1 / 67

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:

Télécharger la présentation

9. Semiconductors Optics

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  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

More Related