1. Gallium Nitride Lisa Reiner April 20, 2005 MSE 630 2. Overview Semiconductor Materials Motivation for Interest in GaN Physical Properties ApplicationsR&D (Companies & Universities) Methods for ProcessingSummary 3. Semiconductor Materials Silicon SiC( 2.86 eV ) Silicon on Sapphire GaAs AlGaAs InGaAs InAlAs InP ZnSe( 2.7 eV ) ZnS( 3.6 eV ) AlGaNGaN ( 3.4 eV ) 4. Motivation General Electric, Philips and Sylvania have spent years trying to develop a pure white LED to replace conventional lighting sources (incandescent, halogen and fluorescent).LEDs are smaller, longer lasting & less expensive light sources.White light (achromatic) requires a combination of ( complementary monochromatic ) colors.Laser diodes 5. What can lasers do? Almost all current optical disc systems (CD and DVD) use GaAs lasers that emit light in thered or infraredpart of the spectrum.CDs that hold ≈700MBof data storage, use a780nmwavelength laser.DVDswith a4.7GBcapacity use a laser with a wavelength of ≈640nm . Blue laserswith a wavelength≈405 nm(technology from Blu-ray and Advanced Optical Disc) can store between23G bytes and 36G bytesper disc.Short Wavelength can writehuge amounts of data. 6. 7. L ightA mplification byS timulatedE mission ofR adiation (LASER) Diagnose cancer -ORNL has developed a blue laser-based technique forlocating tumorsin the intestinal tract by threading an endoscope into the patient's stomach or colon andshining abluelight .Cancerousand precancerouscells fluorescedifferently in this light than do healthy cells, making them easier to spot. Detect chemical and biological weapons -Blue lasers cause certain otherwise invisible chemical and biological agents to fluoresce.Build better printers -Blue laser printers will have at least twice the resolution of today's best models.Medicine/Dentistry - Surgeonsuse lasers as scalpels. Lasers are also used to pulverize gallstones and clear clogged arteries.Ophthalmologistsuse them to repair damaged retinas and blood vessels in the eye.Dentistsuse lasers to drill teeth and harden fillings.Military -Laser targeting guides many of the newsmart weapons .Science -Lasers are used to make a variety of ultraprecise measurements and image supersmall chemical and biological processes.Characterization & metrology 8. 9. Physical Properties Environmentally friendly compared to Arsenic High melting point Bandgap EHP recombination-> blueorUV lightPhoton Emission 10. How to Exploit GaN? What process can be used to create wafers?Standard techniques (Czochralski, Bridgeman, Float Zone) used to make single crystal wafers (GaAs & Si) don't work for GaN.GaN has ahigh melting temperatureand avery high decomposition pressure. The nitrogen evaporates out of the crystal as it grows and thegallium nitrogen atoms won't bond .To keep the nitrogen in, you'd need very high pressures ( more than 1000 MPa ), which are difficult to achieve in a commercial process.Chemical interactions between materials 11. GaN wafers? GaN is difficult to grow.Suitable substrate for epitaxial growth.Factors determining appropriateness include: Crystallography (lattice mismatch) Physical (thermal expansion coefficients, dislocation density) Chemistry (reactions & evaporation) CostAvailability 12. History Japan ( Shuji Nakamura, now at UCSB ) developed the1 st green ,blue ,violet& white LEDs with GaN semiconductors (epitaxialMOCVDon asapphire substrate-1993) the 1 st blue -light semiconductor laser (1995) LEDs are now used in traffic lights, billboards, flashlights 13. Applications DVD Player/Recorder Optical data storage system LEDs Powerful laser diode Field Effect Transistor (FET) Signs and signalsMobile phonesLightingUV emitters Military and aerospaceAutomotiveIndustrialCommunication systems 14. Existing Technology ShortcomingsGaN on Sapphire (lasers):huge lattice mismatch with GaN (-13% misfit).It creates stress in the GaN crystal that causes the GaN atoms to misalignVery largedislocationdensity in GaN epitaxial films on sapphire.Threading dislocations prevalent Poor reliability Low production yield Low power output GaAs( melts at 1238 ºC ) growing GaN on top of GaAs requires a temperature higher than 1000 ºC, too close to GaAs melting point, the material is very soft andreacts with the ammonia gasthat supplies the nitrogen needed to form GaN.SiC mismatch is only -3.1% to GaN TiO 2 ZnO good lattice match, ideal structure, but reacts with gallium & hard to obtain MgAl 2 O 4(spinel) MgO The (111) face of MgO is mismatched by -6.4% to GaN 15. TEM Micrograph showing a distribution ofdislocationsat grain boundaries in Gallium Nitride grown on Sapphire 16. SEM image of GaN film grown at 750 °C; photoelectrochemically etched to reveal the dislocations. 