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1、Blue Semiconductor Lasers,Leo J.SchowalterPhysics,Applied Physics&Astronomy DepartmentRensselaer Polytechnic Institute,Guest Lecture for ScIT,Topics,Why the Interest?What is a semiconductor?Metals,insulators and semiconductorsHow big a band gap energy?How does a semiconductor laser work?Other Applic
2、ations for Wide band gapsWhat is the Future?,Why the Interest?,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 i
3、s now the largest industry in the US,Impact,Automotive industry,Avionics and defense,Traffic lights,Solid state lighting,Electric power industry,Health care,Information technology,Wirelesscommunications,Displays,The Market for GaN Devices,After Strategies Unlimited(1997),Nichia estimates that the LD
4、 market alone will be worth$10B.,%of Compound Semiconductor market,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 greaterMarket expects laser cost to be app
5、rox.$10.,What is a semiconductor?,MetalsMany free electrons not tied up in chemical bondsInsulatorsAll electrons(in intrinsic material)tied up in chemical bonds,Crystal(Perfect),Crystal(Excited),Crystal(Excited),Band Gap,Valence Band,Conduction Band,Band Gap Energy Eg(Minimum Energy needed tobreak t
6、he chemical bonds),Energy,Position,Band Gap,Valence Band,Conduction Band,Energy,Position,photon in,Band Gap,Valence Band,Conduction Band,Energy,Position,Band Gap,Valence Band,Conduction Band,Energy,Position,Crystal(Doped n-type),Plus a little energy,d.,Crystal(Doped p-type),+3,Crystal(Doped p-type),
7、+3,Doped Semiconductors,Energy,n-type,p-type,donor level,acceptor level,Put them together?,p-n junction,n-type,Energy,depleted region(electric field),+,+,+,+,+,+,+,+,-,-,-,-,-,-,-,-,p-n junction,n-type,Energy,depleted region(electric field),+,+,+,+,+,+,+,+,-,-,-,-,-,-,-,-,Vo,What happens if a bias i
8、s applied?,Biased junction,n-type,depleted region(electric field),Negativebias,positivebias,Biased junction,n-type,depleted region(electric field),Negativebias,a Philips Lighting and Agilent Technologies joint venture thats changing the future of light.In the next century,LED-based lighting will qui
9、ckly 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-efficie
10、nt products will also improve the planet,by reducing waste and power consumption.,How does a semiconductor laser work?,Absorption and Emission,E,o,E,1,photon out,photonin,Stimulated vs.Spontaneous Emission,We can now derive the ratio of the emission rate versus the absorption rate using the equilibr
11、ium 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 1950s.,Laser needs a Population Inversion,Biased junction,n-type,depleted region(electric field),Negativebias,History of Lasers,Fi
12、rst operating Laser in 1960(Maser in 1958)Simulated emission concept from Einstein in 1905Townes(1964)and Schawlow(1981)First semiconductor injection Laser in 1962First was Robert Hall(GE)but many competing groupsYear before he had argued it was impossible,Violet Laser Diode,Currently costs about$20
13、00 apiece!,Nichia Laser Diode,Epitaxial Lateral Overgrowth material,10,000 hoursoperation!,10 mW CW405 nm,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
14、,also has:a different c-axis,relatively large thermal expansion mismatch and chemical mismatch at the interface.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
15、will change!)LEO and HVPE GaN films allow fabrication of“quasi-bulk”substrates.Temporary solution until bulk substrates become available?,15 mm Diameter AlN Boule,How information is stored on a DVD disc,Other Applications for Wide band gaps,High Power devicesLarge band gap allows semiconductor to be
16、 used at high voltagesGenerally larger band gap means stronger bonds so material can withstand higher currents and temperaturesHigh Temperature devicesMuch smaller effect of thermal excitation of carriersTougher material,Conclusions,Very intense and fast moving fieldPhysicists are making major contr
17、ibutionsLots more to doVery broad applications but information storage is one of the biggest.,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 develope
18、d 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 t
19、he 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.Schowalte
20、r,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.,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 te
21、chnology 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 tec
22、hnology 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?,Time invariant laws of Physics imply that the rate of absorption must be equal to the rate of spontaneous emission.Thus,if
23、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(Planks constant,1901).,Sapphire:poor crystal structure match
24、,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 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,
