Gallium Nitride

Lisa ReinerApril 20, 2005

MSE 630

Overview

Semiconductor Materials Motivation for Interest in GaN Physical Properties Applications R&D (Companies & Universities) Methods for Processing Summary

Semiconductor Materials Silicon SiC (2.86 eV) Silicon on Sapphire GaAs AlGaAs InGaAs InAlAs InP ZnSe (2.7 eV) ZnS (3.6 eV) AlGaN GaN (3.4 eV)

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

Physical Properties

Environmentally friendly compared to Arsenic High melting point Bandgap EHP recombination → blue or UV light Photon Emission

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

History

Japan (Shuji Nakamura, now at UCSB) developed the

1st green, blue, violet & white LEDs with GaN semiconductors (epitaxial MOCVD on a sapphire substrate -1993)

the 1st blue-light semiconductor laser (1995)

LEDs are now used in traffic lights, billboards, flashlights

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.

SiCmismatch is only -3.1% to GaN

TiO2

ZnOgood lattice match, ideal structure, but reacts with gallium & hard to obtain

MgAl2O4 (spinel) MgO

The (111) face of MgO is mismatched by -6.4% to GaN

TEM Micrograph showing a distribution of dislocations at grain boundaries in Gallium Nitride grown on Sapphire

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/cm2 to 109/cm2 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).

R&DGaN has been the subject of intensive research and product development for the past 12 years.

UCSB, Chalmers, Cornell, RensselaerHitachi, Matsushita, Samsung, SumitomoHRL, GELucent TechnologyDARPA, DOD, ONR, BMDO Northrop Grumman, Raytheon, Boeing

Wide band gap semiconductor technology initiative

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).

Thick GaN layers were grown by hydride vapor phase epitaxy (HVPE); the original LiAlO2 substrate is subsequently removed resulting in a free standing GaN wafer.

Processing Techniques Hydride vapor phase epitaxy (HVPE) one of the primary means of

growing thick GaN films with low defect densities. Samsung has reported defect densities of thick films (750 μm) to be on the order of 2-3(106) /cm2 in 2000.

Metal Organic Chemical Vapor Deposition (MOCVD) one of the most commonly used processes to grow GaN thin films. The basic MOCVD reaction describing the GaN deposition process is: Ga(CH3)3 + NH3 →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.

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 over 100 µm wide and more than 500 µm long with dislocation densities of less than 2 x 105/cm2--an area big enough for a laser diode.

Defect densities of GaN on sapphire substrates are down to about 5 x 106/cm2.

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

Laser diodes

Summary The US & many other governments

continue to heavily finance development of compound semiconductor technology for defense applications, environmental motivation, commercial practicality.

GaN devices are capable of handling frequencies and power levels well beyond those of Si, GaAs, SiC (important factors for amplifiers, modulators & advanced communication networks).

$$$$

Blue, green and white LED technology has continued its large-scale commercial growth resulting in revenues exceeding $3 billion in 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 billion by 2007.

Industry experts estimate the market for blue lasers in the next generation of DVD and CD players alone will exceed $1 billion.