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Blue Semiconductor Lasers

Guest Lecture for ScIT. Blue Semiconductor Lasers. Leo J. Schowalter Physics, Applied Physics & Astronomy Department Rensselaer Polytechnic Institute. Topics. Why the Interest? What is a semiconductor? Metals, insulators and semiconductors How big a band gap energy?

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Blue Semiconductor Lasers

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  1. Guest Lecture for ScIT Blue Semiconductor Lasers Leo J. Schowalter Physics, Applied Physics & Astronomy Department Rensselaer Polytechnic Institute

  2. Topics • Why the Interest? • What is a semiconductor? • Metals, insulators and semiconductors • How big a band gap energy? • How does a semiconductor laser work? • Other Applications for Wide band gaps • What is the Future?

  3. Why the Interest?

  4. Importance of new semiconductor materials and devices for modern civilization Paul Romer (1990s) The wealth is created by innovations and inventions, such as computer chips. 106 - 107 MOSFETs per person in Western World Electronics industry is now the largest industry in the US

  5. Impact Displays Avionics and defense Automotive industry Information technology Solid state lighting Traffic lights Wireless communications Electric power industry Health care

  6. The Market for GaN Devices After Strategies Unlimited (1997) % of Compound Semiconductor market Nichia estimates that the LD market alone will be worth $10B.

  7. Laser Diode Market • Optical Data Storage Market will use over 300M LDs in 1999 (Compound Semicond., March 1999) • HD-DVD will use GaN or SHG laser; will dominate future market with 15GB capacity or greater • Market expects laser cost to be approx. $10.

  8. What is a semiconductor? • Metals • Many free electrons not tied up in chemical bonds • Insulators • All electrons (in intrinsic material) tied up in chemical bonds

  9. Crystal (Perfect)

  10. Crystal (Excited)

  11. Crystal (Excited)

  12. Band Gap Energy Conduction Band Band Gap Energy Eg (Minimum Energy needed to break the chemical bonds) Valence Band Position

  13. Band Gap Energy Conduction Band photon in Valence Band Position

  14. photon out Band Gap Energy Conduction Band Valence Band Position

  15. photon out Band Gap Energy Conduction Band Valence Band Position

  16. +5 +5 Crystal (Doped n-type) Plus a little energy, d.

  17. Crystal (Doped p-type) +3

  18. Crystal (Doped p-type) +3

  19. Doped Semiconductors Energy • Put them together? donor level acceptor level p-type n-type

  20. p-type p-n junction Energy + + - - - - - - + + + + + + - - n-type depleted region (electric field)

  21. p-type p-n junction Energy Vo + + - - - - - - + + + + + + - - n-type depleted region (electric field)

  22. What happens if a bias is applied?

  23. p-type Biased junction Negative bias positive bias n-type depleted region (electric field)

  24. photon out p-type Biased junction Negative bias n-type depleted region (electric field)

  25. a Philips Lighting and Agilent Technologies joint venture that's changing the future of light. In the next century, LED-based lighting will quickly replace conventional lighting in a wealth of commercial, industrial and consumer applications. LumiLeds‘ LED-based solutions will bring irresistible value to lighting solutions of all kinds, earning us a leadership position in a fast-growing and lucrative marketplace. Our long-lasting, energy-efficient products will also improve the planet, by reducing waste and power consumption.

  26. How does a semiconductor laser work?

  27. Absorption and Emission E 1 photon in photon out E o

  28. Stimulated vs. Spontaneous Emission • We can now derive the ratio of the emission rate versus the absorption rate using the equilibrium concentrations of photons and excited atoms: • Derived in 1917 by Einstein. Required stimulated emission. However, a “real” understanding of this was not achieved until the 1950’s.

