ElectronicMaterials - Lecture - GanSemiconductors, Lecture notes of Engineering Physics

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Gallium Nitride

Overview

 Semiconductor Materials

 Motivation for Interest in GaN

 Physical Properties

 Applications

 R&D (Companies & Universities)

 Methods for Processing

 Summary

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

What can lasers do?

 (^) Almost all current optical disc systems (CD and DVD) use GaAs lasers that emit light in the red or infrared part of the spectrum.  (^) CDs that hold ≈ 700MB of data storage, use a 780nm wavelength laser.  (^) DVDs with a 4.7GB capacity use a laser with a wavelength of ≈ 640nm.  (^) Blue lasers with a wavelength ≈ 405 nm (technology from Blu-ray and Advanced Optical Disc) can store between 23G bytes and 36G bytes per disc.  (^) Short Wavelength can write huge amounts of data.

Light Amplification by Stimulated Emission of Radiation (LASER)  (^) Diagnose cancer - ORNL has developed a blue laser-based technique for locating tumors in the intestinal tract by threading an endoscope into the patient's stomach or colon and shining a blue light. Cancerous and precancerous cells fluoresce differently 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 - Surgeons use lasers as scalpels. Lasers are also used to pulverize gallstones and clear clogged arteries. Ophthalmologists use them to repair damaged retinas and blood vessels in the eye. Dentists use lasers to drill teeth and harden fillings.  (^) Military - Laser targeting guides many of the new smart weapons.  (^) Science - Lasers are used to make a variety of ultraprecise measurements and image supersmall chemical and biological processes. Characterization & metrology

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 a high melting temperature and a very high decomposition pressure.  (^) The nitrogen evaporates out of the crystal as it grows and the gallium 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

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) Cost Availability

Applications

 (^) DVD Player/Recorder  (^) Optical data storage system  (^) LEDs  (^) Powerful laser diode  (^) Field Effect Transistor (FET)  (^) Signs and signals  (^) Mobile phones  (^) Lighting  (^) UV emitters  (^) Military and aerospace  (^) Automotive  (^) Industrial  (^) Communication systems

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 large dislocation density 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 and reacts with the ammonia gas that 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

SEM image of GaN film grown at 750 °C;

photoelectrochemically etched to reveal the dislocations.

Defects

 Dislocations can affect device performance and lifetime.

 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

108 /cm^2 to 10^9 /cm^2 and 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).

Military Interest

 (^) Radar & Satellite comm links operating at frequencies ranging from 100 MHz to 90 GHz have large power requirements  (^) No current technology can cope with these frequencies and power demands.  (^) GaN Transistors can withstand extreme heat; Rugged  (^) Currently amplifiers are using Si technology that is roughly 10% efficient; 90% of the power that goes into a transistor is wasted as heat. This means powerful fans and complex circuitry to correct for distortions.  (^) GaN can improve amplifier efficiency to 20 or 30%;

2002 Transistor Power Densities

 (^) GaN transistors can sustain power densities above 10 W/mm of gate width, while amplifying signals at 10 GHz.  (^) Si-based transistors can efficiently amplify signals up to 2-3 GHz.  (^) SiC (experimental devices at Cree) achieved 7.2 W/mm, but at frequencies no higher than 3.5 GHz.  (^) GaAs transistors can handle 10 GHz but withstand a power density of less than 1 W/mm at that frequency.  (^) SiGe devices can handle even higher 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 ).