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9. Semiconductors Optics

9. Semiconductors Optics. Absorption and gain in semiconductors Principle of semiconductor lasers (diode lasers) Low dimensional materials: Quantum wells, wires and dots Quantum cascade lasers Semiconductor detectors. Semiconductors Optics. Semiconductors in optics:

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9. Semiconductors Optics

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  1. 9. Semiconductors Optics • Absorption and gain in semiconductors • Principle of semiconductor lasers (diode lasers) • Low dimensional materials: • Quantum wells, wires and dots • Quantum cascade lasers • Semiconductor detectors

  2. Semiconductors Optics • Semiconductors in optics: • Light emitters, including lasers and LEDs • Detectors • Amplifiers • Waveguides and switches • Absorbers and filters • Nonlinear crystals

  3. The energy bands One atom Two interacting atoms N interacting atoms Eg

  4. Insulator Conductor (metals) Semiconductors

  5. Doped semiconductor p-type n-type

  6. Interband transistion   nanoseconds in GaAs

  7. Intraband transitions   < ps in GaAs n-type

  8. UV Optical fiber communication

  9. InP GaAs ZnSe

  10. Bandgap rules The bandgap increases with decreasing lattice constant. The bandgap decreases with increasing temperature.

  11. Interband vs Intraband C • Interband: • Most semiconductor devices operated based on the interband transitions, namely between the conduction and valence bands. • The devices are usually bipolar involving a p-n junction. V • Intraband: • A new class of devices, such as the quantum cascade lasers, are based on the transitions between the sub-bands in the conduction or valence bands. • The intraband devices are unipolar. • Faster than the intraband devices C

  12. Interband transitions E Conduction band k Valence band

  13. E Conduction band Eg k Valence band Examples: mc=0.08 me for conduction band in GaAs mc=0.46 me for valence band in GaAs

  14. Direct vs. indirect band gap k k GaAs AlxGa1-xAs x<0.3 ZnSe Si AlAs Diamond

  15. Direct vs. indirect band gap Direct bandgap materials: Strong luminescence Light emitters Detectors Direct bandgap materials: Weak or no luminescence Detectors

  16. Fermi-Dirac distribution function E 0.5 1 EF f(E)

  17. Fermi-Dirac distribution function For electrons For holes E 0.5 1 EF kT f(E) kT=25 meV at 300 K

  18. Fermi-Dirac distribution function For electrons For holes E f(E) 0.5 1 EF kT kT=25 meV at 300 K

  19. E Conduction band Valence band

  20. E Conduction band Valence band For filling purpose, the smaller the effective mass the better.

  21. Where is the Fermi Level ? E Conduction band n-doped Intrinsic Valence band P-doped

  22. Interband carrier recombination time (lifetime) ~ nanoseconds in III-V compound (GaAs, InGaAsP) ~ microseconds in silicon Speed, energy storage,

  23. Quasi-Fermi levels E E E Ef e Immediately after Absorbing photons Returning to thermal equilibrium Ef h

  24. E fe # of carriers EF e x = EF h

  25. E Condition for net gain >0 EF c Eg EF v

  26. P-n junction unbiased EF

  27. P-n junction Under forward bias EF

  28. Heterojunction Under forward bias

  29. Homojunction hv N p

  30. Heterojunction waveguide n x

  31. Heterojunction 10 – 100 nm EF

  32. Heterojunction A four-level system 10 – 100 nm Phonons

  33. Absorption and gain in semiconductor g Eg E 

  34. Absorption (loss) g Eg   Eg

  35. Gain g Eg   Eg

  36. Gain at 0 K Eg EFc-EFv g EFc-EFv Eg   Density of states

  37. Gain and loss at 0 K g EF=(EFc-EFv) Eg E=hv 

  38. Gain and loss at T=0 K at different pumping rates g EF=(EFc-EFv) Eg E N1 N2 >N1 

  39. Gain and loss at T>0 K laser g Eg N2 >N1 N1 E 

  40. Gain and loss at T>0 K Effect of increasing temperature laser g Eg N2 >N1 N1 E At a higher temperature 

  41. A diode laser Larger bandgap (and lower index ) materials <0.2m p n <0.1 mm Substrate Cleaved facets w/wo coating Smaller bandgap (and higher index ) materials <1 mm

  42. Wavelength of diode lasers • Broad band width (>200 nm) • Wavelength selection by grating • Temperature tuning in a small range

  43. Wavelength selection by grating tuning

  44. A distributed-feedback diode laser with imbedded grating <0.2m p n Grating

  45. Typical numbers for optical gain: Gain coefficient at threshold: 20 cm-1 Carrier density: 10 18 cm-3 Electrical to optical conversion efficiency: >30% Internal quantum efficiency >90% Power of optical damage 106W/cm2 Modulation bandwidth >10 GHz

  46. Semiconductor vs solid-state Semiconductors: • Fast: due to short excited state lifetime ( ns) • Direct electrical pumping • Broad bandwidth • Lack of energy storage • Low damage threshold Solid-state lasers, such as rare-earth ion based: • Need optical pumping • Long storage time for high peak power • High damage threshold

  47. Strained layer and bandgap engineering Substrate

  48. Density of states 3-D (bulk) E 

  49. Low dimensional semiconductors When the dimension of potential well is comparable to the deBroglie wavelength of electrons and holes. Lz<10nm

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