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
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.
Japan ( Shuji Nakamura, now at UCSB ) developed the
1 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
Optical data storage system
Powerful laser diode
Field Effect Transistor (FET)
Signs and signals
Military and aerospace
14. Existing Technology Shortcomings
GaN on Sapphire (lasers):
huge lattice mismatch with GaN (-13% misfit).
It creates stress in the GaN crystal that causes the GaN atoms to misalign
Very largedislocationdensity in GaN epitaxial films on sapphire.Threading dislocations prevalent
Low production yield
Low power output
GaAs( melts at 1238 C )
growing GaN on top of GaAs requires a temperature higher than 1000 C, too close to GaAs melting point, the material is very soft andreacts with the ammonia gasthat supplies the nitrogen needed to form GaN.
mismatch is only -3.1% to GaN
good lattice match, ideal structure, but reacts with gallium & hard to obtain
MgAl 2 O 4(spinel)
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).
GaN has been the subject of intensive research and product development for the past 12 years.
UCSB, Chalmers, Cornell, Rensselaer
Hitachi, Matsushita, Samsung, Sumitomo
DARPA, DOD, ONR, BMDO
Northrop 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 ).
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.