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Wide Band-gap Semiconductor Group/RensselaerBlue Semiconductor LasersLeo J. SchowalterPhysics, Applied Physics & Astronomy DepartmentRensselaer Polytechnic InstituteGuest Lecture for ScITWide Band-gap Semiconductor Group/RensselaerTopicsWhy the Interest?What is a semiconductor?Metals, insulators and semiconductorsHow big a band gap energy?How does a semiconductor laser work?Other Applications for Wide band gapsWhat is the Future?Wide Band-gap Semiconductor Group/RensselaerWhy the Interest?Wide Band-gap Semiconductor Group/RensselaerImportance of new semiconductor materials and devices for modern civilizationPaul Romer (1990s)The wealth is created by innovations and inventions, such as computer chips.106 - 107 MOSFETs per person in Western WorldElectronics industry is now the largest industry in the USWide Band-gap Semiconductor Group/RensselaerImpactAutomotive industryAvionics and defenseTraffic lightsSolid state lightingElectric power industryHealth careInformation technologyWirelesscommunicationsDisplaysWide Band-gap Semiconductor Group/RensselaerThe Market for GaN DevicesThe Market for GaN Devices19%20%17%12%7%5%4%3%3%2%05001000150020002500300035001997 1998 1999 2000 2001 2002 2003 2004 2005 2006YearSlaes of GaN Devices (US $ Millions)After Strategies Unlimited (1997)Nichia estimates that the LD market alone will be worth $10B.% of Compound Semiconductor marketWide Band-gap Semiconductor Group/RensselaerLaser Diode MarketOptical 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 greaterMarket expects laser cost to be approx. $10.Wide Band-gap Semiconductor Group/RensselaerWhat is a semiconductor?MetalspMany free electrons not tied up in chemical bondsInsulatorspAll electrons (in intrinsic material) tied up in chemical bondsWide Band-gap Semiconductor Group/RensselaerCrystal (Perfect)Wide Band-gap Semiconductor Group/RensselaerCrystal (Excited)Wide Band-gap Semiconductor Group/RensselaerCrystal (Excited)Wide Band-gap Semiconductor Group/RensselaerBand GapValence BandConduction BandBand Gap Energy Eg(Minimum Energy needed tobreak the chemical bonds)EnergyPositionWide Band-gap Semiconductor Group/RensselaerBand GapValence BandConduction BandEnergyPositionphoton inhEgWide Band-gap Semiconductor Group/RensselaerBand GapValence BandConduction BandEnergyPositionphoton outWide Band-gap Semiconductor Group/RensselaerBand GapValence BandConduction BandEnergyPositionphoton outWide Band-gap Semiconductor Group/RensselaerCrystal (Doped n-type)+5+5Plus a little energy,d.Wide Band-gap Semiconductor Group/RensselaerCrystal (Doped p-type)+3Wide Band-gap Semiconductor Group/RensselaerCrystal (Doped p-type)+3Wide Band-gap Semiconductor Group/RensselaerDoped SemiconductorsEnergyn-typep-typedonor levelacceptor levelPut them together?Wide Band-gap Semiconductor Group/Rensselaerp-n junctionn-typep-typeEnergydepleted region(electric field)+-Wide Band-gap Semiconductor Group/Rensselaerp-n junctionn-typep-typeEnergydepleted region(electric field)+-VoWide Band-gap Semiconductor Group/RensselaerWhat happens if a bias is applied?Biased junctionn-typep-typedepleted region(electric field)NegativebiaspositivebiasBiased junctionn-typep-typedepleted region(electric field)Negativebiasphoton outWide Band-gap Semiconductor Group/Rensselaera Philips Lighting and Agilent Technologies joint venture thats 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.Wide Band-gap Semiconductor Group/RensselaerHow does a semiconductor laser work?Wide Band-gap Semiconductor Group/RensselaerAbsorption and EmissionEoE1nnEE1010exp()photon outphotonin 1 / k TBWide Band-gap Semiconductor Group/RensselaerStimulated vs. Spontaneous EmissionWe can now derive the ratio of the emission rate versus the absorption rate using the equilibrium concentrations of photons and excited atoms:wwn pn pemisabs( , )( , ).1lDerived in 1917 by Einstein. Required stimulated emission. However, a “real” understanding of this was not achieved until the 1950s.Wide Band-gap Semiconductor Group/RensselaerLaser needs a Population InversionBiased junctionn-typep-typedepleted region(electric field)Negativebiasphoton outWide Band-gap Semiconductor Group/RensselaerHistory of LasersFirst operating Laser in 1960 (Maser in 1958)pSimulated emission concept from Einstein in 1905pTownes (1964) and Schawlow (1981)First semiconductor injection Laser in 1962pFirst was Robert Hall (GE) but many competing groupspYear before he had argued it was impossibleWide Band-gap Semiconductor Group/RensselaerViolet Laser DiodeCurrently costs about $2000 apiece!Wide Band-gap Semiconductor Group/RensselaerNichia Laser DiodeEpitaxial Lateral Overgrowth material10,000 hoursoperation!10 mW CW405 nmWide Band-gap Semiconductor Group/Rensselaer 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 AlN bulk crystals have 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? Wide Band-gap Semiconductor Group/Rensselaer15 mm Diameter AlN BouleWide Band-gap Semiconductor Group/RensselaerWide Band-gap Semiconductor Group/RensselaerHow information is stored on a DVD discWide Band-gap Semiconductor Group/RensselaerOther Applications for Wide band gapslHigh Power devicespLarge band gap allows semiconductor to be used at high voltagespGenerally larger band gap means stronger bonds so material can withstand higher currents and temperatureslHigh Temperature devicespMuch smaller effect of thermal excitation of carrierspTougher material Wide Band-gap Semiconductor Group/RensselaerConclusionsVery intense and fast moving fieldPhysicists are making major contributionsLots more to doVery broad applications but information storage is one of the biggest.Wide Band-gap Semiconductor Group/RensselaerQuestions1. 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 arent 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.Wide Band-gap Semiconductor Group/RensselaerQuestions (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?Wide Band-gap Semiconductor Group/RensselaerTime 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 Emissionnwnwemisabs10lMinimum packet of energy (photon) that light can have at a particular frequency is h (Planks constant, 1901). 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 AlN bulk crystals have but are difficult to produce presently (will this change?). LEO and free-standing GaN films more expensive than bulk crystal substrates.GaNAlN4H-SiC6H-SiCSapphireCrystal Structurehexagonal(2H)hexagonal(2H)Hexagonal(4H)Hexagonal(6H)rhombohedral3.396.23.263.039.9a=3.189c=5.185a=3.111c=4.978a=3.073c=10.053a=3.081c=15.117a=4.76c=12.99Thermal Conductivity (W/cm-K)1.73.24.94.90.35Band Gap (eV) oLattice Constant(A)
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