17. Defects Dislocations can affectdevice performanceandlifetime .Electrons can collide with dislocations causing the electrons to recombine with holes without creating photons; destroying the lasing action (charge trapping). Laser diodes built on a layer of GaN (directly grown) on a sapphire substrate can have dislocation densities of 10 8 /cm 2to 10 9 /cm 2and lifetimes of less than 100 hours. ( That's not good enough for DVD players )The real breakthrough in laser technology was the dramatic improvement of the LD lifetime in 1997 (10000 hours). 18. R&D GaN has been the subject of intensive research and product development for the past 12 years.UCSB, Chalmers, Cornell, Rensselaer Hitachi, Matsushita, Samsung, Sumitomo HRL, GE Lucent Technology DARPA, DOD, ONR, BMDONorthrop Grumman, Raytheon, Boeing Wide band gap semiconductor technology initiative 19. Military Interest Radar & Satellite comm links operating at frequencies ranging from 100 MHz to 90 GHz have largepowerrequirements No current technology can cope with these frequencies and power demands. GaN Transistors canwithstand extreme heat;Rugged Currently amplifiers are usingSi technologythat is roughly10% efficient ;90%of the power that goes into a transistor iswasted as heat . This means powerful fans and complex circuitry to correct for distortions. GaN can improve amplifier efficiency to 20 or 30%; 20. 2002 Transistor Power Densities GaNtransistors can sustain power densities above10 W/mmof gate width, while amplifying signals at10 GHz. Si-basedtransistors can efficiently amplify signals up to2-3 GHz. SiC( experimental devices at Cree ) achieved7.2 W/mm , but at frequencies no higher than3.5 GHz . GaAstransistors can handle10 GHzbut withstand a power density of less than1 W/mmat that frequency.SiGedevices can handle evenhigher frequencies , cannot withstand high power.Capable of handling frequencies and power levels well beyond those of Si, GaAs, SiC ( important factors for amplifiers, modulators & advanced comm networks ). 21. Thick GaN layers were grown by hydride vapor phase epitaxy (HVPE); the original LiAlO 2substrate is subsequently removed resulting in a free standing GaN wafer. 22. Processing Techniques Hydride vapor phase epitaxy( HVPE)one of the primary means of growingthickGaN films with low defect densities. Samsung has reported defect densities of thick films (750 μm) to be on the order of 2-3(10 6 ) /cm 2in 2000.Metal Organic Chemical Vapor Deposition( MOCVD)one of the most commonly used processes to grow GaNthin films. The basic MOCVD reaction describing the GaN deposition process is: Ga(CH 3 ) 3 + NH 3->GaN Molecular Beam Epitaxy (MBE)This method of epitaxial growth uses solid, elemental sources for the precursors and Ultra High Vacuum (UHV) to reduce the background impurity concentration. 23. Dislocations GaN grows into a hexagonal pyramid shape, resulting in low dislocation density. As the pyramids grow they merge, forming depressions, or pits, where dislocations concentrate.Motoki developed a technique for producing regions with very low dislocation density by forcing the dislocations into a small area, leaving regions of low dislocation density elsewhere.Developed a method for positioning the location of the pits to leave larger areas relatively free from dislocations.By 2002, able to produce areas over100 µm wide and more than 500 µm longwith dislocation densities of less than 2 x 10 5 /cm 2 --an area big enough for a laser diode.Defect densities of GaN on sapphire substrates are down to about 5 x 10 6 /cm 2 . 24. GaN wafers 50 mm wafer GaN = $10000 300 mm wafer Si = $200 Roughly 1000 diodes can be made on a 50 mm wafer. Sony DVD recorder costs $3800 25. Laser diodes 26. Summary The US & many other governments continue to heavily finance development of compound semiconductor technology fordefense applications ,environmental motivation , commercial practicality.GaN devices are capable of handlingfrequenciesandpower levelswell beyond those of Si, GaAs, SiC (important factors for amplifiers ,modulators&advanced communication networks ). 27. $$$$ Blue ,greenand white LED technology has continued its large-scale commercial growth resulting in revenues exceeding$3 billionin 2004.White LEDs are responsible for over 50% of the total GaN related LED market.Sales for Blue laser diodes & LEDs are expected to reach$4.7 billionby 2007. Industry experts estimate the market for blue lasers in the next generation of DVD and CD players alone will exceed$1 billion .