Guest Lecture for ScIT
Blue Semiconductor Lasers
Leo J. Schowalter
Physics, Applied Physics & Astronomy
Department
Rensselaer Polytechnic Institute
Wide Band-gap Semiconductor Group/Rensselaer
Topics
 Why the Interest?
 What is a semiconductor?
 Metals, insulators and semiconductors
 How big a band gap energy?
 How does a semiconductor laser work?
 Other Applications for Wide band gaps
 What is the Future?
Wide Band-gap Semiconductor Group/Rensselaer
Why the Interest?
Wide Band-gap Semiconductor Group/Rensselaer
Importance of new semiconductor
materials and devices
for modern civilization
Paul 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
Wide Band-gap Semiconductor Group/Rensselaer
Impact
Automotive industry
Displays
Avionics and defense
Information technology
Solid state lighting
Traffic lights
Wireless
communications
Electric power industry
Health care
Wide Band-gap Semiconductor Group/Rensselaer
The Market for GaN Devices
After Strategies
Unlimited (1997)
The Market for GaN Devices
Slaes of GaN Devices
(US $ Millions)
3500
3000
2500
20%
% of Compound
Semiconductor market
19%
17%
2000
1500
12%
1000
500
2%
3% 3%
4%
5%
7%
0
1997 1998 1999 2000 2001 2002 2003 2004 2005 2006
Year
Nichia estimates
that the LD market
alone will be
worth $10B.
Wide Band-gap Semiconductor Group/Rensselaer
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.
Wide Band-gap Semiconductor Group/Rensselaer
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
Wide Band-gap Semiconductor Group/Rensselaer
Crystal (Perfect)
Wide Band-gap Semiconductor Group/Rensselaer
Crystal (Excited)
Wide Band-gap Semiconductor Group/Rensselaer
Crystal (Excited)
Wide Band-gap Semiconductor Group/Rensselaer
Band Gap
Energy
Conduction Band
Band Gap Energy Eg
(Minimum Energy needed to
break the chemical bonds)
Valence Band
Position
Wide Band-gap Semiconductor Group/Rensselaer
Band Gap
Energy
Conduction Band
h  Eg
photon
in
Valence Band
Position
Wide Band-gap Semiconductor Group/Rensselaer
Band Gap
Energy
Conduction Band
photon out
Valence Band
Position
Wide Band-gap Semiconductor Group/Rensselaer
Band Gap
Energy
Conduction Band
photon out
Valence Band
Position
Wide Band-gap Semiconductor Group/Rensselaer
Crystal (Doped n-type)
+5
Plus a
little
energy,
+5
d.
Wide Band-gap Semiconductor Group/Rensselaer
Crystal (Doped p-type)
+3
Wide Band-gap Semiconductor Group/Rensselaer
Crystal (Doped p-type)
+3
Wide Band-gap Semiconductor Group/Rensselaer
Doped Semiconductors
Energy
donor level
acceptor level
n-type
p-type
Put them together?
Wide Band-gap Semiconductor Group/Rensselaer
p-n junction
Energy
+
+
+
+
+
+
+
+
--
-
-
-
-
-
-
p-type
n-type
depleted region
(electric field)
Wide Band-gap Semiconductor Group/Rensselaer
p-n junction
Energy
+
+
+
+
+
+
+
Vo
+
--
-
-
-
-
-
-
p-type
n-type
depleted region
(electric field)
Wide Band-gap Semiconductor Group/Rensselaer
What happens if a bias is applied?
Wide Band-gap Semiconductor Group/Rensselaer
Biased junction
Negative
bias
positive
bias
p-type
n-type
depleted region
(electric field)
Biased junction
Negative
bias
photon out
p-type
n-type
depleted region
(electric field)
a Philips Lighting and Agilent
Technologies joint venture that's
changing the future of light. In the next
century, LED-based lighting will
quickly replace conventional lighting
in a wealth of commercial, industrial
and consumer applications.
LumiLeds‘ LED-based solutions will
bring irresistible value to lighting
solutions of all kinds, earning us a
leadership position in a fast-growing
and lucrative marketplace. Our longlasting, energy-efficient products will
also improve the planet, by reducing
waste and power consumption.
Wide Band-gap Semiconductor Group/Rensselaer
How does a semiconductor laser
work?
Wide Band-gap Semiconductor Group/Rensselaer
Absorption and Emission
E
photon
in
1
 n1 
 exp[ ( E1  E0 )]
 n0 
  1/ kBT
photon out
Eo
Wide Band-gap Semiconductor Group/Rensselaer
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:
wemis
wabs


n( p,  )  1

.

n( p,  )
Derived in 1917 by Einstein. Required
stimulated emission. However, a “real”
understanding of this was not achieved until
the 1950’s.
Wide Band-gap Semiconductor Group/Rensselaer
Laser needs a
Population Inversion
Wide Band-gap Semiconductor Group/Rensselaer
Biased junction
Negative
bias
photon out
p-type
n-type
depleted region
(electric field)
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
Wide Band-gap Semiconductor Group/Rensselaer
Violet Laser
Diode
Currently costs about $2000
apiece!
Wide Band-gap Semiconductor Group/Rensselaer
Nichia Laser
Diode
p-contact
n-GaN blocking
layers
Active p-GaN/InGaN
MQW
p-GaN
n-Al 0.15Ga0.85N
SiO2
n-contact
10,000 hours
operation!
p-Al 0.15Ga0.85 N
n-GaN
sapphire substrate
10 mW CW
405 nm
Epitaxial Lateral Overgrowth material
Wide Band-gap Semiconductor Group/Rensselaer
Comparison
 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 match, high thermal conductivity, and the
same 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?
Wide Band-gap Semiconductor Group/Rensselaer
Boule
Wide Band-gap Semiconductor Group/Rensselaer
Wide Band-gap Semiconductor Group/Rensselaer
How information
is stored on a
DVD disc
Wide Band-gap Semiconductor Group/Rensselaer
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
Wide Band-gap Semiconductor Group/Rensselaer
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.
Wide Band-gap Semiconductor Group/Rensselaer
Questions
1. 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.
Wide Band-gap Semiconductor Group/Rensselaer
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?
Wide Band-gap Semiconductor Group/Rensselaer
Stimulated vs. Spontaneous
Emission
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:
 n1  wemis  n0  wabs

Minimum packet of energy (photon) that light
can have at a particular frequency  is h
(Plank’s constant, 1901).
Wide Band-gap Semiconductor Group/Rensselaer
Substrate Alternatives for
Nitride Epitaxy
Crystal Structure
Band Gap (eV)
o
Lattice Constant(A)
Thermal Conductivity
(W/cm-K)
GaN
AlN
hexagonal
(2H)
3.39
a=3.189
c=5.185
1.7
hexagonal
(2H)
6.2
a=3.111
c=4.978
3.2
4H-SiC
6H-SiC
Hexagonal
(4H)
3.26
a=3.073
c=10.053
4.9
Hexagonal
(6H)
3.03
a=3.081
c=15.117
4.9
Sapphire
rhombohedral
9.9
a=4.76
c=12.99
0.35
 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
match, 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.