Blue Semiconductor Lasers

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Blue Semiconductor Lasers
Guest Lecture for ScIT

• Leo J. Schowalter
• Physics, Applied Physics Astronomy Department
• Rensselaer Polytechnic Institute

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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?

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Why the Interest?
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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
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Impact
Displays
Avionics and defense
Automotive industry
Information technology
Solid state lighting
Traffic lights
Wireless communications
Electric power industry
Health care
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The Market for GaN Devices
After Strategies Unlimited (1997)
of Compound Semiconductor market
Nichia estimates that the LD market alone will be
worth 10B.
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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.

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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

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Crystal (Perfect)
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Crystal (Excited)
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Crystal (Excited)
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Band Gap
Energy
Conduction Band
Band Gap Energy Eg (Minimum Energy needed
to break the chemical bonds)
Valence Band
Position
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Band Gap
Energy
Conduction Band
photon in
Valence Band
Position
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Band Gap
Energy
Conduction Band
Valence Band
Position
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Band Gap
Energy
Conduction Band
Valence Band
Position
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Crystal (Doped n-type)
Plus a little energy, ?d.
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Crystal (Doped p-type)
3
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Crystal (Doped p-type)
3
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Doped Semiconductors
Energy
donor level
acceptor level
p-type
n-type

• Put them together?

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p-n junction
Energy


-
-
-
-
-
-






-
-
n-type
depleted region (electric field)
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p-n junction
Energy
Vo


-
-
-
-
-
-






-
-
n-type
depleted region (electric field)
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What happens if a bias is applied?
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Biased junction
Negative bias
positive bias
n-type
depleted region (electric field)
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Biased junction
Negative bias
n-type
depleted region (electric field)
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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
long-lasting, energy-efficient products will also
improve the planet, by reducing waste and power
consumption.
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How does a semiconductor laser work?
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Absorption and Emission
E
1
photon in
photon out
E
o
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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

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

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Laser needs a Population Inversion
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Biased junction
Negative bias
n-type
depleted region (electric field)
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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

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Violet Laser Diode
Currently costs about 2000 apiece!
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Nichia Laser Diode
10,000 hours operation!
10 mW CW 405 nm
Epitaxial Lateral Overgrowth material
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Substrate 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?

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15 mm Diameter AlN Boule
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(No Transcript)
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How information is stored on a DVD disc
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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

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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.

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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.

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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?
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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

• Minimum packet of energy (photon) that light can
  have at a particular frequency ? is h? (Planks
  constant, 1901).

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Substrate Alternatives for Nitride Epitaxy

• 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.