  29. Laser needs a Population Inversion

  30. photon out p-type Biased junction Negative bias n-type depleted region (electric field)

  31. History of Lasers • First operating Laser in 1960 (Maser in 1958) • Simulated emission concept from Einstein in 1905 • Townes (1964) and Schawlow (1981) • First semiconductor injection Laser in 1962 • First was Robert Hall (GE) but many competing groups • Year before he had argued it was impossible

  32. Violet Laser Diode Currently costs about $2000 apiece!

  33. Nichia Laser Diode 10,000 hours operation! 10 mW CW 405 nm Epitaxial Lateral Overgrowth material

  34. Substrate Comparison • Sapphire: poor crystal structure match, large thermal expansion mismatch, poor thermal conductivity. • SiC has high thermal conductivity and close lattice match in the c-plane. • But, also has: a different c-axis, relatively large thermal expansion mismatch and chemical mismatch at the interface. • GaN and AlNbulk crystals have the same crystal structure, excellent chemical match, high thermal conductivity, and the same thermal expansion but are difficult to produce presently(but this will change!) • LEO and HVPE GaN films allow fabrication of “quasi-bulk” substrates. Temporary solution until bulk substrates become available?

  35. 15 mm Diameter AlN Boule

  36. How information is stored on a DVD disc

  37. Other Applications for Wide band gaps • High Power devices • Large band gap allows semiconductor to be used at high voltages • Generally larger band gap means stronger bonds so material can withstand higher currents and temperatures • High Temperature devices • Much smaller effect of thermal excitation of carriers • Tougher material

  38. Conclusions • Very intense and fast moving field • Physicists are making major contributions • Lots more to do • Very broad applications but information storage is one of the biggest.

  39. Questions • 1. We all know that lasers, such as semiconductor lasers, are initially developed for more scientific needs than we are privy to. However, what practical applications might we see from a newly developed semiconductor in devices that we would be able to relate to, such as CD players, DVD players, and the like? What about the coveted "blue laser"? • 2. What is an area where semiconductor lasers aren't being used at the moment, but could be employed in the future? • 3. I would like to know if Dr. Schowalter thinks the semiconductor use of lasers will ever replace magnetic storage devices as our primary source of permanent storage. • 4. What do you believe that next step will be in semiconductor laser development? What other possible uses are being considered? • 5. I would like you to ask the guest lecturer Dr. Schowalter, if there is an eventual limit to the power the lasers will be able to have in the future. • Meaning how far they will go and with what strength.

  40. Questions (cont.) 6. How feasible is it to have a CD-ROM or DVD drive the can read from the top and bottom of the disk at the same time? how would new laser technology affect the answer? 7. Is there any problem or difficulty in making wave lengths smaller to put more data into DVD or CD? 8. What is the next innovation for lasers in the world of entertainment? 9. What is the next innovation that lasers will bring into our homes? 10. What do you see as the next technology that will surpass the laser and CD/DVD technology in data storage in the near future? 11. Do you think there will ever be a push for ultraviolet lasers to use in storage?

  41. Stimulated vs. Spontaneous Emission • Time invariant laws of Physics imply that the rate of absorption must be equal to the rate of spontaneous emission. • Thus, if there was no stimulated emission, population levels of the two energies would be equal. • Principal of detailed balance says: • Minimum packet of energy (photon) that light can have at a particular frequency  is h (Plank’s constant, 1901).

  42. GaN AlN 4H-SiC 6H-SiC Sapphire Crystal Structure hexagonal hexagonal Hexagonal (4H) Hexagonal (6H) rhombohedral ( 2H) ( 2H) 3.39 6.2 3.26 9.9 Band Gap (eV) 3.03 o a=3.189 Lattice Constant(A) c=5.185 Thermal Conductivity (W/cm-K) 0.35 3.2 4.9 4.9 1.7 a=3.111 a=3.073 a=3.081 a=4.76 c=4.978 c=10.053 c=15.117 c=12.99 Substrate Alternatives for Nitride Epitaxy • Sapphire: poor crystal structure match, large thermal expansion mismatch, poor thermal conductivity. • SiC has high thermal conductivity and close lattice match in the c-plane. • But, also has: a different c-axis, relatively large thermal expansion mismatch and chemical mismatch at the interface. • GaN and AlNbulk crystals have the same crystal structure, excellent chemical match, high thermal conductivity, and the same thermal expansion but are difficult to produce presently (will this change?). • LEO and free-standing GaN films more expensive than bulk crystal substrates.

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