light-Emitting Diodes, Fred Schubert.

28. Promise of Solid State Lighting • The use of solid state lighting devices promises huge savings in energy consumption. • The electricity for lighting needs is 60GW, over 24 hrs. • About 24 GWyear is consumed by incandescent lamps with a luminous intensity of 15lm/W. • 36 GWyear is consumed by FL/HID lamps with a luminous intensity of 75lm/W. • Assuming that by year 2020, they are replaced by LEDs with luminous intensity of 150 lm/W, energy savings are 40 GWyear. • That translates to $40 billion in savings. • At 4Mtons / GWyear of coal consumption, net savings lead to 25% less coal consumption, leading to lesser emissions of green house gases. • Global savings are projected to be about $140B. Roland Haitz, Adv. in Solid State Physics