1 / 65

Semiconductor Optics

Semiconductor Optics. Absorption & Gain in Semiconductors: Some Applications Semiconductor Lasers (diode lasers) Low Dimensional Materials: Quantum wells, wires & dots Quantum cascade lasers Semiconductor detectors. More Applications Light Emitters, (including lasers & LEDs)

kibo-hebert
Download Presentation

Semiconductor 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. Semiconductor Optics Absorption & Gain in Semiconductors: Some Applications Semiconductor Lasers (diode lasers) Low Dimensional Materials: Quantum wells, wires & dots Quantum cascade lasers Semiconductor detectors

  2. More Applications • Light Emitters, (including lasers & LEDs) • Detectors, Sensors, Amplifiers, • Waveguides & Switches, • Absorbers & Filters • Nonlinear Optics

  3. Energy Bands One atom 2 interacting atoms N interacting atoms Eg

  4. Insulator Conductor (metals) Semiconductors

  5. Doped Semiconductors p-type n-type

  6. Interband Transitions   nanoseconds in GaAs

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

  8. UV “Bandgap Engineering”

  9. InP GaAs ZnSe

  10. Bandgap “Rules” The Bandgap Increases with decreasing Lattice Constant. The Bandgap Decreases with increasing Temperature.

  11. Interband vs Intraband • 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. C 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 kT = 25 meV at 300 K For holes E f(E) 0.5 1 EF kT

  18. E For filling purposes, the smaller the effective mass the better. Conduction band Valence band

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

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

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

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

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

  24. P-n junction unbiased EF

  25. P-n junction Under forward bias EF

  26. Heterojunction Under forward bias

  27. Homojunction hv N p

  28. Heterojunction waveguide n x

  29. Heterojunction 10 – 100 nm EF

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

  31. Absorption and gain in semiconductor g Eg E 

  32. Absorption (loss) g Eg   Eg

  33. Gain g Eg   Eg

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

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

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

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

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

  39. 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

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

  41. Wavelength selection by grating tuning

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

  43. 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

  44. 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

  45. Strained layer and bandgap engineering Substrate

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

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

  48. 2- dimensional semiconductors: quantum well Example: GaAs/AlGaAs, ZnSe/ZnMgSe Al0.3Ga0.7As GaAs Al0.3Ga0.7As E constant For wells of infinite depth E2 E1 

  49. 2- dimensional semiconductors: quantum well E2c E1c E1v E2v

More Related