1. Wide Band-gap Semiconductor Group/Rensselaer Blue Semiconductor Lasers Leo J. Schowalter Physics, Applied Physics & Astronomy Department Rensselaer Polytechnic Institute Guest Lecture for ScIT

2. Wide Band-gap Semiconductor Group/Rensselaer 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. Wide Band-gap Semiconductor Group/Rensselaer Why the Interest?

4. Wide Band-gap Semiconductor Group/Rensselaer 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. 10 6 - 10 7 MOSFETs per person in Western World Electronics industry is now the largest industry in the US

5. Wide Band-gap Semiconductor Group/Rensselaer Impact Automotive industry Avionics and defense Traffic lights Solid state lighting Electric power industry Health care Information technology Wireless communications Displays

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

7. Wide Band-gap Semiconductor Group/Rensselaer 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. Wide Band-gap Semiconductor Group/Rensselaer What is a semiconductor? Metals pMany free electrons not tied up in chemical bonds Insulators pAll electrons (in intrinsic material) tied up in chemical bonds

9. Wide Band-gap Semiconductor Group/Rensselaer Crystal (Perfect)

10. Wide Band-gap Semiconductor Group/Rensselaer Crystal (Excited)

11. Wide Band-gap Semiconductor Group/Rensselaer Crystal (Excited)

12. Wide Band-gap Semiconductor Group/Rensselaer Band Gap Valence Band Conduction Band Band Gap Energy E g (Minimum Energy needed to break the chemical bonds) Energy Position

13. Wide Band-gap Semiconductor Group/Rensselaer Band Gap Valence Band Conduction Band Energy Position photon in

14. Wide Band-gap Semiconductor Group/Rensselaer Band Gap Valence Band Conduction Band Energy Position photon out

15. Wide Band-gap Semiconductor Group/Rensselaer Band Gap Valence Band Conduction Band Energy Position photon out

16. Wide Band-gap Semiconductor Group/Rensselaer Crystal (Doped n-type) Plus a little energy,  d.

17. Wide Band-gap Semiconductor Group/Rensselaer Crystal (Doped p-type) +3

18. Wide Band-gap Semiconductor Group/Rensselaer Crystal (Doped p-type) +3

19. Wide Band-gap Semiconductor Group/Rensselaer Doped Semiconductors Energy n-type p-type donor level acceptor level Put them together?

20. Wide Band-gap Semiconductor Group/Rensselaer p-n junction n-type p-type Energy depleted region (electric field) ++++++ + + ----- - - -

21. Wide Band-gap Semiconductor Group/Rensselaer p-n junction n-type p-type Energy depleted region (electric field) ++++++ + + ----- - - - VoVo

22. Wide Band-gap Semiconductor Group/Rensselaer What happens if a bias is applied?

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

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

25. Wide Band-gap Semiconductor Group/Rensselaer 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. Wide Band-gap Semiconductor Group/Rensselaer How does a semiconductor laser work?

27. Wide Band-gap Semiconductor Group/Rensselaer Absorption and Emission E o E 1 photon out photon in

28. Wide Band-gap Semiconductor Group/Rensselaer 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: l Derived in 1917 by Einstein. Required stimulated emission. However, a “real” understanding of this was not achieved until the 1950’s.

29. Wide Band-gap Semiconductor Group/Rensselaer Laser needs a Population Inversion

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

31. Wide Band-gap Semiconductor Group/Rensselaer History of Lasers First operating Laser in 1960 (Maser in 1958) pSimulated emission concept from Einstein in 1905 pTownes (1964) and Schawlow (1981) First semiconductor injection Laser in 1962 pFirst was Robert Hall (GE) but many competing groups pYear before he had argued it was impossible

32. Wide Band-gap Semiconductor Group/Rensselaer Violet Laser Diode Currently costs about $2000 apiece!

33. Wide Band-gap Semiconductor Group/Rensselaer Nichia Laser Diode Epitaxial Lateral Overgrowth material 10,000 hours operation! 10 mW CW 405 nm

34. Wide Band-gap Semiconductor Group/Rensselaer 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. the same crystal structure, excellent chemical match, high thermal conductivity, and the same thermal expansion  GaN and AlN bulk 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. Wide Band-gap Semiconductor Group/Rensselaer 15 mm Diameter AlN Boule

36. Wide Band-gap Semiconductor Group/Rensselaer

37. How information is stored on a DVD disc

38. Wide Band-gap Semiconductor Group/Rensselaer Other Applications for Wide band gaps l High Power devices pLarge band gap allows semiconductor to be used at high voltages pGenerally larger band gap means stronger bonds so material can withstand higher currents and temperatures l High Temperature devices pMuch smaller effect of thermal excitation of carriers pTougher material

39. Wide Band-gap Semiconductor Group/Rensselaer 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.

40. Wide Band-gap Semiconductor Group/Rensselaer 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.

41. Wide Band-gap Semiconductor Group/Rensselaer 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?

42. Wide Band-gap Semiconductor Group/Rensselaer 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: Stimulated vs. Spontaneous Emission Minimum packet of energy (photon) that light can have at a particular frequency is h (Plank’s constant, 1901).

43.  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. the same crystal structure, excellent chemical match, high thermal conductivity, and the same thermal expansion  GaN and AlN bulk 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. Substrate Alternatives for Nitride Epitaxy