Wide Band-gap Semiconductor Group/Rensselaer Blue Semiconductor Lasers Leo J. Schowalter Physics,...

Wide Band-gap Semiconductor Group/Rensselaer

Blue Semiconductor Lasers

Leo J. Schowalter

Physics, Applied Physics & Astronomy Department

Rensselaer 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 Applications for Wide band gaps What is the Future?


Why the Interest?


Importance of new semiconductor materials and devices for modern civilizationPaul Romer (1990s)

The wealth is created by innovations and inventions, such as computer chips.

106 - 107 MOSFETs per person in Western World

Electronics industry is 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

The Market for GaN Devices

19%

20%

17%

12%

7%5%

4%3%3%2%0

500

1000

1500

2000

2500

3000

3500

1997 1998 1999 2000 2001 2002 2003 2004 2005 2006

Year

Sla

es

of

GaN

De

vice

s (

US

$ M

illio

ns

)

After Strategies Unlimited (1997)

Nichia estimates that the LD 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 greater

•Market expects laser cost to be approx. $10.


What is a semiconductor?

Metals Many free electrons not tied up in chemical bonds

Insulators All electrons (in intrinsic material) tied up in chemical

bonds


Crystal (Perfect)


Crystal (Excited)


Band Gap

Valence Band

Conduction Band

Band Gap Energy Eg

(Minimum Energy needed tobreak the chemical bonds)

Energy

Position


Band Gap

Valence Band

Conduction Band

Energy

Position

photon in h Eg


Band Gap

Valence Band

Conduction Band

Energy

Position

photon out


Crystal (Doped n-type)

+5 +5Plus a little energy,

d.


Crystal (Doped p-type)

+3


Doped SemiconductorsEnergy

n-type p-type

donor level

acceptor level

Put them together?


p-n junction

n-type

p-type

Energy

depleted region(electric field)

+ + + + + ++

+

--------


p-n junction

n-type

p-type

Energy


+ + + + + ++

+

--------

Vo


What happens if a bias is applied?

Biased junction

n-type

p-type


Negativebias

positivebias

Biased junction

n-type

p-type


Negativebias

photon out


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How does a semiconductor laser work?


Absorption and Emission

Eo

E1

n

nE E1

01 0exp[ ( )]

photon out

photonin

1 / k TB


Stimulated vs. Spontaneous Emission

We can now derive the ratio of the emission rate versus the absorption rate using the equilibrium concentrations of photons and excited atoms:

w

w

n p

n pemis

abs

( , )

( , ).

1

Derived in 1917 by Einstein. Required stimulated emission. However, a “real” understanding of this was not achieved until the 1950’s.


Laser needs a Population Inversion

Biased junction

n-type

p-type


Negativebias

photon out


History of Lasers

First operating Laser in 1960 (Maser in 1958) Simulated emission concept from Einstein in 1905 Townes (1964) and Schawlow (1981)

First semiconductor injection Laser in 1962 First was Robert Hall (GE) but many competing

groups Year before he had argued it was impossible


Violet Laser Diode

Currently costs about $2000 apiece!


Nichia Laser Diode

Epitaxial Lateral Overgrowth material

10,000 hoursoperation!

10 mW CW405 nm

SiO2

n-contact

p-contact

p-GaN

n-GaN

sapphire substrate

p-Al0.15Ga0.85 N

n-Al0.15Ga0.85N

n-GaN blockinglayers

Active p-GaN/InGaNMQW


Substrate Substrate ComparisonComparison

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, 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, the same crystal structure, excellent chemical match, high thermal conductivity, and the excellent chemical match, high thermal conductivity, and the same thermal expansionsame thermal expansion but are difficult to produce presently (but this 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 devices Large band gap allows semiconductor to be used at

high voltages Generally larger band gap means stronger bonds so

material can withstand higher currents and temperatures

High Temperature devices Much smaller effect of thermal excitation of carriers Tougher material


Conclusions

Very intense and fast moving field

Physicists are making major contributions

Lots more to do

Very broad applications but information storage is one of the biggest.


Questions1. 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 developed 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 aren't being used at the moment, but could be employed in the future?

3. I would like to know if Dr. Schowalter thinks the 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. Schowalter, 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 technology 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 technology 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 there was no stimulated emission, population levels of the two energies would be equal.

Principal of detailed balance says:

Stimulated vs. Spontaneous Emission

n w n wemis abs1 0 Minimum packet of energy (photon) that light

can have at a particular frequency is h (Plank’s constant, 1901).

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, 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 the same crystal structure, excellent chemical match, high thermal conductivity, and the same thermal expansionmatch, 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 Substrate Alternatives for Nitride EpitaxyNitride Epitaxy

GaN AlN 4H-SiC 6H-SiC Sapphire

Crystal Structure hexagonal(2H)

hexagonal(2H)

Hexagonal(4H)

Hexagonal(6H)

rhombohedral

3.39 6.2 3.26 3.03 9.9a=3.189c=5.185

a=3.111c=4.978

a=3.073c=10.053

a=3.081c=15.117

a=4.76c=12.99

Thermal Conductivity (W/cm-K)

1.7 3.2 4.9 4.9 0.35

Band Gap (eV) o

Lattice Constant(A)

Wide Band-gap Semiconductor Group/Rensselaer Blue Semiconductor Lasers Leo J. Schowalter Physics,...